Method and Mechanism for Adhesion Recovery of Laser-Treated Aluminum Surface Exposed to Hygrothermal Environment

Abstract

Laser treatment is used to improve the surface adhesion performance of metal materials, including aluminum alloy. However, the adhesion performance of laser-treated aluminum alloy is reduced after the hygrothermal exposure during storage before bonding, obtaining the reliable surface adhesion is a challenge. In this paper, the nanosecond laser-treated aluminum alloy was exposed in a hygrothermal environment with 80°C 95%R.H. for 48h. Scanning electron microscopy, transmission electron microscopy and adhesion strength testing were used to characterize the physical/chemical properties and adhesion reduction of the aluminum surface. Subsequently, a simple and effective method of heat treatment was proposed to recover the adhesion strength. Finally, molecular dynamics (MD) simulation was utilized to explore the underlying recover mechanism. The experimental results revealed a AlOOH layer with ~410 nm thickness was generated on the laser-treated aluminum surface after hygrothermal exposure, which reduced the adhesion strength by 38% (from 30.9MPa to 18.9MPa). After heating at 120 °C for 24 hours, the adhesion strength (30.4MPa) of aluminum surface recovered to the level before hygrothermal exposure. MD results suggested that there are two mechanisms for the adhesion recovery by heat treatment: 1) the atomic kinetic energy of AlOOH increased and the structural order decreased, which strengthened its electrostatic and hydrogen bond interactions with adhesive molecules; 2) the mechanical property of AlOOH were enhanced.