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.
Degassing is very important for aluminum alloys especially for 7xxx series alloys. In the present study, a high shear technology was used to degas 7032 aluminum alloy in order to study its degassing efficiency. The experimental results showed that the high shear technology can significantly degas 7032 aluminum alloy. By applying intensive melt shearing and an Ar injection of 60 seconds, the density index, D,, was reduced from 13.25% to 0.28% and the hydrogen concentration was significantly i＇educed from 0.31 to 0.10 mL/100g AI. Compared with the conventional rotary degassing, high shear technology showed a much higher degassing efficiency, achieving a lower concentration of hydrogen in a shorter time. The water simulation experiment was used to study the mechanism of the high degassing efficiency. The small bubble size and the uniform distribution of Ar bubbles with the application of high shear technology are believed to be the main cause for the high degassing efficiency.
Yu-bo ZuoYi-yao KangYue LinXu-dong LiuChao SunSen-sen YuanJian-zhong Cui
为了了解电磁场对组织细化作用的机理,采用低频电磁铸造方法制备直径200 mm Al-Zn-Mg-Cu-Zr合金铸锭,并对铸造过程中的温度场进行测量。结果表明:施加磁场产生的强制对流使得温度场均匀且低于液相线约6℃。结晶器中熔体温度场的变化显著改变了熔体从开始浇注到完全凝固的热历史,从而有效促进了异质形核,显著减少晶核的重熔,使更多晶核能够生存下来,并最终促进形成均匀、细小的微观组织。
Intensive melt shearing has a significant grain refining effect on some light alloys.However,the persistence of the grain refining effect during isothermal holding and remelting is still unclear,although it is very important for the practical application.In this study,intensive melt shearing was achieved in a twin-screw mechanism to investigate its grain refining effect on AZ91D magnesium alloy.The refinement mechanism was discussed and the persistence of grain refinement after remelting and isothermal holding was also studied.A Zeiss imaging system with polarized light was used for quantitative measurement of grain size.The results show that the intensive melt shearing has a significant grain refining effect on AZ91D magnesium alloy.With the application of intensive melt shearing,the grain size of AZ91D magnesium alloy can be reduced from 530 μm(for a typical as-cast microstructure) to 170 μm,which is about 70% size reduction.The grain refinement achieved by the intensive melt shearing can be partially kept after isothermal holding and remelting.It is believed that the refinement effect was mainly due to the finer and well dispersed oxide particles formed by high intensive shearing.The smaller size of oxide particles and their slow motion velocity in the sheared melt could make important contributions to the remained grain refinement.
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 fiat 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.
