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TH increasing fuel cost, there is a strong demand in the automotive industry to decrease the weight of a vehicle. However, when reducing vehicle weight, the structural performance needs to be maintained. Therefore, stronger steels are desired so that thinner and lighter sheets can be used. The application of advanced high-strength steels (AHSS) has increased significantly in the last decade because this family of steels can satisfy the preceding requirements.[1–5] The AHSS are characterized as having high yield strength and high work hardening rates compared to conventional steels such as mild steel. This allows for the design of thinner components while maintaining the same load bearing capability. Besides weight reduction, AHSS also have several other advantages over conventional steels. They have good fatigue performance and excellent energy absorbing properties, both of which help to improve the durability and safety performance of the final products.[5,6,7] If the formed AHSS parts are subjected to bake hardening operations, sheet strength can increase even higher through precipitation of carbides in the matrix.[1,3,8] These advantages in mechanical and material properties make AHSS very attractive to the automotive industry. However, the application of AHSS has been hindered because of the challenges encountered during welding and forming operations.[9–22] During the welding process, the microstructure in the weld heat-affected zone (HAZ) is modified. For welds produced on a particular AHSS, the HAZ microstructure is determined by the imposed thermal cycle. Higher heat input welds and less efficient heat sinking (which produce slower cooling rates) generally result in lower peak HAZ hardness and greater HAZ softening. The WEI GAN, Project Engineer, S.S. BABU, Technology Leader, and NICK KAPUSTKA, Applications Engineer, are with the Edison Welding Institute, Columbus, OH 43221. Contact e-mail: [email protected] ROBERT H. WAGONER, Professor, is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210-1124. Manuscript submitted May 5, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A

degree of base metal hardness degradation that occurs in the weld HAZ generally increases with the strength grade of the material.[11,14] In addition, AHSS are classified according to their strength level and the combination of phases that give them their unique properties. Therefore, two steels that fall under a particular AHSS designation (such as two transformationinduced plasticity (TRIP) 600 steels produced by different manufacturers) can have significantly different compositions. This may cause distinctive HAZ microstructures because they are dependent on the chemical compositions of the steels. For example, no softening was observed in the HAZ of welds produced on a TRIP steel that contained approximately 1.5 wt pct silicon.[14] For a TRIP steel that contains approximately 1.8 wt pct aluminum and a relatively low silicon content, ferrite was observed in all HAZ regions, along with a continuou