1,721,020 research outputs found
An investigation on primary Si refinement by Sr and Sb additions in a hypereutectic Al-Si alloy
No full textThe effect of Sr and Sb additions as potential primary Si refiners for A390 hypereutectic Al alloy was analyzed. Despite the impurity induced twining (IIT), the experimental results indicate that only twin formation is not responsible for primary Si refinement. Stacking fault, by enforcing the atoms to adjust the sequence, and kinks, by deviating the Si growth from its easy growth direction, cooperate in growth discontinuity. An enhanced IIT theory has been proposed for primary Si evolution where twining, stacking faults and kinks are contributing to control the Si growth. Type of alloying element determines the major contributing mechanism in primary Si evolution. To the best of Authors’ knowledge, the kink formation and its effect on primary Si refinement has yet been reported elsewhere
Effects of bismuth on the characteristics of eutectic Si particles in Na-modified A356 alloys
No full textIn this work, the effect of bismuth over Na-modified eutectic Si particles was investigated in an A356 alloy with different Bi and Na contents. Metallographic and thermal analysis techniques were used to examine the microstructural changes occurring at different Bi and Na levels. The cooling curves and the corresponding derivative curves were plotted to determine the nucleation T-N, minimum T-Min and growth T-G temperatures for the Al-Si eutectic reaction. The results indicate that the T-N, T-Min and T-G values decrease after Na modification. In the unmodified alloy, the eutectic Si shows a fully unmodified lamellar morphology. A fully modified structure is obtained with 55 ppm Na in the alloy containing 20 ppm Bi impurity, while the same level of Na can partially modify the alloy containing 200 ppm Bi. It was also determined that the Bi-Na-rich intermetallic in the microstructure causes counteraction and it can be a binary or triple eutectic system containing more than one compound
The Influence of Sealing Processes and Machining Operations on the Scratch and Wear Resistance of Anodized AlSi9Cu3(Fe) Diecasting Alloy
Full text linkThe effects of hydrothermal and cold sealing processes on the scratch and wear resistance of the anodic layer have been studied. High-pressure diecast AlSi9Cu3(Fe) alloy plates were anodized in a sulfuric acid electrolyte at 16°C and further sealed in boiled water or in a NiF2 solution at 25°C. To analyze the influence of pre-anodizing machining operations, the plates were studied in the as-diecast condition and after milling. Metallographic investigations and image analysis techniques were carried out to study the morphology and thickness of the anodic layer. Hardness, wear, and scratch measurements were also performed to characterize the surface mechanical properties. The results showed that the sealing processes enhanced the wear and scratch resistance of the anodized surfaces because of the precipitation of hydrates that sealed the surface porosity. The thicker oxide layer formed on the milled substrate led to a greater wear resistance compared to the as-diecast surface, owing to reduced wear of the underlying aluminum substrate. Finally, a cracked mechanically mixed layer, which was enriched with fragmented intermetallics and anodic layer debris, was formed on the anodized surfaces at the end of the wear tests
Surface-segregation mediated Mg2Si and Mg(Zn,Cu,Al)2 formation on primary α-Al phase during solidification of the hot-tear sensitive AA 7xxx aluminum alloys
No full textComputer-aided thermal analysis and phase diagram calculation using the ThermoCalc® software were used to study the solidification path of Al7.8Zn2.6Mg2Cu alloy containing average values of 0.1 wt% Si and 0.1 wt% Fe as impurity elements. The Al6(Fe,Cu), Mg2Si, and Mg(Zn,Cu,Al)2 phases were detected as the main secondary phases precipitated during the solidification of the alloy. A four-step surface segregation-mediated description was proposed to explain the formation mechanism of the Mg-bearing, Mg2Si and Mg(Zn,Cu,Al)2, phases: i) surface segregation of Mg atoms form the bulk toward the surface of the primary α-Al phase, ii) adsorption of liquid Si atoms on the Mg-rich surface, iii) nucleation of Mg2Si, and iv) lateral growth of Mg2Si nuclei on the surface. Interfering with the kinetics of any or all of the first three steps may postpone or preclude the Mg2Si formation, alleviating the hot-tearing susceptibility of the alloy. Mg2Si can decrease the non-equilibrium solidus point of the alloy by depleting the source of Mg atoms on the surface of the primary α-Al phase, thereby postponing the formation of the Mg(Zn,Cu,Al)2 phase to lower temperatures. The freezing range is thus enlarged, increasing the hot-tearing susceptibility of the alloy. Therefore, the surface segregation of the primary α-Al phase appears to be a key feature for understanding the hot tearing phenomena of an alloy. A more accurate calculation of the solidification path to be used with Kou’s index of hot-tearing requires taking into account the possibility of surface segregation
The influence of Gd content on the solidification and microstructure of AlSi7Mg0.3 casting alloy
Full text linkThe effects of Gd content on the solidification path and microstructure of an AlSi7Mg0.3 alloy have been studied. Two different Gd levels, 0.1 and 0.5 mass%, respectively, have been investigated while the material has been solidified at low (0.2 °C s−1) and high (1.3 °C s−1) cooling rates. Computer-aided thermal analysis and metallographic techniques have been used to study the solidification and microstructural evolution of the alloy at different Gd contents and cooling rates. The results show how Gd has no effect on the grain refinement at both cooling rates. The eutectic structure is unaffected at Gd level of 0.1 mass%. A concentration of 0.5 mass% Gd promotes the precipitation of the gadolinium phosphide (GdP) phase instead of aluminium phosphide (AlP) compounds, thus suppressing the eutectic plateau during the solidification and serving as nucleant for the GdAl2Si2 phase. The eutectic Si crystals solidified at low cooling rate are refined at 0.5 mass% Gd content, while the morphology of Si crystals is only partially modified at high cooling rate
Laboratory-Scale Procedure for Evaluating the Flux Efficiency on Melt Cleanliness and Tensile Properties of Recycled Aluminum Alloys
No full textThe present study aims to develop a laboratory-scale procedure to evaluate the efficiency of melt-cleaning and drossing fluxes during aluminum alloy recycling, focusing on their effects on melt cleanliness and tensile properties of an AlSi9Cu3(Fe) alloy. X-ray fluorescence and diffraction, as well as differential scanning calorimetry, are used to characterize two different commercial fluxes. The molten metal is treated at 720 and 760 degrees C. Reference melts without flux treatment are also collected. The melt sampling procedure involves H2 concentration monitoring, reduced pressure test (RPT), and casting of tensile specimens. The composition of the generated dross is analyzed too. Differences are encountered in the chemical composition of the analyzed fluxes, while they exhibit similar melting temperatures. The RPT emphasizes the advantages of flux treatment on melt cleanliness. The tensile plastic properties, including ultimate tensile strength and ductility, are influenced by the flux treatment, while the elastic properties, such as yield strength, remain unaffected. The Weibull analysis allows us to discriminate the efficiency between the different processing conditions. Additionally, the three-parameter Weibull statistics highlights the significance of flux treatment and temperature monitoring to increase the threshold value. The dross analysis highlights the importance of flux treatment to minimize metal losses
The Effect of the Holding Time on the Microstructure of Gd-Containing AlSi7Mg Alloys
No full textAmong lanthanides, Gd addition has been expected to show a good achievement in changing the microstructure of the alloys for better performance in high-temperature applications. Nevertheless, there is a lack of information about the casting practice of Gd addition in terms of holding time and fading effect. In this study, the effective holding time and the fading effect of Gd in AlSi7Mg alloy have been investigated. After minor (0.1 wt. %) and major (0.5 wt. %) Gd additions, different holding times in the range from 15 to 200 min were examined. The chemical composition of the molten bath was frequently verified as a function of the holding time. Thermal analysis macro- and microstructural investigations were performed. The results indicate that 45 min of holding time can produce eutectic Si refinement. Longer holding times promote Gd settling leading to grain coarsening and formation of larger eutectic Si flakes
Effect of surface roughness and eutectic segregation on anodising of Al-Si-Cu alloys
No full textTo study the influence of the surface roughness and eutectic silicon segregation on the anodising of diecast Al-Si-Cu alloys, an AlSi11Cu2(Fe) alloy was high-pressure diecast and hard anodised. The microstructure and surface topography of milled and grit-blasted regions were investigated to analyse their effect on the growth of the anodic layer. The surface mechanical properties of the anodised surfaces were also studied. The results showed how high surface roughness and silicon segregation present in the grit-blasted surface hindered the thickening of the oxide layer. After anodising, the milled surface exhibited better mechanical properties than the grit-blasted one. The wear resistance was enhanced by a thicker anodic layer, while the scratch resistance was positively affected by a lower surface roughness
Lanthanides: a focused review of eutectic modification in hypoeutectic Al-Si alloys
Full text linkA modified fibrous-like eutectic structure strongly improves the final mechanical properties of Al-Si foundry alloys, especially ductility. Beside widely used commercial eutectic modifiers such as Sr and Na, lanthanides appear to be a possible alternative in the eutectic Si modification process for hypoeutectic Al-Si casting alloys. All lanthanides have similar physical and chemical properties, such as density, melting point, and fading phenomena, which have been compared in the present review. They also show atomic radii close to the optimal atomic radius for a modifying agent. However, the microstructural evolution of eutectic Si is strictly related to the specific element and content of lanthanides, whose abundance, reserve, mining, production and market situations have been preliminary evaluated in this paper. The eutectic modification mechanisms induced by lanthanides are not well-discussed and clarified yet. The advantages and disadvantages of individual addition of lanthanides for chemical modification of hypoeutectic Al-Si alloys have been here critically reviewed. The use of lanthanides for eutectic Si modification in Al-Si alloys has been discussed from both effectiveness and economical point of views. Nowadays, the actual cost and their efficiency seem to make lanthanides still far to be used in foundry industry for commercial and large-scale applications
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