Brazing Time and Temperature Affected Joints of Brazing Titanium Graphite and TZM Alloy

When the brazing filer metal is selected, process parameters of brazing process is the main reason to impact joint’s organization and performance. Use titanium alloy as brazing filer metal. The temperature of heat preservation, time of heat preservation and quantity of brazing filer metal will affect structure and properties of joint, and this joint is made up by graphite and TZM alloy braze welding. To analysis the mechanism and optimize the process parameters, making joint using temperature increases to 1400 ℃, and joint re-melting temperature is higher than 1600 ℃. Besides, the joint strength is about 141 ~ 150MPa.

Brazing process significantly affects the property and performance of joint. Brazing temperature and time determine the thickness and composition of the brazing layer. Combined with Fick law of diffusion and the research of ceramic and metal activity brazing found in certain reaction system the reaction layer thickness depends on the connection time and temperature. With the increase of the brazing time, temperature increasing, the thickness of the reaction layer is increase, till saturation. If the reaction temperature is low, or the time is short, and the reaction layer is not continuous. Besides, the wetting effect of brazing filler metal on graphite base material is insufficiency. What’s more, the interface bonding force is too small. Analyzed interface reaction layer’s thickness and morphology by SEM found within a certain range, the higher the temperature, the longer the time, the TiC reaction layer is more wider. However, the interface reaction does not increase with time and temperature without limit, when the reaction layer thickness reaches a certain extent will produce dense carbides to separate Ti and graphite. Ti difficult to penetrate or diffuse through the carbide layer to react with graphite, so long temperature preservation can not be significantly improved reaction layer thickness of the joint.

TZM ALLOY PICTURE

In addition, the brazing temperature and time also determines the thickness of the Ti-Mo solid solution layer. The weld of brazing and TZM substrate through diffusion reaction can change the composition of brazing filler metal to form a solid solution, and after isothermal solidification can obtain solid solution. Analyze the connection of TZM by TLP (transient liquid phase) diffusion mechanism. After brazing filler metal was melted, Mo of TZM element through the interface of brazing filler metal dissolves in liquid the brazing filler metal, and uniformly, resulting in solid solution melting point increase, solidification occurs in the brazing temperature. When the brazing filler metal thickness is 0.1mm, at different temperatures between 1700 ~ 1750 ℃, the thickness of the Ti-Mo layer has some differences. Showing from non-steady-state diffusion equation, the brazing temperature and time determines the diffusion amount of Mo to brazing filler metal. After solidification can form Ti-Mo solid solution layer with different thickness and composition. Brazing temperature and time optimized can produce interface reaction layer with appropriate thickness and the solid solution layer. Interface reaction layer of 30 ~ 40um is of TiC, and the joint is made up by 70 ~ 80um Ti-Mo solid solution layer has better performance. Further improve the brazing temperature and time to produce thicker interfacial reaction layer will weaken performance of joint and result in the TZM matrix grains to grow, reducing the mechanical properties.


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