Effect Of Surface Topography Characteristics On Wetting And Brazing Of Titanium Based Alloys

Wetting phenomena have been a hot topic of experimental and theoretical research since the Young's equation initial publication.The wettability of solid surfaces is related to many important technical and industrial applications,such as lubrication,coating,printing,waterproofing and microfluidics.For an ideal solid surface,the wetting characteristics satisfy the interface free energies.However,the true surface of materials in nature,production and manufacturing processes is rough.On an isotropic rough surface,the droplet shape is almost spherical and the apparent contact angle of the droplet is nearly uniform.On the geometrically anisotropic surface structure,the shape of the liquid will be deformed and the apparent contact angle will not be uniform.Anisotropic wettability on the substrate surface has potential applications in microfluidic devices and directional flow control devices.Therefore,studies on anisotropic wettability are of great significance.The surface of the rough solid has microscopic protrusions and grooves.The unevenness of the surface will produce different barriers during the wetting of the liquid,which will affect the wetting.The surface protrusions and grooves have a influence on the formation of hole defects during the brazing process.In this paper,three kinds of Ti-based brazing and wetting systems of AgCu/TiAl,AgCu/TB8 and AgCu/TC4were chosen,and the effect of surface topography(surface roughness and surface wear scar direction)on the brazing and speading of Ti-based alloys were studied.Firstly,P360,P800,P1200 three types of sandpaper were used for directional grinding and disorderly grinding of the selected substrate surface to obtain the surface of the substrate with different surface wear marks and surface roughness Sa.The substrates were polished by polishing paste to obtain a polished surface,then observed and measure the surface of the substrate by used tomic force microscopy.The AgCu/TiAl and AgCu/TC4 systems were subjected to a wetting and spreading experiment under vacuum conditions in a vacuum furnace and analyze the wetting and spreading interface.High-low hot stage was used to perform the spreading of AgCu/TB8 and AgCu/TC4 systems under the high purity argon gas.The real-time monitoring system was used to analyze the influence of surface topography on its spreading dynamics and study the spreading interface microstructure;the experiment to analysis the effects of surface topography on the shear strength of brazed joints of AgCu/TiAl and AgCu/TC4 systems were implemented in vacuum furnace,and study the interface microstructure of brazed joints.The results show that the surface roughness Sa has a great influence on the shear strength of TiAl/AgCu/TiAl brazed joints and TC4/AgCu/TC4 brazed joints.The suitable surface roughness Sa can promote the formation of good brazed joints.The polishing scratch direction has little effect on the shear mechanical properties of the brazed joint;whether under the condition of high purity argon gas protection or vacuum;On the condition of high purity argon gas protection and vacuum,the AgCu solder wettability spread has certain directionality on the anisotropic substrate surface.Spreading preferentially along the direction of the polishing scratch,and with the increase of the surface roughness Sa,this feature become more obvious;for the AgCu/TB8 wetting system,the surface roughness Sa is larger,the faster the solder spreads during the stable spreading stage,The value of n in the exponential exponential model R~n~t become smaller.For three brazing wet systems with strong interfacial reaction in the preceding,the surface roughness Sa and the surface polishing scratch direction have no obvious influence on the brazing joint interface structure and the wetting and spreading interface structure.The analysis of fracture shows that the surface of the substrate with high roughness is likely to cause the generation of voids during the brazing process.