The Experimental Study Of Ultrasonic Shot Peening Based On The Composite Horn With Large Amplitude Ratio

Many high-stress components, such as axles of trains, gears, shafts of cars and aircrafts, etc, are often subjected to the cyclic load at work. In this case, the speed of the crack propagation is very fast, which rapidly shortens the life of parts. It will not only lead to property lost, but also engender casualties, therefore to improve the fatigue strength and the life-span of the machine parts is significant. Shot peening, as a cold processing technique of metal surface, is mainly used to enhance the fatigue strength of metal parts and has been widely used in such demanding industries on reliability and security as aerospace, automotive, electric power and nuclear industry. Shot peening strengthen process leads metal surface to engender plastic deformation, grain refinement, local extension outward and work hardening by using small and high hardness pellets to impact the metal surface of parts at high speed, and then metal surface will generate residual compressive stress layer of 120~1000μm. Deformation strengthening and structure strengthening of shot peening can increase HV hardness of parts, hinder crack propagation and slow down the speed of crack propagation. Shot peening strengthen and higher residual compressive stress are able to stop surface micro-crack, eliminate defects of parts appeared in process and improve the reliability and the life-span of parts, which can slow or prevent the premature failure of metal parts under cycle load.Ultrasonic shot peening can get deeper compressive stress layer than the traditional shot peening, and the surface roughness is also superior to the traditional shot peening strengthen technique. Ultrasonic shot peening has been proved to be able to enhance the fatigue resistance ability. For the current ultrasonic shot penning process, the speed of shot is so small that can't damage the stability of parts. Therefore, enhancing the speed of shot namely improving the magnification factor of the horn is the primary mission under the current development of ultrasonic shot peening equipment. At present, the common horns are exponential horn, conical horn, stepped horn and etc. Different kinds of horns have its own advantages, but none of them is perfect.In this paper, to enhance the amplitude magnification ratio of horn is the fundamental purpose. The design of composite horn is based on analytical method, and finite element software ANSYS is used for the dynamic analysis of the ultrasonic horn to verify reliability of it, while experiment is designed to verify amplitude ability. In addition, shot peening equipment and experiment are designed to bear out the ability of ultrasonic shot peening. The main contents are as follows:1.After discussing theoretically the amplification performance of composite horn, it is known that the amplitude ratio of composite horn is amplification ratio product of two single-horns. The driven horn section and output horn section of composite horn are selected according to the design theory of composite horn and the performance characteristics of single horn.2.Firstly the composite horn, used in experiments of ultrasonic shot peening, is designed according to the theory of single horn and conditions of experiments, and its the parameters are analyzed and calculated. Secondly, according to vibration theory and finite element method, modal analysis for three kinds of horns is carried through by using the finite element software ANSYS11.0. Comparing the modal dynamic parameters and the theory calculation result, it can be concluded that the both results are consistant.3.After the composite horn produced, a set of apparatus for experimental measurements of amplitude ratio is built. Two piezoelectric accelerometers, which are sticked to both ends of the measured composite horn, are used to measure the amplitudes of accelerations in both ends of the composite horn; the signals are amplified by a charge amplifier, then collected and recorded by digital oscillograph. Based on the values of accelerations, the amplitudes of displacement are easily calculated and the amplification factor of the composite horn can be obtained. Comparing the theory calculation result, the finite element analysis and the experimental measuring results, it can be elicited that the finite element analysis and the experimental measuring results both are close.4.Purchase experiment shots of different materials and models, and design experiment device for ultrasonic shot peening. Then the measuring system of ultrasonic shot peening impact force is established. The piezoelectricity force transducer is mounted on the center surface of the metal part to be used to measure the shot impact force, and then dates of impact force are analyzed. The result shows that the effect of shot material on impact force is largest; when the shot material is different, the effect of shot diameter and its distance to sensor is largest, while the effect of output power of ultrasonic generator is lager; the effect of quantity of shots is not great.5.The contact stress of shot peening impact force under different parameters is calculated and the effect of different parameters on contact stress and shot peening strengthen ability is analyzed. The conclusions are as follows: as the height increases, the contact stress of pellet reduces; while in same height, the contact stress of cast steel shots, whose diameter is the same as stainless steel shot, is lager than that of stainless steel shot.