Additive manufacturing of materials inevitably has an impact on their performance and properties. Now a research article published online in the journal Additive manufacturing examines how the properties of stainless steel are affected by additive laser printing methods.
To study: Dislocation Microstructure and Its Influence on Corrosion Behavior in 316L Stainless Steel Additively Made to Laser. Image Credit: NDAB Creativity / Shutterstock.com
How additive manufacturing affects the microstructure of stainless steel
Additive manufacturing is widely used in the manufacturing of materials, but it causes changes in microstructural characteristics that can affect the chemical composition of the material as well as the quality and durability of the finished product.
Laser-based additive manufacturing processes have a layer-by-layer action that causes the formation of very heterogeneous microstructures. Another aspect of the process which causes this is the rapid solidification due to the large thermal gradients. Rapid solidification results in high density dislocations in steel which are vital to the aerospace, marine and automotive industries.
Print orientation showing the direction of the coater in relation to the gas flow; the samples for this study were taken from the columns, which exhibited systematic variations in print speed. Image credit: Sproust, D et al. Additive manufacturing
Common characteristics of industrial grade steels made with processes such as laser powder bed smelting are melting limits and solidification structures at the macro scale, and sub-granular cellular structures at the nanoscale. The cellular dislocation networks produced have implications for the overall quality and mechanical properties of finished steels. Some specific microstructural features of interest include multiscale interfaces, irregular surface chemistry, and bulk chemical heterogeneities.
Microstructural characteristics are intrinsically linked and lead to physical effects such as decreased corrosion resistance, which has implications for the structural integrity and longevity of additively manufactured steels. Defects can lead to the creation of surface electrochemical cells which cause localized corrosion (such as crevice corrosion and pitting) as well as general surface corrosion. The processes that lead to deleterious effects are complex and require intensive research to overcome them.
The realization of these characteristics and their effects has stimulated much research into additively manufactured steels. Recent studies in the field have included research on titanium alloys, aluminum alloys, stainless steels and tool steels.
Understanding microstructural defects and their effects on chemical structure is necessary to elucidate the potential effects of additive manufacturing on the end product. The research team used several multimodal analytical techniques to arrive at the study’s conclusions.
316L powder was used to additively print the stainless steel sample. The system used was a Renishaw AM 250. The main variable was the sweep speed, at 550, 650 and 700 mm / s, respectively. The samples produced were 25 mm cylindrical rods. Five millimeter sections were taken for analysis and corrosion measurements.
Stainless steel samples made with laser printed additives were analyzed by synchrotron X-ray diffraction, imaging techniques, spectroscopic and TEM techniques. The specimens were L-PBF 316L stainless steels which were printed at different frame speeds. The chemical, atomic and microstructural information about the fabricated steel has been elucidated by analytical techniques.
Stitched bright field TEM micrographs of a selected region containing two adjacent grains near a melt pool boundary on the L-PBF 316L sample printed at 700 mm / s showing the underlying cell dislocation network. The corresponding selected area diffraction patterns are shown in i and ii. Image credit: Sproust, D et al. Additive manufacturing
Experimental analysis was also performed to provide information on both localized and uniform corrosion behavior on the two single samples and to elucidate variations in corrosion effects on multiple samples at different print speeds.
Through the team’s characterization studies, data that quantified the chemical distribution and underlying defects in the microstructure provided insight into the corrosion behavior of the material. The results of the study showed that the localized and uniform corrosion behavior was affected by the microstructure and that the chemical heterogeneities specific to the samples showed a dependence on the printing speeds.
How the dislocation microstructure affects corrosion behavior
One of the main results of the study was the implications of microstructural defects on the corrosion behavior of the finished steel product. The corrosion resistance of 316L steel grades is due to Cr which promotes the formation of a passive bipolar film. This film consists of an interior Cr2O3 barrier layer and an outer layer enriched with metal oxyanions. This forms an electrokinetic barrier that prevents anions from entering.
The microstructural characteristics produced during additive manufacturing can prevent a solute from contributing to the formation of a passive film layer. Some studies of duplex stainless steel using synchrotron methods and electrochemical polarization experiments have indicated that there is a correlation between the surface structure of the alloy and compositional heterogeneities at the nanoscale. These heterogeneities influence passivity behavior such as passivity failure.
Studies have also indicated that there is a strong correlation between pitting formation and surface defects at the nanoscale. Knowing the nature of nanoscale defect formation will help inform manufacturing strategies that strengthen additive-printed stainless steel (and other materials) against the risk of corrosion.
Results of quantitative structural characterization as a function of print speed of Rietveld, modified Williamson-Hall (m-WH) and Warren-Averbach (WA) analyzes including (a) micro-stress,, and dislocation density , and (b) coherent diffusion size. Image credit: Sproust, D et al. Additive manufacturing
Defining and categorizing microstructural and atomic defects that result in loss of passivity and increased risk of corrosion in additively manufactured materials such as stainless steel is important to the field of materials science. A better understanding of how the manufacturing process affects the overall quality and durability of these materials will lead to better grades of industrially important metals in the future.
Sproust, D et al. (2021) Dislocation Microstructure and Its Influence on Corrosion Behavior in 316L Stainless Steel Additively Made to Laser [online] Additive manufacturing 47 102263 | Sciencedirect.com. Available at: https://www.sciencedirect.com/science/article/pii/S2214860421004231