Showing papers on "Spot welding published in 2003"

Heat transfer and fluid flow during laser spot welding of 304 stainless steel

Abstract: The evolution of temperature and velocity fields during laser spot welding of 304 stainless steel was studied using a transient, heat transfer and fluid flow model based on the solution of the equations of conservation of mass, momentum and energy in the weld pool. The weld pool geometry, weld thermal cycles and various solidification parameters were calculated. The fusion zone geometry, calculated from the transient heat transfer and fluid flow model, was in good agreement with the corresponding experimentally measured values for various welding conditions. Dimensional analysis was used to understand the importance of heat transfer by conduction and convection and the roles of various driving forces for convection in the weld pool. During solidification, the mushy zone grew at a rapid rate and the maximum size of the mushy zone was reached when the pure liquid region vanished. The solidification rate of the mushy zone/liquid interface was shown to increase while the temperature gradient in the liquid zone at this interface decreased as solidification of the weld pool progressed. The heating and cooling rates, temperature gradient and the solidification rate at the mushy zone/liquid interface for laser spot welding were much higher than those for the moving and spot gas tungsten arc welding.

Ultimate Strength and Failure Mechanism of Resistance Spot Weld Subjected to Tensile, Shear, or Combined Tensile/Shear Loads

Abstract: Strength tests were performed to reveal the failure mechanisms of spot weld in lap-shear and cross tension test samples. It is shown the while the lap-shear (cross tension) sample is subjected to shear (normal) load at the structural level the failure mechanism at the spot weld is tensile (shear) mode at the materials level. Based on the observed failure mechanism, stress distribution is assumed and related to the far field load for the lapshear and cross tension test samples. It appears that the failure load of the cross tension sample is 74 percent of the lap-shear sample based on the classical von Mises failure theory. The theoretical model is further extended to the mixed normal/shear loading condition. Data from strength tests as well as finite element numerical method are used to validate the model. Finally, the utility of the model in accessing the failure strength of spot welds is discussed. @DOI: 10.1115/1.1555648#

Modeling of heat transfer and fluid flow during gas tungsten arc spot welding of low carbon steel

Abstract: The evolution of temperature and velocity fields during gas tungsten arc spot welding of AISI 1005 steel was studied using a transient numerical model. The calculated geometry of the weld fusion zone and heat affected zone and the weld thermal cycles were in good agreement with the corresponding experimental results. Dimensional analysis was used to understand the importance of heat transfer by conduction and convection at various stages of the evolution of the weld pool and the role of various driving forces for convection in the liquid pool. The calculated cooling rates are found to be almost independent of position between the 1073 and 773 K (800 and 500 °C) temperature range, but vary significantly at the onset of solidification at different portions of the weld pool. During solidification, the mushy zone grew significantly with time until the pure liquid region vanished. The solidification rate of the mushy zone/solid interface was shown to increase while the temperature gradient in the mushy zone at...

Method and system for assessing quality of spot welds

Abstract: A system and method for assessing the quality of spot weld joints between pieces of metal includes an ultrasound transducer probing a spot weld joint The ultrasound transducer transmits ultrasonic radiation into the spot weld joint, receives corresponding echoes, and transforms the echoes into electrical signals An image reconstructor connected to the ultrasound transducer transforms the electrical signals into numerical data representing an ultrasound image A neural network connected to the image reconstructor analyzes the numerical data and an output system presents information representing the quality of the spot weld joint The system is trained to assess the quality of spot weld joints by scanning a spot weld joint with an ultrasound transducer to produce the data set representing the joint; then physically deconstructing the joint to assess the joint quality

Finite element simulation of laser spot welding

Abstract: The present work reports on a two-dimensional axisymmetric finite element analysis of heat flow during laser spot welding, taking into account the temperature dependence of the physical properties and latent heat of transformations. An analysis based on conduction heat transfer alone, but using the 'double ellipsoidal' representation of the laser beam, seems to be sufficient to estimate the transition to keyhole formation during laser spot welding, although the 'double ellipsoidal' representation requires an a priori knowledge of the expected weld pool dimensions. Transient temperature isotherms and the weld pool dimensions are predicted using the model; the latter are found to compare well with measurements of weld bead dimensions. The results show that the keyhole mode is stimulated using either a high laser power and low on-time or a low laser power and high on-time. The outcomes are found to be sensitive to the assumed absorptivity and the assumed weld pool depth used to define the 'double ell...

Alloying element vaporization during laser spot welding of stainless steel

Abstract: Alloying element loss from the weld pool during laser spot welding of stainless steel was investigated experimentally and theoretically. The experimental work involved determination of work-piece weight loss and metal vapour composition for various welding conditions. The transient temperature and velocity fields in the weld pool were numerically simulated. The vaporization rates of the alloying elements were modelled using the computed temperature profiles. The fusion zone geometry could be predicted from the transient heat transfer and fluid flow model for various welding conditions. The laser power and the pulse duration were the most important variables in determining the transient temperature profiles. The velocity of the liquid metal in the weld pool increased with time during heating and convection played an increasingly important role in the heat transfer. The peak temperature and velocity increased significantly with laser power density and pulse duration. At very high power densities, the computed temperatures were higher than the boiling point of 304 stainless steel. As a result, evaporation of alloying elements was caused by both the total pressure and the concentration gradients. The calculations showed that the vaporization occurred mainly from a small region under the laser beam where the temperatures were very high. The computed vapour loss was found to be lower than the measured mass loss because of the ejection of tiny metal droplets owing to the recoil force exerted by the metal vapours. The ejection of metal droplets has been predicted by computations and verified by experiments.

Fatigue behavior of spot welded high-strength sheet steels

Abstract: The spot weld fatigue properties of advanced automotive high-strength sheet steels were evaluated and compared to conventional sheet steels.

Friction Processing Technologies

Abstract: The utilisation of friction as an efficient thermo-mechanical source to both weld and process materials in the solid phase has come a long way since the first patent filing by Bevington in the late 19th Century. It is fair to say that up until the early Eighties, rotation was the primary motion used to practice friction welding for most applications on a commercial basis, certainly for metals. Work by Searle in the Seventies with orbital motion gathered momentum to permit the welding of non-round parts. This was followed by the development of a dedicated machine to use linear reciprocating motion for joining. From the late Eighties onwards an ‘explosion’ of friction based technologies were conceived and promoted. Such processes include friction taper stud and stitch welding, friction hydro pillar processing, friction extrusion, friction plunge welding, third-body friction welding and not least friction stir welding, which must be regarded as the major step change for the welding of aluminium and its alloys. Sandwiched between motion and process developments came more detailed studies of friction surfacing and friction seam welding, which were both the subject of a patent filing in 1941. The aforementioned processes are reviewed and selected processes discussed more fully. Attention is drawn to the applications, industrial sectors, etc., to which they can be aligned.

Probing temperature during laser spot welding from vapor composition and modeling

Abstract: Measurement of weld pool temperature during laser spot welding is a difficult task because of the short pulse duration, often lasting only a few milliseconds, highly transient nature of the process, and the presence of a metal vapor plume near the weld pool. This article describes recent research to estimate weld pool temperatures experimentally and theoretically. Composition of the metal vapor from the weld pool was determined by condensing a portion of the vapor on the inner surface of an open ended quartz tube which was mounted perpendicular to the sample surface and coaxial with the laser beam. It was found that iron, chromium, and manganese were the main metallic species in the vapor phase. The concentrations of Fe and Cr in the vapor increased slightly while the concentration of Mn in the vapor decreased somewhat with the increase in power density. The vapor composition was used to determine an effective temperature of the weld pool. A transient, three-dimensional numerical heat transfer and fluid flow model based on the solution of the equations of conservation of mass, momentum and energy was used to calculate the temperature and velocity fields in the weld pool as a function of time. The experimentally determined geometry of the spot welds agreed well with that determined from the computed temperature field. The effective temperature determined from the vapor composition was found to be close to the numerically computed peak temperature at the weld pool surface. Because of the short process duration and other serious problems in the direct measurement of temperature during laser spot welding, estimating approximate values of peak temperature from metal vapor composition is particularly valuable.

Relationships between Quality and Attributes of Spot Welds A spot weld's strength can be determined by the geometric characteristics and mechanical properties of the weldment

Abstract: A resistance spot weld's strength is determined by the physical at- tributes of the weldment. However, it is extremely difficult to establish a universal relationship with experiments between the measurable attributes of a weld and the weld's quality. The large number of variables and experimental uncertainty in- hibit establishing such a relation. A com- puter simulation experiment was con- ducted in this study, using the concept of design of experiments, to overcome the shortcomings of traditional experimental investigations. Quantitative relationships were established to link a weld's geomet- ric and mechanical attributes to its strength under tensile-shear loading.

Application of design of experiments to a spot welding process

Abstract: Design of experiments (DOE) is a scientific method for identifying the critical parameters associated with a process and thereby determining the optimal settings for these process parameters for enhanced performance and capability. It is a powerful technique employed in the improvement phase of the Six Sigma methodology as a means to improve process yield, stability, customer satisfaction and reduce process variability which results in high scrap, rework and warranty costs. This paper presents the application of DOE to a spot welding process in order to discover the key process parameters, which influence the tensile strength of welded joints. Statistical analysis was carried out to identify the process parameters, which affect the mean strength and variability in weld strength. The results of the study encouraged the engineering team within the company to extend the application of DOE to other core processes within the business as a process performance improvement technique.

Prediction of cooling rate and microstructure in laser spot welds

Abstract: Theoretical and experimental investigations have been carried out to examine the influence of laser power and on time on the weld thermal cycle and weld metal microstructure and hardness during laser spot welding of low alloy steel. A transient heat transfer model that takes into account the temperature dependence of material properties and latent heat of phase transformation is employed to simulate thermal cycles and cooling rates experienced by the material under various combinations of power and on times. Two models for predicting the microstructure and hardness of the weld pool metal from the cooling rates are used to evaluate the results.

Experimental study of nugget formation in resistance spot welding

Abstract: Nugget formation mechanism and its effect on the welding process parameters are observed by using a digital high-speed camera.

Relationships between quality and attributes of spot welds

Abstract: A resistance spot weld's strength is determined by the physical attributes of the weldment. However, it is extremely difficult to establish a universal relationship with experiments between the measurable attributes of a weld and the weld's quality. The large number of variables and experimental uncertainty inhibit establishing such a relation. A computer simulation experiment was conducted in this study, using the concept of design of experiments, to overcome the shortcomings of traditional experimental investigations. Quantitative relationships were established to link a weld's geometric and mechanical attributes to its strength under tensile-shear loading.

Welding electrode for aluminum sheets

Abstract: A design of copper welding electrode is disclosed. The electrode can be used in successive spot welding operations on assembled aluminum alloy sheet parts of widely varying thicknesses, e.g., from 0.9 to 4+ mm. The electrode has a round body terminating in a truncated cone with a crowned face. The face has a diameter of about eleven millimeters up to the diameter of the body and the radius of the crowned face is about twenty to thirty millimeters. The dimensions and shape of the electrode provide robust welding performances despite varying part thicknesses and off angle electrode positioning, and the textured surface provides good electrical contact.

Effect of wire feed position on laser welding with filler wire

Abstract: The utilization of the laser welding process with filler wire addition is often considered a complicated and difficult process which has too high accuracy demands for a work shop floor. This study shows the effect of some of the most important welding variables to the quality and efficiency of laser welding when welding with filler wire. The material used was a common structural steel St52/37 of 6 mm in thickness. The joint type used was a butt joint. Acceptable weld quality was achieved with most of the tested parameter combinations. The effect of the wire feeding parameters for the acceptable weld quality was smaller than expected. The variations of weld quality caused by the nonoptimized aiming of filler wire can be compensated by the adjustment of the filler wire feed rate and the heat input. The quality of the welds was compared with developed weld quality index. The increase in heat input will make it possible to accept wire feeding to a lower position or from backwards compared to the welding direction. The misalignment of wire in the transverse direction to welding direction and laser beam optical axis causes problems that may lower the quality of the weld. The comparison of the welds with variable air gap widths showed that the weld width is more dependent on the heat input that on the original gap volume.

Effects of Electrode Degradation on Electrode Life in Resistance Spot Welding of Aluminum Alloy 5182 Degradation of electrode tip faces increased the contact area at the electrode~sheet interface, which resulted in an undersized nugget at the sheet~sheet interface

Abstract: Electrode endurance tests were conducted to investigate the effects of electrode degradation on electrode life in resistance spot welding of 1.5-mm-thick sheet aluminum Alloy 5182 using a medium-frequency direct-current welding machine and electrodes with tip-face diam- eter of 10 mm and radius of curvature of 50 mm. The observed electrode life ranged from about 400 to 900 welds even though all the process conditions were intentionally kept constant. However, despite the large variation, distinct patterns were found to correlate electrode life to electrode degra- dation in terms of the change in nominal electrode tip-face area and contact areas at both electrode/sheet (E/S) and sheet/sheet (S/S) interfaces. The reduction in joint strength occurred because of undersized nugget formation due to increased contact areas and hence reduced current density. The electrode degradation may be moni- tored by the increase in all three areas (nominal tip-face area, and E/S and S/S con- tact areas), but the E/S contact area is be- lieved to be the most suitable because a minimum of extra work is needed to mea- sure it. The button diameter, measured from peel testing, is affected by nugget di- ameter (current density) and possibly other factors, such as weld expulsion and porosity distribution as well.

Effects of electrode degradation on electrode life in resistance spot welding of aluminum Alloy 5182

Abstract: Electrode endurance tests were conducted to investigate the effects of electrode degradation on electrode life in resistance spot welding of 1.5-mm-thick sheet aluminum Alloy 5182 using a medium-frequency direct-current welding machine and electrodes with tip-face diameter of 10 mm and radius of curvature of 50 mm. The observed electrode life ranged from about 400 to 900 welds even though all the process conditions were intentionally kept constant. However, despite the large variation, distinct patterns were found to correlate electrode life to electrode degradation in terms of the change in nominal electrode tip-face area and contact areas at both electrode/sheet (E/S) and sheet/sheet (S/S) interfaces. The reduction in joint strength occurred because of undersized nugget formation due to increased contact areas and hence reduced current density. The electrode degradation may be monitored by the increase in all three areas (nominal tip-face area, and E/S and S/S contact areas), but the E/S contact area is believed to be the most suitable because a minimum of extra work is needed to measure it. The button diameter, measured from peel testing, is affected by nugget diameter (current density) and possibly other factors, such as weld expulsion and porosity distribution as well.

A study on using scanning acoustic microscopy and neural network techniques to evaluate the quality of resistance spot welding

Abstract: This paper shows that the quality of resistance spot welds can be evaluated using scanning acoustic microscopy (SAM). Two-layered coated spot-welded samples are investigated utilising a wide-field short-pulse scanning acoustic microscope with operation frequencies of 25, 50 and 100 MHz. Geometrical parameters, e.g. nugget area, maximum axis of nugget, and minimum axis of nugget, are acquired from C-scan images of weld nuggets using mathematical morphology techniques. These parameters serve as inputs for an artificial neural network (ANN) model to evaluate the quality of spot welds. The output of the model during the training process comprises the results of nugget peeling tests and expert opinions. The ANN can provide suggestions on weld quality with a higher than 95% correctness. A JAVA computer program is developed for image processing, ANN training, and ANN testing. With this model, the computer program can render the quality of spot welds that are close to those achieved using off-line destructive method.

Monitoring of expulsion in small scale resistance spot welding

Abstract: Although there have been many investigations into monitoring and control of resistance spot welding (RSW) of sheet metal having a thickness greater than 0.5 mm, that of thinner components has rarely been investigated. Monitoring of expulsion in a small scale RSW process was carried out via measurement of voltage, electrode displacement, and force change during the welding current pulse. It was found that electrode displacement increased steadily during the current pulse for an expulsion free weld. For welds with visible expulsion, the electrode voltage had a small but readily observed spike; the electrode displacement showed a dip or a decrease in total amplitude; the force change during a welding cycle was of greater magnitude. Since the system uses a constant current power supply, the voltage increase corresponds to an increase in dynamic resistance associated with the expulsion event. It was also observed that the magnitude of the electrode displacement dip was directly related to the volume of...

Simulation of Time-Dependent Pool Shape during Laser Spot Welding: Transient Effects

Abstract: The shape and depth of the area molten during a welding process is of immense technical importance. This study investigates how the melt pool shape during laser welding is influenced by Marangoni convection and tries to establish general qualitative rules of melt pool dynamics. A parameter study shows how different welding powers lead to extremely different pool shapes. Special attention is paid to transient effects that occur during the melting process as well as after switching off the laser source. It is shown that the final pool shape can depend strongly on the welding duration. The authors use an axisymmetric two-dimensional (2-D) control-volume-method (CVM) code based on the volume-averaged two-phase model of alloy solidification by Ni and Beckermann[1] and the SIMPLER algorithm by Patankar.[2] They calculate the transient distribution of temperatures, phase fractions, flow velocities, pressures, and concentrations of alloying elements in the melt and two solid phases (peritectic solidification) for a stationary laser welding process. Marangoni flow is described using a semiempirical model for the temperature-dependent surface tension gradient. The software was parallelized using the shared memory standard OpenMP.

Influence of Welding Machine Mechanical Characteristics on the Resistance Spot Welding Process and Weld Quality How machine stiffness, friction, and moving mass can affect welding results

Abstract: 116 ABSTRACT. Mechanical characteristics of resistance spot welding machines, such as stiffness, friction, and moving mass, have complex influences on the resistance welding process and weld quality. In this paper, these influences are systematically investigated through experiments. The mechanisms of the influences are ex- plored by analyzing process signatures, such as welding force and electrode dis- placement, and other process characteris- tics, such as electrode alignment. A better understanding of the influences is achieved through this analysis. This study shows machine stiffness and friction affect welding processes and weld quality. It also confirms the moving mass does not signif- icantly affect the process and quality of re- sistance spot welding.

Numerical Modelling of Resistance Spot Welding of Aluminium Alloy

Abstract: An electro-thermal and mechanical coupled model is reported for analysing resistance spot welding process of aluminum alloys using curved face electrodes. An in-house software is developed based on finite element method that can predict in-process growth of nugget diameter, penetration and electrode-sheet contact diameter for different combinations of welding current, weld time, electrode force. Non-linear, temperature-dependent, thermo-physical material properties as well as latent heat of fusion/solidification have been considered. An exponential decay of initial contact resistance is considered with different values of initial contact resistance for different surface conditions of aluminium alloys. It is observed that the formation of weld nugget is completed within the very first cycle considering a 50 Hz AC power supply. Increase of welding current enhances the area of molten zone substantially. Change in the initial value of contact resistance does not influence the overall nugget zone significantly. Overall, the present work indicates that such a simulation model can be very useful as an off-line monitoring tool to estimate the influence of process parameters on the final weld dimensions.

Influence of welding machine mechanical characteristics on the resistance spot welding process and weld quality

Abstract: Mechanical characteristics of resistance spot welding machines, such as stiffness, friction, and moving mass, have complex influences on the resistance welding process and weld quality. In this paper, these influences are systematically investigated through experiments. The mechanisms of the influences are explored by analyzing process signatures, such as welding force and electrode displacement, and other process characteristics, such as electrode alignment. A better understanding of the influences is achieved through this analysis. This study shows machine stiffness and friction affect welding processes and weld quality. It also confirms the moving mass does not significantly affect the process and quality of resistance spot welding.