Spot-welded Joints Research Articles

Effects of the Applied Magnetic Field on the Quality of Resistance Spot Welding in Rail Vehicles Using Stainless-Steel

The ring magnet nested on the surface of a spot welding electrode is used as the external magnetic field source. We analyzed the effects of an applied magnetic field on the appearance quality of joints, the nugget diameter of resistance spot welding for rail vehicles using stainless steel through comparative resistance spot welding experiments. A coupled electrical-magnetic finite element model was established to analyze the effect of magnetic forces on the welding current direction. Result show that the appearance of a magnetically assisted spot welding joint was significantly better than that obtained by conventional spot welding. The nugget diameter of the welding spot was also significantly increased. The Lorentz force introduced by applied magnetic field deflects the path of electrons in welding electoral field, which makes nuggets with a larger radius and a lower depth.

Elucidation of fatigue characteristics and fracture mechanism of friction stir spot‐welded tension–shear joint steels

AbstractThe fatigue life and fracture mechanism of friction stir spot welded tension–shear joints using 590‐MPa class steel as a base material under constant‐amplitude conditions were investigated with focus on welding dimension variations caused by tool wear. The fatigue limit of the friction stir spot welding (FSSW) joint used for this study is significantly low compared with the static tensile strength of the joint itself. It was clarified that the FSSW joint in this study exhibited two different failure morphologies regardless of the applied load level: base metal fracture and weld area fracture. Although the welding state changes due to the tool wear phenomenon that produce two types of fracture modes in relation to the welding rip diameter, they have no effect on the fatigue strength, regardless of the applied load.

Tensile-Shear Mechanical Behaviors of Friction Stir Spot Weld and Adhesive Hybrid Joint: Experimental and Numerical Study

In this work, the tensile-shear mechanical behaviors of friction stir spot weld and adhesive hybrid joint were performed from both numerical and experimental viewpoints. Weld through (WT) and flow in (FI) processes were studied in this research. The focus was to evaluate joint defects, tensile-shear failure load (TSFL), failure energy, failure mode and stress distribution of the joint. In FI joints, keyhole and hook defects appeared in the weld zone and the areas of material removed from the base metal were filled with adhesive. In the WT joints, the adhesive layer close to the weld zone was carbonized due to the welding heat. Meanwhile, under the rotating movement of welding tool, the adhesive impurities entered the stirring zone (SZ) and heat affected zone (HAZ) of the weld, which decreased mechanical performances of WT joints. Compared to the friction stir spot welding (FSSW) joint, the TSFL value, stiffness and failure energy of FI joint were increased by 2.7, 1.1 and 8.14 times, respectively. In order to study the stress distribution of the joints, a finite element (FE) model, which considered the weld structure and mechanical properties of weld regions, was implemented. Moreover, the adhesive layer was simplified by the cohesive zone model (CZM). FE results show that the FI process effectively decreases the stress concentration of the weld edge from 243.09 to 15.5 MPa, under the 2 kN tensile load. The weld can block the adhesive crack propagation, and the adhesive optimized the stress distribution of FI joints through a synergistic effect. So, the use of FI process for aluminum alloy connection is strongly recommended, especially in crucial structure areas.

A Review of Technologies for Welding Magnesium Alloys to Steels

We survey various state-of-the-art methods for welding magnesium alloys and steels using different joint configurations. Microstructural characterizations indicate that four microstructures may form at the Mg/steel interface after welding: unwelded gap, metal oxides, solid solutions, or intermetallic compounds. Reaction products at the Mg/steel interface vary with different welding methods, alloying elements in base materials, interlayers or coatings applied, and preparations of base material before welding. Mechanical property characterizations, (a) lap tensile shear testing for lap-welded and spot-welded joints, (b) tensile testing for butt-welded joints and (c) fatigue properties of lap-welded and spot-welded joints are summarized and compared, separately. Reaction products at the Mg/steel interface are correlated with mechanical properties. Finally, ways to enhance Mg/steel joint strength, such as introducing interlocking features during friction stir lap and butt welding, are discussed.

Process analysis and regression modelling of resistance spot welded joints of austenitic stainless steel 304L and low carbon steel sheets by using surface response methodology

Evaluating the strength performance of spot-welded joints of dissimilar material is critical for their continued integration into the automobile and aerospace industries. The effect of weld joint strength is an important consideration in the design of weld structures. The objective of the present work undertaken to investigate the effect of the input process parameters on the strength of the welded joint of dissimilar material. Full factorial design (FFD) has used for designing the experiment matrix. Further, experimental results have used to develop a mathematical model to predict the strength of the spot weld joint. Analysis of Variance (ANOVA) has been applied to establish the correlation between the process parameters and their interaction on the output. The confirmation test case experiments have conducted for validating the developed mathematical model and observed that the developed model is capable of evaluating weld joint strength within the process parameters.

Correlating Finite Element Model of a Car Spot-welded Front-End Module in the Light of Modal Testing Data

Model updating methods can be adopted to improve the correlation level between the finite element model of a spot welded structure and its test model. However, in the presence of contact interfaces in the vicinity of the welded areas, improving the correlation level is problematic and challenging. An approach for correlating the finite element model of a welded structure with contact interfaces using finite element model updating and modal testing is proposed. The proposed approach was tested on a car front-end module structure that consisted of nine components and 76 resistance spot-welded joints used to assemble the components. CWELD and CELAS1 element connectors were used to represent the spot-welded joints and contact interfaces in the finite element modelling and updating. This approach was applied successfully to predict the modal parameters of the car spot-welded front-end module. The total error of the initial finite element model of the structure was reduced from 27.13% to 5.75%. The findings of this work suggest that the proposed approach has a great potential for use in investigating the dynamic behaviour of various spot-welded structures without a significant decline in accuracy.

Effect of multi pulse current treatment on microstructure and microhardness of resistance spot welded dual phase steel DP590

AbstractIn this paper, microhardness and microstructural characteristics of dual phase DP590 steel resistance spot weld joint under two different welding conditions (i. e. single pulse and multi pulse) were studied. It was observed that applying multi pulse currents resulted in quasi‐equiaxed grains of tempered martensite in fusion zone (FZ) of the weldment. The refinement of microstructure in fusion zone using multi pulse current treatment resulted in reduced hardness of the weld joint.

Refill friction stir spot and resistance spot welding of aluminium joints with large total sheet thicknesses (III-1965-19)

Refill friction stir spot welding (RFSSW) is a highly flexible and promising solid-state joining method for aluminium alloys. Alternatively, resistance spot welding (RSW) can be stated as an appropriate joining method which can be automated and used within a high-volume production due to short process times. Both processes do not need any additional elements and a flat surface on both sides of the joints can be realised. In order to meet the modern requirements for crash safety and structural stiffness, thermal and mechanical joining methods are mainly combined by using single-component epoxy resin adhesives. Due to an insufficient knowledge about the application of both thermal joining methods for the abovementioned material combinations combined with additional adhesives, deeper investigations were done regarding the interactions of the polymers and the joining processes. Starting with a brief presentation of the boundary conditions of the investigations and the refill friction stir spot welding and resistance spot welding of high-strength aluminium alloys with sheet thicknesses bigger than 5.8 mm, the paper introduces the process-related joint properties of friction-based and resistance-based welded joints. Afterwards, the paper discusses the influences of the process parameter on the metallographic joint formation and load-bearing capacities for a selected two-sheet and four-sheet material combination. When combining the spot welding technologies with adhesives, the process parameters of the RFSSW process have to be adapted for the two-sheet combination by adding a squeeze-out step, while for RSW, just the preholding time has to be increased. Different challenges for both joining methods are shown. For RFSSW, the gap formation has to be considered when welding big total sheet thicknesses, while for RSW, the shape of the weld nugget is more important for an appropriate joint performance. Additionally, process optimisations for less adhesive incineration will be discussed for both joining processes, and the influences of the adhesive on the joint formation will be addressed with the help of load-bearing capacity evaluations. The paper closes with specific recommendations for the realisation of refill friction stir and resistance spot-welded joints with and without adhesive in the field of Al joints with big total sheet thicknesses which meet the quality demands and an outlook for further research steps will be given.

Bonding dissimilar materials via adhesively bonded spot-welded joints: cohesive zone model technique

This study investigates the single-strap joints (adhesively bonded spot-welded) under static loading. Experimental tensile tests are carried out to validate the numerical analyses. For this purpose, the specimens of dissimilar materials were prepared by joining them with a thin layer of a two-component epoxy-based adhesive at one side and spot welding at the other side. Then, they were tested under tensile-loading conditions. The numerical analyses were performed according to the cohesive zone model (CZM) to estimate the failure loads of the joints. The failure loades of the joints with different dimensions were studied to evaluate the conditions causing the cohesive failure of either the adhesive layer or the weld nugget. It is shown that with minor changes in the bondline and weld designs, the joining process of the dissimilar materials will be more feasible.

Friction stir spot welding of AA6061-T6 and Cu with preheating: Strength and failure behavior at different test temperatures

In the current study, the influence of test temperatures was investigated on lap shear strength and fracture behavior of dissimilar and similar friction stir spot-welded (FSSW) joints between Cu and AA6061-T6 sheets prepared without and with employing additional heating. Formation and distribution of intermetallic compounds (IMCs) in Cu-AA6061 FSSW joints was further appraised. An increase in maximum lap shear force was observed in Cu-AA6061 and Cu-Cu joints when preheating was employed. Lap shear force decreased with increase in test temperature for Cu-AA6061 joints prepared without preheating; however, for joints prepared employing preheating, the lap strength increased from 25 to 50 °C following a slight drop at 100 °C test temperature. For Cu-Cu joints, lap shear force also increased with test temperature for the range studied in the present work. X-ray diffraction, and energy dispersive spectroscopy analysis illustrates the formation of IMCs in the form of Al2Cu and Al4Cu9 in Cu-AA6061 and oxides in Cu-Cu joints. The interface failure from IMC rich regions in dissimilar joints was observed whereas similar joints failed under plug failure. Intermittently, thick IMC layer was observed in dissimilar Cu-AA6061 joint prepared with preheating. Presence of tearing/rupture from oxide dominated intermetallic region observed in Cu-AA6061 FSSW joints whereas dimples and micro-voids were noted in Cu-Cu joints.

Determination of residual stress in resistance spot-welded joint by a novel X-ray diffraction

Abstract The X-ray diffraction, named cosα method based on Debye-ring was utilized to measure the residual stress distribution of resistance spot-welded nugget firstly. The resistance spot-welded stainless steel 0Cr18Ni9 was investigated in this present work. The interesting residual stress distribution in the stainless steel resistance spot-welded joint was obtained by the experiments. For the integral nugget, the surface residual stress transforms gradually from the maximum tensile stress in the nugget center and reduced towards the nugget edge, owing to the thermal gradient in the nugget and the plastic deformation applied by the electrode. However, by cutting the nugget into two parts along the centerline, the core of nugget suffered a slight compression stress, and stress gradually transform from the compression to tensile stress, along with the center to edge, owing to the stress relief.

Fatigue Properties of Resistance Spot Welded Dissimilar Interstitial-Free and High Strength Micro-Alloyed Steel Sheets

This study aims to investigate the influence of nugget diameter on tensile-shear and fatigue properties of the dissimilar resistance spot-welded joints of interstitial free and high strength niobium microalloyed steel sheets. The spot-weldings are done at currents of 7 kA and 9 kA at welding time of 300 ms with electrode force of 2.6 kN. Fatigue tests are done at different load amplitudes, considering the maximum tensile-shear load bearing capacity of the joints. The tests are interrupted before the final failure to understand the crack initiation and propagation paths. The study is supplemented by the microstructural and detailed fractographic analyses. The results indicated that the fatigue strength of the spot-welds increased with a decrease in nugget diameter. This could be attributed to the higher microhardness of heat affected zone (HAZ), presence of compressive residual stress and lower variation in thickness reduction on the IF steel side, welded at 7 kA. However, the tensile-shear load bearing capacity of the spot-welds increased with increase in the nugget diameter. The failure of the spot-welded joint under static loading initiates from the HAZ/base metal interface of the IF steel side. Nevertheless, under cyclic loading crack initiates from the notch root at the interface of two sheets lying in the HAZ of IF steel side. Fractographic investigations indicate intergranular fracture features in combination with striations, secondary cracks on the IF steel side under cyclic loading. Fracture surfaces of the specimens failed under static loading shows however, shear dimples on the IF steel side.

Improvement of mechanical properties of dissimilar spot-welded joints of additively manufactured stainless steels

The dissimilar resistance spot welding of additively manufactured steel to conventional automotive steel has attracted significant attention from automotive manufacturer. However, the mechanical properties of dissimilar spot welds could be affected by the printed properties of additively manufactured steels, limiting the further application of 3D printing process in auto-body assembly line. This paper proposed an approach to improve the mechanical properties of spot-welded joints of additive manufactured steels by the design of binder jetting printed steels with the addition of nanoparticles. Cu-Sn nanoparticles have been injected to the stainless steel via binder jetting process, aiming to fill the voids between steel particles and reduce the microstructure heterogeneity in the spot welds. The microstructure evolution, sintering behavior of nanoparticles and mechanical properties of resistance spot welded stainless steel were characterized and analyzed. The sintering behavior of Cu-Sn nanoparticles during welding process attributes to the formation of transition zone with homogenous microstructure, resulting to the improvement of hardness property and lap-shear strength of spot-welded joints. Compared to the spot welds of selective laser melting printed stainless steels, the resistance spot welded stainless steel via binder jetting process shows better mechanical properties with 48 % increase of energy absorption and 19 % increase of peak load.

Correlation of Local Constitutive Properties to Global Mechanical Performance of Advanced High-Strength Steel Spot Welds

Recent results have shown that the strength of resistance spot-welded joints made from advanced high-strength steels (AHSS) do not increase linearly with their base metal strength. While the martensite tempering and subsequent softening in a narrow heat affected zone (HAZ) of these joints has been reported as a primary cause for this degradation, the quantitative effects of HAZ softening on above nonlinearity in different steels have not been explored. In this research, the role of material heterogeneity on load-displacement characteristics of dual-phase and martensitic AHSS with initial martensite volume fractions of 16, 58, and 100 pct during tension-shear (TS) and cross-tension (CT) testing was modeled with finite element method and compared with experimental measurements of global deformation and fracture behavior. Results from low-strength steels showed that the location of HAZ failure transitions from hardened to softened regions, as the nugget diameter increases from 4 to 6 and 8 mm, even with the presence of softening in the HAZ. At the same time, the results from higher strength steels showed more sensitivity to the softened region in larger nugget diameters. This result elucidates that the nonlinearity in strength of high-strength AHSS spot welds in comparison to lower strength of mild steel, is due to their intrinsic brittleness, as well as the overall geometry of the weld nugget. Our results also suggest that, while HAZ softening plays a detrimental role in DP steels, it helps to improve the ultimate load and global extension in high strength grades. Finally, several uncertainties related to finite element simulations and experiments have been highlighted. The results from this study can help in optimizing the resistance spot-welding process parameters and the design of the part made from AHSS.

Relationship of deformation meso- and macrostructures types with the staging of damage accumulation in welded joints of stainless steel under high-cycle fatigue

ABSTRACT The fatigue fracture mechanism of spot-welded joints of stainless constructional steel at multi-cycle loading condition is studied using the method of digital images correlation. Analysis of deformation meso- and macrostructures was carried out within the framework of the four-stage scheme of deformation and fracture of welded joints identified in the work. Characteristic fields of displacement vectors are presented and quantitative indicators and duration of damage accumulation stages are determined. On the basis of inverse pseudo-images, new types of deformation structures (trickle and cellular hexagonal) have been discovered.

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.

Microstructures Evolution in Refill Friction Stir Spot Welding of Al-Zn-Mg-Cu Alloy

In this study, Al-Zn-Mg-Cu alloy was refill friction spot welded, and the precipitates, dislocation, recovery, and recrystallization characteristics were focused. In the stir zone (SZ), continues dynamic recrystallization occurs under the intense plastic deformation. All the original GP (II) zones and η’ precipitates dissolved into the aluminum matrix under the welding heat input, and the stable η and E precipitates remained. In the thermo-mechanically affected zone (TMAZ), high-density dislocations and subgrains boundaries can be observed. The continued dynamic recrystallization was not activated and only dynamic recovery occurs. Sub-boundaries and high-density dislocations in this zone can be observed. In this zone, the η precipitates are coarsened and dissolution is the main evolution mechanism for the GP (II) zones and η’ precipitates. In the heat-affected zone (HAZ), no dislocation was induced and all the initial precipitates were coarsened under the welding heat input. The HAZ, the TMAZ, and the SZ constitute a soft region in the refill friction stir spot welding (RFSSW) joint, and the minimum value located at the interface between the HAZ and the TMAZ.

Fatigue life prediction of spot-welded joints with a notch stress approach

Spot-welded joints are widely used in the construction of vehicle structures and frequently become critical locations for fatigue failure. Hence, it is essential to have reliable fatigue life prediction method for the spot-welded joints during vehicle structure design. In this paper, a new notch stress approach is developed for fatigue life prediction of the spot-welded joints. Currently, structural stress methods are widely used in automotive industry for fatigue life prediction of spot-welded joints. However, these methods are not well considering local geometry information. This paper introduces a notch stress based method to overcome the limitation of the structural stress methods. In the notch stress method, stress concentration factors for spot-welded joints are calculated from stress intensity factor equations. Then, the notch stress method is validated with fatigue test results of lap-shear and coach peel specimens.

Analysis of Resistance Spot Welding Process Parameters Effect on the Weld Quality of Three-steel Sheets Used in Automotive Industry: Experimental and Finite Element Simulation

In the present research, the effects of spot-welding process parameters on the nugget diameter and electrode penetration depth of spot-welded joints were investigated. To achieve this, a spot-welded joint of three-thin sheet low carbon steels (same thicknesses of 0.8 mm) was simulated as an electerical-thermal-mechanical coupling of 3D finite element model. After validating the finite element simulation presented in this study by comparison with the experimental results for the spot diameter, various cases of spot welds were analyzed based on the design on experiment (i.e., Taguchi method). Six variables including electrode force, electric current, and quadrilateral times (squeeze, up-slope, welding time, and hold) at three different levels were considered as Taguchi algorithm inputs. The results of Taguchi sensitivity analysis showed that the parameters of electrical current (22 %) and welding time (17 %) are the most effective factors on the nugget diameter. Next, Multiple Regression Technique (MRT) was used to present a new equation for calculating spot diameter via the process parameters. The findings of this study showed that the difference between FE results and MRT for predicting spot diameter is less than 13%. Eventually, Response Surface Method (RSM) was utilized to determine the interaction effects of process parameters on the spot weld quality.

Study on Microstructure and Properties of Refill Friction Stir Spot Welding Joints of 2219 Aluminum Alloy with Different Thickness

Study on Microstructure and Properties of Refill Friction Stir Spot Welding Joints of 2219 Aluminum Alloy with Different Thickness