Microstructure quantification and phased array ultrasonic inspection of GH3535 alloy weld

Abstract

The critical problem in accurately characterising the defects in nickel-based alloy weld is the skewing of the ultrasonic beam due to the heterogeneity. This article aims to address this problem by utilising microstructure analysis and mode techniques to account for the skewing of the ultrasonic beam and the formation of root signals in the weld. The macroscopic and microscopic metallography of the weld is quantitatively described, focusing on the GH3535 alloy weld. The mean grain size of the base metal and the weld are 60 and 400 μm, respectively. The local crystal orientation map of the weld is obtained using the electron backscatter diffraction technique, revealing a preferred orientation of the grains in the <100> direction. The propagation of the ultrasonic beam in GH3535 alloy weld was visualised by CIVA software. The beam's degree of skewing and splitting is related to the grain's angle of orientation difference. Phased array ultrasonic tests were conducted using a 64 L linear array probe with a centre frequency of 2.25 MHz to inspect the weld and side-drilled holes with a diameter of 3 mm. The echo signals from side-drilled holes in different depths were analysed in both the simulation and tests. The quantitative deviation of depth at the centre and lower end of the weld is 2.6% and 6%. The signal–noise ratio of the side-drilled holes in the weld in the test is 12.5–15.8 dB. The root signals were observed in the weld but not in the base metal. The forming mechanism of the root signal was elucidated by analysing the vertical orientation of the grains.