Study Of Surface Mechanical Properties Improvement Based On Coupling Bionic And Secondary Indentation

Surface mechanical properties of mechanical product materials are directly related to their service performance and life under service conditions.The cause of surface mechanical properties decline phenomena are complicated and difficult to predict effectively,like stress-induced surface wear and fatigue failure.Improving and enhancing the surface mechanical properties of products is an objective requirement of the continuous development and progress of modern machinery industry.At the same time,traditional mechanical processing and surface embossing processing?such as nanoimprint processing,bionic functional surface preparation,etc.?may cause pile-up phenomenon of material surface and residual stress accumulation effect,thereby further aggravating the decline of surface mechanical properties.Therefore,promoting surface mechanical properties of material is a direct way to improve the working surface in mechanical contact.Based on its unique morphology,microstructure and heterogeneous material properties,natural organisms exhibit excellent surface mechanical properties,such as wear resistance,drag reduction,crack arrest,fatigue resistance.This provides a direct reference for improving mechanical properties of working surface in contact through bionic design and preparation of mechanical products surface.In this paper,by selecting shell and woodpecker beak as biological models,based on the ternary coupling bionic design principle of structure,material and morphology,the porous structure,surface coating material and secondary indentation morphology elements were constructed.Porous structure,heterogeneous material and non-smooth pit shape were selected to carry out bionic design and preparation of magnesium alloy-based aluminum coating bionic material.The nano-indentation and scratch test were used to study wear resistance of material.At the same time,a method of surface planarization and residual stress relief based on secondary indentation was proposed,and the feasibility of this method was verified by finite simulation analysis and experimental characterization.The main research contents of this artical involved the following aspects:?1?Preparation and surface mechanical properties study of bionic coupling sampleBased on the principle and method of multi-element coupling bionic,using shell and woodpecker beak with excellent wear resistance as biological models,porous magnesium alloy substrate was processed.And then functional coating on magnesium alloy substrate was manufactured by using magnetron sputtering technology when aviation aluminum alloy was target material.The binary bionic coupling sample was produced.On this basis,the nano-indentation/scratch responses and surface mechanical parameters?hardness,elastic modulus?of unary?solid magnesium-based aluminum alloy coating?and binary?porous magnesium-based aluminum alloy coating?bionic coupling samples were tested,and the effect of different coupling factors on the mechanical behavior of bionic coupling samples was analyzed.By means of Vickers indentation technology,micrometer-scale concave pit?ternary morphology?was machined.Regarding residual stress accumulation and derived pile-up phenomenon when using press-in processing technology to prepare pit shape coupling element,a secondary indentation method was proposed to achieve the further planarization and residual stress release.The procedures included using a Vickers indenter for initial indentation to create a pit shape coupling element?first-order indentation?,and using a cube-corner indenter to create nano-scale secondary indentation on already formed wedge-shaped surface of the pit.Based on this,the quantitative effect of coupling bionic and secondary indentation on surface mechanical properties of materials was studied.?2?Finite element analysis of secondary indentation on coating surface.Finite element analysis software ABAQUS was used to build secondary indentation simulation model of aluminum alloy material.The effective improvement of surface mechanical properties by secondary indentation method was verified by analyzing stress distribution,stress data of node path and internal energy data of element set after first and second indentation.The analysis results confirmed that secondary indentation induced significant residual stress release and internal energy dissipation on surface,and the correlation between the depth and position of secondary indentation and the residual stress and internal energy release value was established.A 33.03%stress reduction and an internal energy reduction of 6.044×10^-8 J were obtained by 500 nm cube-corner indentation and 4.00mm transverse distance between the indentation position and the sample center.?3?Experimental study on surface planarization and residual stress relief by secondary indentation.Based on the proposed secondary indentation method,the first-level Vickers indentation was prepared by experimental means,and the secondary indentation was prepared on wedge-shaped surface of already formed first-level Vickers indentation by cube-corner indenter.The indentation experiment showed that the residual morphology of secondary indentation was inside the projection of Vickers indentation's morphology,which was adjacent to the Vickers indentation edge.By selecting the symmetry plane perpendicular to the edge of the Vickers indentation as the reference path,combined with the atomic force microscope?AFM?characterization analysis of initiation indentation and secondary indentation edges,the significant pile-up reduction with 368.7±46 nm induced by the secondary indentation was verified.At the same time,a significant reduction in crystallinity and residual stress relief,which was revealed by X-ray microdiffraction?micro-XRD?.Secondary indentation method can release residual stress and promote surface planarization was verified by further experiments.