Flux is commonly used to prevent oxides from forming during brazing. When flux is applied to the joint surfaces before brazing, it creates a chemical barrier that inhibits the formation of oxides. This is important because oxides can interfere with the metallurgical bond in brazing and prevent the filler metal from properly wetting and flowing.
In addition to flux, controlled atmospheres can also be used in furnace brazing to prevent the formation of oxides. These atmospheres remove or reduce oxides on the surface of the base metals, allowing the filler metal to form strong joints. It is important that the brazing atmospheres are compatible with both the base metals and filler metals being used.
Certain metals, such as aluminum, have a natural oxide coating that can inhibit the flow of brazing filler metals. In these cases, it is necessary to suppress the oxide layer before brazing. This can be done through chemical actions, such as using a corrosive flux or acid attack, or through mechanical actions like sanding. It is important to ensure that the brazing temperature is accurately controlled and that there is a homogeneous thermal distribution within the load and on the pieces being brazed.
It is worth noting that not all aluminum alloys can be brazed. Aluminum is highly reactive with oxygen, and if there is any trace of oxygen in the furnace, oxide will form again, preventing proper wetting of the metal to be brazed. Magnesium can be used to reduce the re-formation of the oxide layer and improve the wetting process, flow of the brazing, and quality of the joint.
In the case of titanium, it oxidizes rapidly when heated in any gaseous atmosphere. These titanium oxides cannot be reduced or eliminated in the furnace environment, making the surface of the titanium part impossible to braze. Therefore, titanium should not be brazed in a gaseous atmosphere.
It is important to note that brazing filler metals will not bond or flow with oils, lubricants, dirt, or any oxides on the metal surface. Therefore, it is crucial to prevent oxide formation in the first place to ensure successful brazing.
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