Chemical, mechanical, non-destructive and corrosive properties were investigated of welded joints of 1 mm thick niobium and tantalum sheets, which were either electron-beam welded in a high vacuum or TIG welded in a controlled welding chamber filled with one-atmosphere argon gas containing an impurity gas of air, nitrogen, oxygen or hydrogen.
The following conclusions were obtained ;
(1) Contents of carbon and oxygen in the weld metal of sintered niobium base metal (C : 180 ppm, 0 : 60 ppm) were not changed by TIG welding, while they were reduced considerably by electronbeam welding. Content of nitrogen in the weld metal of electron-beam melted base metal (N: 85 ppm) was increased by TIG welding.
(2) The heat affected zone and weld metal in both niobium and tantalum specimens showed very coarse grains in microstructure. A rapid cooling was slightly effective in preventing grain coarsening.
(3) The hardness of weld metal in the electron-beam melted niobium specimen was greater when TIG welded in 99.99% argon atmosphere than when electron-beam welded, probably because of nitrogen absorption from argon atmosphere.
Air contents over 10
3 ppm in argon atmosphere hardened and embrittled niobium weld metal. The TIG weld metal in sintered tantalum specimen was hardened and embrittled by air contents greater than 200 to 500 ppm and 10
3 ppm, respectively.
Repeated bead welding more than twice with electron-beam softened the sintered niobium weld metal.
Fracture in tensile test of sintered tantalum welded joints with TIG and electron-beam occurred only in weld metal, while fracture in welded joints of niobium, sintered or electron-beam melted, occurred in HAZ or base metal.
(4) The sintered tantalum specimens used in the present study showed no porosity both with TIG and electron-beam welding.
The electron-beam melted niobium specimen showed no porosity witn both welding methods.
However, the sintered niobium specimens showed pbrosities with electron-beam welding, while far less porosities with TIG welding.
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