Diffusion Welding of Molybdenum to Hastelloy Alloy X

Abstract

Diffusion welding between pure molybdenum and heat-resisting alloy hastelloy X has been performed in vacuum at the temperature range of 750°C-1200°C. Tensile strength of the weld joint at room temperature is discussed on the effects of methods in welding technique and a use of Ni insert-metal. The use of two steps welding technique and a Ni insert-metal is appreciable to improve the tensile strength of weld joint, but the fracture had occured always near weld bond.
Metallographical investigations using transmission electron microscopy (TEM), scanning electron miccroscopy (SEM), X-ray diffraction analysis and electron probe X-ray microanalysis have been performed in order to clarify the effects of two steps welding technique and a use of Ni insert-metal on the microstructure near the bonding interface. Results obtained are summarized as follows;
(1) Two steps welding technique including a short time annealing at high temperature (1 minute at 1200°C) increases the real metal-contact between molybdenum and hastelloy X, and takes the improvement of the joint strength. Furthermore, this technique is useful not to enlarge the deformation by welding procedure.
(2) Application of the Ni insert-metal suppresses the formation of the brittle intermetallic compound, P phase, and increases the tensile strength. However the formation of binary compound, δ-MoNi, was observed at the bonding interface.
(3) Fracture of the weld joints by the tensile strength tests has occured always through the intermetallic compounds and also along the grain boundaries of molybdenum base metal.
(4) After the welding procedure, the voids and MoO₂ were found out at the grain boundaries of molybdenum using SEM and TEM techniques.