Holographic imaging system obtains AM melt pool topography

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(a) Two-wavelength holographic set-up for LBM monitoring. (b) Key-hole in melt pool on 316L substrate. (Image: Matthieu Piniard, Beatrice Sorrente, Gilles Hug, and Pascal Picart)

Researchers have developed a two-wavelength holographic system capable of measuring the topography of a melt pool in-situ during laser-based metal additive manufacturing.

The system has been tested on a 316L substrate, with the results being published in Light: Advanced Manufacturing.

According to the researchers, from French Aerospace Lab-ONERA and the Laboratoire d’Acoustique de l’Université du Mans, while several optical monitoring approaches have previously been developed based on melt pool process radiation or with a secondary illumination source, they cannot measure the 3D shape of the metallic melt pool.

The new holographic module was therefore developed, comprising two linearly polarised lasers emitting probe beams in the visible range at 632.8nm and 634.4nm. These beams are co-axially aligned with the high-power 1,080nm fusion laser of an AM system. The surface to be analysed is installed on a motorised stage that permits the scanning of the fusion laser. Holograms are obtained from the reflected probe beams using a high-speed camera, which records the spatially multiplexed two-colour holograms at both the temporal and spatial scales of the laser melting process. 

Using the newly developed system, the scientists were able to observe a deep depression zone characteristic of the key-hole regime at the bottom of the melt pool. During the recording sequence, instabilities of the melt-pool regime that oscillated between the key-hole mode and the transition mode without depression were observed. The key-hole regime is a crucial defect that cannot be identified from thermal radiation monitoring techniques, but it can corrupt the manufactured object and dramatically reduce the productivity of metal additive manufacturing. Therefore it is strategic to be able to detect this defect.

These first tests were carried out at a speed of 100mms-1

‘To the best of our knowledge, these are the first results of in-situfull-field melt pool and track topography measurements in laser beam melting,’ the researchers remarked in their announcement of the results. ‘Although the proof of concept is demonstrated, several improvements are required to increase the performance of the set-up.’ 

One such improvement would be to increase the power of the two probe beams. This would enable the collection of more photons, especially when processing non-cooperative surfaces such as 316L powders that are less reflective than the 316L substrate. Since motion blur can degrade the quality of the measured 3D shape, more powerful probe beams would also minimise this effect so that the exposure time could be reduced and the velocity increased up to that of industrial processes – typically 1ms-1

‘We plan also to simplify the optical set-up to allow its implementation in an industrial additive manufacturing machine,’ concluded the researchers. ‘This work opens the way to new strategies to optimise in-situadditive manufacturing processes and develop reliable numerical models to apprehend the dynamics of the melting process.’

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