Microstructural Evolution of DP980 Steel during Friction Bit Joining
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INTRODUCTION
AUTOMOTIVE spot welding of sheet metal has become more challenging in recent years, with an increasing usage of advanced high-strength steels (AHSSs) and light metals in the fabrication of automotive structures. These efforts are driven by the requirement for the weight reduction and the performance optimization of the vehicles. The use of these materials presents challenges of formability and weldability, because AHSS has higher contents of alloying elements than lower strength steels and because light metals cannot easily be joined to AHSS by traditional resistance spot welding (RSW). The RSW is a well-known method for joining sheet steel components in the automotive industry. However, AHSS can often experience weld property degradation when joined by RSW.[1–5] In resistance spot welds of AHSS, and especially the steels with tensile strengths above 800 MPa, the brittle microstructure is developed in the weld nugget during RSW, which causes interfacial failure under impact loading conditions, resulting in relatively low impact resistance. In addition, the brittle microstructure phases present concerns of hydrogeninduced cracking, which could reduce the durability of a vehicle. Friction stir spot welding (FSSW) is an alternate joining process with RSW, developed from friction stir welding (FSW), which has already been applied to welding of steels.[6–16] This process creates the lap weld T. HUANG, Postdoctoral Student, Y.S. SATO, Associate Professor, and H. KOKAWA, Professor, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Contact e-mail: a6td9508@ stu.material.tohoku.ac.jp. M.P. MILES and K. KOHKONEN, Associate Professors, and B. SIEMSSEN, Student, are with School of Technology, Brigham Young University, Provo, UT 84602. R.J. STEEL and S. PACKER, Research Scientists, are with MegaStir Technologies, Provo, UT 84604. Manuscript submitted March 31, 2008. Article published online October 14, 2009 2994—VOLUME 40A, DECEMBER 2009
by plunging and retracting of the welding tool, which does not accompany the travel of the tool. Compared with RSW, FSSW can reduce the thermal effect to the welded material due to the solid-state process. The FSSW has been already known as an effective and productive method for joining aluminum alloys,[17–20] and it has been recently applied to AHSS sheet metals using the polycrystalline cubic boron nitride tool.[21,22] Feng et al.[21] have shown that FSSW can produce metallurgical bonding for both 600 MPa dual-phase steel and 1310 MPa martensitic steel under 3 seconds of welding time and that the welds exhibited similar microstructure and hardness as the base materials for both steels. However, both the lap shear and cross tension strengths are currently limited due to the relatively small bonding widths, which would be expected to be increased through further process development. In order to resolve some difficulties of spot joining AHSS and dissimilar combinations of AHSS and aluminum, a new concept, fric
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