| Modern aerospace,aircraft,and automobile industries have an urgent demand for the increased payload and fuel efficiency,promoting the development of metal matrix composites with higher strength/weight ratio.Titanium matrix composites are outstanding among various composites because of their low density,high specific strength and specific modulus,and high hardness and creep resistance.As a new technology developed in recent years,laser additive manufacturing is famous for its mould-free,high-precision and rapid prototyping.It has highly non-equilibrium physical metallurgical nature,featured by superhigh cooling rates and strong convection of molten pool,are beneficial to inhibit the growth and uniform distribution of the reinforced phase in the titanium matrix composites,which shows great potential in the preparation of titanium matrix composites.In this paper,the effect of trace C addition on microstructure of titanium and typical Ti-Al-V alloys were studied systematically by laser micro-area metallurgy.And clarify the effect of trace C addition on the solid solution strengthening of matrix and the morphology and distribution of Ti C particles.The morphology of Ti C was optimized by laser solid forming,the mechanical properties of typical composites were evaluated,and the strengthening and fracture mechanisms during the tensile process were discussed.The main results are as follows:(1)Titanium matrix composites were prepared by laser micro-area metallurgy using pure Ti,Ti-4Al-3V and TC4 as matrix and adding trace C.The solid solution strengthening effect and influencing factors of C in different matrix were studied,the results show that with the increase of Al and V content,the solid solubility of C in titanium alloys decrease,and the hardness contribution of the same amount of C decreases gradually.This is because the atomic radius of Al and V is less than Ti.The addition of Al and V makes the lattice collapse of ?-Ti,and the solution ability decreases,which weakens the effect of solid solution strengthening after adding C.(2)The effect of C addition on the microstructure of the matrix was investigated,the results show that the original ? grains and ?-laths of the matrix are significantly refined by adding a small amount of C.When the C content was exceeds 0.12 wt%(solid solubility),the change of ? grain and ?-lath was not obvious,indicating that Ti C has little effect on the microstructure of the matrix.XRD analysis showed that the composites were mainly composed of ? phase and Ti C,and no other phases were found.(3)The effect of C addition on the morphology and distribution of Ti C particles was studied,the results show that there are two types of Ti C in Ti-4Al-3V/Ti C composites,one is eutectic Ti C in granular,clavate or feathery shape,and the symbiotic growth of ?-Ti and Ti C inhibits the branch of Ti C;the other is dendritic primary Ti C with larger size.When the C content is lower than the eutectic composition(about 0.6 wt%),the precipitated phases in the composites are all eutectic Ti C,but after exceeding the eutectic composition,the primary dendrite Ti C begins to appear.(4)The morphology of Ti C was optimized by laser solid forming technology in YAG pulsed laser.The size of precipitated Ti C is about 650 nm,which is due to the high energy density,small molten pool and superhigh cooling rate of the pulsed laser.There is no time to grow up after Ti C precipitated from the molten liquid.(5)The micro mechanical properties of Ti-4Al-3V/Ti C composites were analyzed by micro indentation.The results show that with the increase of C content,the microhardness,elastic modulus and dislocation density of Ti-4Al-3V/Ti C composites gradually increase,and the creep depth and creep rate at room temperature decrease.It shows that the addition of trace C can improve the mechanical properties of titanium alloys to a certain extent,but there are usually differences between the micro and macro properties.(6)The room temperature tensile properties of Ti-4Al-3V/Ti C composites fabricated by laser solid forming were evaluated,the results show that the tensile strength and elongation of Ti-4Al-3V alloy are 774 MPa and 13.7%,respectively.When the C content is0.23 wt%,the strength of the composites is 1067 MPa and the elongation is 11.5%.The strengthening mechanism of C addition is mainly solid solution strengthening,Orowan strengthening,fine grain strengthening and load transfer strengthening.The fracture mechanism is that Ti C leaves micropores after debonding from the matrix,and the micropores rapidly aggregate and expand,leading to the final failure of the composites. |
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