Temperature Field Simulation,defect,microstructure And Mechanical Property Of TC4 Alloy By Selective Laser Melting

The requirements of aerospace parts on lightweight,high-strength,high-precision and complex structure as well as manufacturing and maintenance costs limit the application of traditional forming methods represented by Cast forging welding.Selective laser melting(SLM)is considered as one of the most potential technologies for the manufacture of TC4alloy aerospace complex precision parts due to its capacity of complex parts forming.However,SLM is a layer-by-layer forming method with fast cooling process and complex thermal history.There are obvious differences between SLM forming parts and traditional forming parts in defects,microstructure and mechanical properties.The existing experimental methods and numerical simulation technology can not fully explain the defects in SLM process,the regular pattern of microstructure formation and the differences in mechanical properties of the samples.In this paper,the variation of defects,microstructure and mechanical properties of SLM parts of TC4 alloy with process parameters and scanning strategy was studied with the combination of numerical simulation and experimental research.The control methods of defects,microstructure and properties were discussed.The main research contents and results are as follows:(1)The modified segmented function model of thermophysical parameters and the adaptive mesh technology of Fully Threaded Tree(FTT)are used to solve the problems of the lack of thermophysical parameters and the large number of meshes in the SLM forming process under the mixed states of alloy.The numerical simulation of temperature field in the multi-layer and multi-channel SLM forming process is realized.Based on the analysis of physical parameters of powder,liquid and dense state in SLM forming process,the subsection function model of material physical parameters in SLM forming process is modified,and the problem of lack of thermal physical parameters of powder,liquid,dense state and mixed state in SLM forming process at variable temperature is solved.By using the octree adaptive grid technology,the grid in different regions of SLM forming space is coarsened and refined in real time,which reduced the number of grid and calculation amount without sacrificing the calculation accuracy.The numerical simulation of temperature field in SLM forming process of cube parts with 6 layers and 20 tunnels under different parameters and annular parts with one tunnel and many layers was accomplished.(2)As for study on SLM forming defects of TC4 alloy,the formation law of three kinds of defects,pore defect,thermal stress crack and side sticking powder,are found.The relationship between defects and process parameters are established,the optimal process parameter window is obtained,and the high density test piece is formed.Influence of process parameters and scanning strategy on defect formation was studied by the combination of graphic processing technology,statistical analysis and numerical simulation.The results show that gas bubbles,pit from removing the ejected molten materials,melt pool discontinuity,and less overlap melt pool are the main reasons for pore defect in SLM process.Results also indicated that high energy density is the main factor to produce thermal stress crack,and that adesion of unmelted powder is the main source of side sticking powder.According to the morphology characteristics of pore defects,the process parameters are divided into high energy density region,optimal process parameters region and low energy density region.In the high energy density area,the pore defects have large area,high circularity and small elongation,while in the optimal process parameter area,the number of hole defects is small,the area is small and the circularity is high.In the low energy density region,the pore defects are large in area,small in circularity and large in elongation.In this paper,The range of energy density 19.49-58.48 J/mm~3 belongs to the optimal process window.The densification of formed samples is higher than 99%,and the densification of samples is as high as 99.9%under the condition of 29.24 J/mm~3.(3)As for meso-and micro-structure of SLMed TC4 alloy,the influence of different region thermal history on the transformation process and martensite growth in SLM forming process is clarified.The formation mechanism of stripe pattern,chessboard pattern,original columnarβcrystal structure is revealed.The change rule of three structures with energy density and rotation angle is given,which provides a theoretical basis for the organization control.The mechanism of phase transformation and tissue formation was studied by means of experiment and numerical simulation.Results show that different thermal history results in the different transformation process ofα’→β→L→β→α’(M)in weld pool andα’→β→L→β→L→β→α’(m)in the lap zone,so that causes the difference of martensite size and orientation between the two zones which indicates the microscopic mechanism of the formation of checkerboard pattern;The rotation angle results in the mismatch of scanning paths of different layers,which makes the transition region and overlap region of epitaxial growth divide the weld channel into square region at macro level.The study of martensite size and the proportion of chessboard pattern area shows that the size of martensite increases and the proportion of checkerboard pattern area decreases with the increase of energy density.The samples with phase angle of 90°,energy density of 19.49 J/mm~3 and 21.93 J/mm~3 have fine martensite structure and more than 80%of chessboard pattern area.(4)Based on the research results of defects and meso-and micro-structure,the mechanical properties and fracture behavior of SLM parts of TC4 alloy were studied.Based on in-situ SEM observation,the crack initiation and propagation mode in SLM of TC4 alloy is given,and the influence rules of defects and microstructure characteristics on crack initiation and propagation are verified.Results show that the highest tensile strength and yield strength of TC4 alloy SLM samples are 1184 MPa and 1081 MPa respectively.The crack initiated at the surface defect,internal hole defect,checkerboard pattern boundary and martensite boundary of the specimen.The crack growth can be divided into three stages:rapid growth,stable growth and accelerated growth;the checkerboard pattern boundary and the martensite boundary perpendicular to the direction of crack growth both play a role in preventing crack growth.(5)In addition,SLM and HIP are both powder forming.In order to compare the advantages and disadvantages of these two processes,the defects,microstructure and mechanical properties of SLM and HIP of TC4 alloy are studied.The results show that the strength of SLM parts is higher than that of HIP parts,and the elongation of SLM parts is slightly lower than that of HIP parts.SLM forming is suitable for manufacturing parts with complex shape,high strength,low elongation and no isotropic requirements,while HIP forming is suitable for manufacturing parts with high strength and isotropy.