Thermal Stability and Residual Stresses in Additively Manufactured Single and Multi-material Systems

A sequential coupled thermo-mechanical model was developed for laser-based direct energy deposition of single- IN718 and multi-material IN718-Ti6Al4V systems to monitor the thermal stability, solidification characteristics and origin of the residual stresses in each successively deposited layer in real-time. A qualitative agreement was observed between the model and experimental measurements of temperature field and residual stress in the ten layered build system. The substitution of the IN718 substrate with Ti6Al4V alloy caused remarkable temperature rise (similar to 220 K) in the preliminary deposited layers due to the high thermal energy accumulation in Ti6Al4V, leading to relatively low solidification velocity (2.02 mm/s) and large melt pool (0.95 mm). The heat sink effect of the substrate was effective up to the deposition of five-layers. The calculated solidification parameters, i.e., temperature gradient, G and solidification velocity, R suggested a columnar structured interface for both systems in the solidification map. The primary arm dendritic spacing (PDAS) ranging from 8.9 to 21.7 mu m increased to 10.8 to 24.6 mu m on changing the substrate from IN718 (10IN/IN) to Ti6Al4V (10IN/Ti). The overall tensile residual stress reduced from 655 MPa in the 10IN/IN to 621 MPa in the 10IN/Ti due to the lowered thermal gradient. However, an interesting reversal of maximum tensile residual stress, sigma(11) location from the top (tenth layer) to the first layer occurred on changing the substrate from IN718 to Ti6Al4V due to the substantial difference in the coefficient of thermal expansion (Delta CTE similar to 4.3 x 10(-6) K-1) at the interface.