Correlation between surface characteristics and static strength of adhesive-bonded magnesium AZ31B

The use of adhesive is posed to increase dramatically for application to the next generation of vehicle structures, as is the use of non-ferrous materials (e.g., magnesium and aluminum alloys). In this study, surface polishing treatment of 1.6-mm-thick Mg AZ31B with various sandpaper grits (i.e., 80, 240, 600, and 1200 grit) was utilized to understand the correlation between the surface characteristics and strength of the adhesive-bonded polished magnesium joints. Lap-shear joints were fabricated with an epoxy adhesive (Lord Versilok 253/254) and quasi-static tested. It was found that the strength of the adhesive-bonded Mg AZ31B was decreased by the surface polishing. To understand the effect of surface polishing on the surface characteristics of Mg AZ31B, surface morphology (i.e., surface roughness and surface area), surface chemistry component and surface free energy of the as-received and polished Mg AZ31B substrates were measured and analyzed. Test results revealed that the removal of the surface oxide by polishing resulted in a decrease in actual surface area and content of oxygen element to decrease the surface free energy of the treated Mg AZ31B, and consequently, decreased the strength of the adhesive-bonded Mg AZ31B. Finally, examinations of the test results indicated that although the actual surface area of the bonding region correlated with the joint strength, the surface free energy provided a better index to indicate the strength of the adhesive-bonded Mg AZ31B.