pared to the moderately high amounts of porosity in the welds made under atmospheric Ar conditions. Overall, there was much less porosity in the Ti than in the Ni welds. CT results showed that the morphology of the porosity in atmospheric pressure Ar welds was globular and similar to that of the welds made in Ni under Ar conditions. 4) There is a dramatic difference in the weld pool shape and penetration for laser welds made in Ar vs. those made under reduced atmosphere conditions. For identical welding parameters, the laser weld made in nickel in Ar is wide at the top, with shallow penetration and has a very short keyhole compared to the weld made under reduced pressure. The aspect ratio 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. Similar weld cross-sectional shape differences were observed in Ti, where the aspect ratios changed from 0.77 for the Ti weld made in Ar to 3.48 for the Ti weld made under reduced pressure conditions. 5) A comparison of the porosity between EB and laser welds made in Ni under vacuum showed differences between the amount and types of weld porosity. The EB welds contained an overall amount of porosity of 1.5–2 times that of the corresponding laser weld in vacuum. Some of the pores in the EB weld are similar to the laser weld with widely distributed spherical porosity. However, the EB weld also contained spiking-type porosity at the weld root that was not observed in the laser weld. 6) The geometric shape of the weld fusion zone for laser welds made under reduced atmospheric conditions is very similar to that of the corresponding EB weld. Both showed keyhole shapes with high aspect ratios. However, some differences exist. The EB weld penetrated more than the laser weld (8.9 vs. 7.5 mm), and has a narrower keyhole width (0.94 vs. 1.1 mm) than the laser weld. It is important to note that the weld cross-sectional areas are nearly identical at 9.4 and 9.3 mm2 for the EB and laser welds, respectively, which indicates similar melting efficiencies. In addition, the laser vaccuum weld did not show any root spiking defects like those observed in the EB welds. 7. The CT results were analyzed using statistical methods to determine the size distribution of the 1590 pores analyzed in these welds. The porosity distributions showed a monotonic decrease in frequency with increasing pore size and were fit by a Weibull relationship. The fits to the pore distribution were made by weld type, material type, and an overall grouping of all pores. The results showed that the Weibull shape parameter for the Ni laser welds made in atmospheric Ar were lower than all the other welds with = 0.37. On average, for all 1590 pores, = 0.49, which is similar to the results from a previous study where = 0.54 for 531 measured pores from laser welds in different shielding gases under atmospheric conditions (Ref. 1). Acknowledgments This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52- 07NA27344. The authors would like to thank Richard Watson, Tim Whiteside, and Jessica Opie of AWE for performing CT radiography and reconstruction of the data, Neil Bond and Gail Smith for preparing the metallographic samples, and Andrew Johnson for preparation of the laser welding figures and review of the manuscript. References 1. Elmer, J. W., Vaja, J., Carlton, H. D., and Pong, R. 2015. The effect of Ar and N2 shielding gas on laser weld porosity in steel, stainless steels, and nickel. Welding Journal 94(10): 313-s to 325-s. 2. Kuo, T.-Y., and Lin, Y. D. 2007. 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