激光增材Ni基合金凝固裂纹萌生机制多尺度研究

Focusing on the hot spot of solidification cracks during laser additive manufacturing of Ni-based alloys. A new macro-micro approach including experimental observations, macroscopic calculation and microscopic phase-field modeling is developed to study solidification cracks during laser additive manufacturing of Ni-based alloys, the formation mechanism of solidification cracks is revealed. Firstly, the sensitivity of solidification cracks during laser additive manufacturing of Ni-based alloys are studied by experiments. The metallurgical characteristics of solidification cracks are observed and effects of processing parameters on solidification cracks during laser additive manufacturing are also analyzed. Then, the thermodynamic and temperature field models are established to calculate the stress-strain distribution and temperature field during laser additive manufacturing. Based on the experimental observations and calculated stress-strain distribution, the formation causes of solidification cracks during laser additive manufacturing are analyzed. Finally, the multi-phase field model of solidification cracks is developed considering the dendritic dynamic growth behavior in the molten pool. The complex formation process of solidification cracks during the solidification of the molten pool is quantitatively simulated, and effects of process parameters, solidification conditions, material physical properties, dendritic morphologies and misorientation angle on solidification cracks during laser additive manufacturing are also revealed. Based on the reflux healing theory, experimental observations and computed stress-strain distribution, the formation mechanism of solidification cracks during laser additive manufacturing of Ni-based alloys are clarified.

本项目针对激光增材Ni基合金凝固裂纹热点问题，提出实验测量、宏观计算和微观相场建模的宏-微观多尺度耦合方法，研究激光增材Ni基合金凝固裂纹行为，揭示凝固裂纹的萌生机制，开辟新的揭示凝固裂纹萌生机制的研究途径。首先进行激光增材Ni基合金凝固裂纹敏感性实验，观察凝固裂纹冶金学特征，探索工艺参数对凝固裂纹的影响规律。然后，建立激光增材宏观温度场模型和热力学模型，分别计算温度场和应力应变分布，结合实验数据从宏观力学和冶金学角度分析凝固裂纹的形成原因。最后，基于熔池枝晶的动态生长行为，建立微观尺度凝固裂纹多晶相场模型，定量模拟凝固裂纹显微尺度的复杂形成过程，揭示工艺参数、凝固条件、材料物性参数、晶粒形态和晶界角度对凝固裂纹的影响规律，结合凝固裂纹回流愈合理论，凝固裂纹冶金学特征和宏观计算的应力应变分布，阐明激光增材Ni基合金凝固裂纹的萌生机制。

本文建立了激光沉积IN718合金宏-微观多尺度耦合模型，首先对激光沉积宏观传热过程进行数值求解，发现熔池是一个典型的带有长尾的椭球形状，峰值温度约为2963.12℃，获得了熔合线特征点的温度梯度G，固液界面推进速度Vp和冷却速度G×Vp。利用微观相场模型耦合熔池凝固条件，模拟计算的一次枝晶臂间距从熔合线上部到下部呈现增加趋势。Nb在枝晶间区域发生偏析，最终以液滴形状分布；计算的实际溶质分布系数从0.584下降到5.548，遵循平衡凝固理论；计算的液相线斜率为0.05322wt.%/μm，非常接近理论值0.05394wt.%/μm。激光沉积IN718熔池凝固后形成较长的柱状晶组织，柱状晶间存在较长的液相通道，不利于凝固收缩时液态金属的回流愈合；此外，柱状晶侧枝生长不够强壮，相邻柱状晶间不易搭接，抵抗拉应力的能力较差，具有较高的凝固裂纹敏感性。根据模拟结果，随着各向异性强度的增加，枝晶尖端变得更加稳定，不易形成侧枝，枝晶间不易形成搭接，容易形成较长的液相通道，具有较高的凝固裂纹敏感性。