Yu Bo Zuo

A New Technology for Treating Liquid Metals with Intensive Melt Shearing

Melt quality is crucial for both continuous and shape casting of light alloys. Gas, oxides and other inclusions in the melt usually deteriorate the quality of the casting products. Conventional refining techniques, such as filtration and rotary degassing, can refine the melt by removing the inclusions although they are costly and time-consuming. A new technology for liquid metal treatment through intensive melt shearing was developed recently to improve the melt quality prior to metal casting. The new technology uses a simple rotor-stator unit to provide intensive melt shearing, which disperses effectively the harmful inclusions into fine particles to enhance nucleation during the subsequent solidification processing. Experimental results have demonstrated that the high shear unit can be used for general melt treatment, physical grain refinement, degassing and preparation of metal matrix composites and semisolid slurries. In this paper we offer an overview of the high shear device and its application in processing light alloys.

Grain refinement of DC cast AZ91D Mg alloy by intensive melt shearing

Abstract Melt conditioning by advanced shear technology (MCAST) is a new process for microstructural refinement of both cast and wrought magnesium alloys. Melt conditioned direct chill (MCDC) casting combines the MCAST process with conventional direct chill (DC) casting. In the present work, melt conditioning has been combined with permanent mould casting to simulate the production of DC cast AZ91D billets and slabs. The results show that the MCDC process can achieve significantly finer grain size and more uniform microstructure than conventional DC process for both billets and slabs. Grain refinement in the MCDC process is due to the fine and well dispersed oxide particles produced after processing in the MCAST unit.

Microstructures of DC Cast Light Alloys under the Influence of Intensive Melt Shearing

A new direct chill (DC) casting process, melt conditioned DC (MC-DC) process has been developed for production of high quality ingots and billets of light alloys. In the MC-DC casting process, intensive melt shearing provided by a newly developed rotor-stator unit is used to control the solidification process during the DC casting with a conventional DC caster. Experimental results of DC casting of Al- and Mg-alloys with and without intensive melt shearing have demonstrated that the MC-DC casting process can produce light alloy billets with significantly refined microstructure and substantially reduced cast defects. The effect of intensive melt shearing on grain refinement has been mainly attributed to the enhanced heterogeneous nucleation on well dispersed oxides occurring naturally in the alloy melt.

Grain refinement of DC cast magnesium alloys with intensive melt shearing

A new direct chill (DC) casting process, melt conditioned DC (MC-DC) process, has been developed for the production of high quality billets/slabs of light alloys by application of intensive melt shearing through a rotor-stator high shear device during the DC casting process. The rotor-stator high shear device provides intensive melt shearing to disperse the naturally occurring oxide films, and other inclusions, while creating a microscopic flow pattern to homogenize the temperature and composition fields in the sump. In this paper, we report the grain refining effect of intensive melt shearing in the MC-DC casting processing. Experimental results on DC casting of Mg-alloys with and without intensive melt shearing have demonstrated that the MC-DC casting process can produce magnesium alloy billets with significantly refined microstructure. Such grain refinement in the MC-DC casting process can be attributed to enhanced heterogeneous nucleation by dispersed naturally occurring oxide particles, increased nuclei survival rate in uniform temperature and compositional fields in the sump, and potential contribution from dendrite arm fragmentation.

Modification of a Hypereutectic Aluminium Silicon Alloy under the Influence of Intensive Melt Shearing

A physical modification method with the application of intensive melt shearing during solidification was used to treat a hypereutectic aluminium silicon alloy. The effect of intensive melt shearing on the microstructure of the alloy was studied. Experimental results showed that intensive melt shearing can significantly refine the primary silicon particles, and treatment temperature plays a very important role in the refinement. The optimum condition for refining primary silicon particles of Al-20wt%Si alloy is treating at 660 °C.

Degassing LM25 aluminium alloy by novel degassing technology with intensive melt shearing

Abstract Hydrogen is the only gas that is appreciably soluble in aluminium and its alloys and is the main cause of the gas porosity in aluminium alloy castings. A novel degassing technology with intensive melt shearing and Ar injection has been developed and its degassing effect on aluminium alloy has been studied. The experimental results showed that the new degassing technology can significantly degas LM25 aluminium alloy within a quite short period of time. By applying intensive melt shearing and Ar injection of 60 s, the density index Di was reduced from 10·70 to 0·98%, which means that the hydrogen concentration in the liquid alloy was significantly reduced. The effect of degassing time and isothermal holding time after degassing on the hydrogen concentration level in LM25 alloy melt was also studied and the mechanism of high degassing efficiency with this technology was discussed.

DC Casting of Large Sized Ingot of a High Strength 7xxx Alloy under the Influence of Electromagnetic Field

Hot tearing and cold cracks are major defects during direct chill (DC) casting of large sized ingots of high strength aluminium alloys. In order to solve these problems, based on a low frequency electromagnetic casting (LFEC) process, a new technology, electromagnetic casting with the application of an air blade (EMA) was developed. In the present work, this new technology was used to prepare large sized AA7055 aluminium alloy ingots and the effects of the low frequency electromagnetic field and the air blade on macro-physical fields, microstructure and cracking are studied by numerical and experimental methods. The results show that applying an electromagnetic field can modify the flow direction, increase the velocity of melt flow and homogenize the distribution of temperature in the sump. Applying an air blade can homogenize the distribution of temperature and decrease the stress and strain in the solidified ingot. Furthermore, the microstructure of the ingot is refined remarkably and cracking is eliminated by simultaneously applying the electromagnetic field and the air blade during DC casting.

Melt Conditioned DC (MC‐DC) Casting of Magnesium Alloys

A new melt conditioned direct chill (MC-DC) casting process has been developed for producing high quality magnesium alloy billets and slabs. In the MC-DC casting process, intensive melt shearing provided by a high shear device is applied directly to the alloy melt in the sump during DC casting. The high shear device provides intensive melt shearing to disperse potential nucleating particles, creates a macroscopic melt flow to distribute uniformly the dispersed particles, and maintains a uniform temperature and chemical composition throughout the melt in the sump. Experimental results have demonstrated that the MC-DC casting process can produce magnesium alloy billets with significantly refined microstructure and reduced cast defects. In this paper, we introduce the new MC-DC casting process, report the grain refining effect of intensive melt shearing during the MC-DC casting process and discuss the grain refining mechanism.

The Structure Evolution of a 99.995 Percent High Purity Aluminum during Multi-Forging Process in Room Temperature

In this study, a purity of 99.995percent high purity aluminum was multi-directionally forged up to a maximum cumulative strain of 4.5 at room temperature. The macro and micro structure evolution in the multi-directionally forge process was investigated by structure observations and hardness measurements. The results show that the inhomogeneous deformation of multi-directional forging results in that the structure and hardness is quite different between the easy deformation zone and stagnant zone. Dynamic recrystallization occurs in easy deformation zone of high purity aluminum sample at room temperature as the cumulative true strain is 1.5 (3 forging passes), while the structure in the stagnant zone is still not recrystallizated even at a cumulative true strain of 4.5 (9 forging passes). The recrystallized grain size in the easy deformation zone is reduced with the number of forging passes, and the area of recrystallize grains increase with the number of forging passes.

Degassing of LM24 Al alloy by intensive melt shearing

Abstract Hydrogen can dissolve to a significant extent in molten aluminium and is the main cause of gas porosity in castings. The effect of intensive melt shearing on hydrogen concentration in molten aluminium alloy has been studied by both reduced pressure test and direct hydrogen measurement, and the results show that intensive melt shearing has a significant degassing effect for the LM24 aluminium alloy melt. By applying intensive melt shearing, the hydrogen concentration in the liquid alloy was reduced from 0·16 to 0·08 mL/100 g, and the density index Di was reduced from 12·40 to 2·96%. It was also found that shearing speed, shearing time and holding time after shearing could influence the degassing efficiency. In addition, pressurised melt filtration was also carried out to understand the mechanism of reduced porosity by intensive melt shearing, particularly the role of oxide in the formation of porosity, and the microstructure refinement.