Solidification in Spray Forming
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THE spray forming process is an advanced casting process for the manufacture of billet materials and is shown schematically in Figure 1. Spray forming comprises the sequential steps of (1) the continuous gas atomization of a melt stream to produce a spray of 10- to 500-lm-diameter alloy droplets, (2) droplet cooling at typically 102 to 104 Ks)1 and acceleration to 50 to 100 ms)1 under the action of the atomizing gas, (3) the deposition of droplets at the growing spray-formed billet surface, and (4) the relatively slow cooling at 0.1 to 10 Ks)1 and solidification of any residual liquid in the spray-formed billet.[1] As well as potential near-netshape benefits, the primary advantage of the spray forming process is the ability to manufacture alloy compositions that are problematical in conventional processes such as ingot casting, direct chill casting, and powder metallurgy. Commercial examples of alloys manufactured by spray forming include Al-Si based alloys for cylinder liners,[2] high speed and speciality steel billets,[3] Si-Al alloys for thermal management applications,[4] and Al-Nd alloys for sputtering targets.[5] The disadvantages of spray forming include as-sprayed porosity that requires closing by hot isostatic pressing or other downstream processes, and losses because not all P.S. GRANT, Cookson Professor of Materials, is with the Department of Materials, Oxford University, Oxford OX1 3PH, United Kingdom. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled ‘‘Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunf ’’, which occurred March 13–15, 2006, in the TMS Spring meeting in San Antonio, TX, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee. Article published online February 2, 2007. 1520—VOLUME 38A, JULY 2007
droplets created by atomization end up in the sprayformed billet. These aspects and some of the underlying process physics have been reviewed in Reference 1. Despite many investigations of the relationship between the spray forming conditions, the solidification conditions arising, and the as-sprayed microstructure,[7–16] some uncertainties remain in how the underlying process physics and mechanics give rise to the advantageous spray-formed microstructure that typically comprises the following: (1) equiaxed/polygonal grains of diameter typically in the range 20 to 50 lm and the complete absence of columnar/dendritic morphologies, (2) high levels of microstructural homogeneity and low levels of microsegregation regardless of position in the billet, and (3) the ability to produce this characteristic spray-formed microstructure in all engineering alloy systems. For example, Figure 2(a) shows a 26 kg Al-5.3 Mg-1.2 Li-0.28 Zr alloy billet spray formed at Oxford University, and the corresponding as-spray-formed microstructure is shown in the electron backscattered diffraction (EBSD) orientation map in Figure 2(b). The as-spray-formed grain size was ~15 lm an
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