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Metal inert-gas welding of 7005 aluminum alloy profiles was conducted using ER5356 filler wire. The mechanical properties and microstructures of the welding joint were investigated by OM, SEM, micro-hardness test and tensile test. The results show that the tensile strength, yield strength, and elongation of the welding joint are 288 MPa, 190 MPa and 3.9%, respectively. The weld zone is characterized by cast structure. The tensile fracture morphology of the welding joint is characterized by ductile rupture. The welded grain near the fusion line is columnar crystals along the radiation direction and the grain in the fusion area is coarse. The fibrous microstructure is found in the base metal, while in the heat affected zone, the recrystallization occurs. The soften zone in the heat affected zone (HAZ), forms due to the coarsening of precipitate phase Mg2Si.The longer the natural aging time, the greater micro-hardness. Introduction Stability and reliability have become the basic design requirements for engineering machinery parts. In the future, it will require higher sustainability such as lightweight, low-carbon, green and intelligence. More and more aluminum alloy are used in the course of weight reduction of engineering machinery [1-3].Al-Zn-Mg alloy with low density, high strength, good process ability and excellent welding properties are widely used in the aerospace industry as well as bridges, buildings and other civilian industry [4-6]. 7005 aluminum alloy belongs to the Al-Zn-Mg-Cu medium-high strength aluminum alloys which can be strengthened by aging treatment [7-9]. They also have good weld ability in high-strength aluminum alloy, but they have a certain degree of stress corrosion tendency [10,11].Metal inert-gas (MIG) welding is being widely utilized to join Al-Zn-Mg alloys [12,13]. Aluminum Alloy is able to withstand high temperatures because of being easily oxidized to form oxide film witch. And they also have high thermal conductivity, high thermal expansion and large crystallization interval[14,15], gas holes and small cracks which would seriously affect the performance of welded components[16], are ubiquitous when welding the aluminum or aluminum alloy with MIG. In this paper, the microstructures and mechanical properties of MIG welding joint of 7005 aluminum were studied, and the effect of natural aging and artificial aging on the properties of welding joint were compared. Experimental Procedures Experimental Materials The welding base metal is Al-Zn-Mg-Cu (7005-T5) hollow sections (80mm×50mm×5mm), and the filler metal is ER5356 with a 1.2mm diameter. Chemical compositions of 7005 aluminum alloy and ER5356 filler metal are presented in table 1. 2nd Annual International Conference on Advanced Material Engineering (AME 2016) © 2016. The authors Published by Atlantis Press 334 Table 1 Chemical composition of 7005 aluminum alloy and ER5356 filler metal Material Mass fraction(%) Al Zn Mg Cu Si Fe Mn Cr Ti Zr 7005 aluminum alloy Bal. 4.43 1.63 0.06 0.09 0.15 0.49 0.12 0.03 0.14 ER5356 filler metal Bal. 0.09 4.92 0.015 0.13 0.14 0.138 0.102 0.102 Experimental Equipment and Methods ESAB Aristo Mig 5000i welding machine with the pulse function was used. The optimum synergic-pulsed MIG process parameters were listed in table 2.The welding groove type and angle are V and 90°, respectively. The welding assembly clearance is 2mm. Before MIG welding, the oxidation film and greasy dirt on the surface of substrates were eliminated by a series of mechanical and chemical cleaning methods. After welding, the samples were cut from the MIG welding joint using a line cut machine. Table 2 MIG parameters of 7005 aluminum alloy Welding current Welding voltage Wire feed rate Argon gas flow 160~180(A) 18~22(V) 8.6~9.6(m min) 15~18(L·min) Micro-hardness test were conducted across the welds using aKB30S type Vickers hardness tester with a load of 200g and a dwell time of 10s.Tensile test was performed according to the ISO 4136:2001standard[17].Tensile test were carried out in SHIMADZU 100kN electronic universal testing machine. The microstructures of the samples and the tensile fracture were analyzed by Leica DMI 5000M type Inverted metallurgical microscope and FEI Inspect S50 type scanning electron microscope (SEM), respectively. Results and Discussions Microstructure The microstructure of the welding metal(WM), welding heat affected zone(HAZ) and base metal(BM) are shown inFig.1 (a)、(b) and (c), respectively. As-cast structure with equiaxed shape grains is found in the welding metal (Fig.1 (a)), where the grain size is relatively big. It can be seen from Fig. 1(b) that the columnar grains form in the fusion zone on one side of the welding seam as well as equiaxed crystal on the other side. Most of the heat affected zone retained original structure characteristics of the base metal, and only a fraction of the zone remelted. As seen in Fig.1 (c), the microstructure of 7005 aluminum alloy (BM) is the original rolling state organization. Fig. 1 Microstructures of welding metal(a),fusion zone(b) and base metal(c) Micro-hardness The profile of micro-hardness of the joint is shown in Fig. 2.The micro-hardness is added to study the influence of nature aging time on the mechanical properties of the welded joint. In homogeneity is observed in the MIG welded joint under different NA time. The order of micro-hardness in all NA condition is: BM>HAZ>WM.

Microstructures and Mechanical Properties of MIG Welding Joint of 7005 Aluminum Alloy