Dualscaled-dualphase Structure And Wear Performance Of Al-Sn Bearing Alloy

Along with the development of plain bearing to higher load-carrying capacity, bettertribological properties and environmentally friendly (Pb-free), there are continuousdemands for developing new Al-Sn based bearing alloys combining with excellentmechanical and wear performances. Refining microstructure and high strength of nanophasecomposite Al-Sn bearing alloys are beneficial for their tribological properties. However, astrength-ductility trade-off presenting in nanocrystallined alloy restricts its good wearperformance and engineering applicationin Al-Sn bearing alloy. It was demonstrated thatdual-phase and dual-scale structured alloys obtain a simultaneously enhanced strength andductility. In this dissertation, toughened high-strength nanostructured Al-Sn by creatingdual-scale structure provides a bran-new idea for developing new bearing alloys withexcellent wear performance.Firstly, a dual-phase and dual-scale Al-12wt%Sn alloy consisting of mixtures ofnanocrystalline/ultra-fine grain (NC/UFG) and coarse grain (CG) was produced by acombination of mechanical alloying and conventional powder sintering, and the correlationbetween dual-scale microstructure and mechanical properties was studied. The results showthat the milled NC Al-12wt%Snpowders were mixed with ummilled CG Al-12wt%Snpowders at a certain ratio, and then the powder mixtures were consolidated and sintered. Thefinal bulk alloys comprise of UFG (hard) and discrete CG (soft) regions, and simultaneouslyinclude Al (hard) and Sn (soft) phases in the two above regions, which forme two pairs ofcross-coupling“hard-soft” phases and develop an ideal microstructure of new bearingmaterials. Nano-indentation, micro-hardness and compressive strength are used tocharacterize mechanical properties in accordance to the heterogeneous feature of dual-scalealloys. It was found that the hardness and compressive strength of Al-12wt%Sn decreasegradually with the increasing CG content, but the ductility in opposite. The hardness, strengthand ductility of the dual-scale structured Al-Sn alloys can be adjusted by controlling theUFG/CG ratios. A simultaneously enhanced strength and ductility is achieved at a ratio of Xwt%CG（20≤X≤60）.On this basis, we further to study wear properties of dual-phase anddual-scale structured Al-Sn, Al-Sn-Mg and Al-Sn-Si alloys. Secondly, for the first time, sliding wear properties and wear mechanism of dual-phaseand dual-scale Al-12wt%Sn alloy have been explored, and monolithic UFG and CGAl-12wt%Sn alloys were also investigated for comparison. The results show that the bestwear performance is achieved at a value of HV56hardness for the dual-phase and dual-scaleAl–Sn alloy, which has a wear resistance about1.5times greater than that of the monolithicUFG alloy, about twice that of the monolithic CG alloy. The friction coefficient decreases bynearly13%of the monolithic CG alloy. It also has been found that a dynamic steadytribolayer consisting of fine crystalline oxides plays a dominant role in improving the wearproperties of both the UFG and dual-scale alloys. For the CG alloys, poor wear properties,caused by delamination wear ofthe tribolayer, could not be maintained on the worn surface.However, the damage of the dynamic steady tribolayer is governed by the matching betweenductilityand strength of the matrix.The low ductility of the UFG alloy substrate causes thetribolayer to suffer crack damage rather easily, which limits any further improvement of itswear resistance. In contrast, the dual-scale structured alloy has excellent ductility-strengthmatching in accordance to UFG/CG ratios, and therefore a dynamic stable tribolayer caneasily be maintained on the worn surface, leading to excellent wear performance.Thediscrepant structure of dual-phase and dual-scale also provides a novel approach to controlthe wear properties of Al-Sn alloys.Furthermore, in order to overcome the low strength of sintering and the reducinganti-friction by introducing of CG part, we attempted to further promote microstructure,mechanical and wear properties of the dual-phase and dual-scale structured Al–12wt%Snalloys by Mg and Sn-Si composite structure addition. Mg addition can disrupt the oxide filmof Al through the formation of MgAl2O4, which increases of wet-ability and reduces divorcedeutectics between Al and Sn, and simultaneously improves sintering density and strength. Mgaddition also enhances the transportation and growth of Sn nanophases, includingnaoparticles and nanowires, towards sliding worn and nanoscratch surfaces, which effectivelyreduces the friction coefficient due to their lower shear strength. Simultaneously, both theenhanced strength and heating reaction of Mg addition have an obvious improvement in theformation and features of tribolayer, and cause further promotion of wear resistance. Inaddition, employing milled Sn-12wt%Si to replace CG Sn in dual-phase and dual-scale Al-Sn alloy achieves a simultaneously enhanced microstructure and hardness. In particular, A largeresidual stresses in hard Si particles induced by ball milling act as the drive forces, which areimposed on the Sn phase. It leads to large amounts of Sn nanophases in the worn surface andobtains a significant improvement of wear properties. Therefore, the theories of the presenceof Sn nanowires or whiskers in the worn surface for both Mg and Sn-Si composite structurecontaining alloys were also investigated for comparison.Finally, in order to achieve the engineering application of dual-phase and dual-scaleAl-Sn alloy in sliding bearing. The rolling bonding between the dual-phase and dual-scalestructured Al-12wt%Sn-(Mg) powders and steel with spraying Al coating also has beeninvestigated. After researching the effect of rolling deformation, annealing and sinteringtechnology on microstructure and properties, we obtain an optimum rolling technology.Finally, a homogeneous dual-phase and dual-scale structure in Al-Sn-Mg alloy layer wasprepared. The interfaces of Al-Sn-Mg layer/spraying Al layer/steel can achieve a closebonding. And the rolling strip was also successfully bended to be bearing. It establishes afoundation for the application of dual-phase and dual-scale Al-Sn alloy.