Microstructure And Mechanical Properties Of TiB_w/TC4 Composites In-situ Fabricated By SLM

In-situ titanium matrix composites have a series of advantages such as high specific strength,high specific modulus,excellent high temperature mechanical properties,etc.However,traditional in-situ synthesis technologies,such as casting and powder metallurgy,can not directly fabricate complex structural parts,and it is difficult to realize the uniform dispersion distribution of low content ceramic reinforcement in titanium matrix.In recent years,the rapid development of selective laser melting（SLM）additive manufacturing technology is expected to solve the above problems.At present,the research on SLM titanium matrix composites mainly focuses on process control and reinforcement content.In terms of mechanical properties,the current research is mainly limited to hardness and wear resistance,which is similar to the research of traditional laser cladding coating.However,there is a lack of evaluation on the strength and other comprehensive mechanical properties of bulk-form titanium matrix composites for complex component engineering applications.In view of the above problems,this paper designed the titanium matrix composite powder for SLM powder laying,prepared the bulk-form titanium matrix composites with different reinforcement content through SLM process optimization,proposed an appropriate heat treatment strategy,and analyzed the effects of the phase,surface quality,microstructure as well as the morphology,size and distribution of reinforcement on the mechanical properties of the composites.The coarsening mechanism of nanoscale Ti B ceramic whiskers was also clarified.Thermodynamic calculations showed that the Gibbs free energy and enthalpy of formation in the composite powder are all negative,indicating the in-situ reaction can be spontaneous and exothermic.For the composite powders milled for 3 hours,the fine hard B₄C ceramic particles are evenly embedded on the surface of spherical TC4particles.The particle surfaces become rough due to the collision during ball milling,but it still maintains a regular spherical morphology as a whole.On the basis of ensuring the fluidity of the powder,the uniform mixing of the two powders can be realized and the smooth powder laying process of SLM can be ensured.Ti B_w-reinforced TC4 composites with relative density higher than 99%is realized through SLM process optimization.The effects of SLM process parameters on its density,surface morphology and microstructure and properties were studied in detail.When the laser power is 260 W and the volume energy density is 120 J/mm³,the surface of titanium matrix composites prepared by SLM is flat and the density can reach 99.3%and an excellent combination of strength-plasticity was obtained.The phase and microstructure analysis showed that B₄C in the composite powder completely reacted with titanium matrix to form dispersed nanoscale Ti B whiskers accompanied by a small amount of Ti C nano-particles.The as-bulit TC4 is consist of coarse initialβ-Ti columnar crystals distributed along the stacking direction and a large number of fineα’martensite needles,while the as-bulit TMCs is consist of semi-elliptical molten pools that overlap with each other along the building direction.The typical microstructure of the TMCs is mainly composed of the fine columnar structure in the molten pool area and the coarse equiaxed structure in the bottom area of the adjacent molten pools.The diameter of Ti B whiskers in the as-built composites is ranged from 30 nm to 50 nm,and the parallel Ti B exist in the form of whisker clusters.Compared with TC4 alloy prepared by SLM and other Ti-Ti B composites,the compressive strength,microhardness and nanohardness of the samples are significantly improved.Fracture analysis shows that there is a ductile brittle fracture mode during compression.The effects of different phase region heat treatment on microstructure and mechanical properties of 2vol%Ti B_w/TC4 composite were studied.With the increase of annealing temperature in the biphase region,the matrix microstructure of TC4 and TMC samples coarsened obviously,and a stable（α+β）microstructure was obtained.The width ofα-Ti lath increases from 0.27μm to 4.46μm.With the increase of temperature,the clusters composed of parallel nano Ti B precipitates in TMC disappear and gradually evolve into nanoscale and submicron Ti B whiskers with larger size when the temperature is beyond 850°C.Theβphase zone heat treatment（1050℃/2 h）can completely break the lamellar structure characteristics brought by SLM process and eliminate the initial priorβcolumnar structure of TC4 alloy as well as the molten pool profile in TMC.A new kind titanium-based composite material composed of equiaxedα-Ti grains and uniformly dispersed double-scale Ti B whiskers was obtained.The tensile strength and elongation reached 1131 MPa and11.8%,respectively,achieving a synergistic improvement of strength and toughness.In-situ dynamic heating TEM analysis showed that the coarsening of nanoscale Ti B whiskers follows Ostwald ripening principle.In this process,the small-size nanoscale whiskers gradually disappear,and larger sub-micron Ti B whiskers formed by re-nucleation and growth on the relative large-size whiskers.