橫向穩(wěn)恒磁場對激光熔化增材制造Al-12wt.%Si合金凝固組織的影響

橫向穩(wěn)恒磁場對激光熔化增材制造Al-12wt.%Si合金凝固組織的影響Effect of transverse static magnetic field on microstructure of Al-12wt.%Si alloys fabricated by powder-blow additive manufacturing

利用送粉式激光熔化3D打印工藝，研究了外加橫向穩(wěn)恒磁場對3D打印Al-12%Si合金構(gòu)件凝固組織的影響。結(jié)果表明，在有/無橫向穩(wěn)恒磁場下，激光熔化單道薄壁試樣的宏觀凝固組織未發(fā)生明顯改變，其主要以白亮帶（以α-Al相為主）和灰暗區(qū)（以Al-Si共晶相為主）為基本單元疊加構(gòu)成。而微觀組織分析表明，無磁場時灰暗區(qū)內(nèi)的初生α-Al相呈柱狀枝晶形態(tài)，施加了0.35 T橫向穩(wěn)恒磁場后，試樣灰暗區(qū)內(nèi)的初生α-Al相全部轉(zhuǎn)變?yōu)榈容S枝晶形態(tài)，且枝晶臂發(fā)達?；跓犭姶帕捌銱artman無量綱數(shù)（用于表征穩(wěn)恒磁場對金屬熔體流動抑制作用的參數(shù)）估算分析表明，0.35 T穩(wěn)恒磁場下，作用于初生α-Al枝晶上的熱電磁力可達105 N/m3量級；Hartman數(shù)遠大于10，表明激光熔化微小金屬熔池中強烈的Marigoni以及熱溶質(zhì)對流一定程度上被抑制。分析認為，穩(wěn)恒磁場下凝固組織灰暗區(qū)內(nèi)α-Al相柱狀枝晶向等軸枝晶的轉(zhuǎn)變是固相中熱電磁力（~105 N/m3）的對枝晶的破碎作用導(dǎo)致，而等軸枝晶發(fā)達的枝晶臂則是橫向穩(wěn)恒磁場抑制熔體流動的結(jié)果。

Using powder laser melting, the effect of transverse static magnetic field on the solidified structure of additive manufactured Al-12%Si alloy was studied. The macrostructure was formed by white band (mainly primary α-Al phase) and dark grey area (mainly eutectic phase) and no obvious influence was presented with or without static transverse magnetic field of 0.35 T. However, for the microstructure, the primary α-Al in dark grey area formed as columnar structure without magnetic field was found to transform to dendritic like with developed dendrite arms when under a static transverse magnetic field. The analysis on thermoelectricity and dimensionless Hartman parameter which used to characterize the restriction of static magnetic field on molten flows show that under a static transverse magnetic field of 0.35 T, the thermoelectricity can be as high as a magnitude of 105 N/m3, and Hartman values is far more than 10. The results indicate that the Marigoni and thermosolutal convection in laser melting pool was restricted. The transform from columnar to equiaxed dendrite of primary α-Al in dark grey area under static magnetic field was attributed to the fragmentation by thermoelectric magnetic force (105 N/m3) in solid phase. In addition, the formation of high order dendrite arms was supposed to be caused by the restriction of static magnetic field on the melt.

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