Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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In situ microstructure analysis of Inconel 625 during laser powder bed fusion

2022 , Schmeiser, F. , Krohmer, E. , Wagner, C. , Schell, N. , Uhlmann, E. , Reimers, W.

Laser powder bed fusion is an additive manufacturing process that employs highly focused laser radiation for selective melting of a metal powder bed. This process entails a complex heat flow and thermal management that results in characteristic, often highly textured microstructures, which lead to mechanical anisotropy. In this study, high-energy X-ray diffraction experiments were carried out to illuminate the formation and evolution of microstructural features during LPBF. The nickel-base alloy Inconel 625 was used for in situ experiments using a custom LPBF system designed for these investigations. The diffraction patterns yielded results regarding texture, lattice defects, recrystallization, and chemical segregation. A combination of high laser power and scanning speed results in a strong preferred crystallographic orientation, while low laser power and scanning speed showed no clear texture. The observation of a constant gauge volume revealed solid-state texture changes without remelting. They were related to in situ recrystallization processes caused by the repeated laser scanning. After recrystallization, the formation and growth of segregations were deduced from an increasing diffraction peak asymmetry and confirmed by ex situ scanning transmission electron microscopy.

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Residual stress assessment during cutting tool lifetime of CVD-diamond coated indexable inserts

2022 , Uhlmann, E. , Hinzmann, Daniel

Insufficient coating adhesion limits reproducibility regarding tool lifetime as well as workpiece quality during the application of CVD-diamond coated cutting tools. Depending on the combination of tungsten carbide substrate material, coating thickness as well as coating morphology, individual residual stress conditions exist within CVD-diamond coated cutting tool specifications. The application of these tools is accompanied by coating delamination as primary cutting tool failure. The tool lifetime of the respective cutting tool composition depends on the corresponding residual stress condition until crack development within the CVD-diamond coating initiates tool failure. During external cylindrical turning of hypereutectic aluminium silicon alloy AlSi17Cu4Mg-T6 the residual stress condition of a CVD-diamond coated cutting tool is assessed along the cutting edge, the rake face as well as flank face throughout the respective tool lifetime. Consequently, the progression of the residual stress condition until cutting tool failure regarding coating delamination is observed. During the tool lifetime of the investigated CVD-diamond cutting tools, compressive residual stress ∆σR,c shifts to tensile residual stress ∆σR,t underneath the cutting edge corner. The approximated residual stress difference of ∆σR ≈ 5 GPa indicates stress peak relaxation processes, such as crack initiation, within the CVD-diamond coating.

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Numerical investigation on the process behavior of a closed-loop internal cooling system for turning operations

2021 , Uhlmann, E. , Meier, P.

The development of sustainable manufacturing processes and the associated reduction of operating materials such as cooling lubricants represent key focuses on recent production technology. Thus, a novel tool system with a closed-loop internal cooling system for turning operations is presented in this study. The system design is based on numerical investigations of the heat flow Q⋅ with consideration of the thermomechanical load of the cutting tool and flow characteristics of the cooling fluid. The process behavior of the proposed system shows a significant dependence of the flow rate V⋅ and tool thickness tT in terms of tool temperature reduction DTHT.

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Fully coupled wet cylindrical turning simulation using the Finite-Pointset-Method

2021 , Uhlmann, E. , Barth, E. , Quellhorst, A. , Seifarth, T. , Kuhnert, J. , Nabbout, K. , Sommerfeld, M.

In industrial machining operations, many turning processes are carried out under wet conditions. In contrast, most simulation tools are only suitable for dry cutting processes due to challenges of modeling fluid-structure-interactions (FSI). In this paper, a wet, fully coupled cutting simulation using the Finite-Pointset-Method (FPM) is presented and validated for industrially relevant cylindrical turning. To facilitate the required reduction of the calculation effort, different routines like adaptive numerical discretization are applied. The results indicate different cutting fluid effects like evaporation areas around the chip.

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Acoustic emission-based process monitoring in the milling of carbon fibre-reinforced plastics

2022 , Uhlmann, E. , Holznagel, Tobias

Milling of fibre-reinforced plastics is a challenging task. The highly abrasive fibres lead to high tool wear and coating failures, which cause increasing process forces and temperatures. Machining with a worn tool, in turn, can result in unwanted workpiece damages such as delamination or fibre protrusion. Reliable monitoring of the process must therefore be able to detect damages to the milling tool and the workpiece alike. The presented process monitoring approach measures the acoustic emission generated by the milling tool cutting edge entering the workpiece with a sensor attached to the tool holder. Specific acoustic emission frequency spectra and waveforms are emitted in the cutting zone for different tool wear states. Coating failures as well as other acoustic emission events due to workpiece damages can be robustly detected and distinguished by feature extraction and signal processing as well. The developed setup, the monitoring parameterisation techniques and signal processing algorithms as well as experimental and monitoring results are presented and discussed in this paper.

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Influence of superimposed low frequency oscillations on single-pass honing of long-chipping steel

2022 , Uhlmann, E. , Rozek, André

Single-pass honing is used as a finishing process to meet high demands regarding form and dimensional accuracy of drilled bores. The disadvantages of single-pass honing compared to the conventional long-stroke honing are high process forces and torques as well as an increased risk of chip space clogging of the abrasive stones. A significant reduction in process forces and torques can be achieved by superimposing the axial movement with oscillations. In this work the kinematic basics of different oscillation parameters and their effects on single-pass honing of long-chipping steel are analyzed. It can be concluded that by superimposing low frequency oscillations in single-pass honing, the process forces and torques as well as the specific energy consumption can be reduced significantly without a decline in surface quality and form accuracy.

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Measurement and Modeling of Contact Forces during Robot-guided Drag Finishing

2021 , Uhlmann, E. , Kopp, M.

Robot-guided drag finishing is a free abrasive grinding operation that is used for polishing and deburring of workpieces with complex shaped geometries. The workpiece is attached to a robot and moved through a bulk of abrasive particles. The motion of abrasive particles during contact with the workpiece is the basis of the material removal mechanisms. To investigate the motion of abrasive particles during contact, a force measurement system was used to determine contact forces. The experimental setup was replicated in a numerical simulation based on the discrete element method (DEM). Based on the comparison of experimental and simulative results the qualitative validity of the DEM model was concluded. With the presented DEM model, the characteristic particle behavior during contact with the workpiece can be modeled which allows the prediction of resulting processing marks. Consequently, the DEM model can be used to design free abrasive grinding operations without using the time and cost intensive trial and error approach.

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Numerical investigation into cleanability of support structures produced by powder bed fusion technology

2022 , Campana, G. , Uhlmann, E. , Mele, M. , Raffaelli, L. , Bergmann, A. , Kochan, J. , Polte, J.

Purpose: Support structures used in laser powder bed fusion are often difficult to clean from unsintered powder at the end of the process. This issue can be significantly reduced through a proper design of these auxiliary structures. This paper aims to investigate preliminary the airflow within differently oriented support structures and to provide design guidelines to enhance their cleanability, especially the depowdering of them. Design/methodology/approach: This study investigates the cleanability of support structures in powder bed fusion technology. Digital models of cleaning operations were designed through computer-aided engineering systems. Simulations of the airflow running into the powder entrapped within the thin walls of auxiliary supports were implemented by computational fluid dynamics. This approach was applied to a set of randomly generated geometrical configurations to determine the air turbulence intensity depending on their design. Findings: The resul ts, which are based on the assumption that a relationship exists between turbulence and powder removal effectiveness, demonstrated that the maximum cleanability is obtainable through specific relative rotations between consecutive support structures. Furthermore, it was possible to highlight the considerable influence of the auxiliary structures next to the fluid inlet. These relevant findings establish optimal design rules for the cleanability of parts manufactured by powder bed fusion processes. Originality/value: This study presents a preliminary investigation into the cleanability of support structures in laser powder bed fusion, which has not been addressed by previous literature. The results allow for a better understanding of the fluid dynamics during cleaning operations. New guidelines to enhance the cleanability of support structures are provided based on the results of simulations.

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Optimizing the sharpening process of hybrid-bonded diamond grinding wheels by means of a process model

2022 , Uhlmann, E. , Muthulingam, A.

The grinding wheel topography influences the cutting performance and thus the economic efficiency of a grinding process. In contrary to conventional grinding wheels, super abrasive grinding wheels should undergo an additional sharpening process after the initial profiling process to obtain a suitable microstructure of the grinding wheel. Due to the lack of scientific knowledge, the sharpening process is mostly performed manually in industrial practice. A CNC-controlled sharpening process can not only improve the reproducibility of grinding processes but also decrease the secondary processing time and thereby increase the economic efficiency significantly. To optimize the sharpening process, experimental investigations were carried out to identify the significant sharpening parameters influencing the grinding wheel topography. The sharpening block width lSb, the grain size of the sharpening block dkSb and the area-related material removal in sharpening VâSb were identi fied as the most significant parameters. Additional experiments were performed to further quantify the influence of the significant sharpening parameters. Based on that, a process model was developed to predict the required sharpening parameters for certain target topographies. By using the process model, constant work results and improved process reliability can be obtained.

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Effects on the distortion of Inconel 718 components along a hybrid laser-based additive manufacturing process chain using laser powder bed fusion and laser metal deposition

2021 , Uhlmann, E. , Düchting, J. , Petrat, T. , Krohmer, E. , Graf, B. , Rethmeier, M.

The combination of laser powder bed fusion (LPBF), known for its geometrical freedom and accuracy, and the nozzle-based laser metal deposition process (LMD), known for its high build-up rates, has great potential to reduce the additive manufacturing times for large metallic parts. For the industrial application of the LPBF-LMD hybrid process chain, it is necessary to investigate the influence of the LMD process on the LPBF substrate. In addition, the build plate material also has a significant impact on the occurrence of distortion along the additive manufacturing process chain. In the literature, steel build plates are often used in laser-based additive manufacturing processes of Inconel 718, since a good metallurgical bonding can be assured whilst reducing costs in the production and restoration of the build plates. This paper examines the distortion caused by LMD material deposition and the influence of the build plate material along the hybrid additive manufacturing process chain. Twin cantilevers are manufactured by LPBF and an additional layer is subsequently deposited with LMD. The distortion is measured in the as-built condition as well as after heat treatment. The effect of different LMD hatch strategies on the distortion is determined. The experiments are conducted using the nickel-base alloy Inconel 718. The results show a significant influence of LMD path strategies on distortion, with shorter tool paths leading to less distortion. The remaining distortion after heat treatment is considerably dependent on the material of the build plate.

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