WELDING RESEARCH B Fig. 7 — 3D CT images and volumetric porosity plotted as a function of weld depth in nickel for the following: A, B — The HP laser weld made in Ar; C, D — the HP laser weld made under vacuum conditions. NOVEMBER 2016 / WELDING JOURNAL 425-s the porosity measurements are summarized in Table 3. In addition, a qualitative level of porosity is given as high (H), medium (M), or low (L) based on an arbitrary level of percent porosity per unit volume of weld. Note that the H and M levels of porosity would be considered too high for most weld applications. A total of 1590 pores were identified in the ten welds, and it is interesting to note that some of the welds with the highest number of pores did not contain the highest volumetric porosity. It is clear from these data that Ni has higher porosity than Ti for a given welding parameter, Ar produces higher porosity than vacuum for a given welding parameter, and higher-power welds contain higher amounts of porosity. There are many ways to analyze the porosity distributions, but one of the most useful is a histogram showing the pore frequency vs. pore size. Figure 6 shows this histogram for all 1590 pores. The results show the pore distribution monotonically decreases for all of the welds, much the same as what was observed in a previous study of the effects of shielding gas on laser weld porosity (Ref. 1). Color coding in this figure shows the grouping of porosity frequency by type of weld, where the low- and high-power welds have been lumped together in this plot with all weld types showing similar frequency trends. This behavior shows there are many more small-diameter pores than large pores, and the distribution can be represented by a Wiebull relationship with a beta shape factor less than unity (Ref. 1). The Weibull relationship is described in Equation 1, where the pore size, P, is represented as a continuous function of two parameters, , the shape factor, and athe scaling parameter: The Weibull fit to all of the data is plotted in the inset of Fig. 6 for all of the pores in all of the welds. Each weld type was further fit by the twoparameter Weibull relationship using a statistical data analysis package (Ref. 14), and the results are summarized in Table 5. This table shows that varies from 0.37 to 0.61 each for the different weld and material types, while the fit to the all of the data grouped together has a parameter of 0.49. Note that the overall Weibull parameter of 0.49 is close to that measured in a previous study of 0.54 for laser welds made in different shielding gas conditions at at- ( )= β ( ) ( ) α α (β− ) −( α)β f P P / 1 exp P/ 1 Fig. 8 — Histograms showing the pore size distribution in the Ni welds for the following: A — Laser welds made with Ar shielding gas; B — the laser vacuum welds. The inset plots show the Weibull curve fits to the data, where the pore probability is plotted on the y axis and cumulative pore size on the x axis. A A C D B Argon Vacuum
Welding Journal | November 2016
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