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Vacuum Brazing Technology of Aluminum Alloy Plate Radiator

The so-called vacuum brazing is to clamp the materials to be brazed together with fixtures, heat them in a vacuum furnace to melt the 4004, 4104 or 4N04 aluminum alloy solder into a high temperature liquid state, and fill the welded parts (3003 alloy) under the action of capillary force. ) to achieve metallurgical bonding. Vacuum brazing belongs to ultra-high vacuum airtight welding, which can braze materials and structures that are difficult to connect with general methods, so as to obtain smooth and dense brazed joints with excellent mechanical properties and corrosion resistance. Compared with other welding methods, its advantages are that the deformation of parts is small, the connection of multiple parts can be completed at the same time, and different metal materials can be connected.

Vacuum brazing of aluminum plate radiators has high requirements on the surface roughness, assembly quality, fit tolerance, brazing system, etc. of the parts before brazing, and is prone to false welding, fin bending and lodging, corrosion, brazing discontinuity, etc. Leakage and other defects.

1 The structure of the radiator and its production process

1.1 The structure of the radiator

The structure of the aluminum plate radiator is shown in Figure 1, which is mainly composed of large seals, small seals, 434 composite plate (partition plate) outer fins, inner fins, side plates and guide fins.

1.2 Production process flow

The production process of the radiator: material preparation → acid-base washing → heat sink assembly → vacuum furnace high temperature brazing → shaping → argon arc welding deflector → pound pressure test → rework and repair → surface spraying → finished product packaging.

Material preparation: Cut 434 composite slats, inner fins, and outer fins to prepare large seals, small seals, and side panels.

Acid-base washing: alkali washing→water washing→acid washing→water washing→hot water washing→drying.

The assembly process of the heat sink: side plate→the bottom layer of the composite plate→a layer of outer fins→enlarged seals at both ends→a composite plate→small seals on both sides→a layer of inner fins→a composite plate→a layer of outer fins→ Enlarged seals at both ends→topmost composite board→side board.

Vacuum furnace brazing: The vacuum brazing furnace generally adopts a three-stage heating and heat preservation process (Fig. 2), that is, pre-evacuation → preheating and setting temperature of 400 ~ 425 ℃ in the first stage, heat preservation for 60 ~ 80 min → energy storage and constant temperature in the second stage 560 ~575 ℃, heat preservation for 40 ~ 60 min – the third stage brazing fixed temperature 602 ~ 609 ℃, holding time 30 ~ 40 min → power off and wait for the air temperature in the furnace to drop to 350 ~ 400 ℃.

Reshaping of the heat sink: Due to the deformation of the heat sink after brazing, it is leveled by applying external force shaping method to make the unevenness ≤ 2 mm.

Argon arc welding deflector: the deflector is welded to the large seals at both ends of the radiator.

Pound pressure test: Pass in compressed air of 1.0 MPa, check the leakage of the radiator, and mark the leakage point.

Repair: Saw the leaked radiator → repair welding → re-pressure test.

Spraying process: cleaning → drying → spraying (aluminum silver paint) → drying.

2.Vacuum brazing process system

2.1 Heating characteristics of vacuum brazing furnace

The workpiece in vacuum is mainly heated by radiation, and radiation heat transfer can be expressed by the Stephen Boltzmann formula:

In the formula, Q and α——the heat transferred by radiation and the heat transfer coefficient;

T1, T2 – radiation element, the surface temperature of the radiated object.

At high temperature, even a small temperature difference can produce a high heat transfer rate; under the same conditions, the time required for vacuum heating is about 3 times that of an air furnace and 6 times that of a salt bath furnace. That is to say, if it is heated at a low temperature in a vacuum furnace, the heating speed will be slow. Therefore, when using a vacuum furnace, the heating process of air furnace, salt bath furnace and atmosphere furnace must not be copied.

2.2 Vacuum brazing temperature

There is solder on the fins and thin plates, and the solder needs a certain temperature to melt. The brazing temperature not only affects the wettability of the solder, but also affects the connection strength of the brazed joint. If the temperature is too low, the necessary brazing temperature is not reached, and the temperature of each part is not balanced, the fluidity of the brazing material is poor, which is likely to cause false welding, discontinuous brazing, pores and slag inclusions inside the brazing, which will make the strength of the welded joint weak. If the temperature is too high, the solder will be completely melted, and pores will be easily generated, and the oxidation of the solder will be serious, which will easily cause defects such as solder loss, corrosion, and fin bending.

If the temperature outside and in the middle of the plate body can be kept consistent during vacuum brazing, that is, the temperature rises very slowly to the highest temperature, and then drops slowly after heat preservation, then the brazing quality can be well guaranteed. However, considering the economic benefits, the brazing time should be shortened as much as possible under the premise of ensuring the quality. Wetting of solder and joint formation during brazing is a fast process (about 1 s), so the holding time is mainly determined by the time required for the entire part to be heated to the brazing temperature and the time required for the oxide film to dissipate. Too long, the workpiece is easy to form erosion.

Practice has proved that the length of holding time at each temperature stage has a great influence on the quality of brazing.

2.3 Vacuum degree of brazing furnace

When the vacuum degree of the brazing furnace is low, the oxidizing gases such as O2 and H2O in the furnace chemically react with Al to form a hard Al2O3 oxide film, which is not easy to decompose at ordinary brazing temperature and affects the welding of the solder and the substrate. The brazing furnace adopts multi-temperature zone temperature control, the furnace temperature uniformity is ±5 C, the working vacuum degree is ≤1.3x 10-3 Pa, and the pre-vacuum degree must be in the order of 10-4.

2.4 Weather conditions during vacuum brazing

During vacuum brazing, the ambient temperature and humidity will have a great influence on the brazing quality of the plate body. When assembling under high humidity conditions, more moisture will remain on the fins, separators and seals, and more gas will be released when brazing in a vacuum furnace, reducing the vacuum degree of brazing; at the same time , Since the evaporation of water vapor requires a lot of heat, it will affect the temperature of the plate body. The level of ambient temperature directly affects the degree of oxidation on the surface of the aluminum alloy sheet, thereby affecting the quality of vacuum brazing. Therefore, a certain ambient temperature and humidity should be maintained before pickling, assembly and brazing of the plate body.

Selection of vacuum brazing equipment: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for vacuum brazing of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum tempering furnace, etc., which are widely sold in developed and developing countries.