Fabrication and characterization of corrosion-resistant surface layer on Mg-alloys by using large-pulsed electron beam (LPEB) irradiation process

Abstract

This research was begun for finding an appropriate industrial application of electron beam process.Particularly, since last decade, the surface treatment method using electron beam has been largely investigated. However, in the all published researches for Mg-alloys known for the world lightest-weight metal, the energy density level of the electron beam surface treatment was limited at only 2.5 - 3 J/㎠ and the electrochemical performance has not yet been enhanced for engineering application. In this thesis, the energy density was increased up to 10 J/㎠ with parameter optimization by applying large pulsed electron beam (LPEB) irradiation process on AZ31 plate specimens. Firstly, through mathematical modelling of energy absorptivity of LPEB, the pitch of irradiation pattern was anticipated to adequate value. Appling the prediction model, the temperature profile was simulated by 2-D heat transfer equation. The estimated result was verified by real-time temperature measurement. The process was assessed how to be progressed rapid quenching and tempering. At over 20 cycles, the substrate temperature was above eutectic point (220℃) of Mg-Al alloy, but it was not increased over 300℃ due to self-diffusion. To analyze the surface modification effects mechanically, it was demonstrated brightness, deformation of LPEB treated surface with the results of ball-on-disc wear test. The mechanical characteristics were enhanced by ~30% using LPEB process. For electrochemical analysis, the surface corrosion characteristics were qualitatively and quantitatively evaluated by 3-electrode cell test. Potentiodynamic polarization and electrochemical impedance spectroscopy was applied to evaluation. Then, low-field approximation and equivalent circuit modelling was used to certify the optimum LPEB parameter. At as-received sample, it presented irregular results since the oxide layer; it can protect the bare surface from corrosion, but it was easily damaged than the newly modified surface layer by LPEB process. The result was demonstrated that the electrochemical characteristics were improved by ~45%. In addition, using the scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses, the morphology and the microstructure with chemical composition transformation were detailed discussed by metallurgically. As a result, it was presented that the tool mark was eliminated with new wavy surface morphology and the Al content was increased up to maximum level when the energy density is 5 J/㎠. Consequently, the LPEB irradiation was verified that it can efficiently fabricate nano-grained corrosion-resistant surface layer with activating surface alloying induced by vaporization and re-melting process in Mg-Al binary alloy system. However, the technology was analyzed to require more development because surface defects were appeared by LPEB process due to inhomogeneous evaporation of Mg such as crater, crack, and micro-pole.