Papers by Author: Zhi Hao Zhao

Abstract: Grain refinement is quite important for producing 7050 alloy billet especially in large scale. Low frequency electromagnetic casting (LFEC) process was used to make 7050 aluminum alloy Φ500 mm billets and study the effect of electromagnetic field on the microstructure. The sound Φ500 mm billets of 7050 alloys without any grain refiner can be successfully prepared by the LFEC process. The results show that low frequency electromagnetic field has a significant grain refining effect on 7050 alloy and can effectively eliminate feather grain structure. The microstructures of LFEC ingot from the border to the center of the cross section are all equiaxed grains and are finer and more uniform than that of conventional direct chill (DC) cast billets. The LFEC process also shows a strong power to eliminate hot tearing during casting large sized billet of high strength aluminium alloy.

Abstract: The aluminium alloy bar is mainly obtained by hot extrusion in the past. In order to increase the production efficiency and reduce the cost, a new mold is designed, and horizontal continuous casting is used to produce the aluminium alloy bar in this work. The results show that aluminium alloy bar can be cast by horizontal continuous casting. The asymmetry of the sump is overcome by optimizing the design of the mold. When the temperature of the melt was 730°C, casting speed is 650 mm/min, cooling water is 15 L/min, the quality of lubricating oil is 0.5 ml/min, and the ingot with good surface quality can be cast.

Abstract: 7075 aluminum alloy ingot with the diameter of 100 mm has been produced by horizontal direct chill casting in different casting speed. The effect of casting speed on the sump profile and the ingot surface quality was studied by sump profile observation. It was found that increasing the casting speed results in the deepening of the liquid pool, the adding of segregation knots and the reducing of cold shouts in the ingot surface. It is also found that the depth of the liquid pool is directly proportional to the casting speed and the squared radius of the ingot.

Abstract: The influence of different electromagnetic fields on the horizontal direct chill (HDC) casting of aluminum alloy is studied. 7075 aluminum alloy with 100-mm in diameter is produced by HDC casting process; single electromagnetic field located before or around the mold and an out-phase electromagnetic field (OPEMF) are applied in the HDC casting process. The effect of different electromagnetic fields on the HDC casting solidification behavior and as-cast structure is investigated. The electromagnetic field can effectively make uniform thermal distribution in the molten pool and refine the as-cast structure. The OPEMF is more efficient than the single electromagnetic field in improving the as-cast structure of HDC cast ingots.

Abstract: Based on the non-uniform distribution of magnetic field within the ingot caused by its interactions during the dual-ingot low-frequency electromagnetic semi-continuous casting process, a three-dimensional finite element model was constructed. This model was meshed and calculated with the ANSYS software, and the distribution of magnetic field in low-frequency semi-continuous casting mould was obtained. The influence of the distance and current directions between two coils on magnetic field distribution in the ingot was studied. Calculated results showed that, during dual-ingot low-frequency electromagnetic semi-continuous casting process, whether the current was in the same direction or the reverse, magnetic field density on the remote end should be greater than that on the proximal of the same ingot; when the current directions of nearby coils were reverse, the magnetic intensity on ingot was higher than that with the same directions; as the distance between coils increased, the distal and proximal difference of magnetic flux density declined. Based on the results, a casting mould for dual-ingot electromagnetic semi-continuous casting ø152mm 7075 aluminum alloy was designed and produced. The experimental results showed that when the current directions of nearby coils were reverse, the as-cast macrostructures were a little bit better than that with the same directions. Moreover, when the current directions were reverse, the heterogeneity of the as-cast inner structure could be almost ignored.

Abstract: Hot tear is one of the main defects during the aluminium casting process, and it usually occurs during solidification. In this work, the low frequency electromagnetic field was applied to study the effects of low frequency electromagnetic field on hot tear during the conventional hot-top casting process. The results show that under the effect of the low frequency electromagnetic field, the structure of the ingot is refined, the flow direction is changed, and the depth of the sump becomes shallow. All these factors can decrease the hot tear susceptibility.

Abstract: 7050 aluminum alloy ingots were produced by low frequency electromagnetic casting (LFEC) and direct chill casting (DC) respectively. As-cast microstructures and homogenization behaviors of LFEC and DC ingots were investigated experimentally. The optical microscope and DSC analyses shown that the grain size of LFEC ingot was finer and the content of constituents and eutectic structure was less than that of DC ingot. Accordingly, the homogenization behaviors of the LFEC and DC ingots were significantly different. The remnant constituents of LFEC ingot were less in content and smaller in size than that of DC ingot after homogenization at 480°C for various lengths of time. Similar to the dissolving of constituents, the LFEC ingot exhibited faster diffusion kinetics of alloying elements from grain boundary to inner. The concentrations of Cu, Mg and Zn inside grain of LFEC ingot after homogenization for 12 h were 2.4%, 2.2% and 6.5% respectively, but the DC ingots had not reach the level even for 48 h.

Abstract: The effect of increasing the amount of added grain refiner and combination electromagnetic filed on grain size and morphology has been investigated by die casting of commercial aluminum. The results show the grain size and the grain morphology both are transited with the increasing of the addition of Al-5Ti-B. The grain size continuously decreased with increasing the addition of grain refiner. Without grain refiner additions, an obvious transition in grain size and morphology is observed with the application of low frequency electromagnetic field (LFES). The efficiency in grain refinement of LFES decreases with increasing the amount of grain refiner. However, even when the addition of grain refiner is 2 wt. % Al-5Ti-B, 63 % grain size reduction still is observed by the affect of LFES. This result shows that the LFES can enhance the efficiency in grain refinement of grain refiner.

Abstract: This paper discussed the effects of different cooling rate on the microstructure of the 4Y32 aluminum alloy and the refinement and modification on 4Y32 aluminum alloy by using Na and Al-10%Sr master alloys in various treatment states. And also the effect of Al-10Sr and Al-3Ti-1B compound modification on microstructures of 4Y32 aluminum alloy has also been studied. The results showed that: the primary silicon was eliminated during slow cooling. As the cooling rate increased, the number of primary silicon was also growing, but the size of primary silicon and eutectic silicon were significantly refined; when without modification, the shape of eutectic silicon was needle and flake; 4Y32 aluminum alloy could be effectively refined after Na modification, under the optimum addition amount of 0.8wt%. The best modification was achieved with 15 minutes, that is, the shape of primary silicon changed to small granular or oval and the primary silicon was eliminated; the as-cast structure has been improved after Al-10Sr modification, under the optimum addition amount of 0.2wt. %, the shape of eutectic silicon changed to fine short rod or point and the primary silicon was eliminated; However, in the compound modification, efficiency of Sr was decreased with the increasing additional amount of AI-3Ti-1B master alloy, which was attribute to the interaction of Sr and Ti.

Abstract: The influences of low frequency electromagnetic field on the as-cast structure of horizontal direct chill casting aluminum alloy slab were experimental and numerical studied. The results of numerical analysis show that the interaction of the low-frequency electromagnetic field and the melt can generate an electromagnetically induced forced flow in the melt, which, in turn, changes flow pattern and temperature field in the mold. the as-cast structure of slab can be greatly improved by the changes of flow pattern and temperature field. The results of experimental analysis show that the grains of low frequency electromagnetic casting (LFEC) are fine, uniform, equiaxed, rosette-shaped; the surface of the slab of LFEC has no cold shut and less exudations. Moreover, the low-frequency electromagnetic field can improve the distribution of solute elements in the slab. The macrosegregation such as negative segregation and gravity segregation were reduced, which results in the homogenous distributions of the solute elements over the cross-section of the slab.