The effect of Fe_xNb_y (x = 2,7 and y = 1,6) intermetallics on microstructure and mechanical properties of electron beam welded Nb-1Zr refractory alloy

Highlights

• The electron beam offset of 50:50 and 13 mA current, presented the best.

• Fe₇Nb₆ intermetallic compound in the weld zone adjacent to Nb-1Zr alloy.

• The tensile strength was equal to 60.5% of that of welded Nb-1Zr alloy.

• Nb particles were detached due to the high turbulence of the melt.

• Recrystallization temperature of Nb-1Zr alloy was determined 1810 °C.

Abstract

In this study, weldability and mechanical properties of dissimilar joint between Nb-1Zr and 321 stainless steel produced by electron beam welding were investigated. The results indicated that the optimum parameters to make this dissimilar joint possible are the electron beam offset of 50–50 and the current of 13 mA. Because of high melting temperature of Nb relative to 321 stainless steel and also high thermal conductivity of Nb, the molten weld pool was drawn to 321 stainless steel side containing 85.1%Vol 321 stainless steel and 14.9%Vol Nb. Non uniform regions were observed in the weld zone which were due to melt turbulence and its rapid solidification. The Fe₇Nb₆ and Fe₂Nb intermetallic compounds formed in various regions of the weld zone directly govern the mechanical properties and weldability of the weld. Tensile strength of the weld was 170 MPa which was equal to 60.5% of that in autogenous weld of Nb-1Zr alloy. Tensile strength of the autogenous weld of Nb-1Zr alloy was 281 MPa. According to the Rosenthal's equations and measurements of the weld section, the recrystallization temperature of the Nb-1Zr was identified 1810 °C in the EBW condition.

Introduction

Niobium is a refractory metal which has high thermal stability and good mechanical properties at elevated temperatures [[1], [2], [3], [4]]. Generally, Nb-based alloys are widely used in superconductors, high temperature and corrosive environments [5,6]. These alloys are very reactive at elevated temperatures. Thus, welding of these alloys should be performed in vacuum or protective gas atmosphere. Welding of dissimilar metals is always favorable for industries because of its potential advantages in saving materials and cost, reduction of weight, increase of design flexibility and improving performance of the product [7]. In welding of dissimilar metals, particularly Nb alloys, there are many scientific and technical challenges [8,9]. The techniques which are available for dissimilar welding of Nb alloys include: explosive welding, magnetic pulse welding, friction welding, brazing, laser beam welding, electron beam welding (EBW), GTAW and plasma arc welding among which two techniques of laser and electron beam welding generate better results [10,11]. In dissimilar welding of Nb-alloys to steels, due to high difference in melting and evaporation points of the two metals and also, intensive recrystallization of niobium at elevated temperatures (particularly in HAZ), it is required to use the beams with concentrated energy which can melt both sides of the joint, simultaneously. Formation of brittle and hard intermetallic compounds between iron and niobium in these joints is very important issue which is one of the most important factors making this dissimilar joint complex and difficult [8,12].

EBW is a high power density welding method widely used in aerospace, automobile, electronic and nuclear industries [13]. Due to very high power density and consequently more depth to wide ratio in weld zone, EBW creates very narrow HAZ and minimum residual stress in the part [14,15]. Moreover, because of high concentration of electron beam, the procedure can be controlled precisely. Presence of vacuum chamber in this technique prevents formation of harmful oxide compounds. In addition, fast cooling rate hinders bulky formation of brittle intermetallic compounds [16,17]. Therefore, this technique is one of the most appropriate techniques for dissimilar welding [18,19].

Budkin et al. [20] used EBW to join 2 mm Nb sheet to 12Cr8Ni10Ti steel. They observed a continuous layer of Fe₂Nb at the interface of contact in weld zone that deteriorated the mechanical properties of the weld due to high brittleness and hardness. Zhao [21] used niobium as an intermediate layer in hot-roll bonding of Ti6Al4V alloy and Cr18Ni10Ti stainless steel. A layer of intermetallic compound between Nb and weld zone was formed, in which several cracks were detected. Vacuum soldering of Nb to 316 L was reported by Abhay Kumar [22]. In the soldered sample, no brittle intermetallic layer formed in either side of the joint, but the joint had a low tensile strength (about 120 MPa). Compared to brazing and solid-state welding, fusion welding is suitable for various joint types and obtains higher strength joints at elevated temperature [23].

Laser welding of Nb to 410 stainless steel associated with the Ni intermediate layer formed by electro-spark deposition (ESD), was performed by Baghjari et al. [24]. They use laser beam to join 1 mm sheet of pure Nb to 410 stainless steel. K. Saito et al. [25] developed the coalescence of stainless steel and niobium through hot isostatic pressure (HIP). A. J. Palmer and C. J. Woolstenhulme [26] studied dissimilar joint of Nb and Mo to stainless steel by the brazing method. They used the eutectic alloy of Au-Ni (Au-18Ni) filler to braze the metals. Sabirov et al. [27] investigated the joint of Nb to S.S. created by explosive welding method. They welded Nb tube to S.S. flange through making controlled explosion inside the Nb tube. After treatment of relieving residual stress on welded parts, 250 MPa shear strength was achieved. This special joint is used for accelerating cavities in superconductors.

Generally, in the context of dissimilar joint of Nb alloys to S.S., regarding very important applications that it has, too limited reports have been published. In most of these reports, it is implied that its application is restricted because of the complexity of methods, using a variety of intermediate layers, low speed of the welding process, very costly steps of the process and also impossibility of applying complex geometries. In this work, for the first time the dissimilar joint of Nb-1Zr to 321 S.S. was made by electron beam welding, without using any intermediate layer and only by adjusting the position of focused electron beam over each side of interface and controlling the weld dilution. Offsetting the beam position changes the contribution of base metals in chemical composition of the weld metal in dissimilar beam welding processes and consequently, results in different mechanical properties, microstructure and weldability of welded joint [28].