Strong, highly conductive thermal and mechanical joints and seals
Vacuum Brazing Overview
Vacuum brazed assemblies can be found in a wide array of markets and applications. These assemblies can range from oil coolers for automotive applications to highly complex environmental control systems with reduced weights for the aerospace and defense industries. The vacuum brazing process is especially ideal for products where high quality joints, lightweight assemblies, and high thermal and mechanical performances are required.
Compared to other metallurgical techniques for joining aluminum, vacuum furnace brazing offers numerous technical advantages:
- High-strength, void-free, leak-free joints similar to parent materials
- Proof pressures can reach up to 800 psi and burst pressures up to 1300 psi
- Ability to fill long, otherwise inaccessible joints
- Option for internal joints, even in complex structures, increasing the part’s overall strength
Excellent Thermal Properties:
- Uniform thermal conductivity from using similar parent and braze material properties
- Temperature resistance up to 350°F (176°C)
- Enables highly enhanced surface structures to enhance thermal transfer
Consistent & Clean Results:
- Minimal distortion due to uniform heating and cooling, ideal for complex geometries
- Consistently reproducible results with tightly toleranced joining surfaces
- Extremely clean parts with no residual corrosive flux due to the flux-free process (unlike dip brazing)
- No surface deterioration during processing
Single process production of complex assemblies with multiple joints can 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.
The Vacuum Braze Process
Vacuum brazing is a manufacturing process for joining components by heating a braze alloy between the assembly components. Braze alloys have a lower melting temperature than the parent component material. Prior to the heating process, the braze alloy is applied between close fitting components of the parent alloy that are fixtured into place. To expedite assembly, specially braze-clad components can remove the braze alloy application step. Assembled parts are kept fixtured together to maintain contact during heating and cooling. Assemblies are placed in a vacuum furnace that is heated to at least 450°C, a level that will melt the braze alloy but not the main material. The furnace has a vacuum environment, which eliminates the risk of oxidation and need for flux. The molten braze alloy fills in gaps between components through capillary action so that when the part is cooled, it forms a joint through atomic attraction and diffusion. Vacuum brazing produces a clean, one-piece construction with strong joints with thermal conductivity nearly the same as the parent material.
Uniform heating, tight temperature control, no post cleaning processes, and process repeatability make Vacuum brazing an ideal process for thermal and structural assemblies. Aluminum cold plates, plate-fin heat exchangers, and flat tube heat exchangers are often vacuum brazed.
Vacuum Brazing Preparation & Setup
Before vacuum brazing, cold plate and heat exchanger components are cleaned. Removing grease, oil, dirt, and oxides ensures there is uniform capillary action for the braze alloy to spread. Clean components are required to achieve the highest quality braze joints.
Temporary parts are tack welded to the more complex assemblies to maintain alignment during component expansion and contraction during the heating and cooling cycle. This is essential for heat exchangers, which are built from many layers of smaller components.
Vacuum Brazing & Vacuum Braze Ovens
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.
Aavid, Thermal Division of Boyd Corporation braze furnaces are certified to braze per AWS C3.7 Class A, B, and C.
Vacuum Brazing Post Processing
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 will be added to the furnace to collect remaining oxygen molecules and prevent oxidation. Before assemblies are brazed, engineers define a temperature profile for the vacuum furnace to achieve the highest quality braze for the specific design and application.
After temporary component removal and final assembly cleaning, brazed assemblies 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.
Vacuum Brazed Assembly Quality Assurance
Depending on application requirements of the part and its complexity, a helium leak check can verify the leak tightness of brazed joints. Parts that require T4 or T6 hardness stage undergo heat treatment after leak tight tests. This enables machining at similar feeds and speeds as those for untreated aluminum. Depending on individual part geometries, additional processes such as straightening may allow more effective ways of machining the finished components when required.
Vacuum Brazed Assemblies
Although it can be limited by fixturing or oven size, vacuum brazing is an extremely versatile and clean process. As such, it tends to be the preferred manufacturing process used to produce lightweight, high performance assemblies with increased internal surface areas. These structures can range from plank type assemblies with internal channels to electronic enclosures, with or without internal features such as cold walls.
Aavid specializes in Brazed Heat Exchangers, Liquid Cold Plates, Heat Sinks, Chassis, and Enclosures.
Liquid Cold Plates
Aluminum vacuum-brazed cold plates channel fluid flow within a cavity between two aluminum plates:
Highly specialized geometry allows for uniform cooling on both sides of the cold plate enabling more consistent performance.
Flat Tube Heat Exchangers
Flat Tube Heat Exchangers, also known as oil coolers, have fins between flat tubes. Increase surface area within flat tubes and external fins to maximize thermal performance of your heat exchanger
Tube-Fin Heat Exchangers
Tube-Fin Heat Exchanger utilize tubes passing through a dense fin stack. Typically supported by a mounting frame for added mechanical strength. Also known as finned coil heat exchangers.
Plate Fin Heat Exchangers
Solid plates conduct heat from liquid flow paths to secondary fluid flows and offer increased structural integrity and easy customization within Plate Fin Heat Exchangers.
Common Aluminum Brazing Alloys and Fillers
Brazeable Alloys include:
- Wrought Aluminum 6061, 6063, 6082, 6951 (heat treatable) and 1100
- 3000 Series (non heat treatable)
- Cast Alloys A356, A357 and 443, 700 Series (less typical)
- 2000 and 7000 Series materials not typically brazed due to alloy limitations
- 5000 Series due to magnesium content
- Silicon, copper, zinc and others added in small quantities to aluminum to lower its melting point
- Magnesium for vacuum brazing
- Typical Corrosion Resistant Filler Alloys include 4047, 4343 and 4045
- Comes in sheet, wire and powder forms
- Clad Brazing Sheet (filler alloy embedded on aluminum surface) also used
- No. 21 through 24 can be used for vacuum brazing