1,721,068 research outputs found
Aspetti metallurgici nella superplasticità
No full text"Micrograined" or "structural" superplasticity is the ability of policrystalline materials (metals, ceramics, composites) with very fine grain size (d 0.4 T m, where T m is the absolute melting point) and low strain rates (ε̇ = 10 -5 to 10 -1 s -1). The main microstructural requirements for the observation of superplastic behaviour in high strength materials (the aluminum alloys AA7012, AA7075 and AA7475, the precipitation hardening stainless steel 17- 4 PH [AISI 630] and the nickel-based superalloy IN- 718) are presented and discussed. The chemical compositions of the tested materials are reported in tables 1, 2, and 3, while the microstructures of the as-received alloys are shown in fig. 1. Taking into account the main deformation mechanism (grain boundary sliding), an important role is played not only by the presence of a fine-grained microstructure (stable at high deformation te..
Effect of Different Heat Treatments on Tensile Properties and Unnotched and Notched Fatigue Strength of Cold Work Tool Steel Produced by Powder Metallurgy
Full text linkThe present study investigates the effect of two heat treatments on the microstructure, the tensile and the fatigue properties of a powder metallurgy tool steel that has undergone two heat treatments: quenching and multiple tempering (conventional for powder metallurgy tool steel), and quenching and multiple tempering with an intermediate cryogenic step at −80◦ C (new solution). The findings of the research indicated that the new heat treatment promotes the development of a homogeneous distribution of carbides in the martensitic matrix, with an increase of about 10% in tensile strength and about 7% in elongation to failure. This combination of exceptional strength with a high degree of toughness leads to an improvement in the fatigue behaviour of the steel, which exhibits a higher unnotched and notched fatigue strength (about 15% and 25% respectively) and a lower fatigue notch factor (about 15%) compared to conventionally heat-treated steel. These results highlight that the powder metallurgy tool steel, with the new heat treatment, could be a viable option for the production not only of tools and dies, but also for high-performance automotive components, including even those with complex geometries, such as camshafts or crankshafts
High Temperature Tensile Tests of the Lightweight 2099 and 2055 Al-Cu-Li Alloy: A Comparison
No full textThe present study deals with the high temperature characterization of the unconventional, lightweight AA2099 and AA2055 Al-Cu-Li alloys (density = 2.63 g/cm3and 2.71 g/cm3respectively), which are widely employed for aerospace structural components thanks to their high specific strength at room temperature. The alloys have been characterized through tensile tests at 200°C and 250°C, after different overaging heat treatments, with the aim to simulate the variation of mechanical properties occurring in a component operating at high temperature. At 200°C, AA2099 alloy shows equivalent or superior performance compared with AA2055; therefore, it exhibits advantages in terms of specific strength due to its lower density; T1precipitates, dominating AA2099 after overaging, are considered to provide effective strengthening. The reverse occurs at 250°C operating temperature, at which considerable improvements are offered by the combination of both Ω and θ′ precipitates, which are present in AA2055 matrix in all overaged conditions
CoCr alloy processed by Selective Laser Melting (SLM): effect of Laser Energy Density on microstructure, surface morphology, and hardness
Full text linkSelective Laser Melting (SLM) was used to realize Co-28Cr-6Mo samples. Several process parameters were considered, resulting in a wide range of Laser Energy Density (LED). The study was focused on the investigation of both process and material-related aspects, such as surface morphology, laser tracks dimension and defects formation mechanisms. In addition, macro (HRC) and microhardness (HV0.5) was assessed. A correlation between LED value and density, surface quality, microstructural features and hardness of SLM parts was defined. The final goal was to identify, for the biomedical Co-28Cr-6Mo alloy, the optimal LED window to be considered in order to maximize the overall quality of SLM parts
Mechanical and impact behaviour of (Al2O3)(p)/2014 and (Al2O3)(p)/6061 Al metal matrix composites in the 25-200 degrees C range
No full textThe present work is aimed at studying the impact behaviour of commercially available Aluminium matrix composites, in a temperature interval ranging from 25 degrees C to 200 degrees C. The results of instrumented impact tests and of microstructural and fractographic observations are correlated with the tensile properties of these materials. A description of the phenomena involved (particles cracking, interfacial failure associated to matrix-reinforcement reaction layers, ductile behaviour of the matrix) is given. The effect of testing temperature as well as that of the matrix characteristics are presented and discussed
Development and validation of a probabilistic model for notch fatigue strength prediction of tool steels based on surface defects
Full text linkA new model for the high cycle notch fatigue strength prediction of tool steels subjected to axial loading is proposed, based on previous literatures studies and experimental tests carried out on six different tool steels, including rotating bending fatigue tests on notched specimens, fractographic analyses, hardness, residual stress, and roughness measurements. The novelty is the assumption that surface defects are the main cause of notch fatigue failures of such steels. A probabilistic approach was implemented by modeling size distributions of defects, resulting in the prediction of normal distributions of fatigue strength. Like to other previous models, the effect of steel hardness, surface residual stress, notch severity, and specimen size was also taken into account. Model calibration and validation were performed using the data collected by the experimental activity. Model behavior was investigated by performing a sensitivity analysis, aiming to verify the response to variations of the considered input variables. Prediction errors of only 1.3% (on average) and 3.1% (maximum) resulted from the comparison between model-predicted and experimental notch fatigue strength
A novel heat treatment of the additively manufactured Co28Cr6Mo biomedical alloy and its effects on hardness, microstructure and sliding wear behavior
Full text linkCo28Cr6Mo alloy (ASTM F75 and F1537) is one of the standard biomaterials for permanent orthopedic implants, utilized especially in case of joint replacement, such as knee and ankle prostheses. At the present, innovative Additive Manufacturing (AM) technologies, such as laser-based powder bed fusion (LPBF), also known as selective laser melting (SLM), enable the production of customized medical devices with improved mechanical properties. When dealing with implants for joint replacement, wear resistance is critical and, unlike compressive and tensile properties, the knowledge on wear behavior of the LPBF Co28Cr6Mo alloy is currently limited. Furthermore, the effect of post-process heat treatment on tribological properties, that have to be customized on the peculiar microstructure induced by LPBF, needs to be assessed. In this view, the present work first focuses on a novel direct aging treatment of the LPBF Co28Cr6Mo alloy, performed in the range 600-900 degrees C up to 180 min, and investigates the effects on hardness and microstructural features, with the optimized heat-treated condition found in case of 850 degrees C for 180 min aging treatment. Then, the attention is driven to the dry sliding wear behavior of as-built and heat-treated LPBF Co28Cr6Mo alloy, considering the conventional wrought alloy as benchmark. For testing conditions closer to the in-service ones, the as-built LPBF alloy showed a wear resistance higher than the conventional wrought alloy. The optimized aging treatment significantly modified the as-built LPBF microstructure, it improved the alloy hardness and, in general, it positively affected its friction and wear behavior
Effects of powders and process parameters on density and hardness of A357 aluminum alloy fabricated by selective laser melting
Full text linkAdditive manufacturing processes based on the local fusion of a powder bed, such as selective laser melting (SLM), are a valid alternative to conventional technologies and a growing number of industrial sectors are currently relying on these processes for the production of different components. However, there are still some limits in using SLM and they are often related to the feedstock material. For this reason, in the present work, the effects of powder properties and pre-treatments, as well as process parameters, on the fabrication of aluminum alloy A357 samples were investigated. Two different batches of powder were considered in order to evaluate the effects of particles shape and size in the as-received condition and after two different pre-treatments: 60 °C for 3 h and 200 °C for 1 h. Selective laser-melted samples were produced in the conditions described above and were then characterized in terms of density, phase and chemical composition, defects, and hardness. The results showed a correlation between powder conditions in terms of morphology and pre-treatment on the properties of SLM A357 aluminum alloy components
Optimisation of heat treatment of Al–Cu–(Mg–Ag) cast alloys
Full text linkThe optimisation of heat treatment parameters for Al–Cu–(Mg–Ag) cast alloys (2xxx) having different microstructural scales was investigated. Thermo-Calc software was used to design optimal alloy compositions. Differential scanning calorimetry (DSC), scanning electron microscopy and wavelength-dispersive spectroscopy technique were employed to determine proper solution heat treatment temperature and homogenisation time as well as incidence of incipient melting. Proper artificial ageing temperature for each alloy was identified using DSC analysis and hardness measurement. Microstructural scale had a pronounced influence on time and temperature required for complete dissolution of Al2Cu and homogenisation of Cu solute atoms in the α-Al matrix. Refined microstructure required only one-step solution treatment and relatively short solution treatment of 10 h to achieve dissolution and homogenisation, while coarser microstructures desired longer time. Addition of Mg to Al–Cu alloys promoted the formation of phases having a rather low melting temperature which demands multi-step solution treatment. Presence of Ag decreases the melting temperature of intermetallics (beside Al2Cu) and improvement in age-hardening response. Peak-aged temperature is primarily affected by the chemical composition rather than the microstructural scale
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