The internal friction behavior of Al green power compact duxing the sintering process was studied as a function of temperature. The internal friction measurements were performed from room temperature to 600 °C. T...The internal friction behavior of Al green power compact duxing the sintering process was studied as a function of temperature. The internal friction measurements were performed from room temperature to 600 °C. Two typical internal friction peaks were detected corresponding to heating and cooling processes, respectively. The heating peak corresponds to a recrystallization process of deformed Al particles, which is influenced by many extrinsic parameters, such as measuring frequency, strain amplitude, heating rate, power particle size and compacting pressure. However, the intrinsic nature of the peak is originated from the micro-sliding of the weak-bonding interfaces between Al particles and increased dislocation density induced in compressing. The cooling peak with the activation energy of (1.64±0.06) eV is associated with the grain boundary relaxation, which can be interpreted as the viscous sliding of grain boundaries. The similar phenomena are also found in the Mg green powder compact.展开更多
The effects of temperature （0-500°C） on the compressive strength,hardness,average relative density,and microstructure of Ti6Al4V powder green compacts prepared by magnetic pulse compaction were investigated.The...The effects of temperature （0-500°C） on the compressive strength,hardness,average relative density,and microstructure of Ti6Al4V powder green compacts prepared by magnetic pulse compaction were investigated.The results show that with increasing heating temperature,the compressive strength first increases and then decreases with the maximum value of 976.74 MPa at 400°C.The average relative density and hardness constantly increase,and their values reach 96.11% and HRA 69.8 at 500°C,respectively.The increase of partial welding is found among the junctions of particles inside the compacts; there is no obvious grain growth inside the compacts within the temperature range.展开更多
A new technique, powder compact foaming process for the production of aluminumfoams has been studied in this article. According to this method, the aluminum pow-der is mixed with a powder foaming agent (TiH_2). Subseq...A new technique, powder compact foaming process for the production of aluminumfoams has been studied in this article. According to this method, the aluminum pow-der is mixed with a powder foaming agent (TiH_2). Subsequent to mixing, the powderblend is hot compacted to obtain a dense semi--finished product. Upon heating to tem-peratures within the range of the melting point, the foaming agent decomposes to evolvegas and the semi--finished product expands into a porous cellular aluminum. Foamingprocess is the key in this method. Based on experiments, the foaming characteris-tics were mainly analyzed and discussed. Experiments show that the aluminum--foamwith closed pores and a uniform cell structure of high porosity can be obtained usingthis method by adjusting the foaming parameters: the content of foaming agent andfoaming temperature.展开更多
The relation between friction mechanism and force chains characteristics has not yet been fully studied in the powder metallurgy research area.In this work,a uniaxial compression discrete element model is established ...The relation between friction mechanism and force chains characteristics has not yet been fully studied in the powder metallurgy research area.In this work,a uniaxial compression discrete element model is established based on the compaction process of ferrous powder.Furthermore,the correlation mechanism between force chains and the friction mechanism during powder compaction is investigated.The simulation results reveal a strong correlation between the variation of the friction coefficient and the evolution of force chains.During the powder compaction,the friction coefficient would eventually tend to be stable,a feature which is also closely related to the slip ratio between particles.The side wall friction and the friction between particles would have an important effect on the direction of force chain growth in about one-third of the area near the side wall.The research results provide theoretical guidance for improving the densification process of the powder according to the force chain and friction.展开更多
Ti6Al4V powder compaction was performed by using magnetic pulse compaction in air at 200℃.Effects of process parameters such as voltage,capacitance,discharge times on the microstructure,compressive strength,hardness ...Ti6Al4V powder compaction was performed by using magnetic pulse compaction in air at 200℃.Effects of process parameters such as voltage,capacitance,discharge times on the microstructure,compressive strength,hardness and relative density of compacts were investigated.The experimental results show that the relative density,hardness and compressive strength of compacted specimens increase with increasing voltage.In addition,the relative density and compressive strength of compacted specimens increase with the augmentation of capacitance in the range investigated.The relative density increases,the hardness firstly increases and then tends to be a fixed value;and the compressive strength firstly increases and then decreases from one to five times compaction.Both values of the hardness and compressive strength reach the maxima of HRA 69.1 and 1 062.31 MPa,at three times compaction,respectively.There are pores in and between particles.展开更多
Powder compaction is one of the most crucial processes in initiating explosive device manufacturing. The quality of powder compaction may influence the performances of product directly. Traditional powder compaction m...Powder compaction is one of the most crucial processes in initiating explosive device manufacturing. The quality of powder compaction may influence the performances of product directly. Traditional powder compaction mainly makes use of manual gravitational spiral loading machine or lever loading machine. Potential accident by the strike in powder compaction of initiating explosive device could risk life, property and the environment. To prevent this risk, automatic control system and control strategy can be used to guarantee no strike in pressing process. The scope of this paper is to analyze and review the control strategy for powder compact and discuss the experimental results for the application of the proposed strategy.展开更多
Based on an analysis of the validity of the powder compaction equation of Kawakita,a modified compaction equation is proposed.It is shown by the statistical analysis on the experimental compaction data of various powd...Based on an analysis of the validity of the powder compaction equation of Kawakita,a modified compaction equation is proposed.It is shown by the statistical analysis on the experimental compaction data of various powders that in most cases the proposed equation provides a better description of the compaction data than Kawakita's equation,especially in the cases of the compaction of hard material powders.展开更多
Compaction processes are one the most important par ts of powder forming technology. The main applications are focused on pieces for a utomotive, aeronautic, electric and electronic industries. The main goals of the c...Compaction processes are one the most important par ts of powder forming technology. The main applications are focused on pieces for a utomotive, aeronautic, electric and electronic industries. The main goals of the compaction processes are to obtain a compact with the geometrical requirements, without cracks, and with a uniform distribution of density. Design of such proc esses consist, essentially, in determine the sequence and relative displacements of die and punches in order to achieve such goals. A.B. Khoei presented a gener al framework for the finite element simulation of powder forming processes based on the following aspects; a large displacement formulation, centred on a total and updated Lagrangian formulation; an adaptive finite element strategy based on error estimates and automatic remeshing techniques; a cap model based on a hard ening rule in modelling of the highly non-linear behaviour of material; and the use of an efficient contact algorithm in the context of an interface element fo rmulation. In these references, the non-linear behaviour of powder was adequately desc ribed by the cap plasticity model. However, it suffers from a serious deficiency when the stress-point reaches a yield surface. In the flow theory of plasticit y, the transition from an elastic state to an elasto-plastic state appears more or less abruptly. For powder material it is very difficult to define the locati on of yield surface, because there is no distinct transition from elastic to ela stic-plastic behaviour. Results of experimental test on some hard met al powder show that the plastic effects were begun immediately upon loading. In such mater ials the domain of the yield surface would collapse to a point, so making the di rection of plastic increment indeterminate, because all directions are normal to a point. Thus, the classical plasticity theory cannot deal with such materials and an advanced constitutive theory is necessary. In the present paper, the constitutive equations of powder materials will be discussed via an endochronic theory of plasticity. This theory provides a unifi ed point of view to describe the elastic-plastic behaviour of material since it places no requirement for a yield surface and a ’loading function’ to disting uish between loading an unloading. Endochronic theory of plasticity has been app lied to a number of metallic materials, concrete and sand, but to the knowledge of authors, no numerical scheme of the model has been applied to powder material . In the present paper, a new approach is developed based on an endochronic rate independent, density-dependent plasticity model for describing the isothermal deformation behavior of metal powder at low homologous temperature. Although the concept of yield surface has not been explicitly assumed in endochronic theory, it is shown that the cone-cap plasticity yield surface (Fig.1), which is the m ost commonly used plasticity models for describing the behavior of powder materi al can be easily derived as a special case of the proposed endochronic theory. Fig.1 Trace of cone-cap yield function on the meridian pl ane for different relative density As large deformation is observed in powder compaction process, a hypoelastic-pl astic formulation is developed in the context of finite deformation plasticity. Constitutive equations are stated in unrotated frame of reference that greatly s implifies endochronic constitutive relation in finite plasticity. Constitutive e quations of the endochronic theory and their numerical integration are establish ed and procedures for determining material parameters of the model are demonstra ted. Finally, the numerical schemes are examined for efficiency in the model ling of a tip shaped component, as shown in Fig.2. Fig.2 A shaped tip component. a) Geometry, boundary conditio n and finite element mesh; b) density distribution at final stage of展开更多
Some flow stress models was used to simulate the compaction of a cylindrical specimen.A new flow stress model was presented by analyzing and summarizing the powder compact experiment with different initial densities.C...Some flow stress models was used to simulate the compaction of a cylindrical specimen.A new flow stress model was presented by analyzing and summarizing the powder compact experiment with different initial densities.Compared with other models,the new model fits better with the experiments.The compaction process of a synchronizer hub was simulated with new model and the synchronizer hub shape was improved by the simulation results.The density distribution was measured and compared with the simulation results of splitting the samples into pieces.The comparison shows that the model could be used to simulate the powder compact process in an efficient and accurate manner.And the results indicate that the density distribution is homogenous after optimization.展开更多
The constitutive relation of powder material was derived based on the assumption that metal powder is a kind of elasto-plastic material, complying with an elliptical yield criterion. The constitutive integration algor...The constitutive relation of powder material was derived based on the assumption that metal powder is a kind of elasto-plastic material, complying with an elliptical yield criterion. The constitutive integration algorithm was discussed. A way to solve the elastic strain increment in each iteration step during elasto-plastic transition stage was formulated. Different integration method was used for elastic and plastic strain. The relationship between model parameters and relative density was determined through experiments. The model was implemented into user-subroutines of Marc. With the code, computer simulations for compaction process of a balancer were performed. The part is not axisymmetric and requires two lower punches and one upper punch to form. The relative density distributions of two design cases, in which different initial positions of the punches were set, were obtained and compared. The simulation results indicate the influence of punch position and movement on the density distribution of the green compacts.展开更多
A computer simulation procedure for metal powder die compaction was described. Friction behavior of metal powder during cold compaction was simulated by the finite element method. The movement of powder relative to th...A computer simulation procedure for metal powder die compaction was described. Friction behavior of metal powder during cold compaction was simulated by the finite element method. The movement of powder relative to the die wall was taken into consideration by utilizing the shear friction model. Friction between the powder and the rigid die wall leads to inhomogeneous density distribution during the compaction process. The floating die technique and double punch pressing can attain more homogenous compacts than the fixed die technique can do. The results obtained from numerical analysis agree well with the experimental results. Simulation model was built in MSC.Mentat, and MSC.Marc software was used to calculate the powder compaction process.展开更多
The present study aims to fabricate and evaluate the mechanical properties and wear behavior of Mg metal matrix composite,reinforced by 0,1.5,3,5 and 10 vol.%B4C microparticles.Mg−B4C samples were fabricated at 450℃ ...The present study aims to fabricate and evaluate the mechanical properties and wear behavior of Mg metal matrix composite,reinforced by 0,1.5,3,5 and 10 vol.%B4C microparticles.Mg−B4C samples were fabricated at 450℃ and under different loading rates by using split Hopkinson bar(SHB),drop hammer(DH)and Instron(QS)at strain rates of 1600,800 and 0.008 s–1,respectively.The mechanical properties including microhardness,quasi-static and dynamic compressive strengths and wear behavior of samples were experimentally investigated.The results show that,the hardness of SHB and DH samples is obtained to be 20.2%and 5.7%higher than that of the QS sample,respectively.The wear rate and wear mass loss of Mg–10.0%B4C samples fabricated by SHB were determined lower than those of the QS sample by nearly 33%and 39%,respectively.The quasi-static compressive strengths of Mg−5.0%B4C are improved by 39%,30%and 29%for the SHB,DH and QS samples,respectively,in comparison with the case of pure Mg.Furthermore,it is discovered that the dynamic compressive strength of samples is 51%−110%higher than their quasi-static value with respect to the B4C content.展开更多
The low-voltage-electromagnetic forming was applied to powder compaction.A series of experiments was performed to compact aluminum,copper and tin powders in an indirect working way.Having compacted high-density powder...The low-voltage-electromagnetic forming was applied to powder compaction.A series of experiments was performed to compact aluminum,copper and tin powders in an indirect working way.Having compacted high-density powder parts successfully,the authors analyzed the effects of voltage,capacitance,friction,compaction times,powder size and other factors on the densities of compacted specimens.The experimental results show that lower voltage but larger capacitance are beneficial to increasing the density and homogeneity of the compacted specimens,if the loading velocity and discharging energy are suitable.The higher the voltage,the greater the percentage of energy consumed by friction.If the equipment energy is limited,the iterative compaction is an efficient way to manufacture homogeneous and high-density powder parts.展开更多
This paper describes a new method of surface modification by Electrical Discharge Machining (EDM). By using ordinary EDM machine tool and kerosene fluid, a hard ceramic layer can be created on the workpiece surface wi...This paper describes a new method of surface modification by Electrical Discharge Machining (EDM). By using ordinary EDM machine tool and kerosene fluid, a hard ceramic layer can be created on the workpiece surface with Ti or other compressed powder electrode in a certain condition. This new revolutionary method is called Electrical Discharge Coating (EDC). The process of EDC begins with electrode wear during EDM,then a kind of hard carbide is created through the thermal and chemical reaction between the worn electrode material and the carbon particle decomposed from kerosene fluid under high temperature. The carbide is piled up on a workpiece quickly and becomes a hard layer of ceramic about 20 μm in several minutes. This paper studies the principle and process of EDC systemically by using Ti powder green compact electrode. In order to obtain a layer of compact ceramic film, it is very important to select proper electric pulse parameters, such as pulse width, pulse interval, peak current. Meantime, the electrode materials and its forming mode will effect the machining surface quality greatly. This paper presents a series of experiment results to study the EDC process by adopt different technology parameters. Experiments and analyses show that a compact TiC ceramic layer can be created on the surface of metal workpiece. The hardness of ceramic layer is more 3 times higher than the base body, and the hardness changes gradiently from surface to base body. The method will have a great future because many materials can be easily added to the electrode and then be coated on the workpiece surface. Gearing the parameters ceramic can be created with different thickness. The switch between deposition and removal process is carried out easily by changing the polarity, thus the gear to the thickness and shape of the composite ceramic layer is carried out easily. This kind of composite ceramic layer will be used to deal with the surface of the cutting tools or molds possibly, in order to lengthen their life. It also can be found wide application in the fields of surface repairing and strengthening of the ship or aircraft.展开更多
The compaction of pure Cu powder was carried out through a series of experiments using dynamic magnetic pulse compaction, and the effects of process parameters, such as discharge energy and compacting direction, on th...The compaction of pure Cu powder was carried out through a series of experiments using dynamic magnetic pulse compaction, and the effects of process parameters, such as discharge energy and compacting direction, on the homogeneity and the compaction density of compacted specimens were presented and discussed. The results indicated that the compaction density of specimens increased with the augment of discharge voltage and time. During unidirectional compaction, there was a density gradient along the loading direction in the compacted specimen, and the minimum compaction density was localized to the center of the bottom of the specimen. The larger the aspect ratio of a powder body, the higher the compaction density of the compacted specimen. And high conductivity drivers were beneficial to the increase of the compaction density. The iterative and the double direction compaction were efficient means to manufacture the homogeneous and high-density powder parts.展开更多
High velocity compaction process of atomized Al powders was studied. The green density, the maximal force and the withdraw force of specimen were investigated. The green density of atomized aluminum powder was obtaine...High velocity compaction process of atomized Al powders was studied. The green density, the maximal force and the withdraw force of specimen were investigated. The green density of atomized aluminum powder was obtained to be 2.68 g/cm3 and its relative density is about 99%. The maximal force increased proximately linearly with the compaction energy. The withdraw force was observed ranging between 30 and 70 kN. The radial spring back was less than 0.1%.展开更多
A new method for producing higher density PM parts, high velocity compaction （HVC）, was presented in the paper. Using water atomized pure iron powder without lubricant admixed as the staring material, ring samples w...A new method for producing higher density PM parts, high velocity compaction （HVC）, was presented in the paper. Using water atomized pure iron powder without lubricant admixed as the staring material, ring samples were compacted by the technique. Scanning electron microscopy （SEM） and a computer controlled universal testing machine were used to investigate the morphologies and the mechanical properties of samples, respectively. The relationships among the impact velocity, the green density, the sintered density, the bending strength and the tensile strength were discussed, The results show that with increasing impact velocity, the green density and the bending strength increase gradually, so the sintered density does. In addition, the tensile strength of sintered material is improved continuously with the sintered density enhancing. In the study, the sintered density of 7.545 g/cm^3 and the tensile strength of 190 MPa are achieved at the optimal impact velocity of 9.8 m/s.展开更多
By phenomenological analysis of warm compaction, it is found that, compared with the contribution of particle plastical deformation to densification of powder compact,the particle rearrangement is a dominant densifica...By phenomenological analysis of warm compaction, it is found that, compared with the contribution of particle plastical deformation to densification of powder compact,the particle rearrangement is a dominant densification mechanism for powder warm compaction, and the plastical deformation of particles plays an important role in offering accommodating deformation for particle rearrangement and densifying powder compact at the final stage of pressing.In order to attain density gain as high as possible during warm compaction, six rules for designing warm compacting powder mixtures were proposed in detail.展开更多
Die wall lubrication was applied on warm compaction powder metallurgy in hope to reduce the concentration level of the admixed lubricant since lubricant is harmful to the mechanical property of the sintered materials....Die wall lubrication was applied on warm compaction powder metallurgy in hope to reduce the concentration level of the admixed lubricant since lubricant is harmful to the mechanical property of the sintered materials. Iron-based samples were prepared by die wall lubricated warm compaction at 135 ℃ and 175 ℃, using polytetrafluoroethylene (PTFE) emulsion as die wall lubricant. A compacting pressure of 700 MPa and 550 MPa were used. The admixed lubricant concentration ranging from 0 to 0.6 wt.% was used in this study. Compared with non-die wall lubricated samples, the die wall lubricated samples have higher green densities. Results show that in addition to the decrease in ejection forces, green density of the compacts increased linearly with the decrease in admixed lubricant content. Mechanical property of the sintered compacts increase sharply when the admixed lubricant concentration reduced to 0.125 wt.% or less. Ejection force data indicated that samples with die wall lubrication show lower ejection forces when compared with samples without die wall lubrication. No scoring was observed in all experiments even for samples contain no admixed lubricant. Our results indicated that under experimental condition used in this study, no matter at which compaction pressure, compaction temperature, graphite and lubricant contents in the powder the die wall lubricated warm compaction would give the highest green density and lowest ejection force. It can be concluded that combination of die wall lubrication and warm compaction can provide P/M products with higher density and better quality. It is a feasible way to produce high performance P/M parts if suitable die wall lubrication system was applied.展开更多
Low-voltage electromagnetic compaction （EMC） was used to compact metal powders （Cu） and ceramic powders （TiO2） in the indirect way. It was found that the density of the metal powder parts compacted by low-voltag...Low-voltage electromagnetic compaction （EMC） was used to compact metal powders （Cu） and ceramic powders （TiO2） in the indirect way. It was found that the density of the metal powder parts compacted by low-voltage EMC varied linearly with the discharging voltage in the range investigated. But for ceramic powders, the discharging voltage has an optimal value. Under the value, the density increases as discharging voltage rises, but beyond the value the trend is reverse. The experimental results show that the density of the metal parts decreases gradually along press direction. And the density of the ceramic parts decreases with the advancement of the aspect ratio h/d （height/diameter）. In addition, repetitive compaction can improve the density of both metal and ceramic parts and reduce the effects of aspect ratio on the density.展开更多
基金Project(51301150)supported by the National Natural Science Foundation of ChinaProject(2013KJXX-11)supported by the Special Program of Youth New-star of Science and Technology of Shaanxi Province,ChinaProject(Physics-2012SXTS05)supported by the High-level University Construction Special Program of Shaanxi Province,China
文摘The internal friction behavior of Al green power compact duxing the sintering process was studied as a function of temperature. The internal friction measurements were performed from room temperature to 600 °C. Two typical internal friction peaks were detected corresponding to heating and cooling processes, respectively. The heating peak corresponds to a recrystallization process of deformed Al particles, which is influenced by many extrinsic parameters, such as measuring frequency, strain amplitude, heating rate, power particle size and compacting pressure. However, the intrinsic nature of the peak is originated from the micro-sliding of the weak-bonding interfaces between Al particles and increased dislocation density induced in compressing. The cooling peak with the activation energy of (1.64±0.06) eV is associated with the grain boundary relaxation, which can be interpreted as the viscous sliding of grain boundaries. The similar phenomena are also found in the Mg green powder compact.
文摘The effects of temperature （0-500°C） on the compressive strength,hardness,average relative density,and microstructure of Ti6Al4V powder green compacts prepared by magnetic pulse compaction were investigated.The results show that with increasing heating temperature,the compressive strength first increases and then decreases with the maximum value of 976.74 MPa at 400°C.The average relative density and hardness constantly increase,and their values reach 96.11% and HRA 69.8 at 500°C,respectively.The increase of partial welding is found among the junctions of particles inside the compacts; there is no obvious grain growth inside the compacts within the temperature range.
文摘A new technique, powder compact foaming process for the production of aluminumfoams has been studied in this article. According to this method, the aluminum pow-der is mixed with a powder foaming agent (TiH_2). Subsequent to mixing, the powderblend is hot compacted to obtain a dense semi--finished product. Upon heating to tem-peratures within the range of the melting point, the foaming agent decomposes to evolvegas and the semi--finished product expands into a porous cellular aluminum. Foamingprocess is the key in this method. Based on experiments, the foaming characteris-tics were mainly analyzed and discussed. Experiments show that the aluminum--foamwith closed pores and a uniform cell structure of high porosity can be obtained usingthis method by adjusting the foaming parameters: the content of foaming agent andfoaming temperature.
基金supported by the Natural Science Foundation of Fujian Province,China(Grant No.2020J01869)the Initial Scientific Research Fund in Fujian University of Technology,China(Grant No.GY-Z19123)the Fujian Provincial Science and Technology Guiding Project,China(Grant No.2017H0002)。
文摘The relation between friction mechanism and force chains characteristics has not yet been fully studied in the powder metallurgy research area.In this work,a uniaxial compression discrete element model is established based on the compaction process of ferrous powder.Furthermore,the correlation mechanism between force chains and the friction mechanism during powder compaction is investigated.The simulation results reveal a strong correlation between the variation of the friction coefficient and the evolution of force chains.During the powder compaction,the friction coefficient would eventually tend to be stable,a feature which is also closely related to the slip ratio between particles.The side wall friction and the friction between particles would have an important effect on the direction of force chain growth in about one-third of the area near the side wall.The research results provide theoretical guidance for improving the densification process of the powder according to the force chain and friction.
文摘Ti6Al4V powder compaction was performed by using magnetic pulse compaction in air at 200℃.Effects of process parameters such as voltage,capacitance,discharge times on the microstructure,compressive strength,hardness and relative density of compacts were investigated.The experimental results show that the relative density,hardness and compressive strength of compacted specimens increase with increasing voltage.In addition,the relative density and compressive strength of compacted specimens increase with the augmentation of capacitance in the range investigated.The relative density increases,the hardness firstly increases and then tends to be a fixed value;and the compressive strength firstly increases and then decreases from one to five times compaction.Both values of the hardness and compressive strength reach the maxima of HRA 69.1 and 1 062.31 MPa,at three times compaction,respectively.There are pores in and between particles.
文摘Powder compaction is one of the most crucial processes in initiating explosive device manufacturing. The quality of powder compaction may influence the performances of product directly. Traditional powder compaction mainly makes use of manual gravitational spiral loading machine or lever loading machine. Potential accident by the strike in powder compaction of initiating explosive device could risk life, property and the environment. To prevent this risk, automatic control system and control strategy can be used to guarantee no strike in pressing process. The scope of this paper is to analyze and review the control strategy for powder compact and discuss the experimental results for the application of the proposed strategy.
文摘Based on an analysis of the validity of the powder compaction equation of Kawakita,a modified compaction equation is proposed.It is shown by the statistical analysis on the experimental compaction data of various powders that in most cases the proposed equation provides a better description of the compaction data than Kawakita's equation,especially in the cases of the compaction of hard material powders.
文摘Compaction processes are one the most important par ts of powder forming technology. The main applications are focused on pieces for a utomotive, aeronautic, electric and electronic industries. The main goals of the compaction processes are to obtain a compact with the geometrical requirements, without cracks, and with a uniform distribution of density. Design of such proc esses consist, essentially, in determine the sequence and relative displacements of die and punches in order to achieve such goals. A.B. Khoei presented a gener al framework for the finite element simulation of powder forming processes based on the following aspects; a large displacement formulation, centred on a total and updated Lagrangian formulation; an adaptive finite element strategy based on error estimates and automatic remeshing techniques; a cap model based on a hard ening rule in modelling of the highly non-linear behaviour of material; and the use of an efficient contact algorithm in the context of an interface element fo rmulation. In these references, the non-linear behaviour of powder was adequately desc ribed by the cap plasticity model. However, it suffers from a serious deficiency when the stress-point reaches a yield surface. In the flow theory of plasticit y, the transition from an elastic state to an elasto-plastic state appears more or less abruptly. For powder material it is very difficult to define the locati on of yield surface, because there is no distinct transition from elastic to ela stic-plastic behaviour. Results of experimental test on some hard met al powder show that the plastic effects were begun immediately upon loading. In such mater ials the domain of the yield surface would collapse to a point, so making the di rection of plastic increment indeterminate, because all directions are normal to a point. Thus, the classical plasticity theory cannot deal with such materials and an advanced constitutive theory is necessary. In the present paper, the constitutive equations of powder materials will be discussed via an endochronic theory of plasticity. This theory provides a unifi ed point of view to describe the elastic-plastic behaviour of material since it places no requirement for a yield surface and a ’loading function’ to disting uish between loading an unloading. Endochronic theory of plasticity has been app lied to a number of metallic materials, concrete and sand, but to the knowledge of authors, no numerical scheme of the model has been applied to powder material . In the present paper, a new approach is developed based on an endochronic rate independent, density-dependent plasticity model for describing the isothermal deformation behavior of metal powder at low homologous temperature. Although the concept of yield surface has not been explicitly assumed in endochronic theory, it is shown that the cone-cap plasticity yield surface (Fig.1), which is the m ost commonly used plasticity models for describing the behavior of powder materi al can be easily derived as a special case of the proposed endochronic theory. Fig.1 Trace of cone-cap yield function on the meridian pl ane for different relative density As large deformation is observed in powder compaction process, a hypoelastic-pl astic formulation is developed in the context of finite deformation plasticity. Constitutive equations are stated in unrotated frame of reference that greatly s implifies endochronic constitutive relation in finite plasticity. Constitutive e quations of the endochronic theory and their numerical integration are establish ed and procedures for determining material parameters of the model are demonstra ted. Finally, the numerical schemes are examined for efficiency in the model ling of a tip shaped component, as shown in Fig.2. Fig.2 A shaped tip component. a) Geometry, boundary conditio n and finite element mesh; b) density distribution at final stage of
基金Projects(50325516,50135020) supported by the National Natural Science Foundation of ChinaProject(003019) supported by the Natural Science Foundation of Guangdong Province,China
文摘Some flow stress models was used to simulate the compaction of a cylindrical specimen.A new flow stress model was presented by analyzing and summarizing the powder compact experiment with different initial densities.Compared with other models,the new model fits better with the experiments.The compaction process of a synchronizer hub was simulated with new model and the synchronizer hub shape was improved by the simulation results.The density distribution was measured and compared with the simulation results of splitting the samples into pieces.The comparison shows that the model could be used to simulate the powder compact process in an efficient and accurate manner.And the results indicate that the density distribution is homogenous after optimization.
基金Porject(50325516) supported by the National Natural Science Foundation of China Project(CG2003-GA005) supported by China Education and Research Grid (China Grid) Project(003019) supported by the Natural Science Foundation of Guangdong Province, China
文摘The constitutive relation of powder material was derived based on the assumption that metal powder is a kind of elasto-plastic material, complying with an elliptical yield criterion. The constitutive integration algorithm was discussed. A way to solve the elastic strain increment in each iteration step during elasto-plastic transition stage was formulated. Different integration method was used for elastic and plastic strain. The relationship between model parameters and relative density was determined through experiments. The model was implemented into user-subroutines of Marc. With the code, computer simulations for compaction process of a balancer were performed. The part is not axisymmetric and requires two lower punches and one upper punch to form. The relative density distributions of two design cases, in which different initial positions of the punches were set, were obtained and compared. The simulation results indicate the influence of punch position and movement on the density distribution of the green compacts.
文摘A computer simulation procedure for metal powder die compaction was described. Friction behavior of metal powder during cold compaction was simulated by the finite element method. The movement of powder relative to the die wall was taken into consideration by utilizing the shear friction model. Friction between the powder and the rigid die wall leads to inhomogeneous density distribution during the compaction process. The floating die technique and double punch pressing can attain more homogenous compacts than the fixed die technique can do. The results obtained from numerical analysis agree well with the experimental results. Simulation model was built in MSC.Mentat, and MSC.Marc software was used to calculate the powder compaction process.
文摘The present study aims to fabricate and evaluate the mechanical properties and wear behavior of Mg metal matrix composite,reinforced by 0,1.5,3,5 and 10 vol.%B4C microparticles.Mg−B4C samples were fabricated at 450℃ and under different loading rates by using split Hopkinson bar(SHB),drop hammer(DH)and Instron(QS)at strain rates of 1600,800 and 0.008 s–1,respectively.The mechanical properties including microhardness,quasi-static and dynamic compressive strengths and wear behavior of samples were experimentally investigated.The results show that,the hardness of SHB and DH samples is obtained to be 20.2%and 5.7%higher than that of the QS sample,respectively.The wear rate and wear mass loss of Mg–10.0%B4C samples fabricated by SHB were determined lower than those of the QS sample by nearly 33%and 39%,respectively.The quasi-static compressive strengths of Mg−5.0%B4C are improved by 39%,30%and 29%for the SHB,DH and QS samples,respectively,in comparison with the case of pure Mg.Furthermore,it is discovered that the dynamic compressive strength of samples is 51%−110%higher than their quasi-static value with respect to the B4C content.
文摘The low-voltage-electromagnetic forming was applied to powder compaction.A series of experiments was performed to compact aluminum,copper and tin powders in an indirect working way.Having compacted high-density powder parts successfully,the authors analyzed the effects of voltage,capacitance,friction,compaction times,powder size and other factors on the densities of compacted specimens.The experimental results show that lower voltage but larger capacitance are beneficial to increasing the density and homogeneity of the compacted specimens,if the loading velocity and discharging energy are suitable.The higher the voltage,the greater the percentage of energy consumed by friction.If the equipment energy is limited,the iterative compaction is an efficient way to manufacture homogeneous and high-density powder parts.
文摘This paper describes a new method of surface modification by Electrical Discharge Machining (EDM). By using ordinary EDM machine tool and kerosene fluid, a hard ceramic layer can be created on the workpiece surface with Ti or other compressed powder electrode in a certain condition. This new revolutionary method is called Electrical Discharge Coating (EDC). The process of EDC begins with electrode wear during EDM,then a kind of hard carbide is created through the thermal and chemical reaction between the worn electrode material and the carbon particle decomposed from kerosene fluid under high temperature. The carbide is piled up on a workpiece quickly and becomes a hard layer of ceramic about 20 μm in several minutes. This paper studies the principle and process of EDC systemically by using Ti powder green compact electrode. In order to obtain a layer of compact ceramic film, it is very important to select proper electric pulse parameters, such as pulse width, pulse interval, peak current. Meantime, the electrode materials and its forming mode will effect the machining surface quality greatly. This paper presents a series of experiment results to study the EDC process by adopt different technology parameters. Experiments and analyses show that a compact TiC ceramic layer can be created on the surface of metal workpiece. The hardness of ceramic layer is more 3 times higher than the base body, and the hardness changes gradiently from surface to base body. The method will have a great future because many materials can be easily added to the electrode and then be coated on the workpiece surface. Gearing the parameters ceramic can be created with different thickness. The switch between deposition and removal process is carried out easily by changing the polarity, thus the gear to the thickness and shape of the composite ceramic layer is carried out easily. This kind of composite ceramic layer will be used to deal with the surface of the cutting tools or molds possibly, in order to lengthen their life. It also can be found wide application in the fields of surface repairing and strengthening of the ship or aircraft.
文摘The compaction of pure Cu powder was carried out through a series of experiments using dynamic magnetic pulse compaction, and the effects of process parameters, such as discharge energy and compacting direction, on the homogeneity and the compaction density of compacted specimens were presented and discussed. The results indicated that the compaction density of specimens increased with the augment of discharge voltage and time. During unidirectional compaction, there was a density gradient along the loading direction in the compacted specimen, and the minimum compaction density was localized to the center of the bottom of the specimen. The larger the aspect ratio of a powder body, the higher the compaction density of the compacted specimen. And high conductivity drivers were beneficial to the increase of the compaction density. The iterative and the double direction compaction were efficient means to manufacture the homogeneous and high-density powder parts.
基金National Nature Science Foundation of China (51033026)
文摘High velocity compaction process of atomized Al powders was studied. The green density, the maximal force and the withdraw force of specimen were investigated. The green density of atomized aluminum powder was obtained to be 2.68 g/cm3 and its relative density is about 99%. The maximal force increased proximately linearly with the compaction energy. The withdraw force was observed ranging between 30 and 70 kN. The radial spring back was less than 0.1%.
基金supported by National 973 Program (No.2006CB605207)MOE Program for Changjiang Scholars and Innovative Research Team in Universityof China (No.I2P407)
文摘A new method for producing higher density PM parts, high velocity compaction （HVC）, was presented in the paper. Using water atomized pure iron powder without lubricant admixed as the staring material, ring samples were compacted by the technique. Scanning electron microscopy （SEM） and a computer controlled universal testing machine were used to investigate the morphologies and the mechanical properties of samples, respectively. The relationships among the impact velocity, the green density, the sintered density, the bending strength and the tensile strength were discussed, The results show that with increasing impact velocity, the green density and the bending strength increase gradually, so the sintered density does. In addition, the tensile strength of sintered material is improved continuously with the sintered density enhancing. In the study, the sintered density of 7.545 g/cm^3 and the tensile strength of 190 MPa are achieved at the optimal impact velocity of 9.8 m/s.
文摘By phenomenological analysis of warm compaction, it is found that, compared with the contribution of particle plastical deformation to densification of powder compact,the particle rearrangement is a dominant densification mechanism for powder warm compaction, and the plastical deformation of particles plays an important role in offering accommodating deformation for particle rearrangement and densifying powder compact at the final stage of pressing.In order to attain density gain as high as possible during warm compaction, six rules for designing warm compacting powder mixtures were proposed in detail.
文摘Die wall lubrication was applied on warm compaction powder metallurgy in hope to reduce the concentration level of the admixed lubricant since lubricant is harmful to the mechanical property of the sintered materials. Iron-based samples were prepared by die wall lubricated warm compaction at 135 ℃ and 175 ℃, using polytetrafluoroethylene (PTFE) emulsion as die wall lubricant. A compacting pressure of 700 MPa and 550 MPa were used. The admixed lubricant concentration ranging from 0 to 0.6 wt.% was used in this study. Compared with non-die wall lubricated samples, the die wall lubricated samples have higher green densities. Results show that in addition to the decrease in ejection forces, green density of the compacts increased linearly with the decrease in admixed lubricant content. Mechanical property of the sintered compacts increase sharply when the admixed lubricant concentration reduced to 0.125 wt.% or less. Ejection force data indicated that samples with die wall lubrication show lower ejection forces when compared with samples without die wall lubrication. No scoring was observed in all experiments even for samples contain no admixed lubricant. Our results indicated that under experimental condition used in this study, no matter at which compaction pressure, compaction temperature, graphite and lubricant contents in the powder the die wall lubricated warm compaction would give the highest green density and lowest ejection force. It can be concluded that combination of die wall lubrication and warm compaction can provide P/M products with higher density and better quality. It is a feasible way to produce high performance P/M parts if suitable die wall lubrication system was applied.
基金the National Natural Science Foundation of China(No.50375114)State Key Laboratory of Plastic Forming Simulation and Die & Mould Technology(No.05-03)
文摘Low-voltage electromagnetic compaction （EMC） was used to compact metal powders （Cu） and ceramic powders （TiO2） in the indirect way. It was found that the density of the metal powder parts compacted by low-voltage EMC varied linearly with the discharging voltage in the range investigated. But for ceramic powders, the discharging voltage has an optimal value. Under the value, the density increases as discharging voltage rises, but beyond the value the trend is reverse. The experimental results show that the density of the metal parts decreases gradually along press direction. And the density of the ceramic parts decreases with the advancement of the aspect ratio h/d （height/diameter）. In addition, repetitive compaction can improve the density of both metal and ceramic parts and reduce the effects of aspect ratio on the density.
