Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Gall, Kenneth; Committee Member: McDowell, David; Committee Member: Thadhani, Naresh. Part of the SMARTech Electronic Thesis and Dissertation Collection.

M.Ing. (Mechanical Engineering) / High strength alloys such as titanium are widely used within applications that require specific material properties. These include high strength, high temperature as well as low weight applications. Thus a need arises to investigate the fundamental to understand the mechanics of how these materials are machined. Titanium alloys are known for the difficulties that arise during the machining thereof. Complexities arise due to its inherent material properties, the most important property being the retention of strength at high temperatures. In addition to maintaining its strength, it becomes highly chemically reactive with other materials at increased temperatures. All these factors contribute to extreme temperatures at the tool chip interface contributing to increased tool wear and shortened tool life. The aim of the research is to investigate the effect of machining on various cutting process parameters including cutting force, temperature, tool wear and surface finish for grade 2 and grade 5 titanium alloys during high speed turning. Grade 2 titanium is a commercially grade with lower mechanical properties, while Grade 5 is titanium alloy with substantially higher mechanical properties and is the most widely used titanium alloy. In addition an experimental setup was developed and verified to conduct fundamental research on the high speed machining of titanium alloys. A literature review was concluded with focus on the machining of titanium alloys. This was followed by the development of the experimental setup, measurement and compilation of data. The data was compiled into graphs and compared with the current research available. The research found that for the cuts performed, that cutting forces are independent of cooling applied and that no substantial variation was noted between the two grades. When temperatures were evaluated, dramatic drops in temperature were noted when coolant was applied. As temperatures increased, specifically during un-cooled cutting, the inserts deteriorated having an effect on the quality of the surfaces obtained. When coolant was applied, substantial temperature drops were achieved, improving tool life and directly improving surface finishes. The best surface finish was achieved for higher cutting speeds as and lower feed rates. This phenomenon was found for both grades of titanium evaluated. The largest amount of tool wear was noted for the highest cutting speeds, with increased values noted for Grade 5 in comparison with Grade 2. This phenomenon is noted for crater as well as flank wear.

The successful use of Nickel-Titanium (Nitinol) in biomedical applications requires an accurate control of its unique mechanical properties. The purpose of this study is to analyze the effects of a wide range of heat treatments on the mechanical behavior of hot-rolled and cold-drawn Nitinol. Results comprise an understanding of the effect of heat treatment temperature and time variation on final material response which is imperative for optimization of material properties. Thirty-three heat treatment variations are tested by combining three durations, 10 minutes, 90 minutes, and 8 hours, with eleven different heat treatment temperatures between 200°C and 440°C. Following heat treatment, the Nitinol samples undergo tensile testing with upper plateau strength, lower plateau strength, ultimate tensile strength, strain to failure, and residual elongation compared for all test groups. Heat treatment "power" is used to describe the efficacy of different combinations of heat treatment temperature and duration. When using hot-rolled Nitinol, results show a low heat treatment power does not create significant precipitation hardening or a significant decrease in martensite transformation stress, resulting in a high upper plateau strength, high residual strain values, and evidence of plastic deformation upon unloading. Moderate power treatments lead to sufficient hardening of the material and a decrease in martensite transformation stress resulting in a pseudoelastic response. Increasing to a high treatment power further decreases the transformation stress and increases the martensite transformation temperature leading to a shape-memory response in hot rolled Nitinol. When using cold-drawn Nitinol, low and moderate heat treatment power levels result in the material exhibiting a pseudoelastic response. Increasing heat treatment power shows the same effects on martensite transformation stress and temperature as seen with the hot-rolled material resulting in a material response transition from pseudoelastic to shape memory.

The current study focuses on phase stability and evolution in the titanium-zinc titanium-copper and titanium-antimony systems. The study utilized the Laser Engineering Net Shaping (LENS™) processing technique to deposit compositionally graded samples of three binary system in order to allow the assessment of phase stability and evolution as a function of composition and temperature the material is subjected to. Through LENS™ processing it was possible to create graded samples from Ti-xSb (up to 13wt%) and Ti-xCu (up to 16wt%). The LENS™ deposited gradient were solutionized, and step quenched to specific aging temperature, and the resulting microstructures and phase were characterized utilizing XRD, EDS, SEM, FIB and TEM. The Ti-Zn system proved incapable of being LENS™ deposited due to the low vaporization temperature of Zn; however, a novel processing approach was developed to drip liquid Zn onto Ti powder at temperatures above β transus temperature of Ti (882 ◦C) and below the vaporization temperature of Zn (907 ◦C). The product of this processing technique was characterized in a similar way as the graded LENS™ depositions. From measurements performed on Ti-Sb it seems that Sb could be a potential α stabilizer in Ti due to the presence of a mostly homogeneous α grains throughout the gradient; however, from XRD it can be understood that a titanium antimonide phase is present. From results obtained from the Ti-Zn samples, it can be surmised that the eutectoid reaction seems to be active, i.e. The eutectoid reaction is kinetically fast, as concluded by the presence of pearlitic structures. Finally, for the Ti-Cu system this work has been attempted to prove or disprove the existence of the Ti3Cu through the use of XRD and TEM SAD patterns. From XRD spectra collected there are peaks belonging to the Ti3Cu orthorhombic phase along with Ti2Cu and α-Ti phase. In addition to the Ti-Cu system displayed structures associated with divorced eutectoid decomposition mechanism, and at low undercooling seems to be prone to forming solid state dendrites.

Orientador: Rubens Caram Junior / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-23T09:33:56Z (GMT). No. of bitstreams: 1 Lopes_EderSocratesNajar_D.pdf: 9279894 bytes, checksum: a2ed77cd79f48134e7daf5bc705afb0e (MD5) Previous issue date: 2013 / Resumo: Ligas de Ti do tipo ? metaestável exibem comportamento singular no tocante à possibilidade de manipulação de seu comportamento mecânico. Nessas ligas, a rigidez pode ser alterada por meio do controle das fases presentes, o que permite que esses materiais sejam incluídos no seleto grupo dos materiais com gradientes funcionais (functionally graded materials - FGM). Neste trabalho, objetivou-se projetar, produzir, processar, caracterizar e aplicar ligas Ti-Nb com adições do elemento ? estabilizador de baixo custo Fe. As ligas foram produzidas por fusão a arco voltaico e submetidas a diversas condições de tratamento térmico, incluindo solubilização acima da temperatura ?-transus, resfriamento rápido até a temperatura ambiente e tratamentos térmicos de envelhecimento entre 260 °C e 400 °C. A caracterização envolveu calorimetria diferencial de varredura, difração de raios-X e dureza Vickers em alta temperatura, análises metalográficas e de módulo de elasticidade por técnicas acústicas, ensaios de tração e mapeamento de rigidez por meio de nanoindentação. Os resultados obtidos mostram a retenção completa da fase ? para a liga Ti-30Nb-3Fe solubilizada e resfriada rapidamente e permitem também, compreender a decomposição da fase martensita ?" e a nucleação das fases ? e ?. A adição do elemento Fe tornou a cinética de transformação de fases bem mais lenta, propiciando, sob algumas condições, a supressão da fase ?, que é conhecida por aumentar a dureza em detrimento da ductilidade. Tais resultados foram utilizados no estabelecimento de condições ideais de processamento que permitem obter componentes ortopédicos com rigidez gradual. Finalmente, o conhecimento adquiro foi usado na fabricação de placas e parafusos de osteossíntese a partir da liga Ti-30Nb-3Fe com gradientes de funcionalidade / Abstract: Metastable ? Ti alloys exhibit singular behavior regarding the possibility of manipulating their mechanical behavior. In these alloys, the stiffness can be changed by controlling phases, which allows these materials to be included in the select group of functionally graded materials (FGM). This study aimed to design, produce, process, characterize and apply Ti-Nb alloys with addition of Fe, an inexpensive ? stabilizing element. These alloys were produced by arc melting and subjected to different heat treatment conditions, including solution above the ?- transus temperature, quenching to room temperature and aging heat treatments between 260 °C and 400 °C. Characterization involved differential scanning calorimetry, X-ray diffraction and Vickers hardness at high temperature, metallographic analyzes, elastic modulus by acoustic techniques, tensile test and stiffness mapping via nanoindentation. The results obtained show complete retention of the ? phase in the Ti-30Nb-3Fe alloy after solution in the ? field and rapid cooling to room temperature and also allow understanding martensite ?" decomposition and nucleation of the ? and ? phases. It was found that Fe additions make the phase transformation kinetics much slower, providing, for some conditions, suppression of the ? phase precipitation, which is known to increase the hardness at the expense of ductility. These results were used to establish optimal processing conditions, resulting in orthopedic component with graded stiffness. Finally, the knowledge acquired was used in the manufacturing of osteosynthesis plates and screws with functionality graded from the Ti-30Nb-3Fe / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

Thesis (M. Tech. (Engineering: Mechanical)) -- Central University of Technology, Free State, / The Centre for Rapid Prototyping and Manufacturing (CRPM) at the Central University of Technology, Free State (CUT) manufactures implants using Electro Optical Systems (EOS) titanium Ti-6Al-4V alloy powder (further referred to as EOS Ti64 powder) by means of Direct Metal Laser Sintering (DMLS) process on the EOSINT M 270 machine. For this reason, there is a need to characterise and acquire knowledge of the basic properties of direct metal laser sintered EOS titanium Ti-6Al-4V alloy samples (further referred to as DMLS Ti64 samples) under static tensile loading in order to provide the CRPM with engineering design data. The first objective of this Master’s study is to acquire the characteristics of EOS Ti64 powder in order to ascertain its suitability in the DMLS process. Secondly, the study aims to assess tensile properties and elastic constants of DMLS Ti64 samples produced from the set process parameters of EOSINT M 270 machine. Thirdly, it is to investigate microstructures of DMLS Ti64 samples subjected to different heat treatment techniques which will eventually assist in the determination of a suitable heat treatment technique that will yield higher ductility. Finally, the study aims to validate the static behaviour of DMLS Ti64 samples subjected to the static tensile loading up to a yield point in order to determine failure due to yielding. The samples were manufactured at CRPM Bloemfontein. The metallographic examinations, heat treatment and the determination of mechanical properties were done at the CSIR in Pretoria. Optical Microscope (OM) and Scanning Electron Microscope (SEM) were used to determine microstructures of DMLS Ti64 samples while Energy Dispersive X-Ray (EDX) analyses were performed using SEM. The samples were heat treated at temperatures of 700, 1000 and 1100°C respectively, and subsequently either cooled with the furnace, air or were water quenched. The mechanical property tests included tensile, hardness and determination of elastic constants. The static behaviour of DMLS Ti64 samples under static tensile load up to a yield point was predicted and verified using ABAQUSTM Finite Element Analysis (FEA). The stress-strain curves from ABAQUSTM were interpreted using MDSolid program. The point of interest was Von Mises yield stress at 0.2% offset, in order to determine failure due to yielding. EOS Ti64 powder particles were spherical in shape and the alpha and alpha+beta phases were identified. As-laser sintered samples possess a very fine and uniform alpha case with islands of martensitic plates; samples were brittle and showed low levels of ductility with an average elongation of 2.6% and an area reduction of 3.51%. Ultrasonic test results showed that DMLS Ti64 samples have Young’s modulus of 115 GPa, Shear modulus of 43 GP, a bulk modulus of 109 GPa and Poisson’s ratio of 0,323 while the density was 4.4 g/cm3. Slow cooling of DMLS Ti64 samples from 1000 and 1100oC resulted in a microstructure constituted more by the alpha phase of lower hardness than those from 700oC and as-laser sintered samples. High hardness was obtained by water quenching. The water quenched samples showed martensitic transformation and high hardness when compared to furnace cooled samples. Beta annealing tailored a microstructure of as-laser sintered samples into a lamellar structure with different lath sizes as per cooling rate. Beta annealing improved ductility levels up to 12.67% elongation for samples furnace cooled for 4 hours and even higher to 18.11% for samples furnace cooled for 34 hours, while area reduction increased to 25.94% and 33.39%, respectively. Beta annealing conversely reduced yield strength by 19.89% and ultimate tensile strength was reduced by 23.66%. The calculated maximum Von Mises stresses found were similar to the FEA interpreted results. The average percentage error, without the stress concentration factor, was approximately 8.29%; with the stress concentration factor included, it was 0.07%. The small reaction forces induced in both x-axis and z-axis contributed to this error of 0.07% between the calculations and ABAQUSTM FEA results. Samples that were not heat treated fell outside the Von Mises criterion and failed due to yielding. This justified the brittleness found in the tensile test results where elongation and area reduction were 2.6% and 3.51% respectively. However, all samples that were heat treated fell within the Von Mises criterion. The objectives of this study were achieved; the mechanical properties were similar to those of standard specification for wrought annealed Ti-6Al-4V alloy for surgical implant applications and EOS GmbH manufacturer’s material data sheet. DMLS Ti64 samples must be beta annealed in order to attain higher levels of ductility. A recommendation was made to further investigate the effect of heat treatment on the other mechanical properties. Furthermore, detailed results of basic properties of DMLS Ti64 samples are provided in the appendices in chart format and were written on a CD disc.

Over the last few decades, body-centered-cubic (bcc) beta (β) titanium alloys have largely been exploited as structural alloys owing to the richness in their microstructural features. These features, which lead to a unique combination of high specific strength and ductility, excellent hardenability, good fatigue performance, and corrosion resistance, make these alloys viable candidates for many applications, including aerospace, automobile, and orthopedic implants. The mechanical properties of these alloys strongly depend on the various phases present; which can be controlled by thermomechanical treatments and/or alloy design. The two most important and studied phases are the metastable ω phase and the stable α phase. The present study focuses on the microstructural evolution and the mechanical behavior of these two phases in a model β-Ti alloy, binary Ti-12wt. %Mo alloy, and a commercial β-Ti alloy, β-21S. Microstructures containing athermal and isothermal ω phases in the binary Ti-12wt. %Mo alloy are obtained under specific accurate temperature controlled heat treatments. The formation and the evolution of the ω-phase based microstructures are investigated in detail via various characterization techniques such as SEM, TEM, and 3D atom probe tomography. The mechanical behavior was investigated via quasi-static tensile loading; at room and elevated temperatures. The effect of β phase stability on the deformation behavior is then discussed. Similar to the Ti-12wt. %Mo, the formation and the evolution of the athermal and isothermal ω phases in the commercial β-21S alloy was studied under controlled heat treatments. The structural and compositional changes were tracked using SEM, TEM, HR-STEM, and 3D atom probe tomography (3D-APT). The presence of additional elements in the commercial alloy were noted to make a considerable difference in the evolution and morphology of the ω phase and also the mechanical behavior of the alloys. The Portevin-Le Chatelier (PLC) like effect was observed in iii this alloy at elevated temperature and this has been attributed to the shearing of the ω precipitates and the dynamic precipitation of the α phase within these channels. The formation of the stable α phase in the commercial β-21S alloy due to the influence of precursor phases, like the metastable ω phase, is investigated. It is evident from the microstructural characterization, using SEM, TEM, HR-STEM, and 3D-APT, that the ω phase does play a role on the fine scale α precipitation. The mechanical behavior of the β+α microstructure, investigated via tensile testing, shows that these alloys are ideal candidate for precipitation hardening. The exceptional strength values obtained in this alloy have been attributed to a combination of several factors.