Ultrasonic-assisted Vibration Micro-extrusion Deformation And Finite Element Simulation Analysis Of ZK60 Magnesium Alloy

As we know,magnesium alloy is the lightest metal structure material.However,the magnesium alloy is a packed hexagonal lattice structure.Less independent sliding at room temperature leads to difficulty in plastic deformation.Therefore,exploring new magnesium alloy microforming processes is important for promoting its use in automotive materials and other fields.Due to the conventional micro-forming process faced with many technical problems,a new process was proposed that the ultrasonic vibration was applied to micro bulk forming in this thesis.However,the theory about ultrasonic vibration assisted metal plastic deformation is not mature at present.And ultrasonic vibration was rarely applied to micro-forming field in current research.In this thesis,a series of ultrasonic assisted extrusion microforming experiments on ZK60 magnesium alloy at room temperature was made and the corresponding finite element was analyzed.Meanwhile,the corresponding tests and analysis were conducted for the microstructure and the mechanical properties of micro-forming parts.The experimental results shows that the grain size was basically the same after the traditional extrude for ZK60 magnesium alloy at room temperature,but a large number of deformation twins could be observed inside the grains.When the amplitude of the ultrasonic was 33 ?m,some fined dynamic recrystallization grain appeared at the center of the sample;so the deformation stress was significantly reduced compared with the conventional extrusion.When the amplitude was 39 ?m and 42 ?m,the dynamic recrystallization completely occurred.The grain size was refined from the original 192 ?m to 13 ?m,deformation stress was reduced,and the forming ability was improved.The maximum stress reduction of magnesium alloy under ultrasonic assisted vibration extrusion was about 80%.According to surface principle,when the extrusion diameter was 0.3 mm or 0.5 mm(the original grain size is 0.192mm),the deformation cavity contained an average of 1 to 3 grains,which means that all the grains are on the free surface.Therefore,the deformation stress at different amplitudes was almost the same.But when the extrusion diameter was 0.7mm,a part of the grain was in the internal.The internal grain was constrained so as to cause the stress reduction smaller.At the time,the size effect has occurred.With the increase of ultrasonic-assisted vibration amplitude,the hardness of ZK60 magnesium alloy increased.The dynamic recrystallization occured completely when the amplitude reached 39 ?m.The hardness reached the maximum.But the grain has grown when the amplitude is 42 ?m,so the hardness was slightly reduced.Through the finite element simulation,the stress distribution of the ultrasonic assisted micro-extrusion was improved.The large stress area was significantly reduced,and the large stress was mainly distributed near the extrusion hole.Plastic deformation occured on the surface of specimens under the conventional extrusion.However,under the ultrasonic assisted extrusion,the plastic deformation mainly occured in the extrusion part.The larger the amplitude,the greater the maximum plastic strain.In a word,there was great significance to improve the formability,mechanical properties of micro-forming parts that ultrasonic vibration was applied to micro bulk forming on ZK60 magnesium alloy.