Temperature and residual stress distribution of FGM parts by DED process: modeling and experimental validation

Laser direct energy deposition (DED) is an advanced additive manufacturing technology, which can produce fully dense and functionally graded materials (FGMs) metal parts. Residual stress and distortion are crucial issues in DED process reducing the mechanical strength and the geometrical accuracy of the fabricated components. This work provided a combined approach involving thermo-mechanical model and experimental validation toward two FGM cases fabricated by DED process to reveal the residual stress and distortion distribution. Two fabrication approaches were used: a direct deposition of Cu on SS304L and a structure graded from iron alloy SS316L to nickel alloy In718 to pure Cu based on SS304L substrate. Thermal histories of the substrate and the residual stress on cross-section of the FGM part were measured to calibrate the 3D coupled thermo-mechanical model. The predicted temperature and stress results showed a good agreement with the experimental measurements. The distortion results of both fabricated walls showed an upwards bent trend. Because of the high-temperature gradient induced by the mismatch in the thermal expansion coefficient of different materials, very high distortion was observed at two edge regions of the second printing material of FGM part. From the residual stress standpoint, direct joining Cu on SS304L resulted in extremely high residual stress at the bi-material interface due to mismatch in the thermal expansion coefficient of different materials. By introducing SS316L and In718 buffer layers, defect-free Cu can be successfully deposited on SS304L. This model can be used to predict the stress behavior of products fabricated by DED process and to help with the optimization of design and material chosen of FGMs process.