Study On The Design,Fabrication,Optimization And Mechanical Properties Of 3D Al Based Auxetic Lattice Structures

Auxetic lattice structures are a kind of special lattice structures with negative Poisson’s ratio.i.e.they expand laterally when stretched while shrink when compressed This unique mechanical response behavior provides them many excellent mechanical properties including high shear strength.stiffness,fracture toughness as well as impacting resistance,energy absorption capacity and so on.These excellent properties make auxetic lattice structures promising candidates in a number of engineering fields such as light-weight structures,impact protecting apparatus and sensors.In recent years,auxetic lattice structures have attracted more and more researchers’attention,and have become one of the hot-points in the field of porous materials research.However,the auxetic lattice structures currently available are very simple,and the most are nonmetallic materials with relatively large unit cells due to the limitation of fabrication technologies,unfavorable f-or the studies and applications.In order to overcome these difficulties,a new technique routine was developed in the present study.3D printing technique was firstly used to form nonmetallic patterns of auxetic lattice structures with low melting points,small unit cells,fine struts and complex structures,and then A1 based auxetic lattice structures were fabricated using the printed patterns by investment casting and infiltration technologies.Finally,the compression mechanical behavior,negative Poison’s ratio effect as well as their relationships with the structural parameters were investigated.The following results are obtained1.By CATIA software and 3D printing technique,two auxetic lattice structures with varied structural parameters were designed and printed using low melting point,low activity and easy forming material.Based on the 3D printed patterns,Al-based auxetic lattice structures were fabricated through investment casting and infiltration technologies.The combination of new methods breaks the limitations of traditional technologies to the dimensions of unit cells and strut diameters and diversities of auxetic lattice structures.It also overcomes the shortcomings of 3D printing technique in choosing the strut materials and f’orming efficiency,giving a new guideline for the applications of auxetic lattice structures2.The compression tests show that the stress-strain curves of Al-based re-entrant auxetic lattice structures can be also divided into three stages,that is a linear elastic region at the very beginning of deformation,followed by a so-called plateau region where the stress was slowly increased as the strain was increased,and finally a densification region where the stress rapidly increased.Compared with traditional porous metals,however,they show much higher strength or plateau stress and shorter plateau region.The flow stress,energy absorption capacity per unit volume or per unit weight of Al based re-entrant auxetic lattice structures decrease with increasing the length of struts or the re-entrant angle,but increase with increasing the diameter of struts.The Poisson’s ratio of the auxetic lattice structures changed in the opposite direction.3.The Al based double-V lattice structures exhibited similar stress-strain behavior and energy absorption feature to that of re-entrant lattice structures,but relatively strong dependence of Poisson’s ratio on the strain.The absolute value of Poisson’s ratio increased as the strain was increased.4.It is found that,through examining the deformation mode and evolution of deformed structures,the deformation of Al based re-entrant auxetic lattice structures arisen from the bending and folding of re-entrant struts.As the development of compression,the re-entrant struts moved inward,giving rise to decrease in the transversal dimension of lattice structure and negative Poison’s ratio.The side struts kept horizontal without obvious elongation during the whole compression process.No discernible fracture took place in the struts during the compression.The distribution of deformed struts was uniform over the whole cross section of samples without any macroscopically localized phenomenon,which is significantly different from other porous materials with randomly distributed pores.The compression deformation of 3D double-V lattice structures took place in the form of stretching and bending of long-inclined struts,and compressing and bending of short-inclined struts.As the compression continued,both the long-inclined and short-inclined struts rotated around the strut nodes and meanwhile moved inward,and thus resulting in transversal contraction of lattice structures and negative Poisson’s ratio.5.In order to improve the mechanical properties of Al based re-entrant lattice structures,we added a thin strut in the unit cell to form a support typed cell structure.Compared with original structures,the new lattice structure showed obviously enhanced mechanical properties.The elastic modulus,compressive strength,energy absorption capacity per unit volume or per unit weight of lattice structures increased with increasing the diameter of connecting struts,but they decreased with increasing the re-entrant angle.It should be noticed that,as the diameter of connecting struts increased,the Poison’s ratio gradually changed from negative to positive.This suggests that the connecting struts impeded the deformation of re-entrant struts and thus weakened the negative Poison’s ratio effect of auxetic lattice structures.6.Three new composite structures were produced by filling different polymers in the Al based auxetic lattice structures.The results showed that the composites had significantly increased flow stress,the values of which were largely over the sum of flow stresses of Al based auxetic lattice structure and polymer fillers.The higher the strength of polymer,the larger the strength of composite.Due to the restriction of polymer fillers,the Poisson’s ratio of composites tended to decrease’and the deformation mode changed from typical bending-dominated deformation to stretch-dominated one.This transition would be one reason of enhancement in the strength of composites.7.The other constraint condition was built by squeezing the composites into stainless steel square tubes to observe the mechanical behavior of the lattice structures.It is found that the complex exhibited even high strength,much higher than that of composites and stainless steel square tubes.No obvious yield phenomenon can be seen.