| Porous ceramics is a kind of new ceramic material with excellent performance and wide application because of its special pore structure and special properties.At present,many countries,such as the United States,Europe,Japan and other countries have invested a lot in the field of porous ceramics.China has gradually attached importance to the development and application of this field.Porous ceramics are widely used in many fields,such as vehicle exhaust catalyst carrier,porous ceramic electrode,filtration,sound absorption,heat in sulation,wave permeability materials,and even in biomedical applications.However,porous ceramics materials have high brittleness and are difficult to process and shape,leading to the high cost and high quality of large-size porous ceramics.Therefore,it is particularly important to connect simple porous ceramic parts into complex components with high quality,so as to expand the application of porous ceramics.In this project,Si3N4 porous ceramics with porosity of 50% are used.Porous ceramics are difficult to join for the following reasons: Firstly,ceramic coordination bonds are mainly composed of stable covalent bonds,which are difficult to be wetted by molten metal solder;Secondly,it is necessary to provide additional driving force to overcome the energy barrier of penetration.The penetration of liquid filler metal into porous ceramic pores is a necessary condition for the completion of welding.Ultrasonic-assisted brazing utilizes the action of acoustic energy and thermal energy of ultrasonic to promote the capillary action of the filler metal to drill into the pores.Thus forming a metallized diffusion layer,without the use of active filler metal,which can weld porous materials that are difficult to wetting.Therefore,the wettability of Si3N4 porous ceramics with low temperature Sn9 Zn filler metal and medium temperature Zn5 Al filler metal,the diffusion depth of filler metal and the corresponding influencing factors are studied in the experiment under the action of ultrasound.For porous ceramic materials with good wetting and penetration,ultrasonic assisted brazing was used to connect Si 3N4 porous ceramic structures,and the joints with good properties were obtained.In this paper,the solder tank model with ultrasonic excitation is calculated by ANSYS simulation software,and the harmonic response and transient analysis of the surface vibration field are carried out.The data shows that the amplitude values on both sides of the solder tank are about 2 times of the middle p osition.Therefore,in the wetting test of Si3N4 porous ceramics,the two sides of the solder tank are good wetting positions.The wetting experiment results of Si3N4 porous ceramics show that the Sn9 Zn solder can wetting the base metal,and the penetratio n depth increases with the increase of ultrasonic loading time or ultrasonic amplitude.However,there is a limit of the penetration depth when the strong ultrasonic loading is applied for a long time.When the Zn5 Al solder is used to wetting the porous ceramics,the solder can also drill in,the law is the same as that of Sn9 Zn,but the penetration depth of Zn5 Al pore is shallow,so the wetting parts can be used for subsequent welding experiments.SEM and EPMA tests show that Sn9 Zn solder and base metal are filled with dense and no holes.TEM analysis of the ceramic/solder interface shows that an amorphous layer with a thickness of about 2.5nm is formed at the interface.3-8nm thick amorphous layers are formed at the interface of Zn5 Al solder.Si3N4 porous ceramics can achieve good infiltration state under the action of ultrasonic assistance,for the Sn9 Zn solder and Zn5 Al solder infiltration of porous ceramic blocks,used as welding structural parts.In the welding process,liquid filler metal is precoated and the welding parts are formed by loading ultrasonic and force,and the reliable joints are obtained by changing the experimental process parameters.The joint microstructure interface of the two solders was analyzed.The weld was compact and uniform,and there were no defects such as porosity cracks.Hardness test and shear mechanics test for welded joints were carried out.Among them,the shear strength of the Sn9 Zn solder joint is stable at about 29 MPa,and the fracture is on the side of the filler metal.The XRD analysis of the fracture shows that the main diffraction peak is Sn element.When the Zn5 Al filler metal is used for welding,the shear strength of the welded joint of the Zn5 Al filler metal is about 40 MPa,and the fracture occurs on the ceramic side,which achieves good wetting and welding effect.The connection between the wetting and the connection is established,and the corresponding welded joint is explored under the best experimental parameters.Through EPMA test and analysis of the whole welding seam,the diffusion zone of the filler metal is compact,the joint is reliable,and there is no obvious defect.The two-dimensional schematic diagram of porous ceramics was drawn by CAD,including slender through holes and complex shapes.The threshold value of liquid solder flowing into the pores of porous ceramics was analyzed by pressure formula,and the possibility of solder drilling into the pores was verified from theoretical calculation.In order to realize the structure of connecting porous ceramics with high quality at low temperature,Sn9Zn2 Al with Al as the active element was used for wettability and welding experiments.The strength of the welded joint at 240 ℃could reach 37.27 MPa.The ultrasonic assisted brazing method was adopted,and the joint interface obtained was well combined,meeting the strength requirements proposed in the background of the project.It can realize the bonding performance of Si3N4 porous ceramics at low temperature. |
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