PSI - Issue 17

Liting Shi et al. / Procedia Structural Integrity 17 (2019) 355–362 Jidong Kang/ Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature American Welding Society AWS Conditioning shaping and sizing CSS General Motors GM Heat affected zone HAZ Hot dip galvanized HDG Intermetallic compound IMC Low carbon steel LCS Multi-Ring Domed MRD Resistance spot welds RSWs Scanning electron microscopy SEM

1. Introduction

In response to the increasing demands to improve vehicle fuel economy, multi-material body structures are increasingly utilized in the automotive industry for structural lightweighting purposes. These multi-material structures pose challenges for dissimilar material joining, particularly aluminum to steel. General Motors (GM) developed a new resistance spot welding technique using a multi-ring domed electrode and multiple solidification weld schedules to address these challenges. However, in aluminum-steel resistance spot welds (RSWs), an iron-aluminum intermetallic compound (IMC) layer is formed at the interface of the two metals and its strength affects tensile shear specimen fracture modes. There are two main types of fracture modes in aluminum to steel RSWs, namely the interfacial fracture mode in which cracks propagate along the IMC layer, and pull out fracture mode where cracks propagate through the thickness of the aluminum sheet along with shearing the sheet at the perimeter of the weld nugget (see Fig. 1 ) .

Fig. 1 Images of the (a) interfacial fracture and (b) pull out fracture in stack-ups of 1.2 mm AA6022-2.0 mm HSLA.

It is commonly accepted that there exists a critical weld nugget diameter ensuring pull out fracture mode. Traditionally, the critical weld nugget diameter is believed to be related to the sheet thickness where the fracture occurs. The American Welding Society (1997) thus recommended 4√ as the critical weld nugget size. Over the decades, there have been a variety of modifications proposed to determine the critical weld nugget diameter such as the fracture mechanics based models developed by Smith (1980) and Chao (2003). On the other hand, Pereira et al. (2010), Pouranvari et al. (2007), and Sun et al. (2008) recognized the influence of material inhomogeneity, i.e. differences in shear strength in different regions of a weld on weld nugget size determination. However, they also