Microstructural characterisation of double pulse resistance spot welded advanced high strength steel

Abstract

The effects of single and double pulse resistance spot welding on the microstructures of an advanced high strength automotive steel are presented in this work. The double pulse welding schemes partially remelt the primary weld nugget and anneal the area at the fusion boundary of the nugget. The effects of the annealing treatment on the segregation and the microstructure have been studied by electron probe microanalysis (EPMA) in combination with electron backscatter diffraction (EBSD). Results show that phosphorus has been redistributed at the primary weld nugget edge of the double pulse welds, while the mean block width and ellipticity of the prior austenite grains were smaller in welds subjected to double pulsing compared with single pulse weld. A favourable failure mode was obtained for the double pulse welds although behaviour did not correlate with the measured grain size.

Microstructural characterization of a double pulse resistance spot welded 1200 MPa TBF steel

“…For thin sheets, the cooling mechanism of small welding nuggets is dominated by heat loss toward the cooled electrodes [29]. This can be visualized by the dendrites in the FZ, since they show a preferred orientation along the maximal thermal gradient [11]. In the inner FZ (Fig.…”

“…In the past, several authors recommended a short cooling time between the pulses, leading to recrystallization of the outer FZ and consequently to the transformation of the cast-like structure. Eftekharimilani et al [11] illustrated that the shape of the prior austenite grains (PAGs) changes from an elongated to an equiaxed shape at the edge of the FZ. Furthermore, it is reported that segregations at the edge of the FZ are reduced after a second pulse, and therefore the risk of intergranular failure is reduced [12].…”

“…Moreover, the recrystallization causes an increase of high-angle grain boundaries in the UCHAZ [13]. With this concept, cracking behavior and ultimate strength during cross tension tests were improved significantly [11][12][13][14][15][16][17]. The other concept aims to soften the hard and therefore brittle martensitic FZ.…”

“…Although double pulse RSW of AHSS is well-documented in the literature [11][12][13][14][15][16][17]20], the characterization of the occurring zones in a double pulse welded TBF steel with a tensile strength of 1200 MPa is still missing. The present work shows methods to characterize and distinguish the different zones in a temper pulse welded TBF steel and aims at understanding the ongoing tempering effects during the second pulse.…”

Transient Microstructure Evolutions and Local Properties of Dual-Phase 980 MPa Grade Steel Via Friction Stir Spot Processing

“…Resistance spot welding has been widely used for AHSS to integrate into vehicle body-in-white structures, but some challenges remain to obtaining high weld qualities and mechanical joint properties. For example, segregation of alloying elements such as phosphorous and sulfur into the grain boundary during solidification inevitably deteriorated the toughness of the martensite, leading to low cross-tension strength [ 4 , 5 ]. Therefore, developing suitable welding techniques is essential for transportation industries, and developing new AHSS is essential for steel manufacturers.…”