WELDING RESEARCH Fig. 1 — Schematic of the welding setup. Fig. 2 — Schematic of the realtime monitoring system. found that the droplet transfer frequency increased (Ref. 12). Compared with conventional GTAW, the weld penetration increased by more than one time (Ref. 13), and the welding efficiency was higher. Additionally, the weld grains were refined (Ref. 14), and the strength of the joint could be improved by more than 8% (Ref. 8). Therefore, its application potential is as wide as to cover many arc welding processes. It may find the most attractive applications in welding other positions except for flat with the small welding gun and few process parameters. Another effective but less practiced application of ultrasonic is ultrasonic treatment of cast alloys. The idea of improving the quality of cast alloys by means of elastic oscillations produced by mechanical vibration or shaking applied to solidifying steel was first suggested by Chernov in 1878 (Ref. 15). The effects of ultrasonic on refining the cast microstructure and removal of gases and oxides from the melt have been associated with the occurrence of acoustic cavitation and streaming in the liquid metal (Refs. 16, 17). Increasing 240-s WELDING JOURNAL / JULY 2016, VOL. 95 the applied ultrasonic power generally resulted in smaller, more rounded, and better distributed grains and intermetallic particles (Ref. 18). The Al-Zn-Mg alloys (7XXX series) with high strength and low density are widely used in the aerospace field and other engineering applications (Refs. 19, 20). In all cases, arc welding is the primary joining method. The final properties of the joint are attributed to the welding procedures, welding methods, and welding wire. Aluminum Alloy 7A52 is a medium-strength alloy of the 7XXX series alloys, which gains strength from MgZn2 precipitates and can be fusion welded with few difficulties arising from melting and solidification. The microstructure in the fusion zone is related to the fusion ratio, and there are many problems remaining to be solved, such as coarse grains, joint softening, and element loss (Refs. 21, 22). In this paper, the authors propose the idea of applying the pulsed ultrasonic wave to the GMAW process to better achieve welding the 7A52 aluminum alloy. Compared to continuous ultrasonic, pulsed ultrasonic in pulsed ultrasonic-wave-assisted GMAW (PUGMAW) has the characteristic of higher peak power output. The objective of this study is to prove the concept and feasibility of PU-GMAW of the 7A52 aluminum alloy. Experimental Procedure Material Table 1 shows the chemical compositions of the commercial 7A52 aluminum alloy and consumable ER5356 (1.2 mm) with yield strength of 120 MPa and ultimate tensile strength (UTS) of 260 MPa. The sheet was sheared into sample sizes of 200 100 8 mm3. Single V-groove angles (30 deg) were cut using an abrasive waterjet cutting machine in the plates with 2-mm root faces for a total 60-deg included angle between two plates. Before welding, all of the plates were cleaned with acetone in order to make sure the surface was free from contaminants. Experimental samples were fixed tightly into a fixture to ensure a zero root opening. PUGMAW Procedure The welding setup schematic is Fig. 3 — Dimensions of the flat tensile specimens (in mm). Table 1 — Chemical Compositions of Base Metal (Al 7A52) and Welding Wire (ER5356), (wt%) Compositions Zn Mg Cu Fe Mn Cr Si Ti Al 7A52 4.0–4.8 2.0–2.8 0.05–0.2 0.3 0.2–0.5 0.15–0.25 0.25 0.05–0.18 Bal. ER5356 0.1 4.5–5.5 0.1 0.4 0.05–0.2 0.05–0.2 0.25 0.06–0.2 Bal.
Welding Journal | July 2016
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