1,721,027 research outputs found
The influence of cooling rate and Fe/Cr content on the evolution of Fe-rich compounds in a secondary Al-Si-Cu diecasting alloy
No full textThis study investigates the morphological evolution of primary α-Al(Fe,Mn,Cr)Si phase in a secondary Al-Si-Cu alloy with respect to the initial Fe and Cr contents as well as to the cooling rate. The solidification experiments have been designed in order to cover a wide range of cooling rates, and the Fe and Cr contents have been varied over two levels. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring at different experimental conditions. The morphological evolution of the α-Fe phase has been also analysed by observing deep etched samples. By changing the cooling rate, α-Al15(Fe,Mn,Cr)3Si2 dodecahedron crystals, as well as Chinese- script, branched structures and dendrites form, while primary coarse β-Al5(Fe,Mn)Si needles appear in the alloy with the highest Fe content at low cooling rates
Non-isothermal dissolution modelling of sigma phase in duplex stainless steels
No full textIn this work, the non-isothermal dissolution kinetics of the sigma phase in duplex stainless steels has been studied and modelled. A semi-empirical model is proposed to describe the kinetics of sigma phase precipitation/dissolution during continuous heating starting from the isothermal transformation kinetics. The proposed model, which presumes validity of the additivity rule, is validated by means of experimental investigations. A good agreement is found between experimental and analytical results
Precipitation of primary Fe-rich compounds in secondary AlSi9Cu3(Fe) alloys
No full textThe precipitation of primary Fe-bearing compounds in a secondary AlSi9Cu3(Fe) alloy has been investigated over three levels of iron (0.80, 1.00, 1.20 mass%) and manganese (0.25, 0.40, 0.55 mass%), and two levels of chromium (0.06, 0.10 mass%), as well as different cooling rates (2, 10, 20 °C min-1). Differential scanning calorimetry and metallographic techniques have been used in order to quantitatively evaluate the nucleation temperature and the phase fraction of sludge particles. The temperature of sludge formation increases by increasing the initial concentrations of Fe, Mn and Cr, while an increase in the cooling rate shifts the sludge nucleation towards lower temperature. The amount of sludge particles increases progressively with the sludge factor but even more by decreasing the cooling rate. The combination of Fe, Mn and Cr levels, as well as cooling rate, allows to determine a threshold value that prevents the sludge formation. In the analysed range of composition and cooling conditions, functional equations have been developed in order to accurately predict the precipitation temperature and the phase fraction of sludge in AlSi9Cu3(Fe) die-casting alloys
Trace alloying elements in secondary aluminium alloys
No full textSecondary aluminium and alloys are getting wide acceptance world-wide since recycling process requires lower potential energy than primary aluminium production. The control of type and amount of trace elements during recycling operations can be either difficult and expensive. Furthermore, the role of trace elements in foundry metallurgy is not fully understood, even if it is sometimes a tool actually used to manipulate the microstructure of the alloys. Trace elements can be beneficial, as well as detrimental to the structure and properties of the alloy.
Effects of Chromium and Bismuth additions to an AlSi9Cu3(Fe) secondary diecasting alloy on the microstructure and mechanical properties are investigated. The results indicate that the area fraction and the size of α-Alx(Fe,Mn,Cr)ySiz intermetallic compounds increases by increasing the Cr content, and this affects the ultimate tensile strength and the ductility of the alloy at the highest Cr level. Contrary, the Bi addition seems to not produce significantly changes in the microstructure and mechanical properties of the recycled AlSi9Cu3(Fe) alloy
Evolution of sludge particles in secondary die-cast aluminum alloys as function of Fe, Mn and Cr contents
No full textThe evolution of sludge particles in a secondary high-pressure die-cast AlSi9Cu3(Fe) alloy has been investigated over three levels of iron (0.80, 1.00, 1.20 wt.%) and manganese (0.25, 0.40, 0.55 wt.%), and two levels of chromium (0.06, 0.10 wt.%). Metallographic and image analysis techniques have been used in order to quantitatively evaluate the morphological and dimensional variations of sludge with different Fe, Mn and Cr contents. The results indicate that any increase of the Fe, Mn and Cr level promotes the formation of coarser sludge, with polyhedral and star-like morphologies. Fe and Mn produce an increase on the number of primary Fe-rich particles, while Cr promotes the formation of a large number of secondary Fe-rich particles. No relationship is revealed between the morphological variation of sludge and the Fe, Mn or Cr contents, as well as between the sludge factor and the Fe:Mn ratio. In the analyzed range of composition, the design of experiment methodology and the analysis of variance have been used in order to develop statistical models that accurately predict the average size and number of sludge particles in the AlSi9Cu3(Fe) diecasting alloys
The influence of Sr, Mg and Cu addition on the microstructural properties of a secondary AlSi9Cu3(Fe) die casting alloy
No full textThe influence of Sr, Mg and Cu content on the microstructure of a high-pressure die cast AlSi9Cu3(Fe) alloy is reported. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring at different Sr, Mg and Cu levels. The results reveal that the Sr and Cu increase the amount of microporosity in the die castings, while Mg counteracts this effect. The secondary dendrite arm spacing and the grain size slightly decrease by the addition of the alloying elements. Compared with the base AlSi9Cu3(Fe) alloy, the Sr-modified alloy shows significant refinement and morphological modification of eutectic Si particles in the central regions of the castings. In contrast, such mechanisms at the casting surfaces are substantially driven by more rapid solidification. The Mg and Cu addition annihilates the benefits of refinement of eutectic Si particles by Sr modification, while it seems to not affect their morphology. The combined addition of Cu and Mg determines an increase of Cu- and Mg-rich intermetallic compounds, while the Sr-modified alloy shows lower fraction of intermetallics, even if comparable to the base alloy in terms of other alloying elements
Effects of chromium and bismuth on secondary aluminium foundry alloys
No full textThe effects of impurities such as Chromium and Bismuth on the microstructure and mechanical properties of an AlSi9Cu3(Fe) secondary diecasting alloy are investigated. Metallographic and image analysis techniques have been used to examine the microstructural changes. The results indicate that the area fraction and the size of brittle α-Alx(Fe,Mn,Cr)ySiz intermetallic compounds increase by increasing the Cr content, and this affects the ultimate tensile strength and the ductility of the alloy at higher Cr level. On the other hand, the Bi addition seems to not produce significantly changes in the microstructure and in the average mechanical properties of the secondary AlSi9Cu3(Fe) alloy. However, the Weibull statistics show that the addition of Bi leads to less reliable castings, which may be associated with an increase of oxide film defects
The effects of microstructure heterogeneities and casting defects on the mechanical properties of high-pressure die-cast AlSi9Cu3(Fe) alloys
No full textDetailed investigations of the salient microstructural features and casting defects of the high-pressure die-cast (HPDC) AlSi9Cu3(Fe) alloy are reported. These characteristics are addressed to the mechanical properties and reliability of separately HPDC tensile bars. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes throughout the tensile specimen. The results indicate that the die-cast microstructure consists of several microstructural heterogeneities such as positive eutectic segregation bands, externally solidified crystals (ESCs) cold flakes, primary Fe-rich intermetallics (sludge) and porosities. In addition, it results that sludge particles, gas porosity, as well as ESCs and cold flakes are concentrated toward the casting centre while low porosity and fine-grained structure is observed on the surface layer of the castings bars. The local variation of the hardness along the cross section as well as the change of tensile test results as a function of gauge diameter of the tensile bars seem to be ascribed to the change of porosity content, eutectic fraction and amount of sludge. Further, this behavior reflects upon the reliability of the die-cast alloy, as evidenced by the Weibull statistics
Intermetallic Phase Precipitation in Duplex Stainless Steels: Considerations on the Use of Johnson-Mehl-Avrami- Kolmogorov Equation
No full textDespite the great interest in intermetallic phase precipitation in duplex and super duplex stainless steels, a common controlling factor in the kinetics of its formation has not been reached yet. For example, when the time-evolution of sigma phase precipitation is described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, or its modified forms, very different kinetic parameters were found in the literature, even if duplex stainless steels with very similar chemical composition and initial conditions are used in the experiments. In this work, some critical points that affect the evaluation of the kinetic constants and may cause erroneous interpretation of intermetallic phase precipitation (kinetics) are highlighted and discussed. It was shown that the choice of a high impingement coefficient in the modified JMAK equation leads to a significant improvement in experimental data description. A series of recommendations for the future application of the JMAK equation applied to intermetallic phase precipitation in duplex stainless steels are finally suggested
- …
