Nugget Diameter Research Articles

Multi Response Optimization of Spot Welding Process Parameters by Taguchi Method

Electrode deterioration by deformation and chemical reaction during resistance spot welding of steel sheets confirming to AISI1020 is an issue for class II electrode. In view of this, the present study carried out the impacts of squeeze, weld, current and pressure on the nugget diameter, height, force and waviness of welding joint in electrical resistance spot welding of 0.8mm and 1.5 mm thickness of steel sheets with class II electrode to achieve the benefits of cost saving and quality products. A timer and current controlled electrical resistance spot welding machine having 120 kVA capacities were used. During welding process, current periods of 5, 10, 15, 20 and 25 were selected and it was adjusted by increasing from 8 to 10 kA. The optimum welding parameter for multi objectives was obtained using multi signal to noise ratio and the significant level of the welding parameters was further analysed by analysis of variance for all responses. Based on the confirmation test results, it is found out that the developed model can be effectively used to predict the size of weld zone which can improve the welding quality and performance in GRA. The details of welded joint generated by class II electrode were measured and results were analysed in detail.

Analysis of tensile-shear strength of single and multi-friction stir spot welding joints under fixed welding process conditions

The tensile-shear strength of AA 5052 single and multi-friction stir spot welding joints were analyzed using experimental, numerical, and analytical approaches. Benchmark specimens were designed and manufactured in a similar manner with respect to industrial practice. Under the fixed welding process condition, the failure mechanism of friction stir spot welded specimens under tensile-shear loading was first determined by using macro- and micro-structural analysis. It is shown that increasing the tool shoulder diameter and the number of friction stir spot weldings may nonproportionally increase the strength of the joints. In the linearly arranged multi-friction stir spot welding joints, the strength of these joints was discussed using analytical approach. It is demonstrated that in certain cases, increasing the nugget diameter is preferred than increasing the number of nuggets. This is only applicable to a certain friction stir spot welding failure mechanism. A finite element model prediction tool was developed to predict the tensile-shear strength of friction stir spot welded joints using the material properties obtained from the measurement of experimental hardness.

Mechanical properties of dissimilar Ti-Al resistance spot welds

Abstract In this study, the mechanical behavior of titanium (ASTM Grade 2) alloy sheet, resistance spot welded with 5754 aluminum alloy, was investigated with respect to nugget formation and mechanical properties. The welding operations were performed using resistance spot welding at various welding currents (10, 12.5 and 15 kA), welding times (10, 15 and 20 cycles) and electrode forces (5, 10 and 15 kN). The weld nugget diameter and nugget center height, hardness, and tensile-shear test results of the welded samples were examined, and the mechanical properties were compared. As a result of the tests, it was seen that the welding parameters have a significant effect on the mechanical properties of the titaniumaluminum alloy welding joints. The tensile-shear load of the resistance spot welded joints increased as the welding current increased, and welding time increased for 15 cycles. On the other hand, when the electrode force increased, the tensile-shear load decreased markedly.

Effect of material inhomogeneity on fracture modes in aluminium–steel resistance spot welds

AbstractResistance spot welding (RSW) is attractive for joining dissimilar materials, especially, aluminium to steel in automotive body. The direct joining of aluminium to steel forms an intermetallic compound (IMC) layer at their interface that dominates mechanical behaviour of the joint. A new formula was developed that considers material inhomogeneities such as the different mechanical properties in the weld such as base metal, heat affected zone (HAZ) and the weld nugget to accurately calculate the minimum weld nugget diameter required to enable pull‐out fracture. The shear strengths of weld regions such as the HAZ and IMC were directly measured and used as inputs to this new formula. The new formula was validated using experimental measurements from six combinations of aluminium–steel welds in comparison with analogous aluminium–aluminium welds. The new derivation was able to accurately predict fracture modes for all material combinations.

Impact of process parameters of resistance spot welding on mechanical properties and micro hardness of stainless steel 304 weldments

PurposeResistance spot welding (RSW) is an essential process in the automobile sector to join the components. The steel is the principal material utilized in car generation because of its high obstruction against erosion, toughness, ease of support and its recuperation potential. Due to this, it was planned to study the mechanical properties, hardness and microstructure characteristics of RSW of Stainless steel 304.Design/methodology/approachIn the present research, RSW of 304 stainless steel plates with 1 mm thickness and effect of current intensity, welding time, electrode pressure and holding time on nugget diameter, tensile strength microhardness and microstructure of the joints was investigated. The specimens were prepared according to the dimensions of 30 × 100 mm with 30 mm overlaps joint through the RSW machine. The tensile test of the specimen was carried out on a universal testing machine and microhardness of specimens measured using Vickers’s hardness tester. Taguchi L16 orthogonal array was used to scrutinize the significant parameters for each output.FindingsIt has been observed that the tensile strength of the specimen is affected by the current intensity and nugget diameter, and the weld time has a significant effect on the tensile strength. Microhardness is highly influenced by electrode pressure and holding time, as the increase in both these parameters resulted in the increase of microhardness. This is due to rapid cooling, which is done by the cooling water flowing through the copper electrodes.Originality/valueThis study was carried out using a copper electrode with a flat face with selected parameters and response factors. The study can be useful for researchers working on optimization of welding parameters on stainless steel.

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.

Mechanical properties of dissimilar Ti-Al resistance spot welds

AbstractIn this study, the mechanical behavior of titanium (ASTM Grade 2) alloy sheet, resistance spot welded with 5754 aluminum alloy, was investigated with respect to nugget formation and mechanical properties. The welding operations were performed using resistance spot welding at various welding currents (10, 12.5 and 15 kA), welding times (10, 15 and 20 cycles) and electrode forces (5, 10 and 15 kN). The weld nugget diameter and nugget center height, hardness, and tensile-shear test results of the welded samples were examined, and the mechanical properties were compared. As a result of the tests, it was seen that the welding parameters have a significant effect on the mechanical properties of the titaniumaluminum alloy welding joints. The tensile-shear load of the resistance spot welded joints increased as the welding current increased, and welding time increased for 15 cycles. On the other hand, when the electrode force increased, the tensile-shear load decreased markedly.

Comparative study on resistance and displacement based adaptive output tracking control strategies for resistance spot welding

Abstract To achieve high quality resistance spot welds (RSW) through intelligent manufacturing of automotive body structures, adaptive control for RSW processes becomes imperative and has gained global attention. Tracking ideal process signal profiles predetermined offline is a commonly-used strategy to realize the adaptive control of RSW and is found effective in compensating for the adverse impacts caused by shunting and electrode wear. However, the validity of this control strategy under other types of disturbance is still unproven. In this article, typical abnormal welding conditions, i.e. initial sheet gaps and edge proximity, were designed to test the performance of output tracking control strategies for steel RSW. Two types of process signals including dynamic resistance and electrode displacement were tracked to approach the referenced signal profiles through the adjustment of weld parameters. Comparative studies were conducted based on metallographic analysis and mechanical tests to validate the two tracking control strategies. Results demonstrated that gap and edge proximity conditions decreased the nugget diameter and the tensile-shear strength of the welds. When satisfactory tracking of the reference resistance signal was realized under these two types of abnormal conditions, weld properties are further degraded, owing to a reduction in welding current. On the contrary, tracking the electrode displacement signal mitigated the negative influences from the disturbance through increasing welding current properly. When the disturbance was too severe, the displacement-based strategy raised the risk of expulsion. The findings suggest that tracking the dynamic resistance signal is not optimal for adaptive control of steel RSW, while tracking the electrode displacement signal is more effective. However, to achieve a more satisfactory adaptive control result, the output tracking control should be used in combination with expulsion suppression techniques.

Prediction of the failure mode of automotive steels resistance spot welds

ABSTRACT In this study, the critical nugget size, at which the failure state in tensile shear test changed from the interfacial failure mode to the pull-out failure one, was estimated as a function of nugget and base metal hardness. The proposed approach could address the effect of various parameters involved in resistance spot welding process, such as sheet thickness, base metal chemical composition and physical properties of electrodes and sheets. The reliability of the present model was evaluated using independent experimental results. Based on the obtained results, the effect of steel composition on critical nugget diameter was found to be more important, especially for the sheets thicker than about 1 mm, whereas predicted nugget sizes by previous models could not guarantee the pull-out failure mode.

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.

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.

Effect of the Melting Behavior of the Coating Layer on Weldability in the Bolt Projection Resistance Welding of Al-Si Coated Hot-Stamped Boron Steel

In this study, the effect of Al-Si coating melting behavior and weldability during the resistance spot welding of hot stamped boron steels was investigated. In the case of the Al-Si coated hot-stamped boron steel, high heat was generated on the surface and the oxide layer by the intermetallic coating, and the molten coating layer pushed toward the outer edge of the weld like a film of liquid. The liquid coating layer accumulated at the periphery, preventing current flow and extension of the contact area, and this influenced the height of the nugget growth. As a result of the limited nugget growth, weld strength decreased. However, in the case of bare hot stamped boron steel, relatively low heat was generated due to the large contact area, and the nugget grew in the longitudinal direction. Compared to the nugget diameter of Al-Si coated hot-stamped boron steel, this can result in superior weldability. Key words: Bolt projection welding, Al-Si coated hot stamping steel, Melting behavior, Weldability, Heat generation

Effect of external magnetic field on resistance spot welding of aluminum alloy AA6061-T6

An external magnetic field is applied in resistance spot welding (RSW) of 3 mm thick AA 6061-T6 sheets to improve the weld quality. Under the action of the external magnetic field, an additional circumferential flow is generated, resulting in the reduction of cooling rate and the change of solidification mode, and it finally influences the nugget profiles, microstructural evolutions, and mechanical properties. The profile of the magnetically assisted resistance spot welding (MA-RSW) nugget became more regular and freer of defects, e.g., the internal shrinkage defects and eyelash-like hot cracks. Moreover, a double-layered columnar grain zone and a nearly single-structured equiaxed grain zone were finally obtained. The tensile properties of the MA-RSW welds were significantly improved by increasing nugget diameter, eliminating defects, and complicating the crack propagation path. As a result, the application of the external magnetic field could achieve an energy-saving effect.

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.

To investigate the influence of weld time on joint characteristics of Hastelloy X weldments fabricated by RSW process

In order to explore the influence of weld time on joint strength, nugget size, mode of fracture of Hastelloy X weldments fabricated by resistance spot welding process, an experimental investigation is established to determine the parameters and root cause of the failure by the use of Fractography. The adequate resistance spot welding process parameters for Hastelloy X of 750 µm thickness, welding current periods of 30, 40, 50 and 60 cycles are selected in the welding process. Other parameters like squeeze time, hold time, off time and electrode force is kept fixed. Elevation in weld time led to a significant increase in weld dimension. Also, the relationships between mechanical properties and weld nugget dimensions with weld time is evaluated. Fractographic analysis is conducted using scanning electron microscopy (SEM). Macrographs demonstrated that excellent welding formed continuously along the interface, except for sample welded for 60 cycles which failed during welding. The vital results infer that the nugget diameter broadens with increasing weld time and the strength of resistance spot welded metal improves with increasing weld time up to an optimum value. Beyond an optimum value of weld time, instantaneous crack initiation takes place during welding itself due to excessive heat input and electrode pressure subjected to the metal. The observations on tensile-shear breaking load, nugget diameter, macrographs and SEM fractographs of nugget for tensile-shear application for resistance spot welding are discussed.

Optimization of resistance spot welding process applied to A36 mild steel and hot dipped galvanized steel based on hardness and nugget geometry

Resistance spot welding (RSW), a process intensively used for thin metal sheets assembly in automotive, railway, and aeronautical industries, has always presented technical and economic challenges. Thermal distortions and difficulties caused by the presence of anticorrosive coating on galvanized steels are among the major issues. An evaluation of effects of welding parameters on overall nugget quality will be of great importance in solving these issues. Two grades of steel were used in this study, A36 mild steel and A653 hot dipped galvanized steel. Welding experiments were done in overlap configuration, grade for grade, while following complete factorial plans of three factors at two levels. Micrographic analyses revealed weld microstructure, while micro-indentation hardness tests enabled to establish the hardness profile along weld nuggets. Analyses of variance (ANOVA) were done in order to evaluate the effect of each parameter on nugget hardness and diameter. Response surface models combining hardness in the fusion zone and nugget diameter were plotted as function of welding parameters. These response surfaces showed that current is the predominant parameter and should be maximized in order to achieve strong welds while force and time should be minimized for A36 mild steel and maximized for galvanized steel.

Effect of Hot Stamping Heat Treatment Temperature on Resistance Spot Weldability of Al-10% Si Coated 30MnB5 Steel

Hot stamping is the most efficient manufacturing process to produce high strength automotive parts with reduced springback. Recently, 30MnB5 steel has been developed for hot stamping steel which can provide above 1700 MPa. However, resistance spot weldability of this steel is insufficient due to the need for a large amount of carbon addition. In this study, the effect of the hot stamping heat treatment temperature on resistance spot weldability of Al-10% Si coated 30MnB5 was evaluated to improve the resistance spot weldability of 30MnB5. In terms of nugget diameter, weldable current range, tensile shear strength and cross tension strength, the specimen heat treated at 850 oC achieved better performance than the specimen heat treated at 950 oC. There was a coating microstructure difference between the specimens heat treated at 850 oC and 950 oC whereas both specimens had similar microstructure and mechanical properties of their substrates. The Fe-rich layer of the specimen heat treated at 950 oC was twice as thick as the specimen heat treated at 850 oC because the diffusion rate was much higher at 950 oC. The thick Fe-rich layer deteriorated the resistance spot weldability due to high electrical resistivity during resistance spot welding. Key words: hot stamping, Al-10%Si coating, 30MnB5, resistance spot welding, dynamic resistance

Performances of regression model and artificial neural network in monitoring welding quality based on power signal

In this study, a systematic research was conducted to compare the performances of the regression model and artificial neural network in predicting the nugget diameter of spot-welded joints by monitoring the dynamic power signature. The TC2 titanium alloy with a thickness of 0.4mm was used as the welding material, and a high-frequency precision spot welder was used to join the titanium alloy sheets. The dynamic welding current curve was obtained using the Rogowski coil, while the voltage curve was detected via two leads clipped onto the upper and lower electrodes during the entire welding process. The variations in the welding power signal in the welding process were investigated, and the characteristics of the power signals for different welding currents and electrode forces were analyzed. The power signals of different types of welding joints varied significantly. Five characteristics were extracted from the power signal to describe the shape of the curve. The stepwise regression analysis and back propagation neural network were respectively used to classify the welding joints into three categories: bad welds, good welds, and welds with expulsion. The performances of the two established prediction models were compared, and their behavioral discrepancies were attributed to their own data-mapping capabilities.

Shrinkage Void Formation in Resistance Spot Welds: Its Effect on Advanced High-Strength-Steel Weld Strength and Failure Modes

Among various defects formed in resistance spot welds of advanced high-strength steels (AHSSs), shrinkage void (cavity) is the least addressed topic. Although cavities are formed in numerous sizes and shapes, their influence on the strength and failure modes is yet to be investigated. The present study examines the effect of the cavity size and position on the peak tensile shear load and failure modes in 780 grade dual-phase steel used in automobile body structures. The size and position of the cavity in the nugget were varied carefully by altering the weld parameters and the sheet thickness. The cavity characteristics and its effect on the weld strength were evaluated from shear tensile tests and fracture surfaces. Results show that the size of the cavity increases with sheet thickness and nugget diameter. Further analyses revealed that the shear strength of the weld is influenced by the size and location of the cavity from the interface. A transition from plug-type failure to interfacial failure was observed with an increase in the cavity size. Consequently, the cavities reduce the load-carrying area of the nugget affecting the weld strength and failure mode. The effective nugget area, which is the actual load-bearing area of the nugget without any cavity inferred from the interfacial fracture surface, would be beneficial in determining the interfacial failures in resistance spot welds. Based on the results, a minimum nugget diameter of 5√t was proposed to obtain plug-type failures in AHSS spots with the cavity.

Investigation of RSW process on three-layer joining of steel to 6061-T8 aluminum

ABSTRACTIn this article, the RSW process (Resistance Spot Welding) on 6061-T8 aluminum alloy was studied. Spot resistance welding of two-layer plates is difficult and has very low mechanical properties. That is why this process is not practical for industrial applications. In this paper, a spot welding method was used with a novel three-layer technique, which means that an aluminum sheet was located between two sheets of steel and RSW was performed. The effects of RSW parameters including both current and welding time were investigated on the diameter of nugget, the microstructure of heat-affected zone and reaction layer, and mechanical properties. The results show that this method increases the tensile shear load compared with two-layer joining. Also, increasing of welding current, both tensile shear load and weld nugget diameter increased first, and then reduced. The interfacial reaction layer and weld heat-affected zone (HAZ) consisted of , .