Bong Sun You

Electrodeposition of Copper on AZ91 Mg Alloy in Cyanide Solution

Copper electrodeposition on AZ91 Mg alloy was studied in views of preferential deposition on α- or β-phases and how to achieve uniform deposition over the entire surface on α- and β-phases in a cyanide solution. The inhomogeneous microstructure of AZ91 Mg alloy, particularly α- and β-phases, was found to result in non-uniform deposition of zincate layer, preferential deposition of zincate on β-phases, which leads to non-uniform growth of copper layer during the following electrodeposition process. The preferential depositions of zincate can be attributed to higher cathodic polarizations on the β-phases. Pin-hole defects in the copper electrodeposit were observed at the center of large size β-phase particles which is ascribed to gas bubbles formed at the β-phases. The activation of AZ91 Mg alloy in hydrofluoric acid solution was used to obtain uniform growth of zincate layer on both the α- and β-phases. By choosing an optimum activation time, a uniform zincate layer was obtained on the AZ91 Mg alloy surface and thereby uniform growth of copper was obtained in a cyanide copper electroplating solution.

Effect of Zincate Treatment of As-Cast AZ91 Mg Alloy on Electrodeposition of Copper in a Copper Pyrophosphate Bath

In this work, effect of zincate treatment of AZ91 Mg alloy on the following electrodeposition of copper was examined in a non-cyanide bath containing pyrophosphate ions in view of surface morphology and adhesion of the electrodeposited copper layer. Without zincate treatment, the electrodeposited copper layer showed very porous structure and poor adhesion. On the other hand, the copper layer electrodeposited on the zincate-treated surface showed dense structure and good adhesion. The dissolution rate of AZ91 Mg alloy after the zincate treatment appeared to decrease about 40 times in the copper pyrophosphate bath, as compared to that of the surface without zincate treatment. The porous morphology and poor adhesion of a copper layer on the AZ91 Mg alloy surface without zincate treatment were attributed to small number of nucleation sites of copper because of rapid dissolution of the magnesium substrate in the pyrophosphate bath. Based on the experimental results, it is concluded that the zincate treatment to form a conducting and protecting layer on the AZ91 Mg alloy surface is essential for successful electrodeposition of a copper layer on AZ91 Mg alloy with good adhesion and dense structure in the copper pyrophosphate bath.

Effect of Initial Microstructures on Cold‐Rollability of Re‐Free Magnesium Alloys

The effects of alloying elements and processing parameters on cold-rollability of RE-free magnesium alloys were investigated. Several representative alloys with different microstructural features such as solution or precipitation hardening were selected and extruded to control the initial microstructures by changing extrusion speed and temperature before cold-rolling. The results show that the uniform elongation and the work hardening exponent reflecting inclusive microstructural features of grain size, precipitates and dislocation density is linearly correlated with cold-rollability. It can be concluded that an alloy with weak basal texture can possess improved cold-formability depending on microstructural features effectively affecting work hardening exponent besides basal texture intensity. In particular the fraction of precipitates and deformed grains sensitively affected work hardening exponent and thus the cold-rollability of magnesium alloys. It was confirmed that poor cold-rollability of the precipitation-hardened alloys such as TAZ531 and TAZ711 could be improved by removing precipitates and deformed grains via a homogenization treatment.