PSI - Issue 5

Jidong Kang et al. / Procedia Structural Integrity 5 (2017) 1425–1432 Jidong Kang/ Structural Integrity Procedia 00 (2017) 000 – 000

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Introduction

Automotive manufacturers in North America are facing the challenge of achieving the mandatory fuel efficiency requirements as regulated by Corporate Average Fuel Efficiency (CAFÉ) standards. One approach to achieve fuel efficiency is by reducing vehicle mass, i.e., replacing heavy steel based structural and non-structural components with light structural materials such as aluminum and magnesium alloys, polymer composites and thin gauge, high strength steels. In design and economic perspective, it is desirable to use multi-material structures to fully utilize each individual material ’s functionality. A multi-material structure in vehicles, however, particularly pitches a challenge in terms of establishing robust and reliable joining techniques. Of the light materials, aluminum alloys have attracted substantial attention due to a range of desirable properties that include, low density, high specific strength and excellent corrosion resistance. But the absence of a robust joining technique between aluminum and steel can potentially hinder the use of aluminum alloys in the body structure. Among the commonly used joining techniques, resistance spot welding (RSW) remains the principal joining process in the automotive industry due to its low cost, flexibility, reliability and high throughput. However, joining steels to aluminum alloys by resistance welding presents new challenges mainly due to metallurgical limitations. The formation of brittle intermetallic compounds (IMCs) such as Fe 2 Al 5 and FeAl 3 at the joint interface is well documented and has been shown to substantially reduce the weld strength [1 – 8]. Although the formation of IMCs cannot be completely eliminated during spot welding of aluminum to steel joints, it is possible to obtain improved weld strengths by optimizing the welding process conditions. Qiu et al. [1] used a cover plate made of cold-rolled steel which aided in better weldability of aluminum 5052 sheet to austenitic stainless steel SUS304. Zhang et al. [2] improved the weldability of aluminum 6088-T66 to galvanized high strength steel by optimizing the weld electrode geometry. A smaller electrode tip diameter produced welds with thinner IMCs which improved the weld strength as compared to welds produced with larger electrode tip diameter. Arghavani et al. [4] reported, at lower welding currents, that the presence of thin zinc coatings on galvanized steel obstructed the formation of metallurgical bonding resulting in lower weld strength. With the increase in weld current, the zinc coating melted and in addition reduced the thickness of IMCs layer resulting in higher weld strength. Ibrahim et al. [7] indicated that by using a Al-Mg interlayer between aluminum 6061-T6 and austenitic stainless steel, a thinner, uniform IMC layer was produced resulting in superior weld strength. Beyond IMCs, the presence of an oxide layer on the surface of the aluminum alloy presents a significant challenge in obtaining high quality welds. To overcome this problem, General Motors developed a Multi-Ring Domed (MRD) electrode [9 – 11] and Conditioning, Shaping, and Sizing (CSS) weld schedules [12], which have shown to significantly improve mechanical properties of spot welded aluminum alloys. In the present study, the proprietary GM MRD electrode was used to weld the wrought aluminum AA6022-T4 to IF steel sheet in lap-shear configuration. Lap-shear strength and fatigue behavior of these dissimilar RSWs are presented. The results are then compared with the mechanical performance of similar AA6022-T4 to AA6022-T4 RSW joined using the same GM MRD electrodes. Finally, the feasibility of using the structural stress concept to rationalize the load-fatigue life curves for similar and dissimilar RSWs is discussed. Materials and experimental procedure In the present study, wrought AA6022 – T4 sheet measuring 1.2-mm thick and hot-dip galvanized 2-mm thick rolled IF steel were spot welded in lap-shear configuration. For the comparative study, AA6022-T4 measuring 1.2 mm in thickness was spot welded in lap-shear configuration to a 2.0-mm thick AA6022-T4. A pictorial representing the geometrical features of the test specimen is shown in Fig. 1. The test specimens in both similar and dissimilar material configuration were produced using GM ’s proprietary MRD electrodes and weld schedules. While exact details of the weld schedules used are proprietary, weld force and rms current levels appear in Table 1. While both used MRD electrodes, they did not use the same electrode set in this case. The quasi-static lap-shear test of the specimens was performed on an Instron electromechanical test frame at a cross-head speed of 2 mm/min. Load controlled tension-tension (R=0.1) fatigue test of the RSW specimens were conducted on a servo-hydraulic MTS test 2)