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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 7 A different approach to billet fabri- cation through casting is based on con- trolled cooling during solidification to avoid extensive dendrite growth. The method does not produce a billet with globular morphologies, but rather with very fine dendrites. After reheating, fine dendrites transform to globular forms. The principle is based on spray form- ing, which converts molten alloy di- rectly into a semifinished product, e.g., the Osprey process. In this technique, property benefits arise from rapid so- lidification, which promotes micro- structural refinement and eliminates macrosegregation. Coarse particulate feedstock. The use of metal particulates for consoli- dation into practical product forms has been explored for ages, before furnac- es were developed that could exceed the melting point of metals. This form of feedstock covers relatively coarse particulates several millimeters in size, which have also been used in a variety of chemical, pharmaceutical, military, and metallurgical applications. Partic- ulates are also called chips, granules, and pellets depending on their shape and manufacturing technique [6] . Coarse particulates are either deliberately manufactured by mechanical commi- nuting of cast ingots or collected as a by-product during machining of com- ponents. The latter method offers an effective solution for metal recycling. To manufacture chips, material passes through rolls with cutting teeth locat- ed around their surfaces. After cutting, chips are separated based on size us- ing sieving. An alternative route of man- ufacturing coarse particulates is rapid solidification of a liquid. Coarse particu- lates can be used directly for thixoform- ing, or after solid-state compaction to manufacture larger billets. If compact- ed into bulk billets, the same technique potentially may be used for coarse par- ticulates and powders. Powder feedstock. Powders are defined as particles that are usual- ly less than 1 mm in size; most metal particles used in powder metallurgy are in the range of 5 to 200 µm. Three main methods of powder production in- clude mechanical, chemical, and phys- ical (atomization), leading to different shapes and sizes. Major powder met- allurgy processes include compaction and sintering, powder forging, hot iso- static pressing, metal injectionmolding, and additive manufacturing. In addi- tion, there is growing interest in using fine powder particulates for thixoform- ing [3] . In powder thixoforming, green billets are first obtained by blending and cold pressing a powder mixture, which is partially melted and thixo- formed. In addition to compacted bil- lets, research has been conducted to use powder directly for partial melting and thixoforming. Because thixoform- ing can be used to mix different types and amounts of powder reinforcement, it has great potential for manufacturing composite materials. FEEDSTOCK CHANGES DURING PROCESSING During thixoforming, the billet is subjected to partial melting where, un- like conventional casting, only a frac- tion of the alloy volume is converted into liquid. In contrast to rheo-process- ing, semisolid slurry is not subjected to any formof external agitation. For some techniques, this excludes pressure imposed on slurry to force it to flow into the die or mold cavity. In general, microstructural evolution during par- tial melting is driven by the reduction of interfacial energy between solid and liquid phases and is controlled by dif- fusion [1] . Plastic deformation experi- enced by the material prior to melting affects the coarsening kinetics of solid particles in semisolid slurry. Methods exploring solid-state deformation prior to melting and solidification have some limitations in terms of billet sizes due to the requirement of high, uniform defor- mation over the entire cross-sectional area. However, they generate high qual- ity feedstock for thixoforming with a processing potential for alloys designed for wrought processing and high-melt- ing-temperature alloys such as steel and superalloys. At the same time, the solid-state deformation and recrystalli- zation of conventionally cast dendritic materials by thermomechanical treat- ments are energy and processing inten- sive, increasing component cost. Although there is a difference of orders of magnitude in size between individual precursors of bulk billets, coarse particulates, and fine powders, there is a similarity of microstructure generated after their partial melting (Fig. 5). The key difference in the case of coarse particulates and powders is that deformation is a side effect of their Fig. 5 — Microstructure transformations within Mg-9Al-Zn chips produced by mechanical comminuting (left) and granules produced by rapid solidification (right) during heating. Corresponding temperatures are indicated on simplified phase diagram: A, room temperature; B, heated in solid state; C, heated into semisolid state.