Transmission Electron Microscopy and Nanoindentation Study of the Weld Zone Microstructure of Diode-Laser-Joined Automot
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THE application of transformation-induced plasticity (TRIP) steels in automobile structures may be one of the critical breakthroughs allowing the simultaneous reduction in body weight and improvement in crash worthiness. The TRIP steels have multiphase microstructures consisting of ferrite, bainite, martensite, and carbon-enriched retained austenite.[1,2] The volume fraction of austenite varies with the composition and heat treatment, but it usually is on the order of 5 pct. As discussed in Reference 3, this hard-soft composite structure of TRIP steels leads to a combination of high strength and good uniform elongation. During forming, most of the retained austenite transforms to martensite, further boosting the materialÕs work hardening. Steel automobile bodies and other structural components are assembled almost entirely by welding. Resistance spot welding (RSW) is a mature and established technology, J. CHEN, Research Associate, K. SAND, Undergraduate Research Assistant, C. OPHUS and R. MOHAMMADI, Graduate Research Assistants, and D. MITLIN, Assistant Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, and the National Institute for Nanotechnology, Edmonton, AB T6G 2G6, Canada. Contact e-mail: [email protected] M.S. XIA, Graduate Research Assistant, M.L. KUNTZ, Research Assistant Professor, and Y. ZHOU, Professor, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada. Manuscript submitted February 27, 2007. Article published online February 5, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
currently one of the two dominant methods of auto body assembly. Laser welding is growing in importance as a potentially more productive alternative for automobile assembly due to the fast welding speed, excellent reproducibility of the joints, less distortion, reduced need for refinishing, and high joint rigidity. Similarly, to other fusion welding processes, laser welding generates a weld zone microstructure that significantly deviates from that of the base metal. Studies of these laser-welded TRIP microstructures remain quite limited.[4–6] Han et al. studied 800 MPa class TRIP steel, welded by 4 kW-CO2 laser beam, at welding speeds of 4 to 12 m/min.[4,5] They reported that when the welding speed was 4 m/min, the weld turned to full martensite. When welding speed further increased, resulting in a higher cooling rate, the strength and elongation remained constant. The authors in References 4 and 5 did not specify the cooling rate of the weld, making any processing-microstructure extrapolation quite difficult. Researchers have joined C-Mn steels by using high-power Nd:YAG laser beam at low welding speeds of 0.2 to 0.6 m/min.[6] The authors reported that the weld exhibited the microstructure of primarily acicular ferrite, plus allotriomorphic ferrite. Other phases, including bainite, were also reported present in the weld microstructure. Workers in References 4 through 6 characterized the microstructure using only light optical
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