METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 30A, MARCH 1999—789 Fusion Zone Microstructure and Porosity in Electron Beam Welds of an  +  Titanium Alloy T. MOHANDAS, D. BANERJEE, and V.V. KUTUMBA RAO The effect of electron beam welding parameters on fusion zone (FZ) microstructure and porosity in a Ti - 6.8 Al - 3.42 Mo - 1.9 Zr - 0.21 Si alloy (Russian designation VT 9) has been investigated. It has been observed that the FZ grain width increased continuously with increase in heat input when the base metal was in the  heat-treated condition, while in the  +  heat-treated base metal welds, the FZ grain width increased only after a threshold energy input. The difference is attributed to both the weld thermal cycle and the pinning effect of equiaxed primary alpha on grain growth in the heat- affected zone (HAZ) of  +  heat-treated base metal. Postweld heat treatment (PWHT) in the subtransus and supertransus regions did not alter the columnar grain morphology in the FZ, possibly due to the lack of enough driving force for the formation of new grains by the breaking up of the columnar grains and grain boundary movement for grain growth. As the PWHTs were conducted in a furnace, the role of thermal gradients can be ruled out. Intragranular microstructure in the as- welded condition consisted of hexagonal martensite. The scale of the martensite laths depended on welding speed. The highest porosity was observed at intermediate welding speeds. At low speeds, a majority of pores formed at the fusion boundary, while at high speeds, occurrence of porosity was maximum at the weld center. The trends on porosity can be explained on the basis of solubility of hydrogen in titanium as a function of temperature and the influence of weld thermal cycle on nu- cleation, growth, and escape of hydrogen gas bubbles. The porosity at slow welding speeds is low because sufficient time exists for the nucleation, growth, and escape of hydrogen gas bubbles, while insufficient time exists for the nucleation of gas bubbles at high welding speeds. The effect of pickling of joint surface, vacuum annealing of the base metal, and successive remelting of the weld metal has also been investigated. I. INTRODUCTION POOR fusion zone (FZ) ductilities in gas tungsten arc welds in  +  titanium alloys have been attributed to a large prior beta grain size. [1,2,3] Improved ductilities are re- ported to have been achieved by low energy input welding processes, such as laser [4] and electron beam welding. [5] Loper et al. [6] have reported constancy of average grain width of heat-affected zone (HAZ) and weld metal columnar grains at the fusion boundary up to an energy input of 0.4 kJ/mm in TIG (tungsten inert gas) melt runs of commercial alumi- num. Similar observations were made by Matsuda et al. [7] in commercially pure titanium. The width of FZ columnar grains is reported to increase continuously with increase in energy input. [8] Baeslack [9] reported that the columnar grain morphology of FZ grains in the TIG welds of Ti-6Al-4V and Ti-6Al-6V-2Sn could not be altered by postweld heat treatment (PWHT) of the welds below the beta transus. Early work by Mitchell [10] showed that gas tungsten arc welds of commercially pure titanium are prone to porosity. It was reported that porosity was due to a nonuniform dis- tribution of intrinsic hydrogen (hydrogen present in the starting parent metal). It was further observed that gross porosity can be eliminated by pickling the weld joint in 35 T. MOHANDAS, Scientist, and D. BANERJEE, Director, are with the Defence Metallurgical Research Laboratory, Hyderabad 500 058, India. V.V. KUTUMBA RAO, Professor, is with the Department of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India. Manuscript submitted May 12, 1998. pct HNO 3 + 5 pct HF in water immediately prior to weld- ing. Porosity was reported to decrease with increasing welding speed. [10,11] It was found that porosity did not occur in the absence of a joint. [10,11] Woolcock also reported that the tendency toward porosity increased with increasing al- loy additions. [11] Zamkov and Shevelev [12] and Gurevich et al. [13] showed that electron beam welds are also prone to porosity. Zamkov and Shevelev reported reduction in po- rosity by welding at low welding speeds and by remelting due to the enhanced lifetime of the molten pool, which allows the hydrogen gas bubbles to escape. Gurevich et al. reported porosity to occur in beam on plate welds also. Fatigue properties are prone to scatter and are also reduced due to the presence of porosity. [14,15] The available literature on porosity is restricted to commercial pure titanium and Ti-6Al-4V. A majority of studies are oriented toward re- duction and possible elimination of porosity. The objective of the work reported in this article is to investigate the effect of starting base metal microstructure and heat input on the FZ grain widths in electron beam welds of an  +  titanium alloy Ti-6.8 Al-3.42 Mo-1.9 Zr-0.21 Si (Russian designation VT 9). The effect of sub- transus and supertransus PWHT of the alloy on the FZ grain morphology has also been studied. Effect of welding speed on the intragranular microstructure has been studied. The effect of electron beam welding parameters on porosity also formed a part of the program. For studies on porosity, the variables investigated were speed of welding, remelting, base metal heat treatment condition, and pickling and clean- ing procedures.