In this study, the fundamental mechanism of friction stir welding was investigated to establish the relationship among the three components of the forces acting on the work pieces, the variation of the stirring energy, and the joint characteristics of the materials. A dynamometer designed by Chiou et al., was used to measure the axial force (z-direction), the feed force (x-direction), and the clamping force (y-direction). The output energy of servo motor was monitored by power meter. Experimental results show that with increasing welding speed, the feed force increases obviously, the axial force increases slightly, and the energy almost remains constant for the fixed rotation speed of the spindle. At the rotation speed of spindle of 800 rpm, the spindle angle of 1¢X, the pre-clamping force of 2kN and the welding speed of 60 mm/min, results show that the feed force is about 1kN when the probe is plunged into the specimens but the shoulder does not be in contact with the surface of the specimen. However, when the probe is plunges into the specimens entirely and the shoulder is in contact with the surface slightly, the feed force is reduced to 0.48kN. Moreover, when the shoulder is in contact with the surface heavily, the feed force is reduced to 0.2kN. This result indicates that the contact force between the shoulder and the specimen causes the material to become soft and to backfill into the weld, and then decreases the feed force. After the specimen of the 6061-T6 aluminum has been welding, the micro hardness measurements are made. Results show that the distribution of the hardness is quite consistent along the welding as the feed force approaches to 0.2kN. Furthermore, the appearance on the surface of the weld is quite fine, and thereby it is able to get the high and uniform quality. The spacing distance of the weld surface can be theoretically analyzed. It is found that the spacing distance increases with welding speed and decreases with rotation speed of spindle. The theoretical predictions are in very good agreement with the experimental measurements.

To investigate the fundamental mechanism of friction stir welding to form a butt joint, two additional tests are performed, one using the rotating probe pin only, the other using the rotating shoulder only. In the first case, the pin tool is plunged into the joint interface, but the shoulder is not in contact with the workpiece. When the pin tool is feeding, the material in the vicinity of the pin tool is scratched and piled on the retreating side, but a butt joint is not formed by this test on two thin plates of aluminum alloy 6061-T6. In the second case, when the shoulder is feeding, the plastic shear deformation of the material in the vicinity of the shoulder can be observed and then it is joined together due to the heat generated from the shoulder to cause the material diffusion. According to these additional experiments and the friction stir welding process, the mechanism to form a butt joint is as following. When the probe plunge into the material and the shoulder is in contact with the workpiece, a large amount of frictional heat is generated from the shoulder and the pin. When the tool moves forward, the soft material in front of the pin is squeezed, so that the material is refilled into the space behind the pin by the rotating pin and shoulder. According to the observation of cross-section of butt joint, an interface curve can be found. This curve is formed by the plastic shear deformation of the material in the vicinity of the shoulder and the pin at high frictional temperature. It can be explained by the boundary layer theory.

This Master Thesis was performed in terms of robotic three dimensional friction stir welding with T-butt joint. Friction stir welding (FSW) is a solid state welding method that achieves the weld temperature by friction of a rotating non-consumable tool with the workpiece. Science and technology fast developing requires for higher seam quality and more complex welding joint geometry like 3D welds. In order to acquire high productivity, capacity and flexibility with acceptable cost, robotic FSW solution have been proposed. Instead of the standard FSW machine, using a robot to perform complicated welds such as, three-dimensional. In this report, a solution for weld a 3D T-butt joint, which located in an aluminium cylinder with 1.5 mm thickness using a robot, was developed. Moreover, two new paths were investigated in order to avoid the use of two welds to perform this type of joint. The paths were tested on 2D and on 3D (with a 5050 curvature radius) geometries. Both paths had good results. What is more, the parameter developing methods of FSW process, which is composed of necessary parameter setting, positional compensation was introduced. Specially,the study demonstrates how complicate geometry can be welded using a robot. Also,it shows that TWT temperature control is able to acquire high quality 3D welds. In addition, an analysis of the 2D welding and 3D welding was performed, which exposed that, keeping exactly the same welding conditions, higher lateral forces on the tool were found during 3D welding. Basis on the special case in this paper, when the tool goes like "climbing" the sample, the suffering force of tool decreasing with increasing the height(Z position); nevertheless, when the tool goes like "downhill", the suffering force of tool decreasing with decreasing the height (Z position). What is more, in 2D weld, increasing the downforce (Fz) results increasing the lateral forces which can be Fx and/or Fy. Finally, the future works suggestions were presented in terms of (1) performing the new paths into a real cylinder, (2) performing tensile test on the paths and comparing it with conventional path which weld twice, (3) researching how the downforce (Fz) influence the Fx and Fy during welding of different 3D geometries, (4) how the cooling rate of backing bar influence the seam quality when it is use the same welding parameters and (5) the effect of performing welds in the same welding temperature achieved with different combination of the tool rotational speed and downforce on the material properties

Thesis (M.S.)--Brigham Young University. School of Technology, 2007. / Includes bibliographical references (p. 73-75).

A friction stir weld (FSW) is a solid state joining operation whose processing parameters are currently determined by lengthy trial and error methods. To implement FSWing rapidly in various applications will require an approach for predicting process parameters based on the physics of the process. Based on hot working conditions for metals, a kinematic model has been proposed for calculating the shear strain and shear strain rates during the FSW process, validation of the proposed model with direct measuring is difficult however. Since the shear strain and shear strain rates predicted for the FSW process, are similar to those predicted in metal cutting, validation of the FSW algorithms with microstructural studies of metal chips may be possible leading to the ability to predict FSW processing parameters.

The workpiece materials used in this study are CuNb nano-layered composites (NLC) which are produced in bulk form by accumulative roll bonding (ARB). CuNb NLC panels are of interest because of their increase in strength and radiation damage tolerance when compared to either of their bulk constituents. These increased properties stem from the bi-metal interface, and the nanometer length-scale of the layers. However to be commercially viable, methods to successfully join the ARB NLC which retain the layered structure panels are needed. Friction stir welding is investigated in this study as a possible joining method that can join the material while maintaining its layered structure and hence its properties. Mechanical properties of the weld were measured at a macro level using tensile testing, and at a local level via nano-indentation. The post weld layer structure was analyzed to provide insight into the flow paths. The grain orientation of the resulting weld nugget was also analyzed using electron backscatter diffraction and transmission Kikuchi diffraction. Results from this study show that the nano-layered structure can be maintained in the CuNb NLC by control of the friction stir welding parameters. The resulting microstructure is dependent on the strain experienced during the joining process. A variation in layer thickness reduction is correlated with increasing shear strain. Above a critical level of shear strain, the NLC microstructure was observed to fragment into equiaxed grains with a higher hardness than the NLC panels. Results from this study are also used to further the understanding of the material flow and hot working conditions experienced during the friction stir welding process.

Dissimilar metals joining have been used in many industry fields for various applications due to their technique and beneficial advantages, such as aluminum-steel and magnesium-steel joints for reducing automobile weight, aluminum-copper joint for reducing material cost in electrical components, steel-copper joints for usage in nuclear power plant, etc. The challenges in achieving dissimilar joints are as below. (1) Big difference in physical properties such as melting point and coefficient of thermal expansion led to residual stress and defects. (2) The miscibility issues resulted in either brittle intermetallic compound layer at the welded interface for miscible combinations (such as, aluminum-steel, aluminum-copper, aluminum-titanium, etc.) or no metallurgical bonding for immiscible combinations (such as magnesium-copper, steel-copper, etc.). For metallurgical miscible combinations, brittle intermetallic compounds formed at the welded interface created the crack initiation and propagation path during deformational tests. (3) Stress concentration appeared at the welded interface region during tensile testing due to mismatch in elastic properties of dissimilar materials. In this study, different combinations of dissimilar metals were joined with friction stir welding. Lap welding of 6022-T4 aluminum alloy/galvanized mild steel sheets and 6022-T4 aluminum alloy/DP600 steel sheets were achieved via friction stir scribe technology. The interlocking feature determining the fracture mode and join strength was optimized. Reaction layer (intermetallic compounds layer) between the dissimilar metals were investigated. Butt welding of 5083-H116 aluminum alloy/HSLA-65 steel, 2024-T4 aluminum alloy/316 stainless steel, AZ31/316 stainless steel, WE43/316 stainless steel and 110 copper/316 stainless steel were obtained by friction stir welding. The critical issues in dissimilar metals butt joining were summarized and analyzed in this study including IMC and stress concentration.

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