High Pressure Die Cast AZ91 Research Articles

Corrosion Behaviour of High-Pressure Die-Cast AZ91 Alloy in NaCl Solution: Effects of Friction Stir Process at High Rotational Speed.

The AZ series of Mg alloys have become promising in several industrial fields thanks to its potential microstructure refinement and the β-Mg17Al12 eutectic that controls the mechanical behaviour. Simultaneously, the rapid degradation characterizing Mg alloys makes the investigation of their corrosion behaviour necessary. The present work considers high-pressure die cast (HPDC) AZ91 alloy to evaluate its corrosion behaviour in 1M NaCl solution and investigates how different friction stir process parameters can affect the corrosion responses. No studies analyse the effects induced by the friction stir processed zone, reached using high rotational speeds (>2000 rpm), on the unprocessed HPDC AZ91 alloy. In addition, the morphological analysis of the corroded surfaces having a friction stir processed zone, in which the grain refinement was not obtained, is not present in the literature yet. Microstructural features were investigated by optical microscope and X-ray diffraction analysis before and after the friction stir process. These were subsequently correlated to the corrosion responses after the immersion tests. The results show that HPDC samples with a very smooth surface have the best corrosion resistance with a corrosion rate lower than 3 mm/year, evaluated through the weight loss, compared to the rougher ones. Both the amount of β-Mg17Al12 eutectic and the wt.% Al in the α-Mg matrix, as well as the surface roughness, influence the corrosion behaviour of friction stir processed samples. The best corrosion resistance was obtained with an HPDC alloy processed at 2500 rpm and 50 mm/min.

Strain Rate during Creep in High-Pressure Die-Cast AZ91 Magnesium Alloys at Intermediate Temperatures.

During creep, magnesium alloys undergo microstructural changes due to temperature and stress. These alterations are associated with the evolution of the present phases at a microstructural level, creating different strain rates during primary and tertiary creep, and with the stability of the inter-metallic phase Mg17Al12 formed at these temperatures. In this paper, the results of creep testing of high-pressure die-cast AZ91 magnesium alloys are reported. During creep, continuous and discontinuous precipitates grow, which influences creep resistance. The creep mechanism that acts at these intermediate temperatures up to 150 °C is termed dislocation climbing. Finally, the influence of the type of precipitates on the creep behavior of alloys is determined by promoting the formation of continuous precipitates by a short heat treatment prior to creep testing.

Microstructure evolution and mechanical properties of hot deformed Mg9Al1Zn samples containing a friction stir processed zone

Abstract During the last decade, FSP of magnesium alloys has become more popular due to the potential microstructure refinement of their eutectic phases, and interest has increased around the AZ1 that is one of the most commercially used magnesium alloys. In this work, high pressure die cast AZ91 plates were tensile tested at high temperatures after friction stir processing (FSP), with the stirred region in the middle of the gauge length. Samples deformed at 350 °C revealed an increment of ductility that was doubled as compared to those deformed at 300 °C and a strengthening of the nugget was measured by Vickers microhardness (HV). The correlation of HV average values to local grain size confirmed the validity of the Hall–Petch type equation where stress is replaced by hardness. X-rays diffraction and electrical conductivity highlighted the potential increment of solute atoms in solid solution during FSP. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) investigations performed on as-FSPed and hot deformed samples determined MgAlZn and AlMn particles type evolution through statistical analysis that supported mechanical properties and the strengthening mechanisms.

Relationship Between the 3D Porosity and β-Phase Distributions and the Mechanical Properties of a High Pressure Die Cast AZ91 Mg Alloy

Currently, most magnesium lightweight components are fabricated by casting as this process is cost effective and allows forming parts with complex geometries and weak textures. However, cast microstructures are known to be heterogeneous and contain unpredictable porosity distributions, which give rise to a large variability in the mechanical properties. This work constitutes an attempt to correlate the microstructure and the mechanical behavior of a high pressure die cast (HPDC) Mg AZ91 alloy, aimed at facilitating process optimization. We have built a stairway-shaped die to fabricate alloy sections with different thicknesses and, thus, with a range of microstructures. The grain size distributions and the content of β-phase (Mg17Al12) were characterized by optical and electron microscopy techniques as well as by electron backscatter diffraction (EBSD). The bulk porosity distribution was measured by 3D computed X-ray microtomography. It was found that the through-thickness microhardness distribution is mostly related to the local area fraction of the β-phase and to the local area fraction of the pores. We correlate the tensile yield strength to the average pore size and the fracture strength and elongation to the bulk porosity volume fraction. We propose that this empirical approach might be extended to the estimation of mechanical properties in other HPDC Mg alloys.

High strain rate superplasticity in friction stir processed ultrafine grained Mg–Al–Zn alloys

Friction stir processing of a rolled AZ31 alloy and a high pressure die cast AZ91 alloy produced ultrafine grained materials with average grain size of 0.8μm and 0.5μm, respectively. Tensile testing was done in the temperature range of 210–360°C and strain rate range of 1×10−4–3×10−2s−1. Higher amounts of β-precipitates in AZ91 thermally stabilized the microstructure up to 330°C. Friction processed ultrafine grain AZ91 exhibited high strain rate superplasticity at all tested temperatures. The best elongation of 1251% was achieved at a strain rate of 1×10−2s−1 and temperature of 330°C. The kinetic analysis of the superplastic data revealed a large variation in the kinetic parameter due to difficulty in using proper grain size values because of concurrent grain growth.

Elevated temperature mechanical properties and microstructures of high pressure die cast magnesium AZ91 alloy cast with different section thicknesses

This paper investigates the influence of variations in the microstructure of high pressure die cast AZ91 on the elevated temperature mechanical properties of the alloy. Thinner-walled high pressure die castings show an improvement in elevated temperature strength, ductility and creep resistance. Further improvements to the creep resistance were achieved by ageing the alloy prior to creep testing. It appears that an increased proportion of fine grained ‘skin’ region in the thinner castings contributed to the improved properties. Also, it appeared that the presence of supersaturated solute Al in the eutectic α-Mg contributes to the poor creep properties, probably due to the microstructural instability. Final failure is associated with the growth of voids either from porosity in the alloy or nucleated from discontinuous precipitates.

A Study of Residual Elastic Strain Distribution in an AZ91 Mg Alloy Bar Loaded in Four Point Bending

This paper presents a study of the residual strain field within a high pressure die cast (HPDC) AZ91 Mg alloy bar subjected to four point bending. The technique employed for this purpose is high energy synchrotron X-ray diffraction. Strain scanning using polychromatic X-ray beam allows the collection of multiple peak diffraction patterns and monitoring of small peak shifts as a function of beam position. These shifts allow collective interpretation in terms of the equivalent macroscopic residual elastic strain. Residual elastic strain distributions were studied in the sections subjected to pure bending and also in sections of contact between the sample and the rollers. These experimental results are compared with the predictions from a finite element analysis of contact and deformation. Good agreement is found between the modelled and measured results. It is hoped that these results help improved understanding of complex deformation behaviour of thin-walled HPDC AZ91 components and provide useful background information for lifing assessment of such structures.

Quantitative characterization of Mg17Al12 phase and grain size in HPDC AZ91 magnesium alloy

Quantitative characterization of size and spatial arrangement of β phase particles (β-PP) and grain size by the developed image processing algorithm was studied in this paper. This is expected to be useful to understand the processing–property–microstructure correlation of high-pressure die cast (HPDC) AZ91 alloy. The important parameters of image processing like montage creation, separation of different phases and grains and the characterization procedure are explained. The microstructural quantities such as average size, average nearest neighbour distance (NND), area fraction, grain size and clustering tendency (CT) of β-PP obtained as a function of distance from surface to centre of the casting. The β phase particles are more in number and finer in the surface region compared to the centre of the casting, the average particles size in centre region is one and half times of surface region. There is no considerable change in the area fraction of β phase in different regions throughout the casting and this is a contradictory conclusion from the previous understanding. A minor clustering nature of β-PP is observed in the surface region and the random nature is increasing from the surface to centre of the casting. In addition the grains are very fine in the surface region compared to the centre of the castings.

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 PROPERTIES OF THE SKIN AND BULK OF SEMISOLID PROCESSED AND HIGH PRESSURE DIE CAST AZ91 ALLOY

The corrosion properties of the skin and the bulk of semi-solid processed and high pressure die cast AZ91 alloy were studied in 5% NaCl solution by a) analyzing the corrosion morphology, b) measuring electrochemical polarization curves, c) performing electrochemical impedance spectroscopic measurements, and d) immersion tests at constant pH values. Using X-ray diffraction (XRD), light microscope (LM), scanning electron microscope (SEM) and energy dispersive X-ray (EDX) to evaluate the microstructure of skin and bulk specimens, the corrosion results were related to their respective microstructures of the particular alloys. However, microstructure and corrosion performance of the alloy were found to be similar for both production processes and only slight but distinct differences were found. Generally, the skin corrosion properties are inferior to those of the bulk with the exception of the high pressure die cast AZ91 alloy, where the finer microstructure of the skin layer exhibits a better performance as shown by impedance studies and confirmed in long term immersion tests.Les propriétés en corrosion de la peau et du centre de pièces moulées sous pression en phase semi-solide et en phase liquide de l'alliage AZ91 ont été étudiées dans une solution 5% NaCl : a) en analysant la morphologie de la corrosion, b) en mesurant les courbes de polarisation électrochimique, c) en effectuant des mesures spectroscopiques d’impédance électrochimique, et d) par des essais prolongés d’immersion à des valeurs constantes de pH. Utilisant la diffraction des rayons X (DRX), le microscope optique (MO), le microscope électronique à balayage (MEB) et les rayons X à dispersion d’énergie (XDE) pour évaluer la microstructure de la peau et du centre des échantillons, les résultats d'analyse de la corrosion ont été reliés à leur microstructure respective. Toutefois, il a été montré que la microstructure et les performances en corrosion de l'alliage étaient sensiblement similaires pour les deux procédés de production avec seulement de légères distinctions. Généralement, les propriétés en corrosion de la peau sont inférieures à celles du centre des pièces sauf que pour l’alliage AZ91 moulé à partir de l'état liquide, la microstructure plus fine de la couche superficielle des pièces exhibe une meilleure résistance à la corrosion, comme l’ont montré les études d’impédance et l'ont confirmé les essais prolongés d’immersion.