Papers by Keyword: Al-Mg₂Si

Abstract: The cast Al-Mg₂Si metal matrix composite was prepared by metal model casting process with rare earth element Nd as the modificator. The effects of modification duration and remelting times on microstructure and mechanical properties of the composite were investicated by optical microscope (OM) and electronic universal testing machine. The results show that, after introducing a proper amount of Nd, both primary and eutectic Mg₂Si in the Al-18 wt.%Mg₂Si composite were well modified. The morphology of primary Mg₂Si is changed from irregular or dendritic to polyhedral shape and the morphology of the eutectic Mg₂Si phase is altered from flake-like to very short fibrous or dot-like. Moreover, the effect is of long-lasting and remelting properties. After the composite is modified for 300 min and remelted by 6 times, its primary and eutectic Mg₂Si structures are still in modification state of small block and slices, and the tensile properties of the composite are not significantly affected.

Abstract: The aim of this work is to investigate the effect of different casting speed in DC casting process and solution heat treatment of in situ Al-13.9%Mg₂Si composite. The increasing of DC casting speed not only makes primary Mg₂Si distribute more uniform and restricts segregation of primary Mg₂Si particles, but also it reduces primary Mg₂Si particle size. The DC casting speed significantly alter the eutectic Mg₂Si phase from fibrous to fine dot-like and eutectic phase refines effectively. The results obtained from mechanical testing demonstrated that the increasing of DC casting speed intensifies both hardness and tensile strength values. Then, the billet were subjected to solutionizing at 500^oC for holding time of 4h followed by quenching. The results indicate that the morphology of both primary and eutectic Mg₂Si changes after heat treatment. Solutionizing leads to the dissolution of the Mg₂Si particles and changes morphology from sharp edges to round shape. After solution heat treatment, tensile strength and elongation increase to 238.8MPa and 13.5%.

Abstract: Al-Mg₂Si composites have gained considerable attention because of their attractive properties such as low density, improved wear resistance and good castability. However, when in-situ routes are used for processing of these composites, the formation of coarse Mg₂Si particles with sharp cornersis inevitable and is detrimental for the composite properties. This problem is intensified when a hypereutectic composition such as Al-25wt.% Mg₂Si alloy is processed. In the present study, an innovative semisolid technique termed as the Vibrating Cooling Slope (VCS) has been applied to produce a sample of in-situ Al-25wt.% Mg₂Si composite.This technique combines the conventional cooling slope and vibration casting methods into an integrated one for producing fine and globular structures in the as-cast condition. An inclined plate was prepared from a 10mm thickness copper plate and was coated by boron nitride. This platecould vibrate mechanically in the vertical direction at a predetermined frequency by the aid of four springs and an electric motor. The molten Al-16.5wt.%Mg-9.4%Sialloy with 100^o C superheat was poured on the surface of this cooling slope (set at 45° inclined angle)while it was vibrating at the frequency of 40 Hzandamplitude of 400 μm.The semisolid alloy travelled the length of 40 cm on the slope before being poured into a steel mold (60 mm internal diameter and 35 mm in height). Also for the purpose of comparison, gravity casting (GC) and conventionally still cooling slope casting (CS)were carried outboth using the same mold and with the same superheat.The samples were sectioned, polished and subjected to metallographicstudies,porosity and hardness measurements. It was concluded that CS and VCS techniques resulted in a decrease in the size of Mg₂Si particles by about 50%and 70% respectively when compared with the gravity casting. However, the increased shape factor of Mg₂Si particles in the VCS and CS processed samples was insignificant as compared with GC. Although in comparison with GC, the VCS processed sample showed a higher porosity level, it exhibited a higher hardness value. These results were attributed to the finer and modified microstructure obtained via this newly developed technique.

Abstract: In this work we found that the addition of excess Mg can significantly improve the mechanical properties of pseudo-binary Al-Mg₂Si alloys after high pressure die casting (HPDC). Al-8Mg₂Si-6Mg alloy offered an excellent combination of high strength and reasonable ductility. Excess Mg lowers the Mg₂Si content in the eutectic reaction and promotes the formation of Mg₂Si as the primary phase, and this is believed to be the origin of improved mechanical performance.

Abstract: The transition metals such as chromium and manganese are usually added to 6000 series Al-Mg-Si alloys to control recrystallization and grain size and thus the properties of alloys. In Cr/Mn-addition alloys, Cr or Mn will expense Si to form the dispersoids as AlMnSi or AlCrSi and tend to decrease its aging effect. The aim of this work is to investigate the effect of transition metals (TMs) addition on the hardness and the microstructural features of Al-Mg-Si alloys. Al-Mg-Si alloys, which can be remarked as the quasi-binary alloys of Al-Mg₂Si, are prepared with Cr or Mn addition by laboratory casting. Some other transition metals, such as Co and Ni, are also added to Al-Mg-Si alloys. The grain size of four alloys decreases with TMs addition, which consequently increases the as-quenched hardness of the alloys comparing with that of the Al-Mg₂Si alloy without TMs addition. The difference between Cr/Mn-addition alloy and Co/Ni-addition alloy is that the dispersoids are formed in Co/Ni-addition alloy without expensing Si. Therefore, there is little effect on the aging effect of Si in Co/Ni-addition alloy. Keywords: transition metals, hardness, microstructural, Al-Mg₂Si, dispersoids.

Abstract: The effect of different solution temperatures has been investigated on the tensile properties of Na-modified Al-Mg₂Si in situ composite specimens which were subjected to solutionizing at different temperatures of 480 °C, 500 °C and 520 °C for holding time of 4 h followed by quenching. Tensile test results indicated that elongation value gradually increases upon solution treatment whereas ultimate tensile strength (UTS) reduces. The results of solution treatment also showed that the highest quality index is achieved in 500 °C (354 MPa) and so it is revealed optimum solutionizing temperature level (500 °C) for improving tensile properties.

Abstract: Heat-treatable 6000-series alloys are currently used by automotive body sheets. It is general to improve mechanical properties of the Al-Mg-Si alloy by giving addition of the element and the thermo-mechanical treatment. The electron backscattered pattern (EBSP) technique has been performed in order to analyse individual crystallographic orientation in these alloys. Hardness of the 30%-rolled alloy was higher than that of the 0%-rolled alloy. The Schmid factor of individual crystal grain was calculated by crystallographic orientation. After adding a little deformation, the crack was observed at the interface between higher SF and lower SF grains. It was thought that this crack introduced a fracture from the surface.