Dendrite Coherency Research Articles

Versatile fluidity test model for cast superalloys and comparison between IN718 and IN939

A spiral fluidity test model of superalloys with 10 mm in height and 3 mm in thickness was designed to evaluate the fluidity of two distinct Ni-based superalloys IN718 and IN939. The factors influencing fluidity are ascertained through comparative analysis utilizing methodologies such as JMatPro, differential scanning calorimetry and high-temperature confocal laser scanning microscopy. The results show that under identical testing conditions, the fluidity of the IN939 superalloy surpasses that of the IN718 superalloy. When subjected to the same temperature, the melt viscosity and surface tension of IN939 superalloy are considerably reduced relative to those of IN718 superalloy, which is beneficial to improving the melt fluidity. Furthermore, the liquidus temperature and solidification range for the IN939 superalloy are both smaller compared with those of the IN718 superalloy. This condition proves advantageous in delaying dendrite coherency, thereby improving fluidity.

Relationships Between Macrostructure and Microstructure in Lamellar Graphite Iron Castings

AbstractSpherical sheet steel molds filled with gray iron melts of varying chemical compositions and metallurgical conditions were air-cooled until solid, followed directly by austempering to preserve the austenite grain structure. The castings were studied using a combination of cooling curves and quantitative metallography, in order to clarify control of the austenite grain structure and its impact on the local microstructure. A novel method utilizing fast Fourier transform provided visual overview of macroscopic trends in the scale of the flake graphite structure. Castings inoculated with Sr-containing ferrosilicon featured finer eutectic cell structure but coarser equiaxed structure of austenite, emphasizing that melt treatments applied to control the graphite structure may have unintended effects on the austenite grain structure. In most non-inoculated castings, the microstructure was banded, with alternating layers of coarse and fine flake graphite with distance from the casting surface. The extent of the columnar zone of austenite grains showed no correlation with the graphite structure nor the volume fraction of dendrites. The volume-to-surface ratio of dendrites was more uniform in the columnar zone, but increased toward the center in the equiaxed zone. The casting with the highest carbon equivalent (4.34), featured zones containing finer dendrites and graphite. These zones appear to be gaps in the early solidification structure which filled later by secondary dendritic growth from surrounding austenite. This highlights that high carbon equivalent may lead to poor dendrite coherency which can make the microstructure less uniform and less predictable.

Hot Tearing Susceptibility and Solidification Behavior of Mg-xGd-(4-x)Y-2Ni-0.5Zr Alloys

The hot tearing susceptibility (HTS) of Mg-xGd-(4-x)Y-2Ni-0.5Zr (x = 0, 1, 2, 3, 4, wt%) alloys was evaluated using a T-shape mold method. When the Gd element increased and the Y element decreased, the HTS of the alloys decreases gradually. The Mg-4Y-2Ni-0.5Zr has the highest while Mg-4Gd-2Ni-0.5Zr has the lowest HTS value. Microstructure observation reveals that the grains of the magnesium alloy were refined to a certain extent when the Gd element increased and the Y element decreased. The ‘pinning’ effect of the LPSO phase precipitated along the grain boundary effectively increases the solid skeleton strength of the alloy and reduces the HTS of the magnesium alloy to a certain extent. The thin and uniform liquid film at the fracture can disperse the solidification shrinkage stress. In addition, the binary phase precipitated in the alloy forms a fast channel of residual liquid phase feeding on the liquid film, which is beneficial to feeding hot cracks. Solidification behavior of the alloys by thermal analysis showed that with the increase of the Gd element and the decrease of the Y element, the dendrite coherency temperature of the dendrites of the alloy decreases, and the residual liquid phase feeding is easier. The solid fraction of dendrite coherency increases, the skeleton strength of the alloy increases, and the HTS of the alloy decrease. In Mg-xGd-(4-x)Y-2Ni-0.5Zr (x = 0, 1, 2, 3, 4, wt%) alloys, when x is 1, 2, 3, 4, the HTS of the alloy is reduced by 25%, 49%, 63%, 81%, respectively compared with that when x is 0.

Effect of Ca and rotation speed on microstructure and solidification parameters of AZ91 magnesium alloy produced by semi-solid casting through rotating container process

Microstructure of AZ91 magnesium alloy was modified by using semi-solid rotating container process (RCP) and addition of 1 wt.% Ca. Thermal analysis (CA-CCTA) was performed during semi-solid process in order to determine the relationship between microstructural modification and RCP parameters. The container containing molten AZ91 alloy was rotated by rotation speed of 0, 150, 180 and 210 r/min at 640−580 °C. The cooling curves were plotted to determine the solidification characteristics, solid fraction and dendrite coherency point (DCP). The results indicated that the cooling rate increased with increasing the rotation speed significantly and led to an increase in maximum solidification range of 15.9 and 6.4 °C for AZ91 and AZ91−1Ca (wt.%), respectively. Applying shear force to the slurry at 180 r/min minimized the solid fraction and delayed appearance of DCP. It affected the microstructure characteristics such as grain size and circularity factor. The rotation of 180 r/min at 585 °C resulted in the globular microstructure with a circularity factor of 0.72 and a grain size of 73.4 μm. The addition of 1 wt.% Ca to AZ91 in the optimum condition of RCP increased the circularity factor to 0.81 and reduced the grain size to 53.2 μm.

Effect of Al-5Ti-1B and Zr grain refiners on the solidification characteristics and microstructure of Al-6Mg alloy studied through thermal analysis

Al-Mg based alloys are widely used in industrial applications and grain refining is used for structural refinement to achieve high strength and ductility. The aim of this work is to investigate the effects of two grain refiners of Al-5Ti-1B and Zr on the solidification transformations of Al-6Mg alloy and dendrite coherency point (DCP) using two thermocouple cooling curve thermal analyses (CCTA). The microstructure and intermetallic phases were studied through scanning electron microscopy (SEM), EDS, and X-ray mapping. Microstructural evaluations showed the main secondary phases of Al-6Mg alloy are Al3Fe, Al8Mg5, and Mg2Si. The CCTA results indicated that the addition of Al-5Ti-B and Zr causes an increment of nucleation temperature, TN,α,(from 622.4 to 628.4 and 625.7 °C, respectively). The grain refining potential is an inherent property that depends on the growth restriction factor (GRF) and mostly relates to solute concentration and liquidus line slope. The refining power of Al-5Ti-1B for Al-6Mg is more effective because of its larger growth restriction factor (GRF) in comparison with Zr. Therefore, the grain size of the alloy used Al-5Ti-1B or Zr grain refiner was 105 µm and 185 µm, respectively. This phenomenon has been proved when recalescence undercooling (ΔTR,α) was calculated. It has been shown that ΔTR,α reaches zero when Al-5Ti-1B master alloy is added to Al-6Mg alloy. Zr caused the decrement in the first derivative peak intensity of eutectic phases which may be eventuated from extending the solid solubility by the addition of Zr. These grain refiners also affected the dendrite coherency point (DCP). Although Zr changed the TN-TDCP and fsDCP, Al-5Ti-1B increased the TN-TDCP interval by about 14.8 °C and delayed up the coherency solid fraction by about 89.7 pct which can decrease the casting defects.

Relationship Between Critical Solid Fraction and Dendrite Coherency Point in Al–Si Alloys

Relationship Between Critical Solid Fraction and Dendrite Coherency Point in Al–Si Alloys

Effect of Zn content on hot tearing susceptibility of LPSO enhanced Mg–Znx–Y2–Zr0.06 alloys with different initial mold temperatures

The critical temperature (Thci) corresponding to the load relaxation measured during the solidification of alloy in a ‘T′-shaped mold is introduced into the Clyne-Davies’ model, so that it can be used to predict the hot tearing susceptibility (HTS) of alloy under non-equilibrium cooling. Then, the improved model is applied to study the HTS of Mg–Znx–Y2–Zr0.06 (x = 0.5, 1 and 1.5 at%) alloys under two initial mold temperatures, i.e., two cooling rates. It is found that HTS is very sensitive to both Zn content and initial mold temperature, and the increase in Zn content or initial mold temperature can reduce HTS of the alloy. However, their micro mechanisms are different. The reason for the decrease in HTS with the increase in Zn content is the decrease in the dendrite coherency temperature. Specifically, the ratio of unconstrained feeding stage to intergranular feeding stage increases, and the intergranular liquid phase with eutectic composition increases in the later stage of solidification. The HTS decreases with the increase in the initial mold temperature because the solidification shrinkage stress is relieved. Consequently, the residual liquid phase between the grains can be fed in a more timely manner.

The effects of in-situ ZrB2 particles and Gd on the solidification behavior and mechanical properties of AA6111 matrix composites

In this research work, the new AA6111 matrix composites reinforced by the cooperation of nano ZrB2 particles and Gd had been designed and fabricated via in situ reaction. The addition of Gd made the nano ZrB2 particles uniformly dispersed due to the reduction of solid-particle interfacial energy (σsp) by keeping the low-energy orientation relationship. Meanwhile, it promotes the formation of equiaxed grains whose formation is also based on the nano-sized reinforcing phases as the hindering of dendrite coherency points are related to the growth restriction factor (GRF) and as the pinning effect to the grain boundaries. The fluidity was improved because of uniform viscosity and dendrites avoided meeting with the neighboring ones by the entrapment of nano-sized reinforcing phases like AlGd and ZrB2. The results indicated that AA6111–0.5 wt. % Gd-2 vol. % ZrB2 composites possessed the longest fluidity length of 70 mm. Meanwhile, the composites displayed increases of 32.7 %, 47.3 % and 52.8 % in YS, UTS and elongation, respectively, compared to the matrix alloy. A notable lattice distortion of the matrix near the ZrB2 particles and AlGd phases was proclaimed via Geometric Phase Analysis (GPA) analysis. Strengthening mechanism comprehending yield strength, containing Orowan strengthening, grain refinement strengthening, was integrally elaborated.

In Situ Synchrotron Tomography of the Solidification of an Elektron 21 Mg Alloy

The directional solidification of an Elektron 21 magnesium alloy is investigated by in situ synchrotron radiation tomography. To visualize the solidification process, samples of Elektron 21 are first heated to 800 °C, and the melt is held at this temperature for 5 min, to ensure temperature homogeneity. Subsequently, the samples are cooled with a cooling rate of 10 K min−1, while for every 35 s, one full tomogram is acquired. The evolution of the microstructure can be followed in 3D on the reconstructed tomograms. The contrast between rare‐earth metals and Mg enables to quantitatively analyze the changes in the morphology of the dendritic structure during solidification. At the onset of the detection, the growth of secondary dendrite arms occurs, which ends at the dendritic coherency point. From this temperature on, only the coarsening and coalescence of existing dendrite arms occurs.

Influence of a low-frequency alternating magnetic field on hot tearing susceptibility of EV31 magnesium alloy

The effect of a low-frequency alternating magnetic field (AMF, 0 A 0 Hz, 5 A 10 Hz, 10 A 10 Hz, 15 A 10 Hz) on the hot tearing susceptibility (HTS) of a magnesium alloy (EV31) was systematically studied using a combination of experiment and numerical simulation. By observing the macroscopic hot cracks in hot joints of the “T” samples, the hot tearing tendency of the samples was analyzed. The HTS of the alloy can be predicted via numerical simulation and the crack susceptibility coefficient (CSC). The microstructure and morphology of the hot tearing zone of EV31 were investigated using scanning electron microscopy (SEM). Results show that increasing the magnetic field strength reduces both the alloy solidification temperature range and the dendrite coherency temperature, which increases the feeding time during solidification and decreases the HTS of the alloy. When the magnetic field parameters are 10 Hz 15 A, the EV31 alloy shows the lowest HTS. The main component of the second phase in the microstructure is Mg12Nd. This study also found that the electromagnetic field can effectively refine the grains, purify the melt, and reduce the oxide content in the melt. The obtained simulation results are consistent with the experimental results.

Electromagnetic-thermal coupling behaviors and thermal convection during two-phase zone continuous casting high strength aluminum alloy tube

In this paper, the two-phase zone continuous casting (TZCC) technique was used to prepare 2D12 aluminum alloy tubes and a 3D model of TZCC was established to investigate the electromagnetic-thermal coupling as well as fluid flow behaviors under different mold temperatures. The calculated temperature in the induction heated mold agrees well with the measurement. It was found that the heat flux from the mold to the melt leads to a convex isoliquid fraction line of dendrite coherency point and circular thermal convection in front of the mushy zone. The velocity of the transverse flow increases remarkably with increasing mold temperature while the upward flow velocities at the outer wall are nearly constant. Experimental results indicate that the surface exudations with segregated solute appear at the surface of tubes when the mold temperature is higher than 883 K, and their size and quantity increase with increasing mold temperature. The serious solute redistribution and the thermal convection lead to solute segregation at the surface of tubes. Further, the residual liquid with segregated solutes flows to the surface of the tube and forms the surface exudation due to its lower freezing point than the Al matrix.

Determination of dendrite coherency point characteristics in Al-Si-Mg alloy

ABSTRACT Dendrite coherency or dendrite impingement is an important factor for determining solidification microstructure and casting defects of alloys. In this research, the effect of various cooling rates and Al-5Ti-1B grain refiner contents on the dendrite coherency point (DCP) characteristics of an Al-Si-Mg alloy designated as A357 has been studied. The results showed that both mentioned parameters have a significant influence on DCP of this alloy. Increasing the cooling rate from 0.15 to 1.0 °C/s and the grain refiner content from 0.0 to 3 wt. % increased the solid fraction in where dendrite coherency occurs. Moreover, enhancement of the cooling rate and grain refiner could increase the difference between the liquidus and coherency temperatures (TN–TDCP), about 27 and 26 °C, respectively. The dendrite coherency time decreased by increasing the cooling rate and increased by increasing the grain refiner content.

Assessment of the Microstructure, Solidification Characteristics and Mechanical Properties of AZ61 + xSr Magnesium Alloys

The effect of Sr addition on the solidification parameters of AZ61 alloy, such as liquidus and solidus temperatures, the nucleation under cooling temperature and dendrite coherency point (DCP), was investigated via the cooling curve thermal analysis method. The results revealed that by increasing the Sr content from 0 to 0.75 pct, the nucleation temperature increased about 8.2 °C and nucleation under cooling decreased about 3.5 °C. Also, it was found that increasing the Sr content suspended the dendrite coherency point and increased the solid fraction at this point. Mechanical properties of AZ61 alloy were characterized at room temperature through a compressive test. The results indicated the positive influence of Sr on compressive strength of the alloy. When the Sr content increased from 0 to 0.75 pct, the ultimate compressive stress increased from 186 to 280 MPa. This could be the result of a grain refining effect of Sr on AZ61 alloy and the formation of Al4Sr and Al3Mg13Sr intermetallic compounds.

Effect of Pouring and Mold Temperatures on Hot Tearing Susceptibility of WE43 Magnesium Alloy

The effects of different pouring temperature and mold temperature on the hot tearing susceptibility of WE43 magnesium alloy were studied using computer numerical simulation, hot tearing test and differential thermal analysis experiment. The microstructure of the hot tearing zone of the casting was observed using SEM, and the phase composition was analyzed using EDS. Numerical simulation results show that the change in pouring temperature and mold temperature could affect the hot tearing susceptibility (HTS) of WE43 alloy, and the influence of mold temperature was more significant with the conditions of pouring temperature (Tp) of 710–730 °C and mold temperature (Tm) of 240–320 °C. The experimental results are in good agreement with the simulation results. At the lower pouring temperature, the dendrite coherency began at a higher solids fraction, which could enhance the resistance to hot tearing at the end of solidification and reduce HTS of WE43 alloy. At the same pouring temperature, with the increase in the mold temperature, the number of dispersed eutectic phases in the alloy increased obviously, so that hot tearing could be well filled with the eutectic phase to reduce the HTS of WE43 alloy. In experimental conditions, the WE43 magnesium alloy had the lowest HTS at a pouring temperature of 710 °C and a mold temperature of 320 °C.

Solidification of Mg–Zn–Zr Alloys: Grain Growth Restriction, Dendrite Coherency and Grain Size

The solidification characterization of Mg–xZn–0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys has been extensively investigated through thermal analysis, microstructure characterization and thermodynamic calculations. The impact of Zn content on the grain growth restriction, dendrite coherency and thus the final grain size has been investigated and discussed. Increasing Zn content, the grain size of Mg–xZn–0.5Zr alloy was firstly refined and then coarsened with the finest grain size of ~ 50 μm for the Mg–3Zn–0.5Zr (ZK31) alloy. Significant effects of the grain size on the mechanical properties were observed in the investigated alloys. The combination of growth restriction factor theory and dendrite coherency point provides a reasonable explanation of the grain size results. It helps to further understand the mechanisms of grain refinement and grain coarsening related to solute content, providing reference for alloy design and grain size prediction.

Hot Tearing Behavior of $$\text{Mg}{-}4\text{Zn}{-}x\text{Sn}{-}0.6\text{Zr}$$ Alloys

The effects of different Sn additions (0, 0.5, 1.0, 2.0 and 3.0 wt%) on hot tearing susceptibility (HTS) of Mg–Zn–Zr alloy were studied using a “T-shaped” hot tearing mold in a pouring temperature of 700 °C and a mold temperature of 270 °C. The “Clyne–Davies” hot tearing prediction model and commercial simulation software, Procast, were used to characterize the HTS of alloys. The dendrite coherency temperature was obtained by means of differential thermal analysis. The phases evolution, microstructures and morphology of the crack zone of Mg–4Zn–xSn–0.6Zr alloys were investigated by using X-ray diffraction and scanning electron microscope. The experimental results show that the HTS of Mg–4Zn–xSn–0.6Zr alloys decreases with Sn additions up to 1.0 wt%, and then exhibits a slight increase with further Sn additions up to 3.0 wt%. The Sn additions into Mg–4Zn–0.6Zr alloy can form Mg2Sn phase with high melting point. Appropriate addition of Sn can refine grain size, decrease the dendrite coherency temperature, increase the thickness of liquid film and the feeding ability at the end of solidification, which reduce the HTS of the alloy. However, excessive Sn addition will make Mg2Sn aggregate and grow up, and hinder the residual liquid-phase feeding in the late solidification stage, resulting in an increase in HTS of the alloy. The order of HTS of studied alloys is: CSC(Mg–4Zn–0.6Zr) > CSC(Mg–4Zn–0.5Sn–0.6Zr) > CSC(Mg–4Zn–3Sn–0.6Zr) > CSC(Mg–4Zn–2Sn–0.6Zr) > CSC(Mg–4Zn–1Sn–0.6Zr).

Hot Tearing Susceptibility of AXJ530 Alloy Under Low-Frequency Alternating Magnetic Field

Herein, a hot tearing measured system with external excitation coil and a differential thermal analysis system with applied magnetic field were used to study the effects of low-frequency alternating magnetic field on the solidification behavior and hot tearing susceptibility (HTS) of the AXJ530 alloy under different magnetic field parameters. The hot tearing volume of the castings was measured via paraffin infiltration method. The microstructure of the hot tearing zone of the casting was observed using optical microscopy and scanning electron microscopy, and the phase composition was analyzed using X-ray diffraction and energy depressive spectroscopy. The experimental results show that the solidification interval of AXJ530 alloy was shortened and the dendrite coherency temperature of the alloy decreased with the increase in frequency of alternating magnetic field. Under appropriate magnetic field parameters, the electromagnetic force could enhance the convection in the melt to promote the flow of the residual liquid phase, refine the microstructure, and optimize the feeding channel in the late solidification stage, which reduced the HTS of the alloy. However, when the magnetic field frequency was increased to 15 Hz, the induced current generated excessive Joule heat to the melt. At this time, the thermal action of the magnetic field coarsened the microstructure of the alloy, resulting in an increase in HTS of the alloy.

Effects of Zn Content on Hot Tearing Susceptibility of Mg–7Gd–5Y–0.5Zr Alloy

Mg–7Gd–5Y–0.5Zr alloy has excellent mechanical properties but poor hot tearing resistance. The latter makes it difficult to cast billets, which limits the size of subsequently processed parts. Therefore, the hot tearing susceptibility of Mg–7Gd–5Y–xZn–0.5Zr (x = 0, 3, 5, 7 wt%) alloys was studied. It was found that Zn can significantly reduce hot tearing susceptibility of Mg–7Gd–5Y–0.5Zr alloy, which almost linearly decreased with Zn content. When Zn content was 3 wt%, 5 wt% and 7 wt%, hot tearing susceptibility will be reduced by 27%, 83% and 100%, respectively. It was further revealed that the solid solubility of Gd and Y in α-Mg decreased with the increase of Zn content, and the nucleation temperature decreased accordingly, which resulted in the increase of nucleation rate and the refinement of final grains. On the macro level, it showed that the dendrite coherency temperature decreased, the solidification shrinkage stress of α-Mg slowed down, and the residual liquid channel became shorter and hot tearing susceptibility decreased. It was also found that with the increase of Zn content, the content of Zn, Gd and Y enriched on the grain boundary increased, the content of residual liquid phase between dendrites increased after α-Mg crystallization, and the solidified precipitated second phase also changed from Mg5RE phase to long-period stacking ordered phase + W-phase (a little), long-period stacking ordered phase + W-phase (much) and finally to W-phase only. The feeding effect of sufficient intergranular residual liquid on the shrinkage of α-Mg dendrite and the bridging effect of the precipitated phase at the grain boundary (especially long-period stacking ordered phase which is coherent with the parent phase) also led to the decrease of hot tearing susceptibility.

Influence of Al addition on solidification path and hot tearing susceptibility of Mg–2Zn–(3 + 0.5x)Y–xAl alloys

Abstract Hot tearing susceptibility (HTS) of Mg–2Zn–(3 + 0.5x)Y–xAl (x = 0, 2 and 3 at%) alloys is predicted by using modified Clyne–Davies’ model (CSC*). The solidification path, solidification characteristic temperatures and dendritic coherency solid fraction have been studied by double-thermocouple thermal analysis. The solidification contraction stress vs. temperature (and time) curves are measured by using a “T” type hot tearing permanent-mold. The results reveal that the CSC* prediction values are in good agreement with the experimental results. Moreover, Al2Y phase acts as the heterogeneous nucleation core of α-Mg and significantly influences the grain size. It has been observed that minimum grain size, optimal dendritic coherency and minimum HTS are exhibited by Mg–2Zn–(3 + 0.5x)Y–xAl alloy (x = 2). Furthermore, when Al content was increased to 3 at%, Al2Y phase exhibited a peritectic reaction and transformed into a mixed structure of Al2Y and Al + Al3Y phases, which increased the HTS of the alloy due to reduced fine-grained Al2Y content.

Effect of gas-induced semisolid process on solidification parameters and dendrite coherency point of Al–4.3Cu alloy using thermal analysis

Semisolid processing of Al–4.3%Cu (A206) alloy was carried out using gas-induced semisolid (GISS) process in different conditions. The flow rate of inert gas, 1, 2, and 4 L min−1, starting temperature for gas purging (the temperature of superheated melt), 670, 660, and 650 °C, and the duration of gas purging, 10, 20, and 30 s, were three key process variables which were changed during this investigation. Thermal analysis was successfully implemented through CA-CCTA technique for GISS samples as well as conventionally cast sample. The two-thermocouple thermal analysis technique was utilized to determine the dendrite coherency point of GISS sample and conventionally cast sample. The results showed that gas purging into the melt led to temperature drop of the melt to its liquidus temperature. In fact, copious nucleation is induced by cooling effect of inert gas bubbles. GISS process delays the dendrite coherency point from 644.2 to 637.3 °C which leads to increase the solid percentage from 9 to 21% at this point and therefore enhance the casting characteristics. It is found that inert gas purging into the molten metal, regardless of the process parameters, leads to the microstructural modification from fully dendritic to globular structure. Microstructural evaluation showed that the best sample which included fine grains of 76 μm in average size and with high level of globularity of 0.86 was achieved from a semisolid sample in which the gas purging started at 670 °C and its duration time was 20 s with the gas flow rate of 4 L min−1.