Andrea Di Schino

The use of High Strength Low-Alloy (HSLA) steels in structural applications requires detailed analysis of their microstructure to optimize the strength and toughness of welded joints. During multi-pass welding, thermal cycling generates a complex heat-affected zone (HAZ) with subregions containing brittle phases that can compromise mechanical strength. This study evaluates the effect of vanadium and niobium additions on the HAZ microstructure in 15 mm thick S355 steel welds produced by robotic GMAW welding. The results reveal a heterogeneous microstructure with ferrite of several morphologies, bainite, and martensite/austenite (M/A) islands. The presence of vanadium reduces the solubility of carbon during phase transformations, favoring the formation of ultrafine precipitates (∼11 nm) and reducing the M/A phase in the high vanadium (0.1 wt%) variant. Nevertheless, the micro-alloyed variants improve mechanical strength (yield and ultimate strength) without loss of ductility or fatigue resistance, exhibiting a stress dissipation behavior that favors resistance to crack propagation.

Microstructure design is a main issue when targeting a target material’s properties

Structural applications that use High-Strength Low-Alloy (HSLA) steels require detailed microstructural analysis to manufacture welded components that combine strength and weldability. The balance of these properties depends on both the chemical composition and the welding parameters. Moreover, in multi-pass welds, thermal cycling results in a complex Heat-Affected Zone (HAZ), characterized by sub-regions with a multitude of microstructural constituents, including brittle phases. This study investigates the influence of Vanadium addition on the microstructure and performance of the HAZ. Multi-pass welded joints were manufactured on 15 mm thick S355 steels with different Vanadium contents using a robotic GMAW process. A steel variant containing both Vanadium and Niobium was also considered, and the results were compared to those of standard S355 steel. Moving through the different sub-regions of the welded joints, the results show a heterogeneous microstructure characterized by ferrite, bainite and martensite/austenite (M/A) islands. The presence of Vanadium reduces carbon solubility during the phase transformations involved in the welding process. This results in the formation of very fine (average size 11 ± 4 nm) and dispersed precipitates, as well as a lower percentage of the brittle M/A phase, in the variant with a high Vanadium content (0.1 wt.%), compared to the standard S355 steel. Despite the presence of the brittle phase, the micro-alloyed variants exhibit strengthening without loss of ductility. The combined presence of both hard and soft phases in the HAZ provides stress-damping behavior, which, together with the very fine precipitates, promises improved resistance to crack propagation under different loading conditions.

This paper aimed to investigate the enhancement of the corrosion resistance of a protective system applied on the AZ31 magnesium alloy to be used as an orthopedic biomedical device, composed of three different superimposed layers: (a) magnesium-based oxide, (b) polydopamine, and (c) polylactic acid. Specifically, morphological and chemical analyses, crystallographic, roughness, and micro-hardness were carried out. The electrochemical measurements were performed in Hanks’ Balanced Salt solution at 37 °C. The micro arc oxidation (MAO) treatment involved the classic pancake structure of the oxide with a consequent high extension of the real area.The sealing ofits pores via the polydopamine was well highlighted through the surface roughness analysis. As expected, the magnesium oxide layer reduced the degradation rate.The presence of polydopamine on the oxide layer improved the corrosion resistance of the alloy, showing a pseudo-passivity range in the potentiodynamic polarization curve, due to the filling of oxide pores.The highest impedance modulus in the electrochemical impedance spectroscopy analysis during the temporal observation of 168 h was observed when all coatings were applied on magnesium substrate, due to a synergetic action. Thus, the multilayers should represent a protective system to control the degradation process.

The removal of prosthetic implants, although normally necessary following the recovery of patients, inevitably has disadvantages in terms of costs, further pains and possible post-operative infections. A promising alternative is represented using bioresorbable implant structures. The magnesium alloy-based devices could be ideal candidate due to their poor corrosion resistance and high biocompatibility, but their degradation rate is dissimilar to that of hosting tissues healing. Therefore, a growing interest is addressed to design new magnesium alloys with a slow degradation process or to modify the surface of known magnesium alloys. This paper aims to investigate the enhancement of corrosion resistance of a protective system applied on the AZ31 magnesium alloy composed of three different superimposed layers: a) magnesium-based oxide, b) polydopamine and c) polylactic acid. Morphological and chemical analysis were performed. Roughness and microhardness measurements were carried out. Electrochemical behavior was studied in the Hanks’ solution at 37 °C. As expected, the findings showed that the magnesium oxide layer is able to reduce the degradation rate, in contrast to the polydopamine layer, that negatively affected the barrier protection of the only anodization. However, the presence of polydopamine together with the polylactic acid film improved the corrosion resistance of the alloy. Thus, the multilayers should represent a protective system to control the degradation process.