Search

Magnetic bistability in single-molecule magnets (SMMs) is a potential basis for new types of nanoscale information storage material. The standard model for thermally activated relaxation of the magnetization in SMMs is based on the occurrence of a single Orbach process. Here, we show that incorporating a phosphorus atom into the framework of the dysprosium metallocene [(CpiPr5)Dy(CpPEt4)]+[B(C₆F₅)₄]− (CpiPr5 is penta-isopropylcyclopentadienyl, CpPEt4 is tetraethylphospholyl) leads to the occurrence of two distinct high-temperature Orbach processes, with energy barriers of 1410(10) and 747(7) cm–1, respectively. These barriers provide experimental evidence for two different spin–phonon coupling regimes, which we explain with the aid of ab initio calculations. The strong and highly axial crystal field in this SMM also allows magnetic hysteresis to be observed up to 70 K, using a scan rate of 25 Oe s–1. In characterizing this SMM, we show that a conventional Debye model and consideration of rotational contributions to the spin–phonon interaction are insufficient to explain the observed phenomena.

The present work focuses on the influence of the composition of the protective gas (argon or nitrogen) used in laser‐based powder bed fusion (L‐PBF) on the nitrogen content, microstructure and hardness of AISI 316L austenitic stainless steel. L‐PBF of AISI 316L powder using Ar gas resulted in loss of nitrogen in the final part. On the other hand, L‐PBF using N2 gas resulted inan increase in nitrogen content in the final part, showing that nitrogen is absorbed during L‐PBF manufacturing in N2 gas. The nitrogen absorption implies that the build part is actually AISI 316LN rather than AISI 316L.The microstructures of 316L specimens manufactured in both atmospheres exhibited highly elongated γ‐austenite grains, with acellular structure. The hardness of the part manufactured in N2 gas was systematically higher than the part manufactured in Ar. For the part manufactured in Ar, a clear gradual decrease in hardness was observed with increasing distance from the build plate, whilefor the N2 manufactured part this hardness decrease is first observed at some distance from the build plate.For the part manufactured in Ar, a larger variation in the measured nitrogen content was observed. Moreover, a systematically lower and position dependent micro‐hardness and inhomogeneous etching response of this specimen indicate an inhomogeneous microstructure in the build.The results demonstrate that the nitrogen content of L‐PBF manufactured AISI 316L depends on the composition of the gas atmosphere used in the chamber. It is discussed qualitatively how desorption and absorption of nitrogen from the applied atmosphere play a role on the resulting composition and microstructure of the build part.

Different fabrication methods can be applied for micro fabrication, all characterized by their specific processing time and cost. However, by using the additive manufacturing (AM) technologies for direct micro manufacturing of complex micro geometries the average production time and cost can be drastically reduced. This study investigates an AM-based fabrication method for the production of microstructure surfaces with different feature sizes (50-150 μm). The experiments presented in this work have been carried out in a specifically designed vat photopolymerization AM machine suitable for precision printing developed, built and validated at the Technical University of Denmark (DTU). The AM method was evaluated in terms the capability of the system to produce micro pillars and holes array in different sizes and with increasing geometrical complexity.

Micro polishing is an important process when fine surface quality and low form error are required in the micro scale domain. This process has several differences from conventional polishing and some of the factors can not be neglected any longer. In this work, a study on the thermal behaviour of a micro hexapod-based machine is conducted. The main causes of thermal deformations are addressed and analysed. The contributions are then evaluated and compensation path strategies are discussed. The main criticalities when dealing with micro polishing are due to the small tools used. Micro tools require higher speeds than conventional polishing. High spindle speeds generate heat and therefore thermal deformations. This adds up to the movement of the Hexapod while performing polishing. The observed total deformation in the polishing tool axial direction reached 8.2 μm. Such a large deformation can introduce a potential error in the final material removal up to 50% if not adequately compensated.

Highly miniaturized systems find applications in key technological fields such as health-care, mobility, communications and optics. Required innovations for precision manufacturing of micro components can be achieved through post-process and in-process measurement of process input and output parameters. Hence, it is of critical importance to reduce the measurement and optimization effort, since process and product quality control can take a significant part of the production time in micro manufacturing. To solve this challenge, research is undertaken in order to define, investigate, implement and validate the “Product/Process Micro Manufacturing Fingerprint” concept. In particular, in the Horizon2020 Innovative Training Network “Process Fingerprint for Zero-defect Net-shape MICROMANufacturing”, 9 beneficiaries and 14 industrial partners are collaborating to establish this concept for several manufacturing technologies, such as micro injection and micro ultrasonic moulding, micro mechanical and micro plasma polishing, micro electrical discharge machining, micro electrochecmical machining, micro grinding, micro laser machining, micro extrusion, micro metrology, micro sintering. The project has reached its goals of developing novel methods in micro scale manufacturing process monitoring and control, as well as micro and sub-micro product quality assurance. The overall result has been the establishment of effective micro product and process fingerprints for the considered manufacturing technologies.

Most commercially available laser powder bed fusion (L-PBF) systems are limited to process one material at a time. The ability to spatially apply multiple materials within the same component will strongly expand the available design space for engineers. A typical problem with multi-material components is stress concentration at discrete material interfaces. Functionally graded interfaces could be used to overcome this limitation. In this work, an open-architecture L-PBF system from Aurora Labs was used to mix and process stainless steel 316L and maraging steel MS1 powder. Thereby, continuous and discrete interfaces between both materials were generated and characterized regarding microstructure, micro-hardness, and elemental composition. An L-PBF process window was found to gradually change the composition of 316L to MS1 creating a continuous interface. The controlled mixing of the powders in each layer indicates the versatility of the powder dispensation setup for multi-material combinations. This contribution will further pave the way towards the development of functionally graded L-PBF components.

With the increase in use of L-PBF for functional components, an increase in productivity demands higher laser power and continuous operation. However, both these factors can affect the optics involved. The principal components of a Laser Powder Bed Fusion system are the galvanometer mirrors and the focusing lens. In the present work a method for identification and quantification of thermal lensing is proposed. Laser powder bed fusion experiments of stainless steel 316L powder was carried out at varying scan speeds and scan strategies in order to study the effects, of long exposure times, on the quality of single layer scans. It was observed that the heating of silver-coated mirrors under continuous exposure led to a significant decrease in the beam quality as evidenced by weld track widening and loss in depth of melt pool penetration. Power measurements conducted at various locations along the beam path indicated that the mirrors were absorbing up to 42 % of the total power coming from the laser while also defocusing the beam. Thus demonstrating the importance of including identification and quantification of thermal influence and possible instability of the optical elements as cause for irregular weld tracks.

This paper presents the optimization of a state-of-the-art self-peeling vat for vat photopolymerization (VP) processes. One of the research goals of the Additive Manufacturing (AM) group from the Technical University of Denmark is to achieve the highest resolution, maximum precision and process optimization for the AM machines and the fabricated components. To achieve this goal, it is essential to have control over every single stage of the manufacturing process. One concern mayor concern in VP is the vat performance, which has to withstand with adhesion forces and wear. The previous setup designed by this research group already outperformed industrial vats by 814% [1], by lowering the normal forces induced by the adhesion between the vat and the part with the aid of a flexible membrane system. Although an improvement on the release-capability was achieved, failure was found in several materials with higher material bonding, which ultimately deformed the surface of the parts. In this work, optimization is achieved by increasing the surface area of the vat, diminishing the stress on the membrane. The building area is moved from the center which creates different peeling angles on each side of the part. The angle asymmetry combines easy peeling with low forces, together with an induced passive tilting mechanism that leads to higher quality of the manufactured samples when microfeatures are present.

Portable machine tools are developed to address specific applications needs, such as machining of large components. However, they lack in performance such as stiffness. For such a low-stiffness portable machine tool, in this work, it is analysed the possibility and feasibility of employing the motors absorbed current measurements during actual machining operations as a means to monitor the cutting process and eventually compensate the tool deformations due to the cutting forces. In-process tool-path compensation achieved through a sensorized machine is of extreme interest from the costs point of view, especially for portable or dedicated task machine tool. In this paper, measurements of cutting forces, feed axes and spindle motors absorbed currents were carried out simultaneously for different cutting conditions. It is found that good correlations can be established between specific feature of the cutting force and axis motors absorbed current, the result demonstrates that the absorbed current measurements are reliable indirect measurements of the cutting forces and can therefore be employed in an in-process tool-path compensation.

Continuous Liquid Interface Production (CLIP) is an additive manufacturing (AM) method developed as an evolution of the Digital Light Processing (DLP) principle. Utilizing an oxygen-permeable window below the ultraviolet image projection plane, the step-wise motion of the DLP printer can be avoided by maintaining a persistent liquid interface, thus allowing for a continuous growth of the printed specimen. To investigate the capabilities of this new process, a state-of-the-art CLIP AM machine was utilized to manufacture a series of reference specimens developed following the design of existing metrological transfer standards. The reference specimens were measured by a tactile coordinate measuring machine (CMM) to assess the manufacturing capability of the CLIP printer. Measurands included dimensions of 1D, 2D and 3D geometrical features as well as form errors (flatness, roundness, sphericity). Specifically, a flat hole-plate containing a grid of 25 evenly spaced holes (nominal diameter 5.5 mm, nominal pitch 25 mm) was used to assess the in-plane printing accuracy and precision of the holes’ centres. Additionally, a ball-plate specimen containing 4 spheres (nominal diameter 22 mm) was produced measured to characterise the sphericity, as well as the printing accuracy and precision of the spheres’ centres. The metrological characterization of the two reference specimens by CMM metrology allowed to determine the production capability of the used CLIP machine, and to highlight advantages and limitations of the CLIP method. The traceability of the CMM measurements was established by performing an uncertainty assessment using the calibrated artefacts corresponding to the two additively manufactured CLIP specimens. Eventually, the data collected will enable adjustments for improving the manufacturing accuracy of the CLIP machine and for paving the way towards the quality assurance of production based on the CLIP technique.

High aspect ratio manufacturing in the micro dimensional range by the micro milling process represents some difficulties for diverse applications such as micro moulds, micro channels, micro gears, miniaturized surgical instruments, etc. This study investigates the thin wall machining with different machining approaches and applies 3D finite element modelling (FEM) for the minimum machinable wall thickness to improve the experimental machining and enhance the quality of the machined features. The high stress distribution during machining the thin features are presented and the smallest machined wall is investigated. Finally, the FEM prediction resultsare compared against the experimental tests.

Additive Manufacturing (AM) is a notable procedure for direct production of miniaturized polymer components, mainly due to the ability of the process to produce complex geometries with time and cost effective iteration cycles in the development of new plastic products. This study evaluate the capability of the vat photopolymerization in terms of geometry, when printing features of different sizes and shape. Continuous Liquid Interface Production (CLIP) technology was used for printing the features which is a new method to produce monolithic polymer parts at high speed. For this purpose, two test artifact specimens designed with micro features of different geometries and printed in different batches and evaluated the variability of the results in a single and in various prints.