Solid state welding of medium-entropy CrCoNi with heterogeneous, partially recrystallized microstructures

Abstract

Heterogeneous, partially recrystallized (PRX) microstructures have recently been used to improve strength-ductility combinations in high-entropy alloys. However, these microstructures are incompatible with conventional joining processes that require melting or prolonged exposure to elevated temperatures. This work presents an initial exploration of solid state joining in this challenging condition using vaporizing foil actuator welding (VFAW) applied to PRX equiatomic alloy CrCoNi.

Introduction

Single-phase high- and medium-entropy alloys (HEAs, MEAs) such as the equiatomic CrMnFeCoNi or CrCoNi systems have been recognized for their outstanding ductility, fracture toughness, and high work hardening rates [[1], [2], [3]], but recent work has focused on enhancing their modest yield strengths through thermomechanical processing. Cold-rolling and annealing to produce a partially recrystallized (PRX) microstructure has been shown to enhance the yield strength while preserving acceptable ductility in CrCoNi [4,5], CrMnFeCoNi [6], and (Cr,Co)-rich commercial Ni-base alloy Inconel 740H [7]. High work hardening rates and the success of the PRX strategy are apparently the product of low stacking fault energies, which promote deformation twinning and in some cases deformation-induced phase transformations.

Despite these encouraging improvements in yield strength, the PRX alloys are likely to be incompatible with conventional joining techniques. High yield strengths in these alloys depend intimately on microstructural features generated during prior deformation (e.g., deformation twins, twin/ε-martensite lamellae, and dislocation locks). Exposure to elevated temperatures results in recrystallization of the material, which can begin above approximately 580 °C for heavily cold-rolled CrCoNi [5]. In conventional welding processes, recrystallization in the heat-affected zone and full melting and re-solidification of the weld metal eliminates the carefully engineered PRX microstructures described in recent publications. It is imperative to examine alternative joining techniques if these ultra-high strength materials will be considered for industrial applications. To the authors’ knowledge, no studies have been performed to assess whether PRX microstructures can be retained after joining.

Significant progress has recently been made in developing solid-state joining methods that dramatically reduce thermal input and do not require melting of the base components. Although friction stir welding is the most prominent example [8], the vaporizing foil actuator welding (VFAW) method has also been used to join strong, dissimilar alloys with heat-sensitive microstructures [[9], [10], [11]]. VFAW forms metallurgical bonds using high-velocity impacts between workpieces and imparts substantial deformation and jetting off the nascent surface near the bond line. In addition, such shocks can be used for surface modification, similar to laser shock, or shot peening. For example, the vaporizing foil actuator technique was used to develop a shock wave to introduce extreme twinning and surface hardening in austenitic stainless steel 316L [12]. The potential benefits of the technique for PRX alloys are clear: not only is heat input minimal, but additional deformation imparted at the bond line may make the weld stronger than the PRX base metal. VFAW is typically used with foil or thin sheet flyer plates up to 10 mm which can be joined to targets of arbitrary thickness. Vaporizing Foil Actuators (VFA's) can also be used to generate impulse for numerous other manufacturing techniques such as tube and flange welding, as well as forming, shearing, and embossing of thin sheets [9].

This work explores the use of VFAW to join partially recrystallized, equiatomic CrCoNi. The microstructure and tensile properties of this condition have previously been reported [5], and it exhibits outstanding combinations of tensile yield strength and ductility. In this study, the amenability of PRX microstructures to undergo solid-state joining is assessed for the first time.