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, but graphite molds may be used in some cases. Depending on the size, metal molds may be cast or machined from billets and in either case skilled hand finishing is generally required. Any imperfection in the mold will be reproduced in the mirror image in the casting and may make removal of the casting from the mold more difficult. Permanent molds are generally much more expensive than the tooling used for green sand or dry sand foundry processes, and when changes are made in the design of a casting costly rework of the mold is usually required.
In 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 Simple cavities in the casting may be formed by slightly tapered metal cores that can be withdrawn as soon as the casting has started to solidify.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.
While the initial investment in permanent molds is quite high, if the cost can be amortized over a fairly large number of castings manufacturers will often choose it over sand casting because of the savings in labor and materials costs. While it is not generally used for very large parts, castings the size of a crankcase half for a typical six-cylinder aircraft engine can readily be made by the gravity permanent mold process.
Pressure die casting
The major difference between pressure die casting and the gravity permanent mold process is that in die casting the molten metal is forced into the mold cavity very rapidly at pressures as high as 30,000 psi. The mold is generally machined from steel and is locked in the closed position by either a hydraulic or mechanical system capable of exerting many tons of pressure to prevent leaks of hot metal past the mold faces. Mold temperature is very important because the cyclic rate of the process is in large part dependent on the time required for the casting to cool to a temperature at which it can be ejected from the mold cavity.
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.
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 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.
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.
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.
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.