Materials Science Forum Vols. 638-642

Abstract: The influences of rare earth elements addition on the crystallographic texture and microstructural evolutions are examined during rolling and annealing of Mg-sheets. In case of Nd or Y additions, dynamic recrystallisation is suppressed such that the deformed microstructure is observed after hot rolling with relatively large strain per pass. Cold rolled binary Mg-Nd alloy sheet shows strong texture with splitting of the basal poles in the rolling direction, however, the texture intensity decreases significantly during the recrystallisation annealing. From the comparison of deep drawing behaviours between commercial ZE10 and AZ31 sheets, it is observed that the addition of the rare earth elements and accompanying texture changes result in the improved formability.

Abstract: Magnesium and its alloys are the lightest among the metallic materials which are in common use, having a high strength-to-weight ratio along with an excellent machinability and a high damping capacity.　Because of these characteristics, nowadays they are applied to various structural parts such as aircraft, automobiles, optical and electronic instruments, industrial robot components. 　TIG welded joints of AZ31 magnesium alloy has high joint efficiency which can be obtained by the selection of optimum welding conditions. If the welded joints of magnesium alloy are applied to the structure materials, it is very important not only the strength of under static load but also the strength of under dynamic load. However, there is only few report about impact properties of welded AZ31 magnesium alloy joints. In this research, AZ31 magnesium alloy plates 6 mm in thickness were welded on butt welding of square groove without root gap and filler wire using TIG welding process. AZ31 magnesium alloy was carried out at various welding conditions. The effects of welding conditions and notch location on impact properties were studied. Impact values of welded joints were nearly equal to those of the base metal, regardless of welding current. In case of the notched specimens of welded joints, crack initiation energy showed slightly lower than crack propagation energy, regardless of notch location. But, unnotched specimens of welded joints, crack initiation energy showed remarkably high value than crack propagation energy. This means, crack initiation energy spent the large amount of total absorbed energy.

Abstract: The market share of wrought Magnesium products such as structural and functional components is recently increasing. Extrusion at elevated temperatures is used to produce reliable plastic deformation, since magnesium alloys have limited ductility at room temperature. In order to produce sound extruded products, high quality billets are required. Understanding the influence of direct chill casting conditions on the production properties such as quality, safety, workability and microstructure have a profound importance. Comprehensive computer simulations were used in order to model the casting so that process parameters can be identified and controlled, resulting in significant benefits. The aim of modeling is to provide temperature profiles for a more accurate solidification analysis, predict the solidification time and the effect of cooling on the solidification. The experimental study included castings of several Magnesium alloys, each with 7 (seven) thermocouples that were submerged into the billet. Verification of the simulations was carried out based on the data collected. Complimentary work was conducted on microstructure analysis in as cast and as-extruded states.

Abstract: The effect of annealing treatments on the evolution of the strain rate sensitivity with strain of AZ61 magnesium alloy processed by severe rolling was investigated and related to previous results on normal plastic anisotropy (r-value). The various annealing treatments produce two effects on the microstructure: grain coarsening and slight weakening of the texture. In addition, these treatments produce a noticeable decrease in strain rate sensitivity and an increase of work hardening rate that is related to the decrease of the anisotropy. It is concluded that these effects are related to an enhanced contribution of basal slip as a consequence of the microstructural changes induced by the annealing treatments.

Abstract: Due to the increasing demand of deep drawing applications for magnesium alloys in the future magnesium sheets with good mechanical and forming properties are required. These properties depend on the processing route of the sheet material. The deformation behavior of magnesium alloys is strongly influenced by the texture. Extruded magnesium sheets exhibit a different texture than rolled magnesium sheets. Therefore, the forming properties of the extruded magnesium sheets are supposed to be different compared to rolled sheets. Thin extrusion of the magnesium alloy AZ31 with a thickness of 1.5 and 2 mm were performed. Adjacent the extruded sheets were tested for their microstructure, texture and mechanical properties. The texture stability and evolution after the rolling of extruded magnesium sheets were investigated. Thus some of the 1.5 mm sheets were rolled to 1.0 mm and analyzed by OIM, X-Ray and mechanical testing. Concluding the results were compared to the properties of the just extruded 1.5 mm sheet and conventionally rolled sheet of 1 mm thickness.

Abstract: The AZ61 magnesium alloy was subjected to the differential speed rolling at different rolling conditions including reduction per pass, rotation speed ratio and rolling temperature, and the influences of the rolling conditions on microstructure, texture, mechanical properties and formability of the as-rolled sheets were investigated. Increasing the reduction per pass results in a more homogeneous microstructure and a relatively weaker basal texture. With increasing the rotation speed ratio, the inclination angle of basal pole toward the rolling direction increases and the stretch formability enhances correspondingly. Increasing the rolling temperature has an effect on weakening the basal texture intensity and leads to the enhancement of stretch formability.

Abstract: In order to realize large-scale Mg-Zn-Y alloys with high strength and high heat resistance, we have developed a unique casting process to produce a large homogeneous ingot investigating the mechanical properties of the extruded alloys. First homogeneous ingots (335 mm x 850 mm) were prepared by a unique stir casting process. Then large-scale extruded alloys (100 mm) were prepared at 648 K with the extrusion ratio of 10. The Mg-Zn-Y alloys have exhibited higher yield and fatigue strengths than those of aluminum alloys. The yield strengths of the aluminum alloys have decreased drastically above 473 K, whereas those of the Mg-Zn-Y alloys have not. It is noteworthy that the yield strength (200 MPa) and the fatigue strength (75 MPa) of the Mg-Zn-Y alloys at 523 K are about twice and 1.2-1.4 times as high as those of the aluminum alloys respectively. Moreover, the creep strengths have been equivalent or higher than those of aluminum alloys. From the above results, we have verified that even being made by the large-scale extrusion, the Mg-Zn-Y alloys possess higher strength than those of heat resistant aluminum alloys.

Abstract: Microstructure and tensile creep behaviour of the die-cast AE44 and AJ62 magnesium alloys has been studied at temperatures between 175°C and 200°C and at stresses in the range from 60 to 75 MPa. At the 175°C the AJ62 and AE44 alloys exhibit good creep resistance after 120h creep deformation. At 200°C the AE44 alloy shows still good creep resistance, whereas in the case of AJ62 alloy the rapid decreasing of creep resistance has been observed. TEM observations reveal dislocations cell structure in AE44 alloy after creep test. In AJ62 alloy subgrain formation and decreasing the dislocation density have been observed.

Abstract: The creep behaviour of a creep-resistant AE42 magnesium alloy has been examined in the temperature range of 150 to 240°C at the stress levels ranging from 40 to 120 MPa using impression creep technique. A normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at all the temperatures and stresses employed. The stress exponent varies from 5.1 to 5.7 and the apparent activation energy varies from 130 to 140 kJ/mol, which suggests the high temperature climb of dislocation controlled by lattice self-diffusion being the dominant creep mechanism in the stress and temperature range employed. The creep behaviour of the AE42 alloy has also been compared with its composites reinforced with Saffil short fibres and SiC particles in four combinations. All the composites exhibited a lower creep rate than the monolithic AE42 alloy tested at the same temperature and stress levels and the decrease in creep rate was greater in the longitudinal direction than in the transverse direction, as expected. All the hybrid composites, i.e., the composites reinforced with a combination of Saffil short fibres and SiC particles, exhibited creep rates comparable to the composite reinforced with 20% Saffil short fibres alone at all the temperature and stress levels employed, which is beneficial from the commercial point of view.

Abstract: In the present work, the effect of Nd addition on microstructure change, precipitation behaviours and mechanical properties, of Mg-5Sn-3Ca based alloys were investigated. With increasing of Nd addition, size of the Mg-Sn-Ca ternary phases with fine needle shape (A-type) was decreased and volume fraction of these phase were increased. As Nd was increase from 0.5, 1.0 and 3.0 wt.%, size and volume fraction of Mg2Sn with coarse needle shape phase (B-type) was decreased. However, in 3.0 wt.% Nd containing alloy, size of these Mg2Sn phase was smaller than that of 0.5 and 1.0 wt% Nd addition alloy and volume fraction of these phase was decreased.