Refill Friction Stir Spot Welding (RFSSW) is a new solid-state joining technology, which is suitable for joining similar and dissimilar overlap sheets connections, particularly in aluminium and magnesium alloys. This welding method is expected to have wide applications in joining of body parts in the automotive industry. In the present study, RFSSW has been used to join 1.0 mm gauge sheets of two material combinations: similar AA6111-T4 automotive aluminium alloy joints and a dissimilar aluminium AA6111-T4 to magnesium AZ31-H24 alloy combinations. The performance of the joints was investigated in terms of the effect of the welding parameters (including tool rotation rate, sleeve plunge depth, and welding time etc.) to improve current understanding and allow optimisation of the process for short welding-cycles when joining similar and dissimilar light alloys. The results of the investigations on similar AA6111 welds showed the ability to use a wide window of process parameters that resulted in joints with a successfully refilled keyhole and flat weld surface, even when using a welding time as short as 0.5 s. The joints in the as-welded condition showed strengths as high as 4.2 kN, when using welding parameters of 1500 rpm, 1.0 mm with a range of welding times from 0.55 to 2.0 s. All joints showed a nugget pull-out failure mode when using a sleeve plunge depth of 0.8 mm or more, as a result of increasing the joint area. The strength of the joints further improved and reached peak loads of 5.15 and 6.43 kN after natural and artificial ageing, respectively, for welds produced using optimised welding parameters of a 2500 rpm tool rotation rate, a 1.5 s welding time and a 1.0 mm plunge. This improvement in strength resulted from the improvement in the local mechanical properties in the HAZ and other regions, which results from a minimal HAZ due to the rapid weld cycle and the re-precipitation of GPZs and clustering on natural ageing, or β on artificial ageing. A modification to the RFSSW process was developed in this project to solve the problems faced when dissimilar welding Mg to Al. This modified process involved adding a final brief pin plunge stage to consolidate refill defects and it was successful in producing nearly defect-free joints with improved mechanical properties, using a wide range of the process parameters. The average peak load of the joints increased with increasing tool rotation rate, to reach a maximum value at 2500 rpm due to eliminating the weld defects by increasing the material plasticity. However, increasing the tool rotation rate further to 2800 rpm led to a decrease in the average peak failure load due to eutectic melting at the weld interface. The optimum welding condition was thus found to be: 2500 rpm, 1.0 s, and 1.0 mm, which gave an average peak failure load of 2.4 kN and average fracture energy of 1.3 kN.mm. These values represent an improvement of about 10 % and 27 %, respectively, compared to welds produced with the conventional RFSSW process, and about 112 % and 78 % of the Mg-Mg similar joints produced using the same welding conditions. A FE model developed in this project was successful in increasing understanding of the behaviour of the RFSSW joints when subjected to lap tensile-shear loading. The stress and strain distribution in the modelled samples showed that the highest concentration occurring in the region of the confluence of the SZ with the two sheets. With increasing extension, these regions of highest stress and strain propagated to the outer surfaces of the two sheets and then annularly around the weld nugget. This annular ring of high strain concentration agreed well with the failure path and results in the full plug pull-out fracture mode shown by the experimentally tested samples. The predicted force-extension curves showed high agreement with the experimental results, especially when including the effect of the hook defect and correction of compliance in the experimental results.

Este trabalho teve como principal objetivo a determinação dos parâmetros de soldagem a ponto pelo processo de fricção-mistura (FSSW) em uma liga de alumínio AA2024-T3, pela comparação de resultados obtidos em ensaios de cisalhamento e arrancamento em tração e ensaios de fadiga em juntas sobrepostas. Juntas rebitadas da mesma liga também foram ensaiadas, nas mesmas condições, para comparação de propriedades mecânicas. Os principais parâmetros do processo FSSW são: velocidades de avanço e de rotação da ferramenta, profundidade de penetração e tempo de patamar. Foram explorados principalmente os parâmetros: profundidade de penetração e rotação da ferramenta. Os resultados dos ensaios de cisalhamento em tração possibilitaram a determinação da resistência ao cisalhamento das juntas soldadas a ponto, que apresentaram valores inferiores, mas próximos aos obtidos nas juntas rebitadas. Análises microestruturais e de microdureza foram realizadas para elucidar alguns resultados dos ensaios executados. Os ensaios de fadiga foram executados sob controle de carga, com razão de carga R=0,1, em corpos de prova confeccionados com os parâmetros que geraram os melhores resultados em ensaios de cisalhamento em tração. Para as juntas soldadas, uma técnica de monitoramento de defeitos (MCV monitoramento comparativo de vácuo), baseada na diferença de pressão de vácuo, foi utilizada apresentando bons resultados. Apesar de resultados similares em ensaios de cisalhamento em tração, as juntas soldadas apresentaram vida bastante inferior nos ensaios de vida à fadiga, quando comparadas com as juntas rebitadas. / The main aim of this work was the determination of the parameters governing the Friction Stir Spot Welding (FSSW) of a AA2024-T3 aluminum alloy, by the obtained results comparison in pull-out and shear tensile tests and fatigue tests in lap-joints. Riveted joints of the same alloy were also tested in the same conditions for mechanical properties comparison. The main FSSW process parameters were: plunge rate, dwell time, tool penetration and tool rotational speed. The tool penetration and the tool rotational speed parameters were focused. The shear tensile tests results allowed the lap-joint shear resistance determination, that was lower, but close, of those obtained with riveted lap-joints. Microstructure and micro-hardness analysis was carried out to elucidate some tests perfomed results. The fatigue tests was performed in load control, with load ratio R of 0,1, in a lap joint produced with the same parameters of the specimen which presented the best tensile results. For the welded joints, a monitoring defect technique (CVM comparative vacuum monitoring), difference vacuum pressure based, was used showing good results. Although similar results in shear tensile tests, the welded joints showed so lower life in the fatigue tests, when compared with riveted joints.

Friction stir spot welding (FSSW) is a solid-state welding process which is especially useful for joining precipitation-hardened aluminium alloys that undergo adverse property changes during fusion welding. It also has potential as an effective method for solid-state joining of dissimilar alloys. In FSSW, heat generation and plastic flow are strongly linked, and the scale of the process in time and space is such that it is difficult to separate and control the influence of all the relevant input parameters. The use of modelling is well-established in the field of welding research, and this thesis presents an analysis of the thermal and mechanical aspects of FSSW, principally using the finite element (FE) technique. Firstly, a thermal FE model is shown, which is subsequently validated by reference to experimental temperature data in both aluminium-to-aluminium and aluminium-to-steel welds. Correlations between high-quality welds and temperature fields are established, and predictions are made for peak temperatures reached under novel welding conditions. Deformation and heating are strongly linked in FSSW, but existing modelling tools are poorly suited to modelling flow processes in the conditions extant in FSSW. This thesis discusses the development and optimisation of two novel techniques to overcome the limitations of current approaches. The first of these uses greatly simplified constitutive behaviour to convert the problem into one defined purely by kinematics. In doing so, the boundary conditions reduce to a small number of assumptions about the contact conditions between weld material and tool, and the model calculation time is very rapid. This model is used to investigate changes in the slip condition at the tool to workpiece interface without an explicit statement of the friction law. Marker experiments are presented which use dissimilar composition but similar strength alloys to visualise flow patterns. The layering behaviour and surface patterns observed in the model agree well with observations from these experiments. The second approach extends the FE method to include deformation behaviour without the need for a fully-coupled approach, guided by the kinematic model. This is achieved using an innovative sequential small-strain analysis method in which thermal and deformation analyses alternate, with each running at a very different timescale. This technique avoids the requirement to either remesh the model domain at high strains or to use an explicit integration scheme, both of which impose penalties in calculation time and model complexity. The method is used to relate the purely thermal analysis developed in the work on thermal modelling to welding parameters such as tool speed. The model enables predictions of the spatial and temporal evolution of heat generation to be made directly from the constitutive behaviour of the alloy and the assumed velocity profile at the tool-workpiece interface. Predictions of the resulting temperature history are matched to experimental data and novel conditions are simulated, and these predictions correlate accurately with experimental results. Hence, the model is used to predict welding outcomes for situations for which no experimental data exists, and process charts are produced to describe optimum welding parameters. The methods and results presented in this thesis have significant implications for modelling friction stir spot welding, from optimising process conditions, to integration with microstructural models (to predict softening in the heat-affected zone, or the formation of intermetallics at the interface in dissimilar welds). The technique developed for sequential small strain finite element analysis could also be investigated for use in other kinematically constrained solid-state friction joining processes.

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.

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.

<p>Within the framework of presented PhD, friction stir spot welding (FSSW) of<br />multiple ultrathin sheets of AA 5754 &ndash; H111 (AlMg3) alloy 0.3 mm thick, was<br />studied. The influence of tool geometry and process parameters such as rotational<br />speed and axial load have been analysed using numerous techniques. It has been<br />understood that during the welding at low rotational speeds weld zone undergoes<br />strain hardening, while at high rotational speeds weld zone undergoes thermal<br />softening. It was observed that during FSSW at low rotational speeds a complex<br />layer at weld interface is present, which causes delamination when welded samples<br />are subjected to load.</p> / <p>У оквиру ове докторске дисертације испитивано је тачкасто заваривање трењем са мешањем ултратанких лимова дебљине 0.3 mm од легуре АА 5754 &ndash; H111 (AlMg3). Утицај геометрије алата и параметара као што су угаона брзина и аксијално оптерећење су детаљно анализирани уз помоћ бројних техника. Установљено је да приликом заваривања ниским угаоним брзинама долази до деформационог ојачавања, док на високим угаоним брзинама долази до термичког омекшавања зоне завара. Код узорка завареног са најмањим бројем обртаја долази до формирања комплексног слоја на међуконтактној површини који изазива деламинацију приликом испитивања механичких особина.</p> / <p>U okviru ove doktorske disertacije ispitivano je tačkasto zavarivanje trenjem sa mešanjem ultratankih limova debljine 0.3 mm od legure AA 5754 &ndash; H111 (AlMg3). Uticaj geometrije alata i parametara kao što su ugaona brzina i aksijalno opterećenje su detaljno analizirani uz pomoć brojnih tehnika. Ustanovljeno je da prilikom zavarivanja niskim ugaonim brzinama dolazi do deformacionog ojačavanja, dok na visokim ugaonim brzinama dolazi do termičkog omekšavanja zone zavara. Kod uzorka zavarenog sa najmanjim brojem obrtaja dolazi do formiranja kompleksnog sloja na međukontaktnoj površini koji izaziva delaminaciju prilikom ispitivanja mehaničkih osobina.</p>

Friction bit joining (FBJ) is a new solid-state spot joining technology developed in cooperation between Brigham Young University of Provo Utah, and MegaStir Technologies of West Bountiful Utah. Although capable of joining several different material combinations, this research focuses on the application of FBJ to joining 5754 aluminum to DP 980 steel, two alloys commonly used in automotive applications. The thicknesses of the materials used were 0.070 inches (1.78 mm) and 0.065 inches (1.65 mm), respectively. The FBJ process employs a consumable 4140 steel bit and is carried out on a purpose built research machine. In the first stage of the weld cycle the bit is used to drill through the aluminum top sheet to be joined. After this, spindle speed is increased so that the bit tip effectively forms a friction weld to the steel bottom sheet. Momentary stoppage of the spindle facilitates weld cooling before the spindle is restarted, shearing the bit tip from the bit shank, and retracted. Incorporated into the bit tip geometry is a flange that securely holds the aluminum in place after joint formation is complete. This research consists of several developmental steps since the technology only recently began to be formally studied. Initial joint strengths observed in lapshear tensile testing averaged only 978.5 pounds (4.35 kN), with a relatively high standard deviation for the data set. Final lapshear tensile test results were improved to an average of 1421.8 pounds (6.32 kN), with a significantly lower, and acceptable, standard deviation for the data set. Similar improvements were realized during the development work in cross tension tensile test results, as average strengths increased from 255.8 pounds (1.14 kN) to 566.3 pounds (2.52 kN). Improvements were also observed in the standard deviation values of cross tension data sets from initial evaluation to the final data set presented in this work.