With increasing industrial interest and significance of the selective laser melting the importance for profound process knowledge increases so that new materials can be qualified faster. Also it is the basis for an educated evaluation of possible process innovations. Therefore a 3D numerical model for the selective laser melting process is presented that allows a detailed look into the process dynamics at comparably low calculation effort. It combines a finite difference method with a combined level set volume of fluid method for the simulation of the process and starts with a homogenized powder bed in its initial configuration. The model uses a comprehensive representation of various physical effects like dynamic laser power absorption, buoyancy effect, Marangoni effect, capillary effect, evaporation, recoil pressure and temperature dependent material properties. It is validated for different process parameters using cubic samples of stainless steel 316L and nickel-based superalloy IN738LC. The results show the significance of evaporation and its related recoil pressure for a feasible prediction of the melt pool dynamics. Furthermore a possible way to reduce the times and costs for material qualification by using the simulation model to predict possible process parameters and therefore to reduce the necessary experimental effort for material qualification to a minimum is shown.

Melt pool simulation for the evaluation of process parameters in selective laser melting

The liquation cracking behavior of IN738LC superalloy during low power Nd:YAG pulsed laser welding

The kinetics of the γ′ phase dissolution have been studied at 1473 K and 1523 K (1200 °C and 1250 °C) for CMSX-4® alloy. Interrupted creep tests at 1323 K (1050 °C) and pure thermal aging at 1373 K (1100 °C) have been used to vary the initial γ′ morphology. Subsequent dissolution studies were conducted with or without an applied load. Differences in γ′ dissolution kinetics were observed between the dendrite cores and the interdendritic regions, resulting from the chemical microsegregations remaining after the standard heat treatments. It is shown that the initial γ′ morphology, the relaxation of the coherency stresses and the accumulated plastic strain are critical parameters controlling the dissolution kinetics. An increase of the accumulated plastic strain leads to an increase of the dissolution kinetics, whatever the location in the dendritic structure. In addition, once a given accumulated creep strain is reached prior to the dissolution tests (i.e., a given dislocations density), the dissolution kinetics are the same at 1473 K and 1523 K (1200 °C and 1250 °C). A modified equation of the recently developed Polystar model is proposed to incorporate the role of the accumulated plastic strain on the γ′ dissolution kinetics and to achieve a better predictability of the creep deformation under non-isothermal loading paths.

Strain Effect on the γ′ Dissolution at High Temperatures of a Nickel-Based Single Crystal Superalloy

Transient Liquid Phase Bonding of IN738LC/MBF-15/IN738LC: Solidification Behavior and Mechanical Properties

The heat-affected zone (HAZ) liquation cracking mechanism of nickel-based superalloys with high (Al+Ti) content during the laser solid forming (LSF) process was investigated via laser remelting of an as-deposited IN-738LC superalloy. Microstructural HAZ analysis revealed that cracks consistently propagated from the HAZ to the remelting zone along the grain boundary (GB). The formation of a liquid film during GB liquation was mainly owing to localized melting of the semicontinuous γ-γ′ eutectic distributed along the GB. The solute segregation behavior of the IN-738LC alloy during LSF was analyzed using the Giovanola–Kurz model and Scheil models, revealing that a significant enrichment of γ-γ′ eutectic-forming elements in the residual liquid at the final stage of solidification (solid fraction ~ 0.87) in the molten pool was the main cause of semicontinuous γ-γ′ eutectic formation along the GB. Further, a B enrichment at the GB was identified in LSF-fabricated IN-738LC, which promoted cracking by lowering the GB liquation temperature and promoting wetting of the GB by the liquid film. Unlike the phenomenon observed in the welding of cast IN-738LC, the coherence between the γ′ phases and the γ matrix in LSF-fabricated IN-738LC can suppress the occurrence of constitutional liquation of the γ′ phase. To understand the interaction between the thermal stress and the liquid film in the LSF process, thermal stress as a cracking driving force was also estimated based on the measurement of the residual stress from the substrate to the remelting zone of the IN-738LC deposit by the Vickers micro-indentation method.

HAZ Liquation Cracking Mechanism of IN-738LC Superalloy Prepared by Laser Solid Forming

An investigation on thermal, metallurgical and mechanical states in weld cracking of Inconel 738LC superalloy

Numerical modeling of inconel 738LC deposition welding: Prediction of residual stress induced cracking

Numerical definition of an equivalent GTAW heat source

After experimental identification of the main factors influencing Inconel 738LC superalloy microstructures, a semi-analytical modeling of precipitate dissolution during thermal cycle has been proposed. It was based on an isokinetic model of phase transformation adapted to the presence of high-precipitate density as observed in γ' precipitate hardened IN738LC. Some data required as input for the semi-analytical modeling could be determined by an inverse method applied to isothermal treatments. Finally the semi-analytical modeling was validated through non-isothermal treatments and used to predict the occurrence of hot cracking depending on welding conditions.

https://iopscience.iop.org/article/10.1088/0965-0393/18/8/085003/pdf

Control of Inconel 738LC superalloy microstructure with the aid of a precipitate dissolution semi-analytical modeling

L’utilisation de l’Inconel 738 (superalliage
 base nickel) est aujourd’hui freinee par des problemes de soudabilite qui se traduisent
 par l’apparition de fissures lors des operations de reparation de pieces par
 rechargement ou d’assemblages de pieces par soudage. Un modele de simulation numerique
 du soudage de cet alliage a ete developpe sur la base d’un couplage entre aspects
 thermiques, metallurgiques et mecaniques puis valide sur un cas simple de soudage et sur
 un cas de rechargement.

http://documents.irevues.inist.fr/bitstream/handle/2042/37235/976.pdf

Yann Danis

Papers

An investigation on thermal, metallurgical and mechanical states in weld cracking of Inconel 738LC superalloy

Numerical modeling of inconel 738LC deposition welding: Prediction of residual stress induced cracking

Numerical definition of an equivalent GTAW heat source

Control of Inconel 738LC superalloy microstructure with the aid of a precipitate dissolution semi-analytical modeling

Modélisation numérique du rechargement de pièces en Inconel 738