Innovative lightweight cooling system Open-pored aluminum casting cools power electronics, LEDs, or drives

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Wuxi InfiMotion Technology Co., Ltd.

With open-pored aluminum permanent mold casting, a new class of lightweight materials is available, offering multifunctional and multi-structural properties for power electronics, LEDs, or drives. One component besides aluminum is table salt.

Smaller electronics, while demands for reliability and lifespan of electronic, mechatronic, and partly mechanical systems are increasing, require efficient thermal management.

Heat sinks and heat exchangers made of a lightweight material class are suitable here. They are made of open-pored aluminum casting and differ significantly from conventional solid components with fins, slats, and pins, open-pored aluminum foams, as well as additively manufactured TPMS structures.

Due to their properties, they enable unconventional solutions in thermal management with new technical, economic, and ecological possibilities. The open-pore aluminum die casting is a new class of lightweight construction material with multifunctional and multistructural properties. The material, produced using table salt as an auxiliary substance, is lightweight, mechanically and thermally highly resilient, and can be hybridized in various ways as a mono-material or multi-material system.

Higher material density, thermal conductivity, and compressive strength

The material has up to eight times higher material density, up to eight times higher thermal conductivity, and up to 20 times higher compressive strength than conventional aluminum foams. The pore size can be adjusted application-specifically from 5 µm to several centimeters, and the density from 0.8 g/cm³ to 1.3 g/cm³. Various options are available for different applications to specifically tailor the material properties.

Interesting properties for thermal management include:

• novel, multimodal pore morphology with new heat transfer phenomena,

• direct heat transfer from hot spots to carrier media through a conduction-convection mechanism,

• improved heat conduction, radiation, and convection at smaller dimensions and reduced weight,

• enhanced flow acoustics and direction independence of the cooling circuit,

• a wide variety of possible material combinations, architectures, and designs for a broad range of applications,

• great design freedom for heat sinks, flow circuits, PCB layouts, device designs, and component packages,

• high integrability into installation spaces, hybrid systems, and function bundles,

• optional adjustable capillary effect for rapid removal of the cooling medium from the hotspot,

• optional adjustable lotus effect for protection against splash and condensed water

• optional adjustable vibration damping and muffler function for integrated fans,

• optional adjustable filter function for air and fluid,

• sustainable technology for small, medium, and large-scale production.

Efficient cooling and material properties

The efficiency of cooling directly depends on the material properties, the size of the heat transfer surface, the shape and surface properties of the solid, the fluid properties, the flow pattern, flow rate, viscosity, and thermal conductivity. The heat sinks, named "PORECOOL," are manufactured in any geometry and dimensions from pure aluminum or aluminum alloys.

Compared to conventional heat sinks, they offer the following advantages with a similar construction

• Up to ten times larger surface area for convective heat transfer,

• higher thermal conductivity of pure aluminum with a purity of 99.5 percent,

• higher effective emissivity of the porous cast surface,

• new heat transfer phenomena with a better heat transfer coefficient and

• up to 70 percent less weight.

The heat transfer coefficient

The heat transfer coefficient describes the ability of a gas or liquid to transfer energy across the surface of a material. It depends, among other factors, on the properties of the material, flow properties, geometric conditions, and surface quality. Open-pore aluminum cast structures are open-cell, multimodal metal foams with a special pore morphology and an enormous variety of architectures that determine their thermal and fluid dynamic behavior:

• hourglass-shaped pores with different sizes in the micro, meso, and macro range,

• complex winding of the passages with multiple 1-n, n-1, and n-n connections in one flow direction,

• multiple, spatially distributed micro and macro flows with laminar, turbulent, and mixed areas,

• multiple, spatially distributed velocity ranges of micro and macro flows,

• massive aluminum matrix (material component 30 to 60 percent) with very good thermal conductivity and high effective emissivity and

• adjustable properties for better heat transfer with various gases/fluids.

The novel multimodal aluminum foams are still little explored. The first findings from research projects confirm the following hypotheses:

• the pore morphology is the most important parameter for heat transfer,

• larger surfaces in open-pored metal structures do not automatically mean a better heat transfer coefficient,

• depending on the pore morphology, the heat transfer coefficient can be significantly improved compared to heat sinks with fins, pins, slats, conventional aluminum foams, or TPMS structures,

• with very large surfaces of open-pored metal structures, the thermal conductivity of the material is secondary and

• depending on the pore morphology, up to 50 percent more heat can be transferred with 20 percent less surface area.

Design possibilities and degrees of freedom

Components made of open-pore aluminum casting are traditionally machined and can be brought into any CNC precision shape. They are fully open-pored, regardless of the flow direction of the cooling medium, and can thus be integrated into any free space. This opens up new design possibilities and degrees of freedom for heat sinks, flow circuits, circuit board layouts, device designs, and component packages.

Constructions made of porous aluminum casting are highly durable.

They can take on load-bearing functions, offer further functional integrations such as mounting points, vibration damping, or sound damping, form load-bearing or decorative housing structures, or completely enclose circuit boards with semiconductors as a type of sound-damping cooling box.

New Aluminum Monomaterial Hybrids: Various different aluminum hybrids can be produced: uniformly open-pored, graded open-pored, selectively open-pored, selectively solid, and many more. For example, the thermal contact point to the semiconductor can be made of solid aluminum locally.

New Multi-Material Hybrids and Metal Matrix Composite Materials: Various components made of stainless steel, titanium, copper, or aluminum can be integrated into aluminum mono-material hybrids. This can be done either directly in the casting process or subsequently by pressing, welding, soldering, or gluing. Different foreign materials can be added to the aluminum melt as particles or short fibers. This opens up a new material field of open-pore metal matrix composite materials with a functionalized metal matrix and/or pores.

New Additive Aluminum Hybrids: Components made of open-pored aluminum casting can be functionalized using additive manufacturing processes. Conversely, additively manufactured aluminum components can be integrated into components made of open-pored aluminum casting.

New thermally conductive plastic parts: The pores of porous aluminum structures can be infiltrated with polymers using various manufacturing methods. In this way, efficient, media-tight multi-material systems can be produced for mechanical, fluid dynamics, thermal, chemical, acoustic, decorative, and other tasks. Compared to aluminum, plastics have poor thermal conductivity:

• PP2 = 0.08 W/mK,

• PP6 and ABS = 0.17 W/mK,

• filled casting compounds = 6 W/mK and

• Al99.5 percent = 236 W/mK.

Porous aluminum structures can be integrated into conventional plastic parts in various ways:

1. fully integrated (polymer at hotspot and coolant),

2. with metallic contact to the hotspot,

3. with metallic contact to the coolant,

4. with metallic contact to both the hotspot and coolant,

5. with many other combinations of geometry and material.

Depending on the design and the proportions of porous aluminum, solid aluminum, and polymer, a combined thermal conductivity of the hybrid component results:

• up to 60 W/mK for variant 1 and

• up to 236 W/mK for variant 4.