Castings for Aircraft: Part 2

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...


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 is Dry sand cores are used to form the intake and exhaust ports. Note the large number of deep fins around the exhaust port. & Note the length of threaded engagement between the cylinder barrel and head, providing a secure gas seal and good heat transfer. (Courtesy of Engine Components Inc.)poured into well-prepared molds one can expect castings that are dense and free from porosity and inclusions. Regardless of the foundry process used, castings emerge from the mold in the F or "as cast" temper and are almost always subjected to further heat treatment. The simplest heat treatment is to remove residual stresses, a time-consuming process that may require more than eight hours at carefully controlled furnace temperatures and leaves the casting in the O or annealed condition. The casting is then in its softest, most ductile, most dimensionally stable, and weakest condition. Aircraft castings are almost never used in this form.

The 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.

Steps in making investment shell castings using a liquid ceramic slurry and dry refractory sand as the mold material. (Courtesy of Foundry Management and Technology)When one gets involved in a major overhaul of a certificated engine it soon becomes very evident that practically everything is expensive. While repairs to major castings in an engine are indeed expensive, the cost is generally less than for a new part. There is a point, however, when it becomes questionable whether castings such as cylinder heads that have been subjected to many heating-cooling cycles should be reused beyond several overhauls, particularly if overheating has taken place or they have been subjected to overly rapid cooling during low power descents. Some rebuilders of cast-aluminum engine parts maintain, however, that complete heat treatment, including solution heat treatment and precipitation hardening, of used parts that were not otherwise damaged and re-machining of certain critical areas is an acceptable repair procedure. Others disagree. In the final analysis, the decision rests with the technician who will sign off the repair or overhaul.

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.

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