May_June_AMP_Digital

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 8 manufacturing process. It appears that there is no full analogy between the partial melting behavior of powders and other forms of feedstock. In the case of powder, primary particle size in semisolid slurry is equal to several di- ameters of the original powder, so it is different from coarse particulates form- ing numerous clusters of grains with- in a single chip. Powder thixoforming creates a homogenous microstructure with fine grains, improving the mechan- ical properties of finished products. ALLOYS AND PRODUCTS Alloys for consideration in semisol- id processing must fulfill general crite- ria with a sufficiently wide solidification gap. An additional condition required for thixoforming with a billet prepared by the solid-state route is to have suf- ficient formability to prevent cracking during deformation. Alloys that have been explored over the decades, both at laboratory and industrial scales, are mainly based on Al, Mg, Zn, Cu, Fe (steel), and Co. Experiments with met- al-matrix composites were also carried out. A recent trial included new high en- tropy alloys [7] . The major portion of today’s alloys used for the thermomechanical route of billet preparation are casting grades. The most common ones are aluminum casting alloys A356 and A357, both hav- ing good fluidity and castability due to their high content of Al-Si eutectics. There is interest in exploring alloys de- signed for wrought processing, which offer better performance and high- er mechanical properties than cast grades. Experimental trials show that high-strength wrought aluminum al- loys can be thixoformed. However, wrought alloys are more difficult to pro- cess due to a low content of eutectics and narrow solidification range. More- over, the usual wrought high-strength aluminum alloys, which are typically used in forging, are not well suited for semisolid forming, especially because they have a tendency for hot cracking during solidification. A selection of automotive com- ponents produced over the past few years in Europe by thixoforming with aluminum alloys is shown in Fig. 6. It is concluded that the consistency of mechanical properties of thixoformed aluminum alloys was affected by the quality and consistency of the feedstock material used, tight control of process parameters, and the die design quali- ty [8] . Typical components produced by the semisolid thixo- route includeanauto- motive steering wheel, dashboard elements, exterior elements, and structural beams. There were also efforts in the past to manufacture wheels for motorcycles, including high profile racing styles (Fig. 7). Examples of high-volume au- tomotive parts presently produced from magnesium alloys using semisol- id injection molding are shown in Fig. 8. There is a difference in quality require- ments: For example, as structural parts, the car seat frame and steering wheel require sound mechanical properties, while the dashboard elements require a reproduction of surface details. TECHNOLOGY ASSESSMENT A manufacturing technology must demonstrate clear advantages of supe- rior product properties and competitive cost to achieve commercial acceptance in today’s market. Because an evalu- ation of the advantages of semisolid routes over conventional techniques does not show a single evident ben- efit, an assessment matrix should be created to understand the differences between conventional and novel pro- cessing techniques. The assessment is not straightfor- ward for thixoforming because some Fig. 6 — Examples of thixoformed parts : (a, b) - engine brackets (Stampal SpA -Fiat); (c) – suspension arm (Stampal SpA - TRW); (d) – fuel injection rail (Magneti Marelli); (e) – motor end plate; (f) – motor end plate as die cast (bottom) and as thixoformed (top); (g) – aerospace part thixoformed for Lucas (Al + 17% SiC); (h) – piston thixoformed for T&N (A390 – Al - 17%Si) (Sheffield University); and (i) – wall bracket (Stampal SpA) [8] . Fig. 7 — Components manufactured by thixoforming: (a) motorcycle racing wheel, Durani Racing Components, USA [11] ; (b) pillar for auto front door – Al [12] ; (c) housing with brass inserts – Mg; and (d) automotive steering wheel – Mg [2] .