Influence of transverse load on the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy

A new testing procedure, employing transverse load was adopted to investigate the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy. The tests were conducted with an electro dynamic shaker system by employing specimens fabricated as per ASTM standard. S–N plot was generated from the test results and compared with that of gravity cast AZ91 alloy tested in identical ambience. The influence of transverse load on the fatigue behaviour of these alloys is discussed. As fatigue cracks were found to have initiated in pores in most of the tested samples, pores were assumed as initial cracks as per linear fracture mechanics and the critical stress intensity amplitude (K_cr) was estimated. Structure–fatigue property correlations are discussed using fractographs. Mean stress effect on the fatigue properties and effects of alloying constituents are also discussed.     

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200 °C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200 °C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.     

Damping capacity of sand cast magnesium alloy AZ91

Abstract

Measurements of damping properties have been carried out on sand cast specimens of the most widely used commercial magnesium alloy AZ91. Damping properties of specimens in both as cast and heat treated (T4 and T6) conditions were determined by measurements of the logarithmic decrement of free bending beam vibration at various frequencies. The best damping properties were found in the as cast specimens. This is directly related to the microstructure, which consists of α magnesium, large inclusions of β phase (Mg₁₇Al₁₂), as well as lamellar eutectoid Mg₁₇Al₁₂ precipitating as a result of low cooling rate. For homogenised and annealed specimens, the maxima of the hardness and of the damping capacity as a function of annealing time was found. The measurements of the logarithmic decrement can be explained using a model of Granato and Lücke.     

The effect of skin characteristics on the environmental behavior of die cast AZ91 magnesium alloy

The skin characteristics of die cast AZ91 magnesium alloys and their consequent environmental behavior were evaluated in correlation with die cast wall thickness. The metallurgical examination of the skins was carried out using scanning electron microscopy (SEM), SPX-surface analyses with ESCALAB 250 apparatus, and X-ray diffraction analysis. The corrosion behavior of the skins was evaluated by immersion tests and by potentiodynamic polarization measurements in 3.5% NaCl solution saturated with Mg (OH)₂ at room temperature. The results show that the corrosion resistance of die cast specimens with relatively greater thickness was improved compared to the corrosion resistance of thin wall specimens. This was explained in light of the obtained skin characteristics, mainly in terms of aluminum content, β phase (Mg₁₇Al₁₂) quantity and morphology, and level of porosity.     

Investigation of uniaxial low-cycle fatigue failure behavior of hot-rolled AZ91 magnesium alloy

The uniaxial low-cycle fatigue behavior of hot-rolled AZ91 magnesium alloy was investigated by asymmetric cyclic stress-controlled experiments at room temperature. The effects of the sampling direction, peak stress and stress ratio on the fatigue life were discussed. The fatigue life increases with increasing the stress ratio or decreasing the peak stress. Due to the anisotropic property, the specimen in transverse direction shows superior fatigue resistance. Considering the effects of mean stress on the fatigue strength coefficient and fatigue strength exponent, a modified Basquin model was proposed and validated to evaluate the fatigue life of AZ91 magnesium alloy.     

Fatigue behaviour of cast magnesium alloy AZ91 microstructurally modified by friction stir processing

Friction stir processing (FSP) was applied to cast magnesium alloy AZ91-F to modify the as-cast microstructure, and the effect of FSP on fatigue behaviour was discussed based on microstructural consideration, crack initiation, crack growth behaviour, and fracture surface analysis. Fully reversed axial fatigue tests have been performed using as-cast, T5-aged and their FSPed specimens (as-cast/FSP and T5/FSP). It was found that both FSPed specimens exhibited significantly higher fatigue strength than the as-cast and T5-aged specimens. FSP resulted in the break-up of coarse as-cast microstructure, grain refinement of the matrix, finely dispersed precipitates and increase of hardness, thereby both the crack initiation resistance and the crack growth resistance were considerably enhanced compared with the as-cast and T5-aged specimens, resulting in the improved fatigue strengths of the FSPed specimens.     

Calorimetric analysis of AZ91D magnesium alloy

In order to study the activation energies and microstructure evolution of β-Mg₁₇Al₁₂ and eutectic structure in AZ91D magnesium alloy, calorimetric analyses were carried out using non-isothermal measurements by DSC and scanning electron microscopy (SEM) with EDX. During DSC tests, two peak temperatures of β-Mg₁₇Al₁₂ dissolution/precipitation and eutectic transformation were delayed in the heating/cooling process with the increase of scanning rates due to the occurrence of superheating/supercooling. Activation energies of β-Mg₁₇Al₁₂ dissolution and eutectic transformation were derived using multiple heating rates. Furthermore, the results of SEM and EDX revealed that the size and morphology of eutectic structures in the alloy had a close relationship with the time of β-Mg₁₇Al₁₂ phase dissolution during the heating process. More eutectic structures formed at low scanning rate due to sufficient time for the diffusion of Al in α-Mg regions.     

AZ91C magnesium alloy modified by Cd

In the present work, the effect of Cd on the microstructure, mechanical properties and general corrosion behaviour of AZ91C alloys was investigated. Addition of Cd was found not to be efficient in modifying/refining the microstructure or β-phase. A morphology change in β-phase from fine continuous precipitates to discontinuous β-phase upon the addition of Cd was observed. A marginal increment in mechanical properties was observed. General corrosion behaviour was followed with weight loss measurements, potentiostatic polarisation studies and surface studies in 3.5% sodium chloride solution and 3.5% sodium chloride with 2% potassium dichromate solution. Cd addition deteriorated the corrosion behaviour of AZ91C. This behaviour was attributed to the formation of chunks of β-phase upon the addition of Cd. AZ91C with refined β-phase distribution, performed rather better in the NaCl solutions.     

消失模铸造AZ91镁合金的研究

研究了ZA91镁合金消失模铸造时铸件的厚度,位置和真空度对铸件质量,组织及力学性能的影响,真空度是决定铸件质量的一个关键的工艺因素,无真空时浇注铸件易产生浇不足缺陷,但真空度过大又会导致形成粘砂和气孔等缺陷,真空浇注射明显细化组织,但真空度进一步增大时细化效果野 不明显,铸件显微组织具有很大的壁厚效应,然 位置对组织的影响与是否采用抽真空措施有关,铸件壁厚较小时,铸件的力学性能总体较差,断裂源自Mg/Mg17Al12界面、且主要是以解决理形式的脆性断裂。     

Evaluation of the KIC and JIC fracture parameters in a sand cast AZ91 magnesium alloy

The critical fracture parameters K_IC and J_IC were determined on compact tension (CT) specimens designed according to ASTM standards. The minimum specimen thickness B required for a valid K_IC test is suggested to be directly proportional to the σ_ys/E ratio, which is very high for magnesium alloys. As consequence, the value of B is more than 128 mm. Valid K_IC values were obtained using specimens with B=110 mm. Research on the effect of specimen thickness on the K_Q value was performed. The specimen thickness proposed by ASTM E 399 is overestimated. The experimental results show that not so large specimens are needed for a valid K_IC test. K_IC experimental value was 22 MPa m^1/2, that is two times the value reported in the literature for this alloy. The determination of both K_IC and J_IC has offered also the possibility of evaluating the real efficacy of formulas relating the two parameters: starting from the J_IC experimental data, formulas give some underestimation of the K_IC values. SEM fractographic examinations revealed the influence of grain size on the fracture resistance of AZ91.     

Optical anisotropy of a color-etched AZ91 magnesium alloy

In this paper, the optical anisotropy of a color-etched AZ91 magnesium alloy grain is studied. In the first part, the rinsing and drying conditions after etching were varied in order to improve the contrast between grains. A rinsing solution was selected and ellipsometric measurements were carried out to characterize the optical anisotropy of the etched surface. The wavelength, grain orientation and angle of incidence were varied. It was found that the reflection intensity at oblique incidence and the phase shift between parallel and perpendicular polarizations depend on the orientation of the etched surface of the grains. The optical contrast under diffuse light is explained by the morphology of the surface film deposited upon etching. The optical contrast under polarized light is attributed to form birefringence induced by the film texture. The birefringence, the fast axis, the slow axis, and the optical axis of the etched surface were also determined with the polarizing microscope.     

Damping behaviour of AZ91 magnesium alloy with cracks

The amplitude-dependent damping of a commercial magnesium cast alloy AZ91 was determined at room temperature by measurement of the logarithmic decrement of free decaying vibrations of bending beams clamped at one side. In order to generate cracks in the specimens they were subjected to (1) isochronal heat treatments for 1 h at temperatures above 400 °C with succeeding quenching in cold water and (2) controlled fatigue bending loading in the same equipment also used for the damping measurements. After both treatments, the amplitude-dependent damping curves show a maximum for strains 10⁻⁵ to 10⁻³, which can be correlated with the presence of cracks and can be explained by a simple rheological model based on crack damping. This maximum is enhanced when the number of loading cycles or the quenching temperature is increased which can be explained by crack nucleation. Crack growth with increasing number of loading cycles shifts the maximum to lower strains.     

Electron microscopical investigation of as cast AZ91D alloy

Abstract

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.     

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.     

Organic coatings silane-based for AZ91D magnesium alloy

Organic coatings silane-based containing electron withdrawing group or electron donating group have been synthesized and evaluated as prospective surface treatments for AZ91D magnesium alloy by hydrolysis and condensation reaction of the different silanes. Electrochemical tests were employed to confirm the corrosion resistance ability of the two kinds of organic coatings. The results showed that the coating with electron donating group had better corrosion protection performance. On the basis of the spatial configuration and the density of charge of those silanes molecules which was obtained through Gaussian 03 procedure based on B3LYP and density functional theory, combining experiment results, the rational explanation was provided.     

High cycle fatigue mechanisms in a cast AM60B magnesium alloy

ABSTRACT We examine micromechanisms of fatigue crack initiation and growth in a cast AM60B magnesium alloy by relating dendrite cell size and porosity under different strain amplitudes in high cycle fatigue conditions. Fatigue cracks formed at casting pores within the specimen and near the surface, depending on the relative pore sizes. When the pore that initiated the fatigue crack decreased from approximately 110 µm to 80 µm, the fatigue life increased two times. After initiation, the fatigue cracks grew through two distinct stages before final overload specimen failure. At low maximum crack tip driving forces (K_max < 2.3 MPa√m), the fatigue crack propagated preferentially through the α‐Mg dendrite cells. At high maximum crack tip driving forces (K_max > 2.3 MPa√m), the fatigue crack propagated primarily through the β‐Al₁₂Mg₁₇ particle laden interdendritic regions. Based on these observations, any proposed mechanism‐based fatigue model for cast Mg alloys must incorporate the change in growth mechanisms for different applied maximum stress intensity factors, in addition to the effect of pore size on the propensity to form a fatigue crack.     

Microstructure and mechanical properties of spray-formed AZ91 magnesium alloy

The AZ91 magnesium alloy, preformed with complete shape, has been prepared using spray forming technology under a protective atmosphere. The microstructure and mechanical properties have been investigated. Initially, a homogeneous and equiaxed-grain structure with average grain size of 20 μm was obtained. The tendency for segregation of the divorced eutectic β(Mg₁₇Al₁₂) phase towards the grain boundary was greatly reduced. Further grain refinement was attributed to dynamic recrystallization during extrusion processing. When solution treated at 415 °C and aged at 175 °C, two kinds of β(Mg₁₇Al₁₂) precipitates are formed: the majority are lamellar discontinuous precipitates, in addition to a small amount of dispersed continuous precipitates. The average tensile ultimate and yield strength of the spray-formed and extruded AZ91 magnesium alloy samples were 435 MPa and 360 MPa with a room temperature elongation of 9.2%, indicating an enhanced combination of toughness and strength.     

Comparison of superplastic forming abilities of as‐cast AZ91 magnesium alloy prepared by twin roll casting and WE43 magnesium alloy

This paper describes and compares the superplastic behaviour and microstructural evolution of twin roll cast AZ91 and WE43 rolled sheet alloys. Tests were carried out in uniaxial tension on both alloys across a range of temperatures (300 °C–525 °C) and strain rates (1?10^‐4 s^‐1–1?10^‐1 s^‐1). In the case of WE43 gas bulge testing was employed at 400 °C and 0.6 MPa to offer a better analogy to superplastic forming than uniaxial tensile testing. Elongations of over 400 % were observed within WE43 when tested at 450 °C and 1?10^‐3 s^‐1 strain rate, and over 200 % within AZ91 when tested at 350 °C and 1?10^‐3 s^‐1 strain rate. A peak cone height of 41 mm was achieved with WE43 at a temperature of 400 °C and pressure of 0.6 MPa. Electron back scattered detection technique was employed to analyse the microstructural evolution of the two alloys during the forming process. Both WE43 and AZ91 were observed to undergo dynamic recrystallization during elevated temperature tensile testing and failed at low strain rates mainly by means of coalescence of cavitation, in the case of AZ91 at high strain rates cracking of Al₁₂Mg₁₇ intermetallic particles was the dominating failure mechanism. Both alloys were seen to achieve good levels of superplastic ductility over 200 % elongation, which would be industrially useful in niche vehicle and aerospace manufacturing.