Embossing Formation And Surface Functional Regulation Of Ultrafine-grained Pure Aluminum With Micro-array Structures

The wettability of solid surface,especially the hydrophilicity and hydrophobicity,has been widely used in many fields.Especially the superhydrophobic surface has become a research focus for its excellent performance,such as self-cleaning,drag reduction,heat dissipation,anti-corrosion,anti-icing and so on.Metal materials are widely used in modern industry which belong to hydrophilic materials with higher surface free energy.It has great significance to modify its surface.The more methods used to change the wettability of metal materials are chemical method,such as chemical etching,sol-gel,electrochemical deposition and so on.With the development of technology,sustainable production leads to more and more attention.Micro-embossing processing has the advantages of high precision,low cost,repeatability and environmental protection which belongs to sustainable production.It has potential research and practical application value for applying micro-embossing method to change the wettability of metal surface,but less research has been done on it.In this paper,the micro-array structures with different sizes were constructed on the surface of UFG pure Al through micro-embossing method.Combined with hydrothermal reaction,micro/nano hierarchical structures were obtained.Then the surface with micro/nano hierarchical structures was modified and the superhydrophobic surface was obtained with high adhesion.Dynamical behavior with liquid metal on surface with micro-array structures was discussed.Materials used in this paper was UFG pure Al with grain size of 1 μm which was conducted by ECAP processing through 8 passes.The deformation behavior and filling mechanism was revealed by micro-coining experiment and CPFEM.The dimensional accuracy,surface quality and filling behavior after micro-coining processing of UFG materials at room temperature and elevated temperature were better compared with CG materials.It indicated that the main deformation during plastic deformation process of UFG pure Al was grain boundary deformation which contained grain rotation and grain boundary sliding at room temperature.With the increasing of deformation,the main deformation transferred from grain boundary deformation to intergranular deformation.The main deformation during plastic deformation process of UFG pure Al was grain rotation and grain boundary sliding at elevated temperature.When the temperature increased to 473 K,the dislocation movement become to the main mechanism for plastic deformation of UFG pure Al.The micro-array structures with different sizes were fabricated through micro-embossing method on UFG pure Al and the wettability of processed surface was analyzed through contact angle measurement.The micro hexagonal pillar array structures with good surface forming quality were obtained through micro-embossing.When a/P was lower than 0.45,the forming quality increased with the increasing of sizes where the main influencing factor for forming quality was the number of grains involved in deformation.When a/P was higher than 0.45,the main influencing factor for forming quality was the space ratio.Meanwhile,the CAs of sample surface after micro-embossing had changed which confirmed that the wettability of UFG pure Al surface could be changed by fabricating micro-array structures through micro-embossing.The highest CA of processed UFG pure Al was 114° which was increased 142% compared with original UFG pure Al.The UFG pure Al surface transferred from hydrophilicity to hydrophobicity.Based on the previous obtained samples,the samples were put into boiling water for 10 min to obtain the micro/nano hierarchical structures on the UFG pure Al surface,and then modified by 2.47wt% ethanol solution with stearic acid for 0.5 h at room temperature and then dried at room temperature with 12 h.The CA of the surface was as high as 160° which was increased 142% compared with original UFG pure Al.Meanwhile,the obtained superhydrophobic surface had high adhesion which could be explained by the Cassie impregnating wetting regime.The big gaps between the microscale hexagonal pillar patterns could be wetted by the water droplet.This phenomenon lead to the appearance of large liquid-solid contact area owing to the large scale of microstructures.Hence,the surface has high adhesion because of the van der Waals force produced by the liquid-solid interface between the droplet and the substrate where the water droplet can be pinned.Whereas,the nanoscale structure couldn’t be wetted by water droplet,thus,a large amount of air could be trapped in the nanostructure.Therefore,the obtained surface exhibited superhydrophobic property.The dynamic process of liquid metal droplet impacting on the UFG pure Al surface with micro-array structures was observed by high speed video technology.The influence of different falling height and substrate with different micro-array structures on the dynamic behavior was analyzed to discuss the potential application in the field of heat transfer.The spreading factor and rebounding height of UFG pure Al surface with micro-array structures was higher compared with smooth UFG pure Al surface,especially for the surface with 135 μm-300 μm microstructures.This indicated that the viscous loss of surface with micro-array structures was lower.