Influence of Solid-State Diffusion during Equilibration on Microstructure and Fatigue Life of Superalloy Wide-Gap Brazements

The influence of solid-state diffusion-controlled solute-loss into additive powder particles (APPs), as determined by particles size, during the equilibration stage of wide-gap brazing, on microstructure and fatigue behavior of a brazed aerospace superalloy was studied. The results, which experimentally confirm previously reported numerical model simulation results, show that, in order to avoid degradation of fatigue life of wide-gap brazement, adequate solute-loss into the APPs, which is necessary to prevent their complete melting, but has not been generally considered, is imperative.     

Asymmetric Diffusional Solidification during Transient Liquid Phase Bonding of Dissimilar Materials

A theoretical analysis of diffusional solidification during transient liquid phase (TLP) bonding of dissimilar materials was performed in conjunction with experimental verification. A fully implicit, two-dimensional, finite element numerical simulation model, without the inherent symmetry assumption, was developed and used for the theoretical calculations, and good correlations between the model predictions and experimental results were observed. The study showed that an asymmetric distribution of residual interlayer liquid during a dissimilar joining of polycrystal and single crystal alloys is attributable to a mismatch between their lattice diffusion coefficients or solute solubility, irrespective of enhanced intergranular diffusion as was assumed previously. Also, notwithstanding increased solute diffusivity with temperature, it was found that an increase in bonding temperature can result in the prolongation of processing time t _f that is required to prevent the formation of deleterious eutectic during bonding of dissimilar materials. The occurrence of this seemingly anomalous behavior, however, reduces when a material is coupled with another type that exhibits a higher solute solubility or better capability of accommodating diffusing melting point depressant solute from the liquid interlayer.     

The physical state of nafcillin sodium in frozen aqueous solutions and freeze-dried powders

The purpose of this study was to develop a better understanding of the physical chemistry of freeze drying of lyotropic liquid crystals using nafcillin sodium as a model solute. Solutions and freeze-dried powders of nafcillin sodium were studied by polarized light microscopy, differential scanning calorimetry, x-ray powder diffraction, and water vapor adsorption. Differential scanning calorimetry thermograms of nafcillin sodium solutions contain a melting endotherm at approximately -5.5 degrees C and, depending on the concentration and heating rate, a crystallization exotherm immediately after this endotherm followed by the melting endotherm of ice. When the sample is annealed at -4 degrees C, both the endotherm and exotherm are eliminated, and a new endotherm appears at approximately -1 degree C on the shoulder of the ice-melting endotherm. The data are interpreted as melting of a liquid crystalline phase, followed by crystallization. X-ray powder diffractograms of unannealed freeze-dried nafcillin sodium are consistent with a lamellar liquid crystal. Diffractograms of annealed freeze-dried nafcillin sodium indicate crystalline material which is a different crystal form than the monohydrate starting material. Moisture adsorption isotherms of the freeze-dried annealed (crystalline) and unannealed (liquid crystalline) nafcillin sodium show different affinities for moisture compared to the crystalline starting material. Solid-state stability data demonstrate that the freeze-dried liquid crystalline form of nafcillin sodium is much less stable than the freeze-dried crystal-line material. The literature recognizes two types of solute behavior on freezing, where the solute either crystallizes from the freeze concentrate or remains amorphous. Lyotropic liquid crystal formation during freezing represents a separate category of freezing behavior, the physical chemistry of which is worthy of further investigation.     

HVOF 制备 MCrAlY 涂层过程中 WC 杂质颗粒的演变遗传行为研究

在热喷涂工业生产过程中,喷涂制备不同材料体系的涂层之前需要对送粉路径(送粉罐、送粉盘、搅拌器、送粉管路、送粉针)进行清理,但由于送粉路径内部结构复杂且存在静电吸附效应,使得残留于喷涂系统的粉末无法被完全去除,通常以微量杂质颗粒的形式被带入新涂层,进而影响新涂层的性能。为此,本文研究了在单晶基材表面采用高速火焰(HVOF)喷涂制备MCrAlY涂层过程中,送粉路径残留的WC杂质颗粒(WC-10Co4Cr)在涂层及涂层与基材界面处的遗传演变行为,分别采用SEM、EDS分析了WC杂质在喷涂态、热处理态涂层中的微观组织和相组成。研究结果表明,WC杂质颗粒确实存在于MCrAlY涂层中,并在后期热处理及氧化试验中进一步分解而固溶于涂层中,甚至扩散至单晶基材内部引起含W碳化物的生成,影响涂层及单晶基材的显微组织,改变局部的成分均匀性。同时,本文还采用ThermoCalc软件进行了热力学计算模拟,辅助分析了WC分解及W与C元素在显微组织中的遗传特性。对于WC类粉末和MCrAlY粉末共用的HVOF喷涂设备,建议给MCrAlY粉末配备单独的送粉路径,以确保涂层的纯净度与质量。     

Kinetic study of low-temperature transient liquid phase joining of an aluminum-sic composite

The transient liquid phase (TLP) joining of an aluminum 6061-SiC composite using a gallium interface has been investigated. The observed kinetics of this process suggest that diffusion occurs along interphase and subgrain boundaries at low temperatures (<723 K). An existing TLP model has been modified to account for this behavior.     

Nature of the growth of Al-Cu powder compacts during liquid-phase sintering

Conclusions Arresting the process of liquid-phase sintering of aluminum-copper system powder compacts by rapid cooling does not affect the character of the volume changes experienced by them during subsequent sintering under the same temperature conditions. In the growth stage a decrease in the crystal lattice parameter of aluminum and an appreciable broadening of an x-ray line have been observed, caused by the formation of aluminum base solid solutions. These findings bear out the hypothesis that the growth of aluminum-copper powder compacts above their eutectic melting point is mainly due to diffusion of copper from the liquid phase into aluminum particles.Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 16–19, May, 1988.     

A study of transient liquid-phase bonding of Ag-Cu using differential scanning calorimetry

A novel approach using differential scanning calorimetry (DSC) to quantify interface kinetics in a solid/liquid diffusion couple is applied to characterize the isothermal solidification stage during transient liquid-phase (TLP) bonding of Ag and Cu using a Ag-Cu interlayer. When the DSC results are properly interpreted, the measured interface kinetics are more accurate than those obtained using traditional metallographic techniques. Experimental results are compared to predictions for isothermal solidification given by a selection of analytical models. The comparison yields close agreement with a solution that assumes a moving boundary; but accuracy of the predictions is very sensitive to selection of solute diffusivity. Metallographic inspection of the DSC samples and traditional TLP bonds validates the kinetics measured using this technique, and supports the prediction given by the analytical model. This study shows that the method of using DSC to quantify interface kinetics is valuable in the refinement of process parameters for TLP bonding. Furthermore, simple analytical solutions can be applied to predict the process kinetics of isothermal solidification in simple binary systems with considerable accuracy when the effects of grain boundaries can be neglected, thus reducing the need for complex numerical models when developing process parameters.     

Densification during the supersolidus liquid-phase sintering of nickel-based prealloyed powder mixtures

This article examines densification during supersolidus liquid-phase sintering (SLPS) of a mixture of two nickel-based prealloyed powders. The two powders were of similar compositions, where one powder alloyed with boron had a lower melting-temperature range than the other. Sintering of such binary powder systems is performed at temperatures above the solidus of the low-melting powder, to form a liquid phase that promotes densification. Measurements of shrinkage, sintered density, and melting were used to determine the densification mechanism. An increase in the fraction of the high-melting powder resulted in retardation of sintering. A densification model based on viscous flow was developed using rheological principles. The model is a first step in the extension of the conventional models of densification of single prealloyed powders to ones of mixtures of prealloyed powders. It incorporates the effects of homogenization between the two powders, specifically, the diffusion of boron from the low-melting to the high-melting powder. The densification of the powder mixture was found to depend on the extent of melting of the low-melting powder and the fraction of the high-melting powder.     

A study of the transient liquid phase bonding process applied to a Ag/Cu/Ag sandwich joint

Transient liquid phase (TLP) bonding is a process currently used for joining heat resistant alloys, for example nickel- and cobalt-based superalloys. It involves the formation of a liquid layer between two adjoining pieces and the formation of a solid bond as the liquid disappears during annealing at a suitable constant temperature. In the present study, a model Ag/Cu/Ag sandwich joint associated with a simple eutectic phase diagram was used to study the different stages of this process. The results confirm that the TLP bonding is a diffusional process occurring in clearly distinctive stages. The two most important stages are the widening and homogenization of the previously dissolved liquid interlayer, and the subsequent solidification and shrinking of the interlayer. Whereas the former stage involves diffusional processes both in the liquid phase and in the adjoining solids, the latter is controlled mainly by the diffusion in the solid phase. A modeling approach has been explored which shows that in most eutectic systems there exists an optimal bonding temperature corresponding to the shortest time needed for complete solidification. The results of a study on a Ag/Ag-20 wt pct Cu/Ag sandwich joint provide evidence that the use of an alloy close to the eutectic composition as an interlayer material shortens the TLP process substantially.     

Aluminum volatilization and inclusion removal in the electron beam cold hearth melting of Ti alloys

The electron beam cold hearth melting (EBCHM) process has emerged as either an alternative or a complement to vacuum arc remelting, since it is capable of enhancing the elimination of hard-alpha inclusions by dissolution or sedimentation. The present article describes the use of a mathematical model to simulate the electron beam melting of titanium in a cold hearth. The mathematical model is based on the numerical solution of the coupled momentum, solute, and heat transport equations in a transient regime for a three dimension geometry. The model calculates the velocity, turbulence intensity, temperature, and alloy composition in the liquid and the solid phases. The calculation provides the overall heat balance and the volatilization of metallic elements such as aluminum. A postprocessor numerical tool simulates also the behavior of a hard-alpha inclusion during melting (trajectory and kinetics of dissolution). In order to demonstrate the usefulness of this model, the authors examine the influence of the casting rate and of the beam scanning frequency on the volatilization of aluminum and on the capacity of the process to remove hard-alpha defects.     

Carbon Pickup in Continuous Casting Processes

The quality of the as‐cast strand is influenced by a recarburization reaction since mould powder contains carbon particles necessary for a controlled melting behaviour. Intensive testing reveals a carbon pickup in the surface of the as‐cast strand depending on content and type of the carbon in the mould powder. Using the combination of laboratory trials and numerical simulations the effect of carbon pickup from mould powder to the strand shell was investigated. Laboratory experiments show that the recarburization of the liquid steel through the mould powder can be explained by Marangoni convection. This can happen in the slag rim when the carbon is entrapped by the solidifying steel and starts to diffuse inwards the strand.     

Bonding Mechanism from the Impact of Thermally Sprayed Solid Particles

Power particles are mainly in solid state prior to impact on substrates from high velocity oxy-fuel (HVOF) thermal spraying. The bonding between particles and substrates is critical to ensure the quality of coating. Finite element analysis (FEA) models are developed to simulate the impingement process of solid particle impact on substrates. This numerical study examines the bonding mechanism between particles and substrates and establishes the critical particle impact parameters for bonding. Considering the morphology of particles, the shear-instability–based method is applied to all the particles, and the energy-based method is employed only for spherical particles. The particles are given the properties of widely used WC-Co powder for HVOF thermally sprayed coatings. The numerical results confirm that in the HVOF process, the kinetic energy of the particle prior to impact plays the most dominant role in particle stress localization and melting of the interfacial contact region. The critical impact parameters, such as particle velocity and temperature, are shown to be affected by the shape of particles, while higher impact velocity is required for highly nonspherical powder.     

Development of a Numerical Model for Simulating Transient Liquid Phase (TLP) Bonding Involving Two Solid–Liquid Interfaces that Concurrently Undergo 2D or 3D Migration

A new numerical model is developed for transient liquid phase (TLP) bonding involving two solid–liquid interfaces that concurrently undergo two-dimensional (2D) or three-dimensional (3D) migration in contrast to previous models in the literature where two solid–liquid interfaces are assumed to undergo one-dimensional (1D) migration. The developed model which incorporates variable diffusivity and conserves solute by using a unique hybrid explicit–fully implicit approach and an adaptable space discretization based on Murray–Landis transformation, respectively, is used to investigate the kinetics of the process and major predictions of the model are experimentally validated. It is found that in contrast to the case of 1D migration, despite matching material and bonding conditions, there is a transition from conventional symmetric solidification behavior to asymmetric solidification behavior such that the extent of isothermal solidification is consistently larger on the substrate in which curvature reduces along the direction of solute diffusion. Moreover, aside from what is generally known that the kinetics of isothermal solidification is controlled by diffusivity, equilibrium concentrations at the interface and initial substrate composition, this work shows that when the solid–liquid interface migrates in 2D or 3D, the kinetics is also significantly controlled by the type and degree of curvature at the migrating interface.

     

保护渣晶粒尺寸对传热性能的影响

保护渣在连铸过程中具有重要的作用,其中有效控制钢液向结晶器的传热直接影响了铸坯质量;当保护渣热阻较小、不均匀传热时铸坯容易出现裂纹等缺陷.而保护渣的控热能力主要取决于保护渣的结晶性能,因此,有必要研究保护渣的结晶行为对传热性能的影响.利用双丝热电偶技术通过不同的温度制度,获得不同尺寸的单一结晶相,研究保护渣晶粒尺寸对传...     

Rapid solidification of a droplet-processed stainless steel

Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed. Formerly with Massachusetts Institute of Technology, Cambridge, MA.     

The Influence of Powder Porosity on the Bonding Mechanism at the Impact of Thermally Sprayed Solid Particles

High-velocity oxy-fuel (HVOF) thermal spraying can generate dense depositions without melting the powders during spraying. Our previous study showed that most HVOF-sprayed particles are in solid state prior to impact on the substrate. The deposition of solid particles requires sufficient deformation of the particles as a result of a high impact. This report is a continuation of our previous work to study the bonding mechanism for thermally sprayed solid particles. The same hard material, WC-Co powder, is studied by considering the porosity inside the particles. The detailed deposition mechanism is examined by dynamically tracking the particle impingement using finite element analysis (FEA) models. The results confirm that the deposition of high-speed solid particles is caused mainly by the particle deformation and further implies that deformation is enhanced with increase in porosity alone. Therefore, a possible way to increase the deposition efficiency of hard cermet coating could be to use a properly designed porous powder.     

OBSERVATIONS ON THE SINTERING OF COMPACTSFROM A MIXTURE OF IRON AND COPPER POWDERS

Abstract

Three grades of iron powder-an atomized steel powder, a sponge iron powder reduced from magnetite with carbon, and a powder reduced from mill scale with hydrogen were mixed with 3% of copper powder and pressed into compacts. The diametral dimensional changes of the compacts during sintering below and above the melting point of copper were measured, their microstructures examined, and both related to the characteristics of the powders, particularly their specific surface. During sintering below the melting point of copper, compacts of all three powders shrank. Micrographic examination showed that the copper is transported by solid-state diffusion along the surfacesand grain boundaries of the iron powder particles. During sintering above the melting point of copper, compacts of the atomized and the MH-100 sponge iron powders grew while those of the hydrogen reduced mill-scale powder shrank. This phenomenon is related to the different mode of penetration of liquid copper in the compacts from the three powders, observed in the microstructures of the compacts.     

The free-surface shape and temperature distribution produced in liquid metal droplets by heating coil pulses in the TEMPUS electromagnetic levitation facility

We present the results of analytical and numerical calculations of the free-surface shape and temperature distribution produced in liquid metal droplets processed in the TEMPUS electromagnetic levitation facility. The mathematical models were developed to predict the behavior of liquid metal droplets in containerless experiments used to measure thermophysical properties aboard the Space Shuttle Columbia during the IML-2 mission in July 1994. A normal stress balance model was used to numerically calculate the equilibrium free-surface shapes for various samples produced by a number of induction coil voltages. Analytical and numerical calculations were performed to model the heat transfer in the liquid metal droplets during and following the heating coil pulses. The work illustrates the use of mathematical modeling in the design of microgravity experiments and is applicable to industrial processes such as casting and skull melting.