Metal forming processes

Metal forming Using forces to shape solid metal plastically Avoids problems with solidification which can occur during casting Minimises the high scrap loss of machining processes Removes segregation and defects present in cast ingots Promotes a desirable fibre structure Machined thread Rolled thread

Classification of methods Primary metal working - Processing Forming ingots or other cast forms into simple shapes (plate, sheet, bar) Often performed hot Rolling, extrusion Secondary metal working - Fabricating Producing components from simple shapes. Often performed cold Deep drawing, bending, shearing, machining

Typical stress states

Process analysis Used to determine required forces, for selecting equipment May predict failure Study of yielding behaviour (flow stress) Total work put into working operation = work involved in shape change + work to overcome friction + redundant work. Mathematical & computer modelling

Cold working (forming) Strength is increased and ductility reduced Strength improvements can be dramatic Work hardening rate depends on material Grain structure is distorted Dislocation population is increased From ~104 lines/mm2 in fully annealed metal To ~1010 lines/mm2 in fully cold worked metal Dislocation locking

Cold work advantages Strength, fatigue & wear properties improved by cold working Good surface finish & dimensional control No oxidation Finishing processes may not be needed

Cold work limitations Higher forces and therefore more powerful equipment is required Only a limited amount of cold work can be undertaken before the material fails Brittle materials cannot be cold worked at all (tungsten, silicon carbide, glass) Intermediate anneals may be required Undesirable residual stress may be created

Cold worked material Many materials are available in the cold worked condition The temper designation is the amount of cold work Annealed ‘O’ no cold work 1/4 hard = 25% of the maximum cold work possible 1/2 hard and 3/4 hard designations Fully work hardened = no ductility left

Annealing heat treatment Removes the effect of cold work, Increasing ductility Reducing strength Improves other physical properties Eg conductivity of copper At a temperature of about 0.5 of the melting temperature in degrees Kelvin Applied ONLY to cold worked metals

Effect of annealing New grains are nucleated where deformation is highest The more heavy the cold work, the more grains are nucleated & the finer the grain size The new grains take over the cold worked metal by diffusion annihilating the distorted structure (recrystallisation) Dislocation density is reduced

Grain growth Reduction of grain boundary energy Occurs at higher temperatures or insufficient amounts of deformation Grain boundaries straighten Large grains tend to consume small grains Yield strength and ductility are both reduced Can be inhibited when second phase particles pin grain boundaries

Recrystallisation temperature Depends on amount of initial cold work Less than critical strain, no recrystallisation Critical strain for iron - 10%, for aluminium - 1% High amount of cold work, lower recrystallisation temperature Also depends on the time at temperature. Long times reduce the recrystallisation temperature.

Typical recrystallisation temperatures

Hot working Carried out at a temperature and strain rate at which recrystallisation is simultaneous with deformation. Above about 60% of the absolute melting point New grains are continually formed Material properties (yield strength, ductility) largely independent of the amount of hot work, and are the same as if the material was cold worked and annealed The amount of deformation is limitless

Effects of hot working Crystal structure is refined Original cast structure eliminated Facilitates homogenisation Defects can be welded closed Improves strength, ductility, toughness by refining grain size.

Temperature limits The maximum temperature is determined by the point at which constituents in the material melt Low melting point phases may be present High strain rates cause adiabatic heating Must be above the recrystallisation temperature ‘Hot’ work can be at room temperature (Sn, Pb)

Formability of a metal Load required for yielding Material ductility Reduced by increasing temperature Material ductility Ability to stand tensile stress without cracking Stress system imposed by forming Some processes more suitable than others for less ductile materials

Pure metals Flow stress decreases with melting point Low formability - tungsten Good formability - tin, lead, zinc Ductility increases with number of slip planes fcc - large number of slip planes, Al, Cu bcc - fewer slip planes, Ferrite cph - limited slip planes, Mg

Alloying effect Increasing alloying level and complexity Usually lowers melting point Raises work hardening rate Generally higher alloy levels and more complex alloys are more difficult to form Melting temperature Hot working range Temperature Alloy content %

Forging Localised compression is used to form complex shapes Usually a hot work process, seldom cold Open die forging Simple shapes, larger forgings, slow process Closed die forging (stamping) Small items, large numbers, complex shape, expensive dies Drop hammers (Hammer and anvil) Press forming - more deeply penetrating

Features of forging A batch process, limited productivity Capable of producing a wide variety of shapes Components made by forging have better properties than those made by casting or machining from stock Useful in reducing machining costs Less machining time Less scrap

Forging types Fullering Edging Drawing Gutter Forging Die Flash Swaging (Shaft is rotated) Closed Die

Forged products High quality irregular shapes Coins (cold forgings) Gears, levers, crankshafts, pipe fittings, gas cylinders, rings Fasteners (nuts, screws) Coins (cold forgings)

Rolling By compressing between rolls, a material is reduced in thickness and increased in surface area Can be regarded as a form of continuous open die forging. Cylindrical rolls produce flat products (plate & sheet) Thickness variations by controlling roll spacing Grooved rolls used for long products Angles, channels, tees, beams, columns

Types of roll stands 3-high 4-high 2-high (reversing or Non-reversing) Cluster rolls (12-high) Backing rolls

Hot rolling Breakdown of as-cast shapes (ingots & strands) to slabs, blooms or billets Finished steel sections and plates Simple two- or three-high rolling systems Product has mill scale and has to be finished May require pickling Structural steel can be sand blasted and primed

Cold rolling Improved finish Higher forces needed For better finish and dimensional control, more complex rolls are required

Extrusion Material compressed through a hole in a die to make a product of uniform cross section A mode of deformation that occurs in other working processes, particularly closed die forging Almost always performed hot Flow is complex, with a lot of redundant work (bending and unbending) Friction plays an important part Surface of billet tends to stick to container, extrusion surface is new

Extruded products Long products, uniform cross section. Can be complex sections, which cannot be rolled Reentrant angles Window frame sections Small billets used to make containers Beer cans Ductile materials (aluminium)

Extrusion processes Direct extrusion - solid Indirect extrusion - tube Impact extrusion

Deep drawing and pressing The formation of shapes, such as cups and dishes from sheet material Often undertaken cold to allow work hardening and maintain a high surface quality Stress systems vary over the surface and include biaxial tension, bending and unbending, circumferential and ironing compression. A high work hardening rate is desirable so that distortion is shared over the whole surface

Final forming Deformation less than primary forming Bending is the most common process Includes roll bending of plate, tube and sections Induction bending Local induction heating of increment Spinning of dished heads.

Machining Passing a tool through the metal, which should break off in chips. Cold working with fracture of the chip from the component Ease of machining depends on: Design of tool Lubrication Material being cut (machinability)

Machinability Measured by speed of cutting Strength of material affects the force necessary for machining Ductility affects the type of chip formed and the ease of its removal Very ductile materials such as copper or aluminium spread under the cutting tool, and can pressure-weld to it.

Machinability improved by Low number of slip planes for dislocation glide Compare fcc aluminium to cph magnesium Presence of brittle or weak second phase particles Graphite in cast iron, sulphides in free cutting steel Cold work hardening mechanisms Solid solution, second phases, etc