Tensile Properties and Work Hardening Behavior of Laser-Welded Dual-Phase Steel Joints N. Farabi, D.L. Chen, and Y. Zhou (Submitted June 27, 2010) The aim of this investigation was to evaluate the microstructural change after laser welding and its effect on the tensile properties and strain hardening behavior of DP600 and DP980 dual-phase steels. Laser welding led to the formation of martensite and significant hardness rise in the fusion zone because of the fast cooling, but the presence of a soft zone in the heat-affected zone was caused by partial vanishing and tempering of the pre-existing martensite. The extent of softening was much larger in the DP980-welded joints than in the DP600-welded joints. Despite the reduction in ductility, the ultimate tensile strength (UTS) remained almost unchanged, and the yield strength (YS) indeed increased stemming from the appearance of yield point phenomena after welding in the DP600 steel. The DP980-welded joints showed lower YS and UTS than the base metal owing to the appearance of severe soft zone. The YS, UTS, and strain hardening exponent increased slightly with increasing strain rate. While the base metals had multi-stage strain hardening, the welded joints showed only stage III hardening. All the welded joints failed in the soft zone, and the fracture surfaces exhibited characteristic dimple fracture. Keywords dual-phase steel, fractography, laser welding, micro- structure, tensile property, work hardening 1. Introduction The legislation regarding CO 2 emission reduction via a better fuel economy has urged the car manufacturers to use lighter yet stronger materials which must also meet the safety standards in terms of mechanical properties. In this regard, dual-phase (DP) steel has already gained its popularity as this type of steel has a good combination of higher tensile strength along with significant ductility. The microstructural matrix of DP steel consists of mostly martensitic islands on the ferrite matrix with or without potential bainite and a small amount of retained austenite (Ref 1-5). The ductility of the steel arises from the relatively soft ferrite, and the harder martensite accounts for the strength. Usually DP steel is produced via intercritical annealing followed by rapid cooling (Ref 6, 7). During the intercritical annealing, small pools of austenite are formed in the ferrite matrix, which subsequently transform into martensite upon rapid cooling. The austenite-to-martensite transformation, accompanied by a volume expansion, leads to the occurrence of mobile dislocations into the surrounding ferritic matrix. The mobility of these dislocations is responsible for the high initial work hardening rate and continuous deformation behavior in the DP steels (Ref 6, 8). Therefore, compared with high-strength low-alloy (HSLA) steels, DP steel shows slightly lower yield strength (YS), but the continuous flow behavior in the DP steel results in a larger and more uniform total elongation, and a higher initial work hardening rate along with a considerably higher ultimate tensile strength (UTS) (Ref 9). The automotive applications of DP steels inevitably involve welding and joining. Owing to flexibility and ease of automa- tion, laser welding has been used increasingly as a joining process of both ferrous (Ref 10, 11) and nonferrous materials (Ref 12, 13) to partially replace some commonly used joining operation in automotive industry, e.g., resistance spot welding (RSW) (Ref 14). However, recent studies showed that the welding led to the formation of a soft zone in the subcritical area of the heat-affected zone (HAZ) and the mechanical properties of the welded joints were affected by this area (Ref 15, 16). A lot of investigations have been directed toward the mechanical properties and work hardening behavior of DP steels in relation to their microstructure. The tensile properties of these kinds of steels were found to be dependent on the volume fraction (Ref 17, 18), strength (Ref 19), morphology (Ref 20, 21) of martensite and also other factors. Some empirical laws of stress-strain relationship have been used to explain the work hardening behavior of DP steels (Ref 7, 17, 22-24). Among these most popular are Hollomon analysis (Ref 25), and Crussard-Jaoul (C-J) analysis (Ref 26, 27) based on Ludwik (Ref 28) and Swift (Ref 29) equations, popularly known as differential C-J (DC-J) (Ref 30, 31) and modified C-J (MC-J) (Ref 32, 33) techniques, respectively (Ref 20, 24). However, the effect of laser welding on the tensile properties and hardening characteristics of such steels is still limited. Therefore, this study is aimed at evaluating the tensile properties and work hardening behavior with an emphasis on the failure mechanism of the laser-welded DP steel joints under different strain rates. N. Farabi and D.L. Chen, Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3, Canada; and Y. Zhou, Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. Contact e-mail: dchen@ryerson.ca. JMEPEG (2012) 21:222–230 ÓASM International DOI: 10.1007/s11665-011-9865-8 1059-9495/$19.00 222—Volume 21(2) February 2012 Journal of Materials Engineering and Performance