Microstructural characteristics of 5083 Al alloys processed by reactive spray deposition for net-shape manufacturing

The superplastic forming of Al-Mg alloys is of considerable interest due to its potential for structural applications. In the present study, reactive spray-deposition processing is implemented to reduce grain size to the superplastic range and, thus, to obtain superplasticity for net-shape manufacturing. A typical AI-Mg alloy (5083) was selected for this study and was spray deposited using different N-2-O-2 atomization gas mixtures. The as-deposited microstructures reveal an equiaxed grain morphology, a limited dynamic recovery structure, and a distribution of intermetallic phases and oxide dispersoids. The grain size decreases with increasing atomization gas oxygen content and is affected by changes to the superheat temperature (ST) and by small additions of Zr. In some deposits, gamma-Al2O3 crystallites less than 20 nm were distributed as stringers along grain boundaries; this observation was found to be strongly dependent on the oxygen content. The oxidation kinetics during reactive spray deposition were studied on the basis of the Mott-Cabrera theory of oxidation. A linear growth rate of dX/dt = 2A(0).exp (-Q/kT).exp (k(0).P-1/2/kT) was obtained, which suggests that the rate decreases rapidly with decreasing temperature or oxygen pressure and that, when the temperature (or oxygen pressure) is low, the rate decreases steadily with decreasing oxygen pressure (or temperature). Calculations of the width of oxide stringers as a function of oxygen content and ST show good agreement with the experimental observations. Finally, the volume fraction of oxide phases was estimated on the basis of kinetic considerations.