Prediction of residual stresses in metal LPBF parts through a holistic multiscale simulation approach

Laser Powder Bed Fusion (LPBF) has emerged as a revolutionary additive manufacturing process for the manufacture of complex metallic components, but various challenges still curtail its widespread use by industries. Among these challenges, deformations caused by residual stresses accumulated during printing can lead to severe defects ranging from low geometric accuracy of the as-printed parts to build failure caused by cracking or warpage. Numerical simulation of the LPBF process can help understanding the thermomechanical behavior of a part during printing and predicting its warpage without actually running any costly printing job. In this context, we present a holistic multiscale finite element analysis (FEA) simulation methodology that is able to predict the residual stresses and their associated deformations in metal LPBF parts. A common obstacle in the numerical simulation of the LPBF process is the number of different spatial and temporal scales involved. While part-scale builds usually have a size in the scale of centimeters and take hours to be built, the laser beam operates on a much smaller scale, typically in the scale of micrometers and the timescale of the order of microseconds. Simulating the entire process at the meso-scale level is not tractable in practice and a multiscale simulation methodology is thus warranted. The continuum approach we have developed involves solving two distinct numerical problems at the meso-scale and at the partscale levels, respectively. The meso-scale model is a high-fidelity coupled thermomechanical simulation of the LPBF process that takes into account the dynamics of powder melting and solidification caused by a moving heat source representing the laser beam and its resulting strains and stresses in the material. This problem is solved on a small domain representative of the laser hatch pattern over short time periods (millimeters / seconds) and yields so-called “inherent strains” that are extracted in accordance with the modified Inherent Strain method. The part-scale model uses a layer-by-layer approach, with possible layer lumping, to simulate the LPBF process on full-scale parts. Its simplified physical models give access to the evolution of temperature in the part over the duration of the build and leverages the inherent strains extracted from the meso-scale model to predict the deformations of as-built parts. Our approach will be demonstrated on cantilevers and results will be compared with experimental data.

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