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Some metal flow phenomena arising in axisymmetric flashless forging
0020-7357/00/0001-0000 $02.00"0
Int. J. Mach. Tool Des. Res. Vol. 20. pp. 4 5 53. Pergamon Press Ltd. 1980. Printed in Great Britain.
SOME METAL FLOW PHENOMENA ARISING IN AXISYMMETRIC FLASHLESS FORGING Y. VAN HOENACKER* and T. A. DEAN* (Received 1 November 1979)
Abstract This paper describes aspects of metal flow in flashless forging dies which are either undesirable in manufacture or lead to defectivecomponents.Three differentformsof flowwereidentified ; asymmetricfillingof top and bottom cavitieswhen shapes with peripheral flangesare made and friction is high,splitting of the body of similar shapes of this type, when friction is low and fold formation in shapes with a central boss, under certain geometrical conditions. Velocityfields have been proposed whichdescribe these happenings and enable estimates to be made of the range of conditions under which they will occur.
INTRODUCTION THE ELIMINATIONof flash from drop forgings has been the subject of several recent papers by these authors [1-3]. It is now well established that useful material savings can be made and in addition lower maximum loads and reduced energies may be required. Of necessity, a die cavity for flashless forging comprises basically a container and a punch, in contrast to the two opposed, open faced cavities normally used. This construction can cause differences in material flow which have a significant effect on both the process and the product quality. This paper describes and analyses peculiarities of flow that have been observed in the formation of axisymmetric shapes which typify two ranges of drop forged components. EXPERIMENTAL DETAILS The shape of the two types of forging made and an outline of the die cavity constructions used are shown in Fig. 1. The forgings were all axicircular, and die inserts were used to vary the diameter of the central boss of the type 1 forging and the thickness of the peripheral flange
TYPE 1 FORGING
TYPE
2 FORGING
FIG. 1. Forged shapes and die cavities.
* Department of Mechanical Engineering, University of Birmingham, England. 45
46
Y. VAN HOENACKERand T. A. DEAN T A B L E 1. G E O M E T R I C A L D A T A - TYPE 2 F O R G I N G S
Test number
Web diameter (2 rpm m)
Web thickness (2T mm)
Billet height (Ho mm)
19 20 21
25.40 30.48 35.56
7.62 7.62 7.62
23.75 20.02 15.65
of the type 2 forging. Both types of cavity were symmetrical about their midheight. The maximum diameter of the forgings remained unchanged at 45.7 mm. Two stock materials were used; H30 aluminium alloy, forged at room temperature and M40 080 medium carbon steel, preheated to 1200°C. Room temperature forgings were made, either with clean dry dies, or with dies and billets liberally coated with lanolin as a lubricant. Clean dry dies only were used for the hot forgings. Cylindrical billets were used in all cases. For the type 1 forgings, billets of various heights and diameters were used, but for the type 2 shapes, only the billet height was varied, the diameter being kept the same as that of the cavity, which was 45.7 mm for both forging types. Geometrical details of the three unlubricated, type 2 forging tests referred to later, are given in Table 1. ASYMMETRICAL FLOW Zalesski and Tyurin 1-4] have previously established for shapes similar to those described here that due to friction at the walls, differences in flow into top and bottom die cavities occur. Symmetry can be established by using opposing punches or a "floating" container, but simplicity and ruggedness of design make the arrangement shown in Fig. 1 the most likely to be encountered. With the thin webs formed in the type 1 forgings made in these tests (T/c = 0.22) and the use of billets with diameters less than that of the cavity resulting in contact with the die walls late in the process, the surface areas over which wall friction was effective were relatively small. As a consequence, no asymmetry was recorded, for any test conditions. Metal flow into top and bottom dies was noticeably asymmetric when type 2 forgings were made, using poorly lubricated dies.
Velocity field An Upper Bound solution, using the velocity field shown in Fig. 2, was used to determine "7
E
A
C
F
B
[3
G
t
\
rp f'C
v
FIG. 2. Velocity field for Type 2 forgings.
Some Metal Flow Phenomena Arising in Axisymmetric Flashless Forging
Anotysis - full lines.
47
/
Friction foctor rn=l.
°
c5 E
~.Oy D
o o
~5 o.
o
o
~
@ @
I Or"
@
~1~0
I~'
•
/** : •
I
2 T
Current / Final Web Thickness. ~w Experimental Results Aluminium
O @ •
Hot Steer
[] ~ •
test no.19 test no.20 test no.21 test no.19 test no.20
test no.21
FIG. 3. Asymmetry of flow in Type 2 forgings.
theoretical overall flow patterns in type 2 forgings. In region ABFE and ACDB parallel velocity fields are assumed to exist. In the regions EAIH and FBJG, conical streamlines directed towards the intersection of the tapering walls of the die were used. Optimisation for minimum power dissipation was obtained using the vertical velocity of a particle on BF, as a parameter. A constant friction factor has been assumed over all the forging. Figure 3 shows a comparison of experimental and theoretical results. It is seen that the theoretical analysis is able to predict well the decreasing trend of the ratio of rise top die/bottom die, with deformation. However, the value of this ratio is always over-estimated, particularly for the test in which a small diameter web was formed. It is to be noticed that in all three cases, the height of rise into the top die is greater than that into the bottom, but that the difference decreases with increasing punch diameter. No definable difference between experimental results for hot and cold forgings can be seen, indicating that neither material properties nor temperature significantly affect flow. As a result of the asymmetric flow, very little displacement of the material entering the top cavity occurs. This is particularly evident in the case of the small diameter punch, when the process is akin to a piercing operation. The results of the analysis for a wide range of geometries displayed in Fig. 4, show that the ratio rise in top die/rise in bottom die is increased with the increase of the final web thickness of the forging but that the starting height of the billet has no effect on the initial value of the ratio. Increasing flange thickness (reducing punch radius rp) is again seen to increase the flange height in the top die compared with that in the bottom.
48
Y. VAN HOENACKERand T. A. DEAN I
3.0
~
rp=12.7
j
E o ¢n
c5
2.0
I
Analysis- friction factor m= 1
1.0
I
1.0
2.0
3.0
Current / Final Web Thickness.
No.
rpmm.
3
12.7
21.2 23.7 26.3 28.8
5.1 7.6 10.2 12.7
15.2
8
17.5 20.0 22.6 25.1
5.1 7.6 10.2 12.7
9 10 11 12
17.8
13.1 15.6 18.2 20.7
5.1 7.6 10.2 12.7
/. 5 7
Ho ram.
4.0
T 2"-w
2w ram.
FIG. 4. Theoretical effects of geometry on flow--Type 2 forgings.
Web radius rp = 15~2mm. o
Friction factor m= 1.0
oE -
d=~ °
2o
{3.
"
1"9 "~:~
1.5
u_
1.0 0
I
I
1
I
0.2
0.4
0.6
0.8
Final Web Thickness / Outer Flange Radius. 2w/r c
FIG. 5. Effect of draft angle on asymmetry of flow--Type 2 forgings.
Some Metal Flow Phenomena Arising in Axisymmetric Flashless Forging
49
2.0 £Ci
E o
o E3
1.5
t
{2:
I
1.0
I
m=O
I
2 Current / Finat Web
T 2w
Thickness.
FIG. 6. Effect of friction of asymmetry of flow--Type 2 forgings.
The theoretical curves of Fig. 5 shows that the value of draft angle, within the range likely to be encountered in forging practice, has little influence on asymmetry. Friction however, significantly increases asymmetry, as seen in Fig. 6. If friction is eliminated, equal flow into the top and bottom die cavity is achieved. PERIPHERAL
DEFECTS
Type 1 foroinos
Centralfolds. One way to reduce the energy required for flashless forging is to delay contact between stock and the cavity wall. This can be achieved by increasing the height to diameter ratio of the billet• However, using billets with a large aspect under lubricated forging conditions may result in a bollard shape during the early stages of deformation, due to inhomogeneity of metal flow. Investigation of the incidence and extent of bollarding was carried out using cylindrical billets of H30 aluminium alloy. Five different billet diameters, ranging between 12.7 and 31.7 mm and four different central boss diameters of 8.1, 11.4, 13.7 and 18.3 mm were used. Lanolin was applied generously on both die surfaces and end surfaces of the billets. Deformations corresponding to billet height reductions of 14% and 50% were imposed and the extent of bollarding was measured by the index E, described in Fig. 7. (a)
Undef0rmed
Billet.
(b) BoIlarding Index E .
J~
Do
4-
oI /
2.rma x
iI
I
2 .rmi n E % = rma x - r m i n
,, 100
rmin
FIG. 7. Bollarding of billets--Type 1 forgings. Mrl)R 20.1
D
_1
f
50
Y. VAN HOENACKERand T. A. DEAN 0.9
i
7% ,c
0.8
E=0% 2 . 5 % ~
(~
E -o Q
5 c5
o
IN
0.7
J
0.6
J
0.5
123
0
ca
0.1.
o
0.3
/
\
J
0.2 0
0.2
0.1.
0.6
0.8
1.0
Billet Height / Di0meter.
1.2
1.l.
1.6
Ho Do
FIG. 8. Extent of bollarding at 14% reduction of billet height.
Experimental values of the bollarding index at a height reduction of 14% are shown in Fig. 8. It is seen that E is a function of the initial aspect ratio and also of the ratio of the diameter of the central boss to the initial billet diameter. On the left hand side of the line corresponding to zero E, no bollarding occurred and slight tendencies to barrelling were detected. Generally the bollard type distortion increases with increasing geometric parameters. The tendency to a reversal of this trend, for larger values of the billet aspect ratio, can be attributed to the limitations of the index chosen. It is probable that the angle 0, in Fig. 8, would be a more representative index, but this is not easily measured with accuracy. When the initial billet height was reduced by 50%, it was found that the distortion of billets with a previous E value of less than 3%, had regressed and in some cases had disappeared, but that billets formerly having an E value larger than 5% developed a fold. The geometric limits leading to these conditions are shown in Fig. 9. Here it is seen that the graphical region indicating no defect has enlarged, compared with that associated with the smaller height reduction, particularly in the area corresponding to slender billets and small boss diameters. This indicates that the ratio of boss diameter/billet diameter predominates in the formation of a surface defect in the finished forging. Figure 10 is a photograph of a partially deformed billet and a section of a completed forging, showing the peripheral defect.
Type 2 forgings As discussed earlier, low friction allows symmetrical shapesto be made, but in some cases, during the forging of well lubricated aluminium alloy billets, defects appeared at the periphery of the body of the forging. Depending on the degree of deformation, this defect was found to take one of two different forms. The first consisted of superficial cracks. These generally originated from machining marks remaining on the original billet surface, but even when the surface roughness was reduced to 800/~m, the cracks still formed, as shown in Fig. 11. In these cases the cracks did not correspond to the original machining marks and were seen to develop as deformation progressed. Johnson [5] has observed the formation of defects on smooth surfaces in components of shapes similar to those described here. They were referred to as a "sucking in"
51
Some Metal Flow Phenomena Arising in Axisymmetric Flashless Forging 0.g
08 DEFECT '
'-:
0.7
i5 %
O.6
~5 ~
().5
E ~5
NO DEFECT '
0./.
~
~
~ -
m
~
G.3
0.2 0
0.2
0./.
0.6
0.8
Billet Height / Diameter.
1.0
1.2
1./.
1.6
14o -Do
FIG. 9. Peripheral defects at 50% reduction of billet height.
of metal and a plane strain analogue, consisting of a velocity field in which the surface of the deforming region moved inwards from the wall of the cavity, was proposed. This situation was not recorded in these experiments. It is thought that the existence of a draft angle and the early opening of cracks suppressed such tendencies.
Analysis of crackf o r m a t i o n The axisymmetric velocity field shown in Fig. 12 was constructed as an analogue describing the onset of cracking. It consists of two conical surfaces of velocity discontinuity AK and BK and two rigid blocks AEK and BFK. The point K, normally at the periphery of the forging is allowed to move inwards to K'. Neglecting the power required to form new crack surfaces, the total applied power can be expressed as a function of three parameters; velocity of movement into the flanges, radius to the crack tip r k and height of the crack
FIG. 10. Sections showing peripheral defect
Type 1 forgings.
Y. VAN HOENACKERand T. A. DEAN
52
FIG. 11. Peripheral cracking of Type 2 forging.
opening Zk. The onset of cracking is defined when the minimisation of the applied powei yields a value of rk less than re. The results for various forging conditions are shown in Fig. 13. It is seen that the critical web thickness, at which a crack is likely to form, is a function of both container and flange dimensions and decreases both with increasing friction and increasing flange height. A comparison of the frictionless axisymmetric results, with that from Johnson's plane strain analogue, indicates that the crack is likely to occur much earlier in the process than the "sucking in" type of defect, with the die shape utilised in these tests. Examination of
/ i 2" / / / / / / / / / u
\\
\
E
,t
,,(
\
\
) \
rp rk
[-..-
rc
FIG. 12. Velocity field incorporating crack.
Some Metal Flow Phenomena Arising in AxisymmetricFlashless Forging
53
¢3.
It.
Johnson's ,plane_strain__analo_.que for'sucking in' defect
c( = 00
-6 u ~E c~
1.0
I
I
1.5
2.0
Flange Outer Radius / Web Radius.
rc rp
FIG. 13. Theoretical prediction of crack formation--Type 2 forgings. forged shapes enabled cracks to be seen when the ratio of web to flange thicknesses was about two. N o attempt to establish the exact stage, at which cracking started was made, because of the difficulty of establishing the exact m o m e n t at which a feature is no longer an aspect of surface roughness and constitutes a crack.
CONCLUSIONS F r o m the investigations described in this work, several features of metal flow of concern in the bulk forming of flashless components have been discovered. For forgings with peripheral flanges asymmetrical flow into top and b o t t o m dies can occur. The degree of asymmetry is increased by friction, but is also dependent on the forged shape. Thick webs and thick flanges both increase flow into the top die compared with that into the b o t t o m one. An axisymmetric velocity field has been proposed, which enables predictions of flow to be made for various process conditions. Surface defects can arise in both central boss and flange forgings and those with webs and peripheral flanges. In the former, the defect is due to the folding in of the side of the billet, when it forms a bollard shape early in the deformation. Low friction, slender billets and a large central boss diameter all contribute to the formation of this defect. In forgings with a peripheral flange, defects can occur due to the opposing directions of metal flow at midheight. For conditions of very low friction, cracks can be formed. An axisymmetric velocity field, proposed as a model for this situation, allows predictions of the critical web thickness at which cracking will occur for various forging situations. It is shown that critical thickness depends on both container and flange dimensions.
Acknowledgement The work reported here is part of the outcome of a programme of research sponsored by S.R.C. Y. Van Hoenacker worked as a National Research Council of Canada Scholar.
REFERENCES [1] T. A. DEAN,Metallurgia and Metal Forming 44, 488-498, 542-544 (1977). [2] Y. VAN HOENACKERand T. A. DEAN,Int. J. Mach. Tool Des. Res. 18, 81-93 (1978). [3] T. A. DEAN,I. Mech. E. Proc. Instn mech. Engrs Vol. 193. [4] V. I. ZALrcSSKIand N. I. TYURIN.Kuznechno-shtamp. Proizo. 1, 4-8 (1959). [5] W. JOHNSON,Appl. scient. Res. A. 8, 52-60 (1959)•
Int. J. Mach. Tool Des. Res. Vol. 20. pp. 4 5 53. Pergamon Press Ltd. 1980. Printed in Great Britain.
SOME METAL FLOW PHENOMENA ARISING IN AXISYMMETRIC FLASHLESS FORGING Y. VAN HOENACKER* and T. A. DEAN* (Received 1 November 1979)
Abstract This paper describes aspects of metal flow in flashless forging dies which are either undesirable in manufacture or lead to defectivecomponents.Three differentformsof flowwereidentified ; asymmetricfillingof top and bottom cavitieswhen shapes with peripheral flangesare made and friction is high,splitting of the body of similar shapes of this type, when friction is low and fold formation in shapes with a central boss, under certain geometrical conditions. Velocityfields have been proposed whichdescribe these happenings and enable estimates to be made of the range of conditions under which they will occur.
INTRODUCTION THE ELIMINATIONof flash from drop forgings has been the subject of several recent papers by these authors [1-3]. It is now well established that useful material savings can be made and in addition lower maximum loads and reduced energies may be required. Of necessity, a die cavity for flashless forging comprises basically a container and a punch, in contrast to the two opposed, open faced cavities normally used. This construction can cause differences in material flow which have a significant effect on both the process and the product quality. This paper describes and analyses peculiarities of flow that have been observed in the formation of axisymmetric shapes which typify two ranges of drop forged components. EXPERIMENTAL DETAILS The shape of the two types of forging made and an outline of the die cavity constructions used are shown in Fig. 1. The forgings were all axicircular, and die inserts were used to vary the diameter of the central boss of the type 1 forging and the thickness of the peripheral flange
TYPE 1 FORGING
TYPE
2 FORGING
FIG. 1. Forged shapes and die cavities.
* Department of Mechanical Engineering, University of Birmingham, England. 45
46
Y. VAN HOENACKERand T. A. DEAN T A B L E 1. G E O M E T R I C A L D A T A - TYPE 2 F O R G I N G S
Test number
Web diameter (2 rpm m)
Web thickness (2T mm)
Billet height (Ho mm)
19 20 21
25.40 30.48 35.56
7.62 7.62 7.62
23.75 20.02 15.65
of the type 2 forging. Both types of cavity were symmetrical about their midheight. The maximum diameter of the forgings remained unchanged at 45.7 mm. Two stock materials were used; H30 aluminium alloy, forged at room temperature and M40 080 medium carbon steel, preheated to 1200°C. Room temperature forgings were made, either with clean dry dies, or with dies and billets liberally coated with lanolin as a lubricant. Clean dry dies only were used for the hot forgings. Cylindrical billets were used in all cases. For the type 1 forgings, billets of various heights and diameters were used, but for the type 2 shapes, only the billet height was varied, the diameter being kept the same as that of the cavity, which was 45.7 mm for both forging types. Geometrical details of the three unlubricated, type 2 forging tests referred to later, are given in Table 1. ASYMMETRICAL FLOW Zalesski and Tyurin 1-4] have previously established for shapes similar to those described here that due to friction at the walls, differences in flow into top and bottom die cavities occur. Symmetry can be established by using opposing punches or a "floating" container, but simplicity and ruggedness of design make the arrangement shown in Fig. 1 the most likely to be encountered. With the thin webs formed in the type 1 forgings made in these tests (T/c = 0.22) and the use of billets with diameters less than that of the cavity resulting in contact with the die walls late in the process, the surface areas over which wall friction was effective were relatively small. As a consequence, no asymmetry was recorded, for any test conditions. Metal flow into top and bottom dies was noticeably asymmetric when type 2 forgings were made, using poorly lubricated dies.
Velocity field An Upper Bound solution, using the velocity field shown in Fig. 2, was used to determine "7
E
A
C
F
B
[3
G
t
\
rp f'C
v
FIG. 2. Velocity field for Type 2 forgings.
Some Metal Flow Phenomena Arising in Axisymmetric Flashless Forging
Anotysis - full lines.
47
/
Friction foctor rn=l.
°
c5 E
~.Oy D
o o
~5 o.
o
o
~
@ @
I Or"
@
~1~0
I~'
•
/** : •
I
2 T
Current / Final Web Thickness. ~w Experimental Results Aluminium
O @ •
Hot Steer
[] ~ •
test no.19 test no.20 test no.21 test no.19 test no.20
test no.21
FIG. 3. Asymmetry of flow in Type 2 forgings.
theoretical overall flow patterns in type 2 forgings. In region ABFE and ACDB parallel velocity fields are assumed to exist. In the regions EAIH and FBJG, conical streamlines directed towards the intersection of the tapering walls of the die were used. Optimisation for minimum power dissipation was obtained using the vertical velocity of a particle on BF, as a parameter. A constant friction factor has been assumed over all the forging. Figure 3 shows a comparison of experimental and theoretical results. It is seen that the theoretical analysis is able to predict well the decreasing trend of the ratio of rise top die/bottom die, with deformation. However, the value of this ratio is always over-estimated, particularly for the test in which a small diameter web was formed. It is to be noticed that in all three cases, the height of rise into the top die is greater than that into the bottom, but that the difference decreases with increasing punch diameter. No definable difference between experimental results for hot and cold forgings can be seen, indicating that neither material properties nor temperature significantly affect flow. As a result of the asymmetric flow, very little displacement of the material entering the top cavity occurs. This is particularly evident in the case of the small diameter punch, when the process is akin to a piercing operation. The results of the analysis for a wide range of geometries displayed in Fig. 4, show that the ratio rise in top die/rise in bottom die is increased with the increase of the final web thickness of the forging but that the starting height of the billet has no effect on the initial value of the ratio. Increasing flange thickness (reducing punch radius rp) is again seen to increase the flange height in the top die compared with that in the bottom.
48
Y. VAN HOENACKERand T. A. DEAN I
3.0
~
rp=12.7
j
E o ¢n
c5
2.0
I
Analysis- friction factor m= 1
1.0
I
1.0
2.0
3.0
Current / Final Web Thickness.
No.
rpmm.
3
12.7
21.2 23.7 26.3 28.8
5.1 7.6 10.2 12.7
15.2
8
17.5 20.0 22.6 25.1
5.1 7.6 10.2 12.7
9 10 11 12
17.8
13.1 15.6 18.2 20.7
5.1 7.6 10.2 12.7
/. 5 7
Ho ram.
4.0
T 2"-w
2w ram.
FIG. 4. Theoretical effects of geometry on flow--Type 2 forgings.
Web radius rp = 15~2mm. o
Friction factor m= 1.0
oE -
d=~ °
2o
{3.
"
1"9 "~:~
1.5
u_
1.0 0
I
I
1
I
0.2
0.4
0.6
0.8
Final Web Thickness / Outer Flange Radius. 2w/r c
FIG. 5. Effect of draft angle on asymmetry of flow--Type 2 forgings.
Some Metal Flow Phenomena Arising in Axisymmetric Flashless Forging
49
2.0 £Ci
E o
o E3
1.5
t
{2:
I
1.0
I
m=O
I
2 Current / Finat Web
T 2w
Thickness.
FIG. 6. Effect of friction of asymmetry of flow--Type 2 forgings.
The theoretical curves of Fig. 5 shows that the value of draft angle, within the range likely to be encountered in forging practice, has little influence on asymmetry. Friction however, significantly increases asymmetry, as seen in Fig. 6. If friction is eliminated, equal flow into the top and bottom die cavity is achieved. PERIPHERAL
DEFECTS
Type 1 foroinos
Centralfolds. One way to reduce the energy required for flashless forging is to delay contact between stock and the cavity wall. This can be achieved by increasing the height to diameter ratio of the billet• However, using billets with a large aspect under lubricated forging conditions may result in a bollard shape during the early stages of deformation, due to inhomogeneity of metal flow. Investigation of the incidence and extent of bollarding was carried out using cylindrical billets of H30 aluminium alloy. Five different billet diameters, ranging between 12.7 and 31.7 mm and four different central boss diameters of 8.1, 11.4, 13.7 and 18.3 mm were used. Lanolin was applied generously on both die surfaces and end surfaces of the billets. Deformations corresponding to billet height reductions of 14% and 50% were imposed and the extent of bollarding was measured by the index E, described in Fig. 7. (a)
Undef0rmed
Billet.
(b) BoIlarding Index E .
J~
Do
4-
oI /
2.rma x
iI
I
2 .rmi n E % = rma x - r m i n
,, 100
rmin
FIG. 7. Bollarding of billets--Type 1 forgings. Mrl)R 20.1
D
_1
f
50
Y. VAN HOENACKERand T. A. DEAN 0.9
i
7% ,c
0.8
E=0% 2 . 5 % ~
(~
E -o Q
5 c5
o
IN
0.7
J
0.6
J
0.5
123
0
ca
0.1.
o
0.3
/
\
J
0.2 0
0.2
0.1.
0.6
0.8
1.0
Billet Height / Di0meter.
1.2
1.l.
1.6
Ho Do
FIG. 8. Extent of bollarding at 14% reduction of billet height.
Experimental values of the bollarding index at a height reduction of 14% are shown in Fig. 8. It is seen that E is a function of the initial aspect ratio and also of the ratio of the diameter of the central boss to the initial billet diameter. On the left hand side of the line corresponding to zero E, no bollarding occurred and slight tendencies to barrelling were detected. Generally the bollard type distortion increases with increasing geometric parameters. The tendency to a reversal of this trend, for larger values of the billet aspect ratio, can be attributed to the limitations of the index chosen. It is probable that the angle 0, in Fig. 8, would be a more representative index, but this is not easily measured with accuracy. When the initial billet height was reduced by 50%, it was found that the distortion of billets with a previous E value of less than 3%, had regressed and in some cases had disappeared, but that billets formerly having an E value larger than 5% developed a fold. The geometric limits leading to these conditions are shown in Fig. 9. Here it is seen that the graphical region indicating no defect has enlarged, compared with that associated with the smaller height reduction, particularly in the area corresponding to slender billets and small boss diameters. This indicates that the ratio of boss diameter/billet diameter predominates in the formation of a surface defect in the finished forging. Figure 10 is a photograph of a partially deformed billet and a section of a completed forging, showing the peripheral defect.
Type 2 forgings As discussed earlier, low friction allows symmetrical shapesto be made, but in some cases, during the forging of well lubricated aluminium alloy billets, defects appeared at the periphery of the body of the forging. Depending on the degree of deformation, this defect was found to take one of two different forms. The first consisted of superficial cracks. These generally originated from machining marks remaining on the original billet surface, but even when the surface roughness was reduced to 800/~m, the cracks still formed, as shown in Fig. 11. In these cases the cracks did not correspond to the original machining marks and were seen to develop as deformation progressed. Johnson [5] has observed the formation of defects on smooth surfaces in components of shapes similar to those described here. They were referred to as a "sucking in"
51
Some Metal Flow Phenomena Arising in Axisymmetric Flashless Forging 0.g
08 DEFECT '
'-:
0.7
i5 %
O.6
~5 ~
().5
E ~5
NO DEFECT '
0./.
~
~
~ -
m
~
G.3
0.2 0
0.2
0./.
0.6
0.8
Billet Height / Diameter.
1.0
1.2
1./.
1.6
14o -Do
FIG. 9. Peripheral defects at 50% reduction of billet height.
of metal and a plane strain analogue, consisting of a velocity field in which the surface of the deforming region moved inwards from the wall of the cavity, was proposed. This situation was not recorded in these experiments. It is thought that the existence of a draft angle and the early opening of cracks suppressed such tendencies.
Analysis of crackf o r m a t i o n The axisymmetric velocity field shown in Fig. 12 was constructed as an analogue describing the onset of cracking. It consists of two conical surfaces of velocity discontinuity AK and BK and two rigid blocks AEK and BFK. The point K, normally at the periphery of the forging is allowed to move inwards to K'. Neglecting the power required to form new crack surfaces, the total applied power can be expressed as a function of three parameters; velocity of movement into the flanges, radius to the crack tip r k and height of the crack
FIG. 10. Sections showing peripheral defect
Type 1 forgings.
Y. VAN HOENACKERand T. A. DEAN
52
FIG. 11. Peripheral cracking of Type 2 forging.
opening Zk. The onset of cracking is defined when the minimisation of the applied powei yields a value of rk less than re. The results for various forging conditions are shown in Fig. 13. It is seen that the critical web thickness, at which a crack is likely to form, is a function of both container and flange dimensions and decreases both with increasing friction and increasing flange height. A comparison of the frictionless axisymmetric results, with that from Johnson's plane strain analogue, indicates that the crack is likely to occur much earlier in the process than the "sucking in" type of defect, with the die shape utilised in these tests. Examination of
/ i 2" / / / / / / / / / u
\\
\
E
,t
,,(
\
\
) \
rp rk
[-..-
rc
FIG. 12. Velocity field incorporating crack.
Some Metal Flow Phenomena Arising in AxisymmetricFlashless Forging
53
¢3.
It.
Johnson's ,plane_strain__analo_.que for'sucking in' defect
c( = 00
-6 u ~E c~
1.0
I
I
1.5
2.0
Flange Outer Radius / Web Radius.
rc rp
FIG. 13. Theoretical prediction of crack formation--Type 2 forgings. forged shapes enabled cracks to be seen when the ratio of web to flange thicknesses was about two. N o attempt to establish the exact stage, at which cracking started was made, because of the difficulty of establishing the exact m o m e n t at which a feature is no longer an aspect of surface roughness and constitutes a crack.
CONCLUSIONS F r o m the investigations described in this work, several features of metal flow of concern in the bulk forming of flashless components have been discovered. For forgings with peripheral flanges asymmetrical flow into top and b o t t o m dies can occur. The degree of asymmetry is increased by friction, but is also dependent on the forged shape. Thick webs and thick flanges both increase flow into the top die compared with that into the b o t t o m one. An axisymmetric velocity field has been proposed, which enables predictions of flow to be made for various process conditions. Surface defects can arise in both central boss and flange forgings and those with webs and peripheral flanges. In the former, the defect is due to the folding in of the side of the billet, when it forms a bollard shape early in the deformation. Low friction, slender billets and a large central boss diameter all contribute to the formation of this defect. In forgings with a peripheral flange, defects can occur due to the opposing directions of metal flow at midheight. For conditions of very low friction, cracks can be formed. An axisymmetric velocity field, proposed as a model for this situation, allows predictions of the critical web thickness at which cracking will occur for various forging situations. It is shown that critical thickness depends on both container and flange dimensions.
Acknowledgement The work reported here is part of the outcome of a programme of research sponsored by S.R.C. Y. Van Hoenacker worked as a National Research Council of Canada Scholar.
REFERENCES [1] T. A. DEAN,Metallurgia and Metal Forming 44, 488-498, 542-544 (1977). [2] Y. VAN HOENACKERand T. A. DEAN,Int. J. Mach. Tool Des. Res. 18, 81-93 (1978). [3] T. A. DEAN,I. Mech. E. Proc. Instn mech. Engrs Vol. 193. [4] V. I. ZALrcSSKIand N. I. TYURIN.Kuznechno-shtamp. Proizo. 1, 4-8 (1959). [5] W. JOHNSON,Appl. scient. Res. A. 8, 52-60 (1959)•