An overview of dynamic compaction in powder metallurgy

An overview of dynamic compaction in powdermetallurgy

G. Sethi*1, N. S. Myers2 and Randall M. German3

This paper is a critical review of dynamic compaction as a means to densify metal powders.

Dynamic compaction was discovered in the 1960s. Most of the investigations since then have

focused mainly on the physics dealing with energy, motion and force aspects of the process.

Owing to this, there is a lack of knowledge of the effects of preprocessing and processing factors

on this process. This knowledge gap has created skepticism in the PM community about this

process’ practice. This review attempts to bridge this gap and highlights the powder metallurgical

aspects of dynamic compaction by emphasising the key powder related factors and processing

parameters affecting dynamic compaction. Powder related factors include powder character-

istics and the processing parameters including the machine operating parameters. Attention has

been given to iron, aluminium and copper powders. Through this review, this article paves the

way to design of high density dynamic compaction systems based on the final PM component

requirements, by adjusting the material and processing parameters.

Keywords: Dynamic compaction, High strain rate compaction, High velocity compaction, Powder characteristics, Full density, Iron, Aluminium, Copper,Green density, Apparent density, Powder metallurgy, Consolidation, Springback

IntroductionDensity has a dominant effect on the properties ofpowder metallurgy (PM) components. Residual poros-ity, which is inversely related to density, can be reducedeither during compaction or during sintering. In theprocesses involving densification during compaction,green density is a critical factor. As green densityincreases, target sintered densities are easier to obtain atlower sintering temperatures. Green density gradientswithin the compacts reduce as green density increases.1–3

Figure 1 shows this reduction in green density gradientsrepresented by difference in green density of top andbottom sections of a compact as a function of averagecompact green density. Green density gradients result indimensional distortion, porosity gradients, or both, inthe sintered component. This may lead to cracks,warpage or property gradients in the compact andsintered component.1,4–20 Thus, higher compact greendensity results in less shrinkage and distortion duringsintering, which then leads to improved componentproperties with better dimensional control.4 Doublecompaction, high pressure consolidation,21–23 liquidphase sintering, powder forging and dynamic compac-tion are some of the methods employed to obtain fulldensity components.24 Table 1 lists the various routes

and the corresponding relative densities obtained, in thecase of ferrous powders.25

This paper focuses on dynamic compaction used toproduce full density components, which is variablyknown as high velocity compaction, high speed compac-tion, shock compaction, explosive compaction or highstrain rate compaction depending on the processingequipment. Recently, dynamic compaction has receivedattention for its use in large scale production.26–31

However, the lack of knowledge of this process hascreated skepticism in the PM community while alternateroutes to full density are gaining attention. This reviewexamines dynamic compaction to establish its bench-mark as a public source of scientific knowledge and toenable the design of high density systems based on thistechnique. Specifically, the review first provides thebackground of the dynamic compaction process andthen discusses the preprocessing and processing factorsaffecting it. A majority of the studies and reviews havefocused on the physics dealing with energy, motion andforce aspects of the process. To the knowledge of theauthors, only this review discusses the effects of variousoperating parameters considering present day powdercharacteristics.

Dynamic compaction

BackgroundHigh velocity compaction involves propagation of acompressive stress wave or a deformation wave (shockwave) through a powder to cause its densification. Thisdeformation wave is composed of many small wavelets.The wave is established when a high energy projectile

1261 Materials Research Laboratory, The Pennsylvania State University,University Park, PA 16802-6809, USA2Carbide Process Development, Kennametal Inc., 1600 Technology WayLatrobe, PA 15650, USA3Center for Advanced Vehicular Systems (CAVS), Mississippi StateUniversity, Box 5405, Mississippi State, MS 39762-5405, USA

*Corresponding author, email [email protected]

� 2008 Institute of Materials, Minerals and Mining and ASM InternationalPublished by Maney for the Institute and ASM InternationalDOI 10.1179/174328008X309690 International Materials Reviews 2008 VOL 53 NO 4 219

moving at a high velocity strikes the die punch. Variousconfigurations including guns, rams and explosive unitshave been used to produce such high impact energy.32

Table 2 lists the types of dynamic compaction equip-ments and the corresponding operating parameters.32–34

The impactors used in these equipments have a veryshort life because they operate under very high stresses.

Two typical apparatuses (gas gun based and hydraulicpressed) used for dynamic compaction where animpactor imparts energy to the upper punch are shownin Fig. 2. The impactor in the gas gun based compaction

has a smaller mass and higher velocity than that in thehydraulic pressed compaction. The impactor transfersits momentum to the upper punch. Since the punch isusually lighter than the projectile, it takes on a highervelocity Vpu based on the principle of conservation ofmomentum. This velocity is referred to as the compac-tion velocity. The deformation wave evokes the powderparticles to move at a material speed Vm. If the materialspeed Vm is more than the speed of sound in the powderVsound, a shock wave is established in the powder. Thus,the wave propagation velocity reaches shock wavevelocity Vs causing the powder to densify at a highstrain rate.35 This strain rate is usually greater than103 s21.36

Porosity has a significant effect on shock wavepropagation. Gourdin35 demonstrated the differencebetween wave propagation in solid and porous materi-als. For the same compaction velocity Vpu, the materialspeed Vm is much higher in porous materials than insolids, while the speed of sound is much lower in porousmaterials than in solids. This allows for propagation ofshock waves in porous materials at much lower punchspeeds than in solid materials. As a powder densifies, itscompression behaviour shifts to that of a porousmaterial and finally a wrought material, due to wavepropagation.37 Thus, the Rankine–Hugonoit curve[Rankine–Hugonoit is defined as the locus of all shockedstates (P–V curve) in a material and describes thematerial’s properties. In short, it is the pressure specificvolume path] that explains the compaction behaviourfor a porous material asymptotically approaches thatof a solid material.37,38 Figure 3 shows a typicalRankine–Hugonoit curve that is followed duringdynamic compaction of metal powders.39 The compac-tion behaviour of a powder is primarily dependent onthe material properties, powder characteristics andprocessing parameters.

Densification phenomenaFigure 4 depicts the various densification phenomenathat occur in dynamic compaction, divided simply basedon the compaction velocity of the compaction systemand particle size of the powder used. As the compaction

Table 2 Dynamic compaction machine parameters: different machines have different energy carriers and controllingparameters32–34

Machine type Energy carrier Front end machine parametersBack end machineparameters Advantages

Gun type units Low explosiveCompressed gasInflammable mixtures

Explosive chargeImpactor speedIimpactor massPunch speed Vpu

Punch mass

Base materialDie insulationPowder environment

High speedAutomationIndustrial useForging

Hydrodynamic units Low explosive Piston speedPiston energyLiquid pressurePowder mass

Industrial useSimpleComplex shapesAutomation

High explosiveenergy units

High explosive Explosive chargeGeometry of the chargeCharge distance from powderProperties of intermediate medium

High temperature andhigh speedLow capital investment

Electrical impulseunits

High voltage electricaldischarge in liquidMagnetic fields of highflux density

Impulse currentEfficiency of conversion of electricalenergy into mechanical energy

AutomationComplex shapes

1 Reduction of green body gradients across compaction

sections (top, middle and bottom) with increasing com-

pact green density of compacted water atomised iron

powder3

Table 1 Various powder consolidation routes and relativedensity obtained for iron powder4

Powder consolidation routeRelative density,% theoretical

Quasi-static (conventional die)compaction (P1S1)

90

Warm compaction (WC) 93Double compaction (P2S2) 94High velocity compaction (HVC) 95Powder forging 99

Sethi et al. Overview of dynamic compaction in powder metallurgy

220 International Materials Reviews 2008 VOL 53 NO 4

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velocity is increased, the controlling phenomena ofdynamic compaction change from region B to D that isfrom microkinetic energy to interparticle surfacemelting.40,41

A critical compaction velocity differentiates dynamiccompaction from quasi-static (quasi-static compaction isconventional die compaction at low speed, performed ina mechanical or hydraulic press in the absence of ashock wave) compaction. At this critical compactionvelocity, the material speed of the powders Vm is equalto the velocity of sound in the powder Vsound.33 RegionA in Fig. 4 represents compaction in the absence of ashock wave (quasi-static compaction). In region B,particle deformation is similar to that in quasi-staticcompaction in spite of the shock wave character. This isdue to the poor packing of smaller particles causing highattenuation of the compressive stress waves. Most of theenergy input in this case is used to overcome inter-particle friction. Region C represents dynamic com-paction of a larger particle size powder at highercompaction energy leading to more plastic deformationand workhardening of the metal powder. In region C,the particles are plastically deformed without thermalsoftening.

With a further increase in the compaction energy andparticle size, loss of strength of the plastically deformedvolume (softening) takes place. This is represented by

2 Schematic of high velocity compaction using a gas gun3 and b hydraulic press.25 Projectile hits punch, which propa-

gates shockwave in powder to compact it

3 Typical Rankine–Hugonoit curves for powder, porous

and solid metal.38 Powder Hugonoit approaches porous

Hugonoit and finally solid Hugonoit


International Materials Reviews 2008 VOL 53 NO 4 221



region D. The softening is caused by the localised rise intemperature of the deformed volume.42 This localisedtemperature rise can cause the overall compact tem-perature to rise by ,30uC.43,44 The localised, transienttemperature rise is much higher at the particle contactareas45 due to the adiabatic (an adiabatic process is onein which no heat is gained or lost by the system) natureof the process.36 In cases where the compaction energy ishigh, it is possible to reach the melting point of somemetals at the particle contacts. Table 346,47 shows themelted volume formed in two systems, Alz6%Si48,49

and 4330V steel.50 The melted volume calculatedthrough microstructural analysis was up to 16%.Thermal softening and melting of metal particles causesthem to bond with each other, leading to enhancement

of the compact strength. Further, the melted or softenedvolume will move like a liquid and flow into pores.51 Themain benefit of thermal softening and melting isenhanced strength of the green compact, apart fromincreased green density.49 In addition, heating of theparticles may cause phase transformations to a phasethat has better mechanical properties leading to furtherimprovement of component properties.48 Examplestress–strain curves for HY-TUF steel powders inquasi-static compaction and dynamic compaction areshown in Fig. 5.39 Quasi-static compaction is a nearlyisothermal (an isothermal process is a thermodynamicprocess in which the temperature of the system staysconstant) process. Therefore, in quasi-static com-paction the powder particles deform according to

4 Major densification mechanisms occurring during dynamic compaction.40,41 There is critical limit for loading rate

Vsound at which shockwave propagation occurs through powder causing it to dynamically compact. In dynamic com-

paction, sliding and friction are dominating mechanisms at low loading rate. At higher loading rates, interparticle melt-

ing is dominating densification and strengthening mechanism

Table 3 Comparison of the measured extent of melted volume in Alz6%Si and 4330V steel:46–50 measured extent ofmelting is more in Alz6%Si than in 4330V steel due to its lower melting point; higher packing density alsopromotes thermal softening and melting

SystemMelting range,uC

Particle size,mm

Initial packing density,% theoretical

Stress,GPa

Energy,kJ kg21

Measured extent ofmelting, vol.-%

Alz6%Si 574–632 30 67 1.88 171 0.02.6 236 6.04.0 360 16.0

4330V steel 1400–1455 70 50 2.85 160 2.03.6 206 5.53.86 221 7.0




workhardening and not thermal softening. In dynamiccompaction, the powder particles are deformed accord-ing to thermal softening based on the adiabatic nature ofthe process.

Gourdin and other researchers support the existenceof thermal softening and interparticle melting.35

Figure 6 shows an example of interparticle melting in4349V steel powder leading to its solid state transforma-tion to martensite. At high shock wave characteristicpressing (region D), the major components of totalenergy are the ‘bulk plastic deformation’ and ‘melting’energies (both localized and globalised). Hence, twokinds of energies, mechanical energy and thermalenergy, exist.35,37,52 Quantifying the partitioning of thesetwo pressures (or equivalently energies) is difficult.Various models have made such attempts as enlistedby Gourdin35 and Sawaoka Horie53 and have beenpartially successful. Recently Mamalis et al.54 reviewedthe basic empirical and theoretical models with the aimof understanding and quantifying the dominatingcompaction mechanisms seen in Fig. 4.

The controlling phenomena of dynamic compactionare differentiated by the curves on the graph in Fig. 4.There is a sudden transition across curve 2 due to achange in densification phenomena. However, the transi-tion across curve 3 is smooth, as the phenomena in regionD are an extension of those in region C in Fig. 4. Curve 4in Fig. 4 corresponds to the situation where particledeformation time plus freezing time of interparticlemelting is equal to the time duration of actingpressure.42,55 A straight line represents this conditionbecause the deformation time is directly proportional toparticle size and inversely proportional to speed. Thepositions of curves 2, 3 and 4 depend on the density,hardness and thermal properties of the particles.56–58

EfficiencyMost of the machines performing dynamic compaction,being either forging machines or high speed forming(strain rate) machines such as Hydropulsor,24,25 Petro-forge,59 or Dynapak,60 are designed to generate adiabatic

conditions in the compact. Plastic deformation of thecompact at high strain rates reduces the thermal diffusiondistance. Thus the efficiency of high speed processing isdetermined by the degree of localisation of thermallysoftened particle deformation, which further depends onadiabacity. Adiabacity is defined as the level of localisedthermal build-up during the deformation process. Twonumbers, Peclet number,61 (equation (1)) and Fouriernumber40,61 (equation (2)), quantify adiabacity and relateit to densification. These numbers based on severalmaterial properties as discussed by various authors areapplicable for all metal powders40,58,60,62

Pe~DVm

a(1)

Fo~DVm ln rV2

pu=H� �

a(2)

where Pe is Peclet number, Fo is Fourier number, D ispowder particle size, Vm is material velocity of theparticles, Vpu is the punch velocity (also the compactionvelocity), a is the thermal diffusivity, r is the particletheoretical density and H is the Brinell hardness. Thehigher these numbers, the greater the adiabacity anddensification of the material.

Based on these numbers, it is observed that as theadiabacity of the process increases, the heat evolvedduring the plastic deformation is not transferred toneighbouring regions (deformation localisation).63 Thedeformed region heats up and loses strength, whichleads to further localisation of deformation and betterinterparticle bonding. This results in higher densityand strength in the product. It has been observed thatwith increasing particle size, the tendency towards

5 Quasi-static and dynamic compaction compression

stress–strain curves in HY-TUF steel.39 Quasi-static

compaction is mainly controlled by workhardening of

powder while after plastic flow dynamic shifts to con-

trol by thermal softening of powder. Thermal softening

of powder causes stress on particles to decrease lead-

ing to parabolic stress–strain curve

6 Microstructure of a compact of 4349 vanadium modi-

fied steel powder formed at a compaction pressure of

5?6 GPa (scalebar is 200 mm). As the shockwave propa-

gates through the powder, powder behind the shock-

wave front gets compacted; interparticle melting of the

powder at the shockwave front takes place; the powder

in front of the shockwave is non-compacted. The inter-

particle melting may cause solid state transformations

as shown by the white region (martensite) in the micro-

structure35 above




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deformation localisation grows.50 This was observed incopper powder where the transition from plasticdeformation to thermal softening occurred at highercompaction pressure (or energy) for the smaller particlesize powder v. the larger particle size powder.64

Governing factorsThe performance of dynamically compacted compo-nents depends on its material, preprocessing andprocessing properties. This section provides an overviewof these properties, of which the latter properties will bediscussed in detail in the following section.

Material propertiesThe basic material properties that affect the compressi-bility in dynamic compaction are:

(i) yield and tensile strength65

(ii) strain hardening

(iii) strain rate and temperature sensitivity

(iv) thermal properties such as specific heat, thermalconductivity, latent heat of fusion and meltingtemperature.

The compressibility is inversely proportional to yieldstrength, tensile strength, strain hardening and strainsensitivity, and directly proportional to temperaturesensitivity.4 Table 4 lists the relevant properties of iron,aluminium and copper based on the Johnson and Cookequation66 of stress dependence on strain, strain rate andtemperature. The deformation behaviour of the materi-als is also affected by the crystal structure because thedislocation behaviour is dependent on the same.67–69

Strain hardening of body centred cubic (bcc) metals isusually much more sensitive to strain rate than facecentred cubic (fcc) metals which remain fairly ductileover a wide range of strain rate. Body centred cubicmetals tend to lose their ductility with increasing strainrate and generally undergo a brittle transition.70 Thethermal diffusivity accounts for the different thermalproperties affecting the process. Its effect was mentionedin the section on ‘Efficiency’.

Preprocessing parametersDynamic compaction is a function of powder character-istics and operating parameters. Various types ofpowders have been studied from 1963 to date, but mostof the studies focusing on the effect of powdercharacteristics were performed in the 1960s and 1970s.Following are the key highlights of the literature surveyconducted:

(i) various types of powders have been studied from1963 to date, but most of the studies focusing onthe effect of powder characteristics were per-formed in the 1960s and 1970s. Technologicaladvancements in powder production haveimproved powder purity and altered powdercharacteristics since then. For example, the ironpowder studied by most of the researchers inthe1960s was sponge iron powder (NC100-24),which could be compressed to a maximumdensity of 7?0 g/cm3 whereas present commer-cially available highly compressible water ato-mised iron powders can be compressed to 7?8 g/cm3 (99% theoretical density).71 However, noneof the studies on dynamic compaction from thistime period explicitly described the powdercharacteristics

(ii) in addition, most of the studies that evaluatedpowder characteristics had confounding para-meters. Often powder shape was confoundedwith size or type

(iii) the increasing number of studies on the appro-priate amount of lubricant has equippedresearchers to raise the densification level overthe past 30 years.

Thus, there are no public sources available to enable thedesign of high density systems based on dynamiccompaction.

Processing parametersThe compaction velocity required to establish shockwaves in the powder is reached when the materialvelocity Vm equals the velocity of the sound in thepowder. The velocity of sound in a powdered material isat least a hundred orders of magnitude lower than itscorresponding velocity in the solid form (or the bulksound velocity). Table 5 shows the sound velocity valuesin aluminium, iron and copper.40 The ranges in powdersound velocity are based on the corresponding relativeapparent density of the powder. These are independentof the powder morphology and size but are dependenton the internal lubricant amount, in addition to materialproperties such as Young’s modulus.9,72–74

Assadi et al.75 predicted critical powder velocities thatproduce strong bonding in cold gas spraying. Cold gasspraying can be considered as an analogous process tohigh velocity compaction. In the former, the velocity isimparted to the powder particles to adhere to thesurface, while in the latter, the velocity is imparted to the

Table 4 List of relevant mechanical properties ofaluminium, iron and copper: aluminium is mostcompressible material followed by Cu and Fe;owing to soft nature of Al and Cu, strength ofunloading waves is higher in them than in Fe

Property Al Fe Cu

Crystal structure fcc bcc fccElastic modulus, GPa 70 211 130Tensile strength, MPa 85 280 210Yield strength, MPa 30 130 33Strain hardening exponent n 0.15 0.3 0.5Strain rate sensitivity C 0.0115 0.060 0.025Temperature sensitivity m 1.00 0.55 1.09Melting point Tm, uC 660 1535 1083Density, g/cm3 2.7 7.87 8.92

Table 5 Sound velocities in Al, Fe and Cu

Reference no. Property Al Fe Cu

40 Bulk sound velocity, m s21 5100 4910 3570Powder sound velocity, m s21 – 31–109 26–58Corresponding relative apparent density – 0.20–0.27 0.14–0.20

67 Powder sound velocity, m s21 660 565



surface to hit the powder particles and cause bonding.Table 5 lists the critical velocities for two spherical metalpowders. These critical velocity values should beconsidered as a high estimate for dynamic compaction.

Based on the principle of conservation of momen-tum,76 the punch velocities of different compactionmachines can be estimated by

Vpu~g2mpEp

� �1=2

mpu

(3)

where Vpu is the punch velocity, mpu is the punch mass,mp is the mass of projectile, g is the efficiency of themachine to transfer energy between components and Ep

is the energy of the projectile. The punch velocity is alsoreferred to as the compaction velocity. Figure 7 showsthe effect of projectile mass on the compaction velocityat two different constant energies. These masses andenergies are selected considering both past and presentday machine designs. As the compaction energy isincreased, the effect of projectile and punch massesincreases. The compaction velocity increases with alower punch mass and higher projectile mass. The punchand projectile velocities should be selected such that thesound velocity barrier is crossed.

A majority of the studies and reviews have focusedon the physics of the process, discussing details ofwave propagation based on Rankine–Hugonoit curvesfor porous materials rather than powdered materials.38

To the knowledge of the authors, only this reviewdiscusses the effects of various operating parametersconsidering present day powder characteristics. In thenext section, some basic powder characteristics, powderpurity, density, shape and size are critically analysed.This is followed by an analysis of the processingparameters such as peak compaction pressure, compac-tion energy, impactor/projectile mass and velocity,number of impacts, lubrication and compact tolerance.Note that the graphs depicted while discussing theseparameters are representative graphs for the correspond-ing parameter.

Effect of preprocessing factors

Particle sizeRusnak64 conducted one of the principal studies on theeffect of particle size on air atomised copper powder(irregular in shape). The velocity range in these experi-ments was from 30 to 210 m s21. It was observed thatthe total energy required to obtain a given green densitywas greater for the larger size fraction than for thesmaller size fraction (Fig. 8a) (there was a small dif-ference, ,5%, in the packing density of the sizefractions). In the case of low strain rate (quasi-static)compaction, water atomised iron powder shows theopposite trend. The quasi-static compaction results aresupported by Poquillon et al.77,78 More strain could beput into the larger particles. Also, on conductingdynamic compaction on the same iron powder(Fig. 8b), the trend in Fig. 8a is maintained. Since noother information was given on the powders used byRusnak,64 his studies are inconclusive. Thus, there is stilla need to obtain an optimized window of operation forparticle size.

Roman et al.52,61 also carried out experiments on theeffect of particle size. However, the particle sizedistributions studied61 were very wide and only theirability to dynamically compact was reported. However,their study52 on granular and powder metals indicatedthat particle size is an important factor governing theformation of bonds between the particles duringdynamic compaction. In fact, a smaller sized powderfraction results in a uniform energy distribution over thevolume at high pressure, which can also lead todecompaction of the compact due to the relief orunloading waves (see the section on ‘Impactor massand velocity’). The inefficiency of the process wasobserved in the case where the compaction velocitywas analysed with respect to the relative densityobtained.79 The pressing of fine powders is most closelyrepresented by parameters lying in region B of Fig. 4. Alarger particle size will result in higher adiabacity butparticles may not succeed in being deformed during the

7 Effect of projectile and punch masses on compaction velocity for two constant energies: 1 and 5 kJ




lifetime of the pressure in such a way as to remove all thepores and to form bonds.

Powder shapeIyer et al.80 were the main contributors towards definingthe dependence of powder morphology on dynamiccompaction. They compared spherical and irregularaluminium alloy powders as shown in Fig. 9. This studywas based on a 36 kJ dynamic compaction machine. Apowder size fraction less than 88 mm with an averageparticle size of 55 mm and a tap density of y62% waschosen for air atomised and inert gas atomised Al alloys.The study provided powder composition informationbut no details were given on how the tap densities of thefractions were kept constant. The study concluded thatirregular powder (air atomised) suffers greater bendingstresses and has greater points of stress concentration(energy deposition is more) which causes enhancedlocalised thermal modifications (loss of strength), and

hence lead to better densification than spherical powders(inert gas atomised).33 All the main densificationmechanisms of dynamic compaction were clearly seenin the compressibility curves (Fig. 9). There is a shockwave enhancement that occurs due to paradigm shiftfrom low strain rate compaction to high strain ratecompaction. This is followed by localised plasticdeformation at high strain rate. The last stage is thethermal softening that occurs due to the adiabatic natureof the process. This stage occurs earlier in the irregularparticles due to their shape. Irregular particles suffermore strain and bending moment leading to moresoftening points. For spherical particles, most of theenergy of the shock wave is in fact dissipated over-coming friction and sliding and does not reside indeformational energy. Gourdin35 supported the argu-ment by concluding that as powders become moreirregular, the densification improves. A similar phenom-enon was observed in the case of low speed experiments

8 a Dynamic compaction of two different particle sizes of air atomised copper64 and water atomised iron.3 Trends shown

in studies are contradicting. b Quasi-static compaction of same two particle sizes (197 and 37 mm) of water atomized

of iron as in a




(quasi-static compaction) at both low77,78 and high23

compaction pressures.

Powder type: purity and densityIt is known that powder type affects the final density of acompacted or sintered component.81 Stein et al.82 werethe first researchers to press iron to 99% theoreticaldensity using dynamic compaction. They also investi-gated the effect of type of powder on the densification.Figure 10 shows the comparison for sponge iron andelectrolytic iron powder. The limitation of the compar-ison was that the powder density, purity, size and shapewere confounded. Electrolytic powders are generallypurer than sponge powders. Moreover, sponge ironpowder (7?5–7?7 g/cm3) has lower particle density than

electrolytic powder (7?6–7?85 g/cm3). Hence, increasingthe purity of the sponge powder can lead to higherdensification but may be offset by its low particledensity. Stein et al.82 provided no information on thepurity/particle size/apparent densities or any otherpreprocessing characteristics. Thus, the densificationeffect could solely be based on the particle densityrather than purity.

Further, as seen from Fig. 10, a large gap existsbetween densities attained by the quasi-static anddynamic compaction. This density gap (,10% theore-tical) is almost constant even after inputting high energyinto the system (the assumption in this study is that thespeeds and energies are high enough to propagate shockwaves, but insufficient data were provided to determine

9 Effect of powder shape, irregular and spherical in Al alloy powder80

10 Green density of different powder types of iron powder, which are dynamically and quasi-statically compacted.82

Electrolytic iron powder is more compressible than sponge iron powder. Dynamic compaction is seen to be efficient

at all densities than quasi-static compaction for electrolytic iron powder





the shock wave nature of the experiments conducted). Inother studies (discussed in the section on ‘Compactionpressure/energy’), this gap was not constant but instead,converged and hence a crossover was observed betweenthe high and low speed processing.

Effect of processing factors

Compaction pressure/energyWith the advancement in technological innovations, thecompaction velocity and energy of dynamic compactionhave been increased to new levels in every decade.Raybould56,83 was the first researcher to use a robustnon-explosive machine based on an air cannon with amaximum piston or punch velocity of 600 m s21. Beforethis, velocities in this range could only be reached usingexplosive devices. Although Raybould83 performed highstrain rate compaction on iron and aluminium, theeffects of powder characteristics were not explicitlystudied. Typical compaction energy used in the compac-tion was 3?7 kJ per impact. The peak compactionpressure was measured using a pressure transducer inthe die. Figure 11 shows the effect of compactionpressure or energy on the final green density achievedin iron, aluminium and copper. The dynamic data werefit by the modified Kawakita equation37 (equation (4))and the quasi-static data were fit by the Kawakitaequation (equation (5)).84 Equation (4) is based onthermal parameters and the Mie–Gruneisen coefficient(Gruneisen coefficient is a relationship between pressureand volume in the equation of state in shock waves).Equation (5) is widely accepted in relating the pressure–density data85

P~2C rs=rð ÞP{2r (rs{rs0)2K=2rs0r2

s

� �2C rs=rð Þz1{ r=r0ð Þ

(4)

r

ro

~ az1{a

azbP

� ��1

(5)

where r is the green density, r0 is the apparent density, Pis the compaction pressure, a is a constant representingthe initial porosity of the powder, b is a constantrepresenting the stiffness of powder, c is the Mie–Gruneisen coefficient, K is bulk modulus, rs is the soliddensity and rs0 is the initial solid density. Foraluminium, the dynamic curve is consistently abovethe quasi-static curve (Fig. 11a). The dynamic curve foriron is above the quasi-static curve at high densities, butbelow the curve at low densities (Fig. 11b). A similartransition was observed in copper (Fig. 11c).64 This wasalso supported by other reported literature.2,86 Thistransition occurs due to improvement in the efficiency ofshock propagation as the density of the compact isincreased. The relative inefficiency of the energy transfer(or equivalent compaction pressure) at lower greendensities reflects the inefficiency of transmitting apressure wave through a low density, unbonded mass.As the density of the compact increases, the attenuationof the pressure wave decreases, making high velocitycompaction more effective. Since aluminium is a morecompressible material than iron (Table 4), the transitionin aluminium takes place at a much lower density andcompaction energy. In addition, the pressure trans-mitted was higher in the case of Al than in the case of

Fe. Comparison of the compaction energies of alumi-nium, iron and copper could not be carried out withprevious literature, as it depends on various preproces-sing and processing factors, and especially the machineefficiency. The data support theoretical work37 thatshowed that the modified Kawakita equation fordynamic compaction lies above the Kawakita equationfor quasi-static compaction. The existence of a transitionpoint implies that the compaction conditions in theregion below the quasi-static curve are represented byregions B, E, F or G of the d–u (particle size–loadingrate) diagram (Fig. 4). However, as the total energyinput is increased, conditions start moving towardregions C and D by crossing curves 2 and 3.Thus, thistransition actually depends on the region (and hence thepoint) in the d–u diagram. Conditions closer to curve 2or 3 lower the density at which the transition occurs.Since the d–u curve is material dependent, the transitionpoint will be different for different materials.

Impactor mass and velocityThe impactor mass and velocity control the compactionvelocity. Although pronounced differences exist betweenlow and high compaction velocity pressing, the impac-tor’s effect is marginal at constant energy (decreasing athigh energies).2,64 Once the shock wave is established inthe powder by overcoming the sound velocity barrier, anincrease in velocity will not affect the density if energy isheld constant.64 In most of the studies conducted oneffects of varying impactor (or projectile) velocity, theweights of the impactors were varied by makingimpactors of different lengths. The sketch of wavediagrams (Fig. 12) shows that the variation in theproperties is caused by relief waves from the free endof the projectile and the base plate, where the pressuremust be zero. Three different cases83 were identified, asshown in Fig. 12. The first case occurs when the pressurehas already attained a level sufficient to cause completecompaction before the arrival of relief wave (Fig. 12a).The second case occurs when the shock wave in thepowder does not reach the rigid base before it isovertaken by the relief wave (Fig. 12b). The third caseoccurs when the shock wave in the compact is reflectedto produce a decompacting wave at the rigid base, butbefore it can return to the point of impact, it is met bythe relief wave and its magnitude is diminished(Fig. 12c). Varying the mass of the impactor will thushave an effect on the compaction velocity and on thenature of shock wave and relief wave generated in theimpactor and the specimen. The role of powder fill onwave propagation is discussed later in the paper.

To prevent the formation of relief waves, a splitHopkinson bar87 configuration consisting of a longincident bar and transmitter bar should be used asshown in Fig. 2. The longer length of the bars preventsthe interference of the relief wave with the compactionprocess. In that case, the highest densification can beachieved, as the compaction process will follow theconditions shown in Fig. 12a.

In dynamic compaction of copper powder64 (Fig. 13),no change occurs in the green density due to projectilevelocity at lower energy. However, at higher energies,the green density decreases slightly with increasingvelocity. These results are consistent with otherstudies,33,79 as shown in Fig. 13. This is due tooccurrence of the third case discussed earlier



11 E–D (energy–density) curves for a iron,83 b aluminium83 and c copper powder.64 Each of graphs shows existence of

transition point following which dynamic compaction becomes effective and beneficial. As shown, dynamic compac-

tion is helpful only at high densities




(Fig. 12c). The mass of the impactor only affects thegreen density of powders that require high energy inputfor compaction. For types of metal powder that areeasily compressed, such as pure copper and aluminium,impactor mass has less effect. The same trend wasobserved for Fe infiltrated with three different amountsof Cu.88 As the infiltrated Cu content was increased, theeffect of impactor mass decreased.

Powder fillThe amount of powder filled in the die plays a criticalrole in transference of the shock wave through the

powder. There is a considerable difference in trends ofthe density obtained based on powder fill for quasi-staticand dynamic compaction.60 This difference is shown inFig. 14. The specific energy (compaction energy per unitmass) follows a parabolic trend v. powder fill height toachieve the same density. This parabolic trend can beexplained based on Fig. 12. Initially, with increasingpowder fill, the amount of specific energy requiredincreases because there is a larger amount of powder tocompact. However, as the powder length furtherincreases, the magnitude of the relief wave coming backdecreases because it has to travel a longer distance and

12 Wave diagrams show occurrence of three different cases for relief waves.83 Sources of relief waves are projectile and

base plate. Final density of compact depends on nature of relief wave, as shown above

13 Effect of loading rate at constant energy in air atomised copper powder64 and explosively compacted pure electrolytic

iron powder.79 High velocities are not beneficial, because strengths of unloading waves also increase, leading to

decompaction. To increase compact green density, compaction energy instead of velocity should be increased





thus diminishes early. Thus to derive maximum benefitout of dynamic compaction, the compact should beprecompacted to ensure that the powder fill is not nearthe maxima of required specific energy (Fig. 14). Inaddition, the formation of a preform will improve thewave propagation through it including the requiredsound velocity. This preform should then be compactedto the desired dimensions.25 The trend in quasi-staticcompaction is as expected.1

Number of strokesMultiple strokes have an additive effect on the compactdensity for both dynamic59 and quasi-static89 compac-tion. For constant energy strokes, the increments indensity for each stroke decreased progressively.

Nevertheless, a final density of 95% of theoreticaldensity was achieved (Fig. 15). This would require a10 kN capacity conventional mechanical press toproduce quasi-static compacts of similar density levelswith a single stroke. In addition, the rate at which thegreen density increases with strokes increases with initial(first stroke) green density. This fact is evident from theincreasing slopes for different initial green densities inFig. 15. Sponge iron powder mixed with 1% graphitewas used as the raw material. Typical energy levels in thedynamic study were 2?7–5?4 kJ. Note that this onlyproduces peak compaction pressures from 275 to690 MPa, which is largely dependent on the shock wavevelocity rather than the energy as a whole.90–92

Figure 15 also shows that fewer strokes are required to

14 Specific energy required to produce same density with different powder fills (fill heights) in quasi-static and dynamic

compaction for iron powder.60 No detail on theoretical density of powder was provided. Powder fill should be chosen

to avoid maxima of energy in dynamic compaction

15 Effect of number of impacts in case of dynamically compacted iron powder.59 As number of impacts of same energy

increase, compact green density also increases. Increase is proportional to initial green density





reach 95% theoretical density as the initial green densityis increased. This is due to improvement in thepropagation of the compressive wave as density isincreased (Fig. 11).

As green density increases, the sound velocity barrierrises. Thus, to reach shock wave velocity on each stroke,a higher energy input must be provided to the strokes.However, since the speed of sound in near 90% densematerial is so much higher than in the powder, it isprobably unattainable without explosive compaction.This is clearly not the case in the study discussed. Hence,it can be concluded from Fig. 15 that the benefit ofstrokes may be obtained even if the sound velocitybarrier is not crossed. This can be attributed to highstrain rate of the compaction process, which elucidates aphenomenon similar to creep.36 In addition, it should benoted that the increase in the number of strokes providesa diminishing return in green density and reaches a limit.

Cracking of some compacts near the pressed endbecame a problem when comparatively large energystrokes were delivered to compacts already at highdensity. This is explained in terms of relief waves(Fig. 12). High strength relief waves can crack thesamples once the compaction process is completed.

Lubrication effectsElwakil et al. and Yousuff and Page have investigatedthe friction and lubrication effects in dynamic compac-tion and found that the effect is similar to what isobserved in the case of quasi-static compaction.93–95

Larger amount of internal lubricant hinders thedensification at higher compaction pressures. Die walllubrication is used instead to enhance the densificationat higher compaction pressures.88 In quasi-static com-paction, the limitation is due to the space occupied bythe internal lubricant. This must also apply to dynamic

compaction, and is also detrimental to wave propaga-tion through the powder.3

Compact toleranceThe diameter of a green compact after ejection is alwayslarger than the die bore diameter due to springback.This springback varies with the green density. Figure 16shows this effect for sponge Fe powder with 1%graphite.59 The use of high speeds results in improvedconformity with die dimensions, particularly at highdensities.60 This is because the heat generated duringquasi-static compaction has ample time to dissipateduring low speed compaction, whereas in high speedcompaction, the majority of heat generated is retained inthe compact until pressing is complete. The overall heatinput would be proportional to the energy of compac-tion. Since the overall compact temperature of dynamiccompacts will be higher than that of quasi-staticcompacts, a greater amount of contraction will occuron cooling (less springback). This could be attributed toless springback from the high temperature regions,which are the high deformation regions. Moreover, hightemperature deformation has little or no elastic nature(lower yield strength), so it would have less elasticspringback from those areas.

ConclusionsThis article showed that optimised selection of differentpreprocessing and processing parameters is critical for ahigh performance dynamic compaction system. Forshock wave phenomenon to occur, the material speedVm must be greater than the speed of sound Vsound in thepowder. The speed of sound in the powder is a functionof apparent density of the powder and not the particlesize and shape. Even if the shock wave is not produced,

16 Dimensional control in dynamic and quasi-static compaction.59 Owing to lower springback in dynamic compaction,

dynamic compacts have diameter closer to die diameter than quasi-static compacts




slightly higher densification is achieved due to highstrain rate of the compaction process. However, in thecase of high strain rate compaction, there is no addedadvantage of higher green strength due to the bondingcaused by adiabacity of the process. Dynamic compac-tion provides two benefits due to its adiabatic nature.The adiabatic nature causes localised heat build-up inthe regions where deformation is concentrated in thecompact. As a result, these regions locally reach hightemperatures. This leads to better interparticle bondingand less springback during ejection from the die than incompacts produced by quasi-static compaction.

Once the critical limit of sound speed is reached, theshock wave phenomenon is not solely controlled bycompaction velocity. Energy, not velocity, is the criticalparameter controlling the shock wave phenomenon foreasily compressible materials. If compaction energy isconstant, variations in velocity have no effect on thedensification. However, at higher energy levels (orequivalently higher velocity levels), the velocity hasminor negative effect on the densification. This detri-mental effect is due to high intensity reflection wavesfrom the base, which are decompacting in nature.Although compaction energy is found to be thegoverning factor, there is still no common platform tocompare compaction energies for different studies in theliterature. Different machines are used in the studies,which have different efficiencies. In addition, many ofthe studies do not convert the compaction energy toequivalent compaction pressure. Thus, there is a need todevelop a standard to compare the compaction energies.

Larger particles and more irregular particles formhigher density compacts for a given impact energy. Thehigher particle size increases the dislocation slip distance,leading to better densification. The irregularity in thepowder increases the number and amount of interparticlestress, and friction areas in the powder compact. Thisleads to greater softening of the particles during compac-tion, producing a higher density component.

Dynamic compaction is more efficient at higherapparent and green densities. There generally exists atransition point in efficiency of quasi-static v. dynamiccompaction. This transition point occurs because theshock wave propagation is a function of compact density.The improvement in the density of the compact leads tobetter wave propagation, causing the powder particles tomove more coherently and densify. This transition pointoccurs early in highly compressible materials such as Al atgreen densities of 70% theoretical and around 90%theoretical for low compressible materials such as Fe.The point also occurs at lower compaction energies in thelatter case. Thus there is little benefit from dynamiccompaction unless a machine is used which has thecapacity to operate beyond the transition point.Otherwise, multiple impacts in dynamic compaction canbe used to produce high density as used in quasi-staticcompaction. Even though density returns with number ofimpacts are diminishing, multiple impacts of the sameenergy cause successive increases in the density. Thus thenumber of impacts is a critical factor in design of a highdensity dynamic compaction system.

Acknowledgements

The authors would like to thank the industry membersof the Center for Innovative Sintered Products (CISP) at

the Pennsylvania State University for initial support ofthis investigation. Sustained efforts were provided byCenter for Advanced Vehicular Systems (CAVS) atMississippi State University, Department of EngineeringScience and Mechanics at Pennsylvania State Universityand Kennametal. The authors would like to thank J. L.Johnson, G. Aggarwal and R. Malhotra for theircomments on the manuscript.

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An overview of dynamic compaction in powder metallurgy