Laser Aided Additive Manufacturing Research Articles

Enhanced quasi-static and dynamic tensile properties of stainless steel 316L produced by laser aided additive manufacturing in controlled argon environment

In this paper, laser aided additive manufacturing (LAAM) of stainless steel 316L (SS 316L) in a controlled argon environment was investigated. The controlled argon environment can significantly reduce the oxidation and fundamentally change the process conditions. It was found that after inhibiting oxidation, the surface of the deposited SS 316L showed remarkably high reflectance (71%) to laser radiation, which minimised heat accumulation. Subsequently, stable melt pool temperature and high cooling rate (4.1 × 104 K/s) were also observed. Furthermore, nano-sized oxides were found to be nucleated owing to excessively low internal oxygen content (220 ppm). The unique process condition resulted in enhanced strengthening effect due to cellular structure refinement and dispersed nano-sized oxides. Consequently, a desirable balance of high strength and ductility was achieved for the as-deposited materials under both quasi-static tension (strain rate 10−3 s−1, yield stress 560.0 MPa, ultimate tensile strength 717.2 MPa, and elongation 64.0%) and dynamic tension (strain rate 3150 s−1, maximum flow stress 1241 MPa, and elongation 37.5%).

Oxidation behavior and resultant deposition efficiency and material properties of laser aided additively manufactured stainless steel 316 L

Oxidation is a common issue in directed energy deposition (DED) based additive manufacturing (AM) processes. Severe oxidation is usually considered to be detrimental, which should be avoided. However, it is found that the surface oxidation of the deposited materials can also lower the laser reflectance, thereby increasing the utilization rate of laser energy and improving deposition efficiency. In this study, stainless steel 316 L (SS 316 L) was deposited using laser aided additive manufacturing (LAAM) process in the localized argon gas shielding environment and controlled argon environment, respectively, to investigate the evolution of oxidation on the surface and in the deposited material, as well as the effect of oxidation on the deposition efficiency and material properties. Results showed that severe surface oxidation of the materials deposited in the localized argon gas shielding environment effectively lowered the laser reflectance (14.5 %), which is much lower than that deposited in argon environment (71 %). Subsequently, with the same process parameters, the deposition efficiency in the localized argon gas shielding environment can be 1.57 times of that obtained in the controlled argon environment. Meanwhile, compared with the tensile properties of SS 316 L deposited in the controlled argon environment (YS 560 MPa, UTS 717 MPa, elongation 64 %), the tensile properties of SS 316 L deposited in the localized argon gas shielding environment (YS 456 MPa, UTS 631 MPa and elongation 57 %) are lower，whereas still show a good combination of strength and ductility.

Process optimization and quality prediction of laser aided additive manufacturing SS 420 based on RSM and WOA-Bi-LSTM

Laser aided additive manufacturing (LAAM) is a promising method for surface modification, repair, and direct fabrication 3D components. Nevertheless, the high cost and time-consuming nature of trial-and-error based process development hinder the progress of this technology. Consequently, machine learning has garnered substantial attention from both the academia and industrial sectors due to its ability to expedite process study for both research and industrial applications. This paper presents a study on the LAAM of 420 stainless steel on the surface of Q235 steel using the combined Response Surface Methodology (RSM) and Whale Optimization Algorithm-Bidirectional Long Short-Term Memory (WOA-Bi-LSTM) model. The results show that the process parameters can be effectively optimised to improve the prediction accuracy using the multi-objective whale optimisation algorithm for the fitted equations obtained in response surface analysis. At the same time, with the support of a large number of data, the WOA-Bi-LSTM model can accurately predict the quality of the LAAMed material, which is better than the response surface analysis method.

Microstructure and Mechanical Properties of Hastelloy X Fabricated Using Directed Energy Deposition

Laser-aided additive manufacturing is used for complex shapes and Ni-based superalloy parts. This study aimed to optimize the additive manufacturing process of Hastelloy X alloy to obtain its excellent mechanical properties without pores or cracks in the additively manufactured parts. The additively manufactured Hastelloy X was analyzed by comparing porosity, microstructure, and mechanical properties in as-built and post-heat treatment conditions. In addition, the pores existing inside the as-built specimen considerably decreased after the hot isostatic press (HIP) treatment. Furthermore, cell/columnar microstructures were observed owing to a fast cooling rate in the as-built condition. However, after heat treatment, dendrite structures disappeared, and recrystallized equiaxed grains were observed. The tensile test results showed that there was mechanical anisotropy along the vertical and horizontal directions, and as the microstructure changed to equiaxed grains after heat treatment, the mechanical anisotropy decreased, and the high-temperature properties improved.

Mechanisms of secondary phases formation and resultant mechanical properties of a beta-type Ti-13.3Mo-7.2Al–4Zr-0.25Ni alloy fabricated by laser aided additive manufacturing

The microstructures of Ti alloys are sensitive to the thermal history during the laser aided additive manufacturing (LAAM) process, which inevitably affects the mechanical properties. The dimension of the deposited part is one of the important factors influencing the heat accumulation and dissipation. In this work, the effect of sample size on the LAAM-built β-titanium Ti-13.3Mo-7.2Al–4Zr-0.25Ni (wt.%) was investigated. The results demonstrate that the vertical sample with smaller cross-section area has larger grain size than the horizontal sample with larger cross-section area. In addition, three types of secondary phases, i.e., Ti3Al, Ti2Ni and TiNi, are formed in the vertical sample due to thermal accumulation caused by the smaller cross-section area. The formation of the secondary phases leads to higher Vickers hardness and brittle failure of the vertical sample. However, the horizontal sample with no secondary phase demonstrates excellent combination of high yield strength (923 MPa) and low elastic modulus (68.7 GPa), which is beneficial for structural and biomedical application. Furthermore, the low modulus is mainly ascribed to the crystallographic texture along <001> direction.

Improvement of mechanical properties through inhibition of oxidation by adding TiC particles in laser aided additive manufacturing of stainless steel 316L

Additive manufacturing has gained significant attention in recent years. However, oxidation is hard to be avoided especially for the directed energy deposition (DED) processes, which are normally operated in an open environment with inert gas shielding. Excessive oxidation will negatively affect the mechanical properties of the deposited materials. In this work, inhibition of oxidation was studied through adding TiC particles for laser aided additive manufacturing (LAAM) of stainless steel 316L. Notably, oxygen content can be reduced significantly, as TiC particles functioned as deoxidizers through chemical reaction with oxygen. Meanwhile, the large Si–Cr–Mn oxides can be changed into uniformly distributed fine TiO2 in the deposited stainless steel 316L. Consequently, ductility of the deposited stainless steel 316L was significantly improved by 54.1% with 5.6% increase in yield strength and 11.4% increase in ultimate tensile strength. This study shows a new way to overcome the negative effects of oxidation on the properties of the additively manufactured materials.

Study of the intrinsic mechanisms of nickel additive for grain refinement and strength enhancement of laser aided additively manufactured Ti–6Al–4V

It is well-known that grain refiners can tailor the microstructure and enhance the mechanical properties of titanium alloys fabricated by additive manufacturing (AM). However, the intrinsic mechanisms of Ni addition on AM-built Ti–6Al–4V alloy is not well established. This limits its industrial applications. This work systematically investigated the influence of Ni additive on Ti–6Al–4V alloy fabricated by laser aided additive manufacturing (LAAM). The results showed that Ni addition yields three key effects on the microstructural evolution of LAAM-built Ti–6Al–4V alloy. (a) Ni additive remarkably refines the prior-β grains, which is due to the widened solidification range. As the Ni addition increased from 0 to 2.5 wt. %, the major-axis length and aspect ratio of the prior-β grains reduced from over 1500 μm and 7 to 97.7 μm and 1.46, respectively. (b) Ni additive can discernibly induce the formation of globular α phase, which is attributed to the enhanced concentration gradient between the β and α phases. This is the driving force of globularization according to the termination mass transfer theory. The aspect ratio of the α laths decreased from 4.14 to 2.79 as the Ni addition increased from 0 to 2.5 wt. %. (c) Ni as a well-known β-stabilizer and it can remarkably increase the volume fraction of β phase. Room-temperature tensile results demonstrated an increase in mechanical strength and an almost linearly decreasing elongation with increasing Ni addition. A modified mathematical model was used to quantitatively analyze the strengthening mechanism. It was evident from the results that the α lath phase and the solid solutes contribute the most to the overall yield strength of the LAAM-built Ti–6Al–4V–xNi alloys in this work. Furthermore, the decrease in elongation with increasing Ni addition is due to the deterioration in deformability of the β phase caused by a large amount of solid-solution Ni atoms. These findings can accelerate the development of additively manufactured titanium alloys.

In-process adaptive dimension correction strategy for laser aided additive manufacturing using laser line scanning

Additive manufacturing (AM) technologies have seen rapid growth in the past decade. Achieving high-quality consistency and accuracy remains a challenge in the fabrication of large-format metallic parts using the directed energy deposition AM processes. An efficient dimension correction strategy is required to prevent build failure during the AM process. In this paper, a laser line scanning sensor was integrated into a robot-based laser aided additive manufacturing (LAAM) system to realise the on-machine measurement of the part geometry. With this system, an in-process adaptive dimension correction strategy was proposed. The dimensional deviations in the intermediate layers could be corrected immediately after they were detected during the LAAM process, thus avoiding excessive dimensional deviation leading to build failure. A tool-path generation process for dimension correction was proposed which did not rely on traditional time-consuming CAD reconstruction. Only 3D point cloud was used directly, enabling the quick response of the LAAM system in restoring the dimensional accuracy. The deviated surface could be automatically filled up, and the subsequent deposition processes were resumed after each cycle of the dimension correction. To facilitate the proposed dimension correction strategy, a Robot Operating System (ROS)-based software platform was developed. Experimental comparisons between the part built with and without correction were conducted. The results demonstrated a significant improvement in dimensional accuracy when the correction strategy was applied.

Effect of cyclic heat treatment on microstructure and mechanical properties of laser aided additive manufacturing Ti–6Al–2Sn–4Zr–2Mo alloy

Globular α phases can significantly improve the ductility of titanium alloys. Cyclic heat treatment (CHT) has been proved to be an effective way to induce the formation of globular α phases in α+β titanium alloy Ti–6Al–4V fabricated by laser aided additive manufacturing (LAAM). However, there is no prior research reporting methods for obtaining globular α phases in LAAM-built near-α titanium alloys. This work investigated the cyclic heat treatment (CHT) procedures suitable for the LAAM-built near-α titanium alloy Ti–6Al–2Sn–4Zr–2Mo (Ti6242) to attain the globular α phases. The results show that 980 ​°C is the most suitable upper temperature limit for CHT. However, it is difficult to achieve a high volume fraction of the globular α phases in the LAAM-built Ti6242 alloys through CHT, which is ascribed to the low composition gradient caused by more α-stabilizing elements and fewer β-stabilizing elements. The as-built sample demonstrated elongation of 6.3%, which is lower than the AMS 4919J standard (elongation ≥10%). After 980 ​°C CHT and 980 ​°C CHT with solution heat-treatment, the formation of the globular α phases significantly increased the elongation to 13.5% and 12.9%, respectively. Although the mechanical strength is reduced after heat-treatment, the room-temperature tensile properties still exceed the AMS 4919J standard. Fractography examination showed that the as-built sample exhibited a mixed brittle and ductile fracture behavior, while the 980 ​°C CHT and 980 ​°C CHT with solution heat-treated samples displayed ductile fracture.

Laser aided additive manufacturing of spatially heterostructured steels

Current heterostructured materials demonstrate the high potential to circumvent strength-ductility tradeoff but face challenges with unconfigurable hetero-zone distribution and mechanical anisotropy. This study explored laser aided additive manufacturing (LAAM) of non-laminar spatially heterostructured materials (SHM) with configurable architectures to combine superior properties from the constitutive AISI 420 stainless steel and C300 maraging steel to enhance the overall properties. It is proven that the hatch spacing (h) has a significant effect on the microstructural evolutions and mechanical properties of the SHMs as it affects the layer thickness and inter-dilution regions. The mechanical properties of the SHMs were evaluated at multi-scales. The sample with h of 1.5 mm possesses a high tensile strength of about 1.6 GPa along with a reasonable break elongation of 8.1%, showing a good strength-ductility combination. Micropillar compression tests were conducted to measure localised mechanical properties, which is critical to understand the strengthening mechanism. Additionally, the SHM substantiates a much higher strength than many lamellar and linear functionally graded materials reported in the literature. This can be explained by the rule-of-mixture effect and hetero-deformation induced strengthening (HDIS). Furthermore, in-situ deformation observation reveals multiple deformation bands in SHM, which delays necking and contributed to ductility in tandem with the transformation induced plasticity (TRIP) effect. The findings highlight a novel approach for the development of SHM with a tunable performance by using LAAM of multiple materials following the configurable architectures.

An evolutional algorithm for automatic 2D layer segmentation in laser-aided additive manufacturing

Path planning is an important task in laser aided additive manufacturing (LAAM). The sliced 2D layers usually need to be partitioned into sub-regions such that appropriate filling toolpaths can be designed for different sub-regions. However, reported approaches for 2D layer segmentation generally require manual interaction that is tedious and time-consuming. To increase segmentation efficiency, this paper proposes an autonomous approach based on evolutional computation for 2D layer segmentation. The algorithm works in an identify-and-segment manner. Specifically, the largest quasi-quadrilateral is identified and segmented from the target layer iteration by iteration. Results from case studies have validated the effectiveness and efficacy of the developed algorithm. To further improve its performance, a roughing-finishing strategy is proposed. Via multi-processing, the strategy can remarkably increase the solution variety without significant sacrifice of solution quality and search time, thus providing great application potential in LAAM path planning. The developed segmentation algorithm has also been experimentally validated by comparing with the benchmarking toolpath generated by Powermill. With the developed segmentation algorithm, the quality of the deposited samples could be significantly improved. To the best of the authors’ knowledge, this work is the first to address automatic 2D layer segmentation problem in LAAM process. Therefore, it can be a valuable supplement to the state of the art in this area.

Superior strength-ductility in laser aided additive manufactured high-strength steel by combination of intrinsic tempering and heat treatment

ABSTRACT This work investigated laser aided additive manufacturing (LAAM) high-strength steel by leveraging the intrinsic tempering effect to facilitate the formation of high-fraction of metal carbides (e.g. M23C6 and M7C3) in the as-built samples. The intrinsic tempering effect contributes to a superior mechanical property than traditional manufacturing methods in as-built condition, promoting subsequent heat treatments (HTs) for excellent mechanical properties. The influence of HTs on the microstructures and mechanical properties were characterised in multi-scales. A large number of carbides are intrinsically formed due to the tempering effect during deposition. The high-density dislocations in the as-built sample facilitate the formation of massive nano-twins and carbides during HT. The HTed sample achieves a true tensile stress of about 1.81 GPa together with a true strain of about 21%, achieving an excellent strength-ductility combination compared to wide-range high-strength steels processed by additive manufacturing and conventional methods. The grain and twin boundaries strengthening, precipitation strengthening and dislocation strengthening contribute to the high strength, while the good ductility originates from twinning induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects, and high work-hardening rate, during deformation. The findings imply a potential way to develop AM-customised materials by fully understanding and utilising the IHT effect.

Integrated numerical modelling and deep learning for multi-layer cube deposition planning in laser aided additive manufacturing

ABSTRACT Heat accumulation is a critical problem in continuous multi-layer laser aided additive manufacturing (LAAM) process, resulting in inhomogeneous mechanical properties and non-uniformity in the deposited height which can deteriorate the deposition process. This work presents a new integrated finite element (FE) simulation and machine learning approach to select a multi-layer laser infill toolpath planning strategy for fabricating quadrilateral parts to minimise localised heat accumulation during the deposition process. After one layer deposition simulation, the approach employs a Temperature-Pattern Recurrent Neural Networks (TP-RNN) model to predict the temperature field after the next layer deposition for each of the candidate infill toolpaths, and a process parameters inspired thermal field evaluation method to select the best candidate toolpath. The approach would significantly improve the computational efficiency of the laser infill toolpath planning, which was validated by improving the flatness of the 20-layer cube deposition samples with two dimensions (20 mm × 20 mm and 30 mm × 30 mm).

Influence of oxides on the cryogenic tensile properties of the laser aided additive manufactured CoCrNi medium entropy alloy

The CoCrNi medium entropy alloy with different amounts of oxides was fabricated by laser aided additive manufacturing (LAAM). The cryogenic tensile properties and microstructure evolution during tensile deformation were investigated. For the Sample B with higher oxide content (2.03 vol%), the yield strength (YS), ultimate tensile strength (UTS) and elongation (El) were all inferior to those of the Sample A with much fewer oxides (0.47 vol%). The lower YS of Sample B was mainly attributed to the lower initial dislocation density. The oxides contributed slightly to the increase in YS, while reducing the El significantly. The El of Sample B was comparable at 298 K and 143 K, owing to the compensation effect from twin boundaries (TBs). Higher YS and UTS were obtained at 143 K for both samples. With decreasing temperature from 298 K to 143 K, the YS and UTS of Sample A increased almost linearly, whereas the El decreased. Though a large amount of TBs were formed during the tensile deformation, they were unevenly distributed among the grains near the fractured location. Under higher stress at cryogenic temperature, the interaction of grain boundaries with massive TBs caused micro-voids to initiate more readily along the grain boundaries, resulting in premature failure.

Multiphysics Modeling, Sensitivity Analysis, and Optical Performance Optimization for Optical Laser Head in Additive Manufacturing

Optical laser head is a key component used to shape the laser beam and to deliver higher power laser irradiation onto workpieces for material processing. A focused laser beam size and optical intensity need to be controlled to avoid decreasing beam quality and loss of intensity in laser material processing. This paper reports the multiphysics modeling of an in-house developed laser head for laser-aided additive manufacturing (LAAM) applications. The design of computer experiments (DoCE) combined with the response surface model was used as an efficient design approach to optimize the optical performance of a high power LAAM head. A coupled structural-thermal-optical-performance (STOP) model was developed to evaluate the influence of thermal effects on the optical performance. A number of experiments with different laser powers, laser beam focal plane positions, and environmental settings were designed and simulated using the STOP model for sensitivity analysis. The response models of the optical performance were constructed using DoCE and regression analysis. Based on the response models, optimal design settings were predicted and validated with the simulations. The results show that the proposed design approach is effective in obtaining optimal solutions for optical performance of the laser head in LAAM.

Microstructure and mechanical behavior of laser aided additive manufactured low carbon interstitial Fe49.5Mn30Co10Cr10C0.5 multicomponent alloy

Laser aided additive manufacturing (LAAM) was used to fabricate bulk Fe49.5Mn30Co10Cr10C0.5 interstitial multicomponent alloy using pre-alloyed powder. The room temperature yield strength (σy), ultimate tensile strength (σUTS) and elongation (εUTS) were 645 MPa, 917 MPa and 27.0 % respectively. The as-built sample consisted of equiaxed and dendritic cellular structures formed by elemental segregation. These cellular structures together with oxide particle inclusions were deemed to strengthen the material. The other contributing components include dislocation strengthening, friction stress and grain boundary strengthening. The high εUTS was attributed to dislocation motion and activation of both twinning and transformation-induced plasticity (TWIP and TRIP). Tensile tests performed at −40 °C and −130 °C demonstrated superior tensile strength of 1041 MPa and 1267 MPa respectively. However, almost no twinning was observed in the fractured sample tested at −40 °C and −130 °C. Instead, higher fraction of strain-induced hexagonal close-packed (HCP) ε phase transformation of 21.2 % were observed for fractured sample tested at −40 °C, compared with 6.3 % in fractured room temperature sample.

Additive manufacturing of copper-tool steel dissimilar joining: Experimental characterization and thermal modeling

In high-pressure die casting applications, efficient cooling of the die/mold is critical to reduce cycle time. Tool steel such as H13 is widely adopted to make these components due to its excellent mechanical properties. However, the low thermal conductivity of tool steel largely impedes its cooling capability. Therefore, joining tool steel with copper to enhance the overall thermal conductivity is critical. In this study, pure copper was deposited on H13 substrates using a laser-aided additive manufacturing process. Depositing Cu on H13 directly and via interlayers of a nickel-based alloy Deloro 22 (D22) were employed. Both experimental characterization and computational analysis were conducted to study the properties of these structures. The results highlight the cracking failure in direct depositing copper on H13 while these cracks have been successfully eliminated when the D22 interlayers were introduced. Evidence of strong bonding at the Cu/D22 and D22/H13 interfaces was obtained. A sharp drop of hardness was observed over the Cu/H13 direct joint interface, while this descend was largely smoothed when D22 was inserted as interlayers. The directly joined Cu/H13 structure has a high longitudinal tensile residual stress (~385 MPa) at the Cu/H13 interface. By adding D22 interlayers, a reduced longitudinal tensile residual stress (~192 MPa) is obtained at the Cu/D22 interface. D22 interlayers have mitigated residual stress at the dissimilar joints.

Excellent combination of strength and ductility of CoCrNi medium entropy alloy fabricated by laser aided additive manufacturing

In this study, the CoCrNi medium entropy alloy (MEA) was successfully fabricated by laser aided additive manufacturing (LAAM), a directed energy deposition (DED) process. Results indicate that the ultimate tensile strength (UTS), yield strength (YS), and elongation after fracture (δF) are 873.5 MPa, 620.5 MPa, and 44.8 % respectively. Without any post heat treatment, the as-built state CoCrNi MEA exhibits an excellent combination of strength and ductility. The high YS is mainly attributed to the combination effects from lattice friction stress, boundary strengthening, and dislocation strengthening. Deformation twinning is activated during room temperature tensile deformation, inducing the steady work hardening behavior. Hence, fracture is postponed to higher deformation strain due to the synergistic effect of dislocation gliding and deformation twinning. LAAM is expected to be a versatile and efficient method to fabricate CoCrNi MEA and some other high-performance alloys.

Thermal field prediction for laser scanning paths in laser aided additive manufacturing by physics-based machine learning

Laser aided additive manufacturing (LAAM) is a key metal 3D printing and remanufacturing technology for fabrication of near-net shape parts. Studying thermal field induced by different scanning strategies is important to evaluate and optimize the resultant residual stress and distortion distribution. However, it is very computationally expensive to simulate multi-bead deposition process using existing numerical model to analyze and select appropriate laser scanning strategies. In this paper, we make use of a recently developed and experimentally validated efficient thermal field prediction numerical model for LAAM to generate training data for a physics-based machine learning algorithm. A combined Recurrent Neural Networks and Deep Neural Networks (RNN–DNN) model was developed to identify the correlation between laser scanning patterns and their corresponding thermal history distributions. Subsequently, the developed RNN–DNN model is able to make thermal field prediction for an arbitrary geometry with different scanning strategies. Comparison between the numerical simulation results and the RNN–DNN predictions showed good agreement of more than 95%.

Comparison of carbon-based reinforcement on laser aided additive manufacturing Inconel 625 composites

Abstract In this paper, a comparison study of CNTs and graphite addition in laser aided additive manufacturing (LAAM) Inconel 625 composites was proposed. The difference of microstructure and tensile properties between two modified Inconel 625 alloys was investigated. SEM, EDS and micro-Raman identification results showed that the CNTs structure can be found on columnar dendrite boundaries accompanied with precipitated Laves and γ′ phase in LAAM Inconel 625-CNTs composite. Based on tensile test results, 0.25 wt% of CNTs addition in LAAM Inconel 625 can significantly increase strength and ductility by 20% because of grain refinement and grain boundary pinning reinforcement. However, the graphite addition only raises strength but decreases ductility due to carbon content raise.