Primary Dendrite Spacing Research Articles

CA single track simulation of laser conduction welding with stainless steel 316L (1.4404)

The study presents a coupled cellular automata (CA) approach for a single track microstructure simulation for laser conduction welding. A high-power CO2 laser beam (1000 W) traverses the substrate, with the beam shaped by conventional optics, which produces a Gaussian profile. The process parameters produce a shallow melt phase maintaining a conduction limited weld seam. The stainless steel material 1.4404 was used as substrate material with a grain size of approximately 20 µm without any filler material. The cooling rates and initial grain size influence the grain morphology and were determined. The size of a single crystal was simulated for different cooling rates. The simulation outputs of the melt pool boundary are qualitatively and quantitatively compared to experimental results. The micrograph results have shown that fine sub cells are developed. This fine cellular microstructure was only satisfactorily resolved experimentally along the melt pool boundary. The simulation results are in acceptable agreement with the experiments at the melt pool boundary. They match in terms of cooling rate, grain angle, and primary dendrite spacing. However, the simulated grains are longer which indicates the necessity of homogenous nucleation in the liquid melt pool.

Effect of growth rate on microstructure evolution in directionally solidified Ti–47Al alloy

The microstructures and morphologies of directionally solidified Ti–47Al alloys with different growth rates ranging from 1 to 200 μm/s were investigated using the Bridgman directionally solidified method. The results showed that numerous columnar grains were formed along the growth direction with the onset of directional solidification. With a variation in the growth rate, the solid/liquid interface changed from a flat to cellular and to dendritic interface. The flat-to-cellular interface transition rate of the Ti–47Al alloy varied from 1 to 3 μm/s. When the growth rate was higher than 10 μm/s, the solid/liquid interface showed typical dendritic growth. During the directional solidification process, the main phase of the directionally solidified Ti–47Al alloy was the α phase, which can be attributed to the solute segregation, supercooling of the components, and contamination of the alloy melt by the Y2O3 ceramic shell. After reaching the steady growth state during the directional solidification process, the solidification path of the alloy was: L→α→α+γ→(α2+γ) + γ. With an increase in the growth rate, the primary dendrite spacing (λ) and lamellar spacing (λs) of the alloy decreased gradually.

The heterogeneous band microstructure and mechanical performance in a wire + arc additively manufactured 2219 Al alloy

In this study, a 2219 Al alloy with a heterogeneous band structure characterized by an alternating distribution of equiaxed and columnar α-Al grains was obtained by wire + arc additive manufacturing. The volume fractions of the equiaxed and columnar grains were varied by adjusting the cooling mode of the substrate. The volume fraction of the columnar grains in a water-cooled (WC) deposit was approximately 16% higher than that in an air-cooled (AC) deposit. Moreover, the WC deposit exhibited smaller primary arm dendrite spacing and lower porosity than those of the AC deposit. The results of in-situ tensile testing revealed that the columnar grains could coordinate and distribute strain more effectively than the equiaxed grains. The ex-situ electron back scatter diffraction data showed that the observed deformations of the columnar and equiaxed grains during tensile testing were non-uniform and non-synchronization. Furthermore, numerous dislocations in the columnar grains of the deposit coordinated the deformation process. According to tensile testing data, the ultimate strength, yield strength, and elongation of the WC deposit were 247.7 ± 17.7 MPa, 110.1 ± 6.3 MPa and 11.9 ± 0.7%, which were higher than those of the AC deposit by 10.76%, 15.78%, and 26.78%, respectively. Therefore, the heterogeneous band structure of the equiaxed and columnar grains ensured good synergy between strength and ductility.

Growth competition between columnar dendritic grains – The role of microstructural length scales

The growth competition between columnar dendritic grains is investigated by both phase-field (PF) and cellular automaton (CA) models in a growth regime where the primary dendrite spacing is much larger than the solutal diffusion length. This growth regime favors the formation of highly branched hierarchical dendritic microstructures prevalent in castings. Previous PF and CA studies have shed light on the complex relationship between GB orientation selection and GB bi-crystallography. They showed that, in the CA model, the orientation is governed by the favorably oriented grain (FOG) criterion and the geometrical limit (GL) in the limit of large and small cell size, respectively. The present study focuses on exploring how to quantitatively bridge length scales between PF simulations that resolve the whole solid-liquid interface dynamics and the CA model that resolves the dynamics of the grain envelope under a certain set of assumptions. For this purpose, we study grain boundary (GB) orientation selection as a function of the imposed temperature gradient G under the frozen-temperature approximation, which allows us to vary G-dependent microstructural length scales with a fixed GB bi-crystallography. PF simulations reveal the existence of a transition from FOG to GL dominance with decreasing G at non-degenerate bi-crystallography. Simulations further reveal that this transition can be quantitatively reproduced by the CA model with a choice of the cell size that corresponds, in PF simulations, to the active secondary dendrite arm spacing of the favorably oriented grain preceding the tertiary branching event that gives birth to a new stable primary dendrite. PF simulations are also used to obtain a detailed quantitative characterization of the dynamics of the grain envelope and its internal length scales, thereby providing a quantitative test of the inherent approximations of the existing CA approach and paving the way for its future development.

Formation mechanisms and control method for stray grains at melt-back region of Ni-based single crystal seed

An experimental study together with numerical simulation was conducted to investigate the formation mechanisms and control methods for stray grains at melt-back of seed in this paper. The numerical simulation results showed that the convex solid/liquid (S/L) interface changed very slowly into a concave interface during the initial solidification stage and the undercooling was slightly affected by the S/L curvature change. The formation of stray grains below melt-back interface at seed perimeter could be suppressed with the decrease of clearance between the seed and mold. However when the clearance was further decreased to 0, an orientation deviation layer formed originated from the deformation of un-melted seed caused by different expend coefficient between the seed and mold. The nucleation of stray grains above the melt-back interface at the seed perimeter could be suppressed using the mold with small surface roughness. The isolated un-melted dendrite stem at semi-solid mushy of the seed with the large original primary dendrite spacing transfer to complex network structure, suppressing the formation of the stray grains, when the primary dendrite spacing of seed decreased or the seed underwent solution heat treatment. Then, the formation mechanisms of the stray grains at the melt-back region are discussed based on above results, and a method is proposed to suppress the formation of the stray grains for preparing single crystal components by re-used seed.

Relationship between Microstructure Evolution and Tensile Properties of AlSi10Mg Alloys with Varying Mg Content and Solidification Cooling Rates

This work explored and contrasted the effect of microstructure on the tensile properties of AlSi10Mg alloys generated by transient directional solidification depending on variations in cooling rate and magnesium (Mg) content (i.e., 0.45 and 1 wt.% Mg), with a focus on understanding the dendritic growth and phases constitution. Optical and scanning electron (SEM) microscopies, CALPHAD, and thermal analysis were used to describe the microstructure, forming phases, and resulting tensile properties. The findings showed that the experimental evolution of the primary dendritic spacing is very similar when both directionally solidified (DS) Al-10 wt.% Si-0.45 wt.% Mg and Al-10 wt.% Si-1 wt.% Mg alloys samples are compared. The secondary dendritic spacing was lower for the alloy with more Mg, especially considering the range of high growth velocities. Moreover, a greater fraction of (Al + Si + Mg2Si) ternary eutectic islands surrounding the α-Al dendritic matrix was noted for the alloy with 1 wt.% Mg. As a result of primary dendritic spacings greater than 180 μm related to lower cooling rates, slightly higher tensile properties were attained for the Al-10 wt.% Si-0.45 wt.% Mg alloy. In contrast, combining dendritic refining (<150 μm) and a larger Mg2Si fraction, fast-solidified DS Al-10 wt.% Si-1 wt.% Mg samples exhibited higher tensile strength and elongation. The control of cooling rate and fineness of the dendritic array provided a new insight related to the addition of Mg in slightly higher levels than conventional ones, capable of achieving a better balance of tensile properties in AlSi10Mg alloys.

Influence of the Heat Input on the Dendritic Solidification Structure and the Mechanical Properties of 2.25Cr-1Mo-0.25V Submerged-Arc Weld Metal

The alloy 2.25Cr-1Mo-0.25V is commonly used for heavy wall pressure vessels in the petrochemical industry, such as hydrogen reactors. As these reactors are operated at elevated temperatures and high pressures, the 2.25Cr-1Mo-0.25V welding consumables require a beneficial combination of strength and toughness as well as enhanced creep properties. The mechanical properties are known to be influenced by several welding parameters. This study deals with the influence of the heat input during submerged-arc welding (SAW) on the solidification structure and mechanical properties of 2.25Cr-1Mo-0.25V multilayer metal. The heat input was found to increase the primary and secondary dendrite spacing as well as the bainitic and prior austenite grain size of the weld metal. Furthermore, it was determined that a higher heat input during SAW causes an increase in the stress rupture time and a decrease in Charpy impact energy. This is assumed to be linked to a lower number of weld layers, and therefore, a decreased amount of fine grained reheated zone if the multilayer weld metal is fabricated with higher heat input. In contrast to the stress rupture time and the toughness, the weld metal’s strength, ductility and macro-hardness remain nearly unaffected by changes of the heat input.

Coarsening of dendrites in solution-based freeze-cast ceramic systems

The morphologies of freeze-cast materials are typically controlled and tuned by adjusting the freezing front velocity and temperature gradient. Recently it has been demonstrated that coarsening, similar to that commonly practiced in alloy systems, is also effective for morphological control in freeze-cast materials. However, the underlying coarsening mechanisms and their effect on microstructure evolution are largely unknown. In this study, frozen preceramic polymer/cyclohexane solutions were coarsened at 2 °C and 4 °C for up to 5 h, and the resulting morphologies were characterized using scanning electron microscopy, mercury intrusion porosimetry, and X-ray computed tomography. During coarsening the microstructure evolved from dendritic (primary and secondary pores) to honeycomb-like (large open channels with flat walls). The size of both primary and secondary pores increased linearly with the cube root of coarsening time, consistent with dendritic coarsening in alloy systems. Other important metrics such as primary dendrite spacing, dendrite growth directionality, and the effect of coarsening on the pore-ceramic interface area are reported. These findings provide novel insights into coarsening of freeze-cast systems and can lead to new avenues for microstructure tailorability.

Microstructure features and mechanical/electrochemical behavior of directionally solidified Al−6wt.%Cu−5wt.%Ni alloy

The effects of the addition of 5.0 wt.% Ni to an Al−6wt.%Cu alloy on the solidification cooling rate T˙ growth rate (VL), length scale of the representative phase of the microstructure, morphology/distribution of intermetallic compounds (IMCs) and on the resulting properties were investigated. Corrosion and tensile properties were determined on samples solidified under a wide range of T˙ along the length of a directionally solidified Al−6wt.%Cu−5.0wt.%Ni alloy casting. Experimental growth laws were derived relating the evolution of primary (λ1) and secondary (λ2) dendritic spacings with T˙ and VL. The elongation to fracture (δ) and the ultimate tensile strength (σU) were correlated with the inverse of the square root of λ1 along the length of the casting by Hall−Petch type experimental equations. The reinforcing effect provided by the addition of Ni in the alloy composition is shown to surpass that provided by the refinement of the dendritic microstructure. The highest corrosion resistance is associated with a microstructure formed by thin IMCs evenly distributed in the interdendritic regions, typical of samples that are solidified under higher T˙.

Investigation on the columnar-to-equiaxed transition during laser cladding of IN718 alloy

In the laser cladding process of IN718 alloy, the growth of microstructure has a great impact on the performance of the alloy material. The study on the columnar-to-equiaxed transition (CET) of IN718 can further understand the formation conditions of the microstructure and control the growth of the microstructure. In this work, the columnar-to-equiaxed transition in the laser cladding process of IN718 was calculated using the Macro-Micro-Coupled Model, establishing the finite element model(FEM) for the macroscopic temperature field of laser cladding and simulating the growth of IN718 alloy microstructure by the cellular automata (CA) method (CA-FEM). It shows that CET will be delayed by increasing laser power, reducing scanning speed, or reducing preheating temperature. Increasing the laser power, reducing the scanning speed, or increasing the preheating temperature will increase the primary dendrite spacing (PDS). Reducing the laser power, increasing the scanning speed, or reducing the preheating temperature will increase the volume fraction of columnar crystals. The height of columnar crystal growth can be determined to be about 220−300μm, the volume fraction of columnar crystals is about 17 %–23 %. The maximum error of simulation is 6.72 %, and the minimum error is 1.69 %.

Microstructure and volume shrinkage during directional solidification of peritectic steel

ABSTRACT Directional solidification experiments were carried out on peritectic steels to investigate the microstructure and volume shrinkage. A δ-dendritic structure developed intermittently and was enclosed by γ-austenite at growth rates of 15, 50, and 80 μm s−1. The primary dendritic spacing narrowed and the average spacing of dendrites decreased as the growth velocity increased. The functional relationship between the secondary dendrite arm spacing and cooling rates was established by statistical analysis. The influence of cooling rate on crack formation was analysed as a function of volume shrinkage. Reducing the cooling rate of the solidification delayed a massive type of peritectic transformation away from the ZST–ZDT brittle zone, thus reducing the probability of crack formation.

Primary dendrite array morphology in Al-7 wt% Si alloy samples directionally solidified aboard the International Space Station

Under a NASA (National Aeronautics and Space Agency)-ESA (European Space Agency) collaborative research project, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions), three Al-7 wt% Si samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified aboard the International Space Station (ISS) to determine the effect of mitigating convection on the primary dendrite array morphology. MICAST-6 was processed with a step-increase in the growth speed from 5 to 50 µm s−1, MICAST-7 with a step-decrease from 20 to 10 µm s−1, and MICAST2-12 with constant speed of 40 µm s−1. Nearest-neighbor primary dendrite arm spacings and primary dendrite trunk diameters were measured on transverse sections taken at approximately 5 mm intervals along the length of the MICAST samples. In MICAST samples, especially in the steady-state growth regime, the observed primary dendrite nearest-neighbor spacings show a good agreement with predictions from the Hunt-Lu numerical model. The maximum to minimum (highest 10% to lowest 10%) nearest-neighbor spacing ratio in MICAST samples is 2.10 ± 0.27 in agreement with Hunt-Lu model, which assumes pure diffusive transport. Trunk diameters in MICAST samples also agree well with predictions from a coarsening based model. The terrestrial-grown equivalent samples, on the other hand, display smaller nearest-neighbor spacing, and larger primary dendrite trunk diameters than those predicted from the models. Thermosolutal convection appears to increase the dendrite tip radius, causing the trunk diameter to increase, while simultaneously reducing the primary dendrite spacing. This requires verification-experiments involving directional solidification followed by rapid quenching of the liquid-solid interface in metallic alloys in a low gravity environment. The International Space Station currently does not have on-board hardware to conduct such experiments.

Multiscale prediction of microstructure length scales in metallic alloy casting

Microstructural length scales, such as dendritic spacings in cast metallic alloys, play an essential role in the properties of structural components. Therefore, quantitative prediction of such length scales through simulations is important to design novel alloys and optimize processing conditions through integrated computational materials engineering (ICME). Thus far, quantitative comparisons between experiments and simulations of primary dendrite arms spacings (PDAS) selection in metallic alloys have been mainly limited to directional solidification of thin samples and quantitative phase-field simulations of dilute alloys. In this article, we combine casting experiments and quantitative simulations to present a novel multiscale modeling approach to predict local primary dendritic spacings in metallic alloys solidified in conditions relevant to industrial casting processes. To this end, primary dendritic spacings were measured in instrumented casting experiments in Al–Cu alloys containing 1 wt.% and 4 wt.% of Cu, and they were compared to spacing stability ranges and average spacings in dendritic arrays simulated using phase-field (PF) and dendritic needle network (DNN) models. It is first shown that PF and DNN lead to similar results for the Al-1 wt.%Cu alloy, using a dendrite tip selection constant calculated with PF in the DNN simulations. PF simulations cannot achieve quantitative predictions for the Al-4 wt.%Cu alloy because they are too computationally demanding due to the large separation of scale between tip radius and diffusion length, a characteristic feature of non-dilute alloys. Nevertheless, the results of DNN simulations for non-dilute Al–Cu alloys are in overall good agreement with our experimental results as well as with those of an extensive literature review. Simulations consistently suggest a widening of the PDAS stability range with a decrease of the temperature gradient as the microstructure goes from cellular-dendrites to well-developed hierarchical dendrites.

Additive manufacturing of Inconel625-HSLA Steel functionally graded material by wire arc additive manufacturing

Functional graded materials (FGMs) have been widely applied in many engineering fields, and are very potential to be the substitutions of dissimilar metal welding joints due to their overall performance. In this work, the Inconel625-high-strength low-alloy (HSLA) Steel FGM was fabricated by wire arc additive manufacturing (WAAM). The chemical composition distribution, microstructure, phase evolution and mechanical properties of the FGM were examined. With the increasing of HSLA Steel, the chemical composition appeared graded distribution, and the primary dendrite spacing was largest in graded region with 20%HSLA Steel and then gradually decreased. And the main microstructure of the FGM transformed from columnar dendrites to equiaxed dendrites. Laves phase precipitated along dendrites boundary when the content of HSLA Steel was lower than 70% and Nb-rich carbides precipitated when the content of HSLA Steel exceeded to 70%. Microhardness and tensile strength gradually decreased with ascending content of HSLA Steel, and had a drastic improvement (159HV to 228HV and 355Mpa to 733Mpa) when proportion of HSLA Steel increased from 70% to 80%.

Effect of Process Parameters on Defects, Melt Pool Shape, Microstructure, and Tensile Behavior of 316L Stainless Steel Produced by Selective Laser Melting

Previous studies have revealed that laser power and energy density are significant factors affecting the quality of parts manufactured by selective laser melting (SLM). The normalized equivalent density E0* and dimensionless laser power q*, which can be regarded as a progress on the understanding of the corresponding dimensional quantities, are adopted in this study to examine the defects, melt pool shape, and primary dendrite spacing of the SLM-manufactured 316L stainless steel, because it reflects the combined effect of process parameters and material features. It is found that the number of large defects decreases with increasing E0* due to enough heat input during the SLM process, but it will show an increasing trend when excessive heat input (i.e., utilizing a high E0*) is imported into the powder bed. The q* plays an important role in controlling maximum temperature rising in the SLM process, and in turn, it affects the number of large defects. A large q* value results in a low value of absolute frequency of large defects, whereas a maximum value of absolute frequency of large defects is achieved at a low q* even if E0* is very high. The density of the built parts is greater at a higher q* when E0* remains constant. Increasing the melt pool depth at relatively low value of E0* enhances the relative density of the parts. A narrow, deep melt pool can be easily generated at a high q* when E0* is sufficiently high, but it may increase melt pool instability and cause keyhole defects. It is revealed that a low E0* can lead to a high cooling rate, which results in a refined primary dendrite spacing. Relatively low E0* is emphasized in selecting the process parameters for the tensile test sample fabrication. It shows that excellent tensile properties, namely ultimate tensile strength, yield strength, and elongation to failure of 773 MPa, 584 MPa, and 46%, respectively, can be achieved at a relatively low E0* without heat treatment.

Microstructure and mechanical properties of Ti-52 at% Al alloy synthesized in-situ via dual-wires electron beam freeform fabrication

The in-situ synthesized Ti-52 at% Al alloy was additively manufactured for the first time by electron beam freeform fabrication (EBF3) with pure Ti and Al wires. The chemical compositions, microstructure, crystal orientation and mechanical properties of the Ti-52 at% Al walls were investigated. The results show that the actual Al content was lower than that the designed content due to the evaporation of elements. The microstructure of the as-fabricated walls was composed of γ dendrite which was oriented along the <111> direction. The solidification path was L→ γ phase due to the high cooling rate during EBF3 process. The primary dendrite spacing and the width of the secondary dendrite increased with the building height indicating that the cooling rate decreased. The dendrite disappeared in layer bands due to the repeating thermal cycles. Based on the orientation analysis, the epitaxial growth of the γ phase was found between the dendritic and interdendritice regions and between layers. The microhardness of the dendrite was higher than that of the interdendritic regions. The compressive tests at ambient temperature indicated anisotropic mechanical properties due to the dendrite structure. The compressive strength along the building direction was higher than that along the deposited direction.

Morphology of Intermetallics Tailoring Tensile Properties and Quality Index of a Eutectic Al–Si–Ni Alloy

Herein, the eutectic Al–11 wt% Si–5 wt% Ni alloy, directionally solidified under transient heat‐flow conditions, is comparatively evaluated in relation to the binary Al–11 wt% Si and Al–5 wt% Ni alloys. Correlations are developed relating the solidification cooling rate, primary dendritic spacing, and tensile properties. The Al3Ni intermetallic compounds (IMCs) are shown to have a morphological transition from plate‐like to fishbone for Ṫ &lt; 6 °C s−1. Experimental Hall–Petch‐type equations are proposed relating yield and ultimate tensile strengths to λ1. Moreover, both the quality index (QI), which depends on the determined tensile properties, and the fishbone/plate‐like ratio (FPR) of the Al3Ni IMCs are plotted simultaneously as a function of Ṫ. This procedure permits three different regions to be determined relating solidification operational parameters to microstructural features and a quality parameter, according to the morphologies of the Al3Ni IMCs, namely (I) plate‐like, (II) fishbone rising, and (III) fishbone prevalence.

Microstructure Evolution and Mechanical Properties of FeCoCrNiCuTi0.8 High-Entropy Alloy Prepared by Directional Solidification.

A CoCrCuFeNiTi0.8 high-entropy alloy was prepared using directional solidification techniques at different withdrawal rates (50 μm/s, 100 μm/s, 500 μm/s). The results showed that the microstructure was dendritic at all withdrawal rates. As the withdrawal rate increased, the dendrite orientation become uniform. Additionally, the accumulation of Cr and Ti elements at the solid/liquid interface caused the formation of dendrites. Through the measurement of the primary dendrite spacing (λ1) and the secondary dendrite spacing (λ2), it was concluded that the dendrite structure was obviously refined with the increase in the withdrawal rate to 500 μm/s. The maximum compressive strength reached 1449.8 MPa, and the maximum hardness was 520 HV. Moreover, the plastic strain of the alloy without directional solidification was 2.11%, while the plastic strain of directional solidification was 12.57% at 500 μm/s. It has been proved that directional solidification technology can effectively improve the mechanical properties of the CoCrCuFeNiTi0.8 high-entropy alloy.

Phase-field study of the effects of the multi-controlling parameters on columnar dendrite during directional solidification in hexagonal materials.

The growth of hexagonal columnar dendrite during directional solidification with respect to the multi-controlling parameters such as anisotropy, cooling rate, temperature gradient and orientation angle were investigated by a quantitative phase-field method, respectively. The simulation results show that the increase of anisotropy, cooling rate and temperature gradient can accelerate the solidification velocity of columnar dendrites. Among them, the cooling rate has the most significant effect on the solidification velocity of columnar dendrite. In contrast, the solidification velocity of columnar dendrite slows down with the increase of the orientation angle. Meanwhile, the primary dendrite spacing decreases with the increase of cooling rates and temperature gradient, and the primary dendrite arms are smooth. The primary dendrite spacing increases with the increase of anisotropy and orientation angle, which provides space for the development of secondary dendrite arms. In addition, the effects of cooling rate and temperature gradient on the solid volume fraction were also studied.

Effects of an External Magnetic Field on the Microstructural and Mechanical Properties of the Fusion Zone in TIG Welding

Welding is a widely used process that requires continuous developments to meet new application demands of mechanical projects under severe conditions. The homogeneity of metallurgical and mechanical properties in welded joints is the key factor for any welding process. The applications of external magnetic fields, mechanical vibration, and ultrasound are the fundamental steps to achieve success in improving these properties. The present work aimed at determining suitable processing conditions to achieve the desired balance between metallurgical and mechanical properties of 304L steel in TIG (Tungsten Inert Gas) welding under the application of an external magnetic field. The microstructural characteristics of the weld bead were analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). In order to evaluate the mechanical properties of the welded specimen, its Vickers microhardness map and Charpy impact energy at −20 °C were obtained. In addition, corrosion tests were carried out in the saline medium to compare the corrosion resistance of the joint with that of the base metal and that without the magnetic field. It was found that the external magnetic field decreased the percentage of delta ferrite, improved the filling of the weld pool with the weld metal, and decreased the primary and secondary dendritic spacings. The Vickers microhardness value under the magnetic field was found to be lower than that without the magnetic field, and the Charpy test showed no significant variation in energy absorption. Moreover, the welded joint produced under the external magnetic field manifested less resistance to corrosion.