电磁能对半连续铸造Al-Si-Cu-Mg合金微观组织与成分偏析的影响

Microstructure and Composition Segregation of DC-Casting Al-Si-Cu-Mg Alloy with Electromagnetic Energy

应用新型熔体表面电磁能处理技术对半连续铸造Al-Si-Cu-Mg合金进行电磁能处理,探究电磁能对半连续铸造Al-Si-Cu-Mg合金微观组织与成分偏析的影响。利用光学显微镜(OM)与等离子发射光谱仪对铸锭横断面进行微观组织观察与成分检测。实验结果表明,电磁能作用下,初晶Si减小、形状因子增加,偏聚情况得到改善。α-Al尺寸与二次枝晶臂间距减小。电流增加至40 A,初晶Si尺寸减小29.76%左右,形状因子增加至0.51左右,形状更加接近圆形。心部微观组织中α-Al等积圆直径为18μm,二次枝晶臂间距为11.28μm,边部微观组织中α-Al等积圆直径为11μm,二次枝晶臂间距为7.2μm。以电流40 A为例,电磁能作用下,溶质元素Si, Cu, Mg相对偏析度降低,元素分布更加均匀,溶质元素的成分偏析被显著改善。分析电磁能作用下形核过程,认为电磁能作用合金熔体内的原子集团,改变合金熔体的结构,是电磁能细化合金微观组织的重要原因。

A new type of melt surface electromagnetic energy treatment technology was applied to Al-Si-Cu-Mg alloy direct chill(DC) casting, the influence of electromagnetic energy on the microstructure and composition segregation of Al-Si-Cu-Mg alloy DCcasting was discussed. The experience material was Al-Si-Cu-Mg alloy. The electromagnetic energy grain refinement device was installed directly above the launder. After the DC-casting process was stable, the melt was subjected to electromagnetic energy treatment. The electromagnetic energy treatment temperature was 660 ℃, pulsed electromagnetic field with duty ratio of 20%, frequency of 40 Hz and current of 30 and 40 A. The observation position of the microstructure was the center and the edge of the ingot and the specimens were etched by Keller solution. Hit 5 points in the radial direction of the ingot for composition were tested. Optical microscope(OM) and plasma emission spectrometer were used to observe the microstructure and detect the composition of the cross section of the ingot. Experimental results showed that under the action of electromagnetic energy, the primary Si decreased, the shape factor increased, and the distribution of primary Si was improved:(1) The α-Al size and the distance between the secondary dendrite arms spacing were reduced. When the pulsed electromagnetic field was not applied, there were fine α-Al dendrites in the center and edge microstructures of Al-Si-Cu-Mg alloy, but most of the α-Al dendrites grew disorderly and coarsely, mixed with a large number of secondary dendrites, the growth direction was chaotic. In the center zone, the α-Al equal circle diameter was 26 μm, the secondary dendrite arm spacing was 22.2 μm, the α-Al equal circle diameter in the edge microstructure was 14 μm, and the secondary dendrite arm spacing was 9.17 μm. The segregation of primary Si was serious, the equal circle diameter of the primary Si in the center microstructure was 38.08 μm, and the shape factor was 0.42. Under the action of a 30 A pulsed electromagnetic field, the α-Al equal circle diameter in the center zone was 22 μm, the secondary dendrite arm spacing was 18.26 μm, and the α-Al equal circle diameter in the edge zone was 12 μm. The secondary dendrite arm spacing was 8.56 μm. The segregation of primary Si in the center microstructure was improved, the size of primary Si reduced, the equal circle diameter of primary Si was 31.35 μm, the shape factor increased to 0.49, and the shape of primary Si was closer to a circle. After applying a 40 A pulsed electromagnetic field, the α-Al equal circle diameter in the center zone was 18 μm, the secondary dendritic arm spacing was 11.28 μm, and the α-Al equal circle diameter in the edge zone was 11 μm. The secondary dendrite arm spacing was 7.2 μm. The equal circle diameter of primary Si in the center zone was 26.76 μm, and the shape factor was 0.51, the distribution of primary Si was uniform.(2) Taking a current of 40 A as an example, without applying a pulsed electromagnetic field, the relative segregation degrees of the solute elements Si, Cu and Mg were 1.57%, 2.43% and 2.05%, respectively. After applying a pulsed electromagnetic field, the relative segregation of the solute elements Si, Cu and Mg decreased to 1.18%, 0.94% and 2.00%, respectively. Analyzing the nucleation process under the action of electromagnetic energy, it was believed that electromagnetic energy acted on the atomic clusters in the alloy melt to change the structure of the alloy melt, which was an important reason for the electromagnetic energy to refine the alloy microstructure.Electromagnetic energy treatment of the melt was performed in a certain temperature range above the liquid’s temperature. On the one hand, the energy included by the electromagnetic field could reduce the activation energy. The change of the activation energy would affect the average radius of the atomic clusters, which was one of the main reasons for the nucleation rate increase.