A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Despite continuous technological enhancements of metal Additive Manufacturing (AM) systems, the lack of process repeatability and stability still represents a barrier for the industrial breakthrough. The most relevant metal AM applications currently involve industrial sectors (e.g., aerospace and bio-medical) where defects avoidance is fundamental. Because of this, there is the need to develop novel in-situ monitoring tools able to keep under control the stability of the process on a layer-by-layer basis, and to detect the onset of defects as soon as possible. On the one hand, AM systems must be equipped with in-situ sensing devices able to measure relevant quantities during the process, a.k.a. process signatures. On the other hand, in-process data analytics and statistical monitoring techniques are required to detect and localize the defects in an automated way. This paper reviews the literature and the commercial tools for insitu monitoring of Powder Bed Fusion (PBF) processes. It explores the different categories of defects and their main causes, the most relevant process signatures and the in-situ sensing approaches proposed so far. Particular attention is devoted to the development of automated defect detection rules and the study of process control strategies, which represent two critical fields for the development of future smart PBF systems.

https://iopscience.iop.org/article/10.1088/1361-6501/aa5c4f/pdf

Process defects and in situ monitoring methods in metal powder bed fusion: a review

Significant progress has been made both in experimentation and in theoretical modeling of scanning probe microscopy. The theoretical models used to analyze and interpret experimental scanning probe microscope (SPM) images and spectroscopic data now provide information not only about the surface, but also the probe tip and physical changes occurring during the scanning process. The aim of this review is to discuss and compare the present status of computational modeling of two of the most popular SPM methods---scanning tunneling microscopy and scanning force microscopy---in conjunction with their applications to studies of surface structure and properties with atomic resolution. In the context of these atomic-scale applications, for the scanning force microscope (SFM), this review focuses primarily on recent noncontact SFM (NC-SFM) results. After a brief introduction to the experimental techniques and the main factors determining image formation, the authors consider the theoretical models developed for the scanning tunneling microscope (STM) and the SFM. Both techniques are treated from the same general perspective of a sharp tip interacting with the surface---the only difference being that the control parameter in the STM is the tunneling current and in the SFM it is the force. The existing methods for calculating STM and SFM images are described and illustrated using numerous examples, primarily from the authors' own simulations, but also from the literature. Theoretical and practical aspects of the techniques applied in STM and SFM modeling are compared. Finally, the authors discuss modeling as it relates to SPM applications in studying surface properties, such as adsorption, point defects, spin manipulation, and phonon excitation.

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Theories of scanning probe microscopes at the atomic scale

WHILE the introduction of statistical methods into the analysis of aeronautical experimental data, whether for quality control in production, for the interpretation of the results of structural and aerodynamic laboratory experiments, or for airline operation, has been brought about only in recent years, it may by now be fair to assert that their advantages and even their indispensability are no longer in dispute. Hitherto, investigations on these lines have usually involved, explicitly or implicitly, only the ‘normal curve of error’ and allied considerations; owing, it may be thought, to the controllability of the various manufacturing or laboratory techniques, but also perhaps to the scarcity of data hitherto available. It may well be, however, that with the accumulation of information arising out of investigations planned with particular reference to the statistical analysis of their results the whole range of the apparatus for statistical analysis, usually confined to such fields as those of biology or economics, will be called into full play.

The Statistical Analysis, of Experimental Data: A Fresh Approach Employing a Graphical Method for Investigating Non‐Normal Frequency Distributions

Collisions of atoms and ions with surfaces under grazing incidence

We have discovered an intense, sharp feature in tunneling spectra of the Fe(001) and Cr(001) surfaces that derives from a bcc surface state near the Fermi level. Band-structure calculations show that this state is a general feature of bcc (001) surfaces, and that it originates from a nearly unperturbed $d$ orbital extending out into the vacuum region. This general feature should permit chemical identification with atomic spatial resolution on bcc (001) surfaces. The surface state is in the minority spin band for Fe and Cr, so it should be useful for future spin-polarized tunneling experiments.

Tunneling spectroscopy of bcc (001) surface states.

While much progress has been made using epitaxial growth of Fe/Cr/Fe structures to study magnetic exchange coupling, a number of anomalies have arisen in studies of this model system. Using scanning tunneling microscopy and spectroscopy to investigate Cr growth on Fe(001), we have identified a potential structural cause of these anomalies. We show that Cr growth under layer-by-layer conditions on Fe(001) leads to the formation of a Cr-Fe alloy. We exploit a Cr and Fe surface state to identify single Cr impurities in Fe and evaluate the alloy concentrations with increasing Cr coverage. PACS numbers: 61.16.Ch, 68.55.-a, 75.70.Cn

/pdf/atomic-scale-observations-of-alloying-at-the-cr-fe-001-260oq8ipt3.pdf

Atomic-scale observations of alloying at the Cr-Fe(001) interface.

Being able to characterize the process signatures of powder bed based additive manufacturing process is key to improving the product quality. This paper demonstrates the implementation of a digital fringe projection technique to measure surface topography of the powder bed layers during the fabrication. We focus on developing the metrology tool and observing the types of information that can be extracted from such topographical data. The performance of the system is demonstrated with selected in situ measurements. Experimental results show this system is capable of measuring powder bed signatures including the powder layer flatness, surface texture, the average height drop of the fused regions, characteristic length scales on the surface, and splatter drop location and dimension.

Angela Davies

Papers

Tunneling spectroscopy of bcc (001) surface states.

Atomic-scale observations of alloying at the Cr-Fe(001) interface.

In situ surface topography of laser powder bed fusion using fringe projection

In-Situ Metrology System for Laser Powder Bed Fusion Additive Process

Displacement Uncertainty in Interferometric Radius Measurements