Microstructures of magnetically assisted dual-phase steel resistance spot welds

Abstract

Traditional spot welds and magnetically assisted spot welds were made in 2.25 mm thick galvanised dual-phase steel, and the weld microstructures were compared. The magnetically assisted weld nugget had a 'dog bone' shape, and it had an increased diameter, which indicates a larger load-bearing area that will improve mechanical performance. The fusion zone of the magnetically assisted welds had a finer and less-directional grain structure than the conventional welds, which would improve weld strength, plastic strains, and ductility. Both types of welds contained an unusual soft zone very close to the fusion zone that is thought to be an integral part of the fusion zone. The soft zone of the magnetically assisted welds was wider than the conventional welds.

Transient Softening at the Fusion Boundary of Resistance Spot Welds: A Phase Field Simulation and Experimental Investigations for Al–Si-coated 22MnB5

“…It decreases from 0.2305 wt % at welding time of 433 ms (1) to 0.151 wt % 630 ms (7). Such behavior can be explained by carbon segregation to the liquid phase due to the higher carbon solubility compared to solid phases, which is in good agreement with the suggestions of [10]. However, it is important to notice that the current model does not take the movement of liquid in the weld nugget into account, and thus underestimates the contribution of this mechanism to the carbon redistribution at the fusion boundary.…”

“…Opting for this solution not only causes a broader heat-affected zone (HAZ), but also can lead to softening at the fusion boundary, described for 22MnB5 with Al-Si [5][6][7][8] and Zn coatings [9]. This effect has also been reported for dual phase steels [10]. The presence of the fusion boundary softening for 22MnB5 is argued by Lu et al [11], however their experimental approach was not optimal for detecting the softened region, as will be discussed further.…”

“…According to the results of the present study, the softened region at the fusion boundary is not detected by a commonly used hardness traverse measurement (e.g., [11]) ( Figure 7) due to the small size of the softened region compared to the step of the hardness measurement and under circumstances the size of the indentation. To detect the softened region at the fusion boundary, measurement with a low loading force and small distance between the indentations are required [6,7,10]. Microhardness mapping (Figures 8 and 9) shows a clear evidence of the softened region.…”

“…Several hypotheses exist to explain the softening at the fusion boundary. Li et al [10] suggest carbon depletion due to its segregation to be responsible for the softening at the fusion boundary. Qiao et al [5], as well as Sherepenko and Jüttner [7] suggest δ-Fe formation due to the isothermal soaking of material in a peritectic region is assumed to cause the described effect.…”

Effect of Double Pulse Resistance Spot Welding Process on 15B22 Hot Stamped Boron Steel

“…Also, Mohamadizadeh et al [11] reported that there were two significant softening regions in the HAZ and the fusion boundary. Sherepenko et al [12,13] and Li et al [14] additionally studied the issue of the softening of the fusion boundary. Xia et al [15] concluded that the higher the volume fraction of the martensite, the greater the degree of softening.…”