Welding of Mo-Based Alloy Using Electron Beam and Laser-GTAW Hybrid Welding Techniques
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MO-BASED alloys have excellent high-temperature mechanical properties. Therefore, there are many potential applications of these alloys in compact high-temperature reactors and fusion reactors.[1–3] TZM is a Mo-based alloy with small content of Ti (0.50 wt pct), Zr (0.08 wt pct) and C (0.04 wt pct). This alloy offers approximately twice the strength of Mo at temperatures above 1573 K.[1–11] TZM is an alloy of high liquidus temperature [~2873 K (2600 C)]. In addition, with the increasing temperature beyond 923 K (700 C), formation of volatile MoO3 oxide causes substantial weight loss in TZM. These issues make fabrication of TZM through conventional routes difficult, and therefore, TZM is, generally, fabricated through unconventional routes.[12] TZM produced through these unconventional routes show some variations with respect to the target composition and also show relatively high presence of impurity elements.[13] These compositional variations in TZM are important as excess alloying elements could cause solute segregation due to large immiscibility gaps existing in Mo-Ti and Mo-Zr alloy systems. On the other hand, lower concentration could compromise ANJAN CHATTERJEE, Scientific Officer (G), SANTOSH KUMAR, Scientific Officer (F), RAGHVENDRA TEWARI, Scientific Officer (H), and GAUTAM KUMAR DEY, Head, are with the Materials Science Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra 400085, India. Contact e-mail: raghvendra.tewari@ gmail.com Manuscript submitted April 22, 2015. Article published online December 14, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
high-temperature strength as Ti and Zr provide solid-solution strengthening and most importantly form fine carbide precipitates, which improve creep resistance of the material.[7] Mo forms Mo2C, which improves cohesion of the grain boundaries.[8–10] Beside, carbon decreases segregation of the trace oxygen on the grain boundaries.[8] The segregation of oxygen and nitrogen in the intergranular region is one of the important culprits responsible for poor ductility in this alloy in recrystallized, coarse-grained conditions.[11] Welding of this material is a challenging task; considering its high melting point (more than 2773 K (2500 C)), high thermal diffusivity, and the material being prone to embrittlement by even small degrees of contamination from oxygen and nitrogen.[14] High melting point requires large amount of heat to be deposited at the joint line for fusion welding; but high thermal diffusivity causes heat dissipation at a very rapid rate away from the joint line. Therefore, one requires high-intensity heat sources like electron beam or laser beam or pulsed gas tungsten arc welding (GTAW) system. In the case of laser welding of TZM, there is an additional difficulty as high reflectivity causes significant loss of incident laser beam energy by reflection. Although thermal expansion coefficient of TZM is low at room temperature (~5 9 10 6 K 1), it, however, increases significantly near its melting point (~20 9 10 6 K 1).[14,15] Moderate-to-high values of the coeffici
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