Secondary Dendrite Arm Spacing Values Research Articles

Ablation technology applied to A356 alloys compared with conventional casting processes

Because ablation differs from conventional casting, some conventional metallurgical concepts, such as the relationship among secondary dendrite arm spacing (SDAS), microconstituents and tensile properties, should be revised.To understand the mechanisms governing ablation technology, the most popular of casting alloys, A356 alloys, were selected, and conventional melt treatments, such as degassing, grain refinement and Si modification with Sr, were applied. All ablated as-cast microstructural characteristics (SDAS, grain size, porosity, Si particle morphology and Fe-bearing phases) and the mechanical properties after T6 were compared with two conventional processes: permanent moulding and sand cast manufacturing with the same metal. SDAS values, which are commonly used to predict mechanical properties in conventional casting processes, are no longer valid for ablation, which presents coarse SDAS microstructures, similar to those obtained with the conventional sand casting process, with some isolated areas of fine SDASs. However, the high freezing rate in the last stage of the solidification process obtained by ablation technology generates fine eutectic Si and iron-bearing particles and low porosity. This makes it possible to obtain quality index values of Q = 480 MPa that are significantly higher than those of sand casting (Q = 426 MPa) and similar to those obtained for permanent moulds (Q = 480 MPa).Additionally, the potential advantages of ablation for T5 (Q = 419 MPa) and shorter solution time T6 treatments (Q = 479 MPa) were also investigated in terms of mechanical properties and Si particle size and shape.

Evaluation of tensile properties using uni-axial and ball indentation testing and correlation with microstructure variations across the 316LN stainless steel weld joints

Automated ball indentation (ABI) tests were conducted across the 316LN stainless steel (SS) weld joints at 298 K, and 823 K. Zone wise tensile properties of 316LN SS weld joints such as weld metal, heat affected zone (HAZ), and base metal region are evaluated in this work. Uniaxial tensile tests were conducted to assess the effect of welding processes on the tensile properties. The weld joints were initially fabricated using arc based and laser based welding processes. The variations of tensile properties such as yield strength, ultimate tensile strength and strain hardening component values across the heterogeneous microstructure zones of the weld joints were evaluated. The correspondence between the microstructure variations and the tensile properties have been studied. The variations in grain size, secondary dendrite arm spacing (SDAS) and δ-ferrite content are found to be the primary cause of tensile strength variation across the weld joints. The gradient in tensile strength across the weld joints was higher at 298 K than at 823 K which was attributed to the microstructural changes at higher temperature causing material softening. It was noticed that the 316LN stainless steel weld joint made by hybrid laser MIG welding exhibited the highest values of strength and ductility among all the weld joints. It was attributed to the combined effect of higher δ-ferrite content and finer SDAS values. Therefore, the HLM welding process is recommended for welding of 316LN stainless steel components from the point of view of better tensile properties.

Automatic Determination of Secondary Dendrite Arm Spacing in AlSi-Cast Microstructures

A new procedure for the automatic measurement of the secondary dendrite arm spacing (SDAS) from microscopic images is presented. The individual primary and secondary dendrite arms are identified through suitable segmentation techniques and clustered in such a way that dendritic structures are obtained suitable for SDAS measurement. The algorithms are applied to two different hypoeutectic aluminum cast alloys, and the quality of the measurements obtained is assessed through a comparison to manually measured SDAS values. A good agreement between the automated measurements and the distribution of manual measurements is found for both cast structures considered. In addition, a decrease in computation time is observed which allows for an increase in measurement density that is used to characterize the microstructures.

Casting Microstructure Inspection Using Computer Vision: Dendrite Spacing in Aluminum Alloys

This paper investigates the determination of secondary dendrite arm spacing (SDAS) using convolutional neural networks (CNNs). The aim was to build a Deep Learning (DL) model for SDAS prediction that has industrially acceptable prediction accuracy. The model was trained on images of polished samples of high-pressure die-cast alloy EN AC 46000 AlSi9Cu3(Fe), the gravity die cast alloy EN AC 51400 AlMg5(Si) and the alloy cast as ingots EN AC 42000 AlSi7Mg. Color images were converted to grayscale to reduce the number of training parameters. It is shown that a relatively simple CNN structure can predict various SDAS values with very high accuracy, with a R2 value of 91.5%. Additionally, the performance of the model is tested with materials not used during training; gravity die-cast EN AC 42200 AlSi7Mg0.6 alloy and EN AC 43400 AlSi10Mg(Fe) and EN AC 47100 Si12Cu1(Fe) high-pressure die-cast alloys. In this task, CNN performed slightly worse, but still within industrially acceptable standards. Consequently, CNN models can be used to determine SDAS values with industrially acceptable predictive accuracy.

Evaluation of the microstructure, secondary dendrite arm spacing, and mechanical properties of Al–Si alloy castings made in sand and Fe–Cr slag molds

The microstructure and mechanical properties of as-cast A356 (Al–Si) alloy castings were investigated. A356 alloy was cast into three different molds composed of sand, ferrochrome (Fe–Cr) slag, and a mixture of sand and Fe–Cr. A sodium silicate–CO2 process was used to make the necessary molds. Cylindrical-shaped castings were prepared. Cast products with no porosity and a good surface finish were achieved in all of the molds. These castings were evaluated for their metallography, secondary dendrite arm spacing (SDAS), and mechanical properties, including hardness, compression, tensile, and impact properties. Furthermore, the tensile and impact samples were analyzed by fractography. The results show that faster heat transfer in the Fe–Cr slag molds than in either the silica sand or mixed molds led to lower SDAS values with a refined microstructure in the products cast in Fe–Cr slag molds. Consistent and enhanced mechanical properties were observed in the slag mold products than in the castings obtained from either sand or mixed molds. The fracture surface of the slag mold castings shows a dimple fracture morphology with a transgranular fracture nature. However, the fracture surfaces of the sand mold castings display brittle fracture. In conclusion, products cast in Fe–Cr slag molds exhibit an improved surface finish and enhanced mechanical properties compared to those of products cast in sand and mixed molds.

Building blocks for a digital twin of additive manufacturing

Properties and serviceability of additively manufactured components are affected by their geometry, microstructure and defects. These important attributes are now optimized by trial and error because the essential process variables cannot currently be selected from scientific principles. A recourse is to build and rigorously validate a digital twin of the additive manufacturing process that can provide accurate predictions of the spatial and temporal variations of metallurgical parameters that affect the structure and properties of components. Key building blocks of a computationally efficient first-generation digital twin of laser-based directed energy deposition additive manufacturing utilize a transient, three-dimensional model that calculates temperature and velocity fields, cooling rates, solidification parameters and deposit geometry. The measured profiles of stainless steel 316L and Alloy 800H deposits as well as the secondary dendrite arm spacing (SDAS) and Vickers hardness measurements are used to validate the proposed digital twin. The predicted cooling rates, temperature gradients, solidification rates, SDAS and micro-hardness values are shown to be more accurate than those obtained from a commonly used heat conduction calculation. These metallurgical building blocks serve as a phenomenological framework for the development of a digital twin that will make the expanding knowledge base of additive manufacturing usable in a practical way for all scientists and engineers.

Impact of Interlayer Dwell Time on Microstructure and Mechanical Properties of Nickel and Titanium Alloys

Path planning in additive manufacturing (AM) processes has an impact on the thermal histories experienced at discrete locations in simple and complex AM structures. One component of path planning in directed energy deposition is the time required for the laser or heat source to return to a given location to add another layer of material. As structures become larger and more complex, the length of this interlayer dwell time can significantly impact the resulting thermal histories. The impact of varying dwell times between 0 and 40 seconds on the microstructural and mechanical properties of Inconel® 625 and Ti-6Al-4V builds has been characterized. Even though these materials display different microstructures and solid-state phase transformations, the addition of an interlayer dwell generally led to a finer microstructure in both materials that impacted the resulting mechanical properties. With the addition of interlayer dwell times up to 40 seconds in the Inconel® 625 builds, finer secondary dendrite arm spacing values, produced by changes in the thermal history, correspond to increased yield and tensile strengths. These mechanical properties did not appear to change significantly, however, for dwell times greater than 20 seconds in the Inconel® 625 builds, indicating that longer dwell times have a minimal impact. The addition of interlayer dwell times in Ti-6Al-4V builds resulted in a slight decrease in the measured alpha lath widths and a much more noticeable decrease in the width of prior beta grains. In addition, the yield and tensile values continued to increase, nearly reaching the values observed in the rolled plate substrate material with dwell times up to 40 seconds.

Comparison of Measurement Methods for Secondary Dendrite Arm Spacing

Secondary dendrite arm spacing (SDAS) is most commonly measured by what is referred to as the linear intercept method. However, substantial variation in the technique exists between researchers, and its influence on the measurements has not yet been elucidated. Given the strong correlations between SDAS and material properties, a consistent and accurate technique is essential for interstudy comparability and effective alloy design. In this study, the SDAS of an aluminum alloy cast at two different solidification rates was quantified using five methods typical in the literature. Each method enabled observation of the refinement in dendritic structure associated with faster solidification. Also, each method produced a very similar average SDAS value for castings at high solidification rates, since the error is reduced in fine and uniform microstructures. Yet, significant variation, inaccuracy, and inconsistency in SDAS values were found to be possible between the methods for castings at low solidification rates. The most accurate and precise of the five methods for both coarse and fine microstructures was identified, and its use is recommended to improve SDAS measurement practices in academia and the industry.

Effect of Mold Type on the Microstructure and Tensile Properties of A356 Alloy

The present study was performed on castings obtained from A356 alloy melts subjected to different melt treatments (degassing, modification, grain refining). The castings were prepared using three different processes/molds: lost foam, ASTM standard metallic, and end-chill refractory molds. An examination of samples sectioned from the cylinder head/lost foam casting showed that the secondary dendrite arm spacing (SDAS) depends on the sample location in the cylinder head, with bolt boss and combustion chamber sections exhibiting SDAS values of 52 and 86 µm, respectively, versus 26 µm for the ASTM metallic mold samples, and from 25 to 85 µm with increasing distance from the end-chill, along the height of the refractory mold. Grain refining (Al-4 %B) leads to redistribution of porosity in the casting, as the AlB2 particles of the grain refiner act as sites for the precipitation of hydrogen gas bubbles, resulting in finer pores. This observation is independent of the mold type. In the presence of modifier, Sr–B interaction causes loss in the available Sr and B for modification and grain refining, resulting in a heterogeneous distribution of the eutectic silicon particles. Although increasing the solidification rate reduces the percentage porosity, it also leads to segregation of hydrogen at the solid/liquid interface in the case of directional solidification obtained with the end-chill mold, so that the porosity is the outcome of the solidification rate and the hydrogen content. The tensile properties of the A356 alloy are controlled by the size and morphology of the eutectic silicon particles more than by the addition of grain refiner. The difference in tensile properties obtained using the three molds is essentially attributed to % porosity (shrinkage, gas, and presence of residual pyrolysis) and SDAS. The use of proper melt treatments and the ASTM standard metallic (tensile test bar) mold could produce strength values that are 170 % higher than that obtained from the lost foam castings.

A high thermal gradient directional solidification method for growing superalloy single crystals

The experiments here were conducted at withdrawal rates of 3mm/min and 1mm/min using a CMSX-6 and a CMSX-4 superalloy, respectively. The process was assessed in terms of the thermal gradient (GL), structural refinement, microsegregation and porosity distribution, and compared to those using a Bridgman process. The GL of the process was 200–236K/cm, which was 10–12 times higher than that in the Bridgman process. A more refined microstructure was produced having average primary and secondary dendrite arm spacing values as low as 243μm and 72μm, as well as 272μm and 76μm in the CMSX-6 and the CMSX-4 castings, respectively. The diameter of γ′ phase in the dendrite core of CMSX-6 and CMSX-4 castings was reduced from 0.8μm to 0.3μm and from 1.2μm to 0.6μm, respectively. The average areas of (γ′+γ) eutectic pools became smaller and more homogeneously distributed. The mean pore sizes in the castings were reduced by 57% and 43% for the CMSX-6 and CMSX-4 superalloys, respectively, and the area fractions of the pores in the CMSX-6 and CMSX-4 samples were 16% and 12% of those produced in the Bridgman samples. The segregation coefficients of the major alloying elements were closer to unity than those in the Bridgman process, which indicates that the composition distribution is more uniform.

Influence of SDAS on the High Temperature Tensile Behaviour of the C355 Al Alloy

The aim of the present study was to characterize the high temperature tensile behaviour of the C355 (Al-Si-Cu-Mg) alloy produced under controlled casting condition so as to obtain different secondary dendrite arm spacing (SDAS). C355 samples were produced through a gradient solidification equipment able to produce microstructures with fine (20-25 μm) and coarse (50-70 μm) SDAS values. The as-produced specimens were subjected to hot-isostatic pressing and then T6 heat treated. Microstructural characterization, room and high temperature (200 °C) tensile tests were carried out on the heat treated specimens. The tensile behaviour was related to the different SDAS value of the samples.

Effects of the Delay Between Quenching and Aging on Hardness and Tensile Properties of A356 Aluminum Alloy

The aim of the study was to evaluate the accuracy of heat treatment guidelines, generally followed in industrial practices, about the T6 heat treatment of A356 aluminum alloy. In particular, the effect of the delay between quenching and artificial aging (pre-aging time) on microstructure, hardness, and tensile behavior was studied using specimens extracted from different locations of a cylinder head, characterized by different cooling rates and, consequently, by different secondary dendrite arm spacing values. Hardness and tensile tests confirmed the detrimental effect of pre-aging with a 20% reduction in hardness and strength after approximately 1 h of pre-aging, both for samples with fine and large SDAS. Differential scanning calorimetry analyses on samples that were solutionized, quenched, and pre-aged between 0 and 96 h, suggested that the nature and composition of the clusters formed during pre-aging, rather than their size, influenced the subsequent precipitation process and the final mechanical properties of the alloy.

Effect of cooling rate on microstructure evolution of rapidly cooled high-impurity steels

The effect of cooling rate on secondary dendrite arm spacing (SDAS) and prior austenite grain size (PAGS) has been investigated in high-phosphorous steels (0.1% P) subjected to a rapid cooling process of electron-beam (E-beam) surface remelting. A heat flow calculation was carried out to estimate the cooling rates of surface remelting experiments. The measured SDAS values depended on the cooling rate in good accordance with the Suzuki equation [A. Suzuki, T. Suzuki, Y. Nagaoka, Y. Iwata, J. Jpn. Inst. Met. 32 (1968) 1301–1304] up to the cooling rate of 10 5 K s −1. The classical grain growth modeling (CGM) [N. Yoshida, O. Umezawa, K. Nagai, ISIJ Int. 44 (2004) 547–555] was employed to clarify the effect of the cooling rates on the PAGS. The growth during the continuous cooling process was calculated for each cooling rate by CGM. With increasing the cooling rate, the growth of the prior austenite grains decreased and the PAGS converged to primary dendrite arm spacing (PDAS) when the cooling rates exceeded 10 3 K s −1. An analytic model that can be applied to predict the PAGS for cooling rates from 10 0 to 2 × 10 4 K s −1 was presented based on the present results.

Fatigue behavior of A356/357 aluminum cast alloys. Part II – Effect of microstructural constituents

A quantitative study of the interactions between microstructural features such as secondary dendrite arm spacing (SDAS), eutectic structure, matrix strength, and fatigue behavior of two Al–7% Si–Mg casting alloys with magnesium contents of 0.4% and 0.7%, respectively, has been conducted. In the absence of casting defects, the influence of microstructural features on the fatigue performance becomes more pronounced. The degree and rate of microdamage (microcracking) is strongly affected by the strength of the matrix, and especially by the eutectic particle size, morphology, and distribution (clustering). A soft matrix (under-aged alloys) will generate more local microdamage compared to a peak-aged one. Large and elongated eutectic particles present in unmodified alloys result in lower fatigue lives. A decrease of fatigue life with increasing Mg and Fe content is observed, mainly due to the increased sizes of Fe-rich intermetallic particles. Microstructures with similar eutectic particle size and morphology as the Sr-modified ones, show a minimum fatigue life at intermediate SDAS values (∼60 μm), which is related to the continuity of particles on the dendrite cell boundaries. For coarser microstructures ( SDAS>60 μ m), the increase in fatigue life is attributed to the reduced damage rate along the cell boundaries.