激光立體成形退火態(tài)Zr55Cu30Al10Ni5粉末的晶化行為

激光立體成形退火態(tài)Zr55Cu30Al10Ni5粉末的晶化行為Crystallization behavior of laser solid forming of annealed Zr55Cu30Al10Ni5 powder

將等離子旋轉(zhuǎn)電極法所制粉末在1000K退火處理后作為沉積材料，應(yīng)用激光立體成形技術(shù)沉積Zr55Cu30Al10Ni5塊體非晶合金，考察工藝參數(shù)及退火態(tài)粉末尺寸對熔覆層晶化行為的影響。結(jié)果表明，不同尺寸退火態(tài)粉末組織均由Al5Ni3Zr2、CuZr2和Al2Zr3相組成。以不同激光線能量熔覆后，試樣的熔池區(qū)主要為非晶，晶化區(qū)由熔池底部到熱影響區(qū)依次分布NiZr2納米晶、CuZr2+ZrCu枝狀共晶和CuZr2+ZrCu球粒狀共晶，共晶尺寸隨著距熔池區(qū)距離的增加而減小。當(dāng)激光線能量較低時，熔覆層均保持較高含量的非晶相。隨著激光線能量的增大，尺寸為75-106 μm的退火態(tài)粉末所制試樣的晶化程度無明顯加強(qiáng)，而尺寸為106-150 μm的退火態(tài)粉末所制試樣的晶化程度嚴(yán)重惡化。熔覆層的晶化差異受粉末本身相結(jié)構(gòu)影響較小，主要由熔覆不同尺寸粉末時熔池及熱影響區(qū)的熱歷史決定。

Laser solid forming (LSF) provides an innovative way in building the bulk metallic glasses (BMGs) due to its inherently rapid heating and cooling process and point by point additive manufacturing process, which can eliminate the limitation of critical casting size of BMGs. The annealed powder has been demonstrated to be applicable to the preparation of BMGs with high content of amorphous phase using LSF. In this work, the plasma rotating electrode processed (PREPed) Zr55Cu30Al10Ni5 (Zr55) powders annealed at 1000 K are used for LSF of Zr55 BMGs. The influences of powder size and laser processing parameter on the crystallization characteristic of the deposit are investigated, and the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) is analyzed. It is found that the microstructures of the pre-annealed Zr55 powders are composed of the Al5Ni3Zr2, CuZr2 and Al2Zr3 phases. As the heat input increases from 7.0 J/mm to 15.7 J/mm, the every deposited layer presents a periodic repeating gradient microstructure (amorphous, NiZr2 nanocrystal, CuZr2+ZrCu dendrite-like eutectic, CuZr2+ZrCu spherulite-like eutectic) from the molten pool to the HAZ. The size of the eutectic phase in the HAZ decreases as the increase of distance from the featureless amorphous zone. On condition that the laser heat input is less than 7.0 J/mm, the deposits contain a high content of amorphous phase. As the increase of laser heat input, the crystallization degree of HAZ does not increase obviously for the deposit prepared by the powder with size range of 75-106 μm. However, the crystallization degree of HAZ increases significantly for the deposit prepared by the powder with size range of 106-150 μm. That is because the lower overheating temperature and shorter existing time of the molten pool enhances the heredity of Al5Ni3Zr2 clusters and other intermetallic clusters in remelted alloy melt during LSF of coarser powder, which decreases the thermal stability of the already-deposited layer and induces the severe crystallization. It is deduced that the raw state of annealed powders has a minimal impact on the crystallization behavior of the deposited layers when the content of Al5Ni3Zr2 phase is same in different sizes of annealed powders. The thermal history of RZ and HAZ during deposition is the primary factor to affect the crystallization behavior in the deposits fabricated by different powder sizes.

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