There are a number of factors that influence the development of a brazing cycle. These include such things as base metal and braze alloy composition, mass of the assembly and joint design. 

However, each cycle is comprised of a number of common segments. The illustration below shows the typical profile for a vacuum brazing cycle. During the initial pumpdown, water vapour adsorbed by the parts and furnace is driven off. For most brazing applications, a pumpdown before heating to a vacuum level of 8 x 10-4 torr or better is recommended. A vacuum safety interlock should be programmed into the cycle to ensure this level is reached. After pumpdown, the initial heating rate should not exceed 15ºC (30ºF) per minute. Faster rates may cause paste braze alloy to spall off or distortion of the assembly. Heating continues to a stand off temperature at about 25ºC (50ºF) below the solidus temperature of the braze alloy. The load is then soaked at this temperature to ensure temperature uniformity and to allow vacuum levels to recover. A soak time of 30 minutes is usually sufficient, though the incorporation of a second vacuum safety interlock in the braze cycle program may be desirable.

At the conclusion of the soak, a faster ramp rate of 15ºC to 25ºC (30ºF to 50ºF) per minute is employed to heat the load to the braze temperature. The rate must be fast enough to prevent “liquation” of the braze alloy where lower melting point constituents begin to melt separately. A faster heating rate also reduces the risk of erosion of the base metal.

The brazing temperature should be the lowest possible within the recommended range. For many braze alloys, the minimum brazing temperature will be at least 25ºC (50ºF) above the liquidus temperature. Minimum brazing temperatures are essential when using free-flowing braze alloys, when trying to braze large gaps and when brazing thin materials. At lower temperatures, molten braze alloy will be more sluggish and less reactive with the base metal. The time at brazing temperature should be just long enough to ensure that all sections of a part and all parts within the load reach the desired temperature. This time normally ranges between 5 and 10 minutes but may be longer for heavy loads. When the braze soak is complete, the cooling cycle can begin. Unless a specific heat treatment is required, it is strongly recommended that the load be cooled to a temperature at least 25ºC (50ºF) below the solidus temperature of the braze alloy before gas quenching is initiated. This will ensure that molten braze alloy has re-solidified and will not be blown away from the joint during the quench.

Despite taking all necessary precautions, joint defects will occasionally occur. Fortunately, defects can often be repaired by re-brazing. Because of diffusion and mixing of constituents between braze alloy and base metal, most braze alloys tend to develop a higher re-melt temperature after the initial braze. Rather than attempting to repair the joint defect by re-melting the existing joint, it is better to apply a small amount of additional alloy in the defective area. To prevent re-melting of the existing joint, a re-braze temperature lower than that used in the first braze is preferable, particularly if wide joint gaps are involved. The defective area should be re-inspected for cleanliness before additional braze alloy is applied. The brazing cycle can then be repeated as before, with modifications to the brazing temperature .

Contact us for assistance with your difficult brazing applications.

1. Determining “Brazeability”

2. Selecting filler metal

3. Preparation for Brazing” (ie. cleaning)

For more information on VAC AERO’s Heat Treating Services please click here.

This is the last in a series of four articles on Vacuum Brazing Tecniques. (read part 1) / (read part 2) / (read part 3)