Dendritic Spacing Research Articles

Formation and Control of Central Cracks in Alloy Steel ZKG223

When the molten steel contains high carbon and alloy elements, the continuous casting bloom is prone to internal defects. Soft reduction is an effectively means of improvement and is widely utilized in the continuous casting process. Herein, the generation mechanism of the central cracks in alloy steel ZKG223 is analyzed in detail; that is, the tensile stress is generated by the accelerated solidification and shrinkage in the core of the bloom at the end of solidification, which acts on the dendrites and causes central cracks. In order to reduce the shrinkage stress during the solidification of the bloom, the reasonable reduction parameters of the bloom are determined by establishing a soft reduction model, and the industrial test is carried out. Through the implementation of well‐designed soft reduction, the central cracks in the bloom are significantly bridged, and the secondary dendrite spacing in the center of the bloom is shortened from 315 to 230 μm. The maximum positive and negative carbon segregation indices in the cross section of the bloom are changed from 1.039 and 0.957 to 1.020 and 0.977, respectively, which greatly improve the uniformity of element distribution and the internal quality of the bloom.

Effect of Sc on microstructure and properties of A357 alloy under different casting conditions

The effect of Sc on microstructure and mechanical properties of A357 alloy was investigated using sand molds and metal molds. The results showed that the eutectic Si in A357-0.5Sc alloy showed finer fibrous structure and smaller secondary dendrite spacing than that of A357 alloy in metal mold casting; a significant degeneration of secondary and tertiary dendrites has happened during solidification in A357-0.5Sc alloy using sand molds. UTS and YS of A357-0.5Sc alloy were higher than those of A357 Alloy in both metal molds and sand molds. In metal mold casting, the elongation of A357-0.5Sc alloy was higher than that of A357 alloy. In sand casting, the elongation of A357 and A357-0.5Sc alloys is almost equal. The YS of A357-0.5Sc alloy in sand mold is 7.5% higher than that of A357 alloy in metal mold, and the UTS and EI are almost equal. Taken together, the mechanical properties of A357-0.5Sc using in sand molds shows better than that of A357 using metal molds, which can improve mechanical properties in sand molds to meet the requirements of casting of complex parts in sand molds.The secondary dendrite of A357 and A357-0.5Sc alloys was obvious in metal mold casting. The main factor affecting the mechanical properties of the metal mold alloy was the secondary dendrite spacing. The addition of Sc would degrade the secondary and tertiary dendrites of A357 alloy in sand-casting. Moreover, the main factor affecting the mechanical properties of the sand-casting alloy was grain size.

Understanding the effect of Ni content on microstructures and tensile properties of AlSi10Mg alloy samples under a variety of solidification rates

It is critical to gain a better knowledge of the impact of varying Ni contents on the microstructural changes of the AlSi10Mg alloy solidified at various cooling severities. Understanding the combined features of Si and Al3Ni phases constituting this modified alloy may also have a significant impact on understading its properties. In order to advance in these topics, AlSi10Mg alloys modified with, 1.0, 2.0 and 3.0 wt% Ni are either directionally solidified (DS) at lower cooling rates or laser surface remelted (LSR) at higher rates. α-Al dendritic branches are found in the microstructure of all tested alloys either for DS or LSR specimens, surrounded by the eutectic constituent. Moreover, the dendritic array is defined by the secondary dendritic spacing, λ2, which is found to be 65% lower for the Ni-containing alloys than in the AlSi10Mg alloy. The microstructure of the quaternary alloys is constituted by the α-Al+Si+Al3Ni phases, with the Al3Ni phase having a prevalent fishbone shape and being found in higher proportions as a result of slow cooling and increasing Ni content. Ultimate tensile strength of the AlSi10Mg alloy is 190 MPa for a specified range of λ2 (22–30 µm), which is higher than that of Ni-containing alloys for the same range. However, Ni-containing alloys are more sensitive to λ2, and the DS AlSi10Mg-1Ni alloy, as well as a combination of samples solidified at no less than 3 °C/s, fine dendritic spacing (<18 µm) and fine fishbone+plate-like Al3Ni mixture, shows the best tensile results: 4.6% and 208 MPa. This appears to be the optimal condition for cooperative reinforcement throughout these hardening phases. The laser treated AlSi10Mg-2Ni alloy samples have a superior hardness of 145 HV, much higher than the original substrate, demonstrating the effectiveness of the laser treatment in improving the surface mechanical properties of a Ni-modified AlSi10Mg alloy.

Dscam1 Has Diverse Neuron Type-Specific Functions in the Developing Drosophila CNS.

Two key features endow Drosophila Down syndrome cell adhesion molecule 1 (Dscam1) with the potential to provide a ubiquitous code for neuronal arbor self-avoidance. First, Dscam1 contains three large cassettes of alternative exons, so that stochastic alternative splicing yields 19,008 Dscam1 isoforms with different Ig ectodomains. Second, each neuron expresses a different subset of Dscam1 isoforms, and isoform-specific homophilic binding causes repulsion. This results in even spacing of self-arbors, while processes of other neurons can intermingle and share the same synaptic partners. In principle, this Dscam1 code could ensure arbor spacing of all neurons in Drosophila. This model is strongly supported by studies on dendrite spacing in the peripheral nervous system and studies on axonal branch segregation during brain development. However, the situation is less clear for central neuron dendrites, the major substrate for synaptic input in the CNS. We systematically tested the role of Dscam1 for dendrite growth and spacing in eight different types of identified central neurons. Knockdown of Dscam1 causes severe dendritic clumping and length reductions in efferent glutamatergic and aminergic neurons. The primary cause for these dendritic phenotypes could be impaired self-avoidance, a growth defect, or both. In peptidergic efferent neurons, many central arbors are not formed, arguing for a growth defect. By contrast, knockdown of Dscam1 does not affect dendrite growth or spacing in any of the five different types of interneurons tested. Axon arbor patterning is not affected in any neuron type tested. We conclude that Dscam1 mediates diverse, neuron type-specific functions during central neuron arbor differentiation.

Hypereutectic Zn–Al Alloys: Microstructural Development under Unsteady-State Solidification Conditions, Eutectic Coupled Zone and Hardness

The present study investigates the effects of Al content and solidification thermal parameters on the microstructural development under transient heat flow conditions for two hypereutectic Zn–Al alloys: Zn-6wt.%Al and Zn-11wt.%Al. The alloys were directionally solidified and had experimental cooling profiles monitored permitting cooling rates and growth rates to be determined along the length of the directionally solidified (DS) castings. The microstructure of the Zn-6wt.%Al alloy is shown to be formed by eutectic colonies, constituted by a eutectic mixture of (Zn) and (Al′) phases in the form of lamellae and the Zn-11wt.% Al alloy by the pro-eutectic (Al′) dendrites and the eutectic mixture in the interdendritic regions. Growth laws are experimentally determined relating eutectic and dendritic spacings to the growth rate and cooling rate. A diagram exhibiting the coupled zone of Zn–Al alloys as a function of cooling rate is proposed, which shows different microstructural morphologies influenced by composition and thermal parameters, that is, growth rate and the temperature gradient, synthesized by the cooling rate (Ṫ = G.V). The microhardness of both Zn-6wt.%Al and Zn-11wt.%Al alloys were shown not to depend on the length scale of the resulting microstructure.

Assessing Microstructure Tensile Properties Relationships in Al-7Si-Mg Alloys via Multiple Regression

The development of Al-based alloys presumes a detailed understanding of the microstructure evolution during solidification since the as-solidified microstructure also has effects on the subsequent thermo-mechanical processing. In the present investigation Al-7wt.%Si-xMg (x = 0.5 and 1 wt.%) alloys are subjected to transient directional solidification with a view to characterizing the microstructure evolution, with special focus on both dendritic evolution and the inherent features of the Mg2Si and π-AlSiFeMg intermetallics. Experimental power-type functions relating the primary, secondary and tertiary interdendritic spacings to the solidification cooling rate and growth rate are developed. It is observed that the Mg content added to the Al-7wt.%Si alloy and the consequent increase in the Mg2Si fraction tends to increase the values of the primary dendritic spacing. However, this same behavior is not verified for the growth evolution of dendritic side branches. A multiple linear regression (MLR) analysis is developed permitting quantitative correlations for the prediction of tensile properties and hardness from microstructural parameters to be established. The increase in the Mg alloy content from 0.5 to 1 was shown to promote an increase in both the ultimate tensile strength (σu) and elongation.

Microstructure and Tensile Property of Laser Cladding Assisted with Multidimensional High-Frequency Vibration.

Laser cladding is a promising surface modification technology to fabricate high-performance parts. However, defects such as porosity, cracks and residual tensile stress are easily produced in laser cladding, leading to significant property reduction and poor reliability. In this study, laser cladding with multidimensional high-frequency vibration was investigated. The effects of multidimensional high-frequency vibration on the improvement of microstructure and mechanical properties were analyzed and discussed based on the vibration-assisted laser cladding experiments. In addition, a numerical model was conducted to help understand the significance of the vibration on flow field and temperature field. Results show that 3D vibration led to the primary dendrite spacing reduction from 11.1 to 6.8 μm, microhardness increase from 199 to 221 HV0.2, and a nearly 110% improvement in the elongations. The findings of this study confirmed the significant benefits of multidimensional high-frequency vibration applied in laser cladding and provided a basis to uncover the underlying mechanisms of multidimensional vibration on the rapid melting and solidification.

Effect of Aging on Dendritic Array, Ag3Sn Spacing, and Hardness of a Sn-2Ag-0.7Cu Solder Alloy

Effect of Aging on Dendritic Array, Ag3Sn Spacing, and Hardness of a Sn-2Ag-0.7Cu Solder Alloy

Complex housing partially mitigates low dose radiation-induced changes in brain and behavior in rats

In recent years, much effort has been focused on developing new strategies for the prevention and mitigation of adverse radiation effects on healthy tissues and organs, including the brain. The brain is very sensitive to radiation effects, albeit as it is highly plastic. Hence, deleterious radiation effects may be potentially reversible. Because radiation exposure affects dendritic space, reduces the brain's ability to produce new neurons, and alters behavior, mitigation efforts should focus on restoring these parameters. To that effect, environmental enrichment through complex housing (CH) and exercise may provide a plausible avenue for exploration of protection from brain irradiation. CH is a much broader concept than exercise alone, and constitutes exposure of animals to positive physical and social stimulation that is superior to their routine housing and care conditions. We hypothesized that CHs may lessen harmful neuroanatomical and behavioural effects of low dose radiation exposure. We analyzed and compared cerebral morphology in animals exposed to low dose head, bystander (liver), and scatter irradiation on rats housed in either the environmental enrichment condos or standard housing. Enriched condo conditions ameliorated radiation-induced neuroanatomical changes. Moreover, irradiated animals that were kept in enriched CH condos displayed fewer radiation-induced behavioural deficits than those housed in standard conditions. Animal model-based environmental enrichment strategies, such as CH, are excellent surrogate models for occupational and exercise therapy in humans, and consequently have significant translational possibility. Our study may thus serve as a roadmap for the development of new, easy, safe and cost-effective methods to prevent and mitigate low-dose radiation effects on the brain.

Convective effects on columnar dendritic solidification – A multiscale dendritic needle network study

Gravity-induced buoyancy, inevitable in most solidification processes, substantially alters the dynamics of crystal growth, such that incorporating fluid flow in solidification models is crucial to understand and predict key aspects of microstructure selection. Here, we present a multi-scale Dendritic Needle Network (DNN) model for directional solidification that includes buoyant flow in the liquid, and apply it to a range of alloys and growth conditions. After a brief presentation of the model, we study the selection of stable primary dendrite arm spacings in Al-4at.%Cu and in Ti-45at.%Al alloys under different gravity levels, comparing both applications to published phase-field results and experimental measurements. Then, we simulate the oscillatory growth behavior recently reported via X-ray in situ imaging of directional solidification of nickel-based superalloy CMSX-4. In this last application, the DNN simulations manage to reproduce the oscillatory growth behavior, and hence permit identifying the fundamental mechanisms behind the oscillatory growth regime. In particular, we show that sustained oscillations occur when the average liquid flow velocity is close to the crystal growth velocity, and that primary dendritic spacings also play a crucial role in the oscillatory behavior.

Effect of rare earth elements on the segregation behavior and microstructure of super austenitic stainless steel

Super austenitic stainless steel (SASS) was prepared by non-consumable vacuum arc melting for the first time. The effects of rare earth (RE) elements on its solidification microstructure and segregation behavior were investigated. The σ phase is the main precipitation phase in SASS. The addition of RE elements can refine the solidification microstructure and the dendrite spacing. The addition of RE elements reduced the secondary dendrite spacing at top (S1) by 5.22 μm. The elements as Cr, Mn, and Mo have positive segregation, and elements as Ni and Fe have negative segregation. The addition of RE elements has the trend of reducing element segregation, thereby inhibiting the σ phase precipitation. And the addition of RE reduces the hardness of the secondary phase.

Segregation behavior and precipitated phases of super-austenitic stainless steel influenced by electromagnetic stirring

Super austenitic stainless steel was prepared by vacuum non-consumable arc melting furnace. The influence of electromagnetic stirring on segregation behavior and the precipitates of super-austenitic stainless steel was investigated via a combined thermodynamic calculation and an experimental characterization approach. The σ phase is the main precipitation phase in super austenitic stainless steel, which has been proved by Thermo-Calc software and experimental characterization. Electromagnetic stirring significantly reduces the dendrite spacing and refines the solidification microstructure. In super-austenitic stainless steel, Cr, Mn, and Mo are positively segregated, while Ni and Fe are negatively segregated. Si and Cu have no obvious segregation. The segregation coefficient (K) shows that the segregation of Mo element is the most serious. Compared with no electromagnetic stirring, the segregation of elements in the electromagnetic stirring sample is significantly weakened.

Laser remelting of AlSi10Mg(-Ni) alloy surfaces: influence of Ni content and cooling rate on the microstructure

AlSi10Mg alloys are widely employed in a variety of industries, including aerospace, automotive, and microelectronics. This is because of its low density, acceptable mechanical properties, acceptable corrosion resistance, and inexpensive application cost. Advantageous fluidity, a short solidification period, and minimal volumetric contraction are beneficial characteristics under processing such alloys. Despite being used as commercial alloys, the mechanical properties of the AlSi10Mg alloys still need to be improved. In line with this, the current focus of Al-based alloy development is mostly on modifying commercially available alloys. Under such context, Ni was used as an alloying element in this study to generate the Al3Ni intermetallics, distinguished by its improved mechanical strength. Furthermore, the thermal stability of the Al3Ni may be a benefit, particularly for high-temperature applications. The present study aims to investigate the solidification under low and high cooling rates of four alloys: AlSi10Mg, AlSi10Mg-1Ni, AlSi10Mg-2Ni, and AlSi10Mg-3Ni (wt.%). Samples were obtained by directional solidification (DS) and laser surface remelting (LSR) processes. The cooling rates were calculated for the DS samples and with extrapolation for LSR samples as well as with the use of a model from the literature. After testing several laser conditions, the results also include an examination of microstructural and hardness changes in the treated and untreated zones. The produced gradient of microstructures is fully characterized as well as used to evaluate cooling rates inside the laser molten pools. For energy densities of 400 J/mm2 and 100 J/mm2, the mean dendritic spacings, λ, of the three Ni-containing alloys at the laser molten pool yielded estimated cooling rates of approximately 1.5 × 104 °C/s and 4.7 × 104 °C/s, respectively. A model explaining the reversion of λ across the molten pool will be outlined.

Cooling rate and dendrite spacing control in direct metal deposition printed Cu-Fe alloys

Over the last several decades, additive manufacturing (AM) has been primarily used for rapid prototyping or to create novel geometries that would be difficult or impossible to create by normal manufacturing methods. More recently, research has been focused on expanding the list of materials that can be made through additive manufacturing, opening a greater range of material properties for this manufacturing method. Due to the unusual conditions during AM, including the high cooling rates and voxel by voxel method of production, AM parts often have anisotropic material microstructures and properties. In this investigation, the laser power, composition, and nozzle head speed during direct metal deposition of copper-iron alloys was varied to understand how the grain structure within the printed parts could be changed and controlled. The resulting dendrite spacing was measured and compared to calculated cooling rates from Gaussian beams on flat plates under similar material and laser properties, which resulted in a cooling rate to dendrite spacing relationship following an inverse square root, as is found in other dendritic systems [Young and Kerkwood, Metall. Trans. A 6, 197–205 (1975)]. Thus, it is demonstrated that in the Cu-Fe system, dendrite spacing can be controlled through manipulation of printing parameters.

Effect of synchronous electromagnetic stirring on Laves phase morphology and mechanical property of Inconel625-HSLA steel functionally graded material fabricated by wire arc additive manufacturing

Inconel625- high strength low alloy (HSLA) steel functionally graded material (FGM) specimens have been fabricated by wire arc additive manufacturing (WAAM) system with synchronous electromagnetic stirring (EMS). The chemical composition distribution has been analyzed to prove the graded transition of the FGMs. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM) were applied to analyze the microstructures of the FGMs. By using EMS, the primary dendrite spacing of FGM is reduced by 22.6%, the size of Laves phases in the FGM has been decreased with from about 2 μm to less than 100 nm, and the tensile strength along with building direction has been improved from 512 MPa to 563 MPa.

Multi-phase field model simulation based on MPI+OpenMP parallel: Evolution of seaweed and dendritic structure in directional solidification

A multi-phase model was established to imitate the growth of algal and dendritic grains during directional solidification. We studied the effects of temperature on the growth of bi-crystals and quantitatively analyzed the influence of anisotropic strength, thermal gradient, and pulling velocity on the evolution of bi-crystals. The results show that both weaker anisotropy strength and smaller pulling velocity can maintain the formation of seaweed tissue. The increase in the pulling velocity can degenerate the seaweed grains into dendrites and improve the growth rate of the dendrites, which make grain B produce more spindles, thereby accelerating the elimination of grain A. The thermal gradient is inversely proportional to the average initial spacing of dendrites. When the thermal gradient is too small, dendritic dendrites produce developed secondary dendrite arms, which, in turn, develop into tertiary dendrite arms to occupy the grain boundary, accelerating the elimination of seaweed grains. In addition, the multi-phase field model is solved by using central processing unit serial computation, single MPI (message passing interface) parallel programming method calculation, and MPI+OpenMP hybrid parallel programming structure, and the relevant factors affecting the efficiency of program operation are analyzed and tested. By comparing the computational efficiency of the three methods, it can be seen that the MPI+OpenMP hybrid parallel programming technology can make full use of computing resources in the case of large computing scale, further optimize the MPI parallel model, and obtain a higher acceleration ratio.

Effect of competitive crystal growth on microstructural characteristics of directionally solidified nickel-based single crystal superalloy

Directionally solidified single crystal superalloy test bars were prepared by the spiral grain selection method. The microstructural evolution and orientation characteristics of the starter block and spiral part were studied, and the influence of the competitive growth of crystals on the microstructural characteristics was analyzed. The results show that the divergent grain groups, with small size and randomly oriented grains, appear at the bottom of the start block due to the chilling effect, which is an important area for competitive growth. As the height of the starter block increases, the primary dendrite spacing increases, and the grain density decreases; furthermore, the proportion of grains with an orientation deflection angle less than 10° gradually increases. The <001> texture gradually becomes stronger as the height of the starter block increases, which indicates that the competitive growth of crystals gradually weakens. At the initial stage of the crystal selection in the spiral part, the obstacle of adjacent grains and spiral passage is the main working mechanism. The grains located at the inner side of the front edge of the spiral passage have the growth advantage. The single crystal screening process is achieved at about two-thirds of the spiral height, and the single crystal with the orientation deviation angle of 6.7° from the casting axis is prepared.

Heat-flow parameters affecting microstructure and mechanical properties of Al–Cu and Al–Ni alloys in directional solidification: an experimental comparative study

Abstract A comparative analysis was carried out of the alloys examined, using the results found for thermal variables, macrostructure, dendrite arm spacing, modulus of elasticity and microhardness. Comparing all the results for variables, a predominance was found of higher values for the Al-5.0 wt.% Cu alloy in relation to those for the Al-5.0 wt.% Ni alloy. Although structural transition has occurred in both alloys, the columnar–equiaxed transition position in castings was not altered during the solidification experiments. Addition of Ni solute into pure aluminum favors a significant decrease in tertiary dendrite spacing, while Cu addition leads to a coarser microstructure. Primary dendrite arm spacing, also, was measured along the castings of both alloys. Theoretical approaches such as the well-known Hunt–Lu, Bouchard–Kirkaldy, and Kurz–Fisher models were used to determine quantitatively this particular microstructural parameter. Good agreement was observed between experimental data and predictions by Hunt–Lu and Bouchard–Kirkaldy models, which assume solidification under a transient heat-flow condition. Finally, empirical equations relating tertiary dendritic arm spacing and mechanical properties to the scale of the dendritic microstructure have been proposed. While no relationship was observed between modulus of elasticity and tertiary dendritic arm spacing, addition of Cu into pure aluminum as an alloying element favored a reduction in the average value of modulus of elasticity when compared with those observed for the Al–Ni alloy. It was found that the microhardness decreases with increasing tertiary dendritic arm spacings. On the other hand, solute addition of Cu into commercially pure aluminum favors a microhardness higher than that verified during upward unidirectional solidification of the Al-5.0 wt.% Ni alloy.

NbB refining capability: Effects of slow and rapid solidification on dendritic spacings and grain sizes of a 6201 alloy

NbB inoculants are refiners that might be of interest to the Al industry. Grain size (GS) and secondary dendrite arm spacing (SDAS) are two important microstructural variables to consider when assessing a refiner's performance. Both of these attributes may have an impact on the alloy’s properties. The interplay of grain refiner and cooling rate on grain size, SDAS, and grain shape in the 6201 alloy is investigated in this article. It is demonstrated that GS associated with rapid solidification of the 6201 + NbB alloy was roughly 7 times smaller than that associated with slow solidification. Furthermore, regardless of the processing method (either slow or fast solidification), α-Al cells are generated for GS/SDAS ratio below and equal to 6.

Analyzing effects of temperature gradient and scan rate on metal additive manufacturing microstructure by using phase field-finite element method

Predicting the evolutionary behavior of microstructures with the help of numerical simulation techniques has become an essential tool for studying the solidification process of metal additive manufacturing. As a mesoscopic model based on the diffusion interface theory, phase field method (PFM) can be used to predict the evolution of solidification microstructure. The open-source PFM framework PRISMS-PF can not only efficiently solve systems of equations with billions of degrees of freedom, but also provide a simple adaptive mesh control module. In this paper, based on the open-source PFM framework PRISMS-PF, a phase field-finite element method (PFM-FEM) simulation flow for the solidification process of A356 aluminum alloy additive manufacturing in the two-dimensional case was established. The effects of temperature gradient, scan rate and initial solid-phase morphology on solute concentration, dendrite spacing and dendrite morphology were analyzed and compared with experimental results for verification. Analyzing the results for different temperature gradients and scan rates cases, it was found that the increase of temperature gradient or scan rate made the primary dendrite arm space decrease; as the ratio of temperature gradient to scan rate decreased, the solidification morphology gradually changed from flat crystal to cellular crystal, columnar crystal, and even dendritic structure. Analyzing the results for different initial solid-phase morphology cases, it was found that the influence of initial solid-phase morphology on dendrite growth increased as the ratio of temperature gradient to scan rate decreased. The above influence rules were mainly related to the composition overcooling zone under different conditions. This paper is expected to provide a theoretical support for the effective regulation of solidification microstructure in metal additive manufacturing.