Research On Microstructure And Property Adjustment Of In-situ Nanoparticle Reinforced Aluminum Matrix Composites For Car Body

In recent years,the rapid growth of number of automobiles has caused a large amount of energy consumption and environment pollution.Lightweight of automobile is considered as a viable strategy to eradicate this issue and reduce emissions.In addition,Lightweight aluminum car body plays an important role in automobile lightweight.However,the traditional aluminum alloys are unable to meet the criteria of high strength,high ductility,fatigue resistance,impact resistance and formability because of the higher security requirements for lightweight vehicles.In-situ nanoparticles reinforced aluminum matrix composites(INPRAMCs)have high specific strength and modulus,good heat,friction,and corrosion resistance,and excellent fatigue resistance,and it has become a new lightweight material for automobile,recently.In this regard,INPRAMCs with higher strength and toughness were designed and developed to solve the problem of low strength and poor formability of aluminum alloys used in automobile body.The effects of electromagnetic,ultrasonic fields and hot forming processing on the microstructure and mechanical properties,as well as the corresponding mechanisms,were studied.The strengthening and toughening mechanisms were also revealed.This research has laid a significant theoretical and technological foundation for the design,forming and application of INPRAMCs in lightweight car body.The main contents and results were as follows:The in-situ Al-Zr-B and Al-Zr-B-O systems were designed and developed based on thermodynamic calculation.The in-situ Zr B_{2 np}/AA6111 composites and(Zr B₂+Al₂O₃)_np/AA6111 composites were successfully prepared,respectively.The results showed that the in-situ reaction efficiency was significantly improved,the particle clusters were broken obviously and uniformly distributed in the matrix,as well as the interface between particles and matrix was well bonded.For Al-Zr-B system,the results showed that the produced Zr B₂ particle were of spherical shape with size in the range of 10-80nm and the optimum electromagnetic frequency and ultrasonic power were 10 Hz and1.5 k W,respectively.For Al-Zr-B-O system,the results showed that the bright white and hexagonal square particles were Zr B₂ with a size of 20-80 nm,and the gray white and spherical particles were Al₂O₃ with a size of 5-30 nm;the optimum electromagnetic frequency and ultrasonic power were 15 Hz and 2 k W,respectively.The distribution,morphology and size of nanoparticles were mainly affected by the ultrasonic field,while the electromagnetic field mainly promoted the in-situ reaction to enhance the number density of nanoparticles.The action mechanisms showed that electromagnetic field can improve the wettability between reactants and aluminum melt,promote the mass and heat transfer,as well as the nucleation quantity of in-situ reaction process;Ultrasonic field applied in solidification stage can weak the acoustic flow effect,but greatly enhanced the acoustic cavitation effect which improved particle distribution and morphology,comparing with that applied in reaction stage.The thermal deformation behavior of the composites was studied by thermal simulation technology.The true stress-true strain curves showed that in the early stage of deformation,there was a rapid increase in flow stress dominated by work hardening,followed by steady-state flow stress;at the same time,the flow stress decreased with the increase in temperature and increased with the increase in strain rate.Then constitutive equations for the composites were constructed by selecting steady-state rheological data.The hot processing maps of the composite were constructed based on the power dissipation and instability maps,and the optimal hot processing window of the composites were obtained.Results showed that the deformation temperature in the range of 410-450°C and strain rate in 0.7-1s^-1 range was the optimal hot processing window for the Zr B_{2 np}/AA6111 composites;deformation temperature in the range of410-450°C,strain rate in 0.3-1s^-1 range,and deformation temperature in the range of400-450°C,strain rate in 0.001-0.007s^-1 range were the optimal hot processing windows for(Zr B₂+Al₂O₃)_np/AA6111 composites.Based on the optimal hot processing windows of composites,the influence law of hot extrusion on the microstructure of composites was studied.The results showed that during hot extrusion,the coarse dendritic grains transformed into fine equiaxed grains which distributed near the particles along the extrusion direction,and the distribution of particles was streamline.Meanwhile,the number of dynamically recrystallized(DRX)grains increased with the introduction of in-situ nanoparticles;high-angle grain boundaries and low-angle grain boundaries increased simultaneously;the fiber texture of(113)?extrusion direction changed into fiber texture of(001)?extrusion direction and(111)?extrusion direction;the maximum pole density of inverse pole figures first decreased and then increased with the increase of particle's volume fraction demonstrating the higher maximum pole density in in-situ(Zr B₂+Al₂O₃)_np/AA6111composites.Role of in-situ nanoparticles during hot deformation is as follows:(1)The uncoordinated deformation between the particles dislocating along grain boundaries and the matrix cause a high storage energy appearing near the particles,which promotes the DRX process.Therefore,a large number of fine DRX equiaxed grains appear near the particles and the number of large-angle grain boundaries increases.These particles can also pin the grain boundary motion to refine the grains.(2)Dispersed nanoparticles inside the grains can pin the dislocations which leads to hinder the dislocation movement and the dynamic recovery,resulting in the increase of low-angle grain boundaries.The results of composites showed that the mechanical properties of as-cast composites first increased with increasing the electromagnetic frequencies and ultrasonic powers,and then decreased.The optimum magnetic frequency,ultrasonic power for synthesis of in-situ Zr B_{2 np}/AA6111 and(Zr B₂+Al₂O₃)_np/AA6111 composites were 10 Hz,1.5 k W and 15 Hz,2 k W,respectively.The tensile properties,yield properties and elongation of Zr B_{2 np}/AA6111 and(Zr B₂+Al₂O₃)_np/AA6111 increased12%,20%,17%and 11%,26%,25%,respectively.The mechanical properties of composites revealed that the of the in-situ composites were significantly improved after hot extrusion.The ultimate tensile strength,yield strength and elongation of composites first increased with increase of in-situ nanoparticles content and then decreased.The optimal volume fraction of Zr B₂ particles and Zr B₂+Al₂O₃ particles in two composites were found to be 2 vol.%and 3 vol.%,respectively.The tensile properties,yield properties and elongation of Zr B_{2 np}/AA6111 and(Zr B₂+Al₂O₃)_np/AA6111 were371MPa,308MPa,25.94%and 392MPa,334MPa,27.41%,respectively,correspondingly increasing 16%,18%,27%and 22%,27%,34%over the recpective properties of the AA6111 alloy.The results of formability and impact toughness of extruded composites showed that with the addition of in-situ nanoparticles,?_b,?_u,n,r(key indexes of formability)and impact toughness first increased then decreased,while?_0.2/?first decreased then increased.When the content of in-situ Zr B₂ particles was 2vol.%in Zr B_{2 np}/AA6111 composites and that of Zr B₂+Al₂O₃ particles was 3 vol.%in(Zr B₂+Al₂O₃)_np/AA6111 composites,the formability and impact toughness of composites were the best.The analysis of strengthening mechanisms of the composites results showed that as for the as-cast composites,when the physical field is not applied,the dominant strengthening mechanism in composites were mainly CTE strengthening and Orowan strengthening,as well as the strengthening value of CTE was higher;when the physical field is applied,Orowan strengthening surpasses CTE strengthening and becomes the principle strengthening mechanism.The main strengthening mechanisms for extruded composites include grain boundary strengthening,dislocation strengthening,synergistic strengthening of nanoprecipitates and nanoparticles,as well as texture strengthening.The toughening mechanisms of in-situ nano-PRAMCs composites are as follows:(1)Grains refinement,high bonding strength of interface between nanoparticles and matrix,uniform nanoparticles distribution,fine size and obtuse corner of nanoparticles and precipitate which can effectively reduce the probability of crack initiation.(2)The critical size of crack propagation for composites is larger than that of the matrix,and cracks are more likely to be released because of the yielding surrounding matrix,in addition,the nanoparticles can hinder the crack propagation thereby improving the toughness,plasticity and formability of composites.