WPI’s 145th Commencement ceremony was held on the campus Quadrangle, where 781 Bachelor of Science degrees, 358 master’s degrees, and 26 PhDs were awarded. Students, their families and friends, trustees, and special guests were on hand to experience the messages delivered by key note speaker Eric S. Lander, president and founding director of the Broad Institute of MIT and Harvard, and WPI President and CEO Dennis Berkey. This year, Materials Science and Engineering awarded three Doctor of Philosophy degrees and 33 students received their Master of Science degree.
PhD Graduates 2013
MS Graduate Students
MS Thesis Defense
Friction Stir Welding in Wrought and Cast Aluminum and Magnesium Alloys: Micro-structure, Mechanical Properties, Fatigue Crack Growth, and Novel Applications
Student: Andrew Biro
Advisor: Prof. Diana Lados
Friction stir welding (FSW) is a solid state joining process invented in 1991 at The Welding Institute in the UK. This technology employs frictional heating and mechanical stirring forces to plastically deform and mix metals together without causing melting. Thus, there are many applications in the transportation and aerospace industries where FSW can be implemented, including joining automotive body panels to frames, welding of die castings for engine components, and on aluminum sheets for fuselages or wings on aircraft.
As the technology is relatively young compared to conventional fusion welding, there is significant room for process development, optimization and learning in the areas of aluminum and magnesium alloys. In this study, full characterization of FSW wrought aluminum and cast magnesium alloys were conducted. Microstructural investigations (secondary phase evolution and grain refinement), mechanical property analyses (tensile and fatigue crack growth), and residual stress effects were investigated. Microstructure-property correlations were established for the studied alloys, and fatigue crack growth mechanisms in FSWs and predictive models were developed.
Welding processes are inherently damaging to the base material, even beyond the immediate weld zone (i.e., weld zone + heat affected zones). Post weld heat treatments were performed on FSW aluminum and magnesium alloys to evaluate potential benefits on microstructure and mechanical properties. In addition, the applicability of using FSW for joining dissimilar alloys was studied, with successful joints between different wrought aluminum and cast aluminum alloys. Furthermore, investigations into utilizing FSW for die casting joining, porosity mitigation, crack repair, as well as multiple-pass FSWing were also studied.
Study of Hot Tearing in Cast and Wrought Aluminum Alloys
Student: Qinxin Wu
Advisor: Prof. Makhlouf Makhlouf
During the solidification process in casting, hot tearing may occur. It is a severe defect that normally involves the formation of a macroscopic tear, which generates cracks either on the surface or inside the casting. Over the past decades, many strategies have been developed to evaluate the hot tearing tendency. Unfortunately, most of the tests can only provide qualitative information. Therefore, a reliable and quantitative test to evaluate hot tearing in aluminum alloys is highly desirable. To address this issue, WPI and CANMET MTL (both members of the Light Metal Alliance) jointly developed a quantitative hot tearing test and established a specific methodology. Using a constrained rod mold, the hot tearing formation can be quantitatively studied by measuring the contraction force, time and temperature during solidification for a restrained casting or linear contraction, time and temperature for a relaxed casting. This study investigated cast aluminum alloys A380.1 and A390 and wrought aluminum alloys 6061 and 7075. The results show that wrought aluminum alloys have a much stronger hot tearing tendency than cast aluminum alloys based on a quantitative analysis. Also, the study involves the effects of adding strontium and oxides respectively into the cast aluminum alloy A380.1. Compared with the pure A380.1 alloy, the introduction of strontium decreases the hot tearing tendency, while the inclusion of oxide greatly increases the hot tearing. The information obtained through these tests provides a database of hot tearing phenomenon and establishes a new hot tearing ind
Strengthening Aluminum by Zirconium and Chromium
Student: Yan Shi
Advisor: Prof. Makhlouf Makhlouf
The Al-Zr system is used to form a thermally stable strengthening phase in high temperature aluminum-base casting alloys. These alloys have good strength at elevated temperature due to the precipitation of coherent metastable Al3Zr particles upon decomposition of the supersaturated Al-Zr solid solution by a carefully designed heat treatment. Formation of the Al3Zr particles occurs by a peritectic reaction, which decrees that once formed, the particles cannot be dissolved by a solid-state homogenization process. Accordingly, melting the alloy must serve as the homogenization step of the precipitation hardening process; and solidification during casting must serve as the quenching step. Unfortunately, a prohibitively fast solidification rate is necessary to obtain a solid solution with as little as 0.4% Zr in Al. It is found that adding Cr to Al-0.4wt%Zr binary alloy makes it easier to form the supersaturated solid solution, and the ternary Al-0.4wt%Zr-0.8wt%Cr alloy has better room and elevated temperature tensile properties than the binary Al-0.4wt%Zr alloy. Various one-step and two-step isothermal aging cycles were investigated in order to arrive at the optimum aging schedule for the Al-0.4wt%Zr-0.8wt%Cr. It is found that soaking the alloy at 400C for 24 hours is optimum; and employing a two-step aging schedule reduces the aging time without sacrificing strength. The two-step aging schedule includes soaking the alloy at 375C for 3 hours and then at 425C for an additional 12 hours. Examination of the precipitates that form in the Al-0.4wt%Zr-0.8wt%Cr with High Resolution Transmission Electron Microscopy (HRTEM) shows that they have the L12 crystal structure. Energy Dispersive Spectrometry (EDS) shows that the particles contain only aluminum and zirconium whereas the matrix is a solid solution of chromium in aluminum. Hence, it is suggested that zirconium strengthens the Al-0.4wt%Zr-0.8wt%Cr alloy by a precipitation hardening mechanism and chromium further enhances the strength by solid solution strengthening.
Friction Stir Processing of Aluminum Cast Alloys for High Performance Applications
Student: Ning Sun
Advisor: Prof. Diran Apelian
Friction stir processing (FSP) has been developed based on the basic principles of friction stir welding (FSW), a solid-state joining process originally developed for aluminum alloys. What is attractive about FSP is that it can be incorporated in the overall manufacturing cycle as a post-processing step during the machining operation to provide localized modification and control of microstructures in near-surface layers of metallic components. FSP has emerged as an important post-processing technique, and has been identified as a process that may have a high impact, and perhaps is a disruptive manufacturing process. In this study, FSP has been applied to Al cast alloy A206, which is a high strength, widely used cast alloy in the manufacturing industry. Motivations behind this work are to (1) investigate the feasibility of FSP on manipulating the cast microstructure and strengthening the material, and (2) to explore the viability of FSP to produce a localized particle reinforced zone in cast A206 aluminum components.
The presentation will show that we have optimized FSP for processing of Al alloys to locally manipulate the cast microstructure, eliminate casting defects, and attain grain refinement and second phase homogenization. We have established the mechanism leading to the microstructure evolution and have evaluated the resultant mechanical properties, i.e. hardness, tensile property and fatigue properties. We have also synthesized a localized composite material in the A206 work piece with three different reinforcement materials via FSP. These results will be presented and discussed.
Simulation, optimization and development of thermo-chemical diffusion processes
Student: Yingying Wei
Advisor: Prof. Richard D. Sisson, Jr.
Thermo-chemical diffusion processes play an important part in modern manufacturing technologies. They exist in many varieties depending on the type of diffusing element used and the respective process objectives and procedures.
To improve wear and/or corrosion performance of precisely machined steel components, gas nitriding is selected as the most preferred thermo-chemical surface treatment. Conventional nitriding of steels is a multi-hour, sometimes multi-day hardening process carried out at ferritic temperatures and including a complete heat treatment cycle: normalizing, austenitizing, martensitic quenching and tempering. An alternative, subcritical-temperature austenitic nitriding process is evaluated with the purpose of accelerating the treatment and optimizing the hardness and toughness of nitrided layers while minimizing the distortion of steel parts treated. The alternative process involves liquefied nitrogen cryogenic quenching as well as aging. This study presents results of experimental work on AISI 4140 steel, examining the interplay between the nitriding and tempering conditions and phase transformations in both ferritic (525oC) and subcritical, nitrogen-austenitic (610oC) processes. Thermodynamic models, used to design processing conditions, are applied also in the microstructural interpretation of nitrided layers. Results are verified using the SEM, EPMA and EDS techniques. Kinetics of interstitial diffusion and isothermal martensite transformation, as well as dimensional control of nitrided parts are also presented.
Problems with intergranular oxidation (IGO), energy efficiency and carbon footprint of conventional endothermic atmosphere (CO-H2-N2) carburizing is forcing heat treating and manufacturing companies to move toward increasingly capital- and operating-cost expensive, low-pressure (vacuum furnace) carburizing methods. In response, a new activated and alternate carburizing method (A2A carburizing) has recently been developed, bridging the endothermic atmosphere and vacuum processes, where a plasma-activated, oxygen-free, non-equilibrium nitrogen-hydrocarbon gas blend is utilized. The optimization of industrial A2A carburizing processes involves improvement of case uniformity of parts at different locations in the charge as well as between different sides on the parts. Connected to the optimization, a computational fluid dynamics (CFD) study is conducted for examination of gas flow field inside the furnace and trays holding steel parts treated. To mitigate soot in the atmosphere and minimize the poorly carburized contact area between parts, effects of different combinations of nitrogen-hydrocarbons mixture on soot formation in atmosphere, deposition on metal surface and graphite growth at carburizing temperature are investigated. N2-0.4%C3H8-1%CH4 mixture is proven to be able to provide proper carburizing hardened case with less soot in atmosphere, less coke deposition on metal surface, as well as minimized marginally carburized contact zone. A soot formation mechanism for non-equilibrium atmosphere in A2A carburizing is proposed.
Title: Synthesis of Aluminum-Aluminum Nitride Nanocomposites by Gas-Liquid Reactions
Student: Cecilia Borgonovo
Advisor: Prof. Makhlouf Makhlouf
An innovative method has been developed for synthesizing aluminum-aluminum nitridenanocomposite materials wherein the reinforcing nano-sized aluminum nitride particles are formed in-situ in a molten aluminum alloy. This method, which circumvents most issues associated with the traditional ways of making nanocomposites, involves reacting anitrogen-bearing gas with a specially designed molten aluminum alloy. The method ensures excellent dispersion of the nanoparticles in the matrix alloy, which is reflected in enhanced mechanical properties. In this thesis, the author reviews the limitations of the conventional methods of manufacturing nanocomposites and develops thermodynamic and kinetic models that allow optimizing the in-situ gas-liquid process to produce quality nanocomposite material. Also, in this thesis, the author reports the measured room temperature and elevated temperature tensile properties of materials that were made by the optimized process and compares the measured values to their counterparts obtained for the base alloy. A 75 pct. increase in room temperature yield strength is obtained when the base alloy is reinforced with one pct. nano-size aluminum nitride particles and this significant increase in yield strength is accompanied by only negligible loss of ductility.