Laser Melting Deposition Processed TiC Reinforced Nickel-based Superalloy Composites:Process Control Mechanism And Trans-scale Reinforcing Effect

In the present study,laser metal deposition(LMD)additive manufacturing was used to deposit the pure Inconel 625 alloy and the TiC/Inconel 625 composites with different starting sizes of TiC particles,respectively.The effects of laser energy input per unit length(η)on the densification level,microstructural features,and mechanical properties were investigated.The mechanisms of microstructure evolution and microhardness distinction in the different zones of the individual molten pool which was produced in LMD-processed composites were also elucidated.Meanwhile,the relations of microstructure,microhardness and underlying thermodynamics in molten pools were analyzed.Moreover,the influence of the additive TiC particle and its original size on the constitutional phases,microstructural features,and mechanical properties of the LMD-processed parts was studied.The main conclusions were drawn as follows:LMD process was applied to prepare the TiC/Inconel 625 composites in bulk-form with different laser process parameters.The densification levels of LMD-processed TiC/Inconel 625 composites were influenced by the applied η.When the η of 100 kJ/m was properly settled,a near-full relatively density was gained without any apparent pores.The columnar dendrites of Ni–Cr c matrix obtained a considerably refined microstructure with the increase of the applied η.While,the TiC reinforcements experienced a successive change in its microstructures from agglomeration to uniformly distribution and coarsening as the applied η increased.When the applied η was properly settled in 100 kJ/m,the TiC/Inconel 625 composites ultimately gained the highest microhardness with a mean of 330 HV0.2.Meanwhile,a uniformly distributed COF with a considerably low mean value of 0.41 was gained during the sliding wear tests,decreasing the wear rate to 5.4×10-4 mm~3/(Nm).Based on the above experiments,the laser process parameters were further optimized,thus the nano-TiC/Inconel 625 composite parts with a higher densification level were manufactured by LMD process.It could be observed that the microstructures in the upper part of the molten pool were mainly cellular structures,whereas which in the bottom and edge region were predominantly columnar dendrites.The different size of cellular grains and dendrite spacing were attributed to the varied cooling rates of diverse regions in the molten pool during LMD process.The factors contributing to microhardness variety could be summed up in three aspects,namely size of grains,TiC reinforcing particles and solid solution strengthening.The optimized LMD process was used to deposit the pure Inconel 625 alloy,nano-TiC/Inconel 625 and micro-TiC/Inconel 625 composites,respectively.The results revealed that the incorporation of TiC particles significantly changed the prominent texture of Ni-Cr matrix phase from(200)to(100).The incorporation of nano-TiC particles induced the formation of the significantly refined columnar dendrites with the secondary dendrite arms developed considerably well.With the micro-TiC particles added,the columnar dendrites were relatively coarsened and highly degenerated,with the secondary dendrite growth being entirely suppressed.The cellular dendrites were obviously refined by the additive TiC particles.When the nano-TiC particles were added to reinforce the Inconel 625,the significantly improved microhardness,tensile property,and wear property were obtained without sacrificing the ductility of the composites.