| Magnesium alloy has the characteristics of high specific strength,good thermal conductivity,light weight,low density,etc.It is widely used in aerospace,ship vehicles and other fields.However,the traditional fusion welding is prone to induce cracks,holes and other defects,due to the high temperature creep resistance,heat resistance,corrosion resistance and other properties of conventional magnesium alloys.In recent years,how to improve the performance of magnesium alloys to meet the needs of engineering applications has received widespread attention,and it can improve the defects such as poor heat resistance and poor plastic strength of magnesium alloys that add alloying elements to conventional magnesium alloys.Friction stir welding is not easy to produce cracks,holes,inclusions and other melting welding defects because the welding temperature is usually lower than the melting point of the base metal,the whole welding process is efficient,environmental protection,and it can get high strength of welded joint.In recent years,high-performance Mg-Zn-Y alloys have received extensive attention and have broad application prospects,but there are few studies on welding.In this project we selected5 mm thick Mg-6xZn-xY(x=0.25?0.5?1 at.%)alloy.Through friction stir welding experiment was performed on the Mg-6xZn-xY magnesium alloy,the evolution of microstructure and mechanical properties of friction stir welding joints of Mg-6x Zn-x Y alloys under different welding parameters were studied,and the FSW process parameter combination with better comprehensive performance was optimized,then analyze the relationship between mechanical properties and microstructure of welded joints.And the influence of different heat treatment parameters on the microstructure and mechanical properties of Mg-6x Zn-x Y alloy friction stir welded joints was studied.The main conclusions are as follows:(1)The average grain size of Mg-6x Zn-x Y(x=0.25?0.5?1 at.%)alloy welded joints decreases with the decrease of rotating rate and it also increases with the decrease of the welding speed.And the I-phase phase of the nugget zone still exists through friction stir welding in different process parameters,(2)The tensile strength and yield strength of Mg-1.5Zn-0.25 Y alloy welded joints decrease with increasing of rotating rate,and the elongation increases.As the welding speed increases,the elongation decreases.The tensile strength and yield strength of Mg-3Zn-0.5Y and Mg-6Zn-1Y alloy welded joints increase first and then decrease with the increasing rotating rate,and the elongation increases.As the welding speed increases,it first increases and then decreases,and the elongation decreases.The fracture mode of Mg-6x Zn-xY alloy is a mixed fracture mode in which the ductile fracture and the brittle fracture are combined.With the increase of the rotational rate and the decrease of the welding speed,the fracture mode is closer to the ductile fracture.(3)The comprehensive mechanical properties are good when the welding parameters of Mg-1.5Zn-0.25 Y alloy friction stir welding are 500rpm-50mm/min,the tensile strength is 184 MPa,reaches 73.8% of the base metal,the yield strength is 139 MPa,and the elongation is 5.01%.When the welding parameters of Mg-3Zn-0.5Y alloy are 1100rpm-110mm/min,the comprehensive mechanical properties are good,the tensile strength is 222.5MPa,it reaches 80.9% of the base metal,the yield strength is 173.8MPa,and the elongation after fracture is 4.0%.When the welding parameters of Mg-6Zn-1Y alloy are 1100rpm-110mm/min,the comprehensive mechanical properties are good,the tensile strength is 249 MPa,reaches 81.7% of the base metal,the yield strength is 194 MPa,and the elongation is 3.24%.(4)The hardness of nugget zone which is in the Mg-6xZn-xY alloy friction stir welding joint is higher than that of the parent metal zone,and the hardness of the thermal mechanical affected zone in advancing side is the lowest.With the increase of the rotational rate and the decrease of the welding speed,the hardness of the nugget zone which is in the Mg-1.5Zn-0.25 Y alloy friction stir welding joint gradually decreases In the Mg-3Zn-0.5Y alloy and the Mg-6Zn-1Y alloy,the hardness increases first and then decreases.(5)The comprehensive mechanical properties of the Mg-6Zn-1Y alloy welded joints were good when the annealing mode was 350°C×2h.The tensile strength and yield strength were 287 MPa and 209 MPa,the welded joints were increased by 15% and 8% compared with before,and the elongation was 8.14%.With the increase of the annealing temperature and the increase of the annealing time,the hardness of the whole welded joints has a tendency to decrease,and the hardness value in the thermomechanically affected zone decreases greatly. |
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