On the solidification behaviors of AlCu5MnCdVA alloy in electron beam freeform fabrication: Microstructural evolution, Cu segregation and cracking resistance

Abstract

For most additive manufacturing techniques, it is a challenge to fabricate dense and fully equiaxed Al-Cu alloys. However, the applicability of electron beam freeform fabrication (EBF3) to Al-Cu alloys is not clear due to the complex solidification process. To elaborate the solidification characteristics of Al-Cu alloys during EBF3, this work selected commercial AlCu5MnCdVA alloy as the object and semi-quantitatively investigated solidification behaviors including microstructural evolution, Cu segregation and cracking resistance. Meanwhile, the transient heat transfer model was established. The critical solidification conditions for columnar to equiaxed transformation (CET) and cracking were deduced and obtained. The results show that microstructure is dependent on two solidification processes in completely melted zone and partially remelted zone due to the wider mushy zone with 1.80 ~ 3.88 mm. The fully equiaxed grains are obtained in AlCu5MnCdVA alloy fabricated by the EBF3. The reason is not only that the solidification process in the completely melted zone meets the CET, but also that the existence of a wide partially remelted zone hinders the growth of columnar grains. Additionally, it was found that Cu segregation was decreased due to the solidification in partially remelted zone, which was also theoretically demonstrated by solute redistribution at solid/liquid interface. Moreover, although the AlCu5MnCdVA alloy presents high cracking susceptibility. cracking can be inhibited during EBF3 due to cooling rate with 102 K/s order of magnitude much lower than the cracking condition. Finally, based on the insights into the solidification behaviors during EBF3, this study suggests that EBF3 process is favorable for obtaining fully equiaxed grain, reduce element segregation and inhibit cracking. These characteristics are also suitable for most Al-Cu alloys. Additionally, it is expected the EBF3 is suitable for alloys with a high hot cracking tendency.