Jul 8, 2016 | By Benedict

Researchers from Oak Ridge National Laboratory and the University of Tennessee have discovered that, by employing a simple annealing process, or by using a design algorithm, manufacturers can create 3D printed heat sinks which perform as well as conventionally manufactured alternatives.

The experiment test bench setup

Heat sinks, vital components in many electronic devices, are passive heat exchangers which dissipate heat away from a device in order to keep it cool and therefore functional. For the power electronics industry, heat sinks are especially important because of the increasing power density of power electronics devices. But while some aspects of those devices have advanced dramatically in recent years— higher switching frequencies, better thermal performance—heat sinks have remained virtually the same due to the limitations of their manufacturing processes. Normally created by milling, drilling, and casting, heat sinks are rarely designed with complex, multi-piece structures, since this would increase the likelihood of breakage or leakage.

For some power electronics manufacturers, additive manufacturing appears to present an appealing solution to the heat sink problem: by using a 3D printer, manufacturers are able to design and create heat sinks with far more complex internal structures than would be possible by forging, milling, etc. Moreover, these 3D printed heat sinks can easily be printed as a single piece, reducing the need for assembly and decreasing the possibility of leakage. However, while these 3D printed heat sinks can exhibit superior structures to their conventionally manufactured alternatives, they are often let down by their material properties: metal 3D printing alloys are composed in a different way to regular alloys, and these minor differences in composition can lead to adverse thermal effects.

Tong Wu, of the University of Tennessee and ORNL, recently lead a team of ORNL scientists in a research project in which the performance of 3D printed aluminum heat sinks was compared to that of non-printed aluminum heat sinks. The research paper, “Thermal Response of Additive Manufactured Aluminum”, was last month delivered by Wu at the 3D-PEIM International Symposium on 3D Power Electronics. From the outset, Wu and his team noted the differences in composition between their chosen 3D printing alloy (A1_AM) and conventional alloy (A1_6061). While the 3D printing powder, fabricated by Michigan-based Linear Mold, had a nominal composition of 10% silicon and 0.5% magnesium, the conventional alloy contained less than 1% silicon and 1.5% magnesium.

Microstructural comparison of regular alloy A1_6061 (a) and 3D printing alloy A1_AM (b)

Although the thermal performance of the two heat sinks was similar at high temperatures, the 3D printed sink fared less well at lower temperatures (70°C), with the “chemistry and microstructural differences” between the two alloys causing a 10% performance difference. Replacing the A1_6061 alloy with the 3D printable A1_AM alloy therefore reduced the overall performance of the heat sink. That result, however, was by no means the end of the experiment. Wu and co were hopeful that, by employing a simple annealing process, they could bring the 3D printed heat sink up to a similar level of performance to that of its conventionally manufactured counterpart.

Comparison of thermal conductivity between the two alloys

The process of annealing involves heating a material at a high temperature and then allowing it to cool slowly. It can be used to remove the internal stresses of a material, making it tougher and less brittle. The researchers found that, while annealing the 3D printed alloy at 100°C produced negligible differences in its thermal performance, annealing at 300°C produced a telling difference; so much so, in fact, that the performance of the annealed 3D printed heat sink became equal to that of the non-printed heat sink. This optimized 3D printed heat sink demonstrated a 22.8% reduction in junction temperature and a 33% reduction in weight compared to its untreated predecessor. The researchers were therefore able to demonstrate that, by employing this simple annealing process, they could produce a 3D printed heat sink that performed as effectively as its conventionally manufactured counterpart.

Again, however, that result was not the end of the story. While Wu and co’s annealing experiment showed that the compositional differences between additive manufacturing alloys and conventional alloys could be ironed out, supporting “the candidacy and consideration of AM heat exchangers for power electronic devices”, they did not endeavor to show how 3D printing can be used to create more complex, more efficient internal geometries for heat sinks. Fortunately, Wu had recently conducted further heat sink research, accompanied by ORNL’s Burak Ozpineci and Curtis Ayers (also contributors to the other research project), exploring the “Genetic Algorithm Design of a 3D Printed Heat Sink”. That research paper was presented in March at the 2016 IEEE Power Electronics Conference and Exposition.

For this earlier research project, Wu and his team sought to discover whether a genetic algorithm-based approach to design could be employed to produce optimized and thermally effective heat sinks. While conventional heat sink designing processes are effective for simple heat sink shapes, the researchers proposed a new process for liquid-cooled heat sinks without empirical principles and analytical models, instead focusing on FEA modeling. The new algorithm simply requires the input of maximum heat sink size, device and system loss characteristics, and coolant information. Using this data, the algorithm is able to generate an ideal, 3D printable heat sink design which can improve the thermal performance of a power electronics device while dramatically reducing the design cycle time.

Generating an optimized heat sink design from input data

Temperature is represented by different colors for this heat sink for a 50-kilowatt DC-to-DC converter with red being the hottest.

After conducting tests, Wu, Ozpineci, and Ayers found that their optimized 3D printed heat sinks showed a greater than 15% improvement over their unoptimized counterparts, demonstrating that the algorithm process “provides an automatic way for a unique and better heat sink design without the need for complicated analytical models” while also allowing for a higher degree of design freedom for manufacturers. While these experiments specified the volume of the heat sink and used the algorithm to determine an appropriate design, the researchers plan to try flipping the process around, by specifying the target heat sink temperature and using the algorithm to determine an appropriate heat sink size.

The two research papers demonstrate that 3D printing can function as a viable method of production for heat sinks in power electronics. Wu and his team showed that, by employing an annealing process, manufacturers can raise the performance of 3D printed alloys to that of conventional alloys. At the same time, Wu’s other project shows that a genetic algorithm design process can be used to determine the ideal internal structure of a heat sink for its specific application. Combined, the two studies lay the groundwork for the adoption of 3D printed heat sinks which could soon outperform their conventionally manufactured predecessors.



Posted in 3D Printing Application



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Jesús Esarte wrote at 7/13/2016 10:09:00 AM:

interesting and promising results

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