Microstructure and mechanical properties of resistance spot welded dissimilar thickness DP780/DP600 dual-phase steel joints
Introduction
A dissimilar combination of materials and thickness are specified in order to tailor materials properties for local requirements and more and more widely used in automotive applications [1]. As one of the most common advanced high strength steels (AHSS), dual phase (DP) steel has the higher strength, lower yield rate, higher working hardening rate, higher strain energy absorbing and more excellent forming characteristics than the conventional high-strength low alloy steels with similar strength [2], [3], [4]. DP steel has been used in body-in-white in order to reduce the weight and enhance the safety of automobiles [5], [6]. The joining of unequal thickness and dissimilar metals are often involved in production. Compared to the equal thickness and similar metals of RSW, the unequal thickness and dissimilar metals joints are characterized particularly by compositional gradients and microstructural changes, which yield large variations in chemical, physical and mechanical properties across the joints [7], [8], [9]. However, the majority of investigations on RSW have been carried out on equal thickness and similar metals, and the reports about unequal thickness and dissimilar metals in the literature are limited. Hong-Seok et al. [10] studied the weldabilty of DP780/22MnB5 RSW. They found that IF was caused by the stress concentration resulting from the presence of the sharp notch at the boundary of the nugget as well as by the high hardness and the brittle microstructure of the weld. Marashi et al. [11] studied overload failure behavior of dissimilar thickness RSW. They concluded that final solidification line moves form sheet/sheet interface to the geometrical center of the total thickness of the joint, and this reduces the tendency of a dissimilar thickness spot weld to fail in IF during the tensile-shear test. Khan et al. [12] reported that the RSW fatigue performance of the dissimilar materials HSLA350/DP600 was similar to the fatigue performance of HSLA350/HSLA350.
The objective of the present work is to investigate and characterize the microstructure and mechanical properties of the dissimilar thickness RSW between DP780 and DP600. The distribution of alloying elements and hardness of the joints are reported. Tensile properties of the joints are also discussed.
Section snippets
Experimental details
About 1.2 mm thick hot dip galvanized DP780 and 1.5 mm thick hot dip galvanized HCT600X (DP600) were used as the base metals. The chemical composition and mechanical properties of the base metals are presented in Table 1, Table 2. The carbon equivalent (CE) of base metals as calculated by the equation proposed by Yurioka [13] is included in Table 2. The coat thickness of both base metals is 5 μm. DP steels consist of a ferrite matrix containing a hard martensitic second phase in the form of
Microstructures of weld joints
Fig. 2 shows the cross-section of a typical DP780/DP600 RSW joint under the welding current 8.0 kA, welding time 30 cycles and electrode force 4.0 kN. As can be seen, the joint consists of three different regions including BM, HAZ and FZ, and the HAZ is located between BM and FZ. There is a significant boundary between HAZ and BM. Nugget diameter is approximately 7 mm. Indentation rate (measured as the indentation depth divided by the sheet thickness) is 7.9%. Final solidification line locates in
Conclusions
Microstructure and mechanical properties of RSW dissimilar thickness DP780/DP600 joints were examined. Some results can be concluded as following:
- (1)
The microstructure of FZ consists of large lath martensite and ferrite. The grain size and martensite volume fractions increase in the order of BM, HAZ and FZ. The distribution of alloying elements is uniform in the FZ.
- (2)
The hardness in the FZ is significantly higher than the base metals. There are softening regions in the subcritical HAZ near base
Acknowledgement
The authors wish to thank Pouranvari M for his helpful discussion this study.
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