Abstract
The contour scan strategies in laser powder bed fusion (LPBF) of Ti-6Al-4V were studied at the coupon level. These scan strategies determined the surface qualities and subsurface residual stresses. The correlations to these properties were identified for an optimization of the LPBF processing. The surface roughness and the residual stresses in build direction were linked: combining high laser power and high scan velocities with at least two contour lines substantially reduced the surface roughness, expressed by the arithmetic mean height, from values as high as 30 µm to 13 µm, while the residual stresses rose from ~340 to about 800 MPa. At this stress level, manufactured rocket fuel injector components evidenced macroscopic cracking. A scan strategy completing the contour region at 100 W and 1050 mm/s is recommended as a compromise between residual stresses (625 MPa) and surface quality (14.2 µm). The LPBF builds were monitored with an in-line twin-photodiode-based melt pool monitoring (MPM) system, which revealed a correlation between the intensity quotient I2/I1, the surface roughness, and the residual stresses. Thus, this MPM system can provide a predictive estimate of the surface quality of the samples and resulting residual stresses in the material generated during LPBF.
Highlights
Additive manufacturing (AM) technologies like directed energy deposition (DED) or laser powder bed fusion (LPBF) have been developed for the near-net-shape fabrication of metal components with high material efficiencies [1,2]
In order to investigate the effect of the scan pattern, here, only one parameter set was used per sample (Table 1; for the sake of simplicity, all specimens are listed in Appendix A, Table A1)
The melt pool monitoring (MPM) intensities I1 and I2 (I1 is given in Figure 13a, I2 is omitted for clarity) showed a positive correlation with EV, which was due to raising the laser power P or slowing the scanning velocity v
Summary
Additive manufacturing (AM) technologies like directed energy deposition (DED) or laser powder bed fusion (LPBF) have been developed for the near-net-shape fabrication of metal components with high material efficiencies [1,2]. The process parameters and processing strategies employed in LPBF determine the contour properties significantly This is mostly due to powder particles remaining attached to the resolidified outer surfaces (i.e., secondary roughness), stair-case effects due to discrete layer-wise processing, the melt tracks, and, the contour laser process itself (primary roughness or waviness) [4,5,6,7]. The surface roughness depends strongly on the build angle of the surface, e.g., a side, down- or up-skin surface [5,8]. This has a high impact on the fatigue properties of LPBF coupons or components [4,7,9]
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