Residual stress and strain mitigation in direct laser deposition through adjustment buildup geometry or addition of ductile transition layer

An unfavourable combination of metallurgical factors and high stresses may result in the fracture of additively manufactured parts, especially in the case of high-strength alloys. Frequently in direct laser deposition (DLD) fabrication of large-scale parts, it is impossible to obtain desirable structural-phase composition and required mechanical properties of the depositing metal due to the low inter-pass temperature. Thereby, the stress and strain levels must be reduced to prevent fracture of the build part. The present work seeks to analyse the effectiveness of the proposed approaches for reducing residual stresses and strains by (i) local adjustment of the buildup geometry and (ii) addition of a ductile transition layer. The process conditions were close to those used to produce large-scale parts by the DLD method, resulting in the buildup having the same thermal history. The following cases are analysed: a single-wall buildup of Ti-6Al-4V alloy with flat-faced and concave fillets, and a single-wall buildup with a ductile transition layer of commercially pure titanium between the rigid substrate and rest buildup of the Ti-6Al-4V. A simulation procedure based on the implicit finite element method was adopted for the theoretical study of the stress-strain field evolution. It was found that the addition of flat-faced fillets has little effect on the magnitude of the accumulated plastic strain. On the other hand, concave fillets result in considerable plastic strain reduction compared to buildup without fillets by a factor of 2.8 for 14-mm fillet radius and 5.8 for 28-mm fillet radius. The addition of a transition layer of a less strong and more ductile alloy than the rest buildup causes non-uniform deformation of the buildup. The more brittle buildup part of the Ti-6Al-4V practically does not deform plastically and has a considerably lower peak stress than the buildup entirely composed of the Ti-6Al-4V.