Resistance Spot-welded Joints Research Articles

Local mechanical characterization and fracture prediction modeling for resistance spot-welded joints of advanced high-strength steel

During resistance spot-welding of advanced high-strength steel (AHSS), heat-affected zones (HAZs) are formed and mechanical properties around the welding area are altered, causing a different mechanical response from that of the base metal. This study aimed to develop a finite element modeling method for welded AHSS. Hardness measurements were conducted for JSC590, JSC980, and JSC1180 steels through nano-indentation, and their local mechanical properties were identified using miniature tensile experiments. Then, the correlations between Vickers hardness, tensile strength, and strain were quantified and applied to the shifting of the altered mechanical properties at the HAZ. In the finite element method (FEM) simulations, the nugget parts and HAZ parts of the different FEM models were defined basing on the correlations identified previously. Eventually, the fracture behaviors were elucidated in combination with the modified Mohr–Coulomb (MMC) model. The simulation results indicate that the fracture strain and tensile stress of each material were predicted within a range of 3.3∼17.2 % and 1.4∼6.3 % depending on steel grades. In addition, the behaviors of crack generation and propagation obtained through the simulation were highly consistent with the experimental results. Thus, the fracture behavior of spot-welded AHSS was accurately predicted by the more convenient hardness-based method.

Clinching of High-Strength Steel Sheets with Local Preheating

Clinching is a manufacturing method of mechanically joining two or more materials without the use of heat or additional components. This process relies on high plastic deformation to create a secure bond. Clinching technology is widely used for joining materials of various grades and thicknesses. Especially in the automotive industry, clinching is an alternative to resistance spot welding. However, the load-bearing capacity of clinched joints is comparatively lower when compared to resistance spot-welded joints. This research aimed to increase the load-carrying capacity of clinched joints. To enhance the load-bearing capacity of the clinched joints, localized modification of the microstructure was carried out, primarily focusing on the neck area of the joint. The alteration of the microstructure within the clinched joint was accomplished through the application of localized heating using the resistance spot welding method. The microstructure distribution in the clinched joint region was analyzed using light and scanning electron microscopy, as well as microhardness measurements. Two material grades, micro-alloyed steel HX420LAD+Z and dual-phase ferritic–martensitic steel HCT600X+Z, were tested. Each grade underwent five groups of ten samples, which were subjected to identical experimental conditions of local heating by resistance spot welding (RSW) and clinching. The utilization of RSW on the clinched joint region resulted in an average enhancement of 17% in the load-carrying capacity for material HCT600X+Z, and an average increase of 25% for material HX420LAD+Z.

Comparison of RSW technologies on DP steels with modified instrumented Charpy impact test

In recent decades the toughness of welded structures becomes more and more important. This trend is particularly valid for the advanced high-strength steels (AHSS), which have reduced toughness comparing to the low-strength structural steels. Dynamic characteristics of the welded joints of the newly developed high-strength steel sheets required by the automotive industry have been neglected, with welding procedures being optimized on the static joint properties, mainly on the tensile-shear force. Up to the present time, testing of dynamic properties of the spot-welded joints has been performed by increased testing speed during tensile testing. The authors have developed a new dynamic testing method and designed new testing equipment for impact bending, which can give a numeric result to characterize the resistance of spot-welded joints against dynamic load. In this paper, this method will be used to evaluate resistance spot-welded joints made on DP600 and DP800 steels with three different technological parameters, comparing long-time welding, short-time welding, and two-pulse welding. The different parameters cause different weld nugget sizes, failure modes, impact energies, and force–time diagrams. All test results show that bigger weld nugget diameters cause better impact energies and impact forces, but the differences are not perfectly correlated.

Effect of paint baking on the halo ring and mechanical behavior of 30MnB5 hot-stamped steel resistance spot welding joints

The present work investigates the effect of paint baking (PB) on the resistance spot welding (RSW) joints of 30MnB5 (1.8 GPa grade) hot-stamped (HS) steel, with focus on the halo ring and fracture behavior under cross-tension loading condition. Microstructural and elemental distribution analyses reveal that severe segregation of alloying elements occurred in the fusion zone (FZ) and that a halo ring with a martensitic structure and a width of about 78 μm was formed in the as-welded joint. The PB process resulted in uniform distribution of alloying elements and recarburization in the halo ring region, thereby drastically decreasing the halo ring width to about 33 μm, which changed the fracture behavior of the joint. Therefore, the as-welded joint failed through the halo ring, while in the PB joint, the fracture propagated partly through the FZ and then redirected through the thickness direction. The PB joints exhibited superior mechanical performance to the as-welded joint, especially the energy absorption.

Study on microstructure and mechanical properties of RSW joints of 7075T-6 containing Sc

Currently, resistance spot welding (RSW) is a common welding method used in automobile and other industries, and 7000 series aluminum (Al) alloys have become a very common industrial material because of their high strength and easy machinability. Scandium (Sc) is often used for alloy strengthening; however, there are few studies on the RSW joints of aluminum alloys containing scandium. Here, 7075-T6 alloys containing a small amount of scandium were obtained to investigate the joint of resistance spot weldability. The simulation shows that the effective contact area between the aluminum sheet surface is about 19.5%, which is an important factor in nucleation. It was found that the average grain size of the nugget decreased from 51.51 ± 5.74 to 8.15 ± 1.09 μm when scandium was added from 0 to 0.4 wt%, which is mainly due to Al3Sc particles acting as a heterogeneous nucleus in the melt. The test results show that the tensile shear strength of welded joints with 0.4 wt% scandium addition is 29.2% higher than that of the joints without scandium because of grain refinement strengthening.

Effect of intermetallic compound on the corrosion behaviour of resistance spot welding joints between 5182 aluminium alloy and galvanized DP780 dual-phase steel

Interfacial intermetallic compound (IMC) layer is critical during realizing the galvanic corrosion mechanism and strength degradation of aluminium/steel resistance spot welding (RSW) joints. The effect of IMC layer on the corrosion behaviour of RSW joint between 5182 aluminium alloy and galvanized DP780 dual-phase steel was investigated by immersion corrosion method and electrochemical method. Results demonstrated localized corrosion around Fe-rich phase particles on aluminium nugget and preferential corrosion at the interface front near IMC layer. The IMC layer had the highest open circuit potential value of −0.569 V and low corrosion current density among the investigated nugget and substrates. The potential difference (0.184 V) between the cathodic IMC layer and large area of anodic aluminium nugget was responsible for the preferential initiation of localized corrosion at the interface front near the IMC layer.

EFFECTS OF HOLDING TIME ON MICROSTRUCTURE AND SHEAR STRENGTH OF RESISTANCE SPOT-WELDED JOINTS BETWEEN MILD STEEL AND ALUMINUM WITH ZINC POWDER FILLER

This study aimed to determine effects of holding time on microstructure and shear strength of resistance spot-welded joints between mild steel and aluminum with zinc powder filler. The materials used are 0.9 mm thick mild steel, 1.2 mm thick 3000 series aluminum, and 200 mesh zinc powder. Welding parameters were selected as welding time of 0.5 seconds (s), electric currents of 8000 A and 9000 A, with holding time variations of 1 s, 3 s, and 5 s. Macro and scanning electron microscope (SEM) images follow ASTM E3-01, while ASME IX was used as tensile test standards. The macro images showed that holding time affected quality of the welded joints. Applying a longer holding time was able to reduce defects on the welded joints. SEM energy-dispersive X-ray spectroscopy (SEM-EDX) analysis exhibited that the holding time of 1 s produced a lack of density in the spot-welded joint, resulting in the existence of porosity. In contrast, increasing the holding time to 5 s gives a denser spot-welded joint with no defects found. The tensile test results showed that applying electric current of 8000 A, welding time of 0.5 s, and holding time of 1 s produced the joint with the weakest shear strength of 130 N. The highest joint shear strength of 790 N is obtained with the condition: electric current of 9000 A, welding time of 0.5 s, and holding time of 5 s.

Structural transformations in resistance spot-welded joints from Nimonic 80A superalloys

Joining of Nimonic alloys components delivered as sheets by resistance spot welding process, offers numerous advantages from technical and economic point of view. Knowing the structural modifications induced by the thermal-deformational cycle represents a very important problem in order to avoid the cracks occurrence in the welded joints.In the present paper are analysed the microstructural and the hardness characteristics in the three homogenous welding areas of the nickel super alloy, strengthened by artificial aging. The short time of the welding process and the high power density of the heating source results in a reduced heat-affected zone (HAZ). The dimensional development of the crystal grains from HAZ is negligible due to the low diffusivity of the nickel-based solid solution matrix, having a face-centred cubic lattice that inhibits the quick migration of the grain limits during the welding process.

Effect of Tempering Process on Microstructure and Properties of Resistance Spot-Welded Joints of δ-TRIP Steel

In this paper, a medium-frequency inverter spot welder was used for resistance spot-welding experiments on 980 MPa grade cold-rolled δ-TRIP(Transformation-induced Plasticity) steel. The effects of the tempering process on the morphology, microstructure, element distribution, and properties of spot-welded joints were studied by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), and Electron-Probe MicroAnalysis (EPMA). The microstructure of the nugget zone obtained by single-pulse process was δ ferrite, lath martensite, and twin martensite. After adding tempering under the single-pulse process, the microstructure was δ ferrite and lath martensite. However, the morphology of the microstructure was still dendritic, which remained unchanged. The tensile shear failure of spot-welded joints under the two processes was an interface failure, and the fractures were cleavage fractures. After adding tempering, the interface fracture surface presents two kinds of fracture characteristics: the outer cracks’ growth direction was consistent with the columnar crystal growth direction, and the inner crystal cracks occurred in the nugget core and finally grew along the columnar grain boundary. Due to the significant hardness difference between δ ferrite (283 HV) and martensite (533 HV), the low-strength δ ferrite phase was the main channel of crack propagation. After adding tempering, the hardness distribution of the spot-welded joints was more uniform and the tensile shear force increased (7.4 kN→8.5 kN).

Study on the mechanical properties and fatigue failure mechanism of 7075-T6 aluminum alloy joints under different joining processes

To obtain the theoretical basis and reference basis for the selection of low ductility aluminum alloy joining methods, three kinds of joints were obtained using 7075-T6 aluminum alloy sheets for clinching, self-piercing riveting (SPR), and resistance spot welding (RSW) joints, and the single-lap shear tests were performed on the three kinds of joints, and fatigue tests were conducted on the clinched joints and SPR joints. The fatigue fracture of the SPR joint was analyzed using scanning electron microscopy (SEM) and X-ray energy-dispersive spectroscopy (EDS). The results showed that the SPR joint demonstrated the best static performance, followed by the clinched joint and the RSW joint. In the case of the fatigue test, the SPR joint showed better fatigue performance than that of the clinched joint. The fatigue failure of the SPR joint and clinched joint is associated with fretting wear. Fretting wear of the SPR joint is more severe than that of the clinched joint. The debris produced by fretting led to microcracking phenomena in the plate. Under the action of alternating load, the microcracks continued to expand and generate macrocracks, and the joint was broken and finally failed. This fretting wear is the root cause of the failure of the SPR joint. Thus, the SPR joints exhibited the best mechanical properties.

Glass-box modeling for quality assessment of resistance spot welding joints in industrial applications

Resistance spot welding (RSW) is one of the most relevant industrial processes in different sectors. Key issues in RSW are process control and ex-ante and ex-post evaluation of the quality level of RSW joints. Multiple-input–single-output methods are commonly used to create predictive models of the process from the welding parameters. However, until now, the choice of a particular model has typically involved a tradeoff between accuracy and interpretability. In this work, such dichotomy is overcome by using the explainable boosting machine algorithm, which obtains accuracy levels in both classification and prediction of the welded joint tensile shear load bearing capacity statistically as good or even better than the best algorithms in the literature, while maintaining high levels of interpretability. These characteristics allow (i) a simple diagnosis of the overall behavior of the process, and, for each individual prediction, (ii) the attribution to each of the control variables—and/or to their potential interactions—of the result obtained. These distinctive characteristics have important implications for the optimization and control of welding processes, establishing the explainable boosting machine as one of the reference algorithms for their modeling.

Quantifying influence of LME inner cracks on joint strength of resistance spot weld

It is well known that liquid metal embrittlement (LME) cracking can occur during the resistance spot welding (RSW) of zinc-coated high-strength steels. These cracks can be classified into two types, based on location, namely outer cracks inside or outside the electrode indentation region, and inner cracks around the weld area. Recent research has proposed that the latter cracks can be inhibited by controlling weld schedules, though there have been a few reports concerning especially the effects of these cracks on joint properties. In this study, an X-ray CT scan and a tensile-shear test were carried out consecutively with 10 mm-wide RSW joints in order to visualize the cracks in 3 dimensions, and to clarify their effects on tensile-shear strength (TSS). The cracks in the RSW joints of the zinc-coated ultra-high strength steel could be clearly observed by the scan. The results showed that the TSS decreased to 40% by the small interface cracks even with the maximum depth of 170 µm.

Resistance Spot-Welding of Dissimilar Metals, Medium Manganese TRIP Steel and DP590

Resistance spot-welding of dissimilar metals, medium manganese TRIP steel 7Mn and DP590, is carried out. The effects of single-pulse welding parameters and a double-pulse-tempering current on the quality characteristic parameters and mechanical properties of 7Mn/DP590 spot-welded joints are studied. The welding process parameters are optimized using the control variable method. The results show that the optimal process parameters under a single pulse are as follows: electrode pressure: 4.5 kN, welding current: 9 kA and welding time: 300 ms. The failure mode of the welding joint is partial pull-out failure (PF-TT). The welding parameters have great influence on the nugget diameter and thickness reduction. Expulsion, crack and shrinkage are displayed in the joint under high electrode pressure. Softening occurs in the heat-affected zone due to a strong halo effect in the single-pulse weld. The tempering zone on the DP590 side is 202.49 HV, which is the lowest hardness point, while the hardness of the nugget zone is 450 HV. The addition of the tempering current homogenizes the microstructure with different failure paths and eliminates the stress. The tensile shear force of the joint increases by 17.13%. The 7Mn Steel/DP590 resistance spot-welding joint is from the fusion line to the center of the nugget, and the microstructure is composed of plane crystal, cellular crystal, dendritic crystal and columnar crystal, in turn. The nugget zone is composed of lath martensite and a small amount of residual austenite. Fine quasi-spherical and lamellar interbedded cementites are formed in the tempering zone of the DP590-side heat-affected zone.

Effect of Zinc Aluminum Magnesium Coating on Spot-Welding Joint Properties of HC340LAD + ZM Steel

Zn-Al-Mg (zinc, aluminum and magnesium)-coated steel is gradually replacing traditional hot-dip galvanized steel due to its excellent corrosion resistance, self-healing properties and good surface hardness. However, the effect of Zn-Al-Mg coating on the resistance spot-welding joint properties of HC340LAD + ZM steel plates is not clear, and there are few systematic studies on it. In this paper, L16 (43) orthogonal experiments were designed on Zn-Al-Mg-coated steel HC340LAD + ZM (thickness = 1 mm). In addition, taking the tensile shear force as the main evaluation standard, the optimal spot-welding process properties could be achieved when the welding current, the welding time and the electrode loading were 10 kA, 14 cycles and 2.6 kN, respectively. On this basis, the formation mechanism, microstructure and corrosion properties of two plates of steels, with or without zinc, aluminum and magnesium coating under different welding times, were studied. The presence of Zn-Al-Mg coating slightly affected the mechanical properties of welding joints. However, the corrosion current of the body material containing Zn-Al-Mg plating was 7.17 times that of the uncoated plate.

Optimization in the Resistant Spot-Welding Process of AZ61 Magnesium Alloy

In this paper, an integrated artificial neural network (ANN) and multi-objective genetic algorithm (GA) are developed to optimize the resistance spot welding (RSW) of AZ61 magnesium alloy. Since the stability and strength of a welded joint are strongly dependent on the size of the nugget and the residual stresses created during the welding process, the main purpose of the optimization is to achieve the maximum size of the nugget and minimum tensile residual stress in the weld zone. It is identified that the electrical current, welding time, and electrode force are the main welding parameters affecting the weld quality. The experiments are carried out based on the full factorial design of experiments (DOE). In order to measure the residual stresses, an X-ray diffraction technique is used. Moreover, two separate ANNs are developed to predict the nugget size and the maximum tensile residual stress based on the welding parameters. The ANN is integrated with a multi-objective GA to find the optimum welding parameters. The findings show that the integrated optimization method presented in this study is effective and feasible for optimizing the RSW joints and process.

Nugget formation, microstructural features and strength of resistance spot welded cold-rolled dual-phase steel lap joints for automotive applications

Abstract The main objective of this research paper is to investigate the nugget formation, microstructure and strength performance of DP 800 steel joints developed using resistance spot welding (RSW) process for automotive application. The influence of RSW parameters on nugget formation was studied using one variable at a time approach. The RSW joint showing optimum nugget area was further characterized for microstructural features and strength performance. The microstructural features of nugget were studied using optical and scanning electron microscopy (SEM). The lap tensile shear fractured specimens were analyzed using SEM for optimized condition. The microstructural features were correlated to the tensile shear strength and hardness of RSW joints. Results showed that welding current significantly influences the nugget formation than electrode force and welding time. The DP 800 RSW joints made using a welding current of 5.5 kA, an electrode pressure of 4.0 MPa and a welding time of 2.0 s disclosed defect free weld nugget of 7.28 mm diameter. It showed a higher tensile shear strength of 20 kN and microhardness of 564 HV thereby satisfying the requirement of automotive applications.

Physics-informed machine learning assisted uncertainty quantification for the corrosion of dissimilar material joints

Jointing techniques like the Self-Piercing Riveting (SPR), Resistance Spot Welding (RSW) and Rivet-Weld (RW) joints are used for mass production of dissimilar material joints due to their high performance, short cycle time, and adaptability. However, the service life and safety usage of these joints can be largely impacted by the galvanic corrosion due to the difference in equilibrium potentials between the metals with the presence of electrolyte. In this paper, we focus on Al-Fe galvanic corrosion and develop physics-informed machine learning based surrogate model for statistical corrosion analysis, which enables the reliability analysis of dissimilar material joints under corrosion environment. In this study, a physics-based finite element (FE) corrosion model has been developed to simulate the galvanic corrosion between a Fe cathode and an Al anode. Geometric and environmental factors including crevice gap, roughness of anode, conductivity, and the temperature of the electrolyte are investigated. Further, a thorough Uncertainty Quantification (UQ) analysis is conducted for the overall corrosion behavior of the Fe-Al joints. It is found that the electrolyte conductivity has the largest effects on the material loss and needs to be managed closely for better corrosion control. This will help in designing and manufacturing joints with improved corrosion performance.

Investigation on microhardness and fatigue life in spot welding of quenching and partitioning 1180 steel

Quenching and partitioning (Q&P) steels are among the most critical third-generation steels for realizing vehicle weight reduction and improving safety performance. It is critical for the structure durability to have an excellent fatigue life of the welds. In this study, Q&P1180 steel was laser spot welded (LSW) in order to evaluate the microhardness, mechanical properties, and fatigue behavior of its joints, which were compared with resistance spot welding (RSW). In both types of joining methods, the welded zone is composed of lath martensite with high microhardness value. Due to the martensite tempering, the sub-critical heat affected zone of Q&P RSW welds was soften, resulting in a decrease of 52 HV of microhardness than the LSW joint. The presence of stress concentration in the boundary influenced tensile strength and all-welded samples failed in the boundary. Furthermore, the fatigue limit of the RSW joint (1.139 kN) was about 91.5%t of the LSW joint (1.245 kN). Fatigue specimens of the LSW joint failed between the fusion zone and heat-affected zone because of their higher sensitivity to stress concentration than the RSW joint, resulting in double eyebrow failure and pull-out failure. Fatigue crack originated from the specimen surface and propagated through fatigue striations together with secondary cracks. In addition, it was found that the strength of the base metal has inconsequentially affected the fatigue life of the welds when these welds were produced by the same welding method and have similar geometries.

Robustness and reliability assessment of Single-Sided Spot Welding as a process for sheet to closed profile joining for Body in White vehicle structures

The application of high performance steels in combination with rolled profiles, hydro-formed or tailored tubes (TT) can create a stiffer and safer vehicle structure. However, it presents some challenges regarding the joining process, once accessibility to both sides of the surface to be joined is limited. The Single-Sided Spot Welding (SSSW) is an alternative to Resistance Spot Welding (RSW), as it only requires access to just one side of the joint to weld it. In order to validate SSSW as a robust and reliable welding process for automotive manufacturing, this work encompassed experiments comparing SSSW and RSW under manufacturing conditions and tolerance problems, such as sheets and electrode misalignment, gaps or shunt current conditions, using DX54D steel sheets, and steel profiles made of press-hardened 22MnB5 and S355J2 + N. The experimental parameters in this study were current, electrode force, geometry and misalignment angle. The performed tests were lap shear tensile strength, analyses of weld nugget quality and the measurement of spot diameters by chisel testing. The obtained results show that SSSW can meet RSW requirements regarding mechanical properties and quality standards, overcoming the set manufacturing tolerance issues, without reducing the spot diameter and with similar mechanical properties of RSW joints.

Development of Scanning Acoustic Microscopy System for Evaluating the Resistance Spot Welding Quality

ABSTRACT Scanning acoustic microscopy (SAM) system (TSAM-400) was designed and developed for evaluating the quality of resistance spot welding (RSW) joints of 1.5 mm-thick SUS 316 stainless steel sheets under different welding parameters. Indentation and nugget information (diameter and shape) were used for evaluating the RSW quality. SAM A-scan signals were used to calculate the indentation. The nugget diameter was determined in the C-scan image. Internal information (nugget shape, splashes, and defects) was visualized in the B- and C-scan images. In addition, the spot weld area was treated to examine the surface topography effect. The nugget diameters of treated and untreated areas were measured and compared to conclude that the surface topography effect can be neglected in the quality evaluation by SAM system. By implementing TSAM-400, some specimens were successfully tested with a scanning area of 55 × 15 mm2, and a scanning time of 33.5 s corresponding to a B-scan frame rate of 4.5 Hz, and a resolution of 0.1 × 0.1 mm2. Finally, the good input parameters for welding two 1.5 mm-thick SUS 316 were determined by analyzing the results from TSAM-400.