Printing-Debinding-Sintering Process for Steel Manufacturing

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Jun 29 2022

In an article recently published in the journal Additive Manufacturing, researchers discussed the sintering mechanism for extrusion-based additive manufacturing of stainless steel by using molecular dynamics (MD) simulation.

Background

Metal components are frequently produced using additive manufacturing (AM) and cutting-edge techniques in a variety of industries. It is necessary to investigate an alternate metal AM solution with a lower cost and less energy consumption because the widely used commercial metal AM technologies demand a significant investment for machinery facilities and material feedstocks. Currently, metal part fabrications using fused filament fabrication (FFF) AM are done similarly to metal injection molding (MIM). A Printing-Debinding-Sintering (PDS) technique that uses polymer-based filaments with highly packed metal particles is an efficient way to produce metal parts at a high fabrication rate and a low production cost, in contrast to conventional metal powder-based additive manufacturing (AM) technologies.

The PDS technique for metal AM manufacturing is still a very new area of study. To understand the underlying thermodynamics and kinetics events during the traditional sintering process, research has recently been carried out using experimental and numerical approaches. The discrete element method (DEM) has been developed to mimic the microstructural evolution and mesoscale particle coalescence during the sintering process. Insights into a number of intricate atomic-level physical phenomena, including atomic diffusion, microstructural evolution, and grain boundary migration during the sintering process, can be gained through the use of MD simulation. 316L stainless steel is one of the most significant structural materials for industrial applications.

About the Study

In this study, the authors used a unique Fe-Ni-Cr three-element, three-particle sintering configuration in MD simulation to thoroughly reveal the sintering mechanisms during PDS of stainless steel 316L. By examining the development of particle structure, diffusion activation energy, and interactions between solute elements during the special heating process of PDS, the mechanism of Cr element aggregation at the grain boundaries was made clear. To confirm this aggregation event, the microstructure of the PDS-built 316L samples was also examined.

The team used MD modeling to extensively investigate the sintering mechanisms during PDS of 316L stainless steel.  A revolutionary three-particle, three-element configuration sintering model was established to simulate the coalescence of particles with a distinct temperature profile. Each element's densification mechanism and diffusion behavior in the Fe-Ni-Cr alloy system in 316L stainless steel was described. Additionally, throughout the entire sintering process, the diffusion activation energy, evolution of particle structure, and interaction of each solute ingredient were examined.

The researchers demonstrated the Cr element's grain boundary aggregation process. A scanning electron microscope (SEM) outfitted with energy dispersive spectroscopy (EDS) was used to experimentally analyze the microstructures of the PDS-built 316L specimen and pinpoint the Cr aggregation phenomenon in order to validate the simulation model.

Observations

At the 1,600 K holding stage, it was discovered that the self-diffusion coefficient of the Cr element significantly increased. Due to its lower diffusion activation energy and stronger atomic contacts, the Cr element migrated to the grain boundary and produced severe grain boundary aggregation. The excess energy generated as a result of this aggregation process encouraged greater particle coalescence. A multiscale simulation of the sintering process in PDS of different alloy systems was made possible by the atomic-scale understanding of the sintering behavior of Fe-based alloys constructed in PDS.

At the first holding stage of 1,283 K, the pore structure changed from a triangle to a circle, which signaled the start of densification. At the holding stage of 1,600 K, the total absence of the pore structure revealed the transition from densification to the final sintering body. It was discovered that 1,283 K was the diffusion activation temperature. Each element's centrosymmetric parameter (CSP) and mean square displacement (MSD) value showed a tendency of rapid increase with sintering temperature. The excessive volume diffusion at the 1,600 K holding stage was responsible for the transition from the pore closure to the final sintered body. Additionally, the Cr element's higher self-diffusion coefficient led to more flaws, and its lower diffusion activation energy suggested that it diffused more readily than the Fe and Ni elements.

During the last holding stage of the MD simulation, the cohesive connections between Cr atoms were better than those between Fe and Ni elements. Further promoting particle coalescence and the stability of the sintering body was the energy produced by Cr aggregation. The aggregation behavior of the Cr element was confirmed by the EDS analysis. The Cr element's weight percentage increased near the grain boundaries, according to point analysis and the Cr dot mapping, which demonstrated a higher concentration of dotting sites there. The experimental findings supported the MD simulation of Cr aggregation.

The multi-element diffusion behavior along with the pore evolution during the sintering of alloy materials was highlighted by the atomistic modeling and experimental results. The outcomes also showed that cross-scale simulation was a viable and efficient method for elucidating specific systems.

Conclusions

In conclusion, this study used experimental testing and molecular dynamics simulation to characterize the sintering characteristics of 316L stainless steel during PDS. The link between simulated nanoparticles and experimental micro-scale powders was demonstrated based on the size-dependent melting temperatures of the metal particles. Based on the distinctive PDS-based heating profile, the entire debinding-sintering process was simulated. The methods through which the Cr element aggregates at grain boundaries were discovered.

The authors believe that more atomic-level insights into the optimization of the PDS process can be gained by investigating the link between the anisotropic powder accumulation in printing and the anisotropic grain orientation in sintering.

This study sheds important light on how Fe-based steel alloys sinter, opening the door to a better understanding of how other alloy systems sinter.

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