Ashley Stone, P. Eng.
CTO, Inventor, MAXImolding! Technology GmbH, Siegsdorf, Germany
Magnesium Semisolid Casting Part 2 - Hot and Cold Chamber Die Casting
The most common method of casting magnesium alloys is die casting. In the second part of the article series two types of die casting processes are described: Hot chamber die casting and cold chamber die casting. What is the difference between the two methods? Find the answers in this section.
Hot Chamber Die Casting
The basic difference between hot chamber die casting and cold chamber die casting is that in the hot chamber process, the molten magnesium is held in an enclosed steel crucible in a protective atmosphere. A valve allows a controlled volume of molten metal into the gooseneck, which is immersed in the molten metal. A plunger injects this metal into the cavity of the die through a nozzle. To prevent the metal from freezing, the nozzle is heated through gas, electricity, or induction. The nozzle is often kept full of molten magnesium between the shots to shorten the cycle time. Hot chamber die casting offers some distinct advantages in magnesium casting. Firstly, it limits the contact of the molten magnesium with air, thereby reducing or eliminating the formation of oxides and fires. The sealing of the molten metal in the crucible is protected by cover gas. The cover gas used in older installations in North America is sulfur hexafluoride (SF6) although today, less harmful hexafluoride substitutes are generally used in Europe and China. Several alternative melt protection technologies are commercially available that provide comparable performance to SF6. Some are based on patented AM-cover fluorine-based blend gas technology similar to that used in the refrigeration industry, well known as HFC-134a. Others are specifically tailored to the magnesium die casting industry, like NOVEC 612 fluid, and are often liquid to gas systems that use a fluorinated ketone as the active ingredient and a carrier gas such as carbon dioxide or nitrogen and dry air. Diluted SO2 is also being used by some die casters. By using these substitutes for SF6 the global die casting industry can reduce its greenhouse gas emissions, thereby improving its environmental image. However, this change is happening too slowly, and even these newly formulated cover gases are detrimental to the environment. So why not eliminate need for cover gas altogether?
New and promising technologies in die casting do not use cover gas of any kind, but rather use the process where cold magnesium chips of preferred metallurgical configurations are pre-conditioned and processed in the die casting machine, so that the output is a solid part that does not require cover gas. Solid to solid (S2S) in simple but no simpler one-step, through a safer, more environmentally friendly, less expensive process. This is the future of die casting!
Cold Chamber Die Casting
Cold chamber die casting is the most popular process in the mass production of magnesium castings. The molten metal is still contained in an open holding pot which is placed into a furnace, where it is melted to the necessary temperature. However, this holding pot is kept separate from the die casting machine and the molten metal is transferred from the pot for each casting, usually pumped from the large melting furnace into the shot chamber through a pouring hole. The injection system in a cold chamber machine functions similarly to that of a hot chamber machine; however, it is usually oriented horizontally and does not include a gooseneck channel. A plunger, powered by hydraulic pressure, forces the molten metal through the shot chamber and into the injection sleeve in the die. The typical injection pressure range for a cold chamber die casting machine is from 2.500 (172 bar) to over 25.000 PSI (1.724 bar). After the molten metal has been injected into the die cavity, the plunger remains forward, holding the pressure, while the casting solidifies. After solidification is complete, the hydraulic system retracts the plunger and the clamping unit ejects the part.
Porosity as a Killer
Maintaining molten magnesium at a very high temperature (above 700°C, 1.292°F) is a very energy demanding, unsafe and environmentally unsustainable process. Variation in temperature does not allow for tight quality control of casting parts and x-ray inspection finds a significant number of defects. The biggest disadvantage of the high-pressure cold chamber die casting process for magnesium is the high porosity level of the product, due to entrapped gases resulting from the injection of molten magnesium at very high velocity during injection. Porosity is a killer for high integrity application of parts, yet most automotive magnesium die castings are produced by the cold chamber process, just because there has not been better technology so far.
Where application requires high integrity parts, other processes, like vacuum die casting, are successfully used. However, none of these processes resolve safety and environmental issues that are plaguing the die casting industry and preventing wider use of light metal alloys because all of them include a pot full of molten material that must be kept at a very high temperature and protected from burning!