Vacuum heat treatment is critical for automotive and aerospace component manufacturers

Vacuum heat treatment is critical to automotive and aerospace component manufacturers, as the industry demands the highest levels of quality. Brazing and surface treatment are two process areas that utilize vacuum technology.
Vacuum Brazing
More parts are produced using vacuum brazing than any other process using vacuum furnaces. The transportation (automotive and aerospace) industries have contributed to the increased use of vacuum furnaces for brazing, while the use of lightweight, high-strength materials has also contributed to the popularity of brazing. All brazing requires control of process variables to ensure a strong brazed joint. These variables include: type and characteristics of base metal; type and characteristics of filler metal; design of components; design and gaps of joint; surface preparation; flow characteristics of filler metal; temperature and time; and the rate and manner of heating. Some of these factors affect the behavior of the brazed joint and therefore the ability to obtain a strong joint. Other factors affect the properties of the base metal or the interaction between the base metal and filler metal. Effects on base metals include carbide precipitation, hydrogen embrittlement, heat-affected zone properties, oxide stability, and sulfur embrittlement; on filler metals include vapor pressure, alloying, phosphorus embrittlement, and stress cracking; and on interactions include post-braze heat treatment, corrosion resistance, and combinations of dissimilar metals.
Vacuum furnaces can be either horizontal or vertical designs, and their technical features include:
1) Complex, densely packed components with blind holes can be brazed, which are almost impossible to braze and adequately clean using atmosphere brazing methods.
2) Vacuum furnaces operating at 10-5–10-4 can essentially remove all gases that may hinder the flow of brazing alloys, prevent the formation of difficult-to-remove oxide films, and promote wetting and flow of brazing alloys on vacuum-treated surfaces.
3) Properly treated parts remain clean and bright when they come out of the furnace, and often no further treatment is required.
4) Many types of materials—such as aluminum, cast iron, stainless steel, steel, titanium alloys, nickel alloys, and cobalt-based superalloys—can be successfully brazed in vacuum furnaces without the use of any flux.
Vacuum brazing of aluminum and aluminum alloys
For brazing aluminum parts, a vacuum of 10-5 Torr or better must be maintained. Parts are heated to 575°C–590°C, depending on the alloy type. Temperature uniformity is very important, with typical values ​​of ±5.5°C or better. Zoned temperature controlled furnaces are usually used. Cycle time depends on furnace type, part construction, and part clamping method. Larger parts and denser charges require longer times.
Vacuum brazing of copper and copper alloys
Vacuum brazing can be used for filler metal copper applied to the base metal in the form of paste, foil, cladding, or solid. It is important to note that copper has a high vapor pressure at its melting point, which can generate some vapor inside the furnace and cause undesirable contamination. To avoid this problem, the furnace is first evacuated to a low pressure of 10-2–10-4 Torr to remove trapped air, and then the temperature is raised to approximately 955°C to allow outgassing to remove any surface contamination. Finally, the furnace is brought up to brazing temperature—typically 1,100°C–1,120°C—under an inert gas partial pressure of up to 0.75 Torr to suppress copper volatilization. After vacuum brazing is complete (usually within a few minutes of reaching the temperature set point), the workpieces are slowly cooled to approximately 980°C to allow the filler metal to solidify. A gas quench (typically 2 bar) is then used to rapidly cool the parts.
Nickel-based alloy vacuum brazing
Brazing with nickel-based alloys is typically performed under a vacuum of 10-3–10-5 Torr without any partial pressure. Preheating to 920°C–980°C and holding for a certain period of time is generally performed to ensure uniform heating of large workpieces. After brazing, the furnace temperature may be lowered for additional solution or hardening heat treatment before gas cooling and unloading.