Alexander Gatej

Process observation in fiber laser–based selective laser melting

Abstract. 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.

Self-optimizing assembly of laser systems

Laser assembly can be a tedious task if performed manually. Especially if miniaturization of the laser is desired, robot-based assembly can greatly improve quality, performance and throughput, while self-optimization is regarded as a strategy to reduce planning efforts and increase the robustness of the assembly. An automated laser assembly system has been developed together with a concept to increase the autonomy through a multi-agent system control structure. The multi-agent system allows assembly steps like sequence planning, measurement of components and deviations, selection of components, soldering elements onto a carrier plate and active resonator alignment to be handled by the system itself and enables the assembly system to uniquely plan every laser system and execute its assembly within a flexible robot-based assembly cell.

Thermo-optical (TOP) analysis by coupling FEM and ray tracing

Thermo-optical simulation is a compulsory improvement of classical ray tracing, since many branches of optical and laser technology have to deal with thermal gradients. This paper discusses an approach for coupling FEM and ray tracing simulation tools by processing FE data using scattered data approximation techniques. The implemented interface for two space dimensions is being validated by comparing approximated data to measured values from a CO2 laser application of up to 1.75 kW. Finally, the benefits and further developments of analyzing thermal gradients in optical simulation are being discussed.

Using adaptive weighted least squares approximation for coupling thermal and optical simulation

Thermo-optical simulation is a mandatory enhancement of classical ray tracing, since nowadays many fields in the branch of optical technology have to deal with thermal effects. This paper discusses an approach for coupling the finite element method (FEM) and ray tracing simulation by processing finite element (FE) data using scattered data approximation techniques, particularly with an adaptive weighted least squares approximation algorithm in two dimensions. The validation of the implemented interface is being conducted by comparing approximated data to analytical functions. Finally, FEM data are being processed by the developed algorithm to demonstrate the applicability on appropriate problems.

Tracking the course of the manufacturing process in selective laser melting

An innovative optical train for a selective laser melting based manufacturing system (SLM) has been designed under the objective to track the course of the SLM process. In this, the thermal emission from the melt pool and the geometric properties of the interaction zone are addressed by applying a pyrometer and a camera system respectively. The optical system is designed such that all three radiations from processing laser, thermal emission and camera image are coupled coaxially and that they propagate on the same optical axis. As standard f-theta lenses for high power applications inevitably lead to aberrations and divergent optical axes for increasing deflection angles in combination with multiple wavelengths, a pre-focus system is used to implement a focusing unit which shapes the beam prior to passing the scanner. The sensor system records synchronously the current position of the laser beam, the current emission from the melt pool and an image of the interaction zone. Acquired data of the thermal emission is being visualized after processing which allows an instant evaluation of the course of the process at any position of each layer. As such, it provides a fully detailed history of the product This basic work realizes a first step towards self-optimization of the manufacturing process by providing information about quality relevant events during manufacture. The deviation from the planned course of the manufacturing process to the actual course of the manufacturing process can be used to adapt the manufacturing strategy from one layer to the next. In the current state, the system can be used to facilitate the setup of the manufacturing system as it allows identification of false machine settings without having to analyze the work piece.

Thermo-Optical (TOP) Analysis of Transmissive Elements for Laser- Systems

Increasing laser beam qualities make thermal lensing again a hot topic and demand for a thermo-optical simulation for improving classical ray tracing and enabling optimization possibilities for thermally aberrated optical systems. This paper summarizes the approach for coupling FEM and ray tracing using a weighted least squares approximation algorithm and demonstrates the abilities of the coupled simulation in the case of a CO2 laser system for polishing of glass and plastics. It can be demonstrated that the algorithm can be used for the analysis of higher order aberrations, since the application contains a Gaussian to top-hat conversion lens group which suffers from thermal gradients. Finally, the benefits and further developments of analyzing thermal gradients in optical simulation are being discussed.

Methods for compensation of thermal lensing based on Thermo Optical (TOP) Analysis

The compensation of thermal lensing in laser optics for application in the high power domain is an up-to-date topic and discussed in literature multiple times. This paper combines distinct published approaches with own contributions to enhance current methodologies for the simulation, the measurement and the compensation of thermally induced optical effects. Particularly, a thermal time constant is introduced to characterize the time until steady state is reached. Moreover, a metrological setup is described for thermal lens measurement at high power. Finally, methods for thermal lens compensation and material data acquisition are discussed on the basis of an experimental example.

Approach for self-optimising assembly of optical systems

The alignment of optical components is challenging. This paper proposes self-optimisation as a means for reducing planning efforts and increasing the system’s autonomy. Hence, reduced costs and production times can be achieved. Moreover, self-optimising approaches for laser optics assembly enabling autonomous compensation of manufacturing tolerances are presented. Based on ray-tracing simulation, the assembly system carries out the alignment of optics automatically. During the alignment process the laser beam is monitored and analysed. The ray-tracing simulation is updated in a closed-loop manner for gaining information on required correction movements. This information is passed to the assembly system in order to achieve a valid optical function using tolerance-affected components. The choice of metrology as well as the interfaces between different software packages and hardware components will be discussed. The alignment procedure will be described using the example of a miniaturised optical system suitable for laser beam guiding and shaping.

Robot‐based resistance soldering of optical components

Purpose – Laser systems are becoming more and more a commodity in many fields of application and this is driving a strong trend towards increasingly efficient production technologies and miniaturized products. A central aspect of laser production is the assembly where the majority of cost is due to manual operations. Resistance soldering of optical components is an upcoming technology for automated assembly, especially for high power laser applications. This technology transfers design and packaging concepts from the electronics industry into photonics. The purpose of this paper is to present recent developments in the field of this soldering technology and to show the first experimental results in combination with robot‐based handling.Design/methodology/approach – Soldering results were examined by resistance measurements, shear testing and optical analysis of the melting zone.Findings – The experiments conducted proved the viability of robot‐based automated resistance soldering of optical components. An...

Integrative Business and Technology Cases

In order to strengthen the relevance and integrativity of research in the Cluster of Excellence, current best practice “business and technology cases” were selected. Hereby the theories, hypotheses, predictions and technology projects developed in the Cluster of Excellence are evaluated and advanced in close collaboration with leading production companies in Germany and Europe. To make the work more transparent, different application scenarios will be developed.