Ultrashort-pulse laser-modified surface texturing for heat transfer reduction in die-cast aluminum alloys

The development of battery electric vehicles (BEVs) is actively progressing; however, increasing battery capacity remains difficult, and the required driving range has not been achieved. Consequently, methods to reduce energy losses from various vehicle systems are under investigation. The purpose of this study is to demonstrate the reduction of heat transfer from a powertrain to the external environment by introducing hydrophobic periodic structures on the surface of die-cast aluminum-alloy flat plates (DC Al-alloy test-pieces) and verifying the reduction of heat transfer from lubricating oil to the DC Al-alloy test-pieces. Since the surface roughness of the as-cast DC Al-alloy test-piece is relatively large (Ra > 6.0 μm), picosecond-pulsed laser lapping is applied as a pretreatment, reducing the roughness to Ra < 2.1 μm. Subsequently, a lotus-inspired hierarchical texture (HT) is fabricated on the DC Al-alloy test-piece surface using a femtosecond-pulsed laser, which increases the apparent contact angle of transmission oil from 48.7° to 92.1°. Droplet impact experiments with lubricating oil are conducted on an untreated surface, a surface with chemical treatment (CT), and the surface with HT. Compared with the untreated surface, the surface with HT reduces heat transfer from the lubricating oil to the DC Al-alloy test-piece from 464 J to 347 J, corresponding to a 53.4% reduction in the heat transfer coefficient. This improvement is attributed to the formation of an air cushion generated by the grooved surface structures, together with decreased contact time and area due to enhanced hydro/oleophobicity. These findings demonstrate that the surface with HT fabricated by ultrashort-pulsed laser processing can effectively reduce heat transfer from DC Al-alloy test-pieces to the external environment. In conclusion, the results of this study demonstrate that oleophobic surface texturing by ultrashort-pulsed laser processing can reduce energy losses due to heat transfer in mechanical components.