In this research,a method employing micro-extrusion was designed to produce the micro-scaled barrel-shaped parts with complex geometrical features to study the feasibility of the proposed microforming method and its g...In this research,a method employing micro-extrusion was designed to produce the micro-scaled barrel-shaped parts with complex geometrical features to study the feasibility of the proposed microforming method and its grain size effect on the formability of the complicated internal features in terms of deformation behavior,material evolution,accuracy of dimensions and final components quality.The results reveal that the deformation behavior is highly affected by grain size and becomes unpredictable with increased grain size.In addition,assembly parameters including feature dimension,tolerance and coaxiality also vary with grain size,and the variation of grain size needs to be accommodated by different assembly types,viz.,clearance fit or transition fit.From the microstructural evolution aspect,it was identified there were two dead zones and four shear bands,and the formation of these deformation zones was barely affected by the variation in grain size.Though bulges,cracks,and fracture induced voids were observed on the surface of the final components,tailoring the microstructure of the working material with finer grains could significantly avoid these defects.This study advances the understanding of forming microparts by extrusion processes and provides guidance for microforming of similar microparts.展开更多
As one of the indispensable actuating components in micro-systems,the shafted microgear is in great production demand.Microforming is a manufacturing process to produce microgears to meet the needs.Due to the small ge...As one of the indispensable actuating components in micro-systems,the shafted microgear is in great production demand.Microforming is a manufacturing process to produce microgears to meet the needs.Due to the small geometrical size,there are uncertain process performance and product quality issues in this production process.In this study,the shafted microgears were fabricated in two different scaling factors with four grain sizes using a progressively extrusion-blanking method.To explore the unknown of the process,grain-based modeling was proposed and employed to simulate the entire forming process.The results show that when the grains are large,the anisotropy of single grains has an obvious size effect on the forming behavior and process performance;and the produced geometries and surface quality are worsened;and the deformation load is decreased.Five deformation zones were identified in the microstructures with different hardness and distributions of stress and strain.The simulation by using the proposed model successfully predicted the formation of zones and revealed the inhomogeneous deformation in the forming process.The undesirable geometries of microgears including material unfilling,burr and inclination were observed on the shaft and teeth of gear,and the inclination size is increased obviously with grain size.To avoid the formation of inclination and material unfilling,the punch was redesigned,and a die insert was added to constraint the bottom surface of the gear teeth.The new products had then the better forming quality.展开更多
A physical-based constitutive model was developed to model the viscoplastic flow behavior and microstructure evolution of AZ80 magnesium alloy during the hot working process. The competing deformation mechanisms, incl...A physical-based constitutive model was developed to model the viscoplastic flow behavior and microstructure evolution of AZ80 magnesium alloy during the hot working process. The competing deformation mechanisms, including work hardening, dynamic recovery, and dynamic recrystallization, in an isothermal compression environment were considered in the model. The internal state variables, including the normalized dislocation density and recrystallized volume fraction, were incorporated into the model to articulate the microstructure evolution during hot deformation. The kinetic condition critical for dynamic recrystallization, considering the effects of the deformation temperature and strain rate, was obtained by employing both the Poliak-Jonas criterion and Zener-Hollomon parameter. Microstructure observations indicate that the recrystallized volume fraction increases with decreasing Z parameter at constant strain, which is consistent with the predicted kinetics model. Based on the developed model, a good correlation was also obtained between the predicted and experimental flow stress. The results indicate a good predictability of the model in describing the hot deformation behavior and microstructure evolution of AZ80 magnesium alloy.展开更多
基金funding support to this research from the National Natural Science Foundation of China(Grant No.51835011)the project of ZE1W from The Hong Kong Polytechnic University,and the General Research Fund of Hong Kong Government(Grant No.15223520).
文摘In this research,a method employing micro-extrusion was designed to produce the micro-scaled barrel-shaped parts with complex geometrical features to study the feasibility of the proposed microforming method and its grain size effect on the formability of the complicated internal features in terms of deformation behavior,material evolution,accuracy of dimensions and final components quality.The results reveal that the deformation behavior is highly affected by grain size and becomes unpredictable with increased grain size.In addition,assembly parameters including feature dimension,tolerance and coaxiality also vary with grain size,and the variation of grain size needs to be accommodated by different assembly types,viz.,clearance fit or transition fit.From the microstructural evolution aspect,it was identified there were two dead zones and four shear bands,and the formation of these deformation zones was barely affected by the variation in grain size.Though bulges,cracks,and fracture induced voids were observed on the surface of the final components,tailoring the microstructure of the working material with finer grains could significantly avoid these defects.This study advances the understanding of forming microparts by extrusion processes and provides guidance for microforming of similar microparts.
基金The authors would like to acknowledge the funding support to this research from the projects of ZE1W and BBAT from The Hong Kong Polytechnic University,the National Natural Science Foundation of China(NSFC)(Grant No.51835011)the General Research Fund(GRF)(Grant No.15223520).
文摘As one of the indispensable actuating components in micro-systems,the shafted microgear is in great production demand.Microforming is a manufacturing process to produce microgears to meet the needs.Due to the small geometrical size,there are uncertain process performance and product quality issues in this production process.In this study,the shafted microgears were fabricated in two different scaling factors with four grain sizes using a progressively extrusion-blanking method.To explore the unknown of the process,grain-based modeling was proposed and employed to simulate the entire forming process.The results show that when the grains are large,the anisotropy of single grains has an obvious size effect on the forming behavior and process performance;and the produced geometries and surface quality are worsened;and the deformation load is decreased.Five deformation zones were identified in the microstructures with different hardness and distributions of stress and strain.The simulation by using the proposed model successfully predicted the formation of zones and revealed the inhomogeneous deformation in the forming process.The undesirable geometries of microgears including material unfilling,burr and inclination were observed on the shaft and teeth of gear,and the inclination size is increased obviously with grain size.To avoid the formation of inclination and material unfilling,the punch was redesigned,and a die insert was added to constraint the bottom surface of the gear teeth.The new products had then the better forming quality.
基金the Natural Science Foundation of Beijing Municipality (Grant No. 3182025)the National Natural Science Foundation of China (Grant No. U1730121)+1 种基金the Joint Foundation (general) Project of the Equipment Pre-research of the Ministry of Education (Grant No. 6141A020221)the Postdoctoral Science Foundation of China (Grant No. 2017M620609).
文摘A physical-based constitutive model was developed to model the viscoplastic flow behavior and microstructure evolution of AZ80 magnesium alloy during the hot working process. The competing deformation mechanisms, including work hardening, dynamic recovery, and dynamic recrystallization, in an isothermal compression environment were considered in the model. The internal state variables, including the normalized dislocation density and recrystallized volume fraction, were incorporated into the model to articulate the microstructure evolution during hot deformation. The kinetic condition critical for dynamic recrystallization, considering the effects of the deformation temperature and strain rate, was obtained by employing both the Poliak-Jonas criterion and Zener-Hollomon parameter. Microstructure observations indicate that the recrystallized volume fraction increases with decreasing Z parameter at constant strain, which is consistent with the predicted kinetics model. Based on the developed model, a good correlation was also obtained between the predicted and experimental flow stress. The results indicate a good predictability of the model in describing the hot deformation behavior and microstructure evolution of AZ80 magnesium alloy.