Failure Analysis of Blackening Defect at Joints of NOCOLOK® Brazed Cold Plates

一,Phenomenon Description

The cold plate is brazed in a nitrogen-protected furnace with NOCOLOK® potassium fluoroaluminate flux. Black discoloration appears on the surface of the nozzle (insert/sleeve joint). Leakage is detected during helium leak test and hydrostatic pressure test. Section inspection reveals flux coking, slag inclusion in brazed seams, incomplete brazing and microcracks within the blackened area. Leakage points are concentrated at the root of the nozzle insertion.

二,Blackening Mechanism

1. Excessive flux and impure furnace atmosphere (root cause): Excessively high flux concentration and heavy coating are applied on the nozzle. Inadequate drying leaves organic substances in the flux suspension, which carbonize during furnace heating and form black residues adhering to the outer wall of the nozzle and around the brazed joint. Insufficient nitrogen purity with excessive oxygen and moisture causes re‑oxidation of the aluminum base material. The oxide film reacts with the NOCOLOK® flux to generate dark compounds.
2. Local overheating of temperature field: The wall thickness of the nozzle differs greatly from that of the plate‑fin substrate, leading to inconsistent heat absorption rates. The nozzle zone reaches an excessively high temperature, accelerating flux decomposition and precipitating black residues. Mg in the base material precipitates at high temperature and reacts with NOCOLOK® flux to form dark complex salts, aggravating blackening.
3. Inadequate pre‑brazing cleaning: Oil contamination and cutting residues on the nozzle are not fully removed by ultrasonic cleaning. These organic contaminants carbonize at high temperature and deposit on the joint area.

✅ Blackening and leakage usually share the same source: blackening indicates abnormal flux condition, furnace atmosphere and temperature in this region, accompanied by brazed joint defects. Blackening is merely a superficial phenomenon, while leakage originates from metallurgical defects of the brazed seam.

三,Root Causes of Leakage

1. Uncontrolled assembly clearance: The clearance between the inserted nozzle and the base plate hole is uneven, with local over‑tolerance gaps. Insufficient capillary action prevents the filler metal (4045 / 4343) from filling the gap completely, resulting in incomplete brazing and penetrating micropores that cause leakage during helium test. If the clearance is too small, filler metal fails to flow into the root.
2. Uneven temperature and poor filler metal wetting: The nozzle absorbs heat at inconsistent rates. Some local areas fail to reach the brazing holding temperature, so NOCOLOK® flux cannot fully remove the alumina film. The filler metal fails to wet the base material, leading to discontinuous brazing at the root. Local overheating causes base metal erosion and thinning, forming penetrating micropores for leakage.
3. Flux residue inclusions: Excess molten flux remains at the root of the brazed seam and forms slag inclusions, which interrupt continuous filling of filler metal and create leakage channels.
4. Cooling shrinkage microcracks: Large temperature difference during cooling leads to mismatched thermal shrinkage between the nozzle and cold plate substrate. Residual stress builds up at the joint and generates microcracks in the brazed seam, which leak under pressure test.

四,Investigation and Verification Methods

1. Visual inspection: Visual check plus magnifying glass observation to mark blackened areas.
2. Helium leak location: Pinpoint leakage points.
3. Metallographic sectioning: Examine the cross‑section of brazed seams to confirm defects such as incomplete brazing, slag inclusion, base metal erosion or shrinkage cracks.
4. Black-box temperature measurement: Monitor the actual peak temperature at the nozzle root to verify furnace temperature uniformity.
5. Furnace atmosphere testing: Measure oxygen content and dew point of nitrogen.
6. Flux weighing test: Quantify the flux loading on nozzle areas.

五,Corrective Actions

1. Flux control: Avoid separate heavy coating of high-concentration flux on nozzles. Control the overall flux loading (typically ≤5 g/m²). Ensure thorough drying after spraying to eliminate residual moisture and organic matter.
2. Pre-brazing cleaning: Perform ultrasonic degreasing on nozzles and base plates to remove cutting fluid and oil contamination from surfaces.
3. Assembly optimization: Control the insertion clearance within a reasonable range. Chamfer and deburr the inserts, ensure proper seating, and clamp with positioning fixtures to prevent clearance enlargement caused by thermal deformation during heating.
4. Furnace temperature and atmosphere control: Maintain nitrogen purity ≥99.999% with low dew point. Optimize the heating profile to mitigate local overheating of nozzles. Regulate peak temperature and holding time to avoid base metal erosion. Adopt staged cooling to reduce thermal shrinkage stress.
5. Inspection: Carry out full visual inspection and helium leak test after brazing, with focused sampling inspection on nozzle joints.

六,Summary

Nozzle blackening is mainly caused by excessive flux, carbonization of organic substances, oxygen and moisture in furnace atmosphere, and local overheating. Leakage generally results from improper clearance, poor wetting, slag inclusion, base metal erosion and shrinkage microcracks formed during cooling. Priority control of flux loading, assembly clearance, furnace atmosphere and temperature profile, combined with proper pre‑brazing cleaning, can simultaneously mitigate blackening and leakage issues.