Additive Manufacturing of Metals: Ink Printing System and Enhanced Superhydrophobicity

Abstract

Additive manufacturing (AM) has been greatly developed as it can fabricate complicated metallic architectures with less necessary components, minimizing the manufacturing costs, speeding up the design-production process and tailoring various physical properties. Currently the developments of additive manufacturing of metals are mainly concentrated on the novel design of materials, novel process, novel structure and printing metallic components with structural/functional properties. Directly deposition technologies, e.g. selective laser melting, melt in-suit materials which are then solidified with high heating or cooling rates. Though many breakthroughs are achieved by those direct technologies, two-step approaches (e.g., direct ink writing firstly and then sintering) have a low crack sensitivity, good high surface quality and a high cost efficiency and have not been fully developed.

In this study, a two-step approach combining direct ink writing and sintering was applied in different material systems for two purposes: 1) explore the printing possibility of ‘hard-printing’ alloys using amorphous alloys and high entropy alloys; 2) develop the potential of using AM designs to enhance superhydrophobicity using the conventional metal (i.e., iron).

The ink printing system was initially developed to different materials, including hard-printing and conventional materials. Amorphous alloys/devitrified alloys with dual phase or fully crystallization, as well as high entropy alloys, have many advanced properties in structural, chemical, physical, biomedical and other applications. However, they are seldom additive manufactured with complex structure with the high thermal gradient and rapid temperature change rate of directly AM processes. The ink printing and sintering process is able to fabricate those hard-printing materials, i.e., amorphous alloys/the devitrified alloys and high entropy alloys. The fully crystallized alloys from amorphous states with lattice structure have an excellent specific compressive yield strength. And the 3D printed FeCoNiCr high entropy alloys showed a good mechanical property.

As additive manufacturing or 3D printing is able to fabricate various superhydrophobic structures with high designing flexibility and great potentials, conventional materials (iron) was applied to 3D printing the dual-scale (micro/nano) structures combining printed lattice structure and the in-suit lamellar cementite. The 3D printed lattice showed excellent superhydrophobic properties and good performances on freezing delay and ice adhesion strength.

Then, considering the numerous potentials of 3D printed superhydrophobic structures and to solve several issues (reducing drop bounce contact time, severe loss of superhydrophobicity at severe environments and long freezing delay ability), we propose and realize a new hierarchical structure via 3D printing and selective phase engineering to overcome these challenges simultaneously at multiple length scale and time sequences. This new topology composed of a large number of multiscale confined air pockets (CAPs) which exhibit some exceptional and unexpected properties of enhanced superhydrophobicity and environmental stability from their switchable states. The fundamental physical reaction models in interfaces are elucidated with responding to static, dynamic and severe-environment systems. The solid-liquid-gas reaction model was analyzed by mathematical method and numerical simulation. The overall generated positive force in the surface is ~190 μN and exceeds the adhesion force (~25 μN) which impedes the drop detachment from surface. Though these values change over contact time, the effects from micro/submicron CAPs evidently promote the whole bounce process from initial bounce contact to the final detachment. As results, the bounce process is accelerated with a reduced contact time as low as 39% of its theoretical limit.

Apart from reduced bounce contact time, under a cold and humid environment, the superhydrophobicity of the designed CAPs structures is more stable with almost sustained contact angles and bounce ability, which is not affected by the variable surface chemistry; and the ability of supercooling or delay freezing for an ultralong time (~6 hours) can be achieved at -10 ℃ and relative humidity (RH) of 80%, which is approximately 30 times better than ever reported. Furthermore, this structured surface is scalable to the mass production, applicable to complex geometries and metallurgical bonded between in-suit phases and substrate, and exhibits superior repellency, broad environmental stability and freezing delay abilities; thus, it is a promising topic of interest in areas as diverse as energy transfer, engineering, chemistry, biotechnology and materials science.

To conclude, a two-step approach of additive manufacturing, which is the ink printing and post-sintering, was proposed to be applicable to different material systems with superior properties. It is scalable to the mass production, applicable to complex geometries and metallurgical deification, and exhibits superior physical properties no matter in mechanical properties or superhydrophobicity. The proposed CAPs structures can appear attractive repellency, broad environmental stability and freezing delay abilities. Thus, this is a promising topic in areas as diverse as energy transfer, engineering, chemistry, biotechnology and materials science.

Date of Award	7 Sept 2022
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jian LU (Supervisor)