Vacuum Brazing and its Applications
Like welding, brazing is a method of permanently joining separate metal pieces. Where brazing differs though is that there’s no melting of the pieces being joined. This makes it the preferred process in applications where it’s important that material properties remain unchanged.
Impro gained vacuum brazing capabilities when Impro Aerotek completed the acquisition of Foshan Ameriforge Manufacturing Technology Co., Ltd. (“M-Tech”) in 2022. This blog explains what brazing entails, discusses the benefits of performing it under vacuum, and identifies some applications.
Basics of Brazing
The brazing process entails joining separate metal pieces with a lower melting point filler. The parts are assembled with the filler in the joint. The assembly is then heated until the filler melts. Capillary action draws the liquid through the joint, at which point the assembly is cooled. The filler metal solidifies, forming metallurgical bonds with the component parts.
Filler metals are chosen for their compatibility with the component parts. Their melting point must be lower, and they must be capable of bonding. Most metals can be brazed, and with careful filler selection it’s possible to join those that are dissimilar.
When brazing aluminum components the filler will typically have a melting point of around 840°F (450°C). When the parts are made from alloy or stainless steel, or even superalloys, fillers with melting points of 1,470°F (800°C) or higher may be used.
Brazing Vs. Welding
Welding entails local melting of two separate pieces, usually with the addition of a filler metal. If the metals are dissimilar this melting can lower overall strength, change metallurgical compositions and cause distortion and cracking.
Welding can be especially problematic in alloys with high coefficients of thermal conductivity, pieces with thin sections, and when the metals have high concentrations of alloying elements. All these factors increase the risk of distortion and weld defects.
The Brazing Process
Capillary action requires a stable and consistent gap between the parts being joined. This dictates they undergo precision machining before assembly.
The filler is applied as a paste or solid as the parts are assembled. If brazing takes place in an oxygen rich atmosphere a flux is also applied to the joint. As it heats this initiates a reaction that cleans the metal surfaces and excludes oxygen. Both help ensure a defect-free join.
The assembled parts are moved into an oven and heated until the filler melts. Once enough time has passed for the filler to flow as needed, the parts are taken out of the heat and allowed to cool. The last step is to clean the flux residue from the assembly
An alternative to flux is to braze in a vacuum. This eliminates the flux and flux cleaning steps and eliminates any possibility of oxides forming in the joint.
Vacuum brazing requires an oven that can be sealed and is equipped with radiant heating. Parts are loaded into the oven, the door closed tightly and a vacuum pulled. Only then is the heating turned on and the filler melted. Air is not allowed back into the oven until the filler has solidified.
Benefits of Vacuum Brazing
The process offers numerous advantages over welding.
- Joins dissimilar metals
- No metallurgical changes in the workpieces
- No distortion
- No discoloration – vacuum brazing yields clean, bright finishes
Applications for Vacuum Brazing
The process is an alternative when the alloys involved are difficult to weld or when it’s important to minimize thermal effects. It is used extensively for aerospace components where it’s critical to avoid cracking and distortion. Fuel and hydraulic system components are good applications as brazed joints are reliably leak-tight.
Gas turbine engine components are often assembled with brazing as this avoids any changes to the metallurgy of the alloys involved.
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Impro Aerotek has extensive aerospace component manufacturing expertise, supported by AS9100 and relevant NADCAP certifications. If you’re looking for a reliable source of high quality engineered components, we can help. Our capabilities go from casting and precision machining to assembly, inspection and test. Contact us to learn more.