Ultrasonic Vibration Assisted Severe Plastic Deformation To Refine The Material Grain

Ultrasonic vibration is a kind of high frequency mechanical wave with the frequency larger than20kHz. As a kind of high-intensity energy, the ultrasonic vibration has been widely applied into the metal forming process. This research can be divided into two parts:1) ultrasonic assisted severe plastic deformation to refine material grain and to produce bulk ultrafine-grained (UFG) materials;2) ultrasonic shot peening to produce the nanocrystalline surface.In all the published researches, the ultrasonic vibration was just exerted on the die. With the help of the high frequency vibration of the die, the metal flow and the friction behavior between the surface of the die and the billet were changed. Moreover, it is known that the propagation of the ultrasonic vibration in the solids can increase the dislocation density of the material, decrease the interior friction force and generate heat. Therefore, the high-intensity ultrasonic vibration was applied during the upsetting process, namely, during theupsetting processthe ultrasonic wave propagates in the specimen at the same time.Under the coupling action of the ultrasonic and elastic-plastic deformation, the billets were severely deformed and the grains were refined to the nano-scale. The main researches in this partare shown in the following:(1) The propagation of the ultrasonic vibration in the rod was analyzed by numerical method. The particle displacement and stress equations when the ultrasonic vibration transmits in the rod were obtained, and were solved with the special boundary condition (one side of the rod is fixed and the other side is stress free). Then, the stress distribution along the rod can be got. A3D finite element (FE) model was created to simulate the ultrasonic vibration propagation in the rod. By comparing the results got from the FE simulation and the numerical analysis, the way on how to apply the ultrasonic vibration in the Abaqus software was obtained and verified. Based on this valid FE model, the displacement and stressvariationsof the particles in the bottom surface of the rod were analyzed. When the ultrasonic vibration inject into the rod, it takes effect only if the rod resonated. Therefore, the effects of the outline dimension and interiorhole dimensionof the rod on the self-resonant frequency were also analyzed in this paper.(2) The experiments of ultrasonic vibration assisted upsetting (UAU) and conventional upsetting (CU) of the pure copper cone tip have been carried out. A valid2D FE model was created to analyze the metal flow, forming force and the stress and strain distribution during the UAU and CU process. The deformation mechanism for the UAU and CU process has been completely analyzed.(3) The microstructures of the specimen produced by the UAU and CU processes were observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM), and the grain refinement mechanism during the severe plastic deformation was analyzed. The result shows that, the pure copper grains with the initial grain size of～50μm have been greatly refined to150-300nm after one time UAU process. The microstructures of the specimen produced by UAU and CU process were compared (the height reduction during the upsetting process is the same for both forming processes). The grain just refined to20～30μm for the CU process. This means that the ultrasonic vibration can make the plastic deformation much more severe and improve the grain refinement efficiency. Moreover, base on the TEM test, the effect of the grain boundary on the hardness of the material has been analyzed. Comprehensively considered the effect of the grain size and the grain boundary on the hardness, an equation was obtained to express the material hardness.(3)The experiments of ultrasonic assisted upsetting for the6061Al alloy have been carried out. The main purpose of this research is to investigate the evolution process of the second phase particle Mg2Si during the severe plastic deformation process. Our research shows that the size and hardness of the particle play the critical role on the grain refinement efficiency. If the hardness of the particle is very high, such as the particle of Al2O3, the particle can accelerate the grain refinement because of enhanced dislocation generation and the reduction of the slip distances. However, these particles can be broken into small fragments by the dislocation slip during the severe plastic deformationif they are very soft (such as the Mg2Si). When size of these particles reach to a critical value, the particle can be efficient to pin the grain boundary.The other part of this research is about the ultrasonic shot peening (USP), whichwas applied to strengthen the surface of the low carbon steel1018. The effect of the peening distance, peening time, ball diameter and ball quantity on the thickness of the heavy deformed layer and the surface roughness of the part after the USP process were analyzed. The result shows that with the increasing of the peening time and the decreasing of the peening distance, the deformed layer increases. The relative surface coverage is better to be1S, more or less has a negative effect for USP process. With the increasing of the ball diameter, the deformed layer increased firstly and then decreased. The effect of the peening time, peening distance and ball quantity on the surface roughness is very small. Shot diameter has a significant effect on the surface roughness. With the decreasing of the diameter, the surface roughness will be better. Finally, an improved process was promoted to obtain thicker deformed layer and high surface quality at the same time, which is multi-peening process with the gradient ball diameter.