A. Plotkowski

A Metric for the Quantification of Macrosegregation During Alloy Solidification

A metric for quantifying the degree of solidification macrosegregation is proposed that statistically fits compositional data from experiments and simulations to a three-parameter Weibull distribution. The method for fitting such a distribution is described and examples are presented. The new metrics are compared to existing macrosegregation measures and the Weibull distribution is shown to be the best fit to data. The fitted three-parameter Weibull distribution is generally found to have better agreement with the composition data than a Gaussian distribution, upon which the macrosegregation number is based, because the Weibull better accounts for asymmetry in the dataset. Trends in macrosegregation results are identified using the new metrics, specifically the normalized Weibull deviation, and compared to the trends identified by the macrosegregation number. A grid dependence study is performed using both metrics as tests for convergence. The utility of the Weibull distribution is demonstrated by comparing composition data with different degrees of asymmetry due to different solidification cooling rates. The difference between the values of the two metrics is a measure of the asymmetry in the compositional distribution.

Macrosegregation modeling during direct-chill casting of aluminum alloy 7050

ABSTRACT A fully transient model of the direct-chill casting process is used to predict the macrosegregation development of aluminum alloy 7050. The ingot diameter, casting speed, superheat, secondary cooling, and thickness of pure Al at startup are varied. Predicted radial composition distributions are fit to Weibull probability density functions at each axial location, and the normalized standard deviation describes the macrosegregation level and the time when the process reaches steady state. The sump depth, steady-state height, and macrosegregation level were most affected by changes in casting speed and ingot diameter. The pure Al dilutes the alloy and delays compositional steady state.

Characterisation of the structure and thermophysical properties of solid electroslag remelting slags

The solid structure and thermophysical properties of three electroslag remelting (ESR) slags were characterised. Samples from ESR trials performed by Special Metals Corporation in Burnaugh, KY, were compared to samples solidified in a vacuum induction melter. The microstructures were characterised using a combination of optical and electron microscopy, X-ray diffraction, energy-dispersive spectroscopy and a serial sectioning technique to quantify porosity. The thermophysical properties were measured using differential scanning calorimetry and laser flash diffusivity testing at Thermophysical Property Measurement Laboratory, Inc. in West Lafayette, IN. The two processing routes were found to produce similar phase fractions, but resulted in different properties as a function of phase morphology and porosity content. These results suggest that future modelling and property measurement efforts must account for both the structure and composition of the slags.

Effects of Velocity-Based Packing Criteria on Models of Alloy Solidification with Free Floating Solid

In many solidification processes, particularly in the presence of grain refiner, solid grains initially form as free floating particles suspended within the liquid metal. As these grains continue to grow in number and size, their interactions cause them to form a permeable rigid solid structure. The majority of these models for equiaxed solidification assume that the transition from a slurry of particles to a rigid solid structure occurs at a fixed, uniform solid fraction. This approach does not include the influence of the local velocity field on the likelihood of packing. The present study investigates the effects of three different methods of including the solid particle velocity into packing rules and applies them to simulations of static castings. The advantages and disadvantages of these approaches are discussed in relation to the more common constant packing fraction model.

A New Device to Quantify Human Trunk-Control Measurements

When helping in the rehabilitation of stroke and head trauma patients, physical therapists often find a need to measure the patient’s control of the muscles in their torso. This is called trunk control. Currently, there are two options for the measurement of trunk control. The first is qualitative analysis by the physical therapist, and the second is large, expensive equipment that measures the patient’s balance. The goal of this project was to create a low cost, quantitative means of measuring trunk control. The device used accelerometers placed on the back of the patient’s neck to measure the angle of the patient’s torso from vertical, as compared to acceleration due to gravity, in both left to right and forward to backward directions. The data taken from the accelerometers is stored on a micro-SD card, which is then inserted into a personal computer and analyzed using software built in the lab. The software produces a graphical representation of the data and displays useful calculations. During the course of the project, careful consideration had to be taken to stay within the bounds of professional ethics from a biomedical point of view. This included restricting the testing of the device and taking patient safety as a primary consideration during the entirety of the design process. Future iterations of the device will include technical and aesthetic improvements based on feedback from a group of physical therapy students who are currently testing the quality of the device’s measurements as well as its integration into a clinical setting. Additionally, a group of business students are constructing a business plan for the marketing and sales of this product.Copyright

Geometry-Induced Spatial Variation of Microstructure Evolution During Selective Electron Beam Melting of Rene-N5

High gamma prime (γ′) nickel-based alloys produced by selective electron beam melting are of interest to the turbine industry which requires control of microstructure in relation to loading conditions within complex component geometry. Welding literature predicts cracking and microstructure evolution as a function of alloy composition and process parameters in this class of alloys. In addition, Additive manufacturing causes variations in the above conditions due to the interaction of geometry on processing and heat transfer. The influence of geometry on processing conditions was explored for alloy Rene N5 by characterizing the solidification grain microstructure and solid-state precipitation. A Semi-Analytical Heat Transfer Model was employed to explain the resulting variation in solidification grain morphology that occurred due to the part geometry. The as-built precipitation structure was found to vary as a function of build height and had no correlation to the solidification grain structure or the layer geometry.

Characterization of structure and thermophysical properties of three ESR slags

The structure and properties of electroslag remelting (ESR) slags were characterized. Slags samples of three compositions were obtained from industrial remelting processes at Special Metals Corporation and from casting in a laboratory vacuum induction melter. The structure of the slag samples was observed using optical and electron microscopy, and phases were identified and their relative amounts quantified using X-ray diffraction. Laser flash thermal diffusivity, density, and differential scanning calorimetry measurements for specific heat were performed to determine the bulk thermal conductivity of the samples. Sample porosity was measured as a function of depth using a serial sectioning technique, and a onedimensional computational model was developed to estimate the thermal conductivity of the fully dense slags. These results are discussed in context with previous studies, and opportunities for future research are identified. AFRL Case Number: 88ABW-2015-1871.

Scaling Analysis of Alloy Solidification and Fluid Flow in a Rectangular Cavity

A scaling analysis was performed to predict trends in alloy solidification in a side-cooled rectangular cavity. The governing equations for energy and momentum were scaled in order to determine the dependence of various aspects of solidification on the process parameters for a uniform initial temperature and an isothermal boundary condition. This work improved on previous analyses by adding considerations for the cooling bulk fluid flow. The analysis predicted the time required to extinguish the superheat, the maximum local solidification time, and the total solidification time. The results were compared to a numerical simulation for a Al-4.5 wt.% Cu alloy with various initial and boundary conditions. Good agreement was found between the simulation results and the trends predicted by the scaling analysis.

Refinement of Primary and Eutectic Silicon Particles in Hypereutectic Al-Si Alloys Using an Applied Electric Potential

The effects of applied electric current on the cast microstructure of hypoeutectic Al-13 wt.% Si and Al-20 wt.% Si alloys have been investigated. This involved, with a constant voltage power supply, an application of an electric current density of about 500 mA/cm2 of melt surface area to laboratory-size ingots during solidification in a metal mold. In general, the applied electric current refined the cast microstructure of the hypereutectic Al-Si alloys. Specifically, the electric current did not modify the eutectic silicon particle, but changed the size distribution of the primary silicon particles by increasing the population of comparatively smaller size particles. The extent of the observed cast microstructure refinement was less than the reported effects of applied electric current in the technical literature and is significantly less than the effects of the traditional refinement obtained by addition of strontium and phosphorus to the molten metal prior to casting.