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A clinch-resistance hybrid spot welding method was presented to join Al and steel. The nugget formation was analysed via microstructure observation and numerical simulation. A cup section nugget wa...

An iron sand concentrate was mixed with coal as the reductant and binder to produce a composite pellet by a mini rotating disc. The pellet was inserted into an alumina crucible and coal was added t...

This paper researches the effect of zinc coating of galvanized DP600 steel on the dynamic resistance and the delayed nugget formation of dissimilar DP600 - AISI304 welded joints, obtained with resistance spot welding process (RSW). The RSW evaluations consisted of determining, from the dynamic resistance curves, the time involved in the different stages of the process, particularly the beginning of nugget formation. The experimental results showed that, from the dynamic resistance curves, it is possible to identify 8 distinct stages during the welding of galvanized DP600 steel and AISI304 stainless steel. In the case of the welding of uncoated DP600 steel with AISI304, only 6 stages are identified (except for stages 2 and 3), which are directly related to the heating, softening and melting of the galvanic coating. The energy used in stages 2 and 3, causes a delay in the beginning of nugget formation for welded joints obtained with galvanized DP600 steel compared to uncoated DP600 - AISI304 welded joints, reaching values between 37.28 ms and 52.29 ms for the welding conditions analyzed. Monitoring the time duration of stages 2 and 3, as defined from the analysis of the dynamic resistance curves, could be used as a tool to predict the beginning of nugget formation in the welding of galvanized steels, to avoid undesirable phenomena such as expulsion and to guarantee the quality of the welded joints.

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.

Press-hardened boron steels with ultrahigh strength (above 1500 MPa) are widely used in crash-sensitive safety components in automobiles. Joining such steels to aluminum alloys is challenging due to various factors including the steel’s tenacious Al-Si coating. A novel application of ultrasonic plus resistance spot welding was developed for such dissimilar metal joining. The nugget formation and the interface microstructure especially intermetallics formed were correlated to the joint strength, ductility and failure behavior.

Metallic coatings have been widely used for hot-stamped boron steel in automotive applications. Surface coating in hot-stamped boron steels has improved corrosion resistance. However, a thick coating layer degrades resistance spot weldability. In the case of a Zn-coated hot-stamped boron steel, the high contact resistance and material stiffness with low vaporization temperature of the alloyed layers result in a narrow current passage at the faying surface. Expulsion occurs at low current due to rapid heat development, resulting in a narrow weld current range. The pulsed current can control the heat input by applying a fast cool time during welding to improve the mechanical properties of the joint and acceptable current range. In this study, Zn-coated hot-stamped steels were resistance spot welded using various pulse current procedures, and welding condition was optimized via three pulsed current steps. Step 1: Contact area was controlled without nugget formation. Step 2: Nugget growth was minimized and contact area was maximized. Step 3: The maximum acceptable current range and nugget diameter were achieved. These welding conditions improved the acceptable current range and mechanical properties of spot welds due to increased spot weld size.

In recent years, increasing automotive safety by improving crashworthiness has been a focal point in the automotive industry, employing high-strength steel such as press hardenable steel (PHS). In addition to the improved strength of individual parts in the body of the vehicle, the strength of the resistance-spot-welded joints of these parts is highly important to obtain a safe structure. In general, dimensions of weld nuggets are regarded as one of the criteria for the quality of spot-welded joints. In the presented research, a three-dimensional axisymmetric finite element model is developed to predict the nugget formation in resistance spot welding (RSW) of two types of PHS: the uncoated and AlSi-coated 1.8 mm boron steel after hot stamping. A fully coupled electro-thermo-mechanical analysis was conducted using the commercial software package Abaqus. The FE predicted weld nugget development is compared with experimental results. The computed weld nugget sizes show good agreement with experimental values.

Recently, weight reduction and improvement of crashworthiness of auto bodies have become important issues. At the same time, stiffness of auto bodies is also needed to ensure a smooth ride. Using h...

The selection and application of the type coating to steel sheets in automotive industries depend on factors like corrosion protection, manufacturing compatibility and cost. The ‘first generation’ type primer coating improved the corrosion protection of chromate free pretreatments, however it deteriorated the resistance spot weldability of the sheets. The development of 2nd generation primer coating with additives such as CNT showed dominance in corrosion protection properties along with zinc. The present paper investigates the resistance spot weldability of the CNT added primer coating. The weldability was evaluated by comparing the CNT added primer with 1st generation primer for nugget size and electrode life. At lower welding currents, the nugget size of the CNT added primer was significantly lower than the primer coated steels. The increase in the dynamic resistance of the CNT coated steel after nugget formation led to higher nugget sizes at higher currents. The decrease in aluminum and increment in the Fe concentration at the electrode surface during welding of CNT based primer coated steels would improve the electrode life.

A double-pin tool is adopted in the submerged friction stir lap welding of high density polyethylene sheets in the present investigation. Effects of welding parameters (rotational speed, welding speed, plunge depth) on weld morphology and tensile strength are investigated. The experiment is arranged by taguchi’s L9 orthogonal array. Macromorphology and microstructure of three typical welds are observed and compared. Analysis of variance is adopted to assess the importance of each welding parameter to the joint strength. The results show that joint with low tensile strength presents poor weld formation. Voids and cracks can be observed in the nugget by scanning electron microscope. Joint with high strength shows excellent nugget formation and inverted bowl-shaped weld cross section. Joint strength increases with the increase of rotational speed or the decrease of welding speed. Small or large plunge depth is not beneficial to the joint strength. The joint with maximum tensile strength of 15.3 MPa is obtained at rotational speed of 1300 r/min, welding speed of 20 mm/min and plunge depth of 0.1 mm. Welding speed has the most significant effect on the joint strength, about 75.37%, while plunge depth has the least influence.

Variation in dynamic contact resistance with the change in welding process parameters such as weld current, weld time and electrode force were taken into account for establishing the range of adequate nugget formation parameters. Influence of the welding process parameters on the shear – tensile strength, nugget diameter and the observed failure mode was analysed. The adequate resistance spot welding process parameters for galvannealed high strength interstitial free steel sheets of 1.6 mm thickness were estimated as 8 kA weld current, 250 ms weld time and 3.5 kN electrode force. Increase in the mean dynamic contact resistance led to a significant reduction in nugget diameter. A critical nugget diameter distinguishing between the IF and PF mode was experimentally determined by failure mode analysis. Different numerical models for estimation of critical nugget diameter were evaluated.

Two types of dissimilar AA6061/H70 thermo-compensated resistance-spot-welding joint configuration are designed. Thermal-electric coupled finite element models are built to reveal the temperature field of these two types of joint. An AISI 201 stainless-steel thermo-compensated sheet assists resistance spot welding through heat conduction. The nugget formation is mainly due to conductive heat from this cover sheet when the thermo-compensated sheet is set adjacent to the AA6061 workpiece (type A joint) and is driven by both Joule heat and conducted heat when the cover sheet is set adjacent to the H70 workpiece (type B joint). To ensure the nugget formation, the conducted heat portion requires a long welding time while the Joule heat portion requires a high welding current. The type B joint configuration has higher Cu and Zn contents in the fusion zone, which leads to a high intermetallic content in the Al-Cu-Zn ternary system existing in the nugget. Most type A joints thus have a higher peak load than type B joints in tensile shear tests.

Resistance spot welded joints in different configurations of DP600 and DC54D were investigated to elucidate the nugget formation process and mechanical properties of the resultant joints. Results show that, when the welding time was less than 4 cycles, the fusion zone (FZ) was not formed, but the heat-affected zone (HAZ) occurred with a “butterfly” shape. In 4 cycles, the FZ in dissimilar sheets occurred with an “abnormal butterfly” shape because of nugget shift. When the welding time increased to 14 cycles, the FZ exhibited a “bread loaf” shape and the weld shifted to “dog bones.” The nugget can be divided into three regions, namely, FZ, HAZ1, and HAZ2, and the FZ consisted of lath martensite. The micro hardness of DP600 FZ was lower than that of HAZ because of the dilution of DC54D. The failure mode of B changed from interfacial failure to plug failure during the nugget formation process. The tensile-shear load of sound weld is 6.375, 6.016, and 19.131 kN.

In this study, an effective numerical simulation on resistance spot welding of dual-phase steel was conducted. The electrical, thermal and mechanical behaviours during squeezing, welding and holding stages were analyzed, respectively. Large lath martensite was formed in the fusion zone during the experiment. A good agreement on nugget development was obtained between numerical and experimental results. The size of contact region and pattern of contact pressure distribution at the squeezing stage were found to be force-independent. The variation of contact pressure distribution during the welding process was interpreted combining the temperature and deformation distribution of the workpiece. The contact pressure at both edges of contact interfaces was reconcentrated after nugget formation. The compressive state of axial and radial stress at the weld nugget was explained through the thermal expansion mismatch and the electrode force effect. The plastic annulus and notch between workpieces were also successfully simulated.

Production processes like continuous annealing and continuous galvanizing/galvannealing rely on welding of two strips across their width, for continuous operation. The operational stability of these lines depends highly on the consistency of the welding process. Any failure of weld joint during processing is detrimental to production as the recovery times are very high (~1–3 days). Two instances of similar weld failures having identical chemical composition and dimensions of the strips were investigated to identify the root cause and establish adequate countermeasures. A systematic approach to evaluate weld performance using metallographic examination was carried out. The examinations revealed that poor nugget formation was the root cause of failure. The nugget formation, its orientation and size were also evaluated after carrying out a series of trials with different welding process parameters, and an optimum condition was arrived at. In summary, the weld failure occurred due to insufficient and unbalanced nugget formation which could be rectified by modifying the welding parameters such as current, pressure and speed.

The present paper deals with the joining of 22MnMoB hot-stamping annealed steel carried out by the spot welding process. Microstructural characterization, microhardness testing and tensile-shear testing were conducted. The effects of the welding parameters, including the electrode tip diameter, welding current, welding time and electrode force upon the tensile-shear properties of the welded joints, were investigated. The results showed that a weld size of 9.6 mm was required to ensure pullout failure for the 1.8-mm-thick hot-stamping annealed steel sheet. The welding current had the largest influence upon the tensile-shear properties of the 22MnMoB steel welded joint. The bulk resistance should play an important role in the nugget formation. In pullout failure mode, failure was initiated at the heat-affected zone, where softening occurs owing to the tempering of martensite.

Ni-laterites from the Dominican Republic host rare but extremely platinum-group element (PGE)-rich chromitites (up to 17.5 ppm) without economic significance. These chromitites occur either included in saprolite (beneath the Mg discontinuity) or as 'floating chromitites' within limonite (above the Mg discontinuity). Both chromitite types have similar iridium-group PGE (IPGE)-enriched chondrite normalized patterns; however, chromitites included in limonite show a pronounced positive Pt anomaly. Investigation of heavy mineral concentrates, obtained via hydroseparation techniques, led to the discovery of multistage PGE grains: (i) Os-Ru-Fe-(Ir) grains of porous appearance are overgrown by (ii) Ni-Fe-Ir and Ir-Fe-Ni-(Pt) phases which are overgrown by (iii) Pt-Ir-Fe-Ni mineral phases. Whereas Ir-dominated overgrowths prevail in chromitites from the saprolite, Pt-dominated overgrowths are observed within floating chromitites. The following formation model for multistage PGE grains is discussed: (i) hypogene platinum-group minerals (PGM) (e.g. laurite) are transformed to secondary PGM by desulphurization during serpentinization; (ii) at the stages of serpentinization and/or at the early stages of lateritization, Ir is mobilized and recrystallizes on porous surfaces of secondary PGM (serving as a natural catalyst) and (iii) at the late stages of lateritization, biogenic mediated neoformation (and accumulation) of Pt-Ir-Fe-Ni nanoparticles occurs. The evidence presented in this work demonstrates that in situ growth of Pt-Ir-Fe-Ni alloy nuggets of isometric symmetry is possible within Ni-laterites from the Dominican Republic.

The options for joining aluminum (Al) alloy to ultra-high strength steels are extremely limited due to the great differences in their thermophysical properties. In this work, a novel process, e.g., resistance rivet welding (RRW), was proposed to join Al alloys to press hardened steels (PHS) through combining traditional self-piercing riveting and resistance spot welding. The materials plastic flow, element diffusion and nugget growth characteristics during RRW were studied in depth to reveal the nugget formation mechanism. It was found that in RRW process, the intercepted Al alloy, internal wall of rivet shank and PHS near the faying surface were melted together and produced a unique Al/steel mixed nugget. The microstructure of the mixed nugget was equiaxed ferrite with a large number of B2-structured intermetallic compounds (IMCs) FeAl dispersed within it. The competition between the nugget and Al sheet determined the joint mechanical performance. The nugget dominated joint strength when the heat input is comparatively low while the Al sheet played a significant role with the increase of heat input.

To efficiently recycle valuable metals such as chromium and nickel in stainless steel dust, self-reduction experiments were carried out to study the reduction mechanism of metal oxides in coal composite stainless steel dust hot briquette, which is defined as a CCSB here. Self-reduction of CCSB is proceeded by volatile matter and fixed carbon contained within CCSB. Experiments were performed to study the effects of temperature and carbon to oxygen (C/OCoal) ratio on self-reduction of CCSB. At 1400 and 1450 °C, volatile matter in coal used for experiment could take the place of about 40% of fixed carbon in coal. Under the present experimental conditions, reduction product of chromium appears as FeCr2O4, Cr2O3, Cr7C3, and [Cr] in turn during reduction. To evaluate the formation of metal nuggets in self-reduction process of CCSB, metal nuggets containing chromium and nickel were observed in outside of reduction products under various conditions, and thermodynamic equilibrium calculation was carried out for possible products and formation of molten metal by fixed carbon. SEM and EDS analyses were made for metal nugget and slag in reduced product. The results reveal that it is reasonable to achieve the metal nuggets at 1450 °C, 0.8 of C/OCoal ratio and 20 min of reduction time. The nugget formation can indicate one innovative process for comprehensive utilization of stainless steel dust.

The present study aims at solving weld quality monitoring problem in small scale resistance spot welding of titanium alloy. Typical dynamic resistance curves were divided into several stages based on the weld nugget formation process. A smaller electrode force or lower welding current was found to promote the initial resistance peak. The bulk material heating stage could not be detected under very high welding current condition. Electrode force effect on dynamic resistance and failure load was much smaller than that of welding current. Principal component analysis was made on discrete dynamic resistance values. The first principal component was selected as independent variable in regression analysis for quality estimation. A back propagation neural network model was then proposed to simultaneously predict the nugget size and failure load. The electrode force, welding current, welding time, and first five principal components were designed as network inputs. Effectiveness of the developed model was validated through data training, testing, and validation. The realtime and online quality monitoring purpose could be realized.

It is considered to be available in reducing the weight of automobiles that the aluminum alloy is applied to manufacture them. The spot welding of aluminum alloys is difficult because of high welding current and small electrical resistance. It is necessary to clarify the relation between the nugget formation and the welding conditions. In this study, the relation between the spot welding conditions and the nugget form was investigated, and the joint strength and the fracture mode were investigated. As a result, the nugget area and micro cracks in the HAZ were increased with increase of the welding current. It is found that in case of aluminum alloys the formation of micro cracks was a main factor of reduction of maximum tensile load of the welded zone.

Currently, the use of advanced high strength steels (AHSSs) is the most cost effective means of reducing vehicle body weight and maintaining structural integrity at the same time. However, AHSSs present a big challenge to the traditional resistance spot welding (RSW) widely applied in automotive industries because the rapid heating and cooling procedures during RSW produce hardened weld microstructures, which lower the ductility and fatigue properties of welded joints and raise the probability of interfacial failure under external loads. Changing process parameters or post-weld heat treatment may reduce the weld brittleness, but those traditional quality control methods also increase energy consumption and prolong cycle time. In recent years, a magnetically assisted RSW (MA-RSW) method was proposed, in which an externally applied magnetic field would interact with the conduction current to produce a Lorentz force that would affect weld nugget formation. This paper is a review of an experimental MA-RSW pla...

Annealing under an atmosphere with controlled oxygen partial pressure is an effective way for improving reactive wetting during the continuous galvanizing process. Hence, understanding the effects of this annealing step on further manufacturing processes such as welding is essential. The present work has shown that the formation of internal oxides during a controlled atmosphere galvanizing process reduced the resistance and heat input resulted in delaying nugget growth in resistance spot welding, compared to the as-received condition. Furthermore, it was shown that by annealing and subsequent zinc coating, the tensile-shear peak load was decreased under the same welding parameters. The nugget formation mechanism of different surface conditions showed a good correlation with dynamic resistance profiles.

International audience; An experimental apparatus was designed to reproduce a resistance spot welding operation and allow observation via infrared camera. A specific ex situ device was used to measure the variations of electrical contact resistances, versus pressure and temperature, for the electrode-sheet and sheet sheet interfaces. The high values of the electrical contact resistances associated to the Al-Si coated Press Hardened Steel sheet explains the spark effect observed at these interfaces by infrared thermography. The initiation and the growth of the nugget from the Al-Si coated sheet to the galvanized low carbon thin sheet are also due to these high contact resistances. The different observation methods and measurements concur to explain the nugget formation.

Ti6Al4V titanium alloy is applied extensively in the aviation, aerospace, jet engine, and marine industries owing to its strength-to-weight ratio, excellent high-temperature properties and corrosion resistance. In order to extend the application range, investigations on welding characteristics of Ti6Al4V alloy using more welding methods are required. In the present study, Ti6Al4V alloy sheets were joined using resistance spot welding, and the weld nugget formation, mechanical properties (including tensile strength and hardness), and microstructure features of the resistance spot-welded joints were analyzed and evaluated. The visible indentations on the weld nugget surfaces caused by the electrode force and the surface expulsion were severe due to the high welding current. The weld nugget width at the sheets’ faying surface was mainly affected by the welding current and welding time, and the welded joint height at weld nugget center was chiefly associated with electrode force. The maximum tensile load of welded joint was up to 14.3 kN in the pullout failure mode. The hardness of the weld nugget was the highest because of the coarse acicular α′ structure, and the hardness of the heat-affected zone increased in comparison to the base metal due to the transformation of the β phase to some fine acicular α′ phase.

Resistance spot welding (RSW) is a popular welding process employed for joining sheet metals typically in automobile and aerospace industries. RSW is conventional but not obsolete as it addresses the desirable characteristics of energy efficiency, narrow heat-affected zone (HAZ), simplicity, and automation that make it the first choice of manufacturers. In recent days, there is a perpetual demand for spot welding owing to its suitability for joining components especially in biomedical applications. Austenitic stainless steel of grade AISI 316L, a biocompatible material with extraordinary features and global suitability, caters to several industrial dimensions. In this research paper, an attempt is made to experimentally investigate the quality of the weld prepared by resistance spot welding of austenitic stainless steel type 316L of equal thickness (2.0 + 2.0 mm). Experimental trials based on design of experiments (DOE) are conducted by varying the process parameters such as electrode tip diameter, welding current, and heating time and subsequently measure the nugget size and shape which governs the strength and bond integrity of the resistance spot welds. Subsequently, a coupled structural-thermo-electric analysis is performed to analyse the transient temperature field for predicting the nugget formation during RSW of 316L stainless sheet using a 2D axisymmetric finite element (FE) model. FE-based model of RSW process is developed using ANSYS Parametric Design Language (APDL) module available in FE package, ANSYS. Thermo-physical interactions like heat transfer through the Joule effect, the effect of latent heat of fusion, and the thermal and electrical contact conductance are considered while developing the FE-based model. A good coherence is achieved between numerical predictions of nugget shape and experimental spot-welded results. Finally, the welded specimens are subjected to destructive testing and metallurgical characterizations to evaluate its strength and quality.

Resistance spot welding is used extensively in auto industry. Every commercial vehicle has 4000-5000 spot welds. The weld ability, performance, and reliability are therefore important issues in design. In this paper, we studied weld nugget formation and failure behavior of three-sheet 5052 aluminum alloy resistance spot welds. The Peltier effect between the Cu-Al (the electrode and the Al worksheet) to the nugget formation was noticed. The mechanical strength and fracture mode of the weld nuggets at the upper and lower interfaces were studied using tensile shear specimen configuration. Three failure modes were identified, namely, interfacial, mixed, and pull-out. The critical welding time and critical nugget diameter corresponding to the transitions of these modes were investigated. Finally, an empirical failure load formula for three-sheet weld similar to two-sheet spot weld was developed.

A 2D axisymmetric electro-thermo-mechanical finite element (FE) model is developed to investigate the effect of current intensity, welding time, and electrode tip diameter on temperature distributions and nugget size in resistance spot welding (RSW) process of Inconel 625 superalloy sheets using ABAQUS commercial software package. The coupled electro-thermal analysis and uncoupled thermal–mechanical analysis are used for modeling process. In order to improve accuracy of simulation, material properties including physical, thermal, and mechanical properties have been considered to be temperature dependent. The thickness and diameter of computed weld nuggets are compared with experimental results and good agreement is observed. So, FE model developed in this paper provides prediction of quality and shape of the weld nuggets and temperature distributions with variation of each process parameter, suitably. Utilizing this FE model assists in adjusting RSW parameters, so that expensive experimental process can be avoided. The results show that increasing welding time and current intensity lead to an increase in the nugget size and electrode indentation, whereas increasing electrode tip diameter decreases nugget size and electrode indentation.

In this work, the effects of the coating composition and the welding parameters on the heat generation and nugget formation during resistance spot welding of the hot press forming steels have been evaluated. Two types of coated steels were used, termed as Al–Si and Zn coating. Al–Si coated steel showed rapid nugget growth toward the electrode direction, which is the probable reason of the higher heat conduction from the electrode–sheet surface toward the bulk material. In Zn coated steel, heat was generated and localised at the faying interface and uniformly propagated to the steel substrate. High speed camera images showed that the presence of oxide at faying interface provides inhomogeneous current flow and violent heat generation in the Zn coated steel.

This paper investigates the weld nugget formation in three-sheet aluminum alloy resistance spot welding. The nugget formation process in three equal thickness sheets and three unequal thickness sheets of 5052 aluminum alloy were studied. The results showed that the nugget was initially formed at the workpiece/workpiece interfaces (i.e., both upper interface and lower interface). The two small nuggets then grew along the radial direction and axial direction (welding direction) as the welding time increased. Eventually, the two nuggets fused into one large nugget. During the welding process, the Peltier effect between the Cu-Al caused the shift of the nugget in the welding direction. In addition, the mechanical strength and fracture mode of the weld nuggets at the upper and lower interfaces were also studied using tensile shear specimen configuration. Three failure modes were identified, namely interfacial, mixed, and pullout. The critical welding time and critical nugget diameter corresponding to the transitions of these modes were investigated. Finally, an empirical failure load formula for three-sheet weld similar to two-sheet spot weld was developed.

Weldability of a new lightweight sandwich material, LITECOR®, by resistance spot welding is analyzed by experiments and numerical simulations. The spot welding process is accommodated by a first pulse squeezing out the non-conductive polymer core of the sandwich material locally to allow metal–metal contact. This is facilitated by the use of a shunt tool and is followed by a second pulse for the actual spot welding and nugget formation. A weldability lobe in the time-current space of the second pulse reveals a process window of acceptable size for automotive assembly lines. Weld growth curves are presented together with results of numerical simulations made in the finite element computer program SORPAS® 3D, which is based on an electro-thermo-mechanical formulation. The numerical models are presented together with the specific modeling conditions leading to numerical simulations in good agreement with experimental results in the range of welding parameters leading to acceptable weld nugget sizes. The validated accuracy of the commercially available software proves the tool useful for assisting the choice of welding parameters.

The weld nugget formation in the resistance spot welding (RSW) of aluminum alloy was investigated in the present study. The nugget formation process was directly observed by using a digital high-speed camera. Numerical simulation was also employed to investigate the nugget formation process. The results showed that for the RSW of two aluminum alloy sheets, a nugget was first formed in the workpiece/workpiece (W/W) interface and grew along the radial direction and axial direction of the sheets, and then it became a large elliptical nugget. For the RSW of three aluminum alloy sheets, two small nuggets were firstly formed in two W/W interfaces and grew along the axial direction and radial direction; finally they fused into one nugget. Besides, there existed a critical welding time, after which the nugget size remained nearly unchanged. This indicates that a long welding time is unnecessary for the RSW of aluminum alloy. In addition, the calculated nugget radius was compared with the experimental results, which showed that the simulation results agreed well with the experimental results.

Experiments were made for welding current variation between 6 kA and 12 kA. Microstructure and mechanical performance under tensile-shear tests were recorded and compared. Welding current effect on micro-properties was very slight while nugget size was highly dependent on welding current. Expulsion phenomenon existed at 12 kA welding current and the unsatisfactory partial interfacial failure was detected. Nugget formation and temperature distribution were analysed numerically. Comparisons with experimental results showed that the nugget size deviation was within 10% and the nugget shape was well predicted.

This paper investigates the effect of electromagnetic stirring (EMS) on the nugget formation, microstructure, and mechanical properties of magnesium alloy resistance spot weld (RSW). The EMS was induced by an external magnetic field (EMF), which produced by a pair of permanent magnets. The results showed that the EMS drove the molten metal in the nugget to flow clockwise and generated a centrifugal movement, which brought high temperature molten metal from the center of the growing nugget to its edge. Compared with the traditional magnesium alloy RSW, the weld under the EMS effect (EMS-RSW) exhibited larger weld diameter, higher tensile shear force and energy absorption, and finer equiaxed crystal and columnar crystal. The EMS-RSW had a higher hardness ratio of fusion zone to pullout failure location, which indicated that the EMS-RSW was more sensitivity to experience pullout failure. Finally, an empirical failure load formula for magnesium alloy resistance spot weld was developed.

In this study, a finite element model was developed based on the thermal contact conductance for predicting the nugget formation during the welding process in aluminum alloy 6061-T6. Since the strength of spot welded joints is completely dependent on nugget dimensions, an axisymmetric coupled structural–thermal–electrical finite element model was built and utilized to predict the nugget size. The thermal contact conductance, which is considered as a function of temperature and surface roughness, was applied in the contact area between electrode–workpiece and workpiece–workpiece. The results were validated by the data obtained from the nugget experimental measurement tests on the aluminum alloy and the previous study on steel. The obtained data from finite element model enjoys good agreement with the experimental nugget measurement tests. The results indicate that in the modeling of aluminum alloys spot welding, the thermal contact conductance has a major role in nugget enlargement during the welding process and cannot be ignored. In addition, it was shown that by increasing the welding heat input to the nugget zone, the contact resistance at the interfaces (electrode–sheet and sheet–sheet) will be decreased. As a result of that, the height of the nugget increases with a higher rate comparing to its diameter. Furthermore, in this study, the effects of spot welding parameters such as the welding current and the welding time are investigated on nugget dimensions.

WOS: 000431808800007, In this study, galvanized and austenitic stainless steel sheets were joined to each other by using resistance spot welding. The thicknesses of the galvanized steel and austenitic stainless steel are 0.93 and 1.03 mm, respectively. The experiment parameters are the sheet combination and the weld nugget diameter. Three weld nugget diameters were selected, 4, 5, and 6 mm, which are most widely used in the steel sheet joining industry. Pre-tests were performed to investigate the effect of the welding current on the nugget diameter. By using the pre-test results, series of resistance spot welded steel sheets were obtained with 4, 5 and 6 mm (+/- 0.1) nugget diameters, in the combinations of galvanized - galvanized steel and galvanized - austenitic stainless steel. In the first step of main experiments, spot welded specimens were subjected to tensile tests, and the effect of the sheet combination on the maximal tensile strengths was discussed. In the last step of the investigation, macro photographs of the weld nugget between the steel sheets were taken. The effect of the sheet combination on the nugget formation was discussed and the relationship between the maximal tensile strength and the nugget formation was determined.

Flow phenomenon in spot welding nugget of dissimilar materials was studied. Metallographs in the course of welding nugget formation were observed and analyzed, through which it was indicated that: in the early stage of welding nugget formation, spot welding of dissimilar materials made liquid metals in the nugget produce a violent stirring action because the electrodynamic force overcame the medium resistance, causing unevenness of elements and compositions in the welding nugget; and then the unevenness was represented by vortex flow by means of different resistances to corrosion. It was proposed that the formation law of welding nugget could be recognized according to the formation law of vortex flow, and the vortex flow often moved from later-molten metal to former-molten metal. The property of welded joint with vortex flow was poorer than that without vortex flow.

The objective of this work is to develop a numerical model of the resistance spot welding process with a flat cylindricallower electrode, enabling to predict accurately the spot weld geometry development. A 2D axisymmetric electro-thermo-mechanical finite element (FE) model is developed to study the effect of spot welding parameters on nugget size using ANSYS commercial software package. The nugget formation processes at the various welding conditions were investigated by both experimental measurement and FEM simulation.In order to improve the accuracy, temperature dependent properties of material are taken into account during the simulation. Experiments were carried out to verify the preciseness of simulation. Two different thicknesses of austenitic stainless steel sheets were used in the study. The FE predicted weld nugget sizes agreed well with the experimental results. Overall, the present work indicated that finite element analysis can be very useful as an off-line observation tool to estimate the influence of welding parameters on the welding quality and to predict or improve the welding quality.

The effects of electrical resistance at the workpiece-electrode contact surface or the electrode face on the joint quality, characterized by nugget shape, temperature, cooling rate, and solute concentration responsible for microstructure of the fusion zone during resistance spot welding (RSW), are systematically investigated. The model accounts for electromagnetic force, heat generation due to contact resistance at the faying surface and electrode face, and bulk resistance in the workpieces. Contact resistances composed of different features of constriction and film resistances are functions of hardness, temperature, electrode force, and surface condition. The computed results show that nugget growth and transport processes are independent of the film resistance at the electrode face. An increase in constriction resistance at the electrode face, however, decreases the onset time for nugget formation, cooling rates, and axial and radial heat transfer, and increases the electrode temperature. The workpiece is susceptible to surface melting if the parameter governing constriction resistance at the electrode face is high. Solute concentration and flow patterns affected by constriction resistance at the electrode face are also presented.

In this paper, nugget growth stages during resistance spot welding between aluminum alloy and steel, such as surface breakdown, asperity softening, temperature rising, molten nugget formation, nugget growth and mechanical collapse, was analyzed according to resistance dynamic variation during welding. The effects of work pieces surface condition and electrode force on the occurrence of inter-expulsion and surface expulsion was discussed during resistance spot welding.

Single sided resistance spot welding (SSRSW) developed from RSW is a feasible plan to join vehicle structure to closed-form tube and is increasingly used in automobile manufacturing. During the process of SSSW, large deformation and complex contact status of the workpieces occur because there is no inside support. The complex contact status has directly influence on the size of the nugget which is critical to the quality of welding. In this study, the contact statuses during the current stage of sheet-to-tube spot welding were researched by numerical method. It was found that the widths of the contact regions of electrode to sheet and sheet and sheet to tube have dynamic change during welding process which decided the size and speed of the nugget formation. The results have direct guiding significance to the study on ring nugget formation during sheet-to-tube welding.

Iron nuggets can be obtained from ore–coal composite pellets by high temperature reduction. Alumina in the ore plays a vital role in slag–metal separation during nugget formation, as it increases the liquidus temperature of the slag. In this study, the effect of carbon content, reduction temperature and lime addition on slag–metal separation and nugget formation of varying alumina iron ore fines were studied by means of thermodynamic modelling. The results were validated by conducting experiments using iron ore fines with alumina levels ranging from 1·85 to 6·15%. Results showed that increase in reduction temperature enhances slag metal separation, whereas increasing alumina and carbon content beyond the optimum level adversely affects separation. Carbon below the required amount decreases the metal recovery, and carbon above the required amount reduces the silica and alters the slag chemistry. Optimum conditions were established to produce iron nuggets with complete slag–metal separation using ir...

This paper investigates the source, magnitude and direction of magnetic force on the liquid nugget during resistance spot welding (RSW). High speed photography was used to observe the nugget formation process during half-sectioned RSW of steel (ferromagnetic substance) and aluminium alloy (paramagnetic substance). The induced magnetic force acting on half-sectioned and regular nugget was afterwards analysed and calculated. The results show that in the case of steel RSW, the magnetised workpieces generated a very strong magnetic field around the spot welding zone, while weak magnetic field appeared in the case of aluminium alloy RSW. This strong magnetic field causes a strong convection in the liquid nugget of steel even when the welding current is low. This strong convection will promote that the dimensions of nugget in the steel RSW become larger and closer to rectangle (observed on the cross-section) than in the aluminium alloy RSW.

The aim of this paper is to provide an integrated real-time control system for resistance spot welding (RSW), which is capable of producing welds with predetermined nugget diameters. Nugget diameter is a commonly used criterion for estimating the weld quality; thus, the system can be used for online quality control of RSW. The proposed system consists of two parts: a constant current controller and an online nugget diameter estimator. The constant current controller is used to guarantee the consistency of the nugget formation and growth during the welding process, while the online nugget diameter estimator can be used as a sensor for online estimation of the nugget diameter. The proposed online nugget diameter estimator is a mathematical function of the heat energy absorbed by the weld, which is taken into account only after the time when the first melting point of a weld appears. This time can be obtained by means of the dynamic resistance curve with a high resolution. A mathematical model of the online nugget diameter estimator is proposed and realized based on the experiment results and mathematical analysis. The welding process would be terminated when the difference between the actual nugget diameter and the predetermined desired value is within a given tolerance. According to a series of experiments, the welds used in the experiments had nugget diameters, which were well within the limits and the errors were within the given tolerance.

Nowadays, car manufacturers tried to improve automotive fuel efficiency, and applied many high strength steels such as AHSS or UHSS to car bodies. Therefore, the number of steel combinations for the resistancespot welding are dramatically increased and the need for weldability evaluation of these combinations are also required. In this study, we suggest the lobe curve using FEM simulations for DP780 steel with 1.0t, 1.4t. The lobe curves which could expressed weldablity and optimal welding condition were obtained according to 6 steel combinations. There were two combinations for same steel sheet which were DP780 1.0t, DP780 1.4t. Dissimilar steel sheet combination with different thickness was 1.0t and 1.4t of DP780. Different steel combinations were DP780 1.0t and SPRC440 1.0t, and DP780 1.0t and DP590 1.0t. Finally dissimilar combinations was and DP780 1.0t and DP590 1.4t. The trend of low boundary and high boundaryvariation of lobe curve were analyzed with a viewpoint of the contact resistance and the heat input.Key Words : Resistance spot welding, Advanced high strength steel, Weld lobe curve, Nugget formation, Tensile shear strength

In order to explore the influence of welding parameters and to investigate the Al alloy (AA) nugget formation process, a comprehensive model involving electrical–thermal–mechanical and metallurgical analysis was established to numerically display the resistance spot welding (RSW) process within multiple fields and understand the AA-RSW physics. A multi-disciplinary finite element method (FEM) framework and a empirical sub-model were built to analyze the affecting factors on weld nugget and the underlying nature of welding physics with dynamic simulation procedure. Specifically, a counter-intuitive phenomenon of the resistance time-variation caused by the transient inverse virtual variation (TIVV) effect was highlighted and analyzed on the basis of welding current and temperature distribution simulation. The empirical model describing the TIVV phenomenon was used for modifying the dynamic resistance simulation during the AA spot welding process. The numerical and experimental results show that the proposed multi-field FEM model agrees with the measured AA welding feature, and the modified dynamic resistance model captures the physics of nugget growth and the electrical-thermal behavior under varying welding current and fluctuating heat input.

Resistance spot welding (RSW) is a widely used joining process for fabricating sheet metal assemblies in automobile industry. In comparison with other welding processes, RSW is fast and easy for automation. Spot welding process involves electrical, thermal, and mechanical interactions. These make the whole welding procedure highly nonlinear and difficult to model. This article presents the modelling and simulation of spot welding, using finite element analysing tool, ANSYS. A two-dimensional axisymmetric model was used to model the thermo-electro-mechanical coupling of process. In order to improve accuracy, material properties were defined temperature-dependent in simulation. Through simulation, the thermal history of the process was predicted. Development of weld nugget during process and effect of process parameters on nugget formation were investigated. Also contact area radius was obtained at different welding stages. Weld nugget sizes were obtained by numerical modelling showed good agreement with ex...

In resistance spot welding of thin sheet–thick sheet–thick sheet joint, when the sheet thickness ratio is large (sheet thickness ratio = total thickness of sheet joint/thickness of the thin sheet positioned on the outside of the joint), how to stably secure the nugget between the thin sheet and the adjoining thick sheet is a key issue. If the sheet thickness ratio is so large, nugget formation between the thin sheet and thick sheet is extremely difficult. In order to control of the nugget (position of formation, shape, etc. of the nugget) during welding for three sheets joint with a high sheet thickness ratio, optimum welding process was investigated. The developed ‘two-step force, two-step current’ welding process was suitable for high sheet thickness ratio joint and relaxed the constraints on the sheet thickness ratio. In Step 1 (first part of welding period) of the welding process, a nugget is reliably formed between the thin sheet and thick sheet by applying conditions of low electrode force, short we...

Keywords: dissimilar metals, resistance spot welding, finite difference, numerical simulation Abstract. Established dissimilar metal iron and nickel resistance spot welding nugget formation temperature field mathematical model for the discretization of the finite difference method, using TFRSW numerical analysis software, data processing module based on MATLAB language, study the dissimilar metal welding nugget formation in the process of resistance spot welding, the size of the develop and change. Put forward the dissimilar conductive metal welding process, the heat transfer effect simulation method and the analysis method of contact resistance. Design the corresponding experiment, verify the correctness of the results of numerical simulation, and analysis in dissimilar metal in the resistance spot welding specification for welding quality are studied.