Analysis of Microstructural Changes in the Heat-Affected Zone and Fusion Zone of a Fiber Laser Welded DP980 Steel
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IN the automotive industry, dual phase (DP) steels are quickly becoming one of the most popular materials for structural applications in light weight vehicles.[1] The term ‘dual phase’ indicates that the steel has two distinct phases, i.e., martensite and ferrite. Martensite is a very hard phase with body-centered tetragonal structure, while ferrite is a relatively soft phase with a bodycentered crystal structure. Therefore, DP steels possess high strength and good ductility properties,[2,3] which enable automakers to produce vehicles with improved fuel economy and safety. Welding is a technique widely used in the automotive industry. Despite the fact that laser welding cannot be used on reflective materials, it is still an enabling technology that is versatile, easy to use, and precise. Recently, fiber laser welding (FLW) has been developed and receiving attention due to its multiple advantages such as low maintenance costs and high efficiency, and produces deep penetration at high-welding speed.[4–6] The use of DP steels for automotive structural applications is an effective way to reduce weight while improving the performance of vehicles. The welding of DP steels is an essential procedure during manufacturing. However, one issue in the application of welded DP steels JIANQI ZHANG, Former Graduate Student, ABDUL KHAN, Research Officer, and OLANREWAJU A. OJO, Professor, are with the Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. Contact e-mail: olanrewaju.ojo@ umanitoba.ca NORMAN ZHOU, Professor, is with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON, Canada. DAOLUN CHEN, Professor, is with the Department of Mechanical and Aerospace Engineering, Ryerson University, 350 Victoria Street, Toronto, ON, Canada. Manuscript submitted September 10, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
is that the rapid thermal cycle involved in FLW can significantly change the mechanical properties of the welded materials, especially high strength grade steels such as DP980.[7–9] The evaluation of microstructural changes induced by welding is, nonetheless, vital in order to understand and predict how FLW affects the mechanical properties of the welded materials. However, it is very difficult to properly elucidate microstructural changes that occur during welding as the FLW generates narrow welds that contain large microstructural gradients. Only a few studies have systematically investigated the microstructural changes of fiber laser welded DP980 steel. Therefore, the objective of this study was to study and understand the microstructural changes in welded DP980 steel caused by FLW.
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MATERIALS AND EXPERIMENTAL PROCEDURE
The thickness of the DP980 steel sheets is 1.2 mm. The detailed chemistry composition of DP980 steel is listed in Table I. An IPG Photonics YLR-6000 FLW machine operated at 6 kW was used, with a welding speed of 16 m/min, a beam spot size of 0.6 mm, and a focal length of 30 mm. Welding was performed perpendicular to th
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