Conventional Resistance Spot Welding Research Articles

Indentation-Free Resistance Spot Welding of SUS301L Stainless Steel

Paint-free bodywork has become an attractive alternative for rail vehicles, in the direction of easy maintainability and low manufacturing costs. However, conventional resistance spot welding inevitably leaves indentation marks to detrimentally reduce the optical homogeneity of the paint-free bodywork. In light of this, indentation-free resistance spot welding is proposed for joining SUS301L stainless steel sheets in order to achieve superior surficial integrity. A tiny SUS301L steel ball with a diameter of 1.5 mm was chosen as the intermediate filler between two steel sheets to avoid the formation of surficial indentation. The influence of welding current and welding time on the mechanical properties of joints was studied. The optimal parameters of the mechanical properties were obtained when the welding current was 8.0 kA, the welding time was 150 ms, the electrode pressure was 0.35 MPa, and the electrodes were cylindrical planar electrodes, which was determined by comparing the tensile shear test results. The surficial indentation depth was less than 1% of the plate thickness, and no observable indentations were seen on the surface of the optimized welding spots.

Experimental Analysis on Ultrasonic Resistance Spot Welding of Aluminum Alloys

A recently developed hybrid joining process known as ultrasonic resistance spot welding (URW) was used on various pairs of similar and dissimilar aluminum (Al) alloys with different thicknesses, including AA5182–AA5182, AA6111–AA6111, AA7075–AA6111, and AA7075–AA5182, and comprehensively studied. Compared to conventional resistance spot welding (RSW), URW of the alloys showed consistently enhanced mechanical behavior in lap shear and crosstension tests. This can be attributed to the multiple perspectives on microstructure improvements. For different stacks of Al alloys and welding conditions, nugget formation was promoted with a larger nugget size in URW. In the nugget center, ultrasonically assisted (UA) vibration facilitated the formation of an equiaxed crystal zone. At the nugget boundary, URW showed a narrower coarse columnar zone and partially melted zone, which are associatedwith the lowest hardness in the weld. Specifically in dissimilar Al welds, UA vibration moved the nugget more centered toward the weld interface. These microstructure improvements indicated UA vibration can homogenize temperature and elemental distribution, which modifies solidification behavior.

Magnetic Characterization of the Nugget Microstructure at Resistance Spot Welding

Conventional resistance spot welds are not visible from the outside. Therefore, it is not straightforward to evaluate the joint quality non-destructively. The pulse-echo method of manual ultrasonic is widely used for non-destructive testing. Another option is the passive magnetic flux density testing, which is being developed at Technische Universität Dresden, Germany. The spot weld is magnetized in the normal direction and the residual magnetic flux density is measured on top of the surface of the joint. This method is suitable for spot welds on typical car body steels. Previous investigations show that the magnetic properties of the materials influence the test result. In order to develop this new non-destructive testing method further, it is necessary to know the magnetic properties of the different microstructure regions of a spot weld. This article focuses on methods to measure and evaluate the magnetic properties of these regions, especially of the base material and the weld. Different measuring methods and approaches are presented and compared with each other. Based on the results, recommendations for future measurements for magnetic characterizations are given.

Numerical Simulation and Experimental Verification of Laser Multi-Section Welding

To address the problems of large welding deformation and splashing in the resistance spot welding of the lubricating oil cooler plate, the laser spot welding was employed, instead of the resistance spot welding, and a novel laser spot welding was proposed, i.e., laser multi-section welding. The major processes involved in this study referred to a finite element model of pulsed laser lap welding built by adopting SYSWELD simulation software, as well as the laser welding of various welding methods. The effect of different welding methods on the welding quality was studied, the parameters of the average power and the duty cycle were optimized in line with the comparative analysis of the experimentally achieved results and the numerical simulation. As indicated from the experimentally achieved results, when the new 6-sections welding method was adopted, the resulting welded joint achieved the most uniform heat input and the largest welding fusion area, and the tensile properties exhibited by the welded joints were significantly enhanced, whereas some pores remained. By altering the duty cycle, pores could be eliminated to further improve the quality of the joint. The mentioned process method could tackle down the problems facing conventional resistance spot welding. Furthermore, it was capable of improving the uneven heat input of the laser spot welding.

On the applicability limits of double-sided self-pierce riveting

This paper revisits double-sided self-pierce riveting to discuss the applicability limits related to the thickness of the sheets to be used in hidden lap joint connections. Special emphasis is given to thin sheets with thicknesses that are significantly smaller than those earlier reported in the literature. The overall methodology draws from experimental and numerical simulation to aspects related to the working principle of double-sided self-pierce riveting and geometric scalability of the chamfered tubular rivets. Results show that double-sided self-pierce riveting can be successfully applied in thin sheets and must be seen as an alternative to well-established joining processes such as conventional self-pierce riveting and resistance spot welding. In case of the latter, comparisons are made against double-sided self-pierce riveting regarding the force and energy requirements to assemble and destroy the resulting lap joints. Destruction of the joints is performed by means of shear tests and provides the maximum force and energy that both types of lap joints are capable to withstand without failure.

Dissimilar metal joining of Q235 mild steel to Ti6Al4V via resistance spot welding with Ni–Cu interlayer

Dissimilar metal joining between Q235 mild steel and TC4 titanium alloy was carried out via resistance spot welding with Ni–Cu alloy as filler metal (abbreviated as resistance welding with interlayer). The results were compared with the welded joints obtained by directly conventional resistance spot welding without interlayer (abbreviated as D-RSW). The effect of Ni–Cu alloy on the macro-characteristics, interfacial structure, and microstructure, mechanical properties (including shearing force and microhardness) of the welded joints were investigated. Also, the temperature field in the actual welding process was dynamically simulated using MSC.Marc software, which validity of the numerical results was confirmed by the actual temperature collected at the same location via the infrared thermal imaging. The results showed that the maximum temperature was less than the melting point of TC4, Q235, and interlayer. The reaction layer and diffusion layer were formed between TC4 and Q235 by atomic diffusion due to the introduction of the interlayer, which replaced the welding nugget generated in the D-RSW welding process. Ti2Ni, Cu3Ti2, TiNi, TiCu, and TiFe2 intermetallic compounds (IMCs) could be generated at the welded joints by resistance welding with interlayer. Moreover, there were no other welding defects in the welded joints. The maximum ultimate shearing force was up to 12,726 N in this way, which was more than twice that obtained by D-RSW. Correspondingly, the interfacial fracture was obtained for the former instead of the brittle facture for the latter. There was a narrow range of highest-value microhardness according to the reaction layer.

Experimental analysis of the influence of the embossing and upsetting process on joint strength in resistance element welding with upset auxiliary joining elements

The use of conventional resistance spot welding in the car body shop for joining an aluminium part to a steel part is possible with the process-invention of the resistance element welding (REW) with upset auxiliary joining elements. Within this process, the challenges arise from the novel approach of combining forming process steps in the insertion of the auxiliary joining elements. Thus, the process understanding is to be compiled to the combination of the individual process steps. For this reason, the study deals with the influence of the forming process steps on joint strength of REW with upset auxiliary joining elements. The focus is set to the improvement of the positive locking and force fit considering the results of previous strength analysis. As the key result, the initial relative position of the carrier sheet and the auxiliary joining element is highlighted as the most significant factor on joint strength. The material flow, illustrated in metallographic microsections, is used to explain this effect. Based on the elaborated cause-and-effect-chains, a geometrical model is derived and validated to produce high-quality joints with high joint strength.

Joining of ultra-high-strength steels using resistance element welding on conventional resistance spot welding guns

Single-step joining of dissimilar material combinations between ultra-high-strength steels and high-strength aluminium alloys with sufficient mechanical joint properties by using conventional resistance spot welding equipment has not been reported yet. In this research paper, a novel single-step joining technology, so-called self-penetrating resistance element welding, is introduced. First, the motivation for this novel joining technology, the state of the art in joining, and the process characteristics are presented. In the results section, the welding rivet geometry is first determined using forming simulations and validated by head tensile tests. Followed by the description of the welding process and its characteristics, the mechanical joint properties are reported. The results show that a numerically optimised welding rivet geometry can guarantee sufficient joint strength. By this welding rivet geometry, a thermally assisted penetration of aluminium and therefore welding to steel is possible with and without adhesive. Furthermore, it is shown that the welding process can be designed by means of simulations. Finally, the shear tensile tests prove that an overall sufficient joint strength is ensured.

Experimental Investigation of Damage and Failure Mechanisms of Polymer-Metal Joints assembled by Self-Piercing Riveting

The increased use of polymer-based material in the manufacturing of vehicles structures makes critical the management of multi-material interfaces, and hence the issue of polymer-metal joining. It has been demonstrated in the literature that for large-scale manufacturing, self-piercing riveting (SPR) represents a reliable alternative technique to conventional resistance spot welding (RSW). However, the riveting operation induces, by nature, damages to the joint composite layer due to the steel rivet crossing it. In this study, the damage of the SMC thermoset material during SPR process has been experimentally investigated. Moreover, the influence of the riveting velocity as a major SPR process parameter on the composite layer damage has been identified. Eventually, the failure mechanisms of the polymer-metal joint resulting from failure under lap-shear and pure tension loadings were studied with the support of the numerical simulation tool.

High-productivity and high-strength Fe/Al dissimilar metal joining by spot forge welding

To develop a high-productivity dissimilar joining technique, which is highly sought after in the automobile assembly process, I developed a new method based on spot forge-welding. In the lap joint of 1-mm-thick SPCC steel and A5083 aluminum plates, within the 10-mm forge diameter, a base metal fracture occurred at a reduction ratio R of 1.8. The maximum tensile-shear load reached 3.8 kN during the approximately 80-ms forge welding process. The newly formed surface at the bonded interface exhibited a mesoscopic reaction layer, in which the brittleness was minimal given the short diffusion time. Thus, this approach shows great potential to replace conventional resistance spot welding for next-generation solid-state spot welding of dissimilar joints, for high-throughput welding applications and high-strength joints.

Analysis of the Behavior of Dynamic Resistance, Electrical Energy and Force between the Electrodes in Resistance Spot Welding Using Additive Manufacturing

This work is aimed at the analysis of the dynamic resistance, electrical energy and behavior of the force between electrodes (including thermal expansion) during welding at optimized parameters, referring to the process of spot welding using additive manufacturing (AMSW). For comparative purposes, this analysis also includes the conventional resistance spot welding process (RSW). The experiments were done on low carbon-zinc-coated sheets used in the automotive industry. The results regarding the welding process using additive manufacturing (AMSW), in comparison to the conventional resistance spot welding (RSW), showed that the dynamic resistance presented a different behavior due to the collapse of the deposition at the beginning of the welding, and that a smaller magnitude of electrical energy (approximately <3.35 times) is required to produce a welding spot approved in accordance with the norm. No force of thermal expansion was observed during the passage of the current, in contrast, there was a decrease in the force between the electrodes due to the collapse of the deposition at the beginning of the welding.

Development of a Resistance Spot Welding Process Using Additive Manufacturing

For several decades, the electrical resistance spot welding process has been widely used in the manufacturing of sheet metal structures, especially in automotive bodies. During this period there was no significant development for this welding process. However, in recent years, in order to meet the demand for lighter, economical, and low-cost vehicles, the automotive manufacturing industry is undergoing a revolution in the use of high strength steel sheet combinations, chemical compositions, and of different thicknesses. In this context, the present work focuses on the study and development of a new resistant spot welding technology using additive manufacturing (AMSW) in zinc-coated steel sheets, used in the automotive industry. As a comparison, spot welding was also performed by the conventional resistance spot welding process (RSW). The results showed that the spot welding process using additive manufacturing (AMSW), through the optimized parameters, compared to the conventional resistance spot welding process (RSW), was 34.47% higher in relation to the shear tensile stress, as well as 28.57% higher tensile stress with a perpendicular load to the weld spot. The indentation or thermomechanical mark on the surface of the sheet was imperceptible to the visual inspection, producing a smooth face in the spot region.

In-process monitoring of micro series spot welding using dual accelerometer system

Micro series resistance spot welding (MSRSW) is a common practice to form two neighboring nuggets at the same time in industrial fabrication of precision parts, especially in EV battery pack manufacturing. However, because of the Peltier effect, unbalanced Joule heat generation results in different nugget sizes. Therefore, weld quality monitoring systems are necessary to improve weld quality. Unfortunately, no work has been reported. It is still a challenge to develop a monitoring system. A series of monitoring systems have been developed for conventional resistance spot welding (RSW). Among them, electrode movement caused by heat expansion and contraction during the welding process is an important information. GAP sensor and LVDT are major sensors used in the measurement of electrode movement. However, the physical dimension of these sensors is too huge to be applied in MSRSW. This study proposes an in-process electrode movement monitoring system for MSRSW using dual accelerometers. Two-minute accelerometers were mounted on top of each electrode holders. Thanks to its good sensitivity and frequency response characteristics, an accelerometer signal was successfully picked up and integrated twice to reflect an electrode expansion/contraction movement. The correlation between two signals was analyzed by applying the Lissajous figure technique. Experimental verification of this monitoring system has been carried out. A major finding is that dual accelerometer signal and its double-integrated signal, electrode movement, reflect thermo-physical phenomena during MSRSW process.

Ultrasonic resistance welding of TRIP-780 steel

A hybrid ultrasonic resistance spot welding process is developed, which integrates high-power ultrasonic vibrations into conventional resistance spot welding process. Weldability of galvanized transformation induced plasticity (TRIP-780) steel is studied based on this process. Ultrasonic resistance welds showed nugget pullout failure mode and more ductility compared with conventional resistance spot welds. Optical and scanning electron microscopic analysis revealed the refined microstructure in the fusion zone and heat affected zone of ultrasonic resistance welds. Coalesced bainite is present the fusion zone of conventional resistance spot welds while possible tempered martensite and feathery bainite is observed in ultrasonic resistance welds, which accounts for their higher ductile behavior.

Development of resistance spot welding technology applying multi-stage adaptive control for narrow pitch spot welding

ABSTRACT Narrow pitch spot welding causes decrease in nugget diameter due to shunting to existing- welds under conventional resistance spot welding. This study aims to develop new resistance spot welding technology for ensuring required nugget diameter regardless of weld spacing. Adaptive control based on real-time feedback of heat quantity was applied for welding of mild steel sheets. However, expulsion occurred from steel sheet surface around the contact edge with electrode when weld spacing was shortened to 10 mm. To solve this problem, a new adaptive control method was developed. The method includes 2-stage adaptive control, in which heat quantity is controlled independently in each stage to obtain proper nugget diameter. It is effective to set relatively low heat quantity compared to conventional method in the 1st stage to ensure current pass between steel sheets, and then to apply enough current-flow to obtain proper nugget diameter in the 2nd stage. This method can decrease shunt current flow to existing welds, and excess increase in welding current and over-heating around contact edge with an electrode was suppressed. Appropriate nugget diameter can be consequently ensured without expulsion from steel sheet surface even with 10 mm weld spacing.

Effects of local stiffness on the spot joints mechanical properties: Comparative study between resistance spot welding and resistance spot clinching joints

Different types of aluminum alloy 5052 (AA5052) resistance spot clinching joints with different local stiffness values were designed in this study. The values of the polar moment of inertia in the cross-section of the fusion zones were calculated. Digital image correlation (DIC) technique was used to study the local rotation and in-plane strain distribution during the lap-shear test of half-cut samples. The results show that the joint with higher local stiffness (polar moment of inertia) value experiences the lowest joining zone rotation during the lap-shear test. This leads to higher proportion of normal component of strain/stress at the periphery of the joining zone. According to Tresca or von Mises failure criterion, for a spot joint, high proportion of normal stress at the load-bearing site enables the global joint to sustain high peak load during tensile test. This mechanism indicates that, compared with conventional resistance spot welding process, the resistance spot clinching technique can improve the mechanical properties of AA5052 joints by increasing the local stiffness.

HAZ characterization of welded 42SiCr steel treated by quenching and partitioning

An experimental steel 42SiCr was heat treated with a quenching and partitioning treatment in order to achieve high strength and at the same time acceptable ductility. This crucial combination requires the formation of a martensitic microstructure with a small volume fraction of retained austenite embedded on a microscopic scale. The objective of this work was to investigate the evolution of microstructure and hardness in the heat affected zone (HAZ) of welds in this 42SiCr and its major influence factors. Thermo-physical simulations with a dilatometer yielded samples of each zone in the HAZ and allowed a detailed investigation of HAZ microstructures and properties. A decrease in hardness from 580 HV in the quenched and partitioned base material to the HAZ, with minimum hardness values of 370 HV in a region exposed to temperatures close to the Ac3 temperature, was revealed. Simulation showed that the experienced peak temperature is predominantly determining the HAZ properties. The microstructure after quenching and partitioning treatment consisted of fine martensite with presumed retained austenite. The HAZ regions that were heated above Ac3 temperature can be characterized to have a fine bainitic/martensitic microstructure, while the regions heated to more than 1100 °C show coarser grain, with bainitic/martensitic features.To support the HAZ simulation results sheets of this steel were rolled and resistance spot welds were produced. The welds were characterised with respect to the microstructure in the heat affected zone, as well as the mechanical properties of the joint. Results proved that the data obtained by thermo-physical simulation is in very good agreement with the spot welding results. On a macroscopic scale the welding results indicate very good joint formation but very brittle fracture behaviour using a conventional resistance spot welding process for the quenching and partitioning steel. However, despite the relatively high carbon equivalent of the material, also ductile aspects of fracture were observed.

Development of resistance spot welding technology applying multi-stage adaptive control for narrow pitch spot welding

Narrow pitch spot welding causes decrease in nugget diameter due to shunting to existing- welds under conventional resistance spot welding. This study aims to develop new resistance spot welding technology for ensuring required nugget diameter regardless of weld spacing. Adaptive control based on real-time feedback of heat quantity was applied for welding of mild steel sheets. However, expulsion occurred from steel sheet surface around the contact edge with electrode when weld spacing was shortened to 10mm. To solve this problem, a new adaptive control method was developed. The method includes 2-stage adaptive control, in which heat quantity is controlled independently in each stage to obtain proper nugget diameter. It is effective to set relatively low heat quantity compared to conventional method in the 1st stage to ensure current pass between steel sheets, and then to apply enough current-flow to obtain proper nugget diameter in the 2nd stage. This method can decrease shunt current flow to existing-welds, and excess increase in welding current and over-heating around contact edge with electrode was suppressed. Appropriate nugget diameter can be consequently ensured without expulsion from steel sheet surface even with 10mm weld spacing.

Effect of the laser power on the mechanical performance of the laser spot welds in dual phase steels

Fibre laser spot (circular) welds (LSW) in dual phase DP780 steel grade were produced with fusion areas equivalent to conventional resistance spot welds (RSW) using different fibre laser parameters. Shear tensile properties show significantly higher loads in fibre laser spot welds primarily because of the combined strength of spot size, and hard microstructures in the fusion zone and heat-affected zone (HAZ). Greater HAZ softening is observed at high heat inputs. The transition of failure mode from interfacial to pull-out and the corresponding strength value is primarily controlled by the weld size. The crack initiation site and propagation mechanism were observed to be different for interfacial and pull-out failure. The increased HAZ softening with increase heat input was quantified with nano-indentation hardness measurement technique.

Friction Stir Resistance Spot Welding of Aluminum Alloy to Advanced High Strength Steel

A hybrid friction stir resistance spot welding (RSW) process is applied for joining aluminum alloy 6061 to TRIP 780 steel. Compared with conventional RSW, the applied current density is lower and the welding process remains in the solid state. Compared with conventional friction stir spot welding (FSSW) process, the welding force is reduced and the dissimilar material joint strength is increased. The electrical current is applied in both a pulsed and direct form. With the equal amount of energy input, the approximately same force reduction indicates that the electro-plastic material softening effect is insignificant during FSSW process. The welding force is reduced mainly due to the resistance heating induced thermal softening of materials. With the application of electrical current, a wider aluminum flow pattern is observed in the thermo-mechanically affected zone (TMAZ) of weld cross sections and a more uniform hook is formed at the Fe/Al interface. This implies that the aluminum material flow is enhanced. Moreover, the Al composition in the Al–Fe interfacial layer is higher, which means the atomic diffusion is accelerated.