【作者】 张帅；

【导师】 程昀； 王胜海；

【作者基本信息】 山东大学 ， 材料工程（专业学位）， 2023， 硕士

【摘要】 AlSi10Mg合金由于具有良好的铸造性能被广泛应用于航空航天、汽车轻量化等领域,但粗大的初生α-Al枝晶和针状共晶硅使得AlSi10Mg铸件机械性能较差。选区激光熔化(Selective Laser Melting,SLM)技术以高能激光作为热源,极大地改善了 AlSi10Mg成形件的机械性能,但由于铝合金本身密度低且具有高的激光反射率和高导热性等特性使得SLM成形困难,影响成形件的力学性能。本文针对AlSi10Mg合金的SLM成形,研究了工艺参数对成形质量及熔池分布的影响并对成形工艺参数进行优化,通过原位重熔工艺对熔池微观组织与分布进行调控,提高成形件强度,并研究了多次循环使用后粉末的变异情况,对比粉末不同循环使用次数对常规成形件及重熔成形件力学性能的影响,评估重熔工艺在粉末多次使用后增强效果的稳定性。首先研究了 SLM成形AlSi10Mg过程中激光功率、扫描速率与成形件致密度、表面质量和表面硬度的关系。建立了以能量密度为变量的致密度预测模型,利用图像识别技术,对不同工艺参数下孔隙缺陷形状的变化规律进行定量分析,优化工艺参数窗口。同时研究了熔池的形成机理,发现在高的激光功率下熔池纵横比较大,而扫描速率较大时易获得小尺寸熔池。通过纳米压痕技术对熔池力学性能差异进行分析发现,相较于常规熔池,这两种熔池均具有更高的强度和模量。同时发现在高激光功率下获得的深长形熔池往往存在伴生锁孔,在较大扫描速率下获得小尺寸熔池存在较多的不规则冶金孔,孔隙率的增高使得成形件整体力学性能表现并不突出。为了在不影响成形质量的前提下获得更多精细结构的熔池分布,引入重熔工艺对熔池微观结构进行调控。选择不同的激光功率与扫描速度对SLM成形AlSi10Mg进行重熔单道实验,对熔道和熔池截面的几何形貌变化进行统计分析,发现扫描速度对熔池顶部起伏影响较大,且对重熔熔池纵横比的影响小于激光功率。故确定以重熔激光功率为变量研究了重熔激光功率对AlSi10Mg打印成形件成形质量、显微组织及力学性能的影响,发现在250 W重熔激光功率下实现了高含量具有精细微观结构的熔池分布。此时重熔样品屈服强度为250.82 MPa,抗拉强度为421.83 MPa,延伸率为6.79%,较未重熔样品屈服强度和抗拉强度分别提高了 16.71%、7.57%,但延伸率降低了 10.54%。最后,通过观察不同循环次数的AlSi10Mg粉末特征,发现随着循环次数的增加,粉末中胞状晶比例增加,粉末变异程度加大。对比不同循环次数的AlSi10Mg粉末成形件与250 W激光功率重熔处理后成形件的力学性能,发现随着粉末循环次数的增加,成形样品与重熔样品延伸率略有下降,但不同循环次数下重熔样品力学性能均优于未重熔样品,表明优化的重熔工艺在粉末循环过程中仍保持良好稳定的强度提升作用。更多还原

【Abstract】 The AlSi10Mg alloy is widely used in the fields of aerospace,automobile lightweighting,and others due to its excellent casting performance.However,the coarse primary α-Al dendrites and needle like eutectic silicon in the alloy result in poor mechanical properties of AlSi10Mg castings.Selective laser melting(SLM)technology,which uses high-energy lasers as heat sources,greatly improves the mechanical properties of AlSi10Mg formed parts.However,the low density and high laser reflectivity and thermal conductivity of aluminum alloys make SLM forming difficult,affecting the mechanical properties of the formed parts.In this study,we investigated the effect of process parameters on the quality and distribution of the melt pool for SLM forming of AlSi10Mg alloy and optimized the forming process parameters.The microstructure and distribution of the melt pool were controlled by in-situ re-melting process,which improved the strength of the formed parts.We also studied the variation of the powder after multiple cycles of use,compared the effects of different numbers of powder reuse on the mechanical properties of conventional formed parts and re-melted formed parts,and evaluated the stability of the reinforcement effect of the remelting process after multiple uses of the powder.The relationship between laser power,scanning speed,and the density,surface quality,and surface hardness of formed parts during SLM forming of AlSi10Mg was first studied.A density prediction model based on energy density was established.Using image recognition technology,the variation of pore shape under different process parameters was quantitatively analyzed to optimize the process parameter window.Ultimately,the best forming quality was achieved within the processing window range of laser power 300 W-350 W and scanning speed 750 mm/s-1650 mm/s.The formation mechanism of the melt pool was also investigated.Larger aspect ratio of the melt pool was obtained under high laser power,while smaller melt pools were obtained with higher scanning speeds.Analysis of the mechanical properties of the melt pool using nanoindentation technique revealed that both types of melt pools had higher strength and modulus compared to conventional melt pools.It was also found that elongated melt pools obtained under high laser power were often accompanied by keyhole defects.The presence of irregular metallurgical pores in small-sized melt pools obtained at higher scanning speeds resulted in a higher porosity rate,leading to an overall weaker mechanical performance of the formed parts.In order to obtain a more refined distribution of melt pool structure without affecting the forming quality,a re-melting process was introduced to control the microstructure of the melt pool.Different laser powers and scanning speeds were selected for single-track re-melting experiments of SLM-formed AlSi10Mg,and the geometric morphology changes of the melt pool and cross-section of the remelted track were statistically analyzed.It was found that the effect of scanning speed on the aspect ratio of the remelted melt pool was smaller than that of laser power,and compared with changes in scanning speed,the topography of the melt pool was less affected by laser power.Therefore,the re-melting laser power was determined as the variable,and the effect of re-melting laser power on the forming quality,microstructure,and mechanical properties of AlSi10Mg printed parts was studied.It was found that a high content of melt pool with fine microstructure distribution was achieved at a re-melting laser power of 250 W,and the best comprehensive mechanical properties were obtained.At this time,the yield strength,tensile strength,and elongation of the remelted sample were 250.82 MPa,421.83 MPa,and 6.79%,respectively,which were increased by 16.71%,7.57%,and decreased by 10.54%,respectively,compared with the unremelted sample.Finally,by observing the characteristics of AlSi10Mg powder with different recycling cycles,it was found that the proportion of cellular crystals in the powder increased and the powder variation degree intensified with the increase of recycling cycles.Comparing the mechanical properties of AlSi10Mg powder specimens with different recycling cycles and specimens treated with 250 W laser power re-melting,it was found that the elongation of both the molded and re-melted samples slightly decreased with the increase of powder recycling cycles,but the mechanical properties of the re-melted samples were better than those of the non-re-melted samples at different recycling cycles,indicating that the optimized re-melting process maintains a good and stable strength enhancement effect during powder recycling.更多还原