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Out of the known aerospace metal and alloys, Aluminium (Al) and Titanium (Ti) are important due to their unique combination of properties, such as strength, ductility and corrosion resistance etc. For these reasons, welding of these two materials, especially in the butt and lap configuration, has a significant impact for structural applications. However, welding of Al to Ti is a challenge due to wide differences in their physical properties and properties of the brittle intermetallic that are formed. Such problems in Ti-Al weld can be minimized if the temperature of welding is reduced. Therefore, many solid-state welding processes have been introduced for this system in the past few decades. Amongst these processes, Friction Stir Welding (FSW) is among the most appropriate for dissimilar materials in the butt and lap configuration, as this process involves lower temperature of processing. The present thesis is an attempt to address the issues pertaining to the friction stir welding of commercially pure Al and Ti. Though these commercially pure materials are seldom used in actual applications, where alloys such as Ti-6Al-4V and Al 2219 (and their variants) are used, this work is done to get a fundamental understanding of the underlying mechanisms during Friction Stir Welding (FSW). The study has been extended to the effect of using a thin strip of other metallic materials between Al and Ti. These inserts are likely to play a role in the formation of intermetallic and control the after effects of the formation of these intermetallic. Two metals have been chosen for this purpose, namely Zinc (Zn) and Niobium (Nb). The thesis has 8 chapters that attempts to systematically understand the process of FSW of cp-Al to cp-Ti. In Chapter 1 of the thesis, the FSW process is introduced with an emphasis on important parameters that control the welding process. In addition, a brief introduction of Al-Ti binary system is also given. Literature related to conventional solid state welding processes and friction stir welding process is presented in Chapter 2. In this chapter, previous works on the FSW of various materials is reviewed, with more emphasis on welding of aluminium to titanium. At the end of the chapter the scope and motivation of the present investigation has been outlined Chapter 3 includes the experimental details involved in the present study. In addition to the details of the processes and various characterization techniques used in the present investigation, the basic principles involved in various techniques, names as X-ray tomography, Scanning Electron Microscopy (SEM) with Electron Back-Scattered Diffraction (EBSD), X-Ray Diffraction (XRD) and Electron Probe Micro-Analysis (EPMA) have also been given. Micro-hardness and tensile tests results are also reported in this chapter. A detailed study on FSW of Al and Ti is presented in chapter 4 of the thesis. The effect of process parameters on the evolution of microstructure and mechanical properties has been reported. A bottom-up approach on experimentally determining the “process window” is presented. The results emphasises on the distribution of titanium fragments and intermetallic particles in the nugget zone and their influence on mechanical properties of the weld. The microstructural evolution in the matrix is also detailed. The most noteworthy observation is substantial grain refinement in the nugget zone due to the presence of fine fragments of titanium and intermetallic. Cross-tensile tests of the samples welded under the optimised conditions fail in the retreating side of the aluminium material and has strength more than the parent material. The last section in this chapter deals with thermal stability of the microstructures. Chapter 5 deals with the use of Zn as interlayer between Al and Ti. The microstructural evolution and its effect on the mechanical properties have been examined. The investigations clearly show that FSW of Al and Ti with Zn interlayer has superior mechanical properties compared to Al-Ti welds without interlayer. The resulting microstructure has a better thermal stability. The use of Nb as interlayer has been studied in chapter 6. The microstructural investigation of the nugget zone reveals that Nb interlayer does not readily form solid solution with any of the base materials and Nb gets distributed more heterogeneously compared to Ti itself. This has led to a reduction in the strength of the weld, however, the ductility increases The thermal stability of the microstructure is poor compared to FSW of Al to Ti with Zn interlayer. In chapter 7, salient features of the different micro-mechanism operating during FSW of the investigated combinations has been discussed in detail. Finally, the outcome of the thesis has been summarized and scope for future investigation is outlined in chapter 8.

M.Ing. (Electrical & Electronic Engineering) / Please refer to full text to view abstract

Good welding research requires accurate control of the variables involved,to achieve consistent results. The variables involved in arc welding are of the dependent and independent type. Control of the independent variables such as arc current, arc voltage and welding velocity is necessary to control the independent variables such as penetration, electrode consumption rate, etc. The purpose of this thesis was to design, develop and test an automatic welding machine which controlled the above variables within the limits required for welding research.

Weld quality in the context of process control and internal conditions is studied. Several different alloys are welded including plain carbon steel, high-temperature steels, and several traditional and advanced superalloys. Across all studied weld systems, the following conditions led to stronger welds: higher forces and feedrates, lower temperatures, and moderate or limited upsets. In the best cases, post-weld strengths were nearly equal to basemetal strength. Tradition holds that large and symmetric upsets are necessary for good welds, but this study contradicts that notion. The fundamental requirements for bonding are two sufficiently clean surfaces in intimate contact. Only minimal upset is necessary to achieve that. In welding alloy 718, only 1 mm of feed (or ~0.4 mm of sample upset) was necessary to achieve $>$95\% of basemetal strength. In an advanced superalloy with low ductility, very low upsets were required in order to achieve high joint strength. For this low-ductility alloy, using a containing geometry increased both the internal pressure and ductility of this alloy, leading to a much larger window of sound welding conditions and stronger welds overall. In several dissimilar alloy systems, the relationship between force/feedrate and upset asymmetry varied between each alloy, but a more symmetric upset did not correlate to stronger welds. Advanced process control in FW was also performed with closed-loop temperature control and open-loop predictive cooling rate control. Using this technique, martensitic microstructures associated with a fast natural cooling rate were avoided, and a pearlitic microstructure was obtained. The yield and tensile strength of the weld was not adversely affected, and both were within range of published values for the basemetal.

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Yes / Resistance spot welding of aluminum (Al5754) to magnesium (AZ31B) alloys results in the formation of a variety of solidification microstructures and intermetallic compounds that may affect the in-service performance of the weld. This study evaluates the relationship between the welding parameters and the properties of the weld nugget that is formed, and clarifies the morphological and microstructural evolutions within the weld regions during the low-current “small-scale” resistance spot welding of Al5754 to AZ31B. The investigations included a combination of microstructural characterization and thermodynamic analysis of the weld region. The results show that the welding time and clamping force parameters have significant effects on the properties of the nugget formed. The optimal welding parameters were found to be 300 ms welding time and 800 N clamping force. Weld nuggets formed with lower welding time and clamping force were undersized and contained extensive porosity. Meanwhile, a clamping force above 800 N caused gross deformation of the test samples and the expulsion of the molten metal during the welding process. The most significant microstructural changes occurred at the weld/base metal interfaces due to the formation of Al17Mg12 and MgAl2O4 intermetallic compounds as well as significant compositional variation across the weld pool. The thermal gradient across the weld pool facilitated the formation of several microstructural transitions between equiaxed and columnar dendrites.

The primary objectives of this thesis investigation were: 1. To establish the maxima and minima values, within limits of the welding machine parameters: current and time, for 28 gauge, 24 gauge, 20 gauge, 18 gauge, and 16 gauge of SAE CR 1010 steel. 2. To implement and calibrate the instrumentation necessary to determine accurate values, within limits, of the variables under investigation. Also, to devise suitable controls for those fixed variables not investigated. 3. To correlate values of welding current and time with resulting tensile-shear strengths of the lapped welding joints. 4. To establish the combination of welding current and weld time which gives a maximum value of tensile strength for the metal gauges under investigation. A brief review of the more important literature on resistance spot welding variables was presented. The calibration process of determining voltage, resistance, and current was described. The mathematical prediction models (relationship between weld strength and weld time) were established for each heat control setting for different thicknesses under investigation, based on non-linear regression techniques. From the prediction models and the use of Calculus, the maximum tensile-shear strengths and corresponding maximum weld cycles were computed. The minimum weld strengths and weld cycles were determined by the welding conditions that just produced a weld of measurable strength. / Master of Science

The results of this investigation are based on the statistical and visual analyses of the data collected during this experiment / Master of Science