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A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 9 1 6 with spheroidal structures. This finding constitutes the basis of semisolid metal processing, also referred to as semisol- id metallurgy. PROCESS AND FEEDSTOCK Thixoforming is a general term used to describe net-shape forming from a partially melted billet within a metal die or mold. When the shap- ing process occurs in an open die, it is called thixoforging and when compo- nent shaping is performed in a closed die, it is called thixocasting. Seen as a hybrid process between high-pres- sure die casting (liquid feedstock) and conventional forging (solid feedstock), thixoforming is conducted using a ma- terial with a viscosity comparable to honey. Moreover, shaping can occur through thixoextrusion or thixorolling. In the latter case, twin roll-casting hard- ware needs to be explored. The three stages of thixoforming include manufacturing a feedstock/bil- let with a specific microstructure, pre- heating a billet to a temperature in the solidus-liquidus range, and net shape forming while in the semisolid state. Three forms of feedstock enabling the implementation of thixoforming include bulk billets, coarse particu- lates, and powders (Fig. 3). When using coarse particulates instead of bulk bil- let, net shape forming is accomplished through injection molding/thixomold- ing [2] . Thixoforming using fine powder as the precursor is termed as powder thixoforming or semisolid powder form- ing [3] . The essential requirement of the feedstock is its thixotropic morpholo- gy after partial melting, expressed by spheroidal solid particles suspended in a liquid matrix. Bulk billets. The bulk billet precur- sor for thixoforming is manufactured by either using the liquid-state route where, in principle, all liquid metal en- gineering techniques can be explored, or using solid-state processing (Fig. 4) [4] . The essence of the solid route of billet manufacturing is the thermomechan- ical treatment of as-cast structures. Its origin is traced to the invention by Young et al. in 1983 [5] of a process for preparing a metal composition suitable for forming in a semisolid condition. The technique is generally known as strain or stress-induced melt activation (SIMA). Solid-state deformation prior to melting offers an opportunity to influ- ence the solidification outcome through changing the scenario of how melting progresses. The treatment of an as-cast microstructure in the solid state can in- volve single or multi- step physical pro- cessing using a var- iety of convention- al or severe plastic- deformation techni- ques. The objective is the same as for liquid metal engi- neering, with the dif- ference that the so- lidification outcome is affected by mani- pulating the solid metal prior to its melting, rather than the melt itself. De- formation (either cold or hot) sufficient enough to trigger recrystallization (either static or dy- namic) is the essential requirement for successful dendrite-to-globule transi- tion during melting. The required de- formation and recrystallization out- come is a polygonal/equiaxed micro- structure at the beginning of melting. Fig. 3 — Schematic of processing cycle during thixoforming: Feedstock in the form of bulk billets, mechanically comminuted ships, rapidly solidified granules, and fine powders are heated to semisolid state and formed to net or near-net shape; component microstructure consists of spheroidal particles surrounded by a solidifiedmatrix; in the case of coarse particulates and powders, composites are generated after adding a reinforcement compound. Fig. 4 — Schematic of liquid and solid routes of billet preparation for thixoforming.