Increasing Solidification Rate Research Articles

New insights on dislocation forming mechanism of nickel-based superalloy fabricated by laser powder bed fusion

In this work, the forming mechanism of dislocation in laser powder bed fusion (LPBF) built IN738LC alloy was elucidated by studying the intrinsic relationships between the density and distribution of dislocation and the orientation and size of dendrite. The substructures with different dislocation densities and cell spacings were customized by controlling solidification conditions, and analyzed by multi-scale characterizing methods. The results show that an increase in geometrically necessary dislocation (GND) density from 1.48 × 1014 to 1.82 × 1014m−2 is accompanied by a decrease in cell spacing from 1046 nm to 640 nm as the solidification rate increases. However, the relationship between GND density and cell spacing doesn't follow Ashby's model. This indicates that GNDs are not entirely generated by thermal strain itself. On the other hand, the characterization results of cell structures show that a number of misfit dislocations are formed by the thermal strain induced deflection growth of the dendrites. These prove that the dislocations are comprehensively generated by thermal strain itself and dendritic deflection growth.

Effects of Chemical Composition and Solidification Rate on the Solidification Behavior of High-Cr White Irons

The effects of chemical composition and solidification rate on the solidification behavior of high-Cr white irons were investigated through directional solidification. Increasing the solidification rate in hypoeutectic alloys caused finer dendrite-arm spacing, as expected. The eutectic structure, which formed in the interdendritic region, was comprised of M7C3 and austenite; however, secondary dendrite arms of hypoeutectic alloys contained a few M7C3 particles that solidified prior to the eutectic structure. The transition from cellular to dendritic solidification occurred at a solidification rate between 50 µm/s and 100 µm/s in a near-eutectic alloy. In the near-eutectic alloy with cellular solidification, a directionally arrayed in-situ composite of M7C3/austenite formed within the cell. Speckle-like features appeared in the intercellular region due to M23C6 carbide precipitation during subsequent cooling after freezing. Like dendrite-arm spacing in hypoeutectic alloys, the inter-speckle spacing and the inter-fiber spacing became finer with an increasing solidification rate in the cellular solidification range.

Strength and ductility dual-enhancement of Ni36Co30Fe11Cr11Al6Ti5Nb1 high entropy alloy by directional solidification

Ni36Co30Fe11Cr11Al6Ti5Nb1 high entropy alloys (HEAs) were prepared by directional solidification. The effect of solidification rate on microstructure evolution and mechanical properties was systematically investigated. Results show that the microstructure has a dendritic morphology with uniform orientation. FCC phase rich in Co, Cr and Ni is formed in the dendrite. Laves phase rich in Nb and Ti and FCC phase rich in Ni and Ti are formed between dendrites. With the increase of solidification rate, the dendrites are gradually refined. Tensile results show that the yield strength and elongation increase with the increase of solidification rate. The yield strength and elongation at 100 μm/s are 606.2 MPa and 27.6 %, respectively, achieving an excellent combination of strength and plasticity. Fine-grain strengthening, second-phase strengthening and solid-solution strengthening are the dominant strengthening mechanisms. The dendrites growing along the directional direction reduce the transverse grain boundaries and avoid the rapid propagation of cracks.

Probing effects of solute trapping on the mechanical properties of α-Al in rapidly solidified hypoeutectic Al-10at.%Cu after surface laser melting

After scanned laser surface remelting the resultant rapid solidification microstructures of multicomponent alloys exhibit morphological gradients, with scale refinement and non-equilibrium features, including non-equilibrium solute trapping, which are associated with property changes. Combinations of site-specific mechanical property measurements by nano-indentation and microstructural analyses by electron microscopy have been used to determine microstructure-property relationships for the characteristic features of the rapid solidification microstructure of a hypo-eutectic Al–Cu alloy containing 10 atom percent Cu. A microstructure gradient developed as the solidification rate increased monotonically during the re-solidification process. For a laser scan velocity of 3 mm/s most of the rapid solidification microstructure is comprised of micrometer scale refined dendritic cells of α-Al(Cu) and nanometer scale refined intercellular θ-Al2Cu phase or eutectic. The cellular α-Al(Cu) constituent consistently showed Cu-solute supersaturation and a 36 % hardness increase relative to the as-cast state. The eutectic microconstituent exhibited increases in hardness that correlated quantitatively with the increasing lamellar spacing refinement obtaining for increasing solidification rate. The possible contributions from precipitation, solute and grain-size strengthening mechanisms to the hardness increase in the α-Al(Cu) cells have been assessed. The hardness increase of the α-Al(Cu) cells was attributed to a combination of solute and grain size strengthening.

Model for particle capture by the solid-liquid interface during solidification of metal matrix nanocomposites

A sharp-interface model for solidification of nanocomposites with nanoparticle-interface interaction is presented. The main novelty of the work is that the engulfment or non-engulfment of nanoparticles is modelled as a function of the local interface velocity and a particle size-based critical velocity. This is incorporated through a variable partitioning coefficient while solving the nanoparticle conservation equation which is modelled similar to an alloying constituent. The movement of nanoparticles is governed by nanoparticle-interface interaction and by diffusion due to Brownian motion. The model couples a sharp-interface solidification model, species transport model and nanoparticle transport model. Thus, the engulfment and the resultant distribution of nanoparticles in the solidified region can be predicted for different combinations of initial undercooling and nanoparticle size. The effect of accumulated nanoparticles on the solute transport and solidification rate is also included in the model. Results show that the engulfment of smaller nanoparticles is very low resulting in high accumulation near the interface. This results in a high restriction to solute diffusion and solidification. Combined variation of initial undercooling and particle size shows that the average engulfment values for initial undercooling of 0.3, 0.5, and 0.7 are 2.75 %, 7.75 % and 21.50 % for r* = 0.1, 15.5 %, 53.5 %, and 82.5 % for r* = 0.5, and 38.0 %, 77.75 %, and 93.25 % for r* = 0.9. Thus, the engulfment of nanoparticles into the solid improves with increase in nanoparticle size and with the increase in solidification rate.

Investigation of columnar to equiaxial transition criterion and solidification conditions for Ni-based superalloy in laser powder bed fusion

Laser powder bed fusion (L-PBF) is a pioneering additive manufacturing (AM) technology for superalloys. The study of the columnar to equiaxial transition (CET) of a superalloy in the L-PBF process is important for controlling the grain morphology of the superalloy and achieving the production of a superalloy with a single-crystal microstructure. This study aims to reveal the fundamental law of CET of superalloys in the L-PBF process. An algorithm was developed to successfully extend the application of the CET model to cover the solidification rate interval of the L-PBF process, and the accuracy of the model was experimentally verified. The extended CET model indicated that the critical temperature gradient from columnar epitaxial growth to equiaxial growth does not increase monotonically with increasing solidification rate but exhibits a peak. The temperature gradient at the solidification interface of the L-PBF molten pool was greater than this peak. In addition, the solidification mode of the alloy remained epitaxial after adjusting the laser power and scanning rate, because adjusting these two parameters exerted a weak effect on the temperature gradient at the solidification interface of the L-PBF molten pool. The excessively high temperature gradient of L-PBF also makes it difficult to change the solidification mode from epitaxial to equiaxial at preheating temperatures below 1000 K. Changes in the morphology of the molten pool due to the increased temperature of the deposited layer are of more concern in terms of microstructure formation. The results of this study are expected to provide guidance for designing process parameters for optimizing the microstructures of superalloys fabricated using L-PBF.

Toward sustainability for upcycling SoG-Si scrap by an immersion rotational segregation purification process

Recycling solar grade silicon (SoG-Si) scrap is critical for the sustainability of solar energy and fulfilling to close material loops in circular economics. A major challenge is to control trace impurities efficiently in short time for upgrading scrap to a level of SoG-Si. Here, a new immersion rotational separation purification (IRSP) process is developed to enhance trace impurities removal and the crystal growth from remelted metal by driving the rotating shear flow to improve the efficiency of solute and heat transfer. In a departure from conventional practice that forces extremely low solidification rate limited by the diffusion layer depth at approximately equilibrium solidification interface, here IRSP achieves the deep removal of impurities and the increase of solidification rate by the non-equilibrium rotating solidification interface. In the non-equilibrium solidification process caused by mold rotation, the rotating shear flow not only reduces the diffusion layer depth to improve impurity removal efficiency, but also increases the temperature gradient of the melt at the solidification front that is conducive to the increase of crystal growth rate. The new method is applicable to molten Si and the production of SoG-Si (99.9998%) has been demonstrated, which is able to improve the quality of recycled SoG-Si requiring a low energy input. It is expected that this process helps close high purity Si material loops, reduce resource consumption and the carbon footprint of the photovoltaic industry.

Formation of dendrites and strengthening mechanism of dual-phase Ni36Co30Fe11Cr11Al6Ti6 HEA by directional solidification

In order to reveal the effect of solidification rate on microstructure and improve room and high temperature tensile properties, Ni36Co30Fe11Cr11Al6Ti6 high entropy alloys (HEAs) were prepared by directional solidification at different solidification rates (5, 10, 20, 50 and100 μm/s). Results show that two kinds of dendrite form in the directionally solidified process. One is primary solidified dendrite, which is composed of FCC1 phase enriched in Co, Cr and Fe. Another is secondary solidified dendrite, forming between the primary solidified dendrites and consisting of the FCC2 phase enriched in Ni, Al and Ti. As the solidification rate increases, the primary solidified dendrite spacing decreases, the solute enrichment layer overlaps, and the constitutional undercooling in the inter-dendrite region decreases, increasing the difficulty of the secondary solidified dendrite formation. Tensile results show that the strength of Ni36Co30Fe11Cr11Al6Ti6 HEAs gradually increases with the increase of solidification rate. The alloy with a solidification rate of 100 µm/s exhibits excellent comprehensive room temperature properties, the yield strength, fracture strength and tensile strain are 732 MPa, 1020 MPa and 34.9 %, respectively. Furthermore, the directionally solidified Ni36Co30Fe11Cr11Al6Ti6 HEA still remains high yield strength of 533 MPa at 900 ℃. Fine grain strengthening, solid solution strengthening, second phase strengthening and hysteresis diffusion effects play an important role in enhancing the mechanical properties. By comparing the mechanical properties of other HEAs at room and high temperature, the proposed directionally solidified Ni36Co30Fe11Cr11Al6Ti6 HEAs with excellent mechanical properties show great potential in industrial applications.

Comprehensive microstructure regularization mechanism and microstructure–property stability at 1773 K of directionally solidified Al2O3/GdAlO3 eutectic ceramic composite

Melt grown Al2O3/GdAlO3(GAP) eutectic ceramic composite is considered as a promising ultra–high temperature structural material in fields as aerospace, power generation and so on. In order to tune the properties of directionally solidified Al2O3/GAP eutectic ceramic composite, its microstructure evolution is investigated over a wide range of compositions and solidification rates by laser floating zone melting method (LFZM). With the increase of solidification rate and the enrichment of Al2O3 phase, the eutectic microstructure undergoes a transformation from “Chinese script” irregular morphology to rod–like regular and complex regular morphology. The synergistic effects of composition and solidification rate on the microstructure regularization have been quantitatively characterized by the proportion of regular eutectic. High solidification rate (>200 μm/s) is beneficial to the formation of isothermal interface, partly inhibiting the anisotropic growth of Al2O3 faceted phase, which further promotes the coupling growth of the eutectic phases at the front of the interface and facilitates the microstructure regularization. The stability of eutectic spacing is verified to coincide with the Ostwald ripening relationship when the eutectic ceramic composite is thermally exposed at 1773 K for long–term (250 h). The microstructure coarsening rate is less than 0.003 μm/h, and the hardness slightly decreases about 4%, and the fracture toughness remains almost unchanged, which indicates excellent thermal stability of microstructure and property for the Al2O3/GAP eutectic ceramic composite at high temperature.

The combined effect of rapid solidification and phosphor addition on the microstructure of hypereutectic A390 alloys

The influence of the combined effect of rapid solidification and phosphor addition on the morphology of primary silicon particles in A390 Al-Si alloy has been investigated by using light optical microscope (LOM). The P as modifier element was added in the shape of copper phosphor (Cu3P) master alloy. The results show that as the solidification rate increases, the size of primary silicon particles in A390 alloy reduces and the morphology of these particles evolve from plate-like to the fine nodular shape. The rapid solidification has a significant improvement on the refining performance of P to modify and refine the primary Si particles in A390 alloy. It is suggested that the refining performance of P in Cu3P master alloy is related to the formation the AlP particles which act as a heterogeneous nucleation. This affects the solidification rate and restricts the Si phase's growth.

Effect of Solidification Variables on the Tensile Property of 2.8 wt% C–26 wt% Cr White Iron

The present study aimed to investigate the increasing solidification rate during directional solidification developed from in situ composites of M7C3/austenite eutectic, to in situ composites with a small portion of dendrites, and to partial in situ composites with equiaxed structures. M7C3 fibre aligned along the solidified direction in the in situ composites; however, its orientation and shape became irregular among the dendrites. In situ composite structure has higher tensile strength than partial in situ composite or equiaxed material. Crack initiation in the fibre occurred because the fibre could not accommodate the deformation of the matrix under tensile stress. The tensile fracture was caused by both crack initiations in the M7C3 fibre or at the randomly oriented particles, and the crack propagation to matrix.

Effect of low-temperature cooling on corrosion properties of laser welding Hastelloy C-276/304 stainless steel with filler wire

To suppress the weld defects and improve corrosion resistance of laser welded joints, the low-temperature cooling was applied to assist laser welding Hastelloy C-276/304 stainless steel with filler wire. And the different low-temperature cooling (-5 °C, 5 °C, 20 °C) were selected for experimental research. Results show that adding low-temperature cooling can increase the cooling rate and solidification rate of weld. And the temperature gradient grows with the increase of solidification rate near the fusion line of weld. Consequently, the content of precipitated phase decreases from 1.75% to 0.71%, reducing by nearly 59.0%. The increase of temperature gradient can reduce the unmixed zone, which decreases from 22.1 μm to 11.4 μm, reducing by nearly 48.0%. At the same time the contents of corrosion-resistant elements and the proportion of grain boundaries with small angles increase in the laser weld with low-temperature cooling. As the cooling temperature drops, the effect of microstructure optimization of weld is enhanced. Finally, the corrosion properties of welded joints are obviously improved with the temperature decrease. The corrosion potential of welded joints increases by 24.3%. The corrosion current density of welded joints reduces by 54.3%. The intergranular corrosion sensitivity of welded joints reduces by 76.5%.

ANALYSIS OF HEAT TRANSFER MECHANISMS IN THE SOLIDIFICATION OF PCM WITH DIFFERENT PASSIVE ENHANCEMENT TECHNIQUES FOR FREE COOLING APPLICATIONS

The continuous increase in the level of green-house gas emissions and the depletion of fossil fuels are identified as the major driving forces behind efforts to effectively utilize different sources of renewable energy. Free cooling concept is gaining importance in building cooling applications. The medium which is used to store energy is Phase Change Material. Among the phase change materials available commercial PCM is of great use. The only drawback with the PCM is its thermal conductivity is very low. Various heat-transfer enhancement techniques between the phase change material (PCM) and the heat transfer fluid (HTF) were tried by researchers. Passive enhancement techniques are very simple and gives good heat transfer enhancement which decreases the solidification time and increases the solidification rate. In the present work two types of passive enhancement techniques like increasing the roughness of the heat transfer surface and dimples over the heat transfer surface were tried and it has been observed that the dimpled surface will provide better results compared to rough surface over normal surface. Overall there is decrease in solidification time of 37 min. and increase in solidification rate of 19.37% with dimpled surface over normal surface.

Planar-cellular-dendritic transformation in the fusion zone of a GH909 superalloy weldment

Solidification behaviors and resultant crystalline growth of primary gamma (γ) and γ/Laves phases were systematically investigated in a GH909 superalloy laser weldment. Results showed that horizontal microstructural evolution varied along the thickness direction under Marangoni force and eruption of metal vapor from keyhole. Overall, there existed three distinct microstructures: γ trunks, eutectic γ and Laves distinguished by composition and band contrast map. Shoji-Nishiyama orientation relation was verified between γ and Laves in the eutectic constituents. For γ trunks in mid-penetration, its morphology displayed a planar-cellular-dendritic growth sequence. Texturally, there displayed an increasingly concentrated tendency of [100]-crystal direction along RD and high Schmidt factor was observed in planar and cellular zones. By recording and analyzing practical thermal field, an increasing solidification rate and a decrease of thermal gradient not only produced the planar-cellular-dendritic growth mode, but also increased γ primary spacing and grain size from planar zone to the dendritic. As for γ/Laves constituents, reduced eutectic reaction time resulted in a decreased fraction from planar growth to cellular zone. Tensile test and fracture analysis indicated that high Schmidt factor, relatively large grain size and high fraction of γ/Laves constituents jointly led to the failure in planar and cellular zones of the GH909 laser weldment.

Effects of Solidification Rates on Microstructure Refinement and Elemental Distribution of Ti44Al6Nb1.0Cr2.0V0.1B0.15Y Alloy by Rapid Solidification

In order to refine microstructure and improve mechanical properties, size of lamellar colonies and reinforced phase, distribution of elements, and microhardness with different solidification rates were studied by rapid solidification. Results show that microstructure morphology changes from equiaxed grain to granulated dendrites after rapid solidification. When speed of revolution increases, size of lamellar colonies decreases from 53.5 to 16.9 μm and length of TiB particles decreases from 60.1 to 20.4 μm. The B2 phase exists in lamellar colonies and at the boundary of lamellar colonies before rapid solidification and forms in lamellar colonies after rapid solidification. An increase in solidification rate increases degree of supercooling to increase number of nucleation particles which form before solidification front. The β phase incompletely transforms to the α phase and is retained in α phase after rapid solidification. The microstructure does not have enough time to transform completely and high‐temperature microstructure is retained at a higher cooling rate. Test results show that microhardness increases from 453 to 562 HV when speed of revolution increases from 0 to 800 rpm. Improvement of microhardness results from solution strengthening of Nb, Cr, and V, grain refinement strengthening and reduction of the reinforced phase of TiB.

Phase growth patterns for Al2O3/GdAlO3 eutectics over wide ranges of compositions and solidification rates

Phase selection and growth characteristics of directionally solidified Al2O3/GdAlO3 (GAP) faceted eutectic ceramics are investigated over wide ranges of compositions and solidification rates to explore the eutectic coupled zone. Through the observation of the quenched solid-liquid interface, the competitive growth of primary faceted Al2O3 phase, primary non-faceted GAP phase and Al2O3/GAP eutectic with different morphologies is detected. Microstructure transitions from wholly eutectic to primary Al2O3 (GAP) dendrite plus eutectic and then to wholly eutectic are found in Al2O3-20 mol% Gd2O3 hypoeutectic (Al2O3-26 mol% Gd2O3 hypereutectic) ceramics with the increase of solidification rate. The dendrite growth of faceted Al2O3 and non-faceted GAP phases are well predicted by KGT model, which have introduced appropriate dimensionless supersaturation Ω to characterize the anisotropic growth of dendrites. Based on the maximum interface temperature criterion, the competitive growth of primary phase and eutectic is analyzed theoretically and the predicted coupled zone of Al2O3/GAP eutectic ceramics is in good agreement with the experimental results. Besides, the influence of microstructure with these different morphologies on the flexural strength of Al2O3/GAP eutectic ceramics is studied.

CAFE simulation of directional solidification structure of low nickel austenitic stainless steel

In this paper, through the secondary development of ProCAST&CAFE software, the dynamic boundary conditions of solidification process were established, the temperature field and solid fraction in the directional solidification process of low nickel austenitic stainless steel were simulated, and the effect of the pull rate on solidification microstructure was discussed. The results shows that the withdrawal rate is in the range of 15~200 μm/s, as the solidification speed increases, the isotherm becomes dense, gummy zone narrowing. And as the solidification rate increases, the primary dendrite arm gradually refines and the secondary dendrite arm becomes dense and coarse. In addition, as the withdrawal rate increases in the range of 15 to 200 μm/s, the lateral heat dissipation of the edge of the casting is gradually smaller than that of the central longitudinal heat dissipation, and the axial deviation of the dendrites is reduced from 27.66 ° to 25.964 °. At the same time, the comparative analysis simulation results are in good agreement with the experimental results, and the simulation calculation process is more reasonable.

Fabrication, microstructural characterization and mechanical properties evaluation of Ti/TiB/TiB2 composite coatings deposited on Ti6Al4V alloy by electro-spark deposition method

In this study, briquettes of Ti–TiB–TiB2 composite were produced by mixing Ti and B powders, performing self-propagating high-temperature synthesis process and by applying hydrostatic pressure on the reaction products with a high-capacity press. Ti–TiB–TiB2 was then coated to Ti6Al4V substrate by Electro-Spark Deposition (ESD) technique under two atmospheres of air and argon, single and double-layer coatings and in two energy levels of 40 W and 110 W as input powers. Coatings microstructure were examined by Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS) analyzer. Comparison of the coatings microstructure with the electrode microstructure shows that the coatings have a much finer microstructure than that of the electrode due mainly to the high solidification rate in the ESD process. Microstructural observations also revealed that the main phases in the coatings are needle-shaped TiB and particle-shaped TiB2 in the Ti matrix. The coating microstructure was much finer due to an increase in solidification rate by reducing the input power from 110 to 40 W. In such a situation, two phases of TiB2 and TiB with nanometer sizes were scattered in the Ti matrix. Also, due to the trapping of the gases in low input power of 40 W, a limited number of very small gas pores was created in the microstructure. Interface examination of the coating and the substrate by the Line Scan and X-ray Image show a good coherence of the coating with the substrate. Adhesion results showed that the bond strength between the coating and the substrate was higher than 50 MPa in all coatings. No crack was observed in the coatings after bending tests which were performed, in accordance with the intended standard, on the samples around a mandrel. The hardness of all coatings was increased with decreasing the input energy from 110 to 40 W.

The Role of Silicon Morphology in the Electrical Conductivity and Mechanical Properties of As-Cast B319 Aluminum Alloy

The enhanced performance of automotive B319 aluminum alloys can be realized via the improvement of both strength and conductivity. Yet, vastly dissimilar mechanisms are responsible for each property, and the incomplete understanding of their respective dominant microstructural features impedes effective alloy design. In this study, permanent mold cast B319 alloy was systematically produced with total solidification rates between 0.14 and 5.89 °C s−1 and strontium contents up to 300 ppm to isolate their respective effects on material properties. The as-cast samples were characterized by their dendritic structure, eutectic silicon morphology, porosity content, hardness, tensile strength, ductility, and electrical conductivity. With increasing solidification rate, the refinement of microstructure considerably improved all mechanical properties analyzed. Nonetheless, these properties were found to be independent of strontium content, attributed to the role of the coarse and brittle intermetallic phases in fracture initiation. In contrast, conductivity was minimally affected by solidification rate in the unmodified condition. However, the synergistic silicon modification promoted by increasing both solidification rate and strontium enhanced conductivity by up to 3 pct IACS. The correlations developed with the quantified silicon characteristics establish this phase as dominant in the conductivity of B319 alloy, and they elucidate opportunities for the further enhancement of automotive materials.

Structural and Mechanical Properties of Directionally Solidified Al-Si Alloys

This review covers research aimed at finding the optimum composition and growth rate to obtain a highly modified Al-Si alloy using directional solidification. Investigations of microstructure and mechanical properties as a function of Si content and growth rate are analyzed. These works show that the composition yielding a eutectic microstructure changes considerably with increasing solidification rate in the range of 102-104 μm/s. The increase in ultimate tensile strength with increasing Si content up to that giving a completely eutectic microstructure is explained by a redistribution of volume content of α-Al and eutectic. The increase in tensile strength with increasing rate is explained by a decrease in microstructural scale accompanying the transformation of flake-to-fiber eutectic microstructure. The optimal fine fiber structure without any primary crystals of Al-Si alloy at a given Si content is obtained at the solidification rate giving a completely eutectic microstructure at that composition. Hypereutectic alloys can be fully modified using rapid cooling at such solidification rate that causes coupled growth of the eutectic for given composition of the alloy. Additional Sr modification results in a super-modified structure, high tensile strength and record high elongation.