温态超声冲击强化对AA6061-T6空蚀行为的影响

Effect of Thermal Ultrasonic Impact Treatment on Cavitation Erosion Behavior of AA6061-T6

目的进一步提高超声冲击处理对AA6061-T6抗空蚀性能的强化效果。方法通过不同温度(RT、50、100和150℃)温态超声冲击强化处理,结合冲击强化与动态应变时效(DSA)的优点,获得应变速率更高、密度位错更高的塑性变形强化层,以提高其抗空蚀性能。利用X射线衍射技术(XRD)、金相显微镜研究了强化层的微观组织。利用显微硬度仪研究了强化层深度方向的硬度分布。利用超声振动空蚀平台、扫描电子显微镜(SEM)和激光共聚焦显微镜,研究了强化试样的空蚀性能及空蚀机理。结果随温度的升高,强化后的铝合金近表层分别形成30、50、70、90μm的晶粒细化层,表面择优取向由(200)转变为(111);强化温度为50℃时的表面显微硬度最高,较原始试样提高了108.2%;经300 min空化腐蚀后,常温、50、100、150℃条件下处理的超声冲击试样的抗空蚀性能分别是未处理试样的1.77、2.03、1.49和1.38倍;温态冲击强化后,试样的空化腐蚀损伤机制不仅包括初始的韧性断裂破坏,还增加了脆性及疲劳损伤形式。结论经过温态超声冲击强化处理后,AA6061-T6的抗空蚀性能随温度的升高呈先增后减的趋势,强化温度为50℃时最优。强化试样的抗空蚀性能提高归因于表面硬度增加、晶粒细化和择优取向转变等多种因素的综合作用。

Aluminum alloys have been widely used in the automotive industry because of their good formability and high specific strength.However,due to the harsh working environment,cavitation corrosion is prone to occur,which results in the part failure.In order to improve its cavitation resistance,surface impact strengthening treatments are often used.But these treatments usually rely on a single surface strengthening technology,by which the enhancement of its cavitation resistance is limited.Current researches show that the thermal-mechanical coupled strengthening technology using temperature-assisted surface impact can further improve the performance of the materials.It combines the advantages of impact strengthening and dynamic strain aging,leading to the higher strain rate and dislocation density.Therefore,in this paper,the method of temperature combined ultrasonic impact treatment(UIT)is used to further improve the cavitation resistance of AA6061-T6.The samples were subjected to ultrasonic impact strengthening treatment at different temperatures(RT,50℃,100℃and 150℃).The depth and microstructure of the strengthening layer were studied by metallographic microscope;The surface orientation after impact strengthening was studied by X-ray diffraction(XRD);The hardness distribution in depth direction of the reinforced layer was studied by a microhardness instrument;The cavitation resistance performance was tested by the ultrasonic vibration cavitation platform,and the cavitation corrosion property and mechanism of the strengthened samples were studied by observing the cavitation surface using scanning electron microscope(SEM).The results show that with the increase of temperature,the yield strength of the aluminum alloy surface decreases,and the work hardening effect of the aluminum alloy surface weakens during the strengthening process,so that the plastic deformation can further develop along the depth direction.Therefore,fine grain layers of 30μm,50μm,70μm and 90μm depth are formed beneath the surface of the aluminum alloy,after being treated at RT,50℃,100℃and 150℃,respectively.After UIT,the surface preferred orientation peak of the sample changes from(200)to(111).The surface hardness increases first and then decreases with the increase of coupled temperature(RT-150℃),which are 168.5HV,193.7HV,164.1HV and 134.1HV respectively.The surface microhardness at the strengthening temperature of 50℃is the highest,which is 14.9%higher than that of the sample treated at RT and 108.2%higher than that of the original sample.This phenomenon results from the dynamic strain aging phenomenon occurring under the compound of temperature field,which improves the deformation resistance of the material,However,excessively high temperature will lead to dynamic recovery of internal structure,which offsets part of the cold work hardening effect and causes the decrease of hardness.After cavitation corrosion with 300 min,the cavitation corrosion resistance of the ultrasonic impact samples treated at RT,50℃,100℃and 150℃is 1.77,2.03,1.49 and 1.38 times higher than that of untreated samples,respectively.The variation trend of cavitation corrosion resistance is consistent with that of surface hardness.After thermal ultrasonic impact treatment,the cavitation corrosion damage mechanism of the samples includes not only the initial ductile fracture failure,but also the brittleness and fatigue damage.The improved cavitation resistance of the reinforced specimens is attributed to the combined effect of the increased surface hardness,grain refinement,and preferential orientation transition.