Publication: Diseño y procesado de aleaciones de Titanio mediante técnicas pulvimetalúrgicas avanzadas
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2011
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2011-11-03
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Abstract
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.
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.
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Keywords
Pulvimetalurgia, Aleaciones de titanio, Ensayo de materiales