Browsing by Author "Dr. Afsaneh Rabiei, Committee Chair"

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Development of Closed Cell Metallic Foam Using Casting Techniques
(2004-11-29) O'Neill, Adrian Thomas; Dr. Afsaneh Rabiei, Committee Chair; Dr. Jeffrey Eischen, Committee Member; Dr. William Roberts, Committee Member
The research sited in this paper involves the development of a new metal foam composite material using casting techniques. This work included the design of the material and the development of a process to produce the metal foam. The materials used to produce the foam consisted of low carbon steel hollow spheres and an aluminum alloy. The foam is comprised of steel hollow spheres packed into a random dense arrangement, with the interstitial space between spheres infiltrated with a casting aluminum alloy. Using prefabricated hollow spheres assures a uniform pore size and cell wall thickness. Casting a metal into the interstitial space provides a solid media to add structural support to the foam. The goal of this research has been to develop metal foam that demonstrates improvements in product uniformity and mechanical properties over the currently available foams. To accomplish this goal, the study included the identification of the various technologies used to manufacture metal foams, the assessment of the improvements needed to augment the quality of foamed metals, and the design of a new product and processing technique that substantiates these goals. The experimental equipment was designed and procured, while the raw materials were obtained. Then the hollow sphere foam samples were successfully produced. Using these samples a series of characterization studies was done to qualify and quantify the results. These findings were then compared to presently published data to gauge the relative success of the work. The hollow sphere metal foam developed in this study displayed significant improvements in the measures of compressive strength and energy absorption capacity, all the while maintaining the characteristic properties of cellular metals. The improvements were measured against the next best existing technology. The newly developed foam averaged 67 MPa over a region of 10 – 50% strain, with densification beginning at approximately 50% strain. The value for energy absorption is 30 MJ/m3 at 50% strain. This foam also has a strength to density ratio on level with the best reported results to date. The combination of these properties gives opportunity for use in previously unidentified applications, such as an energy absorption media for buildings subject to seismic motion. This foam can also be designed in such applications as automobile crumple zones, as structural members in air and space craft, and in biomedical prosthesis. Several areas for improvement have been identified for this technology. The bonding strength between sphere and matrix needs improvement, and different material choices and processing changes have been identified in this research to achieve these improvements. The packing density of the spheres can be improved, and a new method of vibrating the sphere arrangement prior to molding may increase the packing density. The porosity of the aluminum matrix can be reduced, and the design of the casting mold and processing conditions can be modified to reduce undesirable porosity. Additional testing methods have been identified to further characterize the foam and reveal insights for further improvement. The iterative process of sampling, characterization, and analysis will continue to improve this product to satisfy the objectives of this research program.
Development of Nano-Structured Thin Film Shape Memory Alloys for MEMS Applications
(2004-07-21) Baldwin, Elizabeth Anne; Dr. Jerome Cuomo, Committee Member; Dr. Afsaneh Rabiei, Committee Chair; Dr Mohammad Noori, Committee Member
The production of thin film TiPdNi shape memory alloys (SMA) using ion beam assisted deposition (IBAD) is being studied as a way to increase the actuation frequencies and transformation temperatures of thin film SMA for micro-actuator applications. The capability to transmit extremely high forces along with a large stroke, large strain memory, and high corrosion resistance makes shape memory alloys prime candidates for use in micro-actuator applications. However, low actuation frequency (~1Hz at macro-scale), and low transition temperature (below 100°C) makes commercially available NiTi incompatible with applications in extreme environments. The transformation temperature and actuation frequency of shape memory alloys can be improved through the production of thin film TiPdNi. Through the substitution of Pd for Ni in equiatomic NiTi, the transformation temperature can be varied from approximately room temperature to 527°C. The composition that has received the most attention is Ti50Pd30Ni20 because of its transformation temperature of over 200°C. However, the shape memory effect of Ti50Pd30Ni20 is adversely affected by the low critical stress needed for slip at high temperatures, which results in unrecoverable strain. Age hardening or thermo-mechanical treatments such as cold rolling have been found to improve the critical stress for slip in bulk form SMA due to an increased density of dislocations. Precipitation hardening, as well as, ion bombardment, is expected to increase the high temperature properties in IBAD deposited Ti50Pd30Ni20 film SMA. Additionally, ion bombardment during deposition can be used to improve film properties such as morphology, density, stress level, crystallinity, as well as, limit defects. Due to the refined grain size, increased density, and reduced defects, IBAD is able to produce films of 1 micron or less, which will greatly reduces the SMA actuation time due to the increased surface area —to — volume ratio. In this study, we have deposited thin film TiPdNi using IBAD with thicknesses of less than 2 microns. It had been suggested in a previous study that ion bombardment could produce films with shape memory properties without the need for additional heat treatment. As-deposited films on unheated substrates were found to be highly amorphous without the martensitic crystalline structure needed for shape memory effect. As a result, post deposition annealing of amorphous films was evaluated and found to cause severe cracking and delamination. When films were annealed with a slow heating and cooling rate, severe cracking was present throughout the surface as a result of decohesion. In contrast, delamination from the film/substrate interface occurred when the heating and cooling rates were increased. The SEM cross sectional analysis after annealing showed the transformation from the flawless cross section before annealing to a porous cross section afterwards. Independent of heating and cooling rates, all attempts at annealing films that had been deposited using IBAD on unheated substrates resulted in film failure from extensive tensile stresses. In this study, moderate compressive stress was found in IBAD deposited films on unheated substrates. Subsequent annealing of the films resulted in extensive tensile stresses. The magnitude of the stress, and the conversion from compressive to tensile stress, lead to the film failure as described above. Deposition, and in-situ crystallization of films deposited on heated substrates, produced a moderate tensile stress. Neither cracking nor delamination was found in these films. As the result, we concluded that only deposition on heated substrates produced both the flawless cross section and crystalline structure needed for high transition temperature thin films SMAs to be used for MEMS actuators. In depth analysis of films deposited on heated substrates showed a highly crystallized twinned B19 martensitic structure through the bulk of the film without the need for post deposition heat treatment. In a 1.5 micron thick film, a 70nm thick transition layer was identified between the bulk film and silicon substrate. The remaining film showed a twinned martensite structure. The transition layer between the substrate and the fully martensite layer consists of a 50 nm crystalline austenite layer and two amorphous layers of 10 nm thicknesses each, at the interface with the substrate. A very thin layer of silicon oxide was observed between the 10nm amorphous layers and the Si substrate. No precipitates were found within the film, although a slight compositional gradient was identified. This study shows that the deposition of thin film shape memory alloys using IBAD on unheated substrates will produce amorphous films that require post annealing to produce shape memory effect. It has also been shown that this post deposition heat treatment has undesirable effects such as formation of a porous microstructure, as well as delamination and cracking as a result of high tensile stresses. On the other hand, deposition of films on heated substrate can produce the desired microstructure needed to achieve shape memory properties while reducing film stresses and decreasing processing time by allowing deposition and annealing occur simultaneously.
Processing and Characterization of Aluminum-Steel Composite Metal Foams
(2009-06-27) Vendra, Lakshmi Jyotshna; Dr. Larry Silverberg, Committee Member; Dr. Mohammed Zikry, Committee Member; Dr. Ronald Scattergood, Committee Member; Dr. Afsaneh Rabiei, Committee Chair
Composite Metal Foam (CMF), a new material belonging to the class of advanced cellular and porous materials, has been successfully processed using Gravity Casting technique for the first time at NC State University. This material comprises of steel hollow spheres and a solid Aluminum alloy matrix. The complete characterization of the material included mechanical testing such as monotonic compression, compression-compression fatigue, micro hardness, nano hardness and higher strain rate compression. The energy absorption behavior of the material under static compression has been studied extensively. Experimental results show that Al-steel CMF not only has a higher energy absorption capability than that of other commercially available metal foams produced from similar materials, but also possess a higher strength to density ratio. The microstructural analysis of the material was used to study and explain the formation of different phases at the Aluminum-Steel interface and their effect on the deformation behavior of the composite foam under compression. The effect of processing temperature on the microstructure of the composite metal foam and specifically on the sphere-matrix interface was studied by experimental means. The mechanical properties of the ternary phases formed in the microstructure of the composite foam were characterized using micro and nano-hardness tests. The phases were chemically characterized and formulated using Energy Dispersive Spectroscopy analysis and Al-Fe-Si alloy ternary phase diagrams. The fatigue behavior of the composite metal foams was studied under compression-compression fatigue loading and the results were compared with those of other closed cell metal foams. The elastic modulus of the foams was evaluated using experimental and analytical techniques and the results were found to be in good agreement. Composite metal foams were also processed using a higher solidification rate with water cooling. The effect of alterations in microstructure on the mechanical properties of the composite metal foams was studied and results presented. As the result of high strength, the increase in energy absorption of the composite metal foam samples ranges over thirty times compared to that of 100% Al foams and over six times compared to that of 100% steel foams.
A Systematic Study on the Mechanical and Thermal Properties of Open Cell Metal Foams for Aerospace Applications
(2004-12-01) Azzi, Wassim Elias; Dr. Jeffrey Eischen, Committee Member; Dr. William Robertrs, Committee Member; Dr. Afsaneh Rabiei, Committee Chair
Jet engine operating temperatures have been on the rise since the inception of the jet engine. High Operating temperatures mean increased efficiency, more power, and lesser emissions. However, operating temperatures of jet engines are limited by the operating temperatures of the turbine material. This temperature must not be exceeded or else it will damage the turbine components. To solve this problem, complex cooling schemes have been adopted. They employ the use of outside air to dilute the high combustion temperatures to make them suitable for turbine section exposure. This leads to a non-uniform temperature profile leaving the combustion area and therefore decreasing the overall engine performance by introducing thermal cycling and inconsistencies in operational temperatures. This research suggests placing an ultra-high temperature open cell metal foam ring in front of the combustion area and directly before the turbine section. Open cell foam with their 3-dimensional geometry help mix the hot and cool gasses entering the turbine area creating a more uniform temperature profile. This increases efficiency by raising the operational temperature of the jet engine. It also contributes to an increased lifetime since the mixing function of the metallic foam will reduce thermal cycling fatigue on turbine vanes and blades. This research presents a full investigation on the feasibility of this idea; it studies the potential of producing ultra-high temperature metallic foam from a list of currently available as well recently engineered materials. This work also investigates the pressure drop and heat transfer associated with the foam for a full assessment of the design. Infrared imaging for heat transfer measurements done on Al foam of 5 and 10 PPI (pores per inch) samples showed the mixing role of the metal foam as a function of thickness; it showed improvement of the temperature profile resulting from the averaging the hot and the cool gasses. Pressure drop testing provided data on pressure drop in Al foams in the compressibility region of air. Up to this point, similar data was not available. Pressure drop testing revealed that with careful design of pore size, ligament thickness and foam thickness, taking into consideration heat transfer capabilities of the foam, a feasible design of an ultra-high temperature open cell foam ring can be achieved with minimal pressure drop and increased heat transfer capabilities.