The framed sealing strip (perimeter closed sealing frame) serves as the core sealing and pressure-bearing component of the plate-fin radiator. Processing defects mainly occur in five key stages: dimensional tolerance of the sealing strip, assembly clearance, cleanliness, weld formation during vacuum brazing, and post-brazing shaping & leak testing. The control points for each process are as follows:
I. Sealing Strip Blanking, Forming and Pre-treatment Process (Gap Control at Source)
- Strict control of dimensional tolerances
- Length tolerance of long/short sealing strips ≤ ±0.3 mm, height tolerance ≤ ±0.02 mm. The four sides of the framed sealing strip shall form 90° right angles at joints with angular deviation ＜ 0.5°.
- Flatness of the brazing contact surface of sealing strip: surface warpage ≤ 0.1 mm. Wave distortion is prohibited. Uneven brazing gaps caused by warpage will lead to incomplete brazing and filler metal penetration.
2. Deburring and edge finishing
After sawing/milling, all burrs on top & bottom surfaces and joint corners of sealing strips shall be removed. Burr height ≤ 0.02 mm. Sharp edges shall be lightly rounded to R0.2 to avoid scratching separators and puncturing fins during assembly.
3. Roughness control of brazing surface
The welding surface of sealing strip mating with separator shall not be over-polished. Moderate micro-roughening (Ra1.6~3.2 μm) is required to improve capillary spreading of brazing filler metal. Smooth surfaces tend to induce shrinkage cavities of brazing filler metal and discontinuous welds.
4. Storage protection
Store sealing strips by type in dry and dust-free transfer containers to prevent moisture exposure and oxidation. The corner grooves of framed sealing strips are prone to dust and aluminum chip accumulation; secondary air blowing cleaning is required before assembly.
II. Ultrasonic Cleaning Process (Prevent Porosity and Oxidation Defects)
The closed frame of the framed sealing strip tends to trap contaminants, so its cleaning standard is higher than that of ordinary split sealing strips:
- Complete cleaning sequence: Alkaline degreasing → Multi-stage pure water rinsing → Vapor degreasing → Hot air drying (constant temperature at 60℃ for ≥5 min).
- Cleanliness inspection: Test with #40 dyne pen. Lines drawn on the four-side brazing surfaces of sealing strips shall be uniform without shrinkage or beading to be deemed qualified. Residual oil and release agent will cause dense porosity and leakage in brazed joints.
- Time limit control: Assembly and furnace loading shall be finished within 24 hours after cleaning and drying. Re-cleaning is required if exceeding the time limit to avoid secondary oxidation on sealing strip surface caused by moisture and dust in ambient air.
- Distinction between vacuum brazing and CAB atmosphere brazing
- Vacuum brazing: No residual moisture is allowed; drying must be thorough.
- NOCOLOK atmosphere brazing: Apply flux evenly on both inner and outer frame surfaces of the sealing strip. Supplement flux spraying at dead corners inside the frame to avoid incomplete brazing caused by local missing flux.
III. Core Stacking & Assembly Process (Key Control Stage for Framed Sealing Strips)
The framed sealing strip forms a closed flow channel. Assembly clearance directly determines the brazing sealing performance, making this a high-defect-occurrence process:
- Personnel and fixture requirements Wear dust-free white gloves throughout the process; bare-hand contact with brazing surfaces is prohibited. Fixtures shall have sufficient rigidity. Multiple tie rods shall be tightened symmetrically and synchronously to prevent core tilting on one side and excessive local clearance of sealing strips.
- Stacking sequence and clearance specifications
- Contact clearance between separator and sealing strip ≤ 0.02 mm. Clearance shall be uniform around the entire frame; uneven tightness (tight on one side, loose on the other) is not allowed.
- Fins must not be pressed under the sealing strip base. A 3~5 mm gap shall be reserved between fin ends and the inner wall of sealing strips to prevent fin collapse and flow channel blockage during brazing.
- After stacking of multi-layer cores, the height difference around the core ≤1 mm. Diagonal dimension deviation shall comply with drawing tolerances.
3. Control of sealing strip joint corners Joint gap between long and short framed sealing strips ≤0.15 mm. The joint seam shall align with the separator plane. Excessively large gaps cause brazing filler metal loss and form through leakage paths. Inspect after assembly to ensure no step offset, misalignment or warpage of sealing strips.
4. Brazing filler metal placement specification For vacuum brazing, foil-type brazing filler metal shall be attached to the upper and lower brazing surfaces of sealing strips. Small pieces of additional filler metal shall be added at the four corners of the frame. Corners concentrate heat and have weak capillary action, which easily leads to insufficient filler metal and incomplete brazing.
IV. Vacuum Brazing Furnace Process Control (Mitigation of Incomplete Brazing, Distortion and Filler Metal Bleed-out)
Framed sealing strips have low overall rigidity. Uneven temperature rise can easily cause frame distortion and weld failure:
- Vacuum level requirements Vacuum degree ≥1×10⁻³ Pa during heating stage; vacuum shall remain stable without fluctuation at brazing holding temperature. Minor furnace leakage will result in black oxidation on sealing strip surfaces. Brazing filler metal cannot wet the base material, leading to circumferential incomplete brazing.
- Segmented heating profile (gradient heating to prevent frame distortion)
- Low-temperature degassing hold at 200~260℃: Fully evacuate moisture and oil volatiles trapped in sealing strip gaps and fins, to avoid porosity generated by sudden gas evolution at high temperature.
- Intermediate temperature hold plateau at 380~400℃: Relieve assembly stress of framed sealing strips, preventing sealing strip warpage and clearance increase caused by stress release at high temperature.
- Brazing hold temperature at 597~605℃, holding time 15~25 min depending on core thickness. For framed designs with multiple corners, slightly extend holding time to ensure brazing filler metal fully fills gaps along four sides and corners. Insufficient holding time causes incomplete brazing; excessive holding time leads to filler metal overflow, flow channel blockage and overheating distortion of base material.
3. Furnace loading and placement requirements
Arrange cores evenly either vertically or horizontally. Avoid stacking multiple parts which may squeeze framed sealing strips. For large radiators, add auxiliary weights on the framed sealing strip area to restrain high-temperature warpage and distortion.
4. Cooling control
Adopt gradient slow cooling after brazing. Forced air cooling is prohibited above 500℃. Rapid cooling will induce stress cracking at the welds between framed sealing strips and separators, resulting in leakage during subsequent pressure testing.
V. Post-brazing Shaping and Machining Process
- Core shaping
Perform cold straightening of framed sealing strip distortion promptly after furnace unloading to restore the flatness of four sides to drawing requirements. Violent hammering is prohibited to prevent cracks in brazed joints.
2. Drilling and tapping machining (Key protection for openings on sealing strip area) If holes need to be made on the side or top of the framed sealing strip:
- Completely remove aluminum chips and cutting fluid after machining; blow the interior of flow channels with high-pressure air guns.
- Conduct secondary visual inspection on welds around holes. Vibration during drilling may induce microcracks in brazed joints at corners.
- Re-dry after machining to eliminate corrosion leakage caused by residual cutting fluid.
VI. Full-Item Inspection Control (Special Inspection Items for Framed Sealing Strips)
- Visual inspection
- The full perimeter weld of the framed sealing strip shall be continuous and fully filled, free of broken welds, holes, and filler metal overflow that blocks flow channels.
- Focus inspection on welds at four corners; no insufficient filler metal, black oxidation or cracks are allowed.
- The sealing strip shall be free of warpage and distortion, and joint gaps shall be completely filled with brazing filler metal.
2. Non-destructive tightness test (Mandatory 100% full inspection)
- Air pressure hold test: Low-pressure preliminary screening to quickly detect large through-leaks of the framed sealing strip.
- Helium mass spectrometer leak detection (Mandatory for high-pressure water-cooled and new energy thermal management products): The full perimeter weld and four corners of the framed sealing strip are key leak-check zones with specified standard leak rate.
- Hydrostatic pressure test: Hold pressure at the product design pressure. Check that no water seepage or sweating occurs on the framed sealing strip welds.
3. Dimensional re-inspection
Re-measure the outer frame length, width, diagonal and height of the framed sealing strip after brazing to confirm no out-of-tolerance distortion. Radiators for high-pressure service conditions shall be rejected directly if distortion exceeds tolerance.
VII. Summary of Common Defect Prevention
- Full-circumference incomplete brazing and leakage of framed sealing strip: Caused by inadequate cleaning, excessive assembly clearance, insufficient supplementary filler metal at corners, and insufficient vacuum level.
- Porosity and incomplete brazing at four corners: Caused by insufficient degassing during heating, lack of supplementary brazing filler metal at corners, and short holding time.
- Warpage and distortion of sealing strip frame after brazing: Caused by uneven fixture tightening, excessively fast heating rate, and absence of intermediate-temperature stress-relief hold.
- Flow channel blockage due to brazing filler metal overflow: Caused by too small assembly clearance, excessive brazing filler metal, and overlong brazing holding time
- Minor leakage during pressure test (microcracks at corners): Caused by rapid cooling after brazing and weld vibration from machining drilling.
