Monitoring system optimises powder jet in laser material deposition

The Fraunhofer Institute for Laser Technology ILT has developed an inline system for testing, qualifying and adjusting the focused powder jet used in laser metal deposition (LMD). The system also enables the jet nozzles to be certified and the caustic of the process to be characterised.

Process parameters such as the speed and volume of the powder feed in LMD – the additive manufacturing technique by which powder is applied to a laser-generated melt pool via a nozzle to clad material layer by layer – are difficult to adjust at an optimal level, therefore the nozzles and caustics of the process must be regularly checked, certified and calibrated. According to Aachen-based Fraunhofer ILT, however, the sequence of these steps has been very complex and cumbersome up until now.

‘An employee applied a powder trace on a metal sheet, which was then checked by an expert. But only a few specialists can perform this task in reproducible quality,’ said Oliver Nottrodt, project manager for process control and system technology at Fraunhofer ILT.

Fraunhofer ILT has developed a new system for measuring the powder jet of laser metal deposition processes. (Image: Fraunhofer ILT)

The engineers of the institute have therefore developed a machine-supported inline process for testing, qualifying and adjusting the focused powder jet of the LMD nozzles.

The solution consists of three main components: a camera module, movable optics and laser sources, all of which are mounted on the LMD machining head. One laser module is used to measure the nozzle, while another, positioned perpendicularly to the powder gas flow, illuminates the jet, allowing its particle density distribution and caustics to be observed and measured via the coaxially arranged camera module through the powder nozzle. The relative position of the laser and machining head are adjusted several times to obtain multiple measurements during the monitoring process, in which approximately 2,000 to 3,000 images are captured by the system.

According to Nottrodt, the evaluation of such images reveals the statistical distribution of the powder particles in a single 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 characterised very precisely in terms of the most important parameters, such as the minimum diameter and the density distribution,’ he explained.

The new measuring system enables users to measure and certify their powder feed nozzles and completely characterise the respective powder jets. It also helps set up the process by taking over a multitude of tasks from users – such as measuring and marking the positions of the processing laser as well as documenting all work steps. Furthermore, the measuring system uses the geometric characteristics of the melt pool to monitor the laser metal deposition process, which it also visualises and documents.

Last year Fraunhofer ILT launched its high-speed LMD process EHLA, which it says has the potential to capture a portion of the chrome plating market worth €2 billion. The new laser cladding technology can reach a deposition rate of up to 500 metres a minute, compared to 0.5 to 2 metres per minute for standard LMD.

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