Design Methods Used to Minimize Residual Stresses in Direct Laser Deposition

BACKGROUND: In the additive manufacturing of hardening alloy products, they are often destroyed. This is caused by an unfavourable combination of metallurgic factors and high processing stresses. When manufacturing large-sized industrial structures by direct laser deposition, it is impossible to ensure a favorable phase structure and mechanical properties of the deposited metal due to the low interpass temperature and high cooling rate. In this case, to prevent structural damage, it is required to reduce stress and deformation. AIM: To empirically analyze the efficiency of local deposit geometry and local chemical composition modification to reduce residual stresses and deposition deformations simulating the manufacturing of large-sized structures by direct laser deposition. METHODS: We analyze wall-type deposits made of Ti-6Al-4V alloy with two types of fillets (flat and concave) on the ends. We also describe a wall-type deposit without fillets with a gradient transition from a soft layer of pure titanium to a significantly stronger Ti-6Al-4V layer. To determine stresses and deformations in deposits, a numerical direct laser deposition model was developed. Interlinked problems of heat conduction in a non-stationary formulation and the quasi-static problem of thermally-induced plasticity were solved by the finite element method. RESULTS: The flat fillets added to the ends of the deposit has little effect on the magnitude of residual stresses and accumulated plastic deformations. Concave fillets significantly reduce plastic deformation, ensuring the manufacturing of defect-free deposits. The most effective approach was to add a less durable but more plastic interlayer between the rigid substrate and the deposit made of a stronger alloy. In this case, the highest level of accumulated plastic deformations is achieved at the ends of the deposit in the soft interlayer region and depends little on the interlayer length. The stronger part of the Ti-6Al-4V deposit has no effective plastic deformation. CONCLUSION: It was demonstrated that numerical modeling using local deposit geometry and chemical composition modifications allow to significantly reduce residual stresses and plastic deformations in deposits manufactured by direct laser deposition.