WELDING RESEARCH JULY 2016 / WELDING JOURNAL 245-s Figure 9 shows the microstructure of the weld joint obtained with the GMAW process under SEM. The distribution of the main compositional phases in the microstructure could be identified. Region 1 was chosen to test the chemical composition of intragranular coarse bulky particles, Region 2 for the intragranular elliptical particle, Region 3 for the matrix phase, and Region 4 for the chemical composition of discontinuous intergranular strip. The SEM/EDS revealed that the coarse bulky particles in Region 1 contained Al, Cu, Mg, and trace Zn, and their weight percentages corresponded to 51.95, 25.86, 11.59, and 10.60%, respectively. The composition was close to the stoichiometric of the Al2CuMg phase (S phase). Thus, it was believed that the coarse second-phase particles were S phase, consistent with the results of XRD. Element Loss It is known that the 7XXX series aluminum alloy achieves its high strength from a series of precipitates such as typical (MgZn2) and T (Al2Mg3Zn) (Ref. 29). The content and gradient of the alloying elements distinctly change in welds and play a primary role in the microstructure and resulting properties. Few studies have examined the softening mechanism from the elemental perspective. Therefore, the alloying elements distribution was focused. Figure 10 is the schematic of the test section. From the top to bottom of the weld axis at intervals of 3 mm, a region was selected to analyze element content by EDS. Table 2 shows the distribution of the major Mg and Zn strengthening elements. It can be seen that, in general, the weld of the PU-GMAW sample has a higher percentage content of Zn element than the weld of GMAW and UGMAW. The primary metallic elements of 7A52 aluminum alloy were Al and Zn, while ER5356 welding wire was Al and Mg. On the premise of the initial alloying elements’ content being constant, if some elements were reduced, it was mainly due to the effect of strong gasification burning. As the boiling point of Zn and Mg were 1180 and 1380 K, respectively, well below the boiling point of 2740 K of Al the gasification burning of the Al element was relatively small. The gasification burning of Zn and Mg elements mainly occurred under the welding heat source. According to experimental results, it can be speculated that the GMAW sample produced a more intense gasification burning of the Zn element, while the pulsed ultrasonic had a significant inhibition in the gasification burning. Tensile Properties In order to evaluate the effect of ultrasonic on the mechanical properties of the welds, tensile testing was performed, including base metal (BM) and the welded sample. All welded samples broke in the weld location. Figure 11 shows the experimental average strength value. The UTS, yield strength, and elongation efficiency of the BM were 525 MPa, 471 MPa, and 13.2%, respectively, in the as-welded condition. The UTS, yield strength, and elongation efficiency of 375.5 MPa, 322 MPa, and 8.75% as-welded condition were obtained in PU-GMAW condition. The GMAW joints showed the lowest tensile strength (305.9 MPa), yield strength (277.6 MPa), and elongation efficiency (5.4%). Compared to the UTS, yield strength, and elongation of the GMAW joint, those of the PU-GMAW joint increased 23, 16, and 62%, respectively, while those of the U-GMAW joint only increased 13, 7, and 37%, respectively. Fracture surfaces of the BM (rolling state without recrystallization) and the different welding joints can be seen in Fig. 12. A scanning electron microscope study of the tensile fracture surfaces was done to investigate the mode of fracture and to understand the effect of ultrasonic treatment on the mode of failure of welding joints — Fig. 12B–D. Tensile fractured surface of the BM was mainly composed of cleavage steps and tearing edges that were characteristic of transgranular fracture — Fig. 12A. The mode of fracture was brittle or locally ductile and in agreement with low (5.4%) elongation efficiency of the GMAW condition. The fracture surfaces were covered with fine dimples and large tearing ridges — Fig. 12B. Comparatively, fracture surface of U-GMAW joints invariably showed few elongated dimples of varying size and shape uniformly distributed over the surface along with secondary cracks — Fig. 12C. Dimples were shallow and more. The mode of fracture was ductile as evident from the dimpled fracture surface. These observations are in agreement with high elongation efficiency of the joint (7.4%). Similar behavior was observed for PU-GMAW joints, which also exhibited a dimpled fracture surface. The fracture surfaces were covered with deeper and larger dimples, as well as a few flat regions — Fig. 12D. Moreover, some deep pits with relatively featureless surfaces can be seen on the fractured surface, which showed the highest elongation efficiency (8.75%) in the as-welded condition. Microhardness Results of the microhardness test for the joints obtained with GMAW, UGMAW, and PU-GMAW were plotted in Fig. 13. The average microhardness was evidently lower in the fusion zone and higher in the heat-affected zone than that in the BM. Hardness of the fusion zone was the highest when the pulsed ultrasonic was used. Thus, corresponding to the three kinds of joint hardness values over the fusion zone were three groups, which ranged from HV118 to HV125, HV100 to HV115, and HV90 to HV100, respectively. The hardness of the BM was within HV140 and HV160. So the hardness of the fusion zones was about 80, 70, and 65% of the BM, respectively. After PUGMAW, the average microhardness in the weld obviously increased by 7.4 HV compared with that of the UGMAW sample. The variation of the hardness values also approximately reflected the precipitation and dissolution of the second-phase particles during welding. The precipitation of the coarse second-phase particles from the weld of the aluminum alloy under the action of large heat input without ultrasonic agitation reduced the lattice distortion of aluminum crystal and was favorable for the movement of the dislocations, resulting in the decreased hardness and producing a wider softening region. In contrary, the dissolution of the coarse second phase increased the lattice distortion of alu-
Welding Journal | July 2016
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