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The microstructure of linear friction welds in single crystal (SX) CMSX 486 superalloy was studied. Gleeble thermomechanical simulation of the welding process was also performed in order to understand the microstructural changes induced in the alloy during the joining process. Microstructural analysis of the welded and Gleeble-simulated specimens showed that extensive liquation occurred in the alloy during joining, which is in contrast to the general assumption that linear friction welding (LFW) occurs exclusively in the solid state. The study revealed the application of the compressive load during the forging stage of LFW induced rapid solidification of the resultant metastable liquid phase. Nevertheless, part of the liquid resulted in a continuous Hf-base oxide phase along the weld line. Possible ways of preventing the formation of the potentially deleterious oxide film and, thus, improve the prospect of applying LFW for the joining of CMSX-486 superalloy are suggested.

A detailed microstructural analysis was performed on a difficult-to-weld nickel-base superalloy, IN 738, subjected to linear friction welding and Gleeble thermomechanical simulation, to understand the microstructural changes induced in the material. Correlations between the microstructures of the welded and simulated materials revealed that, in contrast to a general assumption of linear friction welding being an exclusively solid-state joining process, intergranular liquation, caused by nonequilibrium phase reaction(s), occurred during joining. However, despite a significant occurrence of liquation in the alloy, no heat-affected zone (HAZ) cracking was observed. The study showed that the manufacturing of crack-free welds by linear friction welding is not due to preclusion of grain boundary liquation, as has been commonly assumed and reported. Instead, resistance to cracking can be related to the counter-crackformation effect of the imposed compressive stress during linear friction welding and straininduced rapid solidification. Moreover, adequate understanding of the microstructure of the joint requires proper consideration of the concepts of nonequilibrium liquation reaction and strain-induced rapid solidification, which are carefully elucidated in this work.

Keyhole-induced macro-porosity, which results from the collapse of the keyhole that formed by the reaction forces of metal vapors, is a major problem limiting laser and laser-arc hybrid weldability of age-hardenable aluminum alloys, such as AA2024-T3. The mechanism of porosity suggests that the weld metal solidifies more rapidly than the possible rise velocity of the gas bubbles that formed during keyhole collapse, resulting in severe porosity. The porosity behavior of AA2024-T3 during laser-arc hybrid welding was studied using microscopy and X-ray radiography techniques. Porosity-free welding of the alloy is attainable in the conduction mode welding, whereas porosity increased significantly with increased laser intensity during keyhole mode welding. Porosity was mostly severe when the beam was focused at the surface of the workpiece. The laser beam and the arc decouple from each other with increased laser-wire distance, affecting keyhole depth and porosity. In order to control porosity during laser-arc hybrid welding of aluminum alloys, the role of various welding parameters on the material's response should be balanced with the required weld geometry.

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