Solidification Microstructure Research Articles

Laser Powder Bed Fusion Printing of CoCrFeMnNi High Entropy Alloy: Processing, Microstructure, and Mechanical Properties

AbstractEquiatomic CoCrFeMnNi high entropy alloy (HEA) powder was processed by laser powder bed fusion (LPBF) additive manufacturing (AM). The properties of the spherical pre-alloyed CoCrFeMnNi powder were characterized and its processability using LPBF AM was systematically investigated through the volumetric energy density (VED) based on the surface roughness, defects (micro-cracks and porosity) and densification. After optimization, LPBF processing at a VED of 104 J/mm3 achieved highly dense and crack-free vertical and horizontal test specimens with a porosity fraction lower than 0.01% and micro-pores having a mean size of, respectively, 25.9 μm and 13.4 μm, as determined from X-ray micro-computed tomography (μCT) inspection. Scanning electron microscope (SEM) analysis of the as-built (AB) CoCrFeMnNi processed at a VED of 104 J/mm3 showed a heterogeneous solidification microstructure, consisting of columnar grains with a cellular subgrain structure, and electron backscattered diffraction (EBSD) revealed a crystallographic texture mainly along the < 100 > direction. Post treatment with hot isostatic pressing (HIP) was effective in closing the remnant micro-pores in the bulk volume of the AB CoCrFeMnNi. Also, the cellular sub-grain structure in the AB CoCrFeMnNi completely disappeared after HIP and the resulting microstructure consisted of recrystallized equiaxed grains with annealing twins. The room temperature tensile response was anisotropic for AB CoCrFeMnNi with horizontally built specimens exhibiting higher strength and fracture strains (global and local) compared to vertically built ones; HIP reduced the anisotropy in the tensile properties and led to similar tensile strength with elongation values that were ~ 50% higher than in the AB condition. The HIPed CoCrFeMnNi also displayed higher Charpy impact toughness and absorbed energy at both room and liquid nitrogen temperatures compared to the AB material. Examination of the fracture surfaces after tensile and Charpy impact testing revealed ductile features with characteristic dimpled appearance and pointed to the important role of the remnant micro-pores on failure in the AB CoCrFeMnNi. Tribological assessments pointed to the superior low-stress abrasion resistance of AB and HIPed CoCrFeMnNi compared to 316L stainless steel (SS), which was included in this study to reinforce the analysis. SEM observations revealed that scratching and micro-fracture are the dominant wear mechanisms for the CoCrFeMnNi HEA, whereas ploughing and cutting parallel to the abrasive flow direction are the dominant mechanisms for 316L SS. To the authors’ knowledge, this study is the first to evaluate and report the low-stress abrasion resistance of any high entropy alloy. To understand the corrosion behavior, polarization curves of AB and HIPed CoCrFeMnNi were measured in 3.5 wt% NaCl and 1N H2SO4 solutions, and the results were compared to those of 316L SS. The findings indicate that AB and HIPed CoCrFeMnNi outperform 316L SS in a chloride-containing environment, but not in an acid-containing environment. Additionally, observations of hydrogen permeability revealed that AB CoCrFeMnNi permeates a lower volume of hydrogen atoms (by ~ 5 times) compared to 316L SS, despite its higher (by nearly 3 times) diffusion coefficient. Electrochemical hydrogen permeation data showed that the concentration of atomic hydrogen in the sub-surface of AB and HIPed CoCrFeMnNi was, respectively, about 32 and 26 times lower than in 316L SS. This study provides important material–structure–property data and indicates a promising outlook for LPBF of the CoCrFeMnNi HEA with high-performance.

GrainGNN: A dynamic graph neural network for predicting 3D grain microstructure

We propose GrainGNN, a surrogate model for the evolution of polycrystalline grain structure under rapid solidification conditions in metal additive manufacturing. High fidelity simulations of solidification microstructures are typically performed using multicomponent partial differential equations (PDEs) with moving interfaces. The inherent randomness of the PDE initial conditions (grain seeds) necessitates ensemble simulations to predict microstructure statistics, e.g., grain size, aspect ratio, and crystallographic orientation. Currently such ensemble simulations are prohibitively expensive and surrogates are necessary.In GrainGNN, we use a dynamic graph to represent interface motion and topological changes due to grain coarsening. We use a reduced representation of the microstructure using hand-crafted features; we combine pattern finding and altering graph algorithms with two neural networks, a classifier (for topological changes) and a regressor (for interface motion). Both networks have an encoder-decoder architecture; the encoder has a multi-layer transformer long-short-term-memory architecture; the decoder is a single layer perceptron.We evaluate GrainGNN by comparing it to high-fidelity phase field simulations for in-distribution and out-of-distribution grain configurations for solidification under laser power bed fusion conditions. GrainGNN results in 80%–90% pointwise accuracy; and nearly identical distributions of scalar quantities of interest (QoI) between phase field and GrainGNN simulations compared using Kolmogorov-Smirnov test. GrainGNN's inference speedup (PyTorch on single x86 CPU) over a high-fidelity phase field simulation (CUDA on a single NVIDIA A100 GPU) is 150×–2000× for 100-initial grain problem. Further, using GrainGNN, we model the formation of 11,600 grains in 220 seconds on a single CPU core.

Melting mode driven tuning of local microstructure and hardness in laser powder bed fusion of 18Ni-300 maraging steel

Laser powder bed fusion (LPBF) of 18Ni-300 maraging steel has attracted abundant popularity in both academia and industry due to its superior mechanical properties and printability. Several studies showcased the optimization of process parameters based on relative density, but there is little exploration on tailoring the as-built microstructure. In this study, fully dense maraging steel samples are fabricated with three different melting modes, and they are examined for the cellular solidification microstructure, retained austenite, and the effect of cyclic reheating, the three characteristics that are unique to additively manufactured maraging steel. Microstructural discrepancies among the samples are traced back to the different cooling rate and residual heat, both of which are ultimately determined by the energy input. The findings suggest that a reasonably low energy input yields a microstructure containing fine cell size and little retained austenite, leading to higher hardness. Nanoindentation mapping reveals that the main bodies of all samples are strengthened around 13 %, likely because of the formation of precipitation zone due to four to five layers of subsequent cyclic reheating. This study demonstrates the important opportunity for microstructural tailoring, and consequently, engineering the mechanical properties of maraging steel via LPBF by controlling the solidification and subsequent cyclic reheating, which are both attributed to the printing parameters deployed.

Effect of Cu on Solidification Microstructure and Solidification Process in High Carbon High Speed Steel Type Cast Alloy

Effect of Cu on Solidification Microstructure and Solidification Process in High Carbon High Speed Steel Type Cast Alloy

Effect of Ce and solidification cooling rate on the microstructure and mechanical properties of AA2017 aluminum alloy

Controlling dendritic growth and grain size in new Ce-containing Al-based alloys becomes crucial due to new envisioned applications and Ce rising demand in casting processes. In the present work Ce effect on the 2xxx Al series microstructures at various industrial-scale solidification cooling rates was investigated. The directional solidification technique is crucial in this endeavor since can generate several solidified samples related to several cooling rates. The AA2017 alloy and a modified version containing 3wt.% of Ce were both produced under directional solidification and various microstructure aspects were characterized. The mechanical behavior was analyzed by microhardness and compression tests. The Ce addition reduced primary and secondary dendritic spacing without impacting grain size. Cells were only observed to form on the Ce-containing alloy at cooling rates of approximately 19°C/s; while dendritic configurations dominated all other conditions. While round pockets composed of Al+Al2Cu+Mg2Si ternary eutectic formed the interdendritic zones in the AA2017 alloy, the AA2017-Ce alloy microstructure was mainly constituted by elongated AlCeSi + Al8CeCu4 interdendritic phases. Finally, the addition of Ce favored an increase in the microhardness and compressive strength values of the AA2017 alloy, which is attributed to the refinement of the solidification structure, as well as the formation of a greater fraction of secondary reinforcement phases.

Microstructure evolution of a novel Al-8Si-0.4Mg-0.2Sc-0.2Er alloy solidified under permanent magnet stirring

An experimental study has been conducted to assess the influence of permanent magnet stirring (PMS) at various rotation speeds (ω = 40, 80, 120, and 160 rpm) on the solidification microstructure of a novel Al-8Si-0.4Mg-0.2Sc-0.2Er alloy. The results reveal that the Sc and Er would produce small size Al(Sc,Er)2Si2 precipitates in the front of the Si phase under the increased rotation speed of PMS. Furthermore, it has been observed that PMS generates forced fluid flow with the magnetic Taylor number as large as 9.80 × 108, which significantly reduces the size of Al(Sc,Er)2Si2 and eutectic Si phase, promotes the formation of equiaxed dendrite α-Al grains.

Experimental determination and thermodynamic optimization of the Nb–V–Zr system

A comprehensive thermodynamic description for each phase is essential for designing of the novel Zr-based nuclear materials. The construction of the experimental liquidus surface projection and isothermal section of the Nb–V–Zr system was established through analyzing solidification microstructures and phase constituents. The stability of the τ phase with C14-structure was established by investigating the composition range of Nb in τ, which was measured to be between 19.8 and 40.5 at. % at 1473 K. The solubility of Nb in λ2 with the C15-type was ∼ 16.9 at. % at 1473 K. The liquidus surface projection showed primary solidification areas of bcc(Nb,V,Zr), λ2 and τ. The thermodynamic parameters were derived combining the CALPHAD method with experimental results from phase diagram data in this work and the literatures. Solution phases, including liquid, bcc, and hcp, were regarded as substitutional solutions. Considering the crystal structures and solid solubility of intermetallic compounds, λ2 and τ were modeled as (Nb,V,Zr)2(Nb,V,Zr) using two-sublattice model. A set of self-consistent thermodynamic parameters of the Nb–V–Zr system was obtained, which will provide theoretical guidance for the design of Zr-based nuclear materials.

Single-pass high-power laser-Arc hybrid welding of thick stainless steel clad plates: Microstructure and mechanical properties

In order to solve the existing problem of low efficiency of multi-layer and multi-pass welding methods for thick stainless steel clad plate (SSCP), this paper proposes high-power laser-arc hybrid welding (HP-LAHW) single-pass weld thick SSCP. The effects of wire feeding rate (WFR) on the weld's composition distribution, microstructure and mechanical properties were investigated. The weld morphology is nail-shaped, consisting of a wide arc zone (ArcZ) within the stainless steel layer and a long narrow laser zone (LaserZ), mainly in the carbon steel layer. With the increase of WFR, the dilution rate of the carbon steel layer to the ArcZ decreases first and then increases. The influence of WFR on the microstructure and mechanical properties of the weld is observed mainly in the ArcZ. When the WFR reaches 6 m/min, the composition of the microstructure within the ArcZ will change from martensite, ferrite, and a small amount of granular austenite to ferrite and a small amount of austenite, resulting in the microhardness of the ArcZ decreasing rapidly from 398 HV to 210 HV. The solidification type and microstructure transformation mechanisms within the weld were elucidated based on quantitative calculations of the chromium and nickel equivalent ratios. The microstructure of the LaserZ in all welds is predominantly martensitic due to the rapid cooling rate, which makes the tensile properties of the weld higher than the base metal (BM). The results of this study provide technical support for realizing high-efficiency and high-performance single-pass HP-LAHW of thick SSCP.

Emergence of rapid solidification microstructure in additive manufacturing of a Magnesium alloy

Abstract Bioresorbable Mg-based alloys with low density, low elastic modulus, and excellent biocompatibility are outstanding candidates for temporary orthopedic implants. Coincidentally, metal additive manufacturing (AM) is disrupting the biomedical sector by providing fast access to patient-customized implants. Due to the high cooling rates associated with fusion-based AM techniques, they are often described as rapid solidification processes. However, conclusive observations or rapid solidification in metal AM — attested by drastic microstructural changes induced by solute trapping, kinetic undercooling, or morphological transitions of the solid-liquid interface — are scarce. Here we study the formation of banded microstructures during laser powder-bed fusion (LPBF) of a biomedical-grade Magnesium-rare earth alloy, combining advanced characterization and state-of-the-art thermal and phase-field modeling. Our experiments unambiguously identify microstructures as the result of an oscillatory banding instability known from other rapid solidification processes. Our simulations confirm that LPBF-relevant solidification conditions strongly promote the development of banded microstructures in a Mg-Nd alloy. Simulations also allow us to peer into the sub-micrometer nanosecond-scale details of the solid-liquid interface evolution giving rise to the distinctive banded patterns. Since rapidly solidified Mg alloys may exhibit significantly different mechanical and corrosion response compared to their cast counterparts, the ability to predict the emergence of rapid solidification microstructures (and to correlate them with local solidification conditions) may open new pathways for the design of bioresorbable orthopedic implants, not only fitted geometrically to each patient, but also optimized with locally-tuned mechanical and corrosion properties.

A new complex-regular eutectic in the Fe7(CoNiMn)93-xBx high entropy alloys

Phase constituents and solidification microstructures of as-cast Fe7(CoNiMn)93-xBx (x = 9, 13) HEAs were investigated via electron backscattered diffraction and transmission electron microscopy. It was observed that the phase constituents contain an FCC1 solid solution and an M23B6 compound. The microstructure of hypoeutectic Fe7(CoNiMn)84B9 high-entropy alloy consists of primary face-centered-cubic (FCC1) dendrite and trigonal-shaped complex-regular eutectic, and that of Fe7(CoNiMn)80B13 eutectic high-entropy alloy is fully consisted of trigonal-shaped complex-regular eutectic. The formation of this rare complex-regular eutectic is attributed to the preferential growth of M23B6 in a trigonal-shaped trunk due to its crystallography anisotropy. In addition, the FCC1 eutectic phase has a cube-on-cube orientation relationship with the M23B6 phase where the same orientation but different lattice constants for them leads to the appearance of misfit dislocations at the interface, which enhances the strength of materials. The compression strength for Fe7(CoNiMn)80B13 eutectic high-entropy alloy is higher than that of hypoeutectic Fe7(CoNiMn)84B9 high-entropy alloy.

Simulation and Study of Influencing Factors on the Solidification Microstructure of Hazelett Continuous Casting Slabs Using CAFE Model.

The Hazelett continuous casting and rolling process represents a leading-edge production method for cold-rolled aluminum sheet and strip billets in the world. Its solidification microstructure significantly influences the quality of billets produced for cold rolling of aluminum sheets and strips. In this study, employing the CAFE (Cellular Automaton-Finite Element) method, we developed a coupled computational model to simulate the solidification microstructure in the Hazelett continuous casting process. We investigated the impact of nucleation parameters, casting temperature, and continuous casting speed on the microstructural evolution of the continuous casting billet. Through integrated metallographic analyses, we aimed to elucidate the controlling mechanisms underlying the Hazelett continuous casting process and its resultant microstructure. The results demonstrate that the equiaxed rate of grains increases with an increase in nucleation density, and the grain size decreases under constant cooling strength. With other nucleation parameters held constant, the grain size decreases as undercooling increases, and the columnar crystal zone expands. The nucleation density of the Hazelett continuous casting aluminum alloy has been determined to range between 1011 m-3 and 1013 m-3, and the undercooling ranges between 1 °C and 2.5 °C. The solidified grain structure can be controlled between 35 μm and 72 μm. The grain size of the continuous casting billet increases with an increase in pouring temperature and decreases as the casting speed increases. Elevating the pouring temperature positively impacts the fraction of high-angle grain boundaries and promotes the dendritic to equiaxed grain transition. Moreover, there exists potential for further optimization of continuous casting process parameters.

Solidification and tribological behaviors of CoCrFeNiSi0.5-(Al,Ti) non-equiatomic high entropy alloy coatings fabricated by direct laser deposition

In order to evaluate the difference in the effects of Al and Ti elements on the microstructure and tribological behaviors of CoCrFeNi-based high entropy alloy (HEA) coatings, CoCrFeNiSi0.5-(Al,Ti) non-equiatomic HEAs coatings were prepared using direct laser deposition (DLD). The phase constitution, solidification microstructure and tribological behaviors of the HEA coatings were investigated. The results indicated that CoCrFeNiSi0.5 HEA coating was composed of face centered cubic (FCC) phase with microstructure of columnar and cellular grains. The addition of Al and Ti led to a columnar-to-equiaxed transition (CET) in the HEA coatings. When only Al was added (CoCrFeNiSi0.5Al0.5), the primary phase transformed into a body centered cubic (BCC) phase (94 vol%). Additionally, Cr3Si formed in the interdendrite (ID) region, and a spinodal decomposition structure emerged in the dendrite (DR) region. Conversely, with the inclusion of Ti alone (CoCrFeNiSi0.5Ti0.5), the matrix maintained its FCC phase, but Ni16Ti6Si7 (G phase, 20.9 vol%) and Cr15Co9Si6 (15.1 vol%) were generated in the ID and DR regions, respectively. Incorporating both Al and Ti (CoCrFeNiSi0.5Al0.5Ti0.5) significantly reduced the G phase content in the ID region to 7.9 vol% and led to the formation of dispersed Fe3Al nanoparticles with an average size of 99 nm in the DR region. The CoCrFeNiSi0.5 HEA coating exhibited the lowest hardness (249 HV0.1), the highest friction coefficient (0.778), and the poorest wear resistance. For the HEA coatings containing Al and Ti, the microhardness significantly increased to over 700 HV0.1, and the friction coefficient decreased to around 0.6. However, the wear resistance of the CoCrFeNiSi0.5Al0.5 HEA coating was almost equivalent to that of the CoCrFeNiSi0.5 with a wear loss of 5.1 mg. By contrast, the HEA coatings containing Ti demonstrated notably improved wear resistance, with wear losses of only 2.2 mg and 2.4 mg for the CoCrFeNiSi0.5Ti0.5 and CoCrFeNiSi0.5Al0.5Ti0.5 HEA coatings, respectively.

Hydrogen generation by hydrolysis of Sc–Sn alloy over a wide temperature range as low as 0 °C

A series of Sc–Sn alloys are prepared by the arc-melting method and their hydrogen generation performance of hydrolysis in pure water is investigated. It was found that only Sc element reacts with water in Sc–Sn alloy, while Sn element is retained as an elemental metal. The reaction temperature of Sc with water can be reduced below 0 °C after Sc and Sn are alloyed. The Sc3Sn2 alloy has the best hydrogen generation performance. Its hydrogen conversion rate is up to 90% at 0 °C for 100 min as well as when the water temperature dropped from 60 °C to 0 °C, the average conversion rate in the first 30 min only dropped from 19.95 mL/min∙g to 14.31 mL/min∙g. The hydrogen generation performance of Sc–Sn alloys depends on the reaction activity after Sc and Sn alloying, the effect of micro galvanic cells and the solidification microstructure in the alloy ingot.

Simulation of Solidification Structure in the Vacuum Arc Remelting Process of Titanium Alloy TC4 Based on 3D CAFE Method

Vacuum arc remelting is the main production method of titanium alloy ingots at present. In order to obtain good quality ingots, it is of great significance to study the formation of the solidification structure of ingots via vacuum arc remelting. In order to select and optimize the nucleation parameters for the solidification microstructure simulation of an ingot, a 3D CAFE model for microstructure evolution during vacuum arc remelting was established, taking into account heat transfer, flow, and solute diffusion. The Gaussian distribution continuous nucleation model and extended KGT model were used to describe the grain nucleation and dendrite tip growth rates, respectively. The multi-point mass source and moving boundary method were used to simulate the ingot growth. The results show that there are three typical crystal regions in the solidification structure of vacuum arc remelting titanium alloy ingots, namely the surface fine crystal region, columnar crystal region, and central equiaxed crystal region. The proportion of the columnar crystal region in the solidification structure of an ingot increases gradually with the increase in the undercooling of the maximum bulk nucleation. With an increase in the maximum bulk nucleation density, the equiaxed grain zone gradually increases, and the grain size gradually decreases. The proportion of the columnar crystal region in the solidification structure of an ingot increases gradually with an increase in the undercooling of the maximum bulk nucleation. The maximum volume nucleation variance has no obvious effect on the change in the solidification structure. When the maximum volume nucleation undercooling is 5.5 K, the maximum volume nucleation standard deviation is 4 K, and the maximum volume nucleation density is 5 × 108. The solidification structure simulation results are in good agreement with the experimental results.

A DSC study of undercooling thermodynamics and crystallization kinetics for liquid Ag-Si alloys

The undercooling thermodynamics and crystallization kinetics of pure silver, hypoeutectic Ag-2%Si, and eutectic Ag-3.1%Si alloys were explored with differential scanning calorimetry. Liquid silver was highly undercooled by an extent of 200 K (0.16Tm) within molten B2O3 denucleating agent. The nucleating incubation time and crystallization kinetics were investigated under isothermal condition. It was found that the crystal incubation time of nucleation generally decreased with enhanced undercooling. The analyses based on classical nucleation theory showed that heterogeneous nucleation was still dominant even at substantial undercoolings. The transformed solid fraction was derived to elucidate the crystallization kinetics of liquid silver by Johnson-Mehl-Avrami equation. The Avrami exponent was revealed to vary with liquid undercooling and crystallization extent. High cooling rate and denucleating agent were two effective ways to achieve large undercoolings for liquid Ag-Si alloys. The primary (Ag) dendrites were refined with increased undercoolings in hypoeutectic Ag-2%Si alloy. Irregular (Ag)+(Si) eutectics were the typical solidification microstructures for eutectic Ag-3.1%Si alloy at small undercoolings, while primary (Ag) dendrites displayed a remarkable volume fraction at extended undercoolings.

Multi-physics simulation of non-equilibrium solidification in Ti-Nb alloy during selective laser melting

This paper underscores the critical role of temperature distribution in the molten pool on the resulting solidification microstructure during selective laser melting (SLM) process. A powder-resolved computational fluid dynamics (CFD) model of Ti-Nb alloy is established to investigate heat transfer and fluid flow in the molten pool. When comparing the bulk model to the powder bed model, discrepancies in temperature gradient and cooling rate can reach up to 26.29 % and 18.32 %, respectively. Utilizing the powder bed CFD model, temperature gradient and cooling rate data at the molten pool boundary are integrated into the finite interface dissipation phase-field model. The simulation results illuminate the non-equilibrium solidification microstructure in the SLMed Ti-Nb alloy. Results indicate that the rapid solidification fosters a pronounced solute trapping effect, mitigating solute microsegregation. The microstructure manifests as cellular and dendritic structures, with Nb concentration in cellular cores and depletion in intercellular region. Predicted primary dendrite arm spacing (PDAS) or cell size range between 0.4 to 0.6 μm, consistent with experimental observations. In particular, the study reveals a transition from dendritic to cellular and from cellular to planar structures, with an accompanying analysis of the underlying mechanisms.

Effect of Al–5Ti–1B Addition on Solidification Microstructure and Hot Deformation Behavior of DC-Cast Al–Zn–Mg–Cu Alloy

This study investigated the effect of adding Al–5Ti–1B grain refiner on the solidification microstructure and hot deformation behavior of direct-chill (DC) cast Al–Zn–Mg–Cu alloys. The grain refiner significantly decreased the grain size and modified the morphology. Fine-grained (FG) alloys with grain refiners exhibit coarse secondary phases with a reduced number density compared to coarse-grained (CG) alloys without grain refiners. Dynamic recrystallization (DRX) was enhanced at higher compression temperatures and lower strain rates in the CG and FG alloys. Both particle stimulated nucleation (PSN) and continuous dynamic recrystallization (CDRX) are enhanced in the FG alloys, resulting in decreased peak stress values (indicating DRX onset) at 450°C. The peak stress of the FG alloys was higher at 300-400°C than that of the CG alloys because of grain refinement hardening over softening by enhanced DRX.

Investigation of melt pool dynamics and solidification microstructures of laser melted Ti-6Al-4V powder using X-ray synchrotron imaging

Titanium alloy (Ti-6Al-4V) is widely used in additive manufacturing (AM) industry. However, as laser powder-bed fusion (PBF-L) additive manufacturing (AM) advances towards reliable production of titanium parts, a thorough understanding of the process-structure-properties (PSP) relationships remain to be fully understood. A study of the laser melting was paired with high-speed X-ray synchrotron imaging at the 32-ID beamline of the Advanced Photon Source at Argonne National Laboratory. Simultaneous melting and imaging was carried out on a Ti-6Al-4V powder layer held in a custom device designed to mimic single-track scans of the PBF-L process at different laser power levels, powder size distributions, and cover gas environments (Ar and He) on top of AM Ti-6Al-4V base metal. It was found that the thickness of the powder layer significantly affected the melt behavior: too much powder led to the formation of molten droplets that wetted the surface of the titanium, yet did not contribute to a uniform melting profile. Residual gas pores in the atomized powder were also observed to contribute to the pores observed in the melt pool, with the porosity of the powder (defined as volume of pores divided by total material volume) constant with powder size distribution (i.e., larger particles contained more entrapped gas, which increased final part porosity). When varying Ar or He through the same gas flow meter settings and nozzle, the difference in flow rates likely contributed more to the resultant porosity of the solidified material than did the thermal conductivity of the gasses, with He being the greater contributor to porosity. The microstructure of the heat affected zone contained α′, α, and an increased β phase fraction relative to the base material. The crystallographic texture of the melt pool region adopted that of the base metal.

Solidification behavior of SiCf/Ti17 induced by fixed-point laser surface remelting

Taking advantage of laser additive manufacturing, continuous SiC fiber reinforced titanium matrix composites (SiCf/TMCs) with low density and excellent mechanical properties can be used to hybrid-fabricate large components with complex shape for aerospace applications. However, due to the anisotropy in both microstructure and heat transfer ability, the high-density and rapid heat input from laser during hybrid manufacturing process will cause the non-uniform damage of fibers and various solidification microstructure in the molten pool. In order to understand the solidification behavior of molten pool in SiCf/Ti17 composites, the fixed-point laser surface remelting was used to bring in rapid and symmetrical heat input. With the decrease of temperature from center to edge of the molten pool, both fiber damage degree and the contents of Si and C elements in melt decreased. The solidification microstructure contains TiCx, Ti5Si3 and β-Ti, and their morphology varies according to the melt composition. SiC fibers act as heterogeneous nucleation sites for first solidified phases and promote their preference growth along the radial direction of fibers resulting from the excellent heat transfer ability. The faceted Ti5Si3 grains solidified through peritectic reaction L+TiCx→Ti5Si3 exhibit strong <0001> texture.

Review on manufacturability and strengthening mechanisms of particulate reinforced Mg composites

The further development of lightweight and energy-saving engineering materials is a great challenge for scientists in the 21st century. Magnesium and its composites, as the metal materials with the lowest density among metal structural materials, have good thermal conductivity, good dimensional stability, high specific strength, good biocompatibility, which can meet the requirements of energy-saving, emission reduction, environmental protection. In recent years, magnesium and its composites have found the major applications in various fields such as aerospace, automotive industry, electronics and medical industry. However, the fabrication of magnesium matrix composites is difficult, and the deformation behaviors and strengthening mechanisms of particulate reinforced magnesium matrix composites have unique characteristics. This article reviews the recent research progress of the magnesium matrix composites, including different fabrication methods and strengthening mechanisms. According to the classification of solidification rate, the advantages and disadvantages of various techniques used to fabricate magnesium matrix composites were introduced. The traditional and novel fabrication techniques were discussed. The influence mechanisms of particulates on solidification microstructures, recrystallization behaviors, texture strength and twin behaviors of magnesium matrix composites during solidification and extrusion were summarized, and the roles of particulates in solidification and extrusion of magnesium matrix composites were analyzed. The strengthening mechanisms of particulates on the mechanical properties, corrosion resistance, fatigue resistance and creep resistance of magnesium matrix composites were revealed. Finally, the development prospect and research direction of magnesium matrix composites were prospected. It is hoped that this paper can provide a reference for the further development of magnesium matrix composites.