The present study investigates the fracture toughness of FSW X-80 HSLA steel welds. Weld cooling rate and peak temperature were varied among welds; indirectly manipulated through FSW travel speed, rpm, and weld preheat. Fracture toughness was tested according to ASTM 1820 standard along the weld centerline using surface-notched SEB specimen cooled to -40 °C. This study resulted in a reliable, repeatable process for generating friction stir welds with CTOD’s consistently above that of the original base metal. CTOD and microstructure of friction stir welds can be selected by controlling weld cooling rate and peak temperature. Material properties and microstructure similar to the original base metal can be recreated throughout the weld stir zone. CTOD of FSW X80 has a strong inverse linear correlation with post-weld cooling rate.

Hybrid structures including aluminum-thermoplastic and aluminum-reinforced thermoplastic composite are increasingly important in the near future innovations due to its lightweight and high strength-to-weight ratio. A critical point for metal-polymer application is that sound joining of these materials is difficult to achieve owing to a large difference in surface energy and dissimilar structure between metal and polymer. In practice, two major joining methods for hybrid structures are mechanical joining and adhesive bonding. However, there are some drawbacks of these conventional methods such as stress concentration, long curing time and low reliability joints. A new novel metal-polymer hybrid joining is required to overcome these issues as well as manufacturing and cost perspectives. To this end, this work aims to develop a general methodology to apply friction stir welding techniques to join a wide range of thermoplastics with and without fibers to aluminum alloy sheets. The present work proposed an experimental study to attain insight knowledge on the influences of welding parameters on the quality of hybrid joints in term of the maximum tensile shear strength. This includes the role of tool geometries, welding methodology as well as material weldability in the investigation. The results showed that friction stir welding is a promising technique for joining of thermoplastic to aluminum. Microstructural observation showed that a good mixing between aluminum and thermoplastic as well as defect-free weldments were obtained. Tool geometries and welding speed are two factors that significantly contribute to the quality of friction stir welded hybrid joints. The results also demonstrated that weld fracture modes are associated with material mixing as well as interfacial bonding between aluminum and thermoplastic. An evaluation of the joint strength was benchmarked with the relevant literatures on hybrid joining. The results of proposed technique showed that the maximum tensile shear strength of friction stir welded joints were the same order of magnitude as the joints welded by laser welding.

Additive friction stir deposition is characterized by rotating a consumable feedstock rod that induces severe plastic deformation to deposit material additively without raising the material past its melting point. In this way, additive friction stir deposition differs from traditional additive manufacturing, and new developments in this technology require further investigation of build parameters, tooling, and resultant builds to better understand this printing process and its applications. This thesis evaluated the effect of rotational speed on aluminum 6061 builds using mechanical testing and microstructural investigations. Three different build conditions were evaluated at 180 RPM, 240 RPM, and 300 RPM. Mechanical testing methods were used to determine hardness values, ultimate tensile strength, yield strength, elastic modulus, and density. Imaging techniques including optical microscopy, electron backscatter diffraction, energy dispersive x-ray spectroscopy, and x-ray computed tomography were used to evaluate microstructure, grain size, chemical composition, and porosity.

A two-dimensional axisymmetric thermo-mechanical model of the Refill Friction Stir Spot Welding (RFSSW) process was developed and validated with experimental data. Welding temperatures, tool forces, and material flow including defect formation, were accurately predicted by the model. Qualified repair techniques are critical for successful implementation of a welding process for use on large weldments with a significant number of spot joints, and this work demonstrates a repair technique for RFSSW that is validated both experimentally and numerically. Repaired properties are shown to exceed 90% of the original mechanical properties of the RFSSW process. RFSSW has different process parameters for every combination of material alloy, material thickness, weld duration, and machine force limits. Numerical modeling develops the process parameters for any RFSSW iteration in a fraction of the time with the same amount of accuracy. The model can effectively simulate how to determine the optimal weld duration given any experimental parameters.

This work establishes the relationships between several key Friction Stir Welding process parameters and the resulting microstructural and flexural properties of the welded joint. A series of four single parameter experiments were run. The parameters investigated were pin diameter, feedrate, shoe temperature, and pressure time. Butt welds were made in 6 mm thick stress-relieved extruded polypropylene sheet. Three-point bend tests were used to determine the ultimate flexural strength and coincident strain. The maximum bend angle before failure was used to label the welds as "good or bad." An optical microscope capable of cross polarization was used to examine and photograph the weld microstructure. Welds were evaluated according to 1) DVS bend angle, 2) flexural properties, and 3) weld microstructure. All welds made surpassed the DVS requirements for classification as a "good weld" established for hot-gas, extrusion, and laser welding processes. Most welds met the bend angle requirement for hot-plate welds. Welds created for this work maintained 80-92% of base material flexural strength. In the majority of the welds, the strength was between 85 and 90% of base material. The FSW joints showed a flexural strength of 10500 psi, compared to a base material strength of 12400 psi. Four microstructural zones were found to exist in the FSW joints. These were: 1)advancing interface, 2) retreating interface, 3) bottom disturbance, and 4) central zone. Several common microstructure types and defects were found to exist in the welds. These were: 1) spherulites, 2) voids, 3) root defects, 4) flow lines, and 5) onion skin. A distinct correlation was observed between weld microstructure and flexural properties. Those welds whose microstructure most nearly resembled the base material demonstrated the best flexural properties. This can be accomplished by operating with a low feedrate, a high shoe temperature, and a large pin.

The purpose of this research was to use the Transvalor S.A. product, Forge3, to develop a finite element simulation of the friction stir welding process that improves the predictability of temperature evolution and material flow within the plunge portion of the process. Previous modeling created more heating within the billet than experimental results, probably due to the simplification of the simulation with adiabatic boundary conditions. More realistic tooling temperatures were included in this model as these affect heat evolution which is a determining factor in microcrystalline growth. These results were validated by experimental efforts using a billet and tooling instrumented with thermocouples used to evaluate the temperatures at specific locations over time. Simulation results were compared with previous experiments to validate the predicted material flow.

The longitudinal forces generated from the Friction Stir Welding process are substantial. An understanding of these forces is critical to proper tool design. This study describes a technique to measure the longitudinal force distribution on a friction stir weld pin tool. Total longitudinal forces were recorded on a dynamometer while welding 6061 aluminum with non-threaded pins that varied in length and diameter. A model was developed that characterizes pin force as a function of pin length and diameter. Results suggest that force generally increases with pin length, while forces remain relatively constant with pin diameter. Unexpected force variation was found at large pin lengths, which yielded several possible models of the force distribution. All of the modeled force distributions proved to be non-uniform and increase linearly with pin length, which produces a pin force that increases with the square of the pin length.

Quantitative understanding of frictional phenomena between the tool and the workpiece is essential for accurate modeling of the Friction Stir Welding (FSW) process. Two methods of measuring the tool-workpiece interface are proposed that allow frictional measurements to be made under extreme conditions. The first method uses a cylindrically curved surface in contact with a flat plate. The ranges of temperature, velocity, and normal force used in this method are 100–600°C, 0.38–2.0 m/s (75–400) surface feet per minute (SFM)), and 450–2700 N (100–600 lbf), respectively. Data are gathered at different parameter level combinations to provide enough data to create an empirical model representing the data. Two friction modes with distinct characteristics are observed. One mode, Coulomb-Amonton's friction, has frictional force proportional to normal force, while the other mode, plastic shear deformation friction, has frictional force independent of normal force. A linear statistical model has been developed to characterize each of the frictional modes for the polycrystalline cubic boron nitride (PCBN) tool and 1018 steel work piece interface as functions of temperature, velocity, and normal force. Two linear models were chosen. A statistical method called membership function regression was used to determine the coefficients of these two models. The resulting model has a correlation of (Predicted Force) = 1.0445(Measured Force) with an R^2 value of 0.83. The second method was an attempt to measure friction with a measurable contact area at a range of temperatures, velocities, and normal pressures. This method rubs the end of a cylindrical rod with a concentric cylindrical pocket against a flat plate. This method caused precessions of the tool on the workpiece. As a result of this precession, plastic shear deformation friction measurements are invalid. However, Coulomb-Amonton's friction is still valid. The experiments of the PCBN-stainless steel interface found that Coulomb-Amonton's friction did not depend on temperature and velocity. In addition, no plastic shear deformation friction was identified using this method and this interface combination.

For decades models have been developed for predicting the size of the weld nugget and heat affected zones in fusion welded structures. The basis for these models is the welding heat input, which is fairly well understood for most arc welding processes. However, this traditional approach is not as straightforward for Friction Stir Welding (FSW). To date, there is no definitive relationship to quantify the heat input for FSW. An important step to establish a heat input model is to identify how FSW process parameters affect weld power. This study details the relationship between FSW process parameters and torque for three different aluminum alloys: 7075, 5083 and 2024. A quantitative weld power and heat input model is created from the torque input. The heat input model shows that decreasing the spindle speed or increasing the feed rate significantly decreases the heat input at low feed rates. At high feed rates, feed rate and spindle speed have little effect on the heat input. Process parameter versus heat input trends are verified by measurements of the weld heat affected zones. In addition, this study outlines and validates the use of a variable spindle speed test for determining torque over a broad range of parameters. The variable spindle speed test provided significant improvements over previous methods of determining torque as this new method enabled the torque to be modeled over a broad range of parameters using a minimum number of welds. The methods described in this study can be easily used to develop torque models for different alloys and materials.

Material flow in friction stir welding is largely uncharacterized due to the difficulty in material flow measurement and visualization in metals. This study investigates plasticine for use as an analog for modeling material flow in friction stir welding (FSW) of metals. Qualitative comparisons between welded plasticine and metal sections exhibit many similarities. The transient temperature response of the plasticine also shows the same qualitative behavior as welds conducted in metal. To quantify its similarity to metal, the plasticine is further analyzed through compression tests to characterize its strain, strain-rate, and temperature sensitivities. A detailed analysis is presented which defines the criteria for rigorous mechanical and thermal similarity between metals and analog materials. The mechanical response of the plasticine is quantitatively similar to many aluminum and steel alloys. In addition to the mechanical properties of the plasticine, thermal properties are measured and thermal similarity is investigated. Generally, complete thermal similarity cannot be achieved in FSW. However, given the similarities between other critical parameters, and observed qualitatively similarity, it is possible to satisfy similarity approximately, such that information can be obtained from the physical model and extrapolated to metals. Using plasticine, material flow behavior in FSW is investigated under various operating conditions. The physical model permits visualization and characterization of material flow around a suspended welding tool. Depending on operating conditions, several material flow regimes are observed, including simple extrusion with substantial tool/material slip, defect formation, a region of rotating material adjacent to the tool, and vertical deformation. Material flow and frictional heating in FSW are also investigated using a three-dimensional numerical model. Two mechanical boundary conditions are investigated, including 1) a sticking constant velocity, and 2) a slipping variable shear stress model. The constant velocity model generally over-predicts the extent of material flow in the weld region. The variable shear model predicts simple extrusion of material around the tool, and substantial tool/material slip. Additionally, the variable shear model exhibits a region of diminishing shear stress, velocity, and pressure at the back advancing side of the pin, suggesting formation of an internal void. The limited deformation, low velocities, and indication of void formation agree well with flow visualization studies using plasticine under identical operating parameters.