Grain selection and growth orientation of prior-β phase for Ti-6-4 during additive manufacturing:insights from a modeling perspective

The microstructure of Ti-6-4 components produced by additive manufacturing suffers from the coarse and elongated prior-β grain,which leads to a decrease of the tensile behavior and the occurrence of anisotropy.To understand and control the grain evolution,a multiscale simulation is applied to investigate the relationship between the grain selection,growth orientation,and the molten pool morphology with the different deposition layer numbers and processing parameters.The accuracy of the simulation is validated by experiments in both qualitative and quantitative ways.Results show that when the grain with unfavorable orientation loses the competitive growth with its neighbors,there will be a great chance that the blocked grain is eliminated in the following layer-and-layer deposition,which leads to the increase of the grain width.The size of the molten pool increases remarkably as the layer number increases,which lays a heavy burden on the stability of the molten pool.The analytical relationship between the molten pool morphology and the grain growth orientation is also deduced.The flat molten pool causes the grains with the <001> direction close to the building direction to have greater survival potential.Besides,decreasing the line power energy shows little effect on the stability of the molten pool and the grain growth orientation,especially when the deposited layer number is large.The revealing mechanisms will help in understanding and further controlling the grain evolution.

Evolution of Grain Selection in Spiral Selector during Directional Solidification of Nickel-base Superalloys

The process of grain selection in the spiral selector was investigated by both a ProCAST simulation based on a cellular automaton finite element（CAFE） model and experimental confirmation.The results show that the height of starter block,the spiral diameter and initial angle play an important role in grain selection.The dimension of selector should be maintained in a stable range to optimize the grain orientation and select a single crystal efficiently.A selector which can efficiently select a single crystal had been successfully designed.Grain orientation fluctuation in the spiral part was also studied by means of the variation of thermal condition.

S. E. M. analysis of quartz sand grains from selected Hong Kong and Chinese littoral environments

This pilot study attempts to demonstrate some underlying scanning electron microscopy themes of quartz grain surface textures. A variety of textural patterns and individual features are described for grains selected from various littoral environments. An attempt was made to differentiate samples on surface textures alone, but limitations of using this technique in sedimentological isolation were apparent. Statistical analysis of checklist data and photographic evidence revealed some of the more important feature combinations used in environmental diagnosis. The use of discriminant analysis provided quantitative sample separation.

Relationship Between Plant Type and Grain Quality of Japonica Hybrid Rice in Northern China

Plant type and grain quality are two major aspects in rice breeding. Using canonical correlation analysis and canonical redundancy analysis, the relationship between plant type traits and rice grain quality traits was studied with 100 crosses derived from 10 sterile lines × 10 restorer lines. There was a complex relationship between parts of the traits of the two aspects. The angle of the 2nd leaf from the top and single panicle weight played important roles in plant type system and amylose content and grain length in grain quality system. The angle of the 2nd leaf from the top, plant height and single panicle weight had a great effect on grain quality traits, and amylose content, brown rice rate and translucency were easily influenced by plant type traits. Selection index model indicated that japonica hybrid rice in Northern China with good quality was characterized by broad flag leaf and 2nd leaf from the top, narrow and short 3rd leaf from the top, low plant height, short culm, long and more panicles and low single panicle weight.

Multiscale modelling and simulation of single crystal superalloy turbine blade casting during directional solidification process

As the key parts of an aero-engine,single crystal(SX)superalloy turbine blades have been the focus of much attention.However,casting defects often occur during the manufacturing process of the SX turbine blades.Modeling and simulation technology can help to optimize the manufacturing process of SX blades.Multiscale coupled models were proposed and used to simulate the physical phenomena occurring during the directional solidification(DS)process.Coupled with heat transfer(macroscale)and grain growth(meso-scale),3D dendritic grain growth was calculated to show the competitive grain growth at micro-scale.SX grain selection behavior was studied by the simulation and experiments.The results show that the geometrical structure and technical parameters had strong influences on the grain selection effectiveness.Based on the coupled models,heat transfer,grain growth and microstructure evolution of a complex hollow SX blade were simulated.Both the simulated and experimental results show that the stray grain occurred at the platform of the SX blade when a constant withdrawal rate was used in manufacturing process.In order to avoid the formation of the stray crystal,the multi-scale coupled models and the withdrawal rate optimized technique were applied to the same SX turbine blade.The modeling results indicated that the optimized variable withdrawal rate can achieve SX blade castings with no stray grains,which was also proved by the experiments.

Additive Manufacturing of Nickel-Based Superalloy Single Crystals with IN-738 Alloy

The production of non-weldable nickel-based superalloys,especially single-crystal superalloys,is important for additive manufacturing.Single-crystal specimens of non-weldable nickel-based superalloys were produced by electron beam selective melting using an IN-738 alloy.In this study,single-crystal nickel-based superalloy specimens of diff erent sizes were prepared by a multiple preheating process and tight control of the melting parameters without the need for a grain selector or single-crystal seed for the first time.Electron backscattered diff raction measurements were performed to confi rm the presence of a single crystal.The transition boundaries between the polycrystalline and single-crystal regions at the edges and bottom of the samples were characterized to analyze the formation of single crystals.

Microstructure Characteristics of Inconel 625 Superalloy Manufactured by Selective Laser Melting

Selective laser melting （SLM）, an additive manufacturing process, is capable of manufacturing metallic parts with complex shapes directly from computer-aided design （CAD） models. SLM parts are created on a layer-by-layer manner, making it more flexible than traditional material processing techniques, in this paper, Inconel 625 alloy, a widely used material in the aerospace industry, were chosen as the build material. Scanning electron microscopy （SEM）, electron back scattering diffraction （EBSD） and X-ray diffraction （XRD） analysis techniques were employed to analyze its microstructure. It was observed that the molten pool was composed of elongated columnar crystal, Due to the rapid cooling speed, the primary dendrite arm space was approximately 0.5 μm and the hardness of SLM state was very high （343 HV）. The inverse pole figure （IPF） indicated that the growing orientation of the most grains was 〈001〉 due to the epitaxial growth and heat conduction. The XRD results revealed that the austenite structure with large lattice distortion was fully formed. No carbides or precipitated phases were found. After heat treatment the grains grew into two microstructures with distinct morphological characters, namely, rectangular grains and limited in the molten pool, and equiaxed grains along the molten boundaries. Upon experiencing the heat treatment, MC carbides with triangular shapes gradually precipitated. The results also identified that a large number of zigzag grain boundaries were formed. In this study, the grain formation and microstructure, and the laws of the molten pool evolution were also analyzed and discussed,