Surface alloying of AZ91E alloy by Al-Zn packed powder diffusion coating

A new surface coating technique, namely packed powder diffusion coating (PPDC), for AZ91E magnesium alloy is reported. This new technique uses a powder mixture of aluminium and zinc as diffusion source and produces uniform and thick coatings at temperatures below 420 degrees C. Experimental results showed that zinc in the powder mixture significantly promotes the formation of intermetallic layers on the surface of the magnesium alloy at process temperatures between 350 degrees C and 413 degrees C, which is more than 50 degrees C lower than the previously reported processes. Depending on the temperature and the Zn-content in the powder. X-ray diffraction analysis identified three intermetallic phases and Mg(Al, Zn) solid solution that consist of the surface alloyed layer. The intermetallic compounds are tau-Mg(32)(Al,Zn)(49), phi-Al(5)Mg(11)Zn(4) and beta-Mg(17)(Al,Zn)(12). The hardness of the over 500 mu m thick surface alloyed layers is up to four times higher than the substrate. Both the beta-Mg(17)(Al,Zn)(12) phase and the tau-Mg(32)(Al,Zn)(49) phase show one to two order magnitude higher corrosion resistance than the alpha-phase (solid solution) and the phi-Al(5)Mg(11)Zn(4) phase in 5% NaCl solution. A process parameter window for the layer thickness as well as a schematic model for the formation of the layer is proposed. The PPDC process is a promising technique that provides effective protection of AZ91E alloy from both wear and corrosion. (C) 2011 Elsevier B.V. All rights reserved

Influence of chemical composition of Mg alloys on surface alloying by diffusion coating

A recently developed technique of surface alloying by diffusion-coating has been used to produce coatings on Mg alloys with various Al and Zn contents. The experimental results show that both Al and Zn solutes in the alloy promote the diffusion of alloying elements through grain refinement of the substrate alloys and through reduction of diffusion active energy because of the reduction of melting temperature of the alloys. Therefore, the efficiency of surface alloying increases by diffusion coating. Thick, dense, uniform, and continuous layers of intermetallic compounds, which consist of a τ-phase layer and a β-phase layer, can be produced on the surface of various Mg alloys. The intermetallic compound layers not only have microhardness values that are 4 to 6 times higher than the substrate but also provide effective protection of the Mg alloys from corrosion in 5 pct NaCl solution at room temperature