Multi-field dynamic modeling and numerical simulation of aluminum alloy resistance spot welding

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.

Numerical and Experimental Study on Nugget Formation Process in Resistance Spot Welding of Aluminum Alloy

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.

Application of the wavelet packet and its energy spectrum to identify nugget splash during the aluminum alloys spot welding

Nugget splash during aluminum alloys spot welding has a detrimental effect on weld nugget integrity, strength and durability of the welded joints. This investigation is performed to identify nugget splash from voltage signals because these are easily accessible on-line. In the present work, we propose a novel method based on the wavelet packet transform and its energy spectrum for pattern recognition of splash signal. The result demonstrates that this novel method is more accuracy and a useful way of monitoring the spot welding quality.

Numerical and experimental study of phase transformation in resistance spot welding of 6082 aluminum alloy

Resistance spot welding（ RSW） is an efficient and convenient joining process for aluminum alloy sheet assembly. Because the RSW has the character of energy concentration and quick cooling rate, the microstructure transformation of the base metal can be confined in the least limit. The material properties and the welding parameters have significant effects on the quality of the nugget. To predict the microstracture evolution in the melted zone and the heat-affected zone, an electrical, thermal, metallurgical and mechanical coupled finite element model is described and applied to simulate the welding process of the 6082 aluminum alloy. Experimental tests are also carried out. The comparison between experimental and numerical results shows that the adopted model is effective enough to well interpret and predict some important phenomena in ierms of the phase transformation in spot welding of 6082 aluminum alloy.

Process of friction-stir welding high-strength aluminum alloy and mechanical properties of joint

The process of friction-stir welding 2A12CZ alloy has been studied. And strength and elongation tests have been performed, which demonstrated that the opportunity existed to manipulate friction-stir welding parameters in order to improve a range of material properties. The results showed that the joint strength and elongation arrived at their maximums (331 MPa and 4%) at 37.5 mm/min and 300 rpm. As welding parameters changing, joint tensile strength and elongation had similar development. Hardness measurement indicated that the weld was softened. However, there was considerable difference in softening degree for different joint zone. The weld top had lower hardness and wider softening zone than other zone of the weld. And softening zone at advancing side was wider than that at retreating side.

Identification of acoustic emission signal in aluminum alloys spot welding based on fractal theory

The acoustic emission signal of aluminum alloys spot welding includes the information of forming nugget and is one of the important parameters in the quality control. Due to the nonlinearity of the signals, classic Euclidean geometry can not be applied to depict exactly. The fractal theory is implemented to quantitatively describe the characteristics of the acoustic emission signals. The experiment and calculation results show that the box counting dimension of acoustic emission signal, between 1 and 2, are distinctive from different nugget areas in AC spot welding. It is proved that box counting dimension is an effective characteristic parameter to evaluate spot welding quality. In addition, fractal theory can also be applied in other spot welding parameters, such as voltage, current, electrode force and so on, for the purpose of recognizing the spot welding quality.

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.

Study on asymmetric thermo-physical effect mechanism of intermediate frequency DC resistance spot welding for aluminum alloy

The asymmetric thermo-physical mechanism of the resistance spot welding technique with intermediate frequency(2 kHz)and direct current(RSWIFDC)on the high strength aluminum(Al)alloy TL091 was studied here in view of the Peltier effect.On the basis of the analysis of the electrode cap surface erosion state and the shape-position of the nugget,it was concluded that asymmetric thermo-physical phenomenon occurred on both ends of the nugget,and even had an influence upon the shape-forming coefficient and the vertical position deviation of the nugget,and the erosion degree of the electrode caps.In this work,the relative thermo-physical model of the welding was established combined with the Peltier effect and the spot welding characteristics.Accordingly the relative welding phenomena,such as nugget center deviation and different erosion degree of the electrode cap surface,was explained clearly using the model related with the Peltier effect for the first time.This model provides important theoretical basis for future study and application of Al alloy spot welding,based on which,effective works may be done to promote the quality of the Al alloy welded joints and to obtain favorable control upon parameters of Al alloy welding for electrode caps.

Physical simulation of alloying between copper and aluminum on electrode tip for resistance spot welding of 5A02 aluminum alloy

The applicatio, n of aluminum alloy in the automotive and aviation fields is impeded by the wear and life of electrode for resistance spot welding （RSW）. The alloying interaction between the copper electrode and aluminum alloy sheet is the main reason of making electrode life decrease. The test of alloying interaction is difficult because of the transient in RSW of aluminum alloy. In this paper, the process of alloying between copper and aluminum on the electrode tip is simulated with Gleeble-1500 thermal simulation testing machine. The microstructure and composition of the sample of physical simulation for the alloying interaction between the copper electrode and aluminum alloy are analyzed using scanning electron microscope （SEM） and X-ray diffraction （XRD） respectively. The results indicate that the alloying reaction between copper and aluminum under the different temperature, pressure and time is mainly the eutectic reaction. The reaction result is the eutectic of （ Al ＋ CuAl2 ） , and then Cu9Al4 forms through solid diffusion between the CuAl2 phase and the copper base metal.

The Asymmetric Laplace Law for distribution of aluminum contact resistance in spot welding

This paper investigates the distribution of contact resistance of the aluminum alloy in the squeeze stage. A new method of measuring path＇ s resistance is proposed firstly. Contact resistances are calculated accurately by the systems of 5 linear equations and solutions show that three contact resistances are different. The probability density functions of contact resistance in workpiece/workpiece（ W/W） and upper electrode/workpiece（ E/W） show that the curve shape has steeper peak and heavier tail than that of the normal distribution. Non-parameter hypothesis test is performed and the result shows that R2 , R4 reject the normal distribution using chi-square and kolmogoroo statistic D method. Therefore, the Asymmetric Laplace distribution is fitted to empirical distributions and is applied to quantify the influence of random contact resistance. The result illustrates that AL distribution is very close to contact resistance of W/W and upper E/W and normal distribution has some deviation. The paper is helpful to research the initial nugget conditions, weldability and the transient multi-coupling field.

Analysis of Acoustic Emission Signal by Fractal Theory in Aluminum Alloy Spot Welding

The relation between acoustic emission signal and nugget during aluminum alloy spot welding was investigated in order to evaluate spot welding quality. Due to the nonlinearity of the signals, fractal theory was utilized to quantitatively describe the characteristics of the signals instead of classical Euclidean geometry which cannot describe the acoustic emission signal accurately. Through experiments and computing, the box counting dimension is found distinct from other acoustic emission signals and is a better approach to discriminating weld nugget stages. Results show that fractal dimensions increase from 1.51 to 1.78,and they are related to nugget areas added from non-fusion to over-heated nugget. And the box counting dimension can effectively evaluate the quality of the nugget in the spot welding and can be aoolied with current, displace, and other soot welding parameters.

Material flow behavior and microstructural evolution during refill friction stir spot welding of alclad 2A12-T4 aluminum alloy

In this study, we used the stop-action technique to experimentally investigate the material flow and microstructural evolution of alclad 2A12-T4 aluminum alloy during refill friction stir spot welding.There are two material flow components, i.e., the inward-or outward-directed spiral flow on the horizontal plane and the upward-or downward-directed flow on the vertical plane.In the plunge stage, the flow of plasticized metal into the cavity is similar to that of a stack, whereby the upper layer is pushed upward by the lower layer.In the refill stage, this is process reversed.As such, there is no obvious vertical plasticized metal flow between adjacent layers.Welding leads to the coarsening of S(Al2CuMg) in the thermo-mechanically affected zone and the diminishing of S in the stir zone.Continuous dynamic recrystallization results in the formation of fine equiaxed grains in the stir zone, but this process becomes difficult in the thermo-mechanically affected zone due to the lower deformation rate and the pinning action of S precipitates on the dislocations and sub-grain boundaries, which leads to a high fraction of low-angle grain boundaries in this zone.

Microstructure characterization and quasi-static failure behavior of resistance spot welds of AA6111-T4 aluminum alloy

The microstructure, microhardness and quasi-static failure behavior of resistance spot welds of AA6111-T4 aluminum alloy were experimentally investigated. Optical metallography and high-resolution hardness traverses were utilized to characterize the weld nugget, heat affected zone and base metal. The AA6111 spot welds displayed a softer nugget and hardened heat affected zone, compared with the base metal. The through-thickness hardness of the base metal sheet was not constant and had to be carefully considered to determine the effect of welding on material properties. Quasi-static lap-shear tensile tests were used to determine the failure load and failure mode. All tensile specimens failed through the interfacial fracture. This failure mode is consistent with the observed reduced hardness in the weld nugget.

Microstructure and mechanical properties of Mg-to-Al dissimilar welded joints with an Ag interlayer using ultrasonic spot welding

Lightweight ZEK100-0 Mg alloy and A16022-T43 Al alloy with an Ag interlayer were joined via ultrasonic spot welding(USW),focusing on the microstructural change and tensile lap shear strength of the welded joints in relation to welding energy.Mg/Al interface was superseded by Mg/Ag and Al/Ag interfaces,and unfavorable Mg门A-intermetallic compound was eliminated.Ag foil was observed to be intact in the nugget center,while it was broken or dissolved at the nugget edge at high welding energy levels.The diffusion layer at the Mg/Ag interface consisted of two distinctive sub-layers:Mg3Ag intermetallic compound adjoining Ag foil,and Mg3Ag-l-Mg eutectic structure adjacent to Mg.Only a thin diffusion layer consisting mainly of Ag3Al occurred al lhe Al/Ag interface.The tensile lap shear strength first increased,reached its peak value,and then decreased with increasing welding energy.The shear strength achieved in the present study was〜31%higher than that of the joint without interlayer.Interfacial failure occurred at all energy levels,with Ag foil particles or fragments being stuck on both Mg and Al sides due to its intense interaction with Mg and Al via accelerated diffusion during USW.The results obtained pave the way for the challenging dissimilar welding between Mg and Al alloys.

Interfacial characterization of resistance spot welded joint of steel and aluminum alloy

The dissimilar material resistance spot welding of galvanized high strength steel and aluminum alloy had been conducted. The welded joint exhibited a thin reaction layer composed of Fe2Al5 and Fe4Al13 phases at steel/aluminum interface. The welded joint presented a tensile shear load of 3.3 kN with an aluminum alloy nugget diameter of 5.7 mm. The interfacial failure mode was observed for the tensile shear specimen and fracture occurred at reaction layer and aluminum alloy fusion zone beside the interface. The reaction layer with compounds was the main reason for reduction of the welded joint mechanical property.

Filling technique for keyhole of friction stir spot welding based on the principles of resistance spot welding

Keyhole at the end of a weld prepared by friction stir welding(FSW)is one of the major issues that impede the application of FSW.To address this issue,a keyhole filling technique was proposed in this paper,which is based on the principles of resistance spot welding(RSW).A three-phase secondary rectifier resistance spot welder was applied as the experimental instrument for filling the keyhole in the center of friction stir spot weld(FSSW).The test sheet is a 2024-T4 aluminium alloy with a thickness of 6.0mm.The experiments results show that the filled joint strength is improved by 26.12%since the area is increased for the plug in the keyhole.And there are two kinds of dimples in the tensile fracture-equiaxial dimples and long dimples.The filled joint involves the fusion welding zone(FWZ),pressure welding zone(PWZ),melted plug zone(MPZ),and plastic deformation zone(PDZ).The FWZ and the PWZ is the melting bond and diffusion bond between the plug and keyhole,respectively.The MPZ is the center part and the PDZ is upper or lower part of the plug.

Microstructure and mechanical property of resistance spot welded joint of aluminum alloy to high strength steel with especial electrodes

Dissimilar material joining of 6008 aluminum alloy to H220 YD galvanized high strength steel was performed by resistance spot welding with especial electrodes that were a flat tip electrode against the steel surface and a domed tip electrode upon the aluminum alloy surface. An intermetallic compound layer composed of Fe2Al5 and FeAl3 was formed at the steel/ aluminum interface in the welded joint. The thickness of the intermetallic compound layer increased with increasing welding current and welding time, and the maximum thickness being 7. 0 μm was obtained at 25 kA and 300 ms. The weld nugget diameter and tensile shear load of the welded joint had increased tendencies first with increasing welding current （ 18 -22 kA） and welding time （ 50 - 300 ms）, then changed little with further increasing welding current （ 22 - 25 kA） and welding time （300 -400 ms）. The maximum tensile shear load reached 5.4 kN at 22 kA and 300 ms. The welded joint fractured through brittle intermetallic compound layer and aluminum alloy nugget.

Effects of nugget alloying on microstructures and properties of resistance spot welded joints of aluminum and steel

The resistance spot welding of 6063-T6 aluminum alloy and 16Mn steel was studied by nugget alloying. The results indicated that the Al-steel joint had characteristics of welding-brazing. The nugget zone consisted mainly of α-Al solid solution with dislocations and fine Mg2Si particles. The interface zone had a double-layer structure： Fe2Al5 layer at steel side and Fe4Al13 layer at Al nugget side. The nugget alloying has a significant effect on the joint properties by changing phase composition and refinement of grains. When alloy elements Cu, Zn, Ti and Ni were added, the tensile shear load of Al-steel joints reached 2 780 N, 2 910 N, 2 915 N and 2 929 N respectively, which increased by 24. 1%, 29.9%, 30. 1% and 30. 7% respectively compared with that （2 241 N） of joint without nugget alloying. Therefore, it is an effective way for improving mechanical properties of resistance spot welded Al-steel joints.

Material properties of friction stir spot welded joints of dissimilar aluminum alloys

Mechanical properties and material mixing patterns of friction stir spot welded (FSSW) joints of dissimilar aluminum alloys were investigated.Two aluminum alloys typically used in automotive applications,5052-H32 and 6061-T6,were selected.During the experiment,the process parameters including the z-axis force and torque histories were measured as a function of the tool displacement.The mechanical properties were investigated by microhardness measurements of the joint,and the material mixing in the stir zone was investigated by EPMA.The experimental results illustrate different process parameter histories,material mixing in the stir zone and material properties including microhardness distributions for FSSW joints of dissimilar aluminum alloys,likely due to different mechanical behaviors of the selected aluminum alloys in the FSSW process temperature range.

Recent progress in corrosion protection of friction stir welded high-strength aluminum alloy joints

Friction stir welding （FSW） has been widely used in many industries, with which high-strength aluminum alloys can be well joined. However, the corrosion resistance of FSW high-strength Al alloy joints is relatively poor, which limits their industrial applications. The joints shall be protected against corrosion. In this review, therefore, the current status and development of corrosion protection for FSW high-strength Al alloy joints are presented. Particular emphasis has been given to different protection methods ： lowering heat input, post-weld heat treatment, surface modification and spray coatings. Finally, opportunities are identified for further research and development in corrosion protection of FSW high-strength Al alloy joints.