The relationship between thermal history and microstructure in spray-deposited tin-lead alloys

Gas-atomized spray deposition involves the creation of a spray of droplets by a gas atomizer and the consolidation and solidification of these droplets on a substrate. The present paper describes an investigation of the fundamental characteristics of heat transfer and solidification during spray deposition. Spray deposition was used to manufacture Sn-15 and 38 wt. % Pb preforms using atomizer-substrate distances of 180 and 360 mm, gas flow rates of 2.5 and 3.4 g/s, and melt flow rates of 61 and 35 g/s. Analytical and numerical models were developed to predict the thermal history of the spray deposit for a range of deposit-substrate heat transfer coefficients. A deposit-substrate heat transfer coefficient of ∼10⁴ W m⁻² K⁻¹ was determined by comparing measured and calculated spray-deposit thermal histories both during and after spray deposition. Microstructural analysis of transverse sections of the spray deposits revealed maximum values of spray-deposit density and cell/grain size at specific distances from the deposit-substrate interface. The distance between the density and cell/grain-size maxima and the deposit-substrate interface increased from 0.9 to 10 mm for Sn–15 wt. % Pb and from 2.6 to 11.3 mm for Sn–38 wt. % Pb as the atomizer-substrate distance was increased from 180 to 360 mm and the melt to gas mass flow rate ratio was decreased from 24 to 10. The origin of these microstructural features is described in terms of heat transfer during spray deposition.