To evaluate potential uses for friction stir welding in additive manufacturing, two separate parts were fabricated, one of 2195-T84 and the other 2219-T87, utilizing fixed pin techniques and additive lap welds. The parts were cut into samples, artificially aged and subjected to Rockwell hardness (HRB), Vickers hardness, micrographic photography, and metallographic imaging on both pre- and post- heat treatment. Additionally, tensile testing was performed on the heat-treated samples. A comparisons of test results showed a minimal increase in the yield strength of the 2195-T84 samples compared to as-welded tensile results obtained from a previous project. The ultimate tensile strength was reduced by approximately 16%. Further testing will be required to determine the nature of this reduction. No previous results were available for the as-welded 2219-T87, but UTS of the artificially aged samples was approximately 91% that of the parent material.

Friction stir welding (FSW) is a relatively new solid-state welding technology invented at The Welding Institute of UK in 1991. It is versatile and has been widely adopted to join various materials. There has been strong research activity on revealing the details of the material flow pattern in the nugget zone induced by the conventional shoulder tool. However, there is insufficient understanding on the aspects of the scroll shoulder tool design and the shoulder flow zone formation utilizing this type of tool. The major objective of this study was to conduct experiments, analyse results and then reveal the shoulder flow zone forming mechanism for the scroll shoulder tool. The method used was to identify the flow pattern in the shoulder flow zone using a ‘marker insert’ technique, and then to suggest the forming mechanism of the shoulder flow zone based on the obtained flow pattern; although the ‘marker insert’ technique has never been used to study the shoulder flow zone flow pattern induced by the scroll shoulder tool. Experiments were conducted to examine the thick sections 6061 aluminium ‘marker insert’ welds, which were welded using a scroll shoulder tool at a range of welding parameters. These were followed by quantifying the mass of the accumulated work piece material within the scroll groove (pick up material-PUM), evaluating the effect of welding parameters on the shoulder flow zone formation, and documenting the shoulder flow zone flow pattern. The major finding was that there is a simple banded structure which forms in a layer to layer manner in the bottom portion of the shoulder flow zone, but it disappears in the top portion of the shoulder flow zone. Accordingly, the forming mechanism of the shoulder flow zone for the scroll shoulder tool was suggested as follows. Firstly, the tool pin is plunged into the work piece; the work piece material is extruded by the pin and pushed up into the scroll groove forming the PUM. Secondly, after the tool shoulder is plunged into the work piece to a certain depth, the scroll groove is fully filled up with the PUM. Finally, during the forward movement of the tool, the central portion of PUM is driven downward by the root portion of the pin and then detaches from the pin (tip portion) in a layer to layer manner. It has also found that the thickness of the shoulder flow zone varies with a thicker on the advancing side than on the retreating side, and there is a positive linear relationship between the mass of PUM and the weld quality. This study has revealed for the first time the forming mechanism of the shoulder flow zone, and has improved the understanding of the shoulder flow zone formation using a scroll shoulder tool. It is recommended that a ‘shoulder-breaking’ technique is developed to break the rotating shoulder suddenly and hence embed it into the work piece during FSW, in which a real-time shoulder-work piece couple could be produced for a better three-dimensional examination of the shoulder flow zone.

Ce travail présente le développement d'un outil numérique. Une formulation arbitrairement lagrangienne-eulérienne (ALE) est implémentée dans le logiciel 3D éléments finis FORGE3® pour simuler les différentes étapes du procédé de soudage par frottement malaxage (FSW). Une méthode découplée est utilisée : a) les champs de vitesses, pressions et températures du matériau sont calculés, b) la vitesse de maillage est calculée à partir de l'évolution des frontières du domaine et d'un critère de raffinement adaptatif procuré via une estimation d'erreur, c) les variables nodales et P0 sont transportées. Différentes techniques de calcul de la vitesse de maillage et de transport des variables sont étudiées, apportant des avantages significatifs par rapport à des approches plus standard. L'algorithme de contact a également été enrichi par une procédure de lissage d'outil. Ces améliorations ont été testées et appliquées sur des cas industriels.L'état stationnaire de soudage, tout comme les phases transitoires, sont simulés, montrant une bonne robustesse et une bonne précision de la formulation ALE développée. Dans un premier temps, la simulation de la phase de soudage stationnaire permet d'identifier, par comparaison avec des résultats expérimentaux, les paramètres de frottement. Dans un second temps, un des intérêts majeurs du modèle ALE étant la possibilité de simuler la formation de vide à l'interface outil/matière, la phase de plongée et des phases transitoires sont modélisées. Leurs simulations peuvent ainsi aider à mieux appréhender les mécanismes du phénomène complexe de déposition de matière qui doit avoir lieu à l'arrière du pion de façon à obtenir un joint sans défaut.

Le friction stir welding (FSW) est un procédé de soudage inventé en 1991 par l'institut de soudure anglais, le TWI. Celui-ci suscite un vif intérêt de la part de l'industrie aéronautique par sa capacité de souder les alliages d'aluminium de la série 2XXX et 7XXX, à durcissement structural, réputés pratiquement insoudables. Ce procédé étant relativement récent, il fait encore sujet de recherches actives. Ce travail a pour objectif de prévoir le profil de dureté d'un joint soudé par FSW d'un alliage d'aluminium, le 2024 T3. Cet alliage étant à durcissement structural, il est nécessaire de prévoir l'influence de la température sur l'évolution de la précipitation au cours du procédé pour en déduire sa limite d'élasticité. L'estimation du champ de température durant le régime stationnaire du procédé s'appuie sur des travaux internes au centre SMS. La prévision de la précipitation au cours du soudage est effectuée à l'aide de deux modèles. Le premier modèle, à base d'équivalence temps–températures, est une proposition d'extension aux alliages d'aluminium sous-revenu du modèle de Myhr & Grong (1991) établi dans le cas des alliages d'aluminium sur-revenu. Le deuxième modèle s'appuie sur une discrétisation de la distribution des rayons des précipités, suivant le schéma numérique de Kampmann et Wagner (1983), pour calculer ensuite son évolution. Bien que le premier modèle permette de prévoir l'évolution de la dureté au cours de recuits isothermes, les profils de dureté simulés ne sont pas en accord avec les profils expérimentaux. Seul le deuxième modèle permet une prévision raisonnable de la microstructure, en accord avec les mesures réalisées dans la thèse de Genevois (2004), et des profils de dureté proches des résultats expérimentaux. Finalement, une expression analytique en fonction des paramètres microstructuraux du flux de chaleur lors d'un essai de calorimétrie différentielle (DSC) a été établie. Celle-ci donne la possibilité de simuler un essai de DSC, et de vérifier ainsi la cohérence entre les grandeurs thermodynamiques et cinétiques introduites dans le deuxième modèle de précipitation.

Friction stir welding (FSW) is a low heat input solid state welding technology. It is often used for fabrication of aluminium alloys in transportation applications including railway, shipbuilding, bridge structures and automotive components. In these applications the material is frequently subject to varying load conditions and fatigue failure is a critical issue. In most cases standard codes and fatigue guidelines for aluminium welded joints address only welded structures with conventional welding methods but not those with FSW procedure. In the scope of this thesis fatigue life assessment of friction stir welded components was performed using theoretical approaches along with finite element method (FEM). The further aim of this study was to generate a basis for standardization of fatigue assessment of friction stir welded joints. Friction stir welded hollow aluminium panels of alloy 6005A are investigated. The panels are used for train wall sides, train floors, deck and bridges. Each panel is made of several profiles that are joined with the friction stir welding method. Fatigue bending tests were performed for profiles in these panels. Fatigue cracks and failure appeared at notches in the profiles. With FEM simulations critical positions for crack initiation and failure were identified. The method of critical distance was used to analyse and estimate the fatigue life. It was shown that the failure location and fatigue limit could be predicted for both base metal and weld location. Choice of welding procedure (clamping condition) can significantly influence the fatigue life. It was shown that for some panels the critical distance method was not able to explain the failure in the weld. In this case fracture mechanics together with residual stress analysis were used successfully to predict the failure. Assuming homogeneous material properties throughout the weld and the base material, FEM analysis for T and overlap joints as well can provide a reasonable fatigue prediction. This suggests that the same assumption can be extended to complex components for failure analysis of the friction stir welded joints when using the critical distance method. Fatigue assessment of friction stir welded joints was also performed using standard codes Eurocode 9 and IIW. Fatigue curves of traditional fusion welded joints were used. The results are in reasonable agreement with experimental data and FEM predictions. / QC 20120330

In the friction stir welding (FSW) process, a high-speed rotating tool, which consists of the probe and the shoulder, are employed to plunge into the faying surfaces. By using the friction heating and the stirring action of the material, the solid-state welding is accomplished to joint two pieces of metal by material diffusion to form a densification structure in the weld. According to the experimental results, the mechanism of friction stir welding is as follows: the probe plunge into the sample and the shoulder is in contact with the sample to generate a large amount of friction heat, which causes the materials soft. When the probe moves forward, the soft materials in front of the probe are scratched. The scratched materials are subjected to the rotational and squeeze actions of the shoulder so that they are refilled into the welded surface behind the probe. For the dissimilar metals joint (6061-T6 aluminum and C1100 copper plates), results show that when C1100 copper is located at the advancing side, the measured feed force appears drastic changes and it is also seen that the components of the force for the friction-stir welding of dissimilar metals become more unstable than those for the same metals joint, so that the structure which has been observed by optical microscopy appears to be open with pores and defects so that the welded quality becomes poor. According to the three components of the measured force during FSW process, the friction between the probe and the work piece can be computed. By using the friction theory, the hardness and the yield strength of the materials in front of the probe can be calculated, and then the faying surface temperature is approximately predicted to be 565.5 oC.

In this study, a novel inserted type of friction welding tool was proposed, where the circular rod was embedded in its central axis using the material same as the workpiece, so that it could effectively promote the friction heat quickly and enhance the welding quality. The welding tool was made of the high-speed steel, the workpiece with its embedded material 6061-T6 aluminum alloy. A vertical milling machine equipped with dynamometer, which could measure the power during the friction stir welding, was employed as the experimental apparatus. During the welding process, the K-type thermocouple was used simultaneously in measuring the welding temperature at the interface of joint. The operating conditions of welding were as followings: the welding speed of 800 rpm, the tool inclination of 1¢X and the clamping force 2kN, the tool with 12mm in diameter and 0.21mm in depth under the downward force about 2 kN. The experiment was conducted into two stages. The first stage was a spot welding to investigate the effect of the ratio of the diameter of embedded material (d) to the diameter of welding tool (D) on the temperature of the interface of joint, the thickness of plastic flow, and the failure load of weld. Experimental results revealed that the interface temperature, the plastic flow thickness, and the failure load of weld are directly proportional to d/D. In comparison with the welding tool without insert (d/D = 0), the maximum interface temperature increased about 1.12 times at d/D = 0.83, the plastic flow thickness increased about 1.52 times, and the failure load of weld increased about 1.45 times. In the second stage, the feeding process was included to investigate the influence of the diameter and the thickness of embedded material on the interface temperature, the plastic flow thickness, and the failure load of weld. Experimental results revealed that the plastic flow thickness was less than 2 mm when the thickness of embedded material was less than 3 mm. However, when the thickness of embedded material was larger than 5 mm, the plastic flow thickness could achieve to 3 mm. Hence, the thickness of embedded material should be larger than 5mm. Moreover, the effect of the diameter of embedded material on the interface temperature and the plastic flow thickness using the feeding process was almost the same as the spot welding. However, in comparison with the welding tool without insert, the failure load of weld increased about two times.

A friction stir welding equipment with high rotation speed and constant load is successfully developed in this study to weld the multi-laminated silicon steel sheets widely used on regular transformers. This equipment consists of a spinning unit, a loading unit, and a feeding unit. A WC round rod with 3 mm diameter is used as welding tool. Under different operating conditions, such as the normal load(140~480 N), the spindle speed (12000~24000rpm), the feeding rate (0~1.58 mm/s), the welding characteristics and the welding mechanism of multi-laminated silicon steel sheets, and the welding feasibility of the transformer are investigated. Firstly, the contour map of welding depth in terms of spindle speed,normal load, and depth of point welding is established for dwell welding time 15 seconds. Secondly, based on this contour map, two experimental conditions of the long-pass welding tests are selected to investigate the effect of normal load (Fd), the spindle speed (Ns), and the feeding rate (f) on the failure load of weld under the shear. According to the experimental results, the empirical formula is obtained as Ff =40.6(Fd¡DNs)1.123(f)-0.791. In this formula,(Fd¡DNs)1.123(f)-0.791 is proportional to the frictional work per unit moving distance. With the larger frictional work, this represents the heat generation of the workpiece material is higher with more uniform friction stir, so that the bonding strength of the material increases and the failure load of weld is larger. According to the micrograph observations, the thermo-mechanically affected zone is significantly influenced by high heat action generated from the friction between the tool and the weld surface region, so that the plastic flow of the workpiece material occurs to cause the multi-laminated silicon steel sheets bonding together. Finally, the transformer is successfully welded under the experimental conditions of the long-pass welding tests with the smaller welding depth and the better failure load.

In order to simulate the histories of temperature distributions and plastic flow of the dwell phase during a friction stir welding process, the Newton-Raphson method is used to solve the simultaneous equations of energy and momentum in the cylindrical-coordinate system. Comparing the simulation with the results of experiment, results show that the contact condition between the tool and the workpiece is at pure sliding without plastic flow at the beginning of the dwell phase until the temperature rises to about 300¢XC at the depth of 1.5 mm. In this period, the heat generation comes from the sliding friction between two surfaces. After the plastic flow occurs, the heat generation rises rapidly, and then decreases to a saturated value so that the temperature rise also achieves a constant value. Thermal expansion of the workpiece will increase the plunge force, so that the heat generation and the temperature raise increase. At the steady state condition, with increasing sticking proportion, the heat generation and the temperature quickly achieve a saturated value. For the steady-state condition, results show that the speed of plastic flow and shear strain rate increase with increasing rotational speed. The control of the contact state variable can effectively describe the heat generation and the distribution of plastic flow in different contact conditions. Comparing the simulation with the results of experiment, the contact condition can be identified.

In this study, a novel assembled-type tool was used to weld SS400 low-carbon steel plate using the friction stir spot welding. The welding tool was made of tungsten carbide embedded a circular rod made of the low-carbon steel. The superiority of this embedded material not only could effectively promote the interface temperature of the joint, but also the thickness of stir zone. Compared to previous studies, this novel tool can significantly improve the manufacturing cost and the trimming time. The welding apparatus composed of a vertical milling machine and a welding platform. The operating conditions of welding were as followings: the diameter of embedded material, the welding speed, and the vertical load. During the welding process, the interface temperature of the joint, the tool plunge depth, and the vertical load were simultaneously measured by the K-type thermocouple, a displacement sensor, and a load cell. Experimental results revealed that the interface temperature, the thickness of the stir zone, and the tensile strength of the welding joint was proportional to the diameter of the embedded material. The best welding condition is the embedded material diameter of 10mm, the vertical load of 8kN, the welding speed of 1200rpm, and the welding time of 100 seconds.