Publication: Estudio experimental y numérico del torneado de INCONEL 718 con herramientas PCBN en condiciones de acabado a alta velocidad de corte
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2020-12
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2020-12-17
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Abstract
Los procesos de conformado por arranque de viruta tienen una gran relevancia en la industria de fabricación y siguen presentando grandes desafíos científicos y tecnológicos debido a la gran complejidad de las condiciones termomecánicas producidas en la zona de corte.
Esta tesis se centra en el mecanizado de acabado de Inconel 718. Se trata de una superaleación de base níquel ampliamente usada en la actualidad en distintos campos como el de automoción, aeroespacial, naval etc. Esta aleación se caracteriza por ser un material de muy baja maquinabilidad debido a sus excelentes propiedades mecánicas a elevadas temperaturas, su elevado endurecimiento por deformación y su baja conductividad térmica, que hace que se generen grandes temperaturas y presiones en la interfase entre pieza y herramienta.
Actualmente, las operaciones de acabado de Inconel 718 se realizan empleando herramientas de metal duro con abundante refrigeración. Estas herramientas presentan una elevada tenacidad que permite emplear geometrías de filo agudas y positivas que garantizan un buen control del daño debido a mecanizado. En esta tesis se ha analizado la posibilidad de emplear nuevas herramientas PCBN (Polycrystalline cubic boron nitride) con bajo contenido en CBN, que combinan una elevada dureza a alta temperatura con una tenacidad mayor que la de las herramientas PCBN convencionales. La aplicación de herramientas PCBN permitiría emplear velocidades de corte muy superiores a las de los procesos industriales actuales con herramientas de metal duro.
Se han realizado ensayos de mecanizado con cuatro herramientas PCBN comerciales con distintas características en cuanto a preparación de filo, recubrimiento y contenido de CBN. Asimismo, se hicieron ensayos con herramientas de metal duro para poder realizar un análisis comparativo de los resultados obtenidos con las nuevas herramientas PCBN en relación con los resultados propios de las condiciones de mecanizado empleadas actualmente a nivel industrial. Para confirmar la viabilidad de estos procesos y determinar las condiciones óptimas de mecanizado se consideraron distintas combinaciones de parámetros de corte, así como tres condiciones de refrigeración: seco, aceite soluble a presión convencional y aceite soluble a alta presión.
Para todas las condiciones de mecanizado y herramientas indicadas se determinó y analizó la evolución del desgaste de la herramienta, la rugosidad superficial, la tasa de eliminación de material y las componentes de la fuerza de mecanizado.
Con el fin de profundizar en el conocimiento de estos procesos mediante el análisis de los parámetros térmicos y mecánicos en la zona de corte se desarrolló y validó experimentalmente un modelo numérico basado en elementos finitos para estudiar el efecto en el mecanizado de las dos preparaciones de filo que dieron mejores resultados en los ensayos de desgaste.
Las principales conclusiones extraídas a partir de los resultados obtenidos fueron las siguientes:
- Las herramientas PCBN no son aplicables al acabado en seco de Inconel 718 debido a la inestabilidad del corte en estas condiciones que provoca su rápido desgaste por roturas frágiles.
- Por el contrario, se ha demostrado la viabilidad de emplear herramientas PCBN en condiciones de mecanizado con fluido de corte a presión convencional y alta presión. Aunque la duración de estas herramientas es notablemente inferior a la de las herramientas de carburo, las elevadas velocidades de corte que permiten las herramientas PCBN permiten alcanzar mayores superficies mecanizadas por filo y por unidad de tiempo.
- Las herramientas PCBN tienen un mejor comportamiento que las herramientas de metal duro en cuanto al acabado superficial obtenido. No se observan diferencias significativas entre los resultados de acabado superficial en los procesos con refrigeración a presión convencional y a alta presión.
- En las operaciones de acabado con herramienta PCBN, emplear refrigeración a alta presión incrementó significativamente la vida de las herramientas.
- La preparación del filo tiene una influencia relevante en el comportamiento de las herramientas PCBN en acabado de Inconel 718. Afilados menos agudos provocaron mayores fuerzas de mecanizado y una menor duración de la herramienta. En los ensayos realizados, los mejores resultados se obtuvieron para una herramienta con un afilado de tipo elíptico que combina una geometría robusta con fuerzas de mecanizado reducidas propias de afilados agudos.
- Los resultados numéricos con herramientas con distintas preparaciones de filo muestran que el incremento de fuerzas de mecanizado para las herramientas con afilados menos agudos se debe principalmente a que aumenta la altura del punto de estancamiento y por tanto la cantidad de material que fluye bajo el filo y provoca un mayor empuje por recuperación elástica (ploughing force) Este fenómeno explica el menor desgaste de flanco observado experimentalmente para las herramientas con afilados más agudos. Asimismo, en las simulaciones numéricas con la herramienta con mayor radio de filo aparente se obtuvieron mayores temperaturas en la superficie mecanizada y mayores deformaciones en el material.
- En los modelos numéricos se obtuvieron mayores temperaturas en la herramienta al incrementar el espesor de viruta no deformada. Este resultado es coherente con la aparición de mayores desgastes de cráter en los ensayos realizados con mayor avance y profundidad de pasada.
Machining processes are very relevant in the manufacturing industry and still present high scientific and technological challenges due to the great complexity of the thermomechanical conditions produced in the cutting zone. This thesis is focused on the finishing machining of Inconel 718. This is a nickel-based superalloy widely used today in different fields such as automotive, aerospace, naval, etc. This alloy is characterized by a very low machinability due to the excellent mechanical properties at high temperatures, high strain hardening and low thermal conductivity, which causes high temperatures and pressures at the interface between part and tool. Currently, finishing operations of Inconel 718 are performed using carbide tools with abundant coolant. These tools have a high level of toughness that allows the use of sharpened and positive cutting edge geometries that ensure proper damage control during machining. In this thesis, the possibility of using new PCBN (Polycrystalline cubic boron nitride) tools with a low CBN content has been analyzed. These tools combine high hardness at high temperature with higher toughness than conventional PCBN tools. The application of PCBN tools would allow the use of cutting speeds much higher than those of current industrial processes with carbide tools. Machining tests have been carried out with four commercial PCBN tools with different characteristics in relation to cutting edge preparation, coating and CBN content. Also, tests were carried out with hard metal tools in order to provide a comparative analysis of the results obtained with the new PCBN tools in relation to the results of the machining conditions currently used at industrial level. To confirm the viability of these processes and to determine the optimum machining conditions, different combinations of cutting parameters were considered, as well as three cooling conditions: dry, conventional pressure soluble oil and high pressure soluble oil. For all machining conditions and tools indicated, the evolution of tool wear, surface roughness, material removal rate and machining force components were determined and analyzed. In order to increase our knowledge of these processes by analyzing the thermal and mechanical parameters in the cutting area, a numerical model based on finite elements was developed and experimentally validated to study the effect on machining of the two edge preparations that gave the best results in the wear tests. The main conclusions reached from the results obtained were the following: - PCBN tools are not applicable to finishing Inconel 718 under dry conditions due to the instability of the cut under these conditions which causes rapid wear from fragile breaks. - On the contrary, the viability of using PCBN tools in machining conditions with conventional pressure cutting fluid and high pressure has been demonstrated. Although the duration of these tools are significantly lower than that of carbide tools, the high cutting speeds that PCBN tools allow to reach higher machined surfaces by edge and by unit of time. - PCBN tools have a better behavior than carbide tools in terms of the surface finish obtained. There are no significant differences between the surface finish results in the processes with conventional and high pressure cooling. - In the finishing operations with PCBN tools, the use of high pressure cooling increased significantly the life of the tools. - Cutting edge preparation has a significant influence on the behavior of PCBN tools for finishing Inconel 718. Less sharp edges ended in higher machining forces and shorter tool life. In the tests conducted, the best results were obtained for a tool with an elliptical cutting edge that combines robust geometry with reduced machining forces typical of sharp cutting edges. - The numerical results with different cutting edge preparations show that the increase in machining forces for tools with less sharp cutting edges is mainly due to the higher height of the stagnation point and therefore the amount of material that flows under the edge and causes a higher ploughing force. This phenomenon explains the lower flank wear observed experimentally for tools with sharper cutting edges. Also, in numerical simulations with the tool with the biggest apparent cutting edge radius, a higher temperature on the machined surface and a higher deformation of the material were obtained. - In the numerical models, higher tool temperatures were obtained when increasing the undeformed chip thickness. This result is consistent with the appearance of higher crater wear in the tests carried out with higher feed rates and depth of pass.
Machining processes are very relevant in the manufacturing industry and still present high scientific and technological challenges due to the great complexity of the thermomechanical conditions produced in the cutting zone. This thesis is focused on the finishing machining of Inconel 718. This is a nickel-based superalloy widely used today in different fields such as automotive, aerospace, naval, etc. This alloy is characterized by a very low machinability due to the excellent mechanical properties at high temperatures, high strain hardening and low thermal conductivity, which causes high temperatures and pressures at the interface between part and tool. Currently, finishing operations of Inconel 718 are performed using carbide tools with abundant coolant. These tools have a high level of toughness that allows the use of sharpened and positive cutting edge geometries that ensure proper damage control during machining. In this thesis, the possibility of using new PCBN (Polycrystalline cubic boron nitride) tools with a low CBN content has been analyzed. These tools combine high hardness at high temperature with higher toughness than conventional PCBN tools. The application of PCBN tools would allow the use of cutting speeds much higher than those of current industrial processes with carbide tools. Machining tests have been carried out with four commercial PCBN tools with different characteristics in relation to cutting edge preparation, coating and CBN content. Also, tests were carried out with hard metal tools in order to provide a comparative analysis of the results obtained with the new PCBN tools in relation to the results of the machining conditions currently used at industrial level. To confirm the viability of these processes and to determine the optimum machining conditions, different combinations of cutting parameters were considered, as well as three cooling conditions: dry, conventional pressure soluble oil and high pressure soluble oil. For all machining conditions and tools indicated, the evolution of tool wear, surface roughness, material removal rate and machining force components were determined and analyzed. In order to increase our knowledge of these processes by analyzing the thermal and mechanical parameters in the cutting area, a numerical model based on finite elements was developed and experimentally validated to study the effect on machining of the two edge preparations that gave the best results in the wear tests. The main conclusions reached from the results obtained were the following: - PCBN tools are not applicable to finishing Inconel 718 under dry conditions due to the instability of the cut under these conditions which causes rapid wear from fragile breaks. - On the contrary, the viability of using PCBN tools in machining conditions with conventional pressure cutting fluid and high pressure has been demonstrated. Although the duration of these tools are significantly lower than that of carbide tools, the high cutting speeds that PCBN tools allow to reach higher machined surfaces by edge and by unit of time. - PCBN tools have a better behavior than carbide tools in terms of the surface finish obtained. There are no significant differences between the surface finish results in the processes with conventional and high pressure cooling. - In the finishing operations with PCBN tools, the use of high pressure cooling increased significantly the life of the tools. - Cutting edge preparation has a significant influence on the behavior of PCBN tools for finishing Inconel 718. Less sharp edges ended in higher machining forces and shorter tool life. In the tests conducted, the best results were obtained for a tool with an elliptical cutting edge that combines robust geometry with reduced machining forces typical of sharp cutting edges. - The numerical results with different cutting edge preparations show that the increase in machining forces for tools with less sharp cutting edges is mainly due to the higher height of the stagnation point and therefore the amount of material that flows under the edge and causes a higher ploughing force. This phenomenon explains the lower flank wear observed experimentally for tools with sharper cutting edges. Also, in numerical simulations with the tool with the biggest apparent cutting edge radius, a higher temperature on the machined surface and a higher deformation of the material were obtained. - In the numerical models, higher tool temperatures were obtained when increasing the undeformed chip thickness. This result is consistent with the appearance of higher crater wear in the tests carried out with higher feed rates and depth of pass.
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Keywords
Tecnología de fabricación, Máquinas herramienta, Proceso de mecanizado, Ensayo de materiales, Superaleación de níquel-cromo Inconel 718, Torneado, Método de Elementos Finitos (MEF), Simulación