Modern electronic components such as high-performance LEDs and computer processors produce large amounts of power loss, which manifests itself in waste heat. High power densities in the order of ten to twenty times that of a hotplate are the rule here, which leads to high demands on the efficiency of the cooling solution. It must be able to dissipate the thermal energy quickly, otherwise stable operation is not possible and the service life of the chips is shortened.
In principle, a combination of the metals copper and aluminum is ideal for this, as copper can absorb the energy quickly, dissipate it away from the heat source and transfer it to the connected aluminum component. The aluminum part usually has numerous ribs, over whose large surface the heat dissipates. The combination of aluminum and copper also offers advantages in terms of costs and the overall weight of the component.
Oxygen-free challenges and potential
However, the interface between the joining partners copper and aluminum, which is critical for heat transfer, rarely has optimal properties in commercial coolers. Common mechanical joining methods leave an air gap between aluminum and copper, which is partly filled with thermal paste, but whose thermal conductivity is far worse than that of the two metals. As a result, there is a high thermal resistance at the joint, which impairs the efficiency of the cooling component, so that it has to be oversized.
The solution to this problem is a direct material bond between the different metals, which can be achieved primarily by casting the copper part with molten aluminum (composite casting). The reciprocal diffusion processes required for the material bond depend largely on the surface quality of the joining partners and are negatively influenced by naturally occurring oxide and hydroxide layers.
Oxygen-free production eliminates disruptive oxide layers
The scientists at the IW in the Collaborative Research Center 1368 “Oxygen-free production” have therefore designed a process for composite casting in an oxygen-free atmosphere that makes it possible to produce copper-aluminum components that have no interfering oxide layers at the interface and therefore have a very high thermal conductivity.
The aims of the project are to investigate the mechanisms of action that occur during this process and to clarify the influence of the process parameters on the component properties. This basic research will later make it possible to scale up the casting process to an industrially relevant scale.
Scalable laboratory process to research the fundamentals
Together with mechanical engineering students at Leibniz University Hannover, doctoral student Andreas Fromm has developed a test rig, the heart of which is an automatic tilting casting system with induction heating that can be operated entirely in a glove box. Since conventional shielding gases such as argon still contain residual oxygen even at very high purity, the remaining oxygen content inside the glove box is converted to silicon dioxide by doping with silane, so that an extremely low oxygen partial pressure of < 10-23 bar can be achieved.
The copper parts are deoxidized within the oxygen-free atmosphere, for example by grinding, and placed in the casting mould. The low oxygen partial pressure not only prevents the reoxidation of the ground surfaces, but also the formation of a new oxide layer on the flowing aluminum melt.
During mold filling, the molten aluminium wets the copper insert and thus creates an oxide-free, material bond between the two metals, whereby the properties of the joint can be controlled by the temperature balance of the casting process. To date, relatively high casting temperatures of 750 to 800 °C and mold temperatures of around 400 °C have been used.
From gravity casting to low-pressure casting in an oxygen-free atmosphere
Gravity casting in a permanent mold made of steel was selected for basic research because it is easy to implement. However, the material science relationships that arise here can also be transferred to technical casting processes for more complex components in the next project phase, which has been recently approved for four additional years by the German Research Foundation.
The working group at the IW has planned to investigate the process of oxygen-free low-pressure casting in the future. This offers the advantage of controlling the joint via the variable pressure, so that lower process temperatures can be used in the medium term to reduce energy requirements and at the same time increase important component properties such as conductivity and strength.
Characterization of the cast part properties
After solidification and cooling, the researchers remove the samples from the test stand and carry out various material characterization methods. Of particular interest in the subsequent microscopic analyses is the microstructure of the joining zone between copper and aluminum, as brittle intermediate layers (intermetallic phases) form here. The focus of the analyses is therefore to describe the interactions between the casting temperatures, the duration of the temperature exposure and the microstructure as well as the component properties.
Due to the relatively high mold temperatures, intermetallic phases of aluminum and copper atoms of the AlxCuy type with a thickness of several tens of micrometers are formed at the interface. With a view to later technical applicability, the thickness of the phase seam must be minimized, as this impairs the fatigue strength and thermal conductivity.
One hundred times better thermal conductivity at the interface
Nevertheless, the researchers at the IW are very satisfied with the thermal conductivities of the composite castings achieved to date. This critical characteristic value is calculated from the specific heat capacity of the material and the thermal diffusivity, which is determined using a special measuring device in the IW laboratory.
The aluminum-copper castings produced in the glove box by Andreas Fromm already exhibit high thermal conductivities of over 80 W/m∙K, which is more than a hundred times the thermal conductivity of the air gap found in typical industrially produced heat sinks. The IW team is optimistic that this value can be increased even further in the future by transferring it to the low-pressure casting process and at the same time demonstrating its technical applicability. These investigations will run until 2028 as part of the newly approved second funding period.
Vision for the future: oxygen-free casting production in industrial applications
The vision of the project is to further develop the process for a wide range of casting methods and alloys in order to enable oxygen-free casting production in the long term. In terms of the heat sink, this could mean that it works more efficiently despite its smaller dimensions, which could ultimately save weight, costs and resources in both production and operation.