Microstructural evolution of LMDp Ti-6Al-4V: effect of time and temperature during heat treatment

Abstract

L'objectiu d'aquest treball és analitzar i comprendre com les temperatures isotèrmiques i els temps durant el tractament tèrmic poden afectar el Ti-6Al-4V fabricat amb LMDP (Laser Metall Deposition - powder). Per a aquest treball, es van construir mostres de Ti-6Al-4V LMDp sobre una placa de Ti-6Al-4V forjada. A causa de la microestructura totalment martensítica obtinguda després de la fabricació, es va considerar necessari realitzar tractaments tèrmics. Les mostres es van sotmetre a diferents tractaments tèrmics i es van analitzar mitjançant una sèrie de tècniques de caracterització microestructural, incloent difracció de raigs X, mesures de duresa i anàlisi d'imatges. Es van mesurar la mida de les agulles de fase alfa, la mida de gra β, la fracció de fases, l'alpha case, les tensions residuals, la microduresa i la mida dels cristalls per avaluar les propietats del material. A més, es va desenvolupar una metodologia de simulació per predir la microestructura resultant després del tractament tèrmic. S'ha observat que la temperatura isotèrmica i la velocitat de refredament tenen un paper important en la microestructura resultant, mentre que el temps de manteniment i l'addició d'un tractament de preescalfament tenen, comparativament, un efecte menor.

El objetivo de este trabajo es analizar y comprender cómo las temperaturas isotérmicas y los tiempos durante el tratamiento térmico pueden afectar al Ti-6Al-4V fabricado con LMDp (Laser Metal Deposition - powder). Para este trabajo, se construyeron muestras de Ti-6Al-4V LMDp sobre una placa de Ti-6Al-4V forjada. Debido a la microestructura totalmente martensítica obtenida tras la fabricación, se consideró necesario realizar tratamientos térmicos. Las muestras se sometieron a diferentes tratamientos térmicos y se analizaron mediante una serie de técnicas de caracterización microestructural, incluyendo difracción de rayos X, mediciones de dureza y análisis de imágenes. Se midieron el tamaño de las agujas de fase alfa, el tamaño de grano β, la fracción de fase, la alpha case, las tensiones residuales, la microdureza y el tamaño de los cristalitos para evaluar las propiedades del material. Además, se desarrolló una metodología de simulación para predecir la microestructura resultante tras el tratamiento térmico. Se ha observado que la temperatura isotérmica y la velocidad de enfriamiento desempeñan un papel importante en la microestructura resultante, mientras que el tiempo de mantenimiento y la adición de un tratamiento de precalentamiento tienen, comparativamente, un efecto menor.

Additive manufacturing is quickly becoming an important technology in many industries as a time and cost-effective prototyping tool, a repair technique and even a replacement for conventional manufacturing processes. In order to fully deploy a manufacturing process or material for its industrial use, it is essential to understand the relationship between processing, microstructure and properties of the material. Ti-6Al-4V, a widely used material in the aerospace industry, has been the focus of many studies for its implementation in additive manufacturing applications. It is a versatile material due to the extensive knowledge around its microstructure, properties, processing and alloying elements, which makes it a good candidate to test the possibilities of additive manufacturing as an alternative to conventional methods. Like many other alloys, the microstructure of Ti-6Al-4V is deeply tied to its thermal history. It is essential to understand how all the different manufacturing processes and the subsequent sequential steps involved can affect the microstructure or help modify it. Additive manufacturing poses a new line of study due to the different resulting microstructure and properties obtained compared to conventional methods. The objective of this work is to analyse and understand how isothermal temperatures and holding times during heat treatment can affect LMDp (Laser Metal Deposition – powder) Ti-6Al-4V. For this work, Ti-6Al-4V LMDp samples were built on top of a forged Ti6Al-4V plate. Due to the fully martensitic microstructure obtained post manufacturing, heat treatment was found to be necessary to perform. The samples were subjected to different heat treatments and analysed through a series of microstructural characterization techniques, including X-ray diffraction, hardness measurements and image analysis. Alpha Lath Size, prior β grain size, phase fraction, alpha case, residual stresses, microhardness and crystallite size were measured to benchmark the material’s properties. Additionally, a simulation methodology was developed to predict the resulting microstructure after heat treatment. It has been observed that isothermal temperature and cooling rate play an important role on the resulting microstructure, while holding time and the addition of a pre-heat treatment have, comparatively, a lesser effect.

Outgoing