Failure mode transition in AHSS resistance spot welds. Part I. Controlling factors
Highlights
► Interfacial to pullout failure mode transition for AHSS RSWs is studied. ► An analytical mode is proposed to predict failure mode of AHSS RSWs. ► Hardness characteristics of RSWs plays key role in the failure mode transition.
Introduction
The demand for high strength steel sheets having excellent ductility has been increasing in the automotive industry in order to improve the fuel efficiency of the vehicle, occupant safety and reducing the car body weight. Due to their excellent strength and formability, advanced high strength steels (AHSS) offer the potential for improvement in vehicle crash performance without extra weight increase [1]. AHSS steels include dual-phase (DP); transformation-induced plasticity (TRIP); complex-phase (CP); and hot-stamped or martensitic (M) steel grades.
Resistance spot welding is the dominant process to join sheet metals in automotive industry. Considering development and commercialization of AHSS for application in automotive bodies, there is a need to study the spot welding behavior of these materials. Without a thorough understanding of how to weld AHSSs, their use and benefits within the automotive industry will be restricted.
Automotive structural assemblies use groups of spot welds to transfer load through the structure during crash. Additionally, spot welds can also act as fold initiation sites to manage impact energy [2]. Joint failure, e.g. resistance spot weld (RSW) joint failure, has been identified as one of the key failure types when vehicle crash occurs. Therefore, stiffness, strength and integrity of a car body structure or any structure composed of sheet metals strongly depend on the quality of the resistance spot welds (RSWs) [3].
Failure mode of RSWs is a qualitative measure of the joint quality. Fig. 1 shows a schematic representation of the main fracture path during mechanical testing of spot welds. Basically, spot welds can fail in three distinct different modes described as follows [4], [5], [6]:
- (i)
Interfacial failure (IF) in which, fracture propagates through the fusion zone (FZ) (Fig. 1a).
- (ii)
Pull-out failure (PF) in which, failure occurs via the withdrawal of weld nugget from one sheet (Fig. 1b). In this mode, fracture may initiate in base metal (BM), heat affected zone (HAZ) or HAZ/FZ depending on the base metal and the loading condition.
- (iii)
Partial interfacial mode (PIF) in which, fracture first propagates in fusion zone (FZ) and then is redirected through thickness (Fig. 1c).
Failure mode can significantly affect load bearing capacity and energy absorption capability of RSWs. Generally, the PF mode is the preferred failure mode due to higher plastic deformation and energy absorption associated with it [3], [4]. Thus, vehicle crashworthiness, the main concern in the automotive design, can be dramatically reduced if spot welds fail via the interfacial mode [5]. As a result, it is needed to adjust welding parameters so that the PF mode is guaranteed.
Resistance spot welding behavior of low carbon steels and HSLA steels is well understood through several researches carried out in past years. However, resistance spot welding of AHSS is still a challenging issue due to the reasons summarized below:
- (i)
Complicated microstructure development in the fusion zone (e.g. martensite formation) and the heat affected zone (e.g. martensite formation and HAZ softening due to martensite tempering) [7], [8], [5]. These complex non-equilibrium phase transformations destroy the sophisticated designed microstructure of AHSS and can affect the failure behavior of AHSS RSWs and should be taken into account.
- (ii)
Increased tendency to fail in interfacial failure mode [7], [5], [9].
- (iii)
High susceptibility to the formation of shrinkage voids in FZ due to their rich chemistry in comparison to low carbon steels [10], [11].
- (iv)
Highly prone to expulsion which can lead to reduced peak load and energy absorption [3], [12], [13].
This two-part paper aims at investigating failure mode of resistance spot welds of AHSS. For this purpose, weld fracture during the tensile–shear quasi-static loading condition which serves as a good indicator of failure energy under impact/crash loading conditions [2], [14] is fully discussed. Apart from countless resources available to refer to, there are two main reasons behind choosing the test are as follows:
- (i)
Spot welded structures are usually designed to carry the shear–tensile loads. Even though, spot welds may experience a combined loading condition in service.
- (ii)
RSWs show greater tendency to fail in the interfacial failure mode during the shear–tensile test in comparison to other loading conditions such as peel, coach-peel and cross-tension [10]. RSWs which fail in the PF mode during the test are also expected to fail in the same mode during cross-tension, peel and chisel tests.
The aim of the present part is to provide an analytical basis for the prediction of the failure mode of AHSS RSWs and to enhance our understanding of the factors governing the failure mode of AHSS spot welds. In order to do so, first an overview on the subject is presented, and after that a simplified improved model based on metallurgical factors is introduced to analyze and predict the minimum FZ size in order to ensure PF mode during the tensile–shear test.
In the second paper, 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
Failure mode transition
As mentioned above, one of the major problems regarding resistance spot welding of AHSS is their high probability to fail in interfacial mode [2], [3], [15], [16], [17], [18], [19]. The equation below describes the minimum weld size for a given sheet thickness recommended by various industrial standards to produce pull-out failure:where D is weld nugget diameter in mm, t is sheet thickness in mm and K is a constant ranging from 3 to 6 [14]. In automotive industry, sizing of spot welds in
Analytical model to predict the failure mode transition
In this section a modified model based on metallurgical factors is presented to analyze and predict the minimum FZ size in order to ensure PF mode during the tensile–shear test. First, failure mechanism of spot welds under the tensile–shear loading is considered. Fig. 3 shows a simple model describing stress distribution at the interface and circumference of a weld nugget during the tensile–shear test. Shear stresses are dominant at the interface. At the nugget circumference, stresses are
Factors affecting the failure mode of the spot welds during the tensile–shear test
Although the model proposed here is a simplified model, it provides an adequate understanding to study the failure mode transition of spot welds, qualitatively. The applicability of this model to quantitatively predict the failure mode is discussed in the second part of this paper [39].
According to this model, factors influencing failure mode transition of RSWs include:
- (i)
Fusion zone size:
The FZ size is the most important parameter governing the failure mode of RSWs. The larger the FZ size (i.e.
Conclusion
Failure mode of the AHSS resistance spot welds during the quasi-static tensile–shear test is investigated through analytical approach. The following conclusions can be drawn from this study:
- 1.
Weld fracture of the AHSS spot welds is a complicated phenomenon which involves interaction among the geometrical factors, weld metallurgical properties and loading mode. In the tensile–shear test with great tendency to fail in interfacial mode, the hardness ratio of fusion zone to pull-out failure location
Acknowledgments
The authors would like to thank Islamic Azad University, Dezful Branch for the financial support of this work.
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