Nugget Formation Research Articles

Weld nugget formation and mechanical properties of three-sheet resistance spot welded low carbon steel

In this paper, weld nugget formation and mechanical properties of 1·25/1·25/1·25 mm three-sheet low carbon steel resistance spot welds are investigated. Heat conduction from sheet/sheet interfaces (i.e. places with higher electrical resistance) to the geometrical centre of the joint and heat dissipation via water cooled electrodes play an important role in weld nugget growth of three-sheet resistance spot welding. It was shown that the fusion zone size along the sheet/sheet interface was greater than that of along geometrical centre of the joint leading to high tendency to interfacial failure. Fusion zone size at the sheet/sheet interface proved to be the most important controlling factor of mechanical performance of three-sheet spot weld in terms of peak load and energy absorption.Dans cet article, on examine la formation du noyau de soudure et les propriétés mécaniques de trois tôles d’acier de 1·25–1·25–1·25 mm à faible teneur en carbone, soudées par points par résistance. La conduction de chaleur des interfaces tôle - tôle (c’est-à-dire les endroits ayant une résistance électrique plus élevée) vers le centre géométrique du joint et la dissipation de chaleur par l’intermédiaire d’électrodes refroidies à l’eau jouent un rôle important dans la croissance du noyau de soudure du soudage à trois tôles par points par résistance. On a montré que la taille de la zone de fusion au long de l’interface tôle - tôle était plus grande que la taille de la zone de fusion au long du centre géométrique du joint, donnant naissance à une tendance élevée à la défaillance interfaciale. Il s’est avéré que la taille de la zone de fusion à l’interface tôle - tôle était le facteur de contrôle le plus important du rendement mécanique de la soudure par points à trois tôles en ce qui a trait à la charge de pointe et à l’absorption d’énergie.

Effect of External Constant Magnetic Field on Weld Nugget of Resistance Spot Welded Dual-Phase Steel DP590

The applications of magnetic field in metal casting and welding processes have drawn much attention of researchers. However, the effect of external magnetic field on nugget formation of resistance spot welding (RSW) is still lack of understanding. In this paper, the effect of a radially oriented constant magnetic field generated by a pair of ring-shaped permanent magnets on the weld nugget of RSW was experimentally investigated. The nugget diameter, microhardness, and mechanical performance were compared between welds with and without the external magnetic field. Results showed that, under the action of the external magnetic field, the weld nugget got wider with an increase in diameter by nearly 10%. Besides, the Vickers hardness in fusion zone (FZ) of the welding joint was more uniform. As a result, the RSW joints welded under the constant magnetic field exhibited better mechanical performance in tensile-shear tests. Therefore, the application of the external constant magnetic field could be an effective way to control the weld quality of RSW by affecting its melting and crystallization process.

Numerical and experimental study of nugget size growth in resistance spot welding of austenitic stainless steels

A 2D axisymmetric electro-thermo-mechanical finite element (FE) model is developed to study the effect of welding time and current intensity on nugget size in resistance spot welding process of AISI type 304L austenitic stainless steel sheets using ANSYS commercial software package. In order to improve the accuracy, temperature-dependent properties of materials are taken into account during the simulation. The diameter and thickness of computed weld nuggets are compared with experimental results. The FE predicted weld nugget growth and nugget size agree well with experimental results. The effects of welding time and current intensity on nugget growth are also studied. Generally, increasing welding time and current is accompanied by an increase in the fusion zone size with a decreasing slope. However if expulsion occurs, nugget size reduces due to melt spattering. Except for an initial nugget formation stage, welding time has minor effect on nugget size in comparison with welding current.

A Review on Al-Al/Al-Steel Resistance Spot Welding Technologies for Light Weight Vehicles

The main focuses of automotive industries are to reduce weight, increase fuel efficiency, and reduce air pollution. To fulfill the demands of fuel economy, lowering the mass of the auto structure can be achieved by substituting steels with light weight materials (like aluminum (Al) and magnesium (Mg) alloys1) ) or steels combined with those. There are lots of advantages to use Al instead of steels; corrosion-resistance, nonmagnetic, no color change during heating, and mechanical properties of some of its alloys are superior compared to mild steel. Use of multimaterials in manufacturing process enables low cost production and improvement of joint strength. Al alloys and steels are the most important and indispensable construction materials. Uses of sounds joining technique of those materials ensure the low cost fabrication process to be employed. Welding of steel and Al is not a simple issue to be solved easily. Because conventional welding process resulted in brittle intermetallic compounds (IMC) which are detrimental to joint as it made joint brittle and weak 2) . Since electrical and thermal conductivity of aluminum alloys are significantly high; which required higher current (2.5 to 3 times as of steel), and shorter weld time. Consistency of uniform pressure and current are fundamental needs for Al resistance spot welding. Deterioration of electrodes is vital issue, which may occur due to non uniformity of pressure and current. Electrode erosion eventually leads to undersized welds which should be avoided by using proper surface coatings and treatments on electrodes. Aluminum has high tendency to react with oxygen in the atmosphere and form oxide films on the surface of metal. This oxide films has high resistivity and corrosion protection ability which resists forming weld nugget in RSW. Auhl and Patrick et al. 3) reported that breaking down of oxide layer and ensures proper weld nugget formation usually required high electrode pressure. Many researchers 3-7) demonstrated that surface cleaned with chemicals; resulted thinnest oxide layer which improves the electrode life. Among the other types of chemically treated surface conditions, pretreated/lubricated surface is the most effective to apply for mass production automotive application. Resistance Spot Welding (RSW) is the primary methods of joining automotive, which is beneficial due to low-cost, high speed production, and availability of multi-materials welding at a same time. Few researchers 8-10) reported that the main barrier to use RSW in Al vehicles is the degradation of electrode life. In this review paper, a few techniques of Al-Al/Al-Steel RSW and possible ways of overcoming the difficulties of welding processes were discussed.

Numerical Analysis of Transport Phenomena in Resistance Spot Welding Process

Resistance spot welding (RSW) is a very complicated process involving electromagnetic, thermal, fluid flow, mechanical, and metallurgical variables. Since weld nugget area is closed and unobservable using experimental means, numerical methods are generally used to reveal the nugget formation mechanism. Traditional RSW models focus on the electrothermal behaviors in the nugget and do not have the ability to model mass transport caused by induced magnetic forces in the molten nugget. In this paper, a multiphysics model, which comprehensively considers the coupling of electric, magnetic, thermal, and flow fields during RSW, temperature-dependent physical properties, and phase transformation, is used to investigate the heat and mass transport laws in the weld nugget and to reveal the interaction of the heat and mass transports and their evolutions. Results showed that strong and complicated mass transport appears in the weld nugget and substantially changed the heat transport laws and, therefore, would be able to substantially affect the hardening, segregation, and residual stress of the weld. Compared with the traditional models which could not consider the mass transport, the multiphysics model proposed in this paper could simulate the RSW process with higher accuracy and more realities.

Monitoring of resistance spot weld quality using electrode vibration signals

Electrode vibrations can reflect resistance spot weld nugget formation, but this has not been widely appreciated so far. Although a displacement sensor can be used to detect the electrode vibrations, it is not suitable for industrial applications for its inconvenient installation and high cost. In this paper, an electrode displacement between two electrodes and acceleration signals from the upper and lower electrodes were measured using a displacement sensor and a dual accelerometer respectively, and the vibration amplitude curves were extracted from the collected signals through data processing and the results showed that they are very consistent. The effect of welding current on an electrode vibration curve monitored using the dual accelerometer was studied, and the results showed that electrode vibrations can be used to monitor weld quality, such as no weld, sticking weld, small weld, good weld and expulsion. Moreover, the monitoring of resistance spot weld quality using electrode vibration signals under abnormal welding conditions, such as shunting, small edge distance and greasy surface, was also performed. This research will provide references for monitoring resistance spot weld quality using the accelerometer method.

Micrometer‐ and nanometer‐sized platinum group nuggets in micrometeorites from deep‐sea sediments of the Indian Ocean

Abstract– We examined 378 micrometeorites collected from deep‐sea sediments of the Indian Ocean of which 175, 180, and 23 are I‐type, S‐type, and G‐type, respectively. Of the 175 I‐type spherules, 13 contained platinum group element nuggets (PGNs). The nuggets occur in two distinct sizes and have distinctly different elemental compositions: micrometer (μm)‐sized nuggets that are >3 μm contain dominantly Ir, Os, and Ru (iridium‐platinum group element or IPGE) and sub‐μm (or nanometer)‐sized (<1 μm) nuggets, which contain dominantly Pt, Rh, and Pd (palladium—PGE or PPGE). The μm‐sized nuggets are found only one per spherule in the cross section observed and are usually found at the edge of the spherule. By contrast, there are hundreds of nanometer‐sized nuggets distributed dominantly in the magnetite phases of the spherules, and rarely in the wüstite phases. Both the nugget types are found as separate entities in the same spherule and apparently, nugget formation is a common phenomenon among I‐type micrometeorites. However, the μm‐sized nuggets are seen in fewer specimens (∼2.5% of the observed I‐type spherules). In all, we analyzed four nuggets of μm size and 213 nanometer‐sized nuggets from 13 I‐type spherules for platinum group elements. Chemically, the μm‐sized PGNs contain chondritic ratios of Os/Ir, but are depleted in the more volatile PGE (Pt, Rh, and Pd) relative to chondritic ratios. On the other hand, the nanometer‐sized nuggets contain dominantly Pt and Rh. Importantly, the refractory PGEs are conspicuous by their absence in these nanometer nuggets. Palladium, the most volatile PGE is highly depleted (<1.1%) with respect to chondritic ratios in the μm‐sized PGNs, and is observed in only 17 of 213 nanometer nuggets with concentrations that are just above the detection limit (≥0.2%). Distinct fractionation of the PGE into IPGE (Ir, Os, Ru) and PPGE seems to take place during the short span of atmospheric entry. These observations suggest several implications: (1) The observation of fractionated PGE in an Fe‐Ni system gives rise to the possibility that Earth’s core could contain fractionated PGE. (2) The present data support the processes suggested for the fractionated PGE patterns observed in the ejecta of ancient meteorite impacts. (3) Meteoric metals released in the troposphere could contain fractionated PGNs in large numbers.

Phase Change Effects on Transport Processes in Resistance Spot Welding

ABSTRACTThe effects of distinct properties during phase change on mass, momentum, energy, species, and magnetic field intensity transport in workpieces and electrodes in the course of heating, melting, cooling and freezing periods in AC (alternative current) resistance spot welding are realistically and extensively investigated. Resistance spot welding has been widely used in joining thin workpieces due to its light weight and easy manufacturing. This study accounts for electromagnetic force, heat generations at the electrode-workpiece interface and faying surface between workpieces, and dynamic electrical resistance taking the sum of temperature-dependent bulk resistance of the workpieces and contact resistances at the faying surface and electrode-workpiece interface. The contact resistance is a function of hardness, temperature, electrode force, and surface condition. Instead of dealing with specific materials, this work is a general dimensionless investigation of resistance spot welding of materials with different specific heat and thermal conductivity ratios subject to realistic working parameters. The computed results show that nugget formation is delayed and heat transfer is reduced by increasing solid-to-liquid thermal conductivity and liquid-to-solid specific heat ratio. The corresponding thermal fields and flow patterns are also presented.

Numerical simulation on nugget formation and evolution in spot welding of aluminum alloy

With the squeeze of electrode tips, the oxide film on aluminum (Al) alloy surface is broken and numbers of micro-gaps are formed randomly. The micro-gaps act as conducting spots at the beginning of welding, so the contact resistance is extremely high and unstable in spot welding of Al alloy. In this paper, a new contact resistance model is adopted to simulate the nugget forming process. This model describes the random distribution characteristic of conducting spots. The simulation results indicate that, within the first 5 ms of welding current (AC, 50 Hz), the temperature distribution at the workpieces interface is seriously irregular. In addition, the nugget does not nucleate from the weld center and grow continuously, however, it nucleates randomly from several points almost instantaneously and then merges into an entity quickly. Experimental results agreed with the numerical simulation.

Critical sheet thickness for weld nugget growth during resistance spot welding of three‐steel sheets

One of the most important issues in resistance spot welding of three‐sheet stack joints is the insufficient growth of the weld nugget, which may cause problems in places needing larger weld nuggets (i.e. sheet/sheet interface). In this paper, the effect of sheet thickness on the pattern of weld nugget development during resistance spot welding of three‐steel sheets of equal thickness is studied. Results showed that there is a critical sheet thickness of 1·5 mm at which the fusion zone size at the sheet/sheet interface is nearly equal to the fusion zone size at the geometrical centre of the joint. Increasing the sheet thickness beyond the critical size causes a shift in the location of weld nugget formation from the geometrical centre to the sheet/sheet interfaces. Below the critical sheet thickness, the weld nugget growth in the geometrical centre of the joint is higher than that in the sheet/sheet interface.

Nugget formation and growth during resistance spot welding of aluminium alloy 5182

Surface interaction at the worksheet/worksheet interface during resistance spot welding of aluminium alloy 5182 with spherical tip electrodes was investigated. Oxide layer cracking and nugget formation were focused. Both experimental work and finite element analysis were employed to explain the contact behaviour at this interface. It was found that sheet separation and thus bending occurred during the squeezing phase of the resistance spot welding process and suggested a profound influence on nugget formation. The sheet separation caused enlarged and aligned cracks in the surface oxide layers which led to a good metal‐to‐metal contact near the periphery of the faying surface. High current densities which occurred at the beginning of the current phase caused significant heat generation in this zone. Consequently, the melting at the faying surface started near the periphery and moved in towards the central zone of the contact region to produce a ‘doughnut shaped’ nugget with a filled‐in but thin central region.On a étudié l'interaction de la surface à l'interface feuille de travail‐feuille de travail lors du soudage par points par résistance de l'alliage d'aluminium 5182 avec des électrodes à bout sphérique. On s'est concentré sur la fracture de la couche d'oxyde et sur la formation du noyau. On a utilisé tant le travail expérimental que l'analyse par éléments finis pour expliquer le comportement de contact à cette interface. On a trouvé que la séparation de la feuille, et donc le pliage, se produisait lors de la phase de compression du procédé de soudage par points par résistance, suggérant une influence profonde sur la formation du noyau. La séparation de la feuille résultait en fissures agrandies et alignées dans les couches d'oxyde de la surface, ce qui amenait un bon contact de métal à métal près de la périphérie de l'aire de contact. Des densités élevées de courant, qui se produisaient au début de la phase de courant, résultaient en un dégagement important de chaleur dans cette zone. Conséquemment, la fonte de l'aire de contact commençait près de la périphérie et se déplaçait vers la zone centrale de la région de contact, produisant un noyau en forme d'anneau avec une région centrale remplie, mais mince.

Finite Element Analysis for Transient Thermal Characteristics of Resistance Spot Welding Process with Three Sheets Assemblies

This project built a two dimensional finite element (FE) model for resistance spot welding (RSW) process of Mild steel with three sheets assemblies instead of conventional two sheets stack ups on practical aspect of industry applications. The thermal-electrical coupling method was implemented to investigate transient thermal characteristics during resistance spot welding history with commercial software code, namely ANSYS. Temperature dependency electrical and thermal material properties, phase change as well as convection boundary condition were taken into consideration throughout the coupling procedure. The temperature distribution changes of three sheets assemblies were obtained and transient thermal characteristics were analyzed during RSW thermal-electrical coupled history. Moreover, nugget formation was particularly viewed and discussed under appropriate welding contact condition and other experimental physical parameters.

980MPa級高張力鋼抵抗スポット溶接部におけるナゲット寸法および残留応力の数値シミュレーション

High-strength steel sheets of HT980-class are beginning to be used for automobile bodies in order to reduce body-weight and improve fuel-efficiency. Resistance spot welding is an important method in joining of high-strength steel sheets. The microstructure of welds becomes martenstic structure and high tensile residual stress is generated. It is important to ensure the performance of resistance spot welds in order to use high-strength steels more widely in automobiles. Therefore, the evaluation of characteristics of resistance spot welds is quite important.In this study, residual stress of resistance spot welds of HT980 sheets were investigated by numerical simulation and X-ray stress measurement. First, resistance spot welded test pieces were fabricated and nugget formation process was observed. Residual stress distribution in the welds of the test pieces was measured by X-ray diffraction method with a two-dimensional detector. High tensile residual stress was measured in the spot welds. The measurement results for the three test pieces were comparable. Second, residual stress distribution in the resistance spot welds was calculated by current-temperature-microstructure-mechanics coupled simulation. Comparison between the experiment and the numerical simulation proved that nugget formation process was reproduced by the simulation. Residual stress distribution obtained by the simulation was in good accordance with the measurement results when initial residual stress in HT980 sheet before welding was taken into consideration.

Nugget shifting in resistance spot welding of multi-stackup sheets

In order to investigate the weld nugget shifting phenomenon in resistance spot welding, a combined thermal-mechanical-electrical finite element model was developed to analyze the weld nugget formation process during welding. It was found that when the thickness of the bottom sheet increases, the degree of weld nugget shifting would become serious which will lead to the unqualified weld nugget. For the welding joint with material combination of SAE1004 + SAE1004 + DP600, the critical thickness combination is 0.6 + 1.8 + 2.0 mm. To improve the weld nugget shifting of the multi-stackup sheets joint, different electrode tips were used in the welding process. The effectiveness of this method was validated by the metallographic pictures.

Visualization of Fluid Flow and Heat Transfer in Resistance Spot Weld Nugget

Resistance spot welding is a very complicated process involving electro-magnetic, thermal, fluid flow, mechanical and metallurgical variables. Since weld nugget area is close and unobservable with experimental means, numerical methods are mainly used to reveal the nugget formation mechanism. So far, heat transfer behaviors in the weld nugget have been systematically studied with traditional electro-thermal model, however, the model cannot model the fluid flow caused by induced magnetic field in the molten nugget. In this paper, a multi-physics model, which comprehensively considers the coupling of electric, magnetic, thermal and flow fields during RSW, is used to investigate the fluid flow and heat transfer laws in the weld nugget. Results show that molten metal in the nugget makes high speed rotation motion in four cores, moreover, the rotation motion significantly changes the profile of the nugget and the thermal field evolution, compared with the traditional electro-thermal model. Because the fluid flow and heat transfer jointly determines the crystallization process and final microstructure, as a result, when predicting crystal growth process, the effects of the fluid flow during resistance spot welding should be considered in order to obtain a more accurate prediction results.

Simulação numérica do crescimento da lentilha de solda obtida pelo processo de soldagem a ponto por resistência elétrica no aço inox AISI 304

O crescimento das lentilhas de solda obtidas pelo processo de soldagem a ponto por resistência elétrica em chapas de aço inoxidável AISI 304 foi analisado através de simulação numérica com o método de elementos finitos, sendo utilizado para essa finalidade o programa comercial ANSYS. O modelo matemático utilizado considerou: acoplamento entre os campos elétricos e térmicos em regime transiente; a variação das propriedades elétricas e térmicas do aço AISI 304 com a temperatura e condições de contorno térmicas de convecção. A geometria tridimensional analisada, axissimétrica, permitiu a simplificação para uma geometria bidimensional. Os resultados obtidos com o modelo foram comparados com resultados experimentais apresentados na literatura. Foram consideradas espessuras de chapas de 1, 2 e 3 mm, e diferentes diâmetros de face para os eletrodos. As simulações determinaram os tempos necessários de aplicação de diferentes valores de corrente para obtenção dos diâmetros e penetração das lentilhas de solda recomendados em cada situação. Os resultados mostraram também que com o uso de maiores correntes menos energia é consumida na formação da lentilha de solda.

핫스템핑 공정에서 가열온도 및 유지시간을 고려한 22MnB5의 단일겹치기 저항 점용접 조건 최적화

본 연구에서는 핫스템핑 소재로 사용되는 보론합금강판 22MnB5의 단일 겹치기 점용접에서 용접부의 강도 향상을 위한 최적화를 수행하였다. 최적화 과정은 다구찌 실험계획법에 의해 행해졌으며 공정변수는 전류, 가압력 및 통전시간으로 선정하였고, 잡음인자로서 핫스템핑 시 소재의 가열온도와 유지시간을 고려하였다. 가열조건에 따라 22MnB5 표면의 알루미늄 도금층과 모재 간의 확산반응에 의해 화합물층 두께에 산포가 발생하였으며 이러한 산포는 너겟의 형성에 영향을 미치는 것을 알 수 있었다. 한편 용접부의 인장전단강도를 목적함수로 하였을 때, 이러한 가열조건에 강건한 최적의 용접 조건은 전류 8㎄, 가압력 4kN, 통전시간 18cycle로 선정되었다. 최적 조건의 검증 결과 용접부의 인장전단강도 는 32kN으로서 요구되는 규격인 23kN보다 크게 증가되었다.

Effect of Initial Gap on Tensile Strength of Resistance Spot Welded Joints

It has been proved that the initial gap has obvious influence on nugget formation, but little works focused on the effect of initial gap on the tensile strength of resistance spot welded (RSW) joints. In this paper, a 3D FE model was built for solving this question. The results show that, even though there are some fluctuations of weld diameter and tensile strength of RSW joints with initial gap, the tensile strength and weld diameter of welded joints with initial gap are still larger than that of welded joints without gap, which confirm that the influence of initial gap on tensile shear strength is little significant. The computation results agree well with experiment.

시뮬레이션 및 너겟 성장 곡선을 이용한 자동차 차체용 3겹 강판의 저항점 용접성 분석

최근 자동차 산업은 고효율 자동차의 개발에 박차를 가하고 있으며 이에 따라 차체 경량화 및 내식성 방지를 위한 고강도강 및 도금강의 사용이 증가하고 있다. 이에 따라 본 연구에서는 자동차 차체에 쓰이는 3종의 다른 강판에 대하여, 겹침 순서에 따른 용접 특성을 시뮬레이션 기법을 이용하여 분석하였다. 자동차 차체에 많이 사용되는 EDDQ 급의 도금강판 0.7t와 인장강도 440 MPa급의 440R 1.2t, 그리고 AHSS (Advanced High Strength Steel)인 DP 590 0.2t 판재에 대하여 겹치기 순서에 따른 용접 특성을 시뮬레이션 하였고, 너겟 성장 곡선 및 전류에 따른 너겟의 크기로 그 용접성을 분석하였다. 또한 용접 시 발생되는 각 접촉부의 접촉저항을 이용하여 겹침순서에 따른 너겟의 크기를 분석하였으며, 이를 통해 최적의 겹치기 순서를 제안할 수 있었다. Nowadays, most car manufactures have tried to improve fuel efficiency and corrosion resistance of car body. Therefore, use of high strength steels and coated steel becomes more and more increased. In this study, spot weld characteristics according to lap sequence of sheets were analyzed using simulation method for three different steel sheet of car body which were EDDQ class coated steel with 0.7t, high strength steel 440R with 1.2t and advanced high strength steel DP 590 with 1.0t. Using simulation, weldability was evaluated by nugget size of welded zone according to nugget growth curve and welding current with respect to lap sequence of sheets. Contact resistance of each sheets contact point was used to analyze formation of nugget and optimal lap sequence was suggested.

Nanoparticle factories: Biofilms hold the key to gold dispersion and nugget formation

Biofilms living on gold (Au) grains play a key role in the biogeochemical cycle of Au by promoting the dispersion of Au via the formation of Au nanoparticles as well as the formation of secondary biomorphic Au. Gold grains from Queensland, Australia, are covered by a polymorphic, organic-inorganic layer that is up to 40 μm thick. It consists of a bacterial biofilm containing Au nanoparticles associated with extracellular polymeric substances as well as bacterioform Au. Focused ion beam (FIB) sectioning through the biofilm revealed that aggregates of nanoparticulate Au line open spaces beneath the active biofilm layer. These aggregates (bacterioform Au type 1) resulted from the reprecipitation of dissolved Au, and their internal growth structures provide direct evidence for coarsening of the Au grains. At the contact between the polymorphic layer and the primary Au, bacterioform Au type 2 is present. It consists of solid rounded forms into which crystal boundaries of underlying primary Au extend, and is the result of dealloying and Ag dissolution from the primary Au. This study demonstrates that (1) microbially driven dissolution, precipitation, and aggregation lead to the formation of bacterioform Au and contribute to the growth of Au grains under supergene conditions, and (2) the microbially driven mobilization of coarse Au into nanoparticles plays a key role in mediating the mobility of Au in surface environments, because the release of nanoparticulate Au upon biofilm disintegration greatly enhances environmental mobility compared to Au complexes only.