Mg profiles extruded through porthole die inherently contain longitudinal welds, which can readily induce the abnormal grain growth (AGG) in solution treatment at elevated temperature. Here, a strategy of two-step (primary and secondary) solution is tailored to inhibit AGG. The results indicate that AGG during primary solution can only be suppressed when the temperature drops below 300 ℃, while these samples experienced AGG during secondary solution. It is interesting that the width of abnormal grain after secondary solution significantly decreases from 1,473 to 71 µm with increasing the holding time of primary solution from 3 to 84 h. This inhibiting effect results in notable enhancements in mechanical properties, where the elongation, initial fracture points, and maximum bending force are improved by 88.58 %, 32.63 %, and 128.50 %, respectively. The dislocation density and types of the precipitated phases after primary solution are the main factors for inhibiting AGG. First, the dislocation density decreases with increasing the time of primary solution, reducing the stored energy in grain interior. Second, as the primary solution time is extended, MgZn₂ phases greatly become coarsening, accompanied by a transition in their relationship with α-Mg from coherent to semi-coherent. Moreover, the quantities of Mg₄Zn₇ (non-coherent with α-Mg) and Mg₂Zn₃ (semi-coherent with α-Mg) undergo a substantial increase, contributing to a high interfacial energy that effectively inhibits the grain boundary migration during secondary solution.

The tensile tests of the extruded ZK60 Mg containing a longitudinal weld seam were carried out at room and elevated temperatures, and the effects of induced microstructure inhomogeneity on tensile deformation behavior was clarified. The results show that the deformation mode, dynamic recrystallization (DRX), texture evolution and mechanical properties are strongly affected by the longitudinal weld seam, temperature, and loading direction. The room temperature (RT) deformation of welding zone is controlled by the dislocation slips with the association of some twins, while twinning plays significant roles in the accommodation of c-axis strain of the coarse grains on matrix zone. The deformation at RT stretched along extrusion direction (ED) and transverse direction (TD) are controlled by basal slip/twinning and basal slip/prismatic slip/twinning, respectively. During high temperature tension, the dislocation cross slip of pyramidal slip is activated, and grain boundary sliding occurred in welding zone, leading to the superplastic behavior. With the increase of tensile temperature, the predominant DRX mode is transformed from continuous DRX to discontinuous DRX. Moreover, the basal poles of the grains spread from TD towards ED with the decrease of maximum pole intensity when stretched along ED, while non-basal textures are transformed to 〈10–10〉 fiber texture when stretched along TD. The slip-dominated flow is seen during RT tension along ED, while twinning becomes predominant during RT tension along TD. The fine grain structure causes the superior RT tensile properties along ED of welding zone with ultimate tensile strength of 315 MPa and elongation to failure of 13.8%. With the increase of tensile temperature, the slipping-dominated deformation is transformed into twinning-dominated, causing the decrease of strength and increase of elongation.