| With the development of technology for aeronautics and astronautics,many complex part or component with multiperformance need to be once formed through additive manufacturing,and the composite forming of graded materials or various materials has been the inevitable trend.Laser melting deposition(LMD)technology based on powders became the preferred method of composites and graded materials additive manufacturing,because it could achieve free deploy of material components during the process of maunfacturing part.When the TiC/Ti6Al4 V composite was manufactured,the laser melting deposition could realize the free design of volume fraction of reinforcements and microstructure and properties through controlling the parameters of powders and laser processing,and the microstructure of materials could realize the graded change,which achieve the purpose that the different part of component owned different properties.In this work,the particles reinforced graded TiC/Ti6Al4 V composite was manufactured by LMD based on the own characteristics of TiC and usage requirements of materials.The relationship between the volume fraction of TiC particles,TiC size and the microstructure and properties of composites.The influencing mechanism of properties such as hardness,strength and plasticity of graded composites was studied systematically.Based on the aboved study,the strength and plasticity of composites were improved further through preparing the insitu TiC reinforced Ti6Al4 V composite and heat treatment,which provided the important theoretical foundation of control of microstructure and properties of TiC/Ti6Al4 V composite.The graded TiC/Ti6Al4 V composite with TiC volume fraction ranging from 0 to 50% was prepared by coaxial powder feeding laser melting deposition method firstly in this article,and the problem that the laser melting deposited multilayered TiC/Ti6Al4 V composite with high TiC volume fraction was easy to crack was solved.Due to the partial dissolution of TiC particles during the LMD process,much eutectic and primary TiC microstructure was in-situ synthesised,and these TiC phases together with extra TiC powders were act as the reinforcements of composites.The gradient distribution characteristics of material properties were founded based on the gradient distributed in-situ TiC and TiC powders in the materials.The influence of TiC powders volume fraction on the in-situ TiC was as follows: when the volume fraction was less than 5%,the granular eutectic TiC and chain shaped eutectic TiC were formed,and when the volume fraction was more than 5%,th e eutectic TiC and granular primary TiC and dendritic primary TiC were together coexisted.With the volume fraction of TiC powders increasing,the number of dendritic primary TiC increaed gradually.As the hardness of dendritic primary TiC was larger,the microhardness of graded composites increased continuously with the volume fraction of TiC powders increasing.When the volume fraction of TiC powders increased from 0 to 50% gradually,the hardness of as-deposited composites increased from 379.8HV to 736.7HV gradually.However,the strength and plasticity of materials decreased when the volume fraction of TiC was more than 5%.The analysis of fracture behaviour of TiC/Ti6Al4 V composite showed that when the number of undissolved TiC particles was larger,cracks were easy to generated inside the particles,which decreased the strength.Adding the volume fraction of in-situ TiC was beneficial to improve the strength and plasticity.In order to improve the strength and plasticity of TiC/Ti6Al4 V composites further,the mixed powders with 5% TiC were prepared through ball-milling method using the ultrafine TiC powders with the size less than 10 ?m and Ti6Al4 V powders,and the TiC reinforced powders were fully melted by laser melting deposition,and the full in-situ TiC reinforced titanium matrix composites.Compared with that of conventional particles reinforced TiC/Ti6Al4 V composites,the tensile strength of the in-situ TiC reinforced Ti6Al4 V composites increaed from 1177 MPa to 1231.3 MPa,and the elongation increased from 0.6% to 2.1%.The main reasons of properties improvement of in-situ TiC reinforced Ti6Al4 V composites was that the plenty of granular eutectic TiC were formed,in-situ TiC induced strengthening mechanism of dislocation and the distribution of reinforcements was more homogeneous.As the plasticity of TiC/Ti6Al4 V composites was hard to be improved effectively through adjusting the processing parameters and powder size,the specialized heat treated optimized process was exploited considering TiC/Ti6Al4 V composites by laser melting deposition method in this thesis.When the composites were heat treated at 950? for 10 h and then cooled in air,the elongation of which increased from 2.1% to 4.2%.After heat treatment,the factors that in-situ chain shaped eutectic TiC were granulated to granular eutectic TiC that obtained the greater capability of coordinating the plastic deformation of the matrix,the microstructure of Ti6Al4 V matrix changed from widmannstatten microstructure to basketweave microstructure,and the chain shaped eutectic TiC distributed continuously along the grain boundary of ? Ti were granulated to dispersive granular eutectic TiC made the plasticity of composites improved twice.In conclusion,the graded TiC/Ti6Al4 V composite was successfully prepared by laser melting deposition in this thesis.When the volume fraction of TiC was higher,the composite could be used as the surface wear-resisting layer for its high value of hardness.When the volume fraction of TiC was lower,the composite could be used to prepare the main body of high strength parts for its high strength.The tensile properties of composites could be improved through preparing the in-situ TiC reinforced Ti6Al4 V composites,and the plasticity of composites could be improved through heat treatment control.The related theories and experimental results can be used in the design of process and materials when preparing the parts with high strength and high plasticity. |
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