7. Microstructural Evolution in Spray Forming

Spray forming is a casting process in which the molten metal is directly converted to a solid bulk with unique characteristics. When processed under optimum conditions, spray formed materials typically present microstructures composed of refined polygonal (non-dendritic) grains, uniformly distributed with low levels of micro- and macro-segregation. This set of characteristics is achieved regardless of the alloy system, making spray forming an attractive process to produce alloys where processing by conventional casting techniques is complicated. This chapter is dedicated to presenting the mechanisms that take place when the atomized droplets arrive at the deposit surface, and how the spray-formed microstructures evolve during deposition. It will be seen that spray forming is a self grain-refining casting process and cannot be considered a rapid solidification technique. Section 7.6 will address the main differences between the microstructural evolution in spray forming and other spray deposition or “thermal spray” processes. These processes include plasma spraying, high velocity oxy-fuel, wire arc spraying, detonation gun spraying, etc. In this way, it will show why spray forming is such a unique process. This chapter is also dedicated to presenting how the porosity—an intrinsic feature of spray-formed microstructures—is generated and how the processing parameters affect its type, size and distribution. Furthermore, the generation of other defects related to the solidification and/or to the cooling of the spray formed product after deposition—such as residual stresses and hot cracks—and their influence on the product quality and material properties will be presented. Finally, this chapter will also discuss the effect of the atomization gas (Ar, N₂ or He) on the final product quality in terms of porosity and chemical composition of steels, superalloys, and copper alloys.