PSI - Issue 68

Tsanka Dikova et al. / Procedia Structural Integrity 68 (2025) 99–105 Tsanka Dikova & Natalina Panova / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Biomedical alloys represent about 80% of all materials used for implants, with the longest history in this regard being stainless steels (Yan et al. (2022), Dikici et al. (2015). The austenite stainless steels contain minimum 10.5 % Cr (ferrite stabilizing element) and alloying elements such as Ni, Mn, N, stabilizing austenite structure. If nickel is presented, the chromium content must be a minimum of 18% for an austenitic structure to exist at room temperature. The success of these steels in the production of implants, medical devices and dental appliances is based on the combination of high corrosion resistance and good mechanical properties. The high corrosion resistance is due to both the monophasic austenite structure and the passivating effect of the chromium which leads to formation of thin, dense and self-healing chromium oxide layer on the steel surface (Borgioli (2022). For avoiding formation of chromium carbides in the microstructure and decreasing the intergranular corrosion, the carbon content should be low (0.03-0.08 %), which is shown by L in the steel designation. The austenite stainless steels, mostly used for biomedical applications, include AISI 316 and 316 L types - for coronary stents, orthopedic implants and devices for fracture fixation, and AISI 304 and 304 L types - for orthodontic wires in dentistry. Body fluids are highly corrosive because they contain chloride, hydroxide and other ions, bacteria, proteins and dissolved oxygen. The pH of the body is usually around 7.4, although, after operations or due to hematomas, inflammations and infections, this value can vary in the range of 4 to 9 (Eliaz (2019), Gregorutti et al. (2020). Saliva is a hypotonic solution containing bioactonate, chloride, potassium, sodium, nitrogen compounds and proteins. Its acidity varies from pH 5.2 to 7.8 depending on the conditions in the oral cavity and the food used (Chaturvedi (2008). Such changes can increase the risk of corrosion and therefore the release of metal ions, which in turn cause certain cytotoxic effects (Gregorutti et al. (2020). Therefore, the corrosion resistance of the materials used for manufacturing of implants, medical and dental devices is of great importance for their durability. In recent years, various laser technologies have been widely implemented in the production of many types of biomedical constructions. They include laser cutting, laser welding, laser cladding, laser heat treatment and additive technologies (AT) such as laser rapid manufacturing (LRM), selective laser melting (SLM), direct laser deposition (DLD), direct metal laser sintering (DMLS) etc. (Panova et al. (2023), Alhajhamoud et al. (2022), Ko et al. (2021). All these processes are characterized with high speeds of heating, melting of the surface layer or metal particles and subsequent cooling. During these processes, high speed microstructural transformations run, resulting in a specific unstable fine-grained and more homogeneous microstructure of the surface layer, which can change the corrosion resistance and mechanical properties of the produced details. Many authors have declared that the microstructure of the austenite stainless steels, fabricated by AT, ensures higher corrosion resistance compared to the wrought steels (Ko et al. (2021), Zhang et al. (2021), Kong et al. (2019). The team of Revilla et al. (2020) has revealed that the microstructure of the laser melted layers of 316 L stainless steel, produced by AT, consists of 3D network of cells. Their study has shown enrichment of the cells’ borders with alloying elements (Cr, Ni, Mo, Si and Mn) and decreased Fe content. According to them, the finer microstructure and redistribution of the alloying elements in the laser melted samples define the formation and growth of more compact and stabile oxide layer compared to the wrought steel. The study of Kong et al. (2020) also proposes that the higher corrosion resistance of the laser melted 316 stainless steel is due to the super high sub-grain density, increasing the number of nucleation sites for formation of stabile passive oxide layer. According to the study of Zhang et al. (2016), the Cr-Ni stainless steels, produced by laser cladding, are characterized by pitting corrosion, propagating in horizontal direction, and better corrosion resistance than the 304 L steel. It is found by Laleh et al. (2019) that the inter-granular corrosion along the grain boundaries in the SLM fabricated 316 L stainless steel is lower compared to the commercial one. The electrochemical corrosion of AISI 321 steel after laser surface melting was studied by Dikova et al. (2014) and Panova N. (2024). It was found that the fine-grained microstructure and the improved passive layer on the surface of laser melted layers of AISI 321 steel defined increased resistance to pitting corrosion in Ringer’s solution and artificial saliva (AS) with pH 6.5. However, the resistance to pitting corrosion in AS with higher acidity was lower (Dikova et al. (2015). During the electrochemical corrosion test in AS with pH 5.6, the increased δ-ferrite amount in the laser melted layers resulted in increased austenite/δ-ferrite interfaces ensuring large number of sites for pits formation. From the other hand, the acetic acid exerted additional effect, as the δ-ferrite is not sufficiently resistant to it and the pits could be initiated on its crystals. The team of Tarasov et al. (2019) investigated the corrosion behavior