Semi-solid metal casting ("SSM"), also sometimes referred to as "semi-solid casting", represents a significant recent advance in the process of creating high quality metal castings from certain non-ferrous metals. This process is used mainly to cast complex products with near net shapes and exceptional accuracy.
Semi-Solid Metal Casting
Essentially, semi-solid metal casting depends upon a property known as "thixotropy" observed in certain gels which become fluid when shaken or stirred and semi-solid again when not agitated. Metallurgists applied this observation to the casting process using certain non-ferrous metal alloys containing elements such as copper, aluminum or magnesium.
Semi-solid metal casting requires the use of low-viscosity fluid (yet not molten) materials. Casting occurs at temperature ranges in which some 30% to 65% of the metal remains in a solid state and 70% to 35% has become viscous. Semi-solid metal casting will occur at variable temperatures, based upon the composition of the alloys in use. It combines many of the advantages of casting and forging. It typically produces stronger and less porous parts than die casting within tighter net tolerances.
History and Development
Metallurgists began using semi-solid casting on a selective basis during the 1970s to fabricate very strong metal parts. However, this form of casting did not become widespread until the 1990s, with the introduction of thixocasting in mass-production commercial environments. The new technique involved manufacturing special initial precast billets and then injecting them into a die to obtain parts with reduced porosity. During this century, many manufacturers began using rheocasting, a slightly lower-cost procedure involving the direct casting of parts from a semi-solid slurry.
Today, metal parts fabrication usually relies upon semi-solid casting to generate higher quality components. This casting process works well for the rapid development of strong parts within tight tolerances. It can produce thin walled intricate components displaying pressure tightness, for example. The process of casting semi-solid materials enables manufacturing facilities to combine many of the advantages of die casting with the tempering effects of forging. It can produce finer, more uniform internal grains within metal parts.
Semi-Solid Metal Casting Processes
Manufacturers utilize a variety of casting processes to produce parts using this new technology. Four of the most popular current methods are as follows:
Manufacturers around the world still use thixocasting today to generate very high quality cast metal parts. First, machines form a special pre-cast billet by stirring the molten metal during casting to reduce the presence of air bubbles within the material. A die-casting machine then re-heats the billet to a semi-molten condition before injecting this material directly into a steel die.
Several methods of rheocasting exist. This process uses molten metal to create a slurry of semi-solid material, in some cases by swirling the mixture at designated rates and adding aluminum chips to the substrate. Following casting, the manufacturer can re-use the excess slurry by returning it to molten form in a die casting furnace (or die casting machine).
In this process, manufacturers create a metal slurry by feeding chips into a heated barrel at a designated volume under highly controlled conditions. As the chips reach a semi-solid temperature, machines automatically inject the slurry into a die. Some manufacturers use this process to work with magnesium alloys, for instance.
Strain-Induced and Melt Activated Method ("SIMA")
The Strain-Induced and Melt-Activated process enhances thixomolding today in some settings. Heated metal melts to a designated temperature, then re-crystallizes with a finer internal grain structure within highly controlled environments. Manufacturers cold-roll the material in a partially-tempered condition in order to produce high quality small metal components.
Semi-solid casting holds numerous applications in this century. For instance, automakers often rely on this manufacturing process to create high-strength yet light weight metal parts for vehicles in high volumes. From the production of aluminum suspension mounts to thin-walled magnesium alloy computer frames, this form of casting holds commercial importance. It also frequently plays a role in the generation of rapid prototypes.
Advances in semi-solid metal casting have produced a number of key advantages in metals fabrication settings recently.
This type of casting holds significant economic attractions for many manufacturers. Firms using automated production processes can generate a high volume of high quality metal components in a comparatively brief period of time with very little waste. By using SSM instead of sand die casting, for instance, companies avoid costs associated with environmental site de-contamination and lead-cleanup efforts. The use of lower temperatures places a lesser burden on company facilities. It also usually permits a manufacturer to use a die for a longer span of time. These long term production cost savings may offset initially higher costs associated with implementing thixocasting or automated rheocasting.
Furthermore, the heat-treatable parts generated through semi-solid casting offer all of the benefits of components manufactured using conventional die casting. They typically demonstrate good mechanical performance and will affix to other components within an assembly.
Also, semi-solid metal casting enables the production of very high quality metal parts because of reduced porosity and tight tolerances. These components often display a better finish due to these factors. During thixocasting, for instance, the process of re-heating the billet and forcing it by injection into a second die permits the creation of very strong, finer (and more uniformly grained) internal structures. Companies can create both intricate parts and parts with pressure-tight, thin walls by employing this technology.