Illumination for process observation in laser material processing

Process control for laser-material-processing requires access to characteristic features which qualify the operating-point of the process. For laser-based manufacturing-processes, this can either be achieved by detecting the emission of the process or by detecting geometric properties of the process-workpiece interaction. Such illumination is usually provided by lasers emitting in the near-infrared. Inherent properties like coherence of these sources and complex optical systems prohibit a wide adoption. The technological advance in sources such as led's has the potential to deploy small light sources directly to the processing heads but bears new problems in light-ray-delivery, emitter-protection against secondary radiation and cooling. From a scientific point of view, the properties of illumination sources are compared with a special focus on the specific requirements of process observation

Process observation in fiber laser-based selective laser melting

The process observation in selective laser melting (SLM) focuses on observing the interaction point where the powder is processed. To provide process relevant information, signals have to be acquired that are resolved in both time and space. Especially in high-power SLM, where more than 1 kW of laser power is used, processing speeds of several meters per second are required for a high-quality processing results. Therefore, an implementation of a suitable process observation system has to acquire a large amount of spatially resolved data at low sampling speeds or it has to restrict the acquisition to a predefined area at a high sampling speed. In any case, it is vitally important to synchronously record the laser beam position and the acquired signal. This is a prerequisite that allows the recorded data become information. Today, most SLM systems employ f-theta lenses to focus the processing laser beam onto the powder bed. This report describes the drawbacks that result for process observation and suggests a variable retro-focus system which solves these issues. The beam quality of fiber lasers delivers the processing laser beam to the powder bed at relevant focus diameters, which is a key prerequisite for this solution to be viable. The optical train we present here couples the processing laser beam and the process observation coaxially, ensuring consistent alignment of interaction zone and observed area. With respect to signal processing, we have developed a solution that synchronously acquires signals from a pyrometer and the position of the laser beam by sampling the data with a field programmable gate array. The relevance of the acquired signals has been validated by the scanning of a sample filament. Experiments with grooved samples show a correlation between different powder thicknesses and the acquired signals at relevant processing parameters. This basic work takes a first step toward self-optimization of the manufacturing process in SLM. It enables the addition of cognitive functions to the manufacturing system to the extent that the system could track its own process. The results are based on analyzing and redesigning the optical train, in combination with a real-time signal acquisition system which provides a solution to certain technological barriers

Verfahren und Vorrichtung zur Prozessüberwachung bei der generativen Fertigung von Bauteilen

The invention relates to a method and to a device for process monitoring during the additive manufacturing of components by layer-by-layer solidification of a material (7) using energetic radiation. According to said method, data from at least one area of the work surface used for solidifying the material is recorded using a radiation-sensitive sensor arrangement before and/or after applying a new layer of the material (7). The data is recorded using at least one line sensor (9) as a radiation-sensitive sensor arrangement which is moved over the work surface. The method enables image data to be recorded with a higher resolution and higher measurement rate without interferences from process emissions and without additional non-productive periods

Setup and maintenance of manufacturing quality in CO2 laser cutting

AbstractManufacturing of parts by laser cutting is widely accepted in industry. In the market there are systems which are highly focused on special applications such as robot based 3D cutting in the automotive industry but also all-round machines which cut a diverse set of sheet steels at a variety of thicknesses. Most of these machines use a melt based cutting process where the molten material is ejected by an assist gas.Quality of the laser cut part is determined by a dross free cut with minimal roughness on the cut face. To ensure this quality, several parameters of the cutting process have to be within their defined tolerances. Such parameters are the focal position, the distance of the cutting nozzle to the work piece and the beam quality to name only the most important ones.The paper reports about the approach to equip industrial laser cutting systems with sensors such that manufacturing quality comes to an advanced level. Prior work has shown that such technical systems can be enabled to determine their focal position autonomously. As a next step, active control of the laser focal position has been demonstrated keeping the process at its nominal operating point in this respect. The benefits for manufacture are discussed in relation to machine setup and sustainability.The reported system relies on expert knowledge about the system behaviour in order to reference the measurement signal to the focal position. As this behaviour is closely connected to the current state of all optical elements in the system, an inference from the current control signal to the status of the optical train of the system becomes possible