MSE 258:Physical Metallurgy of Non-Ferrous Metals

Dr. Emmanuel Kwesi Arthur

Department of Materials Engineering,

Kwame Nkrumah University of Science and Technology, Kumasi

2019

Lecture Three

Aluminum(cont’d)

Aluminum is an extremely useful engineering material as:

• Al stays ductile at low temperatures

• Al has a relatively low price

• It is the third most plentiful element on earth (next to oxygen and silicon).

• Al is easily alloyed and many of its alloys are stronger than pure Al.

• Al alloys have an excellent machining, forming, and forging characteristics.

• Al alloys have a relatively high thermal expansion.

• Al alloys have a relatively high thermal conduction.

- Al’s FCC crystal structure retains its strength, ductility and toughness at cryogenic temperatures. This is why we see many cryogenic tanks made from Al.

Aluminum and Aluminum Alloys

Light-weight bike Car bodyEquipment

Aluminum

Disadvantages of Aluminum include:

Al has a low melting temperature so can’t be used at high temperatures (above ~ 400 oC)

Al has low hardness so it is not good for wear resistance.

Because Al’s elastic modulus is 1/3 that of steel, it’s deflection as a structural component may be too great for the application.

Al’s FCC structure work hardens so it may become brittle after plastic deform and fracture easily.

Al’s high thermal expansion sometimes causes problems with its use as an interconnect for electronic devices. Good electronics use Cu alloys and the best use Au.

Aluminum Alloys Aluminum alloys can be subdivided into two major groups based on

their method of fabrication:

Wrought alloys

Cast alloys

Wrought alloys are shaped by plastic deformation and have microstructures and compositions different from the casting alloys because of the differences in manufacturing requirements.

Within each of these two major groups, they are further subdivided into two subgroups:

Heat treatable alloys

Non-heat treatable alloys

Heat treatable alloys are strengthened by “age hardening”, whereas nonheat treatable alloys are strengthened by strain hardening.

Aluminium and aluminium alloys

- Metallurgical (99.5 - 99.8% Al)

- Refined (up to 99.9% Al)

Age Hardening

There are three steps to age harden materials such as Aluminum.

1. Solution Treatment – the alloy is heated above the solvus temperature into a single phase region of the phase diagram to dissolve any secondary phases such as precipitates. The material is held at this temperature until a homogeneous solid solution is produced. Al is usually solution-treated between 500 oC and 548oC.

2. Quench – the alloy is rapidly cooled so the atoms do not have enough time to diffuse to potential nucleation sites. The alloy remains as a single phase material that is supersaturated with alloying elements. If the material is work hardened, the increase in dislocations density can be used as nucleation sites during aging.

Age Hardening

3. Aged – The alloy is heated to a temperature below the solvus so the atoms can diffuse to numerous nucleation sites to produce precipitates. Nucleation of precipitates is enhanced by the presence of dislocations. Ideally, uniformly highly dispersed, ultrafine precipitates give the best effect in age hardening or precipitate strengthening.

nuclei of the qprecipitate, which is eventually forms in Al-Cu Alloys.

The yield strength hardens by the formation of precipitates, which after longer times rippen (get large) and the strength falls off. The strength does not fall off at low aging temperatures.

Requirements for Age Hardening

Not all alloys are age hardenable. Four conditions must be satisfied for an alloy to have an age-hardened response during heat treatment.

1)The alloy system must display a decreasing solid solubility with decreasing temperature. In other words, the alloy must form a single phase on heating above the solvus line, then enter a two-phase region on cooling.

2) The matrix should be relatively soft and ductile, and the precipitate should be hard and brittle. In most age hardenable alloys, the precipitate is a hard,brittle intermetallic compound.

3) The alloy must be quenchable. Some alloys cannot be cooled rapidly enough to suppress the formation of the precipitate. Quenching may, however, introduce residual stresses that cause distortion of the part. To minimize residual stresses aluminum alloys are quenched in hot water, at about 80 oC, i.e., a hot quench.

4) A coherent precipitate must form.

Many important alloys are age-hardenable including stainless steels and alloys based on aluminum, magnesium, titanium, nickel, chromium, iron, and copper.

Wrought Aluminum

Form: 1XXX

1st #- major alloying element

2nd #- modifications of alloy, if different from 0 indicates a variation of the alloy

3rd & 4th #- decimal % of aluminum in the alloy EX: 1050 is aluminum with minimum 99.50% Al Ex: 1090 shows a minimum of 99.90% Al

Classification system of Al and Al-alloys

Cast Aluminum

Form: 1XX.X

1st # indicates the major alloying element.

2nd & 3rd #- minimum amount of aluminum

4th #- indicates Product form, 1 or 2 is ingot (depends upon purity) and 0 is for casting

Cast aluminum: Aluminum can be cast by all common casting processes. Aluminum casting alloys are identified with a unified, four-digit (xxx.x) system.

The first digit indicates the major alloying element. For instance, 100 series is reserved for 99% pure aluminum with no major alloying element used.

The second and third digits in the 100 series indicate the precise minimum aluminum content. For example, 165.0 has a 99.65% minimum aluminum content.

The 200-900 series designate various aluminum alloys, with the second two digits assigned to new alloys as they are registered.

The fourth digit indicates the product form. Castings are designated 0; ingots are designed 1 or 2.

Temper Designation

F: as fabricated (by cold or hot working or by casting)

O: Annealed (from the cold worked or cast state)

H: strain hardened by cold working (for wrought products only)

T: heat treated

W: solution treated only (unstable temper)

Temper Designation System for Aluminum Alloys