Question 3 of 7: Investment-Casting Soundness and Die-Casting Weakness

Question 3: Investment-Casting Soundness and Die-Casting Weakness (20 marks: 10/10)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Why Investment Castings Have Fewer Cracks and Less Porosity

Several features of the investment-casting process work together to produce sound parts. The ceramic mold is typically preheated before pouring, which sharply reduces the thermal gradient between the molten metal and the mold wall; a lower gradient means slower, more uniform cooling and far less thermally induced residual stress, so the part is much less prone to hot tearing/cracking than a casting poured into a cold or poorly conducting mold. The mold cavity itself, formed against a smooth wax pattern and a fine ceramic slurry coating, has a very smooth internal surface, so the melt fills it gently by gravity with little turbulence; low-turbulence, low-velocity filling minimizes air entrapment and oxide-film folding, both of which are major sources of gas and oxide-inclusion porosity in more turbulent processes. Because filling is by gravity (not high-pressure injection), the melt front advances progressively rather than atomizing or splashing, which further suppresses entrapped-gas porosity. Finally, the process readily accommodates well-designed risers/gating for directional solidification, so shrinkage porosity can be fed out of the casting into the riser rather than trapped internally, exactly as in sand casting but with even finer control because of the smooth, dimensionally stable ceramic mold.

(b) Why Die-Cast Parts Are Very Weak

Die casting injects molten metal into a permanent steel die at high pressure and high velocity so the cavity fills in a fraction of a second before the cold die can freeze the melt prematurely. That very speed is the source of the weakness: the high-velocity, turbulent filling entrains air and mold-release gases directly into the melt, and because the die is a permanent, essentially impermeable mold with almost no time for gas to escape before solidification, that entrained gas is frozen in as internal porosity distributed throughout the part. This porosity directly lowers strength and ductility, and it also means die castings generally cannot be heat treated or welded — heating a part with trapped gas pockets lets the entrapped gas expand, causing surface blistering — which removes the usual route (solution treatment/ageing) to raising strength after casting. The cold, fast-chilling die also produces a very fine-grained, hard outer skin over a somewhat weaker, more porous core, so the effective load-bearing cross-section is smaller than the nominal one. Together, entrapped-gas porosity and the loss of post-cast heat treatment are the two main reasons die castings are markedly weaker than parts made by slower, lower-turbulence casting processes such as sand or investment casting.