Spot-welded Specimens Research Articles

Microstructure and mechanical properties of Al-Zn-Mg-Cu alloy joints welded by ultrasonic spot welding with soft specification

The high-strength lightweight Al-Zn-Mg-Cu alloy sheets were welded using ultrasonic spot welding with soft specification. The peak tensile shear failure load reached 7.3 kN, which satisfies the requirements of AWS D17.2 for spot-welded specimens. This is due to the fact that the material undergoes sufficient plastic deformation during the welding process with soft specification, thereby improving metallurgical bonding. Moreover, the dynamic recrystallization resulting from plastic deformation makes the grain size of the interface significantly refined. The grains with a size of 10 μm in the base metal are transformed into recrystallized grains with a size of 2 μm at most, which is in favor of improving the mechanical properties of the joint. Finally, even under soft specification conditions, excessive welding time can lead to macroscopic damage to the joint, initiation of cracks, and destruction of the grain boundary structure.

Effect of welding current on the microstructural evolution and lap-shear performance of resistance spot-welded 340BH steel

In this study, we used experimental characterizations and finite element analysis (FEA) techniques to investigate the effect that welding current exerts on the microstructural features and mechanical properties of bake-hardened (BH) steels. Resistance spot welding (RSW) experiments were conducted using 5.0 and 7.0 kA of welding current. The microstructure across the weld regions was analyzed using optical microscopy (OM) and electron backscatter diffraction (EBSD). FEA was employed to simulate the RSW process, predict nugget dimensions, and determine cooling rates. The results of the OM analysis showed that nugget size increased with an increase in the welding current, and EBSD characterization revealed similar microstructures under both levels of current, but significant variations were observed within weld zones. Lath martensite formed as a result of rapid cooling in the fusion zone (FZ). Whereas, fine and coarse grains were found in the heat-affected zone (HAZ). In addition, the formation of lath martensite in the FZ resulted in high hardness on a 2D map of microhardness. The welding current had no substantial effect on microhardness, but it did impact the mechanical properties under lap-shear test conditions, which resulted in significant changes in the fracture mode. The deformation and fracture behavior of spot-welded specimens was studied using a lap-shear test coupled with 3D digital image correlation (DIC). The experimental results indicated a fracture in the 5.0 kA current samples near the FZ/HAZ boundary, while 7.0 kA welding current resulted in base metal (BM) necking and tensile fracture. FEA was also employed to elucidate the local deformation properties and fracture modes during lap-shear testing. The results of this study show that nugget size has no effect on a failure site during lap-shear loading, but size does affect the nugget load-carrying capabilities. The failure behavior was influenced by the microstructural properties of the distinct weld areas.

Investigations of Microstructure and Mechanical Properties of Lap Jointed Dissimilar Metals by Friction Stir Spot Welding Process

The joining of different materials is required for industrial application to utilize the hydride structures with attractive advantages such as superior strength to weight ratio, low cost, high tensile strength and less weight of the component. Specifically, Al-Cu composite material is widely used in foil conductors of a transformer, electrical connectors, foil windings in capacitors and tubes in heat exchangers. However, joining or spot welding the Al parts to Cu parts are significant challenge owing to variation in mechanical properties and chemical compositions of joining materials. In this present research work, the friction stir spot welded process (FSSWP) is carried out to join the Al and Cu materials. Further, microstructure and mechanical properties of friction stir spot welded specimens (FSSW) are studied at different tool rotational speeds likely from 1000rpm to 1500rpm. The microstructural study is carried out using scanning electron microscope images at the interface and overall welded region. The tensile strength of both single and double spot-welded specimens is analyzed using a universal tensile test machine. The output of this study states that the optimal tool rotational speed is 1500rpm for both single and double spot-welded specimens. Moreover, the double spot-welded specimen exhibits more tensile with a crack-free spot-welded surface than that of the single spot-welded specimen. The tensile strength double spot-welded specimen has a 6.8% higher strength than that of a single spot-welded specimen. Based on the present study, it is concluded that the double spot-welded specimen can be used for different industrial applications to replace Cu material with this Al-Cu material that gives added advantages to those components.

Fatigue Life Estimation Using Single Spot-Welded Joint Fracture Mechanics

The influence of a spot-welded joint on the overlap section of the specimen, which is critical for spot welded joints subjected to uniaxial in plane loads, on fatigue life is explored in this study. This study was carried out using the ANSYS finite element program. The fatigue life properties of spot-welded specimens with various gap values are numerically calculated using a created computer program and several fatigue-life techniques to see how the gap affects the fatigue life of the spot-welded specimens. With rising gap values, it is noticed that maximum stress and strain values increase, resulting in a loss in fatigue life. Maximum stresses and strains, and hence fatigue-life values, are shown to be relatively constant for gap values less than 0.0025mm.

Rapid Evaluation of Hydrogen Embrittlement Resistance for Spot-Welds of High Tensile Strength Steel Sheet by Slow Rate Tensile Shear Test under Hydrogen Charging Conditions

Automobile manufacturers are accelerating adoption of spot welding of Advanced High-Strength-Steels (AHSS) sheets to reduce weight of automobile bodies. Rapid evaluation of the hydrogen embrittlement (HE) resistance for the spot-welds of AHSS sheets is required, since it is worried that the HE resistance of the nugget will deteriorate compared to the base metal due to the difference in microstructure caused by rapid cooling and solidification during spot welding. However, evaluation of the HE resistance for the spot-welds has not been established. In this study, we prepared spot-welded specimens using AHSS sheets and performed tensile shear tests with varying tensile rates under hydrogen charging to evaluate the relationship between diffusible hydrogen content and tensile shear strength. As a result, the tensile shear strength of spot welds decreased as the amount of diffusible hydrogen increased. The quasi-cleavage fracture surface and intergranular fracture surface were observed at the nugget and inside the crack generated at the nugget-heat affected zone interface. Furthermore, as the results of crack growth behavior and hydrogen thermal desorption spectroscopy analysis, hydrogen embrittlement in spot welds can be attributed to the stress-induced diffusion of hydrogen and the hydrogen trapped in dislocation and vacancy clusters at crack tips. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 75–83.

Experimental study on fatigue performance of resistance spot-welded sheet metals

Resistance spot welding is used as a reliable joining process in many engineering applications because of its effectiveness, automation capability, and low cost. The spot-welded sheet metals, on the other hand, are prone to mechanical fatigue failure, especially under cyclic loadings. Therefore, understanding and elucidating the fatigue phenomenon of the spot-welded joints are crucial in terms of estimating and preventing undesired failure conditions. In the design phase, there exist a considerable amount of challenges to overcome; one of the most important challenges is to select optimum working conditions. Hence, in this study, the fatigue phenomenon of the spot-welded sheet metals is investigated experimentally, by taking electrode force into consideration. For this purpose, spot-welded modified tensile shear (MTS) test specimens were utilized. A series of fatigue life tests were conducted to examine the influence of electrode force on fatigue life. The results obtained through an optical microscope were presented and interpreted. Experimental data showed that the number of cycles to failure changes depending on the spot-generating schemes in terms of electrode force and welding schedules. Through the investigation of an optical micrograph of partially failed spot-welded MTS specimens for different groups of spot welds created under different electrode force effects, it is seen that the fatigue failure is dominated by the through-thickness cracking. Comparing both crack formation and also fatigue lives of different groups of spot-welded MTS specimens, it is shown that the electrode force and accordingly thermal interaction play an important role in the fatigue strength of the spot-welded specimens.

An experimental and numerical investigation on impact spot welding of metallic plates using gas mixture detonation technique

In this paper, the impact spot welding of metallic plates was investigated both experimentally and numerically using a single-stage gas mixture detonation apparatus. The impact spot welding process was carried out on aluminum alloy and steel materials using rigid steel projectiles. In this process, the mixture of oxygen and acetylene was detonated in a combustion chamber to launch the projectile. The masses of flat- and spherical-nosed projectiles were 270 and 230 g, respectively. The impact velocity was measured in all experiments. The cross-sections of the weld spots were inspired by a scanning electron microscope to assess the quality of welding. For several experiments, wavy interfaces were observed showing there is a good bonding. For numerical simulation of the process, Abaqus/Explicit software was used and the deformation and failure mechanisms of impact spot-welded specimens were further investigated. The Johnson–Cook thermoplasticity model along with its failure model was utilized to predict the behavior of metallic materials. The numerical simulation results were in good agreement with those obtained from experiments in terms of the deformation mode and failure pattern. The propagation of the wave on the surface of the flyer plate was further studied. The results showed that the stress waves start from the center and propagate to the corners of the plate. To numerically evaluate the welding quality, two parameters of the shear stress at the collision point as well as the equivalent plastic strain for the flyer and target plates were obtained in the numerical simulation. The numerical results showed opposite directions of shear stress for flyer and base plates at the contact point, which can be used as proof for good bonding. Besides, the magnitudes of the equivalent plastic strain for both flyer and base plates were higher than those reported values in the open literature that confirms successful welding.

Simulation of vehicle body spot weld failures due to fatigue by considering road roughness and vehicle velocity

Durability of the vehicle components needs special attention in the design step due to this fact that the loads on a vehicle are dynamic by their nature. Also, fatigue resistance of the vehicle body is quite important as it is the main load-bearing component among others. The main purpose of the present research is to simulate the spot weld failures of the vehicle body structure due to fatigue damage induced on the body during standardized maneuvers. This was accomplished by using a combination of multi-body dynamics and finite element analyses. To enhance the precision of the analysis, a thickness-dependent nugget diameter was utilized to model the spot welds. To validate the finite element simulations, three-sheet spot-welded specimens were tested and their fatigue lives were compared to numerical results. The findings of this study revealed that the damage to the vehicle body is not always proportional to the vehicle velocity and that depends on the degree of road roughness. Also, the results of simulations showed that the damage to the front part of the body is almost twice the rear part in terms of the number of spot weld failures. The effect of nugget diameter on the fatigue life of the spot welds was also investigated and it was found that the nugget diameter can change the fatigue life of the spot welds up to almost 100%.

Structural Stress\u2013Fatigue Life Curve Improvement of Spot Welding Based on Quasi-Newton Method

∆F-N curves are usually used to predict the fatigue life of spot welding in engineering, but they are time-consuming and laborious and not universal. For the purpose of predicting the fatigue life of spot welding accurately and efficiently, tensile–shear fatigue tests were conducted to obtain the fatigue life of spot-welded specimens with different sheet thicknesses combinations. These specimens were simulated by using the finite element method, and the structural stress was theoretically calculated. In the double logarithmic coordinate system, the structural stress–fatigue life (S–N) curve of spot welding was fitted by the least-squares method, based on the quasi-Newton method. The square of the correlation coefficient of the S-N curve was taken as the optimization objective, with the correction coefficients of force, bending moment, spot welding diameter, and sheet thickness as the variables. During the optimization process, three different ways were utilized to get three optimized spot welding S–N curves, which are suitable for different situations. The results show that the fitting effect of the S–N curve is improved, the data points are more compact, and the optimization effect is significant. These S–N curves can be used to predict the fatigue life, which provide the basis for practical engineering application.

Resistance Spot Welding of Dissimilar Interstitial-Free and High-Strength Low-Alloy Steels

The primary aim of this investigation is to examine the resistance spot weldability of dissimilar interstitial-free (IF) and high-strength low-alloy (HSLA) steels. The effect of dynamic contact resistance on the nugget diameter is examined. The mechanical properties of the spot-welded specimens are investigated in both tensile shear and coach peel configurations. These experiments are supplemented by macro- and microstructural examinations, determination of microhardness profiles, and post-failure examinations, including fractography. It is observed that the nugget diameter increases as mean dynamic contact resistance decreases. The heat-affected zone of the HSLA side of the weld joint shows the maximum hardness, whereas the base metal of the IF side shows the minimum hardness. The hardness at the fusion zone of the dissimilar HSLA–IF joint lies in between that of fusion zone of similar HSLA–HSLA and IF–IF joints because of the homogenization of the chemistry of both the grades. The load carrying ability of the IF–HSLA joint is found to be closer to that of IF–IF joint but significantly lower than that of HSLA–HSLA joint; this is because the location of failure for IF–HSLA and IF–IF joint is same, i.e., the HAZ/base metal interface at IF side, while that of HSLA–HSLA joint is the base metal of HSLA steel.

Applications of ultrasonic testing and machine learning methods to predict the static & fatigue behavior of spot-welded joints

Ultrasonic Testing (UT) is one of the well-known Non-Destructive Techniques (NDT) of spot-weld inspection in the advanced industries, especially in automotive industry. However, the relationship between the UT results and strength of the spot-welded joints subjected to various loading conditions is unknown. The main purpose of this research is to present an integrated search system as a new approach for assessment of tensile strength and fatigue behavior of the spot-welded joints. To this end, Resistance Spot Weld (RSW) specimens of three-sheets were made of different types of low carbon steel. Afterward, the ultrasonic tests were carried out and the pulse-echo data of each sample were extracted utilizing Image Processing Technique (IPT). Several experiments (tensile and axial fatigue tests) were performed to study the mechanical properties of RSW joints of multiple sheets. The novel approach of the present research is to provide a new methodology for static strength and fatigue life assessment of three-sheets RSW joints based on the UT results by utilizing Artificial Neural Network (ANN) simulation. Next, Genetic Algorithm (GA) was used to optimize the structure of ANN. This approach helps to decrease the number of tests and the cost of performing destructive tests with appropriate reliability.

The Study of Local Stress Intensity Factors for Kinked (and Branched) Cracks in Commonly Used Spot Weld Specimens

The Study of Local Stress Intensity Factors for Kinked (and Branched) Cracks in Commonly Used Spot Weld Specimens

Friction Stir Spot Vibration Welding: Improving the Microstructure and Mechanical Properties of Al5083 Joint

In this study, a modified version of friction stir spot welding (FSSW) is applied to join Al5083 specimens. In respect to conventional FSSW, this new method leads to better characteristics, finer grain sizes in the stir zone and higher mechanical properties. In this method, the workpiece is vibrated normal to tool axis direction during FSSW. This process is entitled friction stir spot vibration welding (FSSVW). The finite element method (FEM) was used to validate the experimental results. The FEM and experimental results had a good agreement. The microstructure of the welded zone was analyzed by scanning electron microscopy and optical microscopy. The results showed that the presence of vibration during FSSW led to more grain refinement. This was related to more straining of material in the welded zone which enhanced the dynamic recovery and recrystallization and increased the grain refinement. The results showed that grain size of welded region for friction stir spot-welded specimen was about 35% lower than that for friction stir spot-welded specimen. Mechanical properties such as tensile shear load and hardness increased as the vibration was applied. Also, mechanical properties increased as vibration frequency increased from 28 to 38 Hz during FSSVW.

Effects of HAZ softening on the strength and elongation of resistance spot-welded joints in high-strength steel sheet in an in-plane tensile test

ABSTRACTIn this study, we investigated the effects of heat-affected zone (HAZ) softening on the strength and elongation of resistance spot-welded joints in high-strength steel sheet in an in-plane tensile test. The fracture in the softened HAZ had a little effect on the maximum stress of the resistance spot-welded specimen; however, the fracture elongation decreased. The nugget diameter and HAZ softened width had little effect on the fracture elongation of the resistance spot-welded specimen. Also, the fracture elongation decreased slightly with the decrease in the sheet thickness. The major factor affecting the fracture elongation was the HAZ hardness ratio (= Softened HAZ/Base metal × 100%). For the resistance spot-welded specimen with a thickness of 1.6 mm, when the HAZ hardness ratio decreased to less than 80%, the fracture position changed from the base metal to the softened HAZ and the fracture elongation decreased sharply. In addition, with a decrease in the hardness ratio, the fracture elongation decreased.

Experimental and numerical investigation of the overload effect on fatigue behaviour of spot-welded steel sheets

Spot-weld joints are commonly used to fasten together metal sheets in automotive industry. The car frame used in Renault vehicles is a representative example of the usefulness of this method. Most of the spot-welds experience fatigue damaging occurrence due to rough roads or driving conditions which apply periodical overloads to the vehicle. Understanding their fatigue behaviour is crucial from the viewpoint of failure prevention in design. In this study, a series of experiments was conducted to study the fatigue failure of spot-welded tensile-shear specimens made of a deep-drawing steel (XES) and High strength low alloy steels (HE360D and XE360D). Two different types of fatigue tests were performed, the first one with a constant-amplitude sinusoidal loading (loading ratio equal to 0.1) and the second one with one incidental overload cycle introduced per 100 cycles. The experimental results show a favourable effect of overloads for HSLA steel specimens, whereas the effect is the opposite for XES steel specimens. A finite element analysis was carried out using the open-source Salome-Meca platform to determine the stress states within the specimens around the weld spot and explains both failure modes observed on the specimens at high and low loads.

Material Characterization of Dissimilar Friction Stir Spot Welded Aluminium and Copper Alloy

In this research study, material characterization of dissimilar friction stir spot welded Aluminium and Copper was evaluated. Rotational speeds of 800 rpm and transverse speeds of 50 mm/min, 150 mm/min and 250 mm/min were used. The total numbers of samples evaluated were nine altogether. The spot welds were characterised by microstructure characterization using optical microscope (OEM) and scanning electron microscopy technique (SEM) by observing the evolution of the microstructure across the weld’s cross-section. lap-shear test of the of the spot weld specimens were also done. From the results, it shows that welding of metals and alloys using Friction stir spot welding is appropriate and can be use in industrial applications.

Fatigue analysis of spot-welded automobile components considering fatigue damage-induced stiffness degradation in time and frequency domains

In this study, fatigue analysis was performed on a multi-point spot-welded outrigger structure utilized in an automobile engine room, where stiffness degradation from accumulated fatigue damage were considered. The S-N curve was obtained through a fatigue test on a single-point tensile-shear spot-welded specimen under constant amplitude load and finite element analysis based on the Rupp/LBF model. Fatigue analyses were performed in the time and frequency domains on the spot-welded outrigger structure, where changes in the local stress distribution and transfer function due to accumulated fatigue damage to the welding points were considered. Based on these results, it can be stated that the fatigue life of multi-point spot-welded structure under random loading should be evaluated considering the stiffness changes due to accumulated fatigue damage under the frequency domain.

Mechanical behaviour of thermoplastic composites spot-welded and mechanically fastened joints: A preliminary comparison

The in-plane and out-of-plane mechanical behaviour of both ultrasonically spot-welded and mechanically fastened joints was investigated by double-lap shear and pull-through tests, respectively. Spot-welded specimens showed comparable onset failure load and significantly higher joint stiffness compared to mechanical fasteners when carrying shear load. The failure modes and the damage within specimens were analysed after mechanical tests. Intralaminar failure and very limited damage on the out-most ply were found for welded specimens, whereas catastrophic through-the-thickness failure was observed for mechanically fastened joints. Based on the experimental outcomes, the mechanical performance and failure mechanisms of spot-welded joints were critically assessed in comparison to the mechanical fasteners .

Welding Current Dependence of Dynamic Mechanical Properties for Spot-Welded Electric Arc Furnace Steel Sheet

In 2012, a novel production line was developed to produce electric arc furnace steel sheet for automotive structure, as a national project in Japan. From the viewpoint of crashworthiness, the evaluation of joint strength under dynamic loading is essential. In this study, single-lap spot-welded joint specimens are prepared with the variation of welding current. The specimen sheet is 980-MPa grade electric arc furnace steel and 1.0 mm in thickness, which is supplied by Tokyo Steel Co., Ltd. Nugget diameter is recommended by a well-known empirical equation, , where is sheet thickness. The nugget diameter can be adjusted mainly by welding current. In order to find the optimum nugget diameter under dynamic tensile-shear, five values of welding current, 6.1, 6.4, 6.7, 7.0 and 7.3 kA are tried. The values are set for the recommended nugget diameter 5.00 mm, and ±5% and ±10% of the recommended diameter. Experimental results of quasi-static (loading velocity: 2.0 mm/min = 3.3×10-5 m/s) and dynamic (initial loading velocity: 7 m/s) tensile-shear tests shows that, despite of loading velocity and welding current (equivalent to nugget diameter), tensile-shear strengths are almost constant. Absorbed energies up to the strength give complicated tendencies. At the recommended welding current (equivalent to mm), the most stable strengths and energies are obtained.

Postcrack Propagation of Tensile Shear Test: Analyzing the Carbon Steel Welds, Stainless Steel Welds, and Both Steels Mixed Welds in Resistance Spot Welding

Conducting tensile shear test for the spot-welded specimens is a common practice in resistance spot welding as it helps to understand the loading behavior and its corresponding effect. Two categories of weld failures are conventionally regarded, as to distinguish the sound weld over a poor one. This approach has severe drawback from early days and therefore the postcrack propagation (PCP) method is carried out as an alternative in this experimental study. With the PCP experimental results in hand, the tensile shear test is simulated using ANSYS to observe the corresponding strain distribution. Based on the visual inspection of simulation and practical results, three classifications (IF, PF, and TF) are consequently categorized; so that the weld failures of tensile shearing test can now be accurately distinguished by means of cracking patterns. Moreover, some degrees of fractures have regularly been noticed by other fellow researchers within these three categories and thereby the fractures have been named after the degrees of cracking severity. Carbon steel welds, partially corroded carbon steel welds, stainless steel welds, and mixed steel welds have been parallelly analyzed for the PCP modes and the results obeyed the categorization very well.