Mushy Zone Length Research Articles

Crack inhibition to enhance strength-ductility of CM247LC alloy fabricated by laser powder bed fusion

High cracking susceptibility is a critical challenge for the application of high γ′-fraction Ni-base superalloys fabricated by laser powder bed fusion (LPBF). This study systematically investigated the effect of process parameters on the cracking susceptibility and microstructure of LPBF-processed CM247LC alloy. The formation mechanisms of liquation cracks and solidification cracks were revealed. The effects of process parameters on solidification behavior, microstructure evolution and local strain level were discussed. The cracking behavior was significantly affected by the high-angle grain boundaries (HAGBs), elemental segregations (e.g., C and O) and stress concentration at cracking regions. An increase in the laser power increased the length of mushy zone and the depth/width ratio of melt pools, resulting in high solidification cracking susceptibility. The overlap between adjacent melt tracks determined by hatch spacing and layer thickness could heal partial cracks through remelting. Nearly crack-free CM247LC samples with enhanced strength-ductility (yield strength of 921 ± 38 MPa, ultimate tensile strength of 1278 ± 3 MPa, and elongation of 16.1 ± 1.3%) were obtained by optimized process parameters. The zigzag grain boundaries, refined grains and reduced grain aspect ratio in the nearly crack-free samples could relieve the stress concentration along the grain boundaries and enhance the bonding of adjacent grains and thus delay the propagation of cracks. This work may provide essential comprehension for improving the LPBF processability of high γ′-fraction Ni-base superalloys.

Cracking criterion for high strength Al–Cu alloys fabricated by selective laser melting

Abstract Cracking is one of the most serious problems for the development of selective laser melting (SLM) of high strength aluminum alloys. The process parameters applied in the SLM process determine the cracking severity in these alloys. The purpose of this study is to understand the correlation between the process and the solidification cracking during SLM of Al–Cu alloys. A model based on the modified Rosenthal’s equation and Feurer’s criterion was developed to predict the critical scanning velocity for the crack initiation during SLM. The calculated result shows that the molten pool center is the most susceptible to the crack initiation in the conductive molten pool, and the susceptibility increases as the distance from the observed point to the molten pool center increases. The overlapping of the molten pool can remelt the previous track, and thus the crack initiating during the previous scanning may be eliminated after the remelting. Therefore, a small hatch spacing is beneficial for manufacturing the crack-free components. The cracking is mainly affected by the mushy zone length and the cooling rate during the process. Hence, the composition strongly affects the cracking. Not only the solidification temperature range but also other thermo-physical properties affect cracking. The model was verified by two different Al–Cu alloys. The cracking initiation and the crack length were observed and measured to evaluate the applicability of the model. The experimental data and the calculation results agree well. The model can be used for the process control as well as the alloy development for SLM of Al–Cu alloys.

Multi-scale modeling of liquid-metal cooling directional solidification and solidification behavior of nickel-based superalloy casting

Liquid-metal cooling (LMC) process can offer refinement of microstructure and reduce defects due to the increased cooling rate from enhanced heat extraction, and thus an understanding of solidification behavior in nickel-based superalloy casting during LMC process is essential for improving mechanical performance of single crystal (SC) castings. In this effort, an integrated heat transfer model coupling meso grain structure and micro dendrite is developed to predict the temperature distribution and microstructure evolution in LMC process. An interpolation algorithm is used to deal with the macro-micro grids coupling issues. The algorithm of cells capture is also modified, and a deterministic cellular automaton (DCA) model is proposed to describe neighborhood cell tracking. In addition, solute distribution is also considered to describe the dendrite growth. Temperature measuring, EBSD, OM and SEM experiments are implemented to verify the proposed model, and the experiment results agree well with the simulation results. Several simulations are performed with a range of withdrawal rates, and the results indicate that 12 mm·min-1 is suitable for LMC process in this work, which can result in a fairly narrow and flat mushy zone and correspondingly exhibited fairly straight grains. The mushy zone length is about 4.8 mm in the steady state and the average deviation angle of grains is about 13.9° at the height 90 mm from the casting base under 12 mm·min-1 withdrawal process. The competitive phenomenon of dendrites at different withdrawal rates is also observed, which has a great relevant to the temperature fluctuation.

Optimizing process windows for minimizing the pore size of Ni-based single crystal superalloys

Minimizing pore size during directional solidification is critical for reducing the risk of failure for Ni-based superalloys. In this study, the pore size distribution as a function of casting conditions during the entire single crystal casting was obtained by two-dimensional optical metallography and three-dimensional X-ray microtomography. Then, the relationship between the pore size and local thermal conditions was implemented as a subroutine in a finite element model. Both simulation and experimental results show that the temperature gradient and withdrawal rate play the most significant roles in the pore size distribution. Besides, the influences of withdrawal rate and temperature gradient on the mushy zone length, lateral-axial thermal gradient ratios were also investigated. Combining the characterization results with simulation models, a computationally effective method was developed to optimize the processing window of directional solidification and further reduce the pore size.

Effect of a High Magnetic Field on γ′ Phase for Ni-Based Single Crystal Superalloy During Directional Solidification

The effect of a high magnetic field on the γ′ phase of Ni-based single crystal superalloy during directional solidification is investigated experimentally. The results clearly indicate that the magnetic field significantly reduces the γ′ phase size. Further, the quenching experiment is carried out, and the results found that the length of mushy zone is obviously decreased under a high magnetic field. Based on both experimental results and nucleation mechanism, it is found that the decrease of γ′ phase size should be attributed to the fact that a high magnetic field causes the increase of temperature gradient in front of solid/liquid interface and leads to the increase of undercooling of γ′ phase.

Thermo-Calc Prediction of Mushy Zone in AlSiFeMn Alloys

Convection forces can cause significant segregation within the liquid during directional solidification, influencing the structure of the mushy zone and the type and distribution of phases present in the solidified alloy. The solidification behavior of AlSiFeMn alloys with strong convection was investigated via experimental results combined with thermodynamic calculations. Experimental specimens were processed in a directional solidification facility with forced melt flow, resulting in high levels of elemental segregation across samples. The resulting local compositions were located on phase diagrams Al-Si-Fe, Al-Si-Mn and Al-Fe-Mn for prediction of the variation in solidification behavior. Phase mass fraction diagrams created in Thermo-Calc showed the effect of segregation on the characteristic temperatures, mushy zone length and the order of occurring phases precipitating across specimens. These findings were used to create 2D maps for visualization of the mushy zone, mass fraction of α-Al dendrites, β-Al5FeSi, Al15Si2Mn4 and their spatial location. The specimen centers showed enrichment in AlSi-eutectic but for β-Al5FeSi and Al15Si2Mn4 results are ambiguous. Fe-phases start to grow mainly behind the dendrites tips and in general may flow between them. Mn-rich phases start to precipitate at higher temperatures than β and in many places before α-Al and in this way may flow in the melt above the mushy zone.

Mushy Zone Morphology Calculation with Application of CALPHAD Technique

Mushy zone morphology in AlSiMn alloys was studied using directional solidification, and the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) technique was applied for thermodynamic calculations. The specimens solidified with forced convection presented segregation across the sample diameter, and the measured compositions were located on the Al-Si-Mn phase diagram. Scheil-Gulliver calculations for measured compositions were used to determine various solidification paths that may occur in specimens. Property diagrams and solidification paths presented the segregation effect on the characteristic temperatures, mushy zone length and the sequence of occurring phases whilst 2D maps enabled visualization of the mushy zone during directional solidification. Melt stirring was found to change solidification range, as well as mushy zone length and shape, and the dendrite tips formed a rough profile across the specimens. The study revealed mushy zones with dense dendritic structure and liquid channels empty of Mn phases, where intermetallics had no possibility to flow in the liquid, whilst in other samples with channels filled with Al15Si2Mn4, Mn-precipitates also flowed above the α-Al. The melt flow may lead to a mainly dendritic mushy zone or to a mushy zone with dendrites reaching only lower half of mushy length with intermetallics forming and freely flowing above dendrites in the liquid upper half.

Effect of a transverse magnetic field on solidification structure in directionally solidified Al–40 wt% Cu alloys

Abstract

Effect of a transverse magnetic field on solidification structure in directionally solidified Al-Cu-Ag ternary alloys

The effect of a transverse magnetic field on solidification structure in directionally solidified Al-Cu-Ag ternary alloys was investigated experimentally. The results show that the application of the transverse magnetic field significantly modified the solidification structures. Indeed, the magnetic field caused the formation of macrosegregation and the transformation of the liquid/solid interface from cellular to planar. Moreover, it was found that the magnetic field refined the eutectic cell and decreased the mushy zone length. This may be attributed to the thermoelectric magnetic convection between eutectic cells.

Directional Solidification Microstructure of a Ni-Based Superalloy: Influence of a Weak Transverse Magnetic Field

A Ni-based superalloy CMSX-6 was directionally solidified at various drawing speeds (5–20 μm·s−1) and diameters (4 mm, 12 mm) under a 0.5 T weak transverse magnetic field. The results show that the application of a weak transverse magnetic field significantly modified the solidification microstructure. It was found that if the drawing speed was lower than 10 μm·s−1, the magnetic field caused extensive macro-segregation in the mushy zone, and a change in the mushy zone length. The magnetic field significantly decreases the size of γ’ and the content of γ-γ’ eutectic. The formation of macro-segregation under a weak magnetic field was attributed to the interdendritic solute transport driven by the thermoelectric magnetic convection (TEMC). The γ’ phase refinement could be attributed to a decrease in nucleation activation energy owing to the magnetic field during solid phase transformation. The change of element segregation is responsible for the content decrease of γ-γ’ eutectic.

Influence of the solidification temperature range on Gasar structures made from Cu–Mn alloys

Abstract In this study, the influence of the solidification temperature range of an alloy system on Gasar structures is investigated from the aspect of solidification mode and mushy zone length. The solidification mode and mushy zone length for various Cu–Mn alloys during unidirectional solidification are predicted by theoretical analysis. The calculated data indicate that it is reasonable, possible, but difficult to fabricate porous Cu-34.6 wt.% Mn, Cu-24 wt.% Mn, and Cu-46.6 wt.% Mn alloys by the Gasar process, respectively. Accordingly, lotus-type porous Cu-34.6 wt.% Mn alloy with regularly oriented pores was prepared and solidified with a cellular structure. Porous Cu-24 wt.% Mn alloy with directional pores could be fabricated, solidifying with a columnar dendritic structure. Porous Cu-46.6 wt.% Mn alloy with irregular large pores was obtained, and solidified with an equiaxed dendritic structure.

Effect of a transverse magnetic field on solidification structure in directionally solidified Sn–Pb hypoeutectic alloys

Effect of a transverse magnetic field on the macrosegregation and the growth of the Sn dendrite in the directionally solidified Sn–Pb alloys was investigated experimentally. The results indicated that the magnetic field modified the shape of the liquid/solid interface and the dendrite morphology significantly. Indeed, the application of the magnetic field caused the formation of the sloping interface and the refinement of the dendrite. It is also found that the magnetic field decreased the mushy zone length. These effects were enhanced with the increase of the magnetic field intensity and the decrease of the growth speed. Further, the Seebeck thermoelectric force (ES) at the liquid/solid interface in the Sn-20wt%Pb alloy was measured in-situ and the results indicated that the value of the Seebeck thermoelectric force was about 1µV. The modification of the solidification structure during directional solidification under the magnetic field may be attributed to the interdendritic thermoelectric magnetic convection (TEMC).

Phase Field Modeling of β(Ti) Solidification in Ti-45at.%Al: Columnar Dendrite Growth at Various Gravity Levels

At present, our understanding of the interaction between melt flow and solidification patterns is still incomplete. In columnar dendritic growth buoyancy driven flow may alter the dendrite tip and spacing selection and consequently the microsegregation of alloying elements. With the aim of supporting directional solidification experiments under hyper-gravity using a large diameter centrifuge (LDC), phase field simulations of β (Ti) dendrite growth have been performed under various gravity conditions for the binary alloy Ti-45at.%Al. The results show that Al segregation at the growth front causes convection rolls around the dendrite tips. The direction of the gravity vector is an essential parameter. When g is opposite to the direction of dendrite growth, increasing gravity leads to a marked decrease of the primary dendrite spacing and to a decrease of the mushy zone length. When g is aligned parallel to the direction of dendrite growth, the primary dendrite spacing and mushy zone length are almost unchanged, however the secondary dendrite arms grow more prominently as the magnitude of g increases.

Effect of a weak transverse magnetic field on solidification structure during directional solidification

Six alloys were directionally solidified at low growth speeds (1–5μms−1) under a weak transverse magnetic field (⩽0.5T). The results show that the application of a weak transverse magnetic field significantly modified the solidification structure. Indeed, it was found that, along with the refinement of cells/dendrites, the magnetic field caused the deformation of liquid–solid interfaces, extensive segregations (i.e., freckles and channels) in the mushy zone, and a change in the mushy zone length. Further, in situ monitoring of the initial transient of the directional solidification was carried out by means of synchrotron X-ray radiography. It was observed that dendrite fragments and equiaxed grains were moved approximately along the direction perpendicular to the magnetic field. This result shows that a thermoelectric magnetic force (TEMF) acted on the liquid or the solid during directional solidification under a weak magnetic field. The TEMF during directional solidification under a transverse magnetic field was investigated numerically. The results reveal that a unidirectional TEMF acted on the solid and induced thermoelectric magnetic convection (TEMC) in the liquid. Modification of the solidification structure under a weak magnetic field is attributed to TEMC-driven heat transfer and interdendritic solute transport and TEMF-driven motion of dendrite fragments.

Faceted growth of primary Al2Cu crystals during directional solidification in high magnetic field

The high magnetic field is widely used to modify the crystal morphology. In this work, the effect of the magnetic field on growing behavior of faceted crystals in the Al-40 wt. %Cu alloy was investigated using directional solidification technique. It was found that the faceted growth of primary Al2Cu phase was degraded and the primary spacing was reduced upon applying the magnetic field. Additionally, the length of the mushy zone first decreased and then increased with increase of the magnetic field intensity. The quantitative analysis reveals that the shear stress induced by the fluid motion is insufficient to break the atom bonds at the solid-liquid interface. However, both of the thermoelectric magnetic convection (TEMC) and the thermoelectric magnetic force (TEMF) cause dendrites to fracture and reduce the primary spacing. The two effects also weaken the faceting growth. Moreover, the instability of the solid-liquid interface is generated by the TEMF, which further leads to degrade the faceted growth. The length of mushy zone was changed by the TEMC and reached the minimum in the magnetic field of 0.5 T, which is in good agreement with the predicted value (0.83 T).

Finite element method simulation of mushy zone behavior during direct-chill casting of an Al-4.5 pct Cu alloy

In this article, the stresses, strains, sump depth, mushy zone length, and temperature fields are calculated through the simulation of the direct-chill (DC) casting process for a round billet by using a finite-element method (FEM). Focus is put on the mushy zone and solid region close to it. In the center of the billet, circumferential stresses and strains (which play a main role in hot cracking) are tensile close to the solidus temperature, whereas they are compressive near the surface of the billet. The stresses, strains, depth of sump, and length of mushy zone increase with increasing casting speed. They are maximum in the start-up phase and are reduced by applying a ramping procedure in the start-up phase. Stresses, strains, depth of sump, and length of mushy zone are highest in the center of the billet for all casting conditions considered.

Mushy zone morphology during directional solidification of Pb-5.8 wt pct Sb alloy

The Pb-5.8 wt pct Sb alloy was directionally solidified with a positive thermal gradient of 140 K cm−1 at a growth speed ranging from 0.8 to 30 µm s−1, and then it was quenched to retain the mushy zone morphology. The morphology of the mushy zone along its entire length has been characterized by using a serial sectioning and three-dimensional image reconstruction technique. Variation in the cellular/dendritic shape factor, hydraulic radius of the interdendritic region, and fraction solid along the mushy zone length has been studied. A comparison with predictions from theoretical models indicates that convection remarkably reduces the primary dendrite spacing while its influence on the dendrite tip radius is not as significant.

The Effect of Convectional Flow in Vertical Directional Solidification

In order to get a more systematic view of the different effects of buoyant flow induced by gravity in directional solidification of metallic alloys, we made a study of the structure and composition of samples solidified with a parallel orientation relative to gravity on earth. The solute gradient in the melt immediately ahead of the mushy zone is not expected to be a major factor in solute convection for dendritic growth and the solute convection will be mostly dominated by the solute gradient in the interdendritic melt. The change in the length of the mushy zone affects the intensity of the solute convection because the length of mushy zone is not constant even when the steady-state dendritic growth is maintained during upward directional solidification.

Cellular/dendritic array tip morphology during directional solidification of Pb-5.8 Wt Pct Sb alloy

Cellular/dendritic array tip morphology has been examined in directionally solidified and quenched Pb-5.8 wt pct Sb alloy by a serial sectioning and three-dimensional image reconstruction technique. There is a large scatter in the tip radius, the nearest neighbor spacing, and the mushy zone length, even among the immediately neighboring cells and dendrites. This scatter may be caused by the natural convection (in the mushy zone and in the bulk melt at the array tip), which also produces macrosegregation along the length of the directionally solidified samples. Even in the presence of convection, however, the tip radii are observed to be approximately proportional to the square of the primary spacings, and the radii are in a good quantitative agreement with the predictions from the model due to Hunt-Lu.

Effect of Solidification Rate and Temperature Gradient on the Changes of Mushy Zone Length in Pb-36mass%Sn Binary Alloys during Upward Directional Solidification.

Upward directional solidification experiments have been carried out on Pb-36mass%Sn binary alloys with varying the solidification rate and temperature gradient. Results show that due to solute convection, the length of the mushy zone was not constant even when the dendritic growth at a constant temperature gradient was maintained during upward directional solidification. The length of the mushy zone decreased with increasing fraction solidified if solute convection occured. At a constant solidification rate, the greater the temperature gradient is, the greater the change of length of mushy zone is. At a constant temperature gradient, the smaller the solidification rate is, the greater the change in length of the mushy zone is. Effect of solute convection on the length of the mushy zone during upward directional solidification is responsible for the composition change of Sn at ahead of the mushy zone. If the composition at ahead of the mushy zone changes greatly as increasing the fraction solidified, the change of length of the mushy zone is greater.