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Effect of welding speed on strength and microstructure of friction stir welded joint of 7075 aluminum alloy

7075 Al-Zn-Mg-Cu Aluminum Alloy, it is Aluminum Alloy heat treatment, with high specific strength and hardness, good workability, suitable as a structural material under high load, has been widely used in the manufacture of aircraft fuselage, wing beam, cabin siding and other structures, is an important and indispensable material in aerospace industry in. 7075 aluminum alloy mainly uses inert gas shielded arc welding (MIG), but it is easy to crack, heat affected zone softening and stress concentration sensitivity. Friction stir welding (FSW) is a revolutionary method of solid phase bonding, which can better maintain the mechanical properties of the base metal, reduce or avoid the welding defects. In this paper, the effect of welding speed on the strength and microstructure of 7075 aluminum alloy sheet was studied by FSW, which was used as a reference for the selection of process parameters of 7075 aluminum alloy FSW.

The test material is T6 7075 aluminum alloy, its chemical composition (mass fraction,%) is: 1.68Cu, 2.51Mg, 0.26Mn, 0.15Cr, 0.2Fe, 5.75Zn, the margin is Al; its Sigma b=487MPa, sigma 0.2=420MPa, Delta =9.5%. The size of welding test plate is 250mm * 100mm * 2mm. The material of the stirring head is nickel base superalloy DZ125, the stirring needle is cylindrical, and the diameter ratio of the mixing needle to the shaft shoulder is 1: 3. When welding, the angle of the mixing needle is 2 degrees, and the pressure of the shaft shoulder is controlled at 0.15mm. During the test, the rotating speed of the stirring head is constant (950r/min), and the welding speed (V) is 95, 118, 150 and 190mm/min.

The welding speed of FSW joint weld nugget, thermal mechanical affected zone in advancing side, return to the side of TMAZ microstructure and precipitate size, distribution density has great effect on the mechanical properties of the joints, and impact. The appropriate welding speed is beneficial to control the morphology of the heat affected zone, avoid the interface line, and obtain the high performance welding joint. The rotating and stirring stirring head under the bottom of the cladding layer 7075 Aluminum Alloy will be involved in the weld, and the aluminum layer and Aluminum Alloy not fused together, which will affect the mechanical properties of joints. When the rotating speed of the mixing head is 950r/min and the welding speed is 150mm/min, the strength of the welded joint reaches the maximum value, which is 447.1MPa.