毫米级大光斑熔融月壤粉末床的原位观测研究

In-situ observation of lunar regolith powder bed melting by a millimeter-scale laser spot

由于松散粉末床的导热性很差,直接使用低功率能量束熔融打印时,连续的熔道不易形成。为了提高太阳光聚光熔融月壤打印过程中的能量穿透深度,开展了大尺寸光斑熔化模拟月壤粉末床时,熔池形成过程的原位观测研究。为了模拟太阳光汇聚光斑,将激光的光斑尺寸扩束至数毫米,并直接作用于无基底的厚粉末床表面。基于高速成像技术,原位观测了激光作用过程中的熔滴生长情况。实验发现,部分熔化的表层粉末表现出颗粒间的大规模协同作用,称为“卷吸”过程。在该过程中,粉末床表面依次发生表层卷起、与粉末床分离、融入熔滴,导致了熔滴的不连续生长。基于光斑与粉末床作用的能量守恒关系,分析了熔滴尺寸的生长规律,并与实验结果进行了对比。结果表明,低功率大光斑激光产生的熔滴一般不会超过光斑范围,且功率越高,卷起的粉末层熔化越不完全,此时具有更高的能量利用效率。卷吸机制绕开了低导热性的限制,特别是针对月壤熔融打印技术,使得厚粉末床上直接打印熔道在机制上成为可能。

The formation of continuous melt tracks through a low-power beam for powder bed fusion is hindered by the inadequate thermal conductivity exhibited by a loose powder bed.To increase the depth of energy penetration during the solar convergent molten lunar soil printing process,an in-situ observation study of the melt pool formation process was conducted when a large-sized spot melted a simulated lunar soil powder bed.To simulate the concentrating spot of sunlight,the laser spot was expanded to millimeter scale and impacted directly on the surface of a thick powder bed lacking a substrate.Using a technique of high-speed imaging,the development of melting droplets was observed.The partially melted surface powder was discovered to exhibit a large-scale interparticle cooperation referred to as the“wrapping”process.In this process,the powder bed's surface layer was rolled up,separated from the powder bed,and joined to the molten droplet,resulting in a growth that is discontinuous.The growth law of melting droplet size was investigated and compared with experimental results based on the energy conservation relationship between the spot and the heating of the powder bed.The results indicate that melt droplets generated by a low-power,large-spot laser cannot extend beyond the range of the spot.The higher the power density,the less complete the powder melting while a wrapping process occurs,which increases the energy efficiency of this operation.The utilization of the wrapping mechanism effectively overcomes the constraint posed by the low thermal conductivity,particularly in the context of lunar soil powder bed fusion technology.This mechanism enables the direct printing of melting tracks on substantial powder beds.