The 3D CT rendering of the corresponding laser weld made in Ni but under vacuum conditions is shown in Fig. 7C. This weld was considerably deeper than the weld made in Ar, and because of this, the sample was turned on end to prevent complete penetration of the coupon. This weld also contains porosity, but only about 11% (0.167 mm3) as much as the corresponding HP laser weld made in Ar (1.50 mm3). The vacuum weld porosity is also distributed differently and has a spherical morphology as opposed to the globular morphology seen in the weld made in Ar. The porosity is distributed more uniformly throughout the depth in the vacuum weld. This can be seen in Fig. 7D, which plots the porosity as a function of weld depth. This comparison shows the large increase in penetration, significantly reduced porosity, different porosity distribution, and different pore morphology for laser welds made in vacuum. These findings indicate laser welds made in vacuum are significantly different than those made in an inert gas such as Ar. Histograms, plotting the frequency of a given pore size, are shown in Fig. 8 for the Ni laser welds made in Ar and A C vacuum conditions. The histograms are color coded so that the high- and low-power welds can be compared, and the HP welds correspond to the 3D CT images shown in Fig. 7. Each histogram includes insets that show the Weibull fit to the grouped data, and the total porosity for each condition. This comparison shows the Ni laser welds made in Ar contain both the largest pores and the highest volume of pores, whereas the vacuum welds have a higher number of smaller pores and about 10% the total volumetric porosity. The Weibull plots show the best fit lines through the data, where the green band represents the 95% confidence limit of the fit. The results indicate that the parameter for the welds made in Ar of 0.37 is significantly less than that for the weld made in vacuum of 0.61. The lower parameter for the welds made in Ar, combined with the longer upper tail on the Weibull plot corresponds to the higher number of large pores in the Ar shielded welds than in the laser welds made in vacuum. Laser Welds in Titanium Three-dimensional CT renderings of the porosity in the HP laser welds made in Ti are shown in Fig. 9 for Ar and vacuum WELDING RESEARCH B D conditions. Figure 9A shows the porosity in the Ar-shielded weld, which indicates the porosity is again located in the lower part of the weld and has a globular morphology that is very similar to the morphology of the Ni welds made in Ar. Figure 9B plots the porosity in this weld from the top surface (x = 0 mm) to the root of the weld. The plot shows there is little porosity in the top 1 mm of the weld, and the remainder is concentrated between 1 and 3 mm below the surface. The metallographic cross section of this weld, shown in Fig. 5C, does indicate one small pore at the root of the weld, where it would be expected based on the CT results. The 3D CT rendering of the corresponding HP laser weld made in Ti under vacuum conditions is shown in Fig. 9C, which indicates the two small pores in this weld are clustered together near the weld root. The porosity as a function of weld depth is plotted in Fig. 9D, which emphasizes the near elimination of porosity in Ti when welding under vacuum conditions. Histograms of the porosity in Ti laser welds are shown in Fig. 10 for Ar and vacuum conditions, where the HP color-coded data correspond to the 3D CT images in Fig. 9. This comparison highlights the beneficial results of laser welding Ti in vacuum where porosity is nearly eliminated. In vacuum, the three small voids had a total porosity volume NOVEMBER 2016 / WELDING JOURNAL 427-s Fig. 11 — Metallographic cross sections through the laser and EB lowpower welds made in Ni under vacuum. A — Laser LP vacuum weld with 7.5mm penetration; B — EB LP vacuum weld with 8.9mm penetration. Both are high aspect ratio keyhole welds, but the EB is a bit narrower and more spikey at the root. Fig. 12 — 3D CT images and volumetric porosity plotted as a function of weld depth in nickel for: A, B — the LP laser weld made in vacuum; C, D — the LP EB weld made in vacuum. A B Laser- Vacuum
Welding Journal | November 2016
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