Microstructural Evolutions Of V-5Cr-5Ti Alloy And 316L Stainless Steel Prepared By Laser Melting Deposition Technique

As one of the additive manufacturing technologies for high-performance metal components,laser melting deposition(LMD)has attracted extensive interest due to the advantages of time saving,single-step fabrication,and materials waste-free.However,due to the features of fast non-equilibrium solidification and directional solidification during LMD process,there are some common problems in the LMD microstructures,such as segregation of solute/second phase and epitaxial growth of coarse columnar grains,which restrict the application and performance of LMD materials.It is important to understand the microstructure characteristic,and to further obtain effective methods to adjust the microstructure,and thus to promote the developments of LMD technology and the related materials.In this paper,typical alloys of V-5Cr-5Ti(BCC)and 316L(FCC)alloys are studied.The solution and aging treatments have been used to control the segregation of solutes/second phases in V-5Cr-5Ti alloys.The effect of solution and aging temperatures on microstructure and aggregation of precipitates are studied.The ultrasonic vibrations assisted LMD process has been applied to control the epitaxial growth of coarse columnar grains of 316L stainless steel.The effects of ultrasonic powers on the grain morphologies and sizes,the microstructure formation mechanism,and grain growth characteristics of 316L stainless steel are studied.The main conclusions are as follows:V-5Cr-5Ti alloys fabricated by laser melting deposition(LMD)show a large number of Ti-(CNO)lath-like precipitates aggregated along the grain/dendrite boundaries.After solution treated at different temperatures of 800?,1000?,1200?,1400? and 1560?,it shows that as the solution temperature increases,both the density and size of the aggregated precipitates are reduced,however,the grain sizes are not obviously increased.When the solution temperature reaches to 1560?,most impurity elements can diffuse into the matrix and form into a nearly uniform supersaturated solid solution.After aging at different temperatures of 800?,1000? and 1200?,the 1560? solution treated samples show a uniform distribution of precipitates inside grains.As the aging temperature increases,the density of precipitates first increases and then decreases,accompanied by an increase of precipitate length.The shapes of precipitates change from near-spherical to lath-like as the aging temperature increases.Therefore,the aggregations of the precipitates in the LMD samples can be modified by solution and aging treatments.Compared to the LMD and solution-treated samples,the aged samples have higher micro-hardness,due to the precipitation strengthening.However,for the aged samples,the sample with the highest density of lath-like precipitates has the lowest hardness,due to the reduced possibilities for the cross gliding and interactions of dislocations.The microstructure of 316L stainless steel mainly consist of coarse columnar grains due to directional solidification and epitaxial growth during LMD process.The application of ultrasonic vibrations with frequency of 20 kHz and powers of 600 W,800 W and 1000 W during laser melting deposition can break the directional epitaxial growth of large sized columnar grains along[100]direction,resulting in a refinement of microstructure,increases of cumulative misorientation and dislocation density along the long axis of grains.By promoting the convections in the melt pool,and thus reducing the temperature gradient along the deposition direction,the application of ultrasonic vibrations accelerates the transitions of growth directions for columnar grains.The application of ultrasonic vibrations also helps to increase the cooling rate during solidification,resulting in refinements of both columnar grains and the interior columnar dendrites.However,as the vibration power increases from 600 W to 1000 W,the intercept lengths of columnar grains and interior columnar dendrites both increase slightly,probably due to the reduced cooling rate caused by partial conversions of vibration mechanical energy to thermal energy at high vibration powers.