VCSEL heating approach for metal AM reduces risk of thermal stresses in parts

We can count on seeing many exciting 3D printing innovations at the upcoming Formnext event in Frankfurt—both established and new. With 550 companies and groups exhibiting at the expo and with many more visiting, the Formnext center will be a real who’s who of the additive manufacturing industry.

Among some of the new technologies to be presented at Formnext is a new additive manufacturing process developed by researchers from the Fraunhofer Institute for Laser Technology (ILT). Always at the cutting edge of industrial AM processes, Fraunhofer ILT has partnered with the Chair for Technology of Optical Systems TOS at RWTH Aachen University to present its latest metal AM process: laser powder bed fusion (LPBF).

Along with the new process, the research group will present a new heating process for laser-based metal 3D printing which could be a game changer for the technology in terms of reducing part distortion and increasing the height of 3D printed components. The process, in short, incorporates a system in which printed parts are heated with laser diodes in the powder bed.

The novel approach aims to reduce the internal stresses in printed parts which are caused by temperature gradients. Typically in the powder bed fusion process, metal powders are sintered by a laser which melts a specific point of the component, while the rest of the printed part cools rapidly. This difference in temperature can cause problems, including distortion and even cracking.

Current methods for reducing this risk include heating the component from below using a substrate plate, which can be effective to a certain extent but is limited in its success when taller parts are printed.

To overcome these challenges, a team from Fraunhofer ILT have been working with Philips Photonics to develop heating solutions for metal prints at the Digital Photonic Production DPP research campus. Together, they have come up with the approach of heating the component from above.

The method uses an array of six vertical-cavity surface-emitting laser bars (VCSEL) with 400 W each that are installed in the process chamber. With infrared radiation at 808 nm, the laser bars can heat the part up to several hundred degrees Celsius during the printing process. Moreover, the individual bars can be controlled separately, enabling different heating patterns.

In demonstrating the efficiency of heating from above, the research team printed parts out of Inconel 718 which they heated up to 500 °C. Using an infrared camera to monitor the process, the team found that the parts demonstrated significantly reduced distortion. Overall, the researchers explain that the VCSEL system reduces the thermal gradient of the part, which consequently reduces internal stresses and enables the production of even taller parts.

In addition to those benefits, the Fraunhofer ILT team says that the heating technique could make it possible to work with typically challenging materials. For instance, the team will soon test printed parts made from titanium aluminides, which will be heated up to about 900 °C.

These parts could have applications in the making of turbochargers and specifically their hot gas sections. The innovative heating process could also create new possibilities in industrial sectors which require minimal thermally induced stresses.