Materials Science Forum Vols. 783-786

Abstract: In this paper, Low frequency electromagnetic field and air knife are applied simultaneously to produce large-size AA 7055 aluminum alloy ingots during DC casting. Moreover, the effects of low frequency electromagnetic field and air knife on macro-physical fields during DC casting as well as microstructure and crack in the ingots are studied and analyzed by the numerical and experimental methods. Comparison of the calculated results indicates that applying electromagnetic field can modify the flow direction and increase the velocity of melt flow and homogenize the distribution of temperature in the sump, and applying air knife can homogenize the distribution of temperature and decrease the stress and strain in the solidified ingots. Further, the microstructure of the billet is refined remarkably and the crack is eliminated by applying electromagnetic field and air knife during DC casting because of modification of the macro-physical fields

Abstract: In this study, experiments are combined with numerical simulation to study the temperature field and flow field during the casting process of 4045/3004/4045 three-layer composite ingots with section of 500mm×420mm. The effects of casting temperature, casting speed, contacting height and cooling intensity of cooling plate on the casting process were discussed. The macro-morphologies and microstructures of the composite ingots, the temperature distribution and the element distribution in the interface zone were investigated, also the interface bonding strength was measured. The optimal parameters for casting composite ingots were obtained. Results show that the solid supporting layer formed on the cooling plates plays a key role in the casting process of composite ingots. The solid supporting layer can prevent the blending of two melts by resisting the impact of alloy melt, which ensures the stable casting process and casting high quality composite ingots.

Abstract: In this study, the relationship between the structure and properties of commercial purityaluminium (AW-1199) was investigated by applying constrained groove pressing (CGP) method.The refinement of the coarse grain aluminium microstructure to submicrocrystalline size by largeplastic strain at room temperature defined. The impact of various strains upon microstructurechanges is investigated using transmission electron microscopy (TEM) and electron back scatterdiffraction (EBSD). A mixture of subgrains produced by grains subdivision and polygonizedsubgrains formed locally due to dynamic recovery was found in the deformed aluminium. Thetensile properties and resulting hardness are related to microstructural evolution induced by CGP. Asubstantial impact of straining upon the increasing in tensile strength was observed after the firstpass. Further strain increase had an insignificant effect on tensile strength but was accompanied byductility loss. The post deformation annealing effect was then explored with aim to increase theductility. The results indicate that changes in strength and ductility may be related to formation of abimodal structure.

Abstract: According to von-Mises criterion, five kinds of independent slip systems are required for uniform deformation, so it is necessary to activate non-basal slip systems to show good ductility. However, it has not become clear the effect of Zn or Al for non-basal slip systems yet. To investigate deformation behavior of magnesium crystal by non-basal slip and alloying effect for the non-basal slip, pure magnesium and Mg-Al-Zn single crystals were stretched in the [110] direction. While {112}<23> second order pyramidal slip was activated at room temperature in pure magnesium, {101}<23> first order pyramidal slip became active slip at higher temperature. In Mg-Al-Zn alloy single crystal, {101} twin also activated by adding aluminum. These results indicate that active non-basal slip systems and twin in magnesium strongly depend on deformation temperature and alloying elements.

Abstract: The present work presents a comparative study of FSW die cast AM50 Mg alloy. The main focus is the effect of the FSW metallurgy on the corrosion behavior of the alloy. AC and DC polarization tests were carried out on the FSW Mg alloys. The microstructure was examined using electron microscopy (SEM).

Abstract: Due to limited deformability at room temperature, high temperature forming of magnesium alloys appears as an interesting alternative. Superplastic properties can be obtained in the case of fine grained magnesium alloys and in this regime, due to significant damage sensitivity, fracture strain is mainly controlled by nucleation, growth and coalescence of cavities. Magnesium alloys with large grained alloys can also exhibit interesting deformabilities at high temperature since dislocation movements can be controlled by a solute drag effect promoting plastic stability. Examples of such situations are presented in the case of wrought magnesium alloys, the associated damage mechanisms being investigated thanks to 3D X-ray micro tomography performed in continuous mode, namely directly during high temperature deformation tests.

Abstract: Recent studies on Mg-Sr-Mn system exhibited the importance of α-Mn dynamic precipitation during creep on creep strengthening. In the present work, the effect of trace levels of Ce addition on the creep resistance of Mg-Sr-Mn system was investigated. The creep deformation in the quaternary alloys (Mg-Sr-Mn-Ce) were four times lower than the deformation seen in ternary alloys (Mg-Sr-Mn). Dynamic co-precipitation of Mg₁₂Ce and α-Mn was mainly responsible for creep strengthening. Additionally, the presence of a trace amount of Sr in the Mg matrix affected the growth of the creep-induced Mg₁₂Ce precipitates which were found to be lying parallel to the []_Mg direction. This growth orientation is different from the orientation of Mg₁₂Ce precipitates typically observed in Mg-Ce binary and Mg-Ce-Mn ternary alloys where plate-like precipitates lie along []_Mg.

Abstract: Extruded magnesium alloys showed mechanical anisotropy due to the development of strong crystallographic textures during forming processes. In the present study the strain hardening behavior and texture evolution of an extruded AM30 magnesium alloy were studied in compression using cylindrical samples oriented at angles of 0°, 15°, 30°, 45° and 90° from the extrusion direction (ED). The yield strength decreased with increasing angle up to 45° and then increased at 90° from the ED, while the ultimate compressive strength exhibited a reverse trend. Both hardening capacity and fracture strain first increased from 0° to 45° and then decreased at 90° from the ED. The strain hardening behavior was directly related to the texture change and twinning, which played a key role in accommodating the compressive deformation, as the c-axes in most grains were observed to rotate always towards the anti-compression direction, irrespective of the sample orientation.

Abstract: 6 mm thick and 1500 mm wide magnesium alloy AZ31, AZ61, AZ91, AM50 and AM60 sheets were produced by twin roll casting technique. Sheets were homogenized between 350-475oC for 1-24 h. AZ31 sheets were rolled down to 1 mm by symmetrical warm rolling and asymmetric warm rolling. Age hardening was also performed on magnesium alloy AZ91 sheets. Specimens were aged at 100-300oC for up to 100 h. Characterization was performed by light microscope, scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), transmission electron microscopy (TEM) and x-ray diffraction (XRD) after twin roll casting and also after each thermomechanical process including aging. Tensile tests and micro hardness tests were performed for mechanical properties. In addition to the room temperature tests, elevated temperature tensile tests were also performed at 100, 150, 200, 250, and 300oC at various deformation speeds. Forming limit diagram of the material was determined under warm forming condition.

Abstract: Extrusion-torsion simultaneous processing is a very attractive technique for fabricating a rod-shape material with fine grain and random texture. We have proposed a new screw form rolling process combined with preliminary extrusion-torsion simultaneous working. Microstructure evolution and mechanical property change of AZ91D magnesium alloy during extrusion-torsion simultaneous processing was examined through microstructure observation, X-ray diffraction analysis and micro-Vickers hardness measurement. By the addition of torsion, the crystal orientation of AZ91D magnesium alloy workpiece was drastically changed from basal crystalline orientation to the random orientation. Crystal grain occurred through the dynamic recrystallization and tended to coarsen with an increase of extrusion-torsion temperature. Grain refinement under 2 um was achieved at the lowest extrusion-torsion temperature of 523 K. M8 gauge AZ91D magnesium alloy screw was successfully formed at room temperature using the extrusion-twisted workpiece preliminary solution treating at 678 K for 345.6 ks. It was found that the extrusion-torsion temperature of 678 K must be selected to fabricate the good screw without any defects.