Improvement of Vacuum Brazing Technology for Titanium Alloy Plate-Fin Heat Exchanger

A specific process improvement example for multi-layer vacuum brazing of 70-layer titanium alloy (Ti-6Al-4V) plate-fin heat exchangers. The 70-layer structure significantly increases the difficulty of heat transfer, brazing seam consistency requirements and deformation control challenges, so more sophisticated equipment, tooling and thermal cycle optimization are required.
Application scenario of vacuum brazing of 70-layer titanium alloy (Ti-6Al-4V) plate-fin heat exchangerWorkpiece description: Ti-6Al-4V plate (thickness 0.5 mm) and fin (thickness 0.2 mm), 70 layers stacked, total size of about 300×200×150 mm (layer height increased to about 140-150 mm). Brazing material: B-Ti50CuZr amorphous foil (Ti-50Cu-25Zr, thickness 0.05 mm, melting range 950-980℃). Goal: Achieve 70 layers of uniform brazing seams, tensile strength ≥750 MPa, deformation <0.15 mm, and meet the high temperature and high pressure working conditions of aerospace.

1. Vacuum brazing equipment
Original process: vacuum furnace, mechanical pump + Roots pump + diffusion pump.
Improved process: Vacuum system: mechanical pump (limit 0.1 Pa) + Roots pump + two-stage turbomolecular pump (limit 10⁻⁸ Pa), working vacuum degree 10⁻⁶ Pa. Liquid nitrogen cold trap (-196℃) is installed to capture volatiles (such as moisture, Cu vapor) to keep the furnace clean. Heating system: SiC heating rods are used (6 zones: up and down, left and right + front and back), each zone is independently temperature controlled, and the power is 20 kW/zone. Insulation layer: molybdenum reflective screen (8 layers) + high-purity alumina ceramic felt (thickness 25 mm) to reduce heat loss.
Measurement and control system: Infrared thermometer (6-point monitoring, accuracy ±1℃) covers the surface of the workpiece. 10 S-type thermocouples (Pt-Rh) are embedded in the workpiece, one for every 15 layers, to monitor the temperature difference between layers (target <5℃). Advanced PLC+AI algorithm dynamically adjusts power, temperature uniformity ±2°C. Improved advantages: Two-stage turbomolecular pump increases vacuum to 10-⁶ Pa, 6-zone heating solves 70-layer heat distribution problems, and multi-point temperature measurement ensures consistency.

2. Vacuum brazing thermal cycle
Original process: gradually increase the temperature, keep warm and then gas quench. Improved process: vacuum and preheating: mechanical pump to 10 Pa, heat to 200℃ (4℃/min), keep warm for 20 minutes, release moisture and PMMA volatiles. Switch the turbomolecular pump to 10-6 Pa, heat to 500℃ (6℃/min), keep warm for 15 minutes, and remove residual gas. Uniform temperature and vacuum brazing: heat to 900℃ (8℃/min), keep warm for 25 minutes, and ensure that the 70 layers are fully uniformly heated (thermocouple verification temperature difference <5℃). Rapidly increase to 1010℃ (12℃/min, slightly higher than the liquidus 980℃), keep warm for 25 minutes, and ensure that the brazing material melts, wets and fills the 70 layers of brazing seams.
Cooling: Pass 15 bar high-purity argon (99.999%), and quickly cool to 400℃ (rate 180℃/min) to reduce thermal stress. After dropping to 200℃, gradually reduce the pressure to 2 bar, and slowly cool to room temperature (rate 20℃/min) to control deformation. Improved parameters: Brazing temperature: 1010℃ (to adapt to multi-layer heat transfer lag). Vacuum: 10-⁶ Pa (to prevent Ti oxidation and Cu volatilization). Insulation time: 25 minutes (to ensure uniform filling of 70 layers). Heating rate: slow down the initial rate and strengthen the temperature equalization stage. Improved advantages: extend the temperature equalization time to adapt to the thickness of 70 layers, 15 bar gas quenching + slow cooling double stage to control deformation, high vacuum to inhibit volatilization.

3. Vacuum brazing post-treatment
Original process: mechanical removal of flow barrier, heat treatment optional. Improved process: Removal of flow barrier: CO₂ dry ice blasting (pressure 7 bar, distance 15 cm), 8 minutes to remove BN+ZrO₂ residue. Quality inspection: Ultrasonic C-scan (frequency 15 MHz) to detect the integrity of 70 layers of brazing seams (resolution 0.03 mm). X-ray CT scanning (resolution 10 μm) to check internal pores (target <0.2 mm, proportion <0.5%). Laser interferometer to measure deformation (target <0.15 mm). Heat treatment: annealing in a vacuum furnace, cooling to 850℃, keeping warm for 3 hours to eliminate residual stress. Cooling to room temperature with 8 bar argon (rate 100℃/min). Improved advantages: Extended dry ice blasting time to adapt to multi-layer residues, high-resolution CT to ensure internal quality, and extended annealing to optimize stress distribution.

4. Expected process results
Joint performance: tensile strength ≥ 750 MPa, corrosion resistance passed 2000 hours salt spray test. Brazing seam quality: filling rate > 97%, porosity < 0.5%, 70 layers with high consistency. Deformation control: overall deformation < 0.12 mm, meeting aviation standards. Production efficiency: total cycle time is about 150 minutes, 10% higher efficiency than traditional methods.

Vacuum Brazing Furnace