Microstructure Optimization of AM metals through heat treatment and interstitial alloying

Metal additive manufacturing (AM) provides a high degree of geometrical freedom without the need for part-specific tools or dies, making the manufacturing method relevant for a wide range of products and applications. Metal AM parts can exhibit very different properties as compared to parts manufactured by conventional methods such as casting or forging. One reason for the distinctive properties is the complicated thermal history of metal AM parts, especially the high cooling rates associated with some AM methods resulting in non-equilibrium and possibly highly anisotropic microstructures.

This thesis investigates the use of heat treatment and interstitial alloying for optimization of the microstructure and improved properties of additively manufactured metals. The focus is on the most popular metal AM method, which is laser-based powder bed fusion (L-PBF) and two of the most popular metals for AM, alpha/beta titanium alloy Ti-6Al-4V and austenitic stainless steel AISI 316L.

The first study investigates the influence of scanning strategy on microstructure, porosity, hardness, and surface roughness, in an attempt to understand the microstructure of as-built Ti64. The porosity and surface roughness were found to depend strongly on the scanning strategy, which is discussed in detail. The microstructure of L-PBF Ti-6Al-4V (Ti64) is generally α' martensite in elongated β grains, which was also the case in the investigated specimens. The direction of the β grains depends on the scanning strategy, and some α' decomposition was observed for a specific scan strategy.

The tribological properties of Ti64 are poor, but can be significantly improved by surface hardening. Gaseous thermochemical treatments were found to be an excellent fit for L-PBF Ti64, investigating both nitriding and carbo-oxidizing as well as a combination of the two. The hardness and resistance against wear were enhanced significantly, particularly by carbo-oxidizing, but also nitriding. The dramatic improvement in wear resistance is attributed to the formation of interstitial compounds and diffusion zones with dissolved oxygen, nitrogen, and carbon.

As AM is intended as net-shape or near-net-shape manufacturing, thermo-mechanical processes for grain refinement of AM parts are rarely an option. Instead, the use of thermochemical treatment using hydrogen, so-called thermo-hydrogen processing (THP), could be relevant and was investigated on L-PBF Ti64. The microstructural response of hydrogenation and dehydrogenation was investigated. The results show that the ordered α₂ phase forms during hydrogenation at 650 °C. Further, δ-hydride after hydrogenation. Tensile tests illustrated the potential of using THP on L-PBF Ti64, although only minor improvements were made using the current THPs.

Austenitic stainless steel 316L manufactured by L-PBF consists mainly of austenite, sometimes with a small fraction of residual δ-ferrite, but has an internal cellular structure which contributes to significant strengthening of the material. Heat treatment can homogenize the microstructure, and in combination with thermochemical surface engineering, significantly improve the wear and corrosion resistance of L-PBF 316L. Regular austenitization, active austenitization under an applied partial pressure of nitrogen to prevent nitrogen loss from the steel, and high-temperature solution nitriding (HTSN), i.e. deliberate alloying with nitrogen, significantly improved the corrosion resistance of the material, ranked in order of increasing impact. Unfortunately, high temperature treatment is accompanied by annihilation of the cellular structure and results in a lower hardness and thus yield strength. Additional low-temperature surface hardening (LTSN) forms a surface zone of expanded austenite, which significantly enhances the hardness, wear-resistance, and pitting resistance of L-PBF 316L.

Nitriding can also occur during L-PBF, as revealed by comparing 316L parts build in two otherwise identical build jobs on a CO₂-laser-based system: one built using argon and one built using nitrogen as protective gas. L-PBF of 316L in nitrogen leads to nitrogen uptake, while building in argon leads to nitrogen release. The nitrogen atmosphere also counteracts the oxidation during L-PBF, as is evident from the oxygen content of the parts. The changes in nitrogen and oxygen contents result in significantly different corrosion performance. A 316L specimen built in nitrogen performs significantly better than a 316L specimen built in argon.

For further enhanced nitrogen contents, a novel approach was devised and investigated. The method provides nitrogen and chromium simultaneously by mixing Cr₂N into the 316L powder, followed by L-PBF of the powder mixture. The method successfully augmented the nitrogen content to 0.31 wt%, while it still developed the favorable cellular structure in a fully austenitic microstructure. Added nitrogen was confirmed to be largely interstitially dissolved. The enhanced nitrogen content provides solid solution strengthening, as manifested by a hardness enhancement. The specimens built by this method also exhibited significantly better corrosion resistance, as evaluated by mass loss after immersion in a ferric chloride solution. The novel method was further utilized to also enhance the carbon content in the as-built specimens by addition of Cr₃C₂ to the 316L + Cr₂N powder mixture. The added carbon was confirmed to be in solid solution in austenite, and further improved the hardness and corrosion resistance of the as-built specimen. The addition of Cr₃C₂ to the 316L + Cr₂N mixture results in the promotion of favorable deoxidation, as is evident from a lower oxygen and carbon content of as-built parts as compared to the powder mixture.