Francesco Sordetti

The manufacturing of large duplex stainless steel components commonly requires sand casting, which does not ensure sufficiently high cooling rates to prevent the formation of deleterious secondary phases, especially in thick sections. To restore a balanced ferrite-austenite microstructure free of secondary phases, a solubilization annealing treatment is required. This study investigates the role of rare earth elements, particularly cerium, as alloying additions in SAF 2507 (X2CrNiMoN25-7-4) super duplex stainless steel. Small cerium additions were introduced to modify the precipitation kinetics of secondary phases, reduce segregation during solidification and enhance microstructural stability in thick cast components. The first part of the study focused on solubilization heat treatment, evaluating the influence of temperature and holding time on microstructure, mechanical properties and corrosion resistance. A sand-cast SAF 2507 component was produced and specimens were subjected to isothermal heat treatments between 950°C and 1150°C with different holding times. Microstructural characterization was performed using optical and scanning electron microscopy, while image analysis quantified phase fractions. Mechanical behaviour was assessed through tensile, impact and hardness tests, and corrosion resistance through immersion tests. Two additional castings of the same alloy were produced, each with a different rare earth content. All specimens underwent identical heat treatments and characterization procedures, allowing direct comparison with the base alloy. Tre results confirmed the correlation between σ-phase content and the deterioration of mechanical and corrosion properties. Rare earth additions improved microstructural stability and overall performance by reducing deleterious phase formation, demonstrating the effectiveness of combining alloying design and heat treatment optimization.

The manufacturing of large duplex stainless steel components commonly requires sand casting, which does not ensure sufficiently high cooling rates to prevent the formation of deleterious secondary phases, especially in thick sections. To restore a balanced ferrite-austenite microstructure free of secondary phases, a solubilization annealing treatment is required. This study investigates the role of rare earth elements, particularly cerium, as alloying additions in SAF 2507 (X2CrNiMoN25-7-4) super duplex stainless steel. Small cerium additions were introduced to modify the precipitation kinetics of secondary phases, reduce segregation during solidification and enhance microstructural stability in thick cast components. The first part of the study focused on solubilization heat treatment, evaluating the influence of temperature and holding time on microstructure, mechanical properties and corrosion resistance. A sand-cast SAF 2507 component was produced and specimens were subjected to isothermal heat treatments between 950°C and 1150°C with different holding times. Microstructural characterization was performed using optical and scanning electron microscopy, while image analysis quantified phase fractions. Mechanical behaviour was assessed through tensile, impact and hardness tests, and corrosion resistance through immersion tests. Two additional castings of the same alloy were produced, each with a different rare earth content. All specimens underwent identical heat treatments and characterization procedures, allowing direct comparison with the base alloy. Tre results confirmed the correlation between σ-phase content and the deterioration of mechanical and corrosion properties. Rare earth additions improved microstructural stability and overall performance by reducing deleterious phase formation, demonstrating the effectiveness of combining alloying design and heat treatment optimization.

Ti alloys are widely used in aerospace and biomedical fields due to their high mechanical properties under severe loading. Interest in additively manufactured Ti6Al4V has increased, but further research is needed to fully characterize their properties. This work compares the effects of surface properties, internal defects, microstructure, hardness, and Hot Isostatic Pressing (HIP) or Vacuum Heat Treatment (VHT) on the fatigue behavior of Ti6Al4V produced by Selective Laser Melting (SLM) and Electron Beam Melting (EBM). Printing parameters and post-processing were optimized to achieve high density and minimal porosity, providing a solid basis for realistic fatigue comparisons. Samples were characterized in terms of microstructure (optical microscopy and SEM), mechanical properties (hardness mapping), surface texture (confocal microscopy), and internal defects (image-based analysis). Uniaxial fatigue limits were determined by a Dixon-Mood staircase method, and failed specimens were analyzed for fracture surfaces and defect areas. Applied load on flaws was evaluated to identify root causes of fatigue failure. Results showed that fatigue of as-printed samples is governed by surface roughness, while machined specimens are controlled by internal defect size. Machining increased the fatigue limit roughly threefold, and HIP further improved it by 10–20% by reducing internal porosity. In conclusion, with properly optimized melting parameters, both EBM and SLM produce similar mechanical performance at comparable roughness, supporting their use for structural components.

Ti alloys are widely used in several fields, such as aerospace and biomedical, due to their high mechanical properties under severe loading conditions. Recently, the interest in these materials produced by additive manufacturing process has increased, but intensive research should be done to better characterise their properties. This work aims to study and compare the effect of surface properties, internal defects, microstructure, hardness and Hot Isostatic Pressing (HIP) treatment or in-Vacuum Heat Treatment (VHT) on fatigue properties of a Ti6Al4V produced by Selective Laser Melting (SLM) and Electron Beam Melting (EBM) additive manufacturing technologies. The samples were fully characterised using a wide range of techniques, in terms of microstructure (optical microscopy and SEM), mechanical properties (hardness mapping) and surface texture (confocal microscopy). The internal defects were evaluated using an image-based analysis approach. The uniaxial fatigue endurance limit properties were determined by a Dixon-Mood staircase approach and the failed samples near the fatigue limit were characterised by fracture surface and defect area analysis. A study of the applied load on the flaw areas was carried out to unveil the root causes of fatigue failure. The results showed that the fatigue properties of the as-printed samples were mainly determined by the surface roughness, whereas in the machined samples the internal defect dimension ruled the fatigue resistance of the material. The HIP used as a post-printing treatment is effective in substantially reducing the presence of internal pores, although local microstructural changes can take place only in the case of smooth surface. In conclusion, when properly developed in their melted parameters, both EBM and SLM technologies produce similar mechanical performance on comparable roughness levels, thus finding shared fields of application and fully eligible for the production of structural components.

Thermal spray-coated components are widely used as wear-resistant coatings in many applications. However, these coatings have high levels of discontinuities that affect the corrosion resistance of the coated system. To reduce this problem, these coatings are usually sealed with liquid sealants (metals, organic or inorganic). The aim of this work is to seal the surface discontinuities of thermal-sprayed coatings using PVD and/or ALD coatings. To this end, CrN (arc deposition PVD) and TiO2 (ALD) coatings were deposited on thermal-sprayed alumina coatings. The samples produced were then analysed in both cross-sectional and planar views to detect the possible permeation of the thin film coatings into the thermal spray defects. Rf-GDOES measurements were performed to detect the very thin ALD deposit on the surface. The corrosion resistance of the sealed coatings was verified with immersion tests, wherein the OCP was monitored for 24 h, and potentiodynamic tests were performed after 15 min and 24 h immersions. The results showed that the thin films were not able to block the permeation of corrosive media, but they could reduce the permeation of corrosive media with a beneficial behaviour on corrosion resistance.