Study On Preparation Of Surface Functional Structures On Metal Materials By Laser Pulses And The Effects On Their Properties

With the development of laser technique and the beam control technique,rapid advancements have been achieved in preparation of functional surface structures on metal materials.Facing the more industrial applications,it requires faster efficiency and higher accuracy to process functional surface structures.However,the traditional processing methods,namely chemical corrosion and sandblasting,etc.,have been challenged with technical bottlenecks such as poor consistency,complex processes,environmental pollutions and personnel harmness.As a new method for functional structure preparation,laser surface treatment has the advantages of green environmental protection,low cost,non-contact,and low mechanical load.Compared with traditional methods,it seems potential and promising in industrial precision manufacturing,In recent years,laser-induced surface functional structures on metal materials have become important in the field of aviation,aerospace and vehicles.As the laser processing parameters are esay to control,substrate properties including morphology and hardness can be precisely modulated.As a result,laser processing would further broaden the application scope of metal materials by preparation of functional surface structures.By performing surface modifications,the wettability of metal materials is controlled in order to improve the surface energy and bonding strength.By etching the grating structures,optical properties are changed with decreasing reflectivity and increasing absorptivity,which upgrades the corrosion-resistance of the materials.Through laser irradiation,the hardness and wear durability of the substrate is promoted significantly.Laser-induced surface functional structures is applied to typical metal materials,and our theoretical and experiment results are satisfactory for industrial manufacturing applications.The main contents are shown as belows:Firstly,the research on laser surface machining of micro-nano structures on metal materials has been carried out.According to our laser-metal temperature field model,the temperature field distribution of aluminum alloy during laser processing is studied,and the wetting properties of metal materials is analyzed with the surface wetting theory.And then,the effects of laser parameters,including beam energy density,laser frequency,scanning speed and spot spacing,on the morphology,wettability and chemical composition of the metal surface are vertified experimentally.On this basis,the superhydrophilic surface is obtained by changing the laser spot spacing.When the spot spacing become smaller than the size of the laser spot,multiple laser pulses are used to induce micro-nano composite structures on metal surface and increase the contact area between the metal and the adhesive,which is helpful to improve its bonding strength.Secondly,the mechanism and the control method of metal surface coloring has been described.Based on the theoretical analysis and experimental methods for the preparation of micro-nano composite structures,a grating structure is machined on the metal surface and made metal coloring.In order to study the mechanisms of laser coloring,a finite element model for thermal calculation is established,and the temperature field distribution for laser processing is investigated with different laser parameters.With help of theoretical analysis,coloring control method for metal surface is proposed and various colors can be obtained by adjusting single laser parameter.We experimental obtained colorful images on titanium alloy surface by combining different colors,which is promising in the application of metal coloring.Thirdly,the effects of laser machining on physical properties of metal substrate has been studied.In different processing environments such as air and water-spray,laser ablation experiments are comducted and the effects on the metal properties are examined with surface morphology,metallographic structure and micro-Vickers hardness.Based on our finite element model simulating the temperature field,a method by changing the convective heat transfer coefficient is proposed to control the sample surface temperature.In atmospheric environment,the substrate temperature during laser processing is too high due to heat accumulation,which increases the hardness of the substrate and changes the metallographic structure significantly into three zones: near-surface hardening zone,heataffected zone and matrix zone.In water-spray environment,its physical properties,including hardness and metallographic structure,are the same as the original substrate,which shows that laser surface machining provides an alternative method for metal valve cleaning.In this thesis,the preparation of surface structures on metal matrials by means of laser machining and the effects on its properties are investigated systematically.The mechanism of the laser-induced surface micro-nano structures is explained,and the methods to control the wettability,optical properties,hardness and metallographic structure of the substrate are proposed by laser surface modifications.Furtheremore,our theoretical and experiment results for preparing surface functional structures have been applied to industrial sophisticated products,such as pre-treatment before engine bonding,coloring of precision components and degumming of aviation valves,etc.The feasibility of the technology is verified,which lays a solid foundation for the promotion and application of the preparation of metal surface functional structures.