Section 7: Processing
From http://www.efunda.com/processes/processes_home/process.cfm
And The Open University Walton Hall, Milton Keynes MK7 6AA
Processes
Die Casting
Sand Casting
Extrusion
Forging
Powder Metallurgy
Centrifugal / Heat Treatments
Annealing
Tempering
Direct Hardening
Selective Hardening
Diffusion Hardening
Stress Relieving
Overview
Cold Forming
Processes
Cold Rolling
Staking
Burnishing
Impact Extrusion / Surface Treatments
Electroplating
Electroless Plating
Conversion Coating
Thin-Film Coating
Thermal Spraying
High Energy Treatments
Machining
Drilling
Reaming
Turning
Milling
Grinding
Chip Formation / Rapid Prototyping
Stereolithography
Laser Sintering
Fused Deposition
Solid Ground Curing
Ink Jet
Rapid Tooling
Sheet Metal
Processes
Laser Cutting
CNC Fabrication
Bending
Stamping
(Blanking, Drawing, Piercing)
Welding
CASTING
1. Die Casting - (or Pressure Die-casting)
The metal is injected into the mold under high pressure of 10-210Mpa (1,450-30,500) psi . This gives a more uniform part,good surface finish anddimensional accuracy, (around 0.2 % of casting dimensions). Can almost eliminatepost-machining on some parts.Die casting molds (called dies in the industry) are expensive as they are made from hardened steel and take a long time to make. There are two types of die-casting - cold chamber or hot chamber process.
•Cold chamber process:The molten metal is ladled into the cold chamber for each shot. There is less time exposure of the melt to the plunger walls or the plunger. This is particularly useful for metals such as Aluminum, and Copper (and its alloys) that alloy easily with Iron at the higher temperatures (which will wear out the plunger cylinder).
The largest die-castings are about 20 kg for Magnesium (35 kg for Zinc). Large castings tend to have greater porosity problems, due to entrapped air, and the melt solidifying before it gets to the furthest extremities of the die-cast cavity. Vacuum die casting redues porosity.
•Hot chamber process:The pressure chamber is connected to the die cavity is immersed permanently in the molten metal. The inlet port of the pressurizing cylinder is uncovered as the plunger moves to the open (unpressurized) position. This allows a new charge of molten metal to fill the cavity and thus can fill the cavity faster than the cold chamber process. The hot chamber process is used for metals of low melting point and high fluidity such as tin, zinc, and lead that tend not to alloy easily with steel at their melt temperatures.
Common Alloys in Die Casting
High temperature metals such as iron and steel cannot be die-cast. Aluminum, Zinc and Brass alloys are the materials predominantly used in die-casting.
Aluminum is cast at a temperature of 650 ºC. Pure Aluminum shrinks too much, and suffers hot cracking. It is alloyed with Silicon, which increases melt fluidity, reduces machinability. E.g.Silicon 9%,Copper 3.5%. Silicon increases the melt fluidity, reduces machinability, Copper increases hardness and reduces the ductility. High silicon alloy is used in automotive engines for cylinder castings - e.g.17% Silicon for high wear resistance. Common aluminum alloys for die casting are summarized as follows:
Zinc can be made to close tolerances and with thinner walls than Aluminum, due to its high melt fluidity. Zinc is alloyed with Aluminum (4%), which adds strength and hardness. The casting is done at a fairly low temperature of 425 ºC (800 ºF) so the part does not have to cool much before it can be ejected from the die. This, in combination with the fact that Zinc can be run using a hot chamber process allows for a fast fill, fast cooling (and ejection) and a short cycle time. Zinc alloys are used in making precision parts such as sprockets, gears,connector housings, and fine detail such as carburettor bodies. Chrome plateable.
Copper alloys (brass) are used in plumbing, electrical and marine applications where corrosion and wear resistance is important.
Design: The largest die-castings are about 20 kg for Magnesium (35 kg for Zinc). Large castings tend to have greater porosity problems, due to entrapped air, and the melt solidifying before it gets to the furthest extremities of the die-cast cavity. Vacuum die casting redues porosity.
Design: Keep uniform wall thicknesses. Avoid heavy sections, they cause cooling problems like trapped gases and porosity. Use maximum radii on all corners to avoid stress concentration. Allowdraft - see table above. Keep core shapessimple.
Minimum wall thicknesses and minimum draft angles for die casting:
mm / Min. Draft Angle (º)
Aluminum alloys / 0.9 mm / 0.5
Zinc alloys / 0.6 mm / 0.25
Copper alloys (Brass) / 1.25 mm / 0.7
2. Permanent Mould Casting (Gravity Die Casting)
A metal mould, such as cast iron, with cores made from metal or sand. Cavity surfaces are coated with a thin layer of heat resistant material such as clay or sodium silicate. Moulds are pre-heatedto 200 ºC before the metal is poured into the cavity. Good forproduction run of 1000 or more parts. Materials include aluminum, magnesium and brass and their alloys. Typical parts include gears, splines, wheels, gear housings, pipefittings, fuel injection housings, and automotive engine pistons.
Design rules: Minimum wall thicknesses(e.g. 3mm for lengths under 75 mm), radius (inside radius = nominal wall thickness, outside radius = 3 x nominal wall thickness), draft angles (1 to 3º on outside surfaces, 2 to 5º on inside surfaces). Typical tolerances are 2 % of linear dimensions. Surface finish ranges from 2.5 µm to 7.5 µm.Typical part sizes range from 50 g to 70 kg.
· Slush Casting: Pour out the inner motlen metal before it hardens - leaves a hollow casting. Common for ornamental objects such as candlesticks, lamps, statues etc.
· Low Pressure Permanent Mold Casting: Low pressure gas (up to 1 atmosphere)helps to push the metal into the mould. Better quality.
· Vacuum Permanent Mold Casting: A vacuum sucks the metal into the mould. No risers needed - so less wastage (better yield). Castings range in size from 200 g to 4.5 kg (6 oz to 10 lb).
3. Sand Casting
Sand casting is used to make large parts (typically Cast Iron, but also Bronze, Brass, Aluminum). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic).
The cavity in the sand is formed by using a pattern (a slightly larger duplicate of the real part), which are typically made out of wood, sometimes metal.Cores are for holes, made of sand - often hardened by a binder. The riser tells you the mould is full (stop pouring!) and tosupply metal to the shrinking casting as it initially cools.
The drag isfilledwith sand around thepattern, and with the core print, the cores, and the gating system (which are usually near theparting line). The empty cope is then assembled to the drag, and the sand is poured on the cope half, covering everything, andcompacted by vibration or by flinging the sand. Next, the cope is carefully removed from the drag, and the pattern is extracted without damaging the sand mould cavity. (Needs draft around 1° and a smooth pattern surface)
The molten metal is poured in the pouring cup connected by a vertical sprue to the horizontal runner. This connect to the casting by gates - which need to be cut off later. vents are also needed to allow air to escape.
The pattern is made oversize to allow for the metal contraction.
(smaller number for larger sizes) / Min Wall
mm
(inches)
Aluminum / 1.08 - 1.12 / 0.5 to 1.0 % / 4.75 (0.187)
Copper alloys / 1.05 - 1.06 / 0.5 to 1.0 % / 2.3 (0.094)
Gray Cast Iron / 1.10 / 0.4 to 1.6 % / 3.0 (0.125)
Nickel alloys / 1.05 / 0.5 to 1.0 % / N/A
Steel / 1.05 - 1.10 / 0.5 to 2 % / 5 (0.20)
Magnesium alloys / 1.07 - 1.10 / 0.5 to 1.0 % / 4.0 (0.157)
Malleable Irons / 1.06 - 1.19 / 0.6 to 1.6 % / 3.0 (0.125)
Sand castings generally have a rough surface sometimes with surface impurities, and surface variations. A machining (finish) allowance is made for this type of defect.
The process can be used to make complex shapes becasue the sand cores are destroyed to remove the casting. (e.g. Complex 3D shapes such as this automobile cylinder head). Also good for large custom parts like large pump housings or ship propellors.
4. Investment Casting (Lost Wax Process)
The Egyptians used itto make gold jewelry thousands ofyears ago. Intricate shapes can be made with high accuracy, and it can be used for high temperature metals and alloys that are hard to machine.
A wax pattern of the part is made, thendipped in refractory slurry, which coats the wax pattern and forms a skin. This is dried and the process of dipping in the slurry and drying is repeated until a robust thickness is achieved. After this, the entire pattern is placed in an oven and the wax is melted away and the ceramic is baked at high temperature to harden it. This mould can be filled with the molten metal. Since the pattern can be melted out,very intricate parts and undercuts can be made. The wax pattern itself can be made by machining, casting, or rapid prototyping. the pattern can also be made of thermoplastic foam.
Before casting, the mold is pre-heated to about 1000 ºC (1832 ºF) to remove any residues of wax, harden the binder. The pour in the pre-heated mold also ensures that the mold will fill completely. Pouring can be done using gravity, pressure or vacuum conditions.
View this animation: Flash Animation of Investment Casting Process
Tolerances of 0.5 % of length are routinely possible, and as low as 0.15 % is possible for small dimensions. Castings can weigh from a few grams to 35 kg, although the normal size ranges from 200 g to about 8 kg. Normal minimum wall thicknesses are about 1 mm, or even0.5 mm for easy casting alloys.
Metals include Aluminum alloys, Bronzes, tool steels, stainless steels, various cast irons and steels and precious metals. Post machining can sometimes be avoided altogether because of the close tolerances that can be achieved.
5. Centrifugal Casting
This uses a permanent mold rotated about its axis at high speeds (300 to 3000 rpm) as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling.
Only cylindrical shapes can be produced with this process. Size limits are upto 3 mdiameter and 15 mlong, wall thickness can be 2.5 mm to 125 mm. The tolerances on the OD can be as good as 2.5 mm and on the ID can be 3.8 mm.
Typical materials that can be cast with this process are iron, steel, stainless steels, and alloys of aluminum, copper and nickel. Typical applicationsare pipes, boilers, pressure vessels, flywheels, cylinder liners and other large round parts.
6. Casting Microstructure and Defects
When a liquid metal cools and begins to solidify in a mould, grains (crystals) of the metal start to form, both on the mould walls and in the bulk of the liquid metal. The way they grow is shown schematically in Figure 25(a). As the metal solidifies, it forms tree-like dendrites (from dendron: the Greek for tree). This structure is maintained after the casting is fully solidified, as can be seen from Figure 25(b), which shows a typical casting microstructure. (The image is created by polishing the surface of the metal, immersing it for a short while in a dilute acid and viewing it under an optical microscope.) In addition to the dendritic structure, there are two other common defects that can be found in a cast microstructure: particles of impurities known as inclusions, and porosity which is small holes in the casting.
Castings (a) dendritic formation (b) a typical cast microstructure
Some inclusions can be removed by heating the casting to a temperature somewhat below its melting point to anneal it and ‘dissolve’ the inclusions in the metal; but the porosity is more difficult to remove. The porosity occurs because the casting has shrunk on solidification. Most materials contract on solidification (water is one of the few liquids that expands on solidification, so that ice floats on water; bad news for the Titanic, but good news for polar bears) and this shrinkage is not always uniform, so that substantial holes and voids can be left in the casting. This reduces the load-bearing capability of the component, and in highly stressed products, where the full strength of the material is being utilised, voids can lead to failure. The shrinkage on solidification can be large, and is generally a greater effect than the thermal contraction of the solid material as it cools to room temperature.
In many casting processes, runners and risers are used as reservoirs of molten metal to prevent voids from developing in the casting as it solidifies and shrinks.A section through a gravity-die casting (below) clearly shows the effects of this contraction. The chimney-like feature is the runner, down which liquid aluminium alloy was poured into the mould. There is a hollow in the top of the runner caused by liquid flowing from the runner into the mould as the casting solidified. As well as the hollow at the top, you can see some holes in the runner and one hole within the casting itself. The runners and risers will later be cut off and discarded.
Section though a gravity die-cast microscope body
FORMING
Forming processes involve shaping materials which are solid - if the yield stress is low enough.One way to lower a metal’s yield stress is to heat it up (e.g. blacksmith working on a horseshoe).
Stress-strain curves give indication of formability.This described two properties, yield stress (or flow stress) and ductility. Remember that the yield stress is a good measure of the strength of a ductile material.
The effect of progressively straining a material