Microstructure and Strength of Ultrasonic Plus Resistance Spot Welded Aluminum Alloy to Coated Press Hardened Boron Stee
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With an ever-increasing demand of weight reduction and crashworthiness improvement, multi-material design combining ultrahigh strength steels (UHSS) and aluminum alloys are utilized extensively in vehicle structures.[1–3] Among various UHSS, press hardened (or hot formed) boron steels receive especially important use in crash-sensitive safety components as these steels can be hot-formed into complex geometry while achieving ultrahigh strength (e.g., above 1400 MPa). These steels are coated to prevent oxidation during hot forming and in-service corrosion. For instance, Usibor-AS 1500, a representative press-hardened boron steel, has an aluminum–silicon (Ai-Si) coating, which would subsequently transform into complex structure as it alloys with the steel substrate during hot forming. Dissimilar metal joining of aluminum alloy to coated press-hardened boron steel is very challenging due to such steel’s ultrahigh strength and tenacious Al-Si
YING LU, LUKE WALKER, MENACHEM KIMCHI, and WEI ZHANG are with the Department of Materials Science and Engineering, The Ohio State University, 1248 Arthur E. Adams Drive, Columbus, OH 43221. Contact e-mail: [email protected] Manuscript submitted August 29, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
coating. To date, there are limited studies on it. Silva et al.[4] and Ding et al.[5] applied friction stir spot welding (FSSW) and refill FSSW, respectively, for such joining. However, the steel used in those studies was in a soft, prior hardened state with ultimate tensile strength of only 400 to 600 MPa. Oliveira et al.[6] applied friction element welding to join aluminum alloy to press hardened Usibor-AS 1500 using elements (similar to rivets) made of Cr-Mo steel. Resistance spot welding (RSW) is the predominant process to join vehicle body structures in the assembly lines. However, to authors’ best knowledge, there is no published study on robustly joining aluminum to coated press-hardened boron steel based on RSW. In authors’ previous work, a two-step joining process, named ultrasonic plus resistance spot welding (U + RSW), was developed to join aluminum alloys to uncoated low carbon steel[7] and galvannealed (galvanized and annealed Zn-coating) dual phase steel.[8] Note that Al will be used as the abbreviation for aluminum alloy below. Compared to direct RSW of Al to steel, U + RSW process resulted in a superior joint quality due to thin intermetallics (IMCs) formed as well as high resistance to molten metal expulsion at the Al to steel interface. In the present study, a novel use of U + RSW was developed to join AA6022 to Al-Si-coated presshardened boron steel using stainless steel 316 as insert. The base materials used in this study were 1.2-mmthick aluminum alloy AA6022-T4, and 1.4-mm-thick Al-Si-coated press-hardened boron steel, Usibor-AS 1500 (abbreviated as Usibor in the following). The insert used was an AISI austenitic stainless steel 316 (SS316) in annealed condition with two different thicknesses: 0.25 and 0.125 mm. These materials’ chemical compositions are lis
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