Multi-scale Regulation And Reinforcement Of In-situ Titanium Matrix Composites By Laser Melting Deposition

Titanium-based matrix composites（TMCs）have advantages such as high specific stiffness,high specific strength,excellent high-temperature performance,and low thermal expansion coefficient,compared with traditional matrix alloys.Those merits can effectively improve mechanical performance,reduce weight,and increase efficiency.Additive manufacturing（AM）has the characteristics of near-net shape fabrication and high powder utilization rate,which greatly reduces the processing cost of titanium alloys.Therefore,TMCs by AM have broad application prospects in fields such as the aerospace and automotive industries.However,the steep temperature gradient,ultra-fast solidification rate,and lack of undercooling and nucleation events in front of the liquid-solid interface in AM process of titanium alloy easily result in coarse,anisotropic columnar structures.In prior grains,fine lamellar or needle-shaped martensitic exist due to rapid cooling rate.A few equiaxed or fine grains appear at the bottom and top,forming a typical laser-melting structure with extreme heterogeneity.Therefore,promoting the transition from columnar to equiaxed grain has become an important challenge in the AM process.Based on this,this article selected three kinds of TMCs,TiB/TC4,（TiB+La₂O₃）/TC4,and（TiC+Ti₅Si₃）/TC4,as research objects to refine the coarse columnar grain structures in Laser Melting Deposition（LMD）.Firstly,the influence of processing parameters on the microstructure and melt pool morphology of the single deposition layer was studied in the LMD process.Secondly,the kind of in-situ phase,composition,and scale of the in-situ phase was regulated to study the effect on microstructure evolution,grain refinement,control mechanism,and mechanical properties.The specific research content and conclusions are as follows:（1）The single deposition layers of TC4 with different process parameters were prepared,and the morphologies of the melt pool were obtained.A quadratic regression model between process parameters and melt pool dilution rate was established using the Central Composite Design（CCD）of Response Surface Methodology（RSM）,providing a theoretical basis for the determination of processing parameters.The model showed that the effect of laser power,scanning speed,and powder feed rate on the dilution rate was different.Among them,the powder feed rate had the most significant control effect on the dilution rate.At the same time,when the minimum dilution rate was the optimization aim,the compensated optimal processing parameters could be obtained:1500 W,360 mm/min,and 6.0 g/min.In addition,two formations of grain morphology existed in the melt pool:epitaxial columnar and equiaxed grains with heterogeneous nucleation.The microstructure of the TC4 melt pool was columnar grown from the bottom of the melt pool.When adding SiC,the microstructure of the melt pool was a mixture of columnar and equiaxed grains.Moreover,the processing parameters of single deposition layers were close to the optimal process parameters,then the microstructure became finer.Because the temperature gradient was also the smallest under the optimal processing parameters.（2）To obtain the temperature field characteristics under the optimal process parameters,a finite element model of multiple layers under the alternating scanning path of TC4 alloy was established.The model showed that the maximum temperature of the melt pool was 2271℃,and the transition time of the cyclic heating temperature field in the liquid phase zone,β（?）αphase transition zone,andαcoarsening zone could be determined.The solidified structure of TC4 alloy consisted of coarse and columnar priorβ-Ti grains,with an average width of 768±226μm.The internal structure of the prior grain wasα+βbasket-weave structure,and there was also a fewα’martensite structure.This was due to that the temperature gradient during the laser melting deposition process was greater than the martensite transformation temperature,and the time above theαphase transition zone during the cyclic heating process was insufficient,with durations of 6.35 s and 5.09 s under different scanning paths.（3）For microsize TiB₂/TC4 TMCs,the columnar grain of the TC4 alloy is completely replaced by finer equiaxed grains wirth the TiB₂mass fraction increasing from 1 wt.%to 5 wt.%.The in-situ synthesized TiB reinforcements were distributed around theβ-grain boundary,forming a three-dimensional quasi-continuous network（3DQCN）structure.The average grain size decreases from 105±27μm to 23±8μm.Those include the heterogeneous nucleation effect of TiB whiskers and the supercooling provided by boron atoms.Firstly,the misfit dislocations at the TiB/βand TiB/αinterfaces are 1.72%and 4.8%,respectively.TiB can act as an effective heterogeneous nucleation site.Secondly,the quantitative relationship between the boron atom mass fraction in TiB₂and the growth restriction factor（Q）was established using the Interdependent Theory.Meanwhile,the quantitative relationship between grain size and Q was also established.When 5 wt.%TiB₂was added,and boron atoms provided a significant degree of supercooling（60.97 K）.The formation of the in-situ TiB reinforcement phase refines the microstructure and enhances the mechanical properties.Compared with TC4 titanium alloy,the ultimate tensile strength of TMCs with 5 wt.%TiB₂added was increased by 21%,mainly due to the Hall-Petch effect caused by grain refinement and the load-bearing effect of TiB.（4）For microsize（TiB+La₂O₃）/TC4 TMCs,the influence of La₂O₃and coupling（La₂O₃+TiB）on the microstructure was studied.It was found that when adding La₂O₃solely,the solidified microstructure consisted of small columnar grains,with an average column width of 181μm.However,under the coupled regulation of TiB and La₂O₃,the columnar grain completely disappeared,and the microstructure from the bottom to the top was composed of heterogenous 3DQCN and ultrafine equiaxed grains（with an average grain size of 14±10μm）.Based on the component undercooling provided by boron atoms,La₂O₃can act as a heterogeneous nucleation particle and exhibit the size distribution of micro-nano composites.Nanosize La₂O₃can hinder grain growth,and microsize La₂O₃can act as an effective nucleation substrate.According to the Interdependent Theory,the coupling of TiB and La₂O₃effectively shortens the distance that hat grain needs to grow to create sufficient constitutional supercooling to nucleate a grain and the distance to reach the next effective potent nucleation particle,resulting in further grain size refinement.Moreover,when 5 wt.%TiB₂and 0.5 wt.%La₂O₃were added and the ultimate tensile strength was increased by 26.4%compared to TC4.（5）For nanosize SiC/TC4 TMCs,the in-situ reaction spontaneously occurred between SiC and Ti alloy in the melt pool.TiC was uniformly distributed in the matrix as equiaxed particles,and Ti₅Si₃phase was distributed around the priorβgrains in a chain shape,forming a 3DQCN structure.As the SiC reinforcement phase increased from 0.5 wt.%to 3 wt.%,the worn surface changed from rough and wide and deep plow grooves to a smooth surface.The high hardness Ti₅Si₃formed a work-hardened friction layer on the surface,and the wear mechanism changed from brittle wear to adhesive wear.When the SiC mass fraction was 3 wt.%,the hardness increased to 442.1 HV,higher than that of TC4,and the friction coefficient decreased from 0.39 to 0.32,which was due to the in-situ TiC and Ti₅Si₃phases,as well as the high hardness and the 3DQCN matrix.