Wuhan University of Technology

Graduation Project (Dissertation)

Surface Mechanical Properties’ Analysis of Automotive Parts Enhanced by Ultrasonic Impact Treatment

School： School of International Education

Major amp; Class： Automotive Engineering 1301

Student Name： Tian Ye

Mentor Name： Wang Zhou

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Abstract

The fatigue fracture of most metallic materials begins on its surface. The surface properties and structure of the material have a significant effect on the fatigue life of the material. Therefore, it is possible to optimize the surface state of the metal material by increasing the fatigue strength of the metal material. As an efficient surface treatment process, ultrasonic impact treatment (UIT) can produce beneficial compressive residual stress on the material surface, thereby enhancing the fatigue strength of the material and extending the service life of parts. In this dissertation, the basic principle that the ultrasonic impact treatment produces the compressive residual stress layer on the part surface was studied by theoretical calculation and numerical simulation. The influences of the factors such as the diameter of impact needle, the impact velocity, the number of impacts and the coverage on the residual stress field of the 45 steel surface in the UIT process were analyzed in the dissertation. In addition, the advantages and disadvantages of ultrasonic impact treatment and traditional mechanical surface strengthening technology were compared. The research results have some guiding significance for the parameter setting of ultrasonic impact strengthening process, and they also can verify the effectiveness of the finite element model of ultrasonic impact treatment in this dissertation.

In this dissertation, the mechanism of compressive residual stress on the surface of parts strengthened by ultrasonic impact treatment was explained by the theory of Hertz contact and the theory of elastic-plastic mechanics. The calculation formula of residual stress was deduced. Then, the finite element analysis software ABAQUS was used to simulate the process in which 45 steel was strengthened by ultrasonic impact treatment. The residual stress distribution of the material surface was obtained after the simulation. The influences of the factors such as the diameter of the impact needle, the impact velocity, the number of impacts and the coverage on the simulation results were analyzed.

The research results show that compressive residual stress layer could be produced on the surface when 45 steel was treated by ultrasonic impact treatment, which can improve the fatigue strength obviously. And then the service life of parts would be prolonged. The diameter of impact needle, the impact speed, the number of impacts and the coverage will affect the residual stress distribution of the surface after ultrasonic impact treatment. The increase of the impact needle diameter, impact velocity and the number of impacts also can cause the depth of the compressive residual stress layer and the maximum value of the compressive residual stress to increase. The maximum compressive residual stress appeared at a position that is deeper from the pit surface. Furthermore, the increase in coverage has little effect on the location of the compressive residual stress and the location of the maximum compressive residual stress, while it can make the maximum compressive residual stress decrease slightly and the value of the surface compressive residual stress increase significantly. Compared with traditional mechanical shot peening, ultrasonic impact treatment can reduce the surface roughness of the workpiece, which would help improve the mechanical properties of parts whose surface is treated by this technique. Additionally, since various materials have their own strength, hardness and other properties, the effects of ultrasonic impact treatment on them are different, such as the comparison between 45 steel and 50SiMnVB spring steel.

The characteristic of this dissertation is to analyze the influence of the relevant process parameters on the final surface mechanical properties of the material during ultrasonic impact strengthening process. Besides, the advantages and disadvantages of ultrasonic impact treatment and traditional mechanical surface strengthening technology (mainly ordinary shot peening) were compared, and the principle of ultrasonic impact treatment was illustrated in detail.

Key Words: Ultrasonic impact treatment; Residual stress; 45 steel; Finite element simulation; Surface mechanical properties

Contents

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