The effect of cold sprayed coatings on the mechanical properties of AZ91D magnesium alloys

doi:10.1016/j.surfcoat.2014.05.018

Surface and Coatings Technology

Volume 253, 25 August 2014, Pages 89-95

https://doi.org/10.1016/j.surfcoat.2014.05.018 Get rights and content

Highlights

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Cold sprayed Al/Al₂O₃ coatings increase the yield strength of AZ91D up to 20 MPa.
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Cold sprayed Al/Al₂O₃ coatings increase the fatigue limit of AZ91D by 20 MPa.
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The yield strength and fatigue limit increase with the Al₂O₃ fraction in coatings.
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The coating bond strength positively improves the mechanical properties of AZ91D.

Abstract

Although many previous studies have confirmed that cold sprayed Al + xAl₂O₃ (x = 0–75 vol.%) composite coatings on magnesium alloys can effectively improve wear and corrosion resistance, the effect of such coatings on tensile and fatigue properties is still unclear. The present work aims to evaluate the tensile properties and fatigue resistance of an AZ91D alloy with cold sprayed Al + xAl₂O₃ (x = 0, 30, 50 vol.%) coatings. The experimental results showed that although the cold spray coating leads to an increase in yield strength and fatigue limit, the tensile strength is reduced. In-situ examination of the macro-morphology of surfaces of the coated specimens during tensile testing was carried out through video recording. It indicated that once the tensile specimens have yielded, horizontal cracks that were perpendicular to the tensile axis formed on the surface of the specimens. Fractographic analysis of the fracture surfaces of the tensile specimens in a scanning electron microscope revealed that all the cracks within the cold sprayed coatings were suspended at the coating/substrate interface. Based on these experimental observations, it is considered that the improved yield strength is attributed to the constraint effect of the cold sprayed composite coatings on the magnesium substrate. The enhanced fatigue limit is a result of the higher yield strength and the coating/substrate interface barrier to crack propagation. The decrease in tensile strength is attributed to the brittle fracture of the cold sprayed coatings when yielding of the specimens.

Introduction

Magnesium alloys are the lightest metal available for engineering applications and show promise as a structural material in aerospace and automotive industries, because those sectors are seeking increased fuel efficiency through weight reduction. However, because of the low wear resistance and high chemical reactivity of Mg alloys, surface treatment is normally required for Mg alloy components in order to improve their surface durability. In comparison with the currently available surface treatment techniques for Mg alloys, such as micro-arc oxidation [1], chemical conversion [2], plating [3] and chemical vapour deposition [4], cold spray [5] is a relatively new and environmentally friendly technology. It produces dense protective coatings with bonding strength higher than 80 MPa. In the cold spray process, fine metal or ceramic powders (5–50 μm) are accelerated to a velocity of 500–1200 m/s by compressed gas (generally nitrogen or helium) and then impacted onto a substrate, leading to severe plastic deformation at high strain rate (> 10⁶/s) [6], [7]. This leads to fracture of oxidation layers on the surfaces of both the substrate and the particles. This enables contact of fresh metal and the formation of metallurgical bonds between the powder particles and the substrate, and between the particles themselves. Unlike conventional thermal spray, the coating deposited in the cold spray process is formed in the solid state at a lower temperature. Thus, cold spray is mostly suitable for coating on oxygen-sensitive or thermo-sensitive substrates, including magnesium.

In the interest of minimising weight, the deposition of Al/Al₂O₃ composite coatings on magnesium alloys has been recognised and the cold spray process has been well developed [8], [9], [10]. Incorporation of alumina not only significantly improves wear resistance, but also dramatically increases both the density of the coating, and the bond strength between the coating and the substrate, through the “hammering” effect [11], [12], [13], [14], [15] upon impact of the particles onto the substrate. In addition, Spencer, Luzin and Zhang [16], [17] have also measured the residual stress of various cold sprayed coatings on magnesium alloy substrates using neutron diffraction. They detected a compressive stress in cold sprayed Al coatings. Such compression may positively affect the mechanical properties of magnesium alloy components. However, although considerable work has been done to develop high quality coatings with enhanced surface properties, the effect of cold sprayed coatings on the mechanical properties of magnesium alloy substrates is not well understood. This is particularly important in industrial applications, as tensile and fatigue properties are two critical parameters in engineering design. The present work aims to investigate the effect of cold sprayed Al/Al₂O₃ composite coatings on the tensile and high-cycle three-point bending fatigue properties of AZ91D magnesium alloys and to elucidate the mechanisms.

Section snippets

Materials and sample preparation

An as-cast AZ91D alloy was used as the substrate. Before cold spraying, the alloy was solution treated at 413 °C for 16 h followed by ageing at 168 °C for 16 h in an argon atmosphere. The nominal composition of the AZ91D alloy was Mg–9Al–0.6Zn–0.4Mn (wt.%).

Rectangular tensile testing specimens of AZ91D alloys with 30 mm gauge length, 6 mm gauge width and 5 mm thickness were machined in accordance with the ASTM-B557M-07 standard. Specimen surfaces were ground using SiC papers up to a grit of 600#.

Microstructure of the cold sprayed coatings and the as-heat treated AZ91D substrate

After solution treatment and ageing, the microstructure of the AZ91D alloy generally consists of α-Mg solid solution and β (Mg₁₇Al₁₂) precipitate [18]. Microstructures of cross-sectioned, coated samples are shown in Fig. 2. No detectable inter-diffusion between Al and Mg across the coating/substrate interface is observed. Image analysis using the ImageJ software package indicates that the pure Al coating (Fig. 2a) contains 0.36% porosity. Addition of Al₂O₃ particles reduces the porosity to 0.1%

Conclusions

(1)
AZ91D-T6 alloys coated with a cold sprayed Al + xAl₂O₃ (x = 30, 50 vol.%) composite showed enhanced yield strength compared with uncoated alloys. This is attributed to the constraint effect of the cold sprayed coating on the deformation of the substrate. However, the coating also concurrently reduces the tensile strength. This is because after the specimens yield, large numbers of horizontal cracks form within the coating. As a result, the coating cannot sustain any externally applied force, and the

Acknowledgement

Special thanks go to Mr. Stephen Bonner for checking the written English.

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Citation Excerpt :
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View full text

The effect of cold sprayed coatings on the mechanical properties of AZ91D magnesium alloys

Highlights

Abstract

Introduction

Section snippets

Materials and sample preparation

Microstructure of the cold sprayed coatings and the as-heat treated AZ91D substrate

Conclusions

Acknowledgement

Thin Solid Films

Int. J. Electrochem. Sci.

Appl. Surf. Sci.

Electrochim. Acta

Acta Mater.

Acta Mater.

Appl. Surf. Sci.

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Mater. Sci. Eng. A-Struct.

Surf. Coat. Technol.

Acta Mater.

Surf. Coat. Technol.