Solidification Microstructure Research Articles

On the formation of gradient-distributed dendrites in a single crystal nickel-based superalloy directionally solidified under transverse static magnetic field

Directional solidification (DS) of alloys under static magnetic field has attracted extensive attention due to the development of thermoelectric magnetohydrodynamics (TEMHD). Assessing the influence of static magnetic field on solidification microstructures in single crystal (SX) superalloys is of great significance for both the development of SX superalloys and the understanding of external field-assisted solidification. In this contribution, under different transverse static magnetic fields, the evolution of dendritic structure in a SX nickel-based superalloy directionally solidified over a range of solidification velocities is systematically investigated. The results show that applying a transverse static magnetic field (TSMF) during DS does not disrupt the crystal orientation of SX superalloys. The distribution of dendrites or local primary dendrite arm spacings within the cross section of SX superalloys shows a gradient variation in the direction perpendicular to the TSMF. Additionally, the formation of gradient-distributed dendritic structure is closely related to the applied TSMF intensity and solidification velocity. It is easier to form a gradient-distributed dendritic microstructure at low solidification velocities and high magnetic fields. Based on the experiments and simulations, the formation of gradient-distributed dendritic microstructure is supposed to attribute to the variation of interdendritic constitutional undercooling created by the change of solute concentration in interdendritic region, which is induced by the TEMHD. The SX superalloy with a gradient distribution of dendrites may have potential applications due to its gradient properties. Meanwhile, these findings also provide a new method for preparing alloys with gradient microstructures.

Additive manufacturing of Stellite 6 alloy by laser-directed energy deposition: Engineering the crystallographic texture

Additive manufacturing (AM) of Stellite 6 alloy using laser-directed energy deposition (L-DED) technology was explored. The impact of argon and nitrogen as the shielding gas atmosphere and of the laser processing parameters on the microstructural characteristics of the L-DED manufactured single- and double-layers of the Stellite 6 alloy were studied. The microstructural characteristics and mechanical properties of the additively manufactured Stellite 6 alloy were investigated using electron backscatter diffraction (EBSD) analysis and indentation micro-hardness measurements. Also, the tribological behavior of the processed Stellite 6 were studied under different loading conditions. Based on the observed structural evolution including the phase transformations, optimized process parameters were established for the building of the main structural wall of the Stellite 6 alloy. Subsequently, the optimized set of processing parameters was analyzed in connection to the crystallographic textural majority of the manufactured alloy. The overall homogenous solidification microstructure of the constructed wall was characterized along different sections, i.e., bottom, middle, and top regions. At the bottom, the microstructure was dominated by gradients due to the solidification mechanism and the formation of a combined equiaxed/columnar dendritic structural morphology upon rapid cooling, resulting in a heterogeneous distribution of mechanical strength. The microstructure mainly consisted of columnar dendrites, generating a dominant textural component of 100<013> with a J-index of 1.62 in the matrix. After L-DED, the hardness of the alloy was found increased up to 565 HV, with a mean value of about 430 HV, depending on the geometrical location across the manufactured alloy wall. Also, a superior wear resistance of 4.91 × 10−5 mm3/Nm under a maximum load of 50 N was observed as a result of the enhanced mechanical properties. In fact, these findings substantiate an exciting approach to develop material properties with respect to processing design in which an advanced alloy is manufactured in a single step without particular post-processing.

Processability and Solidification Microstructure of Al-10Si-4.5Mg Alloy Fabricated by Laser Powder Bed Fusion

With the aim of developing a high-strength aluminum alloy for laser powder bed fusion (L-PBF), an Al–10Si–4.5Mg alloy with the a-Al/Si/Mg2Si three-phase microstructure was investigated. The Al–10Si–4.5Mg alloy processed by L-PBF exhibited a fine cellular microstructure including fine granular Mg2Si phases, and therefore exhibited a higher hardness of 187 HV0.1 than those of the conventional Al–Si–Mg alloy. However, cracks were macroscopically propagated between the internal fabrication voids along the melt pool boundaries in the L-PBF processed samples, resulting in a limited relative density below 95.5%. The cracking could be attributed to the relatively coarse Mg2Si particles decorated with the eutectic network. Although the improved strength suggests the advantage of strengthening by the Mg2Si phase, further optimization of the processing conditions will be required to manufacture the intact L-PBF parts.

Relationship between solidification path, microstructure evolution and solidification cracking behavior of Mg-Al-Ca alloy during TIG welding

The high-strength and creep-resistant Mg-Al-Ca-Mn alloys have broad application prospects. However, solidification cracking occurs in these alloys in certain conditions and the origin is still unclear. This work investigated the relationship between the solidification path, microstructure evolution and solidification cracking behavior of the Mg-xAl-2Ca-Mn alloys during tungsten inert gas (TIG) welding. Results show that when the fusion zone's Ca/Al mass ratio ranges from 0.4 to 1.64, solidification cracking occurs at a Ca/Al mass ratio of ∼0.7. As the Ca/Al mass ratio approaches this value, the grain size increases, and the Laves phases are reduced gradually. The early formed Laves phases play an important role in promoting dendrite segmentation, refining grain size and enhancing grain boundaries. When a solidification path delays the formation of Laves phases, the Laves phases will be reduced accompanied by grain coarsening. In such a solidifying microstructure, intergranular cavitation is easy to occur, and the resistance of the semi-solid alloy to crack propagation is severely reduced.

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

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

Development and Numerical Testing of a Model of Equiaxed Alloy Solidification Using a Phase Field Formulation

A computational framework is developed to understand the transient behavior of isothermal and non-isothermal transformation between liquid and solid phases in a binary alloy using a phase-field method. The non-isothermal condition was achieved by applying a thermal gradient along the computational domain. The bulk solid and liquid phases were treated as regular solutions, along with introducing an order parameter (phase field) as a function of space and time to describe the interfacial region between the two phases. An antitrapping flux term was integrated into the present phase-field model to mitigate the amount of solute trapping, which is characterized by the non-equilibrium partitioning of the solute. The governing equations for the phase field and the solute composition were solved by the cell-centered finite volume method using the open-source computational tool OpenFOAM. Simulations were carried out for the evolution of equiaxed dendrites inside an undercooled melt of a binary alloy, considering the effect of various computational parameters such as interface thickness, strength of crystal anisotropy, stochastic noise amplitude, and initial orientation. The simulated results show that the solidification morphology is sensitive to the magnitude of anisotropy as well as the amplitude of noise. A strong influence of interface thickness on the growth morphology and solute redistribution during solidification was observed. Incorporating antitrapping flux resulted in the solute partitioning close to the equilibrium value. Simulations show that the grain shape is unaffected by changes to crystallographic orientation with respect to the Cartesian computational grid. Thermal gradients exerted discernible effects on the solute distribution and the dendritic growth pattern. Starting with multiple nucleation events the model predicted realistic polycrystalline solidification and as-solidified microstructure.

Effect of Nickel Addition on Solidification Microstructure and Tensile Properties of Cast 7075 Aluminum Alloy

In order to explore the casting technology of a high–strength aluminum alloy, the effects of nickel on the solidified microstructure and tensile properties of a 7075 aluminum alloy were studied. 7075 aluminum alloys without nickel and with 0.6% and 1.2% nickel were prepared by a casting method. The results showed that the increase of Ni content in the 7075 alloys increased the liquidus temperatures, primary α (Al) grains were refined significantly, and the divorced eutectic structure was gradually formed among α (Al) grains with the preformation of the Al3Ni phase. In comparison, the 7075 alloy with 0.6% nickel content had less intergranular shrinkage porosity, and its elongation and ultimate tensile strength was enhanced 45% and 105% higher than those of the as-cast 7075 aluminum alloy, respectively. When the Ni content was increased to 1.2%, the eutectic phases of the alloy became much coarser compared to the other two alloys, and the mechanical properties obviously reduced too.

Investigation of the Influence of Indirect Squeeze Casting Process Parameters on the Solidification Microstructure and Properties of A356.2 Aluminum Alloy

Investigation of the Influence of Indirect Squeeze Casting Process Parameters on the Solidification Microstructure and Properties of A356.2 Aluminum Alloy

An operando synchrotron study on the effect of wire melting state on solidification microstructures of Inconel 718 in wire-laser directed energy deposition

Directed energy deposition (DED) with a coaxial wire-laser configuration has gained significant attention in recent years for the production of large-scale metallic components because of its low directional dependence, fast deposition rate, high feedstock efficiency, and low manufacturing costs. This work studies the coaxial wire-laser DED process of Inconel 718 alloy under a stable deposition condition with a relatively low input volumetric energy density (55.5 J/mm3). Post characterization reveals a cluster of refined grains at the center-bottom region of the as-printed track. Operando high-energy synchrotron X-ray experiments and multi-physics modeling are applied innovatively to study the fundamental mechanism responsible for the formation of this microstructure. The X-ray diffraction experiment provides direct evidence, which is supported by the simulation, that the feeding wire can reach the melt pool bottom and release solid particles (primarily carbides) near the mushy zone owing to insufficient melting. Consequently, these sub-micron sized particles suppress the growth of large columnar grains and cause the formation of unique microstructural heterogeneity. This discovery offers new opportunities for tailoring the solidification microstructure by controlling the melting state of the feedstock wire in DED process, in addition to commonly known factors such as the thermal gradient and solidification velocity.

Transmission electron microscopy of the rapid solidification microstructure evolution and solidification interface velocity determination in hypereutectic Al-20at.%Cu after laser melting

The evolution of rapid solidification (RS) microstructure and solidification interface velocity has been studied experimentally by in-situ transmission electron microscopy and postmortem characterization for hypereutectic Al-20at.%Cu (37 wt.%Cu) after laser melting. Four morphologically distinct regions formed via growth modes changing from θ-Al2Cu phase dendrites to eutectic cell growth, possibly α-Al dendrite growth (α-cell), banded growth, and α-Al plane front growth. Tendency for faceting and low capacity for solute trapping limited the θ-phase dendrite growth to solidification interface velocity v < 0.07 m/s. Consequently, formation of pro-eutectic micro-constituent was suppressed, and RS microstructure formation was dominated by eutectic, α-cell, and banded morphology grains for the Al-20at.%Cu alloy. Eutectic growth operated for interface velocity of 0.1 m/s ≤ v < 0.3 m/s, with a transition from regular lamellar, 2-λ and 1-λ mode to a dense irregular morphology dominated by α-phase at v = 0.3 m/s. Interface temperature calculations indicated feasibility of α-cell growth mode for 0.3 m/s ≤ v < 0.7 m/s. Banded growth occurred for 0.7 m/s ≤ v < 1.3 m/s. Plane front α-phase growth was evident for interface velocities v ≥ 1.3 m/s. Previous work on rapid solidification microstructure development in hypereutectic Al-Cu alloys reported a regime of α-cell growth subsequently to eutectic and prior to transition to banded growth for Al-19at.%Cu (36 wt.%Cu), while for Al-22at.%Cu (40 wt.%Cu) eutectic growth transitioned directly to α-plane front growth without emergence of a banded regime. Based on the current study the disappearance of the banded growth regime occurs for a composition larger than 20at.%Cu.

Tuning heterogeneous precipitation behavior in Ni35Al35Co5Cr20Cu5 HEAs under 7 GPa high pressure

Pressure, as a thermodynamic parameter, has a significant influence on the solidification process of alloys, including the precipitation behavior of alloys. Based on this, a Ni35Al35Co5Cr20Cu5 high-entropy alloy (HEA) with a precipitated phase was designed in this study. Through a comprehensive investigation of the microstructure and properties of Ni35Al35Co5Cr20Cu5 HEAs solidified under ambient pressure (AP) and 7 GPa, we confirmed the influence of pressure on the precipitation behavior of the precipitated phase. The results indicate that the solidification microstructure of the AP alloy displays a diverse distribution of Cr-rich precipitated phases with varying sizes, ranging from approximately 40 nm to approximately 10 μm. It is also worth noting that as the size of the Cr-rich phase increases, it becomes closer to the grain boundary. However, under 7 GPa, the Cr-rich precipitated phases near grain boundaries disappeared, and heterogeneous precipitation (continuous and discontinuous precipitation) emerged. Meanwhile, the nanophase size decreases significantly to approximately 10 nm and achieves a more uniform distribution throughout the alloy. Moreover, the grains undergo significant refinement. Consequently, substantial improvements in both strength and plasticity are achieved.

Revealing the effects of laser beam shaping on melt pool behaviour in conduction-mode laser melting

Laser beam shaping offers remarkable possibilities to control and optimise process stability and tailor material properties and structure in laser-based welding and additive manufacturing. However, little is known about the influence of laser beam shaping on the complex melt-pool behaviour, solidified melt-track bead profile and microstructural grain morphology in laser material processing. A simulation-based approach is utilised in the present work to study the effects of laser beam intensity profile and angle of incidence on the melt-pool behaviour in conduction-mode laser melting of stainless steel 316L plates. The present high-fidelity physics-based computational model accounts for crucial physical phenomena in laser material processing such as complex laser–matter interaction, solidification and melting, heat and fluid flow dynamics, and free-surface oscillations. Experiments were carried out using different laser beam shapes and the validity of the numerical predictions is demonstrated. The results indicate that for identical processing parameters, reshaping the laser beam leads to notable changes in the thermal and fluid flow fields in the melt pool, affecting the melt-track bead profile and solidification microstructure. The columnar-to-equiaxed transition is discussed for different laser-intensity profiles.

Evolution of primary carbides during accelerated solidification process of high-carbon martensitic stainless steels

In order to evaluate the potential application advantage for development of sub-rapid cooling strip casting of high-carbon martensitic stainless steels, the effects of cooling rate on the solidification microstructure, solute element segregation, and primary carbide precipitation in high-carbon martensitic stainless steels were investigated using an association of high-temperature confocal laser scanning microscopy (HT-CLSM), optical microscopy (OM), scanning electron microscopy (SEM) and electro-probe microanalysis (EPMA). As a result, the microstructure becomes more refined and the secondary dendrite arm spacing (SDAS) decreases greatly by 51 μm from 65.9 μm as the increase of cooling rate from 30 to 6000 °C·min−1. The equation of SDAS versus cooling rate was formulated as λ2 = 191.62·RC−0.317. Solute elements are enriched in the inter-dendritic region, leading to the precipitation of primary carbides. The elemental segregation ratio reduces by more than 20 % for all elements, meanwhile, the size of primary carbides decreases significantly and the mean area fraction of the primary carbides decreases from 4.77 % to 0.82 % with increase of cooling rate from 30 to 6000 °C·min−1. Moreover, the reticulated carbides gradually become less continuous. Despite the type of carbide is not sensitive to the cooling rate, the content of Cr and Fe in the primary carbide is sensitive to cooling rate. Compared to the reported methods, sub-rapid cooling of strip casting shows great advantage in control of primary carbides of high-carbon martensitic stainless steels.

Predictive microstructure distribution and printability maps in laser powder bed fusion for a Ni–Cu alloy

The solidification microstructure of a melt pool under additive manufacturing conditions is highly heterogeneous due to the heterogeneity in the thermal spatio-temporal fields. This work combines a finite element (FE)-based thermal model with a phase field model (PFM) to predict microstructure distribution among the process parameter span in LPBF, which is strongly controlled by local thermal histories. The segregation distribution across the parameter space can be classified into four different microstructure distribution types: (i) fully planar, (ii) bottom dendritic, (iii) top dendritic, and (iv) fully dendritic. Also, the relationship between the thermal histories (the temperature gradient (G) and the growth rate (R)) variation induced by P and V→ and the microstructure distribution is clearly analyzed in the paper. For a Ni-20 at.%Cu alloy, the predicted microstructural distribution is verified experimentally. The parameter space is further divided into homogeneous and heterogeneous regions using the predicted area fraction of cellular–dendritic segregation across the melt pools. The process map is then used to build AM parts with homogeneous microstructures, where only planar microstructure is found experimentally. This methodology will aid in the exploitation of the alloy and processing space to identify alloy-process combinations that yield microstructurally-homogeneous, defect-free parts, provided an unconditionally printable regime can be identified.

Achieving High Self-Lubricating Performance of Al-Bi-Sm-Ti Alloys Based on the Intermetallic Compounds

Al-Bi immiscible alloys, as wear resistance material, have attracted increasing attention in the past. The solidification processing of alloys plays a significant role in the final solidification microstructure and properties of immiscible alloys. Here, we present a strategy to produce a homogeneous microstructure in Al-Bi alloys based on intermetallic compounds. With the addition of Sm and Ti, the effect on the microstructure and properties was discussed in detail. The Al-Bi-Sm-Ti alloys achieve high self-lubricating performance, referring to the values of the coefficient of friction and wear rate. The intermetallic compounds formed were regarded as the main anti-friction and anti-wear factors. The results of numerical simulation indicate that microsegregation in Al-Bi alloys was retarded based on the intermetallic compounds. The coefficient of friction of the Al-Bi-1Sm-2Ti alloy was 46.2% lower than that of the Al-Bi alloys at 300 m. This research provides a new perspective and guidance for designing and fabricating composites with superior self-lubricating properties.

In Situ Observation the Effect of Y on the Solidification Process of 7Mo-SASS under a Low Cooling Rate.

The effects of Y on the solidification process of 7Mo super austenitic stainless steel (7MoSASS) under low cooling rate conditions (10 °C/min) were investigated using high-temperature confocal laser scanning microscopy (HT-CLSM). The in situ observation results indicate that Y samples promote an increase in austenite nucleation density. After 10 s of nucleation, the nucleation density increased by 149.53/mm2 for the Y sample. Furthermore, variance analysis indicated that Y addition improved the uniformity of the 7MoSASS solidification microstructure under low cooling rate conditions. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory results showed that when the solid phase ratio was 0.5, the nucleation mode of the Y sample transitioned from saturation site nucleation to saturation site nucleation + Avrami nucleation. YAlO3 has a low lattice disregistry value with austenite, making it a suitable heterogeneous nucleation core for promoting the early nucleation of austenite. During the late stages of solidification, Y accumulates in the residual liquid phase, providing a greater degree of compositional undercooling. SEM-EDS analysis showed that Y contributed to the refinement of the 7MoSASS solidification microstructure, with the proportion of precipitated phases decreasing by approximately 7.5%. Cr and Mo were the main elements exhibiting positive segregation in 7MoSASS, and the Cr segregation ratio increased in the Y sample, while the Mo segregation ratio decreased.

Effect of Synergistic Alloying of Co and Mo on Solidification Microstructure and Properties of NiAl-Based Eutectic High-Entropy Alloy

Effect of Synergistic Alloying of Co and Mo on Solidification Microstructure and Properties of NiAl-Based Eutectic High-Entropy Alloy

Analysis of Fractures and Microstructures on Different Injection Speeds in High-Pressure Die-Casting Magnesium Alloy

In this study, to clarify the unknown physical properties of the Mg-Al-Th-RE alloy, the relationship between the injection conditions and the internal porosities, and the mechanical properties exerted by the solidification microstructure was investigated. The obtained cast samples were investigated using X-ray CT internal measurements, tensile tests, Vickers hardness tests, and solidification microstructure observations. The tensile strength and the elongation at the injection speed of 5.0 m/s were higher than at 2.0 m/s. The number of porosities affected the tensile strength and the elongation even at the same fracture position. In addition, it was confirmed that segregation affected the destruction smaller the porosity size and the greater the variability of porosity. As the injection speed increased, the amount of heat transferred between the molten metal and the wall surface also increased, resulting in quick freezing and solidification. The tensile strength increased at the injection speed of 5.0 m/s because the interface between the scattered primary crystals and eutectic systems was narrow. On the other hand, at the injection speed of 2.0 m/s, the tensile strength decreased because the molten metal was delayed in solidification and dendrite growth became remarkable.

Development of process-structure linkage for Inconel 718 processed by laser powder bed fusion: a numerical modeling approach

PurposeUnderstanding and tailoring the solidification characteristics and microstructure evolution in as-built parts fabricated by laser powder bed fusion (LPBF) is crucial as they influence the final properties. Experimental approaches to address this issue are time and capital-intensive. This study aims to develop an efficient numerical modeling approach to develop the process–structure (P-S) linkage for LPBF-processed Inconel 718.Design/methodology/approachIn this study, a numerical approach based on the finite element method and cellular automata was used to model the multilayer, multitrack LPBF build for predicting the solidification characteristics (thermal gradient G and solidification rate R) and the average grain size. Validations from published experimental studies were also carried out to ensure the reliability of the proposed numerical approach. Furthermore, microstructure simulations were used to develop P-S linkage by evaluating the effects of key LPBF process parameters on G × R, G/R and average grain size. A solidification or G-R map was also developed to comprehend the P-S linkage.FindingsIt was concluded from the developed G-R map that low laser power and high scan speed will result in a finer microstructure due to an increase in G × R, but due to a decrease in G/R, columnar characteristics are also reduced. Moreover, increasing the layer thickness and decreasing the hatch spacing lowers the G × R, raises the G/R and generates a coarse columnar microstructure.Originality/valueThe proposed numerical modeling approach was used to parametrically investigate the effect of LPBF parameters on the resulting microstructure. A G-R map was also developed that enables the tailoring of the as-built LPBF microstructure through solidification characteristics by tuning the process parameters.

The evolution characteristics of solidification microstructure in laser welding of Ti-6Al-4V titanium alloy by considering transient flow field

During the laser welding of Ti-6Al-4V titanium alloy, the formed microstructure during the solidification process of molten pool is highly related to comprehensive properties of welded joints. A macro–micro coupling model is developed in this paper for solidification process of molten pool by considering transient flow field to investigate the evolution process of microstructure morphology and solute distribution in laser welding of Ti-6Al-4V titanium alloy. Based on the model, the evolution process of microstructure during solidification process of molten pool is calculated and the formed microstructure morphology in the region of molten pool is obtained. Simultaneously, the solute distribution in the different positions of molten pool under the effect of transient flow field at different times is analyzed and discussed in details. The calculated results are found in good agreement with the experimental results, which confirms the validity of the developed macro–micro coupling model for the laser welding of Ti-6Al-4V titanium alloy. Therefore, the proposed method is efficient for improving microstructure performance of welded joints of Ti-6Al-4V titanium alloy laser welding.