Friction stir welding (FSW) is a solid-state joining process that offers advantages over traditional fusion welding. The amount of heat generated during a FSW process greatly influences the final properties of the weld. The heat is generated through two main mechanisms: friction and plastic deformation, with friction being the larger contributor in a FSW process. There is a need to develop better predictive models of the heat generation and heat transfer in FSW. Almost all models seen in the literature validate temperature predictions on only one side of the tool/workpiece interface, thus ignoring possible inaccuracy that comes from incorrect partitioning of heat generated by friction. This work seeks to model and validate both sides of the interface by matching experimental results for both the plunge and steady state phases of FSW for AA 6061-T6. Proper model validation allowed for a study of the sensitivity of the model predictions to changes in the friction coefficient and heat transfer coefficient at the tool/workpiece interface. Most models in the literature use the Coulomb friction law with a fixed friction coefficient, even though the Norton law better incorporates local material behavior. As such, for the plunge phase of FSW, a method for achieving a time dependent friction coefficient was developed and employed to match experimental temperatures, using Norton's viscoplastic friction law. A friction coefficient of 0.65 was used at the start of the plunge phase, decreasing to 0.08 during the steady state phase. This decrease in magnitude from plunge to steady state is similar to the decrease of the Coulomb friction coefficient calculated by Meyghani et al in a 2017 study. Tuning the models resulted in temperature predictions that differed from experimental measurements by no more than 1.5 percent for the non-steady state plunge and by no more than 9 percent for the steady state simulation. For both models, changes in the heat transfer coefficient had a large effect on tool temperature and very little effect on workpiece temperatures. Increasing the friction coefficient led to a proportional increase in temperature for both the tool and workpiece.

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The performance of a coupled viscoplastic model of FSW has been evaluated over a variety of tool RPMs and feed rates. Initial results suggested that further optimization of the material parameters and an additional ability to model the thermal recovery of the material would improve the overall performance of the model. Therefore, an experimental/numeric approach was taken to improve and quantitatively compare the performance of the model based upon the thermal profile of the workpiece. First, an experimental method for obtaining real-time temperature measurements during Friction Stir Processing (FSP) of 304L Stainless Steel was developed. The focus of the method was to ensure that the obtained temperatures were both accurate and repeatable. The method was then used to obtain thermal cycle data from nine welds, each at different operating conditions ranging in tool rotational speed from 300 to 500 RPMs and in feed rate from 0.85 to 2.54 mm/s (2 - 6 in/min). Then a family of nine numerical models was created, each model corresponding to one welding condition. The performance due to improved convergence stability and the added thermal recovery term are also discussed. A gradient following technique was used to optimization and iteratively adjust nine material parameters to minimize the difference between the numerical and experimental temperature for the whole family of models. The optimization decreased the squared error between the numerical and measured temperatures by 76%. Recommendations are also made that may allow the optimization method to return greater dividends.

A Lagrangian finite-element model of the plunge phase of the friction stir welding process was developed to better understand the plunge. The effects of both modeling and experimental parameters were explored. Experimental friction stir plunges were made in AA 7075-T6 at a plunge rate of 0.724 mm/s with spindle speeds ranging from 400 to 800 rpm. Comparable plunges were modeled in Forge2005. Various simulation parameters were explored to assess the effect on temperature prediction. These included the heat transfer coefficient between the tool and workpiece (from 0 to 2000 W/m-K), mesh size (node counts from 1,200 to 8,000), and material model (five different constitutive relationships). Simulated and measured workpiece temperatures were compared to evaluate model quality. As spindle speed increases, there is a statistically significant increase in measured temperature. However, over the range of spindle speeds studied, this difference is only about 10% of the measured temperature increase. Both the model and the simulation show a similar influence of spindle speed on temperature. The tool-workpiece heat transfer coefficient has a minor influence (<25% temperature change) on simulated peak temperature. Mesh size has a moderate influence (<40% temperature change) on simulated peak temperature, but a mesh size of 3000 nodes is sufficient. The material model has a high influence (>60% temperature change) on simulated peak temperature. Overall, the simulated temperature rise error was reduced from 300% to 50%. It is believed that this can be best improved in the future by developing improved material models.

Two steel pipe piles in place in abutments for two different bridge constructions sites were instrumented with strain gauges to measure the magnitude of negative skin friction. The piles were monitored before, during and up to 19 months after construction was completed. The load versus depth and time in each pile is discussed. Maximum observed dragloads ranged from 98 to 127 kips. A comparison with two methods for calculating dragloads is presented. Both comparison methods were found to be conservative, with the Briaud and Tucker (1997) approach more closely estimating the observed load versus depth behavior.

Frictional resistance reduction of liquid flow over surfaces has recently become a more important topic of research in the field of fluid dynamics. Scientific and technological progress and continued interest in nano and micro-technology have required new developments and approaches related to reducing frictional resistance, especially in liquid flow through nano and micro-channels. The application of superhydrophobic surfaces could be very effective in achieving the desired flow through such small channels. Superhydrophobic surfaces are created by intentionally creating roughnesses on the surface and applying a uniform hydrophobic coating to the entire surface. Liquid droplet tests have revealed that because of the trapped air within the cavities such surfaces could have contact angles as high as 179°. Such a property gives superhydrophobic surfaces liquid repelling characteristics making them very suitable for frictional resistance reduction in liquid flow through nano or micro-channels, provided wetting of the cavities could be avoided. This study presents 3-D numerical simulation results of liquid laminar flow over post patterned superhydrophobic surfaces. The research was performed in three phases, 1) pressure-driven flow with square micro-posts, 2) Couette flow with square micro-posts, and 3) pressure-driven flow with rectangular micro-posts at various aspect ratios. In phases (1) and (2) the influences of important parameters such as the cavity fraction, in the range of 0.0-0.9998, and the relative module width, from 0.01 to 1.5, on frictional resistance reduction in the creeping flow regime were explored. Phase (1) also addressed the effect of varying Reynolds number from 1 to 2500 on frictional resistance. Phase (3) was conducted for aspect ratios of 1/8, 1/4, 1/2, 2, 4, and 8 also in the creeping flow regime. The obtained results suggest that important parameters such as cavity fraction (relative area of the cavities), relative module width (combined post and cavity width relative to the channel hydraulic diameter), and the Reynolds number have great influence on the frictional resistance reduction. For pressure-driven flow at cavity fraction 0.9998, reductions as high as 97% in the frictional resistance were predicted compared with the classical channel flow. This reduction is directly related to the significant reduction in liquid-solid contact area. With respect to the effect of relative module width on the overall frictional resistance, a reduction of 93% in the frictional resistance was observed as the relative module width was increased from 0.1 to 1.5. This is indicative of the importance of the relative spacing size of the posts/cavities compared to the channel size in micro-channel liquid flow. The overall frictional resistance for post-patterned superhydrophobic surfaces was found to be independent of the Reynolds number up to a value of nominally 40 after which the non-dimensional frictional resistance increased at high values of the Reynolds number. However, at very high cavity fractions the frictional resistance was independent of Reynolds number only up to about 4. When the driving mechanism was a Couette flow, similar to the pressure-driven flow, as the cavity fraction and the relative module width increased the frictional resistance on the superhydrophobic surface decreased. At a cavity fraction of 0.9998 the reduction in the non-dimensional frictional resistance was approximately 96%, which was only 1% different from the similar pressure-driven scenario. However, a more significant difference was observed between the slip velocities for the two flow types, and it was determined that the pressure-driven flow resulted in greater apparent slip velocities than Couette flow. A maximum difference in normalized slip between the two scenarios of approximately 20% was obtained at relative module width 0.1 and Reynolds number 1. Results for superhydrophobic surfaces with rectangular micro-posts approached those reported in the literature for micro-ribs as the aspect ratio of the posts increased. When the flow was perpendicular to the long side of the posts, and as the aspect ratio increased, the frictional resistance approached previously published transverse rib results. Similarly, when the liquid flow direction was parallel to the long side of the posts, the frictional resistance results also approached those of the previously published longitudinal ribs as the aspect ratio increased.

Friction stir welding (FSW) has been studied extensively for the past two decades. Thermal modeling has been of particular interest, as the quality of the weld is dependent upon the temperature history of the work piece during the process. Since direct temperature measurements of the welded zone are not possible, an analytical model was developed to predict the temperature in this area. This model requires parameters that cannot be easily experimentally determined, so a best fit for these parameters was acquired via regression analysis by comparing the model to experimental data acquired outside of the weld zone. The model was then validated by comparing it to additional temperature data, not including the data used for regression analysis.

In friction stir welding, the temperature of the process zone affects the properties of the resulting weld and has a dramatic effect on tool life in PCBN (polycrystalline cubic boron nitride) tools. Therefore an active control system that changes process parameters to control weld temperature is desirable. Mayfield and Sorensen proposed a two-stage control model that contains an inner loop that controls the spindle speed to keep power constant and an outer loop for setting the desired power based on weld temperature. This work contains the analysis and implementation of a temperature control method based on their work. This research shows that power input to the stir zone leads tool temperature. Due to the inertia associated with the spindle, power control is best achieved by commanding torque rather than spindle speed. Heat transfer in the tool and stir zone is explored and analytical models are developed. It is shown that the temperature response to power is nonlinear. Nevertheless a first-order approximation with time delay is sufficient to select functional controller gains for a PID controller. Standard manual PID tuning techniques can be used to achieve a desired rise time, settling time and overshoot. Gains for an H-13 tool steel FSW tool were tuned to produce a rise time of approximately 7 seconds, settling time of approximately 30 seconds and overshoot of approximately 30%. Welds were run using these gains in various plate thicknesses, commanded temperatures, backing plates and feed rates. In all cases temperature control functioned properly and the commanded temperature was held with a standard deviation of less than one degree Celsius. Similar results are presented for welds run using PCBN tools.

Temperature control has been implemented in friction stir processing and has demonstrated the ability to give improved process control. In order to have optimal control of the process, the parameters of the system to be controlled must be accurately identified. The system parameters change with tool geometry and materials, workpiece materials, and temperature. This thesis presents the use of the relay feedback test to determine the system parameters. The relay feedback test is easy to use and promotes system stability during its use. The results from the relay feedback test can be used to determine controller gains for a PID controller. The use of this method, as well as the quality of the resulting control is demonstrated in this paper.