Laser Metal Deposition is an effective form of additive manufacturing that is particularly useful for things like tool repair and the application of anti-corrosion coatings. Like most forms of additive manufacturing, however, it has its challenges. The result is dependent on how evenly the laser beam applies the powder, since it’s difficult to optimally adjust the process parameters such as the speed and volume of the powder feed into the melt pool. Prior to printing, nozzles and caustics must be regularly checked, certified and calibrated – but these steps are cumbersome and complex.
“An employee applied a powder trace on a metal sheet, which was then checked by an expert,” said graduate engineer Oliver Nottrodt, Project Manager for Process Control and System Technology at Fraunhofer ILT. “But only a few specialists can perform this task in reproducible quality.”
The challenge spurred engineers at Fraunhofer ILT to develop a machine-supported inline process. The solution consists of three main components: a camera module with movable optics and illumination, all mounted on the machining head. The nozzle is measured with a laser module which is placed in the system. Electronics, integrated either separately or in the machine control cabinet, control the two modules.
“For the documentation, it is important to know where the axes of the system are located,” said Nottrodt. “Their exact position can be transferred from the basic machine via common data bus interfaces.”
To detect and measure the particle density distribution and caustics of the powder jet, the jet is illuminated with a laser line perpendicular to the powder gas flow and observed through the powder nozzle by the coaxially arranged camera. The system changes the relative position of the laser and the machining head several times for further measurements. Then the evaluation of 2,000 to 3,000 images shows the statistical distribution of the particles in one plane.
“If I use this method to gradually capture the so-called caustics – i.e. the focusing area in which the powder particle beam is bundled – it can be calculated and characterized very precisely in terms of the most important parameters, such as the minimum diameter and the density distribution,” said Nottrodt.
This system allows users to measure and certify the powder feed nozzles and completely characterize the respective powder jet. It also automates several of the user’s tasks, like measuring and marking the positions of the processing laser as well as documenting all work steps. The measuring system also uses the geometric characteristics of the melt pool to monitor the laser metal deposition process, which it also visualizes and documents.
Fraunhofer ILT will be at CONTROL 2018, an international trade fair for quality assurance, which is taking place in Stuttgart from April 24th to 27th. To learn more about the new system, attendees can visit the joint booth of the Fraunhofer Alliance VISION 6302 in Hall 6. Fraunhofer ILT will present a camera module with integrated lighting mounted on a typical working head, and the engineers who developed the system will demonstrate how the laser module functions on the computer.
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[Images: Fraunhofer ILT]
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