Laser Blank Welding High-Strength Steels
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TRODUCTION
CURRENT government regulations and consumer demand are driving the automotive industry toward continued improvements in fuel efficiency and crash worthiness. Significant improvements in fuel efficiency will come from further weight reductions of vehicles. To achieve these weight reductions while maintaining (or improving) structural integrity, automobile manufacturers are considering the use of tailor-welded blanks involving high-strength steels (HSS). The use of tailorwelded blanks can save weight and materials, reduce development and production cost, simplify design, and improve the structural performance of vehicles. Tailorwelded blanks have been a rapidly growing segment of the automotive industry.[1,2,3] More than 44 million HAIPING SHAO, formerly Senior Engineer, Edison Welding Institute, is Principal R&D Engineer, Cardiac Rhythm Management, Boston Scientific, St. Paul, MN 55112. Contact e-mail: haiping.shao@ guidant.com JERRY GOULD, Chief Engineer, is with the Edison Welding Institute, Columbus, OH 43221. CHARLIE ALBRIGHT, Associate Professor, is with the Welding Engineering Program, The Ohio State University, Columbus, OH 43221. Manuscript submitted February 21, 2006. Article published online April 14, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
blanks are welded each year and this figure is growing rapidly.[4] The basic metallurgy of HSS sheet is well established.[5,6] The cold-rolled HSS of interest for automotive applications can generally be classified into three types according to the specific strengthening mechanism. These include the solid solution-strengthened, grainrefined, and transformation-hardened steels. The solid solution-strengthened steels are similar in composition to low-carbon steels, with the exception of small additions of solid solution-strengthening agents. The most common of these is phosphorus (although silicon is also used), and, as a result, these steels are often referred to as ‘‘rephos’’ grades of steel. Solid solution-strengthened steels typically achieve strength levels on the order of 350 MPa tensile strength. Grain-refined or microalloyed steels typically use small amounts of carbideforming additions (niobium, vanadium, and titanium) to stabilize the grain structure during hot rolling. These provide a substantial reduction in grain size in the final product. Such steels are typically available up to strength levels on the order of 550 MPa. Transformation-hardened steels typically contain higher levels of carbon and manganese. By controlling the levels of these additions, as well as the specific thermal-mechanical VOLUME 38B, APRIL 2007—321
processing, different fractions (and states) of degenerate martensite can be obtained, resulting in a wide range of strength levels. Transformation-hardened steels are available in tensile strengths ranging from 350 to 1400 MPa. These strengthening mechanisms can be, and often are, used in combination to achieve enhanced performance. In tailored-blank production, laser and mash seam welding are two dominant joining processes, although
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