WELDING RESEARCH Fig. 6 — Size distribution histogram for all 1590 pores analyzed in this study. The pores are grouped by weld type, summing the highand lowpower welds together in this plot. The inset plot shows the Weibull fit to all of the pores. and is also a high-power density welding process to compare with the laser vacuum weld results. The welding parameters are summarized in Table 2 where the beam/ material interaction time (beam diameter to travel speed ratio) was kept at about 53 ms for the welds made in Ni. This is identical to welds made in a previous study to produce similar keyhole behavior (Ref. 1). The LP welds in Ni had heat inputs of 265 J/mm, while the HP laser weld in Ni was made at 30% increased heat input of 347 J/mm. The electron beam welds matched the heat input of the LP laser welds in nickel and nearly completely penetrated the 10-mm-thick plate. The Ti laser welds were made at a correspondingly 424-s WELDING JOURNAL / NOVEMBER 2016, VOL. 95 lower heat input by increasing the travel speed to 17 mm/s, and thus reducing the interaction time to 37.6 ms. The reason for this change was to reduce the penetration in the Ti alloy, which would have completely penetrated the 10-mm-thick plate if the higher heat input used on the Ni samples was used. The resulting heat inputs for the Ti welds were 143 J/mm for the lowpower weld, and 187 J/mm for the high-power weld. Figure 5 shows metallographic cross sections through the Ti and Ni HP welds made in vacuum and argon to indicate the effects of reduced pressure on weld pool shape and porosity. The welds made in nickel under Ar and reduced pressure are shown in Fig. 5A, B, respectively. The first, and most obvious, difference is the dramatic change in weld pool shape and penetration. For identical welding parameters, the weld made in Ar is wide at the top, with shallow penetration and a very short keyhole, compared to the weld made under reduced pressure. The aspect ratio, defined here as weld depth to full top width, is 0.89 for the weld made under Ar and 3.5 for the weld made under reduced pressure. This nearly 4× increase in aspect ratio is fully attributable to the change in pressure surrounding the laser weld. In addition to the difference in weld geometry, it is clear the weld made in Ar has a very large void showing up in the center of the keyhole, whereas very little porosity is present in the weld made under reduced pressure. Figure 5C and D shows the corresponding welds made in Ti under Ar and reduced pressure, respectively, having very similar weld cross-sectional shapes as in Ni. Here, the aspect ratio increases from 0.77 for the Ti weld made in Ar to 3.48 for the Ti weld made under reduced pressure conditions. In the case of Ti, much lower porosity is present than in Ni, with only a few small voids showing up at the root of the weld made in Ar. Computed tomography (CT) is becoming an increasingly important tool for understanding porosity distributions in welds because it can quantify porosity in a way that conventional metallography is not able to do (Refs. 1, 13). In this study, 3D CT was performed on each of the welds over an 11.4-mm region of interest that did not include the weld start or stop locations. Results of the CT analysis characterized every pore in each of the welds so that the number of pores, the volume of each pore, and the total volume of porosity were documented. The overall results of Table 5 — Pore Size Distribution Parameters for the TwoParameter Weibull Relationship where is the Scale Parameter, and is the Shape Parameter Material Weld Total Number Weibull Weibull Parameters Condition of Pores Parameters Grouped by Weld Type Ni LaserAr, HP 117 0.00245 0.37464 Ni LaserAr, LP 190 0.00043 0.41923 0.00090 0.37281 Ni EB1Vac 123 0.00217 0.77319 Ni EB2Vac 627 0.00019 0.64001 0.00031 0.5748 Ni LaserVac, HP 218 0.00040 0.61481 Ni LaserVac, LP 214 0.00034 0.72746 0.00047 0.60573 Ni All Ni Pores 1489 0.00045 0.47519 — — Ti LaserAr, HP 58 0.00089 0.54279 0.00075 0.58557 Ti LaserAr, LP 40 0.00055 0.73203 Ti LaserVac, HP 2 N/A N/A N/A N/A Ti LaserVac, LP 1 N/A N/A Ti All Ti Pores 101 0.00075 0.58432 — — Ni + Ti All Pores 1590 0.00051 0.49036
Welding Journal | November 2016
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