Investigation Of Solidification Behavior And Microstructure Evolution In ZTC4 Alloys

ZTC4(Ti–6Al–4V)alloy is a typical moderate strength ?+? titanium alloy with excellent properties,good casting properties and welding performance.ZTC4 is currently the most widely used titanium alloy in the field of aerospace,marine and biomedical.Titanium alloy casting is used to produce complex structural parts that can not be easily fabricated in other processes.It is characterized by high dimensional accuracy,low cost,excellent performance and so on.However,it exists some problems,such as difficulties in forming thin-walled castings,with coarse grain structure and nonuniform microstructure.In order to improve the forming ability of titanium alloy,the influences of countergravity casting process in ZTC4 alloy on filling capacity and solidification characteristics were studied.The relationships between casting process,microstructure and properties were discussed.The properties of the casting are dependent on the microstructure.While the microstructure is determined by the thermal history of the casting alloys from the liquid state to the solid state.The casting microstructure of the titanium alloy can be adjusted by controlling the thermal history of the solidification process of titanium alloy.The mechanical properties of castings can be controlled,and the uniformity of the organization can be improved as well.The effect of isothermal holding treatment on the microstructure evolution of ZTC4 alloy was studied.The mechanism of microstructural evolution was interpreted.The effect of isothermal holding treatment on the microstructures and properties in ZTC4 alloy in countergravity casting was also studied.Then,the properties of castings were improved by heat treatment.The main research contents and conclusions are as follows:(1)The effects of isothermal holding temperature in solidification process on the microstructure evolution were studied.It was found that an “unique bi-modal” microstructure consisting of high aspect ratio ? particles and small colonies with fine secondary lamellar ? appeared in specimens isothermally held from 950°C to 970°C combined with rapid cooling(more than 150 °C/min).The evolution mechanism was discussed.The phenomenon that primary lamellar ? and grain boundary ? broke down into ? particles can be attributed to the Ostwald ripening and boundary splitting mechanisms during isothermal holding process.The phase transformation temperature of the retained ? matrix decreased because of the solute enrichment of V in the retained ? matrix after isothermal holding.The formation of fine secondary lamellar ? can be ascribed to nucleation promotion and growth inhibition.(2)With the applications of numerical simulation and experiment,the evolution of multi-physics field in counter-gravity casting of ZTC4 alloy was studied.The influences of preheated temperature,section size of cast part and hot isostatic pressure(HIP)were discussed.The casting parameters of counter-gravity casting in ZTC4 alloy were determined: pouring temperature 1760 °C,mold temperature 300 °C and 650 °C,filling pressure 60 kPa,pressurization rate 9 kPa/s.(3)The microstructure evolution of counter-gravity casting was studied with the code CAFé.The grain size of ZTC4 alloy cast part was measured.Room temperature tensile properties of cast bars with different diameters were tested.The results showed that cast parts obtained via counter-gravity casting were more compact,meanwhile with improved strength and ductility.The grain size increased with the increasing of the section size,resulted the lower strength and ductility.With the higher preheated mold temperature,the strength was slightly lower.The ductility increased a bit,however.(4)The isothermal holding process was applied to counter-gravity casting to control the microstructure of cast part.Isothermal holding temperature 850 °C and 900 °C were conducted for 30 min respectively.Sound cast parts were obtained.It was found that as for the cast part produced in isothermal holding process,the RM tensile properties were slightly lower than normal counter-gravity casting.However,after HIP,both the strength and ductility were improved: UTS 850 MPa,YS 800 MPa and elongation 10%.The RM tensile properties of cast parts with different section sizes were found to be consistent.(5)Microstructure evolution and corresponding tensile properties were studied in status of HIP,? solution-aging treatment,?+? solution-aging treatment and special heat treatment.After HIP and ? solution-aging treatment,both the strength and ductility of cast parts produced with different mold temperature reached a high level: ultimate tensile strength(UTS)1023 MPa,yield strength(YS)960 MPa,elongation 11.8%.While after HIP and ?+? solution-aging treatment,the comprehensive performance was lower than the former: UTS 962 MPa,YS 899 MPa and elongation 7.9%.With solution treatment in ? and ?+? phase field in sequence,and then aging treatment,the properties can be further adjusted.