Design Of Composite Modified Layer On The Surface Of Cu-Al Alloy And Research On Its Microstructure And Properties

Because of its excellent wear and corrosion resistance,as well as good mechanical properties and ductility,aluminum bronze alloy is widely used in the manufacture of large engineering parts.With the rapid development of modern industrial technology and increasingly harsh application environment,many higher and newer requirements have been put forward for its comprehensive performance and service life.In practical applications,the failure of aluminum bronze alloy is often due to its poor surface properties.Although adopting the whole replacement method can solve the above problems,it will inevitably lead to a serious waste of resources and economy.The composite surface modification technology is a feasible and effective way to construct high performance surface modification layer.In this paper,a simple aluminum-bronze Cu-8 wt.%Al alloy was used as the matrix to prepare the surface modified layer by two techniques,namely surface nanocrystallization+plasma nitriding and laser cladding+plasma nitriding.Besides,through the fine characterization of the structure and the results of the comprehensive evaluation of the properties,the evolution of the tissue properties and the relationship between the composition-structure-properties were analyzed.This will lay the theoretical and practical foundation for the design and development of surface coatings for high performance metallic materials to meet the practical application requirements.（1）This paper describes the effects of ultrasonic shot peening on the microstructure and properties of Cu-Al alloy surface modified layer.The result shows that the phase structure of the alloy surface is not changed by ultrasonic shot peening,but the gradient nanocrystalline layer is formed on the surface of the alloy,and the surface grains are refined obviously.The nanocrystallization mechanism is presented in two ways,which are dislocation movement and mechanical twinning.The early stage is dominated by dislocation motion,while the later stage is dominated by mechanical twinning Ultrasonic shot peening can greatly improve the mechanical properties,tribological properties and corrosion properties of the alloy.When the shot peening time is 15 min,the modification effect is the best,where the hardness is as high as 254.1 HV_0.2,2.4 times of the hardness of the matrix,and the average friction coefficient is only 0.231.Volume wear and volume wear rate were 4.434×10⁷μm³and0.1848μm²/N,respectively.The strengthening mechanism of nano-sized samples is grain refinement and work hardening,and the wear mechanism is abrasive wear and slight oxidation wear.The corrosion performance of nano-sized samples is mainly improved by the formation of dense passivation film on the surface.（2）This paper also studied the effects of ultrasonic shot peening and plasma nitriding on the microstructure and properties of Cu-Al alloys.It was found that no obvious nitride was found in the surface layer after the composite modification.However,the high temperature nitriding environment would destroy the gradient nanocrystalline structure of the surface obtained by ultrasonic shot peening so that the surface grains would grow rapidly and the comprehensive properties of the alloy would return to the matrix level.The nitriding at low temperature（300℃）can better maintain the fine crystal structure of the surface layer that makes the comprehensive properties of the alloy slightly better than that of a single ultrasonic shot peening.It reflects that the surface nanocrystallization has a good potential in reducing the nitriding temperature,increasing the short-circuit diffusion path of nitrogen atoms,and improving the comprehensive properties of the surface.（3）Based on the thermodynamic calculation results of Cu-Al,Cu-Y,Al-Y and Cu-Al-Y systems,the effects of preset powder type and laser cladding power on the microstructure and properties of the surface cladding layer of Cu-Al alloy were analyzed.It was found that the addition of rare earth element Y would change the phase structure of the cladding layer.The cladding layer is mainly composed of Cu Al₂and Cu₉Al₄intermetallic compounds,which change into Al₆Cu₆Y and Cu₉Al₄with a small amount of Cu Al₂.With suitable laser cladding power,thicker cladding layers with good bonding and flat surface can be obtained.The addition of element Y can further increase the thickness and improve the quality of cladding layer.When the laser cladding power is 200 W,the cladding layer quality is the best.In this case,the thickness of Al（Y）cladding layer is about 2 times that of Al cladding layer,the hardness is 160 HV_0.2higher than that of Al cladding layer,and the hardness is 5.5times that of the matrix.The volume wear and volume wear of Al cladding layer are 1and 2 orders of magnitude lower than that of Al cladding layer,respectively.（4）Finally,the paper discussed the effects of composite treatment technology and rare earth element Y on the phase structure,morphology,microstructure and comprehensive properties of the modified layer and the formation and evolution of the microstructure of the composite modified layer was analyzed.Firstly,it calculated the first principles of the dissolution and diffusion of nitrogen atoms.Next,the cladding layers was regarded as the intermediate transition layer,and the composite modified layer was prepared on the surface of Cu-Al alloy by low temperature plasma nitriding technology.The result shows that the addition of rare earth Y makes the dissolution and diffusion of N atoms easier.The composite modified layer composed of rich Al N→rich Cu₉Al₄→rich Al₆Cu₆Y can be formed on the surface of the Cu-Al alloy by the composite modification technology.The layer is closely bonded with the substrate,and the microstructure is uniform,without obvious defects such as cracks and holes.The thickness of the modified layer increases from 37.4μm to 52.4μm with the addition of Y element.In addition,Y element promotes the nitriding process,increases the concentration of nitriding products,and refines the grain of surface modified layer obviously.The comparison of properties of the modified layer proves that the alloy treated by Al（Y）cladding and nitrizing has the best comprehensive properties,where the hardness is as high as 748 HV_0.2,7.1 times than that of the matrix.The volume wear decreases from 9.285×10⁷μm³to 1.454×10⁶μm³,the volume wear rate decreases from 0.3869μm²/N to 0.0061μm²/N.It proves that laser cladding+nitrizing and Y element addition are an effective way to achieve high hardness,good wear resistance and excellent corrosion resistance on the surface of Cu-Al alloy.