Effect of friction between powder and a mandrel on densification of iron powder during cold isostatic pressing

The effects of friction between the powder and the mandrel on densification behavior of metal powder were investigated under cold isostatic pressing. The friction coefficients between the powder and the mandrels with different surface roughness were determined from the relationship between the compaction pressure and the ejection pressure of the mandrel from powder compacts. The elastoplastic constitutive equations based on the yield function of Shima and Oyane were implemented into a finite element program (ABAQUS) to simulate compaction responses of metal powders during cold isostatic pressing. Finite element results were compared with experimental data for pure iron powder under cold isostatic pressing.     

Densification behaviour of ceramic powder

A three-dimensional compaction device has been developed to carry out compaction of a ceramic powder. The details of the device, which provides compaction with various stress ratios, are described. A densification criterion for the powder is proposed; this is similar to that for the case for metal powders, but the role of the hydrostatic stress component appears to be different. Stress-strain rate relations are then derived using the concept of plastic potential; experimental results show that the normality of the strain rate vector to the surface which corresponds to the criterion almost holds.     

Die compaction of copper powder designed for material parameter identification

This article presents uniaxial compaction experiments of a fine copper powder in a cylindrical die. The compaction process consists of monotonic loading and of loading paths with inserted unloading and reloading cycles. An experimental setup that has been developed for determining the axial and radial stresses during the compaction is described and the calibration of the new device using highly accurate p-finite element simulations of the dies response to internal pressure is shown. The experimental results were subsequently used for the identification of the material parameters of a constitutive model for granular materials recently proposed by Bier and Hartmann [A finite strain constitutive model for metal powder compaction using a unique and convex single surface yield function. accepted for publication by European Journal of Mechanics, Series A/Solids 2006.]. The identification of the elasticity parameters was treated with special attention.     

A single cone-cap plasticity with an isotropic hardening rule for powder materials

In this paper, a new single cone-cap plasticity with an isotropic hardening rule is presented for powder materials. A general form is developed for the cap plasticity, which can be compared with some common double-surface plasticity models proposed for powders in literature. The constitutive elasto-plastic matrix and its components are derived based on the definition of yield surface, hardening parameter and nonlinear elastic behavior, as a function of relative density of powder. Different aspects of the model are illustrated and the procedure for determination of powder parameters is described. Finally, the applicability of the proposed model is demonstrated in numerical simulation of triaxial and confining pressure tests.     

Numerical modelling of cold compaction of metal powder

A finite element programme has been developed for the analysis of porosity and stress distributions in a powder compact, based on rate-independent finite strain plasticity theory. The strain hardening versions of the Gurson model (J. Engng. Mater. Technol., 1977, 99, 2-15), the more recent FKM model (J. Mech. Phys. Solids, 1992, 40(5), 1139-1162), developed by Fleck, Kuhn and McMeeking, and a combination of the two models are used. The friction between the mould wall and the metal powder is modelled by a combination of Coulomb friction and a constant friction shear stress, since Coulomb friction is not realistic at high normal pressures. The finite element programme has been used to study the effects of friction, compaction method, and material parameters. Analyses for powder compacts of various geometries are presented to illustrate the method.     

A densification model for mixed metal powder under cold compaction

Densification behavior of mixed copper and tool steel powder under cold compaction was investigated. By mixing the yield functions proposed by Fleck et al. and by Gurson for pure powder in terms of volume fractions of Cu powder and the fraction of contact, a new mixed yield function was employed for densification of powder composites under cold compaction. The constitutive equations were implemented into a finite element program (ABAQUS) to compare with experimental data and with results from the model of Kim et al. for densification of mixed powder under cold isostatic pressing and die compaction. Finite element calculations by using the yield functions mixed by the fraction of contact agreed better than those by volume fractions of Cu powder with experimental data.     

板料屈服行为及强化规律的研究进展

研究板料塑性成形的理论基础是屈服准则、强化规律以及本构模型。随着新材料、新工艺的不断出现,温度和应变速率对塑性成形过程中的影响也不容忽视,原有的塑性理论已无法满足研究和工程应用的需求。从板料屈服准则研究、包辛格效应与强化模型研究、屈服强化规律试验方法研究以及涉及应变速率和温度的板料屈服强化研究4个方面阐述板料屈服行为及强化规律的研究进展,指出常用屈服准则的特点和不足,说明各种强化模型中组合强化模型仍然是研究重点。试验方法主要从研究屈服轨迹的双向拉伸试验及确定强化模型参数试验的2个方面进行介绍。此外,指出针对板料在复杂应力状态下应力张量与应变张量之间的涉及应变率和温度的屈服准则和相应的流动准则的本构关系还有待研究。提出随着新材料、新工艺的不断出现,涉及应变速率和温度的屈服准则和强化规律、试验方法以及在有限元模拟中的应用等研究将是未来的研究热点。     

Determining coefficients of friction in compaction of refractory metal powders

Knowing the coefficients of friction in tool compaction of powders of metals and alloys allows one to rationally design technological equipment for manufacturing powder semifinished products experiencing minimal warping under vacuum or hydrogen sintering. This is of particular significance when consumable electrodes are produced from powders of refractory metals being compacted as rather long fillets that are curved in sintering if any irregularities in the density in the cross section and in the fillet bulk are present. Both well-known and new methods are analyzed for finding the coefficients of friction in powder compaction, in particular in cylindrical containers. Stable and valid measurement results are shown to be unachievable. A new method for experimental determination of the coefficients of friction under powder compaction is described. It consists in comparing the force parameters of one- and two-sided compaction. This method allows finding the coefficient of side pressure and contact friction (on the cylindrical surface of the container) during the formation of briquettes of TsM-2A alloy with different concentrations of plasticizer and solvent. A positive effect of a plasticizer and negative one of a solvent on the coefficient of friction is stated.     

Genetic algorithm-based numerical optimization of powder compaction process with temperature-dependent cap plasticity model

In this paper, a shape optimization technique is presented for the cold and hot isostatic pressing of metal powders based on the genetic algorithm (GA) approach. The GA technique is used to obtain the desired optimal compacted component by changing the boundaries of component and verifying the prescribed constraints. The coupled thermomechanical analysis of hot isostatic pressing is employed for metal powders during densification process. The numerical modeling of hot powder compaction simulation is performed based on the large deformation formulation, temperature-dependent cap plasticity model, and frictional contact algorithm. The modified cap plasticity takes the temperature effects into the numerical simulation of highly nonlinear behavior of metal powder. Finally, numerical examples are analyzed to demonstrate the feasibility of proposed optimization algorithm for designing powder components in the cold- and hot-forming processes of powder compaction.     

A new test method for measuring the galling resistance between metal powders and die tool materials in powder compaction

The friction characteristics and galling resistance between metal powder and die tool material in metal powder compaction is of outmost importance since they will influence the porosity and surface quality of the green body and consequently the porosity, tolerances and surface quality of the final sintered product. In the present study, a new test method for evaluating the tribological performance of die tool materials aimed for powder compaction is presented. The test method is based on controlled scratch testing using a commercial scratch tester but instead of the commonly used Rockwell C diamond stylus a sample holder with a small green body of compacted powder particles is drawn over the surface in a well controlled multi pass linear reciprocating sliding contact. The capability of the test method was evaluated for different types of tool materials including two PVD coatings in contact with different types of metal powders to determine the friction characteristics and the adhesion and material transfer tendency at the sliding interface. Post-test examination of the tool surfaces using FEG-SEM and EDS were performed in order to evaluate the mechanisms controlling the friction behavior and the material transfer tendency. The results show that the proposed test is a simple and fast method to obtain relevant data regarding the friction and galling characteristics of die tool materials in metal powder compaction. The mechanisms prevailing at the green body/die tool material interface, e.g. cold welding, can easily be monitored by the friction and acoustic emission signals. Of the die tool materials investigated the low friction PVD a-C:Cr coating displayed the lowest friction and highest galling resistance.     

Workability studies on Al–5%SiC powder metallurgy composite during cold upsetting

The present technical work reports on the workability performance along with the consolidation behavior of aluminum (Al) and Al–5% silicon carbide (SiC) powder metallurgy composites during cold compaction. An experimental work has been carried out to investigate the powder compaction behavior of Al–SiC metal matrix composites. SiC of particle sizes 120, 75, and 45 μm has been pre-alloyed with Al powders of particle size ranging from ?37 to 75 μm. Various particle size additives of SiC have been used as a second-phase particle in this work with the intension of predicting the mechanical and metallurgical properties of the metal matrix composites studied. The pressure applied for the preparation of compacts have been considered as 220–260 kN in order to prepare the samples of heights in the range of 30 to 32 mm, and the diameter of the compacted sample was 26.11 mm. The densification during compaction is measured by means of the presence of voids in the compacts applying the mass constancy principle. The effect of particle size on the metal matrix composites proposed has been completely investigated under two different stress state conditions such as plane and triaxial.     

Modeling the powder compaction process using the finite element method and inverse optimization

This paper focuses on studying and adapting modeling techniques using the finite element method to simulate the rigid die compaction of metal powders. First, it presents the implementation of the cap constitutive model into ABAQUS FE software using the closest point projection algorithm. Then, an inverse modeling procedure was proposed to alleviate the problems raised by the interpretation of the experimental tests and to more accurately determine the material parameters. The objective function is formed, based on the discrepancy in density data between the numerical model prediction and the experiment. Minimization of the objective function with respect to the material parameters was performed using an in-house optimization software shell built on a modified Levenberg?CMarquardt method. Thus, an integrated simulation module consisting of an inverse optimization method and a finite element method was developed for modeling the powder compaction process as a whole. The simulation and identification module developed was applied to simulate the compaction of some industrial parts. The results reveal that the maximum absolute error between densities is 2.3%. It corresponds to the precision of the experimental method.     

Experimental based finite element simulation of cold isostatic pressing of metal powders

The present work addresses the various ingredients required for reliable finite element simulations of cold isostatic pressing (CIP) of metal powders. A plastic constitutive model for finite deformation is presented and implemented into an explicit finite element (FE) code. The FE implementation is verified so that numerical errors (both temporal and spatial errors) are kept under control. Thereafter, uniaxial die compaction experiments are performed required for determining the material parameters in the constitutive model. Subsequently they are applied for the simulation of a “complex” CIP process. The experimental observations of the complex CIP process were used to validate the overall method by comparing the FE results (final dimensions and average relative density) to the experimental observations. The numerical results (final dimensions and relative density) are in good agreement with the experimental observations.     

Densification behavior of aluminum alloy powder under cold compaction

Densification behavior of aluminum alloy (Al6061) powder was investigated under cold compaction. Experimental data were obtained under triaxial compression with various loading conditions. A special form of the Cap model was proposed from experimental data of Al6061 powder under triaxial compression. The proposed yield function and several other yield functions in the literature were implemented into a finite element program (ABAQUS) to compare with experimental data for densification behavior of Al6061 powder under cold isostatic pressing and die compaction. The agreement between finite element calculations from the proposed yield function and experimental data is very good under cold isostatic pressing and die compaction.     

Constitutive law for AZ31B Mg alloy sheets and finite element simulation for three-point bending

The constitutive law to describe the anisotropic and asymmetric mechanical behavior of AZ31B magnesium (Mg) alloy sheets at room temperature has been developed here for the plane stress condition, based on the orthotropic yield criterion proposed by Cazacu O, Plunkett B, Barlat F. [Orthotropic yield criterion for hexagonal closed packed metals. International Journal of Plasticity 2006;22:1171–94] and different isotropic hardening laws for tension and compression. Experimental procedures to obtain the material parameters of the yield surface and the hardening laws have been discussed for the AZ31B Mg alloy sheet. For verification purposes, finite element simulation results based on the developed constitutive laws have been compared with experimental results for a three-point bending test.     

Densification of mixed metal powder at high temperature

Densification behaviors of mixed metal powder under high temperature were investigated. Experimental data of mixed copper and tool steel powder with various volume fractions of Cu powder were obtained under hot isostatic pressing and hot pressing. By mixing the creep potentials of McMeeking and co-workers and of Abouaf and co-workers originally for pure powder, the mixed creep potentials with various volume fractions of Cu powder were employed in the constitutive models. The constitutive equations were implemented into a finite element program (ABAQUS) to compare with experimental data for densification of mixed powder under hot isostatic pressing and hot pressing. Finite element calculations by using the creep potentials of Abouaf and co-workers agreed reasonably well with experimental data, however, those by the model of McMeeking and co-workers underestimate experimental data as observed in the case of pure metal powders.     

不同强化模型下的板料成形极限

介绍Hill48屈服准则下基于不同强化模型的屈服方程。推导出能够用来确定随动强化模型和混合强化模型中参数的方程。采用单向拉伸曲线上所取得的数据,对所得方程进行拟合,得到参数值,并使用所得参数值得出三种强化模型下的单向拉伸曲线。结果表明采用上述方法能够准确地确定强化模型中的参数。给出随动强化模型和混合强化模型下成形极限的计算方法。基于三种强化模型,针对分散性失稳准则、Hill集中性失稳准则、凹槽失稳准则和平面应变漂移失稳准则,得到简单加载路径下的成形极限图和成形极限应力图。从这些图中可以看出,强化模型对成形极限图和成形极限应力图影响明显。因此应当确定板料在成形过程中的强化规律,选择合适的强化模型进行成形极限预测。     

Effect of processing parameters on the surface durability of steam-oxidized sintered iron

Surface durability has been reported to be the main factor affecting the tribological behavior of steam-oxidized sintered iron. In this paper, the influence of compaction pressure and powder grade on the surface durability of steam-treated sintered iron is analyzed. Specimens prepared from atomized powders in different sizes were compacted using four different pressures, sintered for 30 min at 1120°C and then subjected to a continuous steam treatment at 540°C for 2 h. The tribological characterization was carried out against a hard steel ball in a reciprocating wear test, in which the electrical contact resistance between the sliding surfaces was continually monitored. The processing parameters had a strong influence on the oxide durability, expressed in terms of the sliding distance required to achieve low contact resistance. High durability was always associated with high compaction pressure and smaller powder size.     

The yield behaviour of metal powders

Copper powders have been densified by cold hydrostatic compaction and by cold closed-die compaction. The stress versus strain relations were measured and the subsequent yield surfaces were probed. The shape of the yield surface is found to be sensitive to the type of compaction employed. Hydrostatic compaction results in a yield surface of approximately elliptical shape, while closed-die compaction produces a yield surface which is elongated along the loading direction with a vertex at the loading point. Existing theories of cold compaction are compared against the observed responses.     

基于双流体模型的粉末压制过程气体流动特性仿真研究

利用实验方法很难获得粉体内部气流轨迹和压力分布的详细信息,因此仿真技术是研究粉末压制的重要手段之一。针对粉体床内部在压制过程中的气体流场变化,采用基于双流体模型的二维多相流CFD软件Fluent对粉体床内的气流进行模拟。通过研究压制过程中初期、中期和末期3个时间点的气体体积分布和速度分布,发现压制速度是粉末压制非常重要的因素。由于粉床中的气体主要聚集在靠近压头的上层,因此提出在压头表面设计排气孔,有助于粉末中的空气流出,提高砖坯质量。