WELDING RESEARCH TRUMPF TruDisk 8002 ficial effects of reduced pressure multikW laser welding of structural and stainless steels through experiments (Refs. 6, 7) and modeling (Ref. 8). Commercial laser vacuum systems are just now being developed for industrial applications to take advantage of these benefits. In this study, commercially pure nickel and titanium were laser welded with a high-power CW disk laser under argon shielding gas and reduced pressure conditions. This set of experiments was aimed at demonstrating the beneficial effects of laser keyhole welding under reduced atmosphere for porosity-prone materials. Experimental Procedures Laser welds were made using an 8- kW Trumpf TruDisk at 1.03 micron wavelength with a 200-m fiber optic. The fiber was fed into a three-axis PFO beam scanner from Trumpf that had a 450-mm focusing lens, and was used to scan the beam over the coupons to be welded. The minimum spot size for this optical path is 0.64 mm, and all of the welds were made with the beam sharp focused on the surface of the coupons to be welded. The weld atmosphere 420-s WELDING JOURNAL / NOVEMBER 2016, VOL. 95 was controlled using a chamber that was placed inside the laser welding workstation. Vacuum welds were made under 10–1 mBar using a mechanical pump, while controlled atmosphere welds were made by purging the chamber with Ar at atmospheric pressure. The top of the chamber was fitted with a fused silica window for the vacuum laser welds, which allowed the chamber to be pumped down and at the same time allow the beam to pass inside. The welds were made by scanning the beam over the weld coupon using the PFO optical scanner at speeds of 12 mm/s for the nickel samples, and 17 mm/s for the titanium samples. The higher speed used on the titanium samples was to prevent complete joint penetration because the beam penetrates deeper into titanium than nickel. A schematic drawing of the setup is shown in Fig. 1, where the laser passes through the window into the chamber. The weld was made by scanning the laser beam over the stationary weld coupon. Vacuum was provided by a rotary pump capable of 10–3 mBar; however, many of the welds were made at a reduced pressure of 10–1 mBar where the majority of the laser vacuum benefit was observed in this study. For the inert gas studies, the chamber was pumped down, then filled with Ar at atmospheric pressure, and constantly purged with Ar during welding. Prior to making the welds, the power density distribution of the laser beam was measured using a Primes diagnostic device, confirming the sharp focus position and quantifying power density of the beam at sharp focus. Figure 2 shows the full beam caustic for the optical arrangement used in this study. The measured minimum spot size at the sharpest focus condition is 0.64 mm; other beam quality parameters are indicated as determined by second-moment calculations and are summarized in Table 1. The weld coupons were machined from nickel 201 plate (>99 wt-% Ni) and Grade 2 commercially pure titanium (>99.5 wt-% Ti). The coupons measured 150 mm long × 25 mm wide × 10 mm thick. Weld segments of 30–40 mm length were made using different power levels between 2090 and 4150 W, as summarized in Table 2. Welds made at the higher power setting Fig. 1 — Schematic of the vacuum laser setup. Fig. 2 — Primes diagnostic results for the laser beam showing the caustic information for the PFO optical setup for the laser beam with a minimum spot size of 0.64 mm diameter. Table 1 — Beam Quality Measurements for the 1.03Micron Wavelength Laser Beam and the Electron Beams at 80 and 85 kV Beam Type Focal Length Raw Beam Diameter Focused Beam Diameter BPP M2 Rayleigh Length (mm) (mm) (mm) (mm) (mmmrad) (mm) Laser 450 23.5 0.64 8.4 24.8 12.3 EB1 (80 kV) 369 12.5 0.68 5.6 * >100 EB2 (85 kV) 369 12.5 0.64 5.3 * >100 * M2 values for electron beams are many orders of magnitude higher than lasers due to their small wavelengths.
Welding Journal | November 2016
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