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1、 南京航空航天大学硕士学位论文 i摘 要 激光烧结快速成形技术因其取材广泛性、工艺灵活性，可用于制备具有新颖结构的多孔金属。但目前多孔金属激光烧结制备研究尚处于起步阶段，对其中涉及的孔结构形成与控制等基础理论，以及激光快速定向凝固过程中，温度梯度和凝固速率对显微组织的影响等均有待深入探讨。本文采用数值模拟、理论分析和实验研究相结合的方法，通过对多孔金属气孔形成机理的实验研究，获得相关基础理论用于指导多孔金属的激光制备。 研究了激光定向凝固条件下气孔的形核机理及长大方式，获得了气泡非自发形核的热力学及动力学条件。结果表明，气孔的长大主要有两种方式：金属熔体中气体的析出和凝固界面处气孔的生长；而只
2、有在凝固界面处析出的气体才能与固相一起长大，并形成规则的藕状多孔结构。 通过对烧结熔池内气孔长大方向及速度的实验研究，获得了激光工艺参数与气孔生长方向及速度的关系。研究表明，可通过改变工艺参数，来控制气孔的生长，进而实现对多孔结构的控制。 建立了孔隙率和孔间距的理论计算模型，并将实验数据与理论计算结果进行了比较。结果发现，利用本文建立的孔隙率计算模型，预测结果与实验值吻合良好。 对激光烧结多孔不锈钢试样进行了显微硬度值的测量，定量分析了激光功率、扫描速率对显微硬度值的影响。对烧结试样进行了压缩性能测试，通过分析压缩过程中的变形机理，建立了多孔不锈钢压缩应力应变的数学关系式。对试样进行了拉伸性能
3、测试，获得了适用于多孔不锈钢的孔隙率与抗拉强度的应用公式，为优化激光工艺参数提供了有利依据。 关键词关键词：激光烧结，定向凝固，藕状多孔结构，形核机理，生长方式，孔隙率， 力学性能 激光烧结多孔金属的成形机理及实验研究 iiABSTRACT Rapid Laser Sintering Manufacturing (RLSM), due to its flexibility in feedstock and shapes, exhibit a great potential for developing novel porous metallic material. However, laser
4、 sintering technology is still in its early stage of development. Significant further research and understanding are required in the aspect of basic theories producing porous metals in forming of gas pore and porous microstructure, and during the process of laser rapid solidification, the effect of
5、temperature gradient, velocity of solidification on the microstructure involved in laser sintering. In this paper, by combining theoretical analysis, numerical simulation and experimental study, foaming mechanism of gas pore were analyzed thoroughly, and the basic theory which can control the techni
6、que of porous metallic material were achieved. Detailed the mechanism of nucleation and growth pattern of gas pore for rapid direction solidification in laser sintering, the thermodynamics and kinetic conditions of heterogeneous nucleation were achieved. Analyzed the forming mode of pore, introduced
7、 the gas separate from the melting metal and the surface of solidification, but only the gas came from the surface of solidification could grow with the solid and form into lotus-type porous. The research on the direction and velocity of the growth of gas pore in the sintering pool were studied. The
8、 effect of parameters on the growth of pore and the relationship between parameters and the microstructure were obtained. So, we can control the microstructure by changing the parameters which has the deeply effect on the growth of gas pore. The theoretical modes for calculating porosity and inter-p
9、ore spacing were established. A comparation between the obtained experimental data and the reuslts calculated using the present theoretical models was perfored, showing a good agreement. The microhardness of porous stainless steel samples were measured and qualitative the effect of laser parameters
10、on it. The compressibility of porous stainless steel was stuied, by analyzing the deformation mechanism, a formula of stress-stain curve was established. The tensile strength was measured, through the application to 南京航空航天大学硕士学位论文 iiithe relative theoretical model, with the obtained experimental dat
11、a, found out the formula which fit the relation between tensile strength and porosity of porous stainless steel. Keywords: Laser sintering, Directional solidification, Lotus-type porous structure, Nucleating mechanism, Growth pattern, Porosity, Mechanical properties 激光烧结多孔金属的成形机理及实验研究 vi图表清单 图清单 图清单
12、 图 1.1 球形粉末颗粒的烧结 .9 图 1.2 粉末烧结型多孔铜材料的 SEM 显微照片.9 图 1.3 激光烧结条件下成形不同孔隙率试样的微观组织 .14 图 2.1 气体吸附于表面颗粒形核 .17 图 2.2 非自发形核的过冷度与接触角及晶坯尺寸的关系 .19 图 2.3 气泡脱离夹杂颗粒表面示意图 .21 图 2.4 接触角对非自发形核的影响 .21 图 2.5 凝固界面处柱状孔的形成 .22 图 2.6 烧结池的运动状态下结晶 .25 图 2.7 气孔生长速度与扫描速率间的关系 .26 图 2.8 烧结池内气孔形态的变化 .26 图 2.9 金属-气体定向凝固示意图 .29 图 2.10 凝固界面上圆柱坐标系 .29 图 2.11 界面能示意图 .31 图 3.1 激光烧结成形示意图 .34 图 3.2 激光功率为 650W，扫描速率为 0.04m/s 时烧结试样的显微组织.35 图 3.3 激光功率为 650W，扫描速率为 0.10m/s 时烧结试样的显微组织.36 图 3.4 扫描速率对气孔生长模式的影响 .36 图 3.5 激光功率为 500W，扫描速率为 0.04m/s 时烧结试样的显微组织.37 图 3.6 不同工艺参数下制得的多孔不锈钢孔隙率实验值与理论值的比较 .
