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Cold plate radiator vacuum brazing process

As a cutting-edge technology in the automotive industry, cold plate radiators for automotive autonomous driving systems are gradually moving from technology research and development to market application and have broad market prospects. This product is manufactured using vacuum brazing technology and has the characteristics of small dimensional deformation and clean inner cavity. At the same time, in order to avoid the risk of product leakage, higher requirements are put forward for welding quality. It is required to carry out ultrasonic testing on the welding surface. The welding rate within the 3 mm width range of the flow channel is required to reach 100%; the welding rate of the remaining parts must be above 85%; the blasting strength is required to be above 3 MPa.

Vacuum brazing has extremely high requirements on the welding gap. The larger the brazing gap, the smaller the additional pressure due to surface tension, and the smaller the height that the liquid solder can climb; and the larger the brazing gap requires more solder to fill, when the additional pressure is less than the liquid solder When subjected to the gravity, the liquid solder cannot fill the seams and form unwelded defects. Generally, the gap is required to be less than 0. 05 mm. However, this product requires a 100% welding rate within a 3 mm width range along the circumference of the flow channel, which places higher requirements on the gap. By welding the samples, it was found that to ensure this welding rate requirement, the gap between the welding surfaces must be less than 0. 02 mm.

Key points of process control
Key points of parts design
The bottom of the base is designed with multiple process bosses, which play a supporting role during brazing and are removed by machine after welding to avoid changes in the welding gap caused by deformation of the base. After the inner fin is formed, a leveling process is set up to control the consistency of the wave height size within 0. 02 mm. The wave height is designed and processed to be 0 to 0. 02 mm lower than the runner groove.
The soldering surfaces between the solder piece, the base hole and the faucet are designed with a single-side gap of 0. 02 mm. The thickness of the solder sheet is 0. 6mm, and the flange gap is designed to be 0. 45 mm to solve the problem of increased gap caused by the thinning of the material at the flange opening. The brazing surface adopts a 20° tapered design to solve the assembly difficulty problem under small gaps and no lubrication conditions.

Key points in brazing fixture design
The fixtures are designed to stack 20 sets of products per rack, with graphite plates spaced between products, making full use of the height of the vacuum furnace to increase the furnace loading capacity. The clamp is made of a combination of stainless steel and graphite plates, which not only ensures sufficient rigidity of the clamp, but also minimizes heat capacity.
The clamp uses high-temperature resistant spring pressure material, and the arrangement of the spring should ensure the balance of the product’s force. During the process of thermal expansion and cooling contraction of the product, the spring should provide sufficient pressure to eliminate the gap change caused by the melting of the solder. At the same time, it is also necessary to avoid product deformation caused by excessive clamping force.
When designing the fixture, the dimensional changes of the aluminum material when heated and the solder melted must be taken into consideration. Thermal expansion amount = thermal expansion coefficient × workpiece temperature difference × workpiece size. When determining clamp parameters such as spring length and clamp escape hole size, the clamp design should take into account the difference in thermal expansion coefficients between the workpiece and the clamp to avoid deformation due to interference between the workpiece and the clamp at high temperatures. Experiments show that after the solder melts and solidifies, the thickness of the solder layer will be reduced by about 50%. Therefore, the spring needs a certain amount of preload, which is greater than the thinning amount of the solder layer.

Vacuum brazing parameters
Four stages of temperature rise and fall were adopted: 450 ℃, 565 ℃, 605 ℃, and 595 ℃, and the test results were good. Rapidly heating up the brazing section and shortening the holding time are effective ways to avoid adhesion and corrosion of the inner fins. The brazing holding time of the products in the middle part of the fixture is controlled so that the workpiece temperature reaches two minutes above the solder liquidus line and the temperature difference of the entire product does not exceed 3°C. The vacuum degree of the brazing section is less than 1 × 10-3 Pa, thereby ensuring the quality of product welding. Reliability and consistency.

Vacuum brazing process conclusion
1) For products with high welding rate and high burst strength requirements, the brazing gap is the key to ensuring that the product meets the requirements. The gap must be controlled within 0.02 mm. The design of products, processes, and tooling must be controlled around this key point. Insufficient structural strength of the product will cause the brazing gap to change. When this problem occurs, it can be solved by adding process bosses to the product or adding supports to the fixture.
2) The welding quality of the inner fins plays a decisive role in the blasting strength of cold plate products.

3) The brazing fixtures for cold plate products can be designed with a stacked product structure to maximize production efficiency. The key points of control are to maintain a balanced stress on the product in terms of structural design, and to always maintain appropriate pressure on the product during the process of thermal expansion of the product and dimensional changes caused by melting of the solder.
4) Vacuum brazing parameters should ensure full melting of the solder, and the product temperature difference should not be too large. At the same time, avoid adhesion and corrosion of fins.