Microstructural evolution and bonding mechanisms during multi-pass cold welding of high-strength precipitation-hardened alloys

Joining high-strength precipitation-hardened alloys without degrading their properties poses a significant challenge. This study investigates multi-pass butt cold welding as a heat-free joining solution, using a high-strength (nearly 600 MPa) Cu-Cr-Zr alloy as a model system to explore the underlying microstructural evolution and bonding mechanisms, aiming to provide insights of generic value. High-integrity joints were achieved after three or more welding passes, exhibiting exceptional tensile strength (average 573 MPa, about 96 % of base material) while retaining high electrical conductivity (about 80 percent International Annealed Copper Standard, %IACS). Systematic analysis reveals that severe localized plastic deformation during welding drives significant microstructural changes: the initial about 10 μm fibrous grains refine into nanocrystalline (sub-micron) equiaxed grains at the joint center. Crucially, this severe deformation also induces partial mechano-chemical dissolution of the strengthening Cr precipitates even at room temperature, highlighting a key aspect of phase stability under severe plastic deformation (SPD). The study demonstrates that the substantial strengthening from nanocrystallization effectively compensates for the softening caused by precipitate dissolution, enabling high joint efficiency. The bonding mechanism is shown to be dynamic, initiating via surface film rupture, followed by extensive plastic flow that expels contaminants and consolidates the interface over multiple passes. These findings provide fundamental insights into the interplay between extreme strain, grain refinement, and phase stability in precipitation-hardened alloys under cold welding conditions, highlighting the process's potential for joining advanced materials where thermal effects are detrimental.