稀土复合熔剂对 6111 铝合金熔体净化处理及其铸态组织与力学性能的影响

Aluminum and its alloys find extensive applications in the aerospace，electronic information，and automobile industries owing to their advantages of lightweight，high specific strength，good ductility，excellent corrosion resistance，favorable machinability，and moderate price. However，aluminum and its alloys are highly susceptible to contamination by reacting with water vapor in the air during the melting and casting process. This reaction leads to the production of hydrogen and oxide inclusions. To enhance the metallurgical quality of aluminum alloy melts，flux-based purification treatment technology is commonly employed for removing hydrogen and oxide inclusions from them. Currently there is limited research on the strategy of designing flux compositions for purifying aluminum alloy melts，hence further exploration is required. This study investigated the influence of rare earth composite flux on melt purification treatment，as-cast microstructure，and mechanical properties of 6111 aluminum alloy. Firstly，four types of rare earth composite fluxes（YH-1~YH-4）were developed based on a comprehensive investigation of the thermophysical parameters of molten salt using differential scanning calorimetry（DSC）and molten property measurement instrument. The melting points and viscosities of the fluxes were determined. Secondly，the purification effect，as-cast microstructure，and mechanical properties of 6111 aluminum alloy were comprehensively evaluated through optical microscope（OM），scanning electron microscope（SEM），and mechanical performance testing. Finally，the correlation between the purification effect achieved by rare earth composite fluxes during melt purification treatment and the resulting microstructure and properties of 6111 aluminum alloy was discussed. The DSC results indicated that the melting points of four rare earth composite fluxes（YH-1~YH-4）were approximately 570 ℃，whereas the commercially available flux（ST#）exhibited a melting point around 630 ℃. All four types of rare earth composite fluxes demonstrated lower melting points compared to their commercially available counterparts. Furthermore，measurements of molten properties revealed that the viscosities of all four types of rare earth composite fluxes were also lower than that of the commercially available flux. Metallographic microstructure analysis showed more pores and inclusions in untreated as-cast aluminum alloy samples，treated ones had fewer and smaller ones. Furthermore，statistical analysis of the area fraction of inclusion indicated that the aluminum alloy samples without flux purification treatment had the highest recorded value at 2.96%. Meanwhile，the area fraction of inclusions in the aluminum alloy samples purified by YH-1~YH-4 rare earth composite fluxes and ST# series fluxes are 1.14%，1.45%，0.75%，0.77%，and 1.13%，respectively. The results of metallographic microstructure analysis and inclusion area fraction measurement demonstrated that all four types of rare earth composite fluxes significantly enhanced the metallurgical quality of the aluminum alloy samples. This improvement can be attributed to the ability of fluoride compounds present in rare earth composite flux compositions（such as Na₃AlF₆ and CaF）₂ to dissolve and eliminate oxide impurities，or it may be due to a reaction between rare earth compounds LaF₃ and CeF₃ in the composite fluxes with the molten aluminum alloy，effectively removing hydrogen and oxide impurities from the melt. Furthermore，examination of average grain size revealed that untreated 0# aluminum alloy exhibited the largest average grain size at approximately 120 μm，while treatment with YH-3 rare earth composite flux resulted in a significant reduction in average grain size for the treated 3# aluminum alloy sample to about 65 μm. In summary，grain size，pores，and inclusions had a significant impact on the mechanical properties of aluminum alloy. After undergoing purification treatment with rare earth composite fluxes，there was a reduction in grain size as well as pores and inclusions，consequently improving the hardness，tensile strength，and elongation of the aluminum alloy. Notably，YH-3 rare earth composite flux exhibited an optimal purification effect on aluminum melt by reducing inclusion area fraction from 2.96% to 0.75%，increasing Vickers hardness from HV 61.9 to HV 69.3，lifting elongation rate from 4.3% to 13.5%，and boosting tensile strength from 95.4 to141.5 MPa.