WELDING RESEARCH A B Fig. 3 — EMFC electron beam power density distribution for the following: A — 40mA, 80kV beam with an average beam diameter of 0.68 mm (FWe2); B — 37.5mA, 85kV beam with an average diameter of 0.64 mm (FWe2). The power density color code units are W/mm2. NOVEMBER 2016 / WELDING JOURNAL 421-s are referred to as HP, and those at the lower power setting are referred to as LP. These powers and speeds were chosen to produce keyhole welds that simulated the results of prior laser welding experiments on Ni made with a fiber laser on commercially pure nickel in Ar and N2 shielding gas (Ref. 1). Electron beam welds were also performed in vacuum on a nickel sample to demonstrate the similarity of laser vacuum weld geometries to electron beam weld geometries, and to compare the amounts of porosity produced by each method. The electron beam welds were made using a 150-kV Hamilton Standard machine (No. 605) at 6.9 × 10–5 mBar vacuum with the parameters summarized in Table 2. The electron beam welds were made at 80 kV, 40 mA (3200 W), and 85 kV, 37.5 mA (3190 W), with a long work distance of 305 mm, which corresponds to an effective focal length of 369 mm (Ref. 9). These conditions produced a spot size on the surface of the coupon of 0.68 and 0.64 mm, respectively, as measured by the Enhanced Modified Faraday Cup (EMFC) diagnostic (Refs. 9, 10), which is similar to the laser beam diameter used in this study. The power density distribution of the beam as measured by the EMFC is shown in Fig. 3 as a pseudo color plot. Analysis of the 0.68-mm-diameter electron beam showed it had a peak power density (PPD) of 16.6 kW/mm2, and full width half maximum (FWHM) was 0.38 mm. For the 0.64-mmdiameter beam, PPD = 18.8 kW/mm2, and FWHM = 0.37 mm. Electron beam propagation parameters are summarized in Table 1, based on prior measurements of electron beam gun characteristics (Ref. 9), an assumed raw beam diameter of 12.5 mm and work distance characteristics for the electron beam welding machine (Ref. 11). Both welds produced nearly complete joint penetration of the 10-mm-thick plate when welded at 12 mm/s on top of a stainless steel backing plate. After welding, samples were prepared for computed tomography (CT) measurement of the porosity by machining away most of the base metal to leave coupons that measured 37 mm long, 9 mm wide, and 10 mm thick, with the weld running down the center of the coupon. A small, flatbottomed hole of known volume (0.8 mm diameter by 5 mm deep) was further drilled in one end of the coupon to aid in volumetric quantification of porosity. Three-dimensional x-ray computed tomography inspection was performed using a Metris X-Tek XTH225CT cabinet based CT system. This commercially available machine features an open tube (or vacuum demountable) 225-kV microfocus x-ray source with a Table 2 — Summary of the Parameters Used to Make the Nickel and Titanium Welds(a) Weld Type/ Beam Weld Energy per Beam Diam. Peak Power Average Power Interaction Time Material Power Speed Length (mm) Density Density (d/V) (W) (mm/s) (J/mm) (kW/mm2) (kW/mm2) (ms) Laser/Ti 2090 17 143 0.64 N/A 6.50 37.6 Laser/Ti 2750 17 187 0.64 N/A 8.55 37.6 Laser/Ni 3180 12 265 0.64 N/A 9.89 53.3 Laser/Ni 4150 12 346 0.64 N/A 12.9 53.3 EB1/Ni 3200 12 267 0.68 16,600 8.82 56.7 EB2/Ni 3180 12 265 0.64 18,800 9.89 53.3 (a) All beams were sharp focused on the surface of the coupon being welded. The welds in Ni were made at 12 mm/s, and the welds in Ti were made at 17 mm/s.
Welding Journal | November 2016
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