Near net shape forming of particulate reinforced al-alloys by isothermal forming compared to semi solid forming

Journal of

Materials Processing Technology ELSEVIER

J. Mater. Process. Technol. 45 (1994) 415-420

Forming of P a r t i c u l a t e R e i n f o r c e d A I - A I I o y s by Isothermal Forming Compared to Semi Solid Forming

Near

Net

Shape

T. Witulski a, J.M.M. Heul~en b' A. Winkelmann "' G. Hirt ~, R. Kopp b EFU Gesellschaft for Ur-/Umformtechnik mbH P.O Box 11 80, D-52147 Simmerath, Germany b Institut for Bildsame Formgebung / RWTH Aachen Intzestrage 10, D-52056 Aachen, Germany

Particulate reinforced aluminium alloys produced by powder metallurgy are relatively expensive and difficult to shape by cutting or milling. Therefore near net shape forming of complex shapes is of high economic and technical interest. However, the formability of these MMC's under conventional forming conditions is limited. The potential of isothermal forging under various conditions as well as semi solid forming has been evaluated as a method for component production. Initially the suitable processing window and the flow stress of the material have been determined by isothermal compression tests at various temperatures and strain rates. Furtheron in a number of forming trials an end fitting has been produced by both routes, isothermal as well as semi solid forming.

1. Introduction Particulate reinforced aluminium alloys produced by powder metallurgy show excellent mechanical properties and a high stiffness due to their fine grain size and the very small and well distributed reinforcement particles. The material used in this investigation to produce the end fitting in figure 1 is a matrix alloy 2124 with 25 vol % SiC-particles. Due to the high reinforcement level an ultimate tensile strength of 700 MPa and an elastic modulus of 115 GPa is achievable. Thus the material combines the density of a high strength aluminium alloy with the stiffness of titanium which makes it applicable for stiffness critical aerospace components when weight savings a r e essential. The disadvantages of the material are the currently high price and the necessity to use special machining tools. Therefore the target of this investigation was to produce a near net shape preform by solid state isothermal forming or semi solid forming from a

cylindrical billet thus reducing raw material input and machining It is known that these types of alloy show superplastic behaviour at high temperatures and low strain rates [1]. However, since the forming behaviour is not well known, the flow stresses at different temperatures and strain rates as well as the formability have been studied in a basic investigation.

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ill

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Fig. 1: Demonstrator part, preform (right) and ready machined (left).

0924-0136/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. S SD l 0924-0136(94)00217-7

• I

416

2. Compression Tests

~Oii

A theoretical consideration of metal forming processes presumes a detailed knowledge of the properties of the metal to be formed. The development of new forming concepts and techniques also entails a more exact quantification of the necessary material input data, especially with a view to the increased demands on product properties. Generally, the most important material parameter for forming processes is the f l o w stress. Process parameters like the required force, material flow, contour behaviour of the workpiece or the temperature field within the formed volume may significantly be influenced by f l o w stress [2,3,4]. Cylindrical compression test with constant strain rate under isothermal conditions have been chosen to evaluate the forming behavior. A detailed description of the facility is given in [3,4]. One goal of the investigation was to determine the process w i n d o w , in which superplastic behavior could be expected for the subsequent forming trials. Additionally forming limits with respect to higher strain rates and lower temperatures should be evaluated for economic reasons. Therefore, based on available experience, the following parameters have been chosen for the compression tests: Table 1 Parameters for the compression tests (specimen geometry: Q 1 8 m m x 25mm, specimen axis parallel to the extrusion direction of the billets).

°

25 ~ 20

o

o ......

O.01/s

t 0.1/s

'

I/s

I

5 0 0

O. 2

0.4

0.6

!) ~

Strai,q

Fig. 2: Effect of strain rate on f l o w stress at 515°C Typical results are shown in figure 2 for the case of 515 °C forming temperature. The f l o w stress varies between 3 and 12 N/mm 2 depending on the strain rate (~p) imposed, The curve for 4 = 1Is shows a typical profile, which is characterised by an initial increase of the f l o w stress with a subsequent decrease at higher strains. At lower strain rates ~ ~ 0.1 s -~ the f l o w stress keeps almost constant. This observation implies~ that at high strain rates dislocation generation could be the main deformation mechanism. At lower strain rates however, the f l o w stress remains almost constant over large variations of strain. This would have to be expected, if grain boundary sliding would be the governing deformation mechanism, which is typical for superplastic deformation[5]. However further investigations would be necessary to provide detailed information about the transition range concerning the deformation mechanisms. 120

"

o,ool/s I

c,

o.ol/s

" I

(~ 1[i

is

| I |

30 T Strain Rates (l/s)

~

~n

Temperature

6

1

40 2.

420 °C

0.001

0.01 0.1

1.0

[E

400

450 °C

0.001

0.01 0.1

1.0

515 °C

0.001

0.01 0.1

1.0

"

iL:

450

500

Temperature

,', 550

(°C}

Fig. 3: Effect of temperature and strain rate on f l o w stress in compression tests (strain ~p ~0.4)

417

The effect of temperature on f l o w stress is s h o w n in Figure 3. The determined f l o w stress increases w i t h lower temperatures and increases significantly with higher strain rates. To determine the formability of the material the specimen were compressed to a strain of 1.5. No cracking on the surface could be observed under all testing conditions. To investigate the influence of the extrusion direction, tests at 5 1 5 ° C and 4 5 0 ° C have been performed also w i t h specimen of perpendicular orientation of the billet axis. Also here no cracks on the surface over the same parameter range were observed. Within the limited strain range in the compression tests the orientation of the specimen has no influence on f l o w stress and formability in the determined process w i n d o w . The described investigations have been used to estimate the expected tool loads and to choose promising process conditions for the demonstrator part production. Compression tests at higher temperatures in the semi solid range of the material - were performed to yield an estimation of the required deformation stress. Due to the semi solid state the determined stress is not a conventional f l o w stress as calculated in the tests above [6,7]. H o w e v e r the applicability of the compression test for deformation of semi solid metalls has been investigated in [7]. Table 2 shows the chosen parameter combinations for the tests.

Table 2 Parameters for the (specimen geometry: semi solid forming

The material is heated above the solidus temperature in order to partially remelt the matrix of the MMC. With higher temperatures the fraction of remelted parts increases. Figure 4 shows the calculated stress values at 6 1 0 ° C for different strain rates. Compared to the curves at 5 1 5 ° C the stress is significantly lower. 10 8

:~

"-

4 ~

2

Strain Rates [ l / s ] Temperature 595 °C

1.0

10.0

605 °C

1.0

10.0

6 1 0 °C

1.0

10.0

l

o

o

0.2

0.4

i

i

q

0.6

0.8

I

Strain Fig. 4: Effect 610°C

of strain

rate on stress

at

The evaluation of the stress at various temperatures is s h o w n in Figure 5. It is obvious that the decrease of the stress follows the characteristic course s h o w n in Figure 3. The difference in stress due to various strain rates decreases w i t h higher temperatures. 10 8

--'--

1/s

~

lOis i

I

6

co

2

590

compression tests G = 18mmx25mm),

, :i io/s ]

1/s

6

595

600

605

610

615

Temperature (°C) Fig. 5: Effect of temperature on stress for various strain rates (strain cp = 0.4) H o w e v e r a meaningful interpretation of the results above 6 0 0 °C is difficult due to the limited measurement accuracy and cracks which have been observed in the specimen. In the real process these cracks may be avoided by sufficient hydrostatic pressure in a closed die.

418

In order to determine reliable high temperature stresses the IBF developed an experimental setup to measure the stress in the mushy state of metals. In future these investigations allow to yield values for the numerical simulation of the process [8].

3. Part production 3.1 Isothermal Forging A combined f o r w a r d / b a c k w a r d extrusion process under isothermal conditions was chosen to produce the preform (fig. 1) w i t h o u t a flash thus saving raw material: A cylindrical billet w i t h a diameter of 50 mm and a length of 53 mm is placed into the cavity of the lower die half and a piston, which formes the conical bore, forges the material into the cavity (fig. 6). The material first f l o w s upward and forms the cylindrical part of the fitting and then the material is extruded into the platelike region.

In accordance with the compression tests and in consideration of the expected material f l o w the piston was driven with an exponantial decrease of velocity during forging, following equation (1). V piston

= 80 • k * e (-kt}

(!t

The value of the constant k has been varied between 10 -3 s-1 and 0.1 s -1, which gives the following values for the starting velocity vstart, the velocity at the end of the piston movement Vo,~d and the complete time t to forge the part: k

(s-1) 0.001 0.002 0.005 0.01 0.1

V start

V end

~"

(minis) 0.08 0.16 0.4 0.8 8

(ram/s) 0.02 0.04 0.1 0.2 2

{rain) 23 ~] 5

2.5 0.25

The load-stroke curves and the stroke-time curves have been recorded and all parts were tested by dye penetration test

Variation of forging velocity Fig. 7 shows the load stroke curves of t w o forging operations at 5 1 5 ° C for t w o different k-values. It is obvious that at the l o w forging velocity (k=0=001

s -1) the load is much

lower than at the higher velocity ( k : 0 . 1 s 1t which correlates to the measured f l o w stresses 300 250 z -~ 200

Fig. 6: Schematic drawing of the die Due to this one step forging operation the material f l o w involves large plastic strains. Therefore the forming tests were mainly performed in that part of the processing w i n d o w where a good formability was expected.

-~

150

_J

1 O0

k - 0 i s-I/

5O 0 20

40

Stroke ram, at Fig. 7: Load-stroke curves k-values (T = 515°C)

RO

different

419

Both load curves have a relatively l o w level and only during the last 10 mm of the stroke, when the platelike region is formed, the load rises. At the high velocity the increase of the force is much higher and it reaches the predefined load limit of 300 kN so that the cavity could not be filled completely and due to the bad formability the platelike region of the part bursted (fig. 8).

300

i

,

r

250 ~

"1

,

-- 150

i

;T

=515 ~'

o

0

20

40

60

80

Stroke / m m Fig.9: Load-stroke curves at t w o different temperatures (k = 0.005s -1)

Fig. 8: Platelike region of the fitting formed with a k-value of 0.1 s 4 at 515°C. Also parts which were formed at a lower forging velocity ( k = 0 . 0 0 5 s 1) the dye penetration test showed small surface defects on the top of the platelike region of the part. However if the forging velocity was furthermore reduced to k-<0.002 s -1 sound parts could be produced.

Temperature variation Figure 9 shows the recorded curves of t w o forging trials with a constant k-value of 0.005 s 1 at t w o different temperatures. The influence of the lower temperature on the load curve is much higher than a higher forging velocity (compare fig. 7). At a temperature of 420 °C the load rises to the predefined force limit so the part could not be filled completely and first surface cracks appeared,

Process optimization Isothermal forging with very low velocities results in very long forging times of more than 10 minutes to produce one part thus leading to a bad productivity. To reduce the total time the stroke time curve has been modified as shown in figure 10 by curve B. A t the begining of the stroke, when the load is small the piston moves relatively fast and only at the end of the stroke, when the load increases, the velocity is decreased to a very small level. Using this velocity profile, sound parts could be produced with a 30 times higher productivity compared to the exponential velocity decrease.

60

.

E ~ I I40 50 IE

.

I

.

.

B

.

l----

~

~

-!

2

ii

i i ii

CO 10

o ~"

1 5

L

l 10

15

i 20

Time / s 300 250 Z -~

200

"o 150 ~

100 50 0 0

2O

40

60

Stroke / mm Fig. 10: Stroke-time and load-stroke curves at an exponential velocity decrease (A) and an optimised velocity decrease (B), (T=515°C)

420

3.2 Semi Solid Forming The isothermal forging process described before has the disadvantage of a low productivity and due to the high die temperatures of 515 °C, a high die wear is to be expected. An alternative is the the semi solid forming process. In this process the cylindrical billet is heated above the solidus temperature where the matrix of the MMC partially remelts. Due to the very fine grain size and the small SiC-particles the billet keeps his cylindrical shape and can easily be placed into the die. If the material gets under shear stresses during forming it shows the typical shear thining behaviour and the viscosity is reduced to a small level where a excellent die filling is achievable. Using the same die as shown in figure 6 semi solid forming tests were performed. The billet temperature has been varied between 6 0 5 ° C and 630 °C. The piston velocity was kept constant at 400 mm/s and the die temperature was set to 300 °C.

°°i L//[

F ili]

E 40 _~ 30

0

.

0

0.1

.

0.2

0.4

0.5

s

] . . . .

o

I

o....... 0

r =:610°C

/

:-------J20

IE

2 40

The authors would like to thank the Ministry of Economics and Technology of NorthRhine-Westphalia for the financial support of EFU's research work.

References

.

0,3

1 ime/

200

.

!iii

forming step, the load rises when the die is closed completly and the material is held under a constant metallostatic pressure to feed the residual solidification shrinkage. With this process complete die filling was achieved and only small surface cracks appeared on the end of the fitting. Further investigations will decide whether this process is suitable to form powder metallurgical MMC's. At this time it is not clear if the remelting of the matrix causes any interfacial reactions between the reinforcement particles and the matrix. Due to the very short reheating t i m e and the only partial remelting of the matrix an interfaciat reaction must not necessarily occur. Further investigations will especially include tests of the mechanical properties, t h u s providing more detailed information.

60

Stroke / m m

Figure 11 :Stroke-time and load-stroke curves at t w o different billet temperatures A t the billet temperature of 610 °C a load during forming could be measured but at the higher temperature of 620 °C, when a higher amount of liquid phase is present, the load is almost zero (fig. 11). Only at the end of the

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