Working principle and system composition of vacuum brazing furnace

Ⅰ. What is vacuum brazing and a vacuum brazing furnace?

Vacuum brazing is a precision welding technique that uses heat to melt a brazing filler metal (which has a lower melting point than the parent metal) in a high vacuum environment (typically below 10-3 Pa to 10-4 Pa or even higher) without the use of flux. The liquid brazing filler metal wets the parent metal, fills the joint gap, and diffuses into the parent metal, thereby achieving a joint.

A vacuum brazing furnace is specialized industrial equipment that provides the high vacuum environment and precise heating process required for vacuum brazing. It is the core element for achieving high-quality brazing.

II. The Working Principle of a Vacuum Brazing Furnace: Its basic workflow can be summarized as follows:

1. Loading the Furnace: Load the cleaned assembly workpiece into the furnace.
2. Evacuation: Close the furnace door, activate the vacuum system, and evacuate the air from the furnace chamber to a preset vacuum level (generally 10-1). This step is crucial, as it removes oxygen and water vapor to prevent oxidation of the workpiece during heating.
3. Heating: While maintaining a high vacuum, the workpiece is heated according to the preset brazing process profile (temperature-time curve). The heating rate, brazing temperature, and holding time all require precise control.
4. Vacuum Brazing: When the temperature rises above the melting point of the brazing filler metal, the filler metal melts and fills the brazing joint by capillary action. During the holding stage, the filler metal dissolves and diffuses into the base metal, forming a strong metallurgical bond.
5. Cooling: After the holding stage is complete, the cooling stage begins. The cooling rate is typically controlled (e.g., by staged cooling or forced cooling with high-purity inert gases such as nitrogen or argon) to ensure workpiece performance and prevent deformation.
6. Unloading: Once the furnace temperature has cooled to a safe range (usually 50°C below the solidus of the brazing material), the vacuum system is shut off, atmospheric air is pumped into the furnace, and the furnace door is opened to remove the finished product.

III. Core Components of a Vacuum Brazing Furnace

A typical vacuum brazing furnace consists of the following main systems:
1. Furnace Body: Typically a double-layered, water-cooled jacket, it serves as a vacuum-sealed container, withstanding external atmospheric pressure.
2. Vacuum System: The “heart” of the furnace, responsible for creating and maintaining a high vacuum environment. It typically consists of a mechanical pump (serving as a backing pump), a Roots pump (increasing pumping speed), and a diffusion pump or molecular pump (achieving high vacuum) connected in series.
3. Heating System: Heating elements: Typically made of high-temperature-resistant materials such as molybdenum, graphite, or tungsten, are arranged around the furnace chamber. Insulation: Located between the heating elements and the furnace body, it consists of multiple layers of molybdenum sheets or graphite felt to reduce heat loss and improve thermal efficiency.
4. Temperature Control System: Consisting of thermocouples, a programmable temperature controller, and a power regulator, it accurately executes the preset process profile, controls heating and cooling rates, and ensures temperature uniformity.
5. Gas Insulation System: During the cooling phase, high-purity inert gas is introduced into the furnace to accelerate cooling. 6. Water Cooling System: Provides circulating cooling water for components requiring heat dissipation, such as the furnace body, electrodes, and vacuum pump.
7. Control System: Modern vacuum brazing furnaces typically utilize a PLC (Programmable Logic Controller) and a human-machine interface (HMI touch screen) to achieve automated control and data recording of the entire brazing process.

IV. Main Advantages of Vacuum Brazing

Compared to methods such as flame brazing and induction brazing, which operate in the atmosphere or use flux, vacuum brazing offers unparalleled advantages:
1. No oxidation, high finish: The vacuum environment completely eliminates oxygen, preventing workpiece oxidation. The brazed surface is bright and requires no subsequent cleaning.
2. High-quality welds: The absence of flux eliminates defects such as porosity and slag inclusions, resulting in dense, uniform, and high-strength brazed joints.
3. Compatible with complex structures and difficult-to-weld materials: Multiple, complex welds can be brazed in a single pass, making it particularly suitable for joining easily oxidized materials such as stainless steel, high-temperature alloys, titanium alloys, aluminum alloys, cemented carbide, ceramics, and composite materials.
4. Environmentally friendly and pollution-free: The entire process uses no flux, resulting in no exhaust gas or waste residue emissions, making it a green and clean manufacturing technology.
5. Minimal workpiece deformation: The entire workpiece is uniformly heated and cooled within the furnace, minimizing thermal stress and significantly reducing deformation compared to localized heating methods. 6. High degree of automation: process parameters can be precisely controlled, with good repeatability, stable product quality, and suitable for mass production.

V. Main Application Areas

Vacuum brazing furnaces are key equipment in high-end manufacturing and are widely used in the following applications:
Aerospace: engine blades, honeycomb structures, radar components, heat exchangers, etc.
Automotive: intercoolers, oil coolers, exhaust gas recirculation (EGR) coolers, etc.
Electronics and Power: vacuum electronic devices such as IGBT water-cooled radiators, microwave tubes, and traveling wave tubes.
Tools and Dies: welding of carbide cutting tools, diamond saw blades, and drill bits.
Refrigeration and HVAC: plate-fin heat exchangers, microchannel heat exchangers, etc.
Medical Devices: surgical instruments, implants, etc.