Micro-extrusion-based additive manufacturing with liquid metals and alloys: Flow and deposition driven by oxide skin mechanics

Abstract

Micro-extrusion-based additive manufacturing (μEAM) for metals and alloys carries the potential to enable multi-material manufacturing of micro- and meso-scale products with high resolution and microstructural control. However, realizing μEAM with metals and alloys has been elusive despite the method’s success with plastics and ceramics, due to the substantially different thermofluidic properties of metals. This paper presents a comprehensive study of flow and deposition mechanisms of μEAM with liquid metal alloy eutectic gallium indium (EGaIn), focusing on the influence of the elastic solid skin of gallium oxides encapsulating the extruded filaments. To this end, effect of the nozzle–oxide skin adhesion on the extrusion process and the three-dimensional surface stresses evolving on the oxide skin are experimentally studied. It is shown that the reduced adhesion between the nozzle and the oxide skin substantially improves the process repeatability. The stress analysis showed that the oxide skin experiences axial, circumferential and shear stresses during the process. Continuous filamentary extrusion is achieved within a circumferential stress range that varies as a function of nozzle diameter and printing angle, dictated by the dispensing pressure. During filamentary extrusion, the oxide skin continuously yields axially while experiencing shear depending on the printing angle. The filament diameter is shown to be a function of the shear strain and the nozzle diameter. Finally, effect of dispensing pressure history on the filament diameter and the stresses were studied, demonstrating that decreasing dispensing pressure during the printing process leads to reduction in filament diameter whereas increasing pressure does not increase the filament diameter. These findings will inform the process and equipment design for EAM for liquid phase metals, impacting several applications including microelectronics, biomedical devices and energy storage.