原位纳米颗粒增强AA6016基复合材料超声空化强化后微观组织及性能的变化

Variation of Microstructure and Property of In-situ Nanoparticle-reinforced AA6016 Matrix Composite after Ultrasonic Cavitation Strengthening

目的探索铝基复合材料的新型表面强化方法。方法采用超声空化的方法对原位纳米颗粒增强AA6016铝基复合材料进行强化,使用电子天平、激光共聚焦显微镜、场发射扫描电子显微镜、显微硬度计、X射线衍射仪以及透射电镜对材料质量损失、表面形貌、残余应力、显微硬度、微观组织等方面进行系统地分析。结果试样的质量损失和表面粗糙度随空化时间的延长而增加,在超声空化处理30 s后,试样的表面硬度和残余压应力较原样分别提高了89.8%和57.7%,材料内部发生位错增殖,位错相互缠结,并且晶粒排列的取向差增大;当空化时间达到60 s时,显现的晶界数量增加,在晶界处出现材料剥落现象,残余应力被释放。结论在一定的时间范围内,超声空化可以较为明显地提高材料的表面性能。在空化泡溃灭产生的多向力作用下,铝基复合材料表面晶粒内会迅速产生大量位错,形成加工硬化层。位错缠结和增强颗粒的钉扎作用,促使晶粒内部亚晶界的形成,最终导致晶粒细化。

As a new composite material,in-situ nanoparticle-reinforced AA6016 aluminum matrix composite,has been developed to fulfill the requirements for the manufacturing of aerospace and automotive equipment.Although with high fatigue strength and resistance to external impact,such a material has shortcoming in terms of enduring alternating loads in corresponding applications.Therefore,it is necessary to devise a surface strengthening method to handle this issue.Strengthening through surface deformation serves as one of the commonly used surface modification methods,which can be implemented through pulsed waterjet,laser shock peening and mechanical shot peening.Each of the three methods has its own limitations.Ultrasonic cavitation strengthening is a promising,environmentally friendly surface strengthening method that can significantly improve the surface hardness of the material and produce a layer characterized by a certain depth and residual compressive stress.The work aims to improve the surface properties of in-situ nanoparticle-reinforced AA6016 aluminum matrix composite.The samples were processed into cylinders with a diameter of 185 mm,which were then polished with diamond polishing agent after polishing by sandpaper.An experimental work on these samples was performed with an ultrasonic cavitation experimental device.A constant ultrasonic frequency of 20 kHz was set throughout the experiment.The diameter of the lower end of the ultrasonic tool head was set to(15.9±0.05)mm,the submerged depth 20 mm,and the standoff distance 0.8 mm.The liquid medium of pure water was adopted and its temperature was remained at(25±1)℃through the thermostat method.The mass loss of the samples before and after the cavitation experiment was measured with an EX225DZH electronic analytical balance with an accuracy of 0.1 mg.Three-dimensional morphology of the sample surface was observed by an OlympusVN-4100 laser confocal microscope,and the surface roughness was calculated based on the recorded data.An FEI Nova NanoSEM 450 scanning electron microscope was used to analyze the surface morphology of the samples.The residual stress on the material surface was measured by an X-350a X-ray stress tester.The Vickers hardness was measured by a KB30S-FA automatic microhardness tester.The SmartLab X-ray diffractometer was used to observe the X-ray diffraction(XRD)patterns on surface of the samples.The electron backscattering diffraction(EBSD)technique was used to investigate variation in orientation of grains after the cavitation experiment,which was based on a GeminiSEM 300 field emission scanning electron microscope.Surface microstructure was observed with a JEM-2100PLUS transmission electron microscope.The results showed that mass loss and surface roughness of the tested samples increased as the cavitation time was extended.More specifically,the surface hardness and the residual compressive stress increased by 89.8%and 57.7%,respectively,after 30 s of ultrasonic cavitation treatment.After cavitation of 60 s,the number of recognizable grain boundaries increased remarkably relative to those monitored at previous moments.Ultrasonic cavitation can effectively improve surface property of in-situ nanoparticle-reinforced AA6016 aluminum matrix composite with appropriate processing parameters.Improvement of surface property is closely related to dislocation proliferation and dislocation entanglement.As cavitation bubbles collapse near the surface of tested samples,multi-directional force is exerted on the surface.Consequently,a large number of dislocations are introduced into the material.Due to movement of the dislocations,mutual entanglement and pinning effect of reinforcing particles cause the formation of dislocation cells,and eventually the formation of sub-grain boundaries,resulting in grain refinement.This essentially explains the increase in surface hardness.When plastic deformation of the surface attains a certain degree,cracks expand along grain boundaries,and the residual compressive stress is released.