Mechanical Properties Of Friction Stir Processed Mg-AZ31 Based Composites With Al₂O₃ Particles And Ti Particles

Particles reinforced metal matrix composite(MMC) have been widely used as structure material in aerospace,automobile and other industries,since its excellent strength at high temperatures,good wear resistance,high specific modulus and easy processing.Mg based composites with～35%lower density compared with Al based composites have exhibited the promise as structural materials in engineering applications which require the high special mechanical properties,especially,in the aerospace and automobile industries.However,its application is limited by the problems of fabricating technology.Friction stir welding(FSW) is a new solid phase joining method,which is invented by The Welding Institute (TWI).Compared with the traditional welding method,FSW has many advantages and it has been applied to many industrial fields.Recently,its modification into friction stir processing (FSP)is also applied to modify the microstructure by localized grain size refinements and homogenization of precipitate particles in aluminum and magnesium alloys.The research of the application of FSW is still a significant subject nowadays.In the present dissertation,pure Ti and Al₂O₃ particles reinforced AZ31 based composites were fabricated by friction stir processing(FSP).The stir tool includes two parts: the probe is made of GT35 steel-bonded carbide alloy and the base is made of 45 steel.In the, The parameters in the present friction stir processing(FSP) were selected as following: advancing speed of 45mm/min,rotation speed of 800round/min,stir tool inclination of 1.5°.The mechanical responses of the resulting composites in terms of microstructure and tensile properties of these AZ31/Al₂O₃ and AZ31/Ti composites were examined and compared.The grain refinements and the corresponding hardening mechanisms are also analyzed and discussed.The results show that the composite layer with Al₂O₃ particles and 40vol%Ti particles have higher tensile properties,the composite layer with 5vol%Al₂O₃ particles(measuring 15nm in average) has highest tensile properties(about 220MPa) and good elongation(about 13%) in room temperature,Yield strength with temperature rising was declined.Because of FSP treatment and dynamic recrystalliza-tion(DRX) process the matrix structure of composite layers can be significally refined within 3～5μm obviously,the clusters of the Al₂O₃ particles are about 120～500nm and the fragments of the Ti particles are about 200 nm.However,significant chemical reactions occurred at the Mg/Al₂O₃ interface to form the MgAl₂O₄ phase.The mechanical responses of the resulting composites in terms of hardness and tensile properties of these AZ31/Al₂O₃ and AZ31/Ti composites were examined and compared.Using uniaxial compression iso-strain modle and iso-stress model to calculate the microhardness values of composite layers and the results approximately match the experimental ones.