Papers by Keyword: Microstructure

Abstract: This study assesses the impact of heat treatment on the microstructure and mechanical properties of AlSi10Mg alloy produced using the L-PBF method. The research compares the mechanical properties and microstructure of samples subjected to direct aging (heat treatment at 170 °C/2 h) and stress relief annealing (at 240 °C/2 h), which is below the temperature for silicon network decomposition. These results are then compared with the as-built state (without any heat treatment) after printing, serving as a reference. Tensile and hardness tests were used to determine the mechanical properties, while electron microscopy was employed to analyze the microstructure. The findings indicate that direct aging led to an increase in yield strength, tensile strength and hardness compared to the as-built state. In contrast, samples treated with stress-relief annealing exhibited comparable yield strength to the as-built state, but significantly lower tensile strength and reduced hardness. Notably, contrary to expectations, the ductility did not increase with decreasing strength and hardness; instead, it decreased.

Abstract: In this work, a TiNbCr alloy is proposed for solid-state hydrogen storage applications. The design of the alloy is based on the Hume-Rothery rules and the thermodynamic parameters ΔH_mix and Ω, while the alloy was conceived as single-phase with a BCC lattice. Samples were synthesized from alloy powder using additive technology and the DED method. The prepared samples were printed with different parameters and their structure and phase composition were subsequently analyzed. The possible influence of these printing parameters on the properties of hydrogen storage alloys is discussed.

Abstract: Aluminum alloy 6082 is known for its use in the manufacture of structures that require welding interventions, high mechanical properties, resistance to pressure and corrosion, such as: boilers, truck structures, bicycles and motor boats. The recent research, regarding the cavitation resistance of the this alloy structure, shows a poor behavior of the semi-finished structure and somewhat improved by artificial aging volumetric heat treatment regimes. On the line of increasing the resistance of the this alloy structure, to the erosive demands of cavitation, they sign up of the rechearch results of the this paper, regarding the behavior and resistance of the vibratory cavitation of the aluminum alloy 6082 structure , obtained by WIG remelting. Comparing with the results obtained on the structures in the semi-finished state and through volume thermal treatments of artificial aging state, using the established parameters, recommended by the ASTM G32-2016 norms, a significant increase in the resistance to cyclic cavitation stresses is found, as a result of the increase in the surface hardness value. The novelty of the work consists in motivating the use of the remelting procedure of the surface structure of aluminum alloy 6082, through WIG remelting in order to increase the surface hardness, with a direct effect on increasing this structure resistance to the cyclical fatigue stresses of shock waves and microjets developed through the hydrodynamic mechanism of cavitation.

Abstract: The paper presents the results of the behavior and resistance to the erosion by cavitation of the 2017 A aluminium alloy structure, obtained by the WIG remelted method. The research is in step with the new directions of study and aims to extend the aluminum alloy 2017 A to the manufacture of parts that work in the cavitation regime, such as: pistons and valves of thermal engines, respectively various pump rotors or motor boat propellers. The analysis performed on the basis of the specific curves, constructed according to the indications of the ASTM G32-2016 normas, shows that structure, obtained by the WIG remelting, confers a constant behavior to the cyclic stresses of microjets generated by the hydrodynamics of the vibratory cavitation. The comparison of the results, based on the specific parameters, used in the laboratory and indicated by the ASTM G32-2016 norms shows a resistance to cavitation erosion, clearly superior to the semi-finished structure and those obtained by artificial aging heat treatment at 1800C and 120 0C, with duration of one hour.

Abstract: Lightweight steel structural systems like trusses or built-up beams, made of thin gage steel elements, are highly efficient, with ease of handling and construction. Self-drilling screws are commonly used for connecting thin-walled elements, but the time and manpower required for numerous connections necessitate an improved solution. One possible solution is to use welding technology, but the conventional methods are not suitable for joining thin sheets. Manufacturing defect-free, mechanically sound welding joints remains challenging due to defects like porosity and undesired microstructural phases in the heat-affected (HAZ) and fusion zones (FZ). Conventional welding processes increase heat input, causing difficult challenges. Brazing, a relatively new joining process, offers the advantages of lower heat input for thin and zinc-coated steel sheets. Therefore, the paper aims to present the effect of MIG brazing parameters on the macro-and microstructural properties of Cu-Al-based weld seams manufactured for joining thin sheets with thickens in the range of 0.8-2 mm. The weld seams were manually fabricated using a MEGAPULS FOCUS 330 compact equipped with TBI XP 363S/4m welding torch, focusing on optimal welding regimes. The macro-and microstructures of the joints were evaluated along with the mechanical properties in terms of hardness, confirming that MIG brazing is a promising method for manufacturing lightweight steel structural systems.

Abstract: Depending on the collecting methods, coal combustion by-products are usually divided into fly ash, bottom ash, and flue gas desulfurization product. Because they are a hazardous dust source, they are often stored underwater in ponds or lagoons and are called pond ash. In this investigation, the pond ashes from Ulaanbaatar's 4^th TPS (thermal power station) and Erdenet’s TPS were characterized by XRF, XRD, BET, PSD, SEM and TEM. The pond ash of the 4^th TPS contains more than 20% calcium oxide, while Erdenet’s TPS contains around 4% calcium oxide. PSD of the 4^th TPS shows a bimodule distribution with a maximum of 36 and 260 mm, while Erdenet's pond ash shows an unimodal distribution with a maximum of 74 mm. The main crystalline compounds of the 4^th TPS pond ash were quartz, calcite, hematite, albite, while in the Erdenet pond ash were quartz, mullite, magnetite, and calcite. The mineralogical composition of the pond ashes depends on the used coal type, the power plant’s working principle and the duration of time inside the ash pond. The alkali-activated binder prepared from these pond ashes demonstrated a weak compressive strength of around 1.5-2.5 MPa after 7 days. Notably, the high-calcium pond ash-based alkali-activated paste exhibited slightly higher mechanical properties than the low-calcium pond ash-based paste. The weak mechanical properties of the pond ash-based alkali-activated materials are related to both pond ashes’ porous and high-surface microstructure. High calcium pond ashes could exhibit a partial calcium silicate formation reaction, which is the reason for the higher mechanical properties than low calcium pond ash-based alkali-activated pastes. Furthermore, a brief mechanical activation of these pond ashes for 20 min slightly improved their mechanical properties, reaching up to 3.75 MPa.

Abstract: Welding is the process of permanently joining materials and tungsten inert gas (TIG) welding is widely used due to its precision, controlled heat input, and cost-effectiveness. This study investigates the stress corrosion behavior of TIG-welded 304L stainless steel in a saline environment, analyzing factors contributing to material degradation. The research involved tensile testing and fractographic analysis to characterize fracture modes and determine the key influences on mechanical strength. Additionally, a microstructural analysis of the heat-affected zone (HAZ) was conducted to assess changes induced by welding. The results indicate that exposure to a chloride-rich environment led to a reduction in mechanical properties, primarily due to the formation of corrosion-related compounds and material thinning. Fractographic analysis revealed a transition in fracture modes, highlighting the influence of corrosion on failure mechanisms. Furthermore, microstructural examination showed significant alterations in the HAZ, which affected the overall integrity of the welded joints. These findings contribute to a better understanding of corrosion-induced degradation in welded 304L stainless steel and provide insights for optimizing welding parameters to improve durability.

Abstract: Many factors influence the effectiveness of traditional binders used for soil stabilization, including anions present in the soil and carbonates. Natural pozzolana-lime stabilization is a relatively new technique that has shown promising results. However, no study has specifically evaluated its success in the presence of phosphate for high-carbonate soils. This paper investigates this question using marly soil from Medea, which was pre-contaminated with the common fertilizer monoammonium phosphate at 0, 2, 4, and 6% by dry weight, then stabilized with lime and/or natural pozzolana at 0, 8%, and 20%, respectively, by dry weight. To assess the effect of phosphate, mineralogical and macrostructural changes in these mixtures were analyzed through X-ray diffraction tests and scanning electron microscopy, respectively. Additionally, pH levels were monitored over 90 days, and changes in Atterberg limits between 1 and 30 days of curing were compared. Variations in immediate bearing indexes and compaction parameters were also examined. The study found that lime alone was ineffective in stabilizing the soil due to high carbonate content, with improvements in geotechnical properties only observed when natural pozzolana was added with lime. Phosphate was found to impact the lime-natural pozzolana stabilization technique significantly.

Abstract: The aim of the work was to assess the nature and causes of defects – cracks in the welds of membrane wall panel tubes (MW) of a thermal power plant boiler made of T24 steel. This steel is intended to produce MW evaporators and superheaters for power plant units operating in the area of supercritical steam conditions. Its advantage, compared to the originally used materials, is higher creep resistance. The main expected advantage of the steel was to obtain optimal properties of welded joints without subsequent post-weld heat treatment (PWHT).

Abstract: This paper deals with results of detailed investigations on a failed cast upper ram (63 tons) of a die-forging hammer. Subsurface cast defects primarily represented the “weak areas” responsible for the initiation of fatigue cracks during industrial exploitation. These defects mostly corresponded to gas bubbles. The occurrence of Ti (C,N) particles on the surface of these bubbles indicated that these defects were probably a consequence of release of gas products from the casting mould and/or mould's painting during the pouring of liquid metal. The evidence of fatigue initiation on repair welds was obtained only in one case. It was revealed that in this case the cast defects on the surface of the guiding radius were not completely cut off before repair welding. The remnants of the cast defects in the interface weld/base material initiated the fatigue crack. Basic mechanical properties of the subsurface layer of the upper ram complied with the design requirements.