Single Track Research Articles

In-process monitoring of track geometry as a control approach for laser metal deposition

Laser metal deposition (LMD) is an additive manufacturing process, which is used to generate large parts or for reconditioning machine components. During the process the welding tracks are affected by complex dynamic temperature-time curves that results in geometric deviations, which limits the build-up process. Therefore, complex structures are more difficult to manufacture with constant parameters. The present work describes an in-process monitoring system and first approaches of a control concept for LMD processes. It is set up using a newly developed laser triangulation sensor from Falldorf Sensor GmbH, which is trailing mounted. Due to the resistance to diffused radiation, e.g. from process light, it is possible to place the sensor close to the process zone which offers the possibility of a height control system. For the experiments, single tracks were welded from 316L powder by using a diode laser. The results show that the track width and height increase with rising laser power. Depending on the process parameter the height changes up to 29% and the width up to 58% if only the laser power is modified by 1800 W. The monitoring system forms a good basis for a later control concept, which can be used to make the process more stable in the case of complex structures.

Two-stage calculation model of single pulse vehicle tracking target and flight track

In order to get rid of the tracking of single-pulse guided aircraft, it is necessary to study its guidance method, analyze its flight and speed change laws, and deeply understand its performance. In this paper, the guided aircraft using the single-pulse working mechanism is analyzed from the flight speed change, the speed and distance changes in the flight process with time are analyzed, and the approximate calculation model is established, which is of good help for understanding the flight process of the single-pulse vehicle.

Single-track investigation of additively manufactured mold steel with larger layer thickness processing: Track morphology, melt pool characteristics and defects

Large layer thickness is essential to increase building rate, but excessive layer thickness easily lead to defects which negatively affect component quality. The evolution of track morphology, melt pool characteristics and defects of additively manufactured mold steel by using various larger layer thickness (60 and 90 μm) while also varying the laser energy density was investigated systematically. Results indicated that the morphologies of single tracks shifted sequentially from discontinuousness to continuous irregularity and continuous regularity with the increasing line energy density at 60 μm layer thickness. The width and depth of melt pool increased substantially with the line energy density increased from 0.4 to 1.3 J/mm, while the height of melt pool was nearly constant. Correspondently, transition of the melt pool based on the depth-to-width ratio from balling to conduction and keyhole modes was observed. When the layer thickness increased from 60 to 90 μm, the average build percentage of the single tracks decreased by 43 % with increasing line energy density from 0.26 to 0.51 J/mm, and more spatters around the surface of tracks were found at 0.8 J/mm. Interestingly, the average width of the melt pool decreased by 37 μm, while the height of melt pool increased by 45 μm. More small spherical gas pores (5–15 μm) occurred in bottom of keyhole due to trapped vapor at high energy density. The decrease in melt width with increased layer thickness is most likely due to the weakened Marangoni convection. Finally, formation mechanism of lack-of-fusion defects and gas pores with larger layer thickness processing were analyzed based on melt pool characteristics, such as spheroidization, keyhole and spatters.

Laser-Based Directed Energy Deposition of Ceramic Nanoadditivated AA7075 Powder Alloys

AA7075 is one of the most resistant aluminium alloys, so it is frequently used in very demanding industries as aeronautics or defence. However, the 7075 alloy falls into the non-weldable category thus hardly processable through additive manufacturing processes, and specially on laser-based DED (Directed Energy Deposition). The low absorption together with cracking behaviour remain a challenge for the industrialisation of these processes. Alloying with minor elements or addition of nano-reinforcement have been proven as a successful approach to increase its manufacturability. In this work, the feasibility of printing 7075 with nano-TiC as additive was evaluated. Two compositions with 0.5 and 2% in weight were developed by dry mixing. The powders were characterized by scanning electron microscopy (SEM) and flowability was compared with the unreinforced alloy. With the optimal laser process parameters, 3D coupons were printed to be characterized microstructurally, thermally, and mechanically. Process monitoring using thermal and high-speed cameras was carried out to gain insight into the thermal behaviour of the melt-pool and resulting process stability. After printing, aspect ratio of single tracks was measured, and dilution was also evaluated. Although addition of 0.5% of n-TiC promotes a slight improvement on the alloy, allowing it to be mechanically tested, it still presents some defects as porosity. By increasing the content up to 2%, both the quality and the mechanical performance were enhanced significantly.

Generating functionally graded steel microstructures by laser powder bed fusion

Laser Powder Bed Fusion (L-PBF) is bearing the potential to generate new functionally graded microstructures (FGMi) whose properties are tailored regarding specific applications. However, locally altering the process parameters in order to create such microstructures can easily compromise the relative mass density and therefore the performance of a material. This work presents and investigates in situ heat treatments by double exposure in order to realize FGMi. The results show an increased parameter flexibility without compromising the relative mass density, when compared to a single exposure of the investigated low-alloy steel 30CrMoNb5-2. The systematic investigation regarding the impact of process parameters enables microstructures in the hardness range from approx. 380–510 HV10. Especially, the introduction of cooling breaks between single exposure tracks increases the hardness of the microstructure. Tensile tests show an increase in UTS by 21.3%, when comparing the results with a nonin situ heat treated sample (single exposure). A homogeneously in situ heat treated sample as well as both discrete and continuously graded samples demonstrate the potential of the novel method. Consequently, the presented double exposure approach for in situ heat treatments is offering a new flexibility regarding the creation of FGMi by L-PBF and will therefore support the development of future high-performance materials.

Cracking mechanisms and effect of extensive preheating in CM247LC and IN713LC Ni-base superalloy processed by Laser Powder Bed Fusion

Laser powder bed fusion processing of non-weldable γ’-strengthened Ni-based superalloys is challenging due to various cracking mechanisms that occur during the solidification and cooling stages of the materials. The current study offers an insight into the failure mechanisms of the CM247LC and IN713LC alloys, identifying the pre-heating temperatures which can mitigate cracking phenomena during processing. The adopted methodology involved the alloy processing by a standard Laser powder bed fusion system followed by microstructural characterization. It was confirmed that the development of cracks with a length in the range of 100–500 µm became evident in both alloys. High-temperature preheating strategies were then investigated using a simplified approach based on single laser track remelting. At pre-heating levels up to about 900 °C, corresponding to measured cooling rates of the order of 104 °C/s, hot cracks were still observed in the alloys due to γ’ precipitation and segregation of carbide former elements at the cell boundaries according to both microstructure observations and thermodynamic predictions. Preheating conditions at higher temperatures targeting levels above the γ’-solvus temperature, exceeding 1200 °C could promote slower cooling rates, leading to full suppression of cracks in the investigated γ’-strengthened alloys.

Single track versus bulk samples: Understanding the grain refinement in inoculated ferritic stainless steels manufactured by powder bed fusion-laser beam

One generic challenge in powder bed fusion - laser beam (PBF-LB) is the formation of epitaxially grown columnar grains, which lead to the undesirable anisotropy of mechanical properties. This anisotropy could be rectified by ex-situ or in-situ inoculation in some particular alloy systems. Understanding the grain refinement mechanism caused by in-situ inoculation is, however, complicated by remelting caused by the overlapping between neighboring scan tracks, when printing bulk samples using multiple tracks. Here in this work, a series of single tracks using ferritic stainless steels feedstock powder with and without pre-alloyed inoculant-forming elements, were printed at different scanning speeds to gain refreshed understanding on the mechanism of the observed grain refinement. Interestingly, the grain refinement in single tracks and bulk samples printed from the powder with and without inoculant-forming elements showed an opposite tendency. When using the powder without inoculant-forming elements, the single tracks showed large columnar grains, while the bulk samples showed even larger grain sizes; when using the powder with pre-alloyed inoculant-forming elements, fine equiaxed grains are found at the centers of the melt pools, surrounded by slightly coarser columnar grains at melt pool boundaries, in both single tracks and bulk samples. Noticeably, the mean grain sizes in the bulk samples are however smaller compared to those for single tracks because of remelting. Our work provides new insights on the grain refinement via in-situ inoculation during the PBF-LB process and highlights the importance of studying single tracks to better understand the melting and solidification behavior.

Ständigt slölyssnande

This article discusses the impact of digitalization on music consumption based on interviews with Swedish music consumers about their music consumption. The findings indicate that digitalization allows music to be consumed in more and more situations and in combination with other activities rather than as a stand-alone activity. On the one hand, this development contributes to making music more ubiquitous in contemporary society; on the other hand, it transforms music listening into something that is done with less attention. Digitalization also changes music as a consumption object, e.g. from specific artists and albums to single tracks or new forms of compilations such as play lists adapted to specific moods or situations. Digitalization creates new coordination needs, e.g. when sharing accounts and play lists or determining in what order tracks will be played in social situations. Digitalization also gives rise to new forms of sociality where both individual and joint listening becomes digitally mediated. Taken together, these findings raise questions about the role of music in style formation and identity construction and the paper concludes with some thoughts on music’s changing role in contemporary consumer culture.

Monitoring the degree of dilution during directed energy deposition of aluminum bronze and H13 tool steel using optical emission spectroscopy

Controlling heat transfer in casting tools is a key quality aspect. It can be improved by selectively applying volumetric aluminum bronze (CuAl9.5Fe1.2) sections in the core of the tools and subsequently depositing these cores with hard-facing H13 tool steel. Directed energy deposition (DED) can be used for both additive manufacturing of aluminum bronze and hard-facing by depositing the filler material onto a substrate surface or previously manufactured bodies. A sufficient metallurgical bonding of the deposited filler material and the underlying layer must be ensured. Hence, the dilution is a key factor for quality assurance. However, high dilution of the underlying layer and the filler material negatively affects the desired properties and must be monitored. Optical emission spectroscopy of the DED process emissions is investigated by comparing the emission lines of the individual elements comprising the base and the filler materials. Multiple single tracks using aluminum bronze as the filler material are laser-cladded with varying power, onto the two different types of substrates, i.e., mild steel S355 (1.0570) and hot working tool steel H11 (1.2343). Additionally, single tracks of H13 (1.2344) are deposited with varying laser powers onto an additively manufactured core of aluminum bronze. Both resulting in deposition tracks with varying dilution values. Multiple emission lines of Cr, Fe, Cu, Al, and Mn are detected and measured (line intensity). Line intensity ratios using the element emission lines are calculated and correlated with the respective metallographic results of the deposition tracks (dilution and chemical composition). Deposition tracks with a higher dilution (CuAl9.5Fe1.2 onto S355/H11 as well as H13 onto CuAl9.5Fe1.2) showed an increased line intensity ratio of the underlying material to the filler material. Moreover, this technology was transferred in a multilayer industrial application.

Process development and process adaption guidelines for the deposition of thin-walled structures with IN718 using extreme high-speed directed energy deposition (EHLA3D)

Extreme high-speed directed energy deposition (EHLA) is a modified variant of the laser based directed energy deposition (DED-LB) and is being applied as an efficient coating process for rotational symmetric components. Characteristics of EHLA processes are feed rates of up to 200 m/min, which result in smaller weld bead deposition and thinner layer thicknesses compared to conventional DED-LB. When transferred to additive manufacturing, this characteristic utilizes the potential of depositing thin-walled filigree structures at deposition rates, which are comparable to typical DED-LB processes (EHLA3D). The results of this work were achieved with an EHLA3D machine, which is a modified CNC-type machine capable of operating feed rates with vf = 30 m/min. In this work, process parameters were developed for the deposition of thin-walled filigree structures with the Ni-based superalloy IN718. Single tracks with constant feed rates and a variation in the beam diameter and powder mass flow were deposited and analyzed regarding the resulting weld bead dimension and dilution zone. Then, process parameters were selected and transferred to the deposition of thin walls, and guidelines of the parameter adaption toward thin-walled deposition were defined. Two parameter sets were developed to assess the feasible wall-thicknesses deposited by EHLA3D. Depending on the developed parameter sets, wall thicknesses between 300 and 500 μm are achieved. To characterize the resulting thin-walls, surface roughness measurements and metallographic cross sections were conducted.

Laser cladding: A high-speed-imaging examination of powder catchment efficiency as a function of the melt pool geometry and its position under the powder stream

This paper provides quantitative information about the paths taken by blown powder particles during laser cladding. A proportion of the powder is “wasted” by bouncing off the solid areas surrounding the melt pool. This wastage reduces the productivity and profitability of the process. In this paper, specially developed software was used to analyze high-speed imaging videos of the cladding process, to monitor the directions of powder particle flight toward and away from the melt pool area. This information has been correlated to the geometry and position of the melt pool zone for three different cladding techniques: single track cladding (A tracks), standard overlapping track cladding (AAA cladding), and a recently developed technique called ABA cladding. The results show that the melt pool geometry, and particularly the overlap between the melt pool and the incoming powder stream, has a strong influence on powder catchment efficiency. ABA cladding was found to have considerably better powder catchment efficiency than standard AAA cladding and this improvement can be explained by consideration of the geometries and positions of the melt pools and surrounding solid material in each case. As powder costs are an important factor in industrial laser cladding, the adaption of the ABA technique, and/or control of pool/powder stream overlap (e.g., by making the powder stream not coaxial with the laser beam), could improve the profitability of the process.

Optimization of Laser Direct Structuring Process Parameters for Material Extrusion of Polycarbonate

Laser direct structuring (LDS) is critical in the integration of circuits onto 3D‐shaped plastic parts, such as antennas and radio frequency components. The LDS process encompasses three stages: deposition of 3D parts, laser structuring, and metallization. While laser‐direct structurable parts have been manufactured through plastic injection molding, material extrusion (MEX) is a favored additive manufacturing process for economic low‐volume production and fast prototyping advantages. Although injection‐molded LDS literature is available, 3D‐printed laser‐direct structured components merit further investigation. This study focuses on the MEX of catalyst‐loaded polycarbonate (PC) parts and its LDS process. The parameters from the nanosecond fiber laser, including scan speed, power, and frequency, are thoroughly analyzed to understand the surface property changes and metallization performance of the printed PC parts. The single scan track width, which corresponds to the accuracy of conductive path width and metallization thickness, is employed to elucidate the findings. A process map is built to keep the track width constant aimed at enhancing the uniform metallization of intricate components. Thresholds are established, identifying a minimum track width of 22.1 μm and metallization thickness of 2.5 μm. These delineate clusters of process parameters that yield conductivity levels suitable for various applications.

Magnetic response to microstructure and phase evolution in laser thermal treated FeSiB amorphous alloy

In the current work, laser thermal treatment of the FeSiB amorphous foil with a single linear laser track was carried out. The resultant microstructure and phase evolution were examined with the aid of x-ray diffraction, differential scanning calorimetry, and site-specific transmission electron microscopy. The laser power was kept constant at 100 W, whereas, laser beam scanning speeds were varied in the range of 500–235 mm/s, generating corresponding laser fluences of 0.42–0.91 J/mm2 on the sample surface. Laser fluences of up to 0.48 J/mm2 structurally relaxed the FeSiB foil, retaining the amorphous structure. Laser fluences of higher than 0.48 J/mm2 led to partial crystallization of FeSiB amorphous foils. The crystallite sizes were in the range of 11–31 nm (laser fluence of 0.49–0.91 J/mm2). α-FeSi formed as a major phase of partial crystallization while its quantity steadily increased from 3.6 to 46 vol. % with laser fluence (laser fluence 0.49–0.91 J/mm2). Fe2B formed in recognizable quantities (≥2%) for laser fluences ≥0.53 J/mm2. Laser fluences leading to structural relaxation and evolution of predominantly α-FeSi phase exerted minimal effects on ratios of intrinsic coercivities to saturation compared to the as-cast FeSiB amorphous foil. On the contrary, formation of Fe2B in significant quantities (≥2%) led to the steady increase in intrinsic coercivities and remanence to saturation ratios as a function of laser fluence indicating a loss in soft magnetic characteristics. Nonetheless, continuous increase in fractions of α-FeSi with laser fluence led to a steady improvement in saturation magnetostriction of the FeSiB foil.

Does the Mott problem extend to Geiger counters?

Abstract The Mott problem is a simpler version of the quantum measurement problem that asks: Is there a microscopic physical mechanism – based (explicitly or implicitly) only on Schroedinger’s equation – that explains why a single alpha particle emitted in a single spherically symmetric s-wave nuclear decay produces a manifestly nonspherically symmetric single track in a cloud chamber? I attempt here to generalize earlier work that formulated such a mechanism. The key ingredient there was identification of sites at which the cross section for ionization by a passing charged particle is near singular at ionization threshold. This near singularity arose from a Penning-like process involving molecular polarization in subcritical vapor clusters. Here, I argue that the same Mott problem question should be asked about Geiger counters. I then define a simple experiment to determine if ionization physics similar to the cloud chamber case takes place in the mica window of a Geiger counter and explains the collimation of wavefunctions that are spherically symmetric outside the counter into linear ion tracks inside. The experiment measures the count rate from a radioactive point source as a function of source-window separation. I have performed a proof of concept of this experiment; results are reported here and support the near-singular-ionization picture. These results are significant in their own right, and they may shed light on physical mechanisms underlying instances of the full quantum measurement problem. I illustrate this for the Stern–Gerlach experiment and a particular realization of superconducting qubits. I conclude by detailing further work required to flesh out these results more rigorously.

Scaling laws for the additive manufacturing of the AISI 316 L deposited by laser surface cladding and direct metal deposition methods

This study presents a comparative analysis of the shape and microstructure of individual tracks formed from AISI 316 L stainless steel using laser surface cladding (LSC) and direct metal deposition (DMD) methods. The research aims to identify the relationship between laser radiation parameters (laser spot size, radiation power, scanning speed) and geometric characteristics of created tracks. The result shows that the change in the geometric characteristics of single tracks (depth, width, and height) can be related to two dimensionless parameters: normalized enthalpy H and Peclet number Pe. Based on the experimental data summary, the limiting values of the conditions for realizing keyhole melting (H > 2, Pe < 10) and heat conduction (H < 2, Pe > 10) were determined.

Femtosecond laser assisted selective etching of microchannels in lithium niobate.

Here, we report on the fabrication of cm-long microchannels in LiNbO3 by selective etching of femtosecond laser inscribed tracks using hydrofluoric acid. We achieved a 1 cm long microchannel after 300 h of etching a single track inscribed into the volume along the optical axis of LiNbO3. Furthermore, we investigated the dependence of the etching behavior on various writing parameters. Highly selective etching with a selectivity up to 104 was achieved and a functional relationship between the etched depth and time was found. Thus, our results set the first milestone for future fabrication of 3D-hollow microstructures in the volume of LiNbO3 combining its outstanding physical properties such as the strong nonlinearity as well as the acousto- and electrooptic properties with both microfluidic and photonic structures in a monolithic setup.

In situ observation of melt pool evolution in ultrasonic vibration-assisted directed energy deposition

The presence of defects, such as pores, in materials processed using additive manufacturing represents a challenge during the manufacturing of many engineering components. Recently, ultrasonic vibration-assisted (UV-A) directed energy deposition (DED) has been shown to reduce porosity, promote grain refinement, and enhance mechanical performance in metal components. Whereas it is evident that the formation of such microstructural features is affected by the melt pool behavior, the specific mechanisms by which ultrasonic vibration (UV) influences the melt pool remain elusive. In the present investigation, UV was applied in situ to DED of 316L stainless steel single tracks and bulk parts. For the first time, high-speed video imaging and thermal imaging were implemented in situ to quantitatively correlate the application of UV to melt pool evolution in DED. Extensive imaging data were coupled with in-depth microstructural characterization to develop a statistically robust dataset describing the observed phenomena. Our findings show that UV increases the melt pool peak temperature and dimensions, while improving the wettability of injected particles with the melt pool surface and reducing particle residence time. Near the substrate, we observe that UV results in a 92% decrease in porosity, and a 54% decrease in dendritic arm spacing. The effect of UV on the melt pool is caused by the combined mechanisms of acoustic cavitation, ultrasound absorption, and acoustic streaming. Through in situ imaging we demonstrate quantitatively that these phenomena, acting simultaneously, effectively diminish with increasing build height and size due to acoustic attenuation, consequently decreasing the positive effect of implementing UV-A DED. Thus, this research provides valuable insight into the value of in situ imaging, as well as the effects of UV on DED melt pool dynamics, the stochastic interactions between the melt pool and incoming powder particles, and the limitations of build geometry on the UV-A DED technique.

Influence of droplet size and ejection frequency on molten pool dynamics and deposition morphology in TIG-aided droplet deposition manufacturing

Droplet parameters like diameter and ejection frequency are identified as essential factors in metal-based droplet deposition manufacturing (DDM). A high-fidelity multi-physics model was developed to reproduce the improved DDM process (previously proposed as tungsten inert gas (TIG) - aided DDM) where multiple metal droplets merge with a moving molten pool to form a single track. This paper mainly discusses the influence of droplet parameters on the molten pool behaviors and deposition morphology. Two droplet sizes with different generation frequency were included in the model: high-frequency small droplet and low-frequency large droplet, however, the volume flow rate of the two cases remains same. During deposition, the significant fish-scale ripples on the top surface of deposited layers occurred from the smaller droplets into the larger ones through partial coalescence. For high-frequency small droplet mode, the shape of deposited layer is more beautiful with the surface being continuous, smooth and even. Because of differences in the physical processes involved, the relationship between droplet size mode and deposition morphology was further elaborated based on the molten pool configuration. This work provides a perspective in understanding the physical phenomena involved in TIG-aided DDM and lays the foundation for the optimization of surface macro-quality of deposits.

Prediction of single track clad quality in laser metal deposition using dissimilar materials: Comparison of machine learning-based approaches

Powder-based laser metal deposition (LMD) offers a promising additive manufacturing process, given the large number of available materials for cladding or generative applications. In laser cladding of dissimilar materials, it is necessary to control the mixing of substrate and additive in the interaction zone to ensure safe metallurgical bonding while avoiding critical chemical compositions that lead to undesired phase precipitation. However, the generation of empirical data for LMD process development is very challenging and time-consuming. In this context, different machine learning models are examined to identify whether they can converge with a small amount of empirical data. In this work, the prediction accuracy of back propagation neural network (BPNN), long short-term memory (LSTM), and extreme gradient boosting (XGBoost) was compared using mean squared error (MSE) and mean absolute percentage error (MAPE). A hyperparameter optimization was performed for each model. The materials used are 316L as the substrate and VDM Alloy 780 as the additive. The dataset used consists of 40 empirically determined values. The input parameters are laser power, feed rate, and powder mass flow rate. The quality characteristics of height, width, dilution, Fe-amount, and seam contour are defined as outputs. As a result, the predictions were compared with retained validation data and described as MSE and MAPE to determine the prediction accuracy for the models. BPNN achieved a prediction accuracy of 0.0072 MSE and 4.37% MAPE and XGBoost of 0.0084 MSE and 6.34% MAPE. The most accurate prediction was achieved by LSTM with 0.0053 MSE and 3.75% MAPE.

Effects of processing parameters on pore defects in blue laser directed energy deposition of aluminum by in and ex situ observation

A single track, as the basic unit of laser directed energy deposition (L-DED) process, plays a significant role in the dimensional accuracy and mechanical performances of the ultimate products. However, there is almost no systematic investigation on the formation process and three-dimensional characteristics of the internal pore defects. Here, we used a high-speed camera, laser scanning confocal microscope (LSCM), and synchrotron radiation X-ray computed tomography (SR-CT) to study single tracks of AlSi10Mg alloy fabricated by blue laser directed energy deposition (BL-DED). A comprehensive investigation is conducted on the impact of processing parameters on the sizes, shapes, and formation mechanism of pore defects. Three types of pore defects are examined in single tracks: Type I lack of fusion, Type II spherical gas pores and Type III large irregular pores. Besides, large irregular pores are the transition between other two types. In particular, the results of SR-CT show that porosity decreases gradually with the increment of laser power and scanning speed. Therefore, high laser power accompanying with fast scanning speed will reduce the porosity. The lowest porosity of 0.074% is achieved under the power at 1600 W with scanning speed at 1080 mm/min, which has an obvious improvement over the current infrared L-DED. In addition, the mapping relationship among laser power, scanning speed and pore defects is established, which will provide a fundamental understanding of the origin of the defect and strategies for controlling the defect in L-DED towards high-quality printing.