Refill friction stir spot welding (RFSSW) is an emerging technology, capable of joining thin sheets of aluminum alloys. The present thesis comprises two studies which were conducted to address two challenges faced by RFSSW: the long cycle time traditionally associated with welding and the poor life of existing RFFSW tools. In the first study, welds were made in AA5052-H36, at various cycle times and with various process parameters. It was shown that RPM, cycle time, and material thickness, all have an effect on the machine response. Decreasing RPM or weld duration leads to increased force and torque response during welding. Welds with cycle times below one second were successfully made without severely impacting joint quality, suggesting that prior work may have been limited by machine capabilities rather than by phenomena inherent to the process. On average, the sub-one second welds caused a peak probe force of 9.81 kN, a plunge torque of 26.3 N*m, and showed average lap-shear strengths of 7.0kN; compared to a peak probe force of 5.14 kN, a plunge torque of 17.3 N*m, and lap-shear strength of 6.89kN for a more traditional four-second welding condition. In the second study, the life of a steel toolset was quantified as consecutive welds were made in AA5052-H36 until the toolset seized from material accumulation/growth. At a one-second welding condition, the toolset was only capable of producing 53 welds before seizure. At a two-second welding condition, the toolset was only capable of producing 48 welds. In subsequent temperature experiments, thermocouples were embedded into welding coupons at various locations near weld center, allowing novel temperature data to be collected for welds with varying cycle times and parameters. The collected temperature data shows that as cycle time increases, so does weld temperature. At weld center, temperatures in excess of 500°C were observed in welds with 4 second durations. At these temperatures, Fe-Al intermetallic growth is anticipated as a mechanism limiting the tool life observed. The results suggest that steel is not an appropriate choice for RFSSW tools, and future evaluation of other materials is merited.

<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.

No description available.

Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Integration of AHSS into the automotive architecture has brought renewed challenges for achieving acceptable welds. The varying alloying content and processing techniques has further complicated this initiative. The current study examines resistance spot welding of high strength and advance high strength steels including high strength low alloy (HSLA), dual phase (DP) and a ferritic-bainitic steel (590R). The mechanical properties and microstructure of these RSW welded steel alloys are detailed. Furthermore a relationship between chemistries and hardness is produced. The effect of strain rate on the joint strength and failure mode is also an important consideration in the design of welded structures. Current literature, however, does not explain the effects of weld microstructure and there are no comprehensive comparisons of steels. This work details the relationship between the joint microstructure and impact performance of spot welded AHSS. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower, respectively. Results for elongation, failure load and energy absorption for each material are presented. Failure modes are detailed by observing weld fracture surfaces. In addition, cross-sections of partially fractured weldments were examined to detail fracture paths during static loading. Correlations between the fracture path and mechanical properties are developed using observed microstructures in the fusion zone and heat-affected-zone. Friction stir spot welding (FSSW) has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has also been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn-coated DP600 AHSS (1.2mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process.