Microstructure And Mechanical Properties Of Ultrafine-Grained Materials Prepared Using Intermittent Ultrasonic-Assisted ECAP

ECAP has been proved to be an efficient technique to prepare grain refinement in a wide range of materials.However there are still some flaws during ECAP,for example: the extrusion needs excessive extrusion force;the microstructure of ECAPed specimens scatter heterogeneously depending on the considerable fragmentation of the structure and strain heterogeneity;ultrafine grains have poor thermal stability.The use of femtosecond lasers offers substantial advantages in the micro–nano processing field,enabling the application of high pulse energy densities and ultrashort pulse durations while avoiding thermal damage.As a result,better precision,higher resolution,and smaller feature size can be realized for the ablation of diverse materials.However,few studies have focused on the interaction phenomena of femtosecond lasers and ultrafine-grained metals(grain sizes of100~1000 nm).Therefore,the preparation of ultrafine-grained materials and its interaction with femtosecond lasers are studied.The research contents are listed as below:(1)The flaws during ECAP,such as large extrusion force,heterogeneous grain sizes,poor thermal stability,which needs to be further ameliorated.The intermittent ultrasonic-assisted ECAP(IU-ECAP)approach was developed to address these issues.In this work,ECAP and IU-ECAP were applied to produce ultrafine-grained Al-6061 alloys and ultrafine-grained Copper.The results show that IU-ECAP technique is a reliable and efficient method to prepare ultrafine-grained materials.The influence of different intermittent time and different ultrasonic amplitudes on the true tensile strength of specimens were investigated,and a group of parameters including intermittent time and ultrasonic amplitude were optimized.The optimized parameters were utilized to prepare ultrafine-grained Al-6061 alloys and ultrafine-grained Copper.(2)Ultrafine-grained Al-6061 alloys and ultrafine-grained Copper were preparedusing ECAP and IU-ECAP techniques,respectively.The microstructure,mechanical properties,thermal stability and mold temperature of ultrafine-grained materials were investigated,respectively.Mechanical testing demonstrated that IU-ECAP significantly lowered the extrusion force,whereas the microhardness and ultimate tensile strength of the materials were strengthened.The microstructural characteristic indicated that the grain size of IU-ECAPed materials was smaller,and the microstructure was more homogeneous.The ultrafine-grained copper prepared by IU-ECAP has more subgrain and recrystallized grains.For the ultrafine-grained Al-6061 alloys,Mg2 Si was mainly distributed in the grain boundary,while for original Al-6061 alloys,Mg2 Si was mainly distributed in the grain.Analysis result of the XRD test showed that IU-ECAP weakened the intensities of textures and produced more homogeneous microstructure and larger dislocation densities in specimens.A thermal stability investigation revealed that ultrafine-grained materials by IU-ECAP recrystallized at higher temperature or longer duration time and thus the material exhibited better thermal stability.For IU-ECAP,the mold temperature exhibited a trend similar to square waves during the temperature test,whereas for ECAP,the die temperature remains constant at room temperature.(3)In the interaction experiment of a femtosecond laser and ultrafine-grained copper,the relationship between the ablation threshold and the different grain size was studied.The ablation threshold increased with decreasing grain size,making materials processing more difficult.Simultaneously,the ablation threshold was directly proportional to the reciprocal of the 0.29 th power of the average grain size.(4)The effects of different grain sizes on the groove depth and width of ablated specimens by femtosecond laser were studied.When the pulse energy of the femtosecond laser was the same,the depth and width of the ablated channels gradually decreased with decreasing grain size,which was consistent with the variation of the ablation threshold.(5)The microhardness of the plane groove formed by the ablation of thefemtosecond laser as a function of laser energy fluence was investigated.The microhardness difference of specimens with grooves with planar sizes of 3 mm × 3mm before and after ablation gradually increased with decreasing grain size for constant energy fluence.The microhardness of the specimens after ablation gradually decreased with increasing energy fluence for constant ultrafine grain size.In contrast,no apparent changes in the microhardness of the specimens with grain sizes of 15 ?m were observed with increasing energy fluence.(6)Microstructural characterization of the ablated specimens revealed that the heat generated by the laser ablation caused recrystallization and grain growth of the ultrafine-grained specimens,which are characteristics of "hot processing".In addition,the use of a larger energy fluence resulted in a larger grain size.In contrast,no apparent microstructural changes were observed for the ablated specimens with initial grain sizes of 15 ?m,which is characteristic of "cold processing".