Strong Joints, No Residue, & High Thermal Conductivity
Vacuum brazing is a high-end joining technology that results in parts with extremely strong joints and no residual corrosive flux. It is a process in which two base metals, such as aluminum plates, are joined using a filler metal that has a melting point below that of the base metal. The filler metal, also known as a braze alloy, is drawn into the closely mated parallel surfaces of the aluminum plates by capillary action.
The vacuum brazing process provides the benefits of uniform heating, tight temperature control, no post cleaning processes, and process repeatability. Because of this, aluminum cold plates, plate-fin heat exchangers, and flat tube heat exchangers are often vacuum-brazed.
Aluminum vacuum-brazed cold plates consist of corrugated aluminum fins brazed into a cavity between two aluminum plates. Plate-fin heat exchangers have fin-filled passages that are separated by flat plates. The fin is brazed to the plates, which are multi-layer aluminum sheets with braze alloy on both sides. The braze alloy that is selected for plate-fin heat exchangers depends on the base metal alloys as well as application-specific requirements. Flat tube heat exchangers, also known as oil coolers, have fins between flat tubes.
Before vacuum brazing, cold plate and heat exchanger components must first be cleaned. Removing grease, oil, dirt, and oxides ensures there is uniform capillary action, which is needed to achieve the highest quality braze joints.
After components are cleaned, they are stored in sealed containers and placed in a temperature and humidity controlled room for assembly. This ensures that components are protected from additional oxidation and contamination prior to assembly and brazing. During assembly, protective gloves are worn to further protect components from contamination.
To start the brazing process, parts are assembled and placed into brazing fixtures. Low thermal mass fixtures reduce brazing cycle time. If magnesium is not present in the alloy, it generally will be added to the furnace as a “getter” of any remaining oxygen molecules. Before being placed in the vacuum furnace, a furnace profile is identified for achieving the highest quality braze, this varies by application and design.
The furnace profile specifies the temperature, vacuum level, and cycle time. Brazing of cold plates and heat exchangers usually takes place at approximately 1100°F (593°C) and a vacuum level between 5 to 6 Torr. However, the profile depends in large part on alloys selected, total mass in the furnace, and the vacuum furnace being used. The furnace controller monitors vacuum levels and temperatures and automatically advances to the next segment as programmed in the recipe until the cycle is complete.
Compared to other metallurgical techniques for joining aluminum, vacuum furnace brazing offers numerous technical advantages:
- High-strength, void-free, leak-free joints approaching parent metal strength, with proof pressures up to 800 psi and burst pressures up to 1300 psi
- Temperature resistance up to 350°F (176°C)
- Consistently reproducible results with tightly toleranced joining surfaces
- Uniform thermal conductivity
- Extremely clean parts with no residual corrosive flux due to the flux-free process (unlike dip brazing)
- Ability to fill long, otherwise inaccessible joints
- Minimal distortion due to uniform heating and cooling
- No surface deterioration during processing
Single process production of complex assemblies with multiple joints can also lower cost while increasing product quality. Yields of 80% are typical for the vacuum brazing process, but 98% or better yields are possible in a carefully controlled process. With high yields for complex fabrications, vacuum brazing is the preferred joining process for manufacturing high performance aluminum cold plates and heat exchangers.