Effect of Temperature on Dry Sliding Tribological Behavior of 7A04 Pin - 50CrMo4 Disc Contact Pair

Based on the problems of mold wear in 7xxx aluminum alloy warm forming, the effects of temperature on dry sliding tribological behavior of 7A04 aluminum alloy pin - 50CrMo4 low alloy steel disc friction pair were investigated. The worn surface morphology, subsurface microstructure and wear debris were characterized by the optical microscope, 3D optical profiler and scanning electron microscope, etc. The results showed that the friction coefficient decreased and then increased as the temperature increased. The wear rate of the 7A04 aluminum alloy pin decreased first and then increased and the main wear mechanism was the abrasive wear. In the process of dry sliding wear, the mechanically mixed layer was observed in the worn subsurface of the 7A04 aluminum pin at 125 ℃. In addition, there was an obvious oxidation phenomenon in the mechanically mixed layer, which made the structure of mechanically mixed layer looser. The oxidation was also one of the reasons for the increase in the wear rate of 7A04 aluminum pins. At 225 ℃, the further increasing of the mechanically mixed layer thickness led to the wear rate increasing sharply. The subsurface plasticity of the 7A04 aluminum pin increased slightly due to the deformation and dynamic recrystallization, which also led to increase wear debris size. The wear rate of 50CrMo4 steel increased dramatically first and then decreased to the minimum at 225 ℃, but there was a significant material transfer phenomenon at all temperatures invariably. During the wear process, the 7A04 aluminum pin separated a large amount of wear debris, which was inserted on the 50CrMo4 disc surface under the vertical load. In the process of dry sliding wear, plastically deformed region and dynamic recrystallization zone generate in the worn subsurface of 50CrMo4 steel disc. With the increase of test temperature, the thickness of the deformed grain zone and dynamic recrystallization zone increased, and the appearance of fine equiaxed grains of dynamic recrystallization enhance the plasticity of the worn surface, which led to the transformation of wear mechanism. The thickness of the dynamic recrystallization layer increased to the peak at 225 ℃, which led to the decrease of hardness and the increase of plasticity of the 50CrMo4 disc and accelerated the generation of spiral chips. The wear mechanism of 50CrMo4 steel was the mixture of abrasion, severe adhesion and delamination at room temperature, and it changed into slight abrasion and general adhesion as temperature increased from 125 ℃ to 175 ℃. With the increase in temperature, the bonding strength between two materials decreased, and the delamination debris containing two materials disappeared while the 7A04 aluminum debris was still manifested as large pieces of delamination debris. The changes of the mechanically mixed layer oxidation and plastic deformation layer on the worn surface of 7A04 aluminum pin as well as the dynamic recrystallization on the worn surface of 50CrMo4 steel determine that the two materials had different wear rates and wear mechanisms at different temperatures. Oxidated mechanically mixed layer was formed at 125 ℃, which led to the increase of aluminum surface hardness, the decrease of 7A04 pin wear rate and the increase of 50CrMo4 disc wear rate. With the further increase of temperature, the oxidation of the mechanically mixed layer enhanced obviously, and the structure was looser and easier to delamination. Therefore, the 7A04 pin wear rate increased and the 50CrMo4 disc wear rate decreased. © 2023 Science Press. All rights reserved.