Research On Process-Structure-Performance Integration Of Lattice Structure In Selective Laser Melting

The lattice structure is a new type of ordered porous material with integration of structure and function.It has the advantages such as light weight,low volume density,high specific stiffness and high specific strength.Therefore,it is widely used in aerospace,transportation and biomedical fields.Some simple lattice structures can be manufactured by traditional manufacturing technology.However,for those which have complex topology,it’s difficult to manufacture through traditional manufacturing technology.In recent years,the rapid development of additive manufacturing technology has made the manufacturing of complex lattice structures simpler and more efficient,as it can provide microscale manufacturing and geometric control.The development of additive manufacturing technology provides a good foundation for the research and application of lattice structure,but the design of lightweight lattice structure with complex function is lack of systematic,basic and integrated research.At present,there are several key problems in the manufacturing of Al Si10Mg lattice structure by using Selective Laser Melting（SLM）technology.First,there is a lack of comprehensive research on the mechanism of SLM process parameters on the microstructure and energy absorption of lattice structure.Second,the effect mechanism of heat treatment on phase,microstructure and energy absorption of Al Si10Mg lattice structure manufactured by SLM is still unclear.Third,the influence of random geometric defects in SLM manufacturing process on energy absorption and deformation mechanism of lattice structures is unclear either.Fourth,the unit cell of lattice structure is still dominated by the traditional structure type,and most of the research stays at the material-level while there is no the design-level and application-level research on lattice structure.Therefore,this topic carries out research on above key issues,and the detailed research content is as follows:（1）The influence mechanism of SLM process parameters（laser power and scanning speed）on the micro morphology and energy absorption of the lattice structure was studied.The correspondence between process parameters,micro morphology and energy absorption was established.The change rule of strut size of lattice structure under different process parameters was studied and the reasons were explained.It is found that the forming mechanism of the struts in different directions of lattice structure is different.Then,the pores were classified according to the size and shape in the lattice structure,and the causes of the formation of each type of pores were emphatically discussed.Through the simulation of the temperature field and the size of the melt pool,the mechanism of the process parameters was further clarified.Different process parameters will form different melt pool temperature field and melt pool size,which will affect the forming quality of parts and change the energy absorption of the structure.（2）In order to improve the energy absorption of Al Si10Mg lattice structure manufactured by SLM,the influence mechanism of Artificial Aging（AA）,Solution Heat Treatment（SHT）and Solution Heat Treatment+Artificial Aging（SHA）on the microstructure and energy absorption was studied.By observing the grain orientation of the manufactured samples,it is found that the grains preferentially grow along the manufacturing direction which indicats that the SLM manufactured samples have strong anisotropy.Then,the microstructure evolution of Al Si10Mg lattice structure under different heat treatment temperatures and time was studied.The change rule of Si particle size and density was summarized,and the mechanism of Si particle change was explained.The corresponding relationship between microstructure evolution and energy absorption after heat treatment was revealed according to the change of morphology and microstructure of lattice structure compression fracture.（3）Through X-ray CT scanning and 3D geometric reconstruction,a digital model based on the real morphology of lattice structure was established to study the influence of stochastic geometric defects in the manufacturing process on energy absorption of lattice structure.Considering the different forming mechanisms of struts in different directions of lattice structure,the types and distribution of stochastic geometric defects generated by different struts in the manufacturing process were extracted,and the geometric defects（strut thickness variation and strut waviness）were statistically quantified.The finite element model with geometric defects was established according to the variation of the dispersion and radius of the struts in different directions.Based on the theory of progressive homogenization,it is found that the elastic modulus of the manufacturing lattice structure exhibits obvious spatial inhomogeneity during the elastic stage.The influence of different geometric defects on mechanical response and energy absorption of lattice structures were explained.The proposed method of incorporating geometric defects into the finite element model can predict the energy absorption of manufactured lattice structures more accurately.（4）Three optimization strategies were designed to solve the struts’nodes stress concentration problem of lattice structure,which were adding fillet at nodes,using circular arc transition and changing the angle of nodes,in order to improve the deformation mode and improve the energy absorption capacity of lattice structure.Through simulation analysis and quasi-static compression experiments,the deformation modes of the three optimized structures and the original structure were compared,and the improvement mechanism of different optimization strategies on the stress concentration of the struts’nodes was analyzed.The local stress changes of the strut nodes of the three optimized structures were mainly analyzed.The coefficients C₁ and C₂ obtained by the Gibson-Ashby equation can be verified to estimate the elastic modulus and compressive strength of the Al Si10Mg lattice structure manufactured by SLM,which provides a theoretical basis for future practical applications.（5）A method combining complex proportional assessment（COPRAS）and discrete optimization was proposed to optimize the topology of thin-walled tubes filled with lattice structures to find the best compression performance of the hybrid structure.Four different cross-section configurations of thin-walled tubes and eight different filling positions of lattice structures were designed,and then combined to form hybrid structures with various topological types.The finite element model verified by experiment was used to analyze the compression performance and deformation mode of the hybrid structure.COPRAS method was used to rank the compression performance of hybrid structures and select the optimal topology.Based on the discrete optimization algorithm,the optimal structure was optimized to make the material distribution more reasonable,in order to further improve the compression performance of the structure.This method can be extended to different optimization strategies to select the best solution.