Evaluation of weldability for resistance spot welded single-lap joint between GA780DP and hot-stamped 22MnB5 steel sheets

Abstract

Recently, in the automotive industry, Al-coated boron steel sheets (22MnB5) have been used for hot stamping, and the use of these sheets makes it possible to achieve a tensile strength of over 1,500 MPa, since a metallurgical transformation from austenite to martensite occurs during the process. In this study, resistance spot welding (RSW) experiments were performed in order to evaluate the weldability of single-lap joints between GA780DP and 22MnB5. The effect of the weld current on the nugget diameter and load-carrying capacity was evaluated by observing the nugget diameter and performing a tensile-shear test. Furthermore, the fracture behavior was evaluated by carrying out optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) observations. Ductile regions were observed on the interfacially fractured surface of the weld, and this implies that a high load-carrying capacity can be obtained even when interfacial fracture (IF) occurs. IF is 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; the microstructure is brittle because of the high carbon equivalent (C eq ) and the penetration of Al in the weld.

Investigation of the Failure of Advanced High Strength Steels Heterogeneous Spot Welds

“…The weld nuggets present intermediate hardness levels due to the chemical homogenization discussed above. Based on the preceding and according to literature [18,27], six different hardness zones can be highlighted in these profiles: …”

“…Second, a thermal study of RSW on homogeneous Usibor ® 1500 was done by Hong-Seok [20]. Concerning heterogeneous welding of Usibor ® 1500, results can be found in the literature about a Usibor ® 1500-DP780 steel [27]. However, the combined effect steel grade and thickness heterogeneity was not thoroughly explored to date.…”

“…Such investigations are however necessary in the global context of vehicle structure optimization introduced above. As a consequence, a number of studies have been dedicated since the 2000s to AHSS welding [8,[17][18][19][20][21][22][23][24][25] and their heterogeneous configurations [15,[26][27][28][29]. Several conclusions can be highlighted in this context: (i) the frequent existence of a critical nugget diameter for failure mode transition (depending on sheet thicknesses) [13]; (ii) the fact that the failure mode of spot welds is not a characteristic of their strength [19]; and (iii) the positive deviation of Cross Tension Strength (CTS) observed for AHSS dissimilar spot welding configurations [14].…”

Effect of Weld Schedule on the Residual Stress Distribution of Boron Steel Spot Welds

“…The cause for this sudden drop has been established to be due to tempering of the parent martensitic microstructure. [9][10][11] The microstructural differences between the HAZ and nugget/BM are an influencing factor on failure; however, the location of the HAZ plays an important role as well. [12] The sharp notch tip of a spot weld, located at the weld nugget edge where the sheets start to separate, may cause a stress concentration when the weld is loaded.…”

Characterization of Loading Responses and Failure Loci of a Boron Steel Spot Weld

“…The cause for this sudden drop has been established to be due to tempering of the parent martensitic microstructure. [2][3][4] It is this microstructural mismatch between the hard nugget/ BM and soft HAZ which causes the HAZ to be a critical area, with reduced mechanical properties. [5] The varying material properties have a profound influence on the load-bearing capacity and failure strength of the weld as a whole.…”