Thesis (Ph. D.)--Missouri University of Science and Technology, 2009. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed July 16, 2010) Includes bibliographical references (p. 175-181).

This research project was conducted to increase weld gap tolerance in Friction Stir Welding (FSW) of 8 mm thick aluminum alloy 6082 – T6. Investigation was done on I-STIR PDS platform and a Johnford milling machine. The research project involved tool-pin design with varying re-entrant features and varying parameters as a method of weld gap closing to produce successful welds. Direction of spindle rotation and dwell time were established as part of a preliminary study. Clockwise spindle rotation with 20 seconds dwell time allows sufficient plasticity and improved material flow which assisted in achieving welds with prior 30 percent weld gap of the plate thickness. Final welds were made using three rotational speeds and feed rates with sufficient plunging to prevent root defects. Analysis of the results were detailed which include vickers microhardness test, tensile test and metallographic observation to access the suitability of the weld structure. From the set of tool-pins designed, the flare tool-pin gave a well-defined weld nugget with improved stirring at the weld root. Also, with a concave shoulder, right hand threaded tool-pin and counterclockwise flutes undergoing a clockwise spindle rotation, plasticized material flow was upward which was beneficial in reducing the amount of plate thinning. The right hand thread counter clockwise flute with a flute machined in the foot exhibited superior tensile strength for welds containing 30 percent weld gap.

Friction stir welding was used to join 3 mm Ti-6Al-4V alloy in a butt joint configuration. This research focused on optimization of a tool geometry and the interaction between process parameters and static performance of welded joints. The main parameters varied were tool travel speed and tool rotational speed. The results showed a relationship between heat input as a function of process parameters and static strength. Improved tensile properties correspond to high heat input. The hardness plots revealed an increase in hardness on both the stir zone and heat affected zone despite the small defects on the weld root. The weld microstructure was also evaluated, which showed a variation in microstructure on both the heat affected zone and stir zone in comparison to the parent material. It was also found that the use of MgO as a heat barrier on the backing plate was detrimental to the weld tensile properties of butt-welded plates compared to bead-on-plate welds of which MgO had no influence.

Friction Stir Welding (FSW) is a solid state welding process invented and patented by The Welding Institute (TWI) in 1991, for joining ferrous and non-ferrous materials1. The FSW of Aluminium and its alloys has been commercialised; and recent interest is focused on joining dissimilar materials. However, in order to commercialise the process, research studies are required to characterise and establish process windows. This research work through material characterisation of the welded joints establishes a process window for the Friction Stir welding of 5754 Aluminium Alloy and C11000 Copper. Furthermore, preliminary studies83,85 on the FSW of aluminium and copper have revealed the presence of intermetallic compounds which are detrimental to the weld qualities. This research work is also aimed at establishing process parameters that will result in limited or no intermetallic formation in the weld. The joint integrity of the resulting welds will also be correlated with the input process parameters. Based on the preliminary investigations conducted, a final weld matrix consisting of twenty seven welds was produced by varying the rotational speed between 600 and 1200 rpm, and the feed rate between 50 and 300 mm/min using three different shoulder diameter tools – 15, 18 and 25 mm to compare the heat input into the welds and to achieve the best results. The welds were characterised through microstructural evaluation, tensile testing, microhardness profiling, X-Ray Diffraction analysis, electrical resistivity and statistical analysis – in order to establish the interrelationship between the process parameters and the weld qualities. viii Microstructural evaluation of the weld samples revealed that the interfacial regions are characterised by mixture layers of aluminium and copper; while 33 percent of the tensile samples are within the acceptable range (> 75 percent joint efficiency). High Vickers microhardness values were measured at the joint interfaces, which corresponded with the intermetallic compounds. The Energy Dispersive Spectroscopy analysis revealed the presence of thin layers of intermetallics in nanoscale at the interfacial regions. The diffractograms of the X-Ray Diffraction analysis showed small peaks for intermetallics in some of the welds. Low electrical resistivities were measured at the joint interfaces. The statistical analysis showed that the downward vertical force, (Fz) can significantly influence the resulting weld qualities. An overall summary of the analysis of the weld qualities - with respect to the shoulder diameter tools employed showed that the 18 mm shoulder diameter tool is most appropriate among the three shoulder diameters considered, and a process window of medium spindle speed of 950 rpm and low-to-medium feed rate between 50 and 150 mm/min is established for FSW of Aluminium and Copper. Welds produced at 1200 rpm and 300 mm/min with low heat input did not have intermetallics formed at the joint interface.

A Dissertation for the Master’s Degree in Mechanical Engineering School of Mechanical, Industrial & Aeronautical Engineering Date: 02/02/2017 / The Friction Stir Welding process is a rotating tool, that consists of a specialy designed shoulder and pin, that is plunged into the joining line of the required material and traverses along this line. The friction is induced by the rotating tool causes the workpiece material to rise to an operating temperature of 70% to 90% of the workpiece material's melting temperature and resulting in, no phase change, nor any defects associated with phase change, occurs in the workpiece. The increased temperature of the material causes the shear yield strength to drastically decrease thus allowing the two pieces to plasticise, easily stir around the tool and subsequently join. As the tool traverses along the workpiece, the softened material cools in the wake of the rotating tool and recrystallises, forming a ne grained microstructure. Attempts to develop an innovative tool to correlate the resulting of thermal models with process parameters are scarce. In this work, 6056-T4 and 6082-T6 Aluminum alloy sheets are friction stir welding at different rotational and translational speeds during the experimental aspect and material 2024-T3 for the analytical calculations. The effects of process parameters on the resulting thermal and mechanical properties are investigated. The results show that the use of coolant during the friction stir weld decrease heat generation substantially, this can also affect the force of the weld. It is also observed that the shear strenght of the processed sheet depends strongly on the rotational and translational speeds as weld as the thermal aspect and varies widely within the processed region, this was shown in this study by evaluating the thermal aspects of different weld types namely the Standard tool, Bobbin tool and the innovative tool. In addition. The proposed approach involves determination of the use of the friction stir welding in different thermal conditions and championing the use of an innovative tool. / MT2017

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.

Load controlled cyclic fatigue testing was conducted on base metal (BM) and friction stir welded (FSW) magnesium (Mg) alloy AZ31B compact tension (CT) specimens in laboratory air and a 0.05 wt. % NaCl fog environment in efforts to delineate the effects of salt fog and stir welding on fatigue performance under tension-tension loading conditions. FSW beads were produced on single piece AZ31B sheet product, simulating a pristine friction stir butt weld. Optical and electron microscopy, as well as X-ray diffraction was employed to observe the features and characteristics of fracture surfaces. The resulting stress vs. number of cycles to failure (S-N) curves demonstrated a reduction in fatigue life in a salt fog environment and an increase in fatigue life for FSW specimens compared to equivalently loaded BM specimens. Tensile frame displacement data indicated that the salt fog environment had an immediate effect on the BM samples, with an increased displacement required to meet the load control criteria. Fatigue cracks that propagated in CT samples that were notched in the direction of FSW tool travel consistently propagated towards the retreating side (RS) of the stir zone. / Thesis / Master of Applied Science (MASc)

In an effort to save weight, transportation and aerospace industries have increasing investigated magnesium alloys because of their high strength-to-weight ratio. Further efforts to save on material use and machining time have focused on the use of additive manufacturing. However, anisotropic properties can be caused by both the HCP structure of magnesium alloys as well as by layered effects left by typical additive manufacturing processes. Additive Friction Stir (AFS) is a relatively new additive manufacturing technology that yields wrought microstructure with isotropic properties. In this study, Additive Friction Stir (AFS) fabrication was used to fabricate WE43 magnesium alloy, with both atomized powder and rolled plate as filler material, into multilayered structures. It was found that the WE43 alloy made by AFS exhibited nearly isotropic tensile properties. With aging these properties exceeded the base material in the T5 condition. The toughness measured by Charpy impact testing also showed an increase over the base material. The relationships among tensile properties, Vickers microhardness, impact toughness, microstructure and thermal history are developed and discussed. / Master of Science

L’allègement des structures est actuellement un enjeu industriel majeur. L’utilisation de certains alliages d’aluminium couplés à de nouveaux procédés d’assemblages est une bonne réponse à cette problématique. Le procédé de soudage FSW permet notamment la réalisation d’assemblages multi-matériaux en s’affranchissant des problèmes de fusion. Cette étude, réalisée au sein de l’entreprise TRA-C industrie, s’est intéressée plus particulièrement au cas du soudage FSW hétérogène d’alliages d’aluminium des séries 2xxx (Al-Cu-Mg-Ag) et 7xxx (Al-Zn-Mg), dans une large gamme de paramètres industriels. Les caractérisations des assemblages ont pu mettre en avant de fortes hétérogénéités microstructurales et mécaniques au travers des cordons. Ainsi la présence d’une zone faible, adoucie dans la ZAT du côté de l’alliage 7xxx, amène à favoriser la rupture en traction. Une évolution métallurgique importante déclenchée par le cycle thermique généré explique principalement ce phénomène. D’autre part, cette étude expérimentale a été couplée à des travaux de simulation numérique du procédé en configuration homogène. Le modèle éléments finis intègre, pour la première fois, la géométrie réelle et complexe (filetage, facettes, …) de l’outil de soudage utilisé expérimentalement et est couplé à l’utilisation d’une technique de maillage mobile. Cette technique numérique a permis de s’affranchir intégralement des distorsions de mailles conséquentes souvent rencontrées, ainsi que de décrire fidèlement les effets thermomécaniques engendrés par l’outil de soudage. Une étude de sensibilité aux paramètres de soudage ainsi qu’aux matériaux soudés a démontré une excellente corrélation entre les cinétiques thermiques expérimentales et numériques tout en démontrant l’aspect prédictif du modèle. / The lightweight structures optimisation is one of the main topics in transportation industry. It can be achieved by optimisation of materials as well as induced assembly process. As a solid-state process, Friction Stir Welding (FSW) allows to produce dissimilar materials joining while avoiding fusion defects. This work focused on the dissimilar welding of aluminium alloys from 2xxx (Al-Cu-Mg-Ag) and 7xxx (Al-Zn-Mg) series in an industrial context. Joints characterizations were conducted at multiple scales to understand parameters impact on material flow, joint morphology, and performances. They have shown large heterogeneities in the microstructure as well as the global and local mechanical behaviour. Whatever the welding parameters used, good mechanical performance has been reached. A specific softened zone has been detected in the 7xxx alloy’s HAZ which caused fracture during transverse tensile test. Significant metallurgical evolution induced by thermal cycles mainly explains these phenomena.On the other hand, simulation works were also conducted to simulate the welding process in similar material configuration. The finite elements model integrates, for the first time, the real and complex tool design (thread, flats…). Complex geometry can be used by coupling with a specific moving mesh technique. This numerical development completely overcomes the consequent mesh distortion often encountered in FSW simulation. The current model presents good sensitivity and robustness for several welding conditions and materials. It also demonstrates an excellent correlation between experimental and numerical thermal fields while revealing the predictive aspect of the model.

A l'heure où les matériaux composites prennent une part toujours plus importante dans les structures aéronautiques, l'utilisation de l'alliage d'aluminium-lithium 2050 assemblé par Friction Stir Welding (FSW) est envisagé pour remplacer les structures rivetées en alliages d'aluminium traditionnels. L'objectif global de ce travail de thèse était d'étudier le comportement en corrosion et en corrosion sous contrainte de joints soudés FSW de l'alliage d'aluminium-lithium 2050 mais aussi l'influence d'un post-traitement thermique sur les comportements observés. Dans ce but, la microstructure, le comportement mécanique et le comportement en corrosion et en corrosion sous contrainte de l'alliage 2050 servant de matériau de base à l'élaboration des joints soudés FSW ont d'abord été analysés ; ensuite, nous nous sommes focalisés sur le comportement en corrosion et en corrosion sous contrainte des structures FSW proprement dites, en analysant bien entendu la microstructure complexe de ces joints soudés et les propriétés mécaniques résultantes. Les résultats obtenus montrent que l'alliage 2050, bien que sensible à certaines formes de corrosion, semble bien plus performant que les alliages d'aluminium traditionnellement utilisés en aéronautique. Sa sensibilité à la corrosion sous contrainte notamment est très faible. Les structures FSW, après traitement thermique post soudage, ont également un très bon comportement en corrosion et en corrosion sous contrainte. Les joints soudés FSW de l'alliage d'aluminium-lithium 2050 apparaissent donc comme une solution très prometteuse pour l'industrie aéronautique. / Today, composite materials are more and more used for aircraft structures; however, the aluminium-lithium 2050 alloy joined using a Friction Stir Welding (FSW) process could be used to replace riveted structures made of traditional aluminium alloys. The aim of this PhD work was to study the corrosion behaviour and stress corrosion cracking (SCC) behaviour of FSW joints of aluminium-lithium 2050 alloy and also the influence of a post welding heat treatment on the behaviours observed. Therefore, the microstructure, the mechanical behaviour and the corrosion and SCC behaviour of the 2050 alloy used for the FSW joints were first analyzed; then, the work was focused on the corrosion and SCC behaviours of the FSW joints. The complex microstructure of the FSW joints and the resultant mechanical properties were studied. The results showed that the 2050 alloy was more corrosion resistant than traditional aluminium alloys used for aircrafts even if it was susceptible to intergranular and/or intragranular corrosion. Its susceptibility to SCC was very low. The FSW joints, after a post welding heat treatment, also show a low susceptibility to corrosion and SCC. As a conclusion, the FSW joints of 2050 alloy seem to be a relevant solution to replace riveted structures of traditional aluminium alloys.