Materials Science Forum Vols. 747-748

Abstract: The distribution of shrinkage porosities in sand cast Mg-Y-RE (WE54) alloy castings was characterized through density measurement and calculated by Archimedess principle. The effect of porosity on mechanical properties of sand cast WE54 alloy was investigated through tensile tests and microstructure observation. It was found that the shrinkage porosities distributed mainly in the middle of the plate where the liquid feeding was quite inconvenient. And the porosities were formed along grain boundaries when secondary phases formed at the end of solidification. Hardness tests showed that the vikers hardness declined linearly with increasing porosity volume fraction. While the tensile strength and nominal yield strength declined exponentially as the porosity volume fraction increased. Microstructure observation showed that the fracture cracks propagated along the grain boundaries where porosities and secondary phases gathering together in as-cast WE54 alloy. The tiny porosities distributed in the secondary phases were observed, which could reduce the tensile strength of cast specimens significantly. The heat treatment strengthening effects were significantly weakened by porosities, and even no heat treatment strengthening effect was detected when the porosity volume fraction was higher than 1%. The microstructure observation also proved that no heat treatment strengthening effect existed in samples containing porosities.

Abstract: The microstructures and tensile properties of as-cast, solution-treated and aged Mg-4Zn-2Sn-2Al (wt.%) alloy have been investigated. The microstructure of the as-cast alloy consists of α-Mg, Mg₂Sn and Mg₃₂(Al,Zn)₄₉ phases. The yield tensile strength and the ultimate tensile strength of the as-cast alloy are 87.7 MPa and 241.3 MPa, respectively, and the elongation reaches to 18.8% showing excellent ductility. After solution treatment, all of the Mg₃₂(Al,Zn)₄₉ phase and the majority of Mg₂Sn particles have dissolved into the matrix. During aging process, Mg₂Sn phase precipitates from the α-Mg supersaturated solid solution. With the precipitation strengthening of Mg₂Sn phase, the yield tensile strength and the ultimate tensile strength of the alloy increase to 112.5 MPa and 280.4 MPa, respectively, while, the elongation decreases to 12%.

Abstract: Casting nozzle structure is the decision factor for the formation process of the fluid field of magnesium alloys melt, and the distribution uniformity of the casting nozzle has an important influence on the process stability and the billet quality. In this research, the casting nozzle structures used in producing AZ31 magnesium alloys slab with width of 1500 mm by using horizontal twin-roll continuous casting were taken as research object, and the fluid field of magnesium alloy melt in different casting nozzle structures were simulated, and the influences of the numbers, position, shape and size of the divergent fluid block on the fluid velocity were analyzed. The results show that the fluid velocity distribution obtained by seven divergent fluid blocks were superior to five or three divergent fluid blocks, the streamline divergent fluid blocks were superior to the linear ones, and the small divergent fluid blocks were superior to big ones. At the same time, a new casting nozzle structure was presented, the difference between the maximum and minimum fluid velocity by using the new casting nozzle was 4.69%, and the distribution uniformity of fluid velocity meets the requirement of production.

Abstract: The control of process parameters in the horizontal twin-roll casting is crucial for the quality of sheet and the continuity of the process. A temperature field coupled with flow field mathematical model was established during the horizontal twin-roll casting of AZ31 magnesium alloy sheet with 1500mm in width and 8mm in thickness in this paper. The temperature field in the casting zone was solved by the software ANSYS. The effect of process factors, such as casting speed, pouring temperature and cooling intensity, on casting zone temperature of different process parameters were studied. Based on the solved temperature field, with the hot roll formula and test data of yield strength, the effect of casting speed, pouring temperature and cooling intensity on cast-rolling force in the cast-rolling zone was also dicussed. The results indicate that the casting speed has the greatest effect upon the temperature field and cast-rolling force, while the pouring temperature is the least. In addition, the value of critical cooling intensity increased with the increase of the casting speed. The quality defect called melt sheet or the leakage phenomenon appear when the cooling intensity is lower than the minimal critical cooling intensity value, and crack or rolling suspended appear if the cooling intensity is higher than the maximal critical cooling intensity value. When the casting speed are 1m/min, 2m/min and 3m/min, the minimal and maximal critical cooling intensity are 500, 1200, 2 000 W/(m^2.K) and 2500, 5000, 7500 W/(m^2.K) respectively.

Abstract: The microstructure and the kinetics of β-Mg₁₇Al₁₂ phase transformation during annealing process for warm-rolled AZ91 magnesium alloy were investigated. The results showed that few twins were found in solid state AZ91 magnesium alloy after warm rolling, which provides energy for β-Mg₁₇Al₁₂ particles to nucleate and growth; the β-Mg₁₇Al₁₂ particles were prior to form at α-Mg, twin grains boundary and especially the place of high energy; by the annealing time extending, the quantity of β-Mg₁₇Al₁₂ particles increased. And combined with the experimental data and the JMAK equation, the fitting equation of Kinetics of β-Mg₁₇Al₁₂ phase transformation was built. During annealing process, the twin grains disappeared and the original bulky organization was replaced by recrystallization grain, while the grain refinement was obvious with the grain reduction to 20-60μm.

Abstract: The Mg-1.0Zn-xCa (x=0.2, 0.5, 0.8, 1 wt. %) alloys were prepared by zone solidification and backward extrusion technology. The microstructure and mechanical properties of backward-extruded Mg-1.0Zn-xCa alloys were investigated. The results showed that these backward-extruded Mg-1.0Zn-xCa alloys were mainly composed of equi-axed pentagon-shaped grains and some Mg_0.9Zn_0.03 precipitates. The tensile and compressive strengths of backward-extruded Mg-1.0Zn-xCa alloys were greatly improved. The improved mechanical properties are mostly attributed to fine grain strengthening, solid solution strengthening and precipitate strengthening. The results demonstrated that the micro alloying of Ca element was one of effective method to improve the mechanical properties of Mg-1.0Zn based biomaterials.

Abstract: By multi-axis active load deformation method, the equivalence diameter tee joint was formed on multi-axis numerical control hydraulic press machine. The loading route was determined by numerical simulation. The experiment results showed that two loading method can complete form the parts. The different directions metal flow was observed under multi-axis loading conditions. To simplify analysis, the flow field was divided into several regions. In every region, the metal flow direction was only one. In multi-axis loading, the way of deformation follows priority deformation principle: The metal flows only choose one direction even if the work piece were under the complicated coupling field condition. The mathematical model of the deformation force and metal flow rate was established. The theoretical calculation had been provided.

Abstract: The corrosion mechanisms of as-cast Mg-8Y and Mg-5Y-7Gd-1Nd-0.5Zr Mg alloys in 5% NaCl aqueous solution were investigated by electrochemical testing and SEM observation. The electrochemical results indicated that the corrosion resistance of Mg-8Y and Mg-5Y-7Gd-1Nd-0.5Zr alloys in 5% NaCl aqueous solution gradually deteriorated with increasing of immersion time expect for 2h and 24h immersion. The Cl^- anion led to the initiation and development of the corrosion pits. The cathodic reaction was driven by hydrogen evolution reaction. The presence of filiform corrosion also proved a resistant oxide film naturally formed on the surface of Mg-8Y and Mg-5Y-7Gd-1Nd-0.5Zr alloys. The corrosion resistance of Mg-5Y-7Gd-1Nd-0.5Zr was better than Mg-8Y.

Abstract: A high strength and toughness extruded Mg-Zn-Y alloy based on quasicrystal-strengthening has been studied. The effect of extrusion and heat treatment on the microstructures and mechanical properties of Mg-Zn-Y alloy were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD) and tensile testing. The experimental results indicated that the coarse dendrite crystals were broken through the hot extrusion, and dynamic recrystallization appeared during the hot extrusion, which obviously refined the hot-extruded microstructure to the average grain size about 20μm. A large amount of strengthening phases such as Mg₃Zn₆Y(I-Phase), Mg₁₂ZnY(X-Phase) and MgZn₂, which were massive, grainy and clavate, dispersedly precipitated from the matrix along grain boundary during ageing treatment at 225 after extrusion, and made the sliding of grain boundaries restrained, which resulted in an enhancement for mechanical properties to a great extent. At the same time, the tensile strength and yield strength increased after ageing treatment. After ageing treatment of 225×24h, the highest tensile strength and yield strength of the extruded Mg-Zn-Y alloy were obtained: σ_b=506.7MPa, σ_0.2=373.5MPa, which were increased by 104.8% and 120.4%, respectively, compared with the extruded Mg-Zn-Y alloy, however the elongation decreased to 16.52%.

Abstract: In this study, a Mg-2.1Y-0.5Zn-0.6Cu-0.1Zr (WZC200, in at. %) alloy was prepared by permanent casting method. On the basis of thermal analysis results during solidification process, two temperatures (T₁/763K and T₂/773K) were selected for subsequent solid solution treatments. Microstructure evolution and the tensile property changes after T4 treatment at T₁ and T₂ for different holding time were also investigated in this study. The tensile testing results showed that yield strength and ultimate tensile strength were improved with the fraction increase of the lamellae-shaped LPSO phase in the grain interiors, rather than the total fraction of LPSO phase.