| Laser solid forming(LSF)technology,which is a solid freeform manufacture method,can be employed in near-net-shape fabricating of metallic components.This technology has been used for the fabrication of a variety of sophisticated components,and gradually applied to airplane engine and large loading-carrying structure.As it has been shown,most of the previous studies were focused on the processing parameters,and the quasi-static mechanical properties of TC4 titanium alloy(LSF).Therefore,it is of great theoretical and practical value to study the dynamic shear mechanical behavior of TC4 titanium alloy(LSF).To investigate the dynamic shear mechanical behavior of TC4 titanium alloy(LSF),the double shear specimen,hat-shaped specimen,static testing machine,and separated Hopkinson pressure bar(SHPB)with high temperature synchronous assembly system were used for the compression and shear experiments at different loading direction(the laser scanning direction and the depositing direction),different temperatures(173 K?1173 K)and different strain rates(10-2?105 s-1).The temperature sensitivity and strain rate sensitivity of plastic flow behavior were analyzed.The mechanism of dynamic shear failure was studied by means of microscopic observation and theoretical analysis.The adiabatic shear failure,which is the most common failure mode of metal material under high strain rates,was discussed.In this paper,there are mainly four parts as follows.(1)In order to investigate the wave propagation attenuation and dispersion in SHPB experiment,based on experimental principle of wave propagation coefficient determined by free end impact test of Hopkinson bar,the in-situ testing wave propagation coefficient method based on SHPB experiment was proposed.A novel specimen design for shear test under high temperatures was developed.The specimen's shear zone is close to the pure shear state,and the high strain-rate shear test at room temperature and high temperature can be realized with this specimen and the improved high-temperature SHTB.A novel double-notch shear specimen was developed.The stress and strain of shear zone are homogeneous and the stress,and the stress triaxiality is low.The testing method was optimized accordingly.The high strain-rate(104s-1 order)shear test can be carried out with the newly designed specimen and SHPB.(2)Using cylindrical compression specimen,compression experiments with wide temperature range(173?1173 K)and strain rate range(0.001?5000 s-1)were carried out.The changing law of flow stress with temperature and strain rate was studied.The microstructure characteristics of the original and deformed specimens were analyzed.The results show that there is no obvious macroscopical mechanical anisotropy of this alloy.The compression strength is lower than that of casting TC4 titanium alloy.The microscopic analysis shows that this alloy is prone to adiabatic shear deformation when dynamic compression,but the increase of experimental temperature will inhibit the generation of adiabatic shear band.By the microscopic observation and mechanical analysis,the dynamic compression fracture mechanism is adiabatic shear failure.The voids and cracks form in the shear band,and propagation of cracks along the interface of?/?forms fracture surface.(3)The static and dynamic(104?105 s-1)shear behaviour of this alloy under different loading directions and different temperatures(173?573 K)were systematically studied.The microstructure characteristics of the original and deformed specimens were compared and analyzed.The dynamic recrystallization process of adiabatic shear bands was discussed.the anisotropy of shear properties is not significant regardless of the different loading conditions.There is no adibatic shear band under static shear loading.The results of tests under the dynamic shear loading(104 s-1)show that:the flow stress increases with increasing strain rate and there is approximately linear relationship between them;the fracturing is characterized by ductile fracture behavior;there are two typical microstructures of smooth region and dimple region.The results of tests under the dynamic shear loading(105 s-1)show that:there is obvious temperature sensitivity;the dynamic shear strength of TC4 titanium alloy(LSF)is lower than that of those TC4 alloys fabricated by forging and extrusion,adiabatic shear band can be formed under dynamic shear load;the hardness in the shear band is higher than that of the surrounding matrix,due to the strengthening effect caused by grain refinement;the process of rotational dynamic recrystallization is considered to be the cause of the grain refinement in shear band.(4)The formation mechanism of adiabatic shear band was studied,and the critical condition for the thermoplastic constitutive instability was calculated using Johnson-Cook constitutive model.By adjusting the parameters of constitutive model,the influence of material parameters on the critical condition was discussed.The relationship between the direction of shear zone and the slip line of plastic mechanics was studied by means of continuum mechanics.The distribution and evolution of temperature,strain and stress in adiabatic shear band were studied by finite element method. |
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