A B Fig. 14 — Histogram showing the pore size distribution in the LP welds in Ni for the following: A — EB welds made in vacuum at both accelerating voltages; B — laser weld made in vacuum. The inset plots show the Weibull curve fit to the data, where the pore probability is very much like that of the Ni laser welds made in vacuum with a Weibull parameter of 0.57, which is similar to all of the other weld types except for the laser welds made in Ar, which had an unusually low parameter of 0.37 compared to the rest. The EB welds contained an overall porosity of 1.5–2 times that of the corresponding laser weld in vacuum, with significant differences between the weld made at 80 kV and the weld made at 85 kV. The 80-kV EB weld, with a slightly larger beam diameter of 0.68 mm and lower peak power density (see Tables 1 and 2), contained a similar amount of total porosity as the 85-kV weld, but the pores were significantly fewer in number and larger in average size. The corresponding histogram of pore size distribution for the LP vacuum laser weld made in Ni is shown in Fig. 14B. In this figure, the amount of porosity is small with only 0.097 mm3, and the Weibull distribution has a parameter of 0.72 for this individual set of data. The comparison between the EB and laser welds made in vacuum shows there appears to be differences between the weld porosity, even though considerable attention was paid to make the two beams as identical as possible. The electron beam can be operated at voltages up to 150 kV, but at this voltage the sharp focused beam diameter was much smaller than the laser beam, so the voltage was dropped to 80 and 85 kV, and the work distance increased, to widen the sharp focus spot size to match the laser beam’s diameter of 0.64-mm FWe2. Other beam properties are summarized in Tables 1 and 2, indicating that the EB has a smaller focal length than the laser beam and a smaller estimated beam parameter product (BPP) of 5.3–5.6 mm-mrad compared to 8.4 mm-mrad for the laser. The smaller BPP of the electron beam is the result of a smaller angle of divergence, whereby the electron beam produces a more intense beam as it propagates into the material being welded. Another measure of the beam propagation is the Rayleigh length, which is the distance over which the beam diameter increases by the square root of 2, and the area of the beam doubles. Table 1 shows the Rayleigh length of the electron beam at 80–85 kV is approximately 10× longer than the laser beam, based on previous studies made on this same electron beam welding machine where the Raleigh length was measured to be 160 mm at 100 kV (Ref. 9). Other differences are related to electron vs. photon material interactions, where to the wavelengh of the electrons (on the order of 10–3 nm at 80–85 kV) is orders of magnitude smaller than the laser beam that has a 1.03-m wavelength. These differences aside, it is clear the laser welds made in vacuum are much more like electron beam welds than the laser welds made in atmospheric Ar shielding in terms of their keyhole behavior, porosity formation, and overall fusion zone geometry. Summary and Conclusions Comparisons of porosity and weld WELDING RESEARCH pool geometry were made between reduced atmosphere and inert atmospheric laser welds in Ni and Ti, and EB welds made in vacuum. Results from this study clearly show the benefits of reduced atmosphere laser welding in materials, such as Ni and Ti, that are prone to large amounts of porosity when welded using inert gas at atmospheric pressure. From the results of this study, the following conclusions were made. 1) Three-dimensional computed tomography is an important tool for understanding porosity distributions in welds. This technique allows individual pores to be quantified in volume and morphology in a way that cross-sectional metallography cannot equal, and provides an understanding of laser and EB material interactions along with porosity formation mechanisms. 2) Laser welds made in Ni showed reduced pressure significantly increases weld penetration and reduces the amount of porosity in the weld, but does not completely eliminate the porosity. It is important to note that high amounts of porosity were observed in the welds made under atmospheric Ar conditions. The CT results showed the morphology of the porosity was different for reduced pressure and atmospheric conditions. Pores in welds made in atmospheric pressure Ar were large and had a globular shape, while those made under reduced pressure were much smaller, more numerous, and had a spherical morphology. 3) Laser welds made in Ti showed reduced pressure significantly increases weld penetration and essentially eliminated porosity in the welds when com- NOVEMBER 2016 / WELDING JOURNAL 429-s plotted on the y axis and cumulative pore size on the x axis.
Welding Journal | November 2016
To see the actual publication please follow the link above