Magnesium and its alloys can be processed either through solid or liquid phase. Despite solid phase processing’s benefits it is less commonly used compared to liquid phase processing. This is due to solid phase high processing cost, limitations on thickness, lower ductility and fracture toughness, and required tedious handling of fine powders. Sand casting, high pressure and gravity die casting, squeeze casting, semi-solid metal (SSM) casting, spray forming and melt infiltration method are all examples of liquid phase processes.
Sand casting is the process in which molten metal is poured into a disposable mold created through compacting sand. The solidified metal is removed from the mold once it is cooled. Magnesium reacts with many molding materials and therefore precaution must be taken to reduce metal-mold reactions. This can be done through minimizing the moisture content within the sand as well as using inhibitors in the sand mixture used to create the mold and cores. Depending on the temperature of the molten metal, the type of alloy cast and the thickness of the casting section, the amount of inhibitor added will vary. Inhibitors can be used alone or in combination with one another depending on the alloy that is being cast. Heat treatment can be done on sand and permanent mold castings.
The majority of applications of magnesium alloys are fabricated using high pressure die casting. This is due to the possibility for high production volume and low production cost. Die casting is completed by forcing molten metal through a narrow opening to fill a mold at a very fast rate (ex. 20m/s (65.62ft/s)). A high intensification pressure of about 40-1000MPa (5802-145,038psi) is applied during solidification. This pressure in the final stages of solidification enables areas to be filled with metal ultimately reducing porosity and improving the internal integrity of the part. Die casting is widely used to produce thin-walled parts with intricate shapes due to the high fluidity of molten magnesium. In addition, the resulting die cast magnesium product has good part strength, can minimize machining and is capable of providing good surface and dimensional precision. Typically as the part thickness increases, the strength and ductility is affected inversely. In die casting magnesium, about 40-60% of the metal becomes process scrap metal via the runner system and overflow.
Squeeze casting is a combination of the forging process and casting process. In direct squeeze casting, molten metal is poured slowly with a minimal amount of turbulence into the lower half of a die and an upper punch is pressed down on the metal once the die cavity is filled. The metal solidifies under this high, unidirectional pressure which in turn reduces any internal defects during solidification. Direct squeeze casting however, allows for the trapping of impurities within the metal as it does not have a runner system. Incidentally, this also results in high internal integrity material. Indirect squeeze casting involves molten metal being poured into an encasement. A plunger is then used to control the speed at which the metal flows into the mold which eliminates the gas bubbles within the casting. There is a lower material yield with indirect squeeze casting due to the greater material loss.
Semi-solid metal (SSM) casting:
Many of the disadvantages that result from high pressure die casting are overcome by SSM casting. Thixomolding, rheocasting, thixocasting and thixoforming are all forms of this processing method. SSM allows for low porosity levels, the production of complex shapes and longer die life. A semi-solid slurry having lower heat content and higher viscosity than the liquid form of the same alloy is used. This makes it possible to have faster cooling rates, greater thermal efficiency, and prolonged die life. The higher viscosity also allows for less turbulent flow during filling resulting in lower levels of contained gas. The downside to this method is the high cost of the feedstock that is necessary.
Thixomolding, a type of SSM casting, is the relatively new method used specifically for processing magnesium alloys. This process is similar to that of injection molding for plastics. Magnesium chips at room temperature are put into a feeder that leads to a heated screw which slowly heats them to just below their liquid temperature. The screw slowly pushes the chips while heating them to the semi-solid temperature range. This heat combined with the shear forces induced by the screw creates a semi-solid slurry which is then injected into the die.
Rheocasting is another type of SSM casting which uses molten metal as feedstock. Reinforcements are added to the alloy in the semi-solid stage. A mechanical stirrer is used to create a homogeneous slurry which is then poured into a mold. Rheocast components have several advantages including homogeneous distribution of any porosity that may occur, lowered amounts of shrinkage, a lowered tendency for micro- and macro-segregation and a fine grain structure.
Spray forming/spray casting:
In this process, either an induction or resistance furnace is used to melt the metal. Reinforcing particles are added either outside the furnace or into the molten spray. The mixed metal is then slowly poured in a thin, steady stream towards a substrate. Jets of inert gas or water interrupt the stream and separate the alloy into small droplets which are then collected on the substrate where they solidify and build up.
Melt infiltration method:
A mold used in melt infiltration includes a porous reinforcement body within it. The molten metal is poured slowly into the mold so that it can fill in the pores and create a solid, composite material. The infiltration can take place either with or without the assistance of applied pressure. Pressure will prevent high levels of porosity but it can also result in the deformation or breakage of the reinforcing pre-form.
Magnesium is also used in wrought product form such as extrusions, forgings, sheet and plate. Wrought alloys have high dimensional stability and ease of machining as well as typically better overall mechanical properties. Applications for these mill products range from bakery racks, loading ramps, and hand trucks to concrete finishing tools, computer printer plates and nuclear fuel element containers.