Laser Rapid Solidification Research Articles

Mechanism for corrosion protection of β-TCP reinforced ZK60 via laser rapid solidification

It remains the primary issue to enhance the corrosion resistance of Mg alloys for their clinical applications. In this study, β-tricalcium phosphate (β-TCP) was composited with Mg-6Zn-1Zr (ZK60) using laser rapid solidification to improve the degradation behavior. Results revealed rapid solidification effectively restrained the aggregation of β-TCP, which thus homogenously distributed along grain boundaries of α-Mg. Significantly, the uniformly distributed β-TCP in the matrix promoted the formation of apatite layer on the surface, which contributed to the formation of a compact corrosion product layer, hence retarding the further degradation. Furthermore, ZK60/8β-TCP (wt. %) composite showed improved mechanical strength, as well as improved cytocompatibility. It was suggested that laser rapidly solidified ZK60/8β-TCP composite might be a potential materials for tissue engineering.

Enabling plastic co-deformation of disparate phases in a laser rapid solidified Sr-modified Al–Si eutectic through partial-dislocation-mediated-plasticity in Si

Nano-scale eutectics, such as rapid solidified Al–Si, exhibit enhanced yield strength and strain hardening but plasticity is limited by cracking of the hard phase (Si). Mechanisms that may suppress cracking and enable plastic co-deformation of soft and hard phases are key to maximizing plasticity in these high-strength microstructures. Using a combination of laser rapid solidification and chemical (Sr) modification, we have synthesized fully eutectic Al–Si microstructures with heavily twinned Si nano-fibers that exhibit high hardness up to 2.9 GPa, and high compressive flow strength (∼840 MPa) with stable plastic flow to ∼26% plastic strain. After deformation, the hard Si(Sr) fibers did not exhibit cracks, but a high density of stacking faults were observed in the Si(Sr) fibers suggesting partial dislocation mediated plasticity. Mechanisms for suppression of cracking and activation of partial dislocations in Si deformed at room temperature are discussed in terms of nanoscale fiber geometry with reduced aspect ratio and lowering of the Peierls barrier in chemically-modified, nano-twinned Si fibers.

Three-dimensional morphology of an ultrafine Al-Si eutectic produced via laser rapid solidification

Al-Si alloys processed by laser rapid solidification yield eutectic microstructures with ultrafine and interconnected fibers. Such fibrous structures have long been thought to bear resemblance to those formed in impurity-doped alloys upon conventional casting. Here, we show that any similarity is purely superficial. By harnessing high-throughput characterization and computer vision techniques, we perform a three-dimensional analysis of the branching behavior of the ultrafine eutectic and compare it against an impurity-modified eutectic as well as a random fractal (as a benchmark). Differences in the branching statistics point to different microstructural origins of the impurity- and quench-modified eutectic. Our quantitative approach is not limited to the data presented here but can be used to extract abstract information from other volumetric datasets, without customization.

Microstructural evolution and hardness of rapidly solidified hypereutectic Al-Si surface layers by laser remelting

ABSTRACT Selective laser melting technique is used to manufacture Al-25 wt.%Si and Al-30 wt.%Si surface layers with a thickness of 0.5–0.6 mm followed by a laser rapid solidification process to refine the silicon to a nano-size and increase the hardness. The as-produced alloyed layers were heterogeneous and composed of clustered aggregates of primary Si crystals of size 5–7 µm, decorated with α-Al cells/dendrites, and embedded in a matrix consisting of α-Al dendritic crystals and Al-Si eutectic. However, after several remelting scanning treatments at a fast speed and low power, significant refinement of all phases was achieved, and a dramatic increase in the proportion of the interwoven nano-sized Al-Si fibrous eutectic. The primary Si sizes were reduced to ≤1 µm, while the eutectic spacing decreased and approached as low as 10–15 nm at the highest cooling rate. The high cooling rate significantly reduced the size and the amount of the α-Al cells/dendrites especially those surrounding the primary Si, thereby stimulating ultrafine Al-Si fibrous eutectic to grow adjacent to the primary Si crystals and spread massively, which in turn contributed to the increase in hardness. Transmission electron microscopy revealed block-like primary Si, worm-like Si, and nanoscale fibrous eutectic Si, which was internally nano-twinned.

Microstructure and mechanical properties of Al-Fe-Sc-Zr alloy additively manufactured by selective laser melting

An Al-5Fe-1Mg-0.8Sc-0.7Zr (wt%) alloy was specifically designed for selective laser melting (SLM) additive manufacturing and atomized powder prepared. The design of this non-equilibrium alloy is based on: first, solid solubility of Fe element in Al alloy can be significantly enlarged during laser rapid solidification; second, Sc and Zr elements are used to refine the microstructure so as to prevent micro-crack and improve strength. At an optimized printing parameters, the SLM printed Al-Fe-Sc-Zr samples exhibited a highest density of 99.2% with rare intergranular cracks. Besides, a bimodal grain structure was observed, consisting of ultrafine equiaxed grain at molten pool boundary and coarse columnar grain inside the molten pool. At the boundary of molten pool, many intermetallic particles (Al 6 Fe and Al 13 Fe 4 ) appeared around supersaturated α-Al grains; while the inside of molten pool presented eutectic of α-Al and Al 6 Fe/Al 3 (Sc, Zr). Interestingly, a large number of stacking faults were observed around the precipitated particles by HRTEM, which are also conducive to strengthening of SLM printed Al-Fe-Sc-Zr sample. Thus, excellent tensile strength of 489 MPa was obtained at an optimized volumetric energy density (VED) of 70 J/mm 3 . The current research results have a certain guiding significance for the composition design and microstructure control of additive manufacturing aluminum alloys. • High Fe-content Al-Fe-Sc-Zr alloy was successfully printed by selective laser melting. • The microstructure is composed of supersaturated solid solution and fine precipitates. • An eutectic microstructure of α-Al and Al 6 Fe/Al3(Sc, Zr) phases was found. • YS and UTS of SLMed alloy reached 388 ± 14 MPa and 489 ± 6 MPa, respectively.

Dendritic Growth Transition During Laser Rapid Solidification of Nickel-Based Superalloy

The dendritic growth transition of nickel-based superalloy in the top region of the laser additive manufactured layer has been studied. The results show that the dendritic growth direction in the top region of the layer is changed from the perpendicular to parallel direction to the laser scanning direction. From the finite element simulation, the dendritic growth transition is found to be attributed to the increased deviation angle between the epitaxial dendritic growth direction and local thermal gradient in the top region of the molten pool. The critical transition angle is revealed to be 70°–80°. The origin of the top transition grains is mostly the re-nucleation sites generated in the last remaining liquid in the top region of the molten pool.

Mechanical Behavior of Al–Al2Cu–Si and Al–Al2Cu Eutectic Alloys

In this study, laser rapid solidification technique was used to refine the microstructure of ternary Al–Cu–Si and binary Al–Cu eutectic alloys to nanoscales. Micropillar compression testing was performed to measure the stress–strain response of the samples with characteristic microstructure in the melt pool regions. The laser-remelted Al–Al2Cu–Si ternary alloy was observed to reach the compressive strength of 1.59 GPa before failure at a strain of 28.5%, which is significantly better than the as-cast alloy with a maximum strength of 0.48 GPa at a failure strain of 4.8%. The laser-remelted Al–Cu binary alloy was observed to reach the compressive strength of 2.07 GPa before failure at a strain of 26.5%, which is significantly better than the as-cast alloy with maximum strength of 0.74 GPa at a failure strain of 3.3%. The enhanced compressive strength and improved compressive plasticity were interpreted in terms of microstructural refinement and hierarchical eutectic morphology.

Microstructure evolution and strengthening mechanism of laser-cladding MoFe CrTiWAlNb refractory high-entropy alloy coatings

Abstract The MoFexCrTiWAlNby refractory high-entropy alloys (HEAs) were designed and the correlative coatings were fabricated by laser cladding on a M2 high-speed steel. The microstructure and performance evolution at various annealing temperatures of 600 °C–800 °C for 4 h, was carefully investigated. The as-cladded coatings consist mainly of body-centered-cubic (BCC) solid solution, MC carbide and C14 Laves phase plus small amounts of un-melted W-particles. The phase structures remain basically unchanged although the contents of Fe and Nb are changing. Meanwhile, the as-cladded MoFe1.5CrTiWAlNb1 HEA coatings exhibit high nano- and micro-hardness due to severe lattice distortion, carbides reinforcement together with laser rapid solidification. Therefore, it was selected for high-temperature experiment, which indicates that the dendrites and MC carbides are unaltered even annealed at 800 °C. However, the C14-Laves precipitates begin to form in the BCC matrix while annealing exceeded 600 °C. The hardness and wear resistance of the coating annealed below 700 °C increased slightly. Especially for the 650 °C-annealed coating shows the highest hardness and most excellent wear resistance strengthened by C14 precipitates. Additionally, a dense and thermostable oxide film was formed in an atmospheric environment of 800 °C, indicating a better oxidation resistance, compared with the M2 steels.

Nano-SiC reinforced Zn biocomposites prepared via laser melting: Microstructure, mechanical properties and biodegradability

Zn has been regarded as new kind of potential implant biomaterials due to the desirable biodegradability and good biocompatibility, but the low strength and ductility limit its application in bone repairs. In the present study, nano-SiC was incorporated into Zn matrix via laser melting, aiming to improve the mechanical performance. The microstructure analysis showed that nano-SiC distributed along Zn grain boundaries. During the laser rapid solidification, nano-SiC particles acted as the sites for heterogeneous nucleation, which resulted in the reduction of Zn grain size from 250 μm to 15 μm with 2 wt% SiC (Zn-2SiC). Meanwhile, nano-SiC acted as a reinforcer by virtue of Orowan strengthening and dispersion strengthening. As a consequence, the nanocomposites showed maximal compressive yield strength (121.8 ± 5.3 MPa) and high microhardness (72.24 ± 3.01 HV), which were increased by 441% and 78%, respectively, compared with pure Zn. Moreover, fracture analysis indicated a more ductile fracture of the nanocomposites after the incorporation of nano-SiC. In addition, the nanocomposites presented favorable biocompatibility and accelerated degradation caused by intergranular corrosion. These findings suggested that the nano-SiC reinforced Zn biocomposites may be the potential candidates for orthopedic implants.

Study on the element segregation and Laves phase formation in the laser metal deposited IN718 superalloy by flat top laser and gaussian distribution laser

The element segregation, Laves phase formation, and mechanical properties of the laser metal deposited IN718 superalloy by the flat top laser beam (FTLB) and gaussian distribution laser beam (GDLB) are studied. It is found that the Laves phase formation in the gaussian distribution laser deposited IN718 (GDLD-IN718) is substantially higher than that in the flat top laser deposited IN718 (FTLD-IN718). The higher production of the Laves phase in the GDLD-IN718 contributes to the higher microhardness and lower tensile strength (about 20% reduction) of the as-deposited IN718 than that of the FTLD-IN718. The element redistribution behavior in the laser rapid solidification under both of the lasers are also studied through the finite element simulation. The results show that the severe laser energy concentration in the center of the GDLB produces higher molten pool temperature, lower horizontal thermal gradient to vertical thermal gradient ratio (GX/GZ) of the solid-liquid interface. These typical thermal characteristic of the GDLB generated molten pool eventually results in a lower redistribution coefficient of the alloying elements and as a result, the interdendritic element segregation and Laves phase formation are dramatically improved in the GDLD-IN718. The present comparative study proves that the FTLB is more superior for the laser additive manufacturing than that of the GDLB.

Secondary hardening in laser rapidly solidified Fe68(MoWCrVCoNiAlCu)32 medium-entropy high-speed steel coatings

Novel Fe68(MoWCrVCoNiAlCu)32 (at.%) medium-entropy high-speed steel (ME-HSS) coatings, containing various carbon contents from 0.65 to 1.05 wt%, are prepared by laser rapid solidification. The newly prepared ME-HSS coatings are characterized by a hard martensitic matrix enhanced by secondary hardening, and specifically by coherent nano-sized M2C. The secondary hardening effect is enhanced with the increasing carbon content. The high amount of alloying elements in ME-HSS coatings results in excellent oxidative wear resistance, without leading to serious compositional segregation and coarsening of carbides.

A combined strategy to enhance the properties of Zn by laser rapid solidification and laser alloying.

The orthopedic application of Zn is limited owing to the poor strength and low plasticity. In this study, a novel strategy by combining rapid solidification obtained by selective laser melting (SLM) and alloying with Mg was proposed to improve the mechanical properties of Zn. The microstructures, mechanical properties, as well as in vitro cytocompatibility of SLM processed Zn-xMg (x = 0-4 wt%) were studied systematically. Results shown that SLM processed Zn-xMg alloys consisted of fine equiaxed α-Zn grains with homogeneously precipitated Mg2Zn11 along grain boundaries. More importantly, the grains size of α-Zn was decreased from 104.4 ± 30.4 µm to 4.9 ± 1.4 µm with Mg increasing. And Mg mainly dissolved in α-Zn developing into supersaturated solid solution due to rapid solidification effect. As a consequence, the ultimate tensile strength and elongation were enhanced by 361% and 423%, respectively, with Mg containing up to 3 wt%. Meanwhile, alloying with Mg enhanced the corrosion resistance of Zn, with the degradation rate decreasing from 0.18 ± 0.03 mm year-1 to 0.10 ± 0.04 mm year-1. Furthermore, SLM processed Zn-xMg exhibited good biocompatibility. This research suggested that SLM processed Zn-3Mg alloy was a potential biomaterial for orthopedic applications.

Effects of Different Levels of Boron on Microstructure and Hardness of CoCrFeNiAlxCu0.7Si0.1By High-Entropy Alloy Coatings by Laser Cladding

High-entropy alloys (HEAs) are novel solid solution strengthening metallic materials, some of which show attractive mechanical properties. This paper aims to reveal the effect of adding small atomic boron on the interstitial solid solution strengthening ability in the laser cladded CoCrFeNiAlxCu0.7Si0.1By (x = 0.3, x = 2.3, and 0.3 ≤ y ≤ 0.6) HEA coatings. The results show that laser rapid solidification effectively prevents brittle boride precipitation in the designed coatings. The main phase is a simple face-centered cubic (FCC) matrix when the Al content is equal to 0.3. On the other hand, the matrix transforms to single bcc solid solution when x increases to 2.3. Increasing boron content improves the microhardness of the coatings, but leads to a high degree of segregation of Cr and Fe in the interdendritic microstructure. Furthermore, it is worth noting that CoCrFeNiAl0.3Cu0.7Si0.1B0.6 coatings with an FCC matrix and a modulated structure on the nanometer scale exhibit an ultrahigh hardness of 502 HV0.5.

Synthesis and characterization of refractory TiZrNbWMo high-entropy alloy coating by laser cladding

A novel refractory high-entropy alloy (HEA) coating with composition close to TiZrNbWMo was synthesized on 45 # steel by laser cladding. Constituent phases, microstructure, chemical composition and microhardness of the coatings before and after annealing were investigated by XRD, SEM, TEM, EDS and micro/Vickers hardness test, respectively. Results showed that the TiZrNbWMo coating was ~513μm in thickness. The microstructure of the coatings exhibited typical dendritic and interdendritic structure. The coating was composed of a major body-centered cubic (BCC) solid solution and a small amount of β-TixW1−x precipitated phase due to high-entropy affect and laser rapid solidification. Apart from enhanced fraction of β-TixW1−x precipitation, the structure and BCC solid solution phases remained almost unchanged after annealing at 800°C, 1000°C, and 1200°C for 20h, indicating that the coating had high thermal stability. The microhardness of as-clad HEA coating reached around 700HV0.5, which was three times of 45 # steel substrate. The coating microhardness increased significantly after annealing, which was due to the solid solution strengthening of BCC matrix and formation of β-TixW1−x precipitation. Notably, the highest value was up to 1300HV0.5 after heat treatment at 800°C. Therefore, the HEA coatings showed high microhardness and resistance to softening.

Solidification path of single-crystal nickel-base superalloys with minor carbon additions under laser rapid directional solidification conditions

The solidification path of single-crystal nickel-base superalloys containing minor carbon was investigated under various laser rapid directional solidification (LRDS) conditions. By controlling the solidification rate, LRDS processing can provide the evidence whether some diffusion-controlled phase transformations occur because such transformations will be suppressed under high cooling rates. Results show that the solidification path and final solidification microstructure depend upon the cooling rate; the microstructure without γ-γ′ eutectic can be obtained as long as the cooling rate is high enough. A peritectic transformation in carbon-containing single-crystal superalloys was first experimentally verified by controlling the cooling rate during LRDS processing.

Laser rapid solidification improves corrosion behavior of Mg-Zn-Zr alloy

Rapid solidification process is an effective method to improve the corrosion behavior of Mg alloys. In the present study, laser melting, as a typical rapid solidification process, was used to fabricate Mg–5.6Zn–0.5Zr (ZK60). With the increase of laser energy density, the crystalline structures experienced successive changes: clustered finer dendrites→uniform equiaxed grains→coarsened equiaxed grains. Moreover, the fully divorced eutectic α-Mg + Mg7Zn3 homogeneously distributed along the boundaries of solid solution structure α-Mg. Meanwhile, at a laser energy density of 600 J/mm3, ZK60 was dense with an optimal hardness of 89.2 Hv and hydrogen evolution rate of 0.006 ml cm−2 h−1. The enhanced corrosion resistance was attributed to the refined grain, homogenized microstructure and extended solid solution caused by laser rapid solidification.

A new microsegregation model for rapid solidification multicomponent alloys and its application to single-crystal nickel-base superalloys of laser rapid directional solidification

The microsegregation in single-crystal nickel-base superalloys, which may result in many undesired precipitated phases, can be effectively reduced by rapid solidification. A new microsegregation model for rapid solidification multicomponent alloys was developed in this paper to predict the microsegregation in the single-crystal superalloys of laser rapid directional solidification and study the influence of processing parameters on the microsegregation behavior. To verify the predicted accuracy of the present model, the concentration profiles and microsegregation ranges under different processing conditions were examined and compared with those calculated by different models. Compared with previous models, this multicomponent model not only has definite physical meaning but also takes nonequilibrium solidification and dendritic tip undercooling into account. Therefore, it is suitable for a wide range of solidification conditions and can obtain better predicted results for complex multicomponent alloys. Moreover, although the microsegregation behavior depends primarily on the solidification velocity, a high temperature gradient also contributes to reducing the microsegregation. Consequently, the microsegregation can be effectively controlled by using a set of processing parameters that can obtain high temperature gradient and solidification velocity. Such a model contributes to the control of the microsegregation while determining relevant processing windows for the preparation of superalloy components using a controlled single-crystal laser processing.

Prediction of primary dendritic arm spacing during laser rapid directional solidification of single-crystal nickel-base superalloys

Primary dendritic arm spacing (PDAS) is an important microstructure feature in the nickel-base single-crystal (SX) superalloys by laser rapid directional solidification (LRDS). A combined numerical model was developed in this paper to investigate the influence of laser processing parameters on the PDAS. This model consists of (1) the theoretical PDAS models which relate PDAS to the solidification conditions and (2) the heat-flux calculations of laser processing which provide the solidification conditions as a function of the processing parameters. It is therefore able to immediately relate the PDAS to the processing parameters and obtain corresponding processing–microstructure maps. To verify the predicted accuracy, the PDAS values calculated by different theoretical models were compared with those produced under different processing conditions. Results show that the PDAS firstly decreases and then increases at a lower laser power whereas it decreases with increasing scanning velocity at a higher laser power, leading to nose-shape contour lines. The predicted accuracy depends on appropriate selections of material thermo-physical properties. These processing–microstructure maps can accurately capture the trends of the PDAS variation with the processing parameters and contribute to the control and optimization of dendritic microstructure while determining relevant processing windows for controlled SX laser processing.

Liquid Phase Separation and the Aging Effect on Mechanical and Electrical Properties of Laser Rapidly Solidified Cu100−xCrx Alloys

Duplex structure Cu-Cr alloys are widely used as contact materials. They are generally designed by increasing the Cr content for the hardness improvement, which, however, leads to the unfavorable rapid increase of the electrical resistivity. The solidification behavior of Cu100−xCrx (x = 4.2, 25 and 50 in wt.%) alloys prepared by laser rapid solidification is studied here, and their hardness and electrical conductivity after aging are measured. The results show that the Cu-4.2%Cr alloy has the most desirable combination of hardness and conductive properties after aging in comparison with Cu-25%Cr and Cu-50%Cr alloys. Very importantly, a 50% improvement in hardness is achieved with a simultaneous 70% reduction in electrical resistivity. The reason is mainly attributed to the liquid phase separation occurring in the Cu-4.2%Cr alloy, which introduces a large a

Effect of Layer Thickness in Selective Laser Melting on Microstructure of Al/5 wt.%Fe2O3Powder Consolidated Parts

In situ reaction was activated in the powder mixture of Al/5 wt.%Fe2O3 by using selective laser melting (SLM) to directly fabricate aluminium metal matrix composite parts. The microstructural characteristics of these in situ consolidated parts through SLM were investigated under the influence of thick powder bed, 75 μm layer thickness, and 50 μm layer thickness in various laser powers and scanning speeds. It was found that the layer thickness has a strong influence on microstructural outcome, mainly attributed to its impact on oxygen content of the matrix. Various microstructural features (such as granular, coralline-like, and particulate appearance) were observed depending on the layer thickness, laser power, and scanning speed. This was associated with various material combinations such as pure Al, Al-Fe intermetallics, and Al(-Fe) oxide phases formed after in situ reaction and laser rapid solidification. Uniformly distributed very fine particles could be consolidated in net-shape Al composite parts by using lower layer thickness, higher laser power, and lower scanning speed. The findings contribute to the new development of advanced net-shape manufacture of Al composites by combining SLM and in situ reaction process.