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Dynamic effects during a high-speed sinter-forging process
Int. J. Mach. Tools Manufact. Vol, 36, No. 10, pp. 1109-1122, 1996 Copyright © 1996 Elsevier Science Lid Printed in Great Britain. All rights re.fred 0890-6955/96515.00 + .00
Pergamon
0890-6955(95)00122-0
DYNAMIC EFFECTS DURING A HIGH-SPEED SINTER-FORGING PROCESS A. K. J H A * and S. K U M A R t (Original received 1 June 1995; in final form 10 November 1995)
Abstract--The paper reports on an investigation into the various technological aspects of a high-speed sinterforging process. Experiments were conducted and measurements were made in the development of barrelling, strain variations at the free surface and densification during dry and lubricated high-speed cold forging of sintered copper powder discs. The relationship between the frictional stress at the interface and other process variables is discussed and an analysis is given for the calculation of pressure distribution and die load by the solution of the equations of equilibrium taking barrelling into consideration for axial symmetry and plane strain sinterforging conditions. Results are discussed critically to illustrate the interaction of various processing parameters involved during high-speed sinter-forging process and are presented graphically. Copyright © 1996 Elsevier Science Ltd
1. NOMENCLATURE initial density of the sintered material dimensionless, p/p, real density of the real contact area apparent density of the apparent area specific cohesion of the contact surface flow stress of the sintered material shear stress pressure at the die-workpiece interface constant and a function of the relative density 13 only coefficient of friction diameter of the disc instantaneous length of the strip width of the strip instantaneous thickness of the workpiece constant quantity much greater than unity Cartesian co-ordinates cylindrical co-ordinates radial co-ordinate
Pp Po
P, p.
¢,, p q 2b 2l w
h n
x, y, Z r, O, Z r O
stress
U t £
P g
die velocity time arbitary parameter which determines the amount of barrelling strain die load acceleration due to gravity with dynamic_ ~ without dynamic effects effects = xl00 X_~ with dynamic effect Subscripts
~
1.1. r O z x y
radial circumferential axial longitudinal lateral
*Department of Mechanical Engineering, Institute of Technology, Banaras Hindu University, Varanasi, 221
005, India. i'Department of Production Engineering, BIT, Mesra, Ranchi, 835 215, India. 1109
I 110
A . K . Jha and S. Kumar 2.
INTRODUCTION
Recently sintered metal powder preform forging has been developed as an economic method for producing products of high density and with good engineering properties. Applying this technology allows pressed and sintered metal powder preforms to be used as starting materials in bulk processing. The mechanical and metallurgical properties of sinter-forged products are comparable and in some cases even superior to the wrought products manufactured by conventional material processing techniques [1, 2]. The deformation pattern during the forging of metal powder preform is different from conventional wrought metal forging and the following characteristics of porous materials undergoing deformation must be taken into account: • a change in density occurs during plastic deformation; • the yielding of porous metals is not completely insensitive to the hydrostatic stress imposed during processing. The deformation pattern during the sinter-forging process is influenced by many factors which interact with each other in a complex manner. The main controlling factors are the density of the preform, lubrication conditions at the die-workpiece interface, the flow stress of the sintered preform and the factors related to forging equipment, such as deformation speed and contact times under load. The prediction of defect occurrence in a sinterforged product requires an understanding of the mechanics of the deforming sintered preform and a knowledge of the problems of pressure distribution at the die-workpiece interface and die load. Although a considerable amount of work has recently been reported on the various technological aspects of industrial processing of sintered preforms [3-10], so far no systematic attempt has been made to study the dynamic effects during the high-speed sinterforging process. Moreover, in recent years interest in high-speed metal working has grown considerably because of the need for increased production rates to meet the increased market demand, through the introduction of new and fast deformation techniques. It is therefore appropriate to investigate the effect of inertial forces during the high-speed sinterforging process. The present investigation has been undertaken with a view to study the dynamic effects during high-speed sinter-forging at room temperature. Experiments were conducted with sintered copper powder discs and measurements were made in the development of barrelling, strain variations at the free surface and densification during the sinter-forging process. The relationship between the frictional stress at the interface and other variables is discussed and an analysis is given for the calculation of pressure distribution and die load by the solution of equilibrium equations taking barrelling into consideration for axial symmetry and plane strain sinter-forging conditions. The results are discussed critically to illustrate the interaction of various processing parameters involved during the process and are graphically presented. It is expected that the present paper will be of great practical importance in designing equipment and tooling needed for the high-speed sinter-forging process. 3.
EXPERIMENTAL WORK
In order to study the dynamic effects during the sinter-forging process experiments were conducted on sintered copper powder discs. 3.1.
Powder used
Electrolytic copper powder of greater than 99% purity was used. The physical and chemical characteristics of the copper powder are given below: Sieve analysis (BS Sieves) +100 mesh - 100+150 mesh 150+200 mesh -200+240 mesh -
Weight retained (%) 0 35.00 15.00 14.50
Dynamic EffectsDuring a High-speedSinter-forgingProcess -240+350 mesh - 3 5 0 mesh Apparent density Theoretical density
20.00 14.50 2.60 g cm -3 8.96 g cm -3
Chemical analysis Copper Phosphorus Iron Silicon
Weight (%) 99.80 <0.001 <0.006 <0.002
1111
The physical characteristics were confirmed using an automatic sieve shaker and the chemical analysis was performed using an X-ray fluorescence technique. 3.2.
Preparation of specimens
Electrolytic copper powder was compacted in a closed circular die (bore 045 mm) using a 150 tonf (1471.05 kN) hydraulic press at a recorded pressure. The die was lubricated with zinc stearate. Sintering was carried out at 900°C for 2 h in an endothermic sand atmosphere. In order to minimize non-uniformity of the density distribution, the compacts were wrapped in teflon sheets (acting as a lubricant) and were repressed at the same compaction pressure in the same die and then resintered at the same temperature for the same time. All sintering operations were carried out in a muffle-type silicon-carbide furnace capable of providing sintering temperatures up to 1300°C to an accuracy of +_5°C. Specimens for testing were made by machining the compacts; the specimens being 30 mm in diameter and l0 mm in height. The surface of the specimens was then polished with a fine grade emery paper. The density of the copper powder strips was obtained simply by measuring specimen dimensions and weight, but in order to confirm this density, the hydrostatic (or Archimedes) method was also employed. The relative density of the specimen was obtained by the ratio of the porous copper powder disc density to that of solid copper density (8.96 g cm--3). 3.3.
Experimentalprocedure and measurements
Experiments were conducted on a pneumatic forging hammer using appropriate dies. The copper powder disc of known relative density (0.75) was placed between fiat dies and it was forged at room temperature by applying the impact load. The following important measurements were made: (1) (2) (3) (4)
increase in relative density with decrease in height; increase in diameter with decrease in height; axial and radial strain values; and amount of barrelling developed after forging to various reductions for dry and lubricated conditions (a thin layer of vaseline was applied as a lubricant). 4. RESULTSAND DISCUSSION
During the high-speed sinter-forging process the compressive forces will gradually close the pores and thus the relative density of the sintered material will increase with an increase in the percentage reduction in height. Figure l shows this variation in graphical form. The relative density changes gradually with an increase in percentage height reduction. Lubricated specimens exhibit relatively better densification than unlubricated ones. At point C (47% reduction) cracks start appearing on the equatorial free surface of the copper powder preform disc. Unlubricated specimens show much more severe cracks as compared to lubricated specimens. In an investigation of the plastic deformation of sintered preform during forging it is evident that the cylindrical free surface barrels when friction exists at the interface and cracks gradually appear at the barrelled surface. The amount of barrelling mainly depends on the initial density of the disc and the degree of frictional constraint. Figure 2(a) and
1112
A . K . Jha and S. Kumar Electrolytic copper power disc compacted at 15 kglmm 2 sintered at 9000C
(Preform cracks)
c/
I.O .t3..------
,~ 0.8 o.~
Dry forging condition Lubricated forging condition (vaseline)
-~ 0.4 0.2 0.13 0
I 10
I I 20 30 Reduction (%)
I 40
I 50
Fig. 1. Relative density variation with percentage reduction in height for high-speed forging. 10% Reduction 30% Reduction
0 ,
o
/
51 Disc size I~ 30ram x 10mm Initial relative density 0.75
1 20
Ic 15
I
10
J
5
l
0
I
5
J
ID 15
10
I 20
10% Reduction 30% Reduction
10i
A
,/I 1/11
1 20
IC 15
[
,,/B =0
51 Disc size ~ 30ram x IOmm Initial relative density 0.75
I 10
I 5
I 0
i 5
I 10
[D 15
1 20
Fig. 2. Development of barrelling during high speed forging of an electrolytic copper powder disc in: (a) dry forging conditions; (b) lubricated forging conditions (vaseline).
(b) show the barrelling graphically for various reductions during dry and lubricated highspeed sinter-forging of copper powder disc, respectively. The bulge profiles were traced on a contour projector with a ×10 magnification. Thus, during the sinter-forging process it is clear that with the coefficient of friction ~t, between the disc and die being zero the disc expands homogeneously and thus the change in shape is disregarded, as shown in Fig. 2(a) and (b). On the other hand, when friction exists between the tool and the die surface, the shape of the disc changes to that of barrelling. The value of expansion depends upon the initial relative density, the height reduction and the coefficient of friction. From the curves it is evident that the displacement at the interface with the moving die (AB) is greater than the interface at the stationary die (CD) during high-speed dry forging, while the reverse is true during forging using vaseline as a lubricant. This effect can be attributed to a high relative velocity between the workpiece material and the die surface, high interfacial pressure and/or severe deformation modes, which causes a breakdown of the lubricant film and allows new surfaces to come into contact with the die, hence facilitating the intimate contact essential for adhesion. The basic affinity of the die and the workpiece material then determines the extent of friction. Figure 3 shows the percentage increase in diameter (with reference to moving and stationary die contact surfaces) plotted against the percentage decrease in height for dry and lubricated high-speed forging of a sintered copper powder disc with an initial relative density of 0.75. Figure 4 shows the variation
Dynamic Effects During a High-speed Sinter-forging Process
1113
of principal strains (e.
ez) with percentage reduction at the equatorial free surface, for high-speed deformation with flat dies under dry and lubricated conditions.
(a)
Top (moving) die contact surface
16128
/
/ 4 [
I
(b)
I
20 30 Decrease in height
IO
0
I
I
40
50
Bottom (stationary) die contact surface
24
20
/
12
8
--
4
/
/ / /~
r
/ / /~ I
0
I0
olytic copper power disc ¢ 30mm x 10mm. Initial relative density 0.75 I
I
20 30 Decrease in height
I
I
40
50
Fig. 3. Percentage increase in diameter vs percentage decrease in height during high-speed forging (with reference to moving and stationary die contact surfaces).
0.22
Copper power disc ( ~ 30ram x 10mm) Compacted at 15 kg/mm 2
~
S i n t e r e d at 9 0 0 ° C 0.16
.~_ E¢
0.12 m
o.o8 0.04
0.00
5
10
.... •- o -
Dry forging condition Lubricated forging condition (vaseline)
15
20
25
30
35
40
._q..o.10
-o.20
f-z
-0.30
Fig. 4. Variation of principal strains (E,., e:) with percentage reduction in height.
1114
A.K. Jha and S. Kumar
4.1. Interfacial friction stress In high-speed sinter-forging, the surface of the workpiece is distorted and takes on an impression of the die surface. Therefore the actual contact area, as far as the specific cohesion of the surface is concerned, is not negligible. Friction conditions in forging is essentially different from sliding friction in machine parts. The relative velocity between the workpiece material and the forging die surface together with high interfacial pressure and/or deformation modes will create the conditions essential for adhesion in addition to sliding [11]. During the high-speed sinter-forging process the compressive force supported by impact gradually increases the relative density and hence the real area of contact grows and approaches the apparent one as the force increases. This approach will probably be an asymptotical one [ 12]. The pattern of metal flow during the forging of a sintered preform is such that there exists two zones, an inner one where no relative movement between work-piece and die occurs (the sticking zone), and an outer zone where sliding occurs. Therefore the appropriate interfacial friction law for different deforming conditions are as follows: axial symmetry condition
(1) plane strain condition
[
x=l.tp+po%
{ (Xm 1-
~ -
(2)
where rm and Xm denote the sticking zone radius for axial symmetry and plane strain conditions, respectively, which may be approximated by the relation given by Rooks [13], and n >> 1. Though the contribution of adhesion in Equation (1) and Equation (2) is less pronounced, it constitutes a major drawback in production, because it affects the die life, deformation load and the surface finish of the forged product appreciably. 4.2. Free body equilibrium approach In addition to the usual assumptions during high-speed wrought material forging [14], the following assumptions are made in the analysis: (1) friction due to adhesion is a function of the relative density; (2) yielding of sintered material is sensitive to the hydrostatic stresses imposed during forging. For analysis of pressure distribution and die load, axial symmetry and plane strain highspeed forging conditions are considered separately here. The following equations may be obtained for the high-speed forging of a sintered preform by two fiat rigid dies moving with velocity U towards each other (Fig. 5). 4.2.1. Axial symmetry. Compressibility [7]: 3U~ (1 - 2rl)bU~ + - 0 c)r 2(1 + q)Oz
(3)
Equilibrium of forces in disc element: i)tsr
ar
(so
PP U~
U.
+
-
=0
(4)
Dynamic Effects During a High-speed Sinter-forging Process -U
I
'~
I
+Ul
I 115
i_ I
b
_1 -I
I
Fig. 5. Schematic diagram of an axially symmetric sintered disc during the high-speed sinter-forging process.
Yielding of sintered preform disc: o~
=
-
p
(5)
Or :
(~0 = ~" - - P
Or
br
(6)
or
(7)
Because of symmetry, the velocity field with bulge satisfying the compressibility Equation (3) is (1 - 2 q ) ~ -I~Jh Ur Ur = 2(1 + rl)h(1 - e -1~2~
(1 - e-I~Jh)U (1 - e -~m
U:-
uo = o
(8)
(9)
(1o)
Now substituting Equations (5)-(10) in Equation (4), the die pressure equation is obtained as follows: dp pp(1 - 21"1)~2re-21~Jh [ h(l - e -m2) I~-Jh; ] 2X drr+2(l+~-)h2-~:~) 2 /U2k+ ~ e uj+~-=O where
O-
du
dt
and k = c ash
(1 + 411) 2(1 +1])
(II)
1116
A.K. Jha and S. Kumar
4.2.1.1.
Pressure distribution:
d~+ 2(1 + ~
from Equation (1) and Equation (11) it follows that
-~ ~ ) 2
+ ~ - P+Po(~o
/ilk +
1 -(~-j]=0
(12)
It is a linear differential equation of the first order and its solution is
Arh
PoOo P = -~-
Ah 2
Polo
rm -- 10°~° -- ~ - ~ + 4~t 2
nb
C_ 211r/h
Po~oh r + 2-~
(13)
+
where C is a constant of integration and pp(1 - 211)l]Ze-21~Jh [ h(l-__e-f~/2)ef~jhf_] ] A = 2(1 + 11)-gh~ - - - ~ 2 ) 2 Uzk + By using the boundary condition at the rim, i.e. (~r 0 and p = ~, at r = b the constant of integration C is determined and therefore Equation (1 3) becomes =
p poC?o[r~bb h ] ~,- ~, + 2~tllb- 1 Ah I ~ _ ,
+2~ 4.2.1.2.
+oe
Die load:
P = 21t
(
21a(b-r )
h
Polo r zu
h 2o,h ~)
~e
~
- r
1 n
1)]
] (14)
the die load is given by
(15)
.r.dr
Substituting the value of p from Equation (14) into Equation (15) and subsequent integration and on simplification gives rm
P = rcbZpo~o ~-b + -2~tnb _ 1
4.2.2.
n
~-+A
+2~tz ~
_[(I
---
_ bz
Plane strain.
dex=
+ ---2~t2e 2°h/h 1 -
1 1)] I ~.[1-(P~o)(rm
rm h ~ + 2gnb b+
3n
-
rch3 + -4 ~ 3 b
+--4~t3
h
~ + 21anb - 1)
--
1 n
] 1)
(16)
2~t //]
Compressibility [8]:
+ 2112) - 211~/3(1 - 112)]de: ?i--4q2)
Equilibrium of forces in strip element:
(17)
Dynamic Effects During a High-speed Sinter-forging Process
p~[ 0G u~G ~Ux]
ga~
g
(18)
:--~-z+--~j-~=o
G-G-x +
II 17
Yielding of sintered preform strip: a: = - p
(19)
ax = ~ - p
(20)
~(~x
~P
bx
~x
(21)
or
The velocity field with barrelling satisfying the above compressibility equation [Equation (17)] is txl3U~e-I ~Jh ux - h(l - e -13/2)
(22)
U,, = 0
(23)
and
U~-
(1 - e-f~Ja)u (1 - e -13/2)
(24)
where (1 + 21] 2) -(Z=
21]X/3(1 - 112)
(1 - 41] 2)
Substituting Equations (21)-(24) into Equation (18), we now obtain
(X2~2ppe-2f~JhX[ heaJh(l -e-l~'2)U] dPdx+ hT(i 7 - - ~ [ U2K + j
2X
(25)
(e 13z/h -- 1) dU :and [J = cz dt Pressure distribution: from Equation (25) and Equation (2) it follows that
where K = 1 + 4.2.2.1. dp+
tX2~2ppe-Z~Jhx [ U2K + he'an(1 - e-lV2)L/]
l
(26)
J
It is a linear differential equation of the first order and its solution is Po¢o
p=---~-x,. - Po¢o
_ Po¢oX Po¢oh Mh 2 nl + 2 - ~ - + --4~t2
where C is a constant of integration and
Mxh --+2~t
Ce_Z~/h
(27)
1118
A.K. Jha and S. Kumar O~2l]2ppe-213:.mx ]M= ~i = ~[U2K
+
het3Jh(l -- e-13/2)ij] o¢[3 J
By using the boundary condition at the edge i.e. Ox = 0 and p = ~. at x = 1 the constant of integration C is determined and therefore Equation (27) becomes
)~-
)~ [nl + 2~n~l- 1 1
1
n 4.2.2.2.
+
Die load:
2~
)~ nl + e ~ +le
h
-
! --2g
,,
nl + 2gn~l
-x]
(28)
the die load is given by
I
P=2w
(29) 0
Substituting the value of p from Equation (28) and integrating, we obtain P=--
+
1 -1 -2gnl n n {Xm h t wh~- 2~'[ Po¢o +2wp°O°l \nl + 2gn-l- 1 ] [ --eS, + I
~t
Xm h nl + 2gnl
I n
1
+
( 1 - e22/) + 2g
e~-,
1)1
30,
In an investigation of the plastic deformation of sintered materials it is evident that the density distribution does not seem to be uniform throughout. It is high in the central region and low at the edges. The density distribution will be more uniform for a smaller coefficient of friction [a and a higher initial relative density P of the sintered material [15]. Figure 6 shows the theoretical pressure distribution over the surface of sintered copper powder disc during high-speed forging. The pressure distribution curve is made up of two symmetrical branches for the entire surface of the disc. The pressure distribution at the die-workpiece interface decreases from the centre towards the edge. The decrease in adhesion friction (or relative density) results in a further decrease in pressure. For high pressure values it is noticed that the curve does not meet the y-axis and gradually becomes parallel to the y-axis. This signifies the existence of a sticking zone on the surface of the disc. Because of the similar characteristics of Equation (14) and Equation (28), the pressure distribution curve for sintered strip is also expected to be of the same form. The relationship between an external pressure p and a relative density P is given by the following expression [16]: p 2~-
2 3 loge(1 - p)
(31)
Solving Equation (31) results in p = i
-
e -3p/2x
(32)
To study the density variation (from centre to edge) during high-speed forging of sintered preforms the various values of p/~. in Equation (32) may be considered using Equation (14).
Dynamic Effects During a High-speed Sinter-forging Process
I 119
Electrolytic copper powder Disc ~t3Omm x IOmm p = 0.75 tt ffi 0.30 U=
p: n=
.....
Hil Sic
Zp
polo =O.Ip
I ~ 0 = 0.',
~ao=0.Jp
q 2b Fig. 6. Theoretical pressure distribution over the surface of a disc during sinter-forging.
Figure 7 shows the theoretical variation of relative density from centre to edge for an electrolytic copper powder disc preform with an initial relative density of 0.75, forged at high speed to a 10% height reduction. The average theoretical value of the relative density (from centre to edge) is found to be 0.795. The experimentally measured relative density of the sintered copper powder disc after being forged to a 10% height reduction agrees satisfactorily with the above theoretical value (Fig. 1). Figure 8(a) and (b) shows the theoretical forging load versus percentage reduction curves for high-speed forging of sintered copper powder preforms under axial symmetry and plane strain sinter-forging conditions, respectively. The curves express the results for a Disc size = I~ 30ram x 10ram Initial relative density = 0.75 = 0.2p It = 0.30 n=3 U=
1.0 I00rn/sec .=,'0~)," 8 ~ ~ ~ - u ~ "~ 0.6 -
Av. Theoretical p ffi 0.795
u u
.~
0.4-
~
0.20.0
I I 5 10 Distance from center to edge
I 15
Fig. 7. Variation of relative density from centre to edge for an electrolytic copper powder disc forged to a 10% reduction.
1120
A.K. Jha and S. Kumar
8O 70 6O _
Electrolytic copper p o w d e r disc Size = 13 30ram x IOmm Initial relative density = 0.75 / POlO = 0.2p / It = 0.30 / n=3
~ 50
2O I0 1 I0
0
35 30 25
I
I
I 40
20 30 Height reduction (%)
Electrolytic copper p o w d e r strip Size -- O 30ram x 15mm x lOmm Initial relative d e n s i t y = 0.75 Po13o-- 0.2p = 0.30 n=3
/ /
~20 o
0
~ IO
I I0
I 20
I 30
I 40
I
50
Height reduction (%) Fig. 8. (a) Theoretical forging load vs percentage reduction curves for high-speed forging of a sintered copper powder disc under axial symmetry conditions; (b) theoretical forging load vs percentage reduction curves for high-speed forging of a sintered copper powder strip under plane strain conditions.
particular value of the coefficient of friction ix and the initial relative density of p of the sintered preform for different values of the deformation speed U. The forging load was found to increase with an increase in deformation speed. Now, to illustrate the order of magnitude of dynamic effects, let us calculate the die velocity required to increase the maximum die pressure twice, as compared with a slow static forging. Let us consider the pressure at the centre of the die for the typical case: 13 = 0.483, b = 1.1 cm, h = 1.0cm, ix = 0.30, Poq~o = 0.4p, n = 3, ~, = 812 kg cm -2, pp = 0.007616 kg cm -3 and g = 981 cm s -2 From Equation (14), the pressure is P = 3.4715 + 1.7046D
where D -
(33)
A
The value
ofp/~,
in Equation (33) is doubled as a result of the inertia forces only, when
Dynamic Effects During a High-speed Sinter-forging Process
1121
D = 2.02. For simplicity, considering the effect of velocity only and disregarding the die acceleration terms, we have pp(1 - 2 " q ) [ ~ 2 O 2 k e -21~'Jh 2.02 2(1 + rl)gh2(l - e-1~2)2~,At the die face, y = h/2, therefore U=I50 m s -). It is noted from Fig. 9 that an increase in 13 increases the value of ~ for a constant value of U. Also, for a particular value of 13, an increase in U results in an increase in as here. Therefore it may be concluded that such dynamic effects are important for extremely high-speed forging processes. As an illustration, Fig. 10(a) shows photomicrograph of pores in an electrolytic copper powder disc sintered at 900°C and high-speed forged to 30% reduction without lubricant, whereas Fig. 10(b) shows photomicrograph of pores in an electrolytic copper powder disc sintered at 900°C and high-speed forged to 30% reduction with vaseline acting as a lubricant. 5. CONCLUSIONS
The feasibility of high-speed processing of sintered materials has been demonstrated. During high-speed forging of sintered materials the main controlling factors are the deformation speed, the amount of interfacial friction between the die--workpiece interface, the initial density of the preform and the pressure distribution from the centre to the edge. During the sinter-forging process, the pressure varies from the centre to the edge which in turn affects the density distribution from centre to edge. Density distribution will be more uniform for a higher initial relative density of the preform. Lubricated specimens show relatively improved densification than unlubricated ones. The density of the sinter-forged product increases with an increase in the forging load. The amount of barrelling depends mainly on the degree of densification and friction conditions. Displacement at the interfaces with the moving die is more than that with the interface at the stationary die during high-speed dry sinter-forging, while the reverse is true during forging with the use of a lubricant. This is mainly due to the breakdown of the lubricant film due to severe deforming conditions. It is also concluded that the dynamic effect on the die load is not negligible during high-speed sinter-forging and must be considered while estimating the deformation load. The inertia forces encountered are functions of processing parameters and deformation characteristics of the sintered material. Evidently, such dynamic effects within the forged material itself are important for very fast forging operations. The work was found to be effective for the assessment of the pressure distribution and die load during high-speed forging of sintered materials.
u|
Disc size = ~ 30mm x 10ram
[- ~ ; O . 2 p / ~=o.3o
"F
/// 13
//-/---0.4 ""
0.6
'i o
I
I
I
I
|
1
25
5o
"75
Ioo
t25
15o
U (m/s~) Fig. 9. Effect of die velocity U and barelling parameter [i and ~.
1122
A.K. Jha and S. Kumar
Fig. 10. (a) Photomicrograph of pores in an electrolytic copper powder disc sintered at 900°C and high-speed forged to a 30% reduction without a lubricant; (b) photomicrograph of pores in an electrolytic copper powder disc sintered at 900°C and high-speed forged to a 30% reduction with lubricant (vaseline). REFERENCES [1] G. W. Cull, Mechanical and metallurgical properties of powder forgings, Powder Metall. 13, 156 (1970). [2] P. K. Jones, The technical and economic advantage of powder forged products, Powder Metall. 13, 114 (1970). [3] R. Davies and J. B. Marx, The production of components by forging of powder preforms, Proc. 13th Int. MTDR Conf., Birmingham, p. 463. Macmillan, London (1972). [4] M. Negm and R. Davies, The hot extrusion of metal powder preforms, Proc. 15th Int. MTDR Conf., Birmingham, p. 637. Macmillan, London (1974). [5] A. Singh and R. Davies, Preliminary investigations of the cold extrusion of powder preforms, Proc. 13th Int. MTDR Conf., Birmingham, p. 449. Macmillan, London (1972). [6] S. Del Wakil, A Plane strain analogue for the extrusion of sintered billets, Proc. 20th Int. MTDR Conj., Birmingham, p. 229. Macmillan, London (1979). [7] A. K. Jha and S. Kumar, Deformation characteristics and fracture mechanisms during the cold forging of metal powder preforms, Int. J. Mach. Tool Des. Res. 26, 369 (1986). [8] A. K. Jha and S. Kumar, Deformation characteristics and fracturing of sintered copper powder strips during cold forging, J. Mech. Work. Technol. 16, 145 (1988). [9] B.-B. Hwang and S. Kobayashi, Application of the finite element method to powdered metal compaction processes, Int. J. Mach. Tools Manufact. 31, 123 (1991). [10] G. Sutradhar, A. K. Jha and S. Kumar, Production of sinter-forged components, J. Mater. Process. Technol. 41, 143 (1994). [11] B. V. Deryagin, What is Friction? Izd. Akad. Nauk, Moscow (1952). [12] T. Wanhein, Friction at high normal pressures, Proc. 1st. WCIT, Paper no. F-7, New Delhi (1972). [13] B. W. Rooks, The effect of die temperature on metal flow and die wear during high speed hot forging, 15th Int. MTDR Conf., Birmingham, p. 487 (1974). [14] S. Kumar, Principles o f Metal Working, Oxford IBH, New Delhi (1976). [15] T. Tabata, S. Masaki and K. Hosokawa, A compression test to determine the coefficient of friction in forging P/M preforms, Int. J. Powder Metall. Powder TechnoL 16, 149 (1980). [16] S. Shima and J. M. Alexander, The interrelation of density and hardness in the isostatic compaction of powders, Proc. 13th Int. MTDR Conf., Birmingham, p. 471. Macmillan, London (1972).
Pergamon
0890-6955(95)00122-0
DYNAMIC EFFECTS DURING A HIGH-SPEED SINTER-FORGING PROCESS A. K. J H A * and S. K U M A R t (Original received 1 June 1995; in final form 10 November 1995)
Abstract--The paper reports on an investigation into the various technological aspects of a high-speed sinterforging process. Experiments were conducted and measurements were made in the development of barrelling, strain variations at the free surface and densification during dry and lubricated high-speed cold forging of sintered copper powder discs. The relationship between the frictional stress at the interface and other process variables is discussed and an analysis is given for the calculation of pressure distribution and die load by the solution of the equations of equilibrium taking barrelling into consideration for axial symmetry and plane strain sinterforging conditions. Results are discussed critically to illustrate the interaction of various processing parameters involved during high-speed sinter-forging process and are presented graphically. Copyright © 1996 Elsevier Science Ltd
1. NOMENCLATURE initial density of the sintered material dimensionless, p/p, real density of the real contact area apparent density of the apparent area specific cohesion of the contact surface flow stress of the sintered material shear stress pressure at the die-workpiece interface constant and a function of the relative density 13 only coefficient of friction diameter of the disc instantaneous length of the strip width of the strip instantaneous thickness of the workpiece constant quantity much greater than unity Cartesian co-ordinates cylindrical co-ordinates radial co-ordinate
Pp Po
P, p.
¢,, p q 2b 2l w
h n
x, y, Z r, O, Z r O
stress
U t £
P g
die velocity time arbitary parameter which determines the amount of barrelling strain die load acceleration due to gravity with dynamic_ ~ without dynamic effects effects = xl00 X_~ with dynamic effect Subscripts
~
1.1. r O z x y
radial circumferential axial longitudinal lateral
*Department of Mechanical Engineering, Institute of Technology, Banaras Hindu University, Varanasi, 221
005, India. i'Department of Production Engineering, BIT, Mesra, Ranchi, 835 215, India. 1109
I 110
A . K . Jha and S. Kumar 2.
INTRODUCTION
Recently sintered metal powder preform forging has been developed as an economic method for producing products of high density and with good engineering properties. Applying this technology allows pressed and sintered metal powder preforms to be used as starting materials in bulk processing. The mechanical and metallurgical properties of sinter-forged products are comparable and in some cases even superior to the wrought products manufactured by conventional material processing techniques [1, 2]. The deformation pattern during the forging of metal powder preform is different from conventional wrought metal forging and the following characteristics of porous materials undergoing deformation must be taken into account: • a change in density occurs during plastic deformation; • the yielding of porous metals is not completely insensitive to the hydrostatic stress imposed during processing. The deformation pattern during the sinter-forging process is influenced by many factors which interact with each other in a complex manner. The main controlling factors are the density of the preform, lubrication conditions at the die-workpiece interface, the flow stress of the sintered preform and the factors related to forging equipment, such as deformation speed and contact times under load. The prediction of defect occurrence in a sinterforged product requires an understanding of the mechanics of the deforming sintered preform and a knowledge of the problems of pressure distribution at the die-workpiece interface and die load. Although a considerable amount of work has recently been reported on the various technological aspects of industrial processing of sintered preforms [3-10], so far no systematic attempt has been made to study the dynamic effects during the high-speed sinterforging process. Moreover, in recent years interest in high-speed metal working has grown considerably because of the need for increased production rates to meet the increased market demand, through the introduction of new and fast deformation techniques. It is therefore appropriate to investigate the effect of inertial forces during the high-speed sinterforging process. The present investigation has been undertaken with a view to study the dynamic effects during high-speed sinter-forging at room temperature. Experiments were conducted with sintered copper powder discs and measurements were made in the development of barrelling, strain variations at the free surface and densification during the sinter-forging process. The relationship between the frictional stress at the interface and other variables is discussed and an analysis is given for the calculation of pressure distribution and die load by the solution of equilibrium equations taking barrelling into consideration for axial symmetry and plane strain sinter-forging conditions. The results are discussed critically to illustrate the interaction of various processing parameters involved during the process and are graphically presented. It is expected that the present paper will be of great practical importance in designing equipment and tooling needed for the high-speed sinter-forging process. 3.
EXPERIMENTAL WORK
In order to study the dynamic effects during the sinter-forging process experiments were conducted on sintered copper powder discs. 3.1.
Powder used
Electrolytic copper powder of greater than 99% purity was used. The physical and chemical characteristics of the copper powder are given below: Sieve analysis (BS Sieves) +100 mesh - 100+150 mesh 150+200 mesh -200+240 mesh -
Weight retained (%) 0 35.00 15.00 14.50
Dynamic EffectsDuring a High-speedSinter-forgingProcess -240+350 mesh - 3 5 0 mesh Apparent density Theoretical density
20.00 14.50 2.60 g cm -3 8.96 g cm -3
Chemical analysis Copper Phosphorus Iron Silicon
Weight (%) 99.80 <0.001 <0.006 <0.002
1111
The physical characteristics were confirmed using an automatic sieve shaker and the chemical analysis was performed using an X-ray fluorescence technique. 3.2.
Preparation of specimens
Electrolytic copper powder was compacted in a closed circular die (bore 045 mm) using a 150 tonf (1471.05 kN) hydraulic press at a recorded pressure. The die was lubricated with zinc stearate. Sintering was carried out at 900°C for 2 h in an endothermic sand atmosphere. In order to minimize non-uniformity of the density distribution, the compacts were wrapped in teflon sheets (acting as a lubricant) and were repressed at the same compaction pressure in the same die and then resintered at the same temperature for the same time. All sintering operations were carried out in a muffle-type silicon-carbide furnace capable of providing sintering temperatures up to 1300°C to an accuracy of +_5°C. Specimens for testing were made by machining the compacts; the specimens being 30 mm in diameter and l0 mm in height. The surface of the specimens was then polished with a fine grade emery paper. The density of the copper powder strips was obtained simply by measuring specimen dimensions and weight, but in order to confirm this density, the hydrostatic (or Archimedes) method was also employed. The relative density of the specimen was obtained by the ratio of the porous copper powder disc density to that of solid copper density (8.96 g cm--3). 3.3.
Experimentalprocedure and measurements
Experiments were conducted on a pneumatic forging hammer using appropriate dies. The copper powder disc of known relative density (0.75) was placed between fiat dies and it was forged at room temperature by applying the impact load. The following important measurements were made: (1) (2) (3) (4)
increase in relative density with decrease in height; increase in diameter with decrease in height; axial and radial strain values; and amount of barrelling developed after forging to various reductions for dry and lubricated conditions (a thin layer of vaseline was applied as a lubricant). 4. RESULTSAND DISCUSSION
During the high-speed sinter-forging process the compressive forces will gradually close the pores and thus the relative density of the sintered material will increase with an increase in the percentage reduction in height. Figure l shows this variation in graphical form. The relative density changes gradually with an increase in percentage height reduction. Lubricated specimens exhibit relatively better densification than unlubricated ones. At point C (47% reduction) cracks start appearing on the equatorial free surface of the copper powder preform disc. Unlubricated specimens show much more severe cracks as compared to lubricated specimens. In an investigation of the plastic deformation of sintered preform during forging it is evident that the cylindrical free surface barrels when friction exists at the interface and cracks gradually appear at the barrelled surface. The amount of barrelling mainly depends on the initial density of the disc and the degree of frictional constraint. Figure 2(a) and
1112
A . K . Jha and S. Kumar Electrolytic copper power disc compacted at 15 kglmm 2 sintered at 9000C
(Preform cracks)
c/
I.O .t3..------
,~ 0.8 o.~
Dry forging condition Lubricated forging condition (vaseline)
-~ 0.4 0.2 0.13 0
I 10
I I 20 30 Reduction (%)
I 40
I 50
Fig. 1. Relative density variation with percentage reduction in height for high-speed forging. 10% Reduction 30% Reduction
0 ,
o
/
51 Disc size I~ 30ram x 10mm Initial relative density 0.75
1 20
Ic 15
I
10
J
5
l
0
I
5
J
ID 15
10
I 20
10% Reduction 30% Reduction
10i
A
,/I 1/11
1 20
IC 15
[
,,/B =0
51 Disc size ~ 30ram x IOmm Initial relative density 0.75
I 10
I 5
I 0
i 5
I 10
[D 15
1 20
Fig. 2. Development of barrelling during high speed forging of an electrolytic copper powder disc in: (a) dry forging conditions; (b) lubricated forging conditions (vaseline).
(b) show the barrelling graphically for various reductions during dry and lubricated highspeed sinter-forging of copper powder disc, respectively. The bulge profiles were traced on a contour projector with a ×10 magnification. Thus, during the sinter-forging process it is clear that with the coefficient of friction ~t, between the disc and die being zero the disc expands homogeneously and thus the change in shape is disregarded, as shown in Fig. 2(a) and (b). On the other hand, when friction exists between the tool and the die surface, the shape of the disc changes to that of barrelling. The value of expansion depends upon the initial relative density, the height reduction and the coefficient of friction. From the curves it is evident that the displacement at the interface with the moving die (AB) is greater than the interface at the stationary die (CD) during high-speed dry forging, while the reverse is true during forging using vaseline as a lubricant. This effect can be attributed to a high relative velocity between the workpiece material and the die surface, high interfacial pressure and/or severe deformation modes, which causes a breakdown of the lubricant film and allows new surfaces to come into contact with the die, hence facilitating the intimate contact essential for adhesion. The basic affinity of the die and the workpiece material then determines the extent of friction. Figure 3 shows the percentage increase in diameter (with reference to moving and stationary die contact surfaces) plotted against the percentage decrease in height for dry and lubricated high-speed forging of a sintered copper powder disc with an initial relative density of 0.75. Figure 4 shows the variation
Dynamic Effects During a High-speed Sinter-forging Process
1113
of principal strains (e.
ez) with percentage reduction at the equatorial free surface, for high-speed deformation with flat dies under dry and lubricated conditions.
(a)
Top (moving) die contact surface
16128
/
/ 4 [
I
(b)
I
20 30 Decrease in height
IO
0
I
I
40
50
Bottom (stationary) die contact surface
24
20
/
12
8
--
4
/
/ / /~
r
/ / /~ I
0
I0
olytic copper power disc ¢ 30mm x 10mm. Initial relative density 0.75 I
I
20 30 Decrease in height
I
I
40
50
Fig. 3. Percentage increase in diameter vs percentage decrease in height during high-speed forging (with reference to moving and stationary die contact surfaces).
0.22
Copper power disc ( ~ 30ram x 10mm) Compacted at 15 kg/mm 2
~
S i n t e r e d at 9 0 0 ° C 0.16
.~_ E¢
0.12 m
o.o8 0.04
0.00
5
10
.... •- o -
Dry forging condition Lubricated forging condition (vaseline)
15
20
25
30
35
40
._q..o.10
-o.20
f-z
-0.30
Fig. 4. Variation of principal strains (E,., e:) with percentage reduction in height.
1114
A.K. Jha and S. Kumar
4.1. Interfacial friction stress In high-speed sinter-forging, the surface of the workpiece is distorted and takes on an impression of the die surface. Therefore the actual contact area, as far as the specific cohesion of the surface is concerned, is not negligible. Friction conditions in forging is essentially different from sliding friction in machine parts. The relative velocity between the workpiece material and the forging die surface together with high interfacial pressure and/or deformation modes will create the conditions essential for adhesion in addition to sliding [11]. During the high-speed sinter-forging process the compressive force supported by impact gradually increases the relative density and hence the real area of contact grows and approaches the apparent one as the force increases. This approach will probably be an asymptotical one [ 12]. The pattern of metal flow during the forging of a sintered preform is such that there exists two zones, an inner one where no relative movement between work-piece and die occurs (the sticking zone), and an outer zone where sliding occurs. Therefore the appropriate interfacial friction law for different deforming conditions are as follows: axial symmetry condition
(1) plane strain condition
[
x=l.tp+po%
{ (Xm 1-
~ -
(2)
where rm and Xm denote the sticking zone radius for axial symmetry and plane strain conditions, respectively, which may be approximated by the relation given by Rooks [13], and n >> 1. Though the contribution of adhesion in Equation (1) and Equation (2) is less pronounced, it constitutes a major drawback in production, because it affects the die life, deformation load and the surface finish of the forged product appreciably. 4.2. Free body equilibrium approach In addition to the usual assumptions during high-speed wrought material forging [14], the following assumptions are made in the analysis: (1) friction due to adhesion is a function of the relative density; (2) yielding of sintered material is sensitive to the hydrostatic stresses imposed during forging. For analysis of pressure distribution and die load, axial symmetry and plane strain highspeed forging conditions are considered separately here. The following equations may be obtained for the high-speed forging of a sintered preform by two fiat rigid dies moving with velocity U towards each other (Fig. 5). 4.2.1. Axial symmetry. Compressibility [7]: 3U~ (1 - 2rl)bU~ + - 0 c)r 2(1 + q)Oz
(3)
Equilibrium of forces in disc element: i)tsr
ar
(so
PP U~
U.
+
-
=0
(4)
Dynamic Effects During a High-speed Sinter-forging Process -U
I
'~
I
+Ul
I 115
i_ I
b
_1 -I
I
Fig. 5. Schematic diagram of an axially symmetric sintered disc during the high-speed sinter-forging process.
Yielding of sintered preform disc: o~
=
-
p
(5)
Or :
(~0 = ~" - - P
Or
br
(6)
or
(7)
Because of symmetry, the velocity field with bulge satisfying the compressibility Equation (3) is (1 - 2 q ) ~ -I~Jh Ur Ur = 2(1 + rl)h(1 - e -1~2~
(1 - e-I~Jh)U (1 - e -~m
U:-
uo = o
(8)
(9)
(1o)
Now substituting Equations (5)-(10) in Equation (4), the die pressure equation is obtained as follows: dp pp(1 - 21"1)~2re-21~Jh [ h(l - e -m2) I~-Jh; ] 2X drr+2(l+~-)h2-~:~) 2 /U2k+ ~ e uj+~-=O where
O-
du
dt
and k = c ash
(1 + 411) 2(1 +1])
(II)
1116
A.K. Jha and S. Kumar
4.2.1.1.
Pressure distribution:
d~+ 2(1 + ~
from Equation (1) and Equation (11) it follows that
-~ ~ ) 2
+ ~ - P+Po(~o
/ilk +
1 -(~-j]=0
(12)
It is a linear differential equation of the first order and its solution is
Arh
PoOo P = -~-
Ah 2
Polo
rm -- 10°~° -- ~ - ~ + 4~t 2
nb
C_ 211r/h
Po~oh r + 2-~
(13)
+
where C is a constant of integration and pp(1 - 211)l]Ze-21~Jh [ h(l-__e-f~/2)ef~jhf_] ] A = 2(1 + 11)-gh~ - - - ~ 2 ) 2 Uzk + By using the boundary condition at the rim, i.e. (~r 0 and p = ~, at r = b the constant of integration C is determined and therefore Equation (1 3) becomes =
p poC?o[r~bb h ] ~,- ~, + 2~tllb- 1 Ah I ~ _ ,
+2~ 4.2.1.2.
+oe
Die load:
P = 21t
(
21a(b-r )
h
Polo r zu
h 2o,h ~)
~e
~
- r
1 n
1)]
] (14)
the die load is given by
(15)
.r.dr
Substituting the value of p from Equation (14) into Equation (15) and subsequent integration and on simplification gives rm
P = rcbZpo~o ~-b + -2~tnb _ 1
4.2.2.
n
~-+A
+2~tz ~
_[(I
---
_ bz
Plane strain.
dex=
+ ---2~t2e 2°h/h 1 -
1 1)] I ~.[1-(P~o)(rm
rm h ~ + 2gnb b+
3n
-
rch3 + -4 ~ 3 b
+--4~t3
h
~ + 21anb - 1)
--
1 n
] 1)
(16)
2~t //]
Compressibility [8]:
+ 2112) - 211~/3(1 - 112)]de: ?i--4q2)
Equilibrium of forces in strip element:
(17)
Dynamic Effects During a High-speed Sinter-forging Process
p~[ 0G u~G ~Ux]
ga~
g
(18)
:--~-z+--~j-~=o
G-G-x +
II 17
Yielding of sintered preform strip: a: = - p
(19)
ax = ~ - p
(20)
~(~x
~P
bx
~x
(21)
or
The velocity field with barrelling satisfying the above compressibility equation [Equation (17)] is txl3U~e-I ~Jh ux - h(l - e -13/2)
(22)
U,, = 0
(23)
and
U~-
(1 - e-f~Ja)u (1 - e -13/2)
(24)
where (1 + 21] 2) -(Z=
21]X/3(1 - 112)
(1 - 41] 2)
Substituting Equations (21)-(24) into Equation (18), we now obtain
(X2~2ppe-2f~JhX[ heaJh(l -e-l~'2)U] dPdx+ hT(i 7 - - ~ [ U2K + j
2X
(25)
(e 13z/h -- 1) dU :and [J = cz dt Pressure distribution: from Equation (25) and Equation (2) it follows that
where K = 1 + 4.2.2.1. dp+
tX2~2ppe-Z~Jhx [ U2K + he'an(1 - e-lV2)L/]
l
(26)
J
It is a linear differential equation of the first order and its solution is Po¢o
p=---~-x,. - Po¢o
_ Po¢oX Po¢oh Mh 2 nl + 2 - ~ - + --4~t2
where C is a constant of integration and
Mxh --+2~t
Ce_Z~/h
(27)
1118
A.K. Jha and S. Kumar O~2l]2ppe-213:.mx ]M= ~i = ~[U2K
+
het3Jh(l -- e-13/2)ij] o¢[3 J
By using the boundary condition at the edge i.e. Ox = 0 and p = ~. at x = 1 the constant of integration C is determined and therefore Equation (27) becomes
)~-
)~ [nl + 2~n~l- 1 1
1
n 4.2.2.2.
+
Die load:
2~
)~ nl + e ~ +le
h
-
! --2g
,,
nl + 2gn~l
-x]
(28)
the die load is given by
I
P=2w
(29) 0
Substituting the value of p from Equation (28) and integrating, we obtain P=--
+
1 -1 -2gnl n n {Xm h t wh~- 2~'[ Po¢o +2wp°O°l \nl + 2gn-l- 1 ] [ --eS, + I
~t
Xm h nl + 2gnl
I n
1
+
( 1 - e22/) + 2g
e~-,
1)1
30,
In an investigation of the plastic deformation of sintered materials it is evident that the density distribution does not seem to be uniform throughout. It is high in the central region and low at the edges. The density distribution will be more uniform for a smaller coefficient of friction [a and a higher initial relative density P of the sintered material [15]. Figure 6 shows the theoretical pressure distribution over the surface of sintered copper powder disc during high-speed forging. The pressure distribution curve is made up of two symmetrical branches for the entire surface of the disc. The pressure distribution at the die-workpiece interface decreases from the centre towards the edge. The decrease in adhesion friction (or relative density) results in a further decrease in pressure. For high pressure values it is noticed that the curve does not meet the y-axis and gradually becomes parallel to the y-axis. This signifies the existence of a sticking zone on the surface of the disc. Because of the similar characteristics of Equation (14) and Equation (28), the pressure distribution curve for sintered strip is also expected to be of the same form. The relationship between an external pressure p and a relative density P is given by the following expression [16]: p 2~-
2 3 loge(1 - p)
(31)
Solving Equation (31) results in p = i
-
e -3p/2x
(32)
To study the density variation (from centre to edge) during high-speed forging of sintered preforms the various values of p/~. in Equation (32) may be considered using Equation (14).
Dynamic Effects During a High-speed Sinter-forging Process
I 119
Electrolytic copper powder Disc ~t3Omm x IOmm p = 0.75 tt ffi 0.30 U=
p: n=
.....
Hil Sic
Zp
polo =O.Ip
I ~ 0 = 0.',
~ao=0.Jp
q 2b Fig. 6. Theoretical pressure distribution over the surface of a disc during sinter-forging.
Figure 7 shows the theoretical variation of relative density from centre to edge for an electrolytic copper powder disc preform with an initial relative density of 0.75, forged at high speed to a 10% height reduction. The average theoretical value of the relative density (from centre to edge) is found to be 0.795. The experimentally measured relative density of the sintered copper powder disc after being forged to a 10% height reduction agrees satisfactorily with the above theoretical value (Fig. 1). Figure 8(a) and (b) shows the theoretical forging load versus percentage reduction curves for high-speed forging of sintered copper powder preforms under axial symmetry and plane strain sinter-forging conditions, respectively. The curves express the results for a Disc size = I~ 30ram x 10ram Initial relative density = 0.75 = 0.2p It = 0.30 n=3 U=
1.0 I00rn/sec .=,'0~)," 8 ~ ~ ~ - u ~ "~ 0.6 -
Av. Theoretical p ffi 0.795
u u
.~
0.4-
~
0.20.0
I I 5 10 Distance from center to edge
I 15
Fig. 7. Variation of relative density from centre to edge for an electrolytic copper powder disc forged to a 10% reduction.
1120
A.K. Jha and S. Kumar
8O 70 6O _
Electrolytic copper p o w d e r disc Size = 13 30ram x IOmm Initial relative density = 0.75 / POlO = 0.2p / It = 0.30 / n=3
~ 50
2O I0 1 I0
0
35 30 25
I
I
I 40
20 30 Height reduction (%)
Electrolytic copper p o w d e r strip Size -- O 30ram x 15mm x lOmm Initial relative d e n s i t y = 0.75 Po13o-- 0.2p = 0.30 n=3
/ /
~20 o
0
~ IO
I I0
I 20
I 30
I 40
I
50
Height reduction (%) Fig. 8. (a) Theoretical forging load vs percentage reduction curves for high-speed forging of a sintered copper powder disc under axial symmetry conditions; (b) theoretical forging load vs percentage reduction curves for high-speed forging of a sintered copper powder strip under plane strain conditions.
particular value of the coefficient of friction ix and the initial relative density of p of the sintered preform for different values of the deformation speed U. The forging load was found to increase with an increase in deformation speed. Now, to illustrate the order of magnitude of dynamic effects, let us calculate the die velocity required to increase the maximum die pressure twice, as compared with a slow static forging. Let us consider the pressure at the centre of the die for the typical case: 13 = 0.483, b = 1.1 cm, h = 1.0cm, ix = 0.30, Poq~o = 0.4p, n = 3, ~, = 812 kg cm -2, pp = 0.007616 kg cm -3 and g = 981 cm s -2 From Equation (14), the pressure is P = 3.4715 + 1.7046D
where D -
(33)
A
The value
ofp/~,
in Equation (33) is doubled as a result of the inertia forces only, when
Dynamic Effects During a High-speed Sinter-forging Process
1121
D = 2.02. For simplicity, considering the effect of velocity only and disregarding the die acceleration terms, we have pp(1 - 2 " q ) [ ~ 2 O 2 k e -21~'Jh 2.02 2(1 + rl)gh2(l - e-1~2)2~,At the die face, y = h/2, therefore U=I50 m s -). It is noted from Fig. 9 that an increase in 13 increases the value of ~ for a constant value of U. Also, for a particular value of 13, an increase in U results in an increase in as here. Therefore it may be concluded that such dynamic effects are important for extremely high-speed forging processes. As an illustration, Fig. 10(a) shows photomicrograph of pores in an electrolytic copper powder disc sintered at 900°C and high-speed forged to 30% reduction without lubricant, whereas Fig. 10(b) shows photomicrograph of pores in an electrolytic copper powder disc sintered at 900°C and high-speed forged to 30% reduction with vaseline acting as a lubricant. 5. CONCLUSIONS
The feasibility of high-speed processing of sintered materials has been demonstrated. During high-speed forging of sintered materials the main controlling factors are the deformation speed, the amount of interfacial friction between the die--workpiece interface, the initial density of the preform and the pressure distribution from the centre to the edge. During the sinter-forging process, the pressure varies from the centre to the edge which in turn affects the density distribution from centre to edge. Density distribution will be more uniform for a higher initial relative density of the preform. Lubricated specimens show relatively improved densification than unlubricated ones. The density of the sinter-forged product increases with an increase in the forging load. The amount of barrelling depends mainly on the degree of densification and friction conditions. Displacement at the interfaces with the moving die is more than that with the interface at the stationary die during high-speed dry sinter-forging, while the reverse is true during forging with the use of a lubricant. This is mainly due to the breakdown of the lubricant film due to severe deforming conditions. It is also concluded that the dynamic effect on the die load is not negligible during high-speed sinter-forging and must be considered while estimating the deformation load. The inertia forces encountered are functions of processing parameters and deformation characteristics of the sintered material. Evidently, such dynamic effects within the forged material itself are important for very fast forging operations. The work was found to be effective for the assessment of the pressure distribution and die load during high-speed forging of sintered materials.
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A.K. Jha and S. Kumar
Fig. 10. (a) Photomicrograph of pores in an electrolytic copper powder disc sintered at 900°C and high-speed forged to a 30% reduction without a lubricant; (b) photomicrograph of pores in an electrolytic copper powder disc sintered at 900°C and high-speed forged to a 30% reduction with lubricant (vaseline). REFERENCES [1] G. W. Cull, Mechanical and metallurgical properties of powder forgings, Powder Metall. 13, 156 (1970). [2] P. K. Jones, The technical and economic advantage of powder forged products, Powder Metall. 13, 114 (1970). [3] R. Davies and J. B. Marx, The production of components by forging of powder preforms, Proc. 13th Int. MTDR Conf., Birmingham, p. 463. Macmillan, London (1972). [4] M. Negm and R. Davies, The hot extrusion of metal powder preforms, Proc. 15th Int. MTDR Conf., Birmingham, p. 637. Macmillan, London (1974). [5] A. Singh and R. Davies, Preliminary investigations of the cold extrusion of powder preforms, Proc. 13th Int. MTDR Conf., Birmingham, p. 449. Macmillan, London (1972). [6] S. Del Wakil, A Plane strain analogue for the extrusion of sintered billets, Proc. 20th Int. MTDR Conj., Birmingham, p. 229. Macmillan, London (1979). [7] A. K. Jha and S. Kumar, Deformation characteristics and fracture mechanisms during the cold forging of metal powder preforms, Int. J. Mach. Tool Des. Res. 26, 369 (1986). [8] A. K. Jha and S. Kumar, Deformation characteristics and fracturing of sintered copper powder strips during cold forging, J. Mech. Work. Technol. 16, 145 (1988). [9] B.-B. Hwang and S. Kobayashi, Application of the finite element method to powdered metal compaction processes, Int. J. Mach. Tools Manufact. 31, 123 (1991). [10] G. Sutradhar, A. K. Jha and S. Kumar, Production of sinter-forged components, J. Mater. Process. Technol. 41, 143 (1994). [11] B. V. Deryagin, What is Friction? Izd. Akad. Nauk, Moscow (1952). [12] T. Wanhein, Friction at high normal pressures, Proc. 1st. WCIT, Paper no. F-7, New Delhi (1972). [13] B. W. Rooks, The effect of die temperature on metal flow and die wear during high speed hot forging, 15th Int. MTDR Conf., Birmingham, p. 487 (1974). [14] S. Kumar, Principles o f Metal Working, Oxford IBH, New Delhi (1976). [15] T. Tabata, S. Masaki and K. Hosokawa, A compression test to determine the coefficient of friction in forging P/M preforms, Int. J. Powder Metall. Powder TechnoL 16, 149 (1980). [16] S. Shima and J. M. Alexander, The interrelation of density and hardness in the isostatic compaction of powders, Proc. 13th Int. MTDR Conf., Birmingham, p. 471. Macmillan, London (1972).