The effect of silicon microsegregation on the mechanical properties of high silicon alloyed ductile cast iron under monotonous loading.

High silicon alloyed ductile cast iron (Si-DCI) can show unpredictable brittle fracture which currently prevents a widespread application of this material. The brittleness is associated with local superstructure formation due to silicon segregation which influences the deformation mechanisms of the matrix phase. In order to understand the effect of silicon segregation on the mechanical properties of Si-DCI under monotonous loading, three alloys with different cooling conditions were examined and micromechanical simulations were carried out by using the phenomenological crystal plasticity model. Here, the segregation profiles were determined through multi phase field simulations. The influence of segregation on the mechanical properties was only evident from the model but not from the experimental results. The simulated results show that the toughness of Si-DCI decreases with stronger silicon segregation when ductile damage is considered.

Thermomechanical properties of 3D-printed sand moulds using inorganic binder.

Additive Manufacturing of 3D-printed sand cores using the binder jetting process is well-established in prototype manufacturing. Due to the rising focus on sustainability and the fact that printed cores are shifting to serial production, a transition from organic to inorganic binder systems is taking place. To ensure a stable casting process and reduce the scrap rate accurate simulation tools are required. However, a study about the thermomechanical properties and the anisotropy of 3D-printed sand cores has not yet been conducted. In this work the thermomechanical properties of 3D-printed sand cores in three different printing orientations using inorganic binder are given. In contrast to homogeneous materials like metals, the simulation of sand cores requires new material models due to the dependency to hydrostatic pressure. The Drucker-Prager soil plasticity model is used, and the parameters needed for the Drucker-Prager-Cap model until 500°C are analysed using the three-point-bending test, the indirect tensile test and the uniaxial compression test. In addition to these specific parameters, also general parameters required for mechanical simulation like the Young's modulus, the Poisson's ratio, the density and the thermal expansion coefficient are given. In comparison to the reference binder system for shot cores using inorganic binder, the 3D-printed cores showed a higher mechanical strength. In the tensile region due to the higher binder content and in the compressive region due to the higher AFS number. Furthermore, the binder system for printed cores showed a lower thermostability.

Preheating Influence on the Precipitation Microstructure, Mechanical and Corrosive Properties of Additively Built Al-Cu-Li Alloy Contrasted with Conventional (T83) Alloy.

In this paper, the high strength and lightweight Al-Cu-Li alloy (AA2099) is considered in as-built and preheated conditions (440 °C, 460 °C, 480 °C, 500 °C, and 520 °C). The purpose of this study is to investigate the influence of laser powder bed fusion (LPBF) in situ preheating on precipitation microstructure, mechanical and corrosive properties of LPBF-printed AA2099 alloy compared to the conventionally processed and heat-treated (T83) alloy. It is shown that precipitations evolve with increasing preheating temperatures from predominantly globular Cu-rich phases at lower temperatures (as-built, 440 °C) to more plate and rod-like precipitates (460 °C, 480 °C, 500 °C and 520 °C). Attendant increase with increasing preheating temperatures are the amount of low melting Cu-rich phases and precipitation-free zones (PFZ). Hardness of preheated LPBF samples peaks at 480 °C (93.6 HV0.1), and declines afterwards, although inferior to the T83 alloy (168.6 HV0.1). Preheated sample (500 °C) shows superior elongation (14.1%) compared to the T83 (11.3%) but falls short in tensile and yield strength properties. Potentiodynamic polarization results also show that increasing preheating temperature increases the corrosion current density (Icorr) and corrosion rate. Indicated by the lower oxide resistance (Rox), the Cu-rich phases compromise the integrity of the oxide layer.

Influence of Process Parameter and Alloy Composition on Misoriented Eutectics in Single-Crystal Nickel-Based Superalloys.

The nucleation and the growth of misoriented micro-structure components in single crystals depend on various process parameters and alloy compositions. Therefore, in this study, the influence of different cooling rates on carbon-free, as well as carbon-containing, nickel-based superalloys was investigated. Castings were carried out using the Bridgman and Bridgman-Stockbarger techniques under industrial and laboratory conditions, respectively, to analyze the impact of temperature gradients and withdrawing rates on six alloy compositions. Here, it was confirmed that eutectics could assume a random crystallographic orientation due to homogeneous nucleation in the residual melt. In carbon-containing alloys, eutectics also nucleated at low surface-to-volume ratio carbides due to the accumulation of eutectic-forming elements around the carbide. This mechanism occurred in alloys with high carbon contents and at low cooling rates. Furthermore, micro-stray grains were formed by the closure of residual melt in Chinese-script-shaped carbides. If the carbide structure was open in the growth direction, they could expand into the interdendritic region. Eutectics additionally nucleated on these micro-stray grains and consequently had a different crystallographic orientation compared with the single crystal. In conclusion, this study revealed the process parameters that induced the formation of misoriented micro-structures, which prevented the formation of these solidification defects by optimizing the cooling rate and the alloy composition.

Phase-Field Simulation of Microstructure Formation in Gas-Atomized Al-Cu-Li-Mg Powders.

Al-Cu-Li (2xxx series) powders for additive manufacturing processes are often produced by gas atomization, a rapid solidification process. The microstructural evolution of gas-atomized powder particles during solidification was investigated by phase-field simulations using the software tool MICRESS. The following topics were investigated: (1) the microsegregation of copper and lithium in the particle, and the impact of lithium addition on the formation of secondary phases in Al-2.63Cu and Al-2.63Cu-1.56Li systems, (2) the effect of magnesium on the nucleation and final mass fraction of T1 (Al2CuLi) growing from the melt, and (3) the effect of increased magnesium content on the T1 and S' (AlCu2Mg) phase fractions. It is observed that the addition of lithium into the Al-Cu system leads to a decrease in the solid solubility of copper in the primary matrix; consequently, more copper atoms segregate in the interdendritic regions resulting in a greater mass fraction of secondary precipitates. Our result agrees with findings on the beneficial impact of magnesium on the nucleation and precipitation kinetics of T1 precipitates in the conventional casting process with further thermomechanical heat treatments. Moreover, it is observed that the increase in magnesium from 0.28 wt.% to 0.35 wt.% does not significantly affect the nucleation and the amount of the T1 phase, whereas a decrease in T1 phase fraction and a delay of T1 formation are encountered when magnesium content is further raised to 0.49 wt.%.

Influence of Hf and MC Carbide on Transverse Platform in Single Crystal Grain Selection of 2D Grain Selector.

CM247LC Ni-based components have been widely used in developing hot ends in aero-engines and gas industrial turbines, and these have exhibited promising directional solidification (DS) results. However, the superalloy CM247LC shows defects after adding carbon (C) and hafnium (Hf). In this study, the effects of adding C and Hf on grain selection have been explored to enhance the 2D grain selector's performance and reduce casting costs. The experimental results reveal that the final region of carbide formation is where the dendrite is pushed into the paste region and finally solidifies. The performance requirements of carbide on the alloy can be controlled by changing the paste region and solidification sequence.

Potential of Metallurgical Gradients in the Design of Structural Components Made of Nodular Cast Iron.

The objective of this work is to investigate the use of metallurgical gradients (MG) in the design of structural components made of ductile cast iron (DCI). MG have been realized in this study by locally varying the pearlite fraction of the matrix. Exemplarily, the allowable cyclic load for a drive shaft as well as the allowable static displacement are calculated. The performed calculations are based on static and cyclic strength data of four different DCI with amounts of pearlite ranging from 0% to 96.8%. To show the advantage of the purposeful usage of local MG, ten different configurations are examined by numerical simulation studies of a generic drive shaft comprising a circumferential notch. Four configurations are calculated assuming homogenous material throughout the entire component. In the other six configurations the surface region of the notch root has an increased amount of pearlite. For each configuration the allowable multiaxial cyclic load by combinations of torsion and bending was calculated and subsequently the allowable static bending displacement. The results show that the targeted realization of MG results in a significant increase in the multiaxial fatigue strength of the shaft as well as in a slight improvement of the allowable static bending displacement.

Characterising the Microstructure of an Additively Built Al-Cu-Li Alloy.

Al-Cu-Li alloys are famous for their high strength, ductility and weight-saving properties, and have for many years been the aerospace alloy of choice. Depending on the alloy composition, this multi-phase system may give rise to several phases, including the major strengthening T1 (Al2CuLi) phase. Microstructure investigations have extensively been reported for conventionally processed alloys with little focus on their Additive Manufacturing (AM) characterised microstructures. In this work, the Laser Powder Bed Fusion (LPBF) built microstructures of an AA2099 Al-Cu-Li alloy are characterised in the as-built (no preheating) and preheat-treated (320 °C, 500 °C) conditions using various analytical techniques, including Synchrotron High-Energy X-ray Diffraction (S-HEXRD). The observed dislocations in the AM as-built condition with no detected T1 precipitates confirm the conventional view of the difficulty of T1 to nucleate on dislocations without appropriate heat treatments. Two main phases, T1 (Al2CuLi) and TB (Al7.5Cu4Li), were detected using S-HEXRD at both preheat-treated temperatures. Higher volume fraction of T1 measured in the 500 °C (75.2 HV0.1) sample resulted in a higher microhardness compared to the 320 °C (58.7 HV0.1) sample. Higher TB volume fraction measured in the 320 °C sample had a minimal strength effect.

Microstructural Investigations of Ni-Based Superalloys by Directional Solidification Quenching Technique.

The improvement of the mechanical properties of Ni-based superalloys is achieved in most cases by modifying the chemical composition. Besides that, the processing can be modified to optimize the as-cast microstructure with regard to the mechanical properties. In this context, the present study highlights the solidification mechanism of several Ni-based superalloys by conducting experiments using a modified, laboratory-scale Bridgman-Stockbarger furnace. In that context, the single-crystal rods are partially melted, directionally solidified and quenched sequentially. Several characterization methods are applied to further analyze the influence of the alloying elements and the variation of the withdrawal rate on the as-cast microstructure. Four stages of solidification are distinguished whereby the morphology observed in the different stages mainly depends on the cooling rate and the local concentration of the carbide forming elements. The effect of carbide precipitation and the effect on the as-cast microstructure is investigated by employing energy dispersive X-ray spectrometry (EDX) and electron backscatter diffraction (EBSD) analysis techniques. A local polycrystalline structure is observed in the single-crystal system as consequence of the influence of the carbon content and the cooling rate. The present work aims to develop strategies to suppress the formation of the polycrystalline structure to maintain the single-crystal microstructure.

Using a Three-Dimensional Reduction Method in the High-Efficiency Grain Selector and Corresponding Grain Selection Mechanism.

In the development of a high-efficiency grain selector, the spiral selectors are widely used in Ni-based single crystal (SX) superalloys casting to produce single crystal turbine blades. For the complex three-dimensional structure of the spiral, a 2D grain selector was designed to investigate in this paper. As a result, the parameters of two-dimensional grain selection bond and the corresponding grain selection mechanism were established, and the three-dimensional grain selection bond was designed again by means of two-dimensional coupling optimization parameters.

Investigation of Start Block and Corresponding Influence for Grain Selection during Casting of Single-Crystal Superalloys.

To figure out the impact of the parameters of a starter block (the diameter D and height H) on grain selection and the selecting mechanism, a spiral selector was measured with optical microscopy (OM) and electron backscatter diffraction (EBSD) during the solidification of Ni-based single crystal (SX) superalloys. In this experiment, starter blocks with diameters of 8 mm, 10 mm, 15 mm, and 30 mm and a height of 30 mm were designed to find the best parameters. Recommendations for optimizing starter block geometry are provided.

Development of a High-Efficiency Z-Form Selector for Single Crystal Blades and Corresponding Grain Selection Mechanism.

Single crystal (SX) is widely used in modern turbine blades to improve the creep fracture, fatigue, oxidation, and coating properties of the turbine, so that the turbine engine has excellent performance and durability. In this paper, the single crystal super alloy MM247LC is used as the research material. The evolution of grain structure in a two-dimensional grain selector was studied by directional experiments, and the mechanism of grain selection in the two-dimensional channel during directional solidification was clarified. In order to optimize the production process of single crystal turbine blades, the effects of the geometrical structure of a Z-type separator (i.e., wire diameter and take-off angle) on the crystal orientation, microstructure, and grain efficiency of blades were discussed.

Grain Selection in a High-Efficiency 2D Grain Selector During Casting of Single-Crystal Superalloys.

Using electron backscattered diffraction techniques (EBSD) and optical microscopy (OM), the grain selection and competitive growth in a new-designed high-efficiency two-dimensional (2D) selector during solidification of Ni-based single-crystal (SX) superalloys have been investigated with emphasis on the geometry of the selector part in this article. It is found that the efficiency of the grain selector depends greatly on the thickness and eccentric distance of the selector part. When the thickness is smaller than 3 mm, a single grain can be selected. After reducing this value, the grain selector becomes more effective. When the eccentric distance is larger than 8 mm, one grain can be selected. As the eccentric distance increases, the selector's efficiency is optimized. Recommendations for optimizing the geometry of the selector part are provided.

Freckle Defect Formation near the Casting Interfaces of Directionally Solidified Superalloys.

Freckle defects usually appear on the surface of castings and industrial ingots during the directional solidification process and most of them are located near the interface between the shell mold and superalloys. Ceramic cores create more interfaces in the directionally solidified (DS) and single crystal (SX) hollow turbine blades. In order to investigate the location of freckle occurrence in superalloys, superalloy CM247 LC was directionally solidified in an industrial-sized Bridgman furnace. Instead of ceramic cores, Alumina tubes were used inside of the casting specimens. It was found that freckles occur not only on the casting external surfaces, but also appear near the internal interfaces between the ceramic core and superalloys. Meanwhile, the size, initial position, and area of freckle were investigated in various diameters of the specimens. The initial position of the freckle chain reduces when the diameter of the rods increase. Freckle area follows a linear relationship in various diameters and the average freckle fraction is 1.1% of cross sectional area of casting specimens. The flow of liquid metal near the interfaces was stronger than that in the interdendritic region in the mushy zone, and explained why freckle tends to occur on the outer or inner surfaces of castings. This new phenomenon suggests that freckles are more likely to occur on the outer or inner surfaces of the hollow turbine blades.

Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy.

A nickel-based single-crystal superalloy was employed to investigate the preferred growth orientation behavior of the (γ + γ') eutectic and the effect of these orientations on the segregation behavior. A novel solidification model for the eutectic island was proposed. At the beginning of the eutectic island's crystallization, the core directly formed from the liquid by the eutectic reaction, and then preferably grew along [100] direction. The crystallization of the eutectic along [110] always lagged behind that in [100] direction. The eutectic growth in [100] direction terminated on impinging the edge of the dendrites or another eutectic island. The end of the eutectic island's solidification terminates due to the encroachment of the eutectic liquid/solid interface at the dendrites or another eutectic island in [110] direction. The distribution of the alloying elements depended on the crystalline axis. The degree of the alloying elements' segregation was lower along [100] than [110] direction with increasing distance from the eutectic island's center.

Structure-function relationships in Macadamia integrifolia seed coats--fundamentals of the hierarchical microstructure.

The shells/coats of nuts and seeds are often very hard to crack. This is particularly the case with Macadamia seed coats, known to exhibit astoundingly high strength and toughness. We performed an extensive materials science characterization of the complex hierarchical structure of these coats, using light and scanning electron microscopy in 2D as well as microCT for 3D characterization. We differentiate nine hierarchical levels that characterize the structure ranging from the whole fruit on the macroscopic scale down to the molecular scale. From a biological viewpoint, understanding the hierarchical structure may elucidate why it is advantageous for these seed coats to be so difficult to break. From an engineering viewpoint, microstructure characterization is important for identifying features that contribute to the high strength and cracking resistance of these objects. This is essential for revealing the underlying structure-function-relationships. Such information will help us develop engineering materials and lightweight-structures with improved fracture and puncture resistance.