Investigation Of The Micro-structrues, Properties And Process Reliability Of Elastic Burnishing Surface Of Aluminum Alloy

The investigations showed that the major failure modes of the large-scale structures such as the aircraft or other huge structures were fatigue failure and corrosion failure, those were the key factors restricted the structural safety and life extension. Thereby increasing the fatigue and corrosion resistances of the critical structures are necessary. It has been known that these properties are-closely related to the surface properties of the structures. The surface properties of the key components directly determine the structural safety and life extension of them, thus the optimizing and improving the surface properties of these critical components are urgently needed. The traditional methods for surface forming, such as precision grinding, are of low productivity, high production costs, and will leave marks on the surface of the workpiece, thus hard to obtain the ideal surface quality and the surface properties. The burnishing technique can solve the above problems perfectly, such as can improving the surface quality and the surface performance of the components, and reducing the costs. During the burnishing process, which is a non-chip processing methods, the high pressure of the hard head imposes on the burnished surface to produce a continuous plastic deformation of the metal surface, and the rough surface morphology is smoothed. In order to extent the application of this surface treatment technology so as to improve the surface quality of more key components, optimization of the burnishing parameters and deep understanding of the mechanisms and characteristics of burnished surface properties and micro-structures is necessary. The study in this paper is of important theoretical and practical significance for understanding the performance and the formation mechanism of the burnished surface, finding out the regularity of burnishing process and better optimizing the burnishing parameters.Experimental and theoretical studies of elastic precision burnishing were carried out in this paper. The mechanism of the burnishing process, the domestic and overseas application background and research status of burnishing were summarized in this study. Intensively and systematic investigation of burnishing techniquehas been made. Elastic precision burnishing equipment, parameters of which were controllable, was designed and fabricatesd. Aluminum alloy specimens were elastic burnished with this equipment on different elastic burnishing parameters. The regularity and formation mechanism of the surface roughness, residual stress, micro-structures, micro-hardness and nano-hardness were studied on the burnished surface.The relationship between surface roughness, residual stress and burnishing force were established in this paper, and discussion of the formation mechanisms of the surface roughness and residual stress during burnishing was carried out. It was found that the increasing in the normal elastic burnishing depth, burnishing speed resulted in lower surface roughness, however, the increase of the burnishing feed led to higher surface roughness.Elastic burnishing parameters such as spring stiffness and preload also affect the surface roughness. In general, the effects of these parameters on the surface roughness can be attributed to the influence of the burnishing force. The experimental results showed that burnishing force has the inversely linear relationship with the surface roughness.Meanwhile, it is found that the surface residual stress would also be affected by the normal elastic burnishing depth, burnishing feed, and some other elastic burnishing parameters. These effects can also be attributed to the differences in the burnishing force. Based on these results, the model of the burnishing force during the burnishing process was proposed. This study also analyzed the relationship between the burnishing force and the residual stress, and the effects of the burnishing parameters on these two properties. The experimental results showed that when the normal elastic burnishing depth reached a critical value, which meant the contact area of the burnishing head and the burnished surface were nearly constant. The burnishing force would enter the platform as the increase of the burnishing depth. At this critical burnishing depth the optimization surface residual stress and surface roughness were obtained. Finally, the calculation model of residual stress also showed that the increase of the burnishing force can improve the surface residual stress.In this article deep investigation of the characteristics of the mirco-structrures of burnished surface and subsurface was carried out. It was found that the nano-scale grains of about 50nm and microscopic texture revealed by the two-dimensional surface scanning XRD analysis coexist on the burnished surfaces; the formation mechanisms of these two characteristics were preliminary discussed. The surface properties were characterized both by micro-hardness and nano-hardness in this paper. The results showed that the micro-hardness and nano-hardness of burnished surface were increased compared to unburnished surface of components, and were influenced by traditional burnishing parameters such as normal burnishing depth and burnishing feed, and also influenced by elastic burnishing parameters such as spring stiffness and preload. However, the nanohardness seemed to be more sensitive to the micro-changes of the deformation layer with a limited thickness and the effects of the burnishing parameters, thus more appropriate for characterizing the burnishing surface.Finally, this study established the reliability model for burnishing system, which could be used to quantitative assessment of the quality and the reliability of the burnished surface. This study also compared elastic burnishing with inelastic burnishing process. The results showed that elastic burnishing process was more reliable, and the reliability model could be used to assess the reliability of other machining processes.After all, this study investigated the characteristics of the elastic burnishing surface, which including parameters controlling, regularity of the properties, characterization methods, burnishing mechanisms and reliability assessment. The investigation in this paper further improved the understancing of the forming of elastic burnished surface, and provided the foundation for the burnishing parameters optimization and surface properties improvement.