Technical ReportInvestigation of tension and compression behavior of AZ80 magnesium alloy
Introduction
In many industries, engineers tend to produce structures with a high strength and low weight. For this reason, nowadays magnesium alloys are very attractive for many researchers and are extensively used in many important industries such as automotive and aerospace [1]. In addition, magnesium alloys have other unique properties which make them different in comparison with other metals. Some of these properties are high corrosion resistance, welding ability and good recyclability. AZ80 is one of the most important magnesium alloys and is recommended as a good alloy for forging in ASTM B80 – 09 Standard Specification for Magnesium-Alloy Sand Castings. But there is no complete research about its properties until today.
Many parameters can be found from the tension curves such as elasticity module, yield stress, tensile strength and strain parameters. To achieve this aim, it is necessary to investigate the mechanical and metallurgical phenomena which happen in a tension test. Strain hardening and strain softening phenomena should be considered during the test. Many researchers have studied magnesium alloys properties during the last decades.
Fereshteh-Saniee et al. presented a material model for stress–strain curve behavior of some magnesium AZ series which was able to predict the behavior of those alloys correctly [2]. Mathis et al. investigated the deformation behavior of AZ91, AE42 and AS21 magnesium alloys in a wide temperature range and different strain rates. The effect of strain hardening coefficient on stress was investigated as a criterion of strain hardening and strain softening. Their tests also showed that strain hardening decreases with increasing the temperature and stress [3], [4]. Sivapragash et al. studied the deformation and fracture behavior of ZE41A at different temperatures and strain rates using tension test. This investigation an analytical model was proposed to predict deformation behavior at various temperatures and strain rates [5]. Wen et al. investigated the mechanical behavior of AZ31 and found that “temperature activated plates” cause strain rate sensitivity. They also found that the reason behind the asymmetry in value of the yield stress in tension and especially compression in low temperature was twinning [6]. Abedi et al. investigated tension and deformation behavior of AZ31 at high temperature range and at strain rate of 0.001 s−1 [7]. Masoudpanah et al. investigated tension deformation and micro structure properties of AZ31 alloy after extrusion process and ECAP process. They showed that the ECAP specimens have lower yield stress and higher formability in comparison with the extrusion specimens [8]. Jayamathy et al. studied the effect of reinforcement on compression deformation and impact reaction in AZ92 alloy. They investigated the effect of “Sic” in compression deformation and energy absorption ability in this alloy [9]. Helis et al. did many researches to improve the microstructure of the AZ31 magnesium alloy during axial compression deformation in high temperature [10].
There are some studies about tension process simulation when the necking phenomenon occurs. Ponthot carried out a finite element simulation using an imperfection with light slip which was placed in the middle of specimen. The result of this simulation was in good agreement with experimental results [11].
Strain rate and temperature are two important parameters to predict the mechanical behavior of materials. Song et al. investigated the compression properties of three AM20, AM50 and AM60 alloys in high strain rates. Results showed that strain rate sensitivity increases by increasing the strain rate [12]. Palumbo et al. experimentally and numerically studied AZ31 magnesium alloy at high temperature and steady strain rate. They proposed an equation between strain rate and the griper displacement [13]. Anbuselvan and Ramanathan investigated deformation of ZE41A alloy using compression tests in high temperatures and different strain rates. Their results showed that the optimized deformation parameters of this alloy are 400 °C temperature and 0.1 s−1 strain rate [14]. Raghu et al. also used a ring test to investigate friction coefficient in hot deformation of the ZM-21 magnesium alloy. This investigation is practiced at various temperatures and by different lubricants [15]. Narayanasamy et al. studied buckling deformation of magnesium alloys. They proposed a model to determine the buckling radius. They showed that buckling radius depended on geometric parameters of primal specimen [16].
In this paper T shape method is used to investigation of temperature and strain rate effects on friction factor. Compression tests are conducted to investigate behavior of AZ80 at 250 °C and 300 °C temperature and 0.001 s−1, 0.01 s−1 and 0.1 s−1 strain rates. Also, tension tests are carried out at 250 °C, 275 °C, 300 °C, 325 °C and 350 °C temperature and 0.0005 s−1, 0.001 s−1 and 0.005 s−1 strain rates. It is obvious that when a metal forming process is performed, appropriate temperatures and strain-rates should be chosen to prepare enough strain for that process. The temperatures and strain-rates in this research are chosen so that the results are useful for other metal forming and simulation processes such as deep drawing. Although the behavior of this magnesium alloy specially mechanical behavior have not yet investigated, but it can be found from the other literatures that almost all researchers who have studied the other magnesium alloys have practiced their investigations in same temperatures and strain rates ranges approximately or their tests have conducted in ranges to be able to prepare same strains as this research [6], [10], [17], [18]. Bulge and numerical correction factor are used to correct the results of compression tests. The effects of strain rate and temperature on the bulge and numerical correction factors are studied. Also, T shape test is used to evaluate friction in hot deformation. Bridgman correction factor is used to correct the tension tests results. Tension and compression processes are simulated using finite element analysis. Some microstructure properties of AZ80 are investigated utilizing optical microscope images of compression tests and SEM images of tension tests.
Section snippets
Friction
Two friction laws are used to compute contact friction which exists between specimen and gadgets. Colomb law is an important model to describe friction. This law is shown in Eq. (1).
τ is the friction stress, μ is the friction module and p is the vertical compression on contact surface. This law is suitable for low compression deformation like p/σo < 1.5, where σo is the flow stress of specimen. The second law is the shear friction law which is shown in Eq. (2).which m is the friction
T shape specimens’ properties
AZ80 magnesium alloy is produced by casting. AZ80 alloy compositions are shown in Table 1. Microstructure image of this alloy prior to deformation which is taken by optical microscope is shown in Fig. 1. Cylindrical specimens which have a diameter of 6 mm and a length of 9 mm are used for T shape tests and compression tests. These specimens are produced by machining. A schematic image of T shape compression test is shown in Fig. 2. According to the ASTM A681-08 which is Standard Specification for
Bulge and numerical correction factors
Bulge correction factor is a method to eliminate the effect of friction and determination of material flow stress in compression tests. The schematic image of the test before and after compression load is shown in Fig. 6. It is obvious that buckling is caused by friction between specimen and cast surface. Therefore, a correction factor is used to evaluate the true stress.
The average stress in the midsection (σave) and corrected flow stress (σf) are computed from Eqs. (3) and (4).
Conclusions
Tension and compression Experiment tests at various temperatures and strain rates are carried out on AZ80 alloy. The results can be mentioned as below:
- 1.
From T shape results it can be seen that MoS2 always has the lowest friction and dry condition always has the highest friction. Simulation results are in good agreement with experimental results.
- 2.
It found from T shape method that for a greater strain rate, the necessary forming load becomes larger, although the slope of the linear part of the load
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