| Magnesium and its alloys are the metal structure materials with the lowest density, which have a lot of excellent properties, such as high specific strength and stiffness, excellent damping capacity, better thermal conductivity, and good machinability. Magnesium alloys are widely used in automobile, electronic, aviation and other fields, which are considered as green engineering materials during 21st. With the requirements of saving energy, reducing emissions and structural lightweight, research on magnesium and its alloy materials has become a global hot spot.Semi-solid forming (SSF) technology is a new metal forming method proposed by Professor M.C. Flemings, which combines the advantages of both liquid forming and solid forming. The processing temperature is lower than the liquid forming (Mg alloy can be reduced about 100℃), and the deformation resistance is lower than the solid forming. SSF is very suitable for the processing of magnesium alloys, and is known as the most promising materials processing methods during 21st. The core of the technology is to obtain the alloy slurry with non-dendritic structure between the solidus and liquidus temperature. The solid particles in the semisolid slurry should be small and equiaxed, in order to make the slurry have good formability. Therefore, the preparation of eligible semisolid microstructure is the basic for semisolid processing. Isothermal heat treatment is a good method for the preparation of semisolid slurry, which only needs to hold the alloy in the semisolid temperature range for a while. Isothermal heat treatment process is simple, low cost, and has broad prospects for development.Currently, the magnesium alloy for SSF in commercial application is mainly dominated by Mg-Al system, such as AZ91D, AM60 and AM50. The content of Al in the alloy is up to a maximum 10wt.%, the magnesium alloys with these composition may be not the best alloys for SSF forming. Therefore, we should design new SSF alloys which can meet the requirements of different parts, and fully displayed the advantages of semisolid forming.First of all, the effects of isothermal process parameters on the microstructure evolution of semisolid AZ91D alloy produced by strain-induced melt activation (SIMA) were investigated. The results showed that long isothermal time could make the semisolid particles more globular, but the size of the particles would grow larger; high semisolid isothermal temperature would reduce the solid volume fraction and accelerate the spherical evolution of the solid particles. The mechanism of the particles formation can be divided into two stages. First are recovery, recrystallization and partial melting when the holding time is short or the holding temperature is lower. With the extension of holding time, the amount of liquid increases, the grains separate from each other forming the solid particles and the particles grow large. During this stage coalescence ripening and Ostwald ripening are the dominant mechanisms for structural coarsening. Furthermore, due to the effect of interface curvature, the particle spheroidization takes place in order to decrease the free energy. It was found that the optimal isothermal process parameters should be 570℃and 10～20 min of isothermal temperature and time respectively for the AZ91D alloys with 36.84% compression.In order to study the effects of Al element on the microstructures of as-cast and semisolid Mg-Al alloys, three types of Mg-Al alloys were prepared and characterized by Mg-10Al-Zn, Mg-20Al-Zn and Mg-44A1, respectively. With the increasing of Al, the content of primary a-Mg phases decrease, and the a-Mg dendrites are refined. Furthermore, the content of P-Mg17Al12 phases increasing, and the eutectic mixtures of a-Mg+β-Mg17Al12 become the main phases of the alloy. When the Al content increases to 44wt.%, the phase in the alloy would be onlyβ-Mg17Al12 through the rapid solidification after remelting. The results of micro-hardness indicate that the micro-hardness of the a-Mg solid solution and the eutectic mixture (a-Mg+β-Mg17Al12) in the Mg-20Al-Zn alloy are higher than in the Mg-10Al-Zn alloy. The Al dissolved in the a-Mg has an effect of solid solution strengthening. When the content of Al in the Mg-Al alloys increases, the Al dissolved in the a-Mg solid solution also increases. Therefore, the micro-hardness of the a-Mg solid solution improves. The intermetallic phaseβ-Mg17Al12 is a hard and brittle phase with the micro-hardness about 280 (HV), which is much higher than the a-Mg. So when the content ofβ-Mg17Al12 increases in the eutectic mixture, the micro-hardness of the eutectic mixture increases obviously. The a-Mg dendrites refinement and the increasing of P-Mg17Al12 phases are both good for SSF, so increasing the content of Al in a certain range could help to develop suitable SSF magnesium alloys.During the next study, a new alloy of Mg-14Al-0.5Mn was prepared. And the microstructure evolution of semi-solid Mg-14Al-0.5Mn alloy during isothermal heat treatment was investigated. Furthermore, the effects of original as-cast microstructure, isothermal temperature and time on the semisolid microstructure were discussed. In the Mg-14Al-0.5Mn alloy, as a result of increasing Al content, the amounts of P-Mg17Al12 and eutectic mixtures increase obviously. These intermetallic compound and mixtures with low melting point distribute at the grain boundaries around the primary a-Mg. When the alloy is held at the semisolid temperature, theβ-Mg17Al12 and eutectic mixtures begin to melt immediately. At the bend of a-Mg dendrite arm, the melting point lowers down because of the large curvature. These areas are first melted, making the morphology of a-Mg change from dendrites to irregular solid particles. At the first few minutes, melting plays a major role on the size of the solid particles. With increasing of isothermal time, the amount of liquid increases and the solid particles grow large and become globular. At this stage, spheroidizing and particle coarsening determine the morphology of the solid particles. In the original as-cast microstructure, adequate amount of the second phase with low melting point is needed, so that the semi-solid alloy will achieve an ideal solid fraction in a short isothermal time. The finer the size of the initial grain is, the smaller the size of the semi-solid particles will be. Select high isothermal temperature between the solid-liquid range and short isothermal time to achieve cost savings and improve production efficiency. Mg-14Al-0.5Mn alloy held at 520℃for 5-8 min can obtain good semisolid microstructures.Increasing the content of Al in magnesium alloys in a certain extent could make the alloy have good semisolid formability. But the increasing ofβ-Mg17Al12 phases also decreases the toughness and high temperature performance of the alloy. Therefore, a small quantity of Ce was added in order to suppress the formation of Mg17Al12 and increase the comprehensive performance of the alloys. Based on the above ideas, new Mg-10Al-xCe (x=0, 0.25,0.5,1.0wt.%) alloys for SSF were designed. Found by the comparison, the a-Mg dendrites in as-cast Mg-10Al-0.25Ce, Mg-10Al-0.5Ce and Mg-10Al-1.0Ce alloys are larger and more developed than those in Mg-10Al alloy. It is mainly caused by the decreasing of undercooling in front of the a-Mg tip with Ce addition. Besides, among the Mg-10Al-0.25Ce, Mg-10Al-0.5Ce and Mg-10Al-1.0Ce alloys, the grain sizes decrease gradually with the increasing of Ce content, which is the result of the Al-Ce compounds at grain boundaries restricting grain growth. The initial as-cast microstructures greatly influence the semisolid microstructures after the isothermal heat treatment. Among the Mg-10Al alloys with Ce addition, the Mg-10Al-0.5Ce alloy has the best semisolid rheological parameters. And the suitable isothermal temperature and time for Mg-10Al-0.5Ce alloy are about 560℃and 20min, respectively.The addition of Ce element will coarse the as-cast structure of Mg-Al alloy, while the cost of rare earth elements is high, so in the next study, we consider to use Ca elements replace the rare earth, and prepare Mg-13Al-xCa (x=0,0.3,0.6,1.0,3.0wt.%) alloys. The effects of Ca content on the microstructure evolution during isothermal heat treatment were investigated. It is found that Ca elements addition could refine the microstructure of as-cast Mg-13A1 alloy. However, when the content of Ca is less than lwt.%, due to the overlap of a-Mg dendrite, the size of solid particles is nonuniform, and the shape of the particles is irregular; when the Ca content increased to 3wt.%, the secondary dendrite arm fracture makes the a-Mg dendrite further refined, which make the semisolid microstructure of Mg-13Al-3Ca alloy more desirable. Ca elements addition can also change the semisolid temperature range, when the content of Ca is less than lwt.%, Ca element mainly dissolved in theβ-Mg17Al12 phase, which increased melting temperature ofβ-Mg17Al12 phase, so that the solidus temperature; when the Ca content is more than 3wt.%, Al2Ca phases will also become an important component of the liquid phases in the semisolid slurry, which narrow the range of semisolid temperature. Judging from the thermal analysis, the effective range of semisolid temperature of Mg-13Al-3Ca alloy is about 520～550℃.Finally, from the analysis of above experimental data and thermodynamic calculations, we can further identify the appropriate range of Al content in the SSF Mg-Al alloy, and the effect of other alloying elements on semisolid microstructure evolution. For the Mg-Al binary alloy, in the case of basic performance guarantee, the appropriate range of Al content in Mg-Al alloy for SSF is 3-15wt.%. When the Al content is less than 9wt.%, the alloy is only suitable for thixotropic forming with high solid fraction; when the Al content is more than 9wt.%, the alloy can be processed by either thixotropic forming or rheologic forming. The effects of alloying elements on the semisolid microstructure of Mg-Al alloy can be divided into the following aspects:First, the impact of the original as-cast microstructure, the solid particles in the semisolid slurry will be fine and globular, if the grain of as-cast alloy is small and equiaxed after the addition of alloying elements; Second, the formation of new phases with the addition of alloying elements, in order to reduce the content ofβ-Mg17Al12 phase, we should select the elements which can form compounds with Al, during the isothermal heat treatment, the alloy elements present as the compound state do little effect on the semisolid microstructure evolution, free state alloying elements may affect the solid-liquid interface energy which impact the coarsening rate of solid particles; finally, alloy elements addition could change the semisolid solidification range (△TS-SS), the temperature sensitivity of solid fraction (|df/dT|) and other rheological parameters. The results of this research provide some experimental data and theoretical basis for the composition design of SSF magnesium alloys. It is expected that the preliminary work could be significant in prompting the development of new semisolid magnesium alloys.
|
PDF Full Text Request