Laser Additive Manufacturing Process Research Articles

Laser additive manufacturing of ductile Fe-based bulk metallic glass composite

• Fe-based BMGCs with high strength and record-breaking ductility was fabricated via LAM technique. • An appropriate amount of stainless steel can effectively ductile the Fe-based BMG without seriously sacrificing strength. • Dual-phase structure was achieved by simply transporting the second phase into laser molten pool in the form of powder during LAM process. Laser additive manufacturing (LAM) is a promising technology for processing bulk metallic glass (BMG) with freeform geometries or unlimited size. However, the inherently brittle Fe-based BMGs produced by LAM have always been plagued by the micro-cracking induced by the large thermal stresses during the LAM process. To solve this dilemma, 316L stainless steel (SS) with excellent toughness and similar elemental composition was carefully selected as the second phase to form Fe-based BMG composites (BMGCs). The obtained Fe-based BMGCs are equipped with a heterogeneous structure, i.e., the 316L SS phase is wrapped by the metallic glass and forms a unique "fishbone" structure with a micron - scale. Excitedly, the special structure nicely improves a plastic strain of the Fe-based BMGC with a strength of 2355 MPa to ∼17%, achieving a record-breaking achievement among Fe-based amorphous with critical dimensions over 1mm.

Laser additive manufacturing of Zr-based bulk metallic glass composite improved by TRIP effect

Laser additive manufacturing (LAM), as a fascinating technique, is equipped with the potential to fabricate bulk metallic glasses (BMGs) with arbitrary geometries. However, the serious thermal stress induced by the LAM process always causes the harmful cracks to be left inside the Zr-based BMGs with brittleness characteristics, which will lead to a sharp deterioration in fracture toughness. To solve this dilemma, B2 phase with excellent plasticity, realizing work-hardening through martensitic transformation, was precipitated during the cooling process of Zr 48 Cu 46.5 Al 4 Nb 1.5 BMG composite (BMGC). The deposited Zr-based BMGCs possess composite structures, i.e ., B2 phase is distributed in the partially crystallized glassy matrix. The TRIP effect provided by B2 phase can not only inhibit the generation of cracks, but also effectively avoid premature shearing-off through the sample to greatly improve the fracture strength.

A laser-shock-enabled hybrid additive manufacturing strategy with molten pool modulation of Fe-based alloy

A novel hybrid laser additive manufacturing (LAM) process based on laser shock modulation of molten pool (LSMMP) is proposed in this work. By using a combination of experiment and simulation, the residual stress, heat transfer, and mass transfer behaviors are comprehensively studied. Also, microstructure evolution behavior is analyzed. The relationship between the convection behavior, the evolution of microstructure and the enhancement of residual stress induced by LSMMP is established. Compared with the traditional post-treatment process, the LSMMP assisted LAM process exhibits higher efficiency in residual stress control with the same pulsed laser energy input. The hidden mechanism in microstructure evolution and residual stress enhancement is expected to be possible related to intensified molten pool convection, uniform solute distribution and improved cooling rate induced by LSMMP. The hybrid LAM process strategy based on LSMMP provides a new approach for the application of short pulse laser in hybrid manufacturing.

The Influence of Intralayer Porosity and Phase Transition on Corrosion Fatigue of Additively Manufactured 316L Stainless Steel Obtained by Direct Energy Deposition Process.

A direct energy deposition (DED) process using wires is considered an additive manufacturing technology that can produce large components at an affordable cost. However, the high deposition rate of the DED process is usually accompanied by poor surface quality and inherent printing defects. These imperfections can have a detrimental effect on fatigue endurance and corrosion fatigue resistance. The aim of this study was to evaluate the critical effect of phase transition and printing defects on the corrosion fatigue behavior of 316L stainless steel produced by a wire laser additive manufacturing (WLAM) process. For comparison, a standard AISI 316L stainless steel with a regular austenitic microstructure was studied as a counterpart alloy. The structural assessment of printing defects was performed using a three-dimensional non-destructive method in the form of X-ray microtomography (CT) analysis. The microstructure was evaluated by optical and scanning electron microscopy, while general electrochemical characteristics and corrosion performance were assessed by cyclic potentiodynamic polarization (CCP) analysis and immersion tests. The fatigue endurance in air and in a simulated corrosive environment was examined using a rotating fatigue setup. The obtained results clearly demonstrate the inferior corrosion fatigue endurance of the 316L alloy produced by the WLAM process compared to its AISI counterpart alloy. This was mainly related to the drawbacks of WLAM alloys in terms of having a duplex microstructure (austenitic matrix and secondary delta-ferrite phase), reduced passivity, and a significantly increased amount of intralayer porosity that acts as a stress intensifier of fatigue cracking.

Wire Oscillating Laser Additive Manufacturing of 2319 Aluminum Alloy: Optimization of Process Parameters, Microstructure, and Mechanical Properties

In this study, a wire oscillating laser additive manufacturing (O-WLAM) process was used to deposit 2319 aluminum alloy samples. The optimization of the deposition process parameters made it possible to obtain samples with smooth surfaces and extremely low porosities. The effects of the deposition parameters on the formability and evolution of the microstructure and mechanical properties before and after heat treatment were studied. The oscillating laser deposition of 2319 aluminum alloy, especially the circular oscillation mode, significantly reduced the porosity and improved the process stability and formability compared with non-oscillating laser deposition. There were clear boundaries between the deposition units in the deposition state, the interior of which was dominated by columnar crystals with many rod- and point-shaped precipitates. After the heat treatment, the θ phase was significantly dissolved. The residual dot- and rod-shaped θ ' phases were dispersedly distributed, exhibiting an obvious precipitation-hardening effect. The samples in the as-deposited state had a tensile strength of 245–265 MPa, an elongation of approximately 12.6%, and an 87 HV microhardness. After heat treatment at 530°C for 20 h and aging at 175°C for 18 h, the tensile strength, elongation, and microhardness reached 425–440 MPa, approximately 10%, and 153 HV, respectively. The performance improved significantly without significant anisotropy. Compared with the samples produced by wire arc additive manufacturing (WAAM), the tensile strength increased by approximately 10%, and the strength and microhardness were significantly improved.

Finite element analysis of thermo-mechanical behavior of a multi-layer laser additive manufacturing process

Finite element analysis of thermo-mechanical behavior of a multi-layer laser additive manufacturing process

Cross-Scale Simulation Research on the Macro/Microstructure of TC4 Alloy Wire Laser Additive Manufacturing

A cross-scale model of macro-micro coupling is established for the wire laser additive manufacturing process of the TC4 titanium alloy. The model reproduces the dynamic evolution process of the molten pool shape, reveals the temperature change law in the molten pool, and simulates the microstructure and morphology of different regions of the molten pool. Finally, the model is used to quantitatively analyze the effects of process parameters (laser power, scanning speed) on the growth morphology of dendrites during solidification. The research shows that with the increase in laser power and the decrease in scanning speed, the peak temperature of the molten pool increases rapidly, and the size of the molten pool increases gradually. When the laser scanning speed is greater than 5 mm/s, the molten pool length decreases significantly. After solidification, an asymmetrically distributed equiaxed grain structure is formed at the upper part of the molten pool, the bottom of the molten pool is made up of slender columnar crystals, and the columnar-to-equiaxed transition (CET) occurs in the middle of the molten pool. With the decrease in laser power and the increase in scanning speed, the growth rate of dendrites becomes faster, the arm spacing and the overall morphology of dendrites become smaller, and the arrangement of columnar crystals have a tighter microstructure.

Effect of Molten Pool Spatial Arrangement on Texture Evolution in Pulsed Laser Additive Manufacturing of Inconel 718.

The epitaxial growth of dendrites, which often results in a strong texture, is the most common phenomenon during the laser additive manufacturing process. In this study, the epitaxial growth of dendrites and texture evolution in three directions were studied by changing the z-increment, pulse period, and track offset, respectively. The influence of the molten pool interface on the growth and competition of dendrites is analyzed. Both green grains (<110> // BD) with rotated cube texture in the molten pool overlapping zones and red grains (<100> // BD) with fiber texture in the molten pool center zones coexist for different z-increment samples, forming the typical sandwich texture feature. In a short pulse period, the dendrites can grow directly epitaxially and form the strong fiber texture due to gentle interface and short distance. With the decrease of the track offset, the molten pool morphology changes from flat to narrow and deep. When θ is close to 90°, dendrites grow along the secondary dendrite arms at the overlapping zone, forming V-shape grains. This work also provides a promising method for texture customization for laser additive manufacturing.

Effect of laser intensity profile on the microstructure and texture of Inconel 718 superalloy fabricated by direct energy deposition

Epitaxial growth of dendrites often results in a strong texture, which is a common phenomenon in a laser additive manufacturing process. In this study, pulsed lasers with flat-top and Gaussian intensity profiles were used to modulate the molten pool morphology and the solidification texture with a unidirectional scanning strategy. The melt pool is planar for the flat-top mode and it turns arc-shaped for the Gaussian mode. Long columnar grains are uniformly distributed and a strong fiber texture with a weak cube texture is obtained for the flat-top mode sample due to a uniform heat flux through many deposition layers. In contrast, large grains and fine grains are alternately arranged for Gaussian mode sample. The large grains exhibit a strong rotated cubic texture with a V-pattern in the overlapping area of the melt pool due to side-branching from the curved melt pool boundary; while the fine grains exhibit a weak fiber texture in the center of melt pool with a less restriction of the other two crystallographic axes perpendicular to the build direction. After heat treatment, the intensity of the textures of both samples is reduced. More recrystallized grains with twin boundaries are found under Gaussian mode due to the accumulated residual strain. This work manifests a promising method for texture customization by manipulating the heat flux strategy for laser additive manufacturing.

Modelling and simulation of the fatigue usage factor of γ-TiAl alloy fabricated through Laser Additive Manufacturing (LAM)

Recently, laser additive manufacturing (LAM) technologies are increasingly being applied for producing components with excellent physical and mechanical properties in the aerospace, automotive and energy industries. This work is aimed at modelling the fatigue usage factor of γ-TiAl alloy fabricated through LAM. The modelling and simulation were performed using the COMSOL Multiphysics 5.4 software by developing a y-TiAl alloy microstructure. This was modelled to generate the material properties (density, heat capacity at constant pressure and thermal conductivity) from the microstructure of a unit cell as a general representation of the alloy. The computed properties were used in modelling the LAM process to fabricate γ-TiAl alloy part with subsequent fatigue simulation to determine the usage factor (Ke). From the models, the maximum Von Mises stress and transient temperature were 2.88 x108 Nm-2 and 1510 K respectively, for the LAM fabrication process; while the fatigue usage factor model showed a maximum Von Mises stress of 2.91 x108 Nm-2 and a fatigue usage factor of 0.35.

Low-carbon Modeling and Process Parameter Optimization in Laser Additive Manufacturing Process

Low-carbon Modeling and Process Parameter Optimization in Laser Additive Manufacturing Process

Heat accumulation in laser Additive Manufacturing processes and its effect on local solidification

Laser based additive manufacturing creates parts layer-by-layer and offers increased design freedom in comparison to traditional manufacturing. However, the repeated application of energy frequently revisits previous deposited layers, which leads to heat accumulation and may result in microstructural inhomogeneity and anisotropic mechanical properties. Here we present a theoretical model which is applied to capture the heat evolution and accumulation during laser direct energy deposition and laser powder bed fusion processes. Infrared imaging from literature is used to validate the model. The model captures the effect of the consecutive deposition of the individual layers and shows its influence on heat accumulation. It demonstrates that heat accumulation leads to noticeable change in local solidification rate and temperature gradient, which results in significant variation in local grain structure. The predictions of the model allows for the development of process optimization strategies for the control of local microstructure evolution.

Microstructure and mechanical properties of transition zone in laser additive manufacturing of TC4/AlSi12 bimetal structure

Ti/Al bimetallic structure (BS) has a good development prospect and broader application potential in aerospace engineering. Considering the limitation that dissimilar welding is only applicable to the thin plate, it is necessary to explore a new manufacturing process for Ti/Al BS. In this study, a TC4/AlSi12 BS was prepared by laser additive manufacturing (LAM). TC4 zone, AlSi12 zone and transition zone were formed in the LAM process. Due to the sufficient diffusion reaction, the transition zone with a width of about 0.8 mm was obtained. At the same time, a few micro-cracks were found in the transition zone. The microstructure and phase composition of the transition zone had been emphatically studied. Research results showed that the presence of Si element made the phase composition of the transition zone more complicated. The structure evolution from TC4 to AlSi12 was: α-Ti → Ti3Al → TiAl+(TiAl+Si) → Ti5Si3 → TiAl3+(α-Al+Si) → α-Al+Si+TiAl3+(α-Al+Si) → α-Al+Si+(α-Al+Si). The hardness distribution of BS was uneven, with the highest value reaching 524 HV. The tensile strength of the TC4/AlSi12 BS was about 110Mpa, and the fracture location was located in the transition zone.

Study on the Effects of Laser Additive Manufacturing Process on Heat Source Model Coefficients by Integrating Experimental and Numerical Models

Study on the Effects of Laser Additive Manufacturing Process on Heat Source Model Coefficients by Integrating Experimental and Numerical Models

Optimizing Laser Additive Manufacturing Process for Fe-Based Amorphous Magnetic Materials

Optimizing Laser Additive Manufacturing Process for Fe-Based Amorphous Magnetic Materials

Four-Dimensional Stimuli-Responsive Hydrogels Micro-Structured via Femtosecond Laser Additive Manufacturing.

Rapid fabricating and harnessing stimuli-responsive behaviors of microscale bio-compatible hydrogels are of great interest to the emerging micro-mechanics, drug delivery, artificial scaffolds, nano-robotics, and lab chips. Herein, we demonstrate a novel femtosecond laser additive manufacturing process with smart materials for soft interactive hydrogel micro-machines. Bio-compatible hyaluronic acid methacryloyl was polymerized with hydrophilic diacrylate into an absorbent hydrogel matrix under a tight topological control through a 532 nm green femtosecond laser beam. The proposed hetero-scanning strategy modifies the hierarchical polymeric degrees inside the hydrogel matrix, leading to a controllable surface tension mismatch. Strikingly, these programmable stimuli-responsive matrices mechanized hydrogels into robotic applications at the micro/nanoscale (<300 × 300 × 100 μm3). Reverse high-freedom shape mutations of diversified microstructures were created from simple initial shapes and identified without evident fatigue. We further confirmed the biocompatibility, cell adhesion, and tunable mechanics of the as-prepared hydrogels. Benefiting from the high-efficiency two-photon polymerization (TPP), nanometer feature size (<200 nm), and flexible digitalized modeling technique, many more micro/nanoscale hydrogel robots or machines have become obtainable in respect of future interdisciplinary applications.

Multiscale simulation of rapid solidification of an aluminium–silicon alloy under additive manufacturing conditions

At present, most multiscale simulation approaches to model the temperature evolution of the molten pool, and the resulting microstructure evolution for selective laser melting, assume an equilibrium freezing range and steady state solidification conditions. This is despite the solidification conditions being observed to be highly unsteady and non-equilibrium. These two assumptions lead to inaccurate predictions of the temperature evolution of the molten pool and thus microstructure predictions. To demonstrate this, an approach to scale-bridging computational models of the laser additive manufacturing process is presented, in which the temperature history is passed from a macroscale molten pool simulation to a microscale phase-field simulation. This linkage is achieved by volume mapping of the temperature field from the grid of the molten pool simulation to the grid of the microstructure simulation. To describe the system evolution at the scale of the molten pool, a computational fluid dynamics (CFD) method that captures the laser–metal interaction, vapour production, gas recoil pressure, fluid flow, surface tension, Marangoni flow, and heat conduction, convection, and radiation is applied. To capture the chemical kinetics of the phase-transition, a non-equilibrium CALPHAD-integrated phase-field (PF) model is applied. The discrepancy between the predictions of the solid front isotherm is quantified as ⩾ 100 K for an Al-10Si alloy under the large observed cooling rate. This leads to a spatial discrepancy in the solidification front between the CFD model, which assumes equilibrium freezing behaviour, and the PF model, which does not, of approximately 10 µm over 50 µs in the present case. Under these conditions, present formulations of multiphase CFD cannot accurately predict the solidification behaviour because of the assumption of equilibrium at the solid–liquid interface. Strategies for reconciling this discrepancy for materials that exhibit rapid solidification under large thermal undercooling will need to be developed for multiscale simulation of additive manufacturing to advance.

The Effect of a Slow Strain Rate on the Stress Corrosion Resistance of Austenitic Stainless Steel Produced by the Wire Laser Additive Manufacturing Process

The wire laser additive manufacturing (WLAM) process is considered a direct-energy deposition method that aims at addressing the need to produce large components having relatively simple geometrics at an affordable cost. This additive manufacturing (AM) process uses wires as raw materials instead of powders and is capable of reaching a deposition rate of up to 3 kg/h, compared with only 0.1 kg/h with common powder bed fusion (PBF) processes. Despite the attractiveness of the WLAM process, there has been only limited research on this technique. In particular, the stress corrosion properties of components produced by this technology have not been the subject of much study. The current study aims at evaluating the effect of a slow strain rate on the stress corrosion resistance of 316L stainless steel produced by the WLAM process in comparison with its counterpart: AISI 316L alloy. Microstructure examination was carried out using optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction analysis, while the mechanical properties were evaluated using tensile strength and hardness measurements. The general corrosion resistance was examined by potentiodynamic polarization and impedance spectroscopy analysis, while the stress corrosion performance was assessed by slow strain rate testing (SSRT) in a 3.5% NaCl solution at ambient temperature. The attained results highlight the inferior mechanical properties, corrosion resistance and stress corrosion performance, especially at a slow strain rate, of the WLAM samples compared with the regular AISI 316L alloy. The differences between the WLAM alloy and AISI 316L alloy were mainly attributed to their dissimilarities in terms of phase compositions, structural morphology and inherent defects.

Investigation of molten pool dynamic behaviors in laser additive manufacturing by numerical simulation

Laser additive manufacturing (LAM) can provide innovative approaches for designing a lightweight product with load-adapted structures and be widely used in the automotive and aerospace industries. A numerical model taking into account wire feeding and laser absorption is developed in this paper to investigate the dynamic behaviors of a mixing molten pool in the LAM process. Based on this model, flow field distribution and the evolution of molten pool are numerically calculated. The surface of the mixing molten pool during the LAM process is tracked and the evolution of the molten pool is analyzed. The droplet transfer mode of molten wire adding into the molten pool and the influence of Marangoni convection on the surface evolution of the molten pool are discussed by way of simulation results. It is found that the molten pool dynamic behaviors have great influence on the morphology of the formed layer after molten metal solidification in the molten pool. The calculated height and width of the formed layer are validated by the experimental results and good agreement between them is found. Therefore, the proposed method is of importance for controlling the molten metal flowing in the molten pool and the morphology of the formed layer in LAM.

Prediction of solidification microstructure of titanium aluminum intermetallic alloy by laser surface remelting

By controlling the processing parameters in the laser additive manufacturing (LAM) process, the mechanical properties of titanium aluminum alloy could be modified. However, the difficulty of optimizing the solidification microstructure is to build the relationship between the microstructure and processing parameters. To solve this problem, a numerical model of combined processing parameters with dendrite arm spacing was developed in this paper to predict the solidification microstructure and reveal the effects of laser processing parameters on dendrite arm spacing. A geometry factor (α) was first introduced in the prediction model to correct the simulated molten pool shape to improve prediction accuracy. Moreover, the nonequilibrium solidification of LAM due to fast cooling was taken into consideration in this study. Based on the model, primary dendrite arm spacing (PDAS) and secondary dendrite arm spacing (SDAS) in laser surface remelting of Ti-47Al-2Cr-2V alloy were investigated in combination with simulations and experimental observations. The experimental results indicate that the model calculated using average solidification conditions applies well in predicting the solidification microstructure of titanium aluminum alloy. The dendrite arm spacing slightly decreases at low scanning velocity but increases at high scanning velocity as the laser power increases. In addition, the scanning velocity plays a dominant role in the change in dendrite arm spacing when the scanning velocity is lower than 600 mm/min, and the impact of laser power on dendrite arm spacing gradually increases as the scanning velocity increases.