Tech Focus: R&D Fine-tuning for additive production
At Formnext 2019, Fraunhofer IWS presented a shielding gas nozzle called “Coaxshield” and a light scanner for laser powder build-welding called “Lisec”.
Additive manufacturing systems can generate highly complex components, which could not be produced with conventional machine tools or only with great effort. Nevertheless, such industrial 3D printers are far from being standard equipment in factories. This is not just due to the purchase costs, but also to many other challenges. The Fraunhofer Institute for Material and Beam Technology IWS Dresden has developed particular solutions and presented them at Formnext. Included were “Coaxshield”, a local molten pool shield for laser powder build-up welding and the “Lisec” analysis device for controlling the powder flow in additive manufacturing processes.
Titanium, a material popular in aerospace, oxidises in contact with air at processing temperatures above or equal to 300 °C. As a result, the material properties change. The components become brittle and can crack. If, for example, a laser is used by a robot to manufacture a titanium workpiece additively, a large chamber must be built first around the robot and the component. This chamber is then either flooded with a low-reaction noble gas such as helium or argon or a vacuum must be generated before manufacturing can start. “This kind of process shielding may be suitable for small component sizes, but it causes considerable difficulties for the production of large components in terms of process control and accessibility,” explains Jakob Schneider, researcher additive manufacturing at Fraunhofer IWS. “In addition, the costs for such a chamber increase proportionally with the size of the component to be protected. These are, for example, the expanses for several cubic meters of helium or argon, which may also have to be pumped in and out due to intermediate processing steps.” The same applies to workpieces made of further materials (refractory metals), such as tantalum, niobium or titanium-aluminum compounds.
“Coaxshield” protects titanium components
For this reason the IWS has developed “Coaxshield”, an alternative protective shield designed to direct the shielding gas only to the areas where it is really needed: directly around the laser beam's processing zone, which melts the metal powder and deposits it layer by layer on the component. The nozzle head can be mounted under standard processing optics. It encloses the powder nozzle and forms a protective gas cone “coaxially” around the process zone. This cone thus only protects the hot processing zone, because just here titanium and ambient air can react with each other.
“This solution saves the user a lot of time and money,” emphasises Jakob Schneider. “In addition, very large titanium components can also be additively manufactured.” An example: For the X-ray space telescope “Athena”, the European Space Agency ESA needs a satellite supporting structure made of titanium with a diameter of several meters. In co-operation with Fraunhofer IWS, ESA is developing a process and the associated system technology for additive manufacturing. The “Coaxshield” has been developed in this context .The technology is expected to be ready for the market at the beginning of 2020.
“Lisec” lights the powder stream
While in conventionally used ablative processes such as milling, the calibration of the tools corresponds to the state of the art, in laser powder build-up welding it is still a great challenge. The “Lisec” measuring device was developed at the IWS to solve this challenge and move the limits to technical feasibility. The abbreviation stands for “Light Section” and reveals the principle: A measuring laser scans the powder flow after leaving the nozzle. A right-angle camera is mounted, which records light sections through the powder and forwards them to analysis software. “The three-dimensional distribution of the powder stream can be calculated with high precision,” explains IWS engineer Rico Hemschik. “This allows significantly simplified quality control and provides conclusions about the wear degree of the powder nozzle.”