Metals that cannot be brazed include titanium due to its rapid oxidation and the formation of stubborn titanium oxides that cannot be reduced in a furnace environment, making the surface impossible to braze. Additionally, certain aluminum alloys, particularly those in the 2xxx (Al-Cu) and 7xxx (Al-Zn-Mg) series, are difficult to braze due to their low melting points and the formation of stable aluminum oxide layers that prevent the wetting of filler metals.
Titanium: Titanium oxidizes rapidly when heated, forming a layer of titanium oxides that are extremely resistant to reduction. This oxide layer prevents the bonding of brazing filler metals, rendering the surface unsuitable for brazing. Attempting to braze titanium in a gaseous atmosphere is particularly problematic due to the continuous formation of this oxide layer.
Aluminum Alloys: Aluminum is highly oxidizable, forming a stable aluminum oxide (Al2O3) layer on its surface. This oxide layer is not wetted by filler metals, necessitating the suppression of this layer before brazing. Certain aluminum alloys, especially those in the 2xxx and 7xxx series, are categorized as precipitation-hardened alloys and are difficult to braze due to their low melting points and the challenges associated with suppressing the oxide layer. The suppression of aluminum oxide can be achieved chemically or mechanically, but the process is complex and requires precise control of brazing conditions.
Reactive Elements in Alloys: Alloys containing reactive elements like aluminum and titanium pose brazing challenges due to the formation of high-temperature oxides that impede the flow of brazing filler metals. This is particularly true for nickel-base superalloys, where the severity of the problem varies with alloy composition. These materials often require high vacuum levels and specialized techniques such as brush nickel plating, chemical etching, or the use of aggressive braze filler metals with self-fluxing characteristics to improve brazeability.
Off-gassing of Metallic Elements: Metals containing cadmium, zinc, or lead can release gases when heated to brazing temperatures, potentially contaminating the brazing furnace and increasing the void content of the brazed joint. This off-gassing is a significant concern, especially in vacuum furnaces, and can affect the integrity of the brazed joints.
In summary, metals and alloys that form stable oxide layers, have low melting points, or contain elements that off-gas at brazing temperatures are generally not suitable for brazing without specialized techniques and conditions. Prevention of oxide formation and careful selection of brazing methods and conditions are crucial for successful brazing of these materials.
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