High Velocity Compaction Process Research Articles

Experimental studies and modelling of high-velocity loaded iron-powder compacts

A production technique with the capacity to significantly improve the mechanical properties of powder metallurgy (PM) parts is high-velocity compaction (HVC). To extend the usage of the HVC method, detailed knowledge of the HVC process is important. To facilitate the development of production processes, numerical simulations can be utilised. In the development of high-precision simulation models, constitutive data of HVC specimens at high strain rates are required. In this study, the dynamic compressive properties of cylindrical specimens made by HVC were measured using a split Hopkinson pressure bar (Kolsky bar) assembly. For this technique, a specimen is placed between two elastic bars. The impact loading is achieved by a projectile accelerating inside an air gun, which impacts the end of the input bar and generates elastic-wave propagation.The powder material used for the experiments is a press-ready iron-based premix. Among specimens made by HVC and conventional compaction (CC), the effects of the specimen density and the strain rate on the compressive properties, such as failure stress, Young's modulus and failure behaviour, are investigated. During dynamic compression, the failure behaviour of the specimens was also recorded using a high-speed video camera. The difference in the mechanical behaviour between HVC-pressed specimens and conventionally pressed specimens are also investigated. The stress–strain curves of HVC-pressed specimens are identical to those of conventionally pressed specimens, but the failure behaviour differs are concluded.A well-established numerical method for forming simulations also conducted for powder compaction is the finite element method (FEM). The impact loading of the powder is modelled and simulated using nonlinear three-dimensional FEM. To model the impact process, a constitutive relation for the powder behaviour is proposed, taking into account the strain rate and density variations. To capture the global response caused by cracking during impact, a damage model is implemented. The numerical results in terms of the stress and strain history in the specimen during impact are compared with the experimental measurements. In conclusion, the proposed material model captures the increase in the yield stress due to the higher strain rates and the decrease in stress due to cracking.

Analysis of Density and Mechanical Properties of Iron Powder with Upper Relaxation Assist through High Velocity Compaction

For producing higher density PM parts a new method, High-Velocity Compaction process with additional upper relaxation assist (URA) device is presented in the paper. Using zinc stearate as a die wall lubricant, iron powder was pressed with and without upper relaxation assist device of mass 0.14 Kg focusing to investigate the density and mechanical properties. To explain a compaction process in loading stage a conservation of momentum principle has been introduced during collision of hammer, upper piston and upper relaxation assist device in die compaction. To observe the morphological characteristics and mechanical properties, a scanning electron microscopy (SEM) and computer controlled universal testing machine were used. The experimental results showed that the samples compacted with URA device had an improved green density and mechanical properties compared to the samples compacted in the absence of URA device.

Fabrication of Electronic Packaging Grade Cu–W Materials by High-Temperature and High-Velocity Compaction

High-velocity compaction (HVC) is a production technique that uses a high-speed punching motion on powder materials to achieve superior mechanical properties. However, as the particle size of the metal powder decreases, achieving higher density becomes increasingly challenging. For this reason, the HVC technique cannot be adopted for manufacturing W85-Cu (85wt% W and 15wt% Cu) heat sinks if the particle size of the tungsten powder is very small [Fisher sub sieve sizer (FSSS) 3 μm or finer]. In this paper, the HVC process at elevated temperature [or high-temperature high-velocity compaction (HTHVC)] was studied for W85-Cu heat sinks. Tungsten skeletons prepared by the conventional uniaxial method were further compacted by HTHVC at various elevated temperatures. The compacted tungsten skeletal blanks were then infiltrated with copper at 1350°C for 2 h. It is concluded that the HTHVC process significantly increases the density of the tungsten skeleton. The distribution of copper in the tungsten skeleton is uniform and no significant clustering is observed in the scanning electron microscopy images, and the resultant W85-Cu material possesses a relative density of 99.5%, hermeticicty of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> Pa m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /s, and thermal conductivity of 185 W/(mK). Thus the following conclusions are drawn: 1) overall, the material meets the requirement for heat sinks for electronic packaging applications; and 2) the HTHVC technique is very useful for processing W85-Cu composites for obtaining high-density composites with uniform material distribution and desirable mechanical properties.

High Velocity Compaction: Comparison with Conventional Compaction for New Press Development in Hot Cell Pellet Manufacturing

Within the framework of the technological developments for fuel fabrication in hot cell, the compaction of ceramic powders using both static and dynamic processes has been studied as regards the manufacturing of nearly fully dense ceramics with unique microstructures from precursor ceramic powders. High Velocity Compaction (HVC) is a dynamic compaction without shock wave. The objective of this study [24,[25] was to compare the results of shaping between a conventional and a HVC process, both with hydraulic systems providing the impaction. Various inactive alumina powders were tested. Their characteristics after pressing were assessed to show the differences between green and sintered pellets.

Simulation of high-velocity compaction process with relaxation assists using the discrete element method

The discrete element method is used to investigate the high-velocity compaction process with additional piston supports known as relaxation assists. It is shown that by incorporating the relaxation assists in the piston-die assembly, particles can be better locked during the compaction process. The simulation results reveal that relaxation assists offer; smooth compaction during loading stage, prevention of the particle separation during unloading stage and conversion of higher kinetic energy of hammer into particle deformation. Finally, the influences of various loading elements on compaction process and effects of presence of adhesion during unloading stage are investigated. The results support the findings of experimental work.

The pressure wave analysis in high velocity compaction process

The pressure propagation in high velocity compaction process is simulated based on the discrete element method in this paper. Because the full process is divided into elastic loading, plastic deformation and elastic unloading, the governing equations are established in three stages respectively. With the help of computing software PFC2D, the impacting force through different layers of powders can be obtained, which cannot be observed in experiment. The simulation results show obviously a delay phenomenon and serrate waves with different gradients in loading and unloading processes. Finally, the simulated low-level stress waves are compared with available experimental data, which are consistent with each other.

High-velocity compaction and conventional compaction of metallic powders; comparison of process parameters and green compact properties

High-velocity compaction (HVC) has been used for some years now with commercially available high-energy hydraulic presses. Compact densification is achieved in less than 0.01 s, shorter than with a conventional process (about 1 s) but longer than with explosive compaction (0.001 s). Powders are compacted by means of an adjustable energy instead of force or displacement. This work presents a comparison between conventional compaction (CC) and HVC using an industrial hydraulic press (18 kJ) and a laboratory press (380 J). The velocity of the hammer producing the impact is therefore restricted to 10 m/s. Six powders were compacted using the HVC process and the conventional die compaction process. Differences and similarities are examined considering process characteristics (punch forces, ejection force, and tooling strength) and the characteristics of green compacts (densities, dimensions, and cohesion).

Effect of powder particle size on green properties and stress wave

In the powder metallurgy (PM) industry, high velocity compaction (HVC) technique is a new way to obtain higher density parts. In this study, three reduced pure iron powders with different particle sizes were pressed by HYP35-2 High Velocity Compaction Machine. A computer controlled universal testing machine was used to measure the bending strength of the green body. The relationships among the particle size, the impact velocity, the green density, the stress wave, the bending strength and the radial springback were discussed. The results show that the powder of −200 mesh is the best option among the three powders for the HVC process. At the identical impact velocity, the green density for the powder of −200 mesh is higher than the other two types of powders, while the bending strength and the radial springback for the powder of −300 mesh is the highest. In addition, the stress waves exhibit similar, pulsed waveforms. As the impact velocity rises up, the duration of the first peak decreases gradually, while that of the stress wave increases slowly. The response time for the powder of −200 mesh is shorter than the other two types of powders.

Green Body Behaviour of High Velocity Pressed Metal Powder

High velocity compaction (HVC) is a production technique with capacity to significantly improve the mechanical properties of powder metallurgy (PM) parts. Several investigations indicate that high-density components can by obtained using HVC. Other characteristics are low ejection force and uniform density. Investigated here are green body data such as density, tensile strength, radial springback, ejection force and surface flatness. Comparisons are performed with conventional compaction using the same pressing conditions. Cylindrical samples of a pre-alloyed water atomized iron powder are used in this experimental investigation. The different behaviour of HVC-pressed green bodies compared to conventional pressed green bodies are analysed and discussed. The HVC process in this study resulted in a better compressibility curve and lower ejection force compared to conventional quasi static pressing. Vertical scanning interferometry (VSI) measurements show that the HVC process gives flatter sample surfaces.

Determination of springback gradient in the die on compacted polymer powders during high-velocity compaction

A uniaxial high-velocity compaction process for polymer powder using a cylindrical, hardened steel die and a new technique with relaxation assist was tested with various heights. The influences of the relaxation assist device on the process characteristics are discussed. Two bonded strain gauges and a high-speed video camera system were used to investigate the springback phenomenon during the compaction process. It was found that the relaxation assist improves the compaction of the polymer powder by locking the powder bed in the compacted form. It is shown that the high-velocity compaction process is an interruption process and that the delay times between the pressure waves can be reduced by increasing the height of the relaxation assist device. The delay times between the pressure waves are also strongly dependent on the strain rate. If the height of the relaxation assist device is increased, the first gross instantaneous springback, and the total elastic springback, are reduced. In addition, the density of the powder bed is increased. The relative times of the compacting stage, decompacting stage and the reorganisation of the particles can be also controlled by altering the height of the relaxation assist.

Development of a High-Velocity Compaction process for polymer powders

The High-Velocity Compaction (HVC) process for powder polymers has been studied, with a focus on the compactibility characteristics and surface morphology of the compacted materials, with and without relaxation assists, by increasing compacting quantity and direction. The basic phenomena associated with HVC are explained and the general energy principle is introduced to explain pull-out phenomena during the decompacting stage. Polyamide-11 powders with different particle size distributions have been compacted with the application of different compaction profiles, e.g. different energies and velocities. Scanning electron microscopy (SEM) and image computer board camera (IC-PCI Imaging Technology) have been used to the study the morphological characteristics, the limit of plastic deformation and particle bonding by plastic flow at contact points, and pull-out phenomena. The relative green density is influenced more by the pre-compacting (primary compaction step) than by the post-compacting (secondary compaction step). The pressure and density distribution differences between the upper and lower surface are not uniform. Projectile supports or ‘relaxation assists’ are presented as a new technique to reduce pull-out phenomenon. Experimental results for different compaction profiles are presented showing the effect of varying the opposite velocity during the decompacting stage, and how to improve the homogeneous densification between the upper and lower surface and the evenness of the upper surface of the compacted powder bed by using relaxation assists.

Simulation of high velocity compaction of powder in a rubber mould with characterization of silicone rubber and titanium powder using a modified split Hopkinson set-up

The paper introduces a method for characterization of silicone rubber and titanium powder in high velocity compaction using the split Hopkinson set-up. The impact test data has been used to estimate parameters in constitutive models for rubber and powder. A finite element study has been performed with different geometrical design of the high velocity compaction of titanium powder against an aluminium mandrel using a rubber mould as pressing medium. One goal of this study is to investigate if and how the manufacturing method can be applied for making dental copings. A conclusion of the experimental work is that it is possible to characterize rubber material and powder material for high velocity compaction of metal powder by the use of a modified split Hopkinson pressure bar set-up. The numerical simulation shows qualitatively good agreement with the experience from practical tests. In conclusion, the work shows the possibility to numerically study the geometric design and to optimize the densification behaviour of a complex high velocity compaction process.

High Velocity Compaction of Powder, 3rd Report

This paper is concerned with a theoretical study on high velocity compaction process of metal powder by the pseudo-viscosity method. As well as in the first paper, the powder uniformly packed in a die is struck by a rigid body. The pseudo-viscous pressure used here is such that running wave solutions vary smoothly through the shock region. Comparing with the results obtained in the first paper, the following ones were obtained.1) The results by this method agree well with those by the jump conditions, though there are oscillations of pressure behind the shock.2) The effects of the elastic recovery of the powder are negligible.3) When the initial kinetic energy of the striking body is constant, the assumptions used in the first paper become less satisfactory as the impact velocity of the body is larger.4) The powder is more uniformly and highly compacted as the mass of the body is larger.

High Velocity Compaction of Powder, 1st Report

This paper deals with a high velocity compaction process of metal powder. The powder is assumed to be homogeneous continuum which can be treated as an ideal fluid, and jump conditions at a shock known as the Rankine-Hugoniot equations are employed for the basic equations.As an example, copper powder is used here. This powder is uniformly packed in a die with a constant cross-section, and then its free surface is struck by a rigid body. An equation of state of the powder assumed by a static pressure-density relation, and the conservations of mass and momentum specify a motion of an element of the powder. Furthermore it is assumed that the elastic recovery of the powder can be neglected and that the pressure increases only at a shock front, and thereby the powder between the front and a fixed plate is always at rest and a particle velocity of the powder between the body and the front is always the same as that of the body. Then, the conservation of total momentum including the body is added to the above equations. These four equations determine the pressure p, the density q, the particle velocity of the powder v and the velocity of the frontNumerical results obtained showed that the assumptions used in this theory were satisfactory and copper powder compacted at high velocity impacts had uniform density if the friction of the powder at side plates can be neglected.