PhD degrees

This work focuses on the research and development of an in-process, adaptive, software-based correction method, which is flexibly adaptable for laser remote scanners, along with a sensor concept to stabilize the target process focus position within the interpolation cycle of the scanner control. This ensures adherence to the specified laser beam diameter and the process-relevant intensity distribution at the workpiece during laser material processing, such as laser cutting, welding, ablation, and selective laser melting (3D printing). The correction method for focus position stabilization developed in this work is primarily designed for fiber-coupled laser remote scanners with lens optics for high laser power applications. It provides users with C++-based correction algorithms that can be integrated into scanner control systems. These algorithms, combined with the ABCD focus model introduced in this work, enable more precise adjustment of the process focus position compared to linear focus models and support active correction of focus shift using matrix optics, ray tracing, and PID control.

The developed focus stabilization method was tested and simulated on the 30-kW laser remote scanner system “Dragon,” developed at the Technical University of Hamburg-Harburg (TUHH) at the Institute of Laser and Plant Systems Technology (iLAS), to verify its effectiveness. Implementation involved researching passive and active analysis and correction methods for focus shift, assessing their suitability for the “Dragon” 30-kW laser remote scanner, and presenting the results in tabular form. Furthermore, the relationships of the identified problems with respect to achieving the defined objectives and derived sub-goals were investigated. The influence of the thermal lens and environmental factors on focus shift was analyzed. Thermal effects leading to the thermal lens (thermo-optic effect, stress-optic effect, end effect) were described, and a mathematical model of the thermal lens was implemented using the finite element method (FEM). Additionally, sensor integration and a mathematical model of the air refractive index were implemented to capture process-acquired environmental parameters and incorporate them into the focus stabilization correction algorithms. Finally, an ABCD matrix optics-based approach was developed to describe the complete optical system mathematically and compactly, thereby accelerating computer-based calculations.

As a result of this research and development, the ABCD focus model allows the target process focus position of the 30-kW “Dragon” laser remote scanner to be adjusted significantly more accurately than with the previously used linear focus model. The focus shift can be actively minimized using a discrete-time PID control loop, stabilizing the target process focus position within the interpolation cycle of the scanner control and ensuring that the specified laser beam diameter and the process-relevant intensity distribution at the workpiece are effectively maintained.

The target audience for this work includes laser system developers, manufacturers, and integrators, end-users in laser material processing, as well as scientific and technical stakeholders.

Laser-additively manufactured components are characterized by geometrically free forms but require post-processing due to their rough surfaces. This dissertation addresses the development of an innovative manufacturing process that combines laser additive and laser subtractive manufacturing to overcome this challenge. The results demonstrate that post-processing via laser ablation improves both the dimensional accuracy and surface quality of additively built structures. A process demonstration for the production of stamping tools highlights the technical and economic advantages compared to conventional manufacturing. In the future, the integration of both laser processes offers further automation potential.

This dissertation investigates the potential of metal binder jetting (MBJ) for the production of patient-specific medical implants. It analyzes the integration of MBJ into existing MIM process chains to enhance cost efficiency and manufacturing flexibility. A key focus is on optimizing powder conditioning and curing strategies for titanium powders, particularly Ti-6Al-4V. The work demonstrates that improved flowability and green part strength are critical for process stability. Furthermore, the recycling of titanium powder is evaluated as a key factor for the sustainable and economical use of the technology.

This work aims to provide insights into how powder bed quality can be captured and evaluated, and how these evaluations can be used to assess process capability in powder layer deposition and the resulting manufacturing quality. Within the scope of the research, a structured light projection system was developed for the 3D topographical capture of the powder bed, along with a test rig for manufacturer- and system-independent representation of common layer deposition processes in selective laser melting. Using a multi-step methodological approach for processing and analyzing the high-resolution powder bed image data, the diverse characteristic properties of the powder bed surface were identified, classified, and evaluated based on various defect patterns in layer deposition. On the basis of this powder bed analytics, a full-factorial ex-situ study of the priority process parameters in the powder layer deposition process (deposition principle, deposition unit, deposition speed, powder layer thickness) was conducted to scientifically investigate the effects of different control variables.

Based on the analysis results, parameter windows were identified that can enhance process capability and ultimately increase productivity. These identified parameter windows were subsequently investigated in a full-factorial manner through in-situ experiments, and the results were validated by downstream inspection of the produced test specimens. The holistic approach of the developed powder bed monitoring measurement technology and the automated evaluation of measurement data were finally assessed for their transferability to industrial-scale applications as part of a potential analysis.

This research demonstrated to academic and industrial audiences that the process capability of powder layer deposition depends to varying degrees on the priority control variables in the deposition process and follows certain trends that can be advantageously utilized in process designs which utilize laser melting equipment. Within these investigations, multiple parameter combinations for the aluminum alloy AlSi10Mg were validated for increased productivity and a stable process. Additionally, as a result of the potential analysis, the suitability of structured light projection as a measurement technique for powder bed capture and in-process monitoring at industrial scale across all current PBF-LB/M system sizes was demonstrated.

(Originally in English)

In this doctoral thesis, the influence of increased particle size distributions (PSD) of up to 20-125 μm in laser-based powder bed fusion (LPBF) for aluminium and titanium alloys is investigated. Larger PSDs are generated and analysed regarding their powder properties, LPBF behaviour, required process parameter adjustments, and resulting mechanical properties. By adapting process parameters, the processability of Ti-6Al-4V is ensured, achieving mechanical properties comparable to industrial standards. Atomization yield increases by 44 %, while material costs are reduced by 20 %.

(Originally in English)

This book provides an in-depth exploration of process planning steps required for Directed Energy Deposition (DED) processes and their automation during the data preparation stage. DED is a resource-efficient, tool-less manufacturing method with high deposition rates and larger build volumes. Their flexibility, enabled by 6-axis industrial robots, allows fabrication of complex geometries, but poses challenges in data preparation and build strategy definition due to software limitations and lack of standards. This thesis thereby deals with the development of a STEP file format-based slicing framework with toolpath generation algorithms specific to build geometries clustered under six categories. Thin-wall components cluster, an important part category in the aerospace sector, is being further examined because of its industrial relevance.

Laser additive manufacturing is subject to few geometric constraints, enabling the realization of highly complex designs that cannot be produced using conventional manufacturing techniques. One application of this process is in the production of injection molding tools where laser-additively manufactured tool geometries and design approaches are investigated to analyze their properties and influence on the injection molding process, with the aim of identifying potential innovations in tool design.

To this end, three different cooling strategies are compared and analyzed. First, conventional tool cooling based on subtractive manufacturing is considered, followed by the use of innovative conformal cooling channel systems. Additionally, a cooling strategy employing a water-permeated cooling cavity was developed and implemented, which most closely meets the theoretical physical requirements for uniform and effective temperature control. Based on the investigation results, approaches and potential design principles for innovative cooling systems are derived at the conclusion of the work.

This work presents an application-oriented research study on the development of an acoustic monitoring system for laser powder deposition welding (LPA). The starting point is the problem of critical defects (cracks, delamination) in material-heterogeneous deposition welding and the question of which sensor technology can reliably be used for early defect detection and process control. Through a systematic evaluation of 34 sensor configurations, an acoustic air-emission-based monitoring system was identified as a robust, flexible, and cost-efficient measurement method with high development potential.

The developed monitoring system is divided into two subsystems: Subsystem I analyzes time- and frequency-resolved acoustic process emissions using a directional microphone. Transient events—characteristic at around 12 kHz—are described via short-time Fourier analysis; targeted filters highlight relevant signals. The measurable quality parameter is the degree of delamination, for which a significant correlation with the time until the occurrence of strong transient events was demonstrated, allowing critical defects to be identified. Subsystem II enables spatial localization of these acoustic events within the build volume. The time-of-flight method, combined with a circular arrangement of six sensors and the difference-square method, provides the best localization performance.

Both subsystems were validated under real process conditions, iteratively optimized, and integrated into a process chamber for an LPA system. This book is aimed at researchers and practitioners in additive manufacturing: it describes concept development, signal processing, validation, and industrial relevance of an in-process monitoring system that enables early defect detection, targeted process control, and thereby significant savings in time and costs.