Qingfeng Zhu

Effect of combined application of electromagnetic fields on horizontal direct chill casting of 7050 aluminium alloy

Abstract Influence of combined electromagnetic field application on horizontal direct chill casting of 7050 aluminium alloy is investigated. Temperature measurement and structure observation are carried out to analyse the casting process under different electromagnetic fields. Combined electromagnetic field can reduce the harmful gravitational thermal effect in the horizontal direct chill casting process and improve the ingot properties. With application of combined electromagnetic field, temperature distribution in the melt is more uniform, sump of the ingot becomes flat and symmetric, surface quality of ingot improves markedly, grain morphology changes from feathery grains to equiaxed grains and grain size decreases. Distribution of copper and zinc in the transverse section of the ingot becomes more uniform.

Crystallographic effect of a high magnetic field on a solidified hypoeutectic Zn–Al alloy

Hypoeutectic Zn-4.45 wt% Al solidified under a high magnetic field was investigated crystallographically. With the field, the primary zinc-rich beta phase is distributed homogeneously and orients with the c axis perpendicular to the magnetic field direction. These results are attributed to the magnetic viscosity resistance force and the magnetocrystalline anisotropy of zinc, respectively. The orientation modification also leads to a preferential alignment of the flat-shaped primary beta dendrites. Furthermore, with the field, the eutectic beta phase shows an orientation character similar to that of the primary beta phase. This arises from its continuous growth with the primary beta phase. In addition, a specific crystallographic orientation relationship (\0001\(beta)parallel to\111\(alpha), (beta)parallel to (alpha)) exists in some of the eutectics (between the eutectic zinc-rich beta and aluminium-rich alpha phases). However, this orientation relationship is related to the distribution of primary beta dendrites, which originates from the independent nucleation of the pseudo-primary alpha phase attached to the primary beta dendrites.

Structure uniformity and limits of grain refinement of high purity aluminum during multi-directional forging process at room temperature

Abstract The effect of forging passes on the refinement of high purity aluminum during multi-forging was investigated. The attention was focused on the structure uniformity due to deformation uniformity and the grain refinement limitation with very high strains. The results show that the fine grain zone in the center of sample expands gradually with the increase of forging passes. When the forging passes reach 6, an X-shape fine grain zone is initially formed. With a further increase of the passes, this X-shape zone tends to spread the whole sample. Limitation in the structural refinement is observed with increasing strains during multi-forging process at the room temperature. The grains size in the center is refined to a certain size (110 μm) as forging passes reach 12, and there is no further grain refinement in the center with increasing the forging passes to 24. However, the size of the coarse grains near the surface is continuously decreased with increasing the forging passes to 24.

Effect of electromagnetic field on microstructure and macrosegregation of flat ingot of 2524 aluminium alloy

Low frequency electromagnetic casting (LFEC) process with the application of an induction coil outside the conventional direct chill (DC) casting mould was used to prepare the flat ingot of 2524 alloy and the effect of electromagnetic field on the microstructure and macrosegregation of this alloy was systematically studied. The results show that the flat ingot prepared by the LFEC process has a finer and more uniform as-cast microstructure and the grain morphology is transformed from dendrite and rosette-like to equiaxed structure. The LFEC process also shows a significant effect on macrosegregation, and with the application of electromagnetic field during casting process, the segregation in the centre of the ingot is obviously reduced. The mechanism of these effects was also discussed.

Directional growth behavior of α(Al) dendrites during concentration-gradient-controlled solidification process in static magnetic field

Abstract The large and small sized Cu (solid)/Al (liquid) couples were prepared to investigate the directional growth behavior of primary α (Al) phase during a concentration-gradient-controlled solidification process under various static magnetic fields (SMFs). The results show that in the large couples, the α (Al) dendrites reveal a directional growth character whether without or with the SMF. However, the 12 T magnetic field induces regular growth, consistent deflection and the decrease of secondary arm spacing of the dendrites. In the small couples, the α (Al) dendrites still reveal a directional growth character to some extent with a SMF of ≤5 T. However, an 8.8 T SMF destroys the directional growth and induces severe random deflections of the dendrites. When the SMF increases to 12 T, the α (Al) dendrites become quite regular despite of the consistent deflection. The directional growth arises from the continuous long-range concentration gradient field built in the melt. The morphological modification is mainly related to the suppression of natural convections and the induction of thermoelectric magnetic convection by the SMF.

Effect of low frequency electromagnetic field on as casting structure and surface quality of horizontal direct chill casting 7075 alloy

Abstract Horizontal direct chill (HDC) casting is a well established production route for aluminium alloy ingots; however, the ingots may suffer from inhomogeneous microstructures and serious macrosegregation due to the unbalanced cooling condition and gravitational effect. The present study considers the HDC casting of 7075 aluminium alloy ingot under the influence of a low frequency electromagnetic field (LFEF). Temperature measurement is employed to analyse the effect of LFEF on the HDC casting process. Surface topography and structure observation are conducted to analyse the effect of LFEF on the surface and interior quality of the HDC ingot. The results show that the LFEF can effectively uniformalise the thermal distribution, flatten the sump shape, improve the ingot surface quality, promote a transition from columnar/feathery to equiaxed dendritic growth and has a distinct grain refining effect.

Shear deformation and plate shape control of hot-rolled aluminium alloy thick plate prepared by asymmetric rolling process

Abstract Asymmetric rolling (ASR), as one of severe plastic deformation (SPD) methods to make ultra-fine materials with enhanced performance is mainly used to prepare foil and thin strip. The asymmetrical rolling was achieved by adjusting the diameters of the upper roll and the bottom roll and was used to prepare hot-rolled thick plate of 5182 aluminium alloy. The shear deformation and plate shape control were experimentally studied. The experimental results show that asymmetrical rolling has a significant effect on metal deformation stream and can somehow refine microstructure and improve the uniformity of microstructure and properties. The asymmetrical rolling process can also reduce the rolling force. However, bending of rolling plate often happens during asymmetrical rolling process. The factors affecting the bending were discussed.

As-cast structure of DC casting 7075 aluminum alloy obtained under dual-frequency electromagnetic field

We have experimentally determined the as-cast structures of semi-continuous casting 7075 aluminum alloy obtained in the presence of dual-frequency electromagnetic field. Results suggest that the use of dual-frequency electromagnetic field during the semi-continuous casting process of 7075 aluminum alloy ingots reduces the thickness of the surface segregation layer, increases the height of the melt meniscus, enhances the surface quality of the ingot, and changes the surface morphology of the melt pool. Moreover, low-frequency electromagnetic field was found to show the most obvious influence on improving the as-cast structure because of its high permeability in conductors.

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.

Effects of low frequency electromagnetic field on solidification of 7050 aluminium alloy during hot top casting

Abstract On the basis of conventional hot top casting, low frequency electromagnetic field is applied during conventional hot top casting process. Effects of low frequency electromagnetic field on cold fold, macrostructure, microstructure and sump shape have been studied. The experimental results show that with the application of the low frequency electromagnetic filed, the as cast structure has been refined, the depth of cold fold is decreased, the upstream conduction distance becomes long, and the sump becomes shallow. The reasons for the low frequency electromagnetic field refining the structure of the ingot, making the upstream conduction distance long and the sump shallow were summarised.