Study On Size Effect Of Mesoscopic H62 Thin Plate

In microplastic forming,the mechanical properties and deformation behavior of the material show size effects that are completely different or even opposite to that at the macro scale.It is of crucial theoretical significance and practical reference value for the manufacture of miniature parts and the development of miniature tools to study the size effect of materials.In this paper,H62 brass sheets with thicknesses range from 50 ?m to 300 ?m are used as the experimental material.The annealing effect,tensile test and bending test are carried out to study the size effect of the mesoscale H62 brass sheet as well as analysing the law of size effect.The main research contents and results are as follows:(1)Protective atmosphere annealing processes for H62 brass sheets were developed,and the effect of rolling deformation on the microstructure and mechanical properties of the raw material was eliminated by annealing,and experimental materials for studying the size effect were prepared.Microstructure evolution of H62 brass sheets annealed at different temperatures is discussed.The grain size of H62 brass sheet in meso scale increases by raising annealing temperature,which is similar to that in macroscopic scale.At the same annealing temperature,the grain size increases with the increase of the sample thickness.The microhardness of the cold-rolled H62 brass foils increases and then decreases when the sample thickness decreases,the electrical conductivity decreases with increasing sample thickness.As the annealing temperature increases,the microhardness of the material gradually decreases,the electrical conductivity shows an upward trend.After annealing,the surface roughness of the samples increased,and the surface quality deteriorated.(2)The mechanical properties of H62 brass sheets are obtained by micro-tensile experiment.Based on the Hall-Petch model,surface layer theory,strain gradient plasticity theory and fracture theory,the size effects of mechanical properties for H62 brass sheet at micro-tensile experiment are analyzed.The results of micro-tensile experiment show that the yield strength and tensile strength decrease and then increase when the thicknesses of samples annealed at the same temperature decrease,the first type of size effect of "smaller and weaker" and the second type of size effect of "smaller and stronger" are exhibited.The elongation of H62 brass sheet gradually increases with the increase of the sample thickness,and fracture gradually changes from pure slip separation type to mixed dimple polymerization and slip separation type.It can be obtained that the elongation increases and then decreases with the increase of the annealing temperature.(3)An U-shaped microbending mold was developed,and the bending force size effect and springback size effect during bending forming were investigated by microbending experiments.The result that the falling speed of maximum bending force is greater than that of the sample cross-sectional area have shown a significant size effect.The bending force slightly increases by raising strain rate,and gradually decreases as the annealing temperature increases or the lubrication condition is improved.The springback angle decreases with the increase of the annealing temperature or sample thickness.Meanwhile,the speed of springback angle reduction decreases as the annealing temperature increases,and the thinner sample will lead to the bigger reduction of springback angle.After bending,the surface roughness of the deformed area of the sample increased,and the surface quality deteriorated.(4)The ratio of sample thickness and grain size t/d is introduced as the size factor.The laws of yield strength,tensile strength,elongation and bending spring angle of mesoscopic H62 brass sheet with the change of t/d are summarized.It can be found that the yield strength and tensile strength of H62 brass increase by raising t/d,and the smaller sample thickness can achieve the faster increase of strength.The elongation increases and then decreases with the increase of t/d,thinner sample will result in the more significantly change of elongation and will show a significant size effect.The springback angle increases as t/d increases,and the thinner the sample is,the faster the springback angle increases.