Investigation Of Cold Sprayed Aluminum And Aluminum With Intermetallic Mg₁₇Al₁₂ Composite Coatings On AZ91D Magnesium Substrates

Magnesium （Mg） and its alloys are considered as the lowest densitystructural metallic materials in the world. They have been used in aerospace,transportation, electronic communication devices, home appliances andmilitary components for many years. However, as everyone knows, Mg is akind of material with very active chemical properties. It can react with manykinds of gases, elements, solutions and compounds, even at room temperature.Compared to other industrial metallic materials, Mg alloys usually have arelative lower electrode potential which means they will be preferentiallyattacked in the corrosion medium and acted as a sacrificial anode. A fewmethods have been reported could be employed to improve the surfaceproperties for Mg alloys, but using a coating to protect Mg and its alloys isundoubtedly a simple and economic solution.Considering the poor corrosion resistance of Mg, aluminum （Al） wasused as the principle component in the coating. Two reasons can be used toillustrate why Al was selected in this study. The first is that when exposed inthe air, Al can react with oxygen and generate a dense oxide film which willprevent the further oxidation, and then has a self-repair function. The secondis that no remarkable electrode potential difference was found between Al andMg, therefore, galvanic corrosion or pitting will not supposed to take placebetween these two metals. Various of techniques, such as electroplating,chemical plating, plasma spraying, laser cladding, thermal spray and vapordeposition, have been used for depositing Al on Mg for improving the surface properties. But these traditional coating fabrication methods usually havesome disadvantages, such as high power consumption, low depositionefficiency or environment unfriendliness. It is worthy to note that Al powderis also very active, so the deposition process （like thermal spray or lasercladding） which has high temperature heat resource should be avoid, becausethey will lead to the formation of oxide on the Al surface or the grain growth,which will undermine the comprehensive performances of the coating. Underthese circumstances, cold spray technique was employed to deposite coatingsin this study. The coatings produced using cold spray not only can retain theinitial microstructure of the feedstock, but also decrease or eliminate thedefects which attributed to the high processing temperature.Pure Al coatings, Al composite coatings and Al gradient coatings weredeposited on AZ91D magnesium substrate using cold spray. The effects of thepost heat treatment on the properties of the diffusion-induced intermetallicswhich formed at the coating/substrate interface were investigated. Theinfluences of the volume fraction of the reinforcement particles in thefeedstock on the performances of the composite coatings and the relationshipbetween the coating microstructure and properties were also discussed.Several modern material testing methods were employed in this study,including optical microscopy, X-ray diffraction, ICP chemical compositionanalysis, scanning electro microscopy, transmission electro microscopy,microhardness test, porosity content and bonding strength test of the coating,et. al. The effects of the feedstock preparation, spraying parameters and postheat treatments on the mechanical performances and anticorrosionperformances of the coatings were systematically investigated and discussed,for the purpose of providing a new solution and some useful experimentalresults for improving the anticorrosion properties of Mg surface.The pure Al powders used as feedstock have an average particle size of58.5μm and present a broad size distribution with a diameter ranging from6to170μm. For improving the comprehensive performances of the as-sprayedcoatings, the main parameters in the cold spray process were optimized according to the thickness and porosity content of the corresponding coatings.The results show that pure Al coatings can be deposited on Mg substratessuccessfully by a single pass with a thickness of approximately400μm and aporosity level less than0.5vol.%. Noted that helium was used as bothpropellent and powder carrier gas, thermoplastic nozzle was used as sprayinggun with a gas temperature and pressure of300°C and0.98MPa, respectively.The standoff distance was kept at10mm and the relative traverse velocitybetween the nozzle and the substrate was kept at2mm/s. Heat treated thecoated samples will result in the formation of two intermetallics near theinterface, and then the coating bonding mechanism will turn intometallurgical bonding, instead of physical mechanical bonding. Thediffusion-induced intermetallics have similar characteristics to ceramicswhich present a higher hardness and relative lower plasticity. They will beeasily cracked under shear load and then undermine the coating bondingstrength. The as-sprayed Al coatings which produced using these optimizedparameters have similar anticorrosion properties to the bulk Al, but they havean inferior cohesion strength.In order to improve the bonding strength of the as-sprayed Al coatings,spherical Mg₁₇Al₁₂powders were used as reinforcement particles in the Alfeedstock. Al-Mg₁₇Al₁₂composite coatings were attained using a stainlesssteel nozzle and a powder mixture. The effects of the powder mixturecompositions on the coating performances were investigated. The resultsshow that with an addition of Mg₁₇Al₁₂particles into the original Al powders,the deposition efficiency of the Al particles was significantly increased andthe adhesion strength between the substrate and the coating was alsoimproved, without any remarkable degradation of the anticorrosion properties.The composite coatings are very dense and no reinforcement particles werefound at the interface. Compared to the results proposed by other researchers,it can be concluded that the main functions of the spherical reinforcementparticles are to remove the surface oxidations and contaminations of thesubstrate and particle, increase the substrate roughness and then increase the contact area, improve the deposition efficiency of the Al powders, tamp thepre-deposited coatings and clean the inner surface of the stainless steel nozzle.Two Al-Mg₁₇Al₁₂gradient coatings were deposited layer-by-layer on Mgsubstrates using cold spray. By carefully examination of the coatingmicrostructure, it was found that the gradient coatings have a good bondingwith the substrate and no defects were found at the interface. Moreover, theinterface between different layers was undetectable and no delaminationswere took place which was attributed to the lower Mg₁₇Al₁₂content in thecoating as well as Al was the dominant ingredient. It implied that the gradientcoatings have an improved bonding strength. All the results demonstrated thatcold spray can be used as an alternative method for fabricating gradientcoatings.