Magnesium Semisolid Casting Part 3 - Semisolid Metal Casting Processes
The cold chamber die casting, hot chamber die casting and other processes such as vacuum die casting described in Part 2 cannot solve the safety and environmental problems in the die casting industry. Read about semi-solid metal forming in this section - a near-net shape variant with excellent part integrity.
Semisolid Metal Forming
Semisolid metal forming (SSM semisolid metallurgy) combines the advantages of casting and forging, and is named after the fluid thixotropic property, which is the phenomenon that allows this process to work. Simply, thixotropic fluids flow when sheared, and thicken when standing. At the temperature range between solid and liquid, this SSM is similar to ice slash thixotropic fluid when water and ice are mixed. The potential for this type of process was first recognized in the early 1970s by MIT researchers. There are four semisolid metal casting processes being used today: thixomolding, thixocasting, rheocasting, and stress induced melt activation (SIMA).
Thixomolding is a process in which the magnesium alloy, in the form of metal granules, is fed into a specially modified plastic injection molding machine, similar to the way in which plastic resin is processed. Thixomolding has several advantages compared to conventional die casting, primarily able to form higher integrity parts, safer operation without a melting furnace, and no negative impact on the environment. No melting pot or environmentally disastrous cover gas are required. Thixomolding makes it possible to mold magnesium alloy slurry in a semi-solid state. This results in products with few defects and superior dimensional accuracy. In injection thixomolding process, the microstructure appears to be offering a new level of process control and much less energy per part as compared to high-pressure die casting. As a bonus, there is no adverse environmental price to pay.
Currently, there are few manufacturers of the injection thixomolding machines - and the machines is very complex, demanding intensive and very expensive service and maintenance due to lots of complex moving parts coming into direct contact with molten magnesium. The most expensive machine part is the extruder made from specialty steel alloys suitable for operating at high temperature and high pressure. However, its screw with front installed check valve is notoriously prone to leakage and inconsistent operation. Maintenance on these units, which is necessary after only 500.000 cycles, requires the use of specialty removal tools as well as a clean-up process that employs an environmentally unfriendly hydrochloric or phosphoric acid. While a step forward in high quality parts output, this process is definitely complex and very expensive to operate.
As one of the primary SSM processing options, thixocasting allows the formation of alloys into near-net shaped products with improved mechanical and aesthetic characteristics. This forming process uses partially melted non-dendrite alloy specially prepared proper to casting. This casting process uses a precast billet with a spherical globular microstructure that is then heated to just above solidus temperature and pushed into a closed mold to create high integrity parts. Although now widely used, this processing technology requires strict adherence to key requirements to ensure the quality of the finished products, including liquid fraction, semisolid temperature, injection speed, injection pressure, and die temperature.
The thixocasting process offers a number of advantages, such as improved mechanical properties, good surface finish, near-net shape, and so on. However, it also has a number of disadvantages, such as the need for special feedstock with near spherical primary crystals. To cast such special billets of limited sizes for thixocasting one has to pay a higher than normal premium. The elimination of this additional special feedstock with near spherical primary crystals in new semisolid casting machine would lead to both cost and time savings without adverse impact on our environment.
Compared with conventional casting technologies, thixocasting has a lower forming temperature, significantly longer die life, high part precision, efficient production processes, and comprehensive mechanical properties. In comparison with hot forging technologies, thixocasting has quite a low yield strength, high fluidity, low forming load and low surface roughness. In the thixocasting process, a complex geometric product can be obtained through one-step forming. Experimental results indicate that defects such as micro porosity, micro shrinkage, dendritic solidification in liquid pool, hot tearing in liquid phase, micro segregation at grain boundaries, and liquid segregation in the sharp corner of the die were observed. To avoid these casting defects, the thixocasting process parameters (liquid fraction, semi solid temperature, injection speed, injection pressure and die temperature) must be tightly controlled.
Rheocasting, on the other hand, produces output with thixocast properties, by developing semisolid slurry from the molten metal produced in a typical die casting furnace, rather than by re-heating billets. The resulting feedstock (in the form of typical die casting alloys) is less expensive and directly recyclable.
Stress Induced Melt Activation
The SIMA method is suitable for industrial applications because it is relatively inexpensive and offers an easy way to obtain the non-dendritic feedstock by just heating the highly stressed feedstock to the semisolid state. The SIMA cycle starts from the conventionally cast ingot. The feedstock is hot-worked material, e.g., rolled, forged or extruded in the solid state above recrystallization temperature and then cooled; such feedstock develops fine globular microstructure after being heated up to the semisolid state.