Study On Vacuum Brazing Process And Mechanism Of ZTA Ceramic To TC4 Titanium Alloy

ZrO₂-toughened alumina ceramics(ZTA)have good application prospects in aviation,medical and other fields due to their good abrasion performance,insulating property,corrosion resistance and high temperature resistance.It is difficult to be processed into complex shapes because of its high hardness and brittleness.By brazing the ceramic or to metal into the desired shape,it not only avoids mechanical processing but also exert the advantages of ceramic and metal.TC4 have been widely used in medical,aerospace and other fields because of high specific strength,good heat resistance and good compatibility with human body.In summary,ZTA/TC4 dissimilar joint will have a broad application space in medical,aerospace and other fields.However,when brazing metals to ceramics with common metal brazing fillers,there is often a problem of large residual thermal stress in the joints after welding due to the mismatch of the thermal expansion coefficients of the metals and ceramics,which reduces the joint strength.In order to study the influence of process parameters on the microstructure and mechanical properties of the joint,this subject brazed ZTA ceramics to TC4 titanium alloy with AgCu28 solder firstly.Then,an attempt,by preparing a silver-based composite solder with a lower thermal expansion coefficient and higher strength aluminum borate whisker reinforcement,was made to solve the problem that reduced joint strength due to the difference in thermal expansion coefficient.In view of the problems that the Al₁₈B₄O₃₃w is likely to react with Ti during the brazing process,this topic proposes to improve the connection quality of ZTA/TC4 joint,by controlling the interface reaction and replacing the matrix solder.ZTA ceramic and TC4 titanium alloy were vacuum brazed with powdered AgCu28solder at different temperatures,the microstructure of the joint was characterized and analyzed by means of SEM and EDS.The typical weld microstructure and the effect of brazing temperature on weld microstructure and joint mechanical properties were studied.The result show that the typical microstructure of the ZTA/AgCu28/TC4 joint is:TC4 titanium alloy/?-?Ti/Ti-Cu intermetallic compound/Ag(s,s)/Ti₃(Cu,Al)₃O+Ti_xO_y/ZTA ceramics.As the brazing temperature increase,the thickness of the Ti₃(Cu,Al)₃O+Ti_xO_yreaction layer on the ceramic side increase,the Ti-Cu intermetallic compound gradually aggregates to the titanium alloy side.With the increase of brazing temperature,the shear strength of the joint firstly increased and then decreased,reaching a maximum value of 54MPa at 825?.Both Ti₃(Cu,Al)₃O and Ti-Cu intermetallic compounds are brittle and hard structures,if its thickness is too high,cracks will occur inside the structure and the mechanical properties will be reduced.Therefore,when the brazing temperature is too high,due to excessive growth of Ti₃(Cu,Al)₃O and Ti-Cu intermetallic compounds,the shear strength of the joint decrease.AgCu28+Al₁₈B₄O₃₃w composite solders with different aluminum borate whisker contents were prepared,and ZTA ceramics and TC4 titanium alloys were vacuum brazed with those solders at 875?/10min.The effects of whisker content and brazing temperature on the microstructure of the weld and the mechanical properties of the joint were studied.The experimental results show that as the whisker content increases,the joint properties first increase and then decrease.When the whisker content is 2wt%,the shear strength of the joint is up to 45MPa.It is because too high whisker content,resulting in insufficient interface reaction on the ceramic side.ZTA ceramic and TC4titanium alloy were brazed with AgCu28+Al₁₈B4O₃₃w solder with whisker(2wt%)at different temperatures.SEM and EDS were used to characterize and analyze the joint microstructure.The study found that during the brazing process,Ti react to aluminum borate whiskers,and forming the Ti₃(Cu,Al)₃O phase.which reaction consumed some of the whiskers and reduced the whisker content in the weld.Typical weld structure at825?/10min is:TC4 titanium alloy/?-?Ti/Ti-Cu intermetallic compound/Ag(s,s)+Al₁₈B₄O₃₃w+Ti₃(Cu,Al)₃O/Ti₃(Cu,Al)₃O+Ti_xO_y/ZTA ceramic.With the increase of the brazing temperature,in addition to the decrease of the whisker content in the weld,the increase of Ti₃(Cu,Al)₃O and the gradual accumulation,other changes in the weld is similar to ZTA/AgCu28/TC4 joint.As the brazing temperature increases,the shear strength of the joint increases first and then decreases and reaching a maximum of56MPa at 850°C.The increase of shear strength is not obvious compared with ZTA/AgCu28/TC4 joint(the 54MPa).There are two reasons for this phenomenon:first,the addition of whisker affect the reaction at the ceramic interface,resulting in inadequate reaction at the ceramic interface,so that a reliable connection cannot be formed;second,the reaction at the ceramic interface consume a lot whisker reducing the whisker content in the weld.The surface of aluminum borate whiskers is coated with a layer of Cu by electroless plating to slow down the reaction to Ti atoms during brazing.Replacing AgCu28 solder with AgCuTi2 solder as the matrix solder of the composite solder,AgCuTi2 solder provide more Ti atoms for the ceramic interface to promote the interface reaction.An AgCuTi2+2wt%Al₁₈B4O₃₃w(copper-coated)composite solder was prepared.ZTA ceramic and TC4 titanium alloy were vacuum brazed with AgCuTi2solder and AgCuTi2+Al₁₈B₄O₃₃w(copper-coated)composite solder at different temperatures respectively.After the brazing,the microstructure observation and the shear strength test are done.The effects of copper-coated on the whisker surface,Ti element in the brazing filler metal and brazing temperature on the microstructure and the mechanical properties of the joint were studied.The results show that after the copper plating on the whiskers,even if Ti-containing AgCuTi2 solder is used to brazing,more whiskers are retained in the weld under the same brazing process parameters.As the brazing temperature increase,the change law of the microstructure and shear strength of the ZTA/AgCuTi2/TC4 joint and the ZTA/AgCuTi2+Al₁₈B4O₃₃w(Cu)/TC4 joint are similar to the ZTA/AgCu28+Al₁₈B4O₃₃w/TC4 joint and the ZTA/The AgCu28/TC4 joint.The shear strength of the ZTA/AgCuTi2/TC4 joint and the ZTA/AgCuTi2+Al₁₈B4O₃₃w(Cu)/TC4 joint reached their maximum values(58MPa and79MPa)at 875?and 900?,respectively.Among them,the maximum shear strength of the ZTA/AgCuTi2+Al₁₈B₄O₃₃w(Cu)/TC4 joint is increased by 46%and 41%compared to the ZTA/AgCu28/TC4 joint and ZTA/AgCu28+Al₁₈B₄O₃₃w/TC4joint,respectively.