Study On The Microstructure And Properties Of In-situ ZrB₂ Nanoparticle Reinforced AA6016 Matrix Composites Fabricated By External Field Processing

Due to the positive combination of high specific strength,good impact resistance and fatigue resistance,in-situ nanoparticles reinforced aluminum matrix composites are ideal new materials for automobile lightweighting.However,the materials also confront series of problems during preparation such as nanoparticle agglomeration,nanoscale control,and etc.In order to solve the problems,in-situ nanoparticles reinforced aluminum matrix composites synthesized by external field processing has become important development direction and research hotspot.In this paper,in-situ reaction technology was combined with electromagnetic field and ultrasonic field processing to fabricate high-performance in-situ ZrB₂ nanoparticles reinforced AA6016 matrix composites from an Al-K₂ZrF₆-KBF₄ system.The effects of rotary electromagnetic field frequencies and ultrasonic vibration time on the microstructures and tensile properties of as-cast composites were studied,and thus the optimum parameters of electromagnetic field and ultrasonic field processing were obtained.The effects of ZrB₂ particle content on the microstructures and tensile properties of as-cast and as-extrusion composites were also studied.On the basis of above research,the impact resistance and fatigue resistance of as-extrusion composites with different ZrB₂ particle content fabricated by electromagnetic field and ultrasonic field processing were also investigated.The microstructure analysis of ZrB₂/AA6016 composites showed that when the frequency of rotary electromagnetic field was 12 Hz and the time of ultrasonic vibration treatment was 4min,the particle distribution of the synthesized 3 vol.%ZrB₂/AA6016 composite was the most uniform,only a few clusters of the ZrB₂ particles with the size below 5?m were observed at the intersection of the grain boundaries,the Al/ZrB₂ interfaces were close with atom bonding,and the average sizes of the grains and the ZrB₂ particles were decreased to 25?m and 60 nm,respectively.The parameters were also applicable to fabricate 1 vol.%and 2 vol.%ZrB₂/AA6016 composites with homogeneous particle distribution.Due to the electromagnetic stirring in the melt produced by electromagnetic rotary field and cavitation and acoustic streaming in the melt produced by ultrasonic field,the external field processing can make the particles distribute disperse and control nano-scale.For the as-cast composites,the average grain sizes were decreased and the particle content along the grain boundaries was increased with the particle content increased.During solidification,ZrB₂ particles can act as heterogeneous nucleating substrates for?-Al crystals,moreover these particles can also inhibit the growth of the?-Al crystals.For the as-extrusion composites,the distribution of the ZrB₂particles is along the extrusion direction,which make the distribution of the recrystallized grains inhomogeneous,and thus the average grain sizes of the 1 vol.%ZrB₂/AA6016 composite were the smallest.ZrB₂ particles can not only increase the nucleation rate of recrystallized grains,but also hinder the growth of recrystallized grains.It is indicated that when the frequency of rotary electromagnetic field was 12 Hz and the time of ultrasonic vibration treatment was 4 min,the 3 vol.%ZrB₂/AA6016 composite showed the best tensile properties at room temperature.The tensile testing result was consistent with the microstructure analysis that the particle distribution of 3 vol.%ZrB₂/AA6016 composite fabricated under the same external field parameters was the most uniform,it is further manifested that the external field processing parameters was optimal.With the particle content increased,the ultimate tensile strength of the as-cast composites and the as-extrusion composite was increased but the elongation was decreased.Among the composites,the ultimate tensile strength of the 2 vol.%ZrB₂/AA6016 as-extrusion composite was up to 375 MPa,and the elongation was up to 23%.Compared to the as-cast composites,the as-extrusion composites had higher strength and elongation.This was ascribed to Hall-Patch and texture strengthening,and the fiber-like particle rich zone can bear high stress.Compared to the composites fabricated by conventional method,the impact energy of 3vol.%ZrB₂/AA6016 composite fabricated by electromagnetic field and ultrasonic field processing was improvement by 112.5%,and the fatigue limit was improvement by 40%.The result showed that the impact resistance and the fatigue resistance is related to the ultimate tensile strength and the particle content,and the 2 vol.%ZrB₂/AA6016 composite had the best impact properties and fatigue properties.The impact fractography showed that ZrB₂ particles can effectively reduce the strain rate sensitivity of the composites,and thus the impact toughness of the composites was improved.According to the fatigue fracture analysis,the fatigue cracks of the composites mainly initiated at the surface with inclusions,and the dispersed ZrB₂ particles can effectively hinder the fatigue crack growth,and thus the life of the fatigue crack growth stage was increased.