Additive manufacturing processes are gaining more importance in the industrial production of metal components, as they enable complex geometries to be produced with less effort. The process parameters used to manufacture a wide variety of components are currently kept constant and closed-loop controls are missing. However, due to the part geometry that causes varying heat flow to neighbouring powder and solidified sections or due to deviations in the atmosphere caused by fumes within the work area, there are changes in the melt pool temperature. These deviations are not considered by system control, so far. It is, therefore, advisable to measure the melt temperature with sensors and to regulate the process. This work presents an approach that enables fast process control of the melt pool temperature and combines a closed-loop control strategy with a feedforward approach. The control strategies are tested by proof-of-concept experiments on a bridge geometry and partly powder-filled steel plates. Furthermore, results of a finite element simulation are used to validate the experimental results. Combining closed-loop and feedforward control reduces the temperature deviation by up to 90%. This helps to prevent construction errors and increases the part quality.
The causes of geometrical deviations from the production process and the prediction of application properties, such as noise behavior, wear or material fatigue, are only possible by having detailed information about the gear geometry. The gold standard for the gear quality inspection is represented by dimensional measurements with a tactile sensor system. As a result for industrial applications, the slow serial measurement leads to the compromise of a random inspection of the gear geometry. For the purpose of a faster and more extensive surface acquisition, a laser line triangulation sensor is investigated providing 1280 points at a line width of 25 mm with up to 200 lines/s. The results at the tooth of a large cylindrical involute gear with a pitch circle diameter of 922 mm and a face width of 246 mm show the qualification for fast three-dimensional measurements of the convex and reflective surface. The detection of the complete profile line at once is possible. It is shown that the measurement deviation of laser line triangulation can be minimized by increasing the dynamic threshold. The measurement deviations amount to ± 8.2 µm and can be attributed to random and systematic errors. Compared to the standard gear inspection, an acceleration factor of 5700 was attained. An optical scanning of the complete tooth flank provides the prerequisite for an identification of surface defects in the form of breakouts and blemish.
Laser chemical machining, a non-conventional processing method based on thermally activated electrochemical material dissolution, represents a promising technology for manufacturing metallic dies for micro forming applications. Prior to widespread industrial acceptance the machining quality of laser chemical machining should be characterized. For this purpose, laser chemical machining is compared with micro milling regarding both the dimensional accuracy and the surface quality. Therefore, square micro cavities exhibiting side walls between 100 μm and 400 μm in length and 60 μm in depth are machined with both manufacturing processes into the cobalt-chrome alloy Stellite 21. The geometrical features are investigated using laser-scanning confocal microscopy and scanning electron microscopy. On the one hand, laser chemical machining is more suitable for manufacturing cavities with dimensions < 200 μm due to higher shape accuracy with stable mean edge radii of (11.2 ± 1.3) μm as a result of roughing and finishing steps. On the other hand, the finish quality of micro milling with mean surface roughness Sa of 0.2 μm could not be achieved with laser chemical machining due to in-process induced waviness. Finally, the metallographic analysis of the surface-near layers reveals that both manufacturing processes ensure gentle machining without any noticeable micro structural impact.
The in situ geometry measurement of microstructures in the laser chemical machining (LCM) manufacturing process places high demands on measurement systems because the specimen is submerged in a closed fluid circuit. The steep slopes of the manufactured micro-components and the general lack of accessibility hinder the use of standard techniques such as tactile measurement or conventional confocal microscopy. A technique based on confocal fluorescence microscopy shows promise for increasing the measurability on metallic surfaces with large curvatures. By applying an intensely scattering fluorescent coating to the specimen, the surface position can be determined by the change in fluorescence signal at the boundary between specimen and coating. In contrast to the currently tested thin coatings (\100 lm) the measurements in layers thicker than 1 mm, as required for in situ application at the LCM process, show distinct dependencies on the fluorescent medium in terms of concentration and index of refraction. Hence, a fundamentally different signal evaluation approach based on a physical model of the fluorescence signal is needed to extract the surface position information from the detected fluorescence intensity signal. For the purpose of validation, the measurement of a step geometry is performed under the condition of a thick fluid layer and referenced with a tactile measurement. As a result, the model-based approach is shown to be suitable to detect the geometry parameter step height with an uncertainty of 8:8 lm for a step submerged in a fluid layer with a thickness of 2:3 mm.
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