Cast AZ91D Research Articles

Microstructure of high pressure die cast AZ91D modified with Ca and Ce

High pressure die cast (HPDC) AZ91D (Mg–8.8 wt%Al–0.74 wt%Zn–0.29 wt%Mn) modified with Ca and Ce was investigated. Thermodynamic calculations of liquid/solid equilibrium were confirmed by DTA measurements and solid/liquid equilibrium at various temperatures between the liquidus and solidus temperatures. The solidification path of HPDC AZ91 and AZ91 modified with Ca and Ce was calculated using the Termo-Calc program. The microstructure was investigated utilizing optical and electron (SEM) microscopy. The phases Mg 17Al 12, αMg, Al 8Mn 5, Al 2Ca, and Al 4CeMg 4 were found. Their volume fractions and the composition of the residual melt as functions of the temperature were determined. The microstructure, the distribution of the alloying elements in the matrix, and the precipitation during exposure to elevated temperatures as well as their effect on creep properties are discussed.

Corrosion resistance of aged die cast magnesium alloy AZ91D

The corrosion behaviour of die cast magnesium alloy AZ91D aged at 160 °C was investigated. The corrosion rate of the alloy decreases with ageing time in the initial stages and then increases again at ageing times greater than 45 h. The dependence of the corrosion rate on ageing time can be related to the changes in microstructure and local composition during ageing. Precipitation of the β phase (Mg 17Al 12) occurs exclusively along the grain boundaries during ageing. The β phase acts as a barrier, resulting in a decreasing corrosion rate in the initial stages of ageing. In the later stages, the decreasing aluminium content of α grains makes the α matrix more active, causing an increase in the corrosion rate. Electrochemical testing results also confirm the combined effects of the changes in α and β phases on the corrosion resistance of the aged die cast AZ91D alloy.

Microstructures and dislocations in the stressed AZ91D magnesium alloys

The microstructures and the dislocation arrangements in the die cast AZ91D magnesium alloy as well as the stressed alloys have been investigated using transmission electron microscopy, high-resolution transmission electron microscopy and energy dispersive X-ray (EDX) analysis. Besides the dominant α-Mg and β-Mg17Al12 phases, Al8Mn5 and Mg5Al phases have also been found and studied in the alloy. Dislocation pile-ups have been found in the stressed AZ91D alloys for the first time. They are confined in the slip planes and piled up against the grain boundaries. The dislocation pile-ups increase with deformation till about 2.1% and then remain almost identical even deformed to higher degree. The dislocations pile-ups in the AZ91D magnesium alloy are found to be beneficial to the resistance to stress corrosion cracking of the alloy, and thereby beneficial to the mechanical properties of the alloy. Dislocation networks are found to increase with deformation in all cases. The dislocation networks have also been found in the β-Mg17Al12 phase as well as in the matrix in the deformed AZ91D magnesium alloys.

Corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D alloys

The microstructure and the corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D magnesium alloys have been investigated. Semi-solid processing leads to a structure with large rounded grains of a solid solution of magnesium (α phase) whereas die-cast alloys are more homogeneous. Electrochemical measurements, particularly with impedance spectroscopy, have shown that the semi-solid cast alloy possesses a corrosion rate at least 35% below that of the die-cast alloy. This can be explained with considering the differences between the composition of the primary α phase and the volume fraction of the β phase, in the two types of alloy.

Characterization and performance of laser melted AZ91D and AM60B

Laser melting can be used to alter the surface of alloys and produce a fine microstructure, usually improving corrosion resistance. In the present work, a pulsed Nd:YAG laser was used to melt and rapidly quench the surface of die cast AZ91D and AM60B magnesium alloys with the objective of improving the corrosion resistance in salt-water (NaCl) solution. The corrosion of AZ91D and AM60B was not significantly reduced by laser melting even though the microstructure of both alloys was refined but a close examination of corroded surfaces gave indications about some corrosion enhancing factors. The physical aspect of corroded surfaces and the microstructure of laser melted specimens are presented along with the results of corrosion tests.

Mechanism of Chemical Conversion Coating Film Growth on Magnesium and Magnesium Alloys

The structure and the composition of surface films formed by chemical conversion coating in Dow7 on pure magnesium and magnesium die cast AZ91D have been studied by XRD, XPS, SEM and TEM combined with ultramicrotomy to clarify the growth mechanism. The films are amorphous containing no definite crystallites detectable by XRD . The major constituents of the films are MgF2 and MgOx(OH)y. The content of NaMgF3 increases very much with increasing finishing time. In addition, small amounts of Cr2O3 and NH4+, whose contents also increase with finishing time, are found. In the film formed on AZ91D, a substantial amount of Al, presumably present as spinel (MgAl2O4) or as it’s hydroxide, and small amounts of FeOx(OH)y and Mn4+ are found in addition to above film components on pure magnesium. The film surface of AZ91D shows a granular structure with each granule corresponding to a single grain of the substrate. Cylindrical porous cell structure of chemical conversion coating films on magnesium, which is formed by anodic reaction, has been confirmed for the first time by direct TEM observation. Film growth proceeds mainly by the formation of MgF2 and MgOx(OH)y at film/metal interface by anodic reaction and subsequent film dissolution followed by precipitation of NaMgF3, Cr2O3 and NH4+ in the film. The porous film is composed of cell colonies in the size of sub-microns having central mother pores, which are branching into fine pores.

Electron microscopical investigation of as cast AZ91D alloy

A detailed examination of a magnesium alloy, AZ91D, in as cast condition was carried out by using electron microscopy techniques. Particles of Al17Mg12, known as β and Al8Mn5 precipitates have been identified by electron diffraction. It was found that Al8Mn5 particles adopted coarse angular shapes, while β phase existed in several distinct morphologies, namely grain boundary blocky particles, lamella of the colonies of discontinuous precipitation, irregular ones in the α constituent of the cellular nodules, and two types of intragranular continuous precipitates, i.e. plates and plaques. The crystallographic relationships between the αand β constituents of the discontinuous precipitation, and between the intragranular β and the α matrix have also been determined. It was observed that the morphologies and the crystallographic relationship observed are identical to the one that was reported in the literature for an aged structure. Based on the dark field electron micrographs a model has been proposed to explain the lengthening of plates of β. Basic morphological differences between the structures of the ingot and aged AZ91 are discussed.

Influence of microstructure on the corrosion of diecast AZ91D

The corrosion of die cast AZ91D was studied and related to its microstructure. For comparison and to more fully understand the behaviour of die cast AZ91D, corrosion studies and microstructural examinations were also carried out using slowly solidified high purity AZ91, Mg-2%Al, Mg-9%Al, low purity magnesium and high purity magnesium. Corrosion was studied in 1 N NaCl at pH 11 by (1) observing the corrosion morphology, (2) measuring electrochemical polarisation curves and (3) simultaneously measuring both the hydrogen evolution rate and the magnesium dissolution rate. The skin of die cast AZ91D showed better corrosion resistance than the interior. This is attributed to a combination of (1) a higher volume fraction of the β phase, (2) a more continuous β phase distribution around finerα grains, and (3) lower porosity in the skin layer than in the interior of the die casting. This study showed that the casting method can influence the corrosion performance by its influence on the alloy microstructure.

Creep studies of AZ91D pressure die casting

Creep properties of pressure die cast AZ91D magnesium alloy (9% Al-1% Zn) were investigated in the temperature range of 150–180 °C and load range of 30–100 MPa. The pressure die cast material is characterized by a small grain size of about 10 μm. Creep tests were performed under constant load and under varying load. Creep rates were found to be a few orders of magnitude lower than these of pure magnesium but higher than these found by the authors in previous study for ingot casting. Elongations to fracture were found to be at least 1.5 times than these of pure magnesium. No steady state stage was observed, however a minimum creep rate was reached after approximately one third of the creep life of the specimens. The stress exponent was found to be 6.9 for 150 °C and 5.4 for 180 °C and was independent of temperature and stress. Transmission electron microscopy (TEM) contrast experiments show dislocation existence on other planes than the basal planes.

マグネシウムダイカストＡＺ９１Ｄの化成処理および陽極酸化により生成した皮膜の組成と形態

The Morphology and composition of surfaces and cross sections of films formed by chemical conversion and anodic oxidation on Magnesium Die Cast AZ 91D were studied by Electron Probe Microanalysis (EPMA), X-Ray Diffractometery and X-ray Photoelectron Spectroscopy. The major constituents of the chemical conversion coated film are Cr, O and Mg. It is assumed that Cr(OH)3 is formed at the film/substrate interface and subsequently converted to microcrystalline Cr2O3 with film growth. The components of anodic oxide films are well-crystallized Cr2O3, MgF2 and NaMgF3. These substances occer as relatively homogeneous film.

マグネシウムダイカストＡＺ９１Ｄの化成処理により成長した表面皮膜の構造と生成機構

The structure and the formation mechanism of surface oxide films grown on magnesium die cast AZ91D have been studied. The substrate surface after chemical etching shows grains of Mg solid solution matrix of several μm in size with Mg17Al12 compound at grain boundaries, large and small voids, and cavities between the grains. The film surface after chemical conversion coating has a granular structure, with each granule corresponding to a single grain. Direct cross sectional observation revealed that the film was formed by anodic reaction and had a porous cell structure which was separated from the substrate by a thin barrier layer at the film/metal interface. The porous film is composed of cell colonies in the sub-micron range that have branched fine pores, with central holes (mother pores) sized approximately 50nm. Square shaped holes about 300nm in size observed on the film surface are believed to be cathodic sites.

マグネシウムダイカストＡＺ９１Ｄの陽極酸化により生成した皮膜の構造と成長機構

The structure and the growth mechanism of anodic oxide films grown on magnesium die cast AZ91D have been studied by electron microscopy combined with an ultramicrotomy technique. The anodic films are highly uneven in thickness. This may have been caused by film destruction resulting from breakdown accompanied by gas evolution, and also by uneven anodic reaction owing to the presence of cavities and compositional irregularities at the grain boundaries. Porous cell structure with dispersed MgF2 and NaMgF3 crystalline particles is found in the films. It is assumed that anodic film growth proceeds by the formation of MgF2 and magnesium oxyhydroxide at the metal/film interface and the dissolution of the film at the pore bases. The crystallization of MgF2 and the formation of NaMgF3 subsequently proceed in the porous layer.