Wetting And Spreading Of Ag/Ti Brazing System And The Preparation Of Porous Titanium By Its In-situ Reaction

Titanium and titanium alloys have excellent properties such as low density,high specific strength,excellent corrosion resistance and biocompatibility.They are widely used in aerospace,military equipment,medical treatment,chemical industry and other fields.Brazing is an important means to connect titanium alloy with other metal or ceramic materials.For the brazing connection of titanium and its alloys,Ag-based filler metal is a commonly used filler metal,and its wetting behavior on titanium is very important for the quality of brazed joints.Wetting and spreading are affected by many factors such as temperature,surface morphology of substrate,interface element composition and protective atmosphere（argon or vacuum）.In order to effectively control the brazing process,it is necessary to quantitatively analyze and predict the wetting and spreading process of Ag/Ti system.It was found in the test that the precursor film appeared during wetting and spreading of Ag-based filler metal on the surface of titanium alloy in argon environment;while in vacuum,the wetting of the precursor film on the surface of pure Ti induces the formation of a special porous structure.However,so far,there are few reports on the formation of precursor film and surface porous structure of Ag/Ti alloy wetting system.The relevant mechanism and influencing factors are not clear.In particular,the precursor film and porous structure in wetting and spreading are of great significance to the brazing process and the preparation of biomedical materials.It is of great significance to explore its mechanism and control its process,so as to guide relevant production and application.Therefore,this paper takes Ag/Ti system as the research object,in which pure Ag and Ag Cu Ti are used as filler metals,TA1,TA2,TC4 and TB8 are used as substrates.The in-situ wetting tests under argon and vacuum are carried out respectively.The wetting and spreading mechanism,kinetic and influencing factors of Ag/Ti system were studied;the formation mechanism of precursor film was revealed;At the same time,the wetting and spreading of Ag/Ti system under vacuum and the formation mechanism of porous structure on Ti surface induced by precursor film were explored.In order to understand the formation mechanism of precursor film when Ag-based filler metal is wetted and spread on the surface of Ti alloy,the in-situ wetting and spreading test of Ag Cu Ti on TC4 in argon environment was carried out.The results showed that the precursor film phenomenon of rapid spreading appeared in the front of the bulk liquid（main part of the molten filler metal）in the middle and late stage of spreading process,and there is a clear distinction between them.The kinetic data of the bulk liquid and the precursor film are fitted by Rⁿ～t model.It is confirmed that the surface roughness of substrate has little effect on the spreading of bulk liquid,but promotes the spread of the precursor film evidently,and the spread speed of the precursor film is much higher than that of the bulk liquid.The chemical reaction of Cu-Ti mainly occurs at the wetting interface,and Ti（s,s）,Ag（s,s）and Ti₂Cu are formed at the interface.Due to the diffusion of Ag,the phase transition ofα-Ti toβ-Ti occurred in the center and edge of the wetting interface,β-Ti is formed on the side of the TC4 substrate.The wetting process is mainly controlled by the chemical reaction in the bulk liquid area.The essence of the precursor film is silver spilled from the bulk liquid,which shows similar spreading characteristics of pure Ag on TC4.Based on the rapid absorption then film overflow and Marangoni movement,the formation mechanism of precursor film is revealed.To understand the effects of temperature,substrate composition and special surface texture on the wetting and precursor film of Ag/Ti system,three groups of in-situ wetting and spreading tests in argon environment were designed.Firstly,the wetting and spreading characteristics of Ag Cu Ti/TC4 system at 800°C,850°C,900°C and 950°C were studied.The results show that the spreading speed and maximum spreading area of the bulk liquid of Ag Cu Ti/TC4 system gradually increase with the increase of temperature;There is no precursor film at low temperature;When the temperature reaches 950°C,a rapidly spreading precursor film appears.According to the evolution of wetting interface,with the increase of temperature,Ag in the bulk liquid will segregate and adsorb at the liquid-gas interface and gradually transfer to the vicinity of triple line,and finally overflow to TC4 to form a precursor film;the Cu-Ti reaction is gradually intense,and the interface structure of Ti（s,s）+Ti₂Cu phase is gradually evolved from a variety of Cu-Ti compounds.The interface evolution verifies the formation mechanism of the precursor film,and shows that the formation of the precursor film is determined by specific interface structure and temperature.Secondly,the wetting and spreading characteristics of pure Ag on three different composition and tissue substrates（TA2,TB8 and TC4）were studied.The results show that Ag showed the characteristics of rapid spreading of precursor film on these three Ti alloys,which further verified the essence of precursor film;Among them,Ag has a very fast spreading speed on TA2,but the final spreading area is small.The spreading speed of Ag on TC4 and TB8 is slower,but the final spreading area is significantly larger than that of Ag/TA2.The wetting interface structure of the three systems is obviously different,in which Ag/TA2 have a strong chemical reaction and form an obvious Ti Ag reaction layer,resulting in a faster spreading speed.The interfacial reaction of Ag/TC4 system is weakened and segregation of Ag occurs atβ-Ti,only a small amount of Ti Ag is distributed between Ag（s,s）and Ti（s,s）.The interfacial reaction of Ag/TB8 is further weakened and a large amount of Ag（s,s）was formed.Ag onβ-Ti alloy has better wettability.Finally,the surface textures of micro bumps,micro-pits,micro-grooves and micro-threads were processed on TA2 surface by ultrafast laser technology,and the wetting and spreading experiments of Ag Cu Ti on TA2 surface with different textures were carried out.The results show that special surface texture can effectively change the wetting characteristics of Ag/Ti system,including spreading speed and final spreading area,and affect the structure and composition of wetting interface.Ag Cu Ti filler metals have preferred orientation when wetting on the surface of special texture TA2.In order to further study the wetting characteristics of precursor film（Ag）on titanium surface in vacuum environment,the wetting and spreading test of Ag/TA2system in vacuum was designed.It was found that micron porous structure with dispersed Ag NPs was formed on the surface of TA2 wetted by Ag.With the increase of temperature,the porosity,pore size and distribution depth of porous structure gradually increase,but the content of Ag also decreases.The formation mechanism of the porous structure and Ag NPs were clarified.In short,Ti and Ag react to form Ti Ag and Ti₂Ag,which are finally transformed intoβ-Ti and liquid Ag in-situ at high temperature.Then,both liquid Ag and Ag dissolved in TA2（equivalent to pore forming agent）are removed by volatilization/sublimation effect to form pores in TA2.These pores further grow and aggregate under the internal Ag vapor pressure.In the cooling process,a stable micron porous structure is finally formed,and some Ag vapor will be condensed to form Ag NPs on the porous structure.The porous Ti in this study is easy to prepare and has good biocompatibility,surface wettability and corrosion resistance.The elastic modulus of porous titanium is close to that of human bone,which helps to alleviate the problem of stress shielding.Due to the modification of Ag NPs,porous Ti has excellent antibacterial activity against S.aureus and E.coli.The porous titanium is expected to be a promising Ti-based implant material in biomedical research and orthopedic applications.