Metals- Casting Flashcards
Which forming processes rely on liquid processing for at least part of the process?
Casting, sintering, powder metallurgy processing
Simple steps of casting
Molten metal is introduced to a mould cavity. It solidifies into the shape of the mould. Then there are finishing processes.
Typical cast products
Engine blocks, crankshafts, pistons, valves
Which metals are more or less castable?
Aluminium more than titanium or steel
What do we need to understand about the metal for casting?
How the metal solidifies from the molten state and what the resultant structure will be. How easily the metal flows and fills the mould cavity (viscosity). Is there any shrinkage.
When is casting the best manufacturing route?
If the material is castable. If after casting or subsequent heat treatment the component meets the requirements in terms of properties
What does castability depend on?
Reactivity of the melt. Fluidity of melt. Its freezing range and ability to withstand the stresses associated with solidification.
What defines the casting process being optimised?
Porosity is kept to a minimum. Defects such as inclusions are reduced in number and size. Properties of the material approach that of wrought material
4 main casting processes
Sand casting. Pressure die casting. Gravity die casting. Investment casting.
Applicability, finish and tolerance of sand, permanent mould and investment casting
Sand: wide applicability, poor finish, poor tolerance
PM: limited applicability, best finish and tolerance
Investment: wide applicability, good finish and tolerance
How is mould made for sand casting?
Use sand clay and water. Mould material pressed around the pattern shape (maybe wood) to make an imprint in the sand. Incorporate a gating and running system if required. Pattern is removed and mould is left.
How does sand casting work?
Molten metal fills the mould from the bottom to avoid turbulence. A filter filters out inclusions before hand. Metal allowed to cool and then mould is broken away to leave the component. Uniform temperature distributions and surface finish can be good but dimensional accuracy low.
How does investment casting work?
Also known as lost-wax process. Pattern made of wax or plastic and is coated in a slurry of refractory material, dried in air and heated to remove moisture. Number of moulds can be combined in one larger mould (tree) for pouring. After metal poured, mould broken up. Costly but good surface finish and tolerances achieved. For relatively small and complex shapes.
How does permanent mould casting work?
Uses permanent moulds that can be reused. For pressure casting, metal forced upward into mould and remains under pressure until solidification finished. Die casting similar where metal forced into the die cavity. Hot chamber means can cast high melting point alloys like Cu or ferrous metals. Gravity casting (from top to bottom) involves no external pressure.
Economics of mould, equipment, labour, production rate for sand, investment, PM, die, centrifugal casting
Sand: low, low, L/M, <20piece/hr Invest: M/H, L/M, H, <100 PM: M, M, L/M, <60 Die: H, H, L/M, <200 Centrifugal: M, H, L/M, <50
Zones in a cast piece of metal
Chill zone of very small equiaxed grains in contact with mould. Columnar zone further in with long grains. Equiaxed zone at centre with smaller grains than columnar. Can change shape of grains by controlling direction of heat transfer.
Design of the casting equipment
Pouring basin high up. Connected to vertical column (sprue) which takes molten metal to sprue base. Moves across to filter then into runner bar. Ingates up from runner bar into mould cavity. Runner extension beyond last ingate. Feeder with extra liquid metal above mould cavity.
Why are feeders needed?
There is a volume change on freezing for most metals (normally contraction). Feeders can add extra metal so there are no gaps inside mould.
Which elements expand on freezing?
Sb, Bi and most importantly Si. Also graphite
How to reduce volume contraction on freezing
Add graphite to Fe or Si to Al to cause progressive decrease in amount of shrinkage. Because the additions expand to counteract the contraction of the metals.
What makes feeding metal into mould more difficult?
If it has a wide freezing range. There is an impedance to flow that results when solidification starts occurring everywhere in the casting simultaneously. Means no central channels along which feeding can occur.
What types of defects can form during the casting process?
Metallurgical projections (swelling), cavities (blowholes, porosity), discontinuities (cracks), surface cracking, incorrect dimensions (shrinkage), inclusions (intrinsic, exogenous)
Origin of gas porosity
When metals melted in air, O, N and H gases can be picked up by the melt. Depends on metal and partial pressures as their solubility in the melt is higher than that in the solid. Concentration of these elements during freezing increases until blowholes of microporosity form.
Problems with porosity
UTS and ductility decrease with increased porosity. Pores are easy paths for crack propagation. Cracks easily jump from pore to pore so leads to fracture at lower stress and strain
Ways to reduce effects of porosity
Thoroughly degas before casting. Counteract shrinkage using feeders. Design casting to shift porosity to non-critical regions. Use hot isostatic pressing to close residual porosity.
Scavenging
Form of degassing. Bubble inert gas (Ar or N) through the melt. Gas in melt diffuses to bubbles which transport it to melt surface. Also leads to more uniform temperature distributions and gives time for any large inclusions to float to surface and be removed. Cheap
Vacuum treatment
Form of degassing. Treatment of metal under vacuum moves gas/metal equilibrium in favour of gas evolution because of decrease in external pressure. Gases are released. Also applied to air melts which can undergo subsequent evacuation treatment in a ladle or by streaming droplets from the ladle. More effective and expensive than scavenging.
Why is complete removal of porosity not always desirable?
Widely dispersed microporosity can give more pressure tightness than more localised shrinkage formation leading to voids. Doesn’t greatly reduce mechanical properties. Can help counter linear shrinkage in die cast Al alloys for example which leads to hot tearing.
Mould metal reactions
Reaction of alloy with any moisture in mould can cause hydrogen pick up. Means risk of explosion or could lead to pinholes or other subsurface porosity defects. Moulds heated beforehand to reduce risk of this
What treatments are often done after casting?
Machining, solution treatment (precipitation hardening), hardening, nitriding, carburising, annealing, tempering, quenching, cleaning, finishing
Squeeze casting
Applies high pressure when solidification is taking place in mould to produce fine grains. Can result in microsegregation if not controlled and requires complex equipment. If done well gives good mechanical properties.
Thixoforming
Semi-solid forming. Uses partially solid material as feedstock and then applied pressure to suppress residual gas evolution and shrinkage porosity.
Vacuum die casting
Removes problem occurring in conventional pressure die casting, namely gross internal porosity. Parts of excellent integrity produced
