Castings for Aircraft: Part 1: Wax and sand casting processes.

Part 1: Wax and sand casting processes By George Genevro Aeronautical engineers face an imposing array of problems in designing aircraft that are safe, perform well, and can be built and sold at a competitive price. After the desired...

Castings for Aircraft

Part 1: Wax and sand casting processes

By George Genevro

Aeronautical engineers face an imposing array of problems in designing aircraft that are safe, perform well, and can be built and sold at a competitive price. After the desired aerodynamic elements of the design have been established and the major type of airframe construction - aluminum, tube and fabric, composite, or some combination - has been chosen, the manufacturing and production engineers must make many choices. They must choose from a variety of processes that may include among others, welding, forming, riveting, forging, and casting. For certain parts with unusual or complicated shapes it is often more economically and technically feasible to cast the item and finish it by appropriate heat treating and machining rather than forging or fabricating it.

Casting is a process that dates from man's earliest work with metals. According to some historians, it was first used about 3000 BC in the area that is now Iraq and possibly was also known and used in China and India at about the same time.

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 look at the major casting processes that manufacturers use to make aircraft and engines more efficient, safer, and durable.

The lost wax process
The earliest precision casting method, used primarily by silversmiths and goldsmiths, was the cire perdue, or lost wax process. It is still used extensively to make products such as dental appliances, jewelry, gas turbine blades, and cylinder heads, among others. This process is of intense interest to those in the aircraft and aerospace industries because of its versatility and adaptability to a wide variety of metals.

The head of this aftermarket cylinder for the TCM 0-520 series of engines is cast by the shell mold process, solution heat-treated, and precipitation-hardened. In its most elementary form, lost wax casting consists of making an exact wax model of the part to be cast and attaching sprues and gates, the passages through which the molten metal will flow into the mold. This assembly, either singly or in groups, is then placed in a container, or flask, and a plaster and water slurry is poured in and allowed to set up. The flask is then placed inverted in a furnace to start the burn-out process, first at relatively low temperatures to melt out the wax and then to slowly remove the water from the plaster without causing it to crack. Next, at temperatures well above 1,000 F, the wax residue is burned out and the mold is dried completely. Thorough drying of the mold is important because any moisture left in the plaster will turn to steam with explosive force and destroy the mold when hot metal is poured into it.

The molten metal is then introduced into the hot mold by one of two methods. Simply pouring it into the mold - the static process - works well with castings where the mold cavity is not very complex and the molten metal is quite fluid. The centrifugal process, widely used for unusually shaped, small, and complex parts, requires a machine that carries the hot plaster mold at the end of an arm with the sprue opening facing inward. An amount of hot metal slightly in excess of what is needed to fill the mold cavity is poured into a basin just below the sprue opening and the machine is started. As rotation begins, the rapid acceleration centrifuges the metal into the mold cavity with considerable force, thus completely filling it. The plaster mold is allowed to cool and is carefully broken to retrieve the casting, which is then cleaned and finished as needed.

Highly heat-resistant alloys may be cast by a variation of the lost wax process developed by those who manufacture jet and rocket engine parts and cutters for machine tools. Individual or grouped wax patterns with gates and sprues attached are repeatedly dipped in a highly heat-resistant plaster slurry and then coated with zircon sand. This produces a very durable shell around the wax pattern. The wax is then melted out and the burnout process heats the shell to temperatures of 2,000 F. The alloys cast by use of this method are usually poured at temperatures that may exceed 2,600 F.

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