Failure mode transition in AHSS resistance spot welds. Part II: Experimental investigation and model validation

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Abstract

The objective of this paper is to investigate and analyze the transition criteria from interfacial to pullout failure mode in AHSS resistance spot welds during the tensile-shear test by the use of both experimental and analytical approaches. Spot welds were made on three dual phase steel grades including DP600, DP780 and DP980. A low strength drawing quality special killed (DQSK) steel and AISI 304 austenitic stainless steel were also tested as a baseline for comparison. The microstructure and mechanical strength of the welds were characterized using metallographic techniques and the tensile-shear testing. Correlations among critical fusion zone (FZ) size required to ensure the pullout failure mode, weld microstructure and weld hardness characteristics were developed. It was found that critical FZ size increases in the order of DQSK, DP600, DP980, DP780 and AISI304. No direct relationship was found between the tensile strength of the base metal and the critical FZ size. It was concluded that low hardness ratio of FZ to pullout failure location and high susceptibility to form shrinkage voids are two primary reasons for high tendency of AHSS to fail in interfacial mode. HAZ softening can improve RSW mechanical performance in terms of load bearing capacity and energy absorption capability. This phenomenon promotes PF mode at smaller FZ sizes. This fact can explain smaller critical FZ size measured for DP980 in comparison with DP780. The results obtained from the model were compared to the experimental results and the literature and a reasonable agreement was obtained.

Highlights

► Interfacial to pullout failure mode transition for AHSS RSWs is experimentally studied. ► Relation between failure mode and metallurgical factors of AHSS RSW is studied. ► HAZ softening reduces FZ size require to ensure pullout failure. ► HAZ softening enhances energy absorption capability of AHSS RSW. ► Good agreement between model prediction and experimental results was observed.

Introduction

Analyzing and predicting the resistance spot welds (RSWs) performance and their failure is a challenging problem. There are number of factors affecting this complexity:

  • (i)

    There are different factors including weld geometry, fusion zone/HAZ/base metal properties, test geometry, and the stress state in each weld which have a measurable consequence on the failure mode of spot welds [1], [2], [3].

  • (ii)

    Inhomogeneous microstructure of spot welds resulting from the weld thermal cycle further complicates the failure analysis and prediction of the failure mode [4], [5], [6]. In order to do so, one has to characterize strain hardening and the strain rate sensitivity properties of both fusion zone and heat affected zone.

  • (iii)

    Strength mismatch existing between fusion zone, heat affected zone and base metal causes strain concentration at the zone with lowest strength. Therefore, it is necessary to determine strain gradient prior to predicting the weld failure mode [4].

  • (iv)

    From the geometrical point of view, external crack appears at the joint when a spot weld is made [5]. Moreover, the indentation caused by electrode force during resistance spot welding process creates stress concentration at the indentation wall [6], [7]. Therefore, the stress concentration associated with these two effects, should also be considered in the analysis of the spot weld failure.

Vehicle crashworthiness, which is defined as the capability of a car structure to provide adequate protection to its passengers against injuries in the event of a crash, largely depends on the integrity and the mechanical performance of the spot welds [8], [9]. Therefore, spot welds with high load bearing capacity and high energy absorption capability are needed to maximize load transfer and energy dissipation during a car crash. Failure mode serves as a quantitative measure for spot weld quality in production environment. There are two main views regarding the effect of failure mode on mechanical performance of spot welds. Some mention that the mode of failure should not be considered as the only criteria to judge the tensile-shear test results [10], [11]. On the other side are some researchers [12], [13] who have reached an opposite conclusion. According to Sun et al. [12] for welds made on DP800 and TRIP800, the weld failure mode has a significant influence on both load bearing capacity and the energy absorption capability. In the case of HSLA steels spot welds, Rivett [13] found out that although the tensile-shear maximum force was not influenced by the failure mode, the total failure energy (i.e. the area under the load–displacement curve) increased (about 250%) for pull-out failure mode compared to interfacial mode. The tortuosity of crack path in the pullout mode results in energy dissipation and therefore it is more desirable than the straight crack propagation in interfacial failure mode. So, in order to improve the mechanical performance of the spot welds, the pull-out failure mode must be guaranteed.The significance of the increased energy and the associated delay observed with the pullout failures is worthy of further investigation. As mentioned in Part 1 [14], resistance spot welds of advanced high strength steels (AHSSs) exhibit high tendency to fail in interfacial failure mode (i.e. failure through fusion zone). It was also indicated that the conventional recommendation (based on the sheet thickness) for weld sizing in order to ensure pull-out failure mode of welds are not dominant for AHSS welds. Therefore, there is a need to develop new weld quality criterion for AHSS resistance spot welds. Metallurgical characteristics of welds should also be considered to predict and analyze the weld fracture phenomena more precisely. According to Part 1 [14], based on the failure mechanism of spot welds in the tensile-shear test, the following equation was proposed to predict the minimum FZ size for the occurrence of pull-out failure mode:DC=4tPfHPFLHFZwhere t is the sheet thickness, P is the porosity factor, f is the ratio of shear strength to tensile strength of the FZ (=0.5 according to Tresca criterion), HFZ and HPFL are hardness values (HV) of the fusion zone and pullout failure location, respectively.

In the present part, failure mode transition of spot welds is experimentally investigated. The applicability of the proposed model is also examined for various grades of AHSS spot welds.

Section snippets

Experimental procedure

Dual-phase steel samples from DP600, DP780 and DP980 were resistance spot welded for the tensile-shear testing. A low strength drawing quality special killed (DQSK) steel and AISI 304 austenitic stainless were chosen as baseline comparison samples. The chemical composition and mechanical properties of sheets used in this study are given in Table 1, Table 2, respectively. The sheet thickness of sheets is 1.5 mm. Spot welding was performed using a PLC controlled 120 kVA AC pedestal type resistance

Hardness characteristics

According to Part 1 [14], microstructure and hardness profile of resistance spot welds play an important role in determining weld failure mode. Rapid heating and cooling induced by welding thermal cycles can cause significant alteration in the microstructure of the joint zone. Fig. 2a shows a typical macrostructure of DP780 RSW indicating three distinct zones of fusion zone (FZ), heat affected zone (HAZ) and base metal (BM). Hardness profile of the investigated RSWs is shown in Fig. 2b. It can

Conclusion

Microstructure and mechanical performance of resistance spot welds of some steels used in automotive industries were investigated with attention focused on interfacial to pullout failure mode transition during the tensile-shear test. The following conclusions can be drawn from this study:

  • (1)

    The fusion zone microstructure resistance spot welds made on DQSK and DP steels was almost martensitic due to high cooling rates inherent to the RSW process. The FZ hardness was proportional to carbon

Acknowledgment

The authors would like to thank Islamic Azad University-Dezful Branch for the financial support of this work.

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