Atmospheric Corrosion of Mg Alloy AZ91D Fabricated by a Semi-Solid Casting Technique: The Influence of Microstructure

Abstract

The atmospheric corrosion behavior of alloy AZ91D produced by a semi-solid metal (SSM) technique and by conventional high pressure die casting (HPDC) was investigated for up to 1176 hours in the laboratory. Alloy AZ91D in the SSM state was fabricated using a rheocasting (RC) technique in which the slurry was prepared by the RheoMetal process. Exposures were performed in 95% RH air at 22 and 4 • C. The RC alloy AZ91D exhibited significantly better corrosion resistance than the HPDC material at two temperatures studied. The effect of casting technology on corrosion is explained in terms of the microstructural differences between the materials. For example, the larger number density of cathodic β phase particles in the HPDC material initially causes relatively rapid corrosion compared to the RC material. During later stages of corrosion, the more network-like β phase particles in the RC alloy act as a corrosion barrier, further improving the relative corrosion resistance of the RC material. Conventional magnesium-aluminum (Mg-Al) alloys, i.e., AZ91D, AM50A and AM60B, offer an exceptional combination of ambient temperature strength and ductility, and good die-castability.1-4 Cast components made of Mg-Al alloys are usually cast by conventional high pressure die casting (HPDC). Despite its advantages over many other casting techniques for producing cast Mg-Al components, there are some inherent problems associated with HPDC. For example, there is a tendency for hot tearing during HPDC due to a relatively wide freezing range and a low solidus temperature. Also, a relatively high fraction of trapped air porosity may form during the turbulent die filling, especially in thick-walled components. Additionally, insufficient resistance to atmospheric and aqueous corrosion sometimes limits Mg-Al alloys applications in the fields of automobiles, aerospace, electronics, etc.5-8 During the past two decades, there have been extensive efforts to increase the corrosion resistance of Mg-Al alloys. Much of the effort has concentrated on the use of various coating systems such as chemical conversion coatings, anodizing, gas-phase deposition processes electro-or electro-less plating, and organic coating. [9][10][11][12][13][14][15] Lowering the impurity levels, alloying, rare earth additions, and heattreatment have also been explored to increase the corrosion resistance of these alloys. [16][17][18][19][20] Alternative casting processes are being developed to resolve the mentioned problems and meet the requirements of future applications of Mg-Al alloys. Semi-solid metal (SSM) processing is a promising manufacturing route capable of producing castings with a high level of complexity. 21,22 The main advantage of SSM processing, compared to the conventional casting processes, is the possibility to have a laminar flow of metal during mold filling.23 This is a consequence of the higher viscosity of the semi-solid material, and it reduces air entrapment compared to die-casting. This results in components with enhanced microstructural proper...

The Influence of SO2on the Corrosion of Mg and Mg-Al Alloys

“…Hence, it has been shown that the Al content in α-Mg in alloy AZ91D can be increased by modifying the casting technology, resulting in improved corrosion properties. 62 The presence of ppb levels of ozone and NO 2 strongly increases the SO 2 deposition on AZ91D at 60% RH (see Figs. 9 and 10).…”

A new semi-solid casting technique for fabricating SiC-reinforced Mg alloys matrix composites

“…Based on the reasons presented in the Introduction, it is highly desirable to produce Mg-based MMC cast components with uniform reinforcement distribution and structural integrity. The RC method in combination with the RheoMetal process has already established itself as a promising casting technique enabling the production of high-quality and complex cast Mg alloys with enhanced service performances [22,27,28]. Thus, one expected to detect a relatively low fraction of casting pores, in the range of 1e2% of the total examined area, in the RC AM50 and AZ91D alloys (see Figs.…”

“…The main microstructural constituents are a-Mg grains surrounded by interdendritic regions (also called eutectic a) and b phase particles (Mg 17 Al 12 ). Owing to the presence of Mn, MgeAl alloys in the AM and AZ series also contain a small amount of h phase particles (Al 8 Mn 5Àx Fe x ) [22,28,29]. Moreover, transient phases such as Mg 5 Al have been reported in the microstructure of MgeAl alloys [28].…”

“…Owing to the presence of Mn, MgeAl alloys in the AM and AZ series also contain a small amount of h phase particles (Al 8 Mn 5Àx Fe x ) [22,28,29]. Moreover, transient phases such as Mg 5 Al have been reported in the microstructure of MgeAl alloys [28]. This is a precursor of b-Mg 17 Al 12 and has considerably lower volume fraction than the dominant intermetallic phase particles.…”

The Effect of Vanadate, Phosphate, Fluoride Compounds on the Aqueous Corrosion of Magnesium Alloy AZ31 in Dilute Chloride Solutions

“…Over the pH range examined, CO2 speciates to bicarbonate, HCO3 − and carbonate, CO3 2− . These species can react with magnesium and aluminum hydroxides at a neutral to moderately alkaline pH to produce a mixed Mg-Al hydroxycarbonate that confers a slight protective effect [34,35]. These changes in the polarization response indicate a time-dependent passivation process that produces a marginal increase in resistance to localized corrosion.…”