Study On The Design,Preparation And Mechanical Properties Of Al Based Lattice Materials

Lattice truss materials are a new ordered cellular material with unit cells consisting of spatial strut truss structure.Metallic lattice materials possess excellent mechanical and functional properties,which makes them promising candidates in extensive industries including aerospace,transportation,weapons and electronic devices and so on.Directed against the potential applications in aerospace as well as the shortages of structure designs,fabrication technologies and property studies in metallic lattice materials,studies have been carried out on the preparation method,mechanical properties,deformation behavior,constitutive characteristics and related mechanisms for A1 based lattice materials through computer aided design,3-D printing combined with pressure infiltration,quasi-static compression tests and microstructure observation.The main results are as follows:1.Methods were programmed for the design of lattice structures using CATIA(Computer Aided Tri-Dimensional Interface Application)software,and different pyramid and tetrahedral type unit cells and their topological structures were designed by the methods.In terms of the principles of 3-D printing technology(selective laser sintering and UV laser curing),the raw non-metallic printing materials including polypropylene,nylon and photosensitive resin and printing technologies were tested and optimized.It is shown that,by use of photosensitive resin,not only is the high printing speed,but also the printed patterns have sound microstructures,good surface finish and high mechanical properties,being beneficial for the subsequent preparation of Al based lattice materials and investigations of properties of the materials.2.Since the strut diameter and length was relatively small,as well as there were many truss joints in the lattice structures,the preparation of plaster molds and the filling of molten aluminum in the molds were difficult.Accordingly,investigations were carried out into the pressure infiltration technologies including the composition of plaster slurry,changes in the microstructures and physical properties of plaster slurry during heat treatments,high-temperature strength,collapsibility after the infiltration and control of the infiltration pressure etc.By these studies,an optimized plaster composition and heat treatment technology were obtained that guarantee the plaster slurry and corresponding molds to have better flow-ability,thermal physical properties,mechanical properties and solubility.In addition,an optimized technology was yielded through studies on the effects of pre-heating temperature of plaster mold,the infiltrating temperature and pressure on the flow of molten aluminum in the porous net.Finally,high quality and multi-layered Al based lattice materials were produced with different configurations and structures,the geometric accuracy of which were in good agreement with the original 3-D printed patterns.3.Based on the load bearing features and application conditions,studies have been conducted on the quasi-static compression behavior and the dependence on the lattice configuration and main structural parameters of Al based lattice materials including the cross sectional shape,length and diameter of struts,inclination angle and relative density of lattice structures.The results exhibit that,similar to other porous materials,there were also three regions in the stress-strain curves,i.e.elastic,plateau and densification regions.However,the wideness and height of each region had a nonlinear relation with the relative density of A1 based lattice materials,while had a close relation with the inclination angle between the struts and the base plane of cells,being different from those of common porous materials.For example,that the relative density was increased about one time leads to seven times increased in the flow stress.When the inclination angle was 45°,the plateau stress was around 7MPa,but when the angle was increased to 70°,the plateau stress was increased to around 15MPa.At the similar inclination angle and relative density,the length-diameter ratio of struts for the tetrahedral type lattice materials was smaller than that of pyramid type,and thus the mechanical properties and energy absorption capacities of the former were better than those of the latter.However,as the length-diameter ratio was further decreased.the energy absorption efficiency of tetrahedral type lattice materials will decay although the flow stress,strength and energy capacity were progressively increased.4.In order to find the way to enhance the Al based lattice materials,efforts have been made by examination of different lattice materials with varied matrices and cross sectional shapes of struts.It is found that,compared with commercially pure aluminum,use of high strength 6063 and 6066 aluminum alloys as the matrices made the flow stress of lattice materials be increased pronouncedly,in which the 6066 aluminum alloy had more obvious strengthening effect and the compression strength was increased to about 50MPa when 6066 aluminum alloy was used.Studies on the circular,half circular and U-type struts show that the U-type struts are most beneficial for increasing the elastic modulus and compression strength of lattice materials.For instance,when other parameters were the same,the compression strength of U-type strut lattice materials were two times higher than that of circular strut materials.The reason would be that the U-type struts have the greatest moment of inertia among the three cross sectional shapes,whilst the compression strength of lattice materials is proportional to the moment of inertia of struts.5.Aimed to disclose the physical mechanisms related to the mechanical behavior of Al based lattice materials,both the macroscopic and microscopic deformation modes and evolution of deformed microstructures were examined.It is found that the macroscopic compression deformation was mainly bending and folding of struts without obvious fracturing.The overall deformation mode was dependent on the inclination angle.When the angle was relatively small,for instance 45°.all the struts were bended and then folded at almost the same time and the def'ormed structures were uniformly distributed throughout the whole section.However,as the angle was increased to 70°,the bending of struts took place only in a localized zone and then propagated to other zones as the compression continued until the densification occurred.The two deformation modes were in accordance with the different stress-strain behaviors,that is,the former had a smooth transition between the elastic and plateau regions while the latter exhibited an obvious yielding platform.6.Considering that the deformation mode of A1 based lattice materials was mainly the bending of struts,two constraint methods were utilized to restrict the deformation of struts for increasing the strength of lattice materials.One was pressing the Al based lattice material sample into a 6063 Al alloy square tube to form an outer-restriction,and the other was filling the cells of lattice samples with silicone rubber to build an inner-restriction.The compression tests demonstrated that the lattice material filled square tube inherited the deformation mode of naked tube at the early stage of deformation,while it showed obviously elevated stress-strain curves as the deformation was developed.The decreased stress was larger than the algebraic sum of individually compression stresses of lattice material and the tube,suggesting that an enhancing effect did take place due to the limitation of square tube to the lateral deformation of struts.For the silicone rubber filled Al based lattice materials,the flow stress was not only increased,but there appeared only elastic and plateau regions without densification region.This stress-strain behavior should be favorable for enhancing the strength,energy absorption capacity and efficiency.