Compressionenergy Absorption And Damping Properties Of Al Matrix Composite Foams Reinforced By In-situ Grown Carbon Nanotubes

As a typical structure and function integration material,aluminum foam has been gradually applied in the fields of architecture,rail transit and aerospace,attributing to its excellent energy absorption,thermal,electromagnetic,acoustic and damping properties.However,when the Al foam serves as a structural material alone,the relative low strength seriously restricts its applications.Hence,it is a promising research direction for improving the comprehensive properties of Al foam by preparing the composite foams.Due to the outstanding mechanical,physical and chemical properties,carbon nanotubes?CNTs?are considered as the ideal reinforcement for the Al matrix composite foams?AMCFs?.Whereas,the easy agglomeration of CNTs seriously hampers the development of CNT/Al composite foams of high performance.At the sametime,the present research on CNT/Al composite foams is only limited to the room temperature quasi-static compressive properties,but both the study on compressive properties at elevated temperatures and high strain rates,and the study about damping properties are still deficient.Aim at above problem,the present work is based on experimental research and the uniformly dispersed CNTs reinforced AMCFs aresuccessfully fabricated by combining the in-situ chemical vapor deposition?CVD?,short ball milling and space-holder method.We have studied theeffect of ball milling technique,testing temperature and strain rate on the compressive properties of CNT/Al composite foams.Besides,the effect of pore structure,CNT content and testing conditions on the damping properties of CNT/Al composite foams has also been investigated.The research contents and results are as follows:For the CNT/Al composite powders synthesized in-situ,what we stress studied are about the effect of ball milling technique on the microstructure,pore wall hardness and room temperature compression and energy absorption properties of CNT/Al composite foams.The results show that with the ball milling time increasing,the microstructure uniformity of the composite foams is improved,as a result of the enhanced CNT dispersion in the Al matrix.In addition,compared to the non-ball-milling 3.0 wt%-CNT/Al composite foams,when the ball milling time increases to 90 min,the pore wall hardness,yield strength and energy absorption capacity of the composite foams are increased by 67%,126%and 343%,respectively.When the ball milling time continues to extend to 120 min,the Al powders areseverely cold welded to large particles with an average size of 800?m,resulting in the reduced pore wall compactness and strength of the composite foams.Thus,the optimal ball milling time in this work is 90 min.Elevated temperature compressive properties and energy absorption response of the CNT/Al composite foamsare studied.Besides,the effects of CNT content on the high temperature property and deformation behavior are also explored.The results show that the high temperature compressive properties and energy absorption capacity of CNT/Al composite foams can be significantly enhanced by CNTs,and increase with the increment of CNT content.For the 3.0 wt%-CNT/Al composite foams,the yield stress and energy absorption capacity reach16.8 MPa and 19.8MJ/m³at 150 ^oC,which were¹.9 and².5 times higher than those of pure Al foam matrix,respectively.Furthermore,with the temperature raising,the deformation mode of the CNT/Al composite foams changes from brittle mode combined with plastic mode to plastic mode.The high strain rate dynamic compressive property and deformation behavior of CNT/Al composite foams are investigated by using the split Hopkinson pressure bar?SHPB?and digital image correlation?DIC?.The results show that the compressive strength of composite foams not only increases with the increment of CNT content but also increases with the strain rate increasing,revealing a typical strain rate sensitivity.For the 3.0 wt%-CNT/Al composite foams,the peak stress at 800 s^-1 is45.9 MPa and further increases to 52.5 MPa when the strain rate is 2130 s^-1,which is⁹0%higher than that at 0.001 s^-1?28.2 MPa?.Related to the deformation behavior of Al foam,the pure Al foam under the quasi-static condition,it can be recognized as firstly forms localized drformation bands,followed by the collapse of multi-layer deformation bands.Different from the deformation behavior of pure Al foam which is obviously strain rate dependent,the failure of CNT/Al composite foams is insensitive to the strain rate.Whether under the quasi-static or high strain rate dynamic compressive conditions,the CNT/Al composite foams both manifest as the formation of shear bands.The damping property and related damping mechanisms of CNT/Al composite foams under various temperatures,vibration frequencies and amplitudes are studied by using the dynamic mechanical analysis?DMA?.The results imply that the loss factor values of the composite foams increase with the porosity and CNT content raising,and also increase with the decrease of vibration frequency and increment of amplitude.Besides,the damping property of the Al foam is related to the temperature.The loss factor is nearly independent of temperature at first??200 ^oC?,but on the whole increases when the temperature exceeds the critical value of 200 ^oC.When the temperature is elevated to 390 ^oC,the value of loss factor for the 3.0wt%-CNT/Al composite foams reaches 0.36.The damping mechanisms of the CNT/Al composite foams mainly lie in the dislocation damping,porosity damping,grain boundary damping,the inherent damping of CNTs and the CNT-Al interfacial damping.