What are the processing techniques and difficulties of microchannel flat tubes
Information summary：The processing of microchannel flat tubes is based on aluminum alloy extrusion molding as the core process (suitable for mass production needs), combined with subsequent surface treatment, precision machining, and testing processes. The overall process revolves around "micro channel forming accuracy, structural integrity, and material performance stability". The following is a detailed process bre
The processing of microchannel flat tubes is based on aluminum alloy extrusion molding as the core process (suitable for mass production needs), combined with subsequent surface treatment, precision machining, and testing processes. The overall process revolves around "micro channel forming accuracy, structural integrity, and material performance stability". The following is a detailed process breakdown and analysis of key difficulties:
1、 Core processing technology (taking mainstream aluminum alloy flat tubes as an example)
1. Raw material preparation and pretreatment
Material selection: Priority should be given to aluminum alloy grades such as 6063, 3003, 6061 (with good thermal conductivity, strong squeezability, and corrosion resistance), and the composition should be adjusted according to the application scenario (such as adding magnesium and silicon to optimize strength);
Raw material processing:
Ingot preparation: Produce round ingots (diameter 80-150mm) through semi continuous casting, control grain size (≤ 100 μ m), and avoid defects such as inclusions and pores;
Uniform annealing: Hold at 520-560 ℃ for 4-8 hours to eliminate internal component segregation of the ingot, improve plasticity, and prepare for subsequent extrusion;
Surface treatment: Remove oxide scale and oil stains from the surface of the ingot to prevent impurities from entering the mold during extrusion and affecting product quality.
2. Core process: Hot extrusion molding (the key link that determines the accuracy of the flow channel)
Process principle: Place the preheated ingot into the extruder barrel, apply pressure (100-300MPa) through the extrusion rod, and shape the aluminum alloy billet into a flat tube billet through a mold with a micro channel structure at high temperature (480-520 ℃);
Key steps:
Ingot preheating: Keep at 450-500 ℃ for 2-3 hours to ensure the plasticity of the billet meets the standard and avoid cracking during extrusion;
Mold installation and preheating: Preheat the mold (including the flow channel core and outer jacket) to 400-450 ℃ to reduce the temperature difference stress between the mold and the blank, and extend the mold life;
Extrusion molding: using "forward extrusion" or "reverse extrusion" (reverse extrusion is easier to ensure uniform wall thickness), controlling the extrusion speed (5-15mm/s) to avoid channel blockage or deformation;
Online cooling: The extruded flat tube is rapidly cooled to room temperature by air or water cooling, fixing the structure and enhancing strength (avoiding coarse grains caused by natural cooling).
3. Subsequent precision machining and processing
Fixed length cutting: According to customer needs, high-precision sawing equipment (such as laser cutting, CNC sawing machine) is used to cut into a certain length (error ≤± 0.5mm) to avoid burrs at the end;
Surface Treatment
Anodizing: forming a 5-15 μ m oxide film to enhance corrosion resistance (suitable for outdoor/humid environments such as automobiles and refrigeration);
Coating treatment: Some scenarios (such as fuel cells) require the application of anti-corrosion coatings (such as PTFE) to enhance their resistance to chemical media;
Shaping and straightening: Use a roller straightening machine to correct the straightness of the flat tube (≤ 0.3mm/m), ensuring the accuracy of fitting with the heat exchanger manifold during assembly;
End processing: According to assembly requirements, the end of the flat tube is expanded, chamfered, or welded with a groove to facilitate brazing connection with the manifold.
4. Testing and Quality Control
Size detection: Use optical microscope and laser caliper to detect the size of the flow channel (aperture, wall thickness) and the width/thickness of the flat tube, with an error controlled within ± 0.02- ± 0.05mm;
Defect detection: X-ray inspection and ultrasonic testing are used to identify internal defects such as pores, inclusions, and blockages in flow channels; Verify the sealing of the flow channel through airtightness testing (applicable to pressure bearing scenarios);
Performance testing: testing tensile strength (≥ 200MPa), thermal conductivity (≥ 180W/(m · K)), and corrosion resistance (salt spray test ≥ 500 hours) to ensure compliance with industry standards.
2、 Core processing difficulties and solutions
1. Precision control of micro channel forming (the core difficulty)
Difficulty: The diameter of a single channel is only 0.2-2mm, and there are many channels (10-50). It is necessary to ensure that the aperture of each channel is uniform, the inner wall is smooth, and to avoid shrinkage, blockage, and uneven wall thickness (especially the thickness difference between the side wall of the channel and the outer wall of the flat tube is ≤ 0.1mm);
Solution:
Mold design: Adopting an integral core mold, the flow channel is processed through electrical discharge machining (EDM) or wire cutting (WEDM) to ensure core accuracy (surface roughness Ra ≤ 0.8 μ m); The core is made of heat-resistant steel (such as H13), which undergoes heat treatment to increase hardness (HRC ≥ 55) and reduce wear;
Optimization of extrusion parameters: Strictly control the extrusion temperature (fluctuation ≤ ± 10 ℃) and speed (uniform extrusion) to avoid uneven flow of the billet causing deformation of the flow channel; Adopting the "gradient extrusion" process, the initial speed is slow, and the speed is increased after the billet is completely filled with the mold;
Online monitoring: Real time monitoring of the extrusion process through infrared thermometers and pressure sensors, and timely adjustment of parameters.
2. Mold wear and lifespan issues
Difficulty: During high-temperature extrusion of aluminum alloy, severe friction occurs, and the micro channel core is prone to wear and deformation, resulting in a decrease in product dimensional accuracy and a short mold life (conventional molds can only produce 10000-50000 pieces);
Solution:
Mold surface treatment: Nitriding treatment (hardness increased to HRC ≥ 60) or TiN coating (coating thickness 3-5 μ m) is applied to the core to reduce the friction coefficient;
Optimizing lubrication system: using graphite based or ceramic based high-temperature lubricants, evenly applied to the contact surface between the ingot and the mold to reduce friction and wear;
Mold maintenance: Regularly polish and repair molds, promptly replace severely worn cores, and avoid batch product defects.
3. Balance between material properties and molding stability
Difficulty: High temperature extrusion can easily lead to coarsening of aluminum alloy grains, reducing product strength and thermal conductivity; At the same time, during the extrusion process, the billet may crack and peel (especially at the edge of the flat tube);
Solution:
Raw material optimization: Control the impurity content of the ingot (Fe ≤ 0.3%, Si ≤ 0.6%) to avoid the formation of hard and brittle phases;
Process parameter matching: Correctly control the preheating temperature and extrusion speed of the ingot and mold to ensure that the billet is in a certain plasticity range;
Online cooling: Adopting the "segmented cooling" process, first rapidly cool to around 300 ℃, and then slowly cool to room temperature to refine the grains and improve strength.
4. Consistency control in mass production
Difficulties: During large-scale production, factors such as the composition of different batches of ingots, the stability of extrusion equipment, and the degree of mold wear can easily lead to fluctuations in product size and performance;
Solution:
Establish a standardized production process (SOP) to standardize raw material specifications and process parameters;
Adopting an automated extrusion production line equipped with a PLC control system to achieve accurate closed-loop control of temperature, pressure, and speed;
Combination of full inspection and sampling inspection: 100% inspection of key dimensions (channel aperture, wall thickness), performance indicators are sampled by batch to ensure batch consistency.
5. Special material processing adaptation (stainless steel, copper alloy)
Difficulties: Stainless steel has high hardness and poor plasticity, requiring greater pressure (300-500MPa) during extrusion, and more severe mold wear; Copper alloy has strong thermal conductivity and is prone to heat loss, making it difficult to form flow channels;
Solution:
Material pretreatment: Stainless steel ingots need to undergo solid solution treatment (1050-1100 ℃) to improve plasticity; Raise the preheating temperature of copper alloy ingots to 600-650 ℃;
Mold upgrade: using hard alloy molds (such as WC Co) to improve wear resistance;
Reduce extrusion speed: Control the extrusion speed of stainless steel at 1-5mm/s and copper alloy at 3-8mm/s to avoid cracking.
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