Electron beam welding of Nimonic 80A: Integrity and microstructure evaluation. (May 2018) - Zhang, H.
Abstract: Microstructure, mechanical properties and microhardness of electron beam welded Nimonic 80A sheets and base metal were studied using various tools, i.e. scanning electron microscopy, field emission scanning electron microscope with energy dispersive spectroscopy and electron backscatter diffraction, X-ray diffraction analysis and Vickers microhardness. Very high cycle fatigue behavior of base metal and welded joint under push – pull condition at room temperature was also investigated using ultrasonic fatigue tests. Examination of microstructure shows that weld zone was mainly composed of columnar dendritic, and solidification modes can be changed according to the cooling rate. Moreover, there is little precipitates and MC carbides observed in the weld zone. X-ray diffraction results show that the phase structure of weld zone has not changed after electron beam welding. Additionally, electron backscatter diffraction results show that the weld zone and base metal has a clear difference in grain boundaries characterization and kernel average misorientation. Therefore, microhardness, tensile and very high cycle fatigue behavior of welded joint is lower than base metal. Highlights: EBSD presents the weld zone and base metal has clear difference in grain boundaries and misorientation. XRD proves the phase structure of weld zone has not changed after welding. The tensile and hardness of weld zone are lower than base metal due to strength mismatch. The fatigue strength of weld zone is poor because of the microstructure and stress/strain changes during welding.