Active quality assurance is one of the most important requirements of additive manufacturing (AM) production users, says German AM specialist Concept Laser, whose metal powder laser melting equipment is available in the UK from ES Technology (01865 821818).
For sure, monitoring the key data of a laser melting system, as regards oxygen content, temperature, laser output and powder quality, are part and parcel of today’s quality control systems, but separate to such measurements is melt pool monitoring (the point where the powder is turned molten by the laser beam). With current so-called 2D melt pool monitoring, a comprehensive statement about the quality of the part cannot be made, says Concept Laser, which adds that it has an answer to that, or soon will have.
As of 2016, the company will offer in-situ quality assurance that makes available a “’3D landscape’ of additive parts, with accuracy down to the micron level, in real time”, with Concept Laser’s M1 cusing and M2 cusing machines being first to benefit.
The company’s new patent -pending system, QMmeltpool 3D, identifies two characteristic melt pool parameters, area and intensity, with these linked to corresponding process errors. For example, a low melt pool intensity may indicate inadequate laser output or an excessively high laser scanning speed, both delivering insufficient energy input. In addition, changes in the area of the melt pool may indicate a variation in the oxygen content within the process chamber.
QMmeltpool 3D's in-situ capability (left) gives 3D point data. 2D melt pool analysis (right) can only give layer average information
A complication here, cautions Concept Laser, is that part geometry also has effects on the thermal conditions during the process, so reference samples and a high level of process understanding are required for the variation in data during the process to be interpreted and analysed correctly.
Of course, 2D melt pool analysis also reveals information, but there is a major difference. In the case of 2D melt pool monitoring, signals are supplied as average values per component, per layer, with this permitting only a restricted interpretation of localised defects, says Concept Laser. QMmeltpool 3D records position-related characteristics of the melt pool while the component is being built up, with this data visualised in a 3D landscape and allowing analysis by the user. The analytical tool is said to be comparable to the HD resolution achieved by computer tomography (CT). (See also ‘QMmeltpool 3D compared to the 2D, off-axis approach’ box, below.)
In more detail, a highly accurate 3D landscape of the part is created via a camera and a photodiode that identify characteristic properties of the melt pool to a high level of resolution, as regards location and timing, with these signals correlated with the corresponding positional data of the laser.
This is QMmeltpool 3D’s special characteristic: melt pool signals, such as melt pool area and melt pool intensity, can be visualised and evaluated in three dimensions to a resolution of 35 micron directly after the build process has finished. The user can trace the process of creating each part, with local effects in the part during the build process better detected and analysed.
For example, excessive or inadequate energy input will produce imperfections. Inadequate energy input into the powder bed leads to unmelted powder in the form of irregularly shaped pores; excess energy input can give rise to gas inclusions that are revealed as regular, round pores in micrographs. QMmeltpool 3D captures the variations that lead to these issues.
Of course, process gas flow, the material and many other factors (measured by other quality control technology) can also influence the process and part quality, but in-situ monitoring systems can detect process characteristics in real time, thanks to their high resolution and sampling rates (every 0.1 mm, depending on the scanning speed). Thus they make a significant contribution to detecting process defects at an early stage and avoiding them in future. For the user, QMmeltpool 3D represents a tool that allows for the optimisation of the process, underlines Concept Laser.
As to the benefits, QMmeltpool 3D helps to minimise quality assurance time and work since it can supply local indications of part defects, leading to a reduction in subsequent inspections and tests. But the technology is unable to rectify defects during the build process and, due to the large number of influencing factors that can cause defects in the build process or on the component itself, together with the highly dynamic properties of the process, developing a self-correcting control loop represents a significant challenge, the company concludes.
The practical added value of 3D visualisation with QMmeltpool 3D is not just that it is an original way of providing active quality assurance, but in production and process development, component jobs can be optimised through iterative variation of the parameters. Support structures can be adapted, and, above all, the design of the part can be structured in a more efficient and production-friendly manner. Last but not least, says the company, new possibilities are opened up in material research and validation of materials.
QMmeltpool 3D expands current 2D inspection into the 3D space, offering coordinate-related data acquisition of melt pool values
QMMELTPOOL 3D COMPARED TO THE 2D, OFF-AXIS APPROACH
QMmeltpool 3D expands current 2D inspection into the 3D space, with coordinate-related data acquisition of the melt pool values. The in-situ nature of the technology is also contrasted with that of 2D ex-situ melt pool measurement alternatives. Such classic off-axis inspection systems have a lower resolution and lower detection rate, but do offer easy system integration of machine and infra-red camera. And while an off-axis structure enables statements to be made about the overall fusing and cooling behaviour, it is not possible to derive a detailed statement about the melt pool.
The on-axis/in-situ approach of QMmeltpool 3D is based on a two axial arrangement of detectors, a camera and a photodiode that make use of the same optic as the laser. This coaxial integration permits a high coordinate-related 3D resolution of 35 micron. The detection rate for the system is related to the scanning speed: if it is 1,000 mm/s, the distance covered by each shot is 100 µm; at 2,000 mm/s, the value is 200 micron. As such, detailed analysis of the melt pool characteristics is possible via the QMmeltpool 3D technology, says Concept Laser.
SWIMMING IN THE SAME POOL
- SLM Solutions’ Melt Pool Control module in development employs fast single-point infra-red emission measurements at two wavelengths, with 2D maps of thermal energy resulting. The aim is to be able to dynamically adjust laser output power, achieving closed-loop power control (www.industrial-lasers.com). SLM Solutions is represented in the UK by Laser Lines (01295 672515). No further information has been forthcoming.
- In August last year, EOS announced that it had entered into a partnership with Plasmo to “advance development of the online process monitoring for direct metal laser sintering”. Said Dr Adrian Keppler, sales and marketing director at EOS: “We already offer comprehensive quality assurance processes for our systems upstream and downstream of the manufacturing process. With modular online process monitoring, we are extending this quality assurance by a further module and ensuring an even greater transparency during our quite complex building process. At the same time, we are making a tool available to our customers that they can use to build up their own quality assurance concept.” With no statement since then, an announcement will be made later this year, Machinery was told.
- The UK’s Renishaw says that it is interested in “obtaining data useful to engineers and that is manageable in terms of size and volume, from a data handling perspective”. The company says it will be introducing some technologies on its forthcoming EVO platform, which is: "Renishaw's first additive manufacturing system for industrial production." Read more about EVO here: http://is.gd/IkgWsv
This article was originally published in the August 2015 issue of Machinery magazine.
