Investigation of the effect of alloy elements added to Mg-4Sn-2Al alloy on mechanical and corrosion properties

Magnesium with a density of 1.74 g/cm3, is an alkali metal with a hexagonal close-packed structure. Mg alloys, which are currently one of the lightest commercial alloys, are preferred in areas such as aerospace and biomedical applications due to their important effects such as mouldability, non-magneticity, low density, high specific strength, abundance in nature and non-toxicity. However, some limitations, such as corrosion, restrict the wider use of magnesium. Alloying plays an important role in the development of properties such as limited ductility, high corrosion in aqueous environments and hardness. It is also necessary to add some alloying elements to magnesium to improve mechanical properties, corrosion resistance, strength and ductility. A large part of magnesium alloys are produced by casting. In this study, six different alloys were produced by gravity casting and the microstructure, mechanical and corrosion properties of these alloys were investigated. Alloys were prepared by using high purity Mg, Sn, Al and Ca in Naberthern brand casting furnace and steel crucibles closed to atmosphere. Other alloying elements were obtained by using Mg-5Mn, Mg-20La, Al-6Ti and Mg-20Nd master alloys. The casting temperature was set at 750 ℃ and the mold temperature at 100 ℃. In order to prevent oxidation, the alloys were protected with CO2-2%SF6 gas during the melting and solidification processes. The produced alloys were subjected to the cutting process in the wire erosion machine. Smaller samples were cut in an aqueous medium for investigations of chemical analysis, microstructure, mechanical and corrosion properties. The samples were polished with 1μm alumina solution after sanding with 400, 600, 800, 1000, 1200 mesh silicon carbide (SiC) abrasives. After polishing, the samples were cleaned with ethanol and dried. Etching is required to distinguish the phases formed in alloys, to determine the contrast difference and to reveal it. A solution prepared with 4.5 g of picric acid, 15 ml of acetic acid, 30 ml of distilled water and 75 ml of ethanol was used to etch the alloys. Polished and dried alloys were immersed in an etching agent for 5 seconds and etched. After etching, the samples were washed with water, cleaned with ethanol, and then dried with a device blowing hot air . Microstructure investigations of the alloys were made with SEM device. Chemical analyzes were also carried out with the energy dispersive X-ray spectroscopy (EDS) module of the SEM device. Investigations of the phases and changes in grain sizes depending on the type and amount of alloying elements were made using x-ray diffraction analysis (XRD) device. According to the EDS results of Alloy 1, there are α-Mg and Mg2Sn in the microstructure. In addition to α-Mg and Mg2Sn, MgSnCa triple phase was detected in the microstructure of Alloy 2 obtained by the addition of Ca. No new phase formation was observed in the structure with the addition of Mn. According to the EDS analysis, Al2La, La5Sn3 phases and triple MgSnLa phases appeared in the structure with the addition of La. While no phase between Mg and Ti was observed in Alloy 5, which was obtained by adding 0.15% Ti by weight to Alloy 1, A3Ti particles were encountered. In the EDS examinations of Alloy 6, Nd5Sn3 phase and triple MgSnNd phase were found. Hardness measurements were made using the Brinell measurement method. Brinell hardness (HB) measurements were made using a Struers Duramin-500 instrument with a ball diameter of 2,5 mm and a load of 31,25 kg. 5 readings were made for each sample and the average of these values was accepted as the hardness value.No significant change was observed in the hardness values of the alloys. Tensile tests were carried out in Instron 3367 universal test device, at 1.8 mm/min tensile speed and at room temperature, by producing specimens conforming to ASTM E8M standard by wire erosion processing. 5 samples were used for each alloy group. Considering the mechanical properties, the tensile strength of the master alloy (Mg-4Sn-2Al) was determined as 131 MPa and the elongation was 9.26%. The highest tensile strength and elongation values were observed in Mg-4Sn-2Al-0,15Ti (Alloy 5) alloy. With the addition of 0.15 Ti by weight to the Mg-4Sn-2Al alloy, the tensile strength increased by 24% to 163MPa, and the elongation increased by 69%. With the addition of 2% La by weight, the tensile strength increased by 21% to 159MPa, and the elongation increased by 22% to 11.34. The lowest tensile strength and elongation values were obtained in Mg-4Sn-2Al-0,5Ca (Alloy 2) alloy. With the addition of Ca, the tensile strength value decreased by 29% to 92.5 MPa compared to the Mg-4Sn-2Al alloy, and the elongation decreased by 20% to 7.38. The hardening exponent values decreased with the addition of Ca to Alloy 1 and remained almost the same with the addition of Mn. With the addition of 2% by weight La and Nd, the hardening exponent increased with the increase of intermetallics in the eutectic region and the decrease in grain size due to grain regeneration. The sliding of the grain boundaries during the drawing process was prevented by the decrease in grain size and the increase of intermetallics in the grain boundaries. It is thought that the hardening exponent increases with the addition of Ti to Alloy 1, as the Al3Ti phase in the structure and Ti change the solubility of the alloying elements in the structure. It has been determined that the plastic deformation capabilities of alloys with high elongation values (Alloy 4 and Alloy 5) increase. The highest hardening exponent value of 0.46 was obtained in Mg-4Sn-2Al-0.15Ti alloy (Alloy 5). In addition, fracture surfaces of the alloys were examined by SEM and it was understood that Mg-4Sn-2Al-2La and Mg-4Sn-2Al-0,15Ti alloys with high % elongation were more ductile fractured than other alloys. As a result of the processes, the best mechanical properties were obtained in Mg-4Sn-2Al-0,15Ti alloy. In order to determine the anisotropy coefficient, ASTM E8M subsize tensile test specimens were subjected to deformation by being pulled to a load lower than the load for which the tensile strength was calculated. The sample length and sample width values of the samples before and after the tensile test were measured with an extensometer placed in the device. The anisotropy coefficient (r value) was obtained by the ratio of the true transverse strain εw to the actual strain εt in thickness, as stated in Equation 3.1. Considering the incompressibility of the material, the materials satisfy the equation ϵ_w+ε_t+ε_l=0. While the addition of Nd decreased the anisotropy coefficient, the anisotropy coefficient increased with other alloying elements. Considering that the formability of materials with anisotropy coefficient greater than 1 is better, it can be said that the formability of the Mg-4Sn-2Al alloy improves with the addition of Mn, La and Ti. Density values of alloys obtained using Archimedes principle were calculated. The highest bulk density was obtained in Alloy 6 and the lowest bulk density was obtained in Alloy 2. Densities did not increase much compared to pure magnesium. This is because the proportions of alloying elements added are low. Finally, corrosion tests were carried out using the potentiodynamic polarization method according to ASTM G59-97 standard. All electrochemical measurements were made using the Gamry 300 electrochemical analyzer system. In corrosion experiments using a conventional three-electrode system, an Ag/AgCl/KCl electrode was used as the reference electrode and a graphite electrode as the counter electrode. A solution with a pH of about 6 was prepared using distilled water and 3.5% by weight NaCl (Merck Company). The samples were isolated and only 1.77 cm2 sample surface area was contacted with the solution. All potentiodynamic polarization measurements were performed at ambient temperature, at a scanning rate of 1 mV/s and a scanning range of -0.6 to +0.6 V. Data were analyzed using the Gamry EChem Analyst software package. Corrosion current (Icorr), corrosion potential (Ecorr) and corrosion rate (Rcorr) values were determined. Accordingly, while the Icorr value of Mg-4Sn-2Al main alloy was determined as 90.69 µA and the corrosion rate as 46.60 mpy, with the addition of Ti, the Icorr value decreased to 23.16 µA and the corrosion rate decreased to 11.68 mpy and had the lowest corrosion rate. The addition of Mn, on the other hand, was the second element that reduced the corrosion rate the most, after Ti, with a corrosion rate of 14.65 mpy. The addition of Ca and Nd increased the corrosion rate and had an adverse effect on the corrosion properties. In general, it can be said that intermetallic phases located at the grain boundaries cause intergranular corrosion by forming micro galvanic cells. According to these results, the alloy with the highest corrosion resistance was Mg-4Sn-2Al-0,15Ti alloy with high mechanical properties.