What is vacuum brazing?
Brazing is a metal-joining process in which two or more materials are joined when a filler metal (with a melting point lower than those of the materials themselves) is drawn into the joint between them by capillary action.
Vacuum Brazing Euipments have many advantages over other metal-joining techniques, particularly welding. Since the base metals never melt, brazing allows much tighter control over tolerances and produces a cleaner connection, normally without the need for secondary finishing. Because components are heated uniformly, brazing consequently results in less thermal distortion than welding. This process also provides the ability to easily join dissimilar metals and non-metals and is ideally suited to cost-effective joining of complex and multi-part assemblies.
How does vacuum brazing work?
High Temperature Vacuum Brazing Furnace is carried out in the absence of air, using a specialized furnace, which delivers significant advantages:
Extremely clean, flux-free joints of high integrity and superior strength
Improved temperature uniformity
Lower residual stresses due to slow heating and cooling cycle
Significantly improved thermal and mechanical properties of the material
Heat treating or age hardening in the same furnace cycle
Easily adapted for mass production
In order to obtain high-quality brazed joints, parts must be closely fitted and the base metals must be clean and free of oxides, normally accomplished by either chemical or mechanical (abrasive) cleaning. In the case of mechanical cleaning, proper surface roughness must be maintained as the capillary action of the filler material occurs much more readily on a rough surface than a smooth surface.
Temperature and time are also important factors that contribute to the quality of brazed joints. As the temperature of the braze alloy is increased, the alloying and wetting action of the filler metal increases as well. In general, the temperature selected must be above the melting point of the filler metal, though there are several other factors that influence temperature selection.
Typically, the preferred process will have the lowest possible braze temperature to minimize heat effects on the assembly, keep filler metal/base metal interactions to a minimum and maximize the life of fixtures. Most production braze processes are optimized to minimize Low Temperature Vacuum Brazing Furnace time and the associated costs.
In vacuum processing, the load being heated in the furnace is rapidly cooled at the end of the heat cycle to impart desired physical properties in a process referred to as quenching. Although the metallurgical reasons for quenching vary depending on the process used, in all cases the goal is to quickly cool the load. Gas quenching involves introducing an inert gas (i.e. nitrogen, argon, or helium) into the furnace and rapidly circulating it through the heating chamber under pressure to remove the stored thermal energy from the load. While being circulated, the gas is forced through a Vacuum Heat Treatment Furnace to remove its heat.
There are several interesting and important factors that play a role in the design of a heat exchanger used for quenching and cooling workloads in a vacuum furnace. In order to understand the function of the heat exchanger in the quenching process, we first want to review the fundamentals of the heat exchanger operation.
An Atmosphere Heat Treatment Furnace is a device that transfers heat from one fluid (liquid or gas) to another fluid (liquid or gas) without the two fluids coming in direct contact. The type of heat exchanger typically used in a vacuum furnace is the finned tube type. Heat is first transferred from the hot gas to the fins and then to the tubes by convection, then through the tube wall by conduction and finally from the tube interior to the cold fluid inside the tube, again by convection. The efficiency of a heat exchanger is highly dependent on the mode of heat transfer, with convection being the dominant form of heat transfer in fluids. However, the conductivity of the materials must also be considered.