Research On Gradient Materials Of Laser Additive Manufacturing Based On Bionic Shell

Natural biomaterials such as shells have both high strength and hardness,as well as high plasticity and toughness.The outer surface of shell has a series of spiral patterns,which are always gradually gradient in the thickness direction.And the structure forms a swirling pattern between adjacent slice,when transition from inner layer to outer layer,that is the reason for its high comprehensive performance.This paper prepared multi-layered gradient models based on natural shells by laser additive manufacturing technology,and 45 steel was selected as the matrix material,three Fe-Cr-Ni alloy powders were used as coating materials with different element contents.The matching principles of expansion coefficients and thermal stress of multi-layered gradient structure materials ware established.Meanwhile,interlayer crystallization rules,microstructure evolutions and wear performances were studied.In this study,we obtain alloy powder matching principles and laser additive manufacturing process parameters of multi-layered gradient structure materials with excellent properties,and the research results have certain technical supports and theoretical guidance significances for laser additive manufacturing of wear resistant materials.The main results are as follows:1.Matching principle of adjacent layer materialsThermal stress matching between adjacent layers of gradient structure should meet the following requirements: This principle is to make high crack resistance material be tensile,and brittle or hard materials be compressed.When 45 steel is used as matrix,material 1 is used as transition layer and material 2 or material 3 is used as second layer in bionic gradient structures,it can make the cladding layer,which adjacent to material 1,generated compressive stress,while material 1 withstood tensile stress.2.Crystallization characteristics of gradient layersContent with alloy elements and angle for adjacent layers affect the crystal morphologies of cladding layers.Among those three alloy powders used in this study,the solidification mode of material 2 and material 3,with low Ni content,is FA,while the solidification mode of material 1,with high Ni content,is AF.As a result,when material 1 is solidified or crystallized,finer grains can be obtained.In addition,when included angle of adjacent layer textures is increased from 30° to 90°,the angle of columnar crystals near interface also increases with the increasing angle of adjacent layer textures.Secondary dendrites grow epitaxial from some columnar crystals below the interface and forming "L" type vertical structure after the angle of adjacent layer textures reaches 90°.3.Microstructures of gradient layersAngles of adjacent layers and numbers of gradient layers by bionic gradient materials affect the cladding layers' microstructures evolution.As material 1,the Ni content is high,reaching 11.41 wt.%,so microstructures are composed of austenite after forming.And a large amount of martensite exists on material 2 and material 3,which have a low nickel content of 4.12 wt.%.When included angle of adjacent layer textures is increased from 30° to 90°,contents of alloy elements near the interface decreases accordingly,such as Cr and Ni,and their distributions are flattened gradually.Meanwhile,number of gradient layers affects the microstructures in surface layers on the gradient structure: After the gradient layer is increased from two layers to three layers,austenite increase and the martensite decrease in low Cr/Ni ratio alloys,while the austenite decreases and the martensite increases accordingly in alloys with high Cr/Ni ratio.4.Hardness and wear resistance of gradient layersThe angles of grain between adjacent layers can change hardness of cladding layers in these bionic gradient structures,and the hardness distributions of cladding layers will be changed by number of layers.Hardness of cladding layers is enhanced as the angle between adjacent layers increased from 30° to 90°.As a result,the phenomenon of dramatic increase in hardness above fusion line of single layer structure is alleviated effectively as the number of gradient layers increases from single layer to three layers.Although hardness of material 1 is lower than material 2 and material 3,but it has higher wear resistance in some cases,and wear resistance of material 1 decreased as the angle between adjacent layers increased from 30° to 90°.We also find that,wear resistance of molded part decreases firstly and then rises as the number of gradient layer increases from one single layer to three layers,in addition,these wear surface metals are composed of material 1.However,the change rule of wear resistances is opposite due to material 1 is replaced by material 2 or material 3.