The vacuum brazing process for heat sinks imposes extremely high demands on part precision and parameter control. Furthermore, due to the long weld seams, achieving a a 100% welding qualification rate is very difficult. For localized leakage points, repair welding processes can be employed for remediation. Below, focusing on the specific and challenging process of "TIG Repair Welding for Aluminum Vacuum Brazed Heat Sink Leakage," I will provide a systematic and detailed introduction. This is not merely simple welding but a precision repair technique.
First, it is essential to clarify: Vacuum brazing is a joining process that uses filler metal with a melting point lower than the base material (e.g., aluminum-silicon series) heated uniformly in a vacuum furnace, filling gaps via capillary action. Its characteristics include uniform overall heating, minimal deformation, and smooth weld seams. In contrast, TIG repair welding is a localized welding repair process, which can easily lead to issues such as *stress concentration, deformation, burn-through, and secondary leakage. Therefore, the goal of repair welding is: to minimize thermal impact on the original sound areas while meeting the requirements for sealing and strength.
I. Pre-Welding Preparation
1. Defect Localization and Assessment
Clean suspected areas with alcohol or acetone, and carefully inspect for cracks or leakage points using a high-power magnifying glass or industrial borescope.
Assessment: If the leakage point is located in areas with dense water channels or main load-bearing ribs, repair welding may cause chain-leakage or severe deformation. An evaluation should be made on whether repair is worthwhile.
2. Surface Cleaning
Mechanical Cleaning: Use a stainless steel wire brush to thoroughly remove the oxide film, scale, and contaminants from the weld area until a metallic luster appears. Use files and angle grinders cautiously to avoid excessive removal of base material.
Chemical Cleaning: Use acetone or specialized aluminum cleaners for degreasing. For stubborn oxide layers, a mild alkaline cleaner can be used, followed by thorough rinsing with clean water and drying.
Immediate Welding: The cleaned area should be welded within 2 hours to prevent excessive formation of a new oxide film (Al₂O₃).
3. Groove Preparation
For cracks, it is necessary to create a groove using a micro-mill, small grinding wheel, or scraper.
Groove Type: A U-groove or V-groove is recommended to ensure complete exposure of the groove root, facilitating penetration and gas protection.
Stop Holes: Drill Φ1-2mm stop holes at both ends of the visible crack to prevent crack propagation during welding.
4. Material and Equipment Preparation
Filler Wire Selection
Priority should be given to grades matching the base material. Common heat sink materials are 3003/6063/6061 aluminum alloys, for which ER4043 (good general usability, excellent fluidity, crack-resistant) or ER5356 (higher strength, greater hardness, slightly poorer fluidity) are typically selected.
Filler wire should be stored in dedicated wire boxes, kept clean and dry.
Equipment:
AC/DC Square Wave TIG Welder (AC TIG): Essential. Aluminum welding requires the "cathode cleaning" effect to remove the oxide film, achievable only with Alternating Current.
High-Purity Argon Gas (Purity ≥99.99%), with dry gas lines.
Tungsten Electrode: Ceriated Tungsten (WC-20) is recommended. Diameter is chosen based on current (typically 1.6mm or 2.4mm).
Gas Lens: Replaces the standard ceramic nozzle, providing a more laminar and effective argon shielding gas layer, which is crucial for aluminum welding.
5. Preheating
Overall Preheating: For larger or structurally complex heat sinks, overall preheating in an oven at 200°C is recommended. This significantly reduces welding heat input, minimizes thermal differential stress, prevents hot cracking, and aids hydrogen escape.
Localized Preheating: A gas torch or heat gun can be used for gentle preheating around the repair area. Do not exceed 250°C to avoid damaging the performance of the original brazed joint.
II. TIG Repair Welding Process Parameters and Operational Essentials
1. Parameter Reference (Example for common 2-3mm thickness)
Current: Use AC mode. Current range approximately 80-120A, requiring fine-tuning based on specific location and heat dissipation conditions. Principle: Use the minimum current and highest speed possible while ensuring fusion.
Frequency: Modern welders allow frequency adjustment. Recommended setting: 80-120 Hz. Higher frequency concentrates the arc, reducing the heat-affected zone (HAZ).
Cleaning Width (Balance): In AC, the "cleaning" (electrode positive) percentage can be set to 65-75% to ensure sufficient cathode cleaning action.
Argon Flow Rate: 8-15 L/min, delivered via gas lens.
Tungsten Extension Length: Keep as short as possible, about 3-5mm, to enhance shielding.
Filler Wire Diameter: Commonly Φ1.6mm or Φ2.0mm.
2. Operational Techniques
Posture: Ensure stable angles for the torch, filler wire, and workpiece. The forehand (leftward) technique is commonly used.
Arc Initiation and Termination: Use high-frequency start or lift start to avoid contamination from contact start. Use the welder's current decay function at arc termination to fill the crater and prevent crater cracks.
Heat Input Control: Use intermittent tack welding or rapid segmental welding. Weld a small section (10-20mm), stop immediately to allow heat dissipation, wait for the temperature to drop to a level touchable by the back of the hand (approx. 50-60°C), then weld the next section. This is the core strategy to prevent deformation and burn-through!
Weld Pool Observation: The color change of the aluminum weld pool is not obvious. Observe the formation of surface tension and edge wetting. Add filler wire only after the base material forms a bright, fluid pool.
Filler Wire Feeding: The filler wire should always remain within the argon shielding zone. Add it using a "dabbing" or "continuous feed" method, with gentle and uniform motion.
III. Post-Weld Treatment and Inspection
1. Post-Weld Cleaning: Use a stainless steel brush to clean oxidation discoloration and spatter from the weld surface.
2. Leak Testing:
Pressure Leak Test: Seal all ports of the heat sink, introduce compressed air (or nitrogen) at the required product pressure, submerge in water, and observe for bubbles. This is the most direct test method.
Pressure Holding Test: After pressurization, let it stand for a period and observe if the pressure gauge reading drops.
Fluorescent Penetrant or Helium Mass Spectrometry Leak Testing: Used for extremely high-requirement applications.
3. Non-Destructive Testing (Optional): For critical areas, Dye Penetrant Inspection (PT) can be used to check for surface cracks.
4. Straightening (If Necessary): If minor deformation occurs, cold correction using a mold on a press is acceptable. Avoid hammering.
IV. Common Problems and Countermeasures During TIG Repair Welding
| Common Repair Welding Problem |
Possible Causes |
Corrective Measures |
| Weld collapse or burn-through |
Excessive current, prolonged dwell time, groove too thin |
Reduce current, increase travel speed, add copper backing bar for heat dissipation. |
| Black, porous weld surface |
Insufficient shielding gas, impure gas, drafts, unclean wire/base metal |
Check gas lines, increase flow, set up wind shields, thorough cleaning. |
| Cracks (Hot cracks) |
High restraint stress, material mismatch, improper arc termination |
Strict preheating and segmental welding, use ER4043 filler, utilize current decay. |
| Rough weld bead, lack of fusion |
Current too low, insufficient cleaning, oxide film not removed |
Appropriately increase current, increase cleaning percentage, re-clean with wire brush. |
| "Popping" during welding |
Residual moisture inside material or internal defects/oxidation from brazing parameters |
Thoroughly dry workpiece, ensure groove is completely dry; evaluate repair feasibility if brazing defects are suspected. |
| Severe softening of HAZ |
Excessive heat input, prolonged welding time |
Core strategy: Low current, segmental welding, forced cooling (e.g., wet copper block on backside). |
V. Special Precautions and Recommendations for Repair Welding
1.Safety First: Welding aluminum produces stronger ultraviolet radiation. Wear dark protective lenses (≥ #12), gloves, and flame-resistant clothing. Maintain good ventilation to avoid inhaling aluminum fumes.
2.Mindset: TIG repair welding of aluminum heat sinks is a "precise, embroidery-like" task. Avoid aiming to complete the weld in a single pass. Patience and meticulous preparation are more important than the welding act itself.
3.Alternative Solution Evaluation: For very minor leaks, consider using low-temperature aluminum brazing filler (torch brazing) or specialized epoxy for sealing, which have less thermal impact and deformation. However, strength and durability need evaluation based on operating conditions.
4.Record Keeping: Document defect location, welding parameters, and operational sequence to facilitate experience summarization.
Conclusion: TIG repair welding for vacuum brazed aluminum heat sinks is a precise remanufacturing technique. Its success does not rely on highly difficult welding skills, but rather on meticulous pre-weld preparation, extremely strict heat input control, and a systematic inspection process. For high-value heat sinks, if conditions permit, entrusting the task to an experienced professional technician is a more prudent choice.
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