In-Situ Defect Detection in Laser Powder Bed Fusion by Using Thermography and Optical Tomography—Comparison to Computed Tomography

Among additive manufacturing (AM) technologies, the laser powder bed fusion (L-PBF) is one of the most important technologies to produce metallic components. The layer-wise build-up of components and the complex process conditions increase the probability of the occurrence of defects. However, due to the iterative nature of its manufacturing process and in contrast to conventional manufacturing technologies such as casting, L-PBF offers unique opportunities for in-situ monitoring. In this study, two cameras were successfully tested simultaneously as a machine manufacturer independent process monitoring setup: a high-frequency infrared camera and a camera for long time exposure, working in the visible and infrared spectrum and equipped with a near infrared filter. An AISI 316L stainless steel specimen with integrated artificial defects has been monitored during the build. The acquired camera data was compared to data obtained by computed tomography. A promising and easy to use examination method for data analysis was developed and correlations between measured signals and defects were identified. Moreover, sources of possible data misinterpretation were specified. Lastly, attempts for automatic data analysis by data integration are presented

Review of in-situ process monitoring and in-situ metrology for metal additive manufacturing

Lack of assurance of quality with additively manufactured (AM) parts is a key technological barrier that prevents manufacturers from adopting AM technologies, especially for high-value applications where component failure cannot be tolerated. Developments in process control have allowed significant enhancement of AM techniques and marked improvements in surface roughness and material properties, along with a reduction in inter-build variation and the occurrence of embedded material discontinuities. As a result, the exploitation of AM processes continues to accelerate. Unlike established subtractive processes, where in-process monitoring is now commonplace, factory-ready AM processes have not yet incorporated monitoring technologies that allow discontinuities to be detected in process. Researchers have investigated new forms of instrumentation and adaptive approaches which, when integrated, will allow further enhancement to the assurance that can be offered when producing AM components. The state-of-the-art with respect to inspection methodologies compatible with AM processes is explored here. Their suitability for the inspection and identification of typical material discontinuities and failure modes is discussed with the intention of identifying new avenues for research and proposing approaches to integration into future generations of AM systems

A fundamental work on THz measurement techniques for application to steel manufacturing processes

The terahertz (THz) waves had not been obtained except by a huge system, such as a free electron laser, until an invention of a photo-mixing technique at Bell laboratory in 1984 [1]. The first method using the Auston switch could generate up to 1 THz [2]. After then, as a result of some efforts for extending the frequency limit, a combination of antennas for the generation and the detection reached several THz [3, 4]. This technique has developed, so far, with taking a form of filling up the so-called THz gap . At the same time, a lot of researches have been trying to increase the output power as well [5-7]. In the 1990s, a big advantage in the frequency band was brought by non-linear optical methods [8-11]. The technique led to drastically expand the frequency region and recently to realize a measurement up to 41 THz [12]. On the other hand, some efforts have yielded new generation and detection methods from other approaches, a CW-THz as well as the pulse generation [13-19]. Especially, a THz luminescence and a laser, originated in a research on the Bloch oscillator, are recently generated from a quantum cascade structure, even at an only low temperature of 60 K [20-22]. This research attracts a lot of attention, because it would be a breakthrough for the THz technique to become widespread into industrial area as well as research, in a point of low costs and easier operations. It is naturally thought that a technology of short pulse lasers has helped the THz field to be developed. As a background of an appearance of a stable Ti:sapphire laser and a high power chirped pulse amplification (CPA) laser, instead of a dye laser, a lot of concentration on the techniques of a pulse compression and amplification have been done. [23] Viewed from an application side, the THz technique has come into the limelight as a promising measurement method. A discovery of absorption peaks of a protein and a DNA in the THz region is promoting to put the technique into practice in the field of medicine and pharmaceutical science from several years ago [24-27]. It is also known that some absorption of light polar-molecules exist in the region, therefore, some ideas of gas and water content monitoring in the chemical and the food industries are proposed [28-32]. Furthermore, a lot of reports, such as measurements of carrier distribution in semiconductors, refractive index of a thin film and an object shape as radar, indicate that this technique would have a wide range of application [33-37]. I believe that it is worth challenging to apply it into the steel-making industry, due to its unique advantages. The THz wavelength of 30-300 ¼m can cope with both independence of a surface roughness of steel products and a detection with a sub-millimeter precision, for a remote surface inspection. There is also a possibility that it can measure thickness or dielectric constants of relatively high conductive materials, because of a high permeability against non-polar dielectric materials, short pulse detection and with a high signal-to-noise ratio of 103-5. Furthermore, there is a possibility that it could be applicable to a measurement at high temperature, for less influence by a thermal radiation, compared with the visible and infrared light. These ideas have motivated me to start this THz work

Nondestructive evaluation and in-situ monitoring for metal additive manufacturing

Powder-based additive manufacturing (AM) technologies are seeing increased use, particularly because they give greatly enhanced design flexibility and can be used to form components that cannot be formed using subtractive manufacturing. There are fundamental differences in the morphology of additively manufactured materials, when compared with, for example castings or forgings. In all cases it is necessary to ensure that parts meet required quality standards and that “allowable” anomalies can be detected and characterized. It is necessary to understanding the various types of manufacturing defects and their potential effects on the quality and performance of AM, and this is a topic of much study. In addition, it is necessary to investigate quality from powder throughout the manufacturing process from powder to the finished part. In doing so it is essential to have metrology tools for mechanical property evaluation and for appropriate anomaly detection, quality control, and monitoring. Knowledge of how and when the various types of defects appear will increase the potential for early detection of significant flaws in additively manufactured parts and offers the potential opportunity for in-process intervention and to hence decrease the time and cost of repair or rework. Because the AM process involves incremental deposition of material, it gives unique opportunities to investigate the material quality as it is deposited. Due to the AM processes sensitivity to different factors such as laser power and material properties, any changes in aspects of the process can potentially have an impact on the part quality. As a result, in-process monitoring of additive manufacturing (AM) is crucial to assure the quality, integrity, and safety of AM parts. To meet this need there are a variety of sensing methods and signals which can be measured. Among the available measurement modalities, acoustic-based methods have the advantage of potentially providing real-time, continuous in-service monitoring of manufacturing processes at relatively low cost. In this research, the various types of microstructural features or defects, their generation mechanisms, their effect on bulk properties and the capabilities of existing characterization methodologies for powder-based AM parts are discussed and methods for in-situ non-destructive evaluation are reviewed. A proof-of-concept demonstration for acoustic measurements used for monitoring both machine and material state is demonstrated. The analyses have been performed on temporal and spectral features extracted from the acoustic signals. These features are commonly related to defect formation, and acoustic noise that is generated and can potentially characterize the process. A novel application of signal processing tools is used for identification of temporal and spectral features in the acoustic signals. A new approach for a K-means statistical classification algorithm is used for classification of different process conditions, and quantitative evaluation of the classification performance in terms of cohesion and isolation of the clusters. The identified acoustic signatures demonstrate potential for in-situ monitoring and quality control of the additive manufacturing process and parts. A numerical model of the temperature field and the ultrasonic wave displacement field induced by an incident pulsed laser on additively manufactured stainless steel 17 4 PH is established which is based on thermoelastic theory. The numerical results indicate that the thermoelastic source and the ultrasonic wave features are strongly affected by the characteristics of the laser source and the thermal and mechanical properties of the material. The magnitude and temporal-spatial distributions of the pulsed laser source energy are very important factors which determine not only the wave generation mechanisms, but also the amplitude and characteristics of the resulting elastic wave signals

X-ray computed tomography for additive manufacturing: a review

In this review, the use of x-ray computed tomography (XCT) is examined, identifying the requirement for volumetric dimensional measurements in industrial verification of additively manufactured (AM) parts. The XCT technology and AM processes are summarised, and their historical use is documented. The use of XCT and AM as tools for medical reverse engineering is discussed, and the transition of XCT from a tool used solely for imaging to a vital metrological instrument is documented. The current states of the combined technologies are then examined in detail, separated into porosity measurements and general dimensional measurements. In the conclusions of this review, the limitation of resolution on improvement of porosity measurements and the lack of research regarding the measurement of surface texture are identified as the primary barriers to ongoing adoption of XCT in AM. The limitations of both AM and XCT regarding slow speeds and high costs, when compared to other manufacturing and measurement techniques, are also noted as general barriers to continued adoption of XCT and AM

Reliable irogane alloys and niiro patination—further study of production and application to jewelry

Japanese metalworkers use a wide range of irogane alloys (shakudo, shibuichi), which are colored with a single patination solution (niiro eki). This approach allows different alloys to be combined in one piece and patinated, producing a multi-colored piece of metalwork. At present the niiro patination process is unreliable. In this study we develop a deeper understanding of the effect of patination solution ingredients on color. We have tested a synthetic niiro solution, comparing the color results with traditional niiro solution patination. Surface products have been analyzed to determine how they are influenced by both the niiro solution and cleaning procedures during patination. A large range of shibuichi and shakudo alloys have been produced to determine the full color pallette. This work also explores the use of alternative processes for the patination of irogane alloys, examining the effect of laser marking and anodizing on irogane alloys

A Review of Welding in Space and Related Technologies

Deployment of welding and additive manufacturing (AM) technologies in the space environment has the potential to revolutionize how orbiting platforms are designed, manufactured, and assembled. These technologies offer the option for repair of sustained damage to habitat structures on space missions, as astronauts would be able to manufacture new parts (using welding-derived AM processes suitable for use in the external space environment) and weld cracks. An added benefit is that required repairs can be achieved more economically, as new parts need not be shipped from Earth. With further maturation of in-space welding capabilities, astronauts could operate under given standards and weld damaged structures rather than rely on cargo resupply. This Technical Memorandum (TM) begins by reviewing the available literature relevant to welding in space, focusing on solidification, heat and mass transfer, and fluid flows in microgravity. This survey considers research on the effects of welding in microgravity on a material system. The various in-space welding devices that have been previously designed and tested are examined to determine their capabilities and shortcomings, with a focus on the results of their individual welding experiments. Safety measures are discussed to protect the orbiting International Space Station (ISS) and crew during welding operations. Finally, the state of the art is examined by focusing on current approaches to AM and on-orbit welding that are being developed by several companies in conjunction with NASA

Generation and detection of defects in metallic parts fabricated by selective laser melting and electron beam melting and their effects on mechanical properties.

Application of Additive Manufacturing (AM) technology to fabricate complex three-dimensional components is one promising direction within the manufacturing industry. This approach is rapidly changing the way designers and engineers create objects with desired shape and structures. Selective Laser Melting (SLM) and Electron Beam Melting (EBM) are two common powder bed fusion processes within AM for fabricating metallic parts. In order to give designers and engineers more insights into employing AM, the quality and long-term behavior of SLM- and EBM-produced parts need to be carefully investigated. Thus, this research project aims to understand how processing parameters affect defect generation and distribution during SLM and EBM processes, to study the morphological features of defects, to identify effective non-destructive method(s) to detect these defects, and to characterize the effect of defects on mechanical properties of SLM- and EBM-produced parts. The study began by generating stochastic defects via adjustment to process parameters from optimal parameters to marginal parameters, in order to correlate the porosity to the marginal parameters. Archimedes method was employed to estimate porosity of SLM- and EBM-produced specimens. After this, by using destructive characterization techniques, the defective specimens were sectioned. The morphology of stochastic defects was investigated based on their contour features on the cross sections. Micro CT was primarily used to evaluate the stochastic defects in the SLM and EBM parts and demonstrate their morphological characteristics and porosity in the single slices and reconstructed models. Finally, tensile and fatigue tests were carried out on Ti-6Al-4V parts with identified porosity. The fracture mechanism was analyzed. This study established a fundamental understanding of defects in parts made by SLM and EBM processes. Porosity was quantitatively correlated to the marginal parameters of SLM and EBM processes. Defects were differentiated based on their morphological properties and features. Micro CT was confirmed to be an effective non-destructive method for evaluating stochastic defects in SLM- and EBM-produced parts. The effects of stochastic defects on Ti-6Al-4V parts were determined based on tensile and fatigue tests. It was found that both microstructure and porosity have an impact on the mechanical properties of SLM- and EBM-produced parts