Microstructure and mechanical properties of heat affected zone in multi-pass GMA welded Al–Zn–Mg–Cu alloy

Microstructure and mechanical properties of the heat affected zone (HAZ) in multi-pass gas metal arc (GMA) welded Al–Zn–Mg–Cu alloy plates were investigated, based upon which the mechanical anisotropy and fracture mechanism were analyzed. The microstructure and composition were analyzed by scanning electron microscope (SEM) and energy dispersive spectroscope (EDS). X-ray diffractometer (XRD), transmission electron microscope (TEM) and selective area electron diffraction (SAED) were used to analyze the phase composition. The distribution of microhardness was identified as gradual transition and tensile strength had a tendency to decrease first and then increase. The distribution of nano-sized η(MgZn₂) particles in the α(Al) matrix and Al₂MgCu phase determined the tensile performances along the thickness direction and led to the formation of ductile/brittle composite fracture in the HAZ. The continuous distribution of Al₂MgCu phase in the strip intergranular precipitates gave birth to premature cracks and the brittle fracture region. The precipitated particles coarsening also led to the deterioration of mechanical properties.