Daojie Zhang

Experimental and Numerical Analysis of the 6061-Based Nanocomposites Fabricated via Ultrasonic Processing

There is strong evidence showing that microstructure and mechanical properties of a cast component can be considerably improved if nanoparticles are used as a reinforcement to form a metal-matrix nanocomposite. In this paper, 6061 nanocomposite castings were fabricated using the ultrasonic stirring technology (UST). The 6061 alloy and Al2O3/SiC nanoparticles were used as the matrix alloy and the reinforcement, respectively. Nanoparticles were injected into the molten metal and dispersed by ultrasonic cavitation and acoustic streaming. The applied UST parameters in the current experiments were used to validate a recently developed multiphase computational fluid dynamics (CFD) model, which was used to model the nanoparticle dispersion during UST processing. The CFD model accounts for turbulent fluid flow, heat transfer, and the complex interaction between the molten alloy and nanoparticles using the ANSYS’s fluent dense discrete phase model (DDPM). The modeling study includes the effects of ultrasonic probe location and the initial location where the nanoparticles are injected into the molten alloy. The microstructure, mechanical behavior, and mechanical properties of the cast nanocomposites have been also investigated in detail. The current experimental results showed that the tensile strength of the as-cast-reinforced 6061 alloy with Al2O3 or SiC nanoparticles increased slightly while the elongation increased significantly. The addition of the Al2O3 or SiC nanoparticles in 6061 alloy matrix changed the fracture mechanism from brittle dominated to ductile dominated.

Numerical modeling of the dispersion of ceramic nanoparticles during ultrasonic processing of A356-based nanocomposites

The metal matrix nanocomposites in this study consist of a A356 alloy matrix reinforced with 1.0 wt.% SiC nanoparticles that are dispersed within the molten alloy matrix using ultrasonic cavitation and induction melting technologies. The required ultrasonic parameters to achieve the required cavitation for adequate degassing and refining of the aluminium alloy as well as the fluid flow characteristics for uniform dispersion of the nanoparticles into the 356 matrix are being investigated in this study using an in-house developed magneto-hydro-dynamics model. The magneto-hydro-dynamics model accounts for turbulent fluid flow, heat transfer and solidification, electromagnetic field as well as the complex interactions between the solidifying alloy and nanoparticles using ANSYS Maxwell and ANSYS Fluent dense discrete phase model and a particle engulfment and pushing model. A parametric study was performed which includes the effects of electromagnetic field from the induction coils and the magnitude of the fluid flow.

3D CFD Modeling of the LMF System

A fully transient 3D CFD modeling approach capable of predicting the three phase (gas, slag, and steel) flows and behavior of the slag/steel interface in the argon gas bottom stirred ladle with two off-centered porous plugs (Ladle Metallurgical Furnace or LMF) was developed. The modeling approach consists of using a multiphase VOF-Level Set explicit model in conjunction with energy, momentum, and species transfer models as well as the k-epsilon realizable turbulence model with standard wall functions, and species transfer models available in ANSYS Fluent software. The CFD model can predict the evolution of Ar, steel and slag phases as well as the fluid flow characteristics during both the high-stirring and low-stirring conditions in the LMF system. The model predicted accurately the observed size of the slag eyes for both stirring conditions. The model was then applied to study the effects of key processing parameters including Ar flow rate, processing time, and depth of slag on the three phase flows and behavior of the slag/steel interface. The desulfurization process is also studied.

Advanced Numerical Modeling of the Dispersion of Ceramic Nanoparticles during Ultrasonic Cavitation Processing and Solidification of 6061-based Nanocomposites

The metal-matrix-nano-composites (MMNCs) in this study consist of a 6061 alloy matrix reinforced with 1.0 wt.% SiC 50 nm diameter nanoparticles that are dispersed uniformly within the matrix in large volume using an ultrasonic cavitation dispersion technique (UCDS) available in the Solidification Laboratory at UA. The required ultrasonic parameters to achieve the required cavitation for adequate degassing and refining of the aluminium alloy as well as the fluid flow characteristics for uniform dispersion of the nanoparticles into the 6061 matrix are being investigated in this study by using an in-house developed CFD ultrasonic cavitation model. The multiphase CFD model accounts for turbulent fluid flow, heat transfer and solidification as well as the complex interaction between the solidifying alloy and nanoparticles by using the Ansyss Fluent Dense Discrete Phase Model (DDPM) and a particle engulfment and pushing (PEP) model. The PEP model accounts for the Brownian motion. SEM analysis was performed on the as-cast MMNC coupons processed via UCDS and confirmed the distribution of the nanoparticles predicted by the current CFD model. A parametric study was performed using the validated CFD model. The study includes the effects of magnitude of the fluid flow and ultrasonic probe location (gravity direction).

3D CFD Modeling of the LMF System: Desulfurization Kinetics

A fully transient 3D CFD modeling approach capable of predicting the three phase (gas, slag and steel) fluid flow characteristics and behavior of the slag/steel interface in the argon gas bottom stirred ladle with two off-centered porous plugs (Ladle Metallurgical Furnace or LMF) has been recently developed. The model predicts reasonably well the fluid flow characteristics in the LMF system and the observed size of the slag eyes for both the high-stirring and low-stirring conditions. A desulfurization reaction kinetics model considering metal/slag interface characteristics is developed in conjunction with the CFD modeling approach. The model is applied in this study to determine the effects of processing time, and gas flow rate on the efficiency of desulfurization in the studied LMF system.

3D Stochastic Modeling of Columnar-to-Equiaxed Transition During the Solidification of Alloy 718

An efficient three-dimensional (3D) stochastic model for simulating the evolution of dendritic crystals during the solidification of alloys was developed. The model includes time-dependent computations for temperature distribution, solute redistribution in the liquid and solid phases, curvature, and growth anisotropy. The 3D model can run on PCs with reasonable amount of RAM and CPU time and therefore no parallel computations are needed. 3D mesoscopic computations at the dendrite tip length scale were performed in this study to simulate the formation of the columnar-to-equiaxed transition in alloy 718. Comparisons between microstructure predictions obtained via 2D and 3D stochastic modeling are also presented.Copyright