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Relative distortion results (CM247LC) from offset strategy as 2D process map (logarithmic scale) with a 40 W laser power (second laser)
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Based on SLM parameters from previous works, which guarantee fully dense and crack free CM247LC samples, multi laser beamstrategies have been pursued to reduce residual stresses or rather distortion during LPBF processing. By using a second postheating and non-melting laser source with a defocused laser beam and lateral offset, cantilever distortio...
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Selective Laser Melting (SLM) methods are widely used to manufacture metal parts for functional use in mainly automotive, aerospace and medical industries. Besides many advantages of SLM, manufacturing takes long times and need many costs. In this study, nine samples were manufactured at the same time to set a scenario of multiple samples manufactu...
Citations
... Approaches for the avoidance or healing of cracks should be the topics of future research. For example, altering the beam's intensity distribution [35,36] or multi-laser heating [37] can be used to reduce the residual stress or to remelt and heal cracks. Adaptation of the scan strategy to alter the grain growth orientation (and therefore susceptibility to crack propagation) may have some effect [33,38]. ...
Copper contamination has a negative effect on the tensile properties of certain stainless steel grades due to a weakening of grain boundaries via liquid metal embrittlement. This is especially problematic given current trends in laser powder bed fusion (L-PBF) that elevate contamination risks, such as multi-material processing or the use of recycled materials. As such, it is critical to establish composition limits for use in standard specifications. This study investigates the changes in tensile properties and cracking behavior in stainless steel alloy 316L contaminated with copper alloy CuCr1Zr at concentrations of 0–10 particle percent (pt.%) in horizontal, diagonal, and vertical build orientations. It is found that microcracks are already present at 1 pt.% Cu alloy and increase in density with contamination. The cracks are generally vertically oriented along columnar grain boundaries and are associated with high local Cu content, thus exacerbating the anisotropy of the as-built material. The contamination decreases the elastic modulus, yield strength (YS), ultimate tensile strength (UTS), and uniform elongation, eventually transitioning from ductile to brittle fracture modes. The build orientation relative to the tensile loading axis is shown to be a critical design parameter due to the preferential crack initiation and growth direction. The fracture surfaces at 10 pt.% contamination show regularly spaced, smooth brick-like cleavage patterns that correspond to the columnar grain dimensions. Even so, the measured YS and UTS exceeded the ASTM F3184-16 standard for CuCr1Zr contaminations up to 5 pt.%. As a conservative limit, it is proposed that a maximum content of 1 wt% Cu be specified for L-PBF SS316L.
... Two or more scans can be applied for: (1) remelting, which consists of double melting of the layer, [23,[25][26][27][28][29][30] (2) presintering and melting [31] as well as (3) melting and scanning with a low energy density to reduce the residual stress. [32] The literature reports beneficial effects of remelting in the LPBF process for various materials. Yasa and Kruth [24] have shown that remelting effectively reduced porosity of LPBF 316L steel, achieved a significant drop in porosity. ...
This study investigates the effect of laser power applied for a remelting scan in the laser powder bed fusion process on the formation of a bimodal microstructure and its impact on the mechanical properties of Ni-based Inconel 625 superalloy. Comparison of primary and remelting scans at similar surface energy densities revealed that the melt pools obtained in the remelting scan are smaller than in the primary scan. To achieve comparable remelted melt pool sizes, the 25 pct increase in energy is required. The shape and size of the remelted melt pools significantly affect the microstructure and material texture. The lower surface energy density in laser powder bed fusion favors the formation of a bimodal microstructure with large columnar grains and fine grain bands. Application of higher energy results in the formation of large columnar grains with Goss texture along build direction and separated by a large amount of low angle grain boundaries. Remelting scan also affects reduction of porosity and increasing of the area fraction of nanometric oxide inclusions. The study revealed that the samples subjected to a remelting laser scan and tensile tested along the direction of columnar grains exhibited higher ductility, which was associated with a slight decrease in the ultimate tensile strength compared to the samples that were not remelted. It was demonstrated that the remelting scan in the laser powder bed fusion process offers the possibility of improving the reliability of additively manufactured Inconel 625 superalloy by reducing porosity and tailoring its microstructure towards single-crystal-like, and thus improving the mechanical properties.
Graphical Abstract
... The high heat input of the laser beam produces severe thermal cycles in most of the additive processes with metal powders, generating high residual thermal stresses [48][49][50] that are usually reduced or removed through heat treatments [51] or different scanning strategies [52]. Tucho et al. [53] studied the dissolution of macro-and micro-segregated precipitates in Inconel 718 specimens obtained by L-PBF by varying the temperature and the total time of the heat treatment. ...
In this research, we investigate the dynamic behavior of Inconel 718 fabricated through laser powder bed fusion (L-PBF), addressing a notable knowledge gap regarding the correlation between process parameters and dynamic properties. The process parameters adopted are deducted from an extension of the Rosenthal solution, formulated to increase the process productivity while avoiding the typical production process defects. The dynamic Young modulus and the structural damping of the material are estimated as a function of the process parameters through ping tests reproducing the flexural vibrations of the specimens in as-built, solutioned, and aged conditions. The microstructure and porosity are investigated through metallographic analyses. The results show a substantial influence of the L-PBF process parameters on the dynamic Young modulus, which markedly increases as the energy density is reduced (23%) and progressively becomes more similar to the conventionally produced material. This influence stands in stark contrast to the relatively modest impact of heat treatments, which underlines a negligible effect of the process-induced residual stress. The structural damping remained approximately constant across all test conditions. The elastic response of the material is found to be primarily influenced by the different microstructures produced as the L-PBF process parameters varied, particularly in terms of the dimensions and shape of the solidification structures. The unexpected relationship between the dynamic Young modulus, energy density, and microstructure unveils the potential to fine-tune the material’s dynamic behavior by manipulating the process parameters, thereby carrying substantial implications for all the applications of additively manufactured components susceptible to significant vibratory phenomena.
... Production of functional prototypes is another possibility, alongside small series production [8,9]. However, some problems still need to be fixed, such as microstructure monitoring, porosity which can lead to bad mechanical behaviour [10], geometrical accuracy inducing the need of post-process rectification [11], and crack events during manufacturing, either at substrate-deposit interface or in the clad [12]. ...
Within the large Additive Manufacturing (AM) process family, Directed Energy Deposition (DED) can be used to create low-cost prototypes and coatings, or to repair cracks. In the case of M4 HSS (High Speed Steel), a reliable computed temperature field during DED process allows the optimization of the substrate preheating temperature value and other process parameters. Such optimization is required to avoid failure during the process, as well as high residual stresses. If 3D DED simulations provide accurate thermal fields, they also induce huge computation time, which motivates simplifications. This article uses a 2D Finite Element (FE) model that decreases the computation cost through dividing the CPU time by around 100 in our studied case, but it needs some calibrations. As described, the identification of a correct data set solely based on local temperature measurements can lead to various sets of parameters with variations of up to 100%. In this study, the melt pool depth was used as an additional experimental measurement to identify the input data set, and a sensitivity analysis was conducted to estimate the impact of each identified parameter on the cooling rate and the melt pool dimension.
... An established experimental method to assess the effect of process parameters, scan strategy or chemical composition has been the manufacturing of cantilever samples, which has also been used to validate thermomechanical simulations [18][19][20]. Other geometries such as bridges [21], L-shaped or prisms [22], or warping of entire build platform [23] have been used for the same purpose. ...
... Surface melting and solidification can cause a measurable curvature of the sample, from which the stress in melted area can be derived, when elastic properties of the material are known. This approach has been applied for example in cold sprayed coatings [31] and directed energy deposition [18,19,24]. The curving of such samples is nicely illustrated by Simson et al., who studied residual stresses in laser powder bed fusion [32]. ...
Laser powder bed fusion is an additive manufacturing method that is based on melting and solidification of powder material. Due to the local heating above the melting point, thermal stresses are usually formed in the final part. Mitigation of residual stresses is usually assessed by laser scan strategies and not by alloy tailoring. In this paper a segregation-based residual stress formation mechanism is proposed and assessed computationally. Additionally, an experimental setup for rapid screening of residual stress formation in various alloys is proposed. The results should ease material development of metal alloys tailored for additive manufacturing by allowing the comparison of residual stress formation tendency (e.g., solid state shrinkage) between alloys. The proposed computational method is comparative in nature and forecasting absolute residual stress values would require known temperature dependent elastoplastic properties for the alloys as well as exact thermal history. The proposed experimental method is quantitative but its reliability depends on material properties such as yield strength.
... Recently, the application of multi-laser powder bed fusion (MLPBF) has gained attraction due to its potential to alter the material's microstructure/properties and control residual stress, i.e., residual stress affects the likelihood of crack formation in LPBF. However, most of these studies in MLPBF were focused on residual stress variation due to changes in thermal gradients for various laser configurations [11][12][13][14][15] and studies of the MLPBF on the material's microstructure/properties are scarce and contradictory. The experimental results of the dual scanning strategy (two synchronized lasers) with a slight coaxial offset showed a meaningful improvement in the maximum bending strength [16], although a multi-laser scanning technique showed no effect on the creep response of Inconel [12] and a lack of drastic change in the microhardness and tensile strength [17,18]. ...
The formation of undesirable microstructures and defects hinders the widespread use of additive manufacturing, e.g., the formation of columnar grains in Ti–6Al–4 V leads to undesirable anisotropic mechanical properties. Here, we investigate the application of a novel synchronized circular laser array in the powder bed fusion technique to alter the microstructure of printed parts toward the preferable equiaxed grains. This feat is not achievable with the single laser powder bed fusion technique for Ti–6Al–4 V alloy. The temporal temperature distributions for different process parameters (laser power, scanning speed, and internal distance between lasers in the array) were obtained by an anisotropic heat transfer model, and the Hunt criterion was employed to construct the solidification map. The results revealed that a degree of overlap between lasers is recommended to form a coherent melt pool, avoid degeneracy in surface quality, and maintain adequate resolution for all processing windows. However, laser overlap is not required for low scanning speed and high power scenarios. Finally, microstructure prediction shows that 45% of printed track includes equiaxed grains at a high power regime (500 W). However, the volume fraction of the equiaxed microstructure is reduced by decreasing laser power.
... The information was extracted at the middle of the domain, which was at location "x" as seen in the sub-figure. The dashed line denoted the time step at the temperature of 1250 K, the moment when inplane stress anisotropies began to develop Likewise, Gerstgrasser et al. [49] noted the cracks from a re-melting strategy. Based on these studies, it is critical to design process strategies that consider both defect initiation and residual stress minimization. ...
A multi-laser powder bed fusion additive manufacturing can shorten manufacturing time and enable greater possibilities of process optimization. Nonetheless, previous studies showed that the process-dependent characteristics from the single laser may not fully translate to the multi-laser system. Therefore, the present work utilized a three-dimensional thermomechanical model to study the influence of process conditions on the physical behaviors of the dual-laser system of Ti-6Al-4V. Numerical results revealed that the dual-laser process could reduce the cooling rate and residual stress by approximately 70 and 30%, respectively. In addition, statistical analysis was employed to determine the level of influence and the contribution of each process parameter. Cooling rates and stress magnitudes were mainly controlled by the secondary laser power, and the scan length of the primary laser largely dictated the stress anisotropy. Therefore, although the dual-laser system can provide in-situ residual stress reduction, the understanding of process-controlled thermal and mechanical characteristics was proven essential to achieve desirable responses.
... Recently, the application of multi-laser powder bed fusion (MLPBF) has gained attraction due to its potential to alter the material's microstructure/properties and control residual stress, i.e., residual stress affects the likelihood of crack formation in LPBF. However, most of these studies in MLPBF were focused on residual stress variation due to changes in thermal gradients for various laser configurations [11][12][13][14][15] and studies of the MLPBF on the material's microstructure/properties are scarce and contradictory. The experimental results of the dual scanning strategy (two synchronized lasers) with a slight coaxial offset showed a meaningful improvement in the maximum bending strength [16], although a multi-laser scanning technique showed no effect on the creep response of Inconel [12] and a lack of drastic change in the microhardness and tensile strength [17,18]. ...
The formation of undesirable microstructures and defects hinders the widespread use of additive manufacturing, e.g., the formation of columnar grains in Ti-6Al-4V leads to undesirable anisotropic mechanical properties. Here, the microstructure produced by a novel synchronized circular laser array for application in the powder bed fusion technique is investigated. The temporal temperature distribution for different process parameters (laser power, scanning speed, and internal distance between lasers in the array) were obtained by an anisotropic heat transfer model, and the Hunt criterion was employed to construct the solidification map. The results revealed that a degree of overlap between lasers is recommended to form a coherent melt pool, avoid degeneracy in surface quality, and maintain adequate resolution for all processing windows. However, laser overlap is not required for low scanning speed and high power scenarios. Finally, microstructure prediction shows that 45% of printed track includes equiaxed grains at a high power regime (500 W). However, the volume fraction of the equiaxed microstructure is reduced by decreasing laser power.
... Laser powder bed fusion (LPBF) is one of the most commonly used additive manufacturing processes in industries to build complex-shaped engineering components due to the shape design freedom offered by the technology [5]. It starts with a 3D model sliced into several layers (2D sections), and material addition is carried out in a layer-by-layer sequence by selective fusion of the pre-placed material using high power lasers [8]. LPBF provides the additional advantages of reduced tooling, geometric freedom and mass customization over conventional manufacturing processes (i.e. ...
This paper reports the effect of hot isostatic pressing (HIP) on the porosity, microstructure and mechanical properties of laser powder bed fusion (LPBF) IN625 structures built at a higher layer thickness of 100 µm. It is observed that the process-induced pores/voids of volume fraction (Vf) 0.43% in as-built IN625 structures are reduced significantly to ~ 0.01% after HIP treatment. The microstructure is changed from fine columnar dendrites to coarse equiaxed dendrites. The microstructural analysis of as-built structures reveals the presence of cellular/dendritic growth along with elemental segregation of Nb, Si and C and precipitation of Nb-rich carbides, whereas coarse recrystallized microstructure along with elemental segregation of Si and precipitation of Nb, Mo and Cr rich carbides is observed in hot isostatic pressed (HIP) samples. HIP structures exhibit lower tensile strength, higher ductility and lower anisotropy as compared to LPBF built structures. There is a reduction in the Vickers micro-hardness of IN625 samples after HIP, and the values are observed to be similar to their conventional counterparts. Further, an increase in the energy storage capacity of the material is observed after HIP treatment through Automated Ball Indentation (ABI®) studies. The study paves a way to develop ~ 100% dense, defect-free and isotropic engineering components using LPBF.
... Building parts without defects in processing a specific material involves choosing optimum parameter levels to achieve desired properties. Numerous parameters such as laser power (Spears and Gold, 2016), scanning speed (Esmaeilizadeh et al., 2020), scanning pattern (Liu et al., 2021), the material composition of the powder (Knieps et al., 2021), surrounding environment (Ch et al., 2019), laser beam size (Gerstgrasser et al., 2021), etc., affect the LPBF process quality. During the process, any deviation from the optimum window parameters for any material causes significant changes in the laser-material interaction affecting the melt pool's depth, width, and length. ...
The defective regimes in metal-based Laser Powder Bed Fusion (LPBF) processes can be minimized by deploying in-situ monitoring strategies comprising Machine learning (ML) algorithms and sensing techniques. So far, algorithms trained for monitoring a particular material type cannot be re-used to monitor another material in Additive Manufacturing (AM). This is a topic rarely researched in AM. Inspired by the idea of transfer learning in ML, we demonstrate the knowledge learned by the two native Deep Learning (DL) networks, namely VGG and ResNets, on four LPBF process mechanisms such as balling, Lack of Fusion (LoF) pores, conduction mode, and keyhole pores in stainless steel (316 L) can be transferred to bronze (CuSn8). In this work, the spectrograms computed using Wavelet Transforms (WT) on Acoustic Emissions (AE) during the LBPF process of stainless steel and bronze are used for training the two DL networks. Either network is first trained for classification by spectrograms representing four mechanisms during the processing of stainless steel. The trained model is then re-trained using transfer learning with spectrograms from bronze data for a similar classification task. The accuracy of the two networks during transfer learning shows that it is effectively possible to learn transferable features from one material to another with minimum network training time and dataset collection.
