Dissimilar ultrasonic spot welding of AA6016 alloy-to-DP800 steel: The role of a novel AlSi(Fe) PVD coating

Abstract

Lightweighting in the automotive industry often involves joining dissimilar materials, while ensuring the safety and durability of load-bearing components. However, the joining of dissimilar materials presents significant challenges due to differences in their physical and chemical properties. In this study, a novel AlSi(Fe) physical vapor deposition (PVD) coating was applied on DP800 steel to enhance its compatibility with AA6016 aluminum alloy during solid-state ultrasonic spot welding (USW). The AA6016-to-coated DP800 steel joints, fabricated at welding energies around 1250 J, surpassed the tensile lap shear load requirements specified in the AWS D17.2 standard. The presence of the PVD coating not only enhanced the tensile lap shear strength by 23 % (i.e., from 69 MPa to 85 MPa), but also reduced the welding energy required to achieve optimal joint performance by 37.5 % (i.e., from 2000 J to 1250 J). This reduction in energy consumption further contributed to improved welding efficiency. The AA6016-to-uncoated DP800 steel joints at a welding energy of 2000 J exhibited button pullout failure under tensile loading and high-stress cyclic loading, largely due to excessive thinning of the AA6016 alloy. In contrast, the AA6016-to-coated DP800 steel joints welded at 500 J showed adhesive failure caused by defects at the weld interface. However, at a welding energy of 1250 J, these joints demonstrated superior intermixing between the Al sub-layer of PVD coating and the AA6016 alloy, resulting in a combination of cohesive and adhesive failure modes. The findings reveal the effectiveness of the AlSi(Fe) PVD coating in improving the mechanical properties of the joints while enhancing the energy efficiency of the welding process.