Investment casting - Nguyen Ngoc Ha

1.1. Process Characteristics Size limits: As small as 1/10 oz; usually less than 10 lb Thickness limits: As thin as .025 in, less than 3 in Tolerances: .005 in on the first inch; .002 in per additional inch Draft allowance: none required Surface finish: 50-125 µin 1.2. Process procedure Schematic illustration of the investment casting process 1.2. Process procedure Steps in investment casting: (1) wax patterns are produced, (2) several patterns are attached to a sprue to form a pattern tree 1.2. Process procedure Steps in investment casting: (3) the pattern tree is coated with a thin layer of refractory material, (4) the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid 1.2. Process procedure Steps in investment casting: (5) the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity, (6) the mold is preheated to a high temperature, the molten metal is poured, and it solidifies 1.2. Process procedure Steps in investment casting: (7) the mold is broken away from the finished casting and the parts are separated from the sprue Example of a Wax Injection Mold Example of a Wax Pattern Example of a Coated Pattern Example of Finished Castings 1.3. Advantages • Although the labor and material involved make the lost-wax process costly; 1- It is suitable for casting high-melting alloys 2- With good surface finish and close tolerance. 3- Thus little or no finishing operations are required. 4- This process is capable of producing complex shapes Investment Casting of a Rotor Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly. (b) Ceramic shell around wax pattern. (c) Wax is melted out and the mold is filled, under a vacuum, with molten superalloy. (d) The cast rotor, produced to net or near-net shape. Source: Howmet Corporation Investment and Conventionally Cast Rotors Cross-section and microstructure of two rotors: (top) investment-cast; (bottom) conventionally cast. Source: Advanced Materials and Processes, October 1990, p. 25 ASM International 1.4. Disadvantages  Many processing steps are required  Relatively expensive process 1.5. Uses • The produced parts weighing from 1 g to 35 kg, for a wide variety of ferrous and nonferrous metals and alloys. Typical parts made are components for office equipment and mechanical components such as gears, cams, valves and ratchets. • Common metals: Mainly aluminum, copper and steel; also used with stainless steel, nickel, magnesium and precious metals Products Products Products Products 2. WAX PATTERN MAKING 2.1. Materials for wax pattern Raw Materials • Parafin • Stearin • Ceresine • Ethyl Cellulose • Colophony • Polystirol Pattern Wax • PS 50-50: 50% parefin + 50% stearin • PS 30-70: 30% parefin + 70% stearin • PSE: parafin + stearin + ethyl cellulose • PCE: parafin + ceresine + ethyl cellulose 2.1. Materials for wax pattern • Colophony-polystirol-ceresine 50-30-20 • Colophony-ceresine 2.2. Wax Pattern Making Ejecting Pattern Pattern Assembly (Tree) 3. MOULD MAKING 3.1. Mold Materials Refractories • Silica (SiO2) • Corundum (Al2O3) • Silicate zirconium (ZrSiO4) • Dioxide titanium (TiO2) • Magnesite Grain size: Slurry coating: <0.05mm; Stucco coating: 0.074 – 2mm Binders • Ethyl Silicate • Water Glass 3.2. Mold Making Slurry Coating Slucco Coating Slucco Coating Mold Drying Pattern Meltout (Autoclaved) P= 0.6 – 0.8 MPa 4. POURING 4.1. Mold Preheating T= 600 – 10500C = 1 – 3 h 4.2. Pouring 4. FINISHING 4. FINISHING 4. FINISHING