摘要
采用多道次搅拌摩擦加工的方法制备了高体积分数Cu颗粒增强7075铝合金的表面复合材料,利用电子背散射衍射(EBSD)、X射线衍射(XRD)、背散射电子成像和硬度测试等技术,结合蒙特卡洛模拟的方法,研究了复合材料制备过程中的流动行为及增强体分布规律。研究结果表明,道次数的增加能够细化Cu颗粒尺寸以及改善颗粒分布,同时有效促进原位反应的进行,形成Cu基固溶体和Al2Cu颗粒的双重增强体系。但是由于有效界面反应面积的减少,原位反应的速率在不断减缓并逐渐平稳,说明在采用多道次搅拌摩擦加工制备高体积分数原位复合材料时,很难实现增强体的完全溶解。通过分析多道次加工过程中Cu颗粒的位置演变和材料的块状流动规律,建立了流动模型。此外,为了定量研究Cu颗粒在Al基复合材料中的分布情况,采用一种改进的最近邻指数从试验和蒙特卡洛模拟两个角度对Cu颗粒分布的均匀性进行了分析,结果显示至少需要5个道次才能确保增强体的均匀性。
Al alloy surface metal matrix composites(SMMCs)are widely utilized as favorable structural materials in many industrial fields.The SMMCs on 7075 aluminum alloy reinforced by a high-volume fraction of Cu particles were prepared via multi-pass friction stir processing(FSP)in this work.The microstructure and microhardness of the SMMCs were investigated to reveal in-situ reaction and flow behavior.Besides,a material flow pattern during multi-pass FSP based on Cu particles distribution and material bulk flow direction were established.To determine a suitable processing pass,we introduced the nearest neighbor index(NNI)value which was based on microhardness distribution and metallography images to further quantify the distribution of reinforcement particles.This quantified parameter could be used as a response of the artificial neural network(ANN)system to optimize the sensitivity of the trained system to the reinforcement distribution to help achieve a more even distribution in subsequent experiments.First,the macroscopic metallographic structure of samples with different passes was observed to determine the evolution of SMMCs structure.Furthermore,the microstructures of different deformed regions were characterized by electron backscatter diffraction(EBSD),which revealed the grain’s orientation characteristics and bulk flow.Besides,X-ray diffraction(XRD)and energy dispersive spectrum(EDS)wereused to analyze the influence of processing passes on the Al-Cu in-situ reaction.The kinetic models of chemical reaction were used to quantitatively calculate the degree of in-situ reaction.A microhardness test was carried out on the MH-3 type digital microhardness tester.The hardness values of the whole cross section were measured with a lateral spacing of 0.5 mm and a longitudinal spacing of 1 mm.Meanwhile,Monte Carlo simulation was used to reconstruct the particle distribution.Through the observation of the microstructure,it could be found that the copper particles segregate seriously in the early stage of processing.As the number of passes increases,the uniformity increases significantly.The cross-section could be divided into three parts according to different driving force modes including shoulder affect zone(SAZ),pin affect zone(PAZ)and bottom affect zone(BAZ),respectively.Combining the bulk flow of grains with different deformed structures and the migration behavior of copper particles,it was found that the advanced side(AS)grains showed a downward trend,while the retreating side(RS)grains showed an upward trend.The average grain size of the weld zone is the only0.91μm,while the BAZ grain size reached 2.36μm,mainly because the processing of multiple passes provided more driving force for the dislocation movement in this area and promoted the BAZ grain growth.In addition,in Al matrix,a dual reinforcement system consisting of Cu-based solid solution and Al2Cu particles could be generated after multiple passes of FSP.With the increasing number of the pass,both the size and homogeneity of Cu particles were further modified associated with severe shear deformation.However,the rate of in-situ reaction gradually decreased due to the less area of effective interfacial reaction.Through the hardness test,it was found that the hardness of the stir zone(SZ)increased significantly,which was due to the fine-grain strengthening caused by dynamic recrystallization combine with the dispersed distribution of the in-situ reaction product Al2Cu strengthened phase.And there was an extreme hardness value in this area where copper particles were enriched,which might not be able to fully eliminate by multi-pass FSP.It was mostly related to the gradient distribution of reinforcements and dislocations around Cu particles.To quantify the distribution of Cu particles,a modified nearest neighbor index was established based on the experiments and Monte Carlo simulations.Furthermore,a 4-order average nearest neighbor index showed good identifiability for large-size particle diameters.It was worth to mention that excessively large order will reduce the sensitivity of the parameter.As increasing passes,the NNI kept increasing.After five passes of processing,the NNI was greater than 0.5 and remains basically stable.The results showed that at least five passes were needed to guarantee to ensure the uniformity of reinforcement.Through multi-pass FSP,a Cu/Al surface composite material with complete structure and no defects and good interface bonding was successfully prepared.The increase in the number of passes and the change in the direction of rotation could improve the uniformity of particle dispersion and facilitate grain refinement.Combining the position evolution of Cu particles and the mass flow of the material during the multi-pass processing,it was found that the mixed flow trend of the material in the turbulent process was:RS→SAZ→AS→SZ.The increase in the number of processing passes could effectively promote the in-situ reaction between Cu particles and the aluminum matrix,and form a dual reinforcement system in the aluminum matrix.At the same time,the rate of in-situ reaction was continuously slowing down.On the one hand,it was due to the reduction of the effective interface reaction area.On the other hand,it could be attributed to the inhibitory effect of the product Al2Cu on the in-situ reaction.Combining Monte Carlo simulation with quantitative analysis technology,comparative analysis from the two perspectives of experiment and simulation could more accurately understand the influence of passes on the microstructure and particle distribution of composite materials.For SMMCs with high-volume fraction(20%in this study),at least five passes were required to ensure the uniformity of the reinforcement when using FSP to prepare composite materials.
作者
朱禹龙
曹宇
黄光杰
张成行
李启磊
刘庆
Zhu Yulong;Cao Yu;Huang Guangjie;Zhang Chenghang;Li Qilei;Liu Qing(International Joint Laboratory of Light Alloys(MOE),College of Materials Science and Engineering,Chongqing Univer-sity,Chongqing,400044,China;Light Metal Materials Research Department,Shenyang National Research Center for Materials Science,Chongqing 400044,China;College of Materials Science and Engineering,Nanjing Tech Univer-sity,Nanjing 211816,China)
出处
《稀有金属》
EI
CAS
CSCD
北大核心
2022年第12期1533-1545,共13页
Chinese Journal of Rare Metals
基金
中央高校基本科研业务费项目(2020CDJQY-A003)
国家自然科学基金创新研究群体项目(51421001)资助。
