本文選題：選區(qū)激光熔化 + 介觀模擬��； 參考：《南京航空航天大學(xué)》2017年碩士論文

【摘要】：本文針對AlSi10Mg粉末選區(qū)激光熔化過程,建立了介觀尺度下的顆粒熔化/凝固有限體積模型,模型充分考慮了疏松粉床中顆粒之間氣相的存在,固相和氣相熱導(dǎo)率的差異,粉體不同于實體的激光吸收機(jī)制,以及材料由粉體向?qū)嶓w轉(zhuǎn)變過程中的流體流動等因素,模擬了疏松粉床在三維高斯熱源作用下,顆粒熔化過程的瞬時流動狀態(tài)。在定掃描速度v(400mm/s)變激光功率P(150W,200W,250W,300W,350W)的情況下,獲得了三維熔池的溫度場與表面形貌、激光功率與成形表面質(zhì)量的關(guān)系、粉床熔化過程中的收縮率以及定激光束下燒結(jié)頸的形成演變過程。研究發(fā)現(xiàn):過高和過低的激光功率都會導(dǎo)致較差的成形件表面質(zhì)量,粉床熔化過程中的收縮率與激光能量密度有很大關(guān)系,顆粒表面剛開始熔化時,由于表面張力的作用會在顆粒之間形成燒結(jié)頸。為了驗證模擬結(jié)果的準(zhǔn)確性,本課題利用實驗方法在相同參數(shù)下制備了邊長5mm的立方塊體,對實驗樣品進(jìn)行SEM分析和表面粗糙度測試,發(fā)現(xiàn)熔池尺寸和表面形貌受到激光功率的影響規(guī)律與模擬一致。為了研究激光掃描速度對激光穿透深度的影響,本研究更進(jìn)一步的建立了多層粉床模型,采用帶有殘留孔隙的實體模擬已加工層,在已加工層上生成一層粉末顆粒。模擬時采用定功率P(100W)變掃描速度v(150mm/s,200mm/s,250mm/s,300mm/s,350mm/s,400mm/s)的方式,得到了不同激光掃描速度下粉末熔化程度的多相流截面圖,可以看出,隨著激光掃描速度的降低,粉末熔化越來越充分,激光穿透粉層越深,粉末層間的冶金結(jié)合越牢固。同時,不同掃描速度帶來了不同形貌的殘留孔隙,通過對熔池內(nèi)單個氣泡的運(yùn)動軌跡進(jìn)行追蹤,發(fā)現(xiàn)較低的激光掃描速度有利于前一加工層殘留的氣孔重新浮起到當(dāng)前加工層,從而使前一層更加致密。據(jù)此推論出打印件在靠近表面的位置孔隙率較高,并且會影響零件的力學(xué)性能。最后在實驗中采用和模擬相同的加工參數(shù)打印了塊體試樣,并對試樣進(jìn)行了顯微形貌的觀察和拉伸性能的測試,結(jié)果顯示,試樣表面處的孔隙率高于內(nèi)部,拉伸試驗中表面處的材料抗拉強(qiáng)度比心部低,實驗結(jié)果間接驗證了模型的準(zhǔn)確性。
[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.
【學(xué)位授予單位】：南京航空航天大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TG665

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