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Weldability of Tungsten and Its Alloys

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Weldability of Tungsten and Its Alloys Tungsten and its alloys can be successfully joined by gas tungsten-arc welding, gas tungsten-arc braze welding, electron beam welding and by chemical vapor deposition BY N. C. COLE, R. G. GILLILAND AND G. M. SLAUGHTER ABSTRACT. The weldability of tungsten eliminated all but a small amount of but it should be noted that the CVD and a number of its alloys consolidated porosity and also eliminated the prob­ material contained more than the nor­ by arc casting, powder metallurgy, or lems associated with the high tempera­ mal amounts of fluorine. chemical-vapor deposition (CVD) tech­ tures necessary for welding (such as Various sizes and shapes of tung­ niques was evaluated. Most of the mate­ large grains in the weld and heat-af­ rials used were nominally 0.060 in. thick fected zones). sten and tungsten alloys were joined sheet. The joining processes employed for comparison. Most of them were were (1) gas tungsten-arc welding, (2) Introduction powder metallurgy products although gas tungsten-arc braze welding, (3) elec­ some arc-cast materials were also tron beam welding and (4) joining by Tungsten and tungsten-base alloys welded. Specific configurations were CVD. are being considered for a number of used to determine the feasibility of Tungsten was successfully welded by advanced nuclear and space applica­ building structures and components. all of these methods but the soundness tions including thermionic conversion All materials were received in a fully of the welds was greatly influenced by devices, reentry vehicles, high temper­ cold worked condition with the excep­ the types of base and filler metals ature fuel elements and other reactor tion of the CVD tungsten, which was (i.e. powder or arc-cast products). For components. Advantages of these ma­ example, welds in arc-cast material were received as-deposited. Because of the terials are their combinations of very increased brittleness of recrystallized comparatively free of porosity whereas high melting temperatures, good and large-grained tungsten the materi­ welds in powder metallurgy products strengths at elevated temperatures, al was welded in the worked condition were usually porous, particularly along high thermal and electrical conductivi­ to minimize grain growth in the heat- the fusion line. For gas tungsten-arc ties and adequate resistance to corro­ (GTA) welds in V]0 in. unalloyed tung­ affected zone. Because of the sten sheet, a minimum preheat of 150° sion in certain environments. Since brittleness limits their fabricability, high cost of the material and the C (which was found to be the ductile- relatively small amounts available, we to-brittle transition temperature of the the usefulness of these materials in structural components under rigorous designed test specimens that used the base metal) produced welds free of minimum amount of material consist­ cracks. As base metals, tungsten-rhenium service conditions depends greatly alloys were weldable without preheat, upon the development of welding ent with obtaining the desired in­ but porosity was also a problem with procedures to provide joints that are formation. tungsten alloy powder products. Pre­ comparable in properties to the base heating appeared not to affect weld po­ metal. Therefore, the objectives of Procedure rosity which was primarily a function these studies were to (1) determine Since the ductile-to-brittle transition of the type of base metal. the mechanical properties of joints temperature (DBTT) of tungsten is The ductile-to-brittle transition tem­ produced by different joining methods above room temperature, special care peratures (DBTT) for gas tungsten-arc in several types of unalloyed and al­ must be used in handling and machin­ welds in different types of powder metal­ 1 lurgy tungsten were 325 to 475° C, as loyed tungsten; (2) evaluate the ing to avoid cracking . Shearing compared to 150° C for the base metal effects of various modifications in heat causes edge cracking and we have and that of 425° C for electron beam- treatments and joining technique; and found that grinding and electrodis- welded arc-cast tungsten. (3) demonstrate the feasibility of charge machining leave heat checks Braze welding of tungsten with dis­ fabricating test components suitable on the surface. Unless they are re­ similar filler metals apparently did not for specific applications. moved by lapping, these cracks may produce better joint properties than did propagate during welding and subse­ other joining methods. We used Nb, Ta, quent use. W-26% Re, Mo and Re as filler metals Materials Tungsten, like all refractory metals, in the braze welds. The Nb and Mo 1 caused severe cracking. Unalloyed tungsten in /36 in. must be welded in a very pure atmos­ Joining by CVD at 510 to 560° C thick sheets was the material of most phere of either inert gas (gas tung­ interest. The unalloyed tungsten in this sten-arc process) or vacuum (electron study was produced by powder metal­ beam process)2 to avoid contamina­ N. C. COLE and G. M. SLAUGHTER are with Metals and Ceramics Div., Oak Ridge lurgy, arc casting and chemical-vapor tion of the weld by interstitials. Since National Laboratory, Oak Ridge, Tenn.: deposition techniques. Table 1 shows tungsten has the highest melting point R. G. GILLILAND is with the University of Wisconsin. Milwaukee. the impurity levels of the powder met­ of all metals (3410° C), welding allurgy, CVD and arc-cast tungsten equipment must be capable of with­ Paper presented at the American Welding products as received. Most fall within Society 49th Annual Meeting, Chicago, standing the high service tempera­ April 1-5. 1968. the ranges nominally found in tungsten tures. WELDING RESEARCH SUPPLEMENT | 419-s cu 1 u CD i to § 0> — t- 43 l_ 43 H 3 o 3 P o z ~J Srg "a 2 £ 8 3 a> 12 <u * 33 - D, CD tt ,*J J 1) •2 S"° 53 2 u -^ cd 3: M-l 3: d ^ a c « C/3 tA u M °s 05 E s «« 8 g rt 43 '5 c o c TJ *« -ri 00 ^ 2 P. tfl CD V. o „ •J-. p co '3 c « u -" "3 p o 15 ta P. 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