Thermo-mechanical and microstructural issues in joining similar and dissimilar metals by friction stir welding

Friction stir welding (FSW) is a newly-emerged metals joining method that has made significant advances in the last 15 years. FSW is a solid-state joining process in which a rotating tool, comprised of a shoulder and a pin, is plunged into the joint and is then traversed along the weld line to form a weld. This novel joining which was initially applied for joining aluminum alloys has now rapidly expanded its applications to join steel, titanium, nickel and other high temperature alloys. This technique provides significant advantages compared to the conventional fusion processes as it offers lesser workpiece distortion and improved mechanical properties due to the solid-state joining of metals. FSW process is currently applied in product industries like the aircraft, aerospace, marine shipbuilding, and auto industry, and the process industry like oil and natural gas.; This dissertation describes a combination of the numerical and experimental investigation of the FSW process. This dissertation encompasses the following five areas of research: (1) Simulation of the process, (2) Mechanical testing using tensile and micro-hardness analysis, (3) Materials microstructure characterization, (4) Process development for joining dissimilar materials (Al alloy and mild steel), and (5) Monitoring and sensing the process.; In the numerical investigation, the thermo-mechanical modeling of the process is presented (1) to predict the thermo-mechanical response with adaptive boundary conditions to make the simulation results more accurate when presented with process parameter variations, and (2) to use the material behavior and deformation analysis to understand the complex thermo-mechanical condition that exists in the near field matrix around the tool. The main objective of creating these simulation models is to understand the conditions under which the weld formation takes place during the FSW. Experimental studies are conducted to validate these simulation results.; In the experimental investigation, extensive experimentation for joining similar and dissimilar Al-alloys is performed. The mechanical and microstructural characterization is performed using tensile and micro-hardness testing, scanning electron microscope (SEM) fractography and electron probe micro-analysis (EPMA).; Solid-state joining of Al-alloy to steel is also conducted successfully. A specially designed tool made of Tungsten-Rhenium (W-25Re) with the pin having a tri-flute cutting edge is used to shear the particles of steel and spread them along the interface. The results of the mechanical and the microstructural characterization of the Fe-Al weld indicate the formation of a structurally sound weld.; In-process monitoring of FSW is accomplished by processing the acoustic emission (transient elastic waves) signals using FFT and wavelet transforms to identify the lack of contact between the tool and the workpiece, as well as the tool breakage in real-time.