Ruel A. Overfelt

Influence of solidification variables on the microstructure, macrosegregation, and porosity of directionally solidified Mar-M247

Abstract The solidification microstructure is critical in determining the amount and distribution of porosity that develops during the freezing of castings. As the solidification velocity V s increases, the microstructural length scales (primary and secondary dendrite arm spacings) decrease; thus (1) the mushy zone permeability would be expected to decrease; and (2) nucleated pores would be increasingly isolated. Although the first effect would tend to increase the observed porosity, the second effect would tend to decrease the porosity. To better understand these competitive mechanisms, a series of controlled unidirectional solidification experiments were performed on bars of nickel-base superalloy Mar-M247. Samples were produced with constant dendrite arm spacing throughout an extended length of each cast bar. The axial thermal gradient and withdrawal velocity imposed on each casting were varied between castings to produce a range of microstructures from aligned cellular dendritic to aligned dendritic to mis-aligned dendritic. Macrosegregation effects along the lengths of the bars were evaluated and the resultant impact upon the density along the length of each casting was also characterized. The density measurements were found to be very sensitive to both (1) compositional macrosegregation in these castings and (2) internal porosity. Statistical analyses of microporosity in the castings were based upon metallographic measurements. The development of microporosity in the unidirectionally solidified castings is shown to be dependent upon the hydrogen gas content of the samples and the imposed solidification velocity through the samples cast microstructures. An optimum intermediate withdrawal velocity of 0.005–0.01 cm s −1 was found, which led to closely spaced dendrite arms, a large number of very small pores and a minimum total porosity level. Lower velocities lead to increased porosity from larger pores whereas higher velocities lead to macroporosity due to centerline shrinkage.

Measurement of Specific Heat Capacity and Electrical Resistivity of Industrial Alloys Using Pulse Heating Techniques

The determination of the specific heat capacity and electrical resistivity of Inconel 718, Ti-6Al-4V, and CF8M stainless steel, from room temperature to near the melting temperatures of the alloys, is described. The method is based on rapid resistive self-heating of a solid cylindrical specimen by the passage of a short-duration electric current pulse through it while simultaneously measuring the pertinent experimental quantities (i.e., voltage drop, current, and specimen temperature). From room temperature to about 1300 K, the properties are measured using an intermediate-temperature pulse-heating system by supplying a constant current from a programmable power supply and measuring the temperature using a Pt-Pt:13% Rh thermocouple welded to the surface of the specimen. From 1350 K to near the melting temperatures of the alloys, the properties are measured using a millisecond-resolution high-temperature pulse-heating system by supplying the current from a set of batteries controlled by a fast-response switching system and measuring the temperature using a high-speed pyrometer in conjunction with an ellipsometer, which is used to measure the corresponding spectral emissivity. The present study extends the application of these techniques, previously applied only to pure metals, to industrial alloys.

Influence of directional solidification variables on the cellular and primary dendrite arm spacings of PWA1484

A series of directional solidification experiments have been performed to elucidate the effects of thermal gradient G and growth velocity V on the solidification behavior and microstructural development of the multicomponent Ni-base superalloy PWA 1484. A range of aligned as-cast microstructures were exhibited by the alloy: (i) aligned dendrites with well developed secondary and tertiary arms; (ii) flanged cellular dendrites aligned with the growth direction and without secondary arms; and (iii) cells with no evidence of flanges or secondary arms. The role of the imposed process parameters on the primary arm spacings that developed in the Bridgman-grown samples were examined in terms of current theoretical models. The presence of secondary arms increases the spacings between dendrites and leads to a greater sensitivity of λ1 on G−1/2V−1/4. The exponent of V was analyzed and found to depend upon the imposed gradient G. High withdrawal velocities and low thermal gradients were found to cause radial non-uniformity of the primary dendrite arm spacing. Such behavior was associated with off-axis heat flows.

High temperature deformation behavior of solid and semi-solid alloy 718

The mechanical response of alloy 718 with various microstructures in the solid and semi-solid state has been characterized. The experimental results presented for the lower temperature solid state deformation are in good agreement with published literature values and extend the experimental range to higher temperatures and lower strain rates. When dendrites were aligned along the compression axis, the directionally solidified materials exhibited an activation energy for plastic flow consistent with the activation energy for creep and self-diffusion in nickel, even at temperatures within the mushy zone. However, samples containing non-aligned grains in the semi-solid state exhibited a greater dependence of deformation with temperature; this was associated with lubricated flow of the grains due to the intergranular liquid in the mushy zone.

Measurement of liquid metal viscosity by rotational technique

The rotational measurement technique has been developed to measure the dynamic viscosity of liquid metals. A special procedure is accomplished to eliminate both end and eccentricity effects. The technique was calibrated and tested with liquids of known rheology. The apparent viscosity of low melting alloy composite is measured by means of the technique.

Oscillating cup viscosity measurements of aluminum alloys: A201, A319 and A356

An oscillating cup viscometer was developed to measure the absolute viscosities of molten metals. Previous experiments established the capability of the apparatus to characterize the viscosities of molten nickel-based superalloys. However, modifications to the instrument and its theoretical analysis were required for reliable measurements on molten aluminum alloys, presumably due to their lower densities and lower viscosities. The theoretical literature for the fluid flow inside an oscillating cup is reviewed, and a working equation without any correction factor is developed for the improved viscometer. Some design parameters of the viscometer that directly affect the accuracy of viscosity estimation by using the working equation are discussed. A special vertical furnace was adopted to uniformly heat a longer cylindrical sample (10 mm inner diameter and 120 mm length) with a temperature difference of less than 2°C over the sample length. The measuring procedure was also improved to get more accurate motion parameters. It is estimated that the working equation and improved instrument provide an uncertainty of less than 4%. In addition, applications and experimental data are presented for pure aluminum and three aluminum alloys: A201, A319, and A356.

Electrical and thermal conductivity of A319 and A356 aluminum alloys

A rotational contactless inductive measurement technique has been used to measure the electrical resistivity of A319 and A356 aluminum alloys at both solid and liquid states. The method is based on the phenomena that when a conducting material rotates in a magnetic field, circulating eddy currents are induced and generate an opposing torque, which is proportional to the electrical conductivity of the material. The technique was checked and calibrated with pure aluminum where considerable electrical resistivity data exist in the literature. Wiedemann-Franz-Lorenz law was used to estimate the thermal conductivity of A319 and A356 aluminum alloys in liquid state.

Numerical modeling and experimental verification of mold filling and evolved gas pressure in lost foam casting process

A simple mathematical model is developed to describe a lost foam casting process. Different aspects of the process, such as liquid metal flow, transient heat transfer, foam degradation and gas elimination were incorporated into this numerical model. Fluid velocity, temperature distribution within molten metal and pressure building-up in the mold cavity are predicted as a function of filling time and filling height. The model was verified by comparison of the predicted velocity profiles, temperature fields and back-pressures with the experimental data conducted in this work. Both coated and uncoated foam patterns were used in experimental part of this study. A good agreement between the predictions and the experimental data was found.

Electrical conductivity measurements in liquid metals by rotational technique

The rotational contactless inductive measurement technique has been developed to measure the electrical conductivity of liquid metals. This method is based on the phenomena when a conductor material rotates in a magnetic field, circulating eddy currents are induced and generate a damping torque proportional to the electrical resistivity of the material. The technique was tested to measure the conductivity of five conductors and one low melting composite (LMA-158).

Fraction Solid Measurements on Solidifying Melt

A new indirect method to measure fraction solid in molten metals is presented. The method is based on the phenomena that when a metal sample (solid or liquid) rotates in a magnetic field (or the magnetic field rotates around a stationary sample), circulating eddy currents are induced in the sample, which generate an opposing torque related to amount of solid phase in a solidifying melt between the liquidus and solidus temperatures. This new technique is applied for measuring fraction solid on commercial A319 aluminum alloy. The solidification curves obtained by the proposed method at different cooling rates are in good agreement with predictions made by the Scheil model.