Chapter 75. A Method to Determine the Damping Behaviour of Coatings on Cast Magnesium Alloy AZ91

Summary

Materials damping is a measure of energy dissipated into heat during mechanical vibration under cyclic loading. For magnesium alloys this is mainly due to dislocation motion whereas the damping is reduced by an increasing content of additional elements. Therefore, the commercial cast magnesium alloy AZ91 (Mg-9wt.% Al-1wt.% Zn) which is already established in the automotive industry exhibits relative low damping.

Magnesium is a very reactive metal and needs to be protected against corrosion. Selective surface preparation, chemical treatment and coatings are recommended to prevent galvanic attack of magnesium components. Wax coating or painting are possible alternatives for surface treatment of magnesium parts to improve their appearance and corrosion resistance. The influence of wax coating on the strain dependent damping behaviour of AZ91 was studied. The damping was measured as logarithmic decrement of free decaying bending beam vibrations. A strong increase of the damping after coating of the substrate AZ91 was found. Assuming the superimposition principle of damping and a boundary condition with fully adherent phases the logarithmic decrement of the film could be estimated. The results of the damping measurements allow to select a coating material that combines both corrosion protection and damping rise of the substrate.