Achieving polycrystalline transformation and microstructural segregation reduction of nickel-based single crystal super-alloys by ultrasonic pulse arc welding

Abstract

The sub-grains coarsening and low melting point eutectic are the internal causes of cracking of single crystal super-alloys in welding. Aiming to overcome two internal causes, in this paper, the effect of ultrasonic pulse arc welding on the grains, sub-grains and the precipitations of single crystal super-alloys (IN738) has been comprehensively studied, and the microstructure evolution of IN738 in ultrasonic pulse arc welding has been systematically revealed. The results show that (1) the combined effect of ultrasonic vibration and electromagnetic oscillation not only realizes the synchronous refinement of grains and sub-grains in WMZ (weld metal zone), and the polycrystalline transformation of single crystal super-alloys. A novel 3D schematic model of sub-grains is established to evaluate the size and orientation of sub-grains in 3D direction. (2) The ultrasonic vibration and electromagnetic field induced by ultrasonic pulse current can effectively reduce the microstructural segregation of welds. With the increase of ultrasonic pulse frequency, the size and volume fraction of harmful precipitations (η + δ eutectic, mixed eutectic and MC carbides) decrease greatly, especially the morphology and distribution characteristics of γ′ in HAZ (heat affected zone). Thanks to the above outstanding advantages, ultrasonic pulse arc welding has great application value in single crystal super-alloys.

Keywords

• Ultrasonic pulse arc welding
• Single crystal super-alloys
• Polycrystalline transformation
• Microstructural segregation