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copper bimetallics by explosive cladding and rolling
ELSEVIER
Journal of Materials Processing Technology 44 (1994) 99-117
Journal of Materials Processing Technology
Fabrication of aluminium/copper bimetallics by explosive cladding and rolling A.G. M a m a l i s a'*, N . M . Vaxevanidis a, A. Szalay b, J. P r o h a s z k a c aDept, of Mechanical Engineering, National Technical University of Athens, 42, 28th October Ave., 10682 Athens, Greece METALLTECH Ltd, Budapest, Hungary ¢Hungarian Academy of Sciences, Budapest, Hungary (Received June 21 1993; accepted December 8 1993)
Industrial Summary In the present paper the authors report on the fabrication of bimetallic components consisting of aluminium and copper plates by explosive cladding and subsequent rolling. Such components are used extensively in the electrical and ship-building industries as well as in vessel design, replacing components made from solid materials. The effect of the cladding and the rolling parameters on the sound fabrication and the micro-structural properties of the resulting bimetallics is evaluated in terms of "surface integrity", i.e. surface topography, microhardness variation and metallurgical changes through the thickness of the cladded/rolled plates, and the existence and/or intensification of particular defects encountered in such processing.
Notation Ek
FR hx hAl hcu
he HV rT r'Al rCu /*i
energy density of the flyer plate roll force thickness of the bimetallic thickness of the aluminium layer thickness of the copper layer thickness of the explosive microhardness total thickness-reduction ratio of the bimetallic total thickness-reduction ratio of the aluminium layer total thickness-reduction ratio of the copper layer thickness reduction-ratio of the bimetallic after ith pass
* Corresponding author. 0924-0136/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 3 ) E 0 1 4 4 - 6
100
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99--- 117
R Ra
welding impact ratio mean surface roughness surface roughness of the aluminium layer surface roughness of the copper layer roll torque detonation velocity velocity of the re-entrant jet velocity of the flyer plate width of the aluminium layer width of the copper layer mean yield stress initial set-up inclination angle collision angle
RAt Rcu
TR Ud Up WAI WCu
Y a
1. Introduction
Explosive cladding is the process of forming a bond by explosively impacting a metallic mass on to another such mass under controlled conditions. Various techniques have been developed over the years to meet a steadily growing field of application and for solving particular industrial problems. The explosive-cladding process achieves excellent bonding of similar or dissimilar metals with different hardness, melting points, thermal-expansion characteristics and electrode potentials. Furthermore, the process can be applied to a broad range of thicknesses and area dimensions due to the ability to distribute the high energy of the explosive over the entire welding areas I-1,2]. In modern technology there is an increasing demand for the use of composite metal laminates instead of solid materials due to the need for corrosion resistance at lower cost, improved heat-transfer characteristics, strength and/or stiffness, improved electrical properties and better abrasion- or erosion-resistance. Applications of explosive bonding are therefore numerous and range from the production of "sandwich" plates for coinage to the more sophisticated use of titanium-to-stainless-steel transition joints in spacecraft [-3]. The most common utilization of explosive bonding is in the production of fiat plate-clads from an extremely large number of combinations of dissimilar metals. Explosively cladded plates combining the properties of the constituent metals with a high bond-integrity provide many opportunities for innovative and economical design in the fabrication of thermo-bimetals I-4], transition joints for ship-building applications [5] and clad systems in vessel design 1,6]. Other applications have been investigated but not exploited; see the extensive lists of published work on the topic in [-2, 3, 6-8]. For the manufacturing of near-net-shape bimetallic components, explosive welding is usually followed by hot/cold rolling and/or punching. However, particular difficulties and limitation arise from such processing due mainly to metallurgical problems and defects encountered in explosive cladding, see for example I-2,9]. Extensive experimental research and testing is needed in order to achieve acceptable bonding
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99-117
101
integrity, sufficient overall strength and improved performance under severe working conditions for composite components fabricated by explosive cladding and forming. The present work is the first step of an investigation regarding the possibilities and limitations of successfully applying the above-described processing for the fabrication of near-net-shape bimetallic components. In this paper the authors report on the fabrication of sound bimetallic components consisting of aluminium and copper plates by explosive cladding and subsequent rolling. Such components can be used as electrical transition joints in high current-density electrical systems and as simple transition joints in machine-supporting structures offering improved stiffness and better corrosion resistance. The overall strength of the fabricated bimetallics, on a macroscopic scale, is determined through the upsetting test. Moreover, the influence of the cladding/rolling parameters on the micro-structural properties of the fabricated bimetallics is evaluated in terms of "surface integrity" [10], i.e. surface topography, microhardness variation and metallurgical changes through the thickness and at the interface of the cladded/rolled plates: the existence and/or intensification of particular defects encountered in such processing reveals useful information about the soundness of the fabricated bimetallics.
2. Experimental 2.1. Equipment and procedure 2.1.1. Explosive cladding A detailed description of the equipment and procedure used for the explosive cladding of AI/Cu plates is given in [2,3], and therefore only the main features are outlined below. The flyer plate used was made from a 5000 series aluminium alloy containing 1% magnesium and 0.7% iron, whilst the parent plate was fabricated from a commercial batch of copper containing 0.01% phosphorus. The initial thickness of the plates was 4 mm, whilst their average surface roughness, Ra, values were 0.33 ~tm and 0.27 ~tm for the aluminium and the copper, respectively. Three different explosives were used namely: (i) ANFO (Ammonium Nitrate/Fuel Oil 6%) explosive in granular form, with a packing density of 0.85 x 10a kg/m 3 and a low detonation velocity which mainly depends upon the thickness of the charge layer. Because of its granular form, an unstable detonation front may be formed and, therefore, in order to achieve stable detonation characteristics within a short distance from the point of ignition it was necessary to use a booster charge of some high energy explosive, for instance a PETN (pentaerythritol tetranitrate) pellet [2]. The detonation characteristics of ANFO explosive are given in [11]. (ii) Nitrammite, which is a high ammonium nitrate, detonator-sensitive powder explosive, its thin granular form resulting in a more stable and uniform wave-front
102
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99 117
formation. Its detonation velocity, measured by the Dautriche method, was 3500 m/s at an initial density of 0.98 x 103 kg/m 3. (iii) Paxit (Ammonium Nitrate 77%, TNT 19%), which is a high-energy explosive with a detonation velocity of 4000 m/s at an initial density of 103 kg/m 3, its detonation characteristics being similar to those exhibited by Nitrammite. The experimental set-up for the explosive cladding is shown schematically in Fig. 1 and was used both for the parallel (a = 0 °) and the inclined (a > 0 °) arrangements. ANFO explosive was used together with a high explosive booster (PETN), the firing of the explosive taking place away from the plates in order to obtain stable detonation characteristics and to avoid destruction of the metal plate. Contrarily, when Paxit or Nitrammite were used the firing of the explosive took place in the vicinity of the plates and a high explosive booster was not needed. The accelerating mass of the experimental device consisted of the flyer and the driver plates, the latter being made of mild steel of 1-4 mm thickness, and essentially used for controlling the impact velocity and avoiding major cladding defects. The stand-off distance, see Fig. 1, which greatly affects the impact energy, was selected as one and a half times the thickness of the flyer and driver plates when ANFO was used, reduced to a distance equal to the flyer plate thickness when Paxit or Nitrammite were used. The cladding variables for the present series of experiments, calculated from the detonation velocity and the plates characteristics according to standard techniques [2], are summarized in Table 1.
2.1.2. Rolling After explosive cladding, strips of dimensions 70 x 30 mm, cut from the clad bimetallic plates, were cold rolled in a succession of passes in order to examine the influence of the rolling parameters on the mechanical properties and structural integrity of the bimetallic plates.
Geometry of the collapse of the fryer plate (detail A) stand-off distance ",\ \
fuse detonotor/
exptosive ~
I
~~ J~vp I I
~ / ~ S t
i
/ °
Ol
driver p[ote "+-~
,e, -uo
,oo<
Fig L Experimentalset-upfor explosivecladding
o<
ANFO ANFO ANFO ANFO ANFO ANFO Paxit Nitrammite Nitrammite
215x70xl.6 260 x100 x 4.1 200x60x2.9 200x60x2.9 200x60x2.9 200x58xl.0 210x60x1.3 210x60x1.3
Driver plate (St)
215 x 7 0 x 5 215x70x5 215×70x5 215x70x5 215x70x5 215x70x5 180x45x5 200×50×4 200x50x4
Flyer plate (Al)
Dimensions (mm)
0 0 0 3 6 9 0 0 7
~(°)
Initial angle
Dimensions of the parent plate (Cu) : 210 x 60 x 4 (mm)
1 2 3 4 5 6 7 8 9
Explosive
Table 1
50 40 40 40 40 40 16 18 18
Thickness of explosive h (mm)
1.05
1.05
1.01
1.05 1.05
2.84 1.31 0.86 1.05
R
Welding impact ratio
2298 2035 2035 2035 2035 2035 4000 3500 3500
Detonation velocity vd (m/s)
611 413 315 359 359 359 573 520 520
Velocity of the flyer plate Vp (m/s) 15.3 11.6 8.9 13.1 16.1 19.1 8.2 8.5 15.5
#(°)
Collision angle
2.49 2.21 3.40 2.80 2.80 2.80 3.61 3.22 3.22
Ek(J/m 2 × 10 6)
Energy density of the flyer plate
4596 4070 4070 3171 2557 2155 8000 7000 3844
Velocity of the re-entrant jet v, (m/s)
I
",-4
Thickness of aluminium layer hA~(mm)
4.10 3.72 3.16 2.54 2.13 1.59 1.23 0.74
8.45 7.19 6.81 5.75 5.05 4.12 3.52 2.55 1.65
4.18 3.50 3.29 2.83 2.43 2.05 1.70 1.23 0.80
"Dry" conditions
8.30 7.46 6.48 5.18 4.53 3.46 2.44 1.60
Lubricated conditions
Thickness of bimetallic hr (mm)
Table 2
4.27 3.69 3.52 2.92 2.62 2.08 1.82 1.32 0.85
4.19 3.75 3.32 2.64 2.40 1.87 1.31 0.86
Thickness of copper layer h~ (mm)
0 15 19 32 40 51 58 70 80
0 11 22 38 45 58 71 81
Total thickness reduction of bimetallic rx (%)
0 14.9 5.3 15.6 12.2 18.4 14.6 27.5 35.3
0 11 13 20.6 12.5 23.6 29.5 34.4
ri (%)
Thickness reduction of bimetallic per pass
0 16.3 21.3 32.4 41.8 51 59.3 70 80.9
0 9.5 21 38.2 48.2 61.2 70 81.9
Total reduction of aluminium layer rA1 (%)
0 13.6 17.6 31.5 38.7 51.3 57.3 68.7 80.1
0 10.7 21 37 42.7 55.5 68.7 79.6
Total reduction of copper layer r~ (%)
29.20 29.70 29.80 30.00 30.15 30.45 30.65 31.15 31.50
28.25 29.30 29.45 29.85 30.20 30.50 30.90 31.30
Width of the aluminium layer WAI(mm)
29.50 29.65 29.70 29.85 30.00 30.20 30.35 30.65 31.75
29.25 29.45 29.45 29.70 30.00 30.30 30.80 31.45
Width of the copper layer w~ (mm)
Roll torque per pass TR (Nm)
0 75 61 93 88 96 91 112 127
0 70 78 84 81 99 106 119
-
-
0 0 0 76 578 560 59 368 348 96 919 905 87 688 688 98 914 902 94 710 699 116 1004 998 128 1 1 0 8 1 0 8 6
-
485 592 847 644 918 965 1006
-
meas. calc. meas. calc.
Roll force per pass FR (kN)
0.29 0.24 0.21 0.19 0.15 0.13 0.12
0.21 0.18 0.14
Surface roughness of the copper layer Re. (~tm)
0.33 0.29 0.25 0.24
-
-
Surface roughness of the aluminium layer RAI (lam)
-.q
I
4~ at~
e~
e~
g~
4~
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
105
Rolling of the rectangular plates was performed on an experimental 2-high rolling mill, properly instrumented for roll force and torque measurements, at a constant speed of 4 m/min between two steel rolls of 200 mm diameter and 100 mm barrel length, see [10] for details. Two series of rolling passes were performed, the first under "dry" conditions and the second under lubricated, in the latter case Mobil Prosol 36 emulsion and Shell Sol D100 kerosene being used as lubricants for the copper and aluminium surfaces of the bimetallic strip, respectively. The rolling direction was opposite to that of the cladding direction. Rolling parameters, force and torque measurements and surface-roughness values for all strips rolled are tabulated in Table 2.
2.2. Measuring techniques For the evaluation of the mechanical properties of the cladded bimetallics, the upsetting test was used, upsetting tests being performed on a Schuller hydraulic press connected with a data acquisition system. The velocity of the upsetting anvil was 5 mm/min, the whole process being fully automated and controlled through a computer, providing an overall strain-rate estimated to be 16 x 10- 3 s- 1. For the preparation of the test specimens and the testing procedure, the recommendations of 1-12] were followed. Small cylinders with a height-to-diameter ratio, hiD = 1.5, were cut from the composite plates, their height being parallel to the cladding direction. The flow stress-strain curves obtained for the bimetallics under consideration as well as the relevant stress-strain curves of the initial test materials compressed under the same conditions are presented in Fig. 2, from which the mean yield stress was estimated to be 301,320 and 278 N/mm 2 for the A1/Cu cladded composite and the AI and Cu initial materials, respectively.
500 aLuminium "E E
400
z
300
bimetat
tic
200
~oo
0
I
I
I
0.2
0.4
0.6
Stral
n
Fig. 2. Compressive stress-strain curves for the initial materials and the fabricated cladded bimetallic plate.
106
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99 117
In order to examine the weld quality and the microstructural changes at the interface after cladding, the fabricated bimetallics were sectioned in planes parallel to the detonation direction and standard metallographic preparation and etching procedures were applied. During rolling, after each pass, the total thickness reduction of the strip as well as the reduction of the aluminium and the copper layers were measured using an optical microscope with a micromeasuring device. Changes in specimens width (spread) were monitored also. The microstructural changes of the cladded samples were observed under a metallographic microscope (Union Optical Co) whilst microhardness testing was performed on a Leitz microscope equipped with a Vickers indenter under a testing load of 1N, the microhardness values obtained being the average of five indentations. A "Talysurf" (Taylor Hobson) recorder was used to measure the centre-line average, Ra, surface roughness, the cut-offlength being selected as 0.8 mm whilst the roughness values were the average of at least ten measurements per specimen.
3. Results and discussion
3.1. Explosive cladding Explosive bonding is accomplished by a high velocity oblique impact between two metals. Due to this impact resulting from the detonation waves, the metal plates behave hydrodynamically and come into intimate contact, thus promoting solid-state bonding [3]. Typical of the explosive bonding is a region of severe plastic deformation restricted to a narrow band near to weld interface. Several mechanisms of interface bonding have been proposed, (for a brief review, see [2, 3, 13]) and three distinct types of bond morphologies have been identified, namely a laminar interface (a flat bond), a wavy interface and/or a molten-layer interface [3,9]. In general, under well-controlled cladding conditons, the explosively cladded plates are expected to be adequately bonded over the entire area of the bimetallic composite, except for small areas near to their edges [2,6]. However, during the present investigation, not only patterns encountered often in explosively welded plates with very different densities, but also certain features and defects dependent upon the explosion/collision variables and the kind of explosive used, were identified: these are discussed below (see also [9, 13, t4]). When the driver plate did not overlap the flyer plate or in the absence of the former, spalling of the later occurred at the outer edge of the plate due to reflected tensile waves. Such spalling, together with rather large unbonded areas, leads to defective bimetallic plates (Experiment 1 in Table 1). Moreover, the parallel arrangement of the experimental set-up with the use of the ANFO explosive, (Experiments 2 and 3 in Table 1) resulted, in general, in cladded plates with poor wave formation and large scale defects. At the inner edge of the bimetallic plate, an unbonded area was identified, the length of this area varying from 6 to 10 mm, followed by a zone of
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
107
waveless bonding of approximately the same length. The amount of the intermetallic compound was found to increase and in the middle of the plate, lengthwise, a continuous intermetallic layer was observed, with cracks spreading transversaUy to the interface of the cladded plates, see Fig. 3(a). This zone is also characterized by poor asymmetrical wave formation, with an average wavelength of 1200 txm along the interface, and large cooling cavities surrounded by entrapped intermetallics, see Fig. 3(b). Towards the distal end of the plate the presence of cracks and cavities was intensified, the intermetallic layer appearing to be discontinuous with an increased
Fig. 3. Microstructuralcharacteristics of explosivelycladded aluminium/copperplates using ANFO explosive: (a) and (b), parallel arrangement; (c) and (d), 6° inclined arrangement.
108
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
thickness, whilst the interfacial bonding was reduced, leading to bond breakage near to the outer edge of the plates, see also 1-14]. In order to achieve more stable detonation/collision characteristics for obtaining better bonding integrity, the angle of initial incidence, a, see Fig. 1, was increased from 0 ° to fixed values of 3 °, 6 ° and 9 °. This modification results in an analogous increase of the collision angle, B, which in turns determines the bonding quality at the interface through the jet entrapping mechanism during cladding, according to the physical modelling of the process proposed in [-2,13]. Almost identical microstructural characteristics were revealed for the bimetallic plates cladded with the ANFO explosive and the inclined arrangement (Experiments 4-6 in Table 1). In addition to an unbonded area of length of about 20 mm observed near the ignition point, a "sea wave"-like pattern at the copper interface was revealed, together with a discontinuous layer of intermetallic of thickness varying from 20 to 30 Ixm, see Fig. 3(c), extending for about 60 mm lengthwise along the detonation direction. This then changed to a mixed mode of interfacial structure consisting of interrupted laminates of intermetallics entrapped inside the front vortices of waves in the copper of about 40-60 pm thickness and regions of direct A1/Cu bonding, see Fig. 3(d). Bonding failure occurred at a distance approximately 10 mm from the outer edge of the strip, an overall length of about 100 mm being considered to be of a sound cladding/bonding integrity from a microscopic point of view. Plates were also cladded using high energy explosives, i.e. Nitrammite or Paxit; see Experiments 7-9 in Table 2. Cladding of plates using the parallel arrangement (a = 0 °) resulted in similar characteristics for both explosives. A very thin continuous intermetallic layer at the interface was formed starting at about 15 mm from the ignition point and spreading along the detonation direction, its thickness increasing, see Fig. 4(a): the estimated average thickness was 80 and 60 ~tm for Nitrammite and Paxit explosives, respectively. Cooling cavities and non-molten copper islands inside the intermetallic were identified in both cases. At a distance of 150-160 mm from the ignition point the intermetallic layer started to delaminate through cracking, leading to interface failure, see Fig. 4(b), a sound welded length of 110-120 mm being obtained. Typical structures at the AI/Cu boundary for the bimetallic plate cladded using a 7 ° inclination angle and Nitrammite are presented in Figs. 4(c) and (d). In this case the first welded zone is characterized by direct A1/Cu bonding without any intermetallic compounds. Note that such a wave-form is similar to typical patterns expected during the explosive cladding of dissimilar metals [2,6]. Towards the detonation direction front, vortices developed gradually, their intensity being more severe with increasing distance from the ignition point. The intermetallic compound was, in general, discontinuous a n d appeared around the front vortex. Non-molten aluminium as well as copper from the parent plate were sometimes found entrapped inside the intermetallic islands, see Fig. 4(c). Towards the free end of the plate, cooling cavities around the vortices were intensified, see Fig. 4(d), accompanied by intense microcracking. Due to the occurrence of these phenomena, the bonding strength is weakened, leading to failure at the interface and to the separation of the metallic plates near their extremities. Sound bonded zones of 130 mm in length were obtained from the present series of experiments.
109
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
i ntermetatlic i
entrapped
crack
copper
!
I1 O0 IJm
cooling
~n t e r r n e t a l l i c
•
~
'
i
i¸¸
c/a , v - i t y
i
~;
!
,
Fig. 4. Microstructural characteristics of explosivelycladded aluminium/copper plates using Nitrammite explosive: (a) and (b), parallel arrangement; (c) and (d) 7° inclined arrangement.
The boundary zones observed during the present investigation and discussed above are mainly dependent upon the cladding conditions pertaining at particular points and upon their distance from the ignition point. The existence of a thin cast interlayer was verified in most cases: however, it is difficult to judge whether it was a finely dispersed mixture of particles of the two metals or a pure metastable metallic compound. Microprobe analysis revealed that this interlayer consisted of 68%
110
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
aluminium and 32% copper, corresponding to an eutectic constituent of the A1-Cu system at 548°C. During explosive cladding, due to stress waves and the intense thermal impact, both plates experienced a hardness increase and/or a generalised shock-hardening [2]. A typical microhardness profile of an A1/Cu cladded plate is presented in Fig. 5. Due to the high velocity explosive used, the flyer plate experienced a greater hardness increase than the parent plate. The average hardness of the aluminium plate, measured after cladding, was 100 HV, i.e., about twice that of the initial unworked material (44 HV), whilst for the copper parent plate a slight increase from an initial value of 103 HV to a terminal value of 110 HV was obtained. In addition to the overall increase in hardness, localised hardening at the interface occurred, see Fig. 5, which may be attributed to intense shock pressure on collision and/or to severe plastic deformation in the region of the welded interface. As far as the overall strength of the cladded composites is concerned, from Fig. 2 it may be concluded that a macroscopic acceptable weld was produced, since the cladded plate is stronger, at least, than the weaker of the two components [2]. The mean yield stress of the bimetallic material, modelled as a rigid-perfectly plastic material, can be obtained also from the "two-layer sandwich" modelling of the rolling of bimetallics proposed by the authors; see [15]. Denoting Ya and Yb as the mean yield stress and ha and h b a s the thicknesses of the test materials, it is shown [15] that the mean yield stress of the cladded bimetallic plate, Y* may be derived as: y,_
Yaha-F Ybhb
(1)
ha + hb
Substitution of the corresponding measured values in Eq. (1) yields Y* = 299 N/mm 2, which is almost identical with the experimentally determined value (Y = 301 N/mm2).
125
AL
Cu
> -r
-o-o
--o--o
o-
-o
--
-o-
75 c
2
or-
2
v-v-v-v--v-
v - v
25
inltiat ~nterfoce
- -
mlcro h a rd n e ss
2
2 Distance across
thickness
(ram)
Fig. 5. Typical microhardness profiles of aluminium/copper explosively cladded bimetallics.
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
111
3.2. Rolling
Multiple-pass cold rolling was selected as the post-welding forming operation for shaping cladded plates to their final dimensions. It must be noted that the bimetallic plates were not subjected to stress-relieving heat treatment either before rolling or at intermediate stages between the various passes. In general, failures occurring during compression-like working processes such as rolling are caused predominantly by secondary tensile stresses [,-14]. Moreover, problems and difficulties arising from the previous process, i.e. the explosive cladding, associated with particular structural changes and material properties of the bimetallic plate are sometimes intensified during subsequent rolling. Rolling in passes with a thickness reduction of the bimetallic strip of less than 10% per pass proved to be unsuccessful, since after a first or a second pass, using a thickness reduction of 5-8%, the rolled strip started to split along the middle horizontal plane on the exit side of the roll gap, with the two layers tending to follow the rotation of the respective rolls, i.e. alligatoring occurred, see Fig. 6. This behaviour was accentuated by metallurgical weakness along the centre-line (interface) of the bimetallic strip caused by defects inherent in explosive cladding, leading therefore to complete separation of the two metallic layers during the subsequent rolling pass: see the above discussion and also [,-14]. To overcome this problem, heavy passes were used, with the plastically deformed zone spreading across the whole thickness of the strip [-10], the results obtained being tabulated in Table 2. Since the elongation of the aluminium layer of the bimetallic strip was greater than that of the copper layer, the rolled strips were slightly curved, with the aluminium layer becoming convex, see Fig. 7(a). In addition, distorted end-shapes were developed during the rolling of bimetallic strips. Due to tail-end folding in the direction of the thickness, overlapping was observed at the rear end of the strips, see Fig. 7(b),
at I. ig a t o r i ng Fig. 6. Alligatoringoccurring during the rolling of explosivelycladded bimetallics at small thickness reduction (rl = 5%).
112
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
AL
t
i 50mrn
(b)
l
rolli
t
ng
20mm
direction
Fig. 7. Common defects encountered in the rolling of cladded aluminium/copper bimetallics: (a) curving due to different elongation; and (b) overlapping at the rear end.
whilst a similar defect, i.e. overhanging, was developed in the front end of the strips, see also [14]. Due to the different mechanical properties of the materials of the two layers of the bimetallic strip, differential lateral expansion, i.e. spread, occurred. In general, the spread of the aluminium layer was greater in both cases, the spread increasing with increasing total reduction in thickness, with the difference being less significant under lubricated conditions. Moreover, the thickness reduction of the aluminum layer was systematically greater, see Table 2. Note that spread not only affects the dimensional accuracy of the bimetallic component to be fabricated but also, through the
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99-117
113
development of secondary tensile stresses, may lead to edge cracking, especially in metals of limited ductility, such as the explosively cladded components: such a characteristic defect is demonstrated in Figs. 8(a) and (b), see also [14]. Furthermore, the residual tensile stresses developed in both layers near to the interface [15], enhanced by the inhomogeneous deformation, are superimposed onto metallurgical weakness at
50pm
(o)
I
I
edge-crocks 20mm (b)
I
stip
l
tines 200pm I
(c}
t
intermetottic Fig. 8. Defects encountered in the rolling of explosively cladded AI/Cu bimetallics after heavy reductions: (a) edge cracking (rr = 58%); (b) centre-split, bond breakage and edge cracking (rr = 80%); and (c) slip-lines at the interface of the rolled bimetallic strip (rr = 40%).
114
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
this zone caused by explosive cladding [9,14], leading to centre-splits and laminations, as shown in Fig. 8(b). Metallographic examination also revealed shear-band formation at the interface of the two metals, i.e. the development of slip-lines, see Fig. 8(c), such a phenomenon contributing to intermetallic-compound fragmentation and to the generation of microvoids at these intense shear zones. The variation of microhardness through the thickness of the rolled bimetallic strips is illustrated in Figs. 9(a) and (b). For the copper layer, work-hardening results in a clear hardness increase compared to the hardness of the initial bimetallic plate: compare also with Fig. 5. Under "dry" conditions a sharp increase in microhardness was obtained, with its ultimate value attained at a total reduction of 19%, the microhardness remaining practically unaffected by subsequent heavy reductions. Under lubricated conditions the microhardness increase was more gradual leading,
o rr=O*/o ~, rT=22°/o mrT=3801,
140
x~ d c "o o to
%
~2o
" ~ r =5 B*/o • r~=81"/0
\
100 "2
I[ Cu i
\
At
'-
4
0
'
'
Distcmce a c r o s s thickness(rnrn) (Q)
ofT=O% D~ I
c "o
- rT=19°/o o rT=/,O*/, o~58"/, • rT=BO°/,
140
120
\
/
o t-
\
1-..
/
100
~
Cu i
i
2
Oistonce
A~
i
i
0 across
i
i
2
/.
thickness(ram)
(b)
Fig. 9. Variation of microhardness through the thickness of explosively cladded A1/Cu bimetallic plates rolled with various total thickness reductions, under: (a) lubricated conditions; and (b) "dry" conditions.
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99 117
115
however, to the same limiting value, see Fig. 9(b). The increase in microhardness in the aluminium layer was quite moderate under "dry" conditions, whilst under lubricated conditions "softening" was revealed at the outside surface of the aluminium layer in contact with the roll and also at the adjacent subsurface layers. Inhomogeneity of deformation and the energy stored during rolling together with the residual stresses developed may account for this phenomenon, see also [10]. In general, the directional mechanical process of rolling does not impart any other impressive surface alterations. Quantitatively, these changes are expressed by the surface-roughness measurements summarised in Table 2. With all other rolling parameters kept constant, only the thickness reduction was considered to affect the surface finish. It was found that with increasing total thickness reduction, rT, the surface roughness, Ra, decreases, showing a tendency to approximating the roll surface roughness: see also the similar remarks reported in [10] for the rolling of steels. The variation of roll force and torque with total thickness reduction for a number of "dry" roll passes is presented in Fig. 10 reproduced from [15]. It is shown that both total roll force and total roll torque increase with increasing total thickness reduction, following a slight parabolic law: however, the effect of work-hardening for the materials under consideration is not evident as in the case of the rolling of steels [10]. Moreover, rolling with lubrication leads, in general, to a reduction of roll force, see Table 2. A theoretical prediction of roll force, FR, and torque, TR, can be made based on the "two-layer sandwich" approach of the rolling of bimetallics presented in [15]. For
Total r o l l
force
0 0
0
0
F, (kN)
1
1
T
0 0
i }>
.-.t 0
N
pass 1
Q
"~,e~
pa s ses
c
1-2
< ~ e ~ p o sses
%
3
1-3
~,®
-
\ \ ~
o
7
o° ~ Total r o l l
torque
TR.(kN.m )
Fig. 10. Variation of roll force FR, and roll torque TR, with total thickness reduction rr, of AI/Cu rolled bimetallics, for a number of passes.
116
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
each rolling pass a mean yield stress, Y* is calculated from Eq. (1). By inserting these values in the equations derived for roll force and torque, the magnitudes of FR and TR are obtained, see [15] for details. Calculated values for all "dry" rolling passes performed are tabulated in Table 2 and plotted in Fig. 10, from which it is noted that, in general, the measured and calculated values are in agreement to within ± 6%.
4. Conclusions
Summarising the main features outlined above on the influence of the explosive cladding and rolling parameters on the integrity of fabricated bimetallics, the following conclusions may be drawn: (a) By increasing the collision angle during explosive cladding through the control of the initial inclination, more stable detonation characteristics were obtained, leading to the fabrication of bimetallic plates with more regular wave pattern and better bonding integrity at the interface between the two layers of the composite plates. (b) High energy explosives such as Paxit and Nitrammite were more effective than ANFO for the explosive cladding of dissimilar metals such as aluminium and copper. (c) Characteristic damage phenomena of explosive cladding, such as unbonded areas, bond breakage, the formation of brittle intermetallic compounds at the interface and a generalised shock-hardening, were observed for the metals and the cladding conditions examined. Therefore, a useful indication regarding the selection of the proper processing parameters to fabricate sound components was attained. (d) The application of cold rolling as a post-cladding forming operation requires a careful selection of the rolling variables, the lubricants and the roll-pass schedule, in order to improve the soundness of the bimetallic strips. The influence of rolling parameters on the microstructural properties of the resulted bimetallics was evaluated in terms of "surface integrity". (e) Defects encountered in the rolling of bimetallics, such as curving and alligatoring, could be minimized by selecting appropriate operational parameters, i.e. by avoiding both very small and relatively high reductions, for the materials and the processing conditions examined. (f) The total roll force and torque after a number of passes increases with increasing total thickness reduction, following a parabolic law, whilst lubrication results, in general, in the reduction or roll force and torque. Experimental results pertaining to roll force and torque as well as to the overall strength of the composite plate are in good agreement with theoretical predictions arising from a "two-layer sandwich" modelling of the rolling of bimetallics.
References [1] D.W. Hawes and D.R. Hay, Explosive welding-mechanism and applications, Eng. Digest, 5 (1977) 21 24. [2] B. Crossland, Explosive Welding of Metals and its Applications, Clarendon Press, Oxford, 1982.
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99-117
117
[3] R.A. Patterson, Explosive bonding, in: D.L. Olson, R. Dixon and A.L. Liby (Eds.), Welding: Theory and Practice, Elsevier, Amsterdam, 1990, pp. 265-291. 1,4] R.A. Pruemmer and D. Stoeckel, Thermobimetallic effect in explosively cladded Mo-inconel compound, in: L.E. Murr, K.P. Staudhammer and M.A. Meyers (Eds.), Metalluroical Applications of Shock Wave and High-Strain Rate Phenomena, Marcel Dekker, New York, 1986, pp. 877 886. [5] S. Kuzmanovic, Z. Cligoric and B. Beatovic, Properties of bimetallic bar as transition element in shipbuilding industry, in: Proc. lOth Int. Conf. on HERF, Sept. 18-22, 1989, Ljubljana, Yugoslavia, pp. 774 784. [6] V.D. Linse and N.S. Lalwaney, Explosive welding, J. Metals, 36(5)(1984)62-65. 1,7] T.Z. Blazynski (Ed.), Explosive Welding, Forming, and Compaction, Applied Science Publishers, Barking, 1983. [8] R. Hardwick and F. Weld, Some more recent advances in cladding technology, in: I.A. Yakovlev and V.F. Nesterenko (Eds.), Proc. 9th Int. Conf. on HERF, Aug. 18 22, 1986, Novosibirsk, pp. 270-277. [9] P.V. Vaidyanathan and A.R. Ramanathan, Design for quality explosive welding, J. Mat. Proc. Techn., 32 (1992) 439-448. 1'10] A.G. Mamalis, N.M. Vaxevanidis and A.P. Karafillis, Surface Integrity and Formability of Steel Sheet, VDI Verlag, D~sseldorf, 1990. [11] B. Crossland and J.A. Cave, The explosive properties of ammonium nitrate fuel oil mixtures, in: Proc. 5th Int. Conf. on HERF, Denver, Co, USA, 1975, pp. 4.9.1 4.9.11. [12] K. Pohlandt and K. Lange, Recommendations for an unified upsetting test for determining flow curves, Ann. CIRP, 38(2) (1989) 681-682. 1-13] A. Szecket, D.J. Vigueras and O.T. Inal, The cyclic pressure distribution of explosively welded interfaces, in: L.E. Murr, K.P. Staudhammer and M.A. Meyers (Eds.), Metallurgical Applications of Shock Wave and High-Strain Rate Phenomena, Marcel Dekker, New York, 1986, pp. 887-903. 1-14] A.G. Mamalis and W. Johnson, Defects in the processing of metals and composites, in: M. Predeleanou (Ed.), Computational Methods for Predictin9 Material Processin9 Defects, Elsevier, Amsterdam, 1987, pp. 231-250. [15] A.G. Mamalis, N.M. Vaxevanidis and D.I. Pantelis, On the rolling of bimetallic explosively cladded plates, in: Proc. 4th lnt. Conf. on Techn. of Plasticity, Sept. 5-12 1993, Beijing, China, pp. 874 879.
Journal of Materials Processing Technology 44 (1994) 99-117
Journal of Materials Processing Technology
Fabrication of aluminium/copper bimetallics by explosive cladding and rolling A.G. M a m a l i s a'*, N . M . Vaxevanidis a, A. Szalay b, J. P r o h a s z k a c aDept, of Mechanical Engineering, National Technical University of Athens, 42, 28th October Ave., 10682 Athens, Greece METALLTECH Ltd, Budapest, Hungary ¢Hungarian Academy of Sciences, Budapest, Hungary (Received June 21 1993; accepted December 8 1993)
Industrial Summary In the present paper the authors report on the fabrication of bimetallic components consisting of aluminium and copper plates by explosive cladding and subsequent rolling. Such components are used extensively in the electrical and ship-building industries as well as in vessel design, replacing components made from solid materials. The effect of the cladding and the rolling parameters on the sound fabrication and the micro-structural properties of the resulting bimetallics is evaluated in terms of "surface integrity", i.e. surface topography, microhardness variation and metallurgical changes through the thickness of the cladded/rolled plates, and the existence and/or intensification of particular defects encountered in such processing.
Notation Ek
FR hx hAl hcu
he HV rT r'Al rCu /*i
energy density of the flyer plate roll force thickness of the bimetallic thickness of the aluminium layer thickness of the copper layer thickness of the explosive microhardness total thickness-reduction ratio of the bimetallic total thickness-reduction ratio of the aluminium layer total thickness-reduction ratio of the copper layer thickness reduction-ratio of the bimetallic after ith pass
* Corresponding author. 0924-0136/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 3 ) E 0 1 4 4 - 6
100
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99--- 117
R Ra
welding impact ratio mean surface roughness surface roughness of the aluminium layer surface roughness of the copper layer roll torque detonation velocity velocity of the re-entrant jet velocity of the flyer plate width of the aluminium layer width of the copper layer mean yield stress initial set-up inclination angle collision angle
RAt Rcu
TR Ud Up WAI WCu
Y a
1. Introduction
Explosive cladding is the process of forming a bond by explosively impacting a metallic mass on to another such mass under controlled conditions. Various techniques have been developed over the years to meet a steadily growing field of application and for solving particular industrial problems. The explosive-cladding process achieves excellent bonding of similar or dissimilar metals with different hardness, melting points, thermal-expansion characteristics and electrode potentials. Furthermore, the process can be applied to a broad range of thicknesses and area dimensions due to the ability to distribute the high energy of the explosive over the entire welding areas I-1,2]. In modern technology there is an increasing demand for the use of composite metal laminates instead of solid materials due to the need for corrosion resistance at lower cost, improved heat-transfer characteristics, strength and/or stiffness, improved electrical properties and better abrasion- or erosion-resistance. Applications of explosive bonding are therefore numerous and range from the production of "sandwich" plates for coinage to the more sophisticated use of titanium-to-stainless-steel transition joints in spacecraft [-3]. The most common utilization of explosive bonding is in the production of fiat plate-clads from an extremely large number of combinations of dissimilar metals. Explosively cladded plates combining the properties of the constituent metals with a high bond-integrity provide many opportunities for innovative and economical design in the fabrication of thermo-bimetals I-4], transition joints for ship-building applications [5] and clad systems in vessel design 1,6]. Other applications have been investigated but not exploited; see the extensive lists of published work on the topic in [-2, 3, 6-8]. For the manufacturing of near-net-shape bimetallic components, explosive welding is usually followed by hot/cold rolling and/or punching. However, particular difficulties and limitation arise from such processing due mainly to metallurgical problems and defects encountered in explosive cladding, see for example I-2,9]. Extensive experimental research and testing is needed in order to achieve acceptable bonding
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99-117
101
integrity, sufficient overall strength and improved performance under severe working conditions for composite components fabricated by explosive cladding and forming. The present work is the first step of an investigation regarding the possibilities and limitations of successfully applying the above-described processing for the fabrication of near-net-shape bimetallic components. In this paper the authors report on the fabrication of sound bimetallic components consisting of aluminium and copper plates by explosive cladding and subsequent rolling. Such components can be used as electrical transition joints in high current-density electrical systems and as simple transition joints in machine-supporting structures offering improved stiffness and better corrosion resistance. The overall strength of the fabricated bimetallics, on a macroscopic scale, is determined through the upsetting test. Moreover, the influence of the cladding/rolling parameters on the micro-structural properties of the fabricated bimetallics is evaluated in terms of "surface integrity" [10], i.e. surface topography, microhardness variation and metallurgical changes through the thickness and at the interface of the cladded/rolled plates: the existence and/or intensification of particular defects encountered in such processing reveals useful information about the soundness of the fabricated bimetallics.
2. Experimental 2.1. Equipment and procedure 2.1.1. Explosive cladding A detailed description of the equipment and procedure used for the explosive cladding of AI/Cu plates is given in [2,3], and therefore only the main features are outlined below. The flyer plate used was made from a 5000 series aluminium alloy containing 1% magnesium and 0.7% iron, whilst the parent plate was fabricated from a commercial batch of copper containing 0.01% phosphorus. The initial thickness of the plates was 4 mm, whilst their average surface roughness, Ra, values were 0.33 ~tm and 0.27 ~tm for the aluminium and the copper, respectively. Three different explosives were used namely: (i) ANFO (Ammonium Nitrate/Fuel Oil 6%) explosive in granular form, with a packing density of 0.85 x 10a kg/m 3 and a low detonation velocity which mainly depends upon the thickness of the charge layer. Because of its granular form, an unstable detonation front may be formed and, therefore, in order to achieve stable detonation characteristics within a short distance from the point of ignition it was necessary to use a booster charge of some high energy explosive, for instance a PETN (pentaerythritol tetranitrate) pellet [2]. The detonation characteristics of ANFO explosive are given in [11]. (ii) Nitrammite, which is a high ammonium nitrate, detonator-sensitive powder explosive, its thin granular form resulting in a more stable and uniform wave-front
102
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99 117
formation. Its detonation velocity, measured by the Dautriche method, was 3500 m/s at an initial density of 0.98 x 103 kg/m 3. (iii) Paxit (Ammonium Nitrate 77%, TNT 19%), which is a high-energy explosive with a detonation velocity of 4000 m/s at an initial density of 103 kg/m 3, its detonation characteristics being similar to those exhibited by Nitrammite. The experimental set-up for the explosive cladding is shown schematically in Fig. 1 and was used both for the parallel (a = 0 °) and the inclined (a > 0 °) arrangements. ANFO explosive was used together with a high explosive booster (PETN), the firing of the explosive taking place away from the plates in order to obtain stable detonation characteristics and to avoid destruction of the metal plate. Contrarily, when Paxit or Nitrammite were used the firing of the explosive took place in the vicinity of the plates and a high explosive booster was not needed. The accelerating mass of the experimental device consisted of the flyer and the driver plates, the latter being made of mild steel of 1-4 mm thickness, and essentially used for controlling the impact velocity and avoiding major cladding defects. The stand-off distance, see Fig. 1, which greatly affects the impact energy, was selected as one and a half times the thickness of the flyer and driver plates when ANFO was used, reduced to a distance equal to the flyer plate thickness when Paxit or Nitrammite were used. The cladding variables for the present series of experiments, calculated from the detonation velocity and the plates characteristics according to standard techniques [2], are summarized in Table 1.
2.1.2. Rolling After explosive cladding, strips of dimensions 70 x 30 mm, cut from the clad bimetallic plates, were cold rolled in a succession of passes in order to examine the influence of the rolling parameters on the mechanical properties and structural integrity of the bimetallic plates.
Geometry of the collapse of the fryer plate (detail A) stand-off distance ",\ \
fuse detonotor/
exptosive ~
I
~~ J~vp I I
~ / ~ S t
i
/ °
Ol
driver p[ote "+-~
,e, -uo
,oo<
Fig L Experimentalset-upfor explosivecladding
o<
ANFO ANFO ANFO ANFO ANFO ANFO Paxit Nitrammite Nitrammite
215x70xl.6 260 x100 x 4.1 200x60x2.9 200x60x2.9 200x60x2.9 200x58xl.0 210x60x1.3 210x60x1.3
Driver plate (St)
215 x 7 0 x 5 215x70x5 215×70x5 215x70x5 215x70x5 215x70x5 180x45x5 200×50×4 200x50x4
Flyer plate (Al)
Dimensions (mm)
0 0 0 3 6 9 0 0 7
~(°)
Initial angle
Dimensions of the parent plate (Cu) : 210 x 60 x 4 (mm)
1 2 3 4 5 6 7 8 9
Explosive
Table 1
50 40 40 40 40 40 16 18 18
Thickness of explosive h (mm)
1.05
1.05
1.01
1.05 1.05
2.84 1.31 0.86 1.05
R
Welding impact ratio
2298 2035 2035 2035 2035 2035 4000 3500 3500
Detonation velocity vd (m/s)
611 413 315 359 359 359 573 520 520
Velocity of the flyer plate Vp (m/s) 15.3 11.6 8.9 13.1 16.1 19.1 8.2 8.5 15.5
#(°)
Collision angle
2.49 2.21 3.40 2.80 2.80 2.80 3.61 3.22 3.22
Ek(J/m 2 × 10 6)
Energy density of the flyer plate
4596 4070 4070 3171 2557 2155 8000 7000 3844
Velocity of the re-entrant jet v, (m/s)
I
",-4
Thickness of aluminium layer hA~(mm)
4.10 3.72 3.16 2.54 2.13 1.59 1.23 0.74
8.45 7.19 6.81 5.75 5.05 4.12 3.52 2.55 1.65
4.18 3.50 3.29 2.83 2.43 2.05 1.70 1.23 0.80
"Dry" conditions
8.30 7.46 6.48 5.18 4.53 3.46 2.44 1.60
Lubricated conditions
Thickness of bimetallic hr (mm)
Table 2
4.27 3.69 3.52 2.92 2.62 2.08 1.82 1.32 0.85
4.19 3.75 3.32 2.64 2.40 1.87 1.31 0.86
Thickness of copper layer h~ (mm)
0 15 19 32 40 51 58 70 80
0 11 22 38 45 58 71 81
Total thickness reduction of bimetallic rx (%)
0 14.9 5.3 15.6 12.2 18.4 14.6 27.5 35.3
0 11 13 20.6 12.5 23.6 29.5 34.4
ri (%)
Thickness reduction of bimetallic per pass
0 16.3 21.3 32.4 41.8 51 59.3 70 80.9
0 9.5 21 38.2 48.2 61.2 70 81.9
Total reduction of aluminium layer rA1 (%)
0 13.6 17.6 31.5 38.7 51.3 57.3 68.7 80.1
0 10.7 21 37 42.7 55.5 68.7 79.6
Total reduction of copper layer r~ (%)
29.20 29.70 29.80 30.00 30.15 30.45 30.65 31.15 31.50
28.25 29.30 29.45 29.85 30.20 30.50 30.90 31.30
Width of the aluminium layer WAI(mm)
29.50 29.65 29.70 29.85 30.00 30.20 30.35 30.65 31.75
29.25 29.45 29.45 29.70 30.00 30.30 30.80 31.45
Width of the copper layer w~ (mm)
Roll torque per pass TR (Nm)
0 75 61 93 88 96 91 112 127
0 70 78 84 81 99 106 119
-
-
0 0 0 76 578 560 59 368 348 96 919 905 87 688 688 98 914 902 94 710 699 116 1004 998 128 1 1 0 8 1 0 8 6
-
485 592 847 644 918 965 1006
-
meas. calc. meas. calc.
Roll force per pass FR (kN)
0.29 0.24 0.21 0.19 0.15 0.13 0.12
0.21 0.18 0.14
Surface roughness of the copper layer Re. (~tm)
0.33 0.29 0.25 0.24
-
-
Surface roughness of the aluminium layer RAI (lam)
-.q
I
4~ at~
e~
e~
g~
4~
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
105
Rolling of the rectangular plates was performed on an experimental 2-high rolling mill, properly instrumented for roll force and torque measurements, at a constant speed of 4 m/min between two steel rolls of 200 mm diameter and 100 mm barrel length, see [10] for details. Two series of rolling passes were performed, the first under "dry" conditions and the second under lubricated, in the latter case Mobil Prosol 36 emulsion and Shell Sol D100 kerosene being used as lubricants for the copper and aluminium surfaces of the bimetallic strip, respectively. The rolling direction was opposite to that of the cladding direction. Rolling parameters, force and torque measurements and surface-roughness values for all strips rolled are tabulated in Table 2.
2.2. Measuring techniques For the evaluation of the mechanical properties of the cladded bimetallics, the upsetting test was used, upsetting tests being performed on a Schuller hydraulic press connected with a data acquisition system. The velocity of the upsetting anvil was 5 mm/min, the whole process being fully automated and controlled through a computer, providing an overall strain-rate estimated to be 16 x 10- 3 s- 1. For the preparation of the test specimens and the testing procedure, the recommendations of 1-12] were followed. Small cylinders with a height-to-diameter ratio, hiD = 1.5, were cut from the composite plates, their height being parallel to the cladding direction. The flow stress-strain curves obtained for the bimetallics under consideration as well as the relevant stress-strain curves of the initial test materials compressed under the same conditions are presented in Fig. 2, from which the mean yield stress was estimated to be 301,320 and 278 N/mm 2 for the A1/Cu cladded composite and the AI and Cu initial materials, respectively.
500 aLuminium "E E
400
z
300
bimetat
tic
200
~oo
0
I
I
I
0.2
0.4
0.6
Stral
n
Fig. 2. Compressive stress-strain curves for the initial materials and the fabricated cladded bimetallic plate.
106
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99 117
In order to examine the weld quality and the microstructural changes at the interface after cladding, the fabricated bimetallics were sectioned in planes parallel to the detonation direction and standard metallographic preparation and etching procedures were applied. During rolling, after each pass, the total thickness reduction of the strip as well as the reduction of the aluminium and the copper layers were measured using an optical microscope with a micromeasuring device. Changes in specimens width (spread) were monitored also. The microstructural changes of the cladded samples were observed under a metallographic microscope (Union Optical Co) whilst microhardness testing was performed on a Leitz microscope equipped with a Vickers indenter under a testing load of 1N, the microhardness values obtained being the average of five indentations. A "Talysurf" (Taylor Hobson) recorder was used to measure the centre-line average, Ra, surface roughness, the cut-offlength being selected as 0.8 mm whilst the roughness values were the average of at least ten measurements per specimen.
3. Results and discussion
3.1. Explosive cladding Explosive bonding is accomplished by a high velocity oblique impact between two metals. Due to this impact resulting from the detonation waves, the metal plates behave hydrodynamically and come into intimate contact, thus promoting solid-state bonding [3]. Typical of the explosive bonding is a region of severe plastic deformation restricted to a narrow band near to weld interface. Several mechanisms of interface bonding have been proposed, (for a brief review, see [2, 3, 13]) and three distinct types of bond morphologies have been identified, namely a laminar interface (a flat bond), a wavy interface and/or a molten-layer interface [3,9]. In general, under well-controlled cladding conditons, the explosively cladded plates are expected to be adequately bonded over the entire area of the bimetallic composite, except for small areas near to their edges [2,6]. However, during the present investigation, not only patterns encountered often in explosively welded plates with very different densities, but also certain features and defects dependent upon the explosion/collision variables and the kind of explosive used, were identified: these are discussed below (see also [9, 13, t4]). When the driver plate did not overlap the flyer plate or in the absence of the former, spalling of the later occurred at the outer edge of the plate due to reflected tensile waves. Such spalling, together with rather large unbonded areas, leads to defective bimetallic plates (Experiment 1 in Table 1). Moreover, the parallel arrangement of the experimental set-up with the use of the ANFO explosive, (Experiments 2 and 3 in Table 1) resulted, in general, in cladded plates with poor wave formation and large scale defects. At the inner edge of the bimetallic plate, an unbonded area was identified, the length of this area varying from 6 to 10 mm, followed by a zone of
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
107
waveless bonding of approximately the same length. The amount of the intermetallic compound was found to increase and in the middle of the plate, lengthwise, a continuous intermetallic layer was observed, with cracks spreading transversaUy to the interface of the cladded plates, see Fig. 3(a). This zone is also characterized by poor asymmetrical wave formation, with an average wavelength of 1200 txm along the interface, and large cooling cavities surrounded by entrapped intermetallics, see Fig. 3(b). Towards the distal end of the plate the presence of cracks and cavities was intensified, the intermetallic layer appearing to be discontinuous with an increased
Fig. 3. Microstructuralcharacteristics of explosivelycladded aluminium/copperplates using ANFO explosive: (a) and (b), parallel arrangement; (c) and (d), 6° inclined arrangement.
108
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
thickness, whilst the interfacial bonding was reduced, leading to bond breakage near to the outer edge of the plates, see also 1-14]. In order to achieve more stable detonation/collision characteristics for obtaining better bonding integrity, the angle of initial incidence, a, see Fig. 1, was increased from 0 ° to fixed values of 3 °, 6 ° and 9 °. This modification results in an analogous increase of the collision angle, B, which in turns determines the bonding quality at the interface through the jet entrapping mechanism during cladding, according to the physical modelling of the process proposed in [-2,13]. Almost identical microstructural characteristics were revealed for the bimetallic plates cladded with the ANFO explosive and the inclined arrangement (Experiments 4-6 in Table 1). In addition to an unbonded area of length of about 20 mm observed near the ignition point, a "sea wave"-like pattern at the copper interface was revealed, together with a discontinuous layer of intermetallic of thickness varying from 20 to 30 Ixm, see Fig. 3(c), extending for about 60 mm lengthwise along the detonation direction. This then changed to a mixed mode of interfacial structure consisting of interrupted laminates of intermetallics entrapped inside the front vortices of waves in the copper of about 40-60 pm thickness and regions of direct A1/Cu bonding, see Fig. 3(d). Bonding failure occurred at a distance approximately 10 mm from the outer edge of the strip, an overall length of about 100 mm being considered to be of a sound cladding/bonding integrity from a microscopic point of view. Plates were also cladded using high energy explosives, i.e. Nitrammite or Paxit; see Experiments 7-9 in Table 2. Cladding of plates using the parallel arrangement (a = 0 °) resulted in similar characteristics for both explosives. A very thin continuous intermetallic layer at the interface was formed starting at about 15 mm from the ignition point and spreading along the detonation direction, its thickness increasing, see Fig. 4(a): the estimated average thickness was 80 and 60 ~tm for Nitrammite and Paxit explosives, respectively. Cooling cavities and non-molten copper islands inside the intermetallic were identified in both cases. At a distance of 150-160 mm from the ignition point the intermetallic layer started to delaminate through cracking, leading to interface failure, see Fig. 4(b), a sound welded length of 110-120 mm being obtained. Typical structures at the AI/Cu boundary for the bimetallic plate cladded using a 7 ° inclination angle and Nitrammite are presented in Figs. 4(c) and (d). In this case the first welded zone is characterized by direct A1/Cu bonding without any intermetallic compounds. Note that such a wave-form is similar to typical patterns expected during the explosive cladding of dissimilar metals [2,6]. Towards the detonation direction front, vortices developed gradually, their intensity being more severe with increasing distance from the ignition point. The intermetallic compound was, in general, discontinuous a n d appeared around the front vortex. Non-molten aluminium as well as copper from the parent plate were sometimes found entrapped inside the intermetallic islands, see Fig. 4(c). Towards the free end of the plate, cooling cavities around the vortices were intensified, see Fig. 4(d), accompanied by intense microcracking. Due to the occurrence of these phenomena, the bonding strength is weakened, leading to failure at the interface and to the separation of the metallic plates near their extremities. Sound bonded zones of 130 mm in length were obtained from the present series of experiments.
109
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
i ntermetatlic i
entrapped
crack
copper
!
I1 O0 IJm
cooling
~n t e r r n e t a l l i c
•
~
'
i
i¸¸
c/a , v - i t y
i
~;
!
,
Fig. 4. Microstructural characteristics of explosivelycladded aluminium/copper plates using Nitrammite explosive: (a) and (b), parallel arrangement; (c) and (d) 7° inclined arrangement.
The boundary zones observed during the present investigation and discussed above are mainly dependent upon the cladding conditions pertaining at particular points and upon their distance from the ignition point. The existence of a thin cast interlayer was verified in most cases: however, it is difficult to judge whether it was a finely dispersed mixture of particles of the two metals or a pure metastable metallic compound. Microprobe analysis revealed that this interlayer consisted of 68%
110
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
aluminium and 32% copper, corresponding to an eutectic constituent of the A1-Cu system at 548°C. During explosive cladding, due to stress waves and the intense thermal impact, both plates experienced a hardness increase and/or a generalised shock-hardening [2]. A typical microhardness profile of an A1/Cu cladded plate is presented in Fig. 5. Due to the high velocity explosive used, the flyer plate experienced a greater hardness increase than the parent plate. The average hardness of the aluminium plate, measured after cladding, was 100 HV, i.e., about twice that of the initial unworked material (44 HV), whilst for the copper parent plate a slight increase from an initial value of 103 HV to a terminal value of 110 HV was obtained. In addition to the overall increase in hardness, localised hardening at the interface occurred, see Fig. 5, which may be attributed to intense shock pressure on collision and/or to severe plastic deformation in the region of the welded interface. As far as the overall strength of the cladded composites is concerned, from Fig. 2 it may be concluded that a macroscopic acceptable weld was produced, since the cladded plate is stronger, at least, than the weaker of the two components [2]. The mean yield stress of the bimetallic material, modelled as a rigid-perfectly plastic material, can be obtained also from the "two-layer sandwich" modelling of the rolling of bimetallics proposed by the authors; see [15]. Denoting Ya and Yb as the mean yield stress and ha and h b a s the thicknesses of the test materials, it is shown [15] that the mean yield stress of the cladded bimetallic plate, Y* may be derived as: y,_
Yaha-F Ybhb
(1)
ha + hb
Substitution of the corresponding measured values in Eq. (1) yields Y* = 299 N/mm 2, which is almost identical with the experimentally determined value (Y = 301 N/mm2).
125
AL
Cu
> -r
-o-o
--o--o
o-
-o
--
-o-
75 c
2
or-
2
v-v-v-v--v-
v - v
25
inltiat ~nterfoce
- -
mlcro h a rd n e ss
2
2 Distance across
thickness
(ram)
Fig. 5. Typical microhardness profiles of aluminium/copper explosively cladded bimetallics.
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
111
3.2. Rolling
Multiple-pass cold rolling was selected as the post-welding forming operation for shaping cladded plates to their final dimensions. It must be noted that the bimetallic plates were not subjected to stress-relieving heat treatment either before rolling or at intermediate stages between the various passes. In general, failures occurring during compression-like working processes such as rolling are caused predominantly by secondary tensile stresses [,-14]. Moreover, problems and difficulties arising from the previous process, i.e. the explosive cladding, associated with particular structural changes and material properties of the bimetallic plate are sometimes intensified during subsequent rolling. Rolling in passes with a thickness reduction of the bimetallic strip of less than 10% per pass proved to be unsuccessful, since after a first or a second pass, using a thickness reduction of 5-8%, the rolled strip started to split along the middle horizontal plane on the exit side of the roll gap, with the two layers tending to follow the rotation of the respective rolls, i.e. alligatoring occurred, see Fig. 6. This behaviour was accentuated by metallurgical weakness along the centre-line (interface) of the bimetallic strip caused by defects inherent in explosive cladding, leading therefore to complete separation of the two metallic layers during the subsequent rolling pass: see the above discussion and also [,-14]. To overcome this problem, heavy passes were used, with the plastically deformed zone spreading across the whole thickness of the strip [-10], the results obtained being tabulated in Table 2. Since the elongation of the aluminium layer of the bimetallic strip was greater than that of the copper layer, the rolled strips were slightly curved, with the aluminium layer becoming convex, see Fig. 7(a). In addition, distorted end-shapes were developed during the rolling of bimetallic strips. Due to tail-end folding in the direction of the thickness, overlapping was observed at the rear end of the strips, see Fig. 7(b),
at I. ig a t o r i ng Fig. 6. Alligatoringoccurring during the rolling of explosivelycladded bimetallics at small thickness reduction (rl = 5%).
112
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
AL
t
i 50mrn
(b)
l
rolli
t
ng
20mm
direction
Fig. 7. Common defects encountered in the rolling of cladded aluminium/copper bimetallics: (a) curving due to different elongation; and (b) overlapping at the rear end.
whilst a similar defect, i.e. overhanging, was developed in the front end of the strips, see also [14]. Due to the different mechanical properties of the materials of the two layers of the bimetallic strip, differential lateral expansion, i.e. spread, occurred. In general, the spread of the aluminium layer was greater in both cases, the spread increasing with increasing total reduction in thickness, with the difference being less significant under lubricated conditions. Moreover, the thickness reduction of the aluminum layer was systematically greater, see Table 2. Note that spread not only affects the dimensional accuracy of the bimetallic component to be fabricated but also, through the
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99-117
113
development of secondary tensile stresses, may lead to edge cracking, especially in metals of limited ductility, such as the explosively cladded components: such a characteristic defect is demonstrated in Figs. 8(a) and (b), see also [14]. Furthermore, the residual tensile stresses developed in both layers near to the interface [15], enhanced by the inhomogeneous deformation, are superimposed onto metallurgical weakness at
50pm
(o)
I
I
edge-crocks 20mm (b)
I
stip
l
tines 200pm I
(c}
t
intermetottic Fig. 8. Defects encountered in the rolling of explosively cladded AI/Cu bimetallics after heavy reductions: (a) edge cracking (rr = 58%); (b) centre-split, bond breakage and edge cracking (rr = 80%); and (c) slip-lines at the interface of the rolled bimetallic strip (rr = 40%).
114
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
this zone caused by explosive cladding [9,14], leading to centre-splits and laminations, as shown in Fig. 8(b). Metallographic examination also revealed shear-band formation at the interface of the two metals, i.e. the development of slip-lines, see Fig. 8(c), such a phenomenon contributing to intermetallic-compound fragmentation and to the generation of microvoids at these intense shear zones. The variation of microhardness through the thickness of the rolled bimetallic strips is illustrated in Figs. 9(a) and (b). For the copper layer, work-hardening results in a clear hardness increase compared to the hardness of the initial bimetallic plate: compare also with Fig. 5. Under "dry" conditions a sharp increase in microhardness was obtained, with its ultimate value attained at a total reduction of 19%, the microhardness remaining practically unaffected by subsequent heavy reductions. Under lubricated conditions the microhardness increase was more gradual leading,
o rr=O*/o ~, rT=22°/o mrT=3801,
140
x~ d c "o o to
%
~2o
" ~ r =5 B*/o • r~=81"/0
\
100 "2
I[ Cu i
\
At
'-
4
0
'
'
Distcmce a c r o s s thickness(rnrn) (Q)
ofT=O% D~ I
c "o
- rT=19°/o o rT=/,O*/, o~58"/, • rT=BO°/,
140
120
\
/
o t-
\
1-..
/
100
~
Cu i
i
2
Oistonce
A~
i
i
0 across
i
i
2
/.
thickness(ram)
(b)
Fig. 9. Variation of microhardness through the thickness of explosively cladded A1/Cu bimetallic plates rolled with various total thickness reductions, under: (a) lubricated conditions; and (b) "dry" conditions.
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99 117
115
however, to the same limiting value, see Fig. 9(b). The increase in microhardness in the aluminium layer was quite moderate under "dry" conditions, whilst under lubricated conditions "softening" was revealed at the outside surface of the aluminium layer in contact with the roll and also at the adjacent subsurface layers. Inhomogeneity of deformation and the energy stored during rolling together with the residual stresses developed may account for this phenomenon, see also [10]. In general, the directional mechanical process of rolling does not impart any other impressive surface alterations. Quantitatively, these changes are expressed by the surface-roughness measurements summarised in Table 2. With all other rolling parameters kept constant, only the thickness reduction was considered to affect the surface finish. It was found that with increasing total thickness reduction, rT, the surface roughness, Ra, decreases, showing a tendency to approximating the roll surface roughness: see also the similar remarks reported in [10] for the rolling of steels. The variation of roll force and torque with total thickness reduction for a number of "dry" roll passes is presented in Fig. 10 reproduced from [15]. It is shown that both total roll force and total roll torque increase with increasing total thickness reduction, following a slight parabolic law: however, the effect of work-hardening for the materials under consideration is not evident as in the case of the rolling of steels [10]. Moreover, rolling with lubrication leads, in general, to a reduction of roll force, see Table 2. A theoretical prediction of roll force, FR, and torque, TR, can be made based on the "two-layer sandwich" approach of the rolling of bimetallics presented in [15]. For
Total r o l l
force
0 0
0
0
F, (kN)
1
1
T
0 0
i }>
.-.t 0
N
pass 1
Q
"~,e~
pa s ses
c
1-2
< ~ e ~ p o sses
%
3
1-3
~,®
-
\ \ ~
o
7
o° ~ Total r o l l
torque
TR.(kN.m )
Fig. 10. Variation of roll force FR, and roll torque TR, with total thickness reduction rr, of AI/Cu rolled bimetallics, for a number of passes.
116
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99- 117
each rolling pass a mean yield stress, Y* is calculated from Eq. (1). By inserting these values in the equations derived for roll force and torque, the magnitudes of FR and TR are obtained, see [15] for details. Calculated values for all "dry" rolling passes performed are tabulated in Table 2 and plotted in Fig. 10, from which it is noted that, in general, the measured and calculated values are in agreement to within ± 6%.
4. Conclusions
Summarising the main features outlined above on the influence of the explosive cladding and rolling parameters on the integrity of fabricated bimetallics, the following conclusions may be drawn: (a) By increasing the collision angle during explosive cladding through the control of the initial inclination, more stable detonation characteristics were obtained, leading to the fabrication of bimetallic plates with more regular wave pattern and better bonding integrity at the interface between the two layers of the composite plates. (b) High energy explosives such as Paxit and Nitrammite were more effective than ANFO for the explosive cladding of dissimilar metals such as aluminium and copper. (c) Characteristic damage phenomena of explosive cladding, such as unbonded areas, bond breakage, the formation of brittle intermetallic compounds at the interface and a generalised shock-hardening, were observed for the metals and the cladding conditions examined. Therefore, a useful indication regarding the selection of the proper processing parameters to fabricate sound components was attained. (d) The application of cold rolling as a post-cladding forming operation requires a careful selection of the rolling variables, the lubricants and the roll-pass schedule, in order to improve the soundness of the bimetallic strips. The influence of rolling parameters on the microstructural properties of the resulted bimetallics was evaluated in terms of "surface integrity". (e) Defects encountered in the rolling of bimetallics, such as curving and alligatoring, could be minimized by selecting appropriate operational parameters, i.e. by avoiding both very small and relatively high reductions, for the materials and the processing conditions examined. (f) The total roll force and torque after a number of passes increases with increasing total thickness reduction, following a parabolic law, whilst lubrication results, in general, in the reduction or roll force and torque. Experimental results pertaining to roll force and torque as well as to the overall strength of the composite plate are in good agreement with theoretical predictions arising from a "two-layer sandwich" modelling of the rolling of bimetallics.
References [1] D.W. Hawes and D.R. Hay, Explosive welding-mechanism and applications, Eng. Digest, 5 (1977) 21 24. [2] B. Crossland, Explosive Welding of Metals and its Applications, Clarendon Press, Oxford, 1982.
A.G. Mamalis et al./Journal of Materials Processing Technology 44 (1994) 99-117
117
[3] R.A. Patterson, Explosive bonding, in: D.L. Olson, R. Dixon and A.L. Liby (Eds.), Welding: Theory and Practice, Elsevier, Amsterdam, 1990, pp. 265-291. 1,4] R.A. Pruemmer and D. Stoeckel, Thermobimetallic effect in explosively cladded Mo-inconel compound, in: L.E. Murr, K.P. Staudhammer and M.A. Meyers (Eds.), Metalluroical Applications of Shock Wave and High-Strain Rate Phenomena, Marcel Dekker, New York, 1986, pp. 877 886. [5] S. Kuzmanovic, Z. Cligoric and B. Beatovic, Properties of bimetallic bar as transition element in shipbuilding industry, in: Proc. lOth Int. Conf. on HERF, Sept. 18-22, 1989, Ljubljana, Yugoslavia, pp. 774 784. [6] V.D. Linse and N.S. Lalwaney, Explosive welding, J. Metals, 36(5)(1984)62-65. 1,7] T.Z. Blazynski (Ed.), Explosive Welding, Forming, and Compaction, Applied Science Publishers, Barking, 1983. [8] R. Hardwick and F. Weld, Some more recent advances in cladding technology, in: I.A. Yakovlev and V.F. Nesterenko (Eds.), Proc. 9th Int. Conf. on HERF, Aug. 18 22, 1986, Novosibirsk, pp. 270-277. [9] P.V. Vaidyanathan and A.R. Ramanathan, Design for quality explosive welding, J. Mat. Proc. Techn., 32 (1992) 439-448. 1'10] A.G. Mamalis, N.M. Vaxevanidis and A.P. Karafillis, Surface Integrity and Formability of Steel Sheet, VDI Verlag, D~sseldorf, 1990. [11] B. Crossland and J.A. Cave, The explosive properties of ammonium nitrate fuel oil mixtures, in: Proc. 5th Int. Conf. on HERF, Denver, Co, USA, 1975, pp. 4.9.1 4.9.11. [12] K. Pohlandt and K. Lange, Recommendations for an unified upsetting test for determining flow curves, Ann. CIRP, 38(2) (1989) 681-682. 1-13] A. Szecket, D.J. Vigueras and O.T. Inal, The cyclic pressure distribution of explosively welded interfaces, in: L.E. Murr, K.P. Staudhammer and M.A. Meyers (Eds.), Metallurgical Applications of Shock Wave and High-Strain Rate Phenomena, Marcel Dekker, New York, 1986, pp. 887-903. 1-14] A.G. Mamalis and W. Johnson, Defects in the processing of metals and composites, in: M. Predeleanou (Ed.), Computational Methods for Predictin9 Material Processin9 Defects, Elsevier, Amsterdam, 1987, pp. 231-250. [15] A.G. Mamalis, N.M. Vaxevanidis and D.I. Pantelis, On the rolling of bimetallic explosively cladded plates, in: Proc. 4th lnt. Conf. on Techn. of Plasticity, Sept. 5-12 1993, Beijing, China, pp. 874 879.