Friction stir butt welding of Mg-Zn-Mn alloy ZM21 hot rolled plates (in three thicknesses -5 mm, 10 mm and 25 mm) was investigated. Defect-free, full-penetration welds were produced after careful process parameter optimization. Microstructural studies, hardness tests, tensile tests, and bend tests were carried out. Welds produced in 5 mm thick (5-mm-welds) and 10 mm thick plates (10-mm-welds) showed relatively finer grains in the weld nugget and in the heat-affected zone compared to the welds produced in 25 mm thick plates (25-mm-welds). When compared to the base material, 25-mm-welds showed coarser grains both in the weld nugget and in the heat-affected zone. No significant hardness differences were observed between the welds and the base material. Tensile tests on 5-mm and 10-mm-welds yielded a joint efficiency of more than 75%. Bend performance of the welds was found to be satisfactory, falling only slightly behind the base material. Overall, the results show that friction stir welding can be successfully utilized for joining magnesium alloy ZM21 in various thicknesses.
The present study emphasizes the effect of addition of sodium citrate (C6H5Na3O7.2H2O) and sodium tungstate (Na2WO4.2H2O) to a silicate based electrolyte system on the corrosion behavior of PEO treated ZM21 magnesium alloy. The phase composition of the as-developed coating was evaluated by X-ray diffraction (XRD) analysis, while its surface morphology, thickness and elemental distribution were observed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Potentiodynamic polarization tests were done in 3.5 wt% NaCl solution to analyze the corrosion behavior of the ceramic coatings in simulated marine environment. The results of XRD showed that the phase composition of all coatings comprised of Mg2SiO4 and MgO irrespective of the additive used. In addition to Mg, Si and O, the presence of W, C in EDS spectrum indicated that these elements were incorporated into the coating from the electrolyte system containing tungstate and citrate. The corrosion test results revealed that the PEO coatings obtained in tungstate containing electrolyte solution showed higher corrosion resistance than those formed in citrate containing electrolyte solution.
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