Brief of Vacuum Brazing
Vacuum brazing is a material joining technique that offers significant advantages: extremely clean, superior, flux-free braze joints of high integrity and strength. The process can be expensive because it must be performed inside a vacuum chamber vessel. Temperature uniformity is maintained on the work piece when heating in a vacuum, greatly reducing residual stresses due to slow heating and cooling cycles. This, in turn, can significantly improve the thermal and mechanical properties of the material, thus providing unique heat treatment capabilities. One such capability is heat-treating or age-hardening the workpiece while performing a metal-joining process, all in a single furnace thermal cycle.
Products that are most commonly vacuum-brazed include aluminum cold plates, plate-fin heat exchangers, and flat tube heat exchangers.
Vacuum brazing is often conducted in a furnace; this means that several joints can be made at once because the whole workpiece reaches the brazing temperature. The heat is transferred using radiation, as many other methods cannot be used in a vacuum.
Vacuum brazing is brazing in a furnace using a vacuum atmosphere. Brazing is broadly defined as a group of joining processes that occur above 840°F (400°C) and below the melting point of the base metal.
Benefits of Vacuum Brazing
No oxidation eliminating the necessity for flux
No post-braze washing; guaranteed absolutely clean, inclusion-free joints
Minimized residual stress and distortion
No heat-affected zones that could potentially cause trouble
Allows performing certain heat-treating operations in the same cycle to save time and money
HOW VACUUM BRAZING WORKS
Brazing is a method of joining two or more metals by melting a filler metal, or the brazing alloy.
When alloys are used instead of a pure metal for the process in a vacuum furnace, they do not contain fluxes, there is therefore no need to remove the slag by mechanical methods and the weld appears clean. The alloy must not include elements that can sublimate or evaporate, since the desired joint characteristics will not be achieved and the furnace could be contaminated.
In general alloys do not fuse together at a well-defined temperature as do pure metals, but within a range. When possible therefore eutectic alloys should be sought. These have a similar behavior to pure metals, where the solid and liquid elements co-exist only at the eutectic temperature.
There are a significant number of filler metalsfor vacuum brazing available. In some alloys, generally with high temperature uses, there are still in many cases a situation of a “solidus” and “liquidus” phase being present at a single temperature, made up of more than two forms of metal. Other alloys, commonly used in vacuum furnaces, nevertheless show a broad intermediate “solidus-liquidus” phase with a temperature rise between the two important phases. In this case the time for a complete fusion increases as this takes place at a higher temperature, which will be the brazing process temperature.
WHY BRAZING IN A VACUUM FURNACE
The vacuum furnace can ensure reaching the melting temperature with extreme precision and without over-shoot. Also note that the problems of resistance to oxidation and corrosion of the alloy itself become negligible in the vacuum furnace.
For the brazing to be successful the filler must melt (whether in the form of a paste or as a metallic wire or tape) at its own specific melting temperature and not at a higher temperature. This primarily avoids the liquid being at a temperature at which its surface tension would be lower, and therefore wetting a greater surface area, with the resulting joint lacking the correct filling. The liquid must spread precisely within the joint cavity between the metals, creating an intermediate layer.
Capillary action ensures the alloy penetrates into the joint spaces in its liquid state. These are created by mechanical processing, exactly where the joint is required. Mechanical processing is required to obtain the right tolerances in the elements of the joint and must determine the exact bed dimension for the bond. The gap must be created so as to avoid too restrictive tolerances, in which case the bond could be difficult to fill and, at the other end of the spectrum, a weak joint would result from too great a tolerance, potentially with gaps or porosity. In fact, the alloy is drawn inside the surfaces to be joined at the wettability temperature, against the force of gravity.
An alloy is required with a melting temperature far from the melting temperature of the metals being joined, but at the same time with suitable mechanical characteristics for the joint. Moreover, in order to achieve perfect brazed joints, the joint surface must be clean, without traces of processing oils or greases, in order to have good wettability.
Brazing is simple and easy in a vacuum furnace. Due to the nature of the vacuum, heat is evenly distributed and part production is consistent. But let’s see more in detail the specific features of a vacuum furnace for brazing.