Semi-solid metal processing was developed at MIT in the mid-1970’s. While examining how shear stress affects hot tearing as alloys solidify, researchers observed a condition known as thixotropy in the metallic systems they were evaluating. It was later discovered that the process of shearing changed the grain structure from that of a conventional dendrite into a more spherical form, which resulted in a significant drop in the shear stress and viscosity. Thixotropic behavior was well understood by materials science for polymers and ceramics, but it had not been previously observed with metals.
For almost 20 years, intellectual patents for the technology were closely held by MIT and licensed to ITT and then Alumax Corporation (now Alcoa) which restricted broad industrialization. As the patents expired however, large-scale commercial development finally began in the early 1990’s when major diecast machine manufacturers coupled the technology to horizontal cold chamber machines.
Although injection velocities are significantly lower and final pressures are substantially higher than typical diecasting, the horizontal cold chamber machine is ideally suited to inject semi-solid aluminum loaded as right-cylindrical billets through modified slots in the shot sleeve. Correctly produced semi-solid billets have a high effective viscosity and behave much like a soft solid or “Play-Doh”.
Injection at low speeds minimizes turbulence, essentially eliminating gas porosity while high final pressures are used to consolidate the material and assist feeding to eliminate shrinkage porosity. Today semi-solid thixocasting at Vforge has a direct line to this original MIT research, as well as subsequent MIT-led industrialization efforts, and it continues to deliver the improved mechanical properties first observed some 50 years ago.
