Research On The Effect Of Annealing Treatment On The Microstructure And Mechanical Properties Of AZ31B Magnesium Alloy

AZ31B magnesium alloy has many advantages such as low density,high specific strength,large elastic modulus,better thermal conductivity,excellent machining performance,etc.,and is widely used in many fields such as automobiles and military industries.However,due to the large amount of deformed structure in the structure of the AZ31 B magnesium alloy after plastic deformation,the uniformity of the structure is poor,which affects the comprehensive mechanical properties of the alloy.Annealing treatment can refine grains and produce static recrystallization,eliminate or reduce defects such as residual stress,uneven composition,and unstable structure in the material,thereby improving the mechanical properties of magnesium alloys.In this thesis,the cold-rolled AZ31 B magnesium alloy sheet has been fully annealed,fast-annealed,and fully annealed after pre-stretching,and combined with metallographic microscope,scanning electron microscope characterization and uniaxial tensile experiments,the cold-rolled state and differences are analyzed The microstructure and mechanical properties of AZ31 B magnesium alloy after annealing.The main research contents and results are as follows:1.The cold-rolled AZ31 B magnesium alloy was fully annealed at 150°C,300°C,and 450°C.The microstructure of the cold-rolled and annealed magnesium alloys was observed with a metallurgical microscope,and measured by uniaxial tensile tests.Mechanical properties.the result shows:(1)After annealing at 150°C,only part of the area was recrystallized.After annealing at 300°C,the uniformity of the grain size is improved,and after annealing at450°C,the crystal grains grow abnormally in a short time.The extension of the annealing time slightly improves the uniformity of the structure but the average grain size is larger.(2)Annealing at 150°C can improve the strength of magnesium alloys,and annealing at 300°C can improve the plasticity of magnesium alloys.The annealing temperature has a significant effect on the mechanical properties of the cold-rolled alloy sheet.(3)Compared with the initial state of the sample,the fracture of the sample after annealing at 150? has a significant increase in the number of dimples and a deeper depth.However,there are a small amount of tearing edges and river-like patterns in the fractures of the samples after annealing at 300°C and 450°C.2.The cold-rolled AZ31 B magnesium alloy was subjected to rapid annealing treatments at 450?,475?,and 500? for 0.5min and 3min respectively,and then the microstructures of the cold-rolled and fast-annealed magnesium alloys were observed with a metallurgical microscope.The uniaxial tensile test measures the mechanical properties.the result shows:(1)After rapid annealing at different temperatures and times,the microstructure of the sample is composed of equiaxed grains.The rapid annealing for 0.5min can make the static recrystallization of the material completely occur.As the time is extended to3 min,the crystal grains grow selectively,and the abnormal growth of the crystal grains occurs.(2)Rapid annealing for 0.5min at different temperatures can simultaneously increase the yield strength,tensile strength and elongation of the alloy.After rapid annealing for 3 minutes,the elongation decreased significantly with the increase of temperature,but the tensile strength and yield strength were still improved compared with the rolled alloy samples.(3)After rapid annealing at different temperatures for 0.5 min,the fracture surface of the magnesium alloy has the characteristics of intergranular ductile fracture.As the annealing time increases to 3 min,the fracture surface shows more equiaxed and elliptical pits,indicating The cracks nucleate at the larger grain boundaries and then merge and grow.3.Pre-stretching the cold-rolled AZ31 B magnesium alloy along the rolling direction with a strain amount of 3%,6%,and 9%,followed by annealing at 200°C for4 h and 350°C for 2h,and then using a metallurgical microscope and The electron backscatter diffraction technique was used to observe the microstructure,and the mechanical properties were measured by uniaxial tensile test.the result shows:(1)After the pre-stretched alloy is annealed at 200°C,the recrystallized size in the microstructure is smaller.At this time,the dominant factors for recrystallization are substrate slip and twins.As the annealing temperature increases to 350°C,a large amount of stored energy inside the alloy grains is released,resulting in a large number of abnormal growth of grains during the recrystallization process,and with the increase of strain,the grain size continues to increase.(2)Annealing at 200°C can significantly improve the plasticity of the pre-stretched alloy while retaining the high strength of the pre-stretched alloy.However,after annealing at 350°C,the increase in annealing temperature caused a large amount of energy stored in the pre-stretched alloy to be released,and part of the existing twins was consumed,resulting in a decrease in the strength of the alloy.(3)After the pre-stretched alloy sample is annealed at 200°C,there is no ellipsoidal precipitate in the fracture,and the sliding resistance to the alloy grain boundary is small.After annealing at 350°C,the tough ellipsoidal precipitates in the fracture also increased.The precipitates increased the resistance to dislocations in the alloy,which could lead to more crack sources.