Failure mode transition in AHSS resistance spot welds. Part II: Experimental investigation and model validation
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:where 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.
References (30)
- et al.
Eng. Fract. Mech.
(2005) - et al.
J. Mater. Process. Technol.
(2009) - et al.
Mater. Sci. Eng. A
(2008) - et al.
Eng. Fail. Anal.
(2008) - et al.
Mater. Lett.
(2010) - et al.
Sci. Technol. Weld. Joining
(2007) - et al.
- et al.
J. Manufact. Sci. Eng.
(2006) - S.M. Zuniga, Predicting overload pull-out failures in resistance spot welded, Ph.D. thesis, Stanford University,...
- et al.
J. Mater. Eng. Perform.
(1993)
Sci. Technol. Weld. Joining
Weld. J.
Weld. J.
Cited by (115)
Insights into microstructure evolution and fracture mechanisms of welded joints of high strength steel patchwork components
2024, Journal of Manufacturing ProcessesAchieving excellent properties of resistance spot welded 2GPa-grade press hardened steel and galvanized DP980 steel via double-pulse
2024, Journal of Materials Research and TechnologyImprovement of the mechanical performance of ZnAlMg coated steel brazed joints through precipitation-based strengthening
2023, Materials Science and Engineering: AUnderstanding fusion zone hardness in resistance spot welds for advanced high strength steels: Strengthening mechanisms and data-driven modeling
2023, Journal of Materials Research and TechnologyResistance spot weldability of Fe<inf>66</inf>Cr<inf>16.5</inf>Ni<inf>14.1</inf>Si<inf>3.4</inf> advanced high strength steel using D-optimal design of experiment method
2023, Journal of Materials Research and TechnologyStudy on the joint formation mechanism and performance of resistance rivet welding of Mg/steel dissimilar materials
2023, Journal of Materials Research and Technology