Key Engineering Materials Vol. 1044

Abstract: The longitudinal turning of 304 austenitic stainless steel (ASTSTS) occurred on a lathe using a Tungaloy-made carbide insert (SNMG 12 04 08). The machining parameters were the cutting velocity, feed rate, and depth of cut (DOC). The machining occurs according to the L27 Taguchi design. The strain hardening index (n) and strength coefficient (K) were available by tensile test on the specimen. The chip reduction coefficients (CRC) and von Mises stresses (VMS) were experimentally available. The maximum CRC and the maximum von Mises stresses were for moderate speed, moderate feed, and moderate depth of cut. The SEM observation on chip surfaces at different experimental conditions revealed hardening behaviour for most of the experimental conditions. However, under the specific condition, extensive ductile behaviour was significant, which resulted in maximum von Mises stress generation. The application of design of experiment (DOE) methodology yielded the theoretical model. The trend of CRC found through the theoretical model showed a similarity with the curve-fitting trend from experimental data. However, a fuzzy inference system (FIS) model showed better adequacy in comparison to the other models in the present study.

Abstract: Aluminum 7075 alloy has excellent mechanical properties and exhibits good ductility, strength, toughness, and fatigue resistance. The addition of oxide additives to cast Al7075 has also enhanced tribological properties apart from these properties. In this study, the tribological properties of cast Al7075 with the addition of 2.5% zirconium oxide (Zirconia) and titanium oxide (Titania), produced in a resistance melting furnace followed by die casting, were investigated. The wear test is conducted using a pin-on-disc wear testing machine, as specified by ASTM G99. The comparison is based on two independent process variables: a fixed sliding distance of 1000 m for all samples and applied load variations are 10, 20, 30, and 50 N with 500, 700, and 1000 revolution speeds. Using a scanning electron microscope (SEM), the wear surface morphology of the samples was analyzed, and the wear test results were compared. Further, it was found that oxides added samples showed less wear loss compared to as-cast Al7075 samples. The abrasion mechanism for as-cast Al7075 samples is identified as ploughing and deep wear tracks, while for Al7075 samples with Zirconia and Titania addition, it is characterized by delamination and shallow wear marks that show less wear.

Abstract: Owing to its superior corrosion resistance properties, thin sheet austenitic stainless-steel grade 304 is widely used in the food and beverage industry. Resistance spot welding (RSW) is a preferred joining method for these sheets because of its speed and efficiency, but RSW can alter the microstructure of the weldments, influencing their strength and corrosion resistance. Parameters such as welding current, time, and electrode force influence weld nugget formation, affecting its shape, size, and strength, while the rapid cooling rate in RSW leads to the formation of skeletal, lathy, and acicular delta ferrite in the weld zone. This study investigates the effect of welding parameters on the microstructure and properties of 0.4 mm AISI 304 stainless steel using peel tests, microstructural analysis, and microhardness testing. Peel test results were used to construct the weld lobe curve for process optimization, and nugget formation under various welding conditions was examined using SEM and EDAX analysis. The pseudo binary phase diagram was used to predict the final weld microstructure, and microhardness measurements confirmed an increase in fusion zone hardness after resistance spot welding. The weld lobe curve revealed that optimal defect free welds without expulsion were obtained at 40 kgf with 3000 to 5000 amperes for 1 to 4 cycles, and at 30 kgf with 5000 to 6000 amperes for 3 to 6 cycles, with electrode force exerting a stronger effect on weld quality than weld time. All welds produced below 4000 amperes at 30 kgf failed through interfacial mode, while weld nuggets above 3 mm diameter within the lobe curve exhibited complete pull out failure. Hardness testing was conducted on samples welded at 40 kgf and revealed that variation in current had a greater influence on fusion zone hardness than changes in weld time from 2 to 5 cycles when current was held constant. Specifically, specimens produced at 2000 amperes showed the highest fusion zone hardness, those at 3000 amperes exhibited intermediate hardness, and samples at 4000 and 5000 amperes had hardness values in the welded zone that closely matched the parent metal. The fusion zone hardness in optimal welds increased to around 200 to 210 HV, while the HAZ in most samples exhibited recrystallization, resulting in hardness values slightly higher than the base metal 195 to 210 HV. In a few samples produced at lower welding current, hardness exceeded 220 HV. Microstructural analysis confirmed the presence of skeletal, lathy, and acicular delta ferrite in both the fusion and heat affected zones, depending on welding current, time, and force. Welds formed with higher current or sufficient heat input favored the development of skeletal delta ferrite, while rapid cooling due to lower heat input resulted in lathy and acicular delta ferrite. All welds exhibited columnar grain growth in the direction of the water cooled electrodes and a distinct heat affected zone surrounding the fusion zone.

Abstract: This study proposes the use of an innovative acidified metallic halide solution to address brass corrosion. The research identifies key parameters that contribute to corrosion and demonstrates that the new solution can effectively restore oxidized brass surfaces, serving as a simple and safer alternative corrosion inhibitor. The formulated solution consists of a metallic halide, an organic acid, and alumina as an abrasive. The metallic halide functions as an oxidation inhibitor, preventing the formation of copper (I) oxide (Cu₂O) and zinc oxide (ZnO). Analytical results confirmed the effectiveness of the treatment solution. FTIR spectroscopy showed reduced oxide formation, while EDX revealed a lower oxygen signal and the presence of iodine, indicating successful corrosion inhibition and surface modification. SEM images demonstrated significant surface improvement after treatment, with reduced pitting and oxidation compared to the corroded sample. Contact angle measurements confirmed that treated surfaces transitioned from hydrophilic to hydrophobic states, indicating successful restoration. This process offers a practical, efficient, and safer corrosion mitigation method for brass, addressing a critical need for metal manufacturers and suppliers.

Abstract: Conventional plasma nitriding can induce defects due to direct plasma formation on the surface of the treated material. To address this issue, the screen-assisted direct current plasma nitriding (S-DCPN) method was developed, which generates plasma on both the sample and a surrounding screen, thereby reducing such defects. In this study, S-DCPN was applied to ferritic stainless steel (SUS430) using austenitic stainless steel (SUS304) as the screen material. Treatments were performed at 633 K for 15 hours under gas pressures of 200 and 600 Pa, with varying gas compositions of 75 % N₂ – 25 % H₂, 50 % N₂ – 50 % H₂, and 25 % N₂ – 75 % H₂. To evaluate the effects of gas composition and pressure, a range of analyses was conducted, including X-ray diffraction (XRD), cross-sectional microstructural observations, glow discharge optical emission spectrometry (GD-OES), hardness testing, and corrosion testing. The results revealed the formation of the α_N phase, a supersaturated solid solution of nitrogen in ferrite, under all conditions. Nitrogen diffusion and surface hardness increased with higher hydrogen content, and corrosion resistance was notably enhanced under the 25 % N₂ – 75 % H₂ condition. These findings demonstrate the effectiveness of S-DCPN in improving the surface properties of ferritic stainless steel while maintaining or enhancing corrosion resistance.

Abstract: The purpose of this study is to clarify the mechanical properties of the expanded austenite (S phase) formed in austenitic stainless steel (ASS). A small thin rolled plate of SUS304 with 0.5 mm thickness was used as test sample. The test sample was nitrided by active screen plasma nitriding (ASPN) at low processing temperature of 400 °C and 450 °C during 4 hrs. processing time. S phase was formed on the surface of the test sample. The surface hardness of ASPN sample was higher than that of untreated sample. Furthermore, tensile tests and fracture surface observations revealed that the tensile strength was also improved compared to untreated samples.

Abstract: Austenitic stainless steels are characterised by excellent corrosion resistance and good formability, but their low hardness and fatigue life are limitations in demanding applications. The aim of this study was to analyze the effect of solution annealing and plasma nitriding on the microstructure, hardness and fatigue properties of AISI 304 steel. The experimental material was examined in three states: initial, after solution annealing and after plasma nitriding. Solution annealing resulted in the removal of deformation martensite, giving a homogeneous austenitic structure with a decrease in hardness. On the contrary, plasma nitriding produced a hard nitride layer (1291 HV0.01), while no martensite retransformation took place. The results of the fatigue tests showed that the specimens after plasma nitriding reached the highest fatigue limit (878 MPa), while the specimens in the initial condition had the highest number of cycles to fracture. Fractographic analysis revealed typical fatigue failure characteristics in all conditions. The study highlights the possibility of optimising the fatigue properties of austenitic steels through an appropriate combination of thermal and chemical-heat treatments.

Abstract: This study investigates the tribological behavior of composites based on Al₂O₃–ZrO₂ stabilized with 3 mol. % Y₂O₃ (ZTA – zirconia-toughened alumina), prepared using spark plasma sintering (SPS) technology. The composites were characterized in terms of microstructure, mechanical properties, and wear resistance in a dry ball-on-flat configuration. SEM analysis confirmed a homogeneous and fine-grained microstructure without porosity, with Al₂O₃ grain sizes of 200–400 nm and ZrO₂ grain sizes of 100–200 nm. Measurements revealed high Vickers hardness (1566.7 ± 133.6 MPa), fracture toughness (6.4 ± 0.29 MPa·m¹ᐟ²), nanoindentation hardness (25.94 ± 2.35 GPa), and Young’s modulus (365.9 ± 18.2 GPa). The coefficient of friction ranged from 0.40 to 0.53 depending on the load, and the specific wear rate was extremely low (4.81 × 10⁻⁷ to 5.08 × 10⁻⁷ mm³/Nm). Analysis of the wear track revealed predominantly abrasive wear without significant fragmentation or delamination. The results demonstrate that optimized microstructure, proper phase stabilization, and a high degree of densification enable the preparation of composites with an excellent combination of hardness, toughness, and tribological resistance. These materials are suitable for demanding applications in industry, energy, and biomedicine.

Abstract: In this study, the mechanical and tribological properties of 3 mol. % yttria-stabilized tetragonal ZrO₂ (3Y-TZP) prepared by spark plasma sintering (SPS) were investigated. Nanoindentation revealed a high hardness of 16.51 ± 0.86 GPa and an elastic modulus of 250 ± 8.8 GPa. The low scatter of these values provides strong evidence for a homogeneous, fine-grained microstructure. Vickers microhardness at a 5 N load was 1382 ± 14 and indentation fracture toughness (K_IC, Niihara) was 5.2 ± 0.03 MPa·m¹ᐟ², confirming the material’s high mechanical resilience. Dry reciprocating sliding against a SiC counterface exhibited a stable coefficient of friction (COF) of 0.37–0.39, with a slight decrease to 0.37 at 25 N load attributed to the formation of a thin protective tribolayer. Wear track depth increased from ~0.8 µm (5 N) to ~2.8 µm (25 N), and width from ~1.400 µm to ~ 1.600 µm, while the specific wear rate rose only marginally from 9.28 × 10⁻⁸ to 5.05 × 10⁻⁷ mm³/N·m, demonstrating excellent wear resistance. SEM/EDX analysis revealed predominant abrasive wear with microcracking, alongside tribochemical oxidation layers rich in SiO₂ and carbon that contribute to surface protection. Stabilization of the tetragonal phase and a fine-grained microstructure are key factors enabling the superior hardness, elasticity, and tribological performance of 3Y-TZP for applications demanding low friction and high wear resistance.