| With the improvement of the strengthening degree of the internal combustion engine,the piston works in an increasingly harsh environment,the thermal load of the piston has also increased significantly.To improve the strength and reliability of the piston,aluminum matrix alloys reinforced by ceramic phases has been widely used in the manufacture of pintions.As a potential kind of composites for piston,aluminum borate whiskers(ABOw)reinforced Al-matrix composites presents high specific strength,good abrasion resistance,excellent high-temperature stability and fairly low price.To maximize the benefits and facilitate the fabrication process,only the critical parts,such as the piston throat,have been local enhancedby composites during piston manufacturing.There is a bond zone between the reinforced composites and unreinforced Al-12Si alloys in the local enhanced piston.Because of the very different material properties between the composites and aluminum,the bond zone becomes a potential failure position of piston due to the stress concertration caused by uncoordinated deformation.In the present study,the bond zone between the aluminum borate whiskers reinforced Al-matrix composites and Al-12Si alloys of local reinforced specimens were tested at elevated temperatures.The effects of temperatures on microstructure evolutions,tensile properties,fatigue behaviours and fracture mechanism of the bond zone were investigated.Fatigue life prediction model for the bond zone was established based on the theory of critical plane model and was used to evaluate the fatigue life of the local reinforced pistion.The main content and conclusions of the research are as follows:(1)The effects of temperatures(25,200 and 350 ~oC)on microstructure evolutions,tensile properties and fracture mechanism of the bond zone were investigated.The results showed that the infiltration in the composites is quite well with fine texture,there are no inclusions or defectsin the bond zone.The tensile results indicated that the bonding state of the bond zone was poor at relative low test temperatures while was improved as temperature rising up to 350°C.The static tensile tests at various test temperaturesfollow an obvious regularity that the specimens fracture exactly on the bonding face at room 25 and200°C,while in the unreinforced aluminum alloys at 350°C.(2)The effects of temperatures(25,200 and 350 ~oC)on fatigue behaviours and fracture mechanism of the bond zone were investigated.The results showed that the specimen presented continuous hardening or secondary hardening behavior at room temperature,while showed cyclic softening behavior at 350 ~oC.The specimens all fractured exactly on the bonding face at different temperatures.The crack nucleates preferentially at the cluster of metallic compounds and whiskers at 25 and 200 ~oC.At 350 ~oC,the cracks were initiated near the bulk Si phases and the transition zone formed by precipitates had a delaying effect on the crack propagation,which was beneficial to the improvement of fatigue life.(3)According to the fatigue fracture behaviours,combining with the simulation methods,considering the effects on modulus and deformation compatibility caused by the strengthening mechanism of the bonding face,the fatigue life prediction model for the bond zone were established based on the theory of critical plane model.The model could predict the life of the bond zone accurately by a comparative analysis.(4)The thermal and mechanical boundary conditions were obtained by perfaomance simulation of engine and numerical analysis.The distributions of temperature and stress situation were obtained by the method of finite element simulation.The fatigue life prediction model was used to evaluate the reliability of the bond zone in the local reinforced pistons.The results showed that the reliability satisfied the requirement of reinforced piston. |
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