Microstructure And Mechanical Properties Of AZ91 Magnesium Alloy Were Studied By Sm And Ca

The increase on the serious air pollution makes people be concerned about the exhaust caused by automobile.Therefore,reducing the vehicle’s weights and fuel emissions are the trends of automobile development.AZ91 magnesium alloy has the characteristics of light weight and good shock absorption performance,which has attracted the attention of various automotive industries.Gearbox as an important structure of automobile,the working environment temperature is generally around 120℃to 150℃,The mechanical properties of magnesium alloys are poor at high temperature,so it is important to develop a new type of magnesium alloy with high performance and low cost.In this paper,the effect of AZ91 was investigated by adding a single rare earth element Sm and compound and Sm and Ca element.By means of electronic tensile testing machine,optical microscope analysis,X-ray diffraction analysis and scanning electron microscope analysis,the changes of microstructure and mechanical properties of alloy at room temperature and high temperature（150℃,175℃）under different composition were studied.At the same time,the evolution of alloy microstructure and mechanical properties after ultrasonic treatment is studied.The results show that in the AZ91-xSm（x=0,1,2,3）alloy,no rare earth Sm element is added,andβ-Mg₁₇Al₁₂2 phase is distributed in the coarse and continuous network in the alloy structure.With the change of Sm content,the alloy The grain size decreases,and theβ-Mg₁₇Al₁₂phase in the structure gradually changes from a continuous network to a discontinuous rod-shaped and dispersed particle-like structure.The structure also precipitates a bright white Al₂Sm phase with a good stability and gradually increases.When the content ofβ-Mg₁₇Al₁₂phase was 2%,the mechanical properties at room temperature and high tempreture are best,tensile strength is 215MPa（room temperature）,184.2 MPa（150℃）,159MPa（175℃）,and the elongation is 4.9%（room temperature）,7.4%（150℃）,11%（175℃）,hardness is 80.9 HB.When the Sm content is more than 2%,the Al₂Sm phase in the structure aggregates,which weakens the mechanical properties of the alloy.The room temperature fracture of AZ91-xSm（x=0,1,2,3）alloy is brittle fracture.With the increase of Sm content,the cleavage surface becomes smaller and smaller.At 150°C and 175°C,the fracture is in the dimple The number of the alloy increases and the depth becomes deeper.The toughness of the alloy becomes better.As the Sm content increases,the fracture structure refines and a small amount of intergranular fracture occurs.After the addition of Sm and Ca elements,the bright white Al₂Sm phase precipitated in the alloy structure and the Al₂Ca phase mainly distributed along theβ-Mg₁₇Al₁₂2 phase.The Al₂Ca phase has high temperature stability and improves the melting point and stability of theβ-Mg₁₇Al₁₂2 phase.When the alloy composition is AZ91-2Sm-1.6Ca,the microstructure and mechanical properties of the alloy are the best.and the strength was 224.5 MPa（room temperature）,189 MPa（150℃）,163.23 MPa（175℃）,elongation is 5.34%（room temperature）,9.02%（150℃）,13.5%（175℃）,hardness is 85.57 HB.Theβ-Mg₁₇Al₁₂2 phase in the alloy structure is uniformly distributed in the fractured rod and granular form,and the Al₂Ca phase andβ-Mg₁₇Al₁₂2 phase distribution.The state is consistent,and the Al₂Sm phase is distributed in a small block in the matrix.Ultrasonic treatment of AZ91-xSm-1.6Ca at 600W,1200W and 1800W showed that the mechanical properties of AZ91-2Sm-1.6Ca alloy were the best at 1200W,and the strength was235 MPa（room temperature）,194 MPa（150℃）,170.3 MPa（175℃）,elongation is 5.7%（room temperature）,9.7%（150℃）,13.9%（175℃）,hardness is 95 HB,the alloy of this composition has small crystallites,Theβ-Mg₁₇A1₁₂2 phase in the tissue is fine,the distribution of Al₂Ca phase is dispersed,and the Al₂Sm is distributed on the matrix in small blocks and particles.he fracture profile of the room temperature of AZ91-2Sm-1.6Ca sample is relatively flat,the fracture mainly occurs in the intergranular,and there is a small amount of platform in the fracture at 150°C.At 175°C,the fracture profile is relatively tortuous,mainly along the crystal fracture.In the local structure,theβ-Mg₁₇Al₁₂2 phase is cracked,and the Al₂Sm phase is prone to transgranular fracture.The number of dimples in the fracture increases and the depth becomes deeper.,ductile fracture is obvious.