Study on hot cracks and mechanical properties of T-joints of high-strength aluminum alloy MIG welded under different rolling directions

As the operating speed of high-speed railways increases, higher requirements are imposed on the welding quality of high-speed train bodies. This paper systematically studies the influence of Al-Zn-Mg high-strength aluminum alloy plates with different rolling directions on the hot cracks, microstructure, and mechanical properties of MIG welded T-joints. The research results show that when the welding direction is parallel to the rolling direction, abnormally grown coarse grains appear near the fusion zone. The low melting point eutectic phases Al(FeMnSi) and MgZn located on the grain boundaries are heated and melted to form a liquid film during the welding process, and then liquefaction cracks are formed due to the action of welding thermal stress. Liquefaction cracks are generally present in the fusion zone. When the welding direction is parallel to the rolling direction, the number of long liquefaction cracks in the fusion zone significantly increases. By changing the rolling direction of the base metal, the number of liquefaction cracks is significantly reduced, and the microhardness, tensile strength, and elongation of the welded joint are improved, reaching 99.81 HV, 321.1 MPa, and 37.7% respectively. The fracture of the tensile specimen shows a mixed fracture characteristic of toughness and brittleness．