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Effect of pulsed and continuous wave emission on the densification behaviour in Selective Laser Melting
No full textThe majority of commercially available Selective Laser
Melting (SLM) systems operates with high brilliance fiber laser
sources. These sources are most commonly operated in continuous
wave (CW). On the other hand, a few employ pulsed wave (PW)
emission by fast power switching, resulting in pulses with s level
durations, kHz level repetition rates and low peak powers. No clear
consensus is present in the academic and industrial communities over
the choice of the emission regime. Clearly, the laser temporal
emission mode can have an impact over key quality aspects, namely
part density, geometrical errors, and roughness. The purpose of this
paper is to investigate the effect of laser emission regime on the
densification of AISI 316L stainless steel in the SLM process. In
particular, single track formation was investigated by varying
temporal overlapping of laser pulses from pulsed wave until
continuous wave. A single mode fiber laser installed on a prototype
SLM system constituted the experimental setup. The open hardware
allowed for varying with high flexibility the laser emission. The CW
and PW emission strategies were compared at fixed fluence levels per
single track melting. The effect of duty cycle was evaluated starting
from CW (i.e. 100% duty) moving towards PW. Furthermore, the
densification behaviour was analysed at single and multiple layers,
depicting the molten track stability in terms of volume. Results show
that at fixed fluence, analogous operating conditions can be
determined in the CW and PW processing of single tracks. The
amount of deposited material is significantly higher when single
tracks are produced with CW emission, although a lower variability
may be identified when exploiting power modulation. Therefore,
industrial systems could be more flexible enabling both CW and PW
regime to exploit their peculiar advantages, where required
Influence of pulsed and continuous wave emission on melting efficiency in selective laser melting
Full text linkA wide proportion of the industrial Selective Laser Melting (SLM) systems are operated with high brilliance fiber laser sources. These sources are most commonly operated in continuous wave (CW). A smaller fraction of these systems employs pulsed wave (PW) emission by power modulation, resulting in pulses with μs level durations, kHz level repetition rates and comparable peak powers to CW emission. Clearly, the laser temporal emission mode can have an impact over temperature fields, which in return control the melt pool size, stability and densification behaviour. The aim of this paper is to investigate the effect of laser emission regime on the melting efficiency in SLM. In particular, an analytical model was developed to investigate the process efficiency in single track formation at fixed energy input when employing different emission modes. A single mode fiber laser installed on an in-house developed prototype powder bed fusion system was used as the experimental setup with AISI 316 L metallic powders. The effect of duty cycle was evaluated starting from CW (i.e. 100% duty) moving towards PW, at fixed energy density levels. Results show that at constant energetic input, CW increases process melting efficiency up to 3 times whilst the deposition stability is reduced with lower duty in PW regime. Although less efficient, at stable conditions the use of modulated emission produces narrower tracks providing higher process resolution
Investigating the effect of laser power modulation in transient states during fusion cutting of 10 mm thick AISI304
Full text linkIn fusion laser cutting of intricate geometries, the processing speed often varies significantly from its nominal value due to inherent machine dynamics. These variations can lead to excessive energetic input and drifts from the reference cut quality. To address defect formation during transient conditions, a conventional approach involves power reduction to compensate for speed variations. This help prevent thermal imbalances and material overheating issues, ultimately preserving the final cut’s quality. An alternative solution is based on pulsed wave emission, providing dynamic and precise control over the thermal energy delivered to the workpiece. In this work, pulsed wave emission was employed to ensure process stability during transient conditions for fusion cutting of 10mm AISI304. A 6kW fiber laser was utilized alongside a coaxial camera-based monitoring setup to observe the melt dynamics. Results demonstrate that power modulation effectively reduces defect formation, and process emission images can be exploited to identify successful cuts
Comparative study between CW and PW emissions in selective laser melting
Full text linkThe definition of process parameters depending on the geometry of the workpiece is one of the main challenges for selective laser melting (SLM). The possibility to use different emission modes is an essential feature of the contemporary fiber lasers, which still requires further attention for controlling the melt pool size. Most of the commercially available SLM systems operate with continuous wave (CW) emission lasers. As a consequence, a substantial effort has been directed toward obtaining full densification by considering the variation of process parameters in CW modality. Pulsed wave (PW) emission achieved by power modulation of a fiber laser is preferred by a smaller fraction of the industrial SLM systems. The differences between the two emission regimes, advantages and disadvantages in their use have not been fully understood. Accordingly, this work proposes a comparative study between the two emission regimes in SLM, namely PW and CW. For this purpose, a single-mode fiber laser is coupled to a prototype SLM system composed of an automated powder bed and a scanner head. The laser source is extensively characterized for pulsed wave emission characteristics in a power modulated regime. Conditions providing the same energy content over the single track are determined and their effect on single-track densification is studied. High-speed imaging is used to observe the differences in the melt pool formation in situ. The overall results confirm that CW emission provides a larger and more stable molten pool during the process, resulting in higher deposition rates. On the other hand, under stable conditions, PW emission provides relatively narrow tracks, which might be problematic for porosity formation and at the same time useful for the production of fine geometries
Nonintrusive estimation of subsurface geometrical attributes of the melt pool through the sensing of surface oscillations in laser powder bed fusion
Full text linkMolten pool geometry, whose surface parameters may be extrapolated through direct process observations, has been identified as a fundamental indicator of stability in laser powder bed fusion (LPBF). However, a parameter that cannot be directly measured on industrial systems by means of conventional sensing equipment is the molten pool depth. Indeed, methods based on x-ray imaging demonstrated in the literature have helped to better understand the process. However, retrofitting such solutions to industrial systems does not appear as a viable route currently. Within the present investigation, a nonintrusive sensing method for the indirect measurement of subsurface molten pool geometry based on the detection of surface oscillations is presented. The analysis of frames acquired using a high-speed camera and a secondary illumination light allows the identification of the crests of capillary waves through bright reflections on the surface of the molten pool. The characteristic oscillation frequency of the surface ripples may be correlated with the penetration depth or to other subsurface geometrical parameters. Proof of concept testing of the sensing principle was conducted on two different materials, namely, AISI316L and IN718, by means of single track LPBF depositions. Experiments were conducted at different levels of laser emission power to induce variations in molten pool characteristics. The process was observed by employing an off-axis illumination light and a high-speed camera, which allowed acquisitions with high spatial and temporal resolution. The acquired frames were postprocessed to extract the oscillation indicator, and analysis of the power spectral density of the signal allowed for the identification of the oscillation frequency. Results show that oscillation frequencies range from 3 to 5.5 kHz. Molten pool penetration depth and cross-sectional area could be correlated with the oscillation frequencies for the inline detection of these parameters during LPBF depositions. For both materials, higher oscillation frequencies corresponded to a shallower molten pool and a smaller mass of molten material. Moreover, different characteristic curves of oscillation frequency variations as a function of the melt pool cross-sectional area were determined for IN718 and AISI316L
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
External Illumination Strategies for Melt Pool Geometry Monitoring in SLM
Full text linkCoaxial monitoring in selective laser melting (SLM) can be applied using different configurations in terms of sensor choice and observed bandwidth. Use of external illumination to observe the melt pool geometry by dominating the emission from the process itself is one option, where the melt pool geometry can be visualized independently of changes in the emission behavior. However, the correct choice of the illumination source and the configuration in which it is implemented is an issue that requires further attention. The aim of this work is to investigate methods for direct observation of the molten pool geometry using an external illumination source to dominate the emission from the process. A coaxial imaging system was devised for this purpose, and two different setups for illumination were designed and tested, namely a diode laser beam coaxial to the working laser and a lateral low-coherence laser illuminating the whole build platform. The advantages and flaws of each experimental setup are extensively discussed. External illumination was found to be useful for direct interpretation of the SLM melting conditions. Furthermore, the actual scan position and velocity could be determined by applying an image processing algorithm to the captured frames
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Effects of laser cutting on the chemical composition and phase transformation capacity in Cu-Al-Mn shape memory alloy sheets
No full textApplication of laser cutting operations on thin sheets of Cu-Al-Mn shape memory alloys can be used for shaping, or for producing different types of holes and hole patterns within the SMA sheets. In this paper, the authors evaluate the effects of laser cutting under different process parameters on the phase and chemical composition in Cu-Al-Mn sheet samples. The effects are observed using optical and scanning electron microscopy (OM and SEM), energy-dispersive x-ray spectroscopy (EDS), and differential scanning calorimetry (DSC)
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