Vacuum & Active Metal Brazing

Brazing Education
Electron Gun A102414 Varian, manufactured Altair Technologies-USA

Vacuum brazing and active metal brazing are core processes in our advanced material joining services, particularly well suited to the joining of:

  • Nickel and Iron-based alloys containing aluminum and/or titanium
  • Refractory metals [1]
  • Reactive metals
  • Ceramics & metal to ceramics
  • Exotic base metals such as Aluminum, Titanium, Zirconium, Niobium, Molybdenum, and Tantalum
  • Beryllium brazing
Why Vacuum Brazing versus Hydrogen Brazing

Vacuum brazing deoxidizes the metals being brazed, but more importantly eliminates undesirable reactions that would have occurred in hydrogen brazing. Vacuum brazing is also sometimes favored over hydrogen brazing when brazing very large parts because of the ability to purge occluded, close-fitting areas.

Why use Active Metal Brazing

Active Metal Brazing is when a metal like titanium is added to the braze alloy, which chemically reacts with the surface of the parent ceramic, allowing the braze alloy to wet directly onto the surface. It eliminates process steps, allowing one to braze directly to raw ceramics without having to metalize/plate the surface prior to brazing; the result is time and costs savings.

What Are Refractory Metals

Refractory metals are the classification of metals that common properties and demonstrate very high melting temperatures, up to 3410° for Tungsten, which is more than double of Titanium. They all share high corrosion and abrasion/wear resistance and excellent thermal and mechanical strength properties. Although there are twelve in this class of metals, only five are widely used: Tungsten, Molybdenum, Niobium, Tantalum, and Rhenium. [1]

Vacuum Brazing Summary

Instead of using a protective and deoxidizing inert gas such as the common hydrogen brazing process, it is fluxless and is performed at high temperatures and in a high vacuum atmosphere with pressures typically in the x 10-5 range. Vacuum brazing also deoxidizes the metals being brazed, but more importantly eliminates undesirable reactions that would have occurred in hydrogen brazing and helps to vaporize or remove high vapor pressure materials that are seen as contaminants in high vacuum applications.

Even vacuum firing can be used to remove hydrogen gas from materials like stainless steel, which were previously brazed in hydrogen. Vacuum brazing is also sometimes favored over hydrogen brazing when brazing very large parts because of the ability to purge occluded, close-fitting areas. Many of the same filler metals or braze alloys used in Hydrogen brazing are also used in vacuum brazing and like Hydrogen brazing, we also vacuum braze common materials like silver, copper and stainless.

Our vacuum furnaces are cold-walled retorts having either a graphite or Moly hot zone. Most furnaces use ultra-clean cryopumps, while a few use also very clean turbopumps to achieve a high vacuum. We have vacuum furnaces of all sizes and our largest vacuum furnace has a 48” diameter hot zone.

Active Metal Brazing is Performed in a High Vacuum

Here at Altair Technologies we routinely perform active metal brazing, achieving strong hermetic bonds between ceramic and various metals. Active-Metal braze alloys eliminate process steps, allowing us to braze directly to raw ceramics (with prior clean firing), without having to metalize/plate the surface prior to brazing; the result is time and costs savings. 

Active metal alloys typically include titanium or aluminum, which when correctly heated, form bonds with virtually any ceramic-like material.   These alloys contain an “active” element (typically titanium) which chemically reacts with the surface of the parent ceramic, allowing the braze alloy to wet directly onto the surface. The titanium’s ability to react chemically with oxides, forming a reaction layer of metal-oxides, in effect creates a metalized layer that is wettable by the other constituents of the active-metal alloy. Hence, active metal brazing is a common method to braze ceramic to metal.

[1] https://www.pickpm.com/wp-content/uploads/2016/08/What-Are-Refractory-Metals.pdf