Metals ProcessingChapter 14:

Outline

Metal Forming Techniques Casting Process Miscellaneous Processes: Powder

Metallurgy and Welding Thermal Processing – Metals Hardenability

Metal Fabrication Techniques Overview

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roll

A o

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• Rolling (Hot or Cold Rolling) (I-beams, rails, sheet & plate)

Ao Ad

force

die

blank

force

• Forging (Hammering; Stamping)(wrenches, crankshafts, piston connecting rods )

often atelev. T

Forming

ram billet

container

containerforce

die holder

die

A o

A dextrusion

• Extrusion (rods, tubing)

ductile metals, e.g. Cu, Al (hot)

tensile force

A o

A ddie

die

• Drawing (rods, wire, tubing)

die must be well lubricated & clean

Forming operations (forging, rolling, drawing, extrusion) are where the shape of a metal is changed by plastic deformation.

Forming processes are commonly classified into hot-working and cold-working operations.

Hot-working refers to processes where metals are plastically deformed above their recrystallization temperature. This allows the material to recrystallize during deformation and prevents the materials from strain hardening; the yield strength and hardness are not increased, while ductility is retained.

Hot-working processes: rolling, extrusion or forging typically are used in the first step of converting a cast ingot into a wrought product.

Deformation energy requirements are less than for cold work.

The lower limit of the hot working temperature is determined by its recrystallization temperature. The upper limit for hot working is determined by excessive oxidation, grain growth, undesirable phase transformation.

Hot Working

Recrystallization

Recrystallization is the formation of a new set of strain-free and equiaxed grains that have low dislocation densities (pre-cold work state).

The driving force to produce the new grain structure is the internal energy difference between strained and unstrained material.

The new grains form as very small nuclei and grow until they consume the parent material.

Recrystallization temperature is between 1/3 Tm to 1/2 Tm.

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When cold-working is excessive, the metal will fracture before reaching the final shape.

Cold-working operations are usually carried out in several steps with annealing used to soften the cold-worked metal and restore ductility.

A higher quality surface finish than hot working.

Closer dimensional control of the finished piece.

Cold-working of a metal results in an increase in strength or hardness and a decrease in ductility.

Cold WorkingWatch a Cold-Drawn Tubing Process

View the video.

Grain Flow

Forging is the process where metal (Fe, Ti, Al) is heated and shaped by plastic deformation (compressive forces). The compressive force typically comes from hammer blows or a press.

Forged articles have outstanding grain structures and the best combination of mechanical properties. Forging refines the grain structure and improves physical properties of the metal.

With proper design, the grain flow can be oriented in the direction of principal stresses encountered in actual use. Grain flow is the direction of the pattern that the crystals take during plastic deformation.

Forging http://www.forging.it/

Forging

Physical properties (strength, ductility and toughness) are much better in a forging than in the base metal that has crystals randomly oriented.

Forgings are consistent from piece to piece, without any of the porosity, voids, inclusions and other defects. Also coating operations such as plating or painting are straightforward due to a good surface that needs very little preparation.

The forge or smithy is the workplace of a smith or a blacksmith. A basic smithy contains a forge for heating the metals to a temperature where work hardening ceases to accumulate, an anvil (to lay the metal pieces on while hammering), and a slack tub (to rapidly cool and harden forged metal pieces). Tools include tongs (not thongs) to hold the hot metal and hammers to strike the hot metal. 9

Connecting rods

crankshafts

Alcoa announces that its Alcoa Auto Wheels business is supplying lightweight forged aluminum wheels for the new Ferrari 458 Italia. The front wheel weighs just 22.8 pounds and the rear wheel weighs 25.3 pounds, a result that is achievable only with a forging process. Wheel diameters are increased to 20 inches to accommodate large carbon-ceramic brake discs. Learn more.

Most forging processes begin with open die forging. Open die forging shapes heated metal parts between a top die attached

to a ram and a bottom die attached to a hammer anvil or giant hydraulic press bed.

Metal parts are worked above their recrystallization temperatures (ranging from 1900°F to 2400°F for steel) and gradually shaped into the desired configuration through hammering or pressing.

While impression or closed die forging confines the metal in dies, open die forging is never completely confined or restrained in the dies.

Wrenches, automotive crankshafts and piston connecting rods are typical objects formed by forging.

Some disadvantages of forging are the high cost (dies) and high residual stress produced.

Die Forging

Closed die forging - The shaping of hot metal within the walls of two dies that come together to completely enclose the work piece.

The process of plastically deforming a metal by passing it between rollers; a reduction in thickness results from compressive stresses exerted by the rolls.

This is the most widely used metalworking process because it lends itself to high production and close control of the final product.

After extraction processes, many molten metals are solidified by casting into large ingot molds. The ingots are normally subjected to hot rolling to produce a flat sheet or slab. These are more convenient shapes for subsequent metal forming operations (extrusion, forging, drawing).

Rolling

http://www.titanium.org/chinese/English/Technical%20Data/Manufacturing%20Techniques/rolling.html

The principal rolling processes are hot rolling and cold rolling.

Hot rolling is the most common method of refining the cast structure of ingots and billets to make primary shapes.

Bars of circular or hexagonal cross section like I beams, channels, and railroad rails are produced in great quantity by hot rolling with grooved rolls.

Cold rolling is most often a secondary forming process that is used to make bar, sheet, strip and foil with superior surface finish and dimensional tolerances.

Hot Rolling & Cold Rolling

A bar of metal is forced through a die orifice by a compressive force that is applied to a ram

The extruded piece that emerges has the desired shape and a reduced cross-sectional area.

Extrusion products include rods and tubing, but shapes of irregular cross-sections may be produced form the more readily extrudable metals, like Al.

Extrusion is increasingly utilized in the working of metals difficult to form, like stainless steels, Ni-based alloys, and other high-temperature materials

Extrusion

To produce tubing by extrusion, a mandrel must be fastened to the end of the extrusion ram

The mandrel extends to the entrance of the extrusion die, and the clearance between the mandrel and the die wall determines the wall thickness of the extruded tube

One method of extruding a tube is to use a hollow billet for the starting material

Extrusion of Tubing

Drawing is the pulling of a metal piece through a die having a tapered bore by means of a tensile force that is applied on the exit side

Rod, wire and tubing products are commonly fabricated in this way.

Wiredrawing usually starts with a coil of hot-rolled rod Draw speeds vary from about 30 to 300 ft/min In general, the term wire refers to small diameter products

under 5 mm that may be drawn rapidly on multiple-die machines.

Drawing

http://www.titanium.org/chinese/English/Technical%20Data/Manufacturing%20Techniques/forming.html

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Four typical casting processes: (a) and (b) Green sand molding where clay-bonded sand is packed around a pattern. Sand cores can produce internal cavities in the casting. (c) The permanent mold process where metal is poured into an iron or steel mold. (d) Die casting where metal is injected at high pressure into a steel die. (e) Investment casting where a wax pattern is surrounded by a ceramic; after the wax is melted and drained, metal is poured into the mold.

Casting Methods

Casting a fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape of the mold but experiences some shrinkage.

Casting techniques are used when

1. The finished shape is so large or complicated that any other method would be impractical

2. A particular alloy is so low in ductility that forming by either hot or cold working would be difficult

3. In comparison to other fabrication processes, casting is the most economical.

Casting

Sand can withstand T >1600ºC Sand is inexpensive and easy to mold. A two-piece mold is formed by packing sand around

a pattern that has the shape of the intended casting. Often used for large parts, auto engine blocks (see

images). A mold is formed by using single or multiple patterns. The molding sand (silica sand with binder) used,

results in a stable and refractory mold which is a perfect negative of the pattern.

Sand Casting

http://www.vonroll-casting.ch/en/

Die Casting

The liquid metal is forced into a mold (die) under pressure at a relatively high velocity, then allowed to solidify with the pressure maintained.

A two-piece permanent steel mold is used; when clamped together, the two pieces form the desired shape.

When complete solidification has been achieved, the mold pieces are opened and the cast piece is ejected.

Rapid casting rates are possible, making this an inexpensive method; a single set of molds may be used for thousands of castings.

This technique lends itself only to relatively small pieces and to alloys of low melting points such as Zn, Al and Mg

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Stage I — Mold formed by pouring

plaster of paris around wax pattern. Plaster allowed to harden.• Stage II — Wax is melted and then poured from mold—hollow mold cavity remains.

• Stage III — Molten metal is poured into mold and allowed to solidify.

• Investment Casting (low volume, complex shapes like jewelry, turbine blades, jewelry and dental crowns and inlays, and jet engine impellers)

wax I

II

III

Investment Casting (lost-wax casting)

Plaster die formedaround wax prototype

Continuous Casting Continuous casting (also called strand casting) is

the process whereby molten steel is solidified into a "semi-finished" billet, bloom or slab for subsequent rolling in the finishing mills.

In the continuous casting process, molten metal is poured from the ladle into the tundish and then through a submerged entry nozzle into a mold cavity.

The mold is water-cooled so that enough heat is extracted to solidify a shell of sufficient thickness. The shell is withdrawn from the bottom of the mold at a "casting speed" that matches the inflow of metal, so that the process ideally operates at steady state. Below the mold, water is sprayed to further extract heat from the strand surface, and the strand eventually becomes fully solid when it reaches the ''metallurgical length''.

Blowholes, pinholes, shrinkage cavities, & porosity Blowholes and pinholes are holes formed by gas entrapped

during solidification. Shrinkage cavities are cavities that have a rougher shape

and sometimes penetrate deep into the casting.Shrinkage cavities are caused by lack of proper feeding or non-progressive solidification.

Porosity is pockets of gas inside the metal caused by micro-shrinkage, e.g. dendritic shrinkage during solidification.

Casting Defects — Cavities

Cracks in casting and are caused by hot tearing, hot cracking, and lack of fusion (cold shut) A hot tear is a fracture formed during solidification because of

hindered contraction.

A hot crack is a crack formed during cooling after solidification because of internal stresses developed in the casting.

Lack of fusion is a discontinuity caused when two streams of liquid in the solidifying casting meet but fail to unite.

Rounded edges indicate poor contact between various metal streams during filling of the mold.

Casting Defects — Discontinuities

Dendrites of a shrinkage cavity in an aluminum alloy

• Discontinuities in castings that exhibit a size, shape, orientation, or location that makes them detrimental to the useful service life of the casting.

• Some casting defects are remedied by minor repair or refurbishing techniques, such as welding.

• Other casting defects are cause for rejection of the casting.

The distinctive metallurgical characteristics of castings are acquired during solidification, whereas with wrought materials, they are acquired during mechanical deformation.

The principal metallurgical difference between castings and wrought materials is that castings lack homogeneity. They do not have the benefit of hot work to accelerate the diffusion

of the chemical elements to achieve homogenization.

Cast alloys require significantly longer soaking times to achieve homogenization.

Cast alloys frequently contain more silicon to improve the fluidity of the molten metal.

Solidified castings contain high residual stresses from solid shrinkage, unless they are removed by a stress relief annealing process.

Cast and Wrought Alloys

How Metals are MadeCool Stuff Being Made: How Steel Is Made

                                                      

From ore to sheet, watch how US Steel makes steel products.Learn more.

Watch how Superior Tube rolls its product.

                 

A flat strip is rolled into a tubular shape and the seam is welded, without the use of flux or filler metal. Watch the video.

Watch aluminum foil being made

                                                      

Learn more.

This 1300-pound bronze bell was cast by the Meneely Foundry in West Troy, New York, in 1850. The video shows how a bronze bell is cast. Learn more.

Basic steps in the extraction of steels using iron ores, coke and limestone. (Source: www.steel.org. Used with permission of the American Iron and Steel Institute.)

(Source: www.steel.org. Used with permission of the American Iron and Steel Institute .)

Secondary processing steps in processing of steel and alloys.

Animation shows blast furnace operation in a training video from Corus Steel (now part of Tata Steel). Watch video.

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Metal Fabrication Methods

Nanophase Al-7.5Mg

Powder Evolution during Cryo-Milling

A fabrication technique involves the compaction of powdered metal, followed by a heat treatment to produce a more dense piece.

Powder metallurgy is especially suitable for metals

having low ductilities

having high melting temperatures

Powder Metallurgy

pressure

heat

point contact at low T

densification by diffusion at higher T

area contact

densify

Production of P/M Parts:

Preparation of Metal Powders

Compaction (pressing)

Sintering (densification) at elevated temperature

Forged Inducer & Impellers in Fuel

Turbopump

Formed sheet panels for elevated temperature fairings

Reduce assembly costsReduce life cycle costsReduce fastener weights

Normal Grain Rivet

Submicron Grain Rivet

High Specific Strength Rivets

Undersea Vehicle Hull

Friction Welded Plate

Lightweight, Structural Armor

Rocket Engine:Extruded,

Flow-Formed & Friction

Welded High Pressure

Propellant Ducts

Laminated Foils

Nano-Aluminum Target Applications

In welding, two or more metal parts are joined to form a single piece when one-part fabrication is expensive or inconvenient.

Both similar and dissimilar metals may be welded.

The joining bond is metallurgical (involving some diffusion) rather than just mechanical, as with riveting and bolting.

A variety of welding methods exist, including arc and gas welding, as well as brazing and soldering.

Brazing is a joining process whereby a filler metal or alloy is heated to melting temperature above 450 °C (840 °F).

Soldering is a process where two or more metals are joined together by melting and flowing a filler metal into the joint, the melting point of the filler metal is below 400 °C (752 °F).

During arc and gas welding, the work pieces to be joined and the filler material are heated to a sufficiently high temperature to cause both to melt; upon solidification, the filler material forms a fusion joint between the work pieces.

Welding

The heat-affected zone is the narrow region of the base metal adjacent to the weld bead, which is metallurgically altered by the heat of welding.

The heat-affected zone is usually the major source of metallurgical problems in welding.

The width of the heat-affected zone depends on the amount of heat input during welding and increases with the heat input. If the material was previously cold worked, the HAF may have experienced recrystallization and grain growth, and a diminishment of strength, hardness and toughness.

Heat-Affected Zone (HAF)

Generally, the heat-affected zone varies from 1.5 mm to 6.5 mm wide (0.06 in to 0.25 in)

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diagram of the fusion zone and solidification of the weld during fusion welding.initial prepared joint

weld with filler metal

weld after solidification

For steels, the material in this zone may have been heated to temperatures sufficiently high so as to form austenite. Upon cooling to room temperature, the microstructural products that form depend on cooling rate and alloy composition.

For plain carbon steels, normally pearlite and a proeutectoid phase will be present.

For alloy steels, one microstructural product phase may be martensite, which is ordinarily undesirable because it is so brittle.

Upon cooling, residual stresses may form in this region that weaken the joint.

It can also lead to loss of corrosion resistance in stainless steels and nickel-base alloys.

Microstructural Changes Nearby HAF

With carbon and low-alloy steels, the rapid cooling rate from the welding temperature is similar to quenching in heat treatment operations

The higher the carbon or alloy content, the more easily martensite is formed and the more brittle the martensite is

This situation may easily cause cracking as the steel cools down.

Steels that are susceptible to cracking must be preheated to “cushion” the effects of martensite formation.

They are also post-weld heat treated to temper (improve the toughness) any martensite that is formed and additionally stress relieve the joint.

Stress Relieving - Always done below the transformation temperature of the metal to minimize the welds residual stress. The temperature is held for roughly an hour until the residual stresses are minimized, then cooled very slowly to prevent new stresses from setting up in the metal.  

Preheating and Post-Weld Heat Treatment

The carbon equivalent is a formula based on chemical composition that determines the need to preheat and post-weld carbon and low-alloy steels.

The higher the carbon equivalent, the greater the tendency toward cracking in the heat-affected zone.

Plain carbon steels with a carbon equivalent < 0.4% to 0.5% are considered readily weldable without the need for preheating or post-weld heat treatment.

Carbon Equivalent

Carbon Equivalent (CE)

= %C + %Mn/6 + %Ni/20 + %Cr/10 + %Cu/40 –%Mo/50 – %V/10

Cracking is rarely tolerated and must be removed by grinding

Crack formation is aggravated

by welding fixtures that do not permit contraction of the weld during cooling,

by narrow joints with large depth-to-width ratios,

by poor ductility of the deposited weld metal,

or by a high coefficient of thermal expansion coupled with low-heat conductivity in the parent metal

Cracking in Welding

Hydrogen cracking occurs in the heat-affected zone of some steels as hydrogen diffuses into this region when the weld cools

Hydrogen cracking is caused by atomic hydrogen.

The sources of atomic hydrogen are

organic material,

chemically bonded water in the electrode coating,

absorbed water in the electrode coating,

and moisture on the steel surface at the location of the weld

Hydrogen Cracking

Using low-hydrogen electrodes, which includes baking and storing them in a low-temperature oven.

Preheating the surface of the steel before welding to remove moisture.

Post-weld heat treating immediately to force the hydrogen to escape.

Peening immediately after each pass is also beneficial because it induces compressive stresses and offsets the tendency toward cracking.

Methods of Avoiding Hydrogen Cracking

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Common forms of heat treating processes.

Thermal Processing of Metals

Types of Annealing

• Process Anneal: Negate effects of

cold working by (recovery/

recrystallization)

• Stress Relief: Reduce stresses resulting from:

- plastic deformation - nonuniform cooling - phase transform.

• Normalize (steels): Deform steel with large grains. Then heattreat to allow recrystallization and formation of smaller grains.

• Full Anneal (steels): Make soft steels for good forming. Heat to get , then furnace-coolto obtain coarse pearlite.

• Spheroidize (steels): Make very soft steels for good machining. Heat just below Teutectoid & hold for

15-25 h.

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a) Full Annealing

b) Quenching

c) Tempering:

(Tempered Martensite)

Heat Treatment Temperature-Time Paths

c)

P

B

0%

100%50%

A

A

a)b)

Annealing describes a heating, holding and cooling process to achieve specific metallurgical results.

The Fe-iron carbide phase diagram shows the eutectoid region.

The horizontal line at the eutectoid temp., labeled A1, is the lower critical temperature (LCT). All austenite will have transformed into ferrite and cementite phases below the LCT.

Annealing

The phase boundaries denoted A3

and Acm represent the upper critical temperature lines for hypoeutectoid and hypereutectoid steels. For temperatures above these boundaries, only austenite will exist.

Normalizing

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An annealing treatment called normalizing is used to refine the grains (decrease the average grain size) and produce a more uniform size distribution; fine grained pearlitic steels are tougher than coarse-grained ones.

To normalize, the temperature must be raised roughly 55 degrees above the upper critical temperature (above A3 or Acm depending on composition).

continuous conveyorized normalizing furnace

Har

dnes

s, H

RC

Distance from quenched end

Hardenability -- Steels• Hardenability – measure of the ability to form martensite• Jominy end quench test used to measure hardenability.

Plot hardness versus distance from the quenched end.

Hardness Changes with Distance

Correlation of hardenability and continuous cooling information for and iron-iron carbon alloy of eutectoid composition.

Hardenability vs Alloy Composition

"Alloy Steels" (4140, 4340, 5140, 8640) -- contain Ni, Cr, Mo (0.2 to 2 wt%) -- these elements shift the "nose" to longer times (from A to B) -- martensite is easier to form

T(°C)

10-1 10 103 1050

200

400

600

800

Time (s)

M(start)M(90%)

BA

TE

Hardenability curves for 5 alloys each with 0.4 wt% C.

Hardenability curves for 8600 series alloys where only carbon content is varied.

Hardness increases with carbon content. Also, during production of steel, there is always a minor variation in

composition and average grain size from one batch to another; this results in some scatter of measured hardness values.

Hardenability band for an 8640 steel indicating maximum and minimum limits for hardness.

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• Effect of quenching medium:

Mediumairoil

water

Severity of Quenchlow

moderatehigh

Hardnesslow

moderatehigh

• Effect of specimen geometry: When surface area-to-volume ratio increases: -- cooling rate throughout interior increases -- hardness throughout interior increases

Positioncentersurface

Cooling ratelowhigh

Hardnesslowhigh

Influences of Quenching Medium & Specimen Geometry