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Although the influence of weld process variables on LME cracking was known to be significant, limited studies have been conducted on the effect of process variables with systematic approaches in the equivalent nugget growth behavior and heat input. This study aimed to identify the effect of weld process variables on LME sensitivity with the equivalent nugget diameter and underlying mechanism with induced tensile stress for cracking. Among the welds with equivalent nugget diameters in the combination of different welding current and time, higher LME sensitivity was observed with the high welding current and short welding time combination than that with the low welding current and long welding time combination for the equivalent nugget diameter. Because a high current and short time combination resulted in faster weld nugget growth than the low welding current and long welding time combination, it rapidly increased the surface temperature along with the cooling from the electrode. These combined effects induced a higher thermal gradient and thermally induced tensile stress on the weld surface, satisfying the conditions of the LME cracking. The simulation results also confirmed that the critical weld cycle time of the LME cracking (tc), which is the cross point between the nugget growth diameter and a contact diameter of the electrode, could be different with the combination of the weld process variables with the equivalent weld nugget size. Therefore, tc can be applied for the sensitivity index of LME cracking of the resistance spot weldment considering complex weld variables.

Three types of self-piercing riveting (SPR) joints, i.e., steel/aluminum, carbon fiber reinforced polymer (CFRP)/ aluminum, and aluminum/aluminum, were constructed using three different top sheet materials with the same aluminum alloy as the bottom sheet. The effects of the top sheet material on the joint quality and mechanical behavior were evaluated. The top sheet materials’ characteristics dominate the rivet piercing process and the consequent interlock distance. The high-strength steel top sheet requires a comparatively higher rivet setting force and induces early flaring of the rivet tail, resulting in a larger interlock distance. Though the CFRP needs the highest rivet setting force to penetrate the rivet through the CFRP fibers, the CFRP/aluminum joint exhibits the smallest interlock distance because of the SPR process-induced damages to the CFRP and subsequently less flaring of the rivet tail. In strength tests, the damaged CFRP sheet resulted in rivet head pullout of the CFRP/aluminum joints, which exhibited the lowest lap-shear and cross-tension strengths. In contrast, the steel/aluminum joints demonstrated the highest strengths because of their comparatively larger interlock distances. In addition to the experimental analysis, simulations revealed the rivet penetration and flaring mechanisms with various top sheet materials, and their respective joint quality and strengths.

Among various defects formed in resistance spot welds of advanced high strength steels (AHSS), shrinkage voids are seldomly studied. Apparently, the voids are thought to be formed towards the end of solidification due to the material volumetric shrinkage. Although, the voids are reported to be influenced by rich material chemistry of AHSS, the exact mechanism of its formation in RSW is yet to be understood. Hence, this paper attempts to study the influence of various RSW parameters on the void size and its formation mechanism. Computed tomography (CT) technique was utilized to visualize and quantify the void formed in a nugget and to study the influence of resistance welding parameters such as weld current, squeeze force and non-welding factors such as sheet thickness material strength on the void volume. A comprehensive analysis reveal that the parameters such as weld current (nugget size), sheet thickness and sheet strength had direct influence on the void volume, but the influence of electrode squeeze force was dependent on the nugget size. Besides, an attempt was also made to develop an empirical relation for the prediction of void volume formed in resistance spot welds of AHSS utilizing the results of present study.

This study provides an optimal shape of square nut for better projection welding performance. Both experimental design method and electrical-thermal-mechanical finite element analysis (FEA) are used to investigate the effects of nut shape parameters on the height decrease of nut leg, called setdown. The relationship between the setdown and weld strength is then analyzed. Also, welding time to reach 50% of setdown tset is introduced as another objective function for better welding performance. The optimal nut shape parameters are determined to maximize the setdown, and minimize the tset at the same time via Taguchi method. The difference of setdown values between experiments and FEA is within 12%, which verify the reliability of the FE model. The gap between DIN928 standard for nut leg geometry and the optimized nut leg shape is less than 0.2 mm. The differences of setdown and tset values for two nut shapes are 6% and 9%, respectively.

In this study, the influence of the microstructural evolution of galvannealed (GA) coating and the thermomechanical characteristics of liquid metal embrittlement (LME) cracking during resistance sp...

Electrode degradation during the resistance spot welding (RSW) of coated steels has been a major challenge in automobile manufacturing as it affects the weld quality in terms of nugget size, defects, and mechanical properties. In this work, we investigate the effect of electrode degradation on such a defect, Zn-assisted liquid metal embrittlement (LME) cracking, in RSW by producing 400 consecutive welds of galvannealed TRIP steel. Infrared thermography, carbon imprinting, dynamic resistance, optical imaging, SEM, and EDS were used to characterize various phenomena during consecutive welding. LME crack severity was found to be unaffected in the first 80 consecutive welds. However, the severity declined sharply between 80–200 welds, and further, it remained low in 200–400 welds. The electrode experienced geometrical degradation (increase in the radius of curvature and face diameter) and metallurgical degradation (Cu-Zn-Fe alloy layer formation) during consecutive welding. The geometrical degradation, specifically the radius of curvature, was confirmed as the main factor affecting LME cracking. On the other hand, metallurgical degradation did not affect the LME cracking in the first 200 welds as its influence was overcome by the geometrical degradation. However, a small impact of metallurgical degradation was detected in 200–400 welds in the form of local rise and fall in the temperature and LME crack length.

Infrared thermography technique (IRT) uses longer wavelength electromagnetic radiations emitted by a hot body to compute its temperature. The radiant energy absorbed by the thermographic system is transformed into electronic signals by sensors and into visible signals known as a thermogram. Besides highly sophisticated military applications, the non-contact infrared thermography is extensively used in medical, electrical, mechanical and many other domains. This paper exploits the infrared thermography technique for precise measurement of temperature across the weld interfaces obtained with two different welding processes, viz. nut projection welding and cold metal transfer gas metal arc welding (CMT-GMAW). The recorded temperature was processed to obtain peak temperature, heat generated, heat affected zones, and to understand the effect of coating on the heat generation and defect formation. IRT results reveal that the weld quality and strength is significantly influenced by the weld pool temperature and region of heat generation in CMT and nut projection welding, respectively.

This study aims to explore the mode of Zn transportation and the failure mechanism of cracking associated with liquid metal embrittlement (LME) using fractography as the key technique. A three-point bend test was performed on a TRIP steel resistance spot weld to open the LME crack in the form of a free fracture surface for the fractographic investigation. The presence of liquid Zn on the fracture surface was revealed by the Fe-Zn phase transformation and the spike-like morphology of the residual Zn, confirming that the mode of Zn transportation in LME cracks was liquid penetration through the austenite grain boundary. In addition, the fractography of the bend test samples and electron backscattered diffraction of the cracks revealed the failure mode of the LME crack as a complete intergranular brittle fracture without the generation of any microplasticity. Thus, the underlying failure mechanism of cracking in Zn-LME can be explained by the Stoloff-Johnson-Westwood-Kamdar brittle fracture model induced by the decohesion of interatomic bonds. Overall, a dramatic reduction in the interatomic bond strength by lowering the surface energy of the grain boundary with liquid Zn penetration causes the decohesion-induced intergranular brittle cracking.

Zinc-coated advanced high-strength steels are known to be susceptible to liquid metal embrittlement (LME) cracking during resistance spot welding (RSW). Despite numerous reports with regard to LME during RSW, a systematic approach has not been proposed for the classification of cracks based on the cracking mechanism. The objective of this study was to characterize the LME cracks at various RSW locations, and thereby propose a classification method to identify the mechanism of the LME cracks at each location. The experimental results revealed the LME cracks were concentrated at certain weld locations and exhibited different features in terms of length, number, and orientation, owing to the synergetic effect of temperature, stress, microstructure, time of exposure to liquid zinc, and time of exposure to tensile stress at the corresponding lo-cations. Thus, the LME cracks were classified into four categories, namely type A, type B, type C, and type D, based on the formation location. The effect of time of exposure to liquid zinc and tensile stress on LME cracking revealed the time dependency of LME in RSW. The nature of contact be-tween the electrode and the sheet, and the heat input during welding, were found to be the main reasons for the difference in the thermal, mechanical, and metallurgical characteristics of various crack locations, which caused the formation of various LME crack types.

Among various defects formed in resistance spot welds of advanced high-strength steels (AHSSs), shrinkage void (cavity) is the least addressed topic. Although cavities are formed in numerous sizes and shapes, their influence on the strength and failure modes is yet to be investigated. The present study examines the effect of the cavity size and position on the peak tensile shear load and failure modes in 780 grade dual-phase steel used in automobile body structures. The size and position of the cavity in the nugget were varied carefully by altering the weld parameters and the sheet thickness. The cavity characteristics and its effect on the weld strength were evaluated from shear tensile tests and fracture surfaces. Results show that the size of the cavity increases with sheet thickness and nugget diameter. Further analyses revealed that the shear strength of the weld is influenced by the size and location of the cavity from the interface. A transition from plug-type failure to interfacial failure was observed with an increase in the cavity size. Consequently, the cavities reduce the load-carrying area of the nugget affecting the weld strength and failure mode. The effective nugget area, which is the actual load-bearing area of the nugget without any cavity inferred from the interfacial fracture surface, would be beneficial in determining the interfacial failures in resistance spot welds. Based on the results, a minimum nugget diameter of 5√t was proposed to obtain plug-type failures in AHSS spots with the cavity.

This paper presents an innovative approach that uses a pulse-profile to improve the welding quality of CP1180 steel in resistance spot welding process. Three pulses with two cooling times were used...

The corrosion behavior of fusion zones (FZ-1: 5.39 Mg–0.66Mn, FZ-2: 5.49 Mg–0.35Mn, and FZ-3: 5.72 Mg–0.32Mn) of AA5083 alloy welded with high Mg-containing Al–Mg filler wires has been systematically investigated. An extensive microstructural characterization was performed to evaluate the localized corrosion properties of the fusion zones. The potentiodynamic polarization test and salt spray test showed that the corrosion resistance decreases with increasing Mg content, therefore, the corrosion resistance of FZ-3 is lower than that of the FZ-1 and FZ-2. The electron microscopic analysis after intermittent corrosion test revealed that the eutectic β-phase (Al3Mg2) was responsible for the initiation of corrosion. The corrosion behavior of each microstructural constituents was related to the dissolution potentials of the corresponding microstructures; hence, it forms galvanic couple with the Al matrix. Moreover, high Mg content in the filler wire lead to the formation of additional β-phase in the fusion zone. However, intermetallics such as Al6(Fe,Mn) and Mg2Si were significant only during the later stages of corrosion.

The present work studied the influence of the geometric design of the electrode on Zn-assisted liquid metal embrittlement (LME) cracking during resistance spot welding (RSW). LME cracking of the galvannealed transformation-induced plasticity (TRIP) steel welds, induced by two types of electrodes, a radius type with different radius of curvature (R), and a dome type with variable tip diameter (d), was studied both experimentally and by simulation. The current density decreased and the contact area at the electrode/sheet (E/S) interface increased with the increasing R, resulting in low temperatures and thermal stress, which subsequently led to decreased LME tendency. On the contrary, the current density decreased but the initial contact area at the E/S interface remained unchanged with increasing d, causing only a minor reduction in the temperature and hence less influence on LME cracking. These results suggested that R is the most critical design parameter of the electrode that controls LME cracking. Moreover, the radius type electrode displayed lower LME sensitivity as compared with the dome type electrode. This is attributed to the fact that the radius type electrode provides the benefits of increase in both R and d.

The toughness of the base material (BM) and the coarse-grain heat-affected zone (CGHAZ) of N-controlled fire- and seismic-resistant (FSR) steels (FSR1: 0.03Ti, 0.25Mo, 0.018Nb; FSR2: 0.07Ti, 0.09Mo; and FSR3: 0.14Ti, 0.17Mo) were systematically investigated. FSR1 steel exhibited better toughness than FSR2 and FSR3 steels in the BM and CGHAZ. Fractographic analysis of the BM and CGHAZ revealed the presence of coarse TiN precipitates in fracture initiation sites, which are responsible for the low toughness of FSR2 and FSR3 steels. Coarse TiN precipitates with equivalent diameters of 1 to 6 μm are present in all FSR steels. However, FSR1 steel has relatively lower number density and area fraction of TiN precipitates. The formation mechanism of TiN precipitates is elucidated from the theoretical thermodynamic approach and validated with a quantitative metallographic observation; it was found that the mechanism is the same for both the BM and CGHAZ in all FSR steels. The equilibrium solubility temperature of TiN precipitates in FSR1 steel was lower than the solidus temperature, which resulted in fewer coarse TiN precipitates. The lower boundary of both the Ti/N (hypostoichiometric) ratio and TiN solubility product at the solidus temperature is highly recommended for increased toughness of FSR steels containing Ti, Mo, and Nb.

This study aims at estimating lobe curve in resistance projection welding (RPW) according to material strength. A three-dimensional (3D) fully coupled electrical-thermal-mechanical finite element (FE) model was developed by considering temperature-dependent material properties and projection forming process. Residual stress within the projection after stamping process, which affects the initial contact resistance, increases as projection height and material strength increase. For DP780 steel, an average error of nugget size between welding experiments and FE analyses is less than 10 %. A method for estimating lobe curves with material strength including welding parameters such as electrode force, current, welding time and projection height is proposed based on systematic FE analyses. Lobe curve moves towards a lower current region as the material strength increases.

Resistance nut projection weldability of Al–Si coated hot stamped steel (HSS) was investigated under the viewpoint of weldable current range and joint strength (pull-out load). The microstructural inhomogeneities in the welds were also studied in order to elucidate the factors affecting the joint strength of the welds. The weldability of the given Al–Si coated HSS was compared with the weldability of an identical HSS without the Al–Si coating (Al–Si coating was polished out) and Zn coated dual phase steel. The weldable current range of Al–Si coated HSS was found to be narrower than that of the other materials. Furthermore, the average pull-out load within the weldable current range of the Al–Si coated HSS was the lowest among the three materials. The reason for poor weld mechanical property of the Al–Si coated hot-stamped steel was attributed to the microstructural inhomogeneities such as unmixed Al–Si coating layer at the edge of the nugget and the second phase Fe3(Al, Si) intermetallic compound. The formation of Fe3(Al, Si) phase was attributed to the solidification segregation of Al and Si during the weld solidification and was confirmed with the numerical analysis of solidification segregation.

Fine-grained materials possess great industrial significance as they have exceptional strength, ductility, and superplastic formability. However, the poor thermal stability of these materials leads to an excessive grain growth and property deterioration during welding. In this regard, friction stir welding (FSW) is considered as a potential candidate for the welding of fine-grained materials as it is a solid-state welding process with a lower process temperature. In this work, the microstructure development in the stir zone of a fine-grained Ti-6Al-4V alloy during FSW was investigated. The microstructure, misorientation distribution and crystallography of the base metal and stir zone were characterized using the electron backscatter diffraction technique to explain the microstructural evolution. The microstructure of different layers of stir zone composed of either fine α + β lamellar structure or fine equiaxed α and β grains or the mixture of both. The combination of heating/cooling induced α/β phase transformations and plastic deformation induced continuous dynamic recrystallization (CDRX) was responsible for such microstructural developments in the stir zone. Homogeneous misorientation increase in low angle boundaries was found as the dominant mechanism of CDRX in the stir zone of fine-grained Ti-6Al-4V during FSW. The hardness measurement confirmed the absence any plausible softening though there was a little grain coarsening in the top layer of the stir zone.

The selection and application of the type coating to steel sheets in automotive industries depend on factors like corrosion protection, manufacturing compatibility and cost. The ‘first generation’ type primer coating improved the corrosion protection of chromate free pretreatments, however it deteriorated the resistance spot weldability of the sheets. The development of 2nd generation primer coating with additives such as CNT showed dominance in corrosion protection properties along with zinc. The present paper investigates the resistance spot weldability of the CNT added primer coating. The weldability was evaluated by comparing the CNT added primer with 1st generation primer for nugget size and electrode life. At lower welding currents, the nugget size of the CNT added primer was significantly lower than the primer coated steels. The increase in the dynamic resistance of the CNT coated steel after nugget formation led to higher nugget sizes at higher currents. The decrease in aluminum and increment in the Fe concentration at the electrode surface during welding of CNT based primer coated steels would improve the electrode life.

This paper focuses on the resistance spot welding of low carbon steel/aluminium/stainless steel 3-ply clad sheet. Weldability, microstructure, tensile properties and fracture behaviour of the clad sheet with resistance spot welding were evaluated. A “squeeze-out pulse” was used to melt and squeeze out the core aluminium completely from the vicinity of the weld zone before the application of actual welding current. This enabled the clad sheet to be welded without the formation of any Fe–Al intermetallic compound layer in the weld zone. The fracture mode associated with the peel test of the spot weld was used as the criterion for resistance spot weldability evaluation of the clad sheet. A weldability lobe diagram in the welding current–welding time space based on the fracture mode was constructed and it revealed a wide weldable window especially at a welding time of 18 cy. A tensile shear test with a cross head speed of 1 mm/min was conducted to study the tensile strength and fracture behaviour. It revealed four different fracture modes namely button pull, clad-bond fracture, partial thickness-partial clad-bond fracture and interfacial fracture. The nugget size and melting of clad interface and stainless steel layer were the dominant factors in determining the fracture mode.

Shear fracture load is estimated in resistance projection welded (RPW) sheets with dissimilar materials and thicknesses. Relation between shear fracture load and welding parameters such as nugget size, sheet thickness and tensile strength of base material is obtained based on dimensional analysis. Lap-shear tests are performed to investigate the effects of nugget size and thickness on the shear fracture load. Proposed equations to predict the shear fracture load within an average error of less than 10%. To ensure the occurrence of pullout failure modein welded specimens of high strength steels, a critical nugget size is suggested as 5.5 (tm) 1/2, where tm is an average thickness of two welded sheets.

Zn assisted liquid metal embrittlement (LME) has characteristics that differ from the general characteristics of other LME systems: the formation of Fe-Zn intermetallic compounds and the high solubility of Zn in Fe. Consequently, there is a need to investigate the influence of the Fe-Zn interaction and resulting phase composition on LME behavior. However, little research has been conducted to reveal such relationships. This paper explores the influence of high temperature phases in galvannealed coating, specifically liquid Zn and α-Fe(Zn) on the LME sensitivity of medium Mn steel using a high temperature tensile test with eight combinations of temperatures (700 °C to 900 °C) and hold times (1 s, 50 s, and 100 s). The test results are categorized as LME, microcrack and LME free cases in terms of LME behavior. The study reveals that the LME sensitivity is governed by the fraction of effective liquid Zn, which is a consequence of isothermal solidification owing to the thermodynamic stability of liquid Zn, diffusivity of Zn in Fe and the partitioning coefficient of liquid Zn. Moreover, LME is influenced by the breaking of α-Fe(Zn) layer, which directs the liquid Zn into the substrate steel. Finally, a water-tank analogy is proposed to demonstrate the effect of the fraction of liquid Zn and the breaking of α-Fe(Zn) layer on LME.

Among various defects formed in resistance spot welds (RSW), voids or cavities formed at the center of the nugget are scarcely studied. However, it is widely accepted that such cavities significantly influence the mechanical properties of the spots, especially in advanced high-strength steels (AHSS). Nonavailability of a proper characterization technique to quantify these cavities is one of the reasons for less research in this area. Although conventional metallographic analysis portrays cavities in the nugget, the associated destructive sampling results in erroneous interpretation. To date, there is no reliable technique to quantify the size, shape, and distribution of the voids in the nugget. In this context, the present study uses a computed tomography (CT) technique to visualize and quantify the cavities in spot-welded AHSS nuggets. The images obtained using a CT scan provide a new insight into the cavity and its characteristics, such as volume and surface topology. According to the CT results, the voids formed in AHSS spots increase with the increase in base metal yield strength. Furthermore, the CT results also reveal that the conventional critical diameter of 4 × thickness1/2 to obtain plug failure in RSW is not valid for AHSS spots because of the influence of the cavity in the faying interface.