Effects Of Pulse Current And Heat Treatment On Solidification Structure And Properties Of TiAl Based Alloy

The pulse current solidification technology is a kind of technology which can effectively improve the solidification structure of the alloy.In this paper,the pulse current with different parameters is introduced into the continuous solidification process of Ti-45.5Al-4Cr-2.5Nb(at.%)alloy to observe the microstructure and test the mechanical properties at room temperature of the prepared alloys,and to study the influence of the pulse current with different parameters on the solidification structure and properties of the Ti Al based alloy.In order to analyze the action mechanism of pulse current,the distribution of magnetic field,Lorentz force and temperature field of the alloy under the action of pulse current was analyzed by using the finite element numerical simulation software,and the action mechanism of pulse current on the solidification structure of Ti Al based alloy was analyzed.Finally,in order to further improve the solidification structure and properties of the alloy,the Ti Al based alloys solidified under the action of pulse current with different parameters was heat treated,and the effects of heat treatment on the structure and mechanical properties at room temperature of the alloys was studied.In this paper,Ti-45.5Al-4Cr-2.5Nb alloy ingot was melted in a water-cooled copper crucible electromagnetic induction melting furnace.The material bar was cut from the ingot for continuous solidification experiment under the action of pulse current.The observation results of alloys microstructure show that the macrostructure of the alloys prepared under the action of pulse current has a certain degree of directional characteristics.The microstructure is mainly composed of ?/B2 long strip structure and ?/B2 lamellae between them.The path of solidification and solid-state transformation of Ti-45.5Al-4Cr-2.5Nb alloy is L??+L????+?????+???+?+B2??2+?+B2.The influence of pulse current parameters on the microstructure and properties of the alloy shows that with the increase of pulse current density at 200 Hz,the average deviation angle of the grains decreases,the average width of grain first decreases and then increases,the average width of the lamellar area first decreases and then increases,the tensile strength first increases and then decreases,and the elongation increases.The pulse current density is 52.9 m A/mm2,with the increase of pulse current frequency,the average deviation angle of grain decreases first and then increases,the average width of grain increases first and then decreases,the average width of lamellar area increases,the tensile strength decreases,and the elongation increases first and then decreases.Compared with the master alloy ingot,the tensile strength at room temperature and elongation at room temperature of the alloy prepared by pulse current are increased by 70.7% and 129.5% respectively.The numerical simulation results show that the magnetic induction intensity and Lorentz force on the surface of the alloy melt under the action of pulse current are the largest,and the magnetic induction intensity and Lorentz force decrease with the increase of the distance from the surface.With the increase of pulse current density and frequency,the magnetic induction intensity and Lorentz force in all parts of the melt increase.The temperature in the middle part of the alloy bar is the highest during the solidification process.With the increase of pulse current density and frequency,the temperature distribution of alloy melt is more uniform in radial direction,the temperature intensity in all parts of the alloy bar increase.Pulse current can enhance the directional tendency of solidification structure and refine the structure.The solidification structure of alloy is affected by two kinds of effects results.In order to improve the solidification structure,heat treatment was carried out on the Ti Al based alloys solidified under the action of the above different parameters of pulse current.The heat treatment parameters were 1250 ?/ 2 h/air cooling.The orientation and arrangement of the macrostructure of the heat-treated alloys were similar to those before heat treatment,with certain directional characteristics.Some grains of the microstructure of the alloy recrystallized,and the whole lamellar structure was found in the recrystallized grains.The content of the long strip structure in the non recrystallized grains decreased greatly,and the lamellae of the heat-treated alloy changed from ?/B2 to ?/?2.The content of B2 phase decreased greatly,which improved the structure.Compared with the alloys before heat treatment,the average deviation angle of grain,the average width of grain,the tensile strength and the elongation of the alloy after heat treatment increase.The change rule of the average deviation angle,the average width of grain,the tensile strength and the elongation of the alloy samples solidified under different pulse current is the same as that of the alloy samples before heat treatment Compared with the master alloy ingot,the room temperature tensile strength and elongation of the alloy are increased by 92.7% and 189.3%,respectively.