The demand for molybdenum-based alloys is on the rise specifically for utilization in high-temperature applications owing to its excellent mechanical properties and outstanding thermal shock capabilities at such elevated temperatures. Consequently, research on the joining of the molybdenum-based alloys has become a popular field of interest amongst scholars and researchers worldwide. Friction stir welding, brazing among other solid state metal joining techniques have been highly investigated and found unsuitable due to the intricate welding technology involved. For fusion techniques, segregation of the alloying elements due to depletion of oxygen and nitrogen at the grain boundaries, disqualifies them. Recently published studies have established that electron beam welding can be effectively utilized for welding of refractory metals due to its vacuum atmosphere and extreme energy density. Unfortunately, a qualitative analyses of the oxides produced during the welding process has not been discussed. Furthermore, the influencing mechanism of segregation of oxygen element on grain boundaries as well as the mechanical properties of these joints at the high-temperature has been rarely reported.
Recently, a team of researchers led by professor Ting Wang from the Harbin Institute of Technology at Weihai, Shandong Provincial Key Laboratory of Special Welding Technology, Weihai, China, proposed a study whose main objective was to cross-examine the influence of microstructure evolution on mechanical properties of a molybdenum-titanium-zirconium (TZM) alloy joint welded by electron beam welding. Their goal was to unravel the impacts of the reactions of the alloying elements during the welding process and the influence of the distribution of precipitates on mechanical properties of the joint. Their work is now published in the research journal, Materials Science & Engineering A.
Briefly, the research procedure employed commenced with welding of TZM alloy plates with electron beam welding equipment under a predefined vacuum, at a preset accelerating voltage, a predetermined beam and a welding speed of 350 mm/min. Next, the welded joints were sliced by wire-cut electric discharge machine for the preparation of the metallographic and tensile specimens. The researchers then sanded the metallographic specimens using abrasive paper and then polished and etched them in a reagent. Eventually, the fracture surfaces of the welded joint and BM were observed by scanning electron microscopy and the micrographs of the weld zone and base metal investigated by transmission electron microscope.
The research team showed that the crystallized grains of the weld zone were coarsened severely, but the dispersed distribution of molybdenum carbide and zirconia particles inside grains caused the micro hardness of weld zone to be higher than that of heat affected zone. The authors also noted that dioxo-molybdenum and titania precipitates easily segregated on grain boundaries due to micro-segregation.
In conclusion, Professor Ting Wang and colleagues presented a successful study on the influence of microstructure evolution on mechanical properties. They realized that alloying elements in TZM alloy resulted in residual oxygen elements reacting with each other, after which various precipitates were produced during the metallurgical reaction process. All in all, they made an excellent discovery whereby the noted that the tensile strength of the welded joint was just 50% of that of base metal and the elongation was nearly decreased to zero with a brittle fracture mode.
Fig. 1. EBSD image contained phase and image quality information of (a) WZ and (b) HAZ.
Fig. 2. TEM bright filed microstructure of (a) BM; (b) WZ showing the presence of grain boundary with TiO2 (~100 nm size) and MoO2; and (c) matrix and Mo2C and ZrO2 (~150 nm size); inset representing a selected-area diffraction pattern from (a) Mo, z=[2 _1 _3]; (b) TiO2, z=[0 _4 4] and MoO2, z=[0 12 0], respectively; (c) hexagonal Mo2C, z=[3 _3 _1] and ZrO2,z=[0 0 16], respectively.
Yongyun Zhang, Ting Wang, Siyuan Jiang, Binggang Zhang, Yong Wang, Jicai Feng. Microstructure evolution and embrittlement of electron beam welded TZM alloy joint. Materials Science & Engineering A, volume 700 (2017), pages 512–518
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