Part 2: Additional casting processes
By George Genevro
Almost 100 years ago this ancient process helped mankind enter the era of powered, heavier-than-air flight when it was used to make the complex cast-aluminum crankcase and cylinder block of the engine that Charles Taylor built for the Wright Brothers. So, let's continue to look at the major casting processes that manufacturers use to make aircraft and engines more efficient, safer, and durable.
Permanent mold casting A common characteristic of the different types of permanent mold casting processes is that the metal mold may be reused many times. The molds are generally made of cast gray iron, ductile iron, or steel, butIn one of the two major subdivisions of the permanent mold casting process generally known as gravity permanent molding, the molten metal is poured into the heated mold through a sprue and gating system. Simple cavities in the casting may be formed by slightly tapered metal cores that can be withdrawn as soon as the casting has started to solidify. More complex cavities may be formed with segmented metal cores or with sand cores which must be replaced prior to making each new casting.
For casting most aluminum alloys the mold must be heated to about 800 F and maintained at that temperature in order for the metal to flow into all parts of the mold and for the castings to be free of porosity and cracks. The pouring temperature of the metal must also be closely controlled and will vary somewhat depending on the alloy being cast.
A release agent, usually a refractory material in an aqueous solution, is sprayed into the heated mold prior to making each casting. The refractory material remains on the mold surface after the water flashes off as steam. It protects the surface of the mold, makes removal of the casting easier, and generally improves the surface finish of the casting.
Because of the cost of the molds, heating equipment, the metal injection system, and related machinery, die casting is generally used for products made in fairly large quantities. The die castings found on aircraft engines are usually smaller and simpler items that are not highly stressed, such as rocker arm covers and parts for accessories. The heads, cylinder blocks, and crankcases of two-stroke cycle engines for light aircraft are almost always pressure die castings. Since sand cores cannot be used, cavities are formed by use of steel cores that are inserted and withdrawn hydraulically or mechanically. Complex cavities, particularly re-entrant shapes, are difficult to form without very expensive equipment.
Aluminum castings Rigorous quality assurance procedures and record keeping are a feature of foundries making aircraft quality castings. When clean and properly alloyed metal that has been heated to the correct temperature isThe casting is usually solution heat-treated and naturally or artificially aged to improve its strength and enhance machinability. For example, typical aluminum alloys such as A356 or AA242, which are widely used in aircraft engine crankcases, cylinder heads, and accessory sections, have an "as cast" (F temper) tensile strength of about 19,000 psi. Solution heat treatment and artificial aging to the T6 condition will raise the tensile strength to about 30,000 psi and make the casting more dimensionally stable. For these and similar alloys, solution heat treatment consists of heating to 1,000 F for 10 hours and then quenching in water held at about 175 F. This is followed by artificial aging, also known as precipitation hardening, at 310 F for about four hours, which will bring the material to the T6 condition. The casting is then inspected for cracks and is ready for machining.
Cost
The cost of a casting, whether new or used, is generally determined by
factors such as complexity, availability from more than one source, and, in the case of certificated engines and aircraft, whether there are authorized producers other than the original maker. Another consideration is repairability at overhaul time or when some malfunction or damage has occurred. For example, fretted areas or cracks in a crankcase casting made from A356 or similar high-silicon alloys can be welded and the casting can be stress-relieved and heat treated if it is deemed necessary. The welded areas, mating faces, and bores can then be machined as necessary to bring the casting into compliance with factory specifications so that it can be yellow-tagged as an airworthy part.
Conclusion
The producers of aluminum castings have been challenged by the unique
needs of aviation for nearly a century and have responded with unique solutions to some difficult problems. It all started with the Wright Brothers. They used a cast-aluminum crankcase and cylinder block in the engine that got them aloft, if only for a few feet, and since then practically all aircraft engines with the exception of the rotaries of World War I have used major cast-aluminum components.
Present interest in converting automotive engines for aircraft use in the experimental/homebuilt market is generally directed at engines whose major structures are aluminum castings. The certificated Orenda V-8 also uses an aluminum block, heads, gearcase, and accessory section, and designers are ever mindful that excess weight is the enemy of performance. On engines ranging from simple two-stroke cycle twins on an ultralight to World War II era radials, the use of aluminum castings and the work of countless foundrymen and metallurgists has helped make our aircraft more efficient and safer.
George Genevro, a retired college professor at Cal State Univ. at Long Beach, CA, is an A&P mechanic, pilot, and aircraft owner.