Crack generation mechanism and control method of electron beam welded Nb/GH3128 joint

Highlights

• Electron beam welding of Nb/GH3128 was firstly carried out.

• The relationship among cracks, microstructure, and residual stress of Nb/GH3128 joints is established.

• High longitudinal residual stress was proved to be the critical factor triggering longitudinal cracking of the joint.

• Crack-free welding of Nb/GH3128 dissimilar alloys was realized firstly.

Abstract

Electron beam welding of Nb/GH3128 dissimilar alloys was conducted in this research. Microstructure characterization and finite element simulation were utilized to uncover the cracking mechanism of the joint. The obtained results revealed that a brittle reaction layer, composed of Ni₆Nb₇/Laves phase beside the Nb substrate, was a critical cracking factor triggering the cracking along the reaction layer. The generation of cracks was affected by a certain thermal-mechanical effect. The longitudinal residual stress fluctuated dramatically in the Nb/FZ interface region, resulting in different welding deformation. High transverse residual stress was also detected in the vicinity of Nb/FZ interface, increasing the cracking propensity of Nb/GH3128 joint. This high residual stress combined with the brittle reaction layer illustrated the intrinsic cause of welding crack. Crack-free welding of Nb/GH3128 dissimilar alloys was realized via adopting the beam offset process. The thickness of the brittle reaction layer was dramatically decreased and the brittle Laves phase within FZ was replaced by the γ phase.

Introduction

Ni-based superalloys, as promising materials for aerospace and nuclear industries, have been of great interest owing to their excellent performance at elevated temperatures. To that end, a great number of Ni-based superalloys, from precipitation-strengthened Ni-based superalloys to solution-strengthened Ni-based superalloys, have been designing to satisfy the demands of high-temperature performance and corrosion resistance in the aggressive environments. GH3128, with high specific strength, good oxidation and hot corrosion resistance, is just such material for manufacturing the burner chamber of aerospace engineering (Zhang et al., 2019). To expand its application, the exploration refiring to the welding techniques of GH3128 superalloy should be elevated to a higher degree.

The Ni-based superalloys, with poor weldability, are generally considered difficult to achieve reliable connection, due to their high sensitivity to solidification cracking and liquation cracking. Senkov et al. (2014) carried out the inertia friction welding of Mar-M247/LSHR dissimilar superalloys. Some oxide and carbide particles were detected at the interface of the two base metals, resulting in the generation of a radial crack. To eliminate the welding crack, Song et al. (2009) investigated the effect of welding speed on the friction stir welding of Inconel 600 superalloy. No crack was generated in the weld obtained with a welding speed of 150 and 200 mm/min, and the maximum strength of the weld was 10 % higher than that of the base metal. However, for the welding of complex structures, friction welding exhibited poor flexibility than fusion welding. In this condition, Ahn et al. (2002) observed the generation of γ/NbC and γ/Laves eutectic decorated along grain boundaries promoted the solidification cracking. Ye et al. (2015) conducted TIG welding of Inconel-718 superalloy, using as-cast material and homogenized respectively. Compared to the as-cast alloy, fewer liquation cracks were generated in the HAZ of homogenized base metal. In addition, the segregation of alloying elements like Nb and B as well as the generation of Laves phase were also regarded as the key factors resulting in cracking in the joint. In fact, the above welding defect also occurred in the joint of solution-strengthened Ni-based superalloys. Lippold et al. (2008) evaluated the solidification cracking susceptibility of several Ni-based filler metals and revealed that Inconel 617 was much more sensitive to crack. Pre and post -weld heat treatments have been proved to be effective methods. González et al. (2011) investigated the influence of pre-weld heat treatment on the microstructure of Inconel 939 superalloy. After pre-welding heat treatment, the microstructure distributed more homogeneous and fewer HAZ liquation cracks were produced. Ghasemi et al. (2020) recently studied the solidification cracking of Hastelloy X joint through employing GTAW. They pointed out that all the micro-solidification cracks distributed in the interdendritic zones of the joint could be eliminated after post-weld solution treatment.

Recently, investigations referring to the welding of Ni-based alloy and dissimilar alloys were conducted to extend the application of Ni-based alloy. Chen et al. (2011) tried to join Inconel 718 and Ti-6Al-4 V alloys using fiber laser welding, cracks and porosities generated in the weld as the laser beam focused on the interface of the two. Crack-free welding was realized by offsetting the laser beam to Inconel 718 side with 35 μm. To avoid the formation of brittle intermetallics, Liu et al. (2020) introduced a Nb/Cu composite interlayer as a barrier to improve the metallurgical compatibility of Inconel 718 and Ti-6Al-4V alloys, obtaining a crack-free weld with the microstructure of Nb-based, Cu-based, Ni-based and (α + β Ti, Nb) solutions. Song et al. (2020) carried out the electron beam welding (EBW) of Ti60/GH3128 dissimilar alloys using V/Cu composite interlayer, promoting Ti-Ni intermetallics to be replaced by Ti-based and Cu-based solutions. Sang et al. (2019) carried out the electron beam welding of tantalum and GH3128 dissimilar alloys, and a defect-free joint with the maximum tensile strength of 277 MPa was obtained.

As a refractory metal, Nb had been gradually applied in aerospace owing to its high thermal stability at elevated temperatures. While, there were limited investigations on welding of Nb-based alloys until now. Hajitabar and Naffakh-Moosavy (2018a,b) explored the weldability of Nb-1Zr alloy, and no defect was observed in the weld. In this condition, Hajitabar and Naffakh-Moosavy (2018a,b) tried to join Nb-1Zr and 321 stainless steel via EBW. The results showed that no crack was detected and the strength of the joint reached 170 MPa. Torkamany et al. (2014) investigated the laser beam welding of Nb and Ti-6Al-4V alloy. Though islands of Nb rich and Ti-rich phases were observed, the joint exhibited high strength, which was higher than that of the base metal. To expand the application of Nb-based alloys, more explorations referring to the welding of Nb and other alloys should be conducted, such as superalloys.

In this paper, EBW of Nb/GH3128 dissimilar alloys was carried out. The microstructure and mechanical properties were investigated to evaluate the weldability of the two materials. The detailed analyses about the causes of welding cracks generated in the weld were discussed. In addition, crack-free welding of Nb/GH3128 dissimilar alloys was realized via adopting the electron beam offset process.