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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 5 range of design options. Other negative factors include limita- tions on the use of specific alloys, increased product cost associated with the billet manufacturing pro- cess, and inferior product proper- ties compared with conventional solid-state forming. Optimization of the billet manufacturing pro- cess and its transformations dur- ing heating and cooling to and from the solidus-liquidus range is required to overcome the tech- nology gap. ADVANTAGES OF THIXOTROPY IN METALLURGY The phenomenon of thixo- tropy, discovered in 1923 in non- metallic materials, is defined as the progressive decrease in vis- cosity over time for a constant ap- plied shear stress, followed by a gradual recovery when the stress is removed (Fig. 2). The term thixotropy, which originally re- ferred to reversible changes from fluid to solid-like elastic gel, was introduced by Peterfi in 1927 as a combination of two Greek words: thixis, stirring or shaking, and trepo, turning or changing. Thixot- ropy found its way into metallurgy when researchers at Massachu- setts Institute of Technology re- vealed that applying shear during solidification of the Sn-15% Pb alloy substantially reduced mea- sured stress. The stress at a given tem- perature below the liquidus was orders of magnitude less than when the alloy was cooled to that temperature without shear. De- creasing temperature leads to a rapid increase in viscosity, but the higher the shear rate, the low- er the maximum viscosity val- ue and the shorter the time to reach steady state. Stress reduc- tion is due to an interaction be- tween solid particles related to morphological changes in semi- solid slurry, particularly a replace- ment of dendritic morphologies Fig. 1 — Hardware for executing thixoforming: (a) pilot-scale metal forming facility of CanmetMaterials with Microdyne 1500- ton triple acting and 1200-ton single acting presses; (b) semisolid magnesium injection molding machine with a clamp force of 680 t. Fig. 1b courtesy of SSD-Magnesium. Fig. 2 — Influence of shear rate on viscosity development. Initial state of alloy for t = 0 is fully liquid; cooling conditions are identical in both cases to reach the same temperature for isothermal holding (i.e., the same solid fraction). T oday’s manufacturing indus- try is continually challenged to satisfy requirements of evolving markets. As a result, there is a search for novel tech- nologies enabling large-scale pro- duction of high performance, net shape components. Convention- al high-pressure die casting offers vast advantages in terms of manu- facturing simplicity, net shape ca- pabilities, competitive cost, and low energy consumption. However, product quality in terms of integrity and properties is still not sufficient for use in some of the structural ap- plications required inmodern trans- portation vehicles. Semisolid metal processing, invented in the 1970s, is seen as an intermediate measure that allows elimination of some of the detrimental features of casting. The first experiments with semisolid metal processing invol- ved forming a component from a liquid state precursor in a pro- cess called rheocasting, but later attention shifted to thixoforming, which relies on solid-state precur- sors. Thixoforming was the first method to be commercialized and was seen as an alternative to the complex and costly multistep pro- cesses of solid-state forming [1] . In contrast to casting, it does not rely on molten metal, so it does not re- quire foundry or melting furnaces and product manufacturing can be conducted as a single step in a relatively clean and safe environ- ment (Fig. 1). In industrial practice, thixo- forming faces certain challeng- es along with competition from well-established conventional and novel technologies. This includes rheocasting, which has again be- come the center of research and industrial interest with the devel- opment of new slurry preparation techniques. The main factor lead- ing to reduced interest in thixo- forming is said to be difficulties with manufacturing suitable feed- stock and billet, despite the fact that the method offers a broader