bck9323 posted: " I am currently on co-op with an aerospace company in Georgia, USA. The facility makes blades and nozzles for gas turbines and jet engines used in both military and commercial aircraft. For those unfamiliar with gas turbines, the basic principle is to tak"
I am currently on co-op with an aerospace company in Georgia, USA. The facility makes blades and nozzles for gas turbines and jet engines used in both military and commercial aircraft. For those unfamiliar with gas turbines, the basic principle is to take air coming into the front of the engine and compress it before combining it with a propellent. This mixture is combusted and sent out the back of the engine to make thrust, or run through turbines to create power. To reach the high compression ratios needed for these engines, the engines compressor will consist of several stages of blades. Each blade is uniquely engineered for its stage and must be able to withstand the extreme environment inside of the engine while being as light and efficient as possible. To create these blades, we use a process called investment or lost-wax casting.
Image 1: Cross section view of internal combustion engine. During operation, air moves from left to right.Image 2: Various blades for different stages and engines
Investment casting starts with a mold of the desired blade or nozzle. In this mold, a wax version of the desired part is made. Each wax part is then checked by hand and any small imperfections are corrected. A wax part may be thrown out for having an air bubble the size of the tip of a needle. Also made out of wax is a runner that attaches to a ceramic sprue. Using various small components, several patterns are attached to the runner, by hand, in a predetermined configuration. The assembled parts and components are called the wax assembly.
The completed assembly is hung on a conveyor belt in a temperature-controlled room where it is automatically brought to the ceramics department. Here, a robot picks each mold off of the conveyor belt and dips it into a proprietary ceramic mixture before placing it back on the belt. This is repeated several times with different layers to build up a thick shell of ceramics before the mold is finally placed on a separate drying belt.
Image 3: example of a mold that has been dipped and is ready to be de-waxed
Row of ceramic moulds used for precision investment casting: the process is based on the production of a wax pattern, an exact replica of the finished alloy casting. The wax patterns act as templates for making ceramic moulds; the wax is subsequently melted & drained from the mould to make ready for the final metal casting. Here, individual moulds are formed on separate units, made by coating the various wax patterns in liquid ceramic & drying in air & ammonia gas. Photographed at NGL, Chard, Somerset, who make a range of products from simple steel castings to complex, heat resistant alloy components for aero- engines.
Once the ceramic shell is fully dried, the entire assembly is placed inside of a pressurized steam furnace where the wax is melted out. The ceramic shell and sprue are left behind and are now a mold that will be used to cast the part out of metal. Before any casting can be done, the mold is inspected for cracks and imperfections by filling it with dyed water. The white exterior of the mold quickly reveals any flaws. The mold is wrapped in insulating fibers that will help to slow down the cooling process after metal has been poured in. Slowing the cooling creates a more desirable, uniform crystal structure in the metal.
When the mold is ready, it is brought into the foundry to be cast. The mold is preheated to a temperature of about 2100°F so that it will not break when hot metal is poured in. The preheated mold is put directly into the lower chamber of a vacuum induction furnace. The air is removed from both chambers and replaced with a smaller amount of inert gas that will not react with the molten metal and change its composition. The metals used are highly specialized nickel-cobalt alloys called superalloys. The metal is melted to temperatures over 2700°F using induction (think of an electric stove) and then poured into the mold all inside the vacuum chamber. The mold is then set aside to cool.
After cooling, the mold is broken away from the metal casting using pressurized water and the individual parts are cut away. Any remaining pieces of ceramic are taken off with a light sandblasting Each part is given a unique serial number that can identify everything from what batch of metal was used to cast it to the person that filled the original mold with wax. The part is further finished and visually inspected by highly trained technicians that can correct some flaws with hand tools. The piece goes on to be inspected for quality. It is X-rayed for internal flaws and visually inspected for defects this time using Fluorescent penetrant that glows under a UV light. Each part is placed into a fixture that electronically measures it for dimensional accuracy down to the ten thousandth of an inch (0.0001). Any problems are flagged for the engineering team to further evaluate what went wrong and fix any issues with the process that caused it.
If and only if a part passes all of the rigorous quality evaluations will it be given a part number designated by the customer and shipped. At times, up to 20%-30% of parts produced may be scrapped for not meeting standards. While this may seem excessive, it is necessary to ensure the safety and reliability of the gas turbines that power our homes and aircraft.
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