Inclusion Modification of AISI 316L by Addition of AlN via Laser Powder Bed Fusion

Abstract

Metals and alloys manufactured by laser powder bed fusion (L-PBF) are known for having high oxygen and nitrogen concentration compared with those made by conventional casting. L-PBF is characterized by its highest cooling rate among various additive manufacturing (AM) processes. Owing to rapid cooling and high oxygen concentration, oxide inclusions enriched with deoxidizer elements (Si, Mn) are formed in tens of nanometers. These inclusions dispersed in the steel matrix have been recently highlighted to have comparative advantages in properties. Especially, some innovative trials have been successfully carried out to control the size and number density of nano-sized particles by adjusting process conditions. However, those inclusions have some limits being utilized as a strengthening source or a structure modifying agent: MnSiO₃ is amorphous and highly incoherent to the steel matrix, MnCr₂O₄ can easily be agglomerated in liquid metals due to the higher interfacial tension, and MnS could be the origin of pitting corrosion in sulfuric and hydrochloric solutions. Hence, the new oxide inclusions in AM should be suggested and studied. Nitrogen dissolved in steels manufactured by L-PBF has been focused on solutionizing due to its effective solid solution strengthening and benefit of corrosion resistance. However, there is a lack of utilizing nitride particles. Dispersed fine nitrides such as AlN and TiN might also be the strength improving source. The purpose of this thesis is to modify an inclusion by adding aluminum nitride (AlN) into the stainless steel (316L) and investigate its effect on mechanical properties. The preliminary test was carried out to analyze the inclusions formed in maraging stainless steel containing 1.5 wt% aluminum (CX-steel). Aluminum is affirmative with not only oxygen but also nitrogen. So AlN, as well as Al₂O₃ could be formed during the L-PBF process. To research the nitrogen dissolution and nitride formation, the printing was performed with two different shielding gases: Ar and N₂. Under a nitrogen atmosphere, the mass of nitrogen increased slightly. Almost inclusions were alumina in the both samples. Only some nitride particles were found in the nitrogen case specimen. The main experiment was conducted with the mixed powder: 1 wt% Aluminum nitride (AlN) powder and 316L stainless steel powder were mixed to add aluminum (Al) and nitrogen (N) into 316L. The aluminum content of the as-built specimen printed with mixed powder (316L-AlN) was 0.515 wt% while the nitrogen concentration was 0.13 wt%. The most distinguished result of mixing 316L with AlN was the decrease of oxygen concentration from 489ppm to 244ppm. With decreasing the oxygen content, the number density of inclusions was lower than that of normal 316L and the texture of 316L-AlN was different from that of 316L. The alumina inclusions were dispersed in the 316L-AlN matrix. Characteristics of those inclusions were interpreted with scanning electron microscopy (SEM), transmission electron microscopy (TEM). And mechanical properties were measured with a hardness test and a tensile test. The yield strength of 316L-AlN was higher than that of 316L about 80 MPa. Strengthening mechanisms were investigated by considering Orowan strengthening, Hall-Patch equation, and dislocation density.