The role of phase transformation in electron-beam welding of TiAl-based alloys
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INTRODUCTION
TiAl-BASED alloys are rapidly emerging as the most attractive alternate structural materials for high-temperature applications in gas turbine engines, owing to their high specific strength and modulus, good creep, and oxidation resistance. The effective utilization of the alloys will, in many cases, have to require joining. Recent research[1–11] has demonstrated that a sound joint can be produced either by fusionwelding processes, including electron-beam welding, gastungsten arc welding, laser welding, and linear friction welding, by diffusion bonding, or by brazing, although every process has certain limitations. Patterson et al.[1] investigated the electron-beam weldability of a Ti-48 at. pct-6.5 vol pct TiB2 alloy and observed a susceptibility to solid-state cracking. They suggested that weld cracking could be avoided by selection of welding parameters that would result in the calculated values of average heat-affected zone cooling rates below, approximately, 300 8C/s for their alloy. It was also reported[6] that a crack-free weld could be obtained on a different joining geometry by preheating the workpiece above the brittle-to-ductile transition temperature. Austin et al.[4] observed that welds made at elevated temperatures did not crack in the glovebox upon cooling, but would later crack when removed to normal atmospheric conditions. It was believed that the combination of residual stresses and hydrogen in the moist air could be responsible for the cracking of welds. They suggested a stress-relief treatment prior to exposure to the ambient atmosphere. The present study was a systematic investigation of the microstructures developed in the electron-beam welds of a Ti-45Al-2Nb-2Mn and a Ti-48Al-2Nb-2Mn alloy, with different welding parameters leading to a wide range of Q. XU, Research Associate, and M.C. CHATURVEDI, Professor, are with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6. N.L. RICHARDS, formerly Manager of Materials and Processes Engineering with Bristol Aerospace Ltd., Winnipeg, Canada, is Adjunct Professor, Department of Mechanical and Industrial Engineering, University of Manitoba, and a Consultant. Manuscript submitted March 2, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
cooling rates. Attempt was made to determine the dependence of the solid-state weld cracking susceptibility of TiAlbased alloys on the microstructures of the welds. It was observed that phase transformations that occurred after the welding process played a significant role in controlling the quality of the electron-beam welds. It is, therefore, suggested that the continuous cooling transformation (CCT) diagram could be used as a reference for the selection of welding parameters to control the microstructures and, hence, the cracking susceptibility of welds. II. EXPERIMENTAL The chemical analysis of the two TiAl-based alloys used in this research is given in Table I. The Ti-45Al-2Nb-2Mn alloy was cast and heat treated at 1320 8C followed by air coolin
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