Analysis of Brazing Collapse in Aluminum Plate-Fin Heat Exchangers

Collapse of aluminum plate-fin heat exchangers (a critical issue in aluminum brazing defects commonly found in automotive intercoolers, oil coolers, and aerospace heat exchangers) during vacuum brazing is one of the most intractable failure modes in heat exchanger failure analysis. It generally signifies that at the brazing temperature, the structural strength is insufficient to support the component’s own weight or the fixture pressure, leading to blocked passages or loss of hermeticity. A systematic brazing collapse analysis of the problem is presented below.

1. Collapse Mechanism and Hazards

During the holding stage of vacuum brazing (approximately 595–620 °C), the base metal (mostly 3003 aluminum alloy) softens, while the brazing filler metal (e.g., 4004 aluminum alloy) melts into a liquid. If the fins buckle or the load-bearing capacity of the core is inadequate, collapse occurs under gravity or fixture pressure. In minor cases, the fins become distorted; in severe cases, multiple layers of parting sheets stick together, and the product is directly scrapped—a common outcome in aluminum heat exchanger brazing failures.

2. Cause Analysis of Collapse

Collapse is rarely caused by a single factor; rather, it results from the combined effect of four aspects: temperature, material, structure, and tooling. Effective brazing process optimization requires addressing all four.

2.1 Improper Brazing Process Parameters

(1) Excessively high brazing temperature
When the temperature greatly exceeds the Al-Si eutectic point, grain growth occurs in the base metal, causing a drastic drop in high-temperature strength that can no longer support the load.

(2) Excessively long holding time
Prolonged presence of the liquid filler metal intensifies erosion of the core base metal, weakening the fin strength—a key factor in brazing thermal cycle control.

(3) Excessively slow heating rate
Lingering too long in the high-temperature zone causes premature volatilization of the gettering agent (vacuum brazing relies on magnesium vapor) and simultaneously results in excessive softening, compromising brazing joint integrity.

2.2 Material Compatibility and Core Strength

(1) Recrystallization softening of the core
At the brazing temperature, 3003 aluminum alloy undergoes recrystallization, and its strength plummets from >100 MPa (before annealing) to approximately 10–20 MPa. If the material has a coarse grain structure, softening is even more rapid—a critical consideration in aluminum brazing sheet selection.

(2) Improper fin selection
The high-temperature collapse resistance of the fin material is inadequate. Typically, modified 3003 alloys (grades with higher Zr or Mn content) or composite foils are specified for optimal brazing fin design. If a pure aluminum series (e.g., 1100) is mistakenly used, collapse becomes highly likely. Moreover, different fin geometries offer different collapse resistance—serrated or offset fins exhibit the poorest collapse resistance, an important factor in heat exchanger core assembly.

(3) Erosion of the brazing filler metal layer
The cladding layer on composite fins or composite plates may be too thick, or severe erosion can penetrate the cladding and attack the core, resulting in brazing filler metal erosion defects.

2.3 Component Tolerances and Assembly Quality

(1) Overly tight core assembly
An excessively small assembly gap between the fins and the parting sheets causes the fin ends to be compressed and buckle during thermal expansion.

(2) Overly loose core assembly
An excessively large gap prevents the fins from being effectively constrained in the hot state, causing them to topple.

(3) Side bar height tolerance
When the side bar height is lower than the fin height, the entire fixture pressure is borne by the fins, crushing them directly.

2.4 Structural Design and Fixturing

• Excessively large aspect ratio: Fins that are too tall or too thin (large height-to-thickness ratio) lead to instability as calculated for slender compression struts.

• Excessive clamping force: Excessive pressure applied to correct distortion cannot be sustained by the fins at elevated temperature.

• Insufficient support: Lack of auxiliary support in the middle of a large core leads to central collapse under gravity—a common issue in brazing fixture design.

3. Troubleshooting and Resolution Approaches

It is recommended to conduct brazing defect troubleshooting in the following sequence:

1. Inspect the process curve
   Verify whether the actual furnace temperature deviates from the specification and whether the thermocouple locations are correct. If necessary, moderately lower the brazing temperature (e.g., from 605 °C to 600 °C) or reduce the holding time—a key step in vacuum brazing process control.

2. Confirm the material batch
   Check whether the fin material is a collapse-resistant grade (e.g., 3003 + 1% Zn modified alloy). Samples can be taken for high-temperature compressive strength testing.

3. Evaluate the assembly gap
   Measure the fin height and side bar height before assembly. The ideal condition is an assembly gap of approximately 0.03 mm, ensuring that the fixture pressure is applied predominantly to the side bars rather than to the fins—essential for quality aluminum brazing outcomes.

4. Verify the vacuum level
   A vacuum level that is insufficient (i.e., pressure higher than 10⁻² Pa) allows oxide films to hinder uniform heat transfer, resulting in localized overheating, erosion, and collapse—a critical parameter in controlled atmosphere brazing.