Mechanical behavior and failure mechanism of resistance spot welded DP1000 dual phase steel

Abstract

This paper reports on the microstructural evolution of resistance spot welded 1000MPa dual phase steel under two different welding conditions, and their relation to the mechanical performance and failure mechanisms. It is shown that a double pulse weld scheme leads to an enhancement in cross-tension strength compared to single pulse welding. The second pulse subdivides the initial fusion zone of the first pulse into two zones. The inner part is solidified with a columnar structure after the second pulse, whereas with the second pulse the outer layer becomes recrystallized (named as Rex-zone) leading to the formation of an equiaxed structure of prior austenite grains. Characteristics of martensite formed in the Rex-zone and coarse-grained heat affected zone, where the crack initiated and propagated, were investigated using orientation imaging microscopy. It was found that a change in welding scheme from single to double pulse effectively alters the characteristics of martensitic microstructure of weld zones. The results obtained demonstrate that the Rex-zone has a lower fraction of high-angle grain boundaries and coarser structure of Bain groups as opposed to the coarse-grained heat affected zone with large fraction of high-angle grain boundaries and finer Bain groups. Besides, double pulse welding creates softer sub-critical heat affected zone which reduces stress concentration at the nugget edge during cross-tension test. The better mechanical performance of double pulse weld is attributed to the significant softening at subcritical heat affected zone, formation of thick Rex-zone with lower residual strain and high fraction of high-angle grain boundaries and finer Bain groups in the coarse grained heat affected zone.

The comparison of gas tungsten arc welding and flux cored arc welding effects on dual phase steel

“…The mechanical properties of DP steels depend on martensite volume fraction, martensite islands distribution in the ferrite phase, shape and grain sizes and partitioning of stress-strain between two phases [1,2]. DP steel can be welded using a variety of fusion techniques, including resistance spot [3][4][5][6][7], laser beam [8][9][10][11], plasma arc [12][13][14], friction stir [15][16][17][18] and gas metal arc welding. Xia and Biro investigated the effects of heat input on heat affected zone (HAZ) softening [19], which cannot be avoided in DP steels.…”

“…Wang et al studied laserwelded joints of DP1000 steel, and concluded that they were softened by the tempering and softening of pre-existing martensite and by carbide precipitation [26,27]. Resistance spot-welded DP1000 was studied by Chabok et al, who found that double pulse welding results in a softer HAZ [28]. Tiziani et al evaluated the effects of three types of welding (gas tungsten arc, plasma arc, and electron beam) on the properties of DP600 steel.…”

Microstructure and Tensile-Shear Properties of Resistance Spot-Welded Medium Mn Steel

“…Resistance spot-welding (RSW) is the predominant process for joining steel sheet components in the automobile industry. The spot-weld quality is critical to the reliability of the overall automobile as cars typically contain thousands of spot-welds [12][13][14]. Saha et al [15] reported that the TWIP steel was highly susceptible to the liquation crack, and some elements such as C, Mn, and Ti showed strong segregation behavior of TWIP steel after RSW.…”

Microstructural analysis of weld nugget properties on resistance spot-welded advance high strength dual phase (α+α/) steel joints

“…The weld zone of the DP 800 joints recorded 584 Hv in the nugget zone, which was higher when compared with the base metal hardness of 295 Hv. The higher hardness in the nugget zone was observed due to formation of columnar/ lath /leaf like martensite phase in the NZ as high cooling rate was observed in the process [12].…”