Ensuring high structural integrity and temperature uniformity
A clear step forward in the creation of high integrity castings is achieved by using extruded, cold rolled or mechanically stressed magnesium chips, instead of billets as seen in thixocasting, to create spherical micro structured semisolid slurry for injection into a permanent mold. Ordinary when chipped can be processed directly into semisolid slurry only by reheating chips to semi solidus temperature due to mechanical residual stress in the chip caused by the chipping process.
The fractured magnesium chips are made from standard die casting alloy AZ91D, which is readily available on the market. The oval shape and size of the chips improve feedstock flow and heat energy absorption due to high surface area. The new machine feeds the chips under a stuffer rods. The stuffer rods are pounding chips into the stuffer cylinders used for heating chips.
The stuffer rods with stuffer cylinders are placed co-axially around the injection piston to ensure high structural integrity and temperature uniformity, critical for slurry generation and magnesium processing. Material is heated to semisolid temperature very quickly by resistive or preferably inductive heaters, and slurry transfer under the injection piston is done in the center of the same high thermal mass block to ensure stable process temperature.
Slurry, in the form of ice-water slash, is transferred under a fast-moving piston designed to inject the semisolid slurry into a permanent mold to form the part.
Mold cooling results in process savings
Maintenance of the new semisolid casting machine is simple. No material hang-ups are possible. Semisolid slurry is fully contained within the machine. The mold is now capable of casting millions of parts due to reduced slurry temperatures. A significant advantage over any other die casting process is achieved because a separate and pot for slurry generation is no longer required. All is done in the same machine, within a fast cycle of 15 to 60 seconds.
Mold cooling is accomplished by water mist system (80 percent air, 20 percent water) circulating just below mold surface with very high cooling rates. Water air generation in closed cooling channels absorbs a large quantity of heat from the solidifying part and is in turn used to pre-heat the incoming magnesium chips. This results in significant process savings as well.
State of the art today is, that all the heat removed from the parts is dumped into the environment. Mold heat in all current processes is also dumped into the atmosphere and no recovery of heat is possible. That is due to liquid cooling with a very small differential in temperature, which is inefficient to the heat recovery process because a small differential in temperature requires in turn, a large volume of cooling medium.
Operating at temperatures just above solidus temperature
The part is then subjected to evaporative cooling with a very high rate of heat removal by water vapor and air mixture circulating just below mold surface. This high rate of cooling supports freezing the globular structure of the part. This cold-to-cold and solid-to-solid (S2S) semisolid magnesium-processing casting machine features several key advantages compared to other die casting or magnesium forming machines.
The major advantage is that the new semisolid magnesium machine operates at temperatures just above solidus temperature, for example 480 Degrees Celsius (896 Degrees Fahrenheit) to 580 Degree Celsius (1.076 Degrees Fahrenheit). This results in great energy savings when compared to molten material temperatures at 600 Degree Celsius (1.112 Degrees Fahrenheit) to 700 Degree Celsius (1.292 Degrees Fahrenheit) in processes today.
In this process, the heating of the magnesium begins at room temperature and rises to the processing semisolid temperature. The new semisolid casting machine can also process any specialty alloys with the preferred globular chips.
