<p>A literature survey on modelling of the resistance spot welding process has been carried out and some of the more interesting models on this subject have been reviewed in this work. The underlying physics has been studied and a brief explanation of Heat transfer, electrokinetics and metallurgy in a resistance spot welding context have been presented.\nl\hsLastly a state of the art model and a simplified model, with implementation in the FEM software LS-DYNA in mind, have been presented.</p>

In order to meet the increasing fuel efficiency requirements, the automotive industry has strived for component weight reduction in order to improve the performance of automotive vehicles through the use of light Al and Mg alloys. Resistance spot welding (RSW) currently is the primary joining method in the manufacturing of automotive assemblies. With the increased use of Al and Mg, there is a pressing need for a technology to produce dissimilar Al/Mg joints, and preferably by RSW since this technology is already prevalent in the industry. Direct welding of Al to Mg usually results in formation of hard and brittle intermetallic compounds and poor quality of the welds. Employing an interlayer is a promising approach to overcome this problem. Current literature, however, does not consider the effects of different interlayers on RSW of Al to Mg. This thesis examines effects of different interlayers on microstructure and mechanical properties of Al/Mg joints made by RSW. Effects of three types of interlayers, specifically pure Ni foil, Au-coated Ni foil and Zn-coated were investigated in details. While only brief investigation of joints made with Sn-coated steel, Zn foil, and Cu foil interlayers was conducted. No joints were achieved with a bare Ni interlayer during Al to Mg alloy resistance spot welding, as coupons separated without applying any force. The Ni interlayer remained intact and Al-Mg intermetallic compounds did not form. Addition of Au coating on Ni surface greatly contributed to the metallurgical bonding at the interfaces and welds easily met requirements of the AWS D17.2 standard. Average lap-shear strength reached 90% of that in similar AZ31B resistance spot welds. Acceptable welds were also produced using galvanised Zn-coated steel interlayer, which easily met strength requirements of the AWS D17.2 standard. Average failure load reached 74% of same size similar AZ31B joints. The steel interlayer was not melted which prevented mixing of Al and Mg. The Zn coating on the steel interlayer was melted and squeezed to the nugget periphery, providing a clean steel surface for welding-brazing in the center and acting as a solder metal at the periphery. A feasibility study of Al/Mg RSW with Sn-coated steel, Zn foil and Cu foil interlayers was also conducted. Mechanical properties of welds made with Sn-coated steel interlayer were very similar to those made with Zn-coated steel interlayer. While welds made with only a Zn foil interlayer were much weaker. The Zn foil completely melted during the welding which resulted in formation of brittle Al-Mg-Zn phases. None of the welds made with Zn foil interlayer met requirements of the AWS D17.2 standard. RSW of Al to Mg with Cu foil interlayer also could not produce welds with acceptable strength.

The influence of the tribology during resistance spot welding (RSW) of aluminum alloy 5182 with spherical-tip electrode has been investigated at both the electrode-worksheet (E/W) and faying surface (FS) interfaces. In RSW, electrode life is limited by poor current transport to the FS interface caused by extensive pitting of the electrode tip surface. The primary focus of the present research was to extend electrode life by using the knowledge gained from studying the contact mechanics at both of these interfaces. Series of experiments were conducted and finite element analysis was employed to investigate the contact mechanics at the interfaces. Based on these findings, a practical way to extend the electrode life was developed. In a series of initial experiments, it was found that attempts to alter the worksheet surface roughness caused damage to the surface oxide layer which resulted in decrease of electrical contact resistance at the E/W interface. The oxide layer on the worksheet surface contained aluminum and magnesium oxide regions and abrasion of the worksheet surface reduced the oxide layer thickness and made it more uniform in composition because when the magnesium oxide regions were abraded, a thin layer of aluminum oxide re-formed immediately while it take specific conditions to re-form magnesium oxide. These factors decreased the electrical contact resistance of the E/W interface compared with the as-received surface, thus reducing heat generation and the associated pitting of the electrode surface during RSW. Further experimental investigations and finite element analysis showed that the contact mechanics that occurred during the loaded “squeezing” phase of the welding sequence, but before current was applied to cause RSW, had a significant effect on the electrode pitting behaviour and nugget formation. At the E/W interface, squeezing caused high shear stress and slip at the periphery of the contact region. This slip disrupted the oxide layer and reduced the electrical resistance. At the beginning of the current phase of the weld sequence, the reduced electrical resistance caused current to concentrate near the periphery but constriction resistance still produced enough heat generation to cause alloying, pickup and eventually pitting of electrode in a ring around the contact centre. At the FS interface, experiments and finite element analysis showed that sheet separation and thus bending occurred during the squeezing phase and this had a profound influence on nugget formation. Experimental observations showed that the bending caused enlarged and aligned cracks in the surface oxide layers which promoted good metal-to-metal contact near the periphery of the FS. As at the E/W interface, high current densities occurred at the beginning of the current phase and the constriction resistance caused significant heat generation in this zone due to an increasing constriction resistance. Consequently, the melting at the FS started near the periphery and moved in towards the central zone of the contact region melted to produce a “doughnut-shaped” nugget with a filled-in but thin central region. Low electrical contact resistance at the E/W interface led to longer electrode tip life because less pitting occurred. In addition, higher current densities could then develop at the FS to affect RSW and achieve good nugget formation despite the rather uneven peripheral heat generation. In attempts to reduce the electrical resistance at the E/W interface, several boundary lubricants were placed on the worksheet surface a short time before starting RSW and they altered the tribology. Both increased and decreased electrode degradation rate were found in electrode life tests. One lubricant was found to be particularly effective in lowering the electrode pitting rate. It extended the electrode life to almost double that occurring with as-received (unlubricated) surfaces. Detailed analysis revealed that the effective boundary lubricant had a beneficial chemical influence on the surface of the AA5182 worksheet. The lubricant chemically attacked the oxide layer thus reducing its thickness and reducing electrical contact resistance of the E/W interface at the critical peripheral region. The result was a lower electrode pitting rate and an extended electrode life. The improved understanding of the current flow during the critical initial period and its dependence on the contact mechanics of the E/W and FS interfaces was considered important in developing ways of improving weld strength and increasing electrode life. The finding of a boundary lubricant that acted to reduce oxide layer thickness was considered an important starting point for industrial development of RSW with longer electrode life. It could be employed without interrupting the RSW process and its efficacy was well-supported by the present contact mechanics studies in which the key role of the oxide layer was demonstrated.

The influence of the tribology during resistance spot welding (RSW) of aluminum alloy 5182 with spherical-tip electrode has been investigated at both the electrode-worksheet (E/W) and faying surface (FS) interfaces. In RSW, electrode life is limited by poor current transport to the FS interface caused by extensive pitting of the electrode tip surface. The primary focus of the present research was to extend electrode life by using the knowledge gained from studying the contact mechanics at both of these interfaces. Series of experiments were conducted and finite element analysis was employed to investigate the contact mechanics at the interfaces. Based on these findings, a practical way to extend the electrode life was developed. In a series of initial experiments, it was found that attempts to alter the worksheet surface roughness caused damage to the surface oxide layer which resulted in decrease of electrical contact resistance at the E/W interface. The oxide layer on the worksheet surface contained aluminum and magnesium oxide regions and abrasion of the worksheet surface reduced the oxide layer thickness and made it more uniform in composition because when the magnesium oxide regions were abraded, a thin layer of aluminum oxide re-formed immediately while it take specific conditions to re-form magnesium oxide. These factors decreased the electrical contact resistance of the E/W interface compared with the as-received surface, thus reducing heat generation and the associated pitting of the electrode surface during RSW. Further experimental investigations and finite element analysis showed that the contact mechanics that occurred during the loaded “squeezing” phase of the welding sequence, but before current was applied to cause RSW, had a significant effect on the electrode pitting behaviour and nugget formation. At the E/W interface, squeezing caused high shear stress and slip at the periphery of the contact region. This slip disrupted the oxide layer and reduced the electrical resistance. At the beginning of the current phase of the weld sequence, the reduced electrical resistance caused current to concentrate near the periphery but constriction resistance still produced enough heat generation to cause alloying, pickup and eventually pitting of electrode in a ring around the contact centre. At the FS interface, experiments and finite element analysis showed that sheet separation and thus bending occurred during the squeezing phase and this had a profound influence on nugget formation. Experimental observations showed that the bending caused enlarged and aligned cracks in the surface oxide layers which promoted good metal-to-metal contact near the periphery of the FS. As at the E/W interface, high current densities occurred at the beginning of the current phase and the constriction resistance caused significant heat generation in this zone due to an increasing constriction resistance. Consequently, the melting at the FS started near the periphery and moved in towards the central zone of the contact region melted to produce a “doughnut-shaped” nugget with a filled-in but thin central region. Low electrical contact resistance at the E/W interface led to longer electrode tip life because less pitting occurred. In addition, higher current densities could then develop at the FS to affect RSW and achieve good nugget formation despite the rather uneven peripheral heat generation. In attempts to reduce the electrical resistance at the E/W interface, several boundary lubricants were placed on the worksheet surface a short time before starting RSW and they altered the tribology. Both increased and decreased electrode degradation rate were found in electrode life tests. One lubricant was found to be particularly effective in lowering the electrode pitting rate. It extended the electrode life to almost double that occurring with as-received (unlubricated) surfaces. Detailed analysis revealed that the effective boundary lubricant had a beneficial chemical influence on the surface of the AA5182 worksheet. The lubricant chemically attacked the oxide layer thus reducing its thickness and reducing electrical contact resistance of the E/W interface at the critical peripheral region. The result was a lower electrode pitting rate and an extended electrode life. The improved understanding of the current flow during the critical initial period and its dependence on the contact mechanics of the E/W and FS interfaces was considered important in developing ways of improving weld strength and increasing electrode life. The finding of a boundary lubricant that acted to reduce oxide layer thickness was considered an important starting point for industrial development of RSW with longer electrode life. It could be employed without interrupting the RSW process and its efficacy was well-supported by the present contact mechanics studies in which the key role of the oxide layer was demonstrated.

In order to meet the increasing fuel efficiency requirements, the automotive industry has strived for component weight reduction in order to improve the performance of automotive vehicles through the use of light Al and Mg alloys. Resistance spot welding (RSW) currently is the primary joining method in the manufacturing of automotive assemblies. With the increased use of Al and Mg, there is a pressing need for a technology to produce dissimilar Al/Mg joints, and preferably by RSW since this technology is already prevalent in the industry. Direct welding of Al to Mg usually results in formation of hard and brittle intermetallic compounds and poor quality of the welds. Employing an interlayer is a promising approach to overcome this problem. Current literature, however, does not consider the effects of different interlayers on RSW of Al to Mg. This thesis examines effects of different interlayers on microstructure and mechanical properties of Al/Mg joints made by RSW. Effects of three types of interlayers, specifically pure Ni foil, Au-coated Ni foil and Zn-coated were investigated in details. While only brief investigation of joints made with Sn-coated steel, Zn foil, and Cu foil interlayers was conducted. No joints were achieved with a bare Ni interlayer during Al to Mg alloy resistance spot welding, as coupons separated without applying any force. The Ni interlayer remained intact and Al-Mg intermetallic compounds did not form. Addition of Au coating on Ni surface greatly contributed to the metallurgical bonding at the interfaces and welds easily met requirements of the AWS D17.2 standard. Average lap-shear strength reached 90% of that in similar AZ31B resistance spot welds. Acceptable welds were also produced using galvanised Zn-coated steel interlayer, which easily met strength requirements of the AWS D17.2 standard. Average failure load reached 74% of same size similar AZ31B joints. The steel interlayer was not melted which prevented mixing of Al and Mg. The Zn coating on the steel interlayer was melted and squeezed to the nugget periphery, providing a clean steel surface for welding-brazing in the center and acting as a solder metal at the periphery. A feasibility study of Al/Mg RSW with Sn-coated steel, Zn foil and Cu foil interlayers was also conducted. Mechanical properties of welds made with Sn-coated steel interlayer were very similar to those made with Zn-coated steel interlayer. While welds made with only a Zn foil interlayer were much weaker. The Zn foil completely melted during the welding which resulted in formation of brittle Al-Mg-Zn phases. None of the welds made with Zn foil interlayer met requirements of the AWS D17.2 standard. RSW of Al to Mg with Cu foil interlayer also could not produce welds with acceptable strength.

Resistance spot welding is used frequently as a successful joining method for a variety of work commonly in automotive and other manufacturing processes. Spot weld nugget is generally hidden between two sheets, causing its inspection difficult and expensive. Undersized nuggets, brittle or cracked nuggets, and excessive indentation of electrodes reveals the lack of fusion between the parts that can make the weld sub-standard. Visual inspection, pry testing and physical teardown with chisel and hammer method or a combination of them are being used traditionally. However, this study presents a more effective nondestructive inspection method based upon an ultrasonic pulse-echo technique. The theory of the technique together with the experimental verification are presented and its advantages over the other destructive and nondestructive techniques are considered.

Yes / Resistance spot welding of aluminum (Al5754) to magnesium (AZ31B) alloys results in the formation of a variety of solidification microstructures and intermetallic compounds that may affect the in-service performance of the weld. This study evaluates the relationship between the welding parameters and the properties of the weld nugget that is formed, and clarifies the morphological and microstructural evolutions within the weld regions during the low-current “small-scale” resistance spot welding of Al5754 to AZ31B. The investigations included a combination of microstructural characterization and thermodynamic analysis of the weld region. The results show that the welding time and clamping force parameters have significant effects on the properties of the nugget formed. The optimal welding parameters were found to be 300 ms welding time and 800 N clamping force. Weld nuggets formed with lower welding time and clamping force were undersized and contained extensive porosity. Meanwhile, a clamping force above 800 N caused gross deformation of the test samples and the expulsion of the molten metal during the welding process. The most significant microstructural changes occurred at the weld/base metal interfaces due to the formation of Al17Mg12 and MgAl2O4 intermetallic compounds as well as significant compositional variation across the weld pool. The thermal gradient across the weld pool facilitated the formation of several microstructural transitions between equiaxed and columnar dendrites.

A literature survey on modelling of the resistance spot welding process has been carried out and some of the more interesting models on this subject have been reviewed in this work. The underlying physics has been studied and a brief explanation of Heat transfer, electrokinetics and metallurgy in a resistance spot welding context have been presented.\nl\hsLastly a state of the art model and a simplified model, with implementation in the FEM software LS-DYNA in mind, have been presented.

The main purpose of this thesis is to improve the fundamental knowledge of non-isothermal tempering of martensite phase and its effects on the reduction in hardness (softening) with respect the base metal occurring at the heat affected zone (HAZ) of resistance spot welded dual-phase (DP) steels. This thesis also aims at understanding the influence of HAZ-softening on the joint performance of various DP steel grades. The tempering of martensite occurring at the sub-critical HAZ (SC-HAZ) of resistance spot welded DP600, DP780 and DP980 steels has been systematically evaluated by microhardness testing through Vickers indentation and the degree of tempering has been correlated to the HAZ-softening. From the joint performance analysis of similar and dissimilar steel grade combinations assessed through standardized testing methods, three important issues have been targeted: a) the joint strength (maximum load to failure), b) the location of failure (failure mode), and c) the physical characteristic of the weld that determines certain type of failure (weld nugget size). In addition, a partial tensile test has been conducted in order to evaluate the initiation of failure in dissimilar steel grade combinations. It has been shown that HAZ-softening lowered the weld size at which transition from interfacial to pullout failure mode takes place along with increased load-bearing capacity and higher energy absorption. Thus, it is concluded from mechanical testing that HAZ-softening benefits the lap-shear tensile joint performance of resistance spot welded DP steels by facilitating pullout failures through failure initiation at the SC-HAZ (tempered region). Instrumented nanoindentation testing was employed to further investigate HAZ-softening along the SC-HAZ by evaluating individual phases of ferrite matrix and tempered martensite islands. Although the ferrite matrix presented a slight reduction in hardness at nanoscale, higher reduction in hardness (softening) resulted for tempered martensite; thus confirming that tempered martensite is the major contributor to softening at micro-scale. A comparison between nanohardness and microhardness testing made at different distances from the line of lower critical temperature of transformation (Ac1) allowed revealing the actual extension of the SC-HAZ. In this regard, good correlation was obtained between nanohardness results along the SC-HAZ and the microstructural changes analyzed by electron microscopy (i.e., the tempering of martensite occurring at various distances far from Ac1 was correlated to low temperature tempering of dual phase steels). An in-depth analysis of the tempering of martensite phase at high temperature in DP steel subjected non-isothermal conditions i.e., rapid heating, extremely short time at peak temperature and rapid cooling (resistance spot welding), has been carried out mainly through analytical transmission electron microscopy (TEM). In addition, an isothermal tempering condition (i.e., slow heating and long time at peak temperature) in DP steel has been evaluated for complementing the analysis. Both non-isothermal and isothermal conditions have been correlated to the softening behaviour. TEM analysis of the base metal in the DP steel indicated that the morphology of the martensite phase is dependent on its carbon content, and its tempering characteristics are similar to that of equal carbon containing martensitic steel. The isothermally tempered structure is characterized by coarsening and spheroidization of cementite (θ) and complete recovery of the martensite laths; whereas precipitation of fine quasi-spherical intralath θ-carbides, coarser plate-like interlath θ-carbides, decomposition of retained austenite into elongated θ-carbides, and partial recovery of the lath structure were observed after non-isothermal tempering of DP steel. This difference in tempering behaviour is attributed to synergistic effect of delay in cementite precipitation due to higher heating rate, and insufficient time for diffusion of carbon that delays the third stage of tempering process (cementite coarsening and recrystalization) during non-isothermal. The finer size and the plate-like morphology of the precipitated carbides along with the partial recovery of the lath structure observed after non-isothermal tempering strongly influenced the softening behaviour of DP steel. The chemical analysis of θ-carbides through extraction replicas for three different DP steels revealed that the chemistry of the carbides is inherited from the parent DP steel during non-isothermal tempering at high temperature confirming that non-isothermal tempering DP steel is predominantly controlled by carbon diffusion.

The effects of AC and DC on cooling rate, solute distribution and nugget shape after solidification, which are responsible for microstructure of the fusion zone, during resistance spot welding are realistically and extensively investigated. The finite difference method is used to predict transport variables in workpieces and electrodes during heating, melting, cooling and freezing periods. The model accounts for electromagnetic force, heat generations at the electrode-workpiece interface and faying surface between workpieces, and dynamic electrical resistance including bulk resistance and contact resistances at the faying surface and electrode-wokpiece interfaces, which are function of hardness, temperature, electrode force, and surface condition. The computed results show that in contrast to DC, using AC readily produces the nugget in an ellipse shape. Deficit and excess of solute content occur in a thin layer around the boundary and interior of the nugget, respectively. The effects of dynamic electrical resistance subject to AC (Alternative current) on transport variables, cooling rate, solute distribution and nugget shape after solidification during resistance spot welding are realistically and extensively investigated. The model accounts for electromagnetic force, heat generation and contact resistances at the faying surface and electrode-workpiece interfaces and bulk resistance in workpieces. Contact resistance are comprised of constriction and film resistances, which are functions of hardness, temperature, electrode force and surface condition. The computed results show that the weld nugget readily occurs by increasing constriction resistance and Curie temperature. High Curie temperature enhances convection and solute mixing, and readily melts through the workpiece surface near the electrode edge. Aside from finding the significant effect of Curie temperature on resistance spot welding, this study indicates that any mean (For example, adjusting solute content) to reduce Curie temperature can be a new way to control weld quality.