Investigation On Micro-heading Of Metal Rod By Dynamic Laser Loading In Cold And Warm Condition

Compared with static or quasi-static bulk forming,high-speed bulk forming has the advantages of suppressing deformation concentration,improving material formability,uniform metal flow,good mold cavity filling,and forming difficult-to-form materials.However,the existing high-speed bulk forming is not suitable for micro-forming.In this dissertation,combined with the characteristics of laser shock foil micro-forming with dynamic high-speed loading and suitable for micro-forming,a new method of micro-heading of metal rod by dynamic laser loading was proposed.Both theoretical and experimental investigations were carried out to study the process of micro-heading of metal rod by dynamic laser loading.The main research work and results are as follows:(1)The interaction between laser and matter and the calculation method of laser power density are expounded.The dynamic response of materials under high strain rate is discussed,including the calculation method of material strain rate and dynamic yield strength and the Johnson-Cook material dynamic constitutive model.The dislocation theory,single crystal and polycrystalline plastic deformation theory,and the mechanism of thermoplastic forming are discussed.The theoretical basis studies provided the foundation for subsequent experimental researches for the process.(2)An experimental die for micro-heading is designed,an experimental system for forming process is built,and a micro-heading experiment of metal rods in cold condition was performed.By observing the shape of the middle section of the forming part under different deformations,the formation process and forming mechanism of the micro-forming of heading of metal rods under laser dynamic loading were discussed.It was found that when the deformation is small,the shape of the forming area is obviously mushroom-shaped under the influence of stress wave effect and inertia effect in high-speed forming;when the deformation gradually becomes larger,the friction effect has a significant influence on the forming process,and the shape of the forming area drum-shaped,the diameter of the upper surface of the head and the diameter of the lower surface gradually tend to be consistent.By measuring theforming size and side surface morphology of the head,the influence of the grain size on the forming was studied.It was found that the deformation of the sample increases with the increase of the grain size,which can be explained by the dislocation plug product theory and the surface layer model;the side surface of the forming part of the specimen with a larger grain size exhibited a marked unevenness as compared with the specimen with a smaller grain size,which can be explained by surface layer theory and metal crystal plasticity theory.By measuring the forming size of the head,the influence of the thickness of the absorption layer and the laser energy on the forming was studied.(3)The experimental device related to warm forming was designed and manufactured,and related experimental research was carried out.By measuring the forming size of the head,the effects and differences of laser energy and grain size on the deformation of the T2 copper cylindrical specimen at room temperature and300°C forming temperature were studied.It was found that at room temperature and300? forming temperature,the influence of laser energy on the deformation of the sample is basically the same;the effect of the grain size on the deformation of the sample is different,because the increase in the deformation temperature improves the deformation coordination between the grains,the difference in the deformation of the surface grains and the internal grains decreases,under the same grain size difference,the variation of the sample deformation at the forming temperature of 300? is significantly smaller than that at room temperature.By measuring the forming size of the head,the influence of forming temperature on the sample deformation was studied.It was found that because of the softening effect of temperature,the deformation of the specimen increases with the increase of the forming temperature.By comparing the degree of dispersion of the deformation size of multiple repeated test samples under the same process parameters,the influence of forming temperature and grain size on the flow stress fluctuation of the sample material was studied.It was found that temperature improves the deformation coordination between grains,and reduces the degree of the influence of anisotropy of single grains on plastic deformation.Therefore,at the forming temperature of 300?,the forming size fluctuation of thesample is significantly smaller than that at room temperature,that is,the flow stress fluctuation of the material becomes smaller.When the grain size is small,the number of grains per unit volume increases,the deformation coordination between grains becomes better,and the influence of the deformation difference of each grain on the overall deformation becomes smaller.Therefore,the forming size fluctuation of the sample with a grain size of 6?m is smaller than the forming size fluctuation of the sample with a grain size of 44?m.The process of the micro-heading of metal rod by dynamic laser loading proposed in this dissertation is a new high-speed micro-bulk forming method for metal rods.This process combines the advantages of micro bulk forming and laser impact micro-forming,and provides an effective process method for manufacturing micro-parts.The theoretical and experimental studies carried out in this dissertation lay a foundation for revealing the mechanism and law of micro-heading of metal rod by dynamic laser loading.