Bremen Institute for Applied Beam Technology

Quality assurance typically involves off-line process monitoring because the geometry of fabrication processes often does not allow for in-process measurement. Usually, the direct view of the process area is blocked by processing devices. We therefore propose a concept using lensless digital holography for off-axis full-field measurements. We use a phase space representation to elucidate the requirements for this concept and to address the challenges concerning illumination and sampling. Our method enables aberration-free sideways imaging with high observation angles, allowing for freedoms concerning the camera positioning for in-line process monitoring. Our simulations with off-axis angles up to 45 deg and experiments with angles up to 37 deg indicate that the approach presented here offers an effective solution for in-line control in spatially restricted production environments.

We present a novel approach for securing valuable documents using a complementary approach based on the encryption of computer-generated holograms (CGHs). The proposed approach utilizes the well-known iterative Fourier transform algorithm (IFTA) to generate a phase-only CGH for valuable digital and/or physical documents. The generated CGH is then secured by binary phase randomization, which is implemented using the symmetric encryption technique, exclusive OR (XOR). The reconstruction process for the calculated secured CGHs varied slightly depending on whether the documents were digital or physical. For digital documents, reconstruction was performed using a symmetric decryption key followed by an inverse Fourier transform (IFFT). On the other hand, the reconstruction of the physical document involved two additional processes: printing and scanning. To evaluate the quality of the digital reconstruction, the speckle signal-to-noise ratio (SSNR) was estimated for both printed grayscale and binary CGHs. The security analysis of the XOR-encrypted CGH was quantitatively evaluated to ensure the level of protection against various cryptographic attacks such as plaintext and brute-force attacks. The results revealed that the combination of phase CGHs and the XOR encryption/decryption provides robust cryptographic protection for valuable documents, benefiting document security and anti-counterfeiting.

Phase measuring deflectometry is a shape measurement technique for specular surfaces, based on the deflection of light rays coming from active patterns generated by a display. To obtain sufficient data for shape reconstruction, additional information must be provided for regularization. Active display registration is a regularization technique where the display is moved, and two cameras are used to register its position and shape using stereo matching. For measuring high curvature specular surfaces, the large field of view of both cameras must substantially overlap, which limits the setup geometry. In this work, we propose using only one camera for registration. We achieve this with an algorithm that utilizes a geometric model of the display. We show experimental results and compare both display registration methods.

The publication of this article unfortunately contained mistakes. The acknowledgement was not correct. The corrected acknowledgement is given below. The project on which this report is based was funded by the German Federal Ministry of Education and Research under the funding number 03HY119F. The responsibility for the content of this publication lies with the authors. The original article has been corrected.

In deep penetration laser welding, the quality of the weld seam is strongly correlated with the process dynamics. Pronounced dynamics of the melt pool and the keyhole cause defects like pores and blow-outs which negatively affect the mechanical properties of the seam. The process dynamics are primarily governed by the fluctuations of the keyhole shape. Therefore, in order to avoid the occurrence of defects in the weld seam, it is necessary to gain control over the keyhole stability by understanding the underlying physical processes that contribute to its dynamics. Besides the intensity distribution of the laser beam on the keyhole walls, the keyhole wall temperature is a crucial influencing factor when it comes to the assessment of the local metal evaporation rate and the induced local recoil pressure acting on the keyhole wall. Using small high-melting tantalum probes as measuring channels, temperatures in the vicinity of the Knudsen layer at the keyhole front wall were measured at different depths below the sample surface by means of high-speed pyrometry. For this purpose, bead on plate welding was conducted at low process velocities in the Rosenthal regime using pure aluminium (EN AW-1050A) as substrate material. It is shown for different laser powers and process velocities that the melt temperature in the vicinity of the keyhole wall lies in the range of 3000 K ± 400 K, therefore intermittently exceeding the vaporisation temperature of the substrate material. Significant correlations between the keyhole wall temperature and the laser power as well as the measuring depth could not be identified.

The morphology and size of the grains of the additively manufactured (AM) parts affect the mechanical properties and performance. The present research aimed to analyze the microstructure of AM parts, particularly the grain features, using image processing techniques. AMed aluminum components were fabricated using wire arc additive manufacturing based on cold metal transfer. The AA 5183 tensile samples fabricated using WAAM-CMT showed ductile behavior and the presence of the Lüder bands. Metallographic sections were prepared, and fine equiaxed grain was observed. An automatic image evaluation program for determining the specified grain features was developed. The grain features, including size, area, perimeter, and sphericity, were quantified, and the obtained results were verified, indicating a satisfactory agreement with those obtained from manual measurement. The output of the image processing technique clearly proved that the MATLAB program, as an affordable and swift approach (~ 4.8 s), can be used successfully to determine the microstructural properties. The image processing techniques could significantly improve the reliability and analysis of the microstructure, which would lead to the possibility of characterizing the microstructure in a broader range and having more details, and saving time and cost.

Laser deep penetration welding is characterized by the formation of a vapor capillary (keyhole) and a high overall absorption (coupling rate) of the laser radiation in comparison to heat conduction welding. Due to multiple reflexions within the highly dynamic keyhole, the absorption mechanisms are very complex. The questions, how a laser beam interacts with the existing keyhole, and, if shadowing effects occur, are a matter of concern for several researchers. In this study, the hypothesis that the beam of a laser source can be transmitted to a significant extent through an existing keyhole and acts predominantly at the keyhole bottom was investigated. Experiments were carried out on mild steel and technical pure aluminum, using a specially designed optical system, which allows to coaxially combine two laser beams and to adjust their settings like the energy and the focal plane individually. A combination of a disk laser with a focal diameter of 390 µm and a single-mode fiber laser with a focal diameter of 30 µm was used. Weld depths and seam shapes were analyzed in cross and longitudinal sections. Based on the results, it is shown that the much smaller focal diameter of the single-mode laser beam acted at the bottom of the keyhole and caused a kind of “keyhole-in-keyhole formation” in the form of a local maximum in weld depth whose expression depended on the focal plane of the single-mode laser beam.

Laser melt injection is a technology for producing metal matrix composite (MMC) layers on tools such as skin-pass rolls by injecting hard particles into a laser-induced weld pool. However, low process speeds prevent the application of laser melt injection on a large scale. To overcome this drawback, a new approach is presented: High-speed laser melt injection (HSLMI) is a promising method for generating highly wear-resistant MMC layers on tools with high productivity. For the first time, high process speeds of up to 100 m/min were reached with HSLMI of spherical fused tungsten carbide (SFTC) particles into the steel 1.2362 that is used for skin-pass rolls. In this paper, the influence of the process speed on the microstructure and on the wear resistance of the MMC layer is investigated. The microstructure of the steel matrix changes from a dendritic to a needle-shaped structure when process speeds of 60 m/min or higher are applied. Furthermore, the steel matrix often features cracks. The SFTC particles show a dissolution seam. It was found that both the crack susceptibility and the SFTC dissolution can be reduced significantly by increasing the process speed. The wear behavior of the MMC layers was studied in a pin-on-plate test. It was found that the SFTC reinforcement leads to a significant improvement in wear resistance over the nonreinforced steel substrate. The wear volume was reduced from 3.6 to 0.1 to 0.3 mm3 by an SFTC particle-reinforcement. Abrasion was the substantial wear mechanism.