Papers by Keyword: Heat Treatment

Abstract: During a heat treatment, a material undergoes microstructural changes that result in an alterationof its hardness. In a two-step heat treatment, the material is first adjusted to an initial hardness viaa specified cooling rate. Subsequently, the hardness is reduced through a tempering process, whileits ductility is increased. Depending on the tempering duration, tempering temperature, and initialhardness, different resulting hardness values are obtained. The resulting hardness after a chosen heattreatment has thus far been difficult to predict. This work employs symbolic regression to develop amodel that predicts the hardness evolution of 42CrMo4 steel as a function of cooling rate, temperingduration, and tempering temperature. By describing the model with few parameters, it has alsobeen demonstrated that cooling rates and tempering temperatures leading to a target hardness canbe determined. The overall model achieves a coefficient of determination of R2 = 98.50 % for knownexperimental data and a combined coefficient of determination of R2 = 93.13 % for previouslyunknown cooling rates (forward) and previously unattained resulting hardness values (inverse).Our work shows that the resulting hardness of 42CrMo4 can be predicted using a small numberof parameters. This work is anticipated to establish a foundation for further research endeavors.For instance, the approach using symbolic regression can be further adapted to identify physicallyinterpretable constants. Furthermore, the model description offers the possibility of coupling witha simulation model to accurately predict the hardness of a component.

Abstract: In order to achieve a long service life for highly stressed parts such as shafts for drivetrains, a combination of a bainitic microstructure with compressive surface residual stresses is beneficial. While a bainitic microstructure offers a good balance between strength and toughness, compressive residual stresses especially near the surface have a positive impact on service life. Research has shown that this is due to a shift of the crack initiation towards the core and a reduced crack growth . These properties can be achieved by hot forming as an established method for manufacturing highly stressed parts followed by an adapted cooling strategy. As this general approach was demonstrated for a simplified process in a prior study , the present article is dedicated to the functionalisation for hot forming processes. In detail, a customised spray cooling is presented for a hot impact extrusion process whereby shafts made of AISI 4140 are cooled down from the forming heat in a single step with adjusted cooling rates. In a finite element-based process design, different cooling strategies were investigated and adequate heat treatments to achieve the combined properties were identified. Following this process design, shafts are formed via hot impact extrusion and spray cooled according to the cooling strategy for experimental validation of the numerical model. Additionally, shafts with air cooling are produced as a reference. During forming, force-displacement curves are measured, which are used for the validation of the numerical hot impact extrusion simulation. The resulting plastic strain and temperature distribution significantly influence the following cooling simulation. The final microstructure as well as hardness values of the produced shafts are determined and compared for the varying cooling strategies.

Abstract: The mechanical properties and electrical conductivity of the Al–0.15Fe–0.5Si–0.5Mg–0.2Mn alloy with a Mg/Si ratio of 1 were investigated using optical microscopy, scanning and transmission electron microscopy, tensile testing, Vickers hardness measurements, and specific electrical resistivity measurements. To analyze the electrical conductivity data, the unit % IACS was used, calculated as a percentage relative to the conductivity of annealed copper. The alloy was studied in the as-cast condition, in the deformed condition (following extrusion and drawing), and after heat treatments: HT1 — solution treatment at 530°C and aging at 140°C for 8 hours, and HT2 - solution treatment at 560°C and aging at 175°C for 6 hours. The microstructure of the investigated alloy varied depending on the condition and heat treatment parameters, consisting of an aluminum matrix and strengthening particles with different morphologies and chemical compositions. For rods in the as-cast state, the conductivity was 55% IACS, ultimate tensile strength (UTS) — 150 MPa, and elongation — 14%. After HT1: 51% IACS, UTS — 140 MPa, elongation — 19%. After HT2: 51% IACS, UTS — 195 MPa, elongation — 19%.

Abstract: Steel 22MnB5 is widely used in the automotive industry for manufacturing high-strength structural car body parts. To achieve desired mechanical properties, hot-stamping is used, during which the Al-Si coating plays a critical protective role against oxidation. This study investigates the structural evolution of the Al-Si coating under various austenitization durations at 920 °C. Intermetallic phase formation and coating morphology are analyzed.

Abstract: Nickel-Aluminium Bronze is a copper alloy with excellent corrosion resistance in marine environments. However, there are also applications of NAB in freshwater and corrosion phenomena have been observed in such cases. To explore the effect of microstructure on the corrosion behaviour, heat treatments were applied to NAB samples, which were corrosion tested in electrolytes with a composition typical for freshwater. Depending on the presence of bicarbonate, sulfate, and chloride, different kinds of corrosion attack were observed. The mayor effect lies in minimization of the β-phase amount and increasing the portion of a- and κ-phases. Corrosion promoted by sulfate is the major hazard in fresh water, while the passivating effect of bicarbonate supports localization of the attack. Chloride plays an ambivalent role; it promotes the corrosion attack but limits the progressively penetrating evolution of localized corrosion. Since the composition of freshwater has a stronger impact on the corrosion phenomena of the NAB alloy, the influence of the heat treatments is not clearly evident. Compared to seawater, heat treatments have a lesser effect on the corrosion behaviour in freshwater.

Abstract: In this research work, Ni rich superelastic Nickel-Titanium(NiTinol) alloy rods were joined using a fully automated direct-driven rotary friction welding machine at 1900 rpm. Samples were subjected to heat treatment after the removal of flash bead. Corrosion behavior of the NiTinol samples were carried out using weight loss method and Potentiodynamic Polarization (PDP) technique using 3.5% NaCl and 1N HCl solution in interval of 12h, 24h, 36h, and 48h at different temperature conditions such as 25°C, 35°C, 45°C, and 55°C respectively. Research has been carried out to find the corrosion characteristics for both annealed and cryogenically treated samples. Research findings revealed that, in weight loss method the impact of corrosion has no effect in the welded zone. In PDP method, the corrosion rate is found to be less and insignificant compared to any other alloys. Hence, the material proved as anti-corrosive in nature. This fact is due to the formation of Titanium oxides (TiO₂) and Titanium nitrides passive layers which hinders the rate of corrosion. However, more corrosion resistance was seen in cryogenically treated welded samples compared to the other samples.

Abstract: The impact of heat treatment on the mechanical characteristics of aluminium metal matrix composite (MMC) was examined in this research work. Here the material chosen for matrix was Al7075-T6, which was aluminium alloy that was tempered with T6 configuration and the Al matrix was reinforced with Silicon nitride (Si₃N₄) powder. For the evaluation of mechanical properties totally two samples were fabricated, one was Al7075-T6 itself without any addition of any reinforcement and the other sample was composed of Al7075-T6 + 5% of Si₃N_4. These two samples were fabricated in necessary testing form with the help of stir casting technique. After fabrication and heat treatment of the samples the sample was mechanically tested to evaluate the tensile and impact strength of the samples prepared to find the changes in the mechanical properties due to the reinforcement of Si₃N₄ and due to the heat treatment process. The samples were subjected to heat treatment process at a temperature of around 500°C for 5 hours, after treating the samples with heat sudden quenching process was done by cooling with distilled water and artificial ageing process was conducted at 150°C for 24 hours. After all this process of fabrication and heat treatment the samples were analysed to find the mechanical properties.

Abstract: AZ91 magnesium injection molding is suitable for manufacturing complex-shaped electronic product frames or thin plates. However, the strengthening effect of the Mg₁₇Al₁₂ precipitate in AZ91 is limited, and it tends to dissolve during heat treatment, leading to a lack of particles that can pin grain boundaries and prevent grain growth. To address these challenges, the LAZ561Ca alloy has been developed, offering a reduced density (83% of AZ91), AlLi nanoprecipitates with strong strengthening capabilities, and thermally stable Ca-bearing intermetallics that effectively pin grain boundaries, maintaining a fine-grained structure (~8 μm) even after heat treatment. Experimental results demonstrate that AZ91 undergoes abnormal grain growth after solution treatment at 400°C due to a significant reduction in Zener pinning forces. In contrast, the LAZ561Ca alloy, with stable Al₂Ca precipitates, resists such growth during two-stage heat treatment at 370°C – 400°C. Through the coupling between Thermo-Calc and MICRESS software, multiphase field modeling reasonably reproduced the microstructure evolution during injection molding and heat treatment processes, highlighting its value in establishing digital physical metallurgy models. This study reveals the microstructural mechanisms of magnesium alloys, confirming the critical role of Ca-bearing precipitates in grain growth suppression. It provides a foundation for further optimization of alloy compositions and heat treatment conditions, paving the way for advanced magnesium alloys with enhanced performance in injection molding applications.

Abstract: This study investigates how heat treatment affects the mechanical properties and microstructure of extruded AA2017 aluminum alloy. Quenching (icy water vs. liquid nitrogen) and tempering (T6: 120–160°C; T7: 240°C) significantly alter hardness, tensile strength, and fatigue life. T6 promotes fine, coherent precipitates, enhancing strength and fatigue resistance, while T7 leads to over-aging and property degradation [X]. Icy water quenching improves fatigue life over liquid nitrogen by refining precipitates [Y]. Microstructural analysis reveals elastic adaptation (T6) and plastic shakedown (T7) as fatigue stabilization mechanisms, with fracture modes shifting from ductile (T6) to mixed ductile-brittle (T7) [Z]. These results optimize heat treatment for AA2017 in high-strength, fatigue-critical applications.

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.