Ya. Unigovski

Corrosion fatigue of die-cast and extruded magnesium alloys

Abstract To study the fatigue life of die-cast and extruded AZ91D, AM50 and AZ31 magnesium alloys, corrosion fatigue tests were carried out using a rotating beam type fatigue machine. Corrosive environment (3.5% NaCl) significantly decreases fatigue life of alloys, especially for extruded alloys. The fatigue data at high stresses were analyzed using fitting equations. Extruded alloys show a higher sensitivity to the action of NaCl solution in comparison with die-cast alloys; however, their corrosion fatigue life is longer than that of die-cast alloys. Corrosion fatigue behavior of Mg alloys correlates with their mechanochemical behavior. The latter was studied by the polarization measurements of the dissolution rate of strained alloys. TEM observations confirmed that the maximums on the curves of Mg dissolution rate dependence on the strain in NaCl solution appeared at the work-hardening stages due to the creation and destruction of dislocation pile-ups.

Corrosion fatigue of extruded magnesium alloys

Abstract Corrosion fatigue tests were carried out on extruded AZ31 (3% Al, 1% Zn, 0.3% Mn, Mg—the rest), AM50 (5% Al, 0.4% Mn, Mg—the rest) and ZK60 (5% Zn, 0.5% Zr, Mg—the rest) Mg alloys in air, NaCl-based and borate solutions. N sol / N air ratios (the relative fatigue life) were used for the analysis of the corrosion fatigue behavior of Mg alloys in various environments, where N sol and N air are the numbers of cycles to failure in the solution and in air, respectively. Extruded ZK60 alloy reveals very high fatigue and corrosion fatigue properties in comparison with other alloys. However, it has the lowest relative fatigue life ( N sol / N air ∼10 −3 –10 −2 ) or the highest sensitivity to the action of NaCl-based solutions in comparison with that of AM50 and AZ31 alloys ( N sol / N air ∼10 −2 –10 −1 ). Under the same stress, the corrosion fatigue life of extruded alloys is significantly longer than that of die-cast alloys ( N sol for extruded AM50 in NaCl is two to three times longer than that of die-cast AM50).

Development of semisolid casting for AZ91 and AM50 magnesium alloys

The feasibility of semisolid casting process with AZ91 and AM50 magnesium alloys, which are usually used for hot- and coldchamber die-casting, was investigated in this work. Two casting methods were examined: direct casting from the melting–stirring unit and billets casting. The billets had been precast with thixotropic properties, and were heated again by an induction furnace before the casting. The apparent viscosity of the semisolid magnesium slurry during continuous cooling was found as a function of the casting temperature. Viscosity tests show that the appropriate casting temperatures in the two-phase region were in the range of 595–575 C for AZ91 alloy and 620–614 C for AM50 alloy. At casting temperature of 585 C, AZ91 exhibited maximum density and the highest standard mechanical properties. AM50 alloy demonstrated the relative poor mechanical properties in the two-phase region. The toleration of heat treatment of semisolid AZ91 alloy with a 1.8-fold increase in the elongation-to-fracture was observed. No significant differences of the densities and the standard mechanical properties of the alloys between direct or billet castings were demonstrated. 2002 Elsevier Science Ltd. All rights reserved.

Influence of technological parameters of permanent mold casting and die casting on creep and strength of Mg alloy AZ91D

To increase the strength of cast magnesium alloys operating at elevated temperatures, it is most important to increase their creep resistance. However, the influence of Mg-alloys die casting and permanent mold casting parameters on creep resistance has not been practically studied in the past. It is shown in this work that such casting parameters as metal injection rate into the die, die (mold) temperature and liquid metal temperature exert essential influence on creep resistance and other mechanical properties of the magnesium alloy AZ91D. It is established that in the case of permanent mold cast alloy, creep magnitude is defined by intermetallic β-phase inclusions, whereas in the case of die casting it is defined by the extent of micro- and macro-porosity development in the alloy.

Mechanoelectrochemical behavior and creep corrosion of magnesium alloys

Abstract Both creep and corrosion resistances are significant problems in the application of magnesium alloys. The synergistic effect of corrosion and stress on the viscoelasticity of magnesium alloys named corrosion creep has been studied in die-cast AZ91D (Mg–9%Al–1%Zn) and AM50 (Mg–5% Al–0.4% Mn) alloys in air and in the borate buffer solution. The highest sensitivity to creep in the corrosive environment is observed in the alloy with the highest Al content. An electrochemical study under a tensile strain demonstrates a good correlation with corrosion creep tests.

Surface morphology of a die-cast Mg alloy

Abstract We have studied the microstructure, phase composition and morphology of the intermetallic β-phase in the surface layer of the most widespread magnesium alloy AZ91D (Mg–9 wt.% Al–1 wt.% Zn) used in pressure die casting. Auger electron spectroscopy (AES), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and optical microscopy have been applied. The surface of the AZ91D Mg alloy consists of an oxide-metallic film with a thickness of about 0.15 micron containing Mg and Al oxides. Prolonged aging in air at 200°C promotes an increase in the MgO/Al 2 O 3 ratio from 9±2 to 14±2. The aluminum concentration gradient in the surface layer is rather significant. The external layer of as-cast specimens contains up to 32 at.% Al, whereas α-Mg grains in the bulk contain, on the average, about 5 at.% Al. Aging leads to a substantial increase in the surface concentration of aluminum at the expense of the acceleration of its diffusion and the intensification of supersaturated solid solution decomposition. During creep tests at elevated temperatures the morphology of the β-phase is significantly affected by strain and, to a lesser extent, by the casting temperature.

STATIC AND DYNAMIC CORROSION FATIGUE OF Mg ALLOYS USED IN AUTOMOTIVE INDUSTRY

Abstract Corrosion fatigue tests were carried out on die-cast and extruded magnesium alloys AZ91D, AZ80, AM50, and ZK60 in air, and NaCl-based and borate solutions. N sol /N air ratios (the relative fatigue life) were used for analysis of the corrosion fatigue behavior of Mg alloys in various environments, where N sol and N air are the numbers of cycles to failure in the solution and in air, respectively. Extruded ZK60 alloy reveals very high fatigue and corrosion fatigue properties in comparison with other alloys. However, it has the lowest relative fatigue life ( N sol /N air ∼ 10−3 − 10−2) or the highest sensitivity to the action of NaCl-based solutions in comparison with that of AM50 and AZ31 alloys ( N sol /N air ∼ 10−2 − 10−1). Under the same stress, the corrosion fatigue life of extruded alloys is significantly longer than that of die-cast alloys ( N sol for extruded AM50 in NaCl is two to three times longer than that of die-cast AM50). The synergistic effect of corrosion and stress on viscoelasticity of magnesium alloys called corrosion creep is shown. The highest sensitivity to creep in a corrosive environment is observed in the alloy with the highest Al content.

Mechanoelectrochemical Behavior of Pure Magnesium and Magnesium Alloys Stressed in Aqueous Solutions

A new synergistic effect of corrosion and stress on the viscoelasticity of pure magnesium and magnesium alloys has been shown. This phenomenon named corrosion creep has been studied in 99.9653% Mg, die-cast AZ91D (Mg–9% Al–1% Zn), AM50 (Mg–5% Al–0.4% Mn), and AS21 (Mg–2.3% Al, 0.23% Mn, 1.10% Si) alloys. Creep tests were carried out at 25°C in air and in the borate buffer aqueous solution (pH 9.3). It is found that the highest sensitivity to creep in the corrosive environment is observed in the alloy with the highest Al content. This agrees with the data obtained earlier in the study of mechanoelectrochemical behavior and corrosion fatigue of Mg alloys. However, in air, the creep behavior of all alloys at room temperature is approximately the same. Creep life of pure magnesium and its alloys significantly decreases in a corrosive environment. Corrosion-creep cracks filled with magnesium hydroxide particles were observed by SEM–EDS analysis at the surface of AS21 alloy. The thickness of the oxide layer defined by AES in samples of AZ91D, AM50, and AS21 alloys placed for 2 h into the borate solution amounts to ∼540, ∼1320, and ∼1440 nm, respectively. This can be explained with the account for surface phenomena.

Corrosion creep of magnesium-based alloys

In recent years special attention has been concentrated on Mg alloys as the lightest structural metal. However, their insufficient corrosion resistance and creep resistance limit Mg alloys application in automotive and aerospace industry. Until now these problems have been treated in many works separately as corrosion behavior in non-stressed state and creep in noncorrosive conditions. However, under real operational and environmental conditions mechanical and corrosion processes occur simultaneously, accelerating each other. This inevitably leads to the appearance of new synergistic effects that significantly reduce the lifetime of metals. For example, the failure of AZ91D alloy (Mg-9%Al-1%) exposed to a rural atmosphere while loaded in constant tension of 102 MPa (near 60% of yield strength) occurred within 10–100 days [1]. In the present work the effect of corrosive medium on the creep of some die-cast Mg alloys has been studied for the first time, and a phenomenon named “corrosion creep” has been found. Specimens (5.9 mm in diameter, gauge length of 75 mm) of Mg alloys AZ91D (Mg-8.4% Al, 0.85% Zn, 0.17% Mn), AM50 (Mg-5.1% Al, 0.15% Zn, 0.57% Mn) and AS21 (Mg-2.3% Al, 0.23% Mn, 1.10% Si)) were produced on a die-cast machine with the locking force of 2000 KN. To study the surface layer effect on corrosion creep, a surface layer 0.5 mm thick was removed by mechanical treatment from the surface of some specimens. Creep tension tests were carried out on Model 3 Satec machine at 25 ± 2 ◦C in air and in 0.1N Na2B4O7 buffer solution (pH 9.3) at stress values of 0.85 and 0.89 of tensile yield strength (TYS) for each alloy (Table I). Creep specimen 1 in a glass electrochemical cell 3 was placed into an averaging high-temperature

Effect of die-casting conditions on viscoelastic behavior of Mg alloy

Creep and stress relaxation resistance of the most common die-cast Mg alloy AZ91D (Mg-9% Al-1% Zn) are influenced by both casting temperature and injection rate as well as by die temperature and porosity. Relationships between viscoelastic properties of Mg alloy at 150 °C and the parameters of die-casting technology are obtained and presented as simple contour plots (isograms). These properties can be improved by reducing casting temperature and increasing injection rate. When fabricating complicated thin-wall components, castability of the alloy can be diminished due to a fast temperature drop. In this case, casting temperature should be higher, whereas injection rate can be reduced. Porosity seems to be the most important characteristic of die-cast alloy microstructure affecting its viscoelastic properties. Coarse β-phase precipitates do not affect creep and relaxation characteristics of die-cast AZ91D alloy.