Failure mode transition in AHSS resistance spot welds. Part I. Controlling factors
15 Nov 2011-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing (Elsevier)-Vol. 528, Iss: 528, pp 8337-8343
TL;DR: In this paper, the authors investigated and analyzed the transition between interfacial and pull-out failure modes in AHSS resistance spot welds during the tensile-shear test by the use of analytical approach.
Abstract: Failure mode of resistance spot welds is a qualitative indicator of weld performance. Two major types of spot weld failure are pull-out and interfacial fracture. Interfacial failure, which typically results in reduced energy absorption capability, is considered unsatisfactory and industry standards are often designed to avoid this occurrence. Advanced High Strength Steel (AHSS) spot welds exhibit high tendency to fail in interfacial failure mode. Sizing of spot welds based on the conventional recommendation of 4t0.5 (t is sheet thickness) does not guarantee the pullout failure mode in many cases of AHSS spot welds. Therefore, a new weld quality criterion should be found for AHSS resistance spot welds to guarantee pull-out failure. The aim of this paper is to investigate and analyze the transition between interfacial and pull-out failure modes in AHSS resistance spot welds during the tensile–shear test by the use of analytical approach. In this work, in the light of failure mechanism, a simple analytical model is presented for estimating the critical fusion zone size to prevent interfacial fracture. According to this model, the hardness ratio of fusion zone to pull-out failure location and the volume fraction of voids in fusion zone are the key metallurgical factors governing type of failure mode of AHSS spot welds during the tensile–shear test. Low hardness ratio and high susceptibility to form shrinkage voids in the case of AHSS spot welds appear to be the two primary causes for their high tendency to fail in interfacial mode.
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TL;DR: In this article, the fundamental understanding of structure-properties relationship in automotive steels resistance spot welds is discussed. And a brief review of friction stir spot welding, as an alternative to RSW, is also included.
Abstract: Spot welding, particularly resistance spot welding (RSW), is a critical joining process in automotive industry. The development of advanced high strength steels for applications in automotive industry is accompanied with a challenge to better understand the physical and mechanical metallurgy of these materials during RSW. The present paper critically reviews the fundamental understanding of structure–properties relationship in automotive steels resistance spot welds. The focus is on the metallurgical characteristics, hardness–microstructure correlation, interfacial to pullout failure mode transition and mechanical performance of steel resistance spot welds under quasi-static, fatigue and impact loading conditions. A brief review of friction stir spot welding, as an alternative to RSW, is also included.
369 citations
Cites background from "Failure mode transition in AHSS res..."
...The cooling rate during RSW is governed by the heat dissipation from molten FZ to the surrounding via two mechanisms: heat dissipation through water cooled electrode in axial direction and radial heat dissipation through adjacent cold BM.51,52 Kim and Eagar79 have shown that the ratio of axial heat loss rate to the redial heat loss rate is proportional to the electrode diameter divided by the square of sheet thickness....
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...These geometrical factors along with microstructural heterogeneity reduce the load bearing capacity of the joint compared with the BM.7,17 Therefore, the stress concentration associated with these two effects should be considered in the analysis of the spot weld failure....
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...It has been experimentally shown that in the low carbon steel resistance spot welds, the necking is initiated from the BM.7,37 Similar observations have been made for HSLA steels and DP600 resistance spot welds.8,18 This can be attributed to the low hardness of the BM rather than HAZ and FZ (Fig....
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...Therefore, the contraction of the solidifying metal can be hindered by the BM....
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...Pouranvari et al.(9) investigated failure mode transition of 1?5 mm thick DP600, DP780, DP980, DQSK and AISI 304 steels....
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TL;DR: In this article, the effect of welding current at constant welding time was considered on the weld properties such as weld nugget size, tensile-shear load bearing capacity of welded materials, failure modes, failure energy, ductility, and microstructure of weld nuggets as well.
102 citations
TL;DR: In this article, the microstructure of the fusion zone and the heat affected zone is investigated in resistance spot welding of AISI 430 ferritic stainless steels and the phase transformations which occur during weld thermal cycle were analyzed in details, based on the physical metallurgy of welding of the ferritic steels.
92 citations
01 Jun 2012-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the failure mode transition from interfacial to pullout in similar and dissimilar combinations of DP600 dual phase steel and low carbon steel (LCS) under tensile-shear (TS) and cross-tension (CT) loading conditions was investigated.
Abstract: The paper investigates the failure mode transition from interfacial to pullout in similar and dissimilar combinations of DP600 dual phase steel and low carbon steel (LCS) under tensile-shear (TS) and cross-tension (CT) loading conditions. In both CT and TS loading conditions, a transition in the failure mode from interfacial to pullout was observed with increasing fusion zone size beyond a critical value (DC). The tendency to fail in interfacial mode during the CT loading was increased in the order of LCS/LCS, DP600/LCS and DP600/DP600. It was shown that interfacial to pullout failure mode transition during the CT test is governed by the fracture toughness of the fusion zone and strength of the pullout failure location (i.e. heat affected zone). It was shown that increasing carbon equivalent of the fusion zone promoted interfacial failure mode in CT loading condition. The high carbon equivalent of DP600 steel led to formation of hard and brittle martensite, which in turn promotes crack propagation through fusion zone. In DP600/LCS combination, decreased carbon equivalent and fusion zone hardness through dilution with the LCS promotes pullout failure at smaller weld sizes. DC during the TS loading is increased in the order of DP600/LCS, LCS/LCS and DP600/DP600. No correlation between fusion zone carbon equivalent and the tendency to fail in IF mode during TS loading was found. Failure mode transition during the TS loading is controlled by hardness of fusion zone and stiffness of the joint. The lowest DC for DP600/LCS is a function of its high fusion zone hardness (in comparison to LCS/LCS combination) and its low stiffness (in comparison to DP600/DP600 combination).
83 citations
TL;DR: In this paper, a critical assessment highlights the current understating regarding factors affecting the joint properties and approaches to control the interfacial reaction and unresolved scientific challenges are discussed with the goal of shedding light on the path forward to produce reliable metallurgical bonding between aluminium and steels for automotive application.
Abstract: Dissimilar joining of aluminium and steel, especially using resistance spot welding as a critical process in vehicle manufacturing, is a key challenge for multi-materials lightweight design strategy. Controlling the formation and growth of Al5Fe2 intermetallic is the outstanding issue for producing high strength crash-resistance Al/steel dissimilar resistance spot welds. This critical assessment highlights the current understating regarding factors affecting the joint properties and approaches to control the interfacial reaction. Finally, the unresolved scientific challenges are discussed with the goal of shedding light on the path forward to produce reliable metallurgical bonding between aluminium and steels for automotive application.
76 citations
References
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20 Dec 2005
TL;DR: WELDING METALLURGY Solidification in Resistance Spot Welding Phase Transformations in RSW Cracking References ELECTROTHERMAL Processes of WELDING Introduction Electrical Characteristics of Resistance Welding Thermal Characteristics and Electrode Life Lobe Diagrams References WELD DISCONTINUITIES Classification of Discontinuities as mentioned in this paper.
Abstract: WELDING METALLURGY Solidification in Resistance Spot Welding Phase Transformations in RSW Cracking References ELECTROTHERMAL PROCESSES OF WELDING Introduction Electrical Characteristics of Resistance Welding Thermal Characteristics of Resistance Welding Heat Balance Electrode Life Lobe Diagrams References WELD DISCONTINUITIES Classification of Discontinuities Void Formation in Weld Nuggets Cracking in Welding AA6111 Alloys Cracking in Welding AA5754 Alloys References MECHANICAL TESTING Introduction Shop Floor Practices Instrumented Tests References RESISTANCE WELDING PROCESS MONITORING AND CONTROL Introduction Data Acquisition Process Monitoring References WELD QUALITY AND INSPECTION Weld Quality Attributes Destructive Evaluation Nondestructive Evaluation References EXPULSION IN RESISTANCE SPOT WELDING Influence of Expulsion on Spot Weld Quality Expulsion Process and Detection Expulsion Prediction and Prevention Examples References INFLUENCE OF MECHANICAL CHARACTERISTICS OF WELDING MACHINES Introduction Mechanical Characteristics of Typical Spot Welders Influence of Machine Stiffness Influence of Friction Influence of Moving Mass Follow-Up in a Welding Cycle Squeeze Time and Hold Time Measurement Other Factors References NUMERICAL SIMULATION IN RESISTANCE SPOT WELDING Introduction Coupled Electrical-Thermal-Mechanical Analysis Simulation of Contact Properties and Contact Area Simulation of Other Factors Modeling of Microstructure Evolution Examples of Numerical Simulation of RSW Processes References STATISTICAL DESIGN, ANALYSIS, AND INFERENCE IN RESISTANCE WELDING RESEARCH Introduction Basic Concepts and Procedures Experiments with Continuous Response Experiments with Categorical Responses Computer Simulation Experiments Summary References INDEX
297 citations
"Failure mode transition in AHSS res..." refers background or methods in this paper
...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]....
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...As confirmed by the finite element modeling [14,31], in the PF mode, load is mainly carried by HAZ while the center region of the nugget is mostly stress free....
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...5 rule [14,18,19]....
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...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....
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TL;DR: In this article, the failure mechanism at the spot weld is tensile (shear) mode at the materials level, and stress distribution is assumed and related to the far field load for the lapshear and cross tension test samples.
Abstract: Strength tests were performed to reveal the failure mechanisms of spot weld in lap-shear and cross tension test samples. It is shown the while the lap-shear (cross tension) sample is subjected to shear (normal) load at the structural level the failure mechanism at the spot weld is tensile (shear) mode at the materials level. Based on the observed failure mechanism, stress distribution is assumed and related to the far field load for the lapshear and cross tension test samples. It appears that the failure load of the cross tension sample is 74 percent of the lap-shear sample based on the classical von Mises failure theory. The theoretical model is further extended to the mixed normal/shear loading condition. Data from strength tests as well as finite element numerical method are used to validate the model. Finally, the utility of the model in accessing the failure strength of spot welds is discussed. @DOI: 10.1115/1.1555648#
232 citations
TL;DR: In this article, an analytical model was proposed to predict failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout failure mode in shear tensile test.
Abstract: In the present paper, effects of welding current, welding time, electrode pressure and holding time on the weld nugget size were studied. A failure mechanism was proposed to describe both interfacial and pullout failure modes. This mechanism was confirmed by SEM investigations. In the light of this mechanism, the effect of welding parameters on static weld strength and failure mode was studied. Then, an analytical model was proposed to predict failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout failure mode in shear tensile test. On the contrary to existing industrial standards, in this model, critical nugget diameter is attributed to metallurgical characterisation of material (weld nugget hardness to failure location hardness ratio), in addition to sheet thickness. For a given sheet thickness, decreasing HWN/HFL increases interfacial failure mode tendency. The results of this model were compared with experimental data and also with the literature.
210 citations
25 Jun 2008-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effects of weld expulsion on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy, image analysis and scanning electron microscopy (SEM) together with energy dispersive X-ray spectroscopy (EDS).
Abstract: Microstructural characterization, microhardness tests, tensile shear tests, and fatigue tests of spot-welded hot dipped galvanized (HDG) DP600 steel were conducted. The effects of weld expulsion on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy, image analysis and scanning electron microscopy (SEM) together with energy dispersive X-ray spectroscopy (EDS). The hardness in the weld nugget was observed to be over two times higher than that in the base metal due to the formation of lath martensite. Fatigue tests showed a slightly lower fatigue limit for the joints with the expulsion. The crack initiation was observed to occur basically at the boundary of the weld nugget and at the interface between two sheets. The causes for the interfacial fracture (IF) were discussed in relation to the microstructure and weld parameters.
183 citations
15 May 2008-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the relationship between failure mode and weld fusion zone characteristics (size and microstructure) was studied and it was found that spot weld strength in the pullout failure mode is controlled by the strength and fusion zone size of the galvanized steel side.
Abstract: Resistance spot welding was used to join austenitic stainless steel and galvanized low carbon steel. The relationship between failure mode and weld fusion zone characteristics (size and microstructure) was studied. It was found that spot weld strength in the pullout failure mode is controlled by the strength and fusion zone size of the galvanized steel side. The hardness of the fusion zone which is governed by the dilution between two base metals, and fusion zone size of galvanized carbon steel side are dominant factors in determining the failure mode.
176 citations