Microstructure and abnormal mechanical properties under the coupling effect of strain and temperature for friction-stir-welded joints of Al-Mg-Si alloy

Abstract

In the fields of automotive engineering and shipbuilding, friction stir welding (FSW) is utilized for joining aluminum alloy structural components. However, the nugget zone (NZ) in FSW joints often exhibits anomalous mechanical properties, consequently diminishing the stability of the welded joints. Here, an NZ was subdivided into a high-strain nugget zone (HS-NZ) and a high-temperature nugget zone (HT-NZ). The anomalous mechanical properties of the Al-Mg-Si alloy HS-NZ were systematically investigated at welding speeds of 100 mm/min and 200 mm/min and rotational speeds of 600 rpm and 900 rpm. Compared to the HT-NZ, the average grain size of the HS-NZ reduced by a maximum of 3 μm, the density of geometrically necessary dislocations (GNDs) increased by a maximum of 0.26 ×1014 m−2, and the microhardness showed a maximum increase of 15 HV. However, its shear strength is lower than that of the HT-NZ, with a maximum decrease of 15 MPa, exhibiting an abnormal mechanical performance. The increase in the "soft-oriented" grains in the HS-NZ results in more localized stress concentration between "soft-oriented" and "hard-oriented" grains during the deformation, thereby providing a reasonable explanation for the observed reduction in shear strength in HS-NZ under severe loading conditions. The increase of "soft-oriented" grains is primarily attributed to the promotion of discontinuous dynamic recrystallization (DDRX) facilitated by the coupled effects of high temperature and high strain. The constructed mathematical model also illustrated the effect of the coupled temperature and strain fields on the abnormal shear strength.