Application of upper bound solution for extrusion of PM aluminium alloys

An upper bound solution with a yield criterion for porous materials is employed for hot extrusion of the PM 7XXX series aluminium alloy. In the model, the kinematically admissible displacement increment field has discontinuities not only in the tangential components, but also in the normal component. The latter one is responsible for a change in density when the porous material passes the discontinuity. The theoretical results show the influence of die geometry (die semiangle, reduction ratio), friction shear factor, and density of porous billet on the total extrusion pressure. Power dissipated due to interfacial friction and velocity discontinuities is presented together with conditions for a dead metal zone formation. The model is verified by experimental results relating extrusion pressure to die semiangles. A satisfactory level of correlation between theory and experiment is shown. The work emphasizes the importance of the interfacial friction, which strongly influences the degree of this correlation.     

Study of workability limits of porous materials under different upsetting conditions by compressible rigid plastic finite element method

Workability limits must be considered when designing powder metallurgy (PM) forging processes. This research successfully applied the general upsetting experiment method to the deformation of porous materials. Based on the plastic theory of porous materials, the compressible rigid plastic finite element method is used to simulate the deformation processes of cold upsetting of disks and rings for porous metal materials with a full account of contact friction boundary conditions, the height-to-diameter ratio, the initial relative density, and the die and workpiece geometry. Furthermore, a successful analysis of the cold forging process results in the prediction of the stress, the strain, and the density field. By coupling with the ductile fracture criterion, which is a strain-based criterion obtained by Lee and Kuhn, possible defects leading to material failure have been checked. This research reveals that larger height to diameter and a lesser friction factor can delay the local strain locus to intersect with the Lee and Kuhn’s fracture line and restrain formation of the surface crack. Meanwhile, it reveals that the initial relative density has only a very small influence on the strain to fracture in compression, and it shows the forming behavior of the ring and disk with the curved die. According to Lee and Kuhn’s results, the calculated results agree well with the experimental results.     

半固态金属触变塑性成形上限法

在高固相率时,半固态金属的力学模型可简化为连续多孔体力学模型。针对半固态金属触变塑性成形的特点,本文发展了上限法理论在半固态金属触变塑性成形分析中的应用。提出速度间断值不仅要包括切向分量,还要包括法向分量。速度间断的法向分量是由金属通过速度不连续面时,材料固相率的变化而引起的。由此建立了半固态金属触变塑性成形的上限分析模型和理论方法,导出了上限功率的计算公式。为在实际触变塑性成形工艺分析中的进一步应用奠定了理论基础。     

Yield condition and compression characteristics of semi-solid material by upper-bound method

In semi-solid forging, it is necessary to control the forming variables accurately in order to make near net shape products. Generally, the defects of products may occur due to liquid segregation that can be caused by deformation, strain rate, and condition of friction. The liquid segregation is to be predicted by flow analysis. This paper presents upper bound analysis model using the new yield function proposed by Doraiveluet al. The model has been applied to analyze the simple compression process of semi-solid Sn-15%Pb alloys to confirm the usefulness of the yield function. The radial distribution of the liquid fraction at various strains, strain rates, and friction conditions between die and workpiece has been estimated.     

MODELLING OF P/M FORMING PROCESSES BY FEM

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Densification modelling for nanocrystalline metallic powders

A model for densification of metallic powders is proposed. It involves viscoplastic constitutive equations based on dislocation density evolution and also accounts for effects of porosity using a pressure-dependent critical density yield criterion. The model was applied to the case of cold compaction of nanocrystalline copper under uniaxial compression conditions. Densification behaviour during powder compaction was simulated using an explicit integration method as applied to the dislocation density evolution and the variation of the relative density of the compact. The model was gauged by comparing the experimental data generated by cylindrical die compaction tests on Cu powder with the simulation results. The model accounts for the grain size and the deformation rate dependence on the densification process. The proposed densification model was implemented into a finite element code. The finite element method was applied to simulating room temperature die compaction of nanocrystalline Cu powder in order to investigate the densification behaviour.     

Ni-625 Superalloy Foam Processed by Powder Space-Holder Technique

In this work, porous Ni-625 superalloys were produced by use of a water soluble pore forming agent prior to sintering. Carbamide particles were used as the space-holder materials. After mixing and compaction, the space-holder particles were extracted using warm water leaching over a range of temperatures. The porous green parts were subsequently thermally debinding to remove the paraffin wax under a pure Argon atmosphere, and subsequently sintered at high vacuum. The effects of volume fraction of space-holder particles on density, porosity, and elastic modulus were investigated. Microstructures were captured using optical and scanning electron microscopy. Pore size was quantified using image analysis software integral to the scanning electron microscopy. In addition, compression tests were conducted on the sintered samples.     

On predicting forming limits using hill’s yield criteria

The analysis of localized necking is strongly dependent on the yield function. To predict forming limits, therefore, numerous yield criteria have been postulated to characterize the plastic deformation of sheet materials. Among them Hill’s 1948 and the fourth form of 1979 yield criteria are the most commonly used. A new yield criterion was proposed by Hill in 1993. It uses five independent and easily obtainable material parameters, which makes it flexible in representing the shape of the yield locus for different materials. The present investigation compares these three yield criteria in forming limit predictions based on both the Marciniak and Kuczynski (M-K) approach and the bifurcation analysis. It is observed that the M-K analysis based on Hill’s 1993 yield criterion provides forming limit predictions in agreement with experimental data. The bifurcation analysis based on Hill’s 1948 yield criterion also provides an acceptable prediction of forming limits for aluminum, although they are slightly higher. All three yield criteria are found to provide acceptable predictions for aluminum-killed (AK) steel based on the M-K method. For brass, only the prediction based on the M-K method and Hill’s 1993 yield criterion is close to the trend of experimental data.     

INVESTIGATION ON THE DENSITY UNIFORMITY OF PRESSED PARTS BY LOW-VOLTAGE ELECTROMAGNETIC COMPACTION

1.IntroductionElectromagnetic compaction is a kind of high-energy and high-speed forming method. Magnetic forces have been used for more than two decades in powder compaction[1,2]. Powder Electromagnetic compaction refers to a new efficient forming technology, which uses the strong pulse electromagnetic power to compact powder to pressed density. The corresponding principle is as follows: by dint of magnetism pulse equipments, capacitor groups discharge electricity to generate the pulse m…     

INVESTIGATION ON THE DENSITY UNIFORMITY OF PRESSED PARTS BY LOW-VOLTAGE EI,RCTROMAGNETIC COMPACTION

Powder metallurgy is an efficient approach to fabricate varieties of high performance structure materials, function materials and special materials working under limited conditions. Research and development of new efficient technology to form high-density,high-performance and net shape parts is a key to widen application and development of powder materials. Recently, the low-voltage electromagnetic compaction (EMC) has been used by present authors to compacted copper, tin, aluminum powders and the products with 99% relative density have been acquired. In this work, the research has been extended to investigation on the density uniformity of pressed parts. The analysis results show that the density of the part compacted by low-voltage EMC decreases gradually in press direction as static compaction. But it is higher and more homogeneous. The densit3““ of the top part increases gradually from the center to the outer, which is just reversal of the bottom part. In some extent, the higher the discharging voltage is, the higher the densiO‘ is and the more homogeneous the distribution is. In addition, repetitive compaction can improve the density of powder parts and the distribution uniformity.     

Deformation characterization of powdered metals in compaction

A plasticity theory is developed for a powdered metal compact. The yield criterion used for sintered powdered metals is modified for describing asymmetric behavior of powdered metal compacts in tension and in compression. Three parameters α, δ, and p are involved in the yield function. The two parameters, α and δ, are determined from the behavior of sintered powdered compacts as functions of relative density R. Green strength vs relative density R relationship for powdered metal compacts is used for defining completely the yield function with the effect of relative density. Predictions are made for compaction pressures as functions of relative density for unidirectional, bidirectional and isostatical compaction, and compared with experimental data for copper and iron powders. Also yield surfaces for various density levels are compared with the experimental data with copper powders.     

A mathematical theory of plasticity for compressible powder metallurgy materials — Part III

A new mathematical equation taking into account of all new and relevant parameters to represent the flow behaviour of partially dense P/M materials during the case of simple upsetting-compression test is proposed in this paper. The ratio of the yield stress of the P/M-porous material to the yield stress of 100% dense material is found to be increasing with increase in deformation strain. Further, a theory for simple plastic instability and work hardening rate of porous P/M-material has been developed with strain hardening index and strain rate sensitivity parameters.     

Characterisation of kinematic hardening and yield surface evolution from uniaxial to biaxial tension with continuous strain path change

With respect to multistage forming processes the material behaviour and the history of the strain path during the process is of special interest for the improvement of the numerical prediction of forming processes. While different researchers investigated the Bauschinger effect during a load reversal and biaxial loading with pre-strained specimens, in this paper the yield locus evolution in the first quadrant of the principles stress space under biaxial loading of a modified cruciform specimen without test interruption is presented. The movement of the yield surface centre caused by kinematic hardening is approximated by an alternative approach based on experimental results.     

Fundamental study of surface densification of PM gears by rolling using FE analysis

The complex shape of gears is very attractive to the near-net-shape PM technology. The strength of conventional PM parts is reduced by the residual porosity due to the power law relationship between the density and the mechanical properties. The maximum stresses in gears are found in the tooth root and the flank near or directly at the surface, so that by a local densification of the near surface layer the maximum load carrying capacity of the complete gear can be increased. This surface densification can be achieved by a rolling process. Due to the own elastic-plastic behavior of PM materials and due to the continuous change of the contact conditions during rolling, the process is too complex to be described analytically. To gain a better understanding of the process, the numerical simulation is a capable method. In this paper an FEA model is prepared and verified based on an experimental investigation. Additionally, case studies were carried out in order to analyze the relationship between the process parameters and the densification result. The investigations described in this present paper were sponsored by the WZL Gear Research-Circle.     

置氢TC21合金粉末压制方程研究

为建立置氢TC21钛合金粉末的压制性能评价体系,和置氢TC21合金粉末室温模压成形过程压制方程,测定了置氢TC21合金粉末外加压力、压坯密度及压坯强度与合金粉末置氢量的关系,将试验数据代入推导的置氢TC21钛合金粉末的压制方程之中,采用VB求解超限定方程组,计算出方程中所含的3个参数的值,并最终定量研究了这3个参数与置氢量的关系,确定了占主导地位的成形机制,最终得到了置氢TC21合金粉末压制性能及压制方程,从而为精确控制置氢钛合金粉末模压成形提供了依据。     

Evaluation of tubular materials by a hydraulic bulge test

This paper aims to evaluate the properties of tubular materials by hydraulic bulge tests combined with a newly proposed analytical model. Annealed AA6011 aluminum tubes and SUS409 stainless steel tubes are used for the bulge test. The tube thickness at the pole, bulge height and the internal forming pressure are measured simultaneously during the bulge test. From above experimental data, the effective stress–effective strain relations can be derived by this analytical model assuming the profile of the free bulge region as an elliptical surface. The flow stresses of the tubular materials by this approach are compared with those obtained by the tensile test and Fuchizawa's model. The finite element method is used to conduct the simulations of hydraulic bulge forming with the flow stresses obtained by the above-mentioned models. The analytical results of forming pressures versus bulge heights are compared with the experimental results to validate the approach proposed in this paper.     

Mechanical Response of Triply Periodic Minimal Surface Structures Manufactured by Selective Laser Melting with Composite Materials

It is of significance but remains a pivotal challenge to simultaneously enhance the strength and lightweight levels of porous structures. We provide an innovative strategy to improve the strength of porous structures with unchanged lightweight levels by applied composite materials. Selective laser melting (SLM) is convenient for integral forming of materials and structures. Hence, in this study, the research about the mechanical response of triply periodic minimal surfaces (TPMS) porous structures with 316L and composites fabricated by SLM was conducted. The compression test and finite element method (FEM) were used to characterize mechanical properties. The composite structures exhibit enhanced elastic modulus, yield strength, unvaried lightweight level and refined grain microstructure, which are difficult to realize for porous structures made by pure 316L materials. The elastic modulus, yield strength, plateau stress and energy absorption of composites were 3187.50, 67.73, 15.24 and 17.09 MJ/m³, respectively.     

电磁成形技术在粉末成形中的应用

介绍了电磁成形技术、粉末挤压成形方法以及电磁成形技术在粉末成形中的应用.用电磁成形技术压制粉末材料是获得高密度、高性能粉末冶金制品的一种有效方法,是低成本制造高密度陶瓷零件的新途径.     

Application of computer aided upper bound method in forging and extrusion processes

This paper presents the development of a Computer Aided Upper Bound software for the plain-strain upset forging, and closed impression die processes. The upper bound method is applied by providing the number of plastic sections (triangles). friction conditions of the tool surfaces, and geometry parameters as inputs to a predefined tool and workpiece configuration. The program calculates the forming pressure and provides the upper bound metal flow field and hodograph as outputs. The forming pressure is calculated by an upper bounding algorithm which can be improved by increasing the number of plastic sections. The differences in tool and workpiece surface friction coefficients and variation of die angles can be considered in order to optimize the forming process.     

厚壁管件上小尺寸深锥孔的成形加工及工艺尺寸的预测

在采用上限法的流函数模式对常规厚壁管件冷挤压成形建立理想模型的基础上,定性地分析了常规开式冷挤压成形规律。然后用实验研究的方法研究了厚壁管件上小尺寸深锥孔的成形加工及预测其变形后的工艺尺寸,对用理论计算的结果进行了修正。