A precision casting method in which a wax pattern is coated with refractory and hardened, and melt is poured into the mold left after the wax is melted out. Also called the lost-wax process; used for turbine blades and the like.
Japanese name: インベストメント鋳造
Investment casting appears in 1 practice question across 1 topic, including Materials processing and joining.
In investment casting, wax is first injected into a metal die to make patterns of the same shape as the product, and many of them are attached to wax bars that will become the runners, forming a tree. This is dipped into a slurry containing fine refractory powder, coarse refractory grains are sprinkled on the surface and it is dried, and this is repeated many times to build a shell. The wax is then melted out, for example in a steam autoclave, and the shell is fired at high temperature to strengthen it and burn off remaining wax. Melt is poured while the fired shell is still hot, and after solidification the shell is broken away, the castings are removed, cut from the runners and finished.
Because the pattern is removed by melting, the mold need not be split, and shapes that could not be withdrawn from a mold, fine grooves and thin blade-like forms can be made in one piece. Fine refractory powder copies the pattern surface, so surfaces are smooth and dimensional accuracy is good, greatly reducing the amount of machining. If ceramic cores are set in the wax die so that they end up inside the pattern, and are dissolved away chemically after casting, hollow parts with complex internal cooling passages can also be made. On the other hand, each pattern and shell is made individually, so the process is long, the cost per part is high and making large parts is harder than with sand casting.
Lost foam casting (the full mold process), in which a foamed polystyrene pattern is buried in sand and vanishes in the heat of the melt as it is poured, also uses a disappearing pattern, but investment casting, which removes the pattern before pouring, gives higher surface and dimensional accuracy. In jewelry and dental casting, the pattern is embedded in a plaster-based investment, while ceramic shells are the mainstream for industrial parts. Die casting, which injects melt into metal dies at high pressure, excels in productivity but is unsuited to high-melting alloys and complex hollow shapes.
Turbine blades for jet engines and gas turbines, made of Ni-based superalloys, are a typical example of investment casting. Withdrawing the shell gradually downward from the furnace so that it solidifies from bottom to top gives directionally solidified blades with aligned columnar grains, and adding a narrow passage partway that selects only one crystal gives single-crystal blades. The aim is to eliminate grain boundaries perpendicular to the load, which are weak points in high-temperature creep. Recently, making wax or polymer patterns by 3D printing and casting prototypes and small batches without a die has also spread. The method is also used for medical implants and golf club heads.
Answer: C. Investment casting
In investment casting (the lost-wax process) the pattern is melted out, so the mold does not have to be split to remove it, and complex shapes can be made with good dimensional accuracy and smooth surfaces. The refractory mold withstands high temperatures, so the method is also used for precision parts in Ni-based superalloys, such as jet-engine turbine blades, and directionally solidified and single-crystal blades are made this way as well. Die casting injects melts of aluminum, magnesium and the like into metal dies under high pressure; it is highly productive and suited to thin-walled parts, but not to high-melting steels. Sand casting makes a sand mold from a pattern such as a wooden one and then withdraws the pattern; it can make large parts, but the surface is rough.
They are used with almost the same meaning. Lost-wax process emphasizes that the wax pattern is melted and lost, while investment casting emphasizes that the pattern is wrapped (invested) in refractory. Industry usually says investment casting, while art and craft usually say lost wax.
Because the refractory shell withstands high temperatures, it can be used for high-melting alloys such as steels, stainless steels, Ni-based and Co-based superalloys and titanium alloys. Titanium melt reacts easily, so the inner face of the shell uses an unreactive refractory such as yttria, and the metal is melted and cast in vacuum. Aluminum and copper alloys are cast this way too.
Because patterns of consistent dimensions made in a die are copied by fine refractory powder, dimensional accuracy is higher and surfaces smoother than with other casting methods. Many surfaces can be used as cast, reducing machining. The die dimensions must allow for the shrinkage of wax and metal, however, and accuracy is harder to keep for larger parts.
In investment casting (the lost-wax process) the pattern is melted out, so the mold does not have to be split to remove it, and complex shapes can be made with good dimensional accuracy and smooth surfaces.
Which casting method makes a pattern of the same shape as t…Nickel-based superalloy Carbon equivalent Extrusion Heat-affected zone Hot isostatic pressing Sensitization Shrinkage cavity Springback