Temperature Field and Microstructural Formation of Semi-Solid AlSi_7Mg Alloy

The microstructural formation of the semi-solid AlSi7Mg alloy stirred by electromagnetic field is investigated together withthe tempeatre field of the stirred melt at continuously cooling. A impoat kinetic factor for primny a-Al nucleation is proposed. It isfound that a low temperatUre gradient exists in the electromagnetic stirred melt. This is why the first dendritic arms and secondary de-ndritic arms are refmed. Experimefltal results also show that the root remelting of secontw dendritic arms is an twortat mechanismfor the primary α-Al refmement. Strong electromagnetic stirring greatly reduces the composition supercooling in the melt and eliminatesprefedrig growth of the first dendritic arms. Therefore, many rosettes or spherical Primary α-Al phase particles form finally.

Microstructures of an AlSi_7Mg alloy by near-liquidus casting

Semi-solid ingots of an A1SiTMg alloy were obtained using the method of near liquidus casting. Their microstructures exhibit the characteristics of free, equiaxed, and non-dendrite, which are required for semi-solid forming. The influences of casting temperature, heat preservation time, and cooling rate on the microstructure were also investigated. The results show that in the temperature region near liquidus the grain size becomes small with a decrease in casting temperature. Prolonging the heat preservation time makes grain crassitude at the same temperature. And increasing the cooling rate makes grain fine. The microstructure of the alloy cast with iron mould is freer than that with graphite mould.

Effect of Pouring Process on the Microstructures of Semi-Solid AlSi_7Mg Alloy

The effect of different pouring temperatures and different pouring heights, the distance between the mouth of the pouring ladle and the top of the mold, on the microstructure of AlSi7Mg alloy have been researched in the paper. When the pouring temperature is close to the liquidus temperature, the primary alpha -Al in 'the billets of AlSi7Mg alloy solidified into spherical and nodular fine grains distributed homogeneously. The optimum pouring temperature for semi-solid AlSi7Mg billet with spherical or nodular primary alpha -Al is 615 degreesC. At the same pouring temperature, the higher the pouring ladle, the more easily the spherical and nodular primary alpha -Al obtained in the semi-solid AlSi7Mg billet. When the pouring temperature is close to the liquidus temperature and the pouring ladle is relatively high, it is the great cooling rate, the flow of the molten allay caused by pouring and the large simultaneous solidification region induced by the near liquidus temperature, that promote the formation of spherical or nodular primary cr-Al.

Effect of the Solidification Conditions on the Microstructures of AlSi_7Mg Alloy during Semi-solid Remelting

The effect of the solidification conditions on the microstructures was studied during partial remelting of AlSi7Mg alloy with the help of an electrical pipe-type fumace. The results show that the eutectic is remelted above all and α phases are gradually evolved into spheroidal shape,if the AlSi7Mg alloys shrmd strongly by rotating electromagnetic field during the first solidification are heated again to 589 or 597℃ and have been held for a short time (for example, 5~10 min), and moreover, the higher the holding temperature,the faster the eutectic remelting process and α phase's evolution are. In contrast even though the AlSi7Mg alloy's samples non-stirred with fine dendritic microstructures are heated to the same temperatures as those stirred by rotating electromagnetic field and have been held for 60 min,it is not possible to change all the dendritic a phases to speroidal α phase.

Electro-pulse modification and reheating process for production of thixotropic microstructure in AlSi_7Mg alloy

Electro-pulse modification(EPM) was used to change the dendritic structure of AlSi7Mg alloy to globular one.The effects of the modified temperature,electro-pulse frequency and time on the solidified structure were examined.The results show that these parameters play an important role in the solidified microstructures.That is to say,under the same modified temperature,the solidified microstructure will be improved greatly with the increase of electro-pulse frequency and time,but when they exceed to limit values,the solidified microstructure will become worse,resulting from the decrease of the ratio of nucleation.The experimental results indicate that the suitable modified temperature of AlSi7Mg alloy is 720 °C,and the appropriate electro-pulse frequency and time are 5 Hz and 40 s,respectively.Then the microstructures produced by suitable EPM process were reheated at temperatures between liquidus and solidus,the primary-Al grains ripen further and become more spherical,which is favorable to the semi-solid forming of AlSi7Mg alloy.

Two-stage dynamic recrystallization and texture evolution in Al-7Mg alloy during hot torsion

Hot torsion tests were performed on the Al-7Mg alloy at the temperature ranging from 300 to 500℃ and strain rates between 0.05 and 5 s^(-1) to explore the progressive dynamic recrystallization(DRX)and texture behaviors.The DRX behavior of the alloy manifested two distinct stages:Stage 1 at strain of≤2 and Stage 2 at strains of≥2.In Stage 1,there was a slight increase in the DRXed grain fraction(X_(DRX))with predominance of discontinuous DRX(DDRX),followed by a modest change in X_(DRX) until the transition to Stage 2.Stage 2 was marked by an accelerated rate of DRX,culminating in a substantial final X_(DRX) of~0.9.Electron backscattered diffraction(EBSD)analysis on a sample in Stage 2 revealed that continuous DRX(CDRX)predominantly occurred within the(121)[001]grains,whereas the(111)[110]grains underwent a geometric DRX(GDRX)evolution without a noticeable sub-grain structure.Furthermore,a modified Avrami’s DRX kinetics model was utilized to predict the microstructural refinement in the Al-7Mg alloy during the DRX evolution.Although this kinetics model did not accurately capture the DDRX behavior in Stage 1,it effectively simulated the DRX rate in Stage 2.The texture index was employed to assess the evolution of the texture isotropy during hot-torsion test,demonstrating significant improvement(>75%)in texture randomness before the commencement of Stage 2.This initial texture evolution is attributed to the rotation of parent grains and the substructure evolution,rather than to an increase in X_(DRX).

电磁搅拌对半固态AlSi_7Mg合金初生α-Al的影响规律

研究了AlSi7Mg合金半固态等温电磁搅拌过程和连续降温电磁搅拌合金熔体的温室分布，结果表明：在电磁搅拌条件及低温度梯度下，可增加初生α－Al形核的位置和数目，细化一次臂和二次臂间距：在电磁搅拌条件下，初生α－Al的二次臂根部熔断机制是最重要的非枝晶化和细化机制：激烈的搅拌均匀了熔体温度场．减小了成分过冷．初生α－Al生长时的过冷度△T降低，也均匀了固液界面前沿的溶质场，结果使初生α－Al一次臂的长大失去方向优先性、并使一次臂生长速度减慢，二次臂的长大速度与一次臂的长大速度相近。

Molecular dynamics simulation of relationship between local structure and dynamics during glass transition of Mg_7Zn_3 alloy

The rapid solidification process of Mg7Zn3 alloy was simulated by the molecular dynamics method. The relationship between the local structure and the dynamics during the liquid-glass transition was deeply investigated. It was found that the Mg-centered FK polyhedron and the Zn-centered icosahedron play a critical role in the formation of Mg7Zn3 metallic glass. The self-diffusion coefficients of Mg and Zn atoms deviate from the Arrhenius law near the melting temperature and then satisfy the power law. According to the time correlation functions of mean-square displacement, incoherent intermediate scattering function and non-Gaussian parameter, it was found that the β-relaxation in Mg7Zn3 supercooled liquid becomes more and more evident with decreasing temperature, and the α-relaxation time rapidly increases in the VFT law. Moreover, the smaller Zn atom has a faster relaxation behavior than the Mg atom. Some local atomic structures with short-range order have lower mobility, and they play a critical role in the appearance of cage effect in theβ-relaxation regime. The dynamics deviates from the Arrhenius law just at the temperature as the number of local atomic structures begins to rapidly increase. The dynamic glass transition temperature （Tc） is close to the glass transition point in structure （TgStr）.

TEM microstructure of rapidly solidified Mg-6Zn-1Y-1Ce alloy

Rapidly solidified（RS） Mg-6Zn-1Y-1Ce ribbons were prepared by single roller melt-spinning technique.Transmission electron microscopy and energy dispersive X-ray spectroscopy were employed to characterize the microstructure of RS ribbons.The results show that there is high density of particles distributed within grains and at grain boundaries in the region near wheel side.The particle density is decreased in the middle region and free surface region.The alloy is predominantly composed of supersaturated--Mg solid solution,T phase and W phase;meanwhile,a few icosahedral quasicrystalline and Mg4Zn7 particles are also observed.The T phase is confirmed having a body-centered orthorhombic structure that is transformed from the body-centered tetragonal structure Mg12Ce phase due to the partial substitution of Mg atoms by Zn.

Preparation for Semi-Solid Aluminum Alloy Slurry under Weak Electromagnetic Stirring Conditions

The effects of pouring temperature, short electromagnetic stirring with low strength and then soaking treatment on the microstructure of AISi7Mg alloy were investigated. The results show that if AlSi7Mg alloy is poured at 630 or 650℃ and meanwhile stirred by an electromagnetic field at a low power for a short time, the pouring process can be easily controlled and most solidified primary α-Al grains become spherical and only a few of them are rosette-like. Weak electromagnetic stirring makes the temperature field more homogeneous and makes the primary α-Al grains disperse in a larger region, which leads to the spherical microstructures of primary α-Al grains. When the AISi7Mg alloy is soaked or reheated at the semisolid state, the primary α-Al grains ripen further and they become more spherical, which is favorable to the semi-solid forming of AlSi7Mg alloy.

Preparation of semisolid AlSi7Mg alloy slurry through weak traveling-wave electromagnetic stirring

The semisolid AlSi7Mg alloy slurry with large capacity was prepared by low superheat pouring and week traveling-wave electromagnetic stirring. The effects of electromagnetic stirring power and frequency on the shape and distribution of primary α-A1 grains in the AlSi7Mg alloy slurry were discussed. The experimental results show that the AlSi7Mg alloy slurry with fine and spherical primary α-A1 grains distributed homogeneously can be obtained. Under the condition of low superheat pouring and week traveling-wave electromagnetic stirring, when the pouring temperature is 630℃, raising the stirring power or frequency appropriately can gain a better shape of primary α-Al grains; but if the stirring power or frequency is increased to a certain value （1.72 kW or10 Hz）, the shape of primary α-A1 grains cannot be obviously improved and spherical primary α-Al grains distributed homogeneously can be still obtained.

New method for the preparation of semi-solid AlSi7Mg alloy slurry

The effects of pouring temperature and short electromagnetic stirring with low strength on the microstructures of AlSi7Mg alloy were investigated. The results show that if liquid AlSi7Mg alloy is poured at 630 or 650℃, many primary α-Al grains in the solidified melt are rosette-like, and only a small number of them are spherical. However, if liquid AlSi7Mg alloy is poured at a lower superheat and meanwhile is stirred by an electromagnetic field at a low power for a short time, then most primary α-Al grains in the solidified melt become spherical, and only a few are rosette-like. The theoretical analysis indicates that the strengthened melt flow motion induced by the short electromagnetic stirring makes the temperature field more homogeneous in the melt, which is poured at a lower superheat, and makes the primary α-Al grains deposit in a larger region at the same time, so this new solidification kinetic condition leads to the microstructure of spherical primary α-Al grains. The experiments also demonstrate that pouring at an appropriate superheat and stirring by an electromagnetic field at a low power for a short time is a good new method for preparing the semi-solid slurry or billet of AlSi7Mg alloy.

Effect of pouring height on the solidified microstructure of AlSi7Mg alloy

The effect of different pouring heights and evenly soaking process in the liquidus and solidus range on the solidified microstructure of AlSi7Mg alloy has been studied. The results show that if the pouring temperature is 630 or 650℃ and the pouring height is 40 mm, the microstructure of the solidified melt is not homogeneous and there are many rosette-like primary α-Al grains. But if the pouring height is increased to 400 mm, the solidified microstructure becomes more homogeneous and favorable to obtain spherical primary α-Al grains in the solidified melt. With further being evenly soaked in the liquidus and solidus range for some time, the temperature difference between the melt center and the melt periphery can be controlled within ±2℃ and the primary α-Al grains will evolve into more spherical grains. The theoretical analysis indicates that the higher pouring height promotes the melt flow motion and makes the temperature field in the melt more homogeneous and restrains the large rosette primary α-Al grains. This flow motion can also promote the ripening effect and the primary α-Al grains in the melt are gradually changed into spherical grains. It can be concluded from the experiments that pouring at an appropriate superheat and from a proper height is a good new method for preparing the semisolid slurry of AlSi7Mg alloy, its process control is easy and the preparation cost is lower.

Microstructure and properties of AlSi7Mg alloy fabricated by selective laser melting

The AlSi7Mg alloy was fabricated by selective laser melting(SLM),and its microstructure and properties at different building directions after heat treatment were analyzed.Results show that the microstructure of SLM AlSi7Mg samples containes three zones:fine grain zone,coarse grain zone,and heat affected zone.The fine-grain regions locate inside the molten pool,and the grains are equiaxed.The coarse-grain regions locate in the overlap of molten pools.After T6 treatment,the microstructure at the molten pool boundary is still the network eutectic Si,but the network structure becomes discrete,and is composed of intermittent,chain-like eutectic Si particles.The yield strength at three directions(xy,45°,z direction)of the AlSi7Mg alloy samples fabricated by SLM is improved after T6 heat treatment.The fracture mechanism of the samples is a mixed ductile and brittle fracture before heat treatment and ductile fracture after heat treatment.

Continuous cooling precipitation diagram of cast aluminium alloy Al-7Si-0.3Mg

The precipitation behaviour during quenching of cast Al-7Si-0.3Mg aluminium alloy was investigated by DSC in the cooling rate range of 0.01 K/s to 3 K/s and by quenching dilatometry for higher rates. Two main precipitation reactions were observed during cooling, a high temperature reaction starting almost directly with quenching from 540℃ and a low temperature reaction starting at about 400℃. Quenching with 3 K/s already significantly suppresses precipitation during quenching. Hardness after T6 ageing increases with increasing quenching rate, due to the increasing content of supersaturated solid solution. By dilatometry and hardness results the critical cooling rate can be estimated as about 60 K/s. Quenched Al-7Si-0.3Mg microstructures have been investigated by light microscopy. The microstructures consist of an aluminium-silicon eutectic structure, aluminium solid solution dendrites and precipitates inside the aluminium dendrites, depending on quenching rate.

Effect of withdrawal rate on microstructure of a hypoeutectic Al-Si alloy by low superheat DC casting

To obtain the billet with homogeneous and spheroidized primary grains is the key step in the semi- solid forming process. Among the semi-solid billet preparation methods, the process of low-superheat direct chill （DC） casting is simpler and more effective. In this study, the billets of AISi7Mg alloy were prepared by low- superheat DC casting at various withdrawal rates. The effect of withdrawal rate on the surface quality of the billets was investigated, and the evolution mechanism of the microstructural morphology in the billets was analyzed. The results show that the periodic corrugations and a large quantity of fine shrinkage pits appear in the billet surface when the withdrawal rate is 100 mm.min-1, and the above defects in the billet surface can be eliminated completely when the withdrawal rate is above 150 mm.min-1. But when the withdrawal rate is too fast （250 mm.min-1）, the primary a grains, except the ones in the billet center, have not enough time for ripening due to the high silidification rate, and will solidify as the dendrite structure. When the withdrawal rate is between 150 - 200 mm.min-1, the dendritic growth of the primary a grains is effectively inhibited, and a billet of AISi7Mg alloy with a smooth surface and homogeneous, fine, non-dendritic grains can be obtained.

Microstructral Evolution of the Dendritic AlSi7Mg Alloy during Semi-solid Remelting

The microstructural evolution during partial remelting of dendritic AlSi7Mg alloys was studied with the help of an electrical fumace. The results show that it is difficult to change all the primary a dendrites into the spheroidal a phases, when the dendritic samples of AlSi7Mg alloy are remelted under 589 or (597±1)℃ and held on mis condition for 5 to 120 min. The results also show that the samples remelted can be deformed under the heavy force, if the holding time is longer under higher temperature.

Volta Potential Analysis of Intermetallics in 7A52 Aluminum Alloy

In order to study the effect of intermaetallics on the corrosion behaviour of 7A52 aluminum alloy, the alloy was characterized by means of SEM-EDS and scanning Kelvin probe force microscopy（SKPFM）. The experimental results indicate that there are two different intermetallics：A1-Mn-Fe and Mg2Si. Both intermetallics exhibite the negative volta potential relative to the matrix indicating an anodic behaviour. Hereby, they are easy to be dissolved and corroded under the erosive environment, and there become the corrosion initiation sites. The A1-Mn-Fe intermetallics show stronger anodic behaviour than those of Mg2Si intermetalics. It means that A1-Mn-Fe intermetalics are easier to be corroded.

熔体过热对Al-7%Si-0.50%Mg合金的显微组织和力学性能的影响

通过对Al 7%Si 0 5 0 %Mg合金进行不同温度的过热处理试验 ,研究了熔体过热对其显微组织和力学性能的影响。结果表明 :过热温度为 810～ 85 5℃时 ,即使不经过Sr变质处理 ,也可以明显细化Al 7%Si 0 5 0 %Mg合金的显微组织 ,提高其力学性能。过热温度为 845℃左右时 。

热处理工艺对Al-7% Si-0.2% Mg合金性能的影响

Al-7%Si-0.2%Mg合金经545℃×7 h+180℃×5 h热处理后,各项性能得到大幅提高,电导率41.28%IACS,抗拉强度240.6 MPa,布氏硬度89.1 HB;过饱和铝基固溶体中析出Mg2Si相和Si相,晶格畸变程度明显降低,合金的电导率增幅最高可达28.16%,强度提升比例为50.65%,硬度的增幅达55.22%。