Fraunhofer and MacLean-Fogg develop AM process for large HPDC molds for automotive sector
Researchers from Fraunhofer ILT, Aachen, Germany, and MacLean-Fogg, Mundelein, Illinois, USA, have successfully demonstrated a scalable process for the Additive Manufacturing of large aluminium components. To showcase its potential in the automotive sector, the team produced a complex die casting tool inlay for the transmission housing of a Toyota Yaris Hybrid.
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By using a gantry-based Laser Beam Powder Bed Fusion (PBF-LB) Additive Manufacturing machine developed at the Fraunhofer ILT and L-40 tool steel developed by MacLean-Fogg, the team produced a mold, with near-contour conformal cooling channels, suitable for large-volume high-pressure die casting (HPDC).
The demands on large geometries
As large castings are becoming increasingly established for automotive applications, the demands on the tools used in HPDC are growing. The molds must repeatedly maintain precise component quality at very high quantities and withstand extreme mechanical and thermal loads.
In order to prolong the service life of the tool inlays, they need complex, internal cooling structures. Many of these cannot be made with conventional manufacturing processes.
However, according to Fraunhofer ILT, two key problems have so far limited Additive Manufacturing processes from producing such large-format die casting molds, namely, available build space and material options.
The available build volume of typical PBF-LB machines, for example, is often too small to produce molds greater than 600 × 600 mm in one piece.
The tool steels used to date – in particular H11 (1.2343), H13 (1.2344) or M300 – cannot be processed reliably in the required size range (>20,000 cm³). Even with optimum parameters, there is a risk of cracking, thermal distortion and inadequate mechanical properties. This applies both during laser-based build-up and during downstream heat treatment. The greater the temperature gradients within the component during the manufacturing process, the greater the risk – an effect that is particularly pronounced with large-volume workpieces.
“To overcome these limitations, we need a new generation of machines and materials specifically tailored to the requirements of large-format HPDC tools,” explained Niklas Prätzsch, Group Leader LPBF Process Technology at Fraunhofer ILT. “It was precisely this combination that was the subject of the latest changes we have implemented.”
The new material and AM machine technology make it possible to produce large-volume tools with a free-form cooling structure. This not only allows local temperature peaks in the casting process to be reduced in a targeted manner, it also increases the number of variants while simultaneously increasing service life. This means that different components can be manufactured on one tool platform without having to produce new tools each time.
Crack-free PBF-LB
The gantry-based five-laser PBF-LB machine developed at the Fraunhofer ILT, has a build volume of 1,000 × 800 × 350 mm. In contrast to conventional technology, this AM machine has a movable processing head and local shielding gas guidance, so the build volume can be scaled linearly along the machine axes with the same process boundary conditions (shielding gas flow speed, laser beam deflection angle, etc.). This means that even larger tools than the tool inlay considered in this project can be additively manufactured in the future.
A heatable substrate module was also developed to minimise the temperature gradients that are critical for large-volume tools. The build platform now reaches a temperature of 200°C, which means that each new layer does not cool down to room temperature, but only to a predefined thermal plateau. This approach reduces thermally induced stresses and the risk of cracking during the construction process. According to the researchers, the combination of large installation space, high process stability and active preheating makes this one of the first PBF-LB Additive Manufacturing machines in the world that is suitable for the economical production of near-net-shape die casting molds, even for mega or giga casting.
“The key to success lies in the L-40 material from MacLean-Fogg, which is tailored to the requirements of PBF-LB/M,” added Prätzsch.
This steel is characterised by a significantly reduced tendency to crack compared to conventional tool steels – both during production and during heat treatment. Even in the as-built condition, L-40 is reported to achieve high dimensional accuracy, outstanding properties in terms of hardness (48 HRC), tensile strength (1420 MPa) and notched impact strength (100 J). Comprehensive tests have successfully validated both the parameter transfer to the new machine concept and the performance in complex geometries – for example with round or overhanging cooling channels.
Hybrid production for series tool
As part of the project, the partners produced an additively manufactured tool for a gearbox housing that is already in use at Toyota. The die casting mold contains a complex network of near-contour cooling channels.
For the tool design, the team opted for a hybrid process on a specially manufactured preform that already had vertical cooling channels. The exact positioning and reliable connection of both components placed high demands on machine calibration, precision and process control. Such hybrid structures have the potential to further reduce construction time and costs, as the more cost-intensive PBF-LB process is only used in those component areas where conventional processes fail to work.
The researchers have designed the complex cooling structure in such a way that critical zones of the mold are effectively tempered during the die casting process. This reduces the thermal load, which significantly lengthens the service life of the mold. In previous projects, a comparable additive mold had already achieved a service life up to four times longer than a conventional H13 mold.
After the HPDC mold was built, it was heat treated with stress relief annealing and hardening, and its functional surfaces were milled conventionally.
The future of automotive casting molds
The production of large-format casting molds using Additive Manufacturing processes is intended to address several key challenges in today’s automotive production, particularly in the context of the transformation towards electromobility.
A decisive advantage noted by the researchers lies in the conformal cooling, which can be freely designed for the first time using AM. The cooling channels can be optimally adapted to the thermally stressed zones of the tool. This lowers local temperature peaks, reduces thermomechanical wear and significantly extends the service life of the mold.
At the same time, Additive Manufacturing offers the opportunity to drastically shorten throughput times. Instead of time-consuming machining of several tool components and their assembly, a consolidated, end-to-end additive structure is sufficient. The die-casting mold for Toyota was produced in less than ten days, including all preparatory steps. For OEMs, this means shorter development cycles and faster time-to-market for new vehicle platforms.
The ability to build large-volume tools using hybrid technology creates additional flexibility. Components with defined interfaces can be efficiently added and functionally optimised without having to manufacture the entire component from scratch. This reduces both material usage and costs per tool.
For vehicle manufacturers such as Toyota, who rely on fewer individual parts and more complex structures, these developments offer new possibilities in terms of tool strategy: less effort in tool production, longer running times and the possibility of realising several variants with just one tool.
Fraunhofer ILT and MacLean-Fogg were keen to highlight that the developed process chain is suitable not only for aluminium HPDC tools but also for most other hot- and cold-forming tools and inserts, such as those for stamping, threading or injection molding. Wherever heavily loaded tools with complex cooling and limited batch sizes are required, Additive Manufacturing can offer clear advantages, the team stated.
