Abstract
AbstractAZ91D and MRI153M alloys were produced by thixomolding. Their corrosion resistance is significantly higher than that of similar materials produced by ingot or die‐casting. A corrosion rate smaller than 0.2 mm/year in 5 wt% NaCl solution is measured for the thixomolded AZ91D alloy. The corrosion behaviour was evaluated using immersion tests, electrochemical impedance spectroscopy, hydrogen evolution, glow discharge optical emission spectroscopy, and atomic emission spectroelectrochemistry. A bimodal microstructure is observed for both alloys, with the presence of coarse primary α‐Mg grains, fine secondary α‐Mg grains, β‐phase, and other phases with a minor volume fraction. The amount of coarse primary α‐Mg is significantly higher for the AZ91D compared with the MRI153M. The network of β‐phase around the fine secondary α‐Mg grains is better established in the thixomolded AZ91D alloy. A combination of several factors such as the ratio of primary to secondary α‐Mg grains, localised corrosion or barrier effect due to other phases, as well as regions of preferential dissolution of the α‐Mg due to chemical segregation, are thought to be responsible for the high corrosion resistance exhibited by the thixomolded AZ91D and MRI153M.
Highlights
Mg alloy components are mostly produced through high pressure die‐casting.[1,2] Alloy development as well as optimisation of casting processes can result in materials of superior mechanical properties, especially at elevated temperatures
The solidified melt of the AZ91D alloy produced by thixomolding process formed four different solid constituents: primary α‐Mg grains, secondary α‐Mg grains, and eutectic consisting of tertiary α‐Mg and β‐phase (Mg17Al12) and other phases of minor volume fraction
The contents of β‐phase and other particles are slightly higher for the MRI153M compared with the AZ91D
Summary
Mg alloy components are mostly produced through high pressure die‐casting.[1,2] Alloy development as well as optimisation of casting processes can result in materials of superior mechanical properties, especially at elevated temperatures. The relatively high fraction of porosity due to solidification shrinkage and gas entrapment due to turbulent die filling are some of the main problems in die‐casting and significantly impact the mechanical properties.[3]. The basic feature of semisolid processing is the formation of a thixotropic suspension of solid spheres immersed in a liquid matrix.[4,5,6] The generation of the thixotropic slurries is explained in detail elsewhere.[7]. The rheological properties of the mixture allow the filling of the moulds under laminar flow conditions, which facilitates the use of geometrically complex moulds
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