Sublayer defects

Journal of Mechanical Working Technology, 10 (1984) 77--86 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

77

SUBLAYER DEFECTS

JOHN SCULAC and BETZALEL AVITZUR Institute for Metal Forming, Lehigh University, Bethlehem, PA 18015 (U.S.A.) (Received October 17, 1983; accepted January 4, 1984)

Industrial Summary The phenomenon of sublayer defect propagation during the process of direct extrusion is studied. Effects of the initial length of the feed stock billet and percent reduction in area on the percentage of the billet extruded are considered. It is shown that shorter billets allow larger percentages of the billet to be extruded before the sublayer defect occurs. The depth of the sublayer defect is dependent on the initial length of the billet: for long billets the sublayer is close to the surface, while short billets result in a sublayer buried deep in the extrudate.

Notation % BE Do Df Lo Lf Ro/Rf t t/Rf

P e r c e n t o f billet e x t r u d e d Initial d i a m e t e r Final d i a m e t e r Initial ~ength o f t h e billet Final l e n g t h o f t h e billet Radkts ratio D e p t h o f s u b l a y e r d e f e c t in e x t r u d a t e Relative d e p t h o f t h e s u b l a y e r in e x t r u d a t e

The defect A u n i q u e d e f e c t w h i c h is n o t o b s e r v a b l e b y visual i n s p e c t i o n o f t h e p r o d u c t is t h e s u b l a y e r d e f e c t , usually f o u n d at t h e rear o f t h e e x t r u d a t e . T h e loc a t i o n o f t h e d e f e c t a n d t h e i n h e r e n t inability t o q u i c k l y d e t e c t t h e d e f e c t can easily result in t h e p r o d u c t i o n o f large a m o u n t s o f a p o o r q u a l i t y p r o d uct. F o r e x a m p l e , s u b l a y e r d e f e c t s are d e t r i m e n t a l to t h e c o n d u c t i n g p r o p e r ties o f electrical bus-bars. In this s t u d y t h e causes o f s u b l a y e r d e f e c t s , t h e m e c h a n i s m s b y w h i c h t h e y are i n t r o d u c e d , a n d criteria f o r t h e i r p r e v e n t i o n are t r e a t e d .

0378-3804/84/$03.00

© 1984 Elsevier Science Publishers B.V.

78 The mechanics o f the development of sublayer defects The original surface of the billet to be extruded may contain an impurity or chemically contaminated layer. Figure 1 was made from a modelling plasticine material: (a) shows a section of the initial length of a billet before direct extrusion where the light areas represent a layer of contaminated surface, whilst (b) shows the final length o f a billet and a portion of the resultant extrudate following a 65% reduction in area. The path of the surface defect emerging as a sublayer in the extrudate is noted.

(o)

(b)

Fig. 1. The billet. Figure 2 describes the process of direct extrusion through a square die, also called a shear die (semicone angle of 90 ° ). As the ram advances through the chamber, the billet undergoes plastic deformation and exits through the opening of the die. Normally a dead metal zone that does not flow forms at the corner of the chamber between the fiat surface at the exit end and the cylindrical portion of the chamber. This dead metal in the corner separates from the billet and becomes entrapped, The outer surface of the billet has to pass through the surface F3 and the dead zone before reaching the exit. Thus, it does not show up in the extrudate the instant the extrusion starts: rather, the outer surface of the billet reaches the extrudate later than the core. The contaminated surface, as it enters the interface (F 3) between the dead zone and the material undergoing plastic deformation, proceeds toward the exit along the surface o f discontinuity shown in Fig. 3. It is desirable to prevent the contaminated surface from ever emerging as part of the product, by delaying it in the chamber so t h a t it is entirely contained in the discard. The speed of flow o f the outer surface along the surface of velocity discontinuity

79

EXTRU O D D EDI BILLET

-

-\\~,

Fig. 2. Direct extrusion.

EN0 OF THE STROK~

RAM

~~

V//////////3 Fig. 3. Dead zone and flow patterns.

80

CONE ANGLE a( = 15 ° REDUCTION R o / R f = L5

COl~

~

_

.O/Rf = z.o

IiJitlIX'

CONE ANGLE o~ = 8 0 ° REDUCTION R o / R f = 3.5

g Fig. 4. Intermediate grid deformation pattern.

\_'

_

_

_

CONE ANGLE ~ = ~ 0 °

CONE ANGLE ¢ = 3 0 e

CONE ANGLE ~, = 6 0 ° REDUCTION R o / R f : 2 ; 0

ANGLE = = 15 °

/'~

I

CONE ANGLE ¢ = 6 0 °

fl

coNE ~mLE a =eO °

81 F3 is given by eqn. (1) (eqn. 10.2b of Ref. 1): v = - - v f ( r f / r ) 2 cos

(al)

(1)

The larger the value of ~t (the semicone angle of the dead zone), the slower will be the speed of the contaminated surface and the longer it will stay in t h e chamber. Since for larger reductions the dead zone angle is larger, larger reductions delay the exit of the contaminent into the extrudate. In Fig. 4 (Fig. 3.15 of Ref. 2) it is seen t h a t this effect is more pronounced with larger die angles as well as with large reductions. The crosses (×) in Fig. 3 denote the path along which the contaminated surface advanced into the billet. As the extrusion proceeds and the ram approaches the exit die, the general flow for long billets no longer prevails. The flow starts to resemble radial flow, and the interior of the shorter billet -containing the original contaminated surface -- proceeds toward the die exit. If too short a discard is left unextruded, the original surface will find itself in a sublayer contained in the extrudate, as shown schematically in the bott o m half of Fig. 3 and in Fig. 1.

The process parameters The independent process parameters considered for the experimental phase of the sublayer defect investigation were the percent reduction in area {% RA) and the original length of the billet (L0). The dependent process parameters were the length at which the contaminent first appeared in the extrudate (Lf) and the depth of the contaminent in the extrudate (t). The original length (L0) and the length at which the contaminent first appear in the product were used to calculate the percentage of the billet ext ~ a d e d as seen in eqn. (2): % BE = (1 - L f / L o ) X 100

(2)

Experimental procedure Figure 5 shows an extrusion chamber for a longitudinally sectioned halfbillet. Also shown is a selection of sleeving devices required to vary the percent reduction in area of the billets. The transparent flat window is designed to allow viewing of the progress of the extrusion throughout the entire test. Test billets (see Fig. 1) were formed using blue plasticine modelling material. These billets were then covered with a thin layer of white plasticine representing a contaminated surface. Completed billets were then coated with CaCO3, which served as a lubricant. The lubricated billets were inserted into the main b o d y of the chamber and the ram lowered into position atop the billet. The entire assembly was then placed over the plexiglass cup and the ram was pushed toward the die end o f the chamber.

82

When the extrusion was completed, both discard and extrudate were removed from the chamber and again cut in half to expose the interior of the product (see Fig. 6). Measurements of the lengths L1 and L2 were taken (see Fig. 1) and the length at which the contaminent first appeared in the extru-

Fig. 5. Simulator.

Fig. 6. Sectioned billet.

83 date was calculated using eqn. (3):

Lf = L1 + L2 (Rf/Ro) 2

(3)

With Lf calculated for a given reduction and billet length, the chamber was loaded with a similar length billet and an extrusion of equal reduction performed. The extrusion was terminated at a final billet length of Lf, the discard and billet removed, cut in half and measured to verify the initial calculations of Lf. Extrusions were performed at 34, 53, 65, 70, and 90 percent reductions in area using half-billets of 32 mm and 64 mm lengths. Discussion

The expected length Lf The expected length Lf at which a defective sublayer first emerges into the extrudate was calculated from the theoretical velocity field of eqn. (1}. It was found that in reality the emergence as measured was delayed even more. This further delay can be explained from an observation of the surface F3, where the dead zone interfaces the extruding billet (see insert to Fig. 3). Across a thin boundary layer the flow of the billet undergoes a severe velocity gradient from the velocity at the extrudate side, given by eqn. (1), falling down to zero at the side of the dead zone. Thus a skin-deep layer of the surface itself does not approach the exit at all. Deeper layers approach the exit more quickly. If the extrusion stops in time, the layer does not emerge. Emergence o f the original surface defects If the extrusion continues, the surface defects may emerge as sublayer defects, surface defects, or both. As shown in Fig. 1, at the end of the stroke the accumulated aggregate of defective material branches off and the branched-off flow proceeds toward the exit, at the surface of the extrudate. This phenomenon at a more advanced stage is shown in Fig. 7. Occasionally, for smaller reductions, the defect may appear on the surface before it develops into a sublayer defect. Results The results of the experiment indicate that the original length of the billet for a specified reduction (% RA) affects the length at which the contaminent first appears in the product (Lf) and the depth (t) of the sublayer defect in the extrudate. Results show that a greater percentage of a shorter billet can be extruded for similar reductions before the sublayer defect will appear, as noted in Fig. 8. A minimum in the percentage of the billet extruded was found to occur at about 65% reduction in area. Thus, increasing the percent reduction in area for values above 65% will lead to higher utilization of the feed stock. The depth of the sublayer defect as a function of the original billet length

84

Fig. 7. Surface and subr-rface defects.

bJ 0 n~ I-X W t-hi ..J .J

EXPECTED tlflIlftll PREVENTED

SUBLAYERDEFECT

I00 -

9O

Lo= 3 2 mm 8O

Loft 6 4

mm

~o6(

I

0

20

I

J 40

I

% REDUCTION

I 60

I

I 80

I

I fO0

IN A R E A

Fig. 8. Percent extruded billet vs. percent reduction in area.

and reduction is shown in Fig. 9. As can be seen, the depth o f the sublayer defect is inversely related to the initial length of the billet. In addition, when the plasticine haft-billets are extruded to final billet lengths much less than the calculated Lf, the contaminent appears as a surface defect as well as in the sublayer within the extrudate as shown in Fig. 7.

85 0.7 A £E .%. .iv 0.6 I.(.~ LU LI.. 1.1.1 0.5

rr" ILl >" J m :::) O3 t.L 0 "1I--

OA

0.3 ~=90%RA 0 = 65% RA

0.2-

w

_> I--- O J ._1 Ixl 0.0

I

I 16

0

I

I 32

I

I 48

1

I

64

ORIGINAL L E N G T H OF B I L L E T

( L o , mm)

Fig. 9. R e l a t i v e d e p t h o f s u b l a y e r d e f e c t vs. o r i g i n a l l e n g t h .

Conclusion The prevention of the sublayer defect in direct extrusion can be achieved when the process parameters are properly selected. A major benefit of such a selection is an increase in the a m o u n t of a good quality product produced accompanied by a decrease in a m o u n t of material discard. IOO EXPECTED SURFACE DEFECT xlLval~low4ulx PREVENTED

~-,.~4"~.~= a w

90

n.- 80 lx 1,1

-

.

0 = 65%RA "~ = 9 0 % R A

%% v , . ~ , ,

~ To _J ..J N

SUBLAYERDEFECT

6o-

o

l/l/l/till

PREVENTED

I

5(:

EXPECTED

I 16

I

I 32

ORIGINAL

|

| 48

I

LENGTH,

I 6A

I

L o (ram)

Fig. 1 0 . P e r c e n t e x t r u d e d b i l l e t vs. o r i g i n a l l e n g t h .

86

Figure 10 emphasizes the importance of correctly defining the process parameters. If sublayer defects cannot be tolerated, the percent of the billet extruded must be below the dash-shaded lines. If sublayer defects are allowed, but surface defects are not allowed, the percent of the billet extruded must be below the cross-shaded line, but may be above the dashshaded lines. Acknowledgment Partial support for this study was provided by a Forging Industry Fellowship.

References 1 B. Avitzur, Metal Forming: Processes and Analysis. McGraw-Hill, New York, ]968, and Kriger, Huntington, New York, 1979. 2 B. Avitzur, Handbook of Metal-Forming Processes. Wiley-Interscience, New York, 1983.