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研究了在液氮冷却条件下激光快速熔凝Ni-28 wt%Sn亚共晶合金的组织演化过程.结果显示,熔池从上至下可以分为三个区域:表层为平行激光扫描方向的α-Ni转向枝晶区;中部为近乎垂直于熔池底部外延生长的α-Ni柱状晶区;底部为少量的残留α-Ni初生相和大量的枝晶间(α-Ni+Ni3Sn)共晶组织.激光熔凝区组织受原始基材组织的影响很大,熔池中的α-Ni枝晶生长方向受到了热流方向和枝晶择优取向的双重影响.与基材中存在的层片状、棒状和少量离异(α-Ni+Ni3Sn)共晶的混合组织相比,熔池内的共晶组织皆为细小的规则(α-Ni+Ni3Sn)层片状共晶,皆垂直于熔池底部外延生长,并且从熔池顶部至底部,共晶层片间距逐渐增大.分别应用描述快速枝晶生长的Kurz-Giovanola-Trivedi模型和描述快速共晶生长的Trivedi-Magnin-Kurz模型对熔池表层凝固界面前沿的过冷度进行估算,发现熔池表层α-Ni枝晶和(α-Ni+Ni3Sn)层片共晶的生长过冷度在50.4—112.5 K之间,远大于相应深过冷凝固(α-Ni+Ni3Sn)反常共晶生长的临界过冷度20 K,这说明文献报道的临界过冷度并不是反常共晶出现的充分条件.
The microstructure evolution of Ni-28 wt% Sn hypoeutectic alloy was studied under the conditions of liquid nitrogen cooling.The results show that the weld pool can be divided into three regions from top to bottom: the surface is parallel to the laser scanning direction α-Ni to the dendrite zone. The middle part is an α-Ni columnar zone which is epitaxially grown near the bottom of the melt pool. The bottom part is a small amount of residual α-Ni primary phase and a large amount of inter-dendritic (α-Ni + Ni3Sn) The microstructure of the laser melting zone is greatly affected by the original microstructure of the matrix, and the growth direction of the α-Ni dendrite in the weld pool is influenced by the direction of heat flow and the preferred orientation of the dendrite. (Α-Ni + Ni3Sn) eutectic, the eutectic structures in the weld pool are all thin and regular (α-Ni + Ni3Sn) lamellar eutectics, which are perpendicular to the bottom of the weld pool Epitaxial growth, and eutectic layer spacing increases from the top to the bottom of the weld pool.The Kurz-Giovanola-Trivedi model, which describes rapid dendrite growth, and the Trivedi-Magnin-Kurz model, which describes rapid eutectic growth, The supercooling at the fronts of the surface solidified interface was estimated and it was found that α-Ni dendrite and (α -Ni + Ni3Sn) lamellae is between 50.4 and 112.5 K, which is much larger than the critical undercooling degree of 20 K for the corresponding eutectic eutectic (α-Ni + Ni3Sn) eutectic The reported critical undercooling is not sufficient for the occurrence of anomalous eutectic.