Zhaohua Zhang

Bio: Zhaohua Zhang is an academic researcher from Tianjin University. The author has contributed to research in topics: Ultimate tensile strength & Spot welding. The author has an hindex of 5, co-authored 5 publications receiving 534 citations.

Study on the Microstructures and Tensile Behaviors of Friction Stir Welded T-joints for AA6061-T4 Alloys

Abstract: Friction stir welding (FSW) of T-joints with 3 mm thickness for AA6061-T4 sheets has been performed, and the influences of process parameters on the microstructures, defects, hardness profiles, and tensile strength were discussed specifically. It is found that the macrostructures and microstructures are similar in different process parameters, and the microstructure features in the skin welds have the same patterns as that of butt-joints. The unique microstructures of T-joints are the two fillet zones. Two lower hardness zones in the skin and one lower hardness zone in the stringer are found. Additionally, the tensile properties of T-welded joints are examined along the skin and the stringer plane directions. Results show that the tensile strength is in the range of 170-180 MPa for all the specimens along the skin direction, and the specimens fail in the heat affected zone (HAZ). In the stringer direction, the larger tunnel defects (DA) (the DA is greater than 0.1 mm2) would reduce the ultimate tensile strength of the T-joints and cause the joints to fracture at the bonding interface. The smaller tunnel defects (the DA is less than 0.015 mm2) and the zigzag lines have no pronounced effect on the failed location. In the stringer direction, tensile efficiencies of T-joints could be 83% of the base material (BM) when ω/v = 1541/218 r/mm is applied.

Effect of welding parameters on lap shear tensile properties of dissimilar friction stir spot welded AA 5754-H22/2024-T3 joints

Abstract: In this study, the effect of weld parameters on mechanical properties, such as lap shear fracture load, of friction stir spot welded dissimilar lap joints was studied. The effects of the plate positioning, the plunge depth and the dwell time as well as the tool tilt angle on the joint strength were investigated. The experimental results showed that the positioning of the sheets played an important role on the strength of the joints. The joint strength was higher when AA 5754 sheet is placed on the top, whereas lower strength values were displayed by dissimilar joints produced by placing AA 2024 as the upper sheet. It was also observed that the plunge depth, dwell time and tool tilt angle had a strong effect on the failure mode of the friction stir spot welded lap joints produced. The joints exhibited two different fracture modes, namely cross and pullout nugget under lap shear tensile testing conditions.