Influence Of Rare Earth And Vanadium On The Microstructure And Properties Of The Cast Al-Zn-Mg-Cu Alloys

Aluminum alloy flanges of beams for textile warp knitting machines are the important parts of the textile industry. Through efforts, our research group has successfully developed a new type of cast Al-Zn-Mg-Cu alloy that has been used for producing superior products that serve the demanding needs of our customers by casting instead of forging. At present the high-performance plate flanges of beams are appraised by the customers. Serious hot crack defects occurred in production, so further study for improving hot cracking resistance is necessary. In this paper, effects of V and RE on microstructure, hot cracking tendency and mechanical properties of the alloy for flanges of beams of textile warp knitting machines (cast Al-4.5Zn-l.0Mg-0.8Cu alloy) were studied in detail. To reveal in depth the influence mechanisms of V and RE on hot cracking tendency and hot cracking mechanism, effects of V and RE on microstructure and hot cracking tendency of typical cast Al-7.0Zn-2.5Mg-1.OCu alloy were studied.Secondary dendrite arm spacing of cast Al-7.0Zn-2.5Mg-1.0Cu alloy was finer. Cast Al-7.0Zn-2.5Mg-1.0Cu alloy have more eutectic phase. Secondary dendrite arm spacing of both cast Al-Zn-Mg-Cu alloys decreased first and then increased with increasing V or RE addition. RE had higher refining effect on dendrites of cast Al-4.5Zn-1.0Mg-0.8Cu alloy than V. V or RE could accelerate the formation of eutectic phase, and RE possessed more significantly effect on the amount of eutectic phase. V mainly present in Al11V phase that were distributed in α(Al) grains. The irregular blocky Al11V phases precipitated in aluminum matrix of the alloy with addition of excess V. RE mainly present in Ce2Cu7Al10 phase and AlLaCu4 phase, which particip-ate in forming α(Al)+AlZnMgCuCeLa eutectic phase.Mechanical properties of cast Al-4.5Zn-1.0Mg-0.8Cu alloy increased first and then decreased with increasing V or RE addition. The mechanical properties of the alloy with proper addition of V or RE could meet the application requirement of the alloy for flanges of beams. The improvement of mechanical properties of the alloy with proper addition of RE is superior to proper addition of V.Hot cracking tendency of cast Al-7.0Zn-2.5Mg-1.0Cu alloy was lower. Hot cracking tendency of both cast Al-Zn-Mg-Cu alloys decreased first and then increased with increasing V or RE addition. Adding proper amount RE processed more significantly effect on decreasing hot cracking tendency of cast Al-4.5Zn-1.0Mg-0.8Cu alloy. By comparing three schemes, adding suitable amount of RE separately into cast Al-4.5Zn-1.0Mg-0.8Cu alloy could obtain best reduction effect on hot cracking tendency, adding proper amount of V and RE following behind, adding proper amount of V the third. Considering synthetically effects of RE on mechanical properties of the alloy for flanges of beams of textile warp knitting machines, decreasing effect of adding 0.12wt% RE on hot cracking tendency was the most effective, decreased by 42.86%, and tensile strength and elongation of this alloy increased by 39.85% and 87.34% respectively.Adding proper amount of V or RE could decrease hot cracking tendency of cast Al-Zn-Mg-Cu alloys, there were two reasons. On one hand, it could decrease secondary dendrite arm spacing of alloys, increase the amount of dendritic bridging, improve dendritic deformability, reduce the possibility of dendritic bridging destroyed by solidification shrinkage. On the other hand, it could increase the amount of eutectic phases, improve feeding effect, and heal first cracks.