Die casting
On the one hand, die castings offer many of the qualities of gravity-cast parts: the rigidity, look and feel of solid metal; documented fatigue strength characteristics; excellent sound-damping properties; inherent EMI shielding for electronic applications; and part serviceability and recyclability. Unique to the advanced die casting process, which employs high pressure and temperatures to cast the most intricate part details using premium quality steel dies, castings can be rapidly and consistently produced to close tolerances, often eliminating all machining. Sand castings require a new sand mold with each casting or gate of castings. The permanent mold gravity process uses iron or steel molds, but, like the sand process, is far slower and less precise than die casting. High-tech die casting can produce the highest quality surface finishes, as cast, and parts can also be designed with inherent bearing properties.
High pressure casting and high-pressure die casting are terms used in Europe and countries other than the U.S. for what is referred to in the U.S. simply as the die casting process. The terms low-pressure die casting and gravity die casting are terms used outside the U.S. for what in the U.S. is called low-pressure permanent mold and gravity permanent mold casting. Although they each use metal dies, because of the lower pressures involved they are restricted to heavier section parts, often resulting in higher cost because of the less efficient use of the alloys involved and the slower processing time. They also require a sprayed-on protective coating on the die cavities, which means looser tolerances and rougher surface finishes.
Metal matrix composites (MMC) are mostly aluminum-based alloys reinforced with up to 20% silicon carbide (SiC) particles. The mechanical properties of ASTM test specimens made from these materials typically match or approach many of the characteristics of iron castings and steel, at a lighter weight. Properties can exceed those of most Al, Mg, Zn, or Cu components commonly produced by die casting. Al MMC parts offer higher stiffness and thermal conductivity, improved wear resistance, lower coefficient of thermal expansion, reduced porosity, and higher tensile and fatigue strengths at elevated temperature, with densities within 5% of Al die casting alloys. Al MMC alloys can be cast on conventional die casting machines but the availability of custom die casting production is still limited.
Squeeze casting is a recently introduced high-pressure casting process which, through the use of very large gates and high hydraulic pressure, can cast liquid metal with a minimum of turbulence or gas entrapment. The result is a porosity-free, heat-treatable component with thick walls capable of surviving the critical functional testing essential for structural automotive parts. Production costs will be higher than conventional high-pressure die casting, but squeeze-cast parts have qualified for replacing permanent mold and iron castings in high strength applications. Most production to date has been made with aluminum alloys. The number of casters utilizing squeeze casting machines is relatively limited.
This semi-solid metal casting process (SSM) uses a special cast billet which is cut into heated slugs at special casting machines. The uniform, globular semi-solid microstructure of the heated slugs provides flow characteristics capable of filling the casting die under high pressure: gas entrapment is substantially eliminated and solidification shrinkage reduced. Early qualification of parts for structural automotive applications has been successful. The process can cast thinner walls than squeeze casting and has proven suitable for use with both aluminum and magnesium alloys. Production costs are higher than for conventional high-pressure die castings, with available production facilities still limited.
Thixomolding is the injection molding of a thixotropic semi-solid alloy to mold net shape or near net shape parts. When heated to near melting point and when shearing forces are applied, certain metal alloys exhibit thixotropic properties, becoming a semisolid material in which solid particles are suspended in a liquid matrix, allowing the viscous metal material to flow. Limitations of the process often make it difficult to accurately mold certain intricate net-shape features called for in a product design, which can be produced by advanced die casting technology. The number of producers utilizing thixomolding machines is still limited.
