Spot Welding Research Articles

Influence of heat input on pinless friction stir spot welding of aluminum‑copper dissimilar materials

The composite structures of aluminum/copper dissimilar materials hold significant practical value and potential in advanced technology and industrial applications. This study conducted lap welding experiments on 2 mm thick T2 copper and 1060 pure aluminum rods using the pinless friction stir spot welding process. By varying the dwelling time of the tool, the evolution behavior of the CuAl interface layer and the mechanical properties of the joints were analyzed. At low dwelling times, the heat input was relatively low, resulting in a thinner IMC layer and the best mechanical properties of the joints, with a pull-out force of 2238.2 N and a tensile strength of 28.49 MPa. The fracture modes of the joints under all parameters were brittle fractures. The connection of aluminum/copper dissimilar materials involves two processes: firstly, the diffusion bonding dominated by kinetics at the initial welding stage; and secondly, the solid-liquid instantaneous bonding dominated by thermodynamics. When the welding temperature exceeds the melting point of aluminum, the aluminum/copper interface continuously undergoes transformation-bonding-retransformation processes until the welding process is completed.

Friction Stir Spot Weldability of AA7075-T6 Sheets with a Pinless Tool Providing Enhanced Stirring Effect

Friction Stir Spot Weldability of AA7075-T6 Sheets with a Pinless Tool Providing Enhanced Stirring Effect

Investigation on the mechanical uncertainty of resistance spot weldings and its effect on structure crashworthiness of automotive B-pillars under side impacts

Resistance spot welding (RSW) is the main connection method for the white body structure of automobiles. Understanding the mechanical uncertainty of RSWs and its effect on structure crashworthiness design is of great significance for vehicle safety and lightweight design. In this work, the mechanical uncertainty of RSW was experimentally investigated and its effect on automotive B-pillar crashworthiness under side impacts was numerically analyzed based on a hybrid response surface model (HRSM). Firstly, the mechanical uncertainty of RSWs and associated failure mechanism were investigated and the fluctuation range of RSW strength under different loading conditions were quantified according to experimental results. Then, a reasonable RSW modeling method and subsystem collision simulation approach were demonstrated and the univariate sensitivity analysis was carried out to reveal the effect of RSWs’ mechanical property on B-pillar crashworthiness. Finally, a HRSM integrating Kriging polynomial and deep neural network (DNN) were proposed as surrogate model, based on which the multi-factor analysis was conducted to explore effects of RSW strength uncertainty on structure crashworthiness design.

Modeling and optimization of aluminum–steel refill friction stir spot welding based on backpropagation neural network

Modeling and optimization of aluminum–steel refill friction stir spot welding based on backpropagation neural network

Parameters influence on mechanical properties of resistance spot welding: AISI304L/AISI1005

AbstractWelding dissimilar metals by resistance spot welding offers several advantages, including lighter weight and better mechanical properties for the shipping, aerospace, and automotive industries. The resistance spot welding parameters effects (welding current, squeeze time, welding time, and hold time) on the welding results of AISI 1005 and AISI 304L were investigated in the present research. The joints' tensile shear force, microhardness, and microstructure were examined. The Taguchi design of experiments approach was utilized to analyze the tensile shear force results statistically. A welding current of 7 kA, welding time of 1.4 s, squeeze time of 1.2 s, and hold time of 0.8 s resulted in an optimum tensile shear force of 6.194 kN. Additionally, the nugget zone demonstrated a higher hardness compared to the base metal and heat-affected zones.

Bonding strength of short carbon fiber reinforced PA6 composites ultrasonic welding joints

AbstractShort carbon fiber composites have high specific modulus, high specific strength and excellent formability, which are suitable for short‐term and environmentally friendly ultrasonic welding. In this study, ultrasonic welding experiments were conducted on short carbon fiber reinforced PA6 composites using a servo‐driven non‐metallic ultrasonic welder. The weld specimens were analyzed for spot weld morphologies, interfacial bonding strength, failure modes and compressive shear fracture to establish the correlation between weld energy and weld quality. The results indicate that when the welding energy is 450 J, the short carbon fibers are interlaced at the interface, which has a pinning effect, thereby enhancing the joint strength reaching 2990 N, and the welding efficiency reaches 46%. Besides, the compressive shear section is divided into regions. The proportion of heat and area in different compressive shear regions are quantitatively analyzed, and the calculation model of joint strength is proposed. Meanwhile, the correlation between welding energy and its correlation, as well as three different shear failure modes, is obtained. This work is helpful for the optimization of carbon fiber reinforced thermoplastic (CFRTP) ultrasonic welding process parameters and the evaluation of joint strength.Highlights A special fixture for ultrasonic welding was designed to optimize the welding energy process of ultrasonic welding CFRTP, and the properties of ultrasonic welded joints were analyzed. When the welding energy is 450 J, a high shear strength welded joint of 2990 N was successfully prepared. The shear fracture of the welded joint was successfully obtained by designing a special clamp for compression shearing. The welding area was qualitatively divided according to the temperature measurement results of the interface center. In addition, the heat and area ratio of different zones in the compression‐shearing area under different welding energy were quantitatively analyzed, and the calculation model of joint strength was proposed. At the same time, the welding energy and its correlation, as well as three different shear failure modes were obtained. The short fiber distribution characterization and pores effect on the joint's strength were analyzed. The appearance of porosity defects has a significant weakening effect on the welded joint.

The influence of modification of the geometry of the front surface of the RFSSW tool inner sleeve on the fatigue life of joints during joining clad sheets made of aluminum alloy 2024-T3

AbstractRefill Friction Stir Spot Welding (RFSSW) has a number of advantages that make it a possible alternative to riveting and resistance welding in aerospace structures, the automotive industry and other applications. Adequate determination of technological parameters which ensure the desired properties of welds and their functioning in various operating conditions requires, among others, appropriate fatigue life of connections. The article presents the results of comparative tests of the mechanical properties of welds (load-bearing capacity and fatigue life at selected three load levels) made with a basic tool (G0) and a tool with a modified geometry (G4). The samples were made of 1.27 mm thick clad sheets of 2024-T3 aluminum alloy with an additional oxide anodic coating. It has been shown that the modified geometry of the working surface of the inner sleeve of the RFSSW tool improves the conditions and course of the plasticization and stirring process of the joined materials. The use of a G4 geometry tool allowed for approximately 30% higher joint load-bearing capacity and approximately twice as long fatigue life (at lower load levels) compared to welds made with the G0 tool.

Understanding interfacial failure mechanisms of advanced high strength automotive steels resistance spot welds under opening-mode loading

This paper addresses interfacial failure (IF) of automotive steel resistance spot welds under opening mode loading (i.e., Mode I), simulated through the cross-tension test. Two modeling approaches were developed to predict cross-tension strength (CTS) during IF: one based on structural stress and the other on fracture mechanics. The structural stress model correlates CTS with the hardness of the fusion zone, while the fracture mechanics model links CTS to the fusion zone’s toughness. Our evaluation of these models across various automotive steels reveals that the hardness-based model is suitable for steels with fusion zone hardness below 350 HV, where IF typically results in ductile fracture. In contrast, for higher-grade steels with fusion zone hardness exceeding 400 HV, the toughness-based model is more appropriate, as IF occurs through brittle crack propagation. The transition in the interfacial failure mechanism of spot welds under Mode I loading—from ductile tensile failure to brittle crack propagation—is governed by the ratio of toughness and hardness of the fusion zone. These insights not only refine the prediction of CTS but also inform the design of effective post-weld heat treatments aimed at enhancing CTS without sacrificing strength under shear loading mode particularly through martensite refining strategy.

Challenges and advances in resistance spot welding of ultra-high strength hot-stamped steels: Coating effect, softening phenomena, and mechanical performance

Challenges and advances in resistance spot welding of ultra-high strength hot-stamped steels: Coating effect, softening phenomena, and mechanical performance

Resistance of the Steel Lap Joints Connected by Spot Welding and Brazing

Welding technologies are constantly evolving, and their applicability is expanding, considering also the construction domain. Steel structures benefit from the advances in welding technologies due to the use of thin gauge steel sheets which can now be connected at high quality and automatically. The resistance of the connection between the thin steel sheets is crucial for the durability and safety of a structure made of built-up thin-walled cold-formed steel elements. Generally, self-drilling screws or bolts are used for the connection between thin-walled elements, but the quantity of time and manpower necessary for a large number of connections demands an improved solution. Conventional welding techniques are unsuitable for joining thin sheets, ranging from 0.4 to 1.0 mm to thicker ones measuring 1.0 to 3.5 mm. This article compares the results of an experimental investigation into lap joints connected by spot welding and MIG brazing with the design code provisions. The study examines single-lap joints in steel sheets of 0.8, 1.0, 1.2, 1.5, 2.0 and 2.5 mm thickness, connected using these welding technologies. The results obtained are then compared with analytical relations and processed according to EN 1990. Depending on the thickness of the connected sheets, spot welding can lead to two failure modes: button pull-out fracture and interfacial fracture. MIG brazing, a welding technology that deposits material below the melting point of the base material, is known for its advantages, such as low energy consumption, spatter-free operation, high welding speed, and compatibility with thin sheet metals. However, its application in the structural engineering of cold-formed elements lacks documentation. In the study, the MIG brazed specimens failed in the heat-affected zone of the connection. The results indicate the dependence of the spot weld lap joint resistance on the connected sheets' thickness, while the resistance of the MIG brazed lap joints is influenced by the minimum thickness of the connected steel sheet. The study aims to demonstrate the feasibility of the proposed solutions, evaluate their performance, and establish the limits of their applicability. A statistical interpretation of the results highlights the precision and reliability of joint resistance.

Physical Heat Cycle Measurement of Resistance Spot Welding

Resistance spot welding (RSW) is still the ideal joining method in the automotive industry. Mostly steel sheets are used in the car body, so overlap and layering are required for welding or riveting, as spot welding provides simultaneous clamping force with interfacial welding to ensure the required strength and quality. A fundamental understanding of heating and cooling rates in thermal distributions is essential for predicting microstructure formation in the weld and the heat-affected zones (HAZ) of RSW joints. The ability to measure the heat cycle in the RSW process can be valuable in weld control and welding parameter optimization. RSW parameters can be optimized through tensile shear tests and microscopic investigations. Heat cycle measurement (HCM) demonstrates the welding consequences in terms of the change in mechanical properties and microstructural formations. The accuracy of cooling rate measurements including t8/5 cooling time is very important to predict the microstructural evolution in the HAZ, however, the thermocouple measurement raises numerous challenges due to the high temperature gradient and small weld and HAZ size. During our investigations heat cycle measurement has been conducted experimentally by a K-type thermocouple. The data logger is connected to the output of the thermocouple for recording the voltage to measure the temperature distributions as a function of both time and position during the welding process. Measurement results of 1 mm thick martensitic MS1400 steel overlapped RSW joints are discussed, and the HCM curve of heating and cooling rates of the spot-welding process is presented. The heat cycle during RSW was measured with two different welding parameter combinations. In addition to welding current, welding time, and electrode force, pulsation has shown disparate curves. Numerous experiments have been attempted to measure the heat cycle in HAZ sub-zones due to the difficulty of positioning the thermocouple accurately, uppercritical HAZ, intercritical HAZ, and subcritical HAZ were investigated and measured in both welding parameter combinations. Difficulties were encountered in the experimental work as a result of the instantaneous welding time and the vibration resulting from the passage of alternating electrical current between the two electrodes. A magnetic field is generated that affects the thermocouple measurement and appears as a noisy curve that is filtered out and smoothed. Joule heat, interfacial heat generation, and cooling effects of electrodes are also considered in the experiment.

Welding of Toroidal Cores Made out of Amorphous Alloy Using Capacitor Discharge Spud Welding Technology (CD Welding)

Amorphous alloys exhibit exceptional characteristics not typically associated with other known classes of materials. These alloys are industrially manufactured in the form of ribbons with thicknesses below 50 μm, which presents challenges in their welding process. Resistance spot welding utilizing energy stored in capacitors is employed for thin sheets or foils due to it’s precise energy control during capacitor discharge. This paper presents the findings from experimental research conducted on spot welding metallic amorphous ribbons with a thickness of 30 μm using stored energy in capacitors, aiming to establish effective welding technologies. Several tests with varying parameters were conducted to optimize the welding process and determine the most effective technology for this foil.

Microstructural Investigation and Mechanical Properties of Resistance Spot Welding Joints of Mild Steel Sheets

Due to its high productivity, low cost, and increased work efficiency, resistance spot welding (RSW) can instantly join two or more steel sheets and it is widely used in the automotive industry and lately in the manufacturing of thin-walled cold-formed steel constructions. However, the RSW of zinc-coated mild steel sheets presents metallurgical challenges, especially when different thickness combinations are used. Therefore, in the present paper a resistance welding machine was used which uses direct current (MFDC) inverter technology combined with SMART AUTOSET technology to weld S250 GD and S350 GD galvanised mild steel sheets with different thicknesses. It is well known that, due to the elevated temperature that occurs during the welding process, followed by rapid cooling, defects such as cracking, porosity, lack of fusion, and an increased amount of brittle phases affect the welding quality. Therefore, the influence of the RSW process parameters established by an automatic sequence on the nugget geometry, microstructure, and mechanical properties was investigated. The phase transformations that took place during the heating-cooling cycle were analysed in detail through metallographic studies. The results showed that the microstructure of the weld nuggets was similar, characterised by columnar grains elongated in the direction of heat evacuation. Nevertheless, there were differences in terms of phase dispersion, defects and mechanical properties that have been linked to the RSW process parameters.

Comparative Study on the Liquid Metal Embrittlement Susceptibility of the High-Si Advanced High-Strength Steel with EG and GA Zn Coatings

The Zn-coated high-Si advanced high-strength steel (AHSS) tends to suffer Zn-assisted liquid metal embrittlement (LME) during the resistance spot welding (RSW) process. In this study, the LME behaviors of electrogalvanized (EG) and galvannealed (GA) high-Si steels were comparatively investigated. The maximum lengths of the LME cracks at the shoulder and center of the spot weld were approximately 366.6 μm and 1486.5 μm, respectively, for the EG yet 137.0 μm and 1533.3 μm, respectively, for the GA high-Si steels. Additionally, all EG and GA welded joints were etched to measure the nugget size. It was found that the increased welding current could aggravate the formation tendency of the LME cracks for both the EG and GA high-Si steels. Furthermore, the statistical results revealed that the electrogalvanized high-Si AHSS exhibited a relatively higher LME susceptibility than the galvannealed high-Si AHSS. It was deemed that the internal oxidation produced during the annealing before the Zn coating was the crucial factor that led to the difference in the LME susceptibilities for the EG and GA high-Si steels.

Characterization of physical metallurgy of quenching and partitioning steel in pulsed resistance spot welding: A simulation-aided study

This study mainly focuses on the microstructure evolution of QP980 steel during resistance spot welding and its influence on the mechanical performance of resistance spot welds which is a critical influencing factor on the quality of body-in-white at the service condition. It is observed that the thermomechanically engineered microstructure of QP980 steel changes to form metastable phases such as martensite in the fusion zone and the heat affected zone due to rapid cooling induced by the thermal cycle of the welding. A finite element modeling of the welding process was used to predict the weldment heat distribution, thermal history and microstructure evolution in different welding zones. The modeled thermal history of the weldments shows that the peak temperature in the four-pulse resistance spot welding is delayed because of pulsed welding conditions and holding times between the welding pulses. This heat management approach in pulsed welding prevents void formation. The modeled thermal history and rapid heating, and cooling conditions are discussed here to predict the microstructure evolution and transformation in the fusion and reheated zones. The modeled results were helpful in the prediction of the microstructure at different weld zones. Then the strategic links between the microstructure and mechanical performance of the welded alloy are discussed thoroughly. The microhardness profile of the weld is discussed from a microstructural point of view to disclose the physical metallurgy of the welds. Softening phenomena were not observed in the sub-critical heat affected zone.

Formation of the Interlock Morphology and Its Role in Refill Friction Stir Spot Welding of Aluminum Alloy to Steel

Considering energy conservation and emission reductions, lightweight automobiles have become a research focus in the automotive industry. Steel/aluminum joining is regarded as an ideal lightweight structure, which can not only reduce the energy consumption but also ensure safety and is already attracting extensive attention. In this study, aluminum alloy 6061 and B410LA steel sheets were successfully joined by refill friction stir spot welding. The tensile properties, microhardness distribution and interfacial microstructure characteristics of the steel/Al welded joints were investigated. The maximum tensile load of the steel/Al joint was 4.3 kN. The mechanical properties of the steel/Al refill friction stir spot welded joint were largely determined by the bonding quality of the sleeve-plunging zone. With the stirring of the sleeve and the pin during the refill friction stir spot welding, work hardening occurred in the stir zone (SZ). The microhardness of the SZ was significantly higher than that of the steel base metal (BM) and could be detected on the steel side. The Fe-Al intermetallic compound (IMC) layer was continuously distributed at the interface of the sleeve-plunging zone, revealing good uniformity in the thickness. In particular, a hook-and-vortex-like structure formed during the refill friction stir spot welding process in the sleeve-plunging zone, producing a mechanical interlock effect at the interface. The ideal mechanical properties of the welded joint could be attributed to the good quality of the metallurgical and mechanical bonding at the interface, especially the mechanical interlock effect, thereby depending on the hook-and-vortex-like structure.

Wear mechanisms and failure analysis of a tool used in refill friction stir spot welding of AA6061-T6

Investigating tool wear in refill friction stir spot welding (RFSSW) is essential for understanding its limitations and improving its efficiency. Increasing the tool's service life is important to push the technology's readiness level and transfer the technology from the laboratory to industrial applications. In this study, the quasi-static lap shear performance of AA6061-T6 similar welded spots was investigated and tool wear was continuously monitored until tool failure at 3450 welding cycles. Furthermore, the fundamentals of the wear mechanisms in RFSSW were further elucidated. The investigation shows that it is possible to achieve steady quasi-static lap shear performance of the spot welds over advancing tool wear by adjusting the heat input to compensate for losses in frictional heat generation efficiency (related to tool profile changes by abrasion). A subsequent tool failure case analysis showed the main causes for the continuous wear degradation of the shoulder. Tool wear was driven by plastic deformation of the hot-work tool steel and subsequent break-out of tool steel ridges, introducing big hard particles into the contact region between the moving and rotating tools. In addition, the formation and detachment of Fe-Al intermetallic compounds counteract with the rotating tools and increase tool wear.

Fatigue Failure of Alloy Grade P22 Hot Reheat Auxiliary Steam Pipe

A leak was discovered in the hot reheat auxiliary pipe. The pipe was found to have a bulging condition and a crack at the base metal area. A circumferential crack was discovered near a repair weld spot on the external surface. The failure was investigated using material characterization techniques, including in-situ replication, glow discharge spectroscopy (GDS), microhardness analysis, optical microscopy, and energy dispersive X-ray spectroscopy (EDX). Based on microstructure analysis, numerous cracks were observed to have initiated at the internal surfaces of the pipe. Most of the cracks were found on the parent metal. The cluster of cracks concentrates on the plane of 0⁰ - 180⁰. Microstructure analysis shows that the cracks are multiple, parallel, mainly transgranular, oxide-filled with branching crack tips. It is thought that the cracking is associated with fatigue and, most likely, high cyclic stress is the key contributory factor. The steam oxide cracked gradually as a result of the cyclic stress. As more metal surface was exposed, new oxide formed, causing cracks to propagate into the metal as this process was repeated due to cyclic loading.

Torsional Fatigue Performance of a Spot-Welded Structure: An XFEM Analysis

This study delves into the exploration of the fatigue performance of a structure that has been spot-welded and is being loaded with torsional fatigue. The extended finite element method (XFEM) was applied to simulate the intricate interaction of spot welds in response to cyclic loading. The developed model was validated through experiments. The influences of different parameters, such as the number of spot welds used to join the adherends, the diameters of the spot welds, and the load ratio applied, on the fatigue performance of the box were investigated. The first two parameters studied had a significant influence on the extent of the fatigue failure-affected spot welds, where the crack propagation rate can be decreased by more than 700%.

The effects of tool geometry on static strength and the failure mode in micro-friction stir spot welding

The effects of tool geometry on static strength and the failure mode in micro-friction stir spot welding