铝合金粉末选区激光熔化过程的介观模拟与实验研究

发布时间：2018-06-02 11:14

本文选题：选区激光熔化 + 介观模拟　；参考：《南京航空航天大学》2017年硕士论文

【摘要】：本文针对AlSi10Mg粉末选区激光熔化过程,建立了介观尺度下的颗粒熔化/凝固有限体积模型,模型充分考虑了疏松粉床中颗粒之间气相的存在,固相和气相热导率的差异,粉体不同于实体的激光吸收机制,以及材料由粉体向实体转变过程中的流体流动等因素,模拟了疏松粉床在三维高斯热源作用下,颗粒熔化过程的瞬时流动状态。在定扫描速度v(400mm/s)变激光功率P(150W,200W,250W,300W,350W)的情况下,获得了三维熔池的温度场与表面形貌、激光功率与成形表面质量的关系、粉床熔化过程中的收缩率以及定激光束下烧结颈的形成演变过程。研究发现:过高和过低的激光功率都会导致较差的成形件表面质量,粉床熔化过程中的收缩率与激光能量密度有很大关系,颗粒表面刚开始熔化时,由于表面张力的作用会在颗粒之间形成烧结颈。为了验证模拟结果的准确性,本课题利用实验方法在相同参数下制备了边长5mm的立方块体,对实验样品进行SEM分析和表面粗糙度测试,发现熔池尺寸和表面形貌受到激光功率的影响规律与模拟一致。为了研究激光扫描速度对激光穿透深度的影响,本研究更进一步的建立了多层粉床模型,采用带有残留孔隙的实体模拟已加工层,在已加工层上生成一层粉末颗粒。模拟时采用定功率P(100W)变扫描速度v(150mm/s,200mm/s,250mm/s,300mm/s,350mm/s,400mm/s)的方式,得到了不同激光扫描速度下粉末熔化程度的多相流截面图,可以看出,随着激光扫描速度的降低,粉末熔化越来越充分,激光穿透粉层越深,粉末层间的冶金结合越牢固。同时,不同扫描速度带来了不同形貌的残留孔隙,通过对熔池内单个气泡的运动轨迹进行追踪,发现较低的激光扫描速度有利于前一加工层残留的气孔重新浮起到当前加工层,从而使前一层更加致密。据此推论出打印件在靠近表面的位置孔隙率较高,并且会影响零件的力学性能。最后在实验中采用和模拟相同的加工参数打印了块体试样,并对试样进行了显微形貌的观察和拉伸性能的测试,结果显示,试样表面处的孔隙率高于内部,拉伸试验中表面处的材料抗拉强度比心部低,实验结果间接验证了模型的准确性。
[Abstract]:In this paper, a finite volume model of particle melting / solidification in mesoscopic scale is established for the selective laser melting process of AlSi10Mg powder. The model fully considers the existence of gaseous phase and the difference of thermal conductivity between solid and gas phase in porous powder bed. The laser absorption mechanism of powder is different from that of solid, and the fluid flow in the process of material transition from powder to solid is simulated. The transient flow state of loose powder bed in the process of particle melting under the action of three-dimensional Gao Si heat source is simulated. Under the condition of variable laser power (P ~ (150) W ~ (200) W ~ (200) W ~ (2 +) W ~ (300) W ~ (3 +) W) at a constant scanning speed of 400 mm / s, the relationship between the temperature field and surface morphology, laser power and the surface quality of the three dimensional molten pool has been obtained. The shrinkage rate in the melting process of powder bed and the evolution process of sintering neck under a fixed laser beam. It is found that too high laser power and too low laser power will lead to poor surface quality of the formed parts. The shrinkage rate in the melting process of powder bed is closely related to the laser energy density, and when the particle surface begins to melt, Because of the effect of surface tension, the sintering neck will be formed between the particles. In order to verify the accuracy of the simulation results, the cubic block with side length 5mm was prepared with the same parameters by using the experimental method. The SEM analysis and the surface roughness test of the experimental sample were carried out. It is found that the influence of laser power on the size and surface morphology of the molten pool is consistent with the simulation. In order to study the effect of laser scanning speed on laser penetration depth, a multilayer powder bed model was established in this study. A layer of powder particles was formed on the machined layer by simulating the machined layer with residual pores. In the simulation, by using the constant power PX 100W), the scanning speed is 150mm / s / 200mm / s ~ 250mm / s / s, the multiphase flow profile of the powder melting degree at different laser scanning speeds is obtained. It can be seen that with the decrease of the laser scanning speed, the powder melts more and more fully. The deeper the laser penetrates the powder layer, the stronger the metallurgical bonding between the powder layers is. At the same time, different scanning velocities bring about different morphology of residual pores. By tracing the movement of single bubble in the molten pool, it is found that the lower laser scanning speed is conducive to the re-floatation of the remaining pores in the former machining layer to the current processing layer. So that the front layer is more compact. It is inferred that the porosity of the printed parts near the surface is high and the mechanical properties of the parts will be affected. Finally, the bulk samples were printed with the same processing parameters as those simulated in the experiment. The microstructure and tensile properties of the samples were observed. The results showed that the porosity on the surface of the samples was higher than that on the inside. The tensile strength of the material on the surface is lower than that at the center of the tensile test, and the accuracy of the model is indirectly verified by the experimental results.
【学位授予单位】：南京航空航天大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG665

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