目的 探究Invar 36合金组织结构-孔隙缺陷-热膨胀系数之间的关联机制,最终通过改变激光工艺参数调控Invar 36合金的显微组织与热膨胀性能。方法 本文利用选区激光熔化技术,采用Invar 36合金粉末制备成形试样。通过排水法测得不同工艺参数下制备Invar 36合金致密度并计算孔隙率;通过EBSD、SEM等技术观察表征不同工艺参数下制备Invar 36合金的微观组织;最后测试Invar 36合金的热膨胀系数。以此来系统研究选区激光熔化工艺参数、Invar 36合金沉积态微观组织以及冶金缺陷与低热膨胀性能的关联机制。结果 通过调整工艺参数,发现选区激光熔化成形Invar 36合金致密度随能量密度的增加呈现先增大后减小的变化趋势,能量密度为108.33 J/mm3时,Invar 36合金致密度达到最高。观察显微组织发现,SLM制备Invar 36合金熔池受热输入影响显著,熔池形貌整体如“鱼鳞状”或者“杯状”,熔池内部凝固组织以外延生长的粗大柱状枝晶为主,柱状枝晶内部为平均尺寸约0.7 μm的细小胞状亚结构,高密度位错缠结在胞状亚结构边界。Invar 36合金的热膨胀系数与孔隙率存在明显关联性,平均热膨胀系数随孔隙率降低而增加,高孔隙率可以抵消结构受热膨胀,低膨胀性能受高应力、孔隙与“因瓦效应”的综合影响。结论 Invar 36合金组织与性能显著受工艺参数的影响,通过调节工艺参数改变合金孔隙率,可制备不同热膨胀系数的Invar 36合金。
Abstract
Invar 36 alloy is a typical low thermal expansion alloy, which exhibits an ultra-low thermal expansion coefficient (<2×10-6 ℃-1) at room temperature. The ultra-low thermal expansion behavior of Invar 36 alloy at room temperature is generally attributed to the "magnetostrictive" effect locking inherent to the alloy itself. With the limitations of traditional processing technologies in meeting the emerging application requirements for Invar 36 alloy structures, selective laser melting (SLM) technology has been adopted as an alternative approach. However, the inherent thermal cycling mode of SLM technology can alter the microstructure and properties of Invar 36 alloy. Therefore, it is essential to investigate the effects of SLM on the microstructure and properties of Invar 36 alloy, particularly its thermal expansion properties. Meanwhile, to achieve more straightforward control over the thermal expansion performance of Invar 36 alloy, an approach for artificially regulating the alloy's expansion behavior by controlling its porosity is proposed. It is necessary to first explore the correlation mechanism between the microstructure, pore defects, and thermal expansion coefficient of Invar 36 alloy. Ultimately, the goal is to artificially control the porosity of Invar 36 alloy by adjusting laser process parameters, thereby regulating its thermal expansion coefficient. In this study, Invar 36 alloy samples were fabricated via SLM under different process parameters, and their density and porosity were calculated. The microstructure was characterized using techniques such as EBSD and SEM. Finally, the thermal expansion coefficients of the Invar 36 alloy samples prepared under different laser process parameters were measured. The results indicated that the density of SLM-fabricated Invar 36 alloy first increased and then decreased with the increase of energy density. The maximum density of Invar 36 alloy was achieved when the energy density was 108.33 J/mm3. Microstructural observations revealed that the molten pool morphology of SLM-fabricated Invar 36 alloy was significantly influenced by heat input, with an overall "fish scale" or "cup-like" appearance. The solidification structure inside the molten pool was dominated by coarse columnar grains grown epitaxially. Within these columnar grains, a fine cellular substructure with an average size of approximately 0.7 μm was observed, and high-density dislocations were entangled at the boundaries of the cellular substructure. A significant correlation existed between the thermal expansion coefficient and porosity of Invar 36 alloy: the average thermal expansion coefficient increased with the decrease of porosity. High porosity could offset the thermal expansion of the structure, and the low expansion performance was affected by high stress, porosity, and the "Invar effect". Additionally, it was found that as the temperature increased, the difference in thermal expansion coefficients among Invar 36 alloy samples prepared at different energy densities decreased. This indicated that with increasing temperature, the ability of Invar 36 alloy to offset thermal expansion through hysteretic shrinkage was weakened, and the expansion space of internal pores was gradually reduced. It can be predicted that the growth trend of the thermal expansion coefficient of Invar 36 alloy will tend to be uniform above the Curie temperature. In conclusion, Invar 36 alloys with different thermal expansion coefficients can be fabricated by adjusting the porosity of the alloy through the optimization of SLM process parameters.
关键词
选区激光熔化 /
Invar 36合金 /
热膨胀系数 /
冶金缺陷 /
显微组织
Key words
selective laser melting /
Invar 36 alloy /
coefficient of thermal expansion /
metallurgical defects /
microstructure
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基金
国家重点研发计划（2023YFB4605600）
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