Mechanics Modeling of Residual Stress Considering Effect of Preheating in Laser Powder Bed Fusion

Abstract

This study aimed at the investigation of the effect of substrate temperature on residual stress in laser powder bed fusion using a physics-based analytical model. In this study, an analytical model is proposed to predict the residual stress through the calculation of preheating affected temperature profile and thermal stress. The effect of preheating is super-positioned with initial temperature in the modeling of temperature profile using a moving heat source approach; the resultant temperature gradient is then employed to predict the thermal stress from a point body load approach. If the thermal stress exceeds the yield strength of the material, then the residual stress under cyclic heating and cooling will be calculated based on the incremental plasticity and kinematic hardening behavior of metal. IN718 is used as a material example to pursue this investigation. To validate the predicted residual stress, experimental measurements are conducted using X-ray diffraction on IN718 samples manufactured via laser powder bed fusion under different process conditions. Results showed that preheating of the substrate could reduce the residual stress in an additively manufactured part due to the reduction in temperature gradient and resultant shrinkage stresses. However, the excessive preheating could have an opposite impact on residual stress accumulation. Moreover, the results confirm that the proposed model is a valuable tool for the prediction of residual stress, eliminating the costly experiments and time-consuming finite element simulations.

Effect of Preheating on the Residual Stress and Material Properties of Inconel 939 Processed by Laser Powder Bed Fusion

“…The increase in RS in the study [23] was explained as a consequence of the reduced yield strength due to the coarsening of the grains. The specimens in study [23] and also in this paper were produced with the same laser-related process parameters, and powder bed preheating was the only parameter that was changed. Therefore, it can be assumed that excess heat accumulated in the manufactured specimens.…”

“…However, he also predicted through simulation that preheating temperatures in the range of (250-500) • C will increase RS. The increase in RS in the study [23] was explained as a consequence of the reduced yield strength due to the coarsening of the grains. The specimens in study [23] and also in this paper were produced with the same laser-related process parameters, and powder bed preheating was the only parameter that was changed.…”

“…However, the σx stresses increased from 68 MPa to 77 MPa, which is an increase of 13%. Mirkoohi et al [23] experimentally validated that RS of IN718 gradually decreases when preheating up to 200 °C is applied [23]. However, he also predicted through simulation that preheating temperatures in the range of (250-500) °C will increase RS.…”

“…Therefore, such universal laws are not always valid and inconsistencies can be found in the literature [3,21]. Such inconsistencies are also observed in the preheating of the base plate, where a higher preheating temperature generally reduces the RS [22], but excessive heating can increase RS [23]. Therefore, the optimal setting must be determined for each material.…”

Residual stresses in additively manufactured parts: predictive simulation and experimental verification

“…To mitigate residual stresses in additively manufactured parts, a lot of research has been done, such as, in-situ monitoring and control of temperature distribution, substrate preheating, and postprinting heat treatment to release induced stresses and improve mechanical properties (Buchbinder et al , 2014; Mirkoohi et al , 2021). In general, part design, material properties and process parameters can be optimized to minimize and control residual stresses, but there is no established method available to precisely determine residual stresses in manufactured parts.…”