| NiTi shape memory alloys are widely used in various fields because of their excellent shape memory effect,superelasticity and good biocompatibility.However in the traditional smelting process of nickel-titanium alloy,component segregation,impurities and other factors making the alloy composition is not uniform and ultimately will seriously affect the performance of the alloy.In addition,the high hardening rate of nickel-titanium alloy make the amount of single-channel deformation small in the latter part of the plastic forming process which make the NiTi alloy need for multi-process processing.The powder metallurgy can effectively avoid the shortcomings of traditional smelting and powder metallurgy can achieve the product near net shape,reducing the machining process.Both to save raw materials,but also reduce the production process,improve production efficiency.In this paper,compact Ni Ti alloy prepared by powder sintering with metal nickel powder and titanium powder.The nickel-titanium alloys were prepared by pressing forming(powder-pressing forming-sintering)and gel-casting(powder-slurry-gel casting-sintering).The effects of sintering process and sintering method on the density of the prepared alloy were studied by X-ray diffractometer(XRD),Scanning electron microscopy(SEM)and microhardness.The results are as follows:(1)It was found that the content of NiTi phase in the alloy prepared by conventional powder metallurgy was increased with the increase of the sintering time at 1050℃ and the content of Ni3 Ti and NiTi2 was decreased.The density and hardness of NiTi alloy increase with increasing sintering time.The NiTi alloy Sintered for 2 to 8hours at 1050 ℃ have a density of 5.97~6.10 g.cm-3,a porosity of 5.5%~7.4%,a hardness of 347.5~408.43 HV and grain size of 10~30um.The density of NiTi alloy sintered 6 hours at 1050℃ was 6.1 g.cm-3 and the hardness value is 408.4HV,which was the closest to the theoretical compact NiTi alloy.The density of sintered at 900℃for 2 to 8 hours is from 5.57 g.cm-3 to 5.63 g.cm-3.Sintering 8 hours at 900℃ to get the maximum density of the alloy was 5.63 g.cm-3and the hardness was 284.6HV.The density of the samples sintered at 1050℃ is 0.44 g.cm-3higher than that of the samples sintered at 900℃ and the average hardness is 146 HV higher.However,the grain size growth of the alloy under high temperature sintering is also more obvious.At 1050℃,the sintering time increases from 2 hours to 8 hours the grain size of the alloy increasesfrom 10 um to 20-30 um.(2)NiTi2 and Ni3 Ti phases are preferentially formed by the mutual diffusion of Ni and Ti atoms during the sintering process and then NiTi phases is formed by the transition of NiTi2+Ni→NiTi and Ni3Ti+Ti→NiTi.When Ni and Ti are depleted,the remaining NiTi2 and Ni3 Ti will remain in the structure which is the reason why there is still a small amount of NiTi2 and Ni3 Ti phases in the alloy even sintered for 8 hours at1050℃.(3)The NiTi green body was prepared by gel-casting.During preparation of Ni-Ti slurry it was found that the viscosity value of Ni-Ti slurry increased with the increase of solid content and with the increase of the dispersant content the Ni-Ti slurry viscosity first decrease and then slowly increase.At the milling speed is 250r/min and the ball-to-powder ratio is 10:1,the effect of dispersion is the best when milling for 60 minutes which make the viscosity of the Ni-Ti slurry greatly reduced.Through comprehensive analysis of the experimental results,using 43% solid content of Ni-Ti slurry to gel-casting that included 1wt% dispersion of the mass of metal powder,1.7wt% initiator of the organic monomer and 10% catalyst of the organic monomer volume.By measuring the quality change of NiTi green body during the heating process and analyzing the DSC-TG results of NiTi green body prepared by gel-casting,the drying process of NiTi green body was 60℃ for 24 hours and the degumming process was 300℃ for 1hour and 450℃ for 4 hours.(4)The NiTi mixed powder and NiTi alloy powder prepared by mechanical ball milling.NiTi green bodies prepared by gel-casting with NiTi mixed powder and NiTi alloy powder were vacuum sintered at 1050℃ for 4 hours.After vacuum sintering,it was found that samples prepared by NiTi alloy powder had more NiTi phase and the distribution of phase was more uniform.But the alloy was more oxidation when compared to alloy prepared by NiTi mixed powder.The density of sample prepared by NiTi alloy powder is 5.42 g.cm-3and the porosity is 16%.The density of the sample prepared by NiTi mixed powder is 5.38 g.cm-3 and the porosity is 16.6%.However,the grain size of sample prepared by NiTi alloy powder was smaller than the alloy prepared by NiTi mixed powder.(5)In the same sintering process,Ni Ti alloy was prepared by vacuum sintering and hot isostatic pressing sintering.It was found that the results of the XRD test of the two sintered samples prepared in different sintering methods were same,which indicat that the two sintering methods have no effect on the phase structure of NiTi alloy.Themicrostructure of the two alloys was observed and found that the alloy prepared by hot isostatic pressing have less porosity.The hot isostatic pressing sintered alloy had a density of 5.9g.cm-3,a porosity of 8.5% and hardness of 226.2HV.The vacuum sintered alloy had a density of 5.4g.cm-3,a porosity of 16.3% and hardness of 192.6HV.This shows that hot isostatic pressing sintering can significantly improve the compactness and performance of NiTi alloy. |
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