Materials Science Forum Vols. 783-786

Abstract: The thermal conductivities of binary Mg-CaO and Mg-Zn, and ternary Mg-Zn-CaO alloys have been investigated by evaluating the effect of CaO on pure Mg and Mg-Zn alloys, with an emphasis to develop a new Mg alloy by compromising thermal conductivity, process-ability and mechanical property. The Mg alloys specimens were prepared by casting into a steel mold and then by machining. The thermal conductivities of the alloys were determined by evaluating the thermal properties of specific heat and diffusivity, from room temperature to 200 °C. OM, SEM, and EDS were used to analyze the microstructures and phases. The fluidity was also investigated by using a spiral fluidity mold for improved process-ability during actual die casting.

Abstract: The object of this study was to develop parameters of plastic deformation in hot direct extrusion and die forging process of magnesium alloy MgAlZn. Extrusion process was conducted in the temperature range 350°C÷450°C at different extrusion speeds (ram speed 0.5 mm/s÷3 mm/s). The extrusion tests have showed that for this alloy it is possible to use the temperature of the extruded material ranging from 350°C÷450°C when the process is run at the optimum ram speed. Higher temperature of extrusion is beneficial For higher ram speed of 3mm/s, in this alloy at a temperature of 420÷450°C, cracks began to show in the surface. The same problem were for die forging. Extruded rods and forging parts were characterized by mechanical properties and structure in different heat treatment tempers. This magnesium alloy obtained in the T5 temper higher mechanical properties then T6 temper. The paper also presents research results of investigation of conversion coating on MgAlZn magnesium alloy by anodic oxidation method in non-chromium solutions. It was found that the coating produced in non-chromium solutions show considerable increase of corrosion resistance of tested alloy.Keywords: Mg alloys, extrusion, mechanical properties, structure, corrosion resistance.

Abstract: Magnesium alloys based on Mg-Sn-Ca system have shown improved corrosion and creep properties. In this type of alloys,Sn forms a solid solution with Mg that improves the corrosion resistance while Ca forms thermally stable intermetallic phases in the matrix enhancing the creep resistance. The Sn to Ca ratio is an important variable in deciding the type of intermetallic phases that form in the microstructure.In Mg-3Sn-1Ca alloy (TX31), a single intermetallic phase CaMgSnforms, which is responsible for its improved creep strength.With a view to evaluate its forgeability,isothermal forging experiments of TX31 were conducted on a hydraulic press in the temperature range of 350 °C to 500 °C and at speeds of 0.01 mm s^-1to 10 mm s^-1 using a semi-closed die. Finite-element (FE) simulation of the forging process was also conducted using the software DEFORM 2D to obtain the local variations of strain and strain rate. The effectivestrain values are below2.4 in the forged components and the forging loads predicted using FE simulation correlated well with the experimental data for all the forging conditions. The microstructures of the forgings show that CaMgSn phase is well distributed in the matrix which exhibited dynamically recrystallized microstructure as predicted by the processing map.

Abstract: The dependence of different brushing parameters during surface preparation on the resulting bonding strength for the roll-bonding of Mg-Mg composites was investigated. A parameter study with different brushing parameters was used to analyze the effect on surface roughness as pre-step for roll-bonding trials with different roughness ratio. Furthermore, the influence of different Mg alloys and of a roll-bonded Al-Mg-Al sandwich composite on the obtained bonding strength was investigated.

Abstract: The Mg-Al-Zn alloy AZ91 is age hardenable, forming precipitates of –Mg17Al12 after suitable heat treatment. In this alloy both continuous and discontinuous precipitation canoccur. These modes are in competition and lead to very different microstructures. In this paper,a new model is developed to predict this competitive precipitation. The model is validated usingliterature data and then used to study the competition between precipitation modes and predictthe effect of cooling rate from solution treatment on the microstructure expected in AZ91.

Abstract: In recent years, Mg-Al-Zn system alloy has been used for the parts in the automobile for weight reductions. The age-hardening behavior of Mg-6mass%Al (-1mass%Zn)-0.3mass%Mn alloys sand mold castings were investigated by Vickers hardness measurement and optical microscopic observation. Both alloys were solution-treated and then isothermal-aged at 473, 498 and 523K. For each aging temperature, both alloys were indicated age-hardening obviously, and decreased the value of maximum hardness and shorten time to maximum hardness with heighten aging temperature. Age-hardening curves for both alloys approximately showed the same change of hardness. However, these optical micrographs after aging treatment are observed different microstructure. In case of AM60 magnesium alloy, discontinuous precipitation preferentially occurred in aged sample. The volume fraction of discontinuous precipitation was larger than that of continuous precipitation. On the other hand, in case of AZ61 magnesium alloy, the volume fraction of continuous precipitation was larger than that of discontinuous precipitation. Furthermore, over-aged sample of both alloys were consisted of discontinuous precipitation, continuous precipitation and pre-precipitation area.

Abstract: Microstructures and Vickers microhardness in AZ91 magnesium alloys without and with 1mass%Y addition fabricated by casting were investigated. Vickers microhardness increases with adding 1%Y. Microstructure in AZ91 without Y addition was analyzed to contain mainly α-Mg and Mg₁₇Al₁₂ by X-ray diffraction. Microstructural observations with optical, scanning and transmission electron microscopes show that microstructure consists of α-Mg dendrite, non-equilibrium eutectic Mg₁₇Al₁₂ and lamellar Mg₁₇Al₁₂. The non-equilibrium eutectic Mg₁₇Al₁₂ exists between α-Mg dendrites. The lamellar Mg₁₇Al₁₂ forms near the edge of the α-Mg dendrite arm. The lamellar Mg₁₇Al₁₂ has Burgers orientation relationship for α-Mg matrix. It suggests that the lamellar Mg₁₇Al₁₂ precipitates from Al-supersaturated region within α-Mg dendrite. Addition of Y to AZ91 hardly changes dendrite arm spacing, but decreases a size of region, where longitudinal directions of primary dendrite arms are almost parallel or a single dendrite exists. Y-addition increases nucleation site for dendrite, namely makes the unidirectionally-solidified region fine, resulting in increase in hardness.

Abstract: The deformation and hardening mechanisms of magnesium usually lead to a typical basal orientation of crystals during a production of sheets by forming techniques. The basal texture related anisotropic property behavior and especially the further decrease in formability at room temperature is disadvantageous and undesired for subsequent rolling and final forming processes.The objective of this work is to find methods to improve these texture-related properties and the cold forming ability of magnesium sheets. Firstly, rolling at different temperatures and pass reductions, with the goal of weakening the basal texture component in semi-finished products is investigated, based on the advantageous initial texture and microstructure of twin-roll-cast (TRC) magnesium strips. In this context, texture and microstructure development is examined after a particular multi-pass rolling and heat treatment processes. Twin-roll-cast magnesium strips of alloy AZ31, with an initial thickness of 4.5 mm, rolled to a final thickness of 1.2 mm, are used as feedstock.Secondly, a new thermo-mechanical magnesium strip treatment has been developed in order to completely disintegrate the basal texture and intentionally generate only non-basal orientations with high Schmid-factors for the easy-to-activate basal slip systems. This process, which is designed as a final strip treatment, has been investigated regarding its texture change effect on rolled 1.2 mm AZ31 sheets, which also originate from TRC feedstock.It has clearly been found that the developed rolling technology for TRC feedstock leads to a significantly reduced basal texture due to grain boundary rotation and recrystallization at those rotated regions. The application of the separately developed strip treatment effects a complete elimination of the basal texture in a large volume of the sheet. Applying both technologies on magnesium sheets results in a tremendous increase in formability at room temperature as a consequence of the altered texture.

Abstract: The isothermal forging process of a bracket and its microstructure evolution of Mg-10Gd-2Y-0.5Zn-0.3Zr alloy have been investigated in the present study. The results show that the bracket with thin-web and high-rib is well formed through modifying corners and adding an active damping block into male die. Amounts of lamellae and particles distribute uniformly on the matrix after the isothermal forging process and ageing process. The isothermal forging process has an obvious effect on the precipitation behaviour of secondary phases, while it did not change the grain size greatly. With the increase of ageing time, more secondary phases precipitate from α-Mg matrix until 60h. The optimal ultimate tensile strength and elongation of the peak-aged alloy are 382MPa and 4.03%, respectively. The combined effects of LPO and β′ phases contribute to the high strength of the peak-aged alloy.

Abstract: Compressive creep behavior of hot-rolled (40%) Mg-Y binary and Mg-Y-Zn ternary dilute solid solution alloys are investigated in this study. Creep strength is substantially improved by the addition of zinc. Activation Energy for creep in Mg-Y and Mg-Y-Zn alloys are around 200 kJ/mol at the temperature range from 480 to 570 K. These values are higher than the activation energy for self-diffusion coefficient in magnesium (135 kJ/mol). Many stacking faults, which are planar type defects are observed on the basal planes of the magnesium matrix in Mg-Y-Zn ternary alloys. TEM observation has been revealed that the non-basal a-dislocation slip is significantly activated by these alloys. The rate controlling mechanism of Mg-Y and Mg-Y-Zn dilute alloys are considered to the cross-slip or prismatic-slip controlled dislocation creep with high activation energy for creep, more than 1.5 times higher than the activation energy for creep controlled dislocation climb.