A study of the relationship between thermal expansion and residual stresses in selective laser melting of Ti-6Al-4V

Abstract

Selective laser melting of Ti-6Al-4V is associated with residual stress generation, thermal expansion, and metallurgical changes due to high heating and cooling rates. These manufacturing flaws affect the microstructure and mechanical properties of the parts produced. In this work, process mapping is presented for producing Ti-6Al-4V using the selective laser melting process. Combinations of process parameters that lead to stable melting are experimentally identified. These process maps represent the density, coefficient of thermal expansion, and deflection of parts produced in terms of laser process parameters. As a result, an optimum process window to minimize manufacturing flaws and provide stable melting is presented. The effect of selective laser melting process parameters on the coefficient of thermal expansion of the as-deposited Ti-6Al-4V parts is investigated. A critical laser energy density of 86.8 J/mm3, EC, is introduced based on the stability of the melting process. Stable melting of Ti-6Al-4V occurs at any combination of process parameters that generates laser energy density at EC. This stable melting provides homogeneous microstructure, high density of 99.9 %, and a thermal expansion coefficient in line with that of the wrought material. The induced residual stresses and part deflections at the optimal conditions are studied. Above EC, high tensile residual stresses are observed due to variations in the coefficient of thermal expansion, temperature differential, and material composition. Lack of fusion and void formation occur below EC.