1,721,118 research outputs found
Il consiglio di amministrazione, la rotazione degli amministratori e la performance dell'impresa: l'esperienza italiana in una prospettiva comparata
No full textL’effetto del controllo familiare sulla performance delle imprese: una sintesi dell’evidenza empirica internazionale
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External Illumination Enables Coaxial Sensing of Surface and Subsurface Molten Pool Geometry in LPBF
Full text linkLaser powder bed fusion (LPBF) attracts the attention of high-end manufacturing sectors for its capability of depositing free-form components with elevated mechanical properties. However, due to the intrinsic nature of the feedstock material and the interaction with the laser beam, the process is prone to defect formation and manufacturing inaccuracies. Therefore, the development of a monitoring architecture capable of measuring the geometrical features of the process tool (i.e., the melt pool generated by the laser-material interaction) is of paramount importance. This information may then be exploited to evaluate process stability. In this work, a high-speed camera was implemented coaxially in the optical chain of an LPBF system to extrapolate the geometrical features of the molten pool surface and its oscillatory behaviour, with elevated spatial and temporal resolution. A secondary light source was tested in both coaxial and off-axis configuration to dominate process emission and assess optimal illumination conditions for extracting the molten pool’s geometrical features. Preliminary results showed that the off-axis configuration of the illumination light enabled direct measurement of the molten pool surface geometry. A newly developed image processing algorithm based on illuminated images obtained via the coaxial observation frame was employed to provide automated identification of the melt pool geometry. Moreover, bright reflections of the external illumination over the melt surface could be clearly observed and used to characterise the oscillatory motion of the molten material. This information may therefore be taken as an indirect indicator of the molten pool penetration depth, hence providing information regarding the subsurface geometry. A successive experimental investigation showed the capability of the monitoring architecture to resolve the molten pool’s length, width and area with elevated acquisition frequency. Molten pool surface oscillations in the kHz range could be correlated to the penetration depth while the molten pool width measured via the high-speed imaging setup corresponded to the track width of the depositions. Hence, the methodological approach for the concurrent measurement of the molten pool’s geometry in three spatial dimensions was demonstrated and may be used to track the stability of LPBF depositions
Understanding the effects of temporal waveform modulation of the laser emission power in laser powder bed fusion: Part I - Analytical modelling
Full text linkThe architecture of contemporary fiber laser sources enables users a wide choice in terms of spatial and temporal profiles during the laser powder bed fusion (LPBF) process. Given the range of possibilities, the need for analytical modelling approaches to predict the consequences of waveform modulation in terms of both thermal and fluid-dynamic aspects over the powder bed, process dynamics and resulting part quality is of great interest. Within the present investigation a moving point source analytical model was developed to study the effect of temporally modulated laser beams over the temperature distribution and recoil pressure induced over the molten region during single track LPBF depositions. This study configures as the first part of an investigation on the topic presented with the aim of developing the modeling framework to predict the effects of temporal waveform modulation in the LPBF process. The model developed was implemented numerically to simulate the single track LPBF deposition of stainless steel AISI316L with different waveform shapes ranging from the conventional Square Wave emission to Ramp Up, Ramp Down and Triangle waveforms. Modulation at different amplitude levels and different waveform frequencies were also investigated. Results show that temperature variations followed the temporal profile of the power exposed over the material. Consequently, recoil pressure oscillations over the melt region exhibited a periodic profiles correlated to the waveform modulation of the laser power indicating that melt flow may be controlled by means of such techniques. Peak values of recoil pressure, which might be symptomatic of melt pool instabilities, could be reduced employing higher levels of modulation frequency or lower oscillation amplitudes between non-zero values of the emission power
Understanding the effects of temporal waveform modulation of laser emission power in laser powder bed fusion: Part II - Experimental investigation
Full text linkThe laser powder bed fusion (LPBF) process has historically been operated with high-brilliance fibre laser sources with continuous wave (CW) emission. Nonetheless, temporal waveform modulation of the laser emission power at high-frequency levels can provide a means to enhance the deposition process by modifying the melt dynamics and solidification mechanisms. In order to disclose the effect of different waveform shapes and their parameters, an experimental study using an open LPBF system was conducted. This paper is the second part of an investigation on this topic, which aims to validate the analytical model proposed in the first part of the work. The LPBF system that was developed enabled the power emission profiles to be programmed during single-track depositions. Four different waveform shapes were tested (namely square wave, ramp up, ramp down and triangle wave) at different levels of waveform amplitude (ΔP= 200–400 W) and different frequencies (fw = 2–4–6–8 kHz) during the single-track deposition of stainless steel AISI316L. High-speed imaging acquisitions allowed the melt dynamics to be disclosed and the melt-oscillation frequency to be identified. Larger waveform amplitudes and waveforms with sudden variations of emission power generated melt ejections and process instabilities. Stable conditions could be identified when employing ramp up and triangle waveforms with ΔP = 200. Melt-surface oscillation frequency corresponded to the values imposed via the modulation of the laser emission power, thus validating the analytical model of Part I, which correlated the melt-surface temperature to the recoil pressure induced over the molten pool. Optical microscopy images and metallographic cross-sections confirmed the high-speed video observations. Three-dimensional reconstructions of the depositions via focus variation microscopy allowed the build rates and roughness of the single tracks to be determined. Build rates obtained in stable deposition conditions with waveform modulation are analogous to values obtainable with CW emission, and beneficial effects over the roughness were reported
Effect of in-source beam shaping and laser beam oscillation on the electromechanical properties of Ni-plated steel joints for e-vehicle battery manufacturing
Full text linkLaser welding is a key enabling technology that transitions toward electric mobility, producing joints with elevated electrical and mechanical properties. In the production of battery packs, cells to busbar connections are challenging due to strict tolerances and zero-fault policy. Hence, it is of great interest to investigate how beam shaping techniques may be exploited to enhance the electromechanical properties as well as to improve material processability. Industrial laser systems often provide the possibility to oscillate dynamically the beam or redistribute the power in multicore fibers. Although contemporary equipment enables elevated flexibility in terms of power redistribution, further studies are required to indicate the most adequate solution for the production of high performance batteries. Within the present investigation, both in-source beam shaping and beam oscillation techniques have been exploited to perform 0.2-0.2 mm Ni-plated steel welds in lap joint configuration, representative of typical cell to busbar connections. An experimental campaign allowed us to define process feasibility conditions where partial penetration welds could be achieved by means of in-source beam shaping. Hence, beam oscillation was explored to perform the connections. In the subset of feasible conditions, the mechanical strength was determined via tensile tests alongside electrical resistance measurements. Linear welds with a Gaussian beam profile enabled joints with the highest productivity at constant electromechanical properties. Spatter formation due to keyhole instabilities could be avoided by redistributing the emission power via multicore fibers, while dynamic oscillation did not provide significant benefits
Sensor Selection and Defect Classification via Machine Learning During the Laser Welding of Busbar Connections for High-Performance Battery Pack Production
Full text linkThe transition towards electric mobility requires the development of manufacturing systems capable of realising products with elevated electrical and mechanical performance and in-line qualification. Laser welding of thin sheets is an enabling technology for the production of battery packs. Given the numerosity of the joints and the stringent requirements, in-situ monitoring of the process and advanced data analysis with Machine Learning (ML) algorithms are fundamental tools which need to be explored. The current study presents a methodological approach for the process development and integration of a monitoring architecture for the realisation of dissimilar material busbar connections (0.2 mm Ni-plated steel over 0.6 mm Cu in lap joint configuration) for the production of a high-performance battery pack for an electric racing motorbike. A single mode fiber laser welding system was equipped with different sensors to retrieve data during the laser-material interaction. The monitoring system was composed of three photodiodes positioned off-axis respectively observing the visible, thermal near-infrared and laser back-reflection region. A spectroscope also sampled process emission from an off-axis perspective whilst another photodiode was positioned within the laser source to observe the process coaxially. Following a preliminary phase required to characterise the process and data provided by the sensors, experiments were designed to identify defects and variations with respect to the reference condition. On a single sensor basis, supervised classification machine learning algorithms were trained to discern joints performed on an out of focus workpiece or in the presence of gap between the sheets. Results indicate that photodiodes observing the laser back-reflected light are capable of providing process relevant information which can be exploited to identify drifts from the reference processing condition. ML algorithms exhibited high accuracy classification even with a reduced amount of data
Observing molten pool surface oscillations during keyhole processing in laser powder bed fusion as a novel method to estimate the penetration depth
Full text linkVarious in-situ monitoring techniques have been developed for the detection of process drifts in the Laser Powder Bed Fusion (LPBF) process. Currently, optical emission monitoring can retrieve information regarding molten pool characteristics, such as temperature, width, length and area which provide substantial process signatures. Nonetheless, a fundamental indicator for the retrieval of a complete set of spatially distributed information is missing: the molten pool depth. Within the present investigation, a system for the estimation of the penetration depth based on the detection of molten pool surface oscillations is reported. Initially, the fundamentals of the monitoring technique are presented. The principle relies upon the observation of molten pool surface ripples through the measurement of probe light reflections in the melt area. Proof of concept testing of the sensing principle was conducted through an experimental investigation on a prototypal platform. A monitoring system (consisting of a high-speed camera and a secondary illumination light) was employed to view the process while realising both bead-on-plate material remelting and single track powder bed fusion depositions of AISI316L at different levels of laser emission power. Oscillation frequencies were extracted from the high-speed imaging acquisitions after image processing and signal analysis. The surface wave oscillations were measured to be in the range of 3.5–5.5 kHz in keyhole conditions. Metallographic cross-sections allowed to observe the effective molten pool penetration depth and cross-sectional area and were correlated to oscillation frequencies. Higher values of oscillation indicated shallower penetration and consequently a smaller mass of molten material
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