Additive manufacturing (AM) technology is found to be a promising approach since it renders the potential to economically fabricate customized parts with complicated geometries in a rapid design to manufacturing cycle as well as minimize waste and enhance sustainability through repair and refurbishment of structures. However, the simultaneous occurrence of several intricate phenomena such as transient temperature field, non-uniform cooling rate, solid-state phase transformation, and microstructural changes in different orientations inevitably generates residual stress fields. [1-2]. To overcome the detrimental effect of residual stress namely, delamination, cracking, warping, etc., in additively manufactured parts, there is continued interest in the accurate prediction and minimization of residual stress to prevent its adverse effect on structural stability as well as the mechanical performance of components, especially on fatigue life and onset of premature failure. In the absence of in-situ routes to measure the evolving residual stresses during printing, numerical modeling has been the primary route to predict and engineer desired residual stresses. Among the several process parameters, the scanning strategy is accounted for, in the present study. It allows for an optimized and effective process route for part fabrication by regulating the temperature gradient and associated cooling rate to control the developed residual stresses. From the literature, there is no consensus as to what the optimum scanning strategies for minimizing the residual stress would be. Few of them have contradicting outcomes in terms of the influence of island or chessboard scanning patterns on residual stress [3-5]. In the present work, three different chessboard scanning patterns with varied orientations (shown in Fig. 1) are adopted to deposit multi-layer and multi-track IN718 superalloy system of dimension 16*16*1.6 mm3. Further, the effect of scanning strategy (scan vector length and rotation) on evolved transient temperature field during deposition and subsequent in-process developed stresses or residual stresses is extensively studied for each case. The build area is composed of small blocks in the chessboard pattern that itself reduces the scan vector length by virtue of which residual stress dampens to a certain extent. The finite element-based sequential-coupled thermo-mechanical model is established via in-house developed subroutines in ABAQUS commercial software. 8-noded linear brick element (DC3D8) with diffusive heat transfer characteristics is implemented for thermal analysis and 3D stress (C3D8R) element is imposed for mechanical modeling. The thermal output, i.e., nodal time-temp. extracted from thermal modeling for the whole solution domain is used as input load for static stress analysis. A trend of increasing residual stress is observed for relatively large chessboard block size. Further, it is confirmed that a 90° alternating scanning pattern leads to a reduction in residual stress by ~20% as compared to the 45° pattern. Therefore, it is conclusively stated that small chessboard block size composed of small scan vector length and alternating scanning strategy by rotating it to 90° proved to be the optimized one for achieving minimal residual stress. Detailed analysis and the root cause of the observation are proposed to be done.

Figure 1. Chessboard scanning strategy: (a) with adjacent block scanned in 45°, (b) with adjacent block scanned in 90° rotation.

References

[1] S., Liu, & Y. C., Shin; Materials & Design, 2019, 164, 107552.

[2] Mukherjee, T., Zhang, W., & DebRoy, T.; Computational Materials Science, 2017, 126, 360-372.

[3] Ali, H., Ghadbeigi, H., & Mumtaz, K.; Materials Science and Engineering: A, 2018, 712, 175-187.

[4] Cheng, B., Shrestha, S., & Chou, K.; Additive Manufacturing, 2016, 12, 240-251.

[5] P., Mercelis, & J. P. Kruth; Rapid prototyping journal, 2006.