This dissertation presents the results of an investigation into the corrosion resistance of friction stir welding (FSW) for aerospace structures. Two of the most common aerospace aluminum alloys, 2024 and 7075, were investigated. In the as-welded condition, both alloys were found to be highly susceptible to exfoliation corrosion, and 7075 was found to be susceptible to stress corrosion cracking as well. The goal of this research was to identify proper initial temper selection and postweld aging treatments for enhancing the corrosion resistance of both 2024 and 7075 alloys, and their dissimilar joints. A large number of heat treatments were investigated for 7075 in the T6 and T73 tempers, including retrogression re-aging (RRA). Heat treatments were also investigated for 2024-T3 and 2024-T81. Samples were evaluated for resistance to exfoliation corrosion using optical microscopy. Microhardness, electrical conductivity, tension, and fatigue crack propagation tests were also performed on the samples. Beneficial heat treatments were found for both alloys as well as for their dissimilar joints. / "December 2005." / Thesis (Ph.D.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering

Friction Stir Welding (FSW) is a new technology which joins materials by using frictional heat. Inthe first part of this thesis, a profound literature study is performed. The basic principles, therobotic implementation and possibilities to use FSW for high strength titanium alloys areexamined. In the next phase, a FSW-tool is modelled and implemented on an industrial robot in arobot simulation program. Reachability tests are carried out on car body parts and jet engineparts. By using a simulation program with embedded collision detection, all possible weldinglocations are determined on the provided parts. Adaptations like a longer FSW-tool and amodified design are suggested in order to get a better reachability. In different case studies, thenumber of required robots and the reduction of weight and time are investigated and comparedto the current spot welding process.

In this study, the energy conservation equation in a cylindrical coordinate system and the moving heat source from the tool are used to establish a steady-state three-dimensional heat transfer model for the friction stir welding (FSW). Then, the simplified momentum conservation equation is employed to predict the material flow model for the FSW. Combining the effects of heat transfer and material flow, this numerical model successfully predicts the weld temperature field and the material flow for the FSW. Numerical results show that increasing the welding or translational speed of the tool has the effect of decreasing the magnitude of the temperature within the workpiece, while increasing the rotating speed has the opposite effect. During the feeding process, the material located on the back of the tool pin has higher temperature than that on the front. Moreover, the temperature profile are asymmetrical between the advancing and retreating sides due to the material flow stirred by the tool, and this temperature difference depends on the speed of material flow under the tool shoulder.

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.

Thesis (M.S.)--University of Texas at El Paso, 2007. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (M.S.)--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 April 9, 2009) Includes bibliographical references (p. 118-127).

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

Systematic investigation of the Friction Stir Welding (FSW) process shows that a fixed rotational velocity and feed rate may not yield uniform mechanical properties along the length of a weldment. Nevertheless, correlations between process parameters and post-weld material properties have successfully demonstrated that peak temperature and cooling rate drive post-weld properties. There have been many reported methodologies for controlling friction stir welding, with varying degrees of cost to implement and effectiveness. However, comparing data from uncontrolled FSW of AA 6111-T4 sheet with controlled FSW at temperatures ranging from 375 °C to 450 °C demonstrates that a simplified methodology of a single-loop PID controlling with spindle speed may be used to effectively control temperature. This methodology can be simply used with any machine that already has the ability to actively control spindle speed, and has been previously shown to be able to be auto-tuned with a single weld. Additionally, implementation of this method compared to uncontrolled FSW in AA6111 at linear weld speeds of 1-2 meters per minute showed improved mechanical properties and greater consistency in properties along the length of the weld under temperature control. Further results indicate that a minimum spindle rpm may exist above which tensile specimens did not fracture within the weld centerline, regardless of temperature. This work demonstrates that a straight-forward, PID-based implementation of temperature control at high weld rates can produce high quality welds with auto-tuned gains. This method also shows promise in application to other processes in the Friction Stir family, and preliminary results in an application to the Additive Friction Stir Deposition (AFSD) process are also presented.

Temperature control in friction stir welding (FSW) is of interest because of the potential to improve the mechanical and microstructure characteristics of a weld. Two types of active temperature control have been previously implemented for steady-state friction stir welding conditions: PID Feedback Control and Model Predictive Control. The start-up portion of a weld is an obstacle for these types of active control.To date, only minimal exploratory research has been done to develop an active temperature controller for the start-up portion of the weld. The FSW temperature controller presented in this thesis, a Position-Velocity-Acceleration (PVA) controller implemented with gain-scheduling, is capable of active control during the start-up portion of a weld. The objectives of the controller are (1) to facilitate fully-automated active temperature control during the entire welding process, (2) to minimize the rise time, the settling time, the percentage maximum post-rise error (overshoot calculated as a percentage of the settling band half-width), and the post-settled root-mean-square (RMS) of the temperature error, and (3) to maintain the steady state performance of previous control methods.For welds performed in 6.35 mm plates of 7075-T651 Aluminum with controller gains identified through a manual tuning process, the mean controller performance is a rise time of 10.82 seconds, a settling time of 11.35 seconds, a percentage maximum post-rise error of 69.86% (as a percentage of the 3◦C settling band half-width), and a post-settled RMS error of 0.92◦C.Tuning of the start-up controller for operator-specified behavior can be guided through construction of regression models of the weld settling time, rise time, percent maximum post-rise error, and post-settled RMS error. Characterization of the tuning design space is performed through regression modeling. The effects of the primary controller tuning parameters and their interactions are included. With the exception of the post-settled RMS error model, these models are inadequate to provide useful guidance of the controller tuning, as significant curvature is present in the design space. Exploration of higher-order models is performed and suggests that regression models including quadratic terms can adequately characterize the design space to guide controller tuning for operator-specified behavior.