Diseño y procesado de aleaciones de Titanio mediante técnicas pulvimetalúrgicas avanzadas

En esta tesis doctoral se plantea el diseño de aleaciones base titanio y su procesado mediante técnicas pulvimetalúrgicas tanto convencionales como avanzadas con el objetivo de obtener materiales con propiedades comparables a las aleaciones fabricadas por técnicas convencionales. Las aleaciones elegidas para el estudio han sido la Ti–6Al–4V, típica de la industria aeronáutica, la Ti–6Al–7Nb, característica de las aplicaciones biomédicas, la Ti–3Al–2,5V, normalmente empleada en los dos sectores anteriores, y se ha procesado también titanio elemental como referencia. El primer paso ha sido establecer la forma de obtener las composiciones deseadas puesto que exclusivamente la aleación más conocida, es decir la Ti–6Al–4V, está disponible en forma de polvos prealeados; por lo tanto, ha sido necesario diseñar la forma de añadir los aleantes y comprobar su viabilidad estudiando la mezcla elemental de polvos y el empleo de aleaciones maestras. En segundo lugar se realizó la búsqueda de las materias primas adecuadas, tendiendo en cuenta el contenido de elementos intersticiales, el coste y las características físico–químicas de los polvos y, a continuación, se realizó la caracterización de los polvos, tanto prealeados como los producidos por mezcla elemental o molienda de alta energía, como etapa previa a su procesado. El procesado se ha llevado a cabo utilizando distintas técnicas: (1) compactación uniaxial en frío y sinterización (P&S), (2) compactación isostática en caliente (HIP) y (3) compactación uniaxial en caliente tanto convencional (HP) como inductiva (IHP). La descripción de todos estos pasos se encuentra en el capitulo de procedimiento experimental (Capitulo 2). Para el estudio de la ruta 1 (P&S), se ha realizado un estudio preliminar de sinterabilidad, variando la temperatura de sinterización en el intervalo 900–1400ºC y manteniendo el tiempo de sinterización en 120 minutos, considerando compactos de geometría rectangular para determinar las propiedades de flexión, cuyos resultados se detallan en el capitulo 5. Previamente, se ha estudiado la selección del soporte de sinterización adecuado para evitar o minimizar la interacción con los componentes de titanio (Capitulo 4). A partir del estudio preliminar, se ha realizado un estudio más detallado limitando el intervalo de temperatura de sinterización a 1250–1350ºC pero considerando el efecto del tiempo de meseta a la máxima temperatura (Capitulo 6). En los materiales procesados se determinaron propiedades físicas, químicas, mecánicas, microestructurales, eléctricas y térmicas que se emplean como referencia para la comparación con los resultados obtenidos cuando se procesan las aleaciones mediante técnicas pulvimetalúrgicas avanzadas. En general, la ruta P&S permite obtener aleaciones de titanio con elevada densidad relativa y propiedades de tracción equiparables a las de las respectivas aleaciones obtenidas por metalurgia convencional. La ruta 2 (HIP) se ha empleado como etapa de postprocesado con el objetivo de reducir la porosidad residual de los materiales obtenidos mediante la vía 1 (P&S). La selección de los parámetros de procesado influye notablemente en el comportamiento mecánico debido a los cambios microestructurales asociados a las diferentes condiciones (Capitulo 6). En el caso de la ruta 3 (HP o IHP), el parámetro que se ha modificado es la temperatura de conformado y el objetivo que se persigue es obtener piezas completamente densas a temperaturas menores y tratando de limitar el tamaño de grano de la microestructura (Capitulo 6). Mediante el desarrollo de esta tesis se ha demostrado que el método de aleación maestra permite obtener propiedades equiparables a las de los polvos prealeados, que suelen ser más costosos, y se propone como técnica para poder diseñar aleaciones a medida y fabricar materiales cuya composición no está disponible actualmente en el mercado. Además, el conformado de los polvos mediante las diferentes técnicas pulvimetalúrgicas permite obtener un gran abanico de propiedades mecánicas comparables o superiores a las de las respectivas aleaciones fabricadas por metalurgia convencional ajustables para aplicaciones específicas. ---------------------------------------------------------------------------------------------------------------------------------------------------This PhD thesis deals with the design of titanium based alloys and their fabrication by means of conventional and advanced powder metallurgy techniques with the aims of producing materials with final properties comparable to those of the respective alloys fabricated by ingot metallurgy. More in detail, the materials studied includes the Ti–6Al–4V alloy, which is normally employed in the aerospace industry, the Ti–6Al–7Nb alloy, which was developed for the production of biomedical devices, the Ti–3Al–2.5V conventionally used on both the previous mention industries, as well as elemental titanium manufactured as reference material. First at all it was decided the way to obtain the desired compositions since exclusively the well–known Ti–6Al–4V alloy is available as prealloyed powder; therefore, the way to add the alloying elements had to be planned and checked considering the blending elemental as well as the master alloy addition approaches. Secondly, the search of the right alloying elements on the base of the interstitials content, of the costs and of the powder features followed by the characterisation of the purchased (prealloyed) as well as produced (blended elemental or high–energy milled) powders was done before proceeding with the shaping of the powders. The manufacturing of the components was done by means of different powder metallurgy techniques: (1) cold uniaxial pressing and sintering (P&S), (2) hot isostatic pressing (HIP) and (3) uniaxial hot–pressing either conventional (HP) or inductive (IHP). The description of the production, characterisation and shaping of the powder can be found in the experimental procedure chapter (Chapter 2). For the study of the route number 1 (P&S), a preliminary sinterability study was carried out ranging the sintering temperature in the 900–1400ºC range, keeping the sintering time constant at 120 minutes and considering rectangular shaped specimens to perform three–point bending tests and measure the bending properties, whose results can be found in Chapter 5. Previously, the selection of the appropriate sintering tray to avoid or, at least, minimize the interaction with the titanium components was done (see Chapter 4). On the base of the preliminary sinterability study, a more detailed sinterability study limiting the sintering temperature range (1250–1350ºC) but considering the influence of the dwell time at temperature was carried out (see Chapter 6). Physical, chemical, mechanical and microstructural analyses as well as thermal and electrical properties were measured on the specimens sintered under diverse conditions and the results are kept as reference for the comparison with the properties obtained when processing the materials with advanced powder metallurgy techniques. Generally, the P&S route allows to obtain titanium alloys with high level of relative density and mechanical properties similar to those of the respective alloys obtained by ingot metallurgy. The route number 2 (HIP) was employed as secondary process with the aim of reducing the residual porosity of the specimens attained by means of the P&S technique. The selection of the processing parameters, namely the temperature, the time and the pressure influences significantly the mechanical behaviour due to the microstructural changes that they induce (Chapter 6). On the case of the uniaxial hot–pressing (both HP and IHP) the sintering temperature was changed in order to manufacture fully dense materials at lower temperatures compared to other processes trying to limit the mean grain size of the microstructural features (Chapter 6). By means of the development of this doctoral thesis it could be demonstrated that the master alloy addition approach permits to obtain materials with mechanical properties comparables to those of the prealloyed powders, which are normally more expensive, and it is proposed as production method to design alloy with the desired composition or as manufacturing method of conventional alloys not commercially available. Moreover, the processing of the titanium powders by means of different powder metallurgy techniques permits to obtain a great range of mechanical properties comparable o higher than the respective alloys fabricated by ingot metallurgy and tailorable for specific applications

Low-Cost α+β PM Ti Alloys by Fe/Ni Addition to Pure Ti

Ti and its alloys can deliver a very interesting combination of properties such as low density, high strength, corrosion resistance and biocompatibility and, therefore, are very flexible materials which can be adapted to various applications. Nonetheless, Ti and Ti alloys are only employed in critical applications (i.e. aeronautical and aerospace, nautical, medical, etc.) or in products for leisure. In both of these cases the higher fabrication costs of Ti in comparison to its competitors (i.e. steel and aluminium) is not the limiting factor as it is for many structural applications, especially for mass production (i.e. automotive sector). The use of creative techniques and the decrement of the starting price of Ti have been identified as the two main routes to follow to decrease the fabrication costs. In this study, the production of low-cost α+β Ti alloys has been assessed by combining the addition of cheap alloying elements (in particular a Fe/Ni powder) with the classical powder metallurgy route (pressing and sintering). Physical and mechanical properties as well as microstructural analysis of these low-cost alloys were measured and correlated to the processing parameters used to sinter them. It is found that the low-cost Ti alloys show similar behaviour to conventional α+β Ti alloys and, thus, have the potential to be used for non-critical applications

Prediction of the mechanical properties of isotropic pure metal-based and two-phase alloy-based porous materials using modified analytical models

This work investigates the applicability and accuracy of the five fundamental analytical models commonly used to estimate the thermophysical properties of porous materials for the prediction of the mechanical behaviour, both compressive and tensile, of isotropic pure metal-based and two-phase alloy-based porous materials. In literature, the prediction of the mechanical behaviour of these advanced engineering materials requires the development of semi-empirical models, which are material-specific and, thus, require empirical constants. The significance of the current investigation is the possibility to optimise the mechanical behaviour of porous metallic materials through the rapid and accurate prediction of their mechano-physical behaviour using non-empirical physically-based prediction models. The work is complemented with the derivation of new combined models with increased accuracy prediction of up to approx. 90% with respect to fundamental models. Although developed for porous materials, the derived combined models could be applied for the accurate and rapid prediction of the thermophysical and mechanical properties of multi-phase materials with unknown microstructure such as two-ductile-phase alloys, nanocomposites, hetero- and harmonic-structured materials, and immiscible alloys

Effect of Mn on the Properties of Powder Metallurgy Ti-2.5Al-xMn Alloys.

Titanium alloys are the ideal material for a wide range of structural applications, but their high cost compared to other metals hinders their adoption. Powder metallurgy and cheap alloying elements can be used to create new Ti alloys. In this study, the simultaneous addition of Al and Mn is considered to manufacture and characterise ternary Ti-2.5Al-Mn alloys obtained via pressing and sintering by varying the Mn content (1-10 wt.%). It is found that the addition of the alloying elements reduces compressibility. Consequently, the amount of porosity increases (8.5 → 10.8%) with the amount of Mn as the alloys were processed under the same conditions. The progressive addition of Mn refines the classical lamellar microstructure and, eventually, transforms it into an equiaxed β-grain structure with acicular α grains. The microstructural changes lead to continuous increases in strength (ultimate tensile strength: 694 → 851 MPa) and hardness (225 → 325 HV30) with an associated loss of ductility (elongation to failure: 13.9 → 1.0%). However, the obtained ternary Ti-2.5Al-Mn alloys have similar or better overall mechanical behaviour than most of the binary Ti-Mn alloys obtained through a variety of manufacturing methods

Influence of vacuum hot-pressing temperature on the microstructure and mechanical properties of Ti—2.5V alloy obtained by blended elemental and master alloy addition powders

This study addresses the processing of near-net-shape, chemically homogeneous and fine-grained Ti–3Al–2.5V components using vacuum hot-pressing. Two Ti–3Al–2.5V starting powders were considered. On one side, hydride-dehydride (HDH) elemental titanium was blended with an HDH Ti–6Al–4V prealloyed powder. On the other side, an Al:V master alloy was added to the HDH elemental titanium powder. The powders were processed applying a uniaxial pressure of 30 MPa. The sintering temperatures studied varied between 900 degrees C and 1300 degrees C. The relative density of the samples increased with processing temperature and almost fully dense materials were obtained. The increase of the sintering temperature led also to a strong reaction between the titanium powders and the processing tools. This phenomenon occurred particularly with boron nitride (BN) coating, which was used to prevent the direct contact between titanium and graphite tools. The flexural properties of the Ti–3Al–2.5V samples increased with vacuum hot-pressing temperature and are comparable to those specified for wrought titanium medical devices. Therefore, the produced materials are promising candidates for load bearing applications as implant materials.Financial support from Comunidad de Madrid through the ESTRUMAT (S-2009/MAT-1585) project and from the Spanish Ministry of Education through the R&D MAT2009-14448-C02-02 and MAT2009-14547-C02-02 Projects

Investigation of the factors influencing the tensile behaviour of PM Ti-3Al-2.5V alloy

Titanium, a relatively new engineering metal, has been employed principally in high demanding industries due to its high final cost and it is well known for its biocompatibility. Powder metallurgy (PM) techniques could offer the possibility to reduce the production cost without paying it in terms of mechanical properties, thanks to their intrinsic advantages. In this study the Ti-3Al-2.5V titanium alloy was produced considering two powder production routes and sintered under different temperatures in order to address their feasibility as alternative to the wrought alloy. The results indicate that PM Ti-3Al-2.5V alloys studied have comparable mechanical behaviour as their counterpart obtained by conventional metallurgy and, therefore, are potential candidates to fabricate cheaper titanium products for structural applications as well as biomedical devices. © 2014 Elsevier B.V.The authors want to acknowledge the financial support from the Spanish Ministry of Science through the R&D Projects MAT2009-14448-C02-02 and MAT2009-14547-C02-02, and from Regional Government of Madrid through the ESTRUMAT (S2009/ MAT-1585) projectPublicad

Flexural Properties, Thermal Conductivity and Electrical Resistivity of Prealloyed and Master Alloy Addition Powder Metallurgy Ti-6Al-4V

A comparison between the properties achievable by processing the Ti&-6Al&-4V alloys by means of two powder metallurgy approaches, precisely prealloyed and master alloy addition, was carried out. Prealloyed and master alloy addition hydride&-dehydride powders characterised by an irregular morphology were shaped by means of cold uniaxial pressing and high vacuum sintered considering the effect of the variation of the sintering temperature and of the dwell time. Generally, the higher the temperature and the longer the dwell time, the higher the relative density and, consequently, the better the mechanical performances. Nevertheless, a higher processing temperature or a longer time leads also to some interstitials pick-up, especially oxygen, which affects the mechanical behaviour and, in particular, lowers the ductility. Although some residual porosity is left by the pressing and sintering route, mechanical properties, thermal conductivity and electrical resistivity values comparable to those of the wrought alloy are obtained.The authors want to acknowledge the financial support from Regional Government of Madrid through the ESTRUMAT (S2009/ MAT-1585) project and from the Spanish Ministry of Science through the R&D Projects MAT2009-14547-C02-02 and MAT2009-14448-C02-02. The authors want also to thanks the Fraunhofer IFAM-Dresden Institute for the measurements of the thermal conductivity and electrical resistivityPublicad

Powder metallurgy CP-Ti performances: hydride-dehydride vs. sponge

Titanium is characterised by two contrasting aspects: outstanding combination of properties and high production costs which confine its application to high demanding sectors. The employment of powder metallurgy (P/M) techniques is one creative alternative to lower the final costs of titanium products due to some intrinsic advantages of P/M such as high yield of material and limited machining requirement. In this work the performances of hydride&-dehydride (HDH) and sponge elemental titanium products obtained by cold uniaxial pressing and sintering are compared. It is found that the two materials achieved similar relative density values but HDH shows much better mechanical performances.The authors want to acknowledge the financial support from the Spanish Ministry of Science through the R&D Projects MAT2012-38650-C02-01, and from Regional Government of Madrid through the ESTRUMAT (S2009/MAT-1585) projec

Influence of Sintering Parameters on the Properties of Powder Metallurgy Ti-3Al-2.5V Alloy

The processing of near net shape Ti-3Al-2.5V components using the conventional pressing and sintering route is addressed in this study. The Ti-3Al-2.5V starting powder was obtained considering both the blending elemental and the master alloy addition methods. The powders were uniaxially pressed and sintered in a high-vacuum furnace under various temperature-time combinations. The influence of the processing parameters on the relative density, microstructural features, amount of interstitials, mechanical behaviour, thermal conductivity and electrical resistivity of the sintered materials was evaluated. It was found that the relative density of the samples increases with processing temperature and time, and almost fully dense materials were obtained. The mechanical performance of the Ti&-3Al&-2.5V improves due to the reduction of the residual porosity and are, generally, of the same order of magnitude of those required for titanium biomedical products. Furthermore, the temperatures&-times selected permit to obtain thermal and electrical properties similar to the wrought alloy.The financial support from the Spanish Ministry of Science through the R&D Projects MAT2009-14448-C02-02 and MAT2009-14547-C02-02, and from the Regional Government of Madrid through the ESTRUMAT (S2009/MAT-1585) project is acknowledged. The possibility to perform the measurements of the thermal conductivity and electrical resistivity in the Fraunhofer IFAM-Dresden Institute is really appreciated.Publicad

Evaluation of the mechanical properties of powder metallurgy Ti-6Al-7Nb alloy

Titanium and its alloys are common biomedical materials owing to their combination ofmechanical properties, corrosion resistance and biocompatibility. Powder metallurgy (PM) techniques can be used to fabricate biomaterials with tailored properties because changing theprocessing parameters, such as the sintering temperature, products with different level ofporosity and mechanical performances can be obtained. This study addresses the productionof the biomedical Ti-6Al-7Nb alloy by means of the master alloy addition variant of the PMblending elemental approach. The sintering parameters investigated guarantee that thecomplete diffusion of the alloying elements and the homogenization of the microstructure isachieved. The sintering of the Ti-6Al-7Nb alloy induces a total shrinkage between 7.4% and10.7% and the level of porosity decreases from 6.2% to 4.7% with the increment of thesintering temperature. Vickers hardness (280-300 HV30) and tensile properties (differentcombination of strength and elongation around 900 MPa and 3%) are achieved.The authors want to acknowledge the financial support from New Zealand Ministry of Business, Innovation and Employment (MBIE) through the UOWX1402 research contract (TiTeNZ - Titanium Technologies New Zealand)