Weld Nugget Growth Research Articles

On-line measurement of nugget diameter in automatic resistance spot welding based on embedded ultrasound probe

The inspection of resistance spot welding (RSW) quality is crucial in the sheet metal joining industries. This paper proposes an enhanced approach for the automated real-time measurement of weld nugget diameter in welded joints, utilizing an ultrasonic probe integrated within the electrode. The ultrasonic signals received by the probe are sequentially arranged to generate consecutive ultrasonic B-scan images, which effectively capture the entire weld nugget growth and solidification process. The corresponding maximum time of flight (TOF) for the liquid nugget is extracted from the B-scan image. In the condition of qualified welding quality, a mathematic equation between maximum TOF and weld nugget diameter is established, while the opposite is alarmed. The on-line quality inspection of RSW can autonomously determine the qualification of welding quality, as well as quantitatively evaluate the weld nugget diameter in qualified weld joints. The research is applicable to various processing and manufacturing fields.

Effect of Process Parameters and Nugget Growth Rate on Liquid Metal Embrittlement (LME) Cracking in the Resistance Spot Welding of Zinc-Coated Steels

Although the influence of weld process variables on LME cracking was known to be significant, limited studies have been conducted on the effect of process variables with systematic approaches in the equivalent nugget growth behavior and heat input. This study aimed to identify the effect of weld process variables on LME sensitivity with the equivalent nugget diameter and underlying mechanism with induced tensile stress for cracking. Among the welds with equivalent nugget diameters in the combination of different welding current and time, higher LME sensitivity was observed with the high welding current and short welding time combination than that with the low welding current and long welding time combination for the equivalent nugget diameter. Because a high current and short time combination resulted in faster weld nugget growth than the low welding current and long welding time combination, it rapidly increased the surface temperature along with the cooling from the electrode. These combined effects induced a higher thermal gradient and thermally induced tensile stress on the weld surface, satisfying the conditions of the LME cracking. The simulation results also confirmed that the critical weld cycle time of the LME cracking (<i>t<sub>c</sub></i>), which is the cross point between the nugget growth diameter and a contact diameter of the electrode, could be different with the combination of the weld process variables with the equivalent weld nugget size. Therefore, <i>tc</i> can be applied for the sensitivity index of LME cracking of the resistance spot weldment considering complex weld variables.

Short-Pulse Resistance Spot Welding of Aluminum Alloy 6016-T4 - Part 1

Short-pulse welding parameters for resistance spot welding (RSW) of aluminum alloy AA6016-T4 using mediumfrequency direct current (MFDC) systems were developed to reduce the heat input required for nugget formation. Optimization of current and time parameters is critical during RSW of aluminum alloys for reducing energy requirements and avoiding weld imperfections, such as solidification cracking and expulsion, while maintaining weld quality, particularly given the high electrical and thermal conductivities of the materials. The welding time and the applied current level of the current pulse were varied systematically for thin sheets (1 mm or 0.04 in.) of AA6016-T4. The quality of the welds was evaluated by pull-out testing, ultrasound testing, and metallography techniques. Simulations of the same welding processes were performed with the finite element-based SORPAS® software. The results showed short-pulse MFDC RSW can reduce the energy required for sound welds in this alloy without requiring an increase in welding current. The simulations and experiments also showed the welding process had distinct weld nugget nucleation and growth phases.

Simulating Thermoelectric Effect and Its Impact on Asymmetric Weld Nugget Growth in Aluminum Resistance Spot Welding

Abstract Resistance spot welding (RSW) of aluminum–aluminum (Al–Al) is known to be very challenging, with the asymmetric growth of the weld nugget often observed. In this article, a semicoupled electrical–thermal–mechanical finite element analysis (FEA) procedure was established to simulate the RSW of two layers of AA6022-T4 sheets using a specially designed Multi-Ring Domed (MRD) electrodes. Critical to the modeling procedure was the thermoelectric (including the Peltier, Thomson, and Seebeck effects) analyses to simulate the asymmetric nugget growth in the welding stage. Key input parameters such as the Seebeck coefficients and high-temperature flow stress curves were measured. Simulation results, experimentally validated, indicated that the newly developed procedure could successfully predict the asymmetric weld nugget growth. Simulation results also showed the Seebeck effect in the holding stage. The simulations represent the first quantitative investigation of the impact of the thermoelectric effects on resistance spot welding.

Transient Softening at the Fusion Boundary of Resistance Spot Welds: A Phase Field Simulation and Experimental Investigations for Al–Si-coated 22MnB5

This work gives an insight into the transient softening at the fusion boundary of resistance spot welds on hot stamped steel. Metallographic investigations and hardness mapping were combined with finite phase–field modeling of phase evolution at the fusion boundary. Saturation of weld nugget growth in the welding process was observed. For industrially relevant, long welding times, the fusion boundary of a spot weld is therefore isothermally soaked between the peritectic and solidus temperatures. This leads to a carbon segregation to the liquid phase due to higher carbon solubility and possibly to δ-Fe formation at the fusion boundary. Both results in a local carbon depletion at the fusion boundary. This finding is in good agreement with carbon content measurements at the fusion boundary and the results of hardness measurements.

Study on Nugget Growth in Resistance Spot Welding of Thin Aluminum A1100 Using Welding Simulation

Resistance spot welding (RSW), generally which is one of the most often used to joint metal plate in the automotive and aviation industries. RSW welding process involves electrical, thermal mechanical, metallurgy, and complex surface phenomenon. Unlike the other welding processes, weld joint formation in RSW process occurs very quick (in milli-seconds) and took place between the workpieces overlap each other. Welding simulation allows visual examination of the weld joint without having to perform an expensive experiment. Weld nugget size is the most important parameter in determining the mechanical behavior of welded joints in RSW process. The quality and strength of the weld joint in RSW process is predominantly determined by the shape and size of the weld nugget. Simulation modeling of RSW process performed using ANSYS Parametric Design Language (APDL) module based on the finite element method (FEM), embedded in ANSYS Workbench. Electrical and transient-thermal interaction was developed to study the weld nugget growth on resistance spot welding of aluminum A1100 metal plate with a thickness of 0.4 mm respectively. Weld nugget diameter can be well predicted by using this simulation model from the temperature distribution during the welding process. Welding is performed by varying the weld current (1 kA and 2 kA) and the welding time for each electric current, which are start from 0.5, 1.0, and 1.5 cycle time. Nugget diameter for each of the welding parameters from the simulation modelling were 4,276 mm, 4,372 mm, 4,668 mm, 5,616 mm and 5,896 mm. Weld expulsion occurred for the specimen with welding current 2 kA and welding time 1.5 cycle time, characterized by the decreasing of the tensile-shear strength of the specimen.

Točkasto varjenje jekel DQSK/DP600: rast jedra zvara

The weld-nugget size is the key issue in determining the mechanical properties of resistance spot welds. This paper aims at investigating the weld-nugget growth of dissimilar-resistance spot welding of ferrite-martensite DP600 and drawing-quality special-killed (DQSK) low-carbon steel. It was revealed how the weld-nugget size is influenced by the main welding parameters: welding current, welding time and electrode force. The weldability lobe was established and proper welding conditions for the welds with a sufficient size and without an expulsion were determined. Using the experimental data, an empirical relationship between the weld-nugget size and the welding parameters was developed.

Peak load and energy absorption of DP600 advanced steel resistance spot welds

The paper aims at investigating the microstructure, failure mode transition, peak load and energy absorption of DP600 dual phase steel during the tensile-shear test. It was found that the welding current has profound effect on the load–displacement characteristics. In the low welding current, welds failed in interfacial failure mode. Increasing welding current resulted in sufficient weld nugget growth to promote double-sided pullout failure mode with improved mechanical properties. Further increase in the welding current caused expulsion and failure mode was changed to single-sided pullout with reduced energy absorption capability. It was found that the fusion zone size is the key parameter controlling the mechanical properties of DP600 resistance spot welds in terms of peak load, maximum displacement and failure energy.

Development of a new program control setting of welding current and electrode force for single-side resistance spot welding

For the indirect-type single-side resistance spot welding, the new program control setting of welding current and electrode force were developed in order to increase the welding current range, suppress the expulsion, and promote the weld nugget growth. Welding experiments verified that the occurrence of expulsion and formation of molten nugget were significantly influenced by the contacting conditions at the interface between the electrode tip and upper sheet and at the faying surface between the sheets and the resulted heat generation at the early stage of welding. Focusing on better controlling the contacting conditions at the early stage, the two-stage program control setting of welding current and electrode force was developed and successfully improved the weldability, increasing the welding current range, compared with constant current and force setting. Numerical simulations were conducted to study the welding phenomena and clarified the effect of two-stage current and force setting on the suppression of expulsion and promotion of nugget growth.

A New Characterization Approach of Weld Nugget Growth by Real-Time Input Electrical Impedance

The in-process changes of weld nugget growth during the Resistance Spot Welding were investigated based on the resistance of input electrical impedance. To compute the time varying resistance of input electrical impedance, the welding voltage and current signals are measured simultaneously and then converted into complex-valued signals by using Hilbert transform. Comparing with the dynamic contact resistance as reported in literature, it showed that the time varying resistance of input electrical impedance can be accurately correlated with the physical changes of weld nugget growth. Therefore, it can be used to characterize the in-process changes of weld nugget growth. Several new findings were reported based on the investigation of spot welds under no weld, with and without weld expulsion conditions.

An Experimental Investigation on Spot Weld Growth on Dissimilar Joints of 304L Austenitic Stainless Steel and Medium Carbon Steel (Part 1)

Carbon steels and stainless steels are more frequently welded joints than any other materials because of their weldability characteristics. So the spot welded joining characteristic of these two materials are discussed in this paper. The experiment was conducted on medium carbon steel and 304L austenitic stainless by varying the process controlling parameters; such as welding current, welding time and electrode pressing force. As such two sets of data were collected to characterize the formation of spot weld using pneumatic based 75kVA spot welder. The first set was made for the variation of welding time and current whereas the second was made for the variation of welding current and electrode force. The welded specimens are finally underwent the tensile test, hardness test and metallurgical test to characterize the weld growth. The weld nugget growth was noticed for the welding current and weld time increment except the electrode force. By increasing the electrode force, the process resistances were reduced and consequently the weld nugget was reduced. Moreover the effect of heat imbalance was clearly noticed in the weld nuggets due to different electrical and chemical properties.

An Experimental Investigation on Spot Weld Growth on Dissimilar Joints of 304L Austenitic Stainless Steel and Medium Carbon Steel (Part 1)

Carbon steels and stainless steels are more frequently welded joints than any other materials because of their weldability characteristics. So the spot welded joining characteristic of these two materials are discussed in this paper. The experiment was conducted on medium carbon steel and 304L austenitic stainless by varying the process controlling parameters; such as welding current, welding time and electrode pressing force. As such two sets of data were collected to characterize the formation of spot weld using pneumatic based 75kVA spot welder. The first set was made for the variation of welding time and current whereas the second was made for the variation of welding current and electrode force. The welded specimens are finally underwent the tensile test, hardness test and metallurgical test to characterize the weld growth. The weld nugget growth was noticed for the welding current and weld time increment except the electrode force. By increasing the electrode force, the process resistances were reduced and consequently the weld nugget was reduced. Moreover the effect of heat imbalance was clearly noticed in the weld nuggets due to different electrical and chemical properties.

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.

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.

Experimental study on electrode displacement fluctuation characteristics during resistance spot welding

Electrode displacement has been used to monitor resistance spot weld quality for long, but the fluctuation of the electrode displacement has not been well studied. This paper analysed the mechanism of the displacement fluctuation by collecting the welding current and electrode displacement during welding process and extracted the incremental pulse expansion of the electrode displacement, which was named as the displacement fluctuation peak. Through metallurgical experiments, the relationship of the displacement fluctuation peak with the weld nugget growth during resistance spot welding process was analysed. The whole process can be divided into four successive stages, and five characteristic points of the displacement fluctuation peak curve were selected to online predict the weld quality. The variation in the displacement fluctuation peak curve under different welding currents was studied, and the results showed that the displacement fluctuation characteristics can be used to effectively evaluate the weld quality.

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.

An Analysis of Resistance Spot Weld Growth on Mild and Stainless Steel with 1mm and 2mm Thicknesses

Resistance spot welding (RSW) is an essential welding technology today for joining two or more metals in various manufacturing industries. A statistic shows that one metal assembly out of five is joined using resistance spot welding mechanism, commercially. It uses traditionally two electrodes to hold the metal sheets and forces high current to pass through it. The growth of weld nugget is, at last obtained from a proper set up of its controlling parameters such as current, weld time, pressure of electrodes and also the tip size of electrodes. However, factors such as electrode deformation, dissimilar materials and materials with different thicknesses also affect weld growth. This paper looks into the effects of different thicknesses of two base materials .The materials that were used are mild steel and 302 authentic stainless steel with thicknesses of 1 mm and 2 mm. Mechanical tensile test and hardness test have been carried out to characterize the formation of weld nugget growth for different welding schedules. The results of the experiments showed that the growth of spot weld is strongly affected by the usage of materials with different thickness or types. The macrostructure of weld nugget also shows distinguishable differences in weld growth for the both mentioned cases. The tensile test was carried out on standard size samples but with different thicknesses and materials. It shows difference in yield strength for the same welding schedules. Meanwhile the hardness of welded materials varies from one another significantly but the hardness distribution along the welded areas seemed to almost same for each category of base metals.

Mechanical performance of three thickness resistance spot welded low carbon steel

Resistance spot welding is the dominant process for joining sheet metals in automotive industry. Despite the application of three thickness resistance spot welds in this industry, present guidelines and recommendations are limited to two thickness spot welds. Study towards better understanding of weld nugget growth and mechanical properties is the first step to understanding the welding behaviour and developing proper guidelines for the three thickness resistance spot welding. In this paper, weld nugget growth, mechanical performance and failure behaviour of three thickness low carbon steel resistance spot welds are investigated. Macrostrcutural and microstructural investigations, microhardness tests and quasi-static tensile–shear tests were conducted. Mechanical performance of the joint was described in terms of peak load, energy absorption and failure mode. In order to understand the failure mechanism, micrographs of the cross-sections of the spot welded joints during and after tensile–shear are examined by optical microscopy. Unlike two thickness resistance spot welded joint, weld nugget was formed in the geometrical centre of the joint (i.e. centre of the middle sheet). Weld nugget size along sheet/sheet interface was greater than that of along geometrical centre of the joint. Increasing welding time leads to increases in peak load and energy absorption of the joint and transition of interfacial failure mode to pullout failure mode, primarily due to the enlargement of weld nugget size along sheet/sheet interface.

Nugget growth characteristic for AZ31B magnesium alloy during resistance spot welding

An axisymmetrical finite element model for studying the distribution of temperature for resistance spot welding (RSW) to predict weld nugget growth of AZ31B Mg alloy was developed by employing a contact resistance model based on the microcontact theory. The RSW of a Mg alloy, with regard to nugget formation, consists of the initiation of a nugget in the first cycle, a rapid growth of the nugget in the following 2–3 cycles and a plateau of nugget growth after ∼4 cycles. Because of its high thermal conductivity, low melting point and low volumetric heat capacity, Mg alloy has many characteristics during nugget formation, compared with Al alloy and mild steel. In the RSW of a Mg alloy, the contact resistance in the interface has an important effect on the nugget formation; the welding time is similar to that in Al alloy but smaller than that in low carbon steel; and the welding current lever is required slightly lower than that in Al alloy but higher than that in low carbon steel. The computational simulations based on this model agree well with the experimental data.

Weld growth mechanism of resistance spot welds in zinc coated steel

Whilst resistance welding has been the primary joining technique for automotive body-in-white (B-I-W) assembly since the 1930s, far more welds than necessary are used to fabricate the structure to achieve the required structural performance. This can be attributed due to lack of confidence in weld quality, particularly when joining three or more sheet combinations. To achieve the necessary confidence, there is a need to understand the mechanism of heat development and weld formation, and develop appropriate welding conditions to ensure acceptable weld quality. With the high proportion of zinc coated steels in B-I-W assembly, one of the key issues is to understand the role of the zinc coating in weld nugget development. Consequently, in this study, the mechanisms of weld nugget growth for three thickness joints of both uncoated low carbon and hot dip zinc (GI) coated steels have been investigated using similar welding conditions. The welding conditions selected were based on those recommended for the uncoated low carbon steel. Since these are unlikely to be optimal in the case of the coated steel, further studies were undertaken to establish the effect of electrode force on the mechanism of weld growth. The results obtained indicated that the rate of heat development was reduced in the case of the coated steel compared to the uncoated steel variant. This can be attributed to the lower surface resistance in the presence of the zinc coating. Variations in electrode force indicated flexibility in the selection of welding conditions to produce an acceptable weld quality. Where, an increase in the level of electrode force was used, a significant change in the location of heat development and weld nugget formation was noted compared to that observed for the low force variant. This was attributed to the change in the dominant resistive heating mechanism that took place.