Ashley Stone, P. Eng.
CTO, Inventor, MAXImolding! Technology GmbH, Siegsdorf, Germany
Magnesium Semisolid Casting Part 4 - New Casting Machine Replaces Complete Casting Line
The four semisolid metal casting processes thixomolding, thixocasting, rheocasting and stress induced melt activation, as well as the cold chamber die casting, hot chamber die casting and other processes such as vacuum die casting, did not fulfill end user expectations for quality, simplicity, energy savings, people safety, economy and environmental cleanliness. This is where we step in to develop a new semisolid casting machine.
A recent study from Neue Materialien Fürth GmbH, Germany comparing a number of currently used processes for energy consumption per part or per gram weight of the finished part shows the great advantage of the thixomolding. The new semisolid vertical casting machine without extruder but with fast-moving injection piston and with its mold heat recycling is promising much better results, showing reduced estimated energy consumption of 30 % compared to a 220t-thixomolding machine.
Magnesium Processor for Particulate AZ91D Scrubbing and Conditioning
The magnesium processor should take magnesium feedstock pellets and de-contaminate, degrease, dry and pre-heat them uniformly to a maximum of approx. 200° C. Feedstock temperature uniformity over conditioner volume must stay within ± 5 % from set point temperature as measured by 12 thermocouples placed strategically throughout the working volume of the conditioner. The conditioning process should be under an inert gas (Ag, N2 or CO2 etc.) cover from room temperature to set point temperature. Precise feedstock metering should allow for material dosing within 2 % from set point with repeatability ± 5 grams.
In the second stage of a new semisolid casting machine pre-heated magnesium particulates are fed into main casting machine that by adding heat converts the solid magnesium into semisolid slash of material. This material structure flows like honey and yet is at temperatures often 100° C below current decanting processing temperature resulting in significant energy advantage. The new semisolid casting machine single step process simplifies the semisolid die casting process even further by not using complex extruder as seen in thixomolding machines but globular semisolid slurry is produced by melting mechanically stressed magnesium chips under the sole influence of external heat. No any shear of chips in extruder is necessary. The cold deformed structure of mechanically comminuted chips is of the same nature as that reported for a thixoforming billet, produced by the stress induced melt activation (SIMA).
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 die casting alloy like AZ91D 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.
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 furnace 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 % air, 20 % 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 of 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, 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.
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, i.e. 480° C (896° F) to 580° C (1.076° F). This results in great energy savings when compared to molten material temperatures at 600° C (1.112° F) to 700° C (1.292° F) in high pressure die casting 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!
The cylindrical structure and coaxial arrangements of the piston and the plunger, surrounded by the processing stuffers, allow for processing at very narrow temperature windows and with tight temperature controls. Computer-controlled heating and high-speed injection at pressures of 15.000 PSI (1.034 bar) allow a new level of casting, resulting in parts with properties that meet the requirements for use in high integrity applications. Energy savings are significant when compared to previously described casting processes. The process is fully enclosed solid to solid part and machine could be operating in orinary manufacturing plant. In order to produce large area, thin wall castings (less than 1 mm) multiple injectors above huge mold are used.