Dendrite Arm Research Articles

Phase-Field Modelling of Bimodal Dendritic Solidification During Al Alloy Die Casting

Tracking the microstructural evolution during high-pressure die casting of Al-Si alloys is challenging due to the rapid solidification, varying thermal conditions, and severe turbulence. The process involves a transition from slower cooling in the shot sleeve to rapid cooling in the die cavity, resulting in a bimodal dendritic microstructure and nucleation of new finer dendrite arms on fragmented externally solidified crystals. In this study, a two-dimensional phase-field model was employed to investigate the solidification behaviour of a hypoeutectic Al-7% Si alloy during high-pressure die casting. The model is based on thermodynamic formulations that account for temperature changes due to phase transformation heat, thermal boundary conditions, and solute diffusion in both liquid and solid phases. To replicate the observed bimodal microstructure, solid–liquid interface properties such as thickness, energy, and mobility were systematically varied to reflect the transition from the shot sleeve to the die cavity. The results demonstrated the model’s ability to capture the growth of dendrites under shot sleeve conditions and nucleation and development of new dendrite arms under the rapid cooling conditions of the die cavity.

Quantitative study of thickness debit effect on the microstructure and elemental segregation characteristics in as-cast and heat-treated nickel-based single crystal superalloys

Reducing blade wall thickness has become essential with advancements in blade cooling technologies such as air film cooling, impact cooling, and transient cooling. Heat dissipation is improved by this reduction, which significantly extends the lifespan of the blades. The impact of thickness variations on the microstructure and elemental segregation of as-cast and heat-treated nickel-based single crystal superalloys is investigated in this study. Thin-walled specimens of four different thicknesses (1.513mm, 2.370mm, 3.060mm, and 5.790mm) were directly cast using a shell mold equipped with varying cavity thicknesses. The microstructure and elemental segregation of these samples were quantitatively analyzed using optical microscope (OM), scanning electron microscope (SEM), and energy-dispersive spectroscopy (EDS). This research not only involves thin-walled specimens of various thicknesses but also includes specimens of the same thickness positioned at different distances from the casting core. Experimental data demonstrate that the size of the γ′ phase follows the log-normal distribution, and its circularity conforms to the four-parameter Dagum distribution. In the as-cast specimens, increases of the thickness from 1.513mm to 5.790mm led to 33.7% increase in the average primary dendrite arm spacing, 86% increase in the average percentage of the eutectic structure, 44.2% increase in the average γ′ phase size, and 8% decrease in the average circularity of the γ′ phase. The degree of elemental segregation also increases, particularly for elements such as W and Re that exhibit significant increases in segregation as the sample thickness increases. The differences in casting thickness lead to initial state differences. Approximately 95% of the eutectic structures are eliminated during heat treatment, but the efficacy of the process is constrained by differences in the initial state of the alloy. The greater the initial state heterogeneity, the more stringent the required heat treatment time and conditions, and the more limited the effectiveness of the heat treatment. The final microstructure and elemental distribution are the results of the coupled effects of the initial state and heat treatment. In the heat-treated samples, as the thickness increases from 1.513mm to 5.790mm, the average primary dendrite arm spacing increases by 33.5%, the average percentage of eutectic structures increases by 275%, the average size of the strengthening phase (γ′ phase) increases by 66.7%, and the average circularity of the γ′ phase decreases by 33%. The variations in microstructural parameters for specimens at different distances from the casting core also show regular patterns. Key findings indicate that decrease in casting thickness significantly affects γ' phase characteristics, elemental segregation, and microstructural uniformity. These findings can provide valuable guidance for optimizing the design and manufacturing processes of the nickel- based single crystal alloys.

Performance optimization and compatibility of FeCoNiCrMo0.2 high-entropy alloy coatings laser-cladded onto 4Cr5MoSiV1 steel surfaces

4Cr5MoSiV1 steel, known for its excellent toughness, thermal stability, and high hardenability, finds widespread application in areas such as hot extrusion dies, forging molds, and die-casting molds. However, its operational performance and service life are significantly impacted by limitations in surface hardness, wear resistance, and corrosion resistance. This study focuses on the optimization of laser cladding processes, preparing a FeCoNiCrMo0.2 high-entropy alloy cladding layer on the surface of 4Cr5MoSiV1 steel through laser cladding, and conducts an examination and validation of the microstructure and mechanical properties of the cladding layer. The experiments employed a laser power of 2.5 kW, a beam diameter of 3mm, a scanning speed of 5 mms−1, a gas flow rate of 1.5 Lmin−1, and a powder feed rate of 30g min−1. The results reveal that the top of the cladding layer is dominated by fine and dense equiaxed crystals, the middle by dendritic crystals with secondary dendrite arms, and the bottom by cellular crystals. The cladding layer is primarily composed of a face-centered cubic (FCC) phase structure, with the precipitation of particulate hard σ phases within and between the crystals. The average microhardness of the cladding layer is 360 HV0.2, approximately 63.6% higher than that of the base material. The average coefficient of friction for the cladding layer is 0.6609, with a wear volume of about 27 mg, representing an improvement of approximately 18% over the base material; the wear volume is about 81% of that of the base material. In a 3.5% NaCl solution, the self-corrosion current density of the cladding layer is measured at 9.98 × 10−9 A/cm2, with a self-corrosion potential of −368.3 mV. Compared to the base material, it achieves an enhancement in electrochemical corrosion capability while maintaining relative compatibility. The process and research provide a theoretical foundation and methodological reference for the surface modification of 4Cr5MoSiV1 steel.

Comparative study of gravity effects in directional solidification of Al-3.5 wt.% Si and Al-10 wt.% Cu alloys

Directional solidification experiments of Al-3.5 wt.% Si and Al-10 wt.% Cu alloys were conducted under gravity and microgravity conditions using a 50-m-high drop tube. The solidification morphology of the two alloys is mainly columnar dendrites and equiaxed dendrites, respectively. The dendrite arm spacing (DAS), eutectic content, grain size, and compositional distribution of both alloys exhibit distinct characteristics under gravity and microgravity conditions. The study introduces an innovative perspective by taking solute density and its redistribution behavior into account when discussing the gravity effects during the directional solidification of alloys. The results indicate that the way gravity works on the solidification behavior of alloys depends strongly on the redistribution behavior and density of solute as well as crystallization modes, such as columnar grain or equiaxed grain. These findings are helpful in clarifying the coupling mechanism of gravity and relevant factors on the solidification of alloys, not only contributing to understanding the effect of gravity on solidification better but also offering valuable guidance for eliminating solidification segregation and producing high-performance alloys.

Pulsed Magneto‐Oscillation Processing Improves the Quality of SCM435 Cold Heading Steel Billet and Its Hot‐Rolled Wire Rods

Pulsed magneto‐oscillation (PMO) has a high homogenization effect on continuous castings, but the heritability of it on hot‐rolled wire rods has not been studied. Recently, the effect of pulsed magneto‐oscillation on SCM435 cold heading steel billet and its hot‐rolled wire rod were investigated. A 180 mm × 180 mm ten‐strand continuous casting machine was used, with the first strand treated by pulsed magneto‐oscillation, and billets hot‐rolled to Φ6.5 mm wire rods. The results showed that compared with the billet without pulsed magneto‐oscillation treatment, the proportion of central equiaxed grain zone of pulsed magneto‐oscillation treated billet increased from 26.5% to 37.4%, and the secondary dendritic arm spacing of columnar was reduced. Futhermore, the segregation index ranges of Cr and Mn decreased by 47.1% and 33.3%, respectively. For the hot rolled wire rods, the solutes homogeneity and the mechanical properties of the rods produced by pulsed magneto‐oscillation treated billet were improved. And its banded structure was significantly refined, where the maximum band width was reduced from 35.4 μm to 12.3 μm. This study showed that the properties of wire rod could be significantly improved by improving the homogeneity of the casting billet by pulsed magneto‐oscillation, and a good histogenetic effect was obtained.

Investigation on the Effect of Water Quenching to Microstructure, Crystal Structure, Hardness, and Wear of Gray Iron

Gray cast iron has been one of the most widely used engineering materials since a long time ago. However, the development of casting techniques and methods to produce various models of cast iron products for the domestic market is not followed by improvements in product quality. The intriguing aspect of gray iron products is the diverse morphologies that graphite can assume, leading to distinct variations in mechanical and physical properties. Quenching is a typical heat treatment procedure performed to improve the mechanical properties of a material that entails the rapid cooling of the material from a high temperature to a low temperature. The aim of this study is to investigate the effect of water quenching effects on microstructure, crystal structure, hardness, and wear of gray iron, which undergoes quenching from the austenitizing temperature. Gray cast iron was obtained from the local foundry industry, then thermally treated at 900°C, held for 15 minutes, and rapidly quenched by water. The quenching procedure induces a significant alteration in the overall microstructure, where transition of most dendrite arms to the eutectic phase microstructure is observed. Moreover, the quenching process is attributed to the reduction of crystal size and growth of carbon crystal. The average crystal size of the sample was reduced from 47.833 nm to 17.97 nm, hence improving the hardness from 16.375 HRC to 48.04 HRC, which in turn improved wear resistance under high loading condition from 0.014 g/sec to 0.00042 g/sec.

Effect of 0.20% Beryllium (Be)-Added CuAl10Ni5Fe4 Alloy on Tribological Behavior and Microstructural Properties After Post-Casting Heat Treatment and Forging Process.

This study explored how post-casting heat treatment and forging affected the tribological and microstructural characteristics of 0.20% beryllium (Be)-added CuAl10Ni5Fe4 alloys. The heat-treated CuAl10Ni5Fe4 microstructure exhibits a copper-rich α (alpha)-solid-solution phase, a martensitic β (beta)-phase, and diverse intermetallic κ (kappa)-phases, such as leaf-shaped κI, thin κIII, and black globs. Adding 0.20% beryllium to CuAl10Ni5Fe4 alloys enhanced the dendritic arm thickness, needle-like shape, and κ-phase quantities. Significant κIV- and κII-phase precipitation was observed in the tempered β-phase. Beryllium improves the aluminum matrix's microstructure. Forging greatly reduced the microstructural thickness of CuAl10Ni5Fe4 and CuAl10Ni5Fe4-0.20% Be alloys. The forging process also developed new κIV-phases. Wear resistance and hardness improved with beryllium. The CuAl10Ni5Fe4-0.20% Be alloy had the highest hardness values (235.29 and 255.08 HB) after solution treatment (ST) and tempering (T) after casting and forging (F). The CuAl10Ni5Fe4-0.20% alloy with Be added had the best wear after solution treatment, tempering, and forging. The CuAl10Ni5Fe4-0.20% Be alloy demonstrated a 0.00272 g weight loss, a 1.36 × 10-8 g/N*m wear rate, and a 0.059 friction coefficient at 10,000 m after forging (F).

Evaluation of fatigue behavior of a casting aluminum knuckle considering influences of inhomogeneous microstructure

Until now the fatigue behavior of casting aluminum components is usually evaluated in the safety analysis without considering the inhomogeneity of local properties. It can result in an underestimation of damage risk or an oversizing of the components. The purpose of the work is to quantify and to predict the influence of the inhomogeneity of microstructure and the corresponding local properties on the overall behavior of the selected casting aluminum knuckle under cyclic loading. Since no shrinkage pores are observed in the knuckle produced by the counter pressure casting process, influences of porosity on fatigue property are not taken into account in this study. Tensile specimens are extracted from different positions in the knuckle and tested under quasistatic and cyclic loading. The dependences of the flow stress, the fracture strain, and the S-N curve on position for specimen extraction are evaluated. Metallographic investigations are performed to reveal the relations between microstructure and the mentioned properties. A relationship between the S-N curves and the secondary dendrite arm spacing (SDAS) is derived from the experimental results. The corresponding parameters of the new fatigue damage model are determined and used to simulate the specimens and component tests. The distribution of SDAS in the component is determined by casting simulations and transferred to the FE-model for the fatigue analysis. The standard fatigue tests on knuckles are performed and the damage behavior is analyzed in view of the influence of microstructure. The simulations of the knuckle fatigue tests are performed with SDAS-dependent material parameters. The prediction of the fatigue behavior of the knuckles agrees with the experimental results well.

Investigation of heat transfer, dendrite growth, Nb segregation, competitive growth, and remelting process during wire-arc directed energy deposition of IN718 using a multi-scale, multi-dimension, and multi-area phase field model(M3-PFM)

In this work, a multi-scale, multi-dimension and multi-area phase field model is established to simulate the heat transfer, dendrite growth, Nb segregation, competitive growth, and remelting process during wire-arc directed energy deposition of IN718. The primary dendrite arm spacing increases from 6.09 μm to 8.0 μm with decreasing cooling rate from bottom to top, which aligns closely with the experimental results. The maximum segregation of Nb is about 14.9 wt.% in simulation results, which is consistent with experimental 15.3 wt.%. During competitive growth, the solute concentrations at the tips of the dendrites gradually increase as the grain orientations increase, while the dendrite lengths gradually decrease. After remelting, the grain growth mode can be divided into three types: continuous growth, branched, and eliminated. The primary dendrite arm spacing increases slightly, from 6.54 μm to 7.50 μm, which agrees well with experimental values of 6.71 μm before and 7.14 μm after remelting respectively.

Microstructure Evolution and Cryogenic Impact Toughness Changes in Ce‐Containing Novel Cryogenic High‐Mn Austenitic Steel Weld Metals under Different Heat Inputs

The Ce‐containing novel cryogenic high‐Mn austenitic steel weld metals under different heat inputs are prepared by submerged‐arc welding to provide a satisfactory welding parameter scheme and further optimize its cryogenic impact toughness. The effect of heat input on microstructure, inclusion, and cryogenic impact toughness of weld metals are investigated via optical, scanning electron microscopy, electron backscatter diffraction, electronic probe microanalyzer, and cryogenic impact test. The results show that the dendrite arm spacing, segregation ratio of Mn, Si, and C elements and average effective grain size of the weld metals gradually increase, and the proportion of high‐angle grain boundaries decrease with the heat input increase from 11.91 to 20.24 kJ cm−1. The type of inclusions of weld metal with different heat inputs are all Ce2O3 and Ce2O2S. The number density of inclusions decreases monotonically from 5976 to 4912 mm−2, while the average size increases from 0.32 to 0.45 μm with the heat input increased. The increase in heat inputs consistently decreases the mean cryogenic impact toughness of weld metals from 123 to 102 J. The higher proportion of high‐angle grain boundaries and finer inclusions at lower heat input play the critical role in improving the cryogenic impact toughness.

Modification mechanism of Te and morphology evolution of primary Mg2Si in an Al-20Mg2Si alloy

In the present work, the modification effect of Te and the corresponding growth process of Mg2Si were clarified. With 0.5 wt% Te addition, the dendritic primary Mg2Si transformed to polygons, accompanied by a reduction in size from over ∼165 μm–∼18 μm. However, 2.0 wt% Te addition resulted in degradation of modification effect, and the morphology of Mg2Si changed to dendrite. Three-dimensional morphology of primary Mg2Si indicated that Te altered the morphology of primary Mg2Si from dendrites to polyhedrons surrounded by {111} and {100}. The modification mechanism of Te can be attributed to the heterogeneous nucleation and the doping of Te during the growth process of primary Mg2Si, which greatly contributed to dimension decrease and morphology evolution of primary Mg2Si respectively. With Te corporation, the relative surface energy between the {100} and {111} (r) drops, contributing to the exposure of {100}. Our work shows that the solute concentration under non-equilibrium solidification plays an important role in determining the final morphology of Mg2Si. The local segregation of Mg and Si solutes results in the degradation of dendrite arms along certain directions during the initial stage of crystal growth. Combined with the modifying effect of Te, which alters the surface energy of the crystal, the final crystal morphology deviates from its equilibrium state. Our study demonstrates that ultimate morphology and dimensions of primary Mg2Si can be determined simultaneously with the addition of certain modifiers, which can be achieved by regulating the growth and nucleation of Mg2Si at the same time.

Grain refiner fabricated by spark plasma sintering for hypoeutectic Al–Si alloy cast and its refining ability

Grain refiner fabricated by spark plasma sintering (SPS) for an as-cast hypoeutectic Al–Si alloy were proposed. The proposed grain refiner contains Al3Ti and Na flux particles in an Al-11 mass%Si alloy matrix. Through SPS, the grain refiner were obtained without any reaction between the Al3Ti particles, Na flux particles, and Al-11 mass%Si alloy matrix. Casting tests were conducted using the proposed grain refiners, and the refiners reduced the dendrite cell size and the dendrite arm spacing of the primary α-Al phase, as well as the lamellar spacing of the eutectic structure. The eutectic Si phase changed to a granular shape upon the refinement of the eutectic structure. Furthermore, the tensile strength and ductility of the as-cast Al-11 mass%Si alloy were drastically improved by adding the proposed grain refiner. This improvement was attributed to the refinement and granulation of the eutectic Si phase.

A Simulation Study on the Effect of Supersonic Ultrasonic Acoustic Streaming on Solidification Dendrite Growth Behavior During Laser Cladding Based on Boundary Coupling

Laser cladding has unique technical advantages, such as precise heat input control, excellent coating properties, and local selective cladding for complex shape parts, which is a vital branch of surface engineering. During the laser cladding process, the parts are subjected to extreme thermal gradients, leading to the formation of micro-defects such as cracks, pores, and segregation. These defects compromise the serviceability of the components. Ultrasonic vibration can produce thermal, mechanical, cavitation, and acoustic flow effects in the melt pool, which can comprehensively affect the formation and evolution for the microstructure of the melt pool and reduce the microscopic defects of the cladding layer. In this paper, the coupling model of temperature and flow field for the laser cladding of 45 steel 316L was established. The transient evolution laws of temperature and flow field under ultrasonic vibration were revealed from a macroscopic point of view. Based on the phase field method, a numerical model of dendrite growth during laser cladding solidification under ultrasonic vibration was established. The mechanism of the effect of ultrasonic vibration on the solidification dendrite growth during laser cladding was revealed on a mesoscopic scale. Based on the microstructure evolution model of the paste region in the scanning direction of the cladding pool, the effects of a static flow field and acoustic flow on dendrite growth were investigated. The results show that the melt flow changes the heat and mass transfer behaviors at the solidification interface, concurrently changing the dendrites’ growth morphology. The acoustic streaming effect increases the flow velocity of the melt pool, which increases the tilt angle of the dendrites to the flow-on side and promotes the growth of secondary dendrite arms on the flow-on side. It improves the solute distribution in the melt pool and reduces elemental segregation.

Understanding the deformation creep and role of intermetallic compound-microstructure in Sn-Ag-Cu solders

Tin-based alloys are commonly used in lead-free solder joints in electronic interconnection applications, where creep can limit joint reliability. In this work, directionally solidified bulk solder alloys of different compositions (pure tin, SAC105 and SAC305) are mechanically tested under a constant load for each composition at room temperature to understand mechanical creep performance and quantify the role of different content of Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) in terms of secondary creep strain rate, microstructural evolution, and total amount of accumulated strain. In this work, microstructures are fabricated using directional solidification to promote a systematic variation in IMC sizes, shapes, and distributions within similar crystal orientations of the primary β-Sn matrix. Analysis of creep data indicates that secondary creep is dominated by obstacle-controlled dislocation motion. This is further confirmed by electron backscatter diffraction (EBSD) analysis, which reveals that the formation of subgrains and internal structure, correlating to the initial microstructure of the sample, i.e. changes in secondary dendrite arm spacing (λ2) and eutectic intermetallic spacing (λe). The experimental observations are supported further by crystal plasticity simulations that explicitly model the presence of IMCs within the Sn-based samples and show the dependence of the secondary creep rate upon the IMC content, and therefore be beneficial for the possible future composition design in solders.

Effect of Printing Parameters and Heat Treatments on Microstructure Development of LPBF Manufactured Inconel 718 Alloy

Abstract Processing parameters of the laser powder bed fusion (LPBF) technique strongly govern achieved performances and manufacturing defects of printed alloys. In this work, it was aimed to study the effects of LPBF printing parameters and subsequent heat treatments on resulted microstructure characteristics and tensile properties of Inconel 718 alloy. Inconel samples were fabricated using three different energy densities. Then, microstructure features such as Lave phase, primary dendrite arm spacing, and internal residual stresses as microstrains of both as-built and heat-treated specimens were determined. It was found that in the range of used energy densities, alterations of phase fractions and average sizes of the Laves phase were insignificant. Decreased energy density led to microstructures with smaller primary dendrite arm spacing and thus principally contributed to enhanced yield and tensile strengths of as-printed samples, whereas increased porosity greatly deteriorated elongation. Moreover, their flow stress curves could be significantly increased by direct aging; however, typical cellular and columnar substructures occurring during the LPBF printing remained. Homogenization treatment could entirely eliminate such substructures and otherwise caused different formations of delta phase when it was performed prior to a delta process.

Austenite grain growth in tailored cooling rate experiment designed by numerical simulation for peritectic steel

The heat transfer model for steel solidification in Al2O3 crucial and permanent mold by finite element method was developed and validated by temperature measurement. The multi-gradient operation was novelly designed, and the tailored cooling rate in range of 0.1–10 °C/s was obtained at the target location. The simulation aided experiment with the large-sized samples was carried out, and the local cooling rate was verified by secondary dendrite arm spacing. The size of austenite grain decreases slightly as cooling rate increases from 0.15 °C/s to 0.31 °C/s, and then increases dramatically in the range of 0.31–0.48 °C/s, finally decreases with cooling rate increasing further to 9.76 °C/s. The relationship between austenite grain size and cooling rate was logarithmically regressed in the range of 0.48–9.76 °C/s, in which the kinetic equations on basis of cooling experiment and continuously cast slab can also work. However, the grain size decrease in 0.15–0.31 °C/s and then increase in 0.31–0.48 °C/s cannot be described. Based on the mechanism of peritectic solidification, the austenite grain growth is mainly controlled by diffusion at a low cooling rate smaller than 0.31 °C/s, and the final grain size is less than 1.0 mm. At a medium cooling rate between 0.48 and 3.60 °C/s, austenite grain growth is accelerated by the energy stored during massive type peritectic transformation previously, and the grain size is between 1.7 and 2.9 mm. At high cooling rate larger than 9.76 °C/s, austenite grains grow to about 0.9 mm in columnar shape. The time for grain growth is not adequate, although addition energy storage by massive transformation can also occur. The critical cooling rate for the onset of massive type peritectic transformation in the large-sized sample is between 0.31 and 0.48 °C/s. The growth velocity in the isothermal experiment is much smaller than that in continuous cooling process, confirming that the austenite grain evolution is significantly affected by previous peritectic solidification.

Effects of Pressure on Nitrogen Content and Solidification Structure during Pressurized Electroslag Remelting Process with Composite Electrode

The nitrogen content and solidification structure of the 1Mn18Cr18N ingots produced by the customized laboratory‐scale vacuum induction melting furnace and the pressure electroslag remelting furnace (PESR) with novel composite electrode under different pressure and the same power consumption are compared and studied. The results show that there are perfectly uniform radial chromium and nitrogen profiles during the PESR process. The nitrogen uptake reaction in the PESR process with composite electrode takes place on the liquid metal film on the electrode. Nitrogen uptake could be improved by increasing the nitrogen partial pressure. In addition, the basin depth at a pressure of 0.1 and 1.22 MPa is about 41 and 38 mm, the angle of the grains with respect to the vertical axis is 35° and 31°, respectively. The flat metal basin profile resulted from the thermal resistance at the slag–mold interface decreasing with increasing pressure. Primary and secondary dendritic arm spacing (PDAS and SDAS) variations exhibit an increasing and subsequently decreasing the trend as they move further away from the center in a horizontal direction. Both PDAS and SDAS decrease with increasing pressure from 0.1 to 1.22 MPa.

Effect of hot-wire method on microstructure and mechanical properties of wire arc additive repaired superalloy

This study shows that the hot wire is an effective way to reduce the amount of undesirable Laves phase during the repair of 718Plus components, which are detrimental to the mechanical properties. The complex relationship between microstructure and hotwire was studied in detail. Five damaged components were repaired by depositing the melted 718Plus wire onto the wrought substrate using directed energy deposition-arc (DED-arc). A finite element model was used to provide clues for the thermal process analysis. The hot-wire method enhanced the cooling rate of the molten pool and suppressed the remelting behaviour of secondary dendritic arms (SDA) in the reheated zone. The resultant microstructure contained refined dendrites with more obvious SDA, which allows the production of tiny reticular Nb-rich liquid spots between dendrites during final solidification stage. These spots beyond a threshold of Nb could cause eutectic reaction, leading to the morphologic transformation from long-stripped to granular Laves phase and a lower volume fraction. Moreover, the hot-wire method hindered the dynamic precipitation of γ' phase due to the shortened precipitation time, which was responsible for the inferior mechanical properties. The heat-treatment re-optimized the precipitation of γ' phase and further dissolved Laves phase to release more Nb for γ' phase, which leads to the improvement of strength about 10 %.

Effects of hot isostatic pressing on the micron-scale residual stress of nickel-based single-crystal superalloys

Quantifying the residual stress at micron-scale is crucial for comprehending the trans- and inter-granular deformation mechanisms and the influence of heat treatment, but remains technically challenging. This study utilized focused ion beam and digital image correlation (FIB-DIC) techniques to assess residual stress within the dendrite stem and arm of nickel-based single-crystal superalloys. The influence of hot isostatic pressing (HIP) on the microstructure and residual stress was also elucidated. Our results revealed that the residual stresses in the dendrite stem and arm regions manifest as tensile stress along the x-axis and compressive stress along the y-axis, with a range of -720 MPa to 680 MPa. HIP treatment effectively improved microstructure and regulated residual stress in nickel-based single-crystal superalloys, leading to a rapid reduction in residual stress levels. The present study lays a solid theoretical groundwork for optimizing processing strategies to regulate residual stress and enhance mechanical properties in next-generation single-crystal superalloys.

Uneven distribution of cooling rate, microstructure and mechanical properties for A356-T6 wheels fabricated by low pressure die casting

Complex aluminum alloy part prepared by low pressure die casting (LPDC) usually has unevenly distributed microstructure and mechanical properties, which is related to the cooling conditions. To investigate the uneven characteristics of cooling rate, microstructure and mechanical properties of LPDC-fabricated A356-T6 wheels, the typical positions (including hub, spoke, outer flange, rim and inner flange) were selected to calculate the cooling rate by casting simulation. Then, the microstructure, mechanical properties, X-ray result and element content of these positions were analyzed in detail. The result shows that, the secondary dendrite arms spacing (SDAS), eutectic silicon and β-Fe phase have the largest size at hub, and the eutectic silicon is plate-shaped when the cooling rate is 10 K/min, which results in poor ultimate tensile strength (UTS, 192 MPa) and elongation (2.1 %). While at the cooling rate of 100 K/min, refined microstructure, the disappearance of β-Fe phases and the elimination of defects result in good mechanical properties of the outer flange. The UTS and elongation exceed 250 MPa and 13 %, respectively. The rim is more prone to occur shrinkage defects at high cooling rate, which results in mechanical properties that do not meet expectations. Furthermore, the Si, Mg, and Fe elements also exhibit uneven distribution in different positions of LPDC-fabricated A356-T6 wheels. The maximum deviation of Si element between the actual content and the standard content is as high as 21 %.