Squeeze casting (SC), is a generic term to specify a fabrication technique where solidification is promoted under high pressure within a re-useable die.
It is a metal forming process, which combines permanent mould casting with die forging into a single operation. In this process a die set is placed on a hydraulic press and preheated, and exact amount of molten alloy is poured into the lower half of the open die set, the press closed so that alloy fills the cavity and pressure maintained until complete solidification occurs. The applied pressure is usually in excess of 50 MPa depending on the process and alloy specifications. External undercut forms can be produced, and using retractable side cores, through-holes are possible. Since the as-fabricated components can be readily used in service or after a minor post-fabrication treatment, squeeze casting is also regarded as a net or near net-shape fabrication route.
The most important process parameter is the alloy itself being cast. The composition and physical characteristics of the alloy are of paramount importance due to their direct effects on the die life. These include the melting temperature, and thermal conductivity of the alloy together with the combined effect of heat transfer coefficient and the soldering onto the die material. Furthermore, the alloy dictates the selection of casting parameters such as die temperature which has direct consequence on the die life. Therefore, squeeze casting is usually employed for low melting temperature alloys of aluminum and magnesium.
There are a wide range of squeeze casting machines available for production and research ranging from those manufactured by international companies or the so-called “home-made” designed and fabricated by research centers. Generally, the SC-fabricated engineering components are fine grained with excellent surface finish and have almost no porosity. They come in a variety of shapes and sizes. The mechanical properties of these parts are significantly improved over those of conventional castings and more sophisticated casting routes of pressure or gravity die-casting.
The fine structure and superior mechanical properties of SC-components are due to the following factors:
- changes in undercooling of the molten alloy
- changes in composition and percentages of the forming phases of the solidifying alloy
- changes in heat-transfer coefficient between the metallic mould and solidifying alloy
- changes in density of the alloy due to reduction of porosity.
The research activities embarked by the SMG cover different angles ranging from process mechanics to alloy structure and improvement. As for process mechanics, new concepts for SC machines should develop as well remarked in one of our published work;
"The next step in developing squeeze casting machine may lie in its combination with rheocasting where stirring of the melt and its injection into the mould and application of pressure may be achieved within one single unit. It may be possible to install magnetic stirrers around the shot sleeve of an indirect vertical squeeze casting, VSC, machine to initiate stirring of the molten metal while its temperature is falling. At an appropriate timing when certain solid percentage is achieved injection and pressurisation is carried out simultaneously.
Such machine is yet to be designed and constructed but we envisage its production in not a too distant future. The main advantage of this hybrid metal-forming process is longer die life."
For more information on theoretical, technical and technological “know-how” and future trends for squeeze casting, please contact us.