Active Metal Brazing and the Role of Vacuum Furnaces

Introduction

Active brazing is a method employed when traditional brazing techniques prove insufficient, particularly with challenging materials such as ceramics-including alumina, silicon carbide, sapphire, diamond, and graphite. Enter active brazing, a streamlined, single-stage process that utilizes various forms of braze alloy-foil, paste, wire, or preforms. Often performed in a vacuum furnace, this method creates robust joints where the material usually outperforms the parent materials.

The Crucial Role of Vacuum Furnaces in Active Brazing

The vacuum furnace plays an indispensable role in active brazing. By operating in a controlled environment devoid of oxygen, vacuum furnaces ensure the exclusion of any oxygen that might prematurely react with the active elements in the braze alloy. This controlled environment is essential for achieving the desired intermetallic reactions between the filler metal's active elements, such as titanium, and the oxides, carbides, or nitrides on the surface of the materials being joined. Moreover, the absence of contaminants and the uniform heating provided by vacuum furnaces contribute to the formation of strong and reliable joints.

Brazing Atmosphere Considerations

The brazing atmosphere plays a pivotal role in achieving high-quality joints. Active brazing is best performed in a vacuum to exclude oxygen, preventing premature reactions of the active element with air. Alternatively, an inert atmosphere can be used, provided the components are thoroughly purged of air. It's crucial to note that both oxygen and nitrogen can react with titanium, ruling out nitrogen as a shielding gas when using titanium-containing active braze alloys.

How Does Active Brazing Work?

Unlike traditional brazing, which necessitates oxide removal for wetting to occur, active brazing relies on a reaction between the filler metal's active element-primarily titanium-and surface contaminants like oxides, carbides, or nitrides. This reaction forms compounds like titanium oxide, titanium carbide, or titanium nitride, facilitating the wetting of the ceramic surface. Commercially available active braze alloys may also contain hafnium, vanadium, zirconium, or chromium as active elements. The resulting characteristics of the intermetallic layer depend on the active element concentration and brazing cycle parameters, emphasizing the importance of careful alloy and cycle selection.

Brazing Ceramics to Metals and Metals to Metals

Active brazing extends beyond ceramic joining applications. When brazing ceramics to metals, differences in thermal expansion coefficients between the materials can pose challenges. However, with careful filler metal selection and design considerations, these challenges can be mitigated. Active braze alloys are also highly effective in brazing metals like stainless steel, which has a protective chromium oxide layer easily managed by these alloys. They ensure effective wetting of stainless steels and refractory materials like tungsten carbide.

• 1150°C Maximum Temperature Rating
• Belt Speed Control
• ±1°C Cross Belt Uniformity
• ±1°C PID Control Precision
• ±1.5 °C Cross Belt Uniformity
• Atmosphere Available

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