Q2. What was the effect of increasing the fluence of the laser?
When the laser fluence was increased to 1 J/cm2, the oxide layer formed by the thermal oxidation became, in a large extent, crystalline and its outer part was entirely covered by a continuous layer of magnesium oxide.
Q3. What is the common method of removal of organic contaminants?
Removal of organic contaminants is based on laser beam disintegration or vaporisation of contaminants, so that no significant thermal effect on the substrate is expected.
Q4. What mechanisms exist in laser removal of oxide layers from metallic substrates?
It was known that three mechanisms in laser removal of oxide layers from metallic substrates exist, i.e.thermal ablation, mechanical effect, and combined thermal ablation and mechanical effects.
Q5. What is the reason for the higher corrosion resistance of the laser-cleaned surface?
The higher corrosion resistance for the laser-cleaned surface at 11.3 J/cm2 than that at 7.1 J/cm2 might be the result of thicker oxide layer formed and the excessive melting which might have modified the microstructure underneath the oxide layer, in terms of refinement of second phases.
Q6. What was the result of the faster diffusion of Mg towards the surface?
Within this region, magnesium was also rich, which was believed to be the result of faster diffusion of Mg towards the surface than the other elements during the hot-rolling process.
Q7. What are the advantages of laser cleaning?
Compared with the conventional cleaning techniques, laser cleaning offers advantages including environment-friendliness, better selectivity, better controllability, better flexibility to achieve complete removal of surface contamination with minimal damage to the underlying substrate material and the process can be much faster for welding surface preparations since only localised cleaning is required.
Q8. What is the purpose of laser cleaning?
In recent years, laser cleaning has been considered as a pre-treatment technique for coating, welding and joining of various materials, and also a post-treatment technique to remove stains and discoloration after welding [4-7].
Q9. What was the effect of laser cleaning on the zinc profile?
In addition, the zinc profile showed depletion on the top surface, but became relatively uniform distributing along the depth after laser cleaning.
Q10. What is the main purpose of laser cleaning?
Laser cleaning has been considered as an alternative method to replace conventional cleaning techniques such as wet chemicals and mechanical cleaning involving air abrasives or grinding.
Q11. What was the reason for the increase in surface reflectance after laser cleaning?
It was worth stating here that the increase in the surface reflectance after laser cleaning was mainly attributed to the removal of the oxide layer that was dark in appearance s well as other contaminants.
Q12. What is the atomic ratio of MgAl2O4?
As MgAl2O4 presents the atomic ratio of Mg:Al:O as 1:2:4, it was most likely that all the aluminium attributed to the formation of MgAl2O4, and the excessive amount of magnesium and oxygen may be combined to form other compounds.
Q13. What are the factors that influence the thermal effect of laser cleaning?
Such a thermal effect is dependent on several factors, including laser beam wavelength, pulse width, laser processing parameters, thickness of oxide layers, nature of oxide layers as well as substrate materials.
Q14. What was the likely source of hydrogen in the aluminium oxide layer?
the hydrogen was also likely to be co-existent in the oxide layer as hydrogen has a higher solubility in aluminium oxide than in aluminium matrix.
Q15. What is the SVET current density of the laser-cleaned surface?
4) The SVET current maps showed that the laser-cleaned at 7.1 J/cm2 exhibited much lessanodic activity while anodic current peaks presented on the as-received alloy surface was dramatic.
Q16. What was the effect of laser cleaning on the magnesium and aluminium spectra?
The intensities of oxygen and magnesium peaks after laser cleaning were significantly reduced, providing a further evidence of removal of the oxide layer.
Q17. What was the resistance of the oxide layer on the as-received alloy?
The intermediate values of resistance and capacitance of the mechanically ground surface indicated that the outer layer and oxide layer on the as-received alloy were completely removed by grinding, followed by the formation of a thin, air-formed oxide layer.
Q18. What is the difference between the mechanically ground and the laser-cleaned surface?
The mechanically ground surface exhibited higher corrosion resistance than the as-received, but lower than the laser-cleaned, due to the formation of air-formed oxide layer which was more protective than the MgO+MgAl2O4 layer on the as-received surface, but less protective than the newly formed Al2O3+MgO layer by laser cleaning.