Study On The Fabrication And Tribological Properties Of Al-Bi/In Monotectic Bearing Alloys

Al based bearing alloys,due to their good bearing capacity,compatibility,corrosion resistance and other characteristics,currently are commonly used bearing materials for passenger vehicles.As soft metals,Pb and Sn play an excellent role in solid self-lubrication and wear reduction in aluminum alloys,and are common alloy elements in Al based bearing alloys at present.However,as Pb is toxic,it gradually becomes an acceptable alloy element.Therefore,more attention has been paid to the development and research of lead-free bearing alloys.To explore the preparation process of Al based lead-free bearing alloys,in this paper,Al based bearing alloys containing soft metals Bi and In were prepared and the following major studies were conducted:the effects of Bi and Pb on the microstructure,mechanical and tribological properties of Al-Sn based alloys were compared to explore the possibility of replacing Pb with Bi;based on Sn,Si and Cu alloy elements,low alloy design was adopted for Al-Bi monotectic alloys,and the synergistic effect of low alloy design on alloy microstructure mechanical and tribological properties was studied;the ultrasonic assisted preparation process of homogeneous Al-In monotectic alloys was explored,and the tribological properties were studied.Through comparative study of Bi and Pb on the microstructure,mechanical properties and friction and wear properties of Al-Sn based bearing alloy.It was found that the addition of Bi and Pb can obviously promote the transformation of Sn-rich soft phase morphology of Al₈₂Sn₁₅Si_2.2Cu_0.8 alloy from network to rod,and even the appearance of Bi-rich or Pb-rich spherical phase.Nano-indentation test shows that the addition of Bi increases the hardness of Sn phase in Al₈₂Sn₁₅Si_2.2Cu_0.8 alloy.The addition of Pb has no obviously effect on the hardness of soft phase,but it promotes the appearance of a Pb-rich spherical phase with lower hardness in the alloy.Bi and Pb changed the hardness of the soft phase in the Al₈₂Sn₁₅Si_2.2Cu_0.8 alloy while not significantly affect the macro hardness of alloy and the hardness of Al matrix.Friction and wear studies show that the addition of Bi and Pb at different sliding velocity could increase the wear amount of Al₈₂Sn₁₅Si_2.2Cu_0.8 alloy.This is because the addition of Bi and Pb could decrease the melting temperature of low-melting liquid phase about Al₈₂Sn₁₅Si_2.2Cu_0.8 alloy.The soft phase of the friction surface is easy to liquefy by the friction heat,which will aggravate the wear of the alloy during friction process.However,the liquefied soft phase on the friction surface can play the role of lubrication.Therefore,the addition of Bi and Pb could reduce the friction coefficient of Al₈₂Sn₁₅Si_2.2Cu_0.8 alloy at lower sliding velocity.The low alloying design of Al₉₂Bi₈ monotectic alloy is studied based on Sn,Si and Cu elements.The results show that Sn has refining effect on the Bi-rich phase of Al₉₂Bi₈alloy by narrowing the miscibility gap and decreasing the interfacial tension between the liquid Al-rich and Bi-rich phases,while Cu and Si play an opposite role in coarsening the Bi-rich phase.Nano-indentation test shows that Sn,Si and Cu have no obvious effect on the hardness of Bi-rich phase in Al₉₂Bi₈ alloy,but improved the mechanical properties of Al₉₂Bi₈ alloy in the follow order:Si>Cu>Sn.Sn phase forms and distributes in the Bi-rich phase through the ternary eutectic reaction.Si phase distributes in the Al matrix through the ternary monotectic reaction,and the mechanical properties of the alloy are improved by dispersion strengthening.The dispersive?Al₂Cu and the solid solution strengthening of Cu are responsible for the improvement of mechanical properties of the alloy.Friction and wear studies show that the addition of Sn,Si and Cu has no influences on the friction coefficient of Al₉₂Bi₈ alloy.This is due to the friction coefficient of the alloy is mainly determined by the oxidative wear during the dry friction experiment,and the influence of the alloy composition on the friction coefficient is not obvious.But the addition of elements can promote the wear resistance of alloy in accordance to the sequence of their strengthening and hardening effects.Through the study on the ultrasonic assisted preparation of Al-In monotectic alloys,it was found that ultrasonic cavitation acts on the two-phase interface of Al-In melt,causing the dispersion of In droplets in Al liquid phase.In addition,the acoustic streaming of ultrasonic waves can suppress the sedimentation of In droplets in the Al liquid phase,thus ensuring uniform distribution and large sized of In phase in the solidification structure.The Al-In monotectic alloys prepared by ultrasonic assisted have higher volume fraction of In phase compared with Al-In monotectic alloys prepared without ultrasonic waves.Friction and wear studies show that ultrasonic assisted preparation has no obvious effect on the wear volume and friction coefficient of the Al-In alloy.