The automotive industry is in the midst of a profound upheaval. Cost pressure and the transition to electromobility are forcing many manufacturers to fundamentally rethink their vehicle architecture and production processes. Many manufacturers are currently reducing the number of individual pressed parts and striving for as few but highly complex structural components as possible. Particularly in the case of large aluminum components, such as frame or transmission components, this also increases the demands on the tools: they must be thermally highly resilient, allow variants and be able to be adapted to new geometries as quickly as possible.
This change brings with it new challenges: the required casting molds not only have to be larger than before, but also more resistant, with complex geometries and shorter development times. This is precisely where a project at the Fraunhofer Institute for Laser Technology ILT, together with the L-40 powder manufacturer MacLean-Fogg and Toyota as the end user, comes in.
By using a gantry-based PBF-LB/M machine developed at the Fraunhofer ILT with a scalable build volume and the tool steel developed by MacLean-Fogg for additive manufacturing, very large die casting molds with near-contour cooling could be additively manufactured for the first time - suitable for large-volume high-pressure die casting (HPDC) components.
Massive geometries previously led to residual stresses and critical defects in parts made with PBF-LB/M
As large casting processes are becoming increasingly established, 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 ensure a sufficient service life of the tool inlays, they need complex, internal cooling structures, which cannot be made with conventional manufacturing processes.
Two key problems have so far limited additive processes from manufacturing such large-format die casting molds: Firstly, the available construction volume of classic PBF-LB/M machines is too small to produce die or mold inserts with dimensions of 600 x 600 mm² or more in one piece. Secondly, the tool steels used to date – in particular H11 (1.2343), H13 (1.2344) or M300 – cannot be processed reliably in this 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 limitation, we need a new generation of machines and materials specifically tailored to the requirements of large-format HPDC tools," explains 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 machine technology makes it possible for the first time 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.
Scalable LPBF production for crack-free large components
The gantry-based 5-laser PBF-LB/M machine developed at the Fraunhofer ILT with a current build volume of 1,000 x 800 x 350 mm³ has been further developed for this purpose. In contrast to conventional systems, it has a movable processing head and local shielding gas guidance, so that 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 can be additively manufactured in the future than the tool inlay considered in this project with a volume of over 20,000 cm³ and a bounding box of 515 x 485 x 206 mm³.
A heatable substrate module was also developed in order to minimize 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. The combination of large installation space, high process stability and active preheating makes this system one of the first LPBF systems 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," comments Prätzsch. This steel is characterized 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 achieves high dimensional accuracy, outstanding properties in terms of hardness (48 HRC), tensile strength (1420 MPa) and notched impact strength (>60 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.
All in all, the combination of a scalable PBF-LB/M machine and a specially developed material enables the economical, reproducible production of large-format die casting molds with conformal cooling for the first time. Initial applications show that the service life of tools produced in this way can be significantly extended compared to conventional molds.
Fraunhofer Institute for Laser Technology ILT
