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2.0 Aluminium Alloy: Introduction to Aluminium and its alloys Aluminium is the world‘s most abundant metal and is the third most common element,

comprising 8% of the earth‘s crust. The versatility of aluminium makes it the most widely used

metal after steel. Although aluminium compounds have been used for thousands of years,

aluminium metal was first produced around 170 years ago. In the 100 years since the first

industrial quantities of aluminium were produced, worldwide demand for aluminium has grown

to around 29 million tons per year. About 22 million tons is new aluminium and 7 million tons is

recycled aluminium scrap. The use of recycled aluminium is economically and environmentally

compelling. It takes 14,000 kWh to produce 1 tonne of new aluminium. Conversely it takes only

5% of this to remelt and recycle one tonne of aluminium. There is no difference in quality

between virgin and recycled aluminium alloys. Pure aluminium is soft, ductile, corrosion

resistant and has a high electrical conductivity. It is widely used for foil and conductor cables,

but alloying with other elements is necessary to provide the higher strengths needed for other

applications. Aluminium is one of the lightest engineering metals, having strength to weight ratio

superior to steel. By utilising various combinations of its advantageous properties such as

strength, lightness, corrosion resistance, recyclability and formability, aluminium is being

employed in an ever-increasing number of applications. This array of products ranges from

structural materials through to thin packaging foils.

2.1 History of aluminium

Aluminium is a strongly electro-negative metal and possesses a strong affinity for oxygen; this is

apparent fi·om the high heat of formation of its oxide. For this reason, although it is among the

six most widely distributed metals on the surface of tbe earth, it was not isolated until well into

the nineteenth century.

Alumina (Al2O3) was known, however, in the eighteenth century, and the first unsuccessful

attempts to isolate the metal were made by Sir Humphry Davy in 1807, when the isolation of the

alkali metals had made a powerful reducing agent available. It was not, however, until 1825 that

the Danish Worker, H.C. Oersted, succeeded in preparing aluminium powder by the reduction of

anhydrous aluminium chloride with sodium amalgam; two years later, F. Wohler replaced the

amalgam by potassium, and between 1827 and 1847 discovered and listed many of the chemical

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and physical properties. However, many years passed before the metal could be produced

commercially.

The father of the light metal industry was probably the French scientist, Henri Sainte-Claire

Deville, who in 1850 improved Wohler's method of preparation by replacing potassium by

sodium, and by using the double chloride of sodium and aluminium as his source of the metal,

thus making the production of aluminium a commercial proposition; the price of the metal,

however, was still comparable with that of gold.

The production of aluminium received a further impetus when Robert Bunsen and, following

him, Deville, showed how the metal could be produced electrolytically from its ores.

In 1885, the brothers Cowie produced the first aluminium alloys containing iron and copper,

soon after which the invention of the dynamo made a cheaper supply of electricity available and

resulted, in 1886, in Herault's and Hall's independent French and American patents for the

electrolytic production of aluminium from alumina and molten cryolite (AIF3NaF). Henceforth,

the production of aluminium in Europe centred round the first factory in Neuhausen, while Hall's

process was applied in the U.S.A. in Pittsburgh. Modem production of aluminium begins from

the mineral bauxite, which contains approximately 25% of aluminium. This is converted to

alumina by digestion with a solution of sodium hydroxide under pressure (the Bayer process),

and the purified alumina produced is added to a molten mixture of cryolite and fluorspar. This

mixture is electrolysed in a cell with carbon anodes and the molten mixture is tapped from the

bottom of the cell.

2.2 Properties

The major advantages of using aluminium are tied directly to its‘ remarkable properties. Some of

these properties are outlined in the following sections.

2.2.1 Mechanical Properties

Commercially pure aluminium has a tensile strength of about 90 MPa. Its usefulness as a

structural material in this form is thus somewhat limited. However, by working the metal, as by

cold rolling, its strength can be approximately doubled. Much larger increases in strength can be

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obtained by alloying aluminium with small percentages of one or more other metals such as

manganese, silicon, copper, magnesium or zinc. Like pure aluminium, the alloys are also made

stronger by cold working. Some of the alloys are further strengthened and hardened by heat

treatment so that today aluminium alloys having tensile strengths approaching 700 MPa have

been developed.

A wide variety of mechanical characteristics, or tempers, is available in aluminium alloys

through various combinations of cold working and heat treatment. In specifying the temper for

any given product, the fabricating process and the amount of cold work to which it will subject

the metal should be kept in mind. In other words, the temper specified should be such that the

amount of cold work the metal will receive during fabrication will develop the desired

characteristics in the finished product. At sub-zero temperatures aluminium alloys increase in

strength without loss of ductility or brittle fracture problems, so that aluminium is a particularly

useful metal for low-temperature applications including cryogenics.

Strength to Weight Ratio: Aluminium has a density around one third that of steel and is used

advantageously in applications where high strength and low weight are required. This includes

vehicles where low mass results in greater load capacity and reduced fuel consumption.

2.2.2 Corrosion Resistance

Whilst aluminium and its alloys generally have good corrosion resistance, localised forms of

corrosion can occur, and it is important to understand the factors contributing to these forms of

corrosion.

Corrosion may be defined as the reaction between a metal and its immediate environment, which

can be natural or chemical in origin. The most recognizable form of corrosion is, perhaps, the

rusting of iron. All metals react with natural environments but the extent to which this happens

can vary; for noble metals like gold the amount is insignificant whereas for iron it is

considerable. Aluminium is no exception but, fortunately, it has the propensity of self-

passivation and for many applications corrosion is not a problem.

When the surface of aluminium metal is exposed to air, a protective oxide coating forms almost

instantaneously. This oxide layer is corrosion resistant and can be further enhanced with surface

treatments such as anodising. The excellent corrosion resistance of pure aluminium is largely due

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to its affinity for oxygen; this results in the production of a very thin but tenacious oxide film

which covers the surface as soon as a freshly-cut piece of the metal is exposed to the atmosphere.

This oxide coating is of great significance in the production of practically every type of surface

finish for the metal. It is, of course, the basis of what is probably the most corrosion-resistant

finish of all, namely, that group of finishes which involves the technique of anodic oxidation in

its varied forms. Here, the natural film is, in effect, greatly thickened and strengthened by

electrochemical means.

Pure aluminium displays the highest corrosion resistance, but as purity decreases and alloying

elements are added this resistance decreases. Copper lowers resistance more than other elements,

whilst magnesium has the least effect. The influence of the main alloying elements and

impurities on the corrosion resistance of aluminium is summarised in Tables 2.1 and 2.2.

Table 2.1 General Effect of Major Alloying Elements on the Corrosion Resistance of

Aluminium

On the other hand, the tenacity of the natural oxide film is a serious adverse factor in the

production of other finishes, such as those based on electro-deposition, and also, but to a lesser

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extent, the organic finishes, as it must be removed or transformed before the alternative coating

can be successfully applied. Special techniques have had to be evolved to effect this.

The Oxide Film: When a freshly formed aluminium metal surface is exposed to the atmosphere,

it is immediately covered with a thin film of oxide, and this oxide film quickly reforms when

damaged. An important and beneficial feature of this oxide film is that its molecular volume is

stoichiometrically 1.5 times that of the metal used up in oxidation. This then means that the

oxide film is under compressive stress, and will not only cover the metal continuously, but can

cope with a certain amount of substrate deformation without rupturing. It is to this protective

surface layer that the aluminium industry owes its existence.

Reports of the structure of this low temperature, air-formed film have varied widely although, in

general, it is assumed to be amorphous, with the outer surface being a hydrated aluminium oxide.

At higher temperatures (above 450oC), crystalline γ-Al2O3 is formed, and then, in the molten

state, α-Al2O3 can occur.

The kinetics of oxide growth on pure aluminium are complex. The currently accepted

mechanism has been described recently by Wefers. At ambient temperatures a limiting oxide

film thickness of 2 to 3 mn will be produced within one day; thermal oxidation is controlled by

diffusion of aluminium and oxygen ions at temperatures up to ~ 400oC and, in this temperature

range, asymptotically decaying rate laws are observed. However, when the temperature is raised

towards and above 450°C, the exponential oxidation rate changes to a linear relationship between

weight gain and time. This change in mechanism represents crystallisation to γ –Al2O3, which

will disrupt the continuity of the film. At temperatures above 500°C, it has been reported that the

preparation of the sheet, i.e. both metallurgical and surface roughness features, can alter the

oxidation kinetics.

2.2.3 Electrical and Thermal Conductivity

Aluminium is an excellent conductor of both heat and electricity. The great advantage of

aluminium is that by weight, the conductivity of aluminium is around twice that of copper. This

means that aluminium is now the most commonly used material in large power transmission

lines. The best alternatives to copper are aluminium alloys in the 1000 or 6000 series. These can

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be used for all electrical conduction applications including domestic wiring. Weight

considerations mean that a large proportion of overhead, high voltage power lines now use

aluminium rather than copper. They do however, have a low strength and need to be reinforced

with a galvanised or aluminium coated high tensile steel wire in each strand.

2.2.4 Light and Heat Reflectivity

Aluminium is a good reflector of both visible light and heat making it an ideal material for light

fittings, thermal rescue blankets and architectural insulation.

2.2.5 Toxicity

Aluminium is not only non-toxic but also does not release any odours or taint products with

which it is in contact. This makes aluminium suitable for use in packaging for sensitive products

such as food or pharmaceuticals where aluminium foil is used.

2.2.6 Recycling

The recyclability of aluminium is unparalleled. When recycled there is no degradation in

properties when recycled aluminium is compared to virgin aluminium. Furthermore, recycling of

aluminium only requires around 5 percent of the input energy required to produce virgin

aluminium metal.

The combination of two remarkable properties of aluminium makes the need to recycle the metal

obvious. These first of these factors is that there is no difference between virgin and recycled

aluminium. The second factor is that recycled aluminium only uses 5% of the energy required to

produce virgin material. Currently around 60% of aluminium metal is recycled at the end of its

lifecycle but this percentage can still be vastly improved.

Table 2.2 Advantages of Aluminium Metal and Products

Advantages of Aluminium The metal

Advantages of Aluminium Products

Light Attractive appearance

Strong Suitable for a wide range of finishes

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High strength to weight ratio Virtually seamless

Resilient / tough Easy to fabricate

Ductile at low temperatures Joinable by various methods

Corrosion resistant Suitable for complex, integral shapes

Non Toxic Suitable for easy assembly designs

Heat conducting Produced to precise, close tolerances

Reflective Produced with uniform quality

Electrically conducting Recyclable

Non magnetic Cost effective

Non sparking Provide freedom of design

2.3 APPLICATIONS

The properties of the various aluminium alloys has resulted in aluminium being used in

industries as diverse as transport, food preparation, energy generation, packaging, architecture,

and electrical transmission applications. Depending upon the application, aluminium can be used

to replace other materials like copper, steel, zinc, tin plate, stainless steel, titanium, wood, paper,

concrete and composites. Some examples of the areas where aluminium is used are given in the

following sections:

2.3.1 Packaging

Corrosion resistance and protection against UV light combined with moisture and odour

containment plus the fact that aluminium is non-toxic and will not leach or taint the products has

resulted in the widespread use of aluminium foils and sheet in food packaging and protection.

The most common use of aluminium for packaging has been in aluminium beverage cans.

Aluminium cans now account for around 15% of the global consumption of aluminium.

2.3.2 Transport

After the very earliest days of manned flight, the excellent strength to weight ratio of aluminium

has made it the prime material for the construction of aircraft. These same properties of

aluminium mean various alloys are now also used in passenger and freight rail cars, commercial

vehicles, military vehicles, ships & boats, buses & coaches, bicycles and increasingly in motor

cars. The sustainable nature of aluminium with regards to corrosion resistance and recyclability

has helped drive the recent increases in demand for aluminium vehicle components.

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2.3.3 Marine Applications

Aluminium plate and extrusions are used extensively for the superstructures of ships. The use of

these materials allows designers to increase the above waterline size of the vessel without

creating stability problems. The weight advantage of aluminium has allowed marine architects to

gain better performance from the available power by using aluminium in the hulls of hovercraft,

fast multi-hulled catamarans and surface planing vessels. Lower weight and longer lifecycles

have seen aluminium become the established material for helidecks and helideck support

structures on offshore oil and gas rigs. The same reasons have resulted in the widespread use of

aluminium in oil rig stair towers and telescopic personnel bridges.

2.3.4 Building and Architecture

Aluminium use in buildings covers a wide range of applications. The applications include

roofing, foil insulation, windows, cladding, doors, shop fronts, balustrading, architectural

hardware and guttering. Aluminium is also commonly used as the in the form of tread-plate and

industrial flooring.

2.3.5 Foils

Aluminium is produced in commercial foils as thin as 0.0065 mm (or 6.5 µm). Material thicker

than 0.2mm is called sheet or strip. Aluminium foil is impervious to light, gases, oils and fats,

volatile compounds and water vapour. These properties combined with high formability, heat

and cold resistance, non-toxicity, strength and reflectivity to heat and light mean aluminium foil

is used in many applications. These applications include:

~ Pharmaceutical packaging

~ Food protection and packaging

~ Insulation

~ Electrical shielding

~ Laminates

2.4 Other Applications of Aluminium Alloys

The above applications account for approximately 85% of the aluminium consumed annually.

The remaining 15% is used in a wide variety of applications including:

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~ Ladders

~ High pressure gas cylinders

~ Sporting goods

~ Machined components

~ Road barriers and signs

~ Furniture

~ Lithographic printing plates

2.5 Types of Aluminium Alloys

Some aluminium alloys are heat treatable, some are not. The non-heat treatable alloys contain

small amounts of elements such as manganese, silicon, iron and magnesium in solid solution.

The alloys can be strengthened by cold work or strain hardening, which is by rolling or drawing.

The heat treatable alloys contain elements such as copper, magnesium, zinc and silicon. The

first step in heat treatment is to heat the alloy to a high temperature (~300 – 500oC, depending on

the alloy) to take the alloying elements into solution. After cooling to room temperature, the

metal is reheated to 100 – 200oC to allow second phase particles to form in the microstructure,

which increases the strength (and reduces the ductility)

2.6 Designation of Aluminium Alloys

2.6.1 Families of Aluminum Alloys

There are several families of wrought aluminum alloys. Each family is based on specific major

alloying elements added to the aluminum. These alloying elements have a large influence on the

properties. The different families of alloys and the major alloying elements are:

1xxx: no alloying elements

2xxx: Copper

3xxx: Manganese

4xxx: Silicon

5xxx: Magnesium

6xxx: Magnesium and silicon

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7xxx: Zinc, magnesium, and copper

The first number in the alloy designation indicates the particular alloy family. Within each

family there are different alloys based on the amounts of the major alloying elements present

and the types and amounts of minor alloying elements that have been added. The XXX‘s are

used to indicate the different alloys in each family.

Example, for 1XX.X (Table 2.3), the number 1in the first digit stands for aluminium with high

purity. The second and third digits are used for minimum Al percentage. The fourth digit is used

after decimal point and it is 1 or 2 for cast ingot and 0 for casting.

Table 2. 3 Aluminum Association Numbering System

Wrought Alloys Cast Alloys

Aluminium (Commercially pure, Al > 99.00%)

1xxx Aluminium (Commercially pure, Al > 99.00%)

1xx.x

Copper 2xxx Copper 2xx.x

Manganese 3xxx Silicon, + copper &/or magnesium 3xx.x

Silicon 4xxx Silicon 4xx.x

Magnesium 5xxx Magnesium 5xx.x

Magnesium & silicon 6xxx Unused series 6xx.x

Zinc 7xxx Zinc 7xx.x

Other elements 8xxx Tin 8xx.x

Unused series 9xxx Other elements 9xx.x

2.6.2 Temper Designation System

F: As fabricated. For wrought products, there are no mechanical property limits.

O: Annealed, recrystallised. Softest temper of the wrought products

H: Strain hardened. Strength increased by cold work, which may be followed by a heat

treatment for partial softening. The H is always followed by two or three digits indicating

the treatment and result:

H1: Strain hardened only, no subsequent heat treatment.

H2: Strain hardened, then partially annealed to reduce their strength.

H3: Strain hardened, then heat treated to stabilize the strength – used for alloys

containing magnesium only. The second digit indicates the strength level

achieved for the alloy:

0: Annealed 1: 1/8th

Hard 2: ¼ Hard 3: 3/8th

Hard

4: ½ Hard 5: 5/8th

Hard 6: ¾ Hard 7: 7/8th

Hard

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8: Full Hard (~75% reduction of area in cold work) 9: Extra Hard

e.g. A95052 H34 = alloy 5052, strain hardened and stabilized to half hard, which for this alloy is

0.2% Proof Stress 180 MPa minimum, Tensile Strength 235 – 285 MPa, Elongation 3 – 8%.

Note the strength achieved by the different alloys for e.g. H34 depends on the composition of the

alloy, so is different for each alloy.

Table 2.3: Temper Designation

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The strength of aluminum alloys can be modified through various combinations of cold

working, alloying, and heat treating. All the alloys can be strengthened by cold working

processes such as cold rolling or wire drawing. Except for the 1xxx alloys, additional strength

can be obtained by solid solution strengthening, dispersion strengthening, and precipitation

strengthening. The particular strengthening mechanisms possible depend on the alloy.

Table 2.4 shows the maximum nominal yield and tensile strengths for the different alloy

families and the methods by which the strength is increased. There is a wide range of strengths

possible with aluminum alloys. The yield and tensile strengths possible in the different alloy

families depends on the strengthening mechanisms available.

Table 2.4 Maximum Nominal Yield and Tensile Strengths for the Different Alloy Families

Alloy

series Methods for increasing strength

Yield Strength

ksi (MPa)

Tensile Strength,

ksi (MPa)

1xxx Cold-working 4-24 (30-165) 10-27 (70-185)

2xxx Cold-working, Precipitation 11-64 (75-440) 27-70 (185-485)

3xxx Cold working, solid solution, dispersion 6-36 (40-250) 16-41 (110-285)

4xxx Cold working, dispersion 46 (315) 55 (380)

5xxx Cold working, solid solution 6-59 (40-405) 18-63 (125-435)

6xxx Cold working, precipitation 7-55 (50-380) 13-58 (90-400)

7xxx Cold working, precipitation 15-78 (105-540) 33-88 (230-605)

2.7 Mechanisms for Strengthening Aluminum

This section discusses the different aluminum alloy families and the different methods for

strengthening aluminum. This includes a discussion of cold working, solid solution

strengthening, precipitation strengthening, and dispersion strengthening.

2.7.1 Cold working

Cold working involves the reduction in thickness of a material. Plate and sheet of different

thickness are produced by cold rolling. Wire and tubes of different diameter and wall thickness

are produced by drawing. All aluminum alloys can be strengthened by cold working.

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During the cold working, the strength of a metal increases due to the increase in the number of

dislocations in the metal compared to its pre-cold-worked condition. Dislocations are defects in

the arrangement of atoms within a metal.

The increase in the number of dislocations due to cold working is responsible for the increase in

strength. Pure aluminum at room temperature has yield strength of 4 ksi (30 MPa). In the fully

cold-worked state the yield strength can be as high as 24 ksi (165 MPa).

2.7.2 Solid solution strengthening

Certain alloying elements added to aluminum mix with the aluminum atoms in a way that results

in increased metal strength. This mixture is called a solid solution because the alloying atoms

are mixed in with the aluminum atoms. The extent of strengthening depends on the type and

amount of the alloying elements. Manganese and magnesium are examples of elements added to

aluminum for the purpose of strengthening. Solid solution strengthening occurs in 3xxx and 5xxx

alloys through the addition of manganese (3xxx) and magnesium (5xxx) to aluminum.

2.7.3 Precipitation strengthening

With precipitation strengthening, particles less than 0.001 mm in diameter form inside the metal

as shown in Fig. 2.1. These particles are called precipitates and consist of compounds of

aluminum and alloying elements or compounds of the alloying elements. This figure shows Al-

Cu precipitates in an Al-Cu alloy.

Precipitates form as a result of a series of heat treating processes. The step of the process during

which precipitates form is called aging.

Fig. 2.1 Al2Cu precipitates in an aluminum matrix.

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Precipitation strengthening can increase the yield strength of aluminum from about five times up

to about fifteen times that of unalloyed aluminum. The strength depends on the specific alloy

and the aging heat treatment temperature.

Only certain alloys can be precipitation strengthened. The 2xxx, 6xxx, and 7xxx alloys can be

precipitation strengthened through the formation of Al-Cu (2xxx), Mg-Si (6xxx), and Al-Zn-Mg-

(Cu) (7xxx) precipitates. The 1xxx, 3xxx, 4xxx, and 5xxx alloys cannot be precipitation

strengthened.

2.7.4 Dispersion strengthening

Dispersoid particles form during the aluminum casting process when manganese in 3xxx series

alloys reacts with aluminum and iron and silicon. These particles are less than 0.001 mm in

diameter. Dispersoid particles influence the grain structure that forms during heat treating so

that there is increased strength compared to an alloy without dispersoids. Fully-annealed 1100

aluminum has tensile strength of 13 ksi and yield strength of 5 ksi. Fully-annealed 3003 has

minimum tensile strength of 16 ksi and minimum yield strength of 6 ksi. This increase in

strength is due to the grain structure formed as a result of the presence of dispersoids.

2.7.5 Additive strengthening

Finally, the methods of strengthening aluminum discussed here are often combined to provide

even higher strength alloys. Solid solution strengthened alloys are often cold-worked and

precipitation strengthening is sometimes combined with cold working prior to the aging step.

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3.0 Copper and Copper Alloys: to and Introduction Copper its Alloys

Copper is the oldest metal used by man. It‘s use dates back to prehistoric times. Copper has been

mined for more than 10,000 years with a Copper pendant found in current day Iraq being dated

to 8700BC. By 5000BC Copper was being smelted from simple Copper Oxides.

Copper is found as native metal and in minerals cuprite, malachite, azurite, chalcopyrite and

bornite. It is also often a by-product of silver production. Sulphides, oxides and carbonates are

the most important ores.

Copper and Copper alloys are some of the most versatile engineering materials available. The

combination of physical properties such as strength, conductivity, corrosion resistance,

machinability and ductility make Copper suitable for a wide range of applications. These

properties can be further enhanced with variations in composition and manufacturing methods.

3.1 Structure and Properties

Copper is non-polymorphous metal with face centered cubic lattice (FCC, Fig. 3.1). Pure copper

is a reddish color (Fig. 3.2); zinc addition produces a yellow color, and nickel addition produces

a silver color. Melting temperature is 1083 °C and density is 8900 kg.m-3

, which is three times

heavier than aluminum. The heat and electric conductivity of copper is lower compared to the

silver, but it is 1.5 times larger compared to the aluminum. Pure copper electric conductivity is

used like a basic value for other metals evaluation and electric conductivity alloys

characterization. Copper conductivity standard (IASC) is determined as 58 Mss-1

. The pure metal

alloying decreased its conductivity

Fig. 3. 1. FCC lattice (http://cst-www.nrl.navy.mil/lattice/struk/a1.html)

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Fig. 3.2. Natural copper (http://jeanes.webnode.sk/prvky/med/)

3.1.1 Properties of Copper Alloys

Key Properties of Copper Alloys Copper is a tough, ductile and malleable material. These

properties make copper extremely suitable for tube forming, wire drawing, spinning and deep

drawing. The other key properties exhibited by Copper and its alloys include:

o Excellent heat conductivity

o Excellent electrical conductivity

o Good corrosion resistance

o Good biofouling resistance

o Good machinability

o Retention of mechanical and electrical properties at cryogenic temperatures

o Non-magnetic

3.1.2 Other Properties of Copper Alloys

o Copper and Copper alloys have a peculiar smell and disagreeable taste. These may be

transferred by contact and therefore should be kept clear of foodstuffs, although some

cooking pans do use these metals.

o Most commercially used metals have a metallic white or silver colour. Copper and

Copper alloys have a range of yellow/gold/red colours.

o Melting Point The melting point for pure Copper is 1083ºC.

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3.1.3 Corrosion Resistance

All Copper alloys resist corrosion by fresh water and steam. In most rural, marine and industrial

atmospheres Copper alloys are also resistant to corrosion. Copper is resistant to saline solutions,

soils, non-oxidising minerals, organic acids and caustic solutions. Moist ammonia, halogens,

sulphides, solutions containing ammonia ions and oxidising acids, like nitric acid, will attack

Copper. Copper alloys also have poor resistance to inorganic acids. The corrosion resistance of

Copper alloys comes from the formation of adherent films on the material surface. These films

are relatively impervious to corrosion therefore protecting the base metal from further attack.

Copper Nickel alloys, Aluminium Brass, and Aluminium Bronzes demonstrate superior

resistance to saltwater corrosion.

3.1.4 Electrical Conductivity

The electrical conductivity of copper is second only to silver. The conductivity of Copper is 97%

of the conductivity of Silver. Due to its much lower cost and greater abundance, Copper has

traditionally been the standard material used for electricity transmission applications. However,

weight considerations mean that a large proportion of overhead high voltage power lines now use

Aluminium rather than Copper. By weight, the conductivity of Aluminium is around twice that

of Copper. The Aluminium alloys used do have a low strength and need to be reinforced with a

galvanised or Aluminium coated high tensile steel wire in each strand. Although additions of

other elements will improve properties like strength, there will be some loss in electrical

conductivity. As an example a 1% addition of Cadmium can increase strength by 50%. However,

this will result in a corresponding decrease in electrical conductivity of 15%.

3.1.5 Surface Oxidation/ Patination

Most Copper alloys will develop a blue-green patina when exposed to the elements outdoors.

Typical of this is the colour of the Copper Statue of Liberty in New York. Some Copper alloys

will darken after prolonged exposure to the elements and take on a brown to black colour.

Lacquer coatings can be used to protect the surface and retain the original alloy colour. An

acrylic coating with benzotriazole as an additive will last several years under most outdoor,

abrasion-free conditions.

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3.1.6 Yield Strength

The yield point for Copper alloys is not sharply defined. As a result it tends to be reported as

either a 0.5% extension under load or as 0.2% offset. Most commonly the 0.5% extension yield

strength of annealed material registers as approximately one-third the tensile strength. Hardening

by cold working means the material becomes less ductile, and yield strength approaches the

tensile strength.

3.1.7 Joining

Commonly employed processes such as brazing, welding and soldering can be used to join most

Copper alloys. Soldering is often used for electrical connections. High Lead content alloys are

unsuitable for welding. Copper and Copper alloys can also be joined using mechanical means

such as rivets and screws.

3.1.8 Hot & Cold Working

Although able to be work hardened, Copper and Copper alloys can be both hot and cold worked.

Ductility can be restored by annealing. This can be done either by a specific annealing process or

by incidental annealing through welding or brazing procedures.

3.1.9 Temper

Copper alloys can be specified according to temper levels. The temper is imparted by cold

working and subsequent degrees of annealing. Typical tempers for Copper alloys are

o ~ Soft

o ~ Half-hard

o ~ Hard

o ~ Spring

o ~ Extra-spring.

Yield strength of a hard-temper Copper alloy is approximately two-thirds of the materials‘

tensile strength.

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3.2 Pure copper and high copper alloys

Commercially pure copper and high copper alloys are very difficult to melt and they are very

susceptible to gassing. Chromium copper melting is negatively linked with oxidation loss of

chromium. To prevent both oxidation and the pickup of hydrogen from the atmosphere copper

and chromium copper should be melted under a floating flux cover. In the case of pure copper

crushed graphite should cover the melt. In the case of chromium copper, the cover should be a

proprietary flux made for this alloy. It is necessary to deoxidize the melted metal. For this reason

the calcium boride or lithium should be plunged into the molten bath when the melted metal

reaches 1260 °C. The metal should then be poured without removing the floating cover.

Beryllium coppers can be very toxic and dangerous. This is caused by the beryllium content in

cases where beryllium fumes are not captured and exhausted by proper ventilating equipment. To

minimize beryllium losses beryllium coppers should be melted quickly under a slightly oxidizing

atmosphere. They can be melted and poured successfully at relatively low temperatures. They

are very fluid and pour well.

Copper-nickel alloys (90Cu-10Ni and 70Cu-30Ni) must also be carefully melted. Concern is

caused by the presence of nickel in high percentages because this raises not only the melting

point but also the susceptibility to hydrogen pickup. These alloys are melted in coreless electric

induction furnaces, because the melting rate is much faster than it is with a fuel-fired furnace.

The metal should be quickly heated to a temperature slightly above the pouring temperature and

deoxidized either by the use of one of the proprietary degasifiers used with nickel bronzes or,

better yet, by plunging 0.1 % Mg stick to the bottom of the ladle. This has to be done to prevent

the of steam-reaction porosity from occurring by the oxygen removing. If the gates and risers are

cleaned by shotblasting prior to melting there is a little need to use cover fluxes (R. F. Schmidt,

D. G. Schmidt & Sahoo, 1988).

3.2.1 Brasses

Yellow Brasses (containing 23 - 41 % of zinc) as the major alloying element and may contain up

to 3 % of lead and up to 1.5 % of tin as additional alloying elements. Due to vaporization these

alloys flare, ore lose zinc close to the melting point. The zinc vaporization can be minimized by

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the addition of aluminum (0.15 to 0.35 %) which also increases the melted metals fluidity. In the

case of larger aluminum amount shrinkages take place during freezing; this has to be solved by

use of risers. Except for aluminum problems, yellow brass melting is simple and no fluxing is

necessary. Zinc lost during the melting should be re- added before pouring.

Silicon brasses have excellent fluidity and can be poured slightly above their freezing range. If

overheated, they can pick up hydrogen. In the case of the silicon brasses no cover fluxes are

required.

Red brasses and leaded red brasses are copper alloys, containing 2 - 15 % of zinc as the major

alloying element and up to 5 % of Sn and up to 8 % of Pb as additional alloying elements.

Because of lengthy freezing range in the case of these alloys, chills and risers should be used in

conjunction with each other. The best pouring temperature is the lowest one that will pour the

molds without having misruns or cold shuts. For good casting, properties retaining these alloys

should be melted from charges comprised of ingot and clean, sand-free gates and risers. The

melting should be done quickly in a slightly oxidizing atmosphere. When at the proper furnace

temperature to allow for handling and cooling to the proper pouring temperature, the crucible is

removed or the metal is tapped into a ladle. At this point, a deoxidizer (15% phosphorus copper)

is added. The phosphorus is a reducing agent (deoxidizer) and must be carefully measured so

that enough oxygen is removed, yet a small amount remains to improve fluidity. This residual

level of phosphorus must be controlled by chemical analysis. Only amount in the range 0.010

and 0.020 % P is accepted, in the case of the larger phosphorus amount internal porosity may

occur. Along with phosphorus copper pure zinc should also be added at the point at which

skimming and temperature testing take place prior to pouring. This added zinc replaces the zinc

lost during melting and superheating. With these alloys, cover fluxes are seldom used. In some

foundries in which combustion cannot be properly controlled, oxidizing fluxes are added during

melting, followed by final deoxidation by phosphor copper.

Leaded red brasses alloys have practically no feeding range, and it is extremely difficult to get

fully sound castings. Leaded red brasses castings contain 1 to 2 % of porosity. Only small

castings have porosity below 1 %. Experience has shown that success in achieving good quality

castings depends on avoiding slow cooling rates. Directional solidification is the best for

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relatively large, thick castings and for smaller, thin wall castings, uniform solidification is

recommended (R. F. Schmidt, D. G. Schmidt & Sahoo 1998).

3.2.2 Bronzes

Manganese bronzes are carefully compounded yellow brasses with measured quantities of iron,

manganese, and aluminum. When the metal is heated at the flare temperature or to the point at

which zinc oxide vapor can be detected, it should be removed from the furnace and poured. No

fluxing is required with these alloys. The only required addition is zinc, which is caused by its

vaporization. The necessary amount is the one which will bring the zinc content back to the

original analysis. This varies from very little, if any, when an all-ingot heat is being poured, to

several percent if the heat contains a high percentage of remelt.

White manganese bronzes. There are two alloys in this family, both of which are copper- zinc

alloys containing a large amount of manganese and, in one case, nickel. They are manganese

bronze type alloys, are simple to melt, and can be poured at low temperatures because they are

very fluid. The alloy superheating resulting in the zinc vaporization and the chemistry of the

alloy is changed. Normally, no fluxes are used with these alloys.

Aluminum bronzes must be melted carefully under an oxidizing atmosphere and heated to the

proper furnace temperature. If needed, degasifiers removing the hydrogen and oxygen from the

melted metal can be stirred into the melt as the furnace is being tapped. By pouring a blind sprue

before tapping and examining the metal after freezing, it is possible to tell whether it shrank or

exuded gas. If the sample purged or overflowed the blind sprue during solidification, degassing

is necessary. For converting melted metal fluxes, are available, mainly in powder form, and

usually fluorides. They are used for the elimination of oxides, which normally form on top of the

melt during melting and superheating (R. F. Schmidt, D. G. Schmidt & Sahoo, 1988).

From the freezing range point of view, the manganese and aluminum bronzes are similar to

steels. Their freezing ranges are quite narrow, about 40 and 14°C, respectively. Large castings

can be made by the same conventional methods used for steel. The attention has to be given to

placement of gates and risers, both those for controlling directional solidification and those for

feeding the primary central shrinkage cavity (R. F. Schmidt & D. G. Schmidt, 1997).

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Nickel bronzes, also known as nickel silver, are difficult to melt because nickel increases the

hydrogen solubility; if the alloy is not melted properly it gases readily. These alloys must be

melted under an oxidizing atmosphere and they have to be quickly superheated to the proper

furnace temperature to allow for temperature losses during fluxing and handling. After the

furnace tapping the proprietary fluxes should be stirred into the metal for the hydrogen and

oxygen removing. These fluxes contain manganese, calcium, silicon, magnesium, and

phosphorus.

Silicon bronzes are relatively easy to melt and should be poured at the proper pouring

temperatures. In the case of overheating the hydrogen, picking up can occur. For degassing, one

of the proprietary degasifiers used with aluminum bronze can be successfully used. Normally no

cover fluxes are used for these alloys.

Tin and leaded tin bronzes, and high-leaded tin bronzes, are treated the same in regard to

melting and fluxing. Their treatment is the same as in the case of the red brasses and leaded red

brasses, because of the similar freezing range which is long (R. F. Schmidt, D. G. Schmidt &

Sahoo, 1988).

Tin bronzes have practically no feeding range, and it is extremely difficult to get fully sound

castings. Alloys with such wide freezing ranges from a mushy zone during solidification,

resulting in interdendritic shrinkages or microshrinkages. In overcoming this effect, design and

riser placement, plus the use of chills, are important and also the solidification speed, for better

results the rapid solidification should be ensured. As in the case of leaded red brasses, tin bronzes

also have problems with porosity. The castings contain 1 to 2 % of porosity and only small

castings have porosity below 1 %. Directional solidification is best for relatively large, thick

castings and for smaller, thin wall castings, uniform solidification is recommended. Sections up

to 25 mm in thickness are routinely cast. Sections up to 50 mm thick can be cast, but only with

difficulty and under carefully controlled conditions (R. F. Schmidt & D. G. Schmidt, 1997).

3.2.3 Brasses

Brasses are copper based alloys where zinc is the main alloying element. Besides zinc, also

some amount of impurities and very often some other alloying elements are present in the

alloys. The used alloying elements can improve some properties, depending on their

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application. Due to the treatment, brasses can be divided into two groups: wrought brasses

and cast brasses. One special group of brasses is brazing solder.

The binary diagram of the Cu-Zn system is quite difficult, Fig. 3.3. For the technical praxis

only the area between the 0 to 50 % of Zn concentration is important. Alloys with higher Zn

concentration have not convenient properties, as they are brittle.

Fig. 3.3 Binary diagram copper-zinc

In the liquid state copper and zinc are absolutely soluble. In the solid state solubility is

limited; copper has face centered cubic lattice and zinc has the hexagonal close-packed

lattice (HCP). During solidification the and phases are released. Most of the

phases are intermediary phases characterized by the relation of valence electrons and

amount of atoms. Primary solid solution (Zn in Cu) has the same crystal lattice as pure

copper and dissolves maximum 39% of Zn at temperature 450°C. Decreasing the

temperature, also the zinc solubility in the solution decreases ~ 33%. In the alloys with

higher content of Zn concentration the supersaturated solid solution is retaining at room

temperature. This is why alloys up to 39% Zn concentration have homogenous structure

consisting from the solid solution crystals. After the forming and annealing the brass

structure consists of the solid solution polyhedral grains with annealed twins.

Alloys with Zn concentration from 32% (point B, Fig. 3.3) to 36% (point C) crystallize

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according to the peritectic reaction at temperature 902°C. During this reaction the phase is

created by the reaction of formed primary crystals and the liquid alloy. By decreasing the

temperature in the solid state the ratio of both crystals is changed. This is caused by

solubility changes and the resulting brass structure is created only by the solid solution

crystals. In the case of alloys with the concentration of Zn from 36 to 56% the phase exists

after the solidification. phase has a body centered cubic (BCC) lattice. The atoms of Cu

and Zn are randomly distributed in the lattice. phase has good ductility. By the next

temperature decreasing the random phase is changed to the ordered hard and brittle ´

phase; temperatures from 454 to 468°C. The 39 to 45% Zn containing brass resulting

structure is heterogeneous, consisting of the solid solution crystals and ´ phase crystals.

Only the alloys containing less than 45% of Zn in the technical praxis, apart from small

exceptions, are used. Brasses with less than 40% Zn form single-phase solid solution of zinc

in copper. The mechanical properties, even elongation, increase with increasing zinc

content. These alloys can be cold-formed into rather complicated yet corrosion-resistant

components. Brasses with higher Zn concentration, created by the ´ phases, or ´ phase,

are excessively brittle. Mechanical properties of the brasses used in the technical praxis are

shown in the Fig. 3.4 (Skočovský et al., 2000).

Fig. 3.4. Influence of the Zn content to the brass mechanical properties

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Brasses used in technical praxis consist also of some other elements; impurities and alloying

elements besides Zn, whose influence is the same as in the case of pure copper. Bismuth and

sulphur, for example, decreases the ability of the metal to hot forming. Lead has a similar

influence. On the other hand, lead improves the materials workability. For this reason, lead

is used for heterogeneous brasses in an amount from 1 to 3 %. Homogenous brasses, for

improving strength, contain tin, aluminum, silicon and nickel. Silicon improves the materials

resistance against corrosion and nickel improves the materials ductility.

Brasses heat treatment. Recrystallization annealing is brasses basic heat treatment process.

Combination of recrystallization annealing and forming allows to change the materials grain

structure and to influence the hardening state reached by the plastic deformation. The lower

limit of the recrystallizing temperatures for binary Cu-Zn alloys is ~ 425°C and the upper

limit is limited by the amount of Zn in the alloy.

Stress-relief annealing is the second possible heat treatment used for brasses. This heat

treatment process, at temperatures from 250 – 300°C, decreases the danger of corrosion

cracking (Skočovský et al., 2000).

3.2.4 Wrought brasses

Wrought brasses are supplied in the form of metal sheets, strips, bars, tubes, wires, etc. in

the soft (annealed) state, or in the state (medium, hard state) after cold forming.

Tombacs are brasses containing more than 80% of copper. They are similar to the pure

copper by its chemical and physical properties, but they have better mechanical properties.

Tombac with higher copper content is used for coins, memorial tablets, medals, etc. (after

production the products are gold-plated before distribution). Tombacs with medium or lower

copper content are light yellow colored, close to the color of gold. Because of this, brass

films are used like the gold substituent in the case of decorative, artistic, fake jewelry,

architecture, armatures, in electrical engineering, for manometers, flexible metal tubes,

sieves etc. plating. Tombac with 80% Zn has lower chemical resistance due to the higher Zn

content, and so its usage (same like for other Tombacs) is dependent on the working

conditions.

Deep-drawing brasses contain around 70 % of Cu; for securing high ductility, the

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impurities elements content has to be low. Cu-Zn alloys with the 32 % Zn content have the

highest ductility at high strength which is why those alloys are used for deep-drawing. They

are used, for example, for ball cartridges, musical equipment production and in the food

industry.

Brass with higher Zn content (37% of Zn) is quite cheap because of its lower Cu content. It

is a heterogeneous alloy with small ´ phase content, with lower ductility and good ability

for cold forming. It is used for different not very hard loaded products; for example wiring

material in electrical engineering, automotive coolers, etc. For improving the materials

workability a small amount of lead is added to brasses with higher Zn content.

Brass with 40 % of Zn is heterogeneous alloy with (´) microstructure. Compared to

other brasses it has higher strength properties, but lower ductility and cold forming ability.

This is caused by the ´ presence. It is suitable for forging and die pressing at higher

temperatures (700 – 800°C). This kind of brass is used in architecture, for different ship

forging products, for tubes and welding electrodes. For this brass type, with 60% of Cu, the

tendency to dezincification corrosion and to corrosion cracking is typical. The crack

tendency increases with the increasing Zn content and it is largest at 40 % Zn content. In the

case of zinc content below 15% this tendency is not present. Brasses alloying elements do

not improve the crack tendency. Some of the used alloying elements can decrease this

disadvantage; Mg, Sn, Be, Mn. Grain refinement has the same influence.

3.2.5 Cast brasses

Cast brasses are heterogeneous alloys ´) containing from 58 to 63 % of copper. For

improving machinability they very often also contain lead (1 - 3%). Cast brasses have good

feeding, low tendency to chemical unmixing, but high shrinkage. Because of their structure

cast brasses have lower mechanical properties compared to the mechanical properties of

wrought brasses. These brasses are used mainly for low stressed castings; pumps parts, gas

and water armatures, ironworks for furniture and building, etc.

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3.2.6 Special brasses

This alloy family consists of copper, zinc and one or more elements in addition (aluminum,

tin, nickel, manganese, iron, silicon). The name of the alloy is usually formed according to

the additional element (for example silicon brass is the Cu-Zn-Si alloy). Other elements

addition improves the materials mechanical properties, corrosion resistance, castability and

workability increasing. The change in properties is dependant on the element type and on its

influence on the materials structure.

According to production technology special brasses can be divided into two groups: wrought

and cast special brasses.

Aluminum brasses. Aluminum brasses contain from 69 to 79% of copper; the aluminum

additive content, in the case of wrought alloys, is below 3 to 3.5% to keep the structure

homogeneous. As well as this aluminum content, the structure is also formed by new phases

as are γ phases, which improve the materials hardness and strength, but decrease its

ductility. Aluminum brass containing 70% of Cu and 0.6 to 1.6% Al, with Sn and Mn

addition, is very corrosion resistant and is used for condenser tubes production. Al brass

with higher content of Cu (77%) and with Al from 1.7 to 2.5%, whose application is the

same as that of the previous brass, its corrosion resistance against the see water is higher

because of the larger Al content.

The structure of cast aluminum brasses is heterogeneous. The copper content is in this case

lower and the aluminum content is higher (below 7%), which ensures good corrosion

resistance of the material in sea water. They are used for very hard loaded cast parts;

armatures, screw-cutting wheel, bearings and bearings cases.

Manganese brasses. Wrought manganese brasses contain from 3 to 4% manganese and cast

manganese brasses contain from 4 to 5% manganese. This alloying family has high strength

properties, and corrosion resistance. They are used usually in the heterogeneous structure.

Wrought manganese brasses with 58% of Cu or 57% of Cu with addition of Al have quite

good strength (in medium-hard state 400 to 500 MPa) at large toughness and corrosion

resistance. They are used for armatures, valve seating, high-pressured tubes, etc. Mn brass

with 58% of Cu is also used for decorations (product surface is layered during hot oxidation

process by attractive, durable brown verdigris). Cast manganese brasses have larger

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manganese and iron content and they are used for very hard loaded castings; weapons parts,

screw propellers, turbine blades and armatures for the highest pressures.

Tin brasses. Tin brasses are mostly heterogeneous alloys containing tin below 2.5 %. Tin

has a positive influence on mechanical properties and corrosion resistance. Some Sn alloys

with 60 % of Cu have good acoustical properties and so they are used for the musical/sound

instruments production. Sn brass alloy with 62 % of Cu is used for the strips, metal sheets

and profiles used in the ships or boat constructions.

Silicon brasses. Silicon brasses contain maximum 5 % of Si at large copper content, from 79

to 81 %. As in the other brasses case, silicon brasses can be wrought (max. 4 % Si) or cast.

They have very good corrosion resistance and mechanical properties also at low

temperatures (- 183 °C). Over 230 °C their creeping is extensive, at a low stress level and at

a temperature larger than 290 °C silicon brasses are also brittle. Lead addition, (3 to 3.5 %),

positively affects the materials wear properties and so these alloys are suitable for bearings

and bearings cases casting (Cu 80 %, Si 3 %, Pb 3 %). Silicon brasses are used in boats,

locomotives and railway cars production. Silicon cast alloys with good feeding properties

are used for armatures, bearings cases, geared pinions and cogwheels production.

Nickel brasses. Nickel brasses contain from 8 to 20 % nickel, which is absolutely soluble in

homogeneous brasses and nickel enlarges area. Homogeneous alloys are good cold

formed and are suitable for deep-drawing. Heterogeneous alloys () are good for hot

forming. Nickel brasses have good mechanical properties, corrosion resistance and are easily

polishable. One of the oldest alloys are alloys of 60 % of copper and from 14 to 18 % of

nickel content; they were used for decorative and useful objects. These alloys have many

different commercial names, for example ―pakfong‖, ―alpaca‖, ―argentan‖ etc. they are used

in the building industry, precise mechanics, and medical equipment production and for

stressed springs (high modulus of elasticity).

Brazing solders. Brazing solders are either basic or special brasses with melting

temperature higher than 600°C. They are used for soldering of metals and alloys with higher

melting temperatures like copper and its alloys, steels, cast irons etc. In the case of binary

alloys Cu- Zn with other element addition (Ag or Ni), the solders are marked like silver or

nickel solders. Brasses solders with Cu content from 42 to 45% have the melting

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temperature 840 to 880°C and they are used for brasses soldering. Silver solders contain

from 30 to 50% of copper, from 25 to 52% of Zn and from 4 to 45% of Ag. Lower the Ag

content; lower the melting temperature (to 720°C). Silver solders have good feeding and

give strong soldering joints. Nickel solders (38 % Cu, 50 % Zn and 12 % Ni) have a melting

temperature of around 900°C. They are used for steels and nickel alloys soldering.

Solder with a very low Ag content (0.2 to 0.4 %) with upper melting temperature 900°C has

good electric conductivity and for this reason is used in electrical engineering. Solder with

an upper melting temperature of 850 °C (60 % Cu and low content of Si, Sn) has high

strength and it is suitable for steels, grey cast iron, copper and brasses soldering.

3.2.7 Bronzes

Bronzes are copper based alloys with other alloying elements except zinc. The name of

bronzes is defined according to the main alloying element; tin bronzes, aluminum bronzes,

etc..

Tin bronzes. Tin bronzes are alloys of copper and tin, with a minimal Cu-Sn content 99.3 %.

Equilibrium diagram of Cu-Sn is one of the very difficult binary diagrams and in some areas

(especially between 20 to 40 % of Sn) it is not specified till now. For the technical praxis

only alloys containing less than 20 % of Sn are important. Tin bronzes with higher Sn

content are very brittle due to the intermetallic phases‘ presence. Cu and Sn are absolutely

soluble in the liquid state. In the solid state the Cu and Sn solubility is limited.

Normally, the technical alloys crystallize differently as compared to the equilibrium

diagram. Until 5 % of Sn, the alloys are homogenous and consist only of the solid solution

(solid solution of Sn in Cu) with face centered cubic lattice. In the cast state the alloy

structure is dendritic and in the wrought and annealed state the structure is created by the

regular polyhedral grains. The resulting structure of alloys with larger Sn content (from 5 to

20%) is created by solid solution crystals and eutectic ( ). phase is an electron

compound Cu31Sn8 (e/a = 21/13) with cubic lattice. phase is brittle phase, which has

negative influence on the ductility and also decreases the materials strength in case of higher

Sn content (above 20%). Even though the solubility in the case of technical alloys decreases,

the phase (Cu3Sn with hexagonal lattice; e/a = 7/4) is not created. The phases do not

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occur because the diffusion ability of Sn atoms below 350°C is low. phase also does not

occur at normal temperature with higher Sn content in bronze.

Tin addition has a similar influence on bronzes properties as zinc addition in the case of

brasses. For the forming, bronzes with around 9 % of Sn are used (it is possible to heat those

alloys to single-phase state above 5 % Sn). Tin bronzes are used when bronzes are not

sufficient in strength and corrosion resistance points of view. For casting, bronzes with

higher Sn content are used; up to 20 % of Sn. Cast bronzes are used more often than

wrought bronzes. Tin bronzes castings have good strength and toughness, high corrosion

resistance and also good wear properties (the wear resistance is given by the heterogeneous

structure ( Tin bronzes have small shrinkages during the solidification (1 %) but

they have worst feeding properties and larger tendency to the creation of microshrinkages.

Wrought tin bronzes. Bronze CuSn1 contains from 0.8 to 2% of Sn. In the soft state this

bronze has tensile strength 250 MPa and 33 % ductility. It has good corrosion resistance and

electric conductivity; it is used in electrical engineering. Bronze CuSn3 with 2.5 to 4 % of

Sn has in its soft state tensile strength 280 MPa and ductility 40%. It is used for the chemical

industry and electric engineering equipment production. Bronze CuSn6 with tensile strength

350 MPa and ductility 40% (in soft state) is used for applications where ´, a higher

corrosion resistance is required for good strength properties and ductility; for example

corrosion environment springs. CuSn8 bronze has, from all wrought tin bronzes the highest

strength (380 MPa) and ductility (40 %). It is suitable for bearing sleeves production and in

the hard state also for springs which are resistant to fatigue corrosion.

Cast tin bronzes. Bronze CuSn1 with low Sn content has sufficient electric conductivity and

so it is used for the castings used in electric engineering. CuSn5 and CuSn10 bronzes have

tensile strength 180 and 220 MPa, ductility 15 % and they have good corrosion resistance.

They are used for the stressed parts of turbines, compressors, for armatures and for pumps

runners‘ production. Bronze CuSn12 is used for parts used to large mechanical stress and

wear frictional loading; spiral gears, gear rims. CuSn10 and CuSn12 bronzes are used in the

same way as bearing bronzes. High Sn content (14 to 16 %) bronzes usage have been,

because of their expense, replaced by lower Sn containing bronzes, around 6 %, with good

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sliding properties.

Leaded tin bronzes and leaded bronzes. Leaded tin bronzes and leaded bronzes are copper

alloys where the Sn content is partially or absolutely replaced by Pb. The Pb addition to

copper, improves the alloys sliding properties without the negative influence on their heat

conductivity. Cu-Pb system is characteristic by only partial solubility in a liquid state and

absolute insolubility in a solid state. The resulting structure, after solidification, consists of

copper and lead crystals. At a high cooling rate both the alloy components are uniformly

distributed and the alloys have very good sliding properties. Leaded bronzes are suitable for

steel friction bearing shells casting. They endure high specific presses, quite high

circumferential speeds and it is possible to use them at elevated temperatures (around 300

°C).

Two types of bearing bronzes are produced. Bronzes with lower Pb content (from 10 to

20%) and Sn addition (from 5 to 10%) and also high-leaded bronzes (from 25 to 30%)

without tin. At present, specially leaded bronzes CuSn10Pb and CuSn10Pb10 like bearing

bronzes are used. Lead (additive from 4 to 25%) improves bearing sliding properties, and tin

(from 4 to 10%) improves strength and fatigue resistance. These alloys are used especially

for bearings in dusty and corrosive environments. Second group binary alloys have lower

strength and hardness and they are used for steel shells coatings. With small additions of

Mn, Ni, Sn and Zn (in total 2%) it is possible to refine the structure and to decrease the

materials tendency to exsolution. These are very often used for steel shells coated by thin

leaded bronze layer for the main and the piston rod bearings of internal-combustion engine.

Aluminum bronzes. Aluminum bronzes are alloys of copper, where aluminum is the main

alloying element. For the technical praxis alloys with Al content below 12% are important.

Equilibrium diagram of Cu-Al is complicated and it is similar to Cu-Sn equilibrium diagram.

One part of Cu-Al system for alloys containing up to 14 % of Al is shown on Fig. 3.5.

The solubility of Al in copper is maximum 7.3 % but it grows with temperature increasing to

9.4 % Al. Homogeneous alloys structure is created by solid solution crystals (substituted

solid solution of Al in Cu) with body centered cubic lattice with similar properties as has the

solid solution in brasses. It is relatively soft and plastic phase. In the real alloys the

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absolutely equilibrium state does not occur. In the case of Al content close to the solubility

limit some portion of phase in the structure will occur. The upper limit of Al in

homogeneous structure alloys is dependent on the cooling rate and it is in the range of 7.5 to

8.5 % of Al.

Fig. 3.5. Part of Cu-Al system equilibrium diagram

Alloys with Al content in the range of 7.3 to 9.4 % solidify at eutectic reaction ( ) and

close to the eutectic line they contain primary released phase or and eutectic. (After the

changed at lower temperatures the eutectic disappears and so its influence in the structure

cannot be proven.)

By decreasing the temperature the composition, of and crystals changes according to the

time of solubility change. phase is a disordered solid solution of electron compound Cu3Al

(e/a = 3/2) with face centered cubic lattice. It is a hard and brittle phase. phase, from which

the solid solution is created at lower temperatures, is precipitated from liquid metal at the

Al content from 9.5 to 12% alloys during the crystallization process. During the slow

cooling rate the phase is transformed at eutectoid temperature 565°C to the lamellar

eutectoid ( ). For this reason the eutectoid reaction of phase is sometimes called

―pearlitic transformation‖.

Phase is solid solution of hard and brittle electron compound Cu9Al4 with complicated

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cubic lattice. After the recrystallization in solid state the slowly cooled alloys with Al

content from 9.4 to 12 % are heterogeneous. Their structure is created with solid solution

crystals and eutectoid ( ).

Because of the possibility to improve the mechanical properties by heat treatment

heterogeneous alloys are used more often than homogeneous alloys. From 10 to 12% Al

content alloys can be heat treated with a similarly process as in the case of steels. The

martensitic transformation can be reached in the case when the eutectoid transformation is

limited by fast alloys cooling rate from the temperatures in the or ( ) areas (Fig. 3.5).

After this process the microstructure with very fine and hard needles phase with a body

centered cubic lattice will be reached. By the phase undercooling below the martensitic

transformation temperature Ms, a needle-like martensitic supersaturated disordered solid

solution ´ phase with body centered cubic lattice is created.

Due to the chemical composition aluminum bronzes can be divided into two basic groups:

o elementary (binary) alloys; i.e. Cu-Al alloys without any other alloying elements,

o complex (multicomponent) alloys; besides the Al these alloys contain also other

alloying elements like Fe, Ni, Mg whose content does not exceeds 6 %.

Iron is frequently an aluminum bronzes alloying element. It is dissolved in phase till 2 %

and it improves its strength properties. With Al it creates FeAl3 intermetallic phase which

causes the structure fining.

Manganese is added to the multicomponent alloys because it has deoxidizing effect in the

melted metal. It is dissolved in phase, up to 12% of Mn content, and it has an effect

similar to iron.

Nickel is the most frequent alloying element in aluminum bronzes. It has positive influence

on the corrosion resistance in aggressive water solutions and in sea water. Up to around 5 %

nickel is soluble in phase. Nickel with aluminum creates Ni3Al intermetallic phase which

has a precipitate hardening effect.

Homogeneous aluminum bronzes are tough and are suitable for cold and also hot forming.

Heterogeneous alloys are stronger, harder, but they have lower cold forming properties

compared to the homogeneous alloys. They are suitable for hot forming and have good cast

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properties. Aluminum bronzes are distinguished by good strength, even at elevated

temperatures, and also very good corrosion resistance and wear resistance. Aluminum

bronzes are used in the chemical and food industry for stressed components production.

These alloys are used in the mechanical engineering for much stressed gearwheels and worm

wheels, armatures working at elevated temperatures etc. Production due to the treatment the

aluminum bronzes are divided into two groups; cast and wrought aluminum bronzes.

Aluminum bronzes with Al content from 4.5 to 11 % are used for forming elementary or

complex. Al content from 7.5 to 12 % are used for casting only complex aluminum bronzes

CuAl15 bronze is used for cold forming. It is supplied in the form of sheets, strips, bars,

wires and pipes. In the soft state this alloy can reach the tensile strength 380 MPa, ductility

40 % and hardness 70 to 110 HB. It is used in the boats building, chemical, food and paper

making industry.

Complex aluminum bronzes are normally used for hot forming. CuAl9Mn2 is used for the

armatures (bellow 250°C) production. CuAl9Fe3 is used for the bearings shells, valve seats

production, etc. CuAl10Fe3Mn1.5 alloy has heightened hardness and strength; it is suitable

for shells and bearings production; it is replacing leaded bronzes up to temperature 500 °C,

sometimes also till 600 °C, the CuAl10Fe4Ni4 where Ni is replacing Mn is used. Nickel

positively affects materials mechanical and corrosion properties. After the heat treatment the

alloy has the tensile strength of 836 MPa and ductility 13.4 %. In the sea water corrosion

environment this bronze reached better results compared to chrome-nickel corrosion steels.

It is resistant against cavitational corrosion and stress corrosion. CuAl10Fe4Ni4 is used for

castings, also used for water turbines and pumps construction, for valve seats, exhaust valves

and other components working at elevated temperatures and also in the chemical industry.

Besides CuAl19Ni5Fe1Mn1 the nickel alloy consists also a higher content of manganese. It

is suitable for cars worm wheels, compressing rings of friction bearings for high pressures

etc.

Silicon bronzes. The silicon content in this type of alloys is in the range from 0.9 to 3.5 %.

The Si content should not exceed 1 % when higher electric conductivity is required. Silicon

bronzes more often in the form of complex alloys Cu-Si-Ni-Mn-Zn-Pb are produced; binary

alloys Cu-Si only rarely are used. Manganese is dissolved in the solid solution; improving

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strength, hardness and corrosion properties. Zinc improves the casting properties and

mechanical properties, as same as Mn. Nickel is dissolved in the solid solution but it also

creates Ni2Si phase with silicon, which has a positive influence on the materials warm

strength properties. Lead addition secure sliding properties.

Silicon bronzes have good cold and hot forming properties and are also used for castings

production. They are resistant against sulphuric acid, hydrochloric acid and against some

alkalis. Because of their good mechanical, chemical and wear properties, silicon bronzes are

used for tin bronzes replacing; they outperform tin bronzes with higher strength and higher

working temperatures interval. Formed CuSi3Mn alloy has in the soft state tensile strength

380 MPa and ductility 40 %. It is used for bars, wires, sheets, strips, forgings and stampings

production. Casting alloys have normally higher alloying elements content and Si content

reaches 5 % very often.

Beryllium bronzes. Beryllium is in copper limitedly soluble (max. 2.7%) and in the solid

state the solubility decreases (0.2% at room temperature). The binary alloys with low

beryllium content (0.25 to 0.7%) have good electric conductivity, but lower mechanical

properties, they are used rarely. More often alloys with higher Be content and other alloying

elements as Ni, Co, Mn and Ti are produced. Cobalt (0.2 to 0.3 %) improves heat resistance

and creep properties; nickel improves toughness and titanium affects like grain finer. The

main group of this alloy family is the beryllium bronzes with 2 % of Be content due to the

highest mechanical properties after the precipitin hardening.

Beryllium bronzes thermal treatment consists of dissolved annealing (700 to 800 °C/1h) and

water quenching. The alloy after heat treatment is soft, formable and it can be improved only

by artificial aging. Hardening is in progress at temperature from 280 to 300°C. After the

hardening the tensile strength of the alloy is more than 1200 MPa and the hardness 400 HB.

By cold forming, applied after the cooling from the annealing temperature, the materials

tensile strength can be improved. Beryllium bronzes usage is given by their high tensile

strength, hardness, and corrosion resistance which those alloys do not lose, even not in the

hardened state. They are used for the good electric conductive springs production; for the

equipment which should not sparkling in case of bumping (mining equipment) production;

form dies, bearings, etc.

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Nickel bronzes. Copper and nickel are absolutely soluble in the liquid and in the solid state.

Binary alloys are produced with minimal alloying elements content. Complex alloys, ternary

or multi components, are suitable for hardening. Nickel bronzes have good strength at

normal and also at elevated temperatures; good fatigue limit, they are resistant against

corrosion and also against stress corrosion, and they have good wear resistance and large

electric resistance.

Binary alloys Cu-Ni with low Ni content (bellow 10%) are used only limitedly. They are

replaced by cheaper Cu alloys. Alloys with middle Ni content (15 to 30%) have good

corrosion resistance and good cold formability. 15 to 20 % Ni containing alloys are used for

deep-drawing. Alloys with 25 % Ni are used for coin production and alloys with 30 % Ni are

used in the chemical and food industry.

Complex Cu-Ni alloys have a wider usage in the technical praxis compared to the binary

alloys. CuNi30Mn with Ni content from 27 to 30 %, Mn content from 2 to 3 % and

impurities content bellow 0.6 % is characterized by high strength and corrosion resistance

also at elevated temperatures. Because of its electric resistance this alloy is suitable for

usage as resistive material till 400 °C. CuNi45Mn constantan is alloy with Ni content form

40 to 46 %, Mn content from 1 to 3 % and impurities content below 0.5 %. From the Cu-Ni

alloys, this one has the largest specific electric resistance and it is used for resistive and

thermal element material.

Most often the Cu-Ni-Fe-Mn alloys are used. Iron and manganese addition improve the

corrosion properties markedly, especially in the seas water and overheated water steam.

CuNi30 alloy with iron content in the range from 0.4 to 1.5 % and manganese content from

0.5 to 1.5 % is used for seagoing ships condensers and condensers pipes production. In the

new alloys also the niobium as an alloying element is used and the nickel content tends to be

decreased because of its deficit. An alloy CuNi10Ge with nickel content from 9 to 10 % and

Fe content from 1 to 1.75 % and maximally 0.75 % of Mn, which is used as the material for

seagoing ships condensers.

3.3 Copper Usage

The building industry is the largest single consumer of Copper alloys. The following list is a

breakdown of Copper consumption by industry on an annual basis:

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~ Building industry – 47%

~ Electronic products - 23%

~ Transportation - 10%

~ Consumer products - 11%

~ Industrial machinery - 9%

There are around 370 commercial compositions for Copper alloys. The most common alloy

tends to be C106/ CW024A - the standard water tube grade of Copper.

World consumption of Copper and Copper alloys now exceeds 18 million tonnes per annum.

Copper and Copper alloys can be used in an extraordinary range of applications. Some of the

applications for Copper include:

~ Power transmission lines

~ Architectural applications

~ Cooking utensils

~ Spark plugs

~ Electrical wiring, cables and busbars

~ High conductivity wires

~ Electrodes

~ Heat exchangers

~ Refrigeration tubing

~ Plumbing

~ Water-cooled Copper crucibles

The largest end use for Copper is in the building industry. Within the building industry the use of

copper based materials is broad. Construction industry related applications for Copper include:

o ~ Roofing

o ~ Cladding

o ~ Rainwater systems

o ~ Heating systems

o ~ Water pipes and fittings

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o ~ Oil and gas lines

o ~ Electrical wiring

In addition, there are many more applications for the Copper Alloys - Brass and Bronze

3.4 Recycling

Copper alloys are highly suited to recycling. Around 40% of the annual consumption of Copper

alloys is derived from recycled Copper materials. The recycling rate for Free Machining Brass

(CZ121/CW614N) is particularly high with clean/dry swarf having a high value, which

contributes to the cost-benefit calculations in material selection.

3.5 Copper Designations

Designation systems for Copper are not specifications, but methods for identifying chemical

compositions. Property requirements are covered in EN, ASTM, government and military

standards for each composition. The alloy designation system used in the UK and across Europe

uses a 6 character alpha-numeric series. The 1st letter is C for Copper-based material. The second

letter indicates the product form:

B = Ingot for re-melting to produce cast products

C = Cast products

F = Filler materials for brazing and welding

M = Master Alloys

R = Refined unwrought Copper

S = Scrap

W = Wrought products

X = Non-standard materials

The method for designating Copper alloys is an expansion upon the system developed by the US

Copper and Brass industry using five digits preceded by the letter C. There is then a letter

indicating the Copper or alloy grouping, also shown in the table

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Casting Copper Alloys

The nature of the casting process means that most cast Copper alloys have a greater range of

alloying elements than wrought alloys.

Wrought Copper Alloys

Wrought copper alloys are produced using a variety of different production methods. These

methods including processes such as rolling, extrusion, drawing and stamping. Such processes

may be followed by annealing (softening), cold working, hardening by heat treatments or stress

relieving to achieve the desired properties.

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4.0 Magnesium and Magnesium Alloys

4.1 Introduction

Magnesium is an excellent metal as it is readily available commercially and it is the lightest

of all the structural metals having a density of 1.7g/cm3; it also has good heat dissipation,

good damping and good electro-magnetic shield. It is most commonly found in the earth‘s

ocean. At room temperature magnesium and its alloys are difficult to deform due to the

crystal structure which is hexagonal close packed (Figure 4.1).

Figure 4.1: Example of Hexagonal Close Packed Crystalline Structure

This structure restricts its ability to deform because it has fewer slip systems at lower

temperatures. Magnesium has a moderately low melting temperature making it easier to

melt for casting. Additionally it is relatively unstable chemically and extremely susceptible

to corrosion in a marine environment. It is thought that the corrosion is due more to

impurities in the metal versus an inherent characteristic. Finally magnesium powder ignites

easily when heated in air and must be handled very carefully in a powder form. The rest of

this section will review the advantages and disadvantages to magnesium use in engineering

applications. In addition, alloy types and an introduction to coating protections will be

discussed.

4.2 History and properties of magnesium alloys

In the past, magnesium was used extensively in World War I and again in World War II but

apart from use in niche applications in the nuclear industry, metal and military aircraft,

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interest subsequently waned. The most significant application was its use in the VW beetle

but even this petered out when higher performance was required. The requirement to reduce

the weight of car components as a result in part of the introduction of legislation limiting

emission has triggered renewed interest in magnesium. In 1944 the consumption had

reached 228,000 tonnes but slumped after the war to 10,000 tonnes per annum. In 1998 with

renewed interest it has climbed to 36,000 tonnes per annum at a price of US$3.6 per kg. The

growth rate over the next 10 years has been forecast to be 7% per annum.

The advantages of magnesium and magnesium alloys are listed as follows:

o Lowest density of all metallic constructional materials;

o high specific strength;

o good castability, suitable for high pressure die-casting;

o can be turned/milled at high speed;

o good weldability under controlled atmosphere;

o much improved corrosion resistance using high purity magnesium;

o readily available;

o compared with polymeric materials:

better mechanical properties;

resistant to ageing;

better electrical and thermal conductivity;

recyclable

One of the reasons for the limited use of magnesium has been some poor properties exacerbated

by a lack of development work. The disadvantages of magnesium are presented based on the

following:

o low elastic modulus;

o limited cold workability and toughness;

o limited high strength and creep resistance at elevated temperatures;

o high degree of shrinkage on solidification;

o high chemical reactivity;

o in some applications limited corrosion resistance.

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It is not possible to use conventional alloying techniques to improve some of the properties, e.g.

elastic constants. Here one must resort to fibre reinforcement. The solubility of alloying

elements in magnesium is limited, restricting the possibility of improving the mechanical

properties and chemical behaviour.

The crystal structure of magnesium is hexagonal which limits its inherent ductility. The only

alloying element, which causes a useful phase change to bcc, in this respect, is lithium. The lack

of large-scale applications of magnesium alloys in the past has resulted in limited research and

development. Consequently, there are few optimised casting alloys available and even fewer

wrought alloys. The production techniques have been adapted from those for other low melting

point alloys, e.g. aluminium. No experience is available on new production and working

techniques and the know-how accumulated in the past has largely disappeared. The renewed

demand has recently started to change this. The number of primary producers has increased and

it is hoped that, when demand increases further, magnesium will be available at reasonable

prices. Figs. 4.2 and 4.3 show the magnesium apportioned to metallurgical applications.

Fig.4.2 Capacity primary magnesium Fig.4.3. Magnesium apportioned to metallurgical applications

4.3 Properties of the Magnesium Alloys

4.3.1 Specific strength

The vast majority of magnesium applications are covered by AZ91, a die-casting alloy. This

alloy has insufficient creep resistance for many desirable applications at temperatures above

130°C. The aluminium system forms the basis albeit with much lower Al contents for the

development of high specific strength wrought alloys. A binary alloy of 6% Al provides the

optimum combination of strength and ductility. Fig. 4.4 shows further development of Mg– Al

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for die-casting Mg – Al– Mn and for wrought alloys Mg – Al– Zn and for sand casting alloys

with Mg– Si, Mg– Al– Ca–(RE). The development of Mg– Li– X is a development of super

light alloys. Addition of Li decreases strength but increases ductility. Mg– Li alloys can be

aged hardened, although they tend to average at about 60°C. Alloy additions aim at

improving the strength and delay overaging. There are numerous applications for high

specific strength materials as automobile constructional parts, components and machine tool

parts undergoing rapid acceleration and retardation.

Fig. 4.4. Directions of alloy development

Fig. 4.5. Increase in pressure die-cast components in USA and Europe from 1991 to1997

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4.3.2 Ductility of Magnesium Alloy

The previous section emphasised the need for improving ductility but concentrated on a

combination of high strength with reasonable ductility. There is a need for a series of alloys

with very high ductility capable of being formed by thermomechanical treatment. Ductility

is determined by the number of operative slip systems. Mg being hexagonal slips at room

temperature on the base plane (0001)(11 ̅0) and secondary slip on vertical face planes

(10 ̅ 0) in the (11 ̅ 0) direction. This limits ductility at low temperatures. At elevated

temperatures slip also occurs in the (11 ̅0) direction on the (10 ̅1) pyramidal planes. This

behaviour is influenced by alloying, but as long as the structure remains hexagonal the effect

is limited. In addition to the development of new alloys based on Mg– Si and Mg– Al– Ca–

(RE) and Mg– Li– X, which offer the possibility of having a phase mixture of bcc and hcp

phases, there has been punch work into the development of fine grain material. One such

technique, which has been proved successful in the case of aluminium and copper based

alloys and steels, is spray forming fine grain material. This has the additional advantage of

being homogeneous and can be further worked at high temperatures by forging or extrusion.

It was possible to extrude Al– 25Si alloys in this state without difficulties.

4.3.3 Creep Resistance of Magnesium Alloy

Magnesium melts at 650°C. Consequently, it is to be expected that there will be problems

preventing creep in stressed components. Alloys containing Thorium, e.g. HZ22 show at

623K the highest service temperatures of magnesium alloys and, incidentally, compared to

melting point, the highest of any material. The radioactivity of thorium has however resulted

in its exclusion as an alloying element. There are various upper limits for service

requirements, e.g. max. 150°C, 175°C, 200°C etc. This problem can be reduced by

achieving creep resistance without room temperature strength and ductility at an acceptable

price and without making fabrication difficult. The castability (fluidity) of die-casting

alloys, for example, is impaired by using rare earth (RE) elements to improve the creep

resistance. In all probability some solutions will be found in the paths suggested for the

lower temperature applications, otherwise replacing die-casting by other methods, e.g.

squeeze casting will be necessary. The development with scandium containing alloys is

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proving to be successful but despite reducing the amount of scandium required, a dramatic

fall in the price of the alloys remains expensive.

4.3.4 Corrosion Resistance of Magnesium

The disadvantage to using pure magnesium is that it is extremely susceptible to corrosion.

When alloyed, the corrosion resistance is improved, but specific alloys have been proven to

be more corrosion resistant than others. Magnesium is susceptible to different types of

corrosion one type, galvanic corrosion, can sometimes be designed out of the part. Two

ways to protect from galvanic corrosion are:

1) to minimize the chemical potential difference between the magnesium/magnesium

alloys and the dissimilar materials and

2) maximize the circuit resistance.

This corrosion susceptibility was greatly reduced with the discovery that small additions

(0.2%) of manganese gave increased resistance. There are also metallurgical factors that

affect the corrosion performance of a magnesium part which are composition and its

corresponding microstructure and the alloy temper/heat treatment. Each of the different

alloys has specific characteristics that are beneficial to different uses. Some alloys such as

AZ91E, WE43B and Elektron 21 are corrosion resistant alloys. Incorporating these into the

design is beneficial for having a part with a longer life.

While the alloys provide a significant improvement to corrosion resistance, an additional

method to protect the surface of magnesium and its alloys is to coat the magnesium part.

This is specifically beneficial in cases where the part is in contact with other metal parts and

could cause galvanic corrosion. Some examples of protective coatings are fluoride

anodizing, chemical treatments, electrolytic anodizing, sealing with epoxy resins, standard

paint finishes, vitreous enameling, electroplating and cold spray.

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4.4 Material Trends in Car Body Manufacturing

The development of new production techniques, e.g. laser technology and the development

of new materials, steels, aluminium, magnesium and plastics is influencing the ways cars are

made and the materials used. Fig. 4.6 shows this diagrammatically. The development of

constructive method is shown in Fig. 4.7. The diagram illustrates actual and possible

developments starting from the steel unibody of the 1950‘s. In the drive to reduce the

exhaust emissions it is necessary to reduce the weight of the car, improve design

streamlining and improve engine efficiency. Other demands on car designers include power

and comfort, air comfort, reduced noise vibration as well as improved safety, environmental

protection, and corrosion protection result. This will result in an actual increase in weight.

Fig. 4.6. Material trends in car body building. Fig. 4.7. Development of constructional materials in car manufacturing.

The impact of fuel savings by minimising weight increases and improving running

efficiency can be demonstrated by a simple calculation. Assuming new registrations in

Germany in 1997 offer 5% fuel savings over previous models and have an average

consumption of 8.5l/100 km an average mileage of 20,000 km, this means a fuel saving of

300,000,000 per annum. The impact on the environment is self-evident. The introduction of

sulphur-free petrol will completely eliminate sulphur dioxide pollution, which at present is

not re- moved by catalytic converters.

Long-term benefits can only be achieved by consistent basic research. Previous studies on

the development of creep- resistant magnesium alloys have resulted in formulation of rules

of selection for alloying elements. The low melting point of magnesium sets a natural limit

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to the operating temperature. Consequently, if magnesium is to remain competitive with

aluminium alloys it is necessary to raise the operating temperature by alloying techniques.

The search for suitable alloying elements is based on the following premises. The alloying

elements should show sufficient solubility in magnesium at high temperatures, which

decreases with decreasing temperature so that age hardening becomes possible through

precipitation from the supersaturated solution. The precipitates should contain a high

magnesium content thereby increasing the volume fraction of precipitated phase thus

reducing the required amount of alloying element. Several alloying additives should be used

to increase the number of precipitates and by forming complex precipitates improve the

properties of the precipitate. Elements should exhibit a low rate of diffusion in magnesium

and thus reduce the tendency to over-ageing and dislocation climb.

4.5 Alloy development

The property profiles demanded by automobile and other large-scale potential users of

magnesium have revealed the need for alloy development. A straight transfer of ‗high

performance‘ aircraft alloys is not possible not only on economic grounds but often the property

profiles do not coincide. Fig. 4.7 shows the different trends in alloy development depending on

the main requirement.

The Physical Properties of the Alloys Are Influenced by Their Chemical Composition. In

general the constituent elements have the following effects.

Aluminum has a favorable effect on magnesium. It is used up to 10 wt%, with optimum

strength and ductility at approximately 6%. Aluminum improves strength and hardness. It

widens the melting range, which makes the alloy easier to cast. With aluminum content

higher than 6%, the alloy is heat treatable.

Beryllium is used in small amounts (up to 0.01wt%) to decrease surface oxidation when

melting, casting, and welding alloys. It is successfully used in die-cast and wrought

products but must be used judiciously in sand-casting be- cause it coarsens the grain.

Calcium is added in small amounts to help metallurgical control because it increases

grain refinement. It is added just prior to pouring to reduce oxidation. It improves

rolling of sheet products, where it is used below 0.3 wt% so the product can be welded

without cracking. It improves thermal and mechanical properties of the alloy,

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including creep resistance. There is interest in magnesium-zinc-calcium alloys for

medical device applications because these three elements are found naturally in the human

body and are biocompatible and biodegradable.

Cerium improves corrosion resistance, in- creases plastic deformation capability such as

elongation, increases work hardening rates, and reduces yield strength.

Copper improves room temperature and high- temperature strength, but in quantities

greater than 0.05 wt% it adversely affects corrosion resistance and ductility.

Iron is one of the most harmful impurities as it significantly reduces the corrosion

resistance of magnesium alloys. For maximum corrosion resistance, the upper limit of iron

content is specified at 0.005 wt%. Commercial grade alloys where corrosion is not a prime

concern may contain iron as high as 0.01 to 0.03 wt%.

Lithium is relatively soluble in magnesium, so it has attracted interest for making ultra-

light structural materials; lithium has a solid density of 0.53 g/cm, 30% the weight of

magnesium. Lithium increases the ductility of magnesium alloys, thus improving

formability, but it de- creases strength.

Manganese increases saltwater corrosion resistance of aluminum and aluminum-zinc

alloys by capturing iron and other heavy metals in intermetallic compounds that can be

removed dur- ing melting. Commercial alloys rarely contain over 1.5 wt% Mn; in the

presence of aluminum the solubility of Mn is reduced to approximately 0.3 wt%.

Nickel increases yield and ultimate strength at room temperature, but negatively affects

ductility and corrosion resistance in even small amounts. Like iron, for commercial grades

where corrosion is not a concern, Ni content can average 0.01 to 0.03 wt%, but for

maximum corrosion resistance the upper limit of Ni content is specified at 0.005 wt%.

Neodymium improves material strength.

Rare earth metals increase high-temperature creep and corrosion resistance and

strength. They improve castability by narrowing the freezing range of the alloys,

which reduces po- rosity. They also reduce weld cracking. Rare earths are added to

alloys in the form of mis- chmetal or didymium. Mischmetal is a natural mixture of 50

wt% cerium with the remainder being lanthanum and neodymium. Didymium is a

natural mixture of approximately 85% neo- dymium and 15% praseodymium. Check

with industry standards, such as ASTM, for the exact material specifications by

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product form.

Silicon can increase molten alloy fluidity. It is only used in high-pressure die-casting

alloys. It improves elevated temperature properties, especially creep resistance. It

decreases corrosion resistance if iron is also present in the alloy.

Strontium is used in conjunction with other elements to enhance creep performance.

Silver improves mechanical properties by increasing the response to age hardening.

Thorium was used to increase creep strength at elevated temperatures. It improved

weldability of alloys also containing zinc. It is no longer used because of its

radioactivity.

Tin is useful when used with small amounts of aluminum to improve ductility, and it

reduces the tendency to crack during processing, such as forging. It is not a major

alloying element.

Yttrium has relatively high solubility in magnesium and enhances high-temperature

(up to 300°C, 570°F) strength and creep performance when combined with other rare

earth metals.

Zinc is second to aluminum as the most effective and commonly used alloying metal

with magnesium. In conjunction with Al, it increases room-temperature strength.

Additions of 1 wt% or greater when Al is 7 to 10 wt% tend to make the alloy prone to

hot cracking. Zinc increases alloy fluidity in casting. When added to magnesium alloys

with nickel and iron impurities, it can improve corrosion resistance. In combination

with Zr and rare earth metals, it produces precipitation-hardenable alloys with good

strength.

Zirconium has a powerful grain-refining effect in sand and gravity castings.

Zirconium is added to alloys containing zinc and rare earth metals (not combined with

alloys containing aluminum or manganese) when it serves as a grain refiner.

4.6 Designation Systems

No designation system has universal acceptance. Names of alloys have evolved from trade

names of the pioneering companies to chemical and numerical systems.

The ASTM Standard Alloy Designation System is widely used by the industry. Details of the

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ASTM system are given in Table 4.1. As an example of how this alphanumeric system works,

consider magnesium alloy AZ91E-T6. The first part of the designation, AZ, signifies that

aluminum and zinc are the two principal alloying elements. The second part, 91, gives the

rounded-off percentages of aluminum and zinc (9 and 1, respectively). The third part, E, indi-

cates that this is the fifth alloy standardized with approximately 9% Al and 1% Zn as the

principal alloying additions. Letters are used in alphabetic order, except for O and I, which are

not used. The fourth part, T6, denotes that the alloy is solution treated and artificially aged. The

common tempers are listed in Table 4.1.

Pure magnesium (98.8% Mg or higher) is designated by the required minimum amount of

magnesium. Several grades are commercially available for metallurgical and chemical uses.

These are rarely used for structural engineering applications. The grades are designated 9880A

(UNS M19980) and 9880B (UNS M19981) for 98.80% min; 9990A (UNS M19990) for 99.90%

min, 9995 (UNS M19995) for 99.95% min, and 9998A (M199980) for 99.98% min. The

Unified Numbering System (UNS) is a complementary designation system of ASTM and the

Society of Automotive Engineers (SAE). It is not a specification because it does not establish

requirements such as mechanical properties or heat treatment, but it provides identifying

numbers that are useful for searching literature. All magnesium metals and alloys have UNS

numbers starting with M, but the M category is defined as ―miscellaneous nonferrous metals

and alloys,‖ so several UNS M alloy numbers are not magnesium- based.

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Using the ASTM alphanumeric designation system encourages grouping magnesium alloys by

principal alloy composition:

• Magnesium-manganese (M)

• Magnesium-aluminum-manganese (AM)

• Magnesium-aluminum-zinc-manganese (AZ)

• Magnesium-zirconium (K)

• Magnesium-zinc-zirconium (ZK), with rare earth (ZE)

• Magnesium–rare earth metal–zirconium (EZ)

• Magnesium-silver–rare earth metal–zirco- nium (QE)

• Magnesium–yttrium rare earth metal–zirco- nium (WE)

• Magnesium-zinc-copper-manganese (ZC)

• Magnesium-aluminum-silicon-manganese (AS)

• Magnesium-aluminum-strontium (AJ)

A selection of magnesium alloys and characteristics are described in Table 2.

Table 2: Select Magnesium Alloys and Characteristics

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International and Commercial Designations and Standards

Industrial and government standards are convenient means of ensuring consistent performance

of the material and identifying alloys. Standards include mandatory requirements and may

include nonmandatory typical values and information. See Table 2 for the major ASTM and

International Standards devoted to magnesium.

A comparison of the designation of magnesium alloys used by the various organizations is

found in Table 3. This also has several designations (British Standards, BS) that may be useful

for interpreting older technical literature and test results.

ISO Standard 16220:2005, Magnesium and magnesium alloys—magnesium alloy ingots

and castings, last reviewed in 2015, provides chemical compositions of magnesium alloy

castings and mechanical properties of separately cast samples and samples cut from castings. A

new version is under development. ISO 3116:2007, last reviewed in 2013, provides chemical

compositions and mechanical properties for wrought magnesium. There are also ISO standards

for unalloyed magnesium (ISO 8287:2013) and magnesium used for anodes (ISO 26202:2007).

EN 1753:1997, Magnesium and magnesium alloys—magnesium alloy ingots and castings,

provides similar information from CEN, the European Committee for Standarization.

Table 2 Magnesium standards

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Table 3 Similar magnesium alloy designation

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Available Product Forms

The thermal properties of magnesium alloys promote cost-effective casting. A majority of

the alloys are created for casting processes. Most are amenable to sand, permanent mold,

and in- vestment casting. A smaller number are best for high-pressure die-casting, which is

the most used casting process. Together, magnesium alloys are the third most popular

nonferrous casting material, behind aluminum and copper-based alloys. Another subset of

magnesium alloys are de- signed for the wrought products such as wire, rod, hollow tubes,

shapes, sheet and plate, and forgings.

High-Pressure Die-Casting Alloys. The die-casting process is ideally suited to high-volume

production where the high cost of the die can be amortized by the large production volume.

Magnesium alloys allow for high production rates due to their relatively low melting

temperatures, thermal conductivity, and other factors. Traditionally, material is injected into

the die in liquid form, but the use of semisolid injection, thixo- molding, is increasing.

Figure 2: Magnesium die cast part

Die-casting alloys are mainly of the Mg-Al- Zn type (AZ), for example, AZ91. Two

versions of this alloy from which die castings have been made for many years are AZ91A

and AZ91B. The only difference between these two versions is the higher allowable copper

impurity in AZ91B, which can be made from scrap magnesium. The AZ91D version is a

high-purity version of the alloy in which the nickel, iron, and copper impurity levels are

very low and the iron-to-manganese ratio in the alloy is strictly controlled. This high-purity

alloy shows a much higher corrosion resistance than the earlier grades and has good

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mechanical and physical properties. The nominal composition and properties of the die-

casting alloys are given in Tables 4 and 5, respectively.

Table 4 Nominal compositions of magnesium casting alloys for die casting

Table 5 Summary of selected die cast properties

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When greater ductility is needed, the Mg- Al-Mn (AM) alloy is used. AM60B has greater

toughness and more elongation than AZ91D, while retaining good corrosion resistance.

The Mg-Al-Si-Mg (AS) alloys are used for elevated temperatures (up to 175°C, 350°F)

where superior creep strength is needed, while retaining good corrosion resistance.

Table 6 Nominal compositions for sand, investment, and permanent mold castings

magnesium alloys

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Table 7 Summary of sand, investment, and permanent mold cast mold characteristics

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More recent work has produced alloys such as AJ52A and AJ62A containing strontium,

with the aim of improving the high-temperature properties with good corrosion resistance.

AE44 alloy, containing rare earth metals, has also been introduced. This type of magnesium

alloy is increasingly being used in the automotive industry.

Sand, Permanent Mold, and Investment Casting. Several alloying systems are used for these

processes. In general, alloys that are normally sand cast are also suitable for permanent mold

casting. The exceptions to this are the Mg- Zn-Zr alloys (for example, ZK51 and ZK61A)

that exhibit strong hot-shortness tendencies and are unsuitable for permanent mold casting.

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Nominal compositions of these cast alloys are found in Table 6. General characteristics of

the cast alloys are in Table 7.

The Mg-Al and Mg-Al-Zn alloys are generally easy to cast but are limited in certain

respects. They exhibit microshrinkage when sand cast.

They are not suitable for applications in which temperatures of over 95 °C (200 °F) are

experienced. The Mg-RE-Zr alloys were developed to overcome these limitations. A small

amount of zirconium is a potent grain refiner. The two Mg- Zn-Zr alloys originally

developed, ZK51A and ZK61A, exhibit high mechanical properties but suffer from hot-

shortness cracking and are not weldable. Hot-shortness is a high-temperature cracking

mechanism that is mainly a function of how metal alloy systems solidify and is typically

observed during welding or hot-working opera- tions. This cracking mechanism is also

known as hot cracking, hot fissuring, solidification crack- ing, and liquation cracking.

For normal, fairly moderate temperature ap- plications (up to 160 °C, 320 °F), the two al-

loys ZE41A and EZ33A are finding the most use. They are very castable and can be used to

make very satisfactory castings of considerable complexity. A further development aimed at

improving both room temperature and elevated temperature mechanical properties produced

an alloy designated QE22A. In it, silver replaced some of the zinc, and the high mechanical

prop- erties were obtained by grain refinement with zirconium and by heat treatment.

The more recent alloys emerging from re- search contain yttrium in combination with other

rare earth metals (i.e., WE43A, WE43B, and WE54A). These alloys have superior elevated

temperature properties and a corrosion resis- tance almost as good as the high-purity Mg-Al-

Zn types (AZ91D). The latest alloy is Elektron 21 (coded EV31A), which has good elevated

temperature performance, good corrosion resis- tance, and improved ease of casting. The

alloys used for investment casting are very similar to those used for the sand casting process

(Ref 3).

Wrought Products include bars, extruded shapes, tube, rods, wires, sheet, plate, and forg-

ing.

Extruded bars and shapes are made of several types of magnesium alloys (Table 8). For nor-

mal strength requirements, one of the Mg-Al-Zn (AZ) alloys is usually selected. The

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strength of these alloys increases as aluminum content in- creases. Alloy AZ31B is a widely

used moder- ate-strength grade with good formability; it is used extensively for cathodic

protection. Alloy AZ31C is a lower-purity commercial variation of AZ31B for lightweight

structural applica- tions that do not require maximum corrosion resistance. The M1A and

ZM21A alloys can be extruded at higher speeds than AZ31B, but they have limited use

because of their lower strength. Alloy AZ10A has a low aluminum content and thus is of

lower strength than AZ31B, but it can be welded without subsequent stress relief. The

AZ61A and AZ80A alloys can be artificially aged for additional strength (with a sacrifice

in ductility); AZ80A is not available in hollow shapes. AZ21X1 is designed especially for

use in battery applications.

Alloy ZK60A is used where high strength and

good toughness are required. This alloy is heat treatable and is normally used in the

artificially aged (T5) condition. ZK21A and ZK40A alloys are of lower strength and are

more readily extrudable than ZK60A; they have had limited use in hollow tubular strength

requirements.

Alloy ZC71 is a member of a new family of magnesium alloys containing neither aluminum

nor zirconium. The alloy can be extruded at high rates and exhibits good strength properties.

The corrosion resistance of ZC71 is similar to that of AZ91C, but it falls short of that of

AZ91E.

Sheet and plate are rolled magnesium- aluminum-zinc (AZ and photoengraving grade, PE)

and magnesium-zinc-rare earth (ZE) (Table 8).

AZ31B is the most widely used alloy for sheet and plate and is available in several grades

and tempers. It can be used at temperatures up to 100

°C (212 °F).

Alloy PE is a special-quality sheet with excellent flatness, corrosion resistance, and etch-

ability. It is used in photoengraving. ZE10A is a newer grade that can we welded without

the need of postweld stress relief.

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Good formability is an important requirement for most sheet materials. When correct

tempera- tures and forming conditions are employed, all magnesium alloys can be deep

drawn to about equal reduction.

Forgings are made of AZ31B, AZ61A, AZ80A, and ZK60A; the compositions and

properties of these alloys are listed under extruded bars and shapes in Table 8. Alloy AZ31B

may be used for hammer forgings (whereas the other alloys are almost always press forged).

The AZ80A alloy has greater strength than AZ61A and requires the slowest rate of

deformation of the magnesium-aluminum-zinc alloys. ZK60A has essentially the same

strength as AZ80A but with greater ductility. To develop maximum properties, both AZ80A

and ZK60A are heat treated to the artificially aged (T5) condition; AZ80A may be given the

T6 solution heat treatment, followed by artificial aging to provide maximum creep stability.

Hydraulic and mechanical processes are both used for forging magnesium. A slow and

controlled rate of one deformation is desirable because it facilitates control of the plastic

flow of metal; therefore, hydraulic press forging is the most commonly used process.

Magnesium, which has a hexagonal crystal structure, is more easily worked at elevated

temperatures. Consequently, forging stock (ingot or billet) is heated to a temperature

between 350 and 500 °C (650 and 950 °F) prior to forging.

Applications of magnesium alloys

The use of magnesium alloys in the European auto- mobile industry encompasses parts such

as steering wheels, steering column parts, instrument panels, seats, gear boxes, air intake

systems, stretcher, gearbox housings, tank covers etc.

Non-automotive applications

Magnesium based alloys have been used for numerous applications in hobby equipment e.g.

bicycle frames. Interesting applications in communication engineering are shown in Fig. 17.

In this case, light weight is required as well as screening against electro– magnetic radiation

which plastic materials cannot offer.

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Fig. 17. Mg parts used in communication engineering (UNITECH).

Fig. 18. Production of high strength structural materials in the 20th century.

Magnesium is used in a wide variety of applications from medical and metallurgical to

chemical and pyrotechnic. Although the main focus of this book is on the structural

applications of magnesium, other uses of magnesium alloys are also addressed.

Structural. The high strength-to-weight ratio of magnesium alloys is usually a prime reason

for considering these materials in engineering designs. High stiffness-to-weight, castability,

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machinability, and excellent damping are desirable properties of magnesium alloys that

factor into the material selection process. The unique- ness of the magnesium alloys is

illustrated in an Ashby diagram of Young‘s modulus against density among engineering

materials (Fig. 1, Ref 3). The position at a corner of the triangular shape representing all

engineering alloys and its position shared by engineering composites highlight the special

qualities of magnesium alloys. The thermal properties of magnesium factor into the

castability of the alloys and serve in application. On the other side of the ledger, the strong

galvanic potential of magnesium and its weak surface oxidation make corrosion behavior a

major consideration. Fortunately, good design practices and preventive measures are

available to ameliorate environmental degradation.

Structural applications include automotive, industrial, materials handling, commercial, and

aerospace equipment. The automotive applications include clutch and brake pedal support

brackets, steering column lock housings, and manual transmission housings. In industrial

machinery, such as textile and printing machines, magnesium alloys are used for parts that

operate at high speeds and must be lightweight to minimize inertial forces. Materials-

handling equipment includes dockboards, grain shovels, and gravity conveyors. Commercial

applications include handheld tools, luggage, computer housings, and ladders. Magnesium

alloys are valuable for aerospace applications because they are lightweight and exhibit good

strength and stiffness at both room and elevated temperatures.

Pyrotechnics. The first applications of magnesium powder were components of fireworks,

flares, and other incendiary devices to produce brilliant white light. Fine magnesium wire

was used for photographic flash bulbs. Magnesium is still used in fire starters for survival

kits.

Metallurgical. Magnesium is used as an alloying element in nonferrous alloys, such as

aluminum, zinc, and lead. It is used as an oxy- gen scavenger in nickel and copper alloys

and as a desulfurizer in iron and steel production. Magnesium improves the toughness and

ductility of cast iron by making the graphite particles nodular. This is the greatest use of

magnesium by weight.

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Electrochemical Applications. Magnesium is highest on the electromotive series among

metals in salt water, making it desirable as a sacrificial anode for cathodic protection. Con-

structive uses of this mechanism are employed in batteries.

Medical. Magnesium alloys are used in portable medical equipment where light weight is

advantageous. It is also employed for wheel- chairs used in sporting activities (where every

ounce is critical). Because of magnesium‘s bio- compatibility and bioabsorbability, alloys

with other biocompatible elements (such as calcium) are being evaluated for cardiovascular

stents and orthopedic devices for internal bone fixation.

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Titanium and Titanium Alloys

Learning Objectives

• Participants should get an overview of titanium based materials

• Understanding titanium‘s metallurgy, phases, microstructures and resulting properties

• Understanding differences between titanium and aluminium alloys or steels

• After the course, participants should be enabled to identify the most suitable titanium alloy in

the right heat treatment state for their product and have knowledge about advantages and risks

during titanium application

1. Introduction

Because of their excellent corrosion resistance and high specific strength (strength/density),

titanium and its alloys have been widely used for chemical, electric power, and aerospace

industries as major metal materials by taking advantage of their characteristics. On the other

hand, their applications to automobile industry have been limited except for racing cars and

special-purpose cars because of their high cost despite the strong interest shown in titanium

materials by the industry in terms of lightweight, fuel efficiency, and performances.

In recent years, however, titanium and its alloys have come to be actively used for various parts

of the general mass-produced cars due to the following factors:

1)The demand for lightweight parts has become increasingly strict for the prevention of global

warming though reduction of CO2 emission;

2)Remarkable progress has been made in the development of technology for the manufacture of

low-cost titanium; and

3)The appearance and fashionableness peculiar to titanium have come to appeal to the public.

Nippon Steel has also been developing related technology in compliance with the above trend as

summarized in the previous report1).

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Since the examples of actual applications of titanium to automobile parts and the principal

advantages in the use of this metal material were introduced in the previous report, they are

omitted from this paper. This paper deals concretely with part of the company‘s recent activity.

Structure

The crystal structure of titanium at ambient temperature and pressure is close-packed hexagonal

(α) with a c/a ratio of 1.587. At about 890oC, the titanium undergoes an allotropic transformation

to a body-centred cubic β phase, which remains stable to the melting temperature.

Figure 3: allotropic transformation

Properties

Titanium is lightweight, strong, corrosion resistant and abundant in nature. Titanium and its

alloys possess tensile strengths from 210 to 1380 MPa, which are equivalent to those strengths

found in most of alloy steels. The density of titanium is only 56 percent that of steel, and its

corrosion resistance compares well with that of platinum. Of all the elements in the earth‘s crust,

titanium is the ninth most plentiful. Titanium has a high melting point 1725°C. This melting

point is approximately 220°C above the melting point of steel and approximately 1100°C above

that of aluminium.

Table 1: Some properties of commercially pure metals

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The world production of titanium is very small, hundreds of thousands of tonnes, which really

is small, compared to steel at 800 million tonnes per annum (Table 1). 80% of all the titanium

produced is used in the aerospace industries. Car suspension springs could easily be made of

titanium with a great reduction in weight but titanium is not available in the large quantities

needed and certainly not at the price required for automobile applications. The target price for

titanium needs to be reduced to about 30% of its current value for serious application in

massmarket cars.

Pure titanium has excellent resistance to corrosion and is used widely in the chemical industries.

There is a passive oxide film, which makes it particularly resistant to corrosion in oxidising

solutions. The corrosion resistance can be further improved by adding palladium (0.15 wt%).

Alloying Elements of Titanium

The alloying elements can be categorised according to their effect on the stabilities of the α and β

phases (Figure 4). Thus, Al, O, N and Ga are all α–stabilisers. Mo, V, W and Ta are all β–

stabilisers. Cu, Mn, Fe, Ni, Co and H are also β–stabilisers but form the eutectoid. The eutectoid

reaction is frequently sluggish (since substitutional atoms involved) and is suppressed.

Molybdenum and vanadium have the largest influence on β-stability and are common alloying

elements. Tungsten is rarely added due to its high density. Cu forms TiCu2 which makes the

alloys age–hardening and heat treatable; such alloys are used as sheet materials. It is typically

added in concentrations less than 2.5 wt% in commercial alloys. Zr, Sn and Si are neutral

elements.

α-Stabilisers

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α-stabilisers are more soluble in the α-phase and raise the β transus temperature. Figure 4.a

typifies the binary phase diagram formed by addition of an α–stabiliser (such as aluminium,

oxygen, nitrogen or carbon) to titanium. Oxygen is added to pure titanium to produce a range of

grades having increasing strength as the oxygen level is raised. Aluminium is the only other α–

stabiliser used commercially and is a major constituent of most commercial alloys. It is a very

effective α–strengthening element at ambient and elevated temperatures up to about 550°C. The

low density of aluminium is an additional advantageous feature but the amount that can be added

is limited because of the formation of a brittle titanium-aluminium compound at aluminium

contents exceeding about 8% by weight.

The α–phase is also strengthened by the addition of tin (Sn) or zirconium (Zr). These metals have

appreciable solubility in both α– and –phases and as their addition does not markedly influence

the transformation temperature they are normally classified as neutral additions. As with

aluminium, the beneficial ambient temperature hardening effect of tin and zirconium is retained

at elevated temperatures. Figure 4.b demonstrates schematically the phase diagram for titanium

and a neutral element.

-Stabilisers

Elements that depress the transformation temperature, readily dissolve in and strengthen the –

phase and exhibit low α–phase solubility are known as –stabilisers. They can be divided into

two categories according to their constitutional behaviour with titanium:

• -isomorphous elements,

• -eutectoid elements.

-Isomorphous Elements

–isomorphous elements exhibit complete mutual solubility with –titanium. Increasing addition

of the solute element progressively depresses the transformation temperature to give the

characteristic phase diagram shown in Figure 4.c. Molybdenum and vanadium are the most

important –isomorphous elements, while niobium and tantalum have also found application in

some alloys. -Eutectoid Elements –eutectoid elements have restricted solubility in beta

titanium and form intermetallic compounds by eutectoid decomposition of the –phase. A

representative phase diagram is illustrated in Figure 4.d. Elements of the –eutectoid type can be

further subdivided into sluggish and active elements. Commercially important metals in the

sluggish category are iron, chromium and manganese. Eutectoid decomposition of –phase in the

titanium-iron, titaniumchromium and titanium-manganese systems is so slow that intermetallic

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compound formation does not occur during normal commercial fabrication and heat treatment or

during service and, therefore, for practical purposes the behaviour of iron, chromium and

manganese can be likened to that of –isomorphous elements.

In contrast, copper and silicon form active eutectoid systems where below the eutectoid

temperature the –phase decomposes to α and intermetallic compounds within commercially

acceptable times. As a result, controlled precipitation of the intermetallic compounds can be

utilised to enhance the strength of titanium alloys containing appropriate concentrations of

silicon or copper. In addition to strengthening the –phase, –stabilisers have two other

important advantages as alloying constituents. –titanium has an inherently lower resistance to

deformation than the α– modification and therefore elements which increase and stabilise the –

phase tend to improve alloy fabricability during both hot and cold working operations. Addition

of sufficient – stabiliser to titanium compositions also confers a heat treatment capability which

permits significant strengthening to be achieved by controlled decomposition of –phase to α–

phase during the heat treatment process.

Main materials used in high temperature applications

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•Ti base alloys

•Superalloys

•Intermetallics

•Ceramic matrix composites

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Refractory Metals

Renowned for their high melting points, the refractory metals tungsten, molybdenum, and

tantalum are widely used in the construction of high-temperature furnaces as elements, insulation

screens, and furnace furniture. Various applications using the materials are discussed.

Unfortunately, all refractory metals oxidize in air above 250oC (480

oF) so their uses are limited

to vacuum or inert atmosphere applications. Nevertheless, tungsten elements can be used up to

2800oC (5070

oF) in vacuum. Reactions with ceramic insulating materials are discussed, and

methods for the construction of hot zones are reviewed. In the future, available anti-oxidation

coatings may allow usage of these materials in oxidizing conditions.

Refractory metals defined

The term ―refractory metals‖ is generally applied to metals that have melting points greater than

2000oC (3630

oF). Metals in the group are shown in Table 1. Principal materials in the group are

tungsten, tantalum, molybdenum, and niobium, all of which are routinely used in the furnace

industry.

Table 1 – Melting points of refractory metals

Although its melting point is 1852oC (3365

oF), zirconium is also included in the refractory

metals group. Other refractory metals are rhenium, iridium, hafnium, and osmium. Iridium and

osmium also belong to the platinum group of precious metals, and all three are very expensive to

produce, precluding their use to very specialized applications such as nozzles for ceramic fiber

production.

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Rhenium is brittle, very expensive, and difficult to fabricate, so it is mainly used as an alloying

element with tungsten or molybdenum for high-temperature thermocouples and some high

stability components in the electronic valve industry. Ruthenium is extremely hard, brittle and

difficult to fabricate, so its uses are limited to an alloying element with platinum and palladium

to increase wear resistance, with titanium to improve corrosion resistance, and for electroplating

onto other materials for corrosion resistance. Hafnium is found as an impurity in zirconium and

its main use is in the nuclear industry due to its ability to absorb neutrons.

Because of their affinity for oxygen

o oxidation occurs above 250oC (480

oF) for tantalum and niobium, above 300

oC (570

oF)

for molybdenum, and above 400oC (750oF) for tungsten

o their use is confined to protective atmosphere or vacuum furnace applications, where they

may be used for resistance or induction heating elements, radiation screens, and furnace

furniture such as skids, supports, and boats. These metals have good electrical properties

and a high vapor pressure making them suitable for a wide range of applications, such as

annealing, sintering, melting, and brazing. Due to their low electrical resistance,

refractory metal elements operate under high current / low voltage conditions requiring a

variable output transformer or a thyristor-controlled transformer with a current limiting

device to limit the power surge when a cold element is switched on.

Selection criteria

Metals commonly used for furnace interior manufacture are tungsten, molybdenum, and

tantalum where they are used as resistance heating elements and screens/shields. The metals are

also used for the racks, skids and the boats used for material processing such as sintering, powder

reduction, annealing, and vacuum brazing. Advantages of refractory metals include good

electrical properties, low vapor pressures, low electrical resistivity, and low thermal capacity.

This latter property is of advantage as refractory metal furnaces have a low thermal mass leading

to rapid heat up and cooling. Energy savings result since heat is not wasted heating up masses of

refractory insulating material such as bricks. Tungsten and molybdenum also have excellent

resistance to molten glass and quartz, so are used for dies and mandrels as well as electrodes in

the melting process. When choosing a refractory metal for a furnace application, operating

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conditions as well as the properties of the product being processed should be considered. Table 2

shows the compatibility of the most frequently used refractory metals with furnace atmospheres;

Table 3, compatibility with common refractories. All refractory metals are readily fabricated by

conventional techniques. Tantalum and niobium are the easiest, and tungsten is the most

difficult. Tungsten is useful in very high-temperature applications up to 2800oC (5070

oF) in high

vacuum or protective, reducing atmosphere. At these high temperatures, insulation of the hot

zone is a problem and it is normal to use tungsten in high vacuum or a low partial pressure of

hydrogen.

Table 2 – Compatibility of refractory metals with selected atmospheres

Table 3 – Refractory metals compatibility with common refractories

• Heating elements. Hot zones usually consist of a refractory metal element (single, two, or three

phase) constructed of sheet, plate, rods, stranded wire or woven wire mesh surrounded by

radiation screens. Rings of tungsten or molybdenum can be used as susceptors in induction

furnaces. Typical high-temperature heating elements made of refractory metals are shown in

Figures 1–3.

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• Thermal insulation. The choice of thermal insulation depends on the operating temperature

and furnace atmosphere. Hydrogen is a good protective atmosphere for tungsten and

molybdenum, but unfortunately it is an excellent conductor of heat, rendering radiation screens

ineffective. When a hydrogen atmosphere is required for applications such as sintering, it is

normal to use refractory bricks as insulating materials. Similarly, bricks or ceramic fiber

insulation should be used with other gaseous atmospheres.

• Hot zones. In certain applications — especially in the production of carbides — graphite fiber

felt can be used. However, if this material is used with moist hydrogen or if water vapor is

formed as part of the process (for example, a reduction process of metal oxides), the oxygen in

the water vapor reacts with the carbon in the felt to form carbon monoxide, which in turn reacts

with the product being processed with possibly disastrous results. Consequently, when

processing products that react with carbon monoxide at high temperatures, all metal hot zones

should be used.

All metal hot zones rely on preventing heat loss by radiation only and can only be used in

vacuum or low partial gas pressure applications. Screens are made from sheet metal, which can

withstand the temperature at that particular point and can be made of tungsten, molybdenum,

tantalum, or stainless steel. The number of screens required depends on the operating

temperature. A useful ―rule of thumb‖ is to use one radiation screen for each 200oC (360oF). For

instance, a furnace operating at 2400oC (4350oF) would require 12 radiation screens, separated

by refractory metal or ceramic spacers between the element and the water-cooled wall of the

vacuum chamber. In this case, the three inner screens would be made from tungsten, the next six

from molybdenum, and the outer screens from stainless steel. The degree of insulation depends

on the screen separation and inter screen contact where heat is transmitted by conduction

between screens. Separation can be small spacers of sheet metal bent into a tube. Another

method is to rigidize the metal so that the screens only touch at the peaks of the ridges. A typical

rigidized tantalum element and radiation screen pack is shown in Fig. 4.

Which refractory metal?

In selecting a refractory metal for use in a vacuum furnace, the degree of vacuum should also be

considered in addition to the operating temperature. For instance, molybdenum should not be

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used at temperatures greater than 1700oC (3090oF) in a vacuum higher than 1 × 10-4

mbar

because of evaporation problems. When produced, all the refractory metals exhibit fibrous

microstructures, but these change to an equiaxed crystalline structure on heating above the

recrystallization temperature. Tantalum and niobium remain ductile in this condition, but

molybdenum and tungsten become brittle. This explains why welding should be avoided during

furnace construction using these materials especially for load-bearing components, which should

be of riveted construction. The recrystallization temperature of tungsten and molybdenum can be

increased by doping with lanthanum or zirconium oxides. The former raises the recrystallization

temperature of molybdenum from about 1000 to 1900oC (1800 to 3450oF). When

recrystallization occurs, the material recrystallizes into long interlocking grains and not normal

equiaxed grains, thereby maintaining a good degree of ductility. Tantalum and niobium screens

can be welded, as the welds in these materials are ductile, but tungsten and molybdenum

elements or screens are normally riveted or, in the case of screens of very thin sheet, ―stitched‖

with wire. Riveted constructions are not very gas-tight so it is normal practice to plasma spray

the riveted areas with molybdenum to effect a gas seal.

Furnace furniture

In addition to heating elements and radiation screens, refractory metals are used for charge

carrying racks, skids, and boats for containing the products being processed. Hooks for

supporting molybdenum, tungsten rod, or braided wire elements in refractory brickwork are also

made from molybdenum rod or heavy wire. A typical rack and boats manufactured from

lanthanated molybdenum are shown in Fig. 5.

Design considerations

Selected properties of refractory metals useful in design appear in Table 4. Properties like

surface loading depend on the actual operating conditions, such as mounting the element as free

radiating or attaching it to a refractory brick. The density of the element material is important

especially when designing current lead-ins, since tungsten mesh elements can be rather heavy.

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Table 4 – Selected properties of refractory metals*

(a) (b)

Fig. 6: (a) Coated molybdenum electrode resists oxidizing in air at 1100°C, while its uncoated

counterpart readily oxidizes. (b) Furnace assembly with braided molybdenum wire element.

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Future trends

One of the major problems of using refractory metals as heating element materials is that they all

oxidize at low temperatures, so use is limited to an inert atmosphere or vacuum. Today, however,

anti-oxidation coatings based on silicon and boron are now available to prevent oxidation by

coating the surface with a layer of silica. These can be applied by a slurry dipping process,

plasma spraying, or by chemically reacting the metal surface with silicon tetrafluoride. This

latter process is carried out by H.C. Starck Ltd under the proprietary name of Muride ―SP‖ and

―T‖ coatings. Being integral with and chemicaly bonded to the surface, the coating resists

thermal shock and spalling. Primarily developed for use in the aero- space and glass industries, it

may prove useful in coating element materials for use in air at temperatures up to 2000oC

(3630oF). Such elements are currently in the experimental stage and will require further

investigation before being offered commercially. When heated in air to 1100oC (2010oF), a

Muride coated glass melting electrode outperforms an uncoated electrode at the same

temperature (Fig. 6). The uncoated electrode oxidizes rapidly by subliming molybdenum oxide.

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