Study On Spherical Indentation Behavior Of Selective Laser Melting TC4

Due to the advantages of high specific strength,excellent corrosion resistance,low density and high temperature resistance,TC4 alloy has become one of the most important structural materials in the aerospace industry.In recent years,because of the advantages of rapid and precise manufacturing of complex and hard materials,selective laser melting technology(SLM)has shown great application value in the processing of TC4 alloy.Current research shows that SLM-TC4 has a complex structure,such as multi-scale needle martensite,original?-phase columnar grains which change the distribution of martensite,and unavoidable processing defects,which have an important impact on its mechanical properties.However,due to the lack of powerful mechanical characterization methods,the current research on the mechanism of various levels of structures and the influence between structures at different scales is still relatively poor.Therefore,to clarify the effect of SLM-TC4 multi-scale structures on mechanical behavior has important scientific significance and industrial value.In this paper,the nano-indentation technology with high load and displacement accuracy,small damage and high efficiency was used,combined with finite element simulation,to study the effect of different SLM-TC4 microstructure and defects on spherical indentation behavior.And comparatively studied the difference between the stress-strain relationship of the SLM-TC4 under the two test methods of triaxial stress field indentation and uniaxial stress field tension,guiding the engineering application of spherical indentation.The main findings are as follows:1.Spherical indentation experiment to analyze the effect of microstructure.Using two sizes(R_i1=10?m,R_i2=100?m)of spherical nanoindentation,the micromechanical properties of SLM-TC4 with different sections were studied,it was found that the spherical nanoindentation behavior has obvious tissue sensitivity.As the main contributor to the strength of the material the martensite can reach hardness above 3.273 GPa.The presence of the original?phase grain boundary region and porosities lead to a reduction of hardness about 47.14%.Meanwhile,the hardness of the section perpendicular to the processing direction is lower than one parallel to the processing direction,about 4.19%.2.Finite element analysis of the effect of original?phase grain boundaries region.Through the analysis of the grain boundary characteristics and the indentation hardness test,a finite element simulation model contain boundary affect zone is established.The load-depth curve and the maximum normal stress at the initial contact are analyzed.The results of the finite element simulation experiment and the theoretical calculation are consistent,which proves that the finite element simulation model is reasonable.The simulation analysis results verified the contribution of the original?phase grain boundary region to the tissue sensitivity of the spherical nanoindentation behavior.The presence of grain boundary region will reduce the hardness test value and result in a lower hardness perpendicular to the machining direction.At the same time,the contribution of the grain boundary area is related to the relative distance away from the indentation centre.3.Finite element analysis of effect of porosity.Base on the analysis of porosity characteristics,the finite element simulation model is established contains different size,position and shape of pore defects.The analysis verified the contribution of porosity to the tissue sensitivity of spherical nanoindentation behavior.The existence of porosity will cause stress concentration,resulting in a lower hardness test value,and the effect is affected by the size and spatial location of the hole defects.Differences in hole defect geometry and spatial orientation will result in lower hardness perpendicular to the machining direction.4.Comparative study of the two stress-strain curves.By analyzing the theoretical model of empirical physics,establishing a finite element simulation model and designing two size nanoindentation experiments,the nanoindentation stress-strain curve and the tensile stress-strain curve of SLM-TC4 were comparatively analyzed.Studies have shown that the indenter size and calculation methods of contact radius will significantly affect the results of nanoindentation stress-strain curve.Choosing smaller nanoindentation experiments can effectively replace the macroscopic tensile characterization results.The confidence can reach more than 86%.