Phase-field simulation of lack-of-fusion defect and grain growth during laser powder bed fusion of Inconel 718

The anisotropy of the structure and properties caused by the strong epitaxial growth of grains during laser powder bed fusion(L-PBF)significantly affects the mechanical performance of Inconel 718 alloy components such as turbine disks.The defects(lack-of-fusion Lo F)in components processed via L-PBF are detrimental to the strength of the alloy.The purpose of this study is to investigate the effect of laser scanning parameters on the epitaxial grain growth and LoF formation in order to obtain the parameter space in which the microstructure is refined and LoF defect is suppressed.The temperature field of the molten pool and the epitaxial grain growth are simulated using a multiscale model combining the finite element method with the phase-field method.The LoF model is proposed to predict the formation of LoF defects resulting from insufficient melting during L-PBF.Defect mitigation and grain-structure control during L-PBF can be realized simultaneously in the model.The simulation shows the input laser energy density for the as-deposited structure with fine grains and without LoF defects varied from 55.0–62.5 J·mm^(-3)when the interlayer rotation angle was 0°–90°.The optimized process parameters(laser power of 280 W,scanning speed of 1160 mm·s^(-1),and rotation angle of 67°)were computationally screened.In these conditions,the average grain size was 7.0μm,and the ultimate tensile strength and yield strength at room temperature were(1111±3)MPa and(820±7)MPa,respectively,which is 8.8%and10.5%higher than those of reported.The results indicating the proposed multiscale computational approach for predicting grain growth and Lo F defects could allow simultaneous grain-structure control and defect mitigation during L-PBF.

Erratum to:Phase-field simulation of lack-of-fusion defect and grain growth during laser powder bed fusion of Inconel 718

The original online version of this article unfortunately contained a mistake.The reference[24]in the original online version is incorrect.The correct version is given below:Y.H.Cheng,Numerical Simulation and Experimental Research of Selective Laser Melting on Nickel Based Alloy Powder GH4169[Dissertation],North University of China,Taiyuan,2016.

Mechanical properties and constitutive behaviors of as-cast 7050 aluminum alloy from room temperature to above the solidus temperature

The mechanical properties and constitutive behaviors of as-cast AA7050 in both the solid and semi-solid states were determined using the on-cooling and in situ solidification approaches, respectively. The results show that the strength in the solid state tends to increase with decreasing temperature. The strain rate plays an important role in the stress–strain behaviors at higher temperatures, whereas the influence becomes less pronounced and irregular when the temperature is less than 250°C. The experimental data were fitted to the extended Ludwik equation, which is suitable to describe the mechanical behavior of the materials in the as-cast state. In the semi-solid state, both the strength and ductility of the alloy are high near the solidus temperature and decrease drastically with decreasing solid fraction. As the solid fraction is less than 0.97, the maximum strength only slightly decreases, whereas the post-peak ductility begins to increase. The experimental data were fitted to the modified creep law, which is used to describe the mechanical behavior of semi-solid materials, to determine the equivalent parameter fGBWL, i.e., the fraction of grain boundaries covered by liquid phase.

A new imaging mode based on X-ray CT as prior image and sparsely sampled projections for rapid clinical proton CT

Proton computed tomography(CT)has a distinct practical significance in clinical applications.It eliminates 3–5%errors caused by the transformation of Hounsfield unit(HU)to relative stopping power(RSP)values when using X-ray CT for positioning and treatment planning systems(TPSs).Following the development of FLASH proton therapy,there are increased requirements for accurate and rapid positioning in TPSs.Thus,a new rapid proton CT imaging mode is proposed based on sparsely sampled projections.The proton beam was boosted to 350 MeV by a compact proton linear accelerator(LINAC).In this study,the comparisons of the proton scattering with the energy of 350 MeV and 230 MeV are conducted based on GEANT4 simulations.As the sparsely sampled information associated with beam acquisitions at 12 angles is not enough for reconstruction,X-ray CT is used as a prior image.The RSP map generated by converting the X-ray CT was constructed based on Monte Carlo simulations.Considering the estimation of the most likely path(MLP),the prior image-constrained compressed sensing(PICCS)algorithm is used to reconstruct images from two different phantoms using sparse proton projections of 350 MeV parallel proton beam.The results show that it is feasible to realize the proton image reconstruction with the rapid proton CT imaging proposed in this paper.It can produce RSP maps with much higher accuracy for TPSs and fast positioning to achieve ultra-fast imaging for real-time image-guided radiotherapy(IGRT)in clinical proton therapy applications.

A novel AMH variant at the prehelix loop impairs the binding to AMHR2 and causes persistent Mullerian duct syndrome

Dear Editor,Persistent Mullerian duct syndrome(PMDS;Online Mendelian Inheritance in Man[OMIM]:#261550)is a rare autosomal recessive inherent difference in sex development in males with Mullerian duct derivatives,including the uterus,fallopian tubes,and upper part of the vagina.'Approximately 88%of PMDS patients were found to have biallelic variants in the anti-Mullerian hormone(AMH;OMIM:*600957)or AMH type 2 receptor(AMHR2;OMIM:*600956)genes,and the rest were referred as idiopathic.I The serum AMH levels are usually very low or undetectable in PMDS patients with AMH gene mutations but normal or elevated in those with AMHR2 gene mutations.Here,an adult Chinese patient with PMDS carrying biallelic variants of the AMH gene was reported to have unexpectedly normal serum AMH level.Please refer to Supplementary Information of Patient and Methods for methodologies.

Advances in Studies and Applications of Centroidal Voronoi Tessellations

Centroidal Voronoi tessellations(CVTs) have become a useful tool in many applications ranging from geometric modeling,image and data analysis,and numerical partial differential equations,to problems in physics,astrophysics,chemistry,and biology. In this paper,we briefly review the CVT concept and a few of its generalizations and well-known properties.We then present an overview of recent advances in both mathematical and computational studies and in practical applications of CVTs.Whenever possible,we point out some outstanding issues that still need investigating.

Comparisons of interface microstructure and mechanical behavior between Ti/Al and Ti-6Al-4V/Al bimetallic composites

The microstructure, interface thickness, element distribution and interfacial mechanical behavior of Ti-6Al-4V/Al couples prepared by an insert moulding method were investigated in depth in this paper. Moreover, Ti/Al bonding was also given as a comparison for understanding the interface bonding mechanism. It is shown that there is much thinner compact sub-layer for the interface of the Ti-6Al-4V/Al joint, whose morphology is obviously different from that of the Ti/Al joint. The Ti-6Al-4V/Al interface has been proven to contain a slight content of vanadium. Moreover, both the shear strength and the interface reaction rate of Ti-6Al-4V/Al compound materials are lower than those of the Ti/Al ones.

Interfacial microstructure and mechanical properties of Ti-6Al-4V/Al7050 joints fabricated using the insert molding method

Ti-6Al-4V/Al7050 joints were fabricated by a method of insert molding and corresponding interfacial microstructure and mechanical properties were investigated. The interfacial thickness was sensitive to holding temperature during the first stage, and a good metallurgical bonding interface with a thickness of about 90 μm can be obtained at 750°C. X-ray diffraction, transmission electron microscopy, and thermodynamic analyses showed that the interface mainly contained intermetallic compound TiAl_3 and Al matrix. The joints featured good mechanical properties, i.e., shear strength of 154 MPa, tensile strength of 215 MPa, and compressive strength of 283 MPa, which are superior to those of joints fabricated by other methods. Coherent boundaries between Al/TiAl_3 and TiAl_3/Ti were confirmed to contribute to outstanding interfacial mechanical properties and also explained constant fracture occurrence in the Al matrix. Follow-up studies should focus on improving mechanical properties of the Al matrix by deformation and heat treatment.

An Explicit-Implicit Predictor-Corrector Domain Decomposition Method for Time Dependent Multi-Dimensional Convection Diffusion Equations

The numerical solution of large scale multi-dimensional convection diffusion equations often requires efficient parallel algorithms.In this work,we consider the extension of a recently proposed non-overlapping domain decomposition method for two dimensional time dependent convection diffusion equations with variable coefficients. By combining predictor-corrector technique,modified upwind differences with explicitimplicit coupling,the method under consideration provides intrinsic parallelism while maintaining good stability and accuracy.Moreover,for multi-dimensional problems, the method can be readily implemented on a multi-processor system and does not have the limitation on the choice of subdomains required by some other similar predictor-corrector or stabilized schemes.These properties of the method are demonstrated in this work through both rigorous mathematical analysis and numerical experiments.

Laser-RF synchronization based on digital phase detector

Laser oscillator synchronization with RF reference signal is ultra-important for a modern light source based on the accelerator.For Tsinghua Thomson scattering X-ray source,we have synchronized the mode-locked laser oscillator to RF reference signal with 48.2 fs RMS relative jitter.Both fundamental and harmonic signals derived from photo diode detection are used for laser-RF synchronization in our scheme.The fundamental signal is for coarse laser-RF synchronization and multiple laser oscillator synchronization.The harmonic signal is for high precise phase locking.The digital phase detector is implemented in the synchronization scheme for less noise,replacing the mixing to DC phase detection scheme.The digital processing algorithm for synchronization is commonly used in low-level RF control field.In order to test the phase locking loop logic without damaging the real laser oscillator,a laser oscillator emulator was developed for phase locking.This paper will report the laser-RF synchronization scheme and its performance.The laser oscillator emulator system will also be introduced here.

A Discontinuous Galerkin Method with Penalty for One-Dimensional Nonlocal Diffusion Problems

There have been many theoretical studies and numerical investigations of nonlocal diffusion(ND)problems in recent years.In this paper,we propose and analyze a new discontinuous Galerkin method for solving one-dimensional steady-state and time-dependent ND problems,based on a formulation that directly penalizes the jumps across the element interfaces in the nonlocal sense.We show that the proposed discontinuous Galerkin scheme is stable and convergent.Moreover,the local limit of such DG scheme recovers classical DG scheme for the corresponding local diff usion problem,which is a distinct feature of the new formulation and assures the asymptotic compatibility of the discretization.Numerical tests are also presented to demonstrate the eff ectiveness and the robustness of the proposed method.

Persistent Müllerian duct syndrome in an assisted reproductive patient: a novel variant impairs the biosynthesis and secretion of anti-Müllerian hormone (AMH)

Dear Editor,Persistent Müllerian duct syndrome(PMDS,MIM 261550)is an autosomal recessive inherent disorder of sex development in normally masculinized subjects with Müllerian duct derivatives,including the uterus,fallopian tubes,and upper part of the vagina.1 PMDS is very rare with presentations of bilateral cryptorchidism(55%),unilateral cryptorchidism with contralateral hernia(20%),or transverse testicular ectopia(25%).

ADC evaluation boards design and test framework for LCLS-Ⅱ precision receiver

In the low-level RF control field,ADC acquisition accuracy and noise set the boundary of our control ability,making it important to develop low-noise acquisition systems.From the design to test stage,all the noise terms should be understood and characterized.The specific need addressed here is the precision acquisition system for the second Linac Coherent Light Source(LCLS-Ⅱ),led by SLAC National Accelerator Laboratory.Test circuit boards for the LTC2174 and AD9268 ADCs are designed and fabricated by LBNL.An ADC test framework based on FPGA evaluation board to assess performance has been developed.The ADC test framework includes both DSP(Digital Signal Processing) firmware and processing software.It is useful for low-level RF control and other synchronization projects.Investigating the clock jitter between two channels give us an understanding of that noise source.Working with the test framework,the raw ADC data are transferred to a computer through a Gigabit Ethernet interface.Then short-term error signal can be calculated based on a sine wave fit.By changing low-pass filter bandwidth,relative long-term performance can also be obtained.Amplitude jitter and differential phase jitter are the key issues for ADCs.This paper will report the test results for LTC2174 and AD9268 chips.The integral amplitude jitter is smaller than 0.003%,and the integral phase noise is smaller than 0.0015°(measured at 47 MHz RF,100 MHz clock,bandwidth 1 Hz to 100 kHz) for both ADC chips.

Preface to the Focused Issue on Fractional Derivatives and General Nonlocal Models

Fractional calculus,which has two prominent characteristics-singularity and nonlocality,comprises the integration and differentiation of any positive real(and even complex)order.It has a more than three-hundred-year history and can be traced back to a letter from Leib-niz to UHopital,dated 30 September 1695,in which the meaning of the one-half order derivative was first discussed and some remarks about its feasibility were made.Abel was probably the first who rendered an application of fractional calculus.He used the derivatives of arbitrary order to solve the tautochrone(isochrone)problem in 1823.Fractional calculus underwent two periods:from its beginning to the 1970s,and after 1970s.During the first period,fractional calculus was studied mainly by mathematicians as an abstract field containing only pure mathematical manipulations of little applications,except for sporadic applications in rheology.During the second period,the paradigm shifted from pure mathematical research to applications in various realms,including anomalous diffusion,anomalous convection,power laws,allometric scaling laws,history dependence,long?range interactions,and so on.

Adaptive waveform design based on Morlet wavelet for ultra-wideband MIMO radar

For the issue of deterioration in detection performance caused by dynamically changing environment in ultra-wideband（UWB） multiple input multiple output（MIMO） radar, this paper proposes a novel adaptive waveform design which is aimed to improve the ability of discriminating target and clutter from the radar scene. Firstly, a sequence of Morlet wavelet pulses with frequency hopping and pulse position modulation by Welch-Costas array is designed. Then a waveform optimization solution is proposed which is achieved by applying the minimization mutual-information（MI） strategy. After that, with subsequent iterations of the algorithm, simulation results demonstrate that the optimal waveform design method brings an improvement in the target detection ability in the presence of noise and clutter.

Revealing excited states of rotational Bose-Einstein condensates

Rotational Bose-Einstein condensates can exhibit quantized vortices as topological excitations.In this study,the ground and excited states of the rotational Bose-Einstein condensates are systematically studied by calculating the stationary points of the Gross-Pitaevskii energy functional.Various excited states and their connections at different rotational frequencies are revealed in solution landscapes constructed with the constrained high-index saddle dynamics method.Four excitation mechanisms are identified:vortex addition,rearrangement,merging,and splitting.We demonstrate changes in the ground state with increasing rotational frequencies and decipher the evolution of the stability of ground states.

On Ginzburg-Landau Vortices of Superconducting Thin Films

In this paper, we discuss the vortex structure of the superconducting thin films placed in a magnetic field. We show that the global minimizer of the functional modelling the superconducting thin films has a bounded number of vortices when the applied magnetic field hex 〈 Hc1 ＋ K log ｜ logε｜ where Hc1 is the lower critical field of the film obtained by Ding and Du in SIAM J. Math. Anal., 2002. The locations of the vortices are also given.

Investigation of flow characteristics in a rotor-stator cavity under crossflow using wall-modelled large-eddy simulation

Rotor-stator cavities are frequently encountered in engineering applications such as gas turbine engines.They are usually subject to an external hot mainstream crossflow which in general is highly swirled under the effect of the nozzle guide vanes.To avoid hot mainstream gas ingress,the cavity is usually purged by a stream of sealing flow.The interactions between the external crossflow,cavity flow,and sealing flow are complicated and involve all scales of turbulent unsteadiness and flow instability which are beyond the resolution of the Reynolds-average approach.To cope with such a complex issue,a wall-modeled large-eddy simulation(WMLES)approach is adopted in this study.In the simulation,a 20°sector model is used and subjected to a uniform pre-swirled external crossflow and a stream of radial sealing flow.It is triggered by a convergent Reynoldsaveraged Navier-Stokes(RANS)result in which the shear stress transport(SST)turbulent model is used.In the WMLES simulation,the Smagoringsky sub-grid scale(SGS)model is applied.A scalar transportation equation is solved to simulate the blending and transportation process in the cavity.The overall flow field characteristics and deviation between RANS and WMLES results are discussed first.Both RANS and WMLES results show a Batchelor flow mode,while distinct deviation is also observed.Deviations in the small-radius region are caused by the insufficiency of the RANS approach in capturing the small-scale vortex structures in the boundary layer while deviations in the large-radius region are caused by the insufficiency of the RANS approach in predicting the external crossflow ingestion.The boundary layer vortex and external ingestion are then discussed in detail,highlighting the related flow instabilities.Finally,the large-flow structures induced by external flow ingress are analyzed using unsteady pressure oscillation signals.

First results on ^(76)Ge neutrinoless double beta decay from CDEX-1 experiment

We report the first results on ^(76)Ge neutrinoless double beta decay from stage one of the China dark-matter experiment(CDEX).A p-type point-contact high-purity germanium detector with a mass of 994 g has been installed to detect neutrinoless double beta decay events, as well as to directly detect dark matter particles. An exposure of 304 kg d has been analyzed over a wide spectral band from 500 keV to 3 MeV. The average event rate obtained was about 0.012 counts per keV per kg per day over the 2.039 MeV energy range. The half-life of ^(76)Ge neutrinoless double beta decay derived based on this result is T^(0ν)1/2>6.4×10^(22) yr(90% C.L.). An upper limit on the effective Majorana-neutrino mass of 5.0 eV has been achieved.