| The use of Mg alloys has been focus increasingly in many industries, suchas automotive,3C (Computer, Communication, Consumer electronics) andaeronautical/aerospace industry, because of their superior physical andmechanical properties such as low density, good heat dissipation, high specificstiffness, good electro-magnetic shielding and ability to be recycled. However,their poor corrosion resistance hinders their use on a larger scale. Coating is astraightforward and perhaps one of the most cost-effective corrosion protectionmeasures. Currently, many coating techniques have been proposed or developedto protect the Mg alloys form corrosion for Mg alloys. These technologies forMg alloy can be divided into two categories. One has been applied in industries,such as chemical conversion treatment and organic coating. The other is still inthe explored stage of experiment. However, the chemical conversion coatingsand organic coatings can be degraded by some factors from the produce process.The effects for these factors for the chemical conversion coatings and organiccoatings on AZ91D Mg alloys have yet to be investigated. We aim to study theinfluence of typical factors from productive process on the properties in detail.The results of our investigations were expected to provide a theoretical basis anddata support for commercial components.In this paper, the influences of mold release agent sprayed during thedie-casting, filling regions, and contamination from various stages of produce onchemical conversion coatings and sequent organic coatings were investigation using stereoscopic microscope,3D optical profiler, SEM, EDS, XRD, FTIR,computer simulation, polarization curve, SST, adhesion testing, acceleratedenvironmental tests, and so on. The mechanisms of influence of various factorswere also discussed in detail.The chemical conversion coating and organic coating were influencedobviously by quantity of mold release agent sprayed during die casting. Afterchemical conversion treatment, the discoloration was formed on both specimens.Compared with the specimen sprayed little agent (A specimen), the surface ofspecimen with excess mold release agent (B specimen) appeared white grey. Theresults from micro-analysis indicate that the baked mold release agent remainedstill on the surface of the specimen. While the surface of A specimen is deepgrey. The results from micro-analysis suggest that a compact and continuouscoating on the surface of the specimen is formed. The corrosion resistance wasalso measured by potentiodynamic polarization, hydrogen evolution and saltspray testing (SST). The results show that excess mold release agent degradedseriously both the corrosion resistance after conversion treatment and adhesionof organic coating of the manufactured component for the B specimen.For die-casting components with a complex geometry, the microporositycontent was related to specific regions of the component relative to the castingdie. Thus microporosity was higher near the end of the filling （#2specimen）than that in the first filling regions（#1specimen）. Several formationmechanisms for microporosity were discussed:(1) air entrapment in the moltenmetal during DCP;(2) explosive expansion of dilution water containing thelubricant when the molten metal enters the die cavity; and (3) metal contractionduring solidification. Relative to specimen#1, corrosion protection was poorerfor conversion-coated specimen#2because of the microporosity-induceddiscontinuous coating. Finally, adhesion of the organic coating was lost at areaswith higher microporosity (2#), demonstrating that microporosity can also reduce adhesion under different environments.The commercial die casting AZ91D Mg alloy components were applied toinvestigate the effect of contamination on blistering behaviors of organic coating.Artificial perspiration was applied to simulate the contamination source duringvarious product processes and/or service. An accelerated environmental test andthe natural weathering test were subsequently carried out on the organic-coatedcomponents. It was found that the blistering only occurred on specimens Ⅱwhich they were spread with the artificial perspiration on the conversion surfaceprior to painting. Further, the blistering initiated because of in situ loss ofadhesion at the organic coating/substrate interface, caused by contaminationand/or corrosion. This was related to the exposure conditions. In the early stagesin a high-temperature/humidity environment, the hydrophilicity of the chloridecontaminants led to in situ adhesion loss; simultaneously, the volume of thecoating increased, and the associated compressive stress caused the coating tobulge. After long-term exposure to the high-temperature/humidity environment,the underfilm AZ91D Mg-alloy substrate corrosion was occurred because of thepresence of chloride anions. The simultaneous corrosion products (consisting ofMgO, Mg(OH)2and Mg2(OH)3Cl) filled up the blisters.Additionally, this paper also investigated the spraying coatings of metals ondie casting AZ91D Mg alloy to expand their application.Aluminum coating was prepared on the surface of die-casting AZ91Dmagnesium alloy specimens through electro-arc spraying technology. Thespecimens were heat treated in different temperature for different time to obtaindiffusion coating. The experimental results show that there are three differentdiffusion layers:(1) β-Mg17Al12phase diffusion layer, with a little γ-Mg2Al3phase,(2) γ-Mg2Al3phase layer and β-Mg17Al12phase layer,(3) β-Mg17Al12phase layer and α-Mg+β-Mg17Al12biphasic diffusion layer. Comparing theAZ91D Mg alloy substrate, the microhardness, corrosion property and abrasiveresistance of the all diffusion layers is obviously increased, also, the both single-phane diffusion layers are better than the biphase (α+β) diffusion layer.The present method, which can generate a compound coating upon AZ91DMg alloy, combined for the first time cold spraying with sequent die casting.The mechanical interlocking and metallurgical bonding, formed during diecasting process, promise the bonding strength and reliability of cold sprayingcoating on substrates. Interestingly, the coating is significantly harder than diecasting AZ91D Mg alloy, and offers high wear resistance. The compoundcoating not only significantly decreased the corrosion rate of Mg alloy, but alsoexhibited a passive behavior and no appreciable hydrogen evolved duringimmersion in5%NaCl solution for72h. Furthermore, it was proved to be animplementable die casting technique to manufacture Mg alloy components withthe improved surface properties for industrial applications... |
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