Analysis of Leakage Causes in Vacuum Brazing of Aluminum Plate-Fin Heat Exchanger

Abstract:

This article introduces the vacuum brazing process for aluminum alloy plate-fin heat exchangers. It analyzes the impacts of material cladding layer thickness, surface roughness, heat exchanger assembly, vacuum brazing temperature and holding time, vacuum level, and environmental factors on the brazing quality of the heat exchanger. Corresponding process measures are proposed, and positive results have been achieved in practice.

Keywords:

Plate-fin heat exchanger; Vacuum brazing; Process

Vacuum brazing is a method of brazing performed in a vacuum atmosphere without the use of flux. One of the fundamental principles of vacuum brazing involves utilizing capillary action to draw the filler metal into the contact surfaces between the parts to be joined, thereby forming a bond between the metals being brazed. Compared to other welding methods, brazing offers advantages such as minimal deformation, the ability to join multiple components simultaneously, and the capacity to join dissimilar metals. Aluminum alloy plate-fin heat exchangers are classified as pressure vessels and must meet corresponding pressure-bearing requirements during application. Due to their structural characteristics, the joining process can only be achieved through brazing.

The core structure of an aluminum plate-fin heat exchanger consists only of long seal bars, short seal bars, parting sheets, outer fins, inner fins, and side plates.

1.Material Cutting: Shaping and sizing of parting sheets, fins, seal bars, and side plates.

2.Surface Treatment: Ultrasonic cleaning.

3.Assembly: Mechanical assembly and forming of parting sheets, fins, seal bars, side plates, etc.

4.Vacuum Furnace Brazing: Vacuum brazing generally employs a three-stage heating and soaking process:

Preliminary vacuum evacuation.

Stage 1 Preheating: Held at 380–470°C.

Stage 2 Energy Storage: Held at 560–575°C.

Stage 3 Brazing: Held at 598–603°C for 15–25 minutes.

Heating stops; the workpiece is removed from the furnace after cooling to the specified temperature.

5.Core Straightening: Mechanical correction of deformation in the heat exchanger core after vacuum brazing.

6.End Cover Welding: End covers are welded onto both ends of the heat exchanger core using TIG (Tungsten Inert Gas) welding.

7.Pressure Testing: Compressed air at 1.6 MPa is introduced to check for leakage in the heat exchanger and identify leak points. After passing the airtightness test, compressed air at 3.0 MPa is introduced for a pressure resistance test; the product must show no significant deformation.

8.Repair: Leaking heat exchangers are cut open, repaired by welding, and then subjected to another pressure test.

9.Coating: The heat exchanger is organized, coated, and dried to improve surface appearance.

10.Packaging and Delivery

The brazing performance of the parting sheet is reflected in its fluidity, wettability, gap-filling capability, erosion characteristics, and joint strength. The silicon (Si) content in the cladding layer not only determines the melting point of the alloy but also affects its fluidity, wettability, and erosion behavior towards the base alloy. Higher Si content leads to better fluidity and gap-filling capability. However, if Si diffuses to the base metal interface and causes the solid-phase composition to reach the brazing composition, it can lead to melting of the solid phase and cause erosion. Magnesium (Mg) in the cladding alloy is an essential metal activator and getter for ensuring vacuum brazing quality. Magnesium in the filler metal begins to evaporate significantly above 550°C, creating a magnesium atmosphere within the brazing chamber. This magnesium vapor combines with residual oxygen or oxygen from water vapor in the brazing atmosphere, protecting the heated part surfaces from re-oxidation, and can also penetrate and consume any remaining oxide film not completely removed from the part surfaces. Non-uniform thickness of the parting sheet cladding layer can lead to defects such as insufficient brazing, burn-through, discontinuous brazing, and leakage. Therefore, the magnesium content in the filler metal is generally controlled between 1.0% and 2.0%. Additionally, ensuring the effective thickness of the filler metal is necessary for full brazed fillets and enhancing the product's pressure-bearing capacity. Typically, the cladding layer thickness on each side of the parting sheet is 0.1–0.15 mm, which has proven highly effective in practice.

Before heat exchanger assembly, fins, parting sheets, and seal bars require cleaning to remove dirt, oil, and surface oxide layers. Oil residue decomposes during vacuum brazing, reducing the vacuum level inside the furnace and causing oxidation of the fins, parting sheets, and seal bars. The oxide layer on aluminum alloys is very dense and has a melting point higher than that of the base metal; it does not melt easily during brazing, thereby affecting brazing quality. To ensure the quality of the brazed components, strict control over the pre-brazing surface treatment of workpieces and filler metal is necessary, along with minimizing the assembly time before brazing.

The surface roughness of structural components affects capillary force. Generally, an overly smooth surface makes it difficult for the filler metal to distribute evenly across the entire contact area, and the resulting voids can reduce brazing strength. This is particularly true for the surface roughness of seal bars. To ensure uniform distribution of the filler metal along the contact joint, the brazing surfaces of the seal bars should have appropriate roughening.

The quality of component assembly is closely related to the final brazing quality of the product and warrants significant attention. First, component height tolerances must be controlled. According to national standards, the height tolerance for fins is -0.02 mm to +0.05 mm, and for seal bars is -0.03 mm to +0.03 mm. In practice, assemblies with negative tolerance fins paired with positive tolerance seal bars should be avoided; a zero-gap fit between fin and seal bar tolerances is generally considered optimal. The heat exchanger is clamped with fixtures after assembly. Due to the difference in thermal expansion coefficients between aluminum and stainless steel fixtures, excessive clamping force can easily cause fin bending or collapse after brazing, while insufficient clamping force can lead to insufficient brazing or loosening of the fins.

The parting sheet is clad with filler metal, which requires a specific temperature to melt. Brazing temperature affects not only the wettability of the filler metal but also the strength of the brazed joint. If the temperature is too low, the required brazing temperature is not reached, and temperatures across different areas are uneven. This results in poor filler metal fluidity, potentially causing insufficient brazing, discontinuous brazing, internal porosity, and slag inclusions, leading to reduced joint strength and increased risk of leakage, potentially even severe blistering or tearing. This phenomenon is occasionally observed in production and is often linked to the gap fit between the internal fins or passages and the seal bar. Practice has shown that if a 100 mm long section of internal fins or passages is not brazed, it can tear under pressure below 2.0 MPa. If the temperature is too high, the filler metal melts completely, which can easily lead to porosity. Additionally, severe filler metal oxidation can occur, causing defects such as filler metal run-out, erosion, and bending. During heating, a temperature gradient exists between the component surface and its core. If the internal and external temperatures of the plate-fin core can be maintained consistently throughout vacuum brazing, brazing quality can be well assured. If the temperature gradient is too small, the required heating and soaking time becomes longer, and excessive soaking can easily cause erosion. Conversely, if the temperature gradient is too large, it inevitably leads to unevenness in the brazed joints (i.e., some areas properly brazed, others not) between the interior and exterior of the assembly. Instances of internal erosion have occurred. Post-mortem analysis revealed that erosion spots were primarily located on the outermost two layers (top and bottom) of the product, appearing as dots or streaks, mostly occurring at the seams between fins. Preventive measures include controlling the brazing furnace temperature, especially ensuring the maximum external temperature of the product is not too high. After the filler metal undergoes phase transformation, the external temperature can be increased appropriately. The main goal is to prevent erosion caused by excessive external temperature or prolonged high-temperature exposure. Using zone control during brazing, where heating zones are turned off sequentially once the product in that zone reaches the brazing temperature (stopping zones as they reach temperature), is also an effective method to prevent erosion.

In vacuum brazing, interactions between the filler metal and certain vapor components can significantly affect brazing characteristics. When the vacuum level in the brazing furnace is low, oxidizing gases such as oxygen and water vapor react chemically with aluminum, forming hard aluminum oxide films. These films are difficult to decompose at typical brazing temperatures, hindering the bonding between the filler metal and the base metal. When the brazing temperature is below 400°C, the protective nature of the oxide film makes it less susceptible to environmental factors within the furnace. Therefore, the presence of some air during the preheating stage is permissible, and the process within this temperature range is carried out while vacuum pumping is ongoing. When the temperature exceeds 400°C, certain elements begin to evaporate significantly. At this point, contaminants react rapidly with the surface, forming oxide layers and simultaneously degrading brazing characteristics. Consequently, a higher vacuum level is required, necessitating effective limitation of the partial pressures of oxygen and water. Typically, the processes of vacuum pumping and heating are interrelated; the duration for each varies depending on the size and weight of the workpiece. Generally, for temperatures below 450°C, the vacuum level should be controlled below 0.05 Pa. During the brazing stage, it should be below 0.005 Pa. A good vacuum level has a significant impact on the quality of the brazed joint.

During vacuum brazing, the ambient atmospheric temperature and humidity can significantly affect the brazing quality of plate heat exchangers. Assembly under high humidity conditions results in more moisture adhering to the fins, parting sheets, and seal bars. When placed in the vacuum furnace for brazing, this moisture releases more gas, lowering the brazing vacuum level. Additionally, the evaporation of water vapor requires a substantial amount of heat, which can affect the temperature of the heat exchanger core. Ambient temperature directly influences the degree of surface oxidation on the thin aluminum alloy plates, thereby impacting vacuum brazing quality.

Based on the above analysis, the following measures should be implemented to reduce or lower the leakage rate of heat exchangers after vacuum brazing:

1.When ordering raw materials, they should be purchased from reputable, specialized manufacturers to ensure material quality and performance.

2.Assemblies with negative tolerance fins paired with positive tolerance seal bars should be avoided; a zero-gap fit between fins and seal bars is generally optimal.

3.Strictly adhere to the process procedures for material preparation, cleaning, and assembly.

4.In practice, optimize and strictly control process parameters such as vacuum brazing temperature, holding time, and vacuum level; adjust the brazing process based on varying internal and external conditions.

5..Control ambient humidity.