Ultrasonic vibration assisted laser welding of nickel-based alloy and Austenite stainless steel

Abstract

In the nuclear and petrochemical industries, there were still several problems in the dissimilar welding of nickel-based alloy and austenite stainless steel by laser welding (LW), such as the appearances of unmixed zone, secondary phase, and uneven element distribution. In this paper, 20 kHz ultrasonic vibration assisted laser welding (ULW) was used to solve these problems in laser welding of Hastelloy C-276 and austenite stainless steel 304 dissimilar materials. The analysis of morphology, microstructure, element distribution and microhardness were carried out to investigate the effect of ultrasonic vibration on the dissimilar weld. The results indicated that with the addition of ultrasonic vibration, the width of unmixed zone and the amount of secondary phase were reduced, and the element distribution was homogeneous. With the increase of ultrasonic intensity, penetration depth was slightly increased, and the dilution level of 304 base metal was correspondingly promoted. The width of unmixed zone was reduced due to the micro-turbulence formed near the fusion boundary caused by the cavitation effect. With the injection of ultrasonic energy, the main texture in the weld metal (WM) shifted from {211} <453> to {111} <213>, and the ratio of misorientation in the range of 55°∼60° was significantly improved. The inter-granular secondary phase in the WM was confirmed to be p phase, and amount of which was decreased from 2.44% without ultrasonic vibration to 0.72% with ultrasonic output power of 500 W, because the segregation of element Mo was suppressed at higher ultrasonic intensity. The cavitation and acoustic streaming effects accelerated both the molten pool convection and element diffusion, and thus, with the increase of ultrasonic intensity, the element macro-distribution in the WM became more and more homogeneous. Because the grain size in the weld metal was not obviously refined, the microhardness value of the WM with ULW was not significantly enhanced.