Nugget Size Research Articles

Investigation of thin sheet stainless steel resistance spot welds: Effect of weld current on nugget failure and microstructure

Thin stainless-steel sheets commonly used in food and beverage industry is resistance spot welded in the process of making leak proof containers. The spot-welding parameters are optimized to obtain maximum nugget size and weld strength. In contrast, least importance is given to the temperature developed at the sheet/electrode interface and subsequent oxidation of the weld. It is well known that the oxidation of the stainless-steel results in the depletion of the chromium in the material affecting its corrosion resistance properties. In this context, this paper is aimed at investigating the effect of resistance spot welding current on the chromium depletion and subsequently on the microstructure of 0.4 mm 304 stainless steel sheet weldments. Comparative study between the microstructures of the heat affected and weld zones were carried using optical microscope and scanning electron microscope. The effect of weld current on the morphology of the delta ferrite formed in the weld zone is presented in the paper with help of pseudo-binary phase diagram. An attempt is made to correlate the local chromium depletion with the microstructure and defect formation.

Resistance spot welding of dissimilar AISI-1008 steel/Al-1100 alloy lap joints with a graphene interlayer

Resistance spot welding of dissimilar materials (AISI-1008 steel and aluminium-1100 alloy) by adding an interlayer of graphene nanoplatelets (GNPs) is reported in this study. This GNP layer is incorporated in the steel plate by drop-coating technique and the coated steel and the aluminium plates were joined by the resistance spot-welding method. The peak loads of these dissimilar lap joints processed at different welding current/time conditions were obtained by lap-shear tests and a comparative study was carried out between the GNP coated and bare specimens. While the weld strength depends predominantly on the time/current used for the welding, a maximum enhancement of ∼124 % is obtained for GNP coated samples in comparison to that of the bare ones. Fractography was performed by SEM and it was observed that a mixture of both brittle and ductile fractures was the major cause of failure. Interfacial failure and metal expulsion were also observed at lower and higher processing conditions respectively. Nugget size variation with different processing parameters was also studied. Dissimilar metal welding formed Al-Fe rich intermetallics (FeAl3, Fe2Al5, and Fe4Al13) which enhanced some of the properties of the nano-composites formed in the nugget zone. Detailed microstructural characterization was carried out by light microscopy, AFM, SEM, TEM, XRD and Raman spectroscopy. The enhancement of strength of the composite was done by numerous strengthening mechanisms as a result of the incorporation of GNPs. Micro-hardness investigation across the weld cross-section was carried out and it was found that an enhancement in hardness due to the formation of intermetallics and graphene entrapment was reported.

Effect of Cr-Ni coated Cu-Cr-Zr electrodes on the mechanical properties and failure modes of TRIP800 spot weldments

Resistance spot welding (RSW) is a highly preferred welding process in the automotive and other industries because of its faster and economical processes and also it has higher quality. However, during the joining process, the temperature at the electrode tip reaches the highest value and, in the meantime, electrode tips are deformed. For this reason, the electrode wears down with increasing weld numbers, which changes the thermal, electrical, and mechanical contact conditions at the electrode and sheet interfaces. The coating is generally applied onto the surface of Cu–Cr–Zr electrodes for extended electrode life during RSW process. In this study, the tip surfaces of the electrodes were coated with Cr–Ni filler metal by using electro-spark deposition (ESD) coating method under 40, 80, and 120 V. Consequently, the effects of the coating on electrode characteristics and properties of the TRIP800 resistance spot weldment are investigated. The mechanical behaviors of the joined samples under static load were investigated by applying a tensile shear test and cross tension test. Tensile-shear strength, cross-tension strength was determined, and the failure modes of the samples was investigated. The nugget size was measured by stereo microscope. Microstructure investigations have also been carried out by optic microscope and scanning electron microscopy (SEM).

Effect of adhesive sealant on resistance spot welding of 301L stainless steel

To solve the poor sealing performance of stainless-steel metro car bodies, a non-curing sealant is introduced into the resistance spot welding (RSW) process. Effects of the sealant on the weldability, mechanical property, and failure mode of 301L austenitic stainless steel (ASS) RSW welds were systematically investigated in this work. Under the action of electrode pressure, the sealant gathers around the quasi-nugget zone and leaves a thin residual sealant layer between the substrates. The insulating sealant leads to an earlier nugget initiation, a bigger nugget size and a higher heat utilization ratio than the traditional RSW process. The thermal decomposition products spread to the nugget and exist in the form of pores and inclusions. The proportion of residual δ-ferrite is decreased due to the long-lasting heat preservation effect of the surrounding sealant. The tensile strengths and failure mode of resistance spot welds with sealant (S-RSW) are undesired, resulting from the local softening and dispersedly distributed gas pore and inclusion defects. However, the adverse effects of the sealant on mechanical performances could be eliminated by appropriate increasing welding current. Considering the markedly improved sealing performance, S-RSW would be a useful and valuable process in engineering applications.

Process characteristics and influences of production-related disturbances in resistance element welding of hybrid materials with steel cover sheets and polymer core

Resistance element welding (REW) is an appropriate thermal-based joining method for joining hybrid materials with other structural steel grades, e.g. high-strength steels. Due to insufficient knowledge on the application of this joining process in combination with these new hybrid materials, deeper investigations were done regarding the suitability of the process and specific boundary conditions. This article introduces by giving a summary of the material properties, the boundary conditions and the welding process itself. Afterwards, the pre-assembly step of the elements and the resulting handling strength of the pre-assembled elements are discussed for two different hybrid material thicknesses. Weldability lobes are compared for REW with and without additional adhesives so that appropriate process parameters are defined. Due to a possible industrial application, relevant production-related disturbance variables are addressed as well. Therefore, the impact on the resulting joint properties, e.g. nugget size, is evaluated for different offsets of the electrodes and different gap sizes between the sheets as well as for both orientations of the polarity. The article closes with a comparison of the resulting load-bearing capacities of the REW-joints with those manufactured by resistance spot shunt welding and self-pierce riveting with semi-hollow rivets.

Resistance spot weldability of TBF steel sheets with dissimilar thickness

This paper presents an experimental study on weldability of TBF steel sheets with dissimilar thickness. Nominal thickness of TBF sheets were 0.95 and 1.55 mm. Optical microscope was used to observe the cracks formed in the weld zone. The indentation depths were determined by ultrasonic technique. Tensile shear tests were applied to the welded specimens in order to determine the mechanical properties. Higher weld current and time resulted in higher nugget size and indentation depth. This increase in nugget size and indentation depth with increasing of weld current and weld time was almost linearly. The liquid metal embrittlement crack sensitivity in the HAZ was very high on TBF spot welds. The liquid metal embrittlement cracks were much deeper and wider along the weld periphery regions compared to the nugget regions. These cracks were occurred easily with a higher heat input and the crack depth increased almost linearly with increasing of heat input. Weld strength of the specimens was governed by crack formation in the HAZ. Lower weld currents and weld times resulted in higher tensile shear properties. The highest tensile shear load (18.50 kN) and energy absorption (37.5 J) were achieved at 6 kA for 20 cycles. This weld exhibited interfacial failure mode.

Correlation Among the Input Thermal Parameters and Thermography Measurements Data of the Resistance Seam Welding

Abstract Resistance Seam Welding (RSEW) is commonly used in continuous steel processing for joining steel coils. Experimental studies focused on the welding process quality controls are justified by their wide application and influence on the performance of steelmaking processes. The present study focuses on the continuous annealing line welding process using as sample, a low carbon steel (0.07%C) with 0.21 mm thickness. The work proposes a correlation of thermal input, electric current parameter and thermogram monitoring during RSEW procedures. A methodology was developed to determine the real thermal profiles values by means of association of the thermograms with the sample emissivity curve. Three heat input levels were used, 3.64 J.mm-1, 5.09 J.mm-1, and 6.38 J.mm-1, respectively, for typical C-Mn steel. The thermal welding profile obtained experimentally exhibited linear correlation coefficients (R) of 0.99 with the electric current welding parameter and (R) of 0.99 with the calculated thermal input, respectively. The welds metallurgical characterizations results, expressed through the ductility and welds nuggets analysis, were conducted to correlate the welds quality with the welding thermal input. The Erichsen cupping test showed that the higher the thermal input corresponds to higher ductility of the welds. Nevertheless, the heat input of 5.09 J.mm-1 indicated better-balanced properties: ductility, weld nugget discontinuity, nugget size, and no expulsion of the melt material.

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.

Effect of Al-Si Coating on Weldability of Press-Hardened Steels

Resistance spot welding (RSW) of Al-Si-coated PHS was undertaken, and the effect of Al-Si coating on nugget formation and mechanical properties was investigated. Press-hardened steel (PHS) has long been applied to automotive body structure construction to support mass and corresponding greenhouse gas emission reductions. PHS materials are often combined with an Al-Si coating applied as an oxidation barrier though unfortunately, the Al-Si coating poses a challenge to the resistance spot welding (RSW) of PHS containing stack-ups. As a newly developed coating for the hot stamping process, the property of Al-Si coating is different from base metal and traditional coatings, and the influence mechanism of Al-Si coating on the welding process is not clear. It would remain at nugget edge and might cause severe stress concentration. To investigate this problem, RSW of 1.5-mm Al-Si-coated PHS was undertaken, and the results indicate that a large portion of the Al-Si coating is extruded and forms a sharp notch close to the nugget edge during the welding process. During the post-weld cooling stage, a thin layer of residual coating is formed between the nugget and notch root. The mechanical performance of the welded joints is limited by the thin residual Al-Si layer which acts as a preexisting crack and supports the interfacial fracture. The presence of the Al-Si coating at the faying interface also significantly delays nugget formation, though it contributes to a larger nugget size by inhibiting expulsion events at the faying interface.

Shunting effects on the resistance spot welding parameters of DP600

Abstract In this study, the effects of spot welding parameters on the weld quality of DP600 steel were experimentally investigated. The welding parameters such as welding current and time were determined based on microstructural characterization, microhardness tests and tensile-shear tests. The joints were welded with two parallel welds perpendicular to the tensile-shear test direction. The shunting effect was evaluated based on microhardness and nugget size. A microstructural examination was performed using optical microscopy. The results show that parallel welding increased current diversion along the shunt path, reducing hardness and nugget size in the second weld. Additionally, higher welding current and welding time caused increased hardness and a higher tensile load due to the formation of lath martensite.

Influence of cooling water temperature on ME20M magnesium alloy submerged friction stir welding: a numerical and experimental study

Submerged friction stir welding on magnesium (Mg) alloy is investigated with limits in the past. ME20M is an important lightweight Mg alloy with enhanced yield strength and heat resistance that merits further research. In this paper, submerged friction stir welding of ME20M Mg alloy was carried out in different temperatures of cooling water. Three-dimensional numerical was employed to analyze the thermal field under the same weld conditions, and the numerical predictions were compared with the experimental results. The macrostructure, microstructure, tensile properties, and hardness are tested. The results show that the numerical results and the experimental results exhibits the same trends. By increasing the cooling water temperature, the grain size of the weld nugget increased, the tensile strength of the joint decreased, and the microhardness of the weld joint decreased. The largest tensile strength was 170.5 MPa, which was ~ 71.04% of the base metal. The highest and the lowest hardness values of the weld joint were obtained at the cooling water temperature of 15 °C and 75 °C, respectively, in the weld nugget and heat-affected zones.

Mechanical Performance and Microstructure of Resistance Element Welds of Dissimilar Metals Created with a Headless Rivet

Resistance Element Welding (REW) is a novel processing technology developed to join dissimilar materials such as aluminum and steel. It uses an auxiliary steel rivet (element) inserted into an aluminum sheet that forms a resistance spot weld with the steel sheet, and the aluminum work piece is mechanically joined by the interlocking rivet. The versatility and agility of REW is advantageous, especially in the automotive industry. However, the head of the rivet often extends out externally over the work piece, and the misalignment of the electrode with the head can be problematic since inserting the head and the weld are performed in separate stages. In the present paper, we performed REW using a headless rivet which has minimal to no overhanging part above the aluminum surface. The lap-shear strength of REW is higher than self-piercing rivets, which makes REW a promising solution for extending applications of multi-material structures. The microstructure of the joint involves a fusion zone which mainly consists of martensite, and the failure occurs near the heat affected zone. This study finds that the electrode-rivet alignment needs to be optimized to manage the optimal nugget size and to avoid current arcing through the aluminum work piece. (Received August 12, 2019; Accepted September 30, 2019)

Enhancing nugget size and weldable current range of ultra-high-strength steel using multi-pulse resistance spot welding

ABSTRACTThis paper presents an innovative approach that uses a pulse-profile to improve the welding quality of CP1180 steel in resistance spot welding process. Three pulses with two cooling times were used in the developed multi-pulse welding (MPW) schedule. The experimental results show that the first pulse increases the contact area between the sheets to improve the current flow pattern. The second pulse was designed to extend the sheet-to-sheet contact area and corona bond for preventing rapid nugget growth. Using these designs, the nugget size was maximised through the third pulse. The maximum nugget size using the designed MPW schedule was 18.5% greater than that of the single-pulse welding schedule and the weldable current range was extended by 130%.

Fatigue Behaviors of Resistance Spot Welds for 980 MPa Grade TRIP Steel

The fatigue life of the resistance spot weld of 980 MPa grade transformation induced plasticity (TRIP) steel was investigated and failure modes and fracture surfaces according to the fatigue load were analyzed. The fatigue life according to the nugget size was observed by using two electrodes with face diameters of 8 mm and 10 mm. When an electrode face diameter with 10 mm was used, the nugget size was large, and the fatigue life was further increased. After the fatigue test, three types of failure modes were observed, namely pull-out, plug, and heat affected zone (HAZ) failure, depending on the fatigue load. The fracture surfaces in each failure mode were analyzed. In all failure modes, a crack was initiated in the HAZ region, which is the interface between the two materials in all failure modes. In the case of pull-out failure, the crack propagates as if it surrounds the nugget at the outer edge of the nugget. In the case of HAZ failure, the crack propagates in the thickness direction of the material and outward in the nugget shell. Plug failure occurs with pull-out failure and HAZ failure mixed. The propagation patterns of cracks were different for each failure mode. The reason why the failure mode and the fracture surface are different according to the fatigue load is that the propagation speed of the fatigue crack is fast when the fatigue load is relatively large and is slow when the fatigue load is low.

Adaptive Control of Resistance Spot Welding Based on a Dynamic Resistance Model

Resistance spot welding is a process commonly used for joining a stack of two or three metal sheets at desired spots. Such welds are accomplished by holding the metallic workpieces together by applying pressure through the tips of a pair of electrodes and then passing a strong electric current for a short duration. This kind of welding process often suffers from two common drawbacks, namely, inconsistent weld quality and inadequate nugget size. In order to address these problems, a new theoretical approach of controlling resistance spot welding processes is proposed in this paper. The proposed controller is based on a simplified dynamical model of the resistance spot welding process and employs the principle of adaptive one-step-ahead control. It is essentially an adaptive tracking controller that estimates the unknown process parameters and adjusts the welding voltage continuously to make sure that the nugget resistance tracks a desired reference resistance profile. The modeling and controller design methodologies are discussed in detail. Also, the results of a simulation study to evaluate the performance of the proposed controller are presented. The proposed control scheme is expected to reduce energy consumption and produce consistent welds.

Effects of Surface Roughness and Force of Electrode on Resistance Spot Weldability of Aluminum 6061 Alloy

The effects of electrode surface roughness and force on the resistance spot weldability and sticking of the electrode during resistance spot welding (RSW) of aluminum 6061-T6 alloy were investigated. RSW was carried out using an as-received electrode and an abraded electrode polished with sandpaper, and the nugget size and properties such as tensile shear strength and hardness of the resulting welds were investigated at two different electrode forces. In addition, a continuous RSW process was performed on the alloy to observe the effect of the electrode surface roughness on electrode sticking. When RSW was performed using the abraded electrode, which had a rough surface, the contact resistance decreased because of the effective removal of the oxide film from the surface of the aluminum alloy; consequently, the heat generated by the resistance on the surface was reduced. In addition, the growth rate of the weld nuggets formed with the abraded electrode in the thickness direction was lower than that of the nuggets formed with the as-received electrode, and the sticking of the abraded electrode was comparatively less. Also, the influence of the electrode force on the sticking of the electrode was greater in the case of the as-received electrode.

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.

Resistance ceramic-filled annular welding of thin steel sheets

To meet the requirements of automobiles for high-strength steel welding and thin-sheet welding, a composite ceramic-filled annular electrode (A-electrode) was designed by controlling welding energy distribution. By embedding a circular non-conductive ceramic rod in the center of a conventional copper electrode, the welding heat changed from centralization in the center of the nugget of traditional spot welding to a more dispersed annular distribution. As a result, an annular nugget that corresponds to the annular copper end face was formed. Given the energy dispersion, a higher current was required to bond the sheets by the A-electrode than that by a traditional electrode. However, the indentation depth on the surface of the nugget decreased effectively. The stress distribution indicated that the stress was concentrated at the edge of the nugget under the load, and the unconnected part in the middle did not affect the strength. The size of the annular nugget was designed by adjusting the structure of the A-electrode. The strength of the nugget could be improved by forming a large nugget area.

Criterion for predicting expulsion in resistance spot welding of steel sheets

Resistance spot welding (RSW) is frequently accompanied by expulsion, an undesirable event taking place in manufacturing processes, including car body production. Many variables are involved in RSW, all of which can affect expulsion. This investigation aimed at developing an analytical model for predicting expulsion in RSW. Accordingly, based on force and moment balances, a criterion was established. It was proposed that expulsion took place when the molten metal ejected as a result of the pressure inside nugget. The model presented here considered that the pressure inside the nugget was mainly due to melting transformation and liquid thermal expansion. Hence, the model took into account the effects of nugget diameter, electrode force, base material yield stress, sheet thickness, and joint fit-up to predict the occurrence of expulsion. To assess the model, a number of experiments were done, where the effects of welding current, electrode force, sheet thickness, and galvanization coating of the steel were studied. Regarding the effects of electrode force and welding current, the prediction made by the analytical model was consistent with the results obtained from the experiments. However, it was observed that expulsion in the galvanized steel sheets occurred at larger nugget diameters. The macroscopic examination of the welded samples showed that, with galvanization the ability for a better electrode fit-up was increased, and according to the model this could result in thereby increasing the critical nugget size.

Effect of Rotational Speed on Microstructure and Mechanical Properties in Submerged Friction Stir Welding of ME20M Magnesium Alloy

Submerged friction stir welding of magnesium alloys has not been well investigated to date. ME20M is an important lightweight magnesium (Mg) alloy with enhanced yield strength and heat resistance that merits further research. In this paper, submerged friction stir welding of the ME20M Mg alloy was carried out using different parameters for the underwater cooling. Defect-free weld joints were produced, and the macrostructure, microstructure, tensile properties, and hardness were investigated. The results show that by increasing the rotational speed, the grain size of the weld nugget increased, the tensile strength of the joint decreased, and the microhardness of the different weld zones decreased. The finest obtained grain size was about 3.5 µm in the weld nugget at a rotational speed of 1100 rpm. The highest tensile strength achieved was 183.2 MPa, which was ~ 76.32% of the base metal. The highest and lowest hardness values of the weld joint were obtained at rotational speeds of 1100 and 1600 rpm, respectively, in the weld nugget and heat-affected zones.