In the research project ProLMD, funded by the German Federal Ministry for Education and Research (BMBF), teams are developing new hybrid processes that combine conventional production methods with laser material deposition (LMD) to form a new manufacturing approach.
In 2016 a research idea was born. “The aim was to develop economical and robust system technology for the LMD process, based on a jointed-arm robot, and to integrate it into a process chain for hybrid manufacturing,” says Jan Bremer, a scientist at the Fraunhofer Institute for Laser Technology ILT in Aachen, looking back. “We are moving along the process chain for robot-based hybrid additive manufacturing and researching various technologies required for this. The spectrum of content covers everything – from processing heads, robot and shielding gas systems to welding processes, quality assurance and software.”
What hybrid actually means in practice is demonstrated by three applications from the project partners – MTU Aero Engines (addition of functional elements on an engine component), Airbus (component reinforcement by 3D ribbing) and Mercedes-Benz (adaptation of a press tool in body production). The project focuses on locally reinforcing or modifying conventionally manufactured components, but the technologies developed also make repair applications possible.
“These examples show what we understand by hybrid manufacturing,” says scientist Bremer. “It is the flexible combination of advantages from different manufacturing processes, as it combines any conventional manufacturing process with LMD to form a continuous process chain.” This flexibility can also be seen in their work with the industrial partners, who manufacture their demonstrators prior to LMD functionalization by roll forming (Airbus), casting (Mercedes-Benz) or forging (MTU).
For the scientist, hybrid manufacturing also provides a good example of how complex variant diversity can be simplified in production. “For example, you always start by punching and trimming a basic part in the same way,” explains Bremer. “The variants are then later produced using LMD. The user can, therefore, continue to use his punching machine, but then additively apply reinforcements to the component, for example. Thanks to the LMD process and the technologies developed in ProLMD, we can act flexibly and use automation to a great extent. This is in line with our guiding principle: additive manufacturing – but only in those parts of the process chain where it results in added value.”
With this goal in mind, the Aachen-based engineers at Fraunhofer ILT, together with a total of seven industrial partners, are working on a highly efficient, modular LMD cell that can be integrated into an existing process chain with little effort.
To take advantage of maximum application flexibility, the partners are developing processes using both wire and powder as an additive material. Among other things, Fraunhofer ILT has developed a processing optic for generating a ring beam for coaxial laser material deposition, which is being further developed and used in the joint ProLMD project. This optic generates a ring with a uniform intensity distribution, thus offering directional independence during welding. In the project, processes with deposition rates in the range of 1 to 2 kg/h at high geometric resolution are being developed.
But why use a multi-axis KUKA robot? “What speaks in its favor is its very large building space, its flexibility and its easy accessibility,” explains Bremer. “In the test facility, we can use up to eight axes to access a component of almost any complexity from all sides. The machine technology can be implemented at an amazingly low cost by means of robots.” The project focuses on machining large, complex components. “Components weighing up to 1.2 tons and with a diameter of two meters can be processed on the robot system,” reports the scientist.
The division of tasks among the other project partners is clearly defined: The Lasertec business unit of the KUKA site in Würselen is responsible for project management and cell integration of the robot, while Laserline GmbH from Mülheim-Kärlich is in charge of the design and development of the beam source and optics. M. Braun Inertgas-Systeme GmbH, Garching (near Munich), is responsible for the construction of a shielding gas cell, while Dortmund-based BCT Steuerungs- und DV-Systeme GmbH is developing the software and machine-integrated measuring technology.
The plug-in solution is flexible because it works on the black box principle. “We are not interested in what happens to the component before or after,” emphasizes Bremer. "We don't just work off a static CAD model, but also use the real geometry in adaptive processes thanks to robust system technology and software. Intelligent algorithms also enable the cell to adapt to and compensate for extreme component deviations. For the hardware and software, the emphasis has been placed on robustness – from the laser head and robot to the flexible inert gas design and suitable adapted path-planning algorithms. This is an indication that Fraunhofer ILT not only continues to develop the welding process, but also researches other important aspects such as the influence of robot accuracy on process reliability and component quality.
While the project manager KUKA in Würselen has the robot weld materials sensitive to oxidation, such as titanium, in a flexible shielding gas cell reliably, the Aachen-based engineers are using another robot system to weld nickel- and iron-based materials without a shielding gas cell, but with shielding gas flowing locally from the nozzle. If the deposition rate is higher, they also use an additional shielding gas nozzle only a few centimeters in size, as required. “In this way, all three solutions get by with significantly less expensive shielding gas,” says Bremer. “This reduces operating costs considerably.” Innovative processes are not only the subject of research, but also part of everyday life at Fraunhofer ILT. “With the first versions of the local inert gas nozzle we had thermal problems at higher deposition rates,” reports the researcher. “However, since our colleagues have been working on processing copper in laser powder bed fusion (LPBF) for several years now, their support has helped us additively manufacture the component out of copper with internal cooling structures on a research machine and thus solve the problem.” For him, it is an example of what becomes possible when an institute develops interdisciplinary solutions.
More about the success of proactive teamwork can be seen by taking a look inside the development halls at Fraunhofer ILT. In Aachen there is one large and one more compact robotic cell for additive manufacturing. The project participants are particularly proud of this latest development: With additional financial support from the BMBF, they are creating a less expensive version of the ProLMD robot system for small and medium-sized enterprises (SMEs). “We have scaled down the solution from a 3.1 m long robot arm with 90 kg load capacity to about 2 m and 60 kg load capacity,” the scientist reports. “On the large robot, we can demonstrate a flexible changing system with wire and powder-based processing heads, while the small cell is all about powder-based LMD, machine-integrated geometry measurement and the new CAM module.”
With the new cell, the Aachen-based research institute, in cooperation with the project partners, is proving that it is also possible to create a compact cell for SMEs, which costs considerably less than a typical machining center.
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