Solidification Pressure Research Articles

Delving Deep into the Influence of Pressure on Columnar‐to‐Equiaxed Transition in High‐Nitrogen Steel Ingot by Thermodynamics and Kinetics

Based on the analysis of thermodynamic driving force and solidification kinetics under pressure, the influence mechanism of solidification pressure on columnar‐to‐equiaxed transition (CET) of high‐nitrogen steel is clarified. It is observed that increasing the solidification pressure from 0.5 to 2 MPa results in a shift of the CET positions toward the center. This is attributed to the fact that higher solidification pressure can promote the growth of columnar dendrites by increasing the solidification rate, temperature gradient, and cooling rate. Meanwhile, increasing the solidification pressure shortens the length of the diffusion zone ahead of the advancing columnar front and reduces the supercooling. As a result, it becomes more difficult for equiaxed dendrites to nucleate ahead of the advancing columnar front, leading to CET positions closer to the center. This indicates that the main influencing factor for the change in CET caused by changing solidification pressure is solidification kinetics, that is, changes in the nucleation and growth environment of equiaxed dendrites, while thermodynamic driving forces are not the primary factor causing CET position changes.

Metastable water at several compression rates and its freezing kinetics into ice VII

Water can be dynamically over-compressed well into the stability field of ice VII. Whether water then transforms into ice VII, vitreous ice or a metastable novel crystalline phase remained uncertain. We report here the freezing of over-compressed water to ice VII by time-resolved X-ray diffraction. Quasi-isothermal dynamic compression paths are achieved using a dynamic-piezo-Diamond-Anvil-Cell, with programmable pressure rise time from 0.1 ms to 100 ms. By combining the present data set with those obtained on various ns-dynamical platforms, a complete evolution of the solidification pressure of metastable water versus the compression rate is rationalized within the classical nucleation theory framework. Also, when crystallization into ice VII occurs in between 1.6 GPa and 2.0 GPa, that is in the stability field of ice VI, a structural evolution over few ms is then observed into a mixture of ice VI and ice VII that seems to resolve apparent contradictions between previous results.

A counter-gravity casting system for in situ high-speed synchrotron x-ray imaging characterization.

Counter-gravity casting (CGC) aims to eliminate turbulent melt flow and defect formation during filling and subsequent solidification by pushing high-temperature melt into the mold cavity against gravity with regulated pressure. However, limited by the opaqueness of molten metals and the complexity of the CGC apparatus, it is extremely difficult to directly quantify the high-velocity mold filling and pressurized solidification in real-time. Here, we report the design and characterization of a CGC system capable of in situ monitoring of mold filling and subsequent solidification processes in the synchrotron beamlines by deploying a high-energy, high-speed synchrotron x-ray imaging technique. The high-velocity melt flow and dendrite growth during pressurized solidification have been quantified for systematical process parameter analysis by investigating time-resolved x-ray images of an exemplary Al-Cu alloy. The high-speed imaging results demonstrate that the in situ CGC system provides a useful way to better understand the fundamentals of mold filling, pressurized solidification, and experimental inputs for high-fidelity modeling in scientific and industrial applications.

Effect of solidification pressure on the corrosion behavior and mechanical properties of Al-7Si-3Cu-(0.4 Mg)-(0.5Ge) alloys

Developing practical microstructural solutions that simultaneously enable excellent corrosion resistance and mechanical properties in Al-Si-Cu-based alloys has been increasingly in demand. In this study, we attempt to tailor the microstructural evolution to manipulate the mechanical performance and corrosion resistance of Al-7Si-3Cu-(0.4 Mg)-(0.5Ge) alloys fabricated by GPa-level pressure combined with solution and/or aging treatments. It was found that as the solidification pressure increased, the dendritic growth tendency of the α-Al phase became less pronounced, and the modification of eutectic Si became increasingly significant. Complete solid solution alloys were even achieved under 6 GPa. The corresponding kinetic and thermodynamic mechanisms of pressure-induced microstructural evolution were explored in detail. Upon aging treatment, dense irregular Si precipitates emerged from the supersaturated matrix in Al-7Si-3Cu alloys solidified at 5 GPa and 6 GPa, while denser and finer Si precipitates growing along the 〈100〉Al directions dominated the matrix of Al-7Si-3Cu-0.4 Mg-0.5Ge alloys, which resulted in considerable enhancement of mechanical properties. The electrochemical corrosion behavior was evaluated in the as-cast and heat-treated conditions. The underlying corrosion mechanisms were discussed in terms of pressure, supersaturated solutes and precipitates. These results reveal the feasibility of designing high-strength and corrosion-resistant Al-Si-Cu series alloys via the manipulation of pressure and heat treatment.

A CALPHAD thermodynamic model for multicomponent alloys under pressure and its application in pressurized solidified Al–Si–Mg alloys

High pressure technology has been utilized as an important means to regulate phase structure and improve the properties/performance of alloys. The CALPHAD approach based on accurate databases has great advantages in efficient alloy design. However, the application of CALPHAD in high pressure field is hindered by the lack of reliable thermodynamic model/database for multicomponent alloys under pressure. In this paper, a phenomenologically thermodynamic model for multicomponent alloys under pressure is first developed by separating the contribution into two parts, one is at atmosphere pressure and the other is caused by an increase in pressure, and then successfully applied to establish the pressure-dependent thermodynamic database of ternary Al–Si–Mg system. The calculated phase equilibria/thermodynamic properties of pressure dependence in related alloys are in good agreement with the limited experimental data in the literature, validating the reliability of the obtained thermodynamic database. After that, a CALPHAD design framework for pressurized solidified alloys is proposed by integrating the present pressure-dependent thermodynamic model/database, CALPHAD-type calculations/simulations, and previously developed high-throughput calculation platform Malac-Distmas. Such a framework is finally applied to predict the pressurized solidification and high pressure heat treatment behaviors in different Al–Si–Mg alloys. The predicted microstructure, phase transitions and phase equilibria after pressurized solidification and high pressure heat treatment are consistent with the experimental data. Furthermore, the insights into effect of pressure on the thermodynamic essence of alloys are gained, which may definitely facilitate the advancement of alloy design under high pressure technology.

Function of Solidification Pressure on the Rising and Breakup of Nitrogen Bubbles in Molten Steel

Function of Solidification Pressure on the Rising and Breakup of Nitrogen Bubbles in Molten Steel

Inhibiting the shrinkage porosity in 30Cr15Mo1N ingots by vanadium-nitrogen synergy during pressurized solidification

Shrinkage porosity in high-nitrogen stainless steel ingots can be eliminated through pressurized solidification. However, in practical industrial settings, achieving the necessary solidification pressure to inhibit shrinkage porosity is often time-consuming, labor-intensive, and challenging. In this study, the area ratio, formation height, type, and morphology of shrinkage porosity were investigated in the ingots with the different vanadium and nitrogen contents by experimental observations and statistical analysis. Then, the inhibition mechanism of the vanadium-nitrogen synergy on shrinkage porosity is revealed via an analysis that amalgamates the feeding resistance and critical feeding driving force. This indicated that the difficulty in generating shrinkage porosity intensified in the 0.2 V ingot compared with that in the 0 V ingot, and the primary type of shrinkage porosity changed from net shrinkage porosity to island shrinkage porosity and gas-shrinkage porosity. Gas-shrinkage porosity were more readily formed and relied on the presence of nitrogen pores. In the 0.4 V ingot, the shrinkage porosity virtually disappeared completely. This could be attributed to the suppression of the nucleation and growth of nitrogen pores with increasing vanadium content. Meanwhile, the dendritic growth restriction factor increased through the vanadium-nitrogen synergistic effect, inhibiting dendrite growth and resulting in a decrease in feeding resistance. This reduced the critical feeding driving force and promoted the feeding flow between the dendrites, ultimately inhibiting the formation of shrinkage porosity. This study revealed the mechanism by which the synergistic effect of vanadium and nitrogen inhibits the formation of shrinkage porosity during the solidification of 30Cr15Mo1N ingots, providing an efficient method for improving the solidification quality of high-nitrogen stainless steel, which is extremely helpful for its manufacture.

Enhanced mechanical properties of high pressure solidified CoCrFeNiMo0.3 high entropy alloy via nano-precipitated phase

CoCrFeNiMo0.3 high entropy alloy was prepared by vacuum arc melting. The effects of different solidification pressures on the microstructure and mechanical properties of CoCrFeNiMo0.3 high entropy alloy were investigated. All ambient pressure (0.1 MPa) Mo0.3, 4 GPa Mo0.3 and 7 GPa Mo0.3 alloys exhibit two kinds of structures. The FCC structure is the solid solution matrix, and the precipitated phase enrich in Cr and Mo (σ phase) is distributed in the grain boundary of the matrix phase. With the increase of the test pressure, the size of the precipitated phase gradually decreases and the volume fraction increases obviously, and tends to form nano-scale particles. Meanwhile, the precipitated phase gradually diffused into the matrix due to the increase of the pressure. The results of nanoindentation tests show that the hardness of the alloy increases significantly with the increase of solidification pressure, especially that of 7 GPa Mo0.3 alloy, which is about 2 times that of 0.1 MPa Mo0.3 alloy. The strengthening mechanism of CoCrFeNiMo0.3 high entropy alloy with increased pressure is mainly nano-precipitated strengthening. This present findings not only pave the way for designing the structure and property regulation of high pressure solidified HEAs but also promote the application orientation of various property combinations. It provides the theoretical basis for its application and is expected to be used as structural materials in various fields.

Alkali silica reaction in concrete - Revealing the expansion mechanism by surface force measurements

Alkali-silica reaction (ASR) is a major cause for concrete deterioration worldwide. However, the mechanism leading to cracking has not been identified yet. In this study, the extended Surface Force Apparatus (eSFA) has been used to determine the surface forces of alkali-silica solutions between two atomically smooth mica surfaces. The setup imitates the situation present in reactive concrete aggregates with negatively charged silicate surfaces and negatively charged polynuclear silica in solution. The distance of strong electrostatic repulsion increases with increasing concentration of dissolved silica, leading to the buildup of pressure up to 6 MPa. When the precipitation of ASR products in confined conditions is triggered by the addition of CaCl2 to the alkali-silica solution, the resulting solidification pressure forces the mica platelets apart and reaches 6–13 MPa. The eSFA experiments shows that solidification pressure is the mechanism leading to aggregate cracking and expansion of ASR-affected concrete.

Effect of pressure on the microstructure refinement, solidification mechanism and fracture behavior of wrought Al-Zn-Mg-Cu alloys prepared by squeeze casting

Squeeze casting process can effectively resolve the casting difficulties for wrought aluminum alloys by pressurized solidification. The effects of applied pressure (0.1–125 MPa) on the microstructure refinement, non-equilibrium eutectic phase aggregation and fracture behavior of Al-Zn-Mg-Cu alloys prepared by squeeze casting were investigated through metallography, scanning electron microscopy and energy dispersive spectroscopy. The results showed that the primary α-Al grains transformed from coarse dendrites to refined equiaxed grains with increasing pressure. Considerable primary refinement was achieved at 100 MPa, with a significant decrease by 63% in the average grain size from 273 µm to 101 µm; the non-equilibrium eutectic phases were homogenized and refined, and almost dissolved into the Al matrix after T6 heat treatment. The mechanism of solidification behaviors in pressurized solidification was elucidated. Firstly, the applied pressure increased the thermodynamic driving force to promote primary grain nucleation. Secondly, the solute diffusion and constitutional undercooling at the front of the solid/liquid interface front were inhibited by pressure, and grain growth in a dendritic structure was thus restrained. Thirdly, interdependence theory and thermodynamic calculations were conducted to quantitatively describe the primary grain size changes under pressurized solidification. Lastly, the ternary eutectic reaction L → α-Al + T-Mg (Al, Zn, Cu)2 + Al2Cu and binary eutectic reaction L→ α-Al + Al7Cu2Fe were suppressed by the reduced solute segregation. Furthermore, when the pressure increased from 0.1 MPa to 125 MPa, the ultimate tensile strength was raised from 428 MPa to 538 MPa, almost approaching that of samples by forging process. The refined eutectic phases decreased stress concentration and improved fracture behaviors.

Prediction of Part Shrinkage for Injection Molded Crystalline Polymer via Cavity Pressure and Melt Temperature Monitoring

During an injection molding process, different parts of the molded material are subjected to various thermal–mechanical stresses, such as variable pressures, temperatures, and shear stresses. These variations form different pressure–temperature paths on the pressure–volume–temperature diagram. If these paths cannot converge at a specific target volume value during ejection, it often leads to different levels of shrinkage and associated warping, which pose a significant challenge for molders during mold trials and part quality control. The situation is particularly complicated when molding crystalline polymers because the degree of crystallinity depends on the processing conditions and may vary across different locations. In this study, we propose an innovative and practical approach to improving part shrinkage when molding crystalline polymers. For the first time, we utilized melt temperature profile monitoring rather than the previous mold temperature measurement to detect the crystallization process and determine the time taken to complete the crystallization at different melt and mold temperatures. In addition, we used response surface methodology to build a crystallization time prediction model. The feasibility of the prediction model was verified by determining the warpage of parts molded at various cooling times. Based on this model, we varied the packing pressure, packing time, and melt temperatures to determine the correlation with part shrinkage. Through regression analysis, the time-averaged solidification pressure values can accurately control part shrinkage. Two prediction models provide reasonable accuracy and efficiency for part shrinkage control, as demonstrated by subsequent verification experiments.

Microstructural Evolution and Mechanical Properties of SiC/Al-40Si Composites Fabricated by High Pressure Solidification.

The microstructure and mechanical properties of SiC/Al-40Si composites prepared under high pressure were studied. As the pressure increases from 1 atm to 3 GPa, the primary Si phase in the Al-40Si alloy is refined. With increasing pressure, the composition of the eutectic point increases, the solute diffusion coefficient decreases exponentially, and the concentration of Si solute at the front of the solid-liquid interface of the primary Si is low, which contributes to the refining of the primary Si and inhibiting its faceted growth. The bending strength of SiC/Al-40Si composite prepared under 3 GPa was 334 MPa, which was 66% higher compared to the Al-40Si alloy prepared under the same pressure.

Nano-Phase and SiC-Si Spherical Microstructure in SiC/Al-50Si Composites Solidified under High Pressure.

The formation of coarse primary Si is the main scientific challenge faced in the preparation of high-Si Al matrix composites. The SiC/Al-50Si composites are prepared by high pressure solidification, which allows the primary Si to form a SiC-Si spherical microstructure with SiC, while the solubility of Si in Al is increased by high pressure to reduce the proportion of primary Si, thus enhancing the strength of the composites. The results show that the high melt viscosity under high pressure makes the SiC particles almost "fixed" in situ. The SEM analysis shows that the presence of SiC in the growth front of the primary Si will hinder its continued growth and eventually form SiC-Si spherical microstructure. Through aging treatment, a large number of dispersed nanoscale Si phases are precipitated in the α-Al supersaturated solid solution. The TEM analysis shows that a semi-coherent interface is formed between the α-Al matrix and the nanoscale Si precipitates. The three-point bending tests shows that the bending strength of the aged SiC/Al-50Si composites prepared at 3 GPa is 387.6 MPa, which is 18.6% higher than that of the unaged composites.

Uncovering the effects of local pressure and cooling rates on porosity formation in AA2060 Al-Li alloy

The third generation of Al-Li alloys (e.g. AA2060) have been widely used in aerospace engineering. The formation of porosity during casting is a major obstacle for improving its mechanical properties due to the 10 times the equilibrium hydrogen concentration in the liquid than the traditional aluminum alloys. In this study, the effects of two fundamental parameters are investigated: cooling rate and pressure on the formation of porosity. The centrifugal casting process was used to apply external pressure during solidification and quantify the 3D porosity morphology as a function of the external parameters using X-ray computed tomography (X-CT). Subsequently, a Cellular Automata (CA) model was used to predict porosity as a function of pressure and thermal boundary conditions. It was found that the externally applied compressive pressure contributes to porosity closure within certain limits. In addition, the cooling rate not only refines the grain size but also minimizes porosity defects by increasing their number density, and the experimental results validated the predictions by the CA model. By increasing the solidification pressure and cooling rate, a high-performance AA2060 Al-Li alloy with a tensile strength of 490 MPa and an elongation of 6.1% was obtained.

Effect of high pressure on solidification microstructure evolution of near-eutectic Al-11.5Si-0.5Fe-0.2 Mg alloy

The microstructures evolution of near-eutectic Al-11.5Si-0.5Fe-0.2 Mg alloy solidified under different pressures (1 GPa and 3 GPa) were investigated. The results show that the increase in solidification pressure leads to a transition from the typical eutectic structure to the hypoeutectic structure of the alloy by increasing the melting point, and a change in the morphology of the eutectic Si phase from a needle-like structure to a short sheet-like structure and contact with each other. The increase of solid solubility drives the disappearance of the π-Al8FeMg3Si6 phase at 1 GPa, the disappearance of the α-Al8Fe2Si phase, and the occurrence of the δ-Al4FeSi2 phase at 3 GPa. The α-Al8Fe2Si phase and β-Al5FeSi phases are separated from each other in the matrix, indicating that these two phases are formed by independent reactions. In contrast, the β-Al5FeSi and δ-Al4FeSi2 phases can remain stable during solidification at higher pressures.

Elimination Mechanism of Shrinkage Porosity During Pressurized Solidification Process of 19Cr14Mn4Mo1N High-Nitrogen Steel Ingot

Elimination Mechanism of Shrinkage Porosity During Pressurized Solidification Process of 19Cr14Mn4Mo1N High-Nitrogen Steel Ingot

Property manipulation of CoCrFeNiAlCu high-entropy alloy under GPa high pressure

In this study, we investigated the corrosion resistance and mechanical properties of a CoCrFeNiAl0.2Cu0.8 high-entropy alloy (HEA) that was subjected to high-pressure solidification at 4 and 7 GPa. In addition, the mechanism of the effect of Cu segregation regulation on the properties was described. Results indicated that the volume fraction of the Cu-enriched FCC2 phase segregated at the grain boundary decreased from 8.17 % to 5.42 % under high pressure. We also observed that high-pressure solidification can lead to the formation of nano-precipitates in the FCC2 phase with better refinement and distribution uniformity. The reduction in the Cu-enriched FCC2 phase led to a decrease in the corrosion rate by approximately 85.6 % with an increased solidification pressure of 7 GPa, thereby improving corrosion resistance. Finally, we achieved a superior hardness and yield strength combination via fine-scale nano-precipitates and solution strengthening.

Effect of Mechanical Vibration on the Mechanical Properties and Solidification Feeding in Low-Pressure Sand Casting of Al-Cu-Mn-Ti Alloy.

The shrinkage defects of Al-Cu-Mn-Ti alloy seriously hinder its application in high-performance engineering. In this study, mechanical vibration was introduced to low-pressure sand casting (LPSC) by a waveguide rod to eliminate shrinkage defects and improve mechanical properties. Four LPSC castings were performed by changing the solidification conditions: 20 kPa solidification pressure without and with 14 Hz vibration and 40 kPa without and with 24 Hz (the natural frequency of a casting system) vibration. The shrinkage defects, microstructures, and mechanical tensile properties at room temperature and at 2 mm/min tensile rate were investigated. X-ray detections showed that applying vibration was more effective than increasing solidification pressure in improving solidification feeding, while the most effective method was applying both simultaneously, which eliminated the shrinkage defects and increased the density by 2.7%. Microstructures exhibited that the average size of primary α-Al grains were reduced by 29.5%; mechanical tests showed that the ultimate tensile strength and the elongation increased by 21.7% and 7.8%, respectively, by applying vibration and increasing the solidification pressure simultaneously, as compared to the casting with 20 kPa solidification pressure without vibration. Mechanical vibration was conducive to mass feeding by refining the primary grains, to interdendritic feeding by reducing the threshold pressure gradient, and to burst feeding by collapsing the barrier.

High pressure solidification of alloying substitution and promotion of hydrogen storage properties in AB2-type Y-Zr-Ti-Fe based alloys

Metastable phase has great potential in searching hydrogen storage alloys. In this work, high pressure solidification has been successfully used to realize the supersaturation of Ti in YFe2 based C15 Laves phase to obtain a metastable Y0.7Zr0.24Ti0.06Fe2 alloy. The segregation of Ti, Y, Zr and related multi-phase formation were substantially suppressed and the obtained HPS alloy contained a primary C15 phase(C15–1) and a secondary C15 phase(C15–2) with 77.1% and 22.9% of the total content, respectively. Meanwhile, the hydrogen absorption and desorption capacities increased substantially. This study proves that the combination of high-pressure solidification and alloying is a very effective method for exploring new hydrogen storage materials with metastable structure and high performance.

The evolution of microstructure, micromechanical and magnetic properties of FeCoNiAlSi alloys with peritectic structure processed by high-pressure solidification

In this work, we examined the microstructure evolution, as well as the micromechanical and magnetic properties of FeCoNiAlSi high-entropy alloys (HEAs) under high pressure solidification (HPS). According to the results, FeCoNiAlSi alloy solidified at 0.1 MPa (atmospheric pressure) contains three kinds of phases: grain boundary (GB)phases, peritectic phases, and primary phases. As the solidification pressure increases to 7 GPa, the volume fraction of GB phases and peritectic phases decreases, while the volume fraction of primary phase increases by 82.8%. In addition, the Fe of the primary phase increases by 63.6 %, which enhances the ferromagnetism and exchange coupling properties of the alloy. Furthermore, HPS also reduces the magneto-crystal anisotropy of the GB phases and the pinning effect of boundaries on domain movement. Additionally, with the increasing solidification pressure, the FeCoNiAlSi HEAs shows stronger ferromagnetic. When solidified at 7Gpa, a reduction of 51.5 % in intrinsic coercivity(H MC ) was observed, as was an increase of 106 % in magnetic polarization strength (Js). As well, HPS significantly increased the high temperature magnetic properties of FeCoNiAlSi alloy as well as the microhardness(i.e. from 8.25 GPa to 10.15 GPa). According to these results, HPS is an effective method for developing new functional applications for FeCoNiAlSi HEAs. • The Js (HMC)of the FeCoNiAlSi HEAs increased (decreased) by 106% (51.5%) after HPS at 7Gpa, The Tc are also improved. • The pinning effect of boundary on magnetic domain movement were reduced after HPS and exchange coupling was enhanced. • The volume fraction of primary phase and the content of Fe in primary phase increased when solidified under 7GPa. • The promoted solution strengthening after HPS significantly improved the microhardness (i.e. from 8.25GPa to 10.15GPa).