Spray forming, also known as spray casting, spray deposition and in-situ compaction, is a method of casting near net shape metal components with homogeneous microstructures via the deposition of semi-solid sprayed droplets onto a shaped substrate. In spray forming an alloy is melted, normally in an induction furnace, then the molten metal is slowly poured through a conical tundish into a small-bore ceramic nozzle. The molten metal exits the furnace as a thin free-falling stream and is broken up into droplets by an annular array of gas jets, and these droplets then proceed downwards, accelerated by the gas jets to impact onto a substrate. The process is arranged such that the droplets strike the substrate whilst in the semi-solid condition, this provides sufficient liquid fraction to 'stick' the solid fraction together. Deposition continues, gradually building up a spray formed billet of metal on the substrate.
The gas atomised spray forming (GASF) process typically has a molten alloy flow rate of 1–20 kg/min-1, although twin atomizer systems can achieve metal flow rates of up to 80 kg/min-1. Special steel billets of 1 tonne or more have been produced by spray forming on a commercial basis, together with Ni superalloy ring blanks of up to 500 kg and Al alloy extrusion billets of up to 400 kg.
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