Post-heat Treatment Effects on Cold-Sprayed Aluminum Coatings on AZ91D Magnesium Substrates

Commercially pure aluminum (CP-Al) powder was deposited by the cold spray process onto AZ91D magnesium (Mg) substrates that had been subjected to three different heat-treatment conditions: namely, as-cast (F), homogenized (T4), and artificially aged (T6). The substrate hardness was measured to be 80.7 +/- A 1.8, 73.7 +/- A 4.0, and 103.6 +/- A 7.4 HV0.025 for the F-, T4-, and T6-Mg alloy substrates respectively. Thick (similar to 400 mu m) and dense (below 1% porosity) Al coatings have been obtained. After post-deposition heat treatment at 400 A degrees C, the intermetallic Mg17Al12 (beta) and Al3Mg2 (gamma) phases with different thicknesses were found to have formed at the coating/substrate interface depending on the holding time. While no significant thickness differences of the intermetallic layers were detected in the cases of F- and T6-AZ91D substrates, thicker layers formed on the T4-AZ91D substrate. It is believed that the higher Al concentration in the T4-AZ91D solid solution within the alpha-Mg could diffuse and contribute more easily to the growth of the intermetallic phases. The hardness of the beta- and gamma-phase was measured to be 260.5 +/- A 10.7 HV0.025 and 279.6 +/- A 13.7 HV0.025, respectively. Shear strength test results revealed lower adhesion strength after heat treatment, which is attributed to the presence of brittle intermetallic layers at the coating/substrate interface.