Contactless quality inspection of additive manufactured components

By means of additive manufacturing technology, such as selective laser sintering, geometries which are not possible using established manufacturing processes can be achieved. The surface of such additive manufactured components often has a high roughness and requires mechanical reworking depending on the respective use, for example by slide grinding, blasting or polishing. To monitor the surface quality throughout the entire process chain, innovative 3D measurement technology from Confovis GmbH is used. This provides the measurement results which are the basis for product and process optimisation quickly and in a contactless and high-resolution manner.

With selective laser sintering (SLS) or with selective laser melting (SLM), complex geometries are melted based on a powdery source material layer by layer through exposure to laser beams. Whether in medical engineering, in tool-making and mechanical engineering or in automotive and aerospace industry sectors: Thanks to the new process, options which are not possible with conventional manufacturing processes or only possible with a great deal of effort are available to design engineers. Any three-dimensional geometry — even cavities, complex internal channel geometries or undercuts— can be created with this process. A main disadvantage of the process, however, is the inhomogeneous surface quality depending on the shape.

Roughness and contour measurement

For process-related reasons, the source material melts differently depending on the shape of the finished object. Horizontal planes have a higher quality than vertical planes, since the laser works vertically to the surface. If the finished product is to have a homogeneous surface, it must be reworked accordingly. In practice, this rework is carried out by means of different processes, such as slide grinding, blasting or polishing, to mention just a few.
For the surface analysis — from the powder to the finished component — the contactless measurement technology from Confovis is used. With only one measurement system, faults, such as incomplete melting or material inclusions as well as roughness can also be measured and assessed in three dimensions in addition to shape and contour characteristics. Here, the measurement system has a stabilised measurement noise of 2.1 nm (determined according to the “Fair Data Sheet / Optoasyst”). Roughness can be assessed using the patented structured illumination technology from Confovis, not only according to DIN EN ISO 4287/4288, but also according to DIN EN ISO 13565 and DIN EN ISO 25178. This is necessary, since the surfaces of components produced by means of SLM in particular cannot be characterised sufficiently using conventional profile sections.

Extensive measurement for more detailed information

If, for example, individual spheres of the powder on the surface are partially melted during the manufacturing process, they can rarely be recorded using an individual profile section and, as a result, lead to an indifferent statement about the surface quality. It is therefore essential to record and assess the surface of additive manufactured components in three dimensions in order to identify partially melted powder residues and thus describe the surface quality of the components produced in terms of quantity and quality.
Up to now, the source material — in this case, the powder which is melted using the laser — has only been characterised in two dimensions and on the basis of image processing. With the Confovis measurement system, the powder, too, is measured in three dimensions. With the 3D data obtained, additional parameters, such as the volume, spherical radius, form factor or aspect ratio of the powder particles can then be determined.

Measurement technology for a wide variety of materials

Measurement systems which are to measure the entire range of additive manufactured components must be able to cope with different reflective properties. Thanks to the integration of two measuring processes (focus variation and structured illumination), Confovis measurement systems record both reflective and strongly absorbing surfaces. This is necessary to monitor and control the process chain from the powder up to the final product. To record and assess steep edges, the measurement systems use the focus variation method in addition to the high-resolution structured illumination. This is especially advantageous for measuring shapes and contours. Here, the acceptance angle is higher than 85°. Thanks to the combining of the measuring processes, only one measurement system has to be purchased. This saves on investment costs and the expense of operator training.

Artefact-free measurements for reliable measuring results

Confovis measurement technology uses our own specially developed and patented structured illumination technology. It is based on confocal microscopy. However, the measuring process does not require a pinhole disc and uses a LED light source instead of a laser. The advantage: Measurement artefacts are reduced significantly, since there are hardly any undesired interference phenomena, such as coherence and speckle effects.
With structured illumination, an optical grid is projected in the focus level of the microscope. Due to the phase-shifted presentation of the grid, the contrast between the two images can be assessed. In order to determine the topography, the height is varied on the Z-axis, as is the case with conventional confocal measurement technology, which results in optical sections.