Effect of severe plastic deformation and heat treatment on the structural, mechanical and tribological properties of Cu-9Al-2Mn bronze

Abstract

Introduction. Severe plastic deformation (SPD) is one of the most promising methods for modifying the structure of non-ferrous alloys, enabling them to be transformed into ultrafine-grained (UFG) and nanocrystalline states. At the same time, for the practical implementation of such materials in machine-building equipment components, the issues of their thermal stability and wear resistance under extreme loads during contact interaction in scratching and sliding friction remain critically important. The purpose of the work is a comprehensive study of the effect of various thermal and severe plastic deformation conditions on the structural, mechanical and tribological characteristics of Cu-9Al-2Mn bronze. The paper studies samples of Cu-9Al-2Mn bronze in different initial states: annealed at 700°C, quenched from 900°C and aged at 400°C. To form a UFG structure and enhance strength properties, the workpieces were subjected to deformation processing by multi-axial forging at room temperature followed by rolling. In order to study recrystallization processes and the thermal stability of the UFG alloy, annealing was performed at 400°C for 30 minutes and at 500°C for 10 minutes. Materials and methods. Metallography, transmission electron microscopy and X-ray diffraction analysis were used as the primary methods for characterizing the structure and phase composition. Mechanical properties were evaluated based on Vickers microhardness measurements and static tensile tests. Resistance to localized surface damage was determined by scratch testing under a linearly increasing load from 0.5 to 30 N. Wear resistance and coefficient of friction were investigated under dry sliding friction using a pin-on-disc scheme (counterbody — steel 52100) under a load of 20 N and at a constant speed of 0.1 m/s. Monitoring of the tribosystem dynamics was carried out using vibrometry and recording acoustic emission (AE) signals. The surface topography of wear tracks and balls was studied by confocal laser scanning microscopy. The change in structure beneath the friction surface was also investigated using metallography. Results and discussion. The effect of heat treatment, severe plastic deformation (SPD), and subsequent annealing on the structure, mechanical, and tribological properties of Cu-9Al-2Mn bronze is studied. It is shown that SPD (multi axial forging and rolling) produces an ultrafine grained (100–300 nm) structure with high dislocation density and deformation martensite, resulting in a yield strength of 998 MPa and a microhardness of 3.24 GPa. Low temperature annealing at 400°C further increases the yield strength to 1,095 MPa owing to polygonization and precipitation hardening. Annealing at 500°C induces recrystallization, partially restoring ductility. Tribological tests under dry sliding friction revealed that SPD significantly enhances wear resistance while maintaining an unchanged coefficient of friction. Scratch depth in scratch testing is reduced by ~40%, and the damage mechanism changes from ductile ploughing to microcutting. Acoustic emission and vibration correlate with the change in wear mechanisms.