Metals- Solid State Processes Flashcards
What do solid state deformation processes do?
Induce a shape change in a solid metal workpiece by plastic deformation under forces applied by tools and dies. Workpiece generally produced initially by casting.
Primary and secondary solid state processes
Primary: rolling, extrusion, forging
Secondary: wire drawing, machining
Features of forging
Produces discrete parts with a set of dies
Finishing operations usually required
Elevated temperatures usually needed
Die and equipment costs relatively high
Features of rolling
Produces flat plates or structural shapes (e.g I-beams)
The piece gets longer and thinner but not wider
High speed of production
High capital investment required
Features of extrusion
Production of long lengths of constant cross section
Round billet placed into chamber and forced through a die opening by a ram (with dummy block in between)
Die dictates final shape of product
Normally carried out at elevated temperatures
Pieces then cut to length
Batch process
What is rolling?
The process of reducing the thickness (gauge) or cross section of a workpiece by compressive forces applied through a set of rolls.
Flat rolling produces a plate or sheet. Can be done at room temperature or at elevated temperatures
Why is hot rolling used?
The metal is softer at higher temperatures so it deforms with a smaller load than at cooler temperatures
Cluster mill
Used for rolling. The work roll is connected to two intermediate rolls on either side which are connected to driven rolls. These are next to bearing shafts and the bearing backing in the housing. Very rigid and used for rolling thin sheet of high strength materials like stainless steel
Steckel mill
Used for rolling. Coiled metal is passed back and forth through a roll until it is the right thickness. Can apply a tension this way to reduce the load requirements in the roller
Rolling force equation
F=χwrt(R(h0-hf))σflow χ is friction factor w is width of material deforming R is radius of rolls h0 and hf are initial and final thicknesses of material σflow is flow strength of material
How does increasing temperature affect the rolling load?
Decreases flow stress so decreases rolling load
Surface defects in rolling
May be present on plate or sheet resulting from inclusions in the material, scale, rust or roll markings.
Can reduce defects caused by scale by prior treatment of the ingot
Structural defects from rolling
Affect integrity of the product such as wavy edges, cracking, centreline porosity or alligatoring (where material explodes while rolling).
Wavy edges solved by pulling metal apart by edges.
Other reduced by better control of homogenisation prior to rolling
How does punch extrusion work?
A punch is pressed into the billet which extrudes around it
Condition for extrusion for steel and aluminium
Aluminium 350-550°C
Steel 1000-1200°C
Direct and indirect extrusion
Direct: billet slides relative to the container wall
Indirect: die moves towards the billet so no relative billet/container motion
Friction conditions are different
Energy requirements and surface quality better with indirect
What is impact extrusion used for?
Similar to indirect and used to making hollow shapes
Hydrostatic extrusion
Chamber is filled with a fluid that transmits the pressure to the billet. No friction at the container walls
How to see flow pattern of the billet experimentally
Billet is halved and a grid machined onto one side. Halves then joined back together and extruded. Subsequent splitting of the two halves reveals the flow pattern. Quite expensive and limited information gained
Extrusion process variables
Container temperature, pad temperature, die temperature, friction conditions at billet/tooling interfaces
How else can the extrusion process be modelled?
Using finite element modelling. Can validate experimentally. Can use for data that is otherwise difficult to obtain. Input data like hot working data and friction characteristics. Can find stress, strain and temperature distributions
Formulae for extrusion
Extrusion ratio: R=A0/Af (Original and final CSA)
Extrusion pressure: P/σ=a+bln(R) (a and b constants)
Extrusion strain rate=6vln(R)/D (v extrusion velocity, D billet diameter)
Problem with extrusion
Only works with certain alloys that are more extrudable than others
Defects with extrusion
Surface cracking if temperatures too high or low or if extrusion speed is too high or low. Need to analyse closely to find which cause. Piping occurs when impurities in billet moves towards centre. Can solve by changing flow pattern or machining the billet surface prior to extrusion. Or could leave a discard by not extruding 10% of the billet.
Components produced by forging
Crankshafts, gears, wheels, turbine blades and many other structural components
Forgeability
Capability of a metal to undergo deformation without severe surface cracking. Good forgeability means able to be shaped by low forces without cracking. Can be quantified using upsetting tests or hot twist tests
Upsetting force/ forging load formula
P=σ(1+2μr/3h)
μ is friction factor
r is effective radius of metal
h is effective height of metal
Open die forging
Deformation of a cylindrical specimen between two platens. Sometimes used as initial step in more complex forging processes. Barrelling takes place due to friction at the platen/metal interface and when cold metal deformed between hot dies. Material at interface cools rapidly so shows greater resistance to deformation than material at centre
Impression die forging
Material acquired the shape of the die cavities whilst being deformed. Some material flows sideways to form a flash which cools quickly and helps the rest of the workpiece fully fill the die. Because high friction encourages the filling of the entire die cavity and metal assumes correct shape. Flash is recycled.
Steps of impression die forging
Start with blank bar stock. Edging, blocking, finishing, trimming.
Closed die forging
No flash formed and workpiece completely surrounded by dies. Proper control of volume of material important to achieve final dimensions. Can do near net shape forging where part formed is very close to final required dimensions and dies are machined to greater accuracy than in other operations. Means less machining needed after forging. Aluminium and magnesium suited to this as forging loads are low and there is little die wear.
Isothermal forging
Dies heated to same temperature as workpiece. Cooling of specimen eliminated and low flow stress maintained. Can be carried out over very narrow temperature range at longer times. Much greater control of microstructural and mechanical properties. Very good dimensional accuracy but is expensive compared to other routes.
Defects with forging
Surface cracking leading to problems with fatigue failure or corrosion. Excess material can cause buckling or lapping. Complet filling of a die can be hard so dies design must try to eliminate small radii where the material can fold over itself.
What is a problem with all 3 deformation processes?
Anisotropy
