Metal Powder Compaction Research Articles

A constitutive formulation and numerical procedure to model rate effects on porous materials at finite strains

A constitutive formulation for viscoplastic porous materials subjected to finite deformations is developed. To account for porosity effects on the viscous response it is proposed an apparent viscosity, that is, it is assumed that the viscosity depends on the relative density evolution. Using an associative Perzyna-type formulation the constitutive equations as well as the density-corrected consistent tangent modulus are obtained from a suitable extension of the elastoplastic porous model. The proposed constitutive formulation is implemented inside the Finite Element Method context and it is numerically assessed by the analysis of some classical examples, as the isostatic compression, simple shear, and die pressing testing. In addition, a more complex simulation is presented. It is shown that in a relaxation test the viscoplastic porous material recovers the rate-independent response as an asymptotic limit only when some conditions are satisfied. Numerical results show that the proposed constitutive formulation is able to capture the rate effects when porous materials are subjected to high deformation speeds. This type of constitutive formulation is convenient for high-velocity metal powder compaction simulation, where rate effects play an important role on the material response and final properties.

Examination of Electrical Conduction of Carbonyl Iron Powder Compacts

In this study, we report the electrical conduction of carbonyl iron powder compacts when a constant voltage/current source is supplied, especially under a typical voltage 9.7V of electric current activated sintering (ECAS). The potential driving forces for resistance drop during stressing, including bulk temperature-dependent and -independent, were discussed. By using a proposed competition mechanism and percolation theory, it can successfully describe electrical behavior in metal powder compacts. The results indicate bulk temperature rise resulting from Joule heating is the main cause for the formation and growth of metallic contact at particle interfaces during ECAS. [doi:10.2320/matertrans.M2014311]

The role of the native oxide shell on the microwave sintering of copper metal powder compacts

Successful microwave sintering of several metal powders had been reported by many researchers with remarkable improvements in the materials properties and/or in the overall process. However, the concept behind microwave heating of metal powders has not been fully understood till now, as it is well known that bulk metals reflect microwaves. The progress of microwave sintering of copper metal powder compacts was investigated via combining both in-situ electrical resistivity and dilatometry measurements that give important information about microstructural changes with respect to the inter-particle electrical contacts during sintering. The sintering behavior of copper metal powders was depending on the type of the gas used, particle size, the initial green density, the soaking sintering time and the thin oxide layer on the particles surfaces. The thin copper oxide native layer (ceramics) that thermodynamically formed on the particles surfaces under normal handling and ambient environmental conditions had a very critical and important role in the microwave absorption and interaction, the sintering behavior and the microstructural changes. This finding could help to have a fundamental understanding of why MW’s interact with copper metal powder in a different way than its bulk at room temperature, i.e. why a given metal powder could be heated using microwaves while its bulk reflects it.

Structural Description of Powder Metallurgy Prepared Materials

The compaction of powder metals and alloys is very difficult field due to preserving of unique properties of initial materials. One of few possible method of succesful compaction is plasma sintering. To describe detailed structure os powder metallurgy materials, it is necesary to use advanced microscopy methods such as SEM and TEM. In this study, the structure of NiAl intermetallic compaoud is described. The material was at first produce by reactive sintering from pure elements. Subsequently, the NiAl porous master alloy was milled and compacted by spark plasma sintering (SPS) technique.The particle size of NiAl powder was compareable to the grain size of compact material, which exhibited low porosity. It was proven that the interconnection on NiAl particles is performed by thin layer of nanocrystalline oxides.

Mechanical properties of heat treated iron based compacts

In this study, powder metal (PM) compacts were prepared by mixing iron powder with 1–6wt.% ratio of Cu and 0.2–0.8wt.% ratio of graphite. The 20 different PM compacts compositions were prepared with a 0.8wt.% ratio of zinc striate as the lubricant. The dimension of compacts were 10×10×55mm. All the compacts were pressed at 600MPa and, sintered at 1120°C for 30min using a conventional sintering furnace and then boronizing, steaming and carbo-nitriding heat treatments were applied. The results of all compacts for the bending test, microstructure images and HV microhardness results were compared. As a result of the experiments, it was concluded that, the highest bending strength was obtained at carbo-nitrided samples, the highest surface hardness was measured at boronized samples and all these heat treatments increase the densities of iron based samples.

Magnetic-Thermal Analysis and Rapid Consolidation of FE–3 wt.% CU Powder Metal Compacts Sintered by Medium-Frequency Induction-Heated System

In this paper, the results of numerical and experimental studies concerning the effect of mediumfrequency induction sintering of Fe–3 wt.% Cu powder metal (PM) compacts are discussed. Several sintering durations from 30 to 180 sec are studied. The mixed powders are compacted at 600 MPa and sintered by medium-frequency induction system (30–50 kHz frequency and 12 kW power). Magnetic and thermal analysis, mechanical properties, and densities of copper–iron PM compacts are presented.

Ultra high frequency induction welding of powder metal compacts

The application of the iron based Powder Metal (PM) compacts in Ultra High Frequency Induction Welding (UHFIW) were reviewed. These PM compacts are used to produce cogs. This study investigates the methods of joining PM materials enforceability with UHFIW in the industry application. Maximum stress and maximum strain of welded PM compacts were determined by three point bending and strength tests. Microhardness and microstructure of induction welded compacts were determined.

Investigation of Conventional- and Induction-Sintered Iron and Iron-Based Powder Metal Compacts

Induction sintering was developed as an alternative method to conventional sintering to sinter iron-based powder metal (PM) compacts. Several compositions of compact such as pure iron, 3 wt.% copper mixed iron, or 3 wt.% bronze mixed iron were sintered by using induction sintering machines with 12 kW power and 30 kHz frequency. The mechanical properties, microstructural properties, densities, and microhardness values were investigated for both processes. Iron-based PM compacts sintered at 1120°C by induction in 8.33 min (500 s) were found to be similar to those sintered conventionally in 30 min. The results were compared with the experimental studies.

Analysis of Densification Process and Structure of PM Al-Mg-Si-Cu-Fe and Al-Zn-Mg-Cu-Sn Alloys

Abstract The paper is focused on the role of the pressing pressure on the densification behaviour of PM aluminium alloys. Commercially aluminium based powders Al-Mg-Si-Cu-Fe and Al-Zn-Mg-Cu-Sn were used as materials to be investigated. The apparent density of the powder mixes was determined according to MPIF St. 04. A set of cylinder test specimen 55x10x10 mm3 was uniaxially pressed in a floating hardened steel die. Compaction pressures ranged from 50 MPa up to 700 MPa. Considering the densification of metal powders in uniaxial compaction, quantification of aluminium compaction behaviour was performed. The compressibility behaviour was evaluated, considering the effect on specimens, as well as on their microstructure. The development of compressibility values with pressing pressure enables to characterize the effect of particles geometry and matrix plasticity on the compaction process.

Properties of Boronized, Carbonitrided and Steamed Iron-Based Compacts

Abstract In this study, powder metal (P/M) compacts were prepared by mixing iron powder with 0–1 wt.-% Al, 2–3 wt.-% Cu and 0.2–0.8 wt.-% graphite. The 16 different P/M compacts compositions were prepared with 0.8 wt.-% zinc stearate as lubricant. The compacts' dimensions were 10 × 10 × 55 mm. All compacts were pressed at 600 MPa and sintered at 1120 °C for 30 minutes using a conventional sintering furnace and then boronizing, steaming and carbonitriding heat treatments were applied. The results of all the compacts in regard of the effects of corrosion endurance, density, and HV microhardness results were compared.

Spatio-temporal dynamics behind the shock front from compacted metal nanopowders.

Laser ablated shock waves from compacted metal nanoenergetic powders of Aluminum (Al), Nickel coated Aluminum (Ni-Al) was characterized using shadowgraphy technique and compared with that from Boron Potassium Nitrate (BKN), Ammonium Perchlorate (AP) and Potassium Bromide (KBr) powders. Ablation is created by focused second harmonic (532 nm, 7 ns) of Nd:YAG laser. Time resolved shadowgraphs of propagating shock front and contact front revealed dynamics and the precise time of energy release of materials under extreme ablative pressures. Among the different compacted materials studied, Al nanopowders have maximum shock velocity and pressure behind the shock front compared to others.

Dynamic compaction of titanium powder by vaporizing foil actuator assisted shearing

Electrically driven rapid vaporization of thin metallic foils is known to create very high pressures, which can be harnessed for mechanical work. Recently, vaporizing foil actuators (VFA) have been applied to a variety of impulse-based metal working operations such as collision welding, embossing, shearing, shape calibration, and closed-die forming. Here, a variation of VFA has been used for dynamic compaction of metallic powders. Holes with a diameter of 10mm were sheared into Cu110 sheets using foil actuators operating at input electrical energies from 4kJ to 10kJ. During the impulse shearing operation, the sheared plugs were found to accelerate up to velocities of 1.4km/s within a few millimeters of travel distance. These sheared plugs were used as pistons to compress milled commercially pure titanium (CP-Ti) and Ti–6Al–4V alloy (Ti–6–4) powders, both of which had tap densities of approximately 25–28%. Cylindrical green compacts weighing ~2g were produced by this process. The densities of the compacts, measured using the Archimedes principle, were found to increase with input electrical energy. A maximum density of 97% was obtained with CP-Ti powder, while Ti–6–4 powder could be compacted to 93% dense. When compared to a quasistatic cold compaction process, a significant gain in densification with same pressures was observed.

Mechanical properties and hardness results of the medium frequency induction sintered iron based powder metal bushings

In this study; iron based powder metal (PM) bushing compacts were sintered via medium frequency induction heating system. Iron based powders were mixed with 2 wt.% copper (Cu), 0.3 wt.% carbon (C) and 1 wt.% zinc stearat (Zn-st) via V-Type mixer. PM compacts were sintered by 2 different sintering processes. One of them was conventional sintering method other was medium frequency induction sintering method. In conventional sintering process, PM bushing compacts were sintered under inert environmental at 1120?C for 30 minutes. In medium frequency induction sintering process, PM bushing compacts were sintered between 30-50 kHz frequency (Medium frequency) at 2 kW, 1120?C for 1 or 3 minutes under atmospheric environmental. Mechanical properties, densities and micro hardness? were investigated for all sintered processes. SEM and micro structural images were taken from polished broken surface of the sintered PM bushing compacts. 3 minutes induction sintered compacts were reached the compressive strength, microhardness and density values of the conventional sintered PM bushing compact.

Evolution of elliptic-cylindrical and circular-cylindrical voids inside power-law viscous solids

The evolution of voids inside power-law viscous solids is investigated. A representative volume element (RVE) model of an infinite matrix containing an isolated void is applied. In the RVE model, the void is assumed to be elliptic-cylindrical or circular-cylindrical, and the matrix is considered as the isotropic and incompressible power-law viscous material. To obtain the velocity field of RVE, a Ritz procedure is developed using the method proposed by Lee and Mear (1992). Moreover, the results obtained from the Ritz procedure are verified by the finite element simulations. Based on the data obtained from RVE models, the effects of material Norton exponent, remote stress field and void aspect ratio on the changing rate of void aspect ratio are discussed. Especially, when the void’s principal axes are parallel to the principal axes of the remote stress, the mathematical models are proposed to relate the changing rate of void aspect ratio to the void aspect ratio and material Norton exponent. The results show that the material Norton exponent, remote stress field and void aspect ratio have a great influence on the changing rate of void aspect ratio. For the remote shear stress and uniaxial compression stress fields, the changing rate of void aspect ratio increases with the increase of void aspect ratio and material Norton exponent. Furthermore, the relationships between the changing rate of void aspect ratio and the void aspect ratio can be represented as the parabolic function and linear function for the remote shear stress field and uniaxial compression stress field, respectively. While the relationships between the changing rate of void aspect ratio and material Norton exponent can be expressed as the first order exponential function for these two remote stress fields. Besides, the changing rate of void aspect ratio can also be expressed as a unified function of void aspect ratio and material Norton exponent. For the remote biaxial compression stress field, the relationships between the changing rate of void aspect ratio and the void aspect ratio can be represented as the parabolic function, in which the coefficients can be expressed as functions of material Norton exponent and remote stress field. The findings of this study can be mainly used to evaluate the aspect ratio of voids inside large ingots during hot working, as well as to model the final densification stage of powder metal compacts.

State of the art of electromagnetic energy for welding and powder compaction

The innovation of advanced materials and their application to engineering structures can be related to their ease of fabricating defect-free parts. The identification of various energy sources has resulted in various new processing techniques in manufacturing and electromagnetic energy is one which offers promising qualities for high-velocity forming, welding, and compaction of metals. Bimetallic joint designs were proposed in automobiles to reduce the weight and to take advantage of properties of different metals in the same construction. Joining by fusion is impossible in certain cases due to huge differences in melting point, thermal conductivity, volumetric specific heat, and coefficient of thermal expansion. In the present study, magnetic-pulse welding of a Cu/Al joint and an ODS alloy is investigated. The formation of intermetallic phases and interface morphology is presented along with a quantitative analysis for temperature in the Al/Cu joint. Also, initial investigations on the application of electromagnetic energy for compaction of aluminum powders are reported.

High-Energy Forming of Monolithic and Powder Materials by Pressure

Well-established and new processes for the high-energy shaping of materials are analyzed along with the equipment they use. Most of the attention is given to the latest methods of high-energy compaction of metallic and nonmetallic powders on equipment made by the Swedish company Hydropulsor. Results are presented from studies performed on metal powders by scientists at the Swedish companies Hoganas and Sinterheat and Dalarna University, on powder ceramics (Al2O3) by French scientists, on copper and iron powders by Chinese scientists, and on carbon powders by the authors of the article.

Improvement in robustness and computational efficiency of material models for finite element analysis of metal powder compaction and experimental validation

In metal powder compaction, density nonuniformity can be a source of flaws. Material models in finite element analysis for the prediction of density distribution still lack robustness and are computationally expensive. In this work, the Drucker–Prager cap (DPC) material model is implemented into the commercial finite element (FE) software ABAQUS/Explicit using the user-subroutine VUMAT. Yield functions in this material model are pressure-dependent and the curvature of the cap yield surface is high. This can cause numerical instability. We implemented a sub-increment technique to address this instability problem in a previous paper (Kashani et al., Trans North Am Manuf Res Inst SME 38:623–631, 2010). The DPC model is also a non-smooth, multi-yield surface material model which has instability problems at the intersection of the yield surfaces; we adopted the corner region in the DPC material models for soils in order to remove instability and the results of which were presented in (Kashani et al., Trans North Am Manuf Res Inst SME 38:623–631, 2010). The computational efficiency of the DPC material model was improved using a novel technique to solve the constitutive equations analytically which was shown in a previous paper by the authors (Kashani et al., Trans North Am Manuf Res Inst SME 38:623–631, 2010). In this paper, experimental tests were conducted where cylindrically shaped parts were compacted from Distaloy AE iron-based powder to 7.0 g/cm3 using 592 MPa of pressure. To measure local density, metallography and image processing were used to find the void area fraction of the surface. The FE results were compared to experimental results and it was shown that the FE analysis predicted local relative density within 3 % of the actual experimental measurements.

Elastoplastic–viscoplastic modelling of metal powder compaction: application to hot isostatic pressing

A constitutive model is proposed for the densification of metal powder during hot isostatic pressing. The model considers an inelastic deformation resulting from time dependent (viscoplastic) and time independent (plastic) mechanisms during loading. With employing the Abouaf’s formalism for viscoplastic part, the paper is focused mainly on the plasticity contribution including the hardening effects of both relative density and the equivalent plastic strain. The proposed plastic–viscoplastic model is summarised to an equation expressing the densification rate under hydrostatic loading. The gas atomised 316LN stainless steel powder was used in this study. Model parameters necessary for simulation of HIP process during a given pressure ramp at constant temperature are identified using previously published data. The identified model is then verified using experimental data obtained from HIP trials performed at another fixed temperature but with varied pressure ramp rates. A good agreement was found between the model prediction and the experimental results.

Modelling of three powder compaction laws for cold die pressing

Abstract Three models to describe the relationship between the green porosity of compacts and the applied external pressure during the compaction of metal powders are proposed in order of growing complexity. Only in the third model do all of the parameters have a clear physical meaning and become measurable. These parameters are related to the plastic behaviour of the powder particle material, the friction coefficient between the powder and die walls, as well as the interparticle friction. The three proposed models include a parameter representing a certain value related to the initial porosity of the powder mass; however, only in the third model is this value clearly defined by the tap porosity of the powders. The proposed models have been experimentally verified by compressibility curves obtained from metal powders of different types. The agreements between the second and third models and the experimental data are reasonable over the tested pressure range.

Fractographic analysis of green compacts of metal powders

The paper examines the mechanisms that contribute to the strength of green compacts produced and tested in different thermal conditions (different homological temperatures of Sn, Zn, Cu, Ni, and Mo powders compacted and tested in normal conditions). The role of powder particle shape is demonstrated using scanning electron microscopy to analyze images of the particles and fracture patterns of the compacts. Partial plastic fracture is found only in the Sn powder sample. When plastic powders with irregular, branched particles (Cu, Ni) are compacted, the mechanism of adjusting particle surfaces to one another prevails and leads to all three mechanical components: interlocking, entangling, and seizing. When plastic powders with spherical particles are compacted, deformation of particles prevails. The ratio of the tensile strength of compacts (σb.i) determined indirectly to the tensile strength (σb) of particulate material shows a semilogarithmic dependence on the ratio of compacting pressure (P) to the yield stress (σ0.2) of the particulate material. The dependence of the ratio of σb.i to the elastic modulus on the homological temperature of compacting and testing divides into two linear dependences for spherical and nonspherical particles.