Secondary Dendrite Arm Spacing Research Articles

Development and benchmarking of numerical model in OpenFOAM® for the prediction of channel segregation during columnar alloy solidification

• Benchmark sn-5 wt%Pb alloy solidification problem is simulated using OpenFOAM solver. • Mesh sensitivity analysis is performed to resolve the flow in channels segregates. • Effect of microsegregation and mushy zone models on channels formation is studied. • Scheil's model predicts more negative and positive segregation than Lever rule. • Inertial drag in mushy zone significantly affects the channel segregates formation. In the past, most of the numerical studies on the prediction of macrosegregation and mesosegregation (e.g. channel type segregation) were performed using in-house codes and commercial software, both of which have limitations such as huge license costs, non-availability for general public research etc. In this study, a dedicated CFD solver is developed in an open-source platform – OpenFOAM® for the prediction and characterization of channel segregates in castings. The solver accounts for the fluid flow, heat transfer, species transport, multiscale segregation, solidification and mushy zone drag phenomena that occur during alloy solidification. The developed solver is extensively validated with the reported benchmarked experimental and numerical results for the sn-Pb alloy solidification. Direct numerical simulations (DNS) are performed to resolve the flow field in and around the channels (which are of mesoscopic length scale) for the first time. It is observed that mesh size ≤ 2 d 2 (secondary dendritic arm spacing) is required to resolve the flow in channels and accurately predict their morphology and locations. Further, the effect of different microsegregation and mushy zone drag models on the segregation and morphology of channel segregates is thoroughly investigated. The predicted extent of negative and positive segregation is found to be more (negative segregation ∼ 7.7 wt%Pb and positive segregation ∼ 18.7 wt%Pb) in Scheil’s microsegregation model compared to Lever rule. Furthermore, the inertial drag in the mushy zone significantly affects the convective transport of solute and thereby the morphology and the number of channel segregates.

Thermal, Metallurgical and Mechanical Determinants of Laminar Nickel/Aluminum Dissimilar Alloys during Laser-material Interaction Part I: Nickel-based Superalloy

To develop in-depth understanding of metallurgical phenomena for completeness, mechanical heterogeneity is kinetically and thermodynamically explained by thorough microstructure characterization of engineering materials, e.g. polycrystalline nickel-based superalloy, for alleviation of weldability-related problems during laser-induced keyhole fabrication. Because of nonequilibrium solidification behavior inside weld pool, finer γ phase dendrite substructure is confined to partition-resistant keyhole bottom part, coarser dendrite is crystallography-independently circumscribed at partition-vulnerable neck transition region of full penetration weld, and thus microstructure is lack of homogeneity. Abundance of segregation-driven eutectic phase or intermetallic phase in the interdendrite area throughout weld is essentially attributable to nonequilibrium solidification conditions, and morphologically increases susceptibility to mechanical properties deterioration. As a result of γ phase instability, Niobium-aided Laves/γ eutectic reaction in the vicinity of dendrite interstices at terminal stage of solidification contributes to severe dendrite boundaries brittleness, impairs mechanical properties, which is consistent with metallography and fractography results, and is more deleterious to weldability, since solute redistribution and supersaturation adversely exacerbate segregation behavior in the residual interdendrite liquid, especially asymmetric weld pool shape. There is inverse parabolic relationship between secondary dendrite arm spacing and solute partition coefficient, when location varies from nail-shaped weld upper site to bottom site. Chemical, microstructural and mechanical heterogeneities are more geometrically favorable in the curvature-related neck transition region. In addition, the mechanism of thermal, metallurgical and mechanical inhomogeneities, which are attributed to asymmetric weld pool shape, is consequently proposed. Untoward metallurgical phenomena, such as microstructure heterogeneity and brittle Niobium-rich Laves/γ eutectic phases mitigate strength, ductility and toughness of weld. In order to macroscopically and microscopically satisfy superior mechanical properties requirement, chemistry and microstructure of high quality weld are metallurgically controlled. Fruitful metallurgical information and mechanical data further support the reasonable explanations. It is imperative to progressively advance welding metallurgy, weldability and fabricability of intricate shape for welding defects minimization, suppress segregation and further develop mechanical properties through viable design and control of laser processing, simultaneously.

Dendrite morphology in Al-20 wt%Cu hypoeutectic alloys in 24 T high magnetic field quantified by ex-situ X-ray tomography

The morphology of the primary α-Al crystal in a solidifying Al-20 wt%Cu alloy under the high static magnetic field (HMSF) was studied by using the ex-situ X-ray computed tomography technique. The side arms were the seaweed and the dendrite under 0 T and 24 T HSMF, respectively. They were identified by 2D slices and 3D reconstruction of dendrites. The secondary dendrite arm spacing increased and the interface curvatures decreased under the HSMF. The growth angle between the secondary dendrite arms and their stems was close to 90° in the case of the 24 T HSMF. These were attributed to the decrease of constitutional supercooling caused by magnetohydrodynamic effect.

Diffusion-limited Nonequilibrium Phase Transformation of Nickel/Aluminum Dissimilar Materials during Laser Welding Part I: Precipitation-strengthened Polycrystalline Nickel-based Superalloy

Polycrystalline heterogeneities of grain growth, phase transformation and tensile properties are discussed by metallography, spectroscopy and fractography when choosing laser welding with butt-joint configuration to satisfactorily minimize effect of inappropriate weld pool shape on microstructure and mechanical properties of γʹʹ precipitation hardable nickel-based superalloy in aerospace industry. There is parabolic relationship between weld penetration and diffusion-limited nucleation, growth and coarseness of secondary dendrite arm spacing alongside fusion boundary through nonequilibrium solidification process, which is dendritically susceptible to grain growth variation and metallurgical discontinuities. The amount, size, morphology and distribution of nonequilibrium intermetallic phase near the dendrite boundaries are kinetically and thermodynamically rely on weld pool shape whose crystal structure is incoherent with γ solid solution austenite phase and increases preference for crack initiation and propagation of brittle intergranular and ductile dimple fracture failures to enormously contribute to losses of strength and ductility. Unsymmetrical keyhole weld is thermometallurgically inconvenient for amelioration of microstructure and mechanical properties on either side, and adversely mitigates resistance to hypereutectic-aided dendrite embrittlement. Beneficial grain refinement and suppression of detrimental nonequilibrium intermetallic phase at the same time are challenging. This problem is an integral part of inhomogeneous microstructure and mechanical properties. The finer grain size occurs, the larger grain boundary area is available for Laves/γ eutectic-type reaction and vice versa. Contributions of coarse grain kinetics and metallurgical reaction thermodynamics to weld disintegration and fracture failure mechanism of tensile properties are explained by microstructure characterization of multicomponent and multiphase weld. Finally, it is imperative to dendritically balance these important factors to minimize inevitable interface solidification products and anomalous substructure growth, and reasonably advance superior mechanical properties of reliable weld.

Comparative study on autogenous diode laser, CO2 laser-MIG hybrid and multi-pass TIG welding of 10-mm thick Inconel 617 superalloy

The present study investigated comparative welding performance of single-pass autogenous diode-laser (DLW) and CO2+ MIG laser-hybrid welding (LHW) processes with multi-pass conventional TIG welding in 10-mm thick Inconel 617 with comprehensive correlation of bead profile, microstructure with mechanical performance of weldments employing FESEM, XRD, high-speed Nano Indentation, uniaxial tensile and impact testing. Results indicated that welds produced by LHW and DLW constituted ‘wine-cup’ and ‘Y” shaped profiles respectively with reduced fusion zone (FZ) and heat-affected-zone (HAZ), high depth-to-width aspect ratio and low distortion as compared to that of ‘bath-tub’ shaped TIG weld on account of low heat input and high-coupling efficiency of laser. Indeed, inhomogeneous microstructures were observed in both LHW and TIG welds due to dual heat sources in LHW (laser + MIG) and multiple passes in TIG as compared to that of DLW, wherein, refined microstructure with fine secondary-dendritic arm spacing and low levels of micro-segregation in interdendritic regions of FZ. XRD confirms presence of higher amounts of M23C6 and M6C carbides in TIG FZ as compared to that of LHW and DLW counterparts and attributed to enhanced micro-segregation due to reduced cooling rate and repetitive thermal cycling effect of multiple passes. Micro and Nano-Indentation hardness profiling indicated that the hardness of FZ of TIG welds is comparatively lower to that of DLW and LHW on account of enhanced segregation and grain coarsening effect. Weld joint efficiency obtained in DLW and LHW are slightly higher than TIG weldments with highest elongation observed in DLW. Overall, joint efficiency of LHW and DLW was found adequate with vast reduction in distortion and improved adoptability for applications in thermal power plant.

Microstructure Analysis of Al-7 wt% Si Alloy Solidified on Earth Compared to Similar Experiments in Microgravity

During ground-based solidification, buoyancy flow can develop by the density difference in the hypoeutectic type of the alloys, such as Al-7 wt% Si alloy. Buoyancy flow can affect the thermal field, solute distribution in the melt, and the position and amount of the new grains. As solidification is a very complex process, it is not very easy to separate the different effects. Under microgravity conditions, natural convection does not exist or is strongly damped due to the absence of the buoyancy force. Therefore, experiments in microgravity conditions provide unique benchmark data for pure diffusive solidification conditions. Compared to the results of the ground-based and microgravity experiments, it is possible to get information on the effect of gravity (buoyancy force). In the framework of the CETSOL project, four microgravity solidification experiments were performed on grain refined (GF) and non-grain refined Al-7 wt% Si alloy onboard the International Space Station in the Materials Science Laboratory. These experiments aimed to study the effect of the solidification parameters (solid/liquid front velocity vSL, temperature gradient GSL) on the grain structure and dendritic microstructures. The microgravity environment eliminates the melt flow, which develops on Earth due to gravity. Four ground-based (GB) experiments were performed under Earth-like conditions with the same (similar) solidification parameters in a vertical Bridgman-type furnace having four heating zones. The detailed analysis of the grain structure, amount of eutectic, and secondary dendrite arm spacing (SDAS) for different process conditions is reported and compared with the results of the microgravity experiments. GB experiments showed that the microstructure was columnar in the samples that do not contain GF material or in case the solid/liquid (vSL front velocity was slow (0.02 mm/s)). In contrast, in the sample which contained GF material, progressive columnar/equiaxed transition (PCET) was observed at vSL = 0.077 mm/s and GSL = 3.9 K/mm. The secondary (SDAS) dendrite arm spacing follows the well-known power law, SDAS=K[t0]13, where K is a constant, and t0 is the local solidification time for both GB and µg experiments.

Investigation on influence of Ta on microstructure evolution of directionally solidified Ni-based superalloys

The microstructure evolution in directionally solidified Ni-based superalloys DZ411 with different Tantalum (Ta) compositions (2.72, 3.10, and 4.00 wt%) was investigated to develop Ta-containing hot corrosion superalloys. It is found that the addition of Ta promote the element segregation in the interdendritic region, thus, leading to the increase in volume fraction of MC carbides and eutectics, while the morphology of them are not obviously changed. In addition, both the volume fraction of regions increases with increasing Ta composition, which is attributed to the reason that Ta is the main forming element of γ' phase. Besides, the previous controversy on the dependence of both the primary and secondary dendrite arm spacing on Ta composition was solved. The increase of dendrite arm spacing with increasing Ta composition can be attributed to increasing solidification range of these superalloys through both JMatPro and Different Scanning Calorimetry (DSC) analysis. Furthermore, the atomic arrangement of γ and γ' phases in alloys was characterized by spherical aberration corrected transmission electron microscopy. Both the saturation of refractory elements in γ matrix and the lattice constant of γ matrix was found to increase with increasing Ta composition, leading to the increase of lattice mismatch of γ/γ' phase and more regular shape of γ' phase which has also been confirmed by JMatPro analysis.

Solidification structure and central segregation of 6Cr13Mo stainless steel under simulated continuous casting conditions

Central macrosegregation and coarse microstructure are the main defects in ingot castings of high alloy steels. Rapid cooling associated with continuous casting could be beneficial in improving the internal quality of ingot castings. This work evaluated the effects of superheat and cooling intensity on the solidification structure and macrosegregation in 6Cr13Mo stainless steel billet. Numerical modeling was performed to determine the cooling conditions of a continuous casting billet. The simulated cooling conditions were used to solidify 6Cr13Mo steel samples in a thermal simulator. Experimental results suggest that low superheat of the melt combined with an increased cooling intensity reduce central segregation and form refine microstructure. Relationships between the macrosegregation index, (Km_i), of solute elements in the final solidification zone and secondary dendritic arm spacing (λ2), which inversely proportional to cooling rates, conform to simple monotonic equations. Such monotonic relationships indicate that increasing cooling rates is an effective way of reducing central macrosegregation and refining microstructure of 6Cr13Mo steel billet.

Numerical investigation on the effect of axial magnetic field on metallurgical quality of ingots during vacuum arc remelting process

The electromagnetic stirring force generated by the axial magnetic field (AMF) interacting with the process current greatly affects the solidification process and consequently the metallurgical quality of ingots, but there are few studies about the effect of AMF on the multi-physics field involving solute transport during VAR process. Based on this, a three-dimensional full-size transient model has been developed to investigate the role of AMF on the metallurgical quality evaluated by the segregation defect and solidification quality in this paper. All essential considerations of the fluid flow, heat transfer and solute transport are incorporated in the model. Both the local solidification time (LST) and secondary dendrite arm spacing (SDAS) are adopted as indexes to characterize the solidification quality. The results show that the stirring force caused by the AMF greatly promotes the downward heat transfer and thus leads to deeper molten pool. The final value of volume-averaged segregation extent is greatly reduced by 24.13% as the AMF intensity increases from 0 to 50 G, while the difference of this value between 50 G and 100 G is only 1.96%. The volume-averaged values of the LST and SDAS are all reduced with the increase of AMF intensity, in which both values change from 1088 s to 190 μm without AMF to 890 s and 182 μm under 100 G. These results indicate that the application of AMF can significantly improve the metallurgical quality of ingots, but this effect becomes more and more limited with the increase of AMF intensity.

Effect of Magnetic Stirring on Microstructures and Properties of Ag-1.5Cu-1.0Y Alloy.

The microstructure of alloys is an important factor that affects their application. In this work, the Ag–1.5Cu–1.0Y alloys were prepared by the permanent magnet stirring method at different rates. The secondary dendrite arm spacing, dendritic segregation, density, microhardness, electrical resistivity, and sulfuration corrosion resistance were analyzed to investigate the effects of different rotation speeds on the microstructures and properties of the Ag–1.5Cu–1.0Y alloy. The results showed that the primary dendrite was refined and the secondary dendrite arm spacing decreased with the increase in the stirring rate of the permanent magnets. The Ag–1.5Cu–1.0Y alloys prepared with a 900 r/min stirring rate had the largest microhardness, relatively high density, and the best sulfuration corrosion resistance. However, the stirring rate had little effect on the electrical resistivity of the Ag–1.5Cu–1.0Y alloys. To sum up, the Ag–1.5Cu–1.0Y alloy had the best comprehensive properties when the permanent magnet stirring rate was 900 r/min, including the most refined dendrites, relatively high density, the largest microhardness, and the best sulfuration corrosion resistance. The study of the effects of permanent magnet stirring speed on the microstructures and properties of the Ag–1.5Cu–1.0Y alloy provides an experimental basis for future alloy casting optimization and property improvement of silver-based alloys.

Shape-Dependent Strength of Al Si9Cu3FeZn Die-Cast Alloy in Impact Zone of Conformal Cooling Core.

This article presents the results of shape-dependent strength analyses in die-castings from traditional (straight-drilled) and conformal core-cooling moulds. Cores with a traditional cooling layout were made of H13 steel using machining, and the working sections of the conformal cores were made using the selective laser melting method (SLM). Two series of casts were produced in the same mould. For Series A, the mould was fitted with traditional cooling cores, and for Series B, the same mould was fitted with conformal ones. The cast specimens were subjected to two weeks of natural ageing. The strength testing of the casts determined the levels of the destructive forces. The destructive forces in the core-cooling impact zones were approximately 28% higher in the B samples than they were in the A samples. The impact of the alloy’s porosity, density, and microstructure on the strengths of the casts was demonstrated. The alloy densities in the central (broken-off) fragments of the casts from Series A were 2.6646 g/cm3; these were 2.6791 g/cm3 in the cases of casts from Series B. The values of the secondary dendrite arm space (SDAS) ranged from 6 to 13 μm in the analysed cross-section of the set of the A casts, and between 3 and 12 μm in the same zone of the set of the B casts. The results of the experimental determinations of the casts porosity levels and SDAS parameters were compared with the results of numerical simulations that were carried out in ProCAST software.

Influence of high-intensity horizontal mould vibration on the density, grain refinement and mechanical characteristics of a die cast aluminium alloy (LM21)

The present investigation analysed the effect of mould vibration using an indigenous vibrating setup on the physical, metallurgical and mechanical properties of die cast A308 alloy. The alloy was vibrated at a constant amplitude of 31µm at 75, 100, 125 and 150 Hz. The work aims to generate more nucleation sites during melt solidification by fragmenting dendrites and Si particles. To examine the changes in metallurgical characteristics, α-Al, secondary dendritic arm spacing, length, width and aspect ratio of eutectic Si particles were studied using optical microscopy, scanning electron microscopy and X-ray diffraction analysis. It was observed that the size of α-Al, secondary dendritic arm spacing, length, width and aspect ratio of eutectic Si particles were decreased by 47%, 53%, 65%, 12% and 61%, respectively. Whereas, the shape factor and density of the alloy were enhanced by 25% and 1.62% compared to stationary cast samples because of mechanical vibration treatment at an optimum frequency of 100 Hz. Mechanical testing findings demonstrated a 13%, 20%, 15% and 20% increase in the parameters such as yield strength, ultimate tensile strength, percentage elongation and microhardness at 100 Hz when compared to the stationary cast sample. Grain refinement, uniformity of structure, improvement in the morphology of metallurgical characteristics, decreased porosities and subsequent increase in density were all factors that contributed to the improvement. Tensile test samples with scanning electron microscopy fractography demonstrated transgranular brittle fracture and a few ductile rips.

Numerical study of thermal fluid dynamics and solidification characteristics during continuous wave and pulsed wave laser welding

Pulsed wave (PW) mode laser welding is often preferred under conditions where specific heat input and tailorable cooling rate are required. Previous works on numerical modelling of laser welding mainly focus on continuous wave (CW) mode, while physical understanding of PW mode from numerical investigation is scarce. In this work, a 3D multi-physics thermal fluid model is developed considering both CW and PW modes laser welding. The main physical factors involving heat transfer, thermal fluid flow, Fresnel absorption, recoil pressure induced by metal evaporation, Marangoni effect driven by surface tension, free surface tracing, and laser multiple reflections inside the keyhole are included in this model. The model is tested and validated against the corresponding experimental results for laser welds of stainless steel 316 L. The simulated weld fusion cross sections and surface molten pool agree well with the experimentally observed results for both CW and PW modes. The results show that PW mode exhibits a higher depth of penetration and a smaller molten pool at the same heat input with CW mode. In CW mode laser welding, the shape of keyhole is unstable, while its depth keeps relatively stable. There are two main flow patterns in the molten pool of CW mode welding: an outward flow from keyhole outlet to the rear weld pool and a clockwise flow from the keyhole tip to the rear weld pool. While for PW mode, keyhole and molten pool show periodic behaviors with a period of the laser energy output used. The periodic fluid flow pattern is mainly affected by the gradually changed surface tension and suddenly changed recoil pressure. Furthermore, it is found that a finer microstructure with a smaller secondary dendrite arm spacing is obtained in PW mode due to a higher cooling rate from both the calculated and experimentally measured results.

A Study on the Interrelationship between the Microstructural Features and the Elevated Temperature Strength of Multicomponent Al-Si-Cu-Ni Casting Alloys

The elevated temperature strength of multicomponent Al-Si alloys is greatly affected by the volume fraction and the interconnectivity of hard phases formed upon solidification. In the present investigation, such influences were examined for two Al-Si-Cu-Ni alloys with different total volume fractions of hard phases. To control the microstructural features related to the size of the phase, the specimens were prepared with and without ultrasonic melt treatment (UST) at different cooling rates. The microstructures of the alloys were composed of primary Si, eutectic Si, (Al,Si)3(Zr,Ni,Fe), Al9FeNi and Al3(Cu,Ni)2 phases. The microstructural features, such as the size and aspect ratio of each phase, changed with UST and cooling rate, and accordingly, the elevated temperature strength at 350 oC was changed. The alloy with a high volume fraction of about 30 vol.% exhibited increased elevated temperature strength at 350 oC when ultrasonic melt treated, and the alloy having a volume fraction as low as about 18 vol.% exhibited the opposite results. Considering the microstructural features of the multi-component Al-Si alloy, a hexagonal shear-lag model was suggested, based on the well-known shearlag model proposed by Nardone and Prewo (Scr. Metall. 20;1986:43-48). Using the 2-D microstructural factors such as the size, aspect ratio of the phase and secondary dendrite arm spacing, the elevated temperature strength was calculated and compared with the measured value. Based on the hexagonal shear-lag model, the influence of microstructural factors on the elevated temperature strength was discussed for multi-component Al-Si-Cu-Ni alloys.

Secondary dendritic arm spacing and cooling rate relationship for an ASTM F75 alloy

The solidification evolution of an ASTM F75 alloy was studied for samples quenched during directional cooling. These samples were obtained using five extraction rates (1.2–18 mm/min) and two temperature gradients (6 and 12 K/mm), leading to a 25-fold change in the cooling rate (0.13–3.31 K/s). The solidification range for this alloy is observed to be approximately 200 °C. The solid grows more rapidly in the first 100 °C of this interval, where a solid fraction of approximately 0.8–0.9 is reached. Coarsening of the Secondary Dendrite Arm Spacing, SDAS, occurs mainly during this growth period. This behavior defines an effective temperature interval for the SDAS coarsening instead of the whole solidification interval. Consideration of this effective temperature interval enables us to reconcile the measured SDAS-cooling rate relationship with that obtained from the data for the cast samples. A model of the SDAS coarsening during anisothermal solidification of multicomponent alloys is proposed. The predictions of this model compare well with the present experimental results.

Microstructure, hardness and slurry erosion-wear behaviors of high-speed laser cladding Stellite 6 coatings prepared by the inside-beam powder feeding method

Stellite 6 alloy has been considered a promising coating for hydro-turbine parts owing to excellent erosion resistance. In the present work, Stellite 6 coatings have been prepared by the high-speed laser cladding technique with the inside-beam powder feeding method. Microstructure, hardness and erosion-wear behaviors of the Stellite 6 coatings have been systematically investigated. The results indicate that, there are no visible defects in all the Stellite 6 coatings; the coating thickness gradually decreases with the cladding speed while it increases with the laser power, ranging from 429.35 to 842.19 μm. The coatings are mainly composed of γ-Co and carbides, and the microstructure of γ-Co gradually displays planar, columnar, dendrite and equiaxed crystals from the substrate interface to coating surface. The second dendrite arm spacing (SDAS) of γ-Co shows positively linear relationship with the line energy density (LED). The dilution rate of Stellite 6 coating increases with the laser power and cladding speed, while the microhardness shows the opposite variation trend. The highest hardness of 610.26 HV can be obtained when the laser power and cladding speed are 1600 W and 4000 mm/min. The erosion-wear resistance of Stellite 6 coating is approximately 2–5 times higher than that of the 45# steel substrate, and it mainly depends on the coating hardness. Microcutting and impacting from abrasive particles are the predominant damage mechanisms while the corrosion attack from 3.5% NaCl solution can be negligible. Thus, the erosion resistance of Stellite 6 laser cladding coating can be further improved by enhancing the coating's hardness.

Fast scanning calorimetry study of Al alloy powder for understanding microstructural development in laser powder bed fusion

We examined the variation in the solidification microstructure of AlSi10Mg alloy powder over a wide range of cooling rates controlled by using the fast scanning calorimetry (FSC) technique. Two exothermic peaks derived from the primary solidification of the α-Al phases and the α-Al/Si eutectic reaction were detected. These exothermic peaks became broader and shifted to lower temperatures at higher cooling rates. Focus ion beam (FIB) milling enables microstructural characterization of AlSi10Mg powder solidified at controlled cooling rates above 102 °C·s−1 in the FSC equipment. The sample solidified at 4 × 104 °C·s−1 exhibited a fine microstructure consisting of a primary α-Al phase surrounded by lamellar-shaped Si phases. The fraction of the Si phase continuously decreased with increasing cooling rate, indicating higher content of solute Si element in the α-Al phase solidified at a higher cooling rate. The secondary dendrite arm spacing (λ) of the primary α-Al phase decreased in the sample solidified at a higher cooling rate (dT/dt). The relation follows a general equation of λ = A (dT/dt)-n (A: 38.4, n: 0.33) in wide range of cooling rate (10-1 ∼ 104 °C·s−1). The results were utilized to discuss the cooling rate of Al alloys during the laser powder bed fusion (L-PBF) process.

Effects of mold electromagnetic stirring on heat transfer, species transfer and solidification characteristics of continuous casting round billet

In this study, the temperature distribution, species transfer, and solidification behavior in round billet continuous casting with mold electromagnetic stirring (M-EMS) was numerically investigated. The joule heat induced by the M-EMS and the liquid fraction profiles in the mushy zone were observed, whereby it was found that increasing current intensity caused further increase in electromagnetic force, which reduced the impact depth of liquid steel while accelerating the dissipation of superheat. The high liquid fraction in the mushy zone at the exit of the secondary cooling zone were 68.1% (case 1) and 77.1% (case 6). The central carbon contents of the round billets were 0.337 wt% (case 1) and 0.335 wt% (case 6). Moreover, the proportion of solidification microstructure and secondary dendrite arm spacing of the round billet samples were compared, which revealed that increasing the current intensity, casting speed, and cooling intensity, were all conducive to improving the uniformity of carbon distribution and compactness of the solidification structure.

Effect of Ce content on the microstructure and mechanical properties of squeeze-cast Al–5Mg-2.2Si-0.6Mn alloys

The effect of Ce in amount of 0–0.5 wt.% on the microstructure and mechanical properties of the squeeze-cast Al–5Mg-2.2Si-0.6Mn alloys produced by squeeze casting technique was explored. The microstructure was observed by optical microscopy and scanning electron microscopy. The hardness and mechanical properties of the alloys were measured by Vickers hardness and tensile test. The microstructure of the squeeze-cast Al–5Mg-2.2Si-0.6Mn alloys is composed mainly of α-Al, Al–Mg2Si eutectic cells, the bulky lath-shaped Al8CeMn4 phase, Al2Mn3 and Al5Mn2Si5 phases after Ce is added. The results show that the addition of Ce leads to the grain refinement, which can improve the mechanical properties. When adding superfluous Ce, the formation of coarse and brittle Al8CeMn4 phase seriously reduces the mechanical properties of the alloy. With the increase of Ce content, the secondary dendrite arm spacing of different alloys decreases first and then increases, hence the hardness and tensile properties increase first and then decrease. The most remarkable improvement in the mechanical properties was acquired in the alloy with 0.10 wt.% Ce addition, the hardness reaches 48.1 HV, the ultimate tensile strength, yield strength and elongation are 265.5 MPa, 185.8 MPa and 5.8%, respectively.

Effect of Ultrasonic-Assisted Modification Treatment on the Microstructure and Properties of A356 Alloy.

Ultrasonic treatment was applied to an A356 aluminum melt with different modifiers, and the effects of ultrasonic treatment on the structure and properties of the A356 alloy were studied. The results showed that α-Al was effectively refined with different ultrasonic modification treatments. In particular, ultrasonic treatment showed the most obvious refinement with macroscopic grains of unmodified alloy and optimized the refinement of secondary dendrite arm spacings in the Sr/Ce synergistic alloys. The eutectic Si of the unmodified A356 alloy had no obvious change after the ultrasonic treatment, but the branch diameter of the eutectic Si reduced in the Sr and Sr/Ce modification alloys after the ultrasonic treatment. The ultrasonic treatment significantly improved the ultimate tensile strength and elongation of the as-cast A356 alloy with the unmodified material, which was due to refinement of the α-Al grains by the ultrasonic treatment. After the T6 heat treatment, the ultimate tensile strength values of the alloys showed no obvious change due to the ultrasonic treatment, but the plasticity of the alloy was significantly improved. Mg2Si precipitation was the dominant strengthening mechanism during the T6 heat treatment, while the plasticity was determined by the size and distribution of the eutectic Si. Acoustic cavitation caused by the ultrasound-activated impurities and the induced heterogeneous nucleation and supercooled nucleation in the groove melt was the main cause of the α-Al refinement, the eutectic Si modification and the improvement in the mechanical properties.