Greta Lindwall

Application of Finite Element, Phase-field, and CALPHAD-based Methods to Additive Manufacturing of Ni-based Superalloys

Numerical simulations are used in this work to investigate aspects of microstructure and microseg-regation during rapid solidification of a Ni-based superalloy in a laser powder bed fusion additive manufacturing process. Thermal modeling by finite element analysis simulates the laser melt pool, with surface temperatures in agreement with in situ thermographic measurements on Inconel 625. Geometric and thermal features of the simulated melt pools are extracted and used in subsequent mesoscale simulations. Solidification in the melt pool is simulated on two length scales. For the multicomponent alloy Inconel 625, microsegregation between dendrite arms is calculated using the Scheil-Gulliver solidification model and DICTRA software. Phase-field simulations, using Ni-Nb as a binary analogue to Inconel 625, produced microstructures with primary cellular/dendritic arm spacings in agreement with measured spacings in experimentally observed microstructures and a lesser extent of microsegregation than predicted by DICTRA simulations. The composition profiles are used to compare thermodynamic driving forces for nucleation against experimentally observed precipitates identified by electron and X-ray diffraction analyses. Our analysis lists the precipitates that may form from FCC phase of enriched interdendritic compositions and compares these against experimentally observed phases from 1 h heat treatments at two temperatures: stress relief at 1143 K (870 °C) or homogenization at 1423 K (1150 °C).

The Effect of Oxygen on Phase Equilibria in the Ti-V System: Impacts on the AM Processing of Ti Alloys

Oxygen is always a constituent in “real” titanium alloys including titanium alloy powders used for powder-based additive manufacturing (AM). In addition, oxygen uptake during powder handling and printing is hard to control and, hence, it is important to understand and predict how oxygen is affecting the microstructure. Therefore, oxygen is included in the evaluation of the thermodynamic properties of the titanium-vanadium system employing the CALculation of PHAse Diagrams method and a complete model of the O-Ti-V system is presented. The β-transus temperature is calculated to increase with increasing oxygen content whereas the extension of the α-Ti phase field into the binary is calculated to decrease, which explains the low vanadium solubilities measured in some experimental works. In addition, the critical temperature of the metastable miscibility gap of the β-phase is calculated to increase to above room temperature when oxygen is added. The effects of oxygen additions on phase fractions, martensite and ω formation temperatures are discussed, along with the impacts these changes may have on AM of titanium alloys.

The effect of nitrogen on the coarsening rate of precipitate phases in iron-based alloys with chromium and vanadium: experimental and theoretical investigations

Abstract A comparison of the coarsening of nitrogen-rich vanadium precipitates and the coarsening of carbon-rich vanadium precipitates is presented. The precipitate phases are studied experimentally, via fabrication of model alloys, and theoretically, via simulations utilizing the DICTRA software. The experimental investigations indicate that the nitrogen-rich precipitates exhibit a slower coarsening behaviour than the carbon-rich precipitates. Analysis using thermodynamic and kinetic modelling shows that this can be explained by the higher thermodynamic stability of the nitrogen-rich precipitate compared to the carbon-rich precipitate. The calculated coarsening rates are compared with the measured rates, and found to be in satisfactory agreement using reasonable values for the interfacial energies. The investigations are motivated by the fine precipitate size distribution of nitrides and carbonitrides characteristic for high nitrogen alloyed tool steels produced by means of powder metallurgy.

Simulation of TTT Curves for Additively Manufactured Inconel 625

The ability to use common computational thermodynamic and kinetic tools to study the microstructure evolution in Inconel 625 (IN625) manufactured using the additive manufacturing (AM) technique of laser powder-bed fusion is evaluated. Solidification simulations indicate that laser melting and re-melting during printing produce highly segregated interdendritic regions. Precipitation simulations for different degrees of segregation show that the larger the segregation, i.e., the richer the interdendritic regions are in Nb and Mo, the faster the δ-phase (Ni3Nb) precipitation. This is in accordance with the accelerated δ precipitation observed experimentally during post-build heat treatments of AM IN625 compared to wrought IN625. The δ-phase may be undesirable since it can lead to detrimental effects on the mechanical properties. The results are presented in the form of a TTT diagram and agreement between the simulated diagram and the experimental TTT diagram demonstrate how these computational tools can be used to guide and optimize post-build treatments of AM materials.

Untersuchungen zur Vergröberung von Ausscheidungen in PM Werkzeugstahlsorten – der Einfluss von Stickstoff

We present experimental and theoretical comparisons of the coarsening rate for the MX (X = C, N or C + N) precipitate in five different tool steel grades produced by Powder Metallurgy (PM). The alloys contain different amounts of nitrogen and carbon; ranging from high carbon/low nitrogen to high nitrogen/low carbon. Common for the grades is a high amount of precipitate forming alloying elements resulting in a high fraction of hard phase. Experimentally we have found out that the nitrogen rich precipitates coarsen slower, both for higher and lower carbon content, than the carbon rich precipitates. Coarsening simulations are performed utilizing the DICTRA software and an agreement between calculations and measurements can be concluded.ZusammenfassungWir präsentieren experimentelle und theoretische Untersuchungen zur Vergröberungsrate von MX (X = C, N oder C + N) Ausscheidungen in fünf unterschiedlich hergestellten pulvermetallurgischen Werkzeugstahlsorten. Die Legierungen beinhalten einen unterschiedlichen Anteil an Stickstoff und Kohlenstoff; dieser reicht von hohem Kohlenstoff/niedrigem Stickstoff bis zu hohem Stickstoff/niedrigem Kohlenstoff. Üblich für diese Sorten ist die große Menge an ausscheidungsbildenden Legierungselementen, was zu einem hohen Anteil an Hartphasen führt. Experimentell haben wir herausgefunden, dass die stickstoffreichen Ausscheidungen sowohl für einen hohen bzw. niedrigen Kohlenstoffgehalt, langsamer vergröbern als die kohlenstoffreichen Ausscheidungen. Simulationen zur Vergröberung wurden mit der Software DICTRA durchgeführt. Es konnte eine Übereinstimmung zwischen den Berechnungen und den Messungen gefunden werden.

Coarsening Investigations of Precipitates in PM Tool Steel Grades: an Effect of Nitrogen

We present experimental and theoretical comparisons of the coarsening rate for the MX (X = C, N or C + N) precipitate in five different tool steel grades produced by Powder Metallurgy (PM). The alloys contain different amounts of nitrogen and carbon; ranging from high carbon/low nitrogen to high nitrogen/low carbon. Common for the grades is a high amount of precipitate forming alloying elements resulting in a high fraction of hard phase. Experimentally we have found out that the nitrogen rich precipitates coarsen slower, both for higher and lower carbon content, than the carbon rich precipitates. Coarsening simulations are performed utilizing the DICTRA software and an agreement between calculations and measurements can be concluded.ZusammenfassungWir präsentieren experimentelle und theoretische Untersuchungen zur Vergröberungsrate von MX (X = C, N oder C + N) Ausscheidungen in fünf unterschiedlich hergestellten pulvermetallurgischen Werkzeugstahlsorten. Die Legierungen beinhalten einen unterschiedlichen Anteil an Stickstoff und Kohlenstoff; dieser reicht von hohem Kohlenstoff/niedrigem Stickstoff bis zu hohem Stickstoff/niedrigem Kohlenstoff. Üblich für diese Sorten ist die große Menge an ausscheidungsbildenden Legierungselementen, was zu einem hohen Anteil an Hartphasen führt. Experimentell haben wir herausgefunden, dass die stickstoffreichen Ausscheidungen sowohl für einen hohen bzw. niedrigen Kohlenstoffgehalt, langsamer vergröbern als die kohlenstoffreichen Ausscheidungen. Simulationen zur Vergröberung wurden mit der Software DICTRA durchgeführt. Es konnte eine Übereinstimmung zwischen den Berechnungen und den Messungen gefunden werden.

Coarsening Investigations of Precipitates in PM Tool Steel Grades: an Effect of Nitrogen@@@Untersuchungen zur Vergröberung von Ausscheidungen in PM Werkzeugstahlsorten – der Einfluss von Stickstoff

We present experimental and theoretical comparisons of the coarsening rate for the MX (X = C, N or C + N) precipitate in five different tool steel grades produced by Powder Metallurgy (PM). The alloys contain different amounts of nitrogen and carbon; ranging from high carbon/low nitrogen to high nitrogen/low carbon. Common for the grades is a high amount of precipitate forming alloying elements resulting in a high fraction of hard phase. Experimentally we have found out that the nitrogen rich precipitates coarsen slower, both for higher and lower carbon content, than the carbon rich precipitates. Coarsening simulations are performed utilizing the DICTRA software and an agreement between calculations and measurements can be concluded.ZusammenfassungWir präsentieren experimentelle und theoretische Untersuchungen zur Vergröberungsrate von MX (X = C, N oder C + N) Ausscheidungen in fünf unterschiedlich hergestellten pulvermetallurgischen Werkzeugstahlsorten. Die Legierungen beinhalten einen unterschiedlichen Anteil an Stickstoff und Kohlenstoff; dieser reicht von hohem Kohlenstoff/niedrigem Stickstoff bis zu hohem Stickstoff/niedrigem Kohlenstoff. Üblich für diese Sorten ist die große Menge an ausscheidungsbildenden Legierungselementen, was zu einem hohen Anteil an Hartphasen führt. Experimentell haben wir herausgefunden, dass die stickstoffreichen Ausscheidungen sowohl für einen hohen bzw. niedrigen Kohlenstoffgehalt, langsamer vergröbern als die kohlenstoffreichen Ausscheidungen. Simulationen zur Vergröberung wurden mit der Software DICTRA durchgeführt. Es konnte eine Übereinstimmung zwischen den Berechnungen und den Messungen gefunden werden.