How to solve the problem of deformation and collapse of radiator after vacuum brazing

The quality of vacuum brazing, the core manufacturing process for aluminum heat sinks, directly determines the product’s performance and reliability. Deformation and collapse are the two most common and fatal defects in this process. The following is a brief introduction to these two defects.

1.Core Deformation
1. Description of the Phenomenon:
Deformation refers to the deviation of the overall macroscopic geometry of the heat sink from the design drawings. Common manifestations include:
Warping: The entire heat sink baseplate is uneven, forming a pot-lid or wavy shape.
Bending: The long or short sides of the heat sink develop a bow-like bend.
Twisting: The heat sink exhibits a spiral distortion.

2.Causes and mechanisms of aluminum stress
(1) Thermal stress and residual stress:
Mechanism: Aluminum has a high thermal conductivity and a large expansion coefficient. During the heating and cooling process, there are temperature differences and cross-sectional differences between the various components of the radiator (thick side panels, thin fins, seals), resulting in asynchronous thermal expansion and contraction, which in turn generates huge thermal stress. After cooling, this part of the stress remains in the form of “residual stress”. When the stress is released unbalancedly, it causes overall deformation. The thicker the side panels and the more asymmetrical the structure, the more prominent the problem.
Speciality of aluminum: Aluminum alloys have extremely low strength at brazing temperatures and are more likely to undergo plastic flow under stress.
(2) Uneven heating and cooling:
Mechanism: The temperature field in a vacuum furnace cannot be absolutely uniform. Uneven furnace temperature, too close or improper placement of radiators will result in different heating/cooling rates for different parts of the product, exacerbating thermal stress and causing distortion or warping.
Speciality of aluminum: Aluminum has a large specific heat capacity, and achieving uniform temperature requires more precise temperature control. Workpiece weight and improper support:
Mechanism: At brazing temperature (about 600°C), the strength of aluminum alloy is only a few percent of that at room temperature. If the support points are unreasonable or the spacing is too large, creep will occur under the action of its own weight, resulting in sagging or bending.
Speciality of aluminum: Aluminum has a low melting point (~660°C), and the brazing temperature is very close to its solidus, so the loss of high-temperature strength is particularly serious.
(3) Material and structural design issues:
Mechanism: The thickness matching of the core (fins/partitions) and the side panels is unreasonable. The thick side panels “constrain” the weak core, generating huge internal stress during thermal cycles. Asymmetric structural design can also lead to uneven stress distribution.

Collapse

Description of the phenomenon: Collapse refers to the local failure of the internal structure of the radiator, especially the fins losing stability and collapsing, bending or sticking, resulting in blockage or narrowing of the fluid channel, seriously affecting the heat dissipation performance and flow resistance. This is a more microscopic and fatal defect than deformation. 2. Causes and mechanisms of collapse for aluminum materials:

(1) Excessive solder and improper vacuum brazing process: Mechanism: This is the main cause of collapse. Aluminum radiators usually use Al-Si brazing filler metals (such as 4004/4104, etc.), which have a lower melting point than the base material (such as 3003/6061). If the amount of solder foil used is too much, the coating layer is too thick, or the brazing temperature is too high or the holding time is too long, the excess liquid solder will be sucked into the root of the fin under capillary action. Capillary action and dissolution: A large amount of liquid solder will not only fill the weld, but will also be sucked into the narrow fin gap through capillary action. At the same time, the liquid solder dissolves aluminum on the surface of the parent material, further increasing the amount of liquid metal. This creates a large bridge of liquid solder between two adjacent fins.

Imbalance between static pressure and surface tension: When the volume of the liquid bridge is large enough, its own static pressure exceeds the yield strength of the aluminum at high temperatures and the surface tension of the liquid metal. Under the influence of this static pressure, the soft, hot fins are either “pushed apart” or “pulled inward,” causing structural instability and, upon cooling, permanent collapse.

(2) Poor control of vacuum brazing temperature curve: Overheating: When the temperature is too high, the dissolution of the base material is intensified, and the fluidity of the brazing material is too strong, which can easily lead to over-brazing and collapse. Excessive heat preservation: Prolonging the existence time of the liquid brazing material provides a longer time for capillary action and dissolution, increasing the risk of collapse.

(3) Insufficient fin structural strength: Mechanism: The fin itself is too thin and too high (the height-to-thickness ratio is too large), and its compression and bending resistance at high temperatures are inherently poor. When subjected to the static pressure of the liquid brazing material or the slight preload during assembly, it is easy to buckle. Excessive assembly gap or insufficient preload: Mechanism: The assembly gap between the components (partition-fin-seal) is too large, requiring more brazing material to fill, which indirectly leads to excessive brazing material. At the same time, if the preload provided by the brazing fixture is insufficient and cannot tightly fix the fin when the brazing material melts, the fin is prone to displacement and collapse under the action of the liquid brazing material.

3. Prevention and solution measures for vacuum brazing
1. General measures for deformation and collapse:
(1) Optimize the vacuum brazing process curve: strictly control the heating rate, vacuum brazing temperature (try to use the lower limit temperature while ensuring the brazing rate) and holding time.
(2) Use special vacuum brazing fixtures (fixtures): design reasonable fixtures to provide uniform and appropriate pressure and effectively restrain deformation during the cooling stage.
(3) Ensure furnace temperature uniformity: regularly check the furnace temperature and place the workpieces reasonably to ensure uniform heating.
2. Special measures for deformation
(1) Optimize structural design: try to use a symmetrical structure and reasonably match the thickness and heat capacity of the side plate and the core.
(2) Optimize support: set reasonable support points in the furnace, especially in the parts where the workpiece is prone to sagging.
(3) Stress annealing: after vacuum brazing, add a low-temperature stress relief annealing process (be careful to avoid affecting the brazing seam strength).
(4) Mechanical correction: for products that have been deformed, cold correction or hot correction can be used, but there are risks. 3. Special measures for collapse
(1) Accurately control the amount of brazing material: This is the most critical measure. Determine the optimal brazing foil thickness or coating thickness through calculation and experimentation.
(2) Optimize fin design: Appropriately increase fin thickness to reduce the height-to-thickness ratio; use fins with corrugations, serrations, or ribs to improve their high-temperature stiffness.
(3) Strictly control assembly clearance: Ensure part processing accuracy and minimize and even assembly clearance.
(4) Strengthen fixture preload: Ensure that the fixture can still provide sufficient clamping force at brazing temperature to firmly fix the fin.