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Journal of Manufacturing Processes 47 (2019) 244–253
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Microstructure and mechanical properties of simultaneously explosivelywelded Steel/Cu pipes and Al/Cu pipe/rod Guoan Zhoua,b, Junfeng Xua, Zhaowu Shena, Honghao Maa,c,
T
⁎
a
CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230027, PR China Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong 999077, PR China c State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Russian-doll-like experimental arrangement Explosive welding Microstructure Mechanical properties
Through an unique manufacturing process which we call it “the Russian-doll-like experimental arrangement”, Steel/Cu pipes and Al/Cu pipe/rod are simultaneously explosively welded in a single experiment. After that, optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), longitudinal compression, transversal compression and compression-shear tests are conducted to analyze samples’ microstructure and mechanical properties. Results show that bonding interfaces of Al/Cu and Steel/Cu couples change from unstable ones, via regular/wavy ones, to final flat ones along the detonation direction. AlCu and Al2Cu are identified around the Al/Cu interface via EDS analysis, and atomic ratios of steel and copper around the Steel/Cu interface varies from 47/53 to 75/25. Samples’ yield strength/strain in longitudinal compression and transversal compression for Al/Cu couple are respectively 340 MPa/4.8% and 120 MPa/4.5%. Steel/Cu specimens’ yield strength/strain in longitudinal compression and compressive strength/failure strain in transversal compression are 598 MPa/5.8% and 90.97 MPa/21.0%, respectively.
1. Introduction Bimetallic composites are expected to be used widely at urban construction, petroleum transportation, aerospace field etc., since they can perfectly combine parent metals’ mechanical -chemical advantages [1,2]. I.e., Steel/Cu bimetallic pipes can used in the blast furnace as cooling staves and Al/Cu bimetallic composites are excellent candidates for air conditioners and cathode conductive heads. Normally considered as a solid-state process, explosive welding is one of the advanced manufacturing processes that are able to fabricate bimetallic composites. It works by detonating an explosive coating and further accelerating the flying plate/pipe to a quite high velocity towards the base plate/pipe/rod [3–5]. The whole process would eventually cause severe, but localized, plastic flow and lead to a linear or wavy interface between two parts. Based on the collision velocity Vc and collision angle β (or the impact velocity Vp), the “weld-ability window” is widely used to predict possible explosively-welded results of metals. Under the condition of Vp=Vc×sin β, two of these three parameters are dependent on detonation velocity of explosives, thickness of flying plate/pipe, and the stand-off distance. It has been decades since the concept of explosive welding is
proposed, but there are still quite a lot of researchers who focus on improving traditional explosive welding methods. I.e., Bataev et al. welded 21-layer Al-Ti laminate composites at a time with 4200 m/s ammonite 6 GV (commercially available in Russia) and 4600 m/s emulsion explosive [6]; Shiran et al. investigated the effect of time and temperature on the explosively-welded multilayer Cu/Al/Cu sample and proved that both layer thickness and types of intermetallic compounds at interfaces altered during the hear treatment process [7]; Yang et al. used the aluminum honeycomb configuration to hold the weight of all parts and proposed a “self-restrained setup” to fabricate explosively-welded plates with higher energy efficiency [8]. Just in 2019, Satyanarayan et al. discussed the effects of heat input and depth of water on the welding interface of tin-copper via underwater explosive welding technique [9]; Parchuri et al. investigated the bending strength of explosively-welded Nb/Cu and Ta/Cu couples with and without an intermediate layer [10]. Compared with papers mentioned above, there seems to be fewer works focused on the explosively-welded pipes/rod and less innovation in this area. Specifically, Shiran et al. used different compounds of ammonium nitrate/T.N.T to weld 321 austenitic stainless steel and 1230 aluminum tubes. They further investigated the effects of stand-off
⁎ Corresponding author at: CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230027, PR China. E-mail address: [email protected] (H. Ma).
https://doi.org/10.1016/j.jmapro.2019.10.004 Received 24 August 2019; Received in revised form 26 September 2019; Accepted 1 October 2019 Available online 11 October 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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distance and heat treatment on the microstructure and mechanical properties of welded samples [11,12]. Mendes et al. studied the influence of explosive characteristics on the weld interfaces of stainless steel AISL 304L to low alloy steel 51CrV4 in a cylindrical configuration [13]. Based on Mendes et al.’s work, Hokamoto et al. used the similar hollow cylindrical emulsion-explosive configuration to fabricate unidirectional porous-structured aluminum [14]. Different from the traditional way to explosively weld pipes, namely, accelerating an inner pipe to a proper velocity to impact outer pipes, Mendes et al. discussed the method of using hollow cylindrical emulsion-explosive configuration to compress outer pipes to impact the inside rod [13]. Nevertheless, this specific processing technology requires a high-strength external constraint outside of explosives to reduce the weakening effect of sparse wave on explosives’ propagation. And these external constraints are doomed to fail after several experiments. In Hokamoto et al.’s work, they waived the high-strength external constraint, but the thickness of core emulsion-explosive setup exceeds 25 mm, much beyond the critical detonation diameter (dcr) of the chosen explosives [14]. In other words, the weakening effect of sparse wave on explosives is still rather serious, and they have to add additional explosives (increasing thickness) to ensure the propagation of explosives. Therefore, to further improve the energy efficiency of explosives and waive the necessity of high-strength external constraints, doomed to fail at last but still required in the explosively-welded pipe/rod area, here we introduce an unique manufacturing process which we call it “the Russian-doll-like experimental arrangement” to weld Steel/Cu pipes and Al/Cu pipes/rod at a time. With properly matched diameters of pipes/rod, this novel experimental arrangement can help us obtain two explosively-welded couples simultaneously in a single experiment.
Table 1 Chemical composition of the chosen metals. Materials
Steel-Q235 Copper-T2 Aluminum1060
Chemical composition (wt.%) Fe
Mg
C
Mn
Si
S
Cu
Al
Balanced 0.005 0.35
– – 0.03
0.2 – –
0.5 – 0.03
– – 0.25
0.05 0.005 –
– Balanced 0.05
– – Balanced
5 g FeCl3+20 ml HCl+100 ml alcohol, and 2 ml HF + 3 ml HCl+5 ml HNO3+190 ml H2O, respectively. Since Crossland et al. has stated in detail that detonation velocity of explosives used in the explosive welding area should not exceed 1.2 times the minimum bulk sound speed of all metal parts [15], and explosives with detonation velocity between 2000 m/s–3000 m/s are frequently used to weld Steel/Cu and Al/Cu couples [16,17], here we choose HGMs-sensitized emulsion explosives whose detonation velocity is about 2815 m/s to fill the tubular emulsion-explosive configuration (part no.3, shown in Fig. 3) and the hollow truncated cone emulsionexplosive setup (part no.6, shown in Fig. 3). Chemical compositions and detonation properties of the chosen explosives are given in Table 2, and detailed parameters of this kind of HGMs-sensitized emulsion explosives are discussed by Zhou et al. at length [18].
2.2. Experimental layout and size design As shown in Fig. 2, the proposed arrangement to weld Steel/Cu pipes and Al/Cu pipe/rod at a time requires that all pipes/rod should be placed concentrically and the tubular emulsion-explosive configuration (no.3) is placed exactly between the T2 copper pipe (no.2) and the 1060 aluminum pipe (no.4). Designed with sufficient thickness to hold the high pressure coming from explosives, part no.1 is a Q235 steel pipe, and no.5 is a T2 copper rod. Once the tubular emulsion-explosive configuration (no.3 with thickness of 10 mm, while that value in Hokamoto et al.’s work exceeds 25 mm [14]) is detonated through a hollow truncated cone emulsion-explosive setup (no. 6) and a detonator (no. 7), as shown in Fig. 3, high-pressure detonation products coming from part no.3 shall accelerate the T2 copper pipe (no.2) to impact the outer Q235 steel pipe (no.1) and compress the 1060 aluminum pipe
2. Experimental section 2.1. Preparation of materials A Q235 steel pipe (outer) coupled with a T2 copper pipe (inner), and a 1060 aluminum pipe (outer) coupled with a T2 copper rod (inner) are chosen to weld. Their corresponding optical microscope images before the experiment and chemical composition are respectively given in Fig. 1 and Table 1. To be specific, the etchant solutions for SteelQ235, copper-T2 and aluminum-1060 are 4 ml HNO3+100 ml alcohol,
Fig. 1. OM images of pipes/rod before experiment: (a: steel-Q235, pipe; b: copper-T2, pipe; c: aluminum-1060, pipe; d: copper-T2, rod). 245
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continuous melted layer among two parts, and high enough impact pressure at the collision point to exceed the yield stress of materials preparing for welding (equations of these four limit lines are discussed by Mendes et al. at length) [13], calculated points of this study are respectively inside the Steel/Cu and Al/Cu weld-ability window (as shown in Fig. 4).
Table 2 Details of the chosen HGMs-sensitized emulsion explosives (HGMs: hollow glass micro-spheres). Chemical compositions (wt.%) Emulsion matrix
59.1
HGMs C18H38 3.2
NaNO3 7.9
C14H44O6 1.6
H2O 7.0
21.2
2.4. Characterization methods
Others −3
Bulk density / (g·cm
0.74
)
Detonation velocity / (m·s−1)
Explosive brisance /mm
2815
7.5
After the welding experiment, a scanning electron microscope operated at 15 kV (SEM, machine model: Gemini SEM 500) equipped with an energy dispersive X-ray spectrometer (EDS), and an optical microscope (OM, machine model: Leica DM4M) are firstly used to examine samples’ welding quality along the detonation direction (the RD direction). On the other hand, longitudinal compressive tests, transversal compressive tests and compression-shear tests of samples are performed on MTS 810 under the condition of quasi-static loading with strain rate control (10−3 s-1) at room temperature. To be specific, the heights of samples (Al-Cu/Steel-Cu) for longitudinal compression, transversal compression and compression-shear tests are 25/× mm (nearly equals to the external diameter), 13/43 mm (about half of the external diameter) and 5/5 mm, respectively, where × means that we seek an alternative way to evaluate the longitudinal compression properties of Steel/Cu samples (stated at length in Section 3.4).
(no.4) to impact the inner T2 copper rod (no.5) at the same time. Apart from a suitable detonation velocity of emulsion explosives, this unique fabrication method also requires a proper control of all pipes/rod’s thicknesses and the distance between each part. As shown in Table 3, in order to popularize this fabrication method in the explosive welding area, here we only choose from those mass-produced pipes/rod with standard diameters and thickness (no additional mechanical processing is required). With respect to the stand-off distance between each part, Yu et al. proposed Eq.(1) in the explosively-welded Al-Steel coaxial pipes situation [19], where S and δ represent the ideal stand-off distance and the thickness of flying pipe, respectively. S ≈ (0.5-1.0)× δ
3. Results and discussion
(1)
Macroscopically explosively-welded results and their profiles are shown in Fig. 5. Due to the jet produced by part no. 6 (geometric reason), there is a spherical groove (about 3 mm in diameter, marked in Fig. 5(a)) on top of the Al/Cu explosively-welded rod. Because of this, the section 8.5 mm from the top (Al/Cu couple) is not welded, while the Steel/Cu couple is not affected.
2.3. Weld-ability windows of Steel/Cu and Al/Cu couples As mentioned in the introduction part, explosively-welded quality is highly affected by the impact velocity Vp (or the collision angle β, calculated by Eq. (2) [13]) and the collision point velocity Vc (numerically equals to explosives’ detonation velocity), so here we calculate and present these three values of this study in Table 4.
3.1. Microstructural observation of the Al/Cu couple
1/4
sin
β 1 (Tm C0)1/2 ⎛ kCp C0 ⎞ = ⎜ ⎟ 2 N 2Vc2 ⎝ ρhf ⎠
Along the RD direction, SEM/BSE images of the Al(no.4, pipe)/Cu (no.5, rod) couple’s bonding interface are given in Fig. 6, where (b), (d) and (f) are enlarged images from (a), (c) and (e), respectively. As shown in Fig. 6(a, c and e), the bonding interface of Al/Cu couple changes remarkably: quite unstable at the beginning, having a wavelength varies from 125 μm (circled in yellow, two rather close peaks) to 425 μm at a relative constant amplitude of about 100 μm, and even a wave is found opposite the RD direction (circled in red); relatively, the metal/metal interface observed from the middle part is much more regular, with a period of about 400 μm and amplitude of about 120 μm; nevertheless, this regular wavy interface gradually becomes a flat one in the last third of the sample, with more than 450 μm in wavelength but less than 80 μm in amplitude. Microcracks shown in Fig. 6(b, d and f), caused by rapid
(2)
where N is a constant: 0.11; Tm is the melting temperature of flying pipe (℃); C0 is the bulk sound speed of flying pipe (cm·s−1); Vc is the collision point velocity (cm·s−1); k is the thermal conductivity of flying pipe (erg· cm−1·℃ −1); Cp is the thermal capacity of flying pipe (erg·g−1·℃ −1); ρ is the flying pipe’s density in (g·cm-3) and hf is the thickness of flying pipe (cm). Compared with “the weld-ability window” of Steel/Cu (proposed by Wang et al. [20]) and Al/Cu (proposed by Loureiro et al. [16]), where the leftmost, rightmost, upper and lower limit lines of “the weld-ability window” are respectively linked to the formation of a metal/metal wavy interface, a jet at the collision point, purpose of avoiding a
Fig. 2. Layout of the Russian-doll-like experimental arrangement:(a, b) and five-storey Russian dolls: (c). 246
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Fig. 3. Profiles of all parts: (a, with no. 7: a detonator) and size design of part no.3 and no.6: (b).
We attribute these unusual microcracks to the part no. 6. Due to its specific geometric shape (hollow truncated cone), there shall be a jet immediately after its detonation (discussed above in Fig. 5(a)). After that, all detonation products coming from no. 6 will continue to move downwards, axially compress part no. 5 and further make it radially expand, which means in this case part no.5 (the copper rod) has an opposite velocity moving towards part no.4 (the aluminum pipe). As the distance increases (along the RD direction), this compression effect caused by the high pressure of detonation products decreases significantly. This makes the relative impact pressure at the first third higher than that at the middle part. In order to identify the intermetallic compounds around the bonding interface, we picked six different points in Fig. 6 for the EDS analysis. As seen in Fig. 7, results of selected points indicate that atomic ratios of aluminum and copper around the interface are about 54/46 and 65/35. These chemical compositions can be referred to the Al-Cu phase diagram and point out the presence of AlCu and Al2Cu phases.
Table 3 Size design of metal pipes/rod for the welding experiment. Part
Material
Function
External diameter (thickness), length / mm
S/δ (mm)
no. no. no. no.
Steel Q235 Copper T2 Aluminum1060 Copper T2
Parent pipe Flying pipe Flying pipe Parent rod
83 60 30 22
2/3
1 2 4 5
(9.5), 105 (3), 105 (2), 105 (11), 105
2/2
Table 4 Calculated values of this experiment. Parameters
hf /mm
Vc =Vd /(m·s−1)
Vp/(m·s−1)
β/°
Steel/Cu pipes Al/Cu pipe/rod
3 2
2815 2815
527.5 747.5
10.8 15.4
solidification and further cooling of the dissimilar partners, are also found rather unusual. Previous researchers have proved that the impact pressure between a parent pipe and a flying pipe increases along the RD direction [19], and micro-cracks are more likely to form around the bonding interface if the impact pressure is huger [21]. But here, for these Al/Cu couples fabricated through this experimental arrangement, we find that there are more microcreaks near the bonding interface at the first third than these at the middle third, and even no fewer than these at the last third, indicating that the impact pressure between aluminum pipe (no.4) and copper rod (no.5) at the first third is quite likely higher than that at the middle part.
3.2. Mechanical properties of the Al/Cu couple Longitudinal compression (along the RD direction) properties of Al/ Cu specimens and blank trails (pure copper cylinders: Φ25 mm × 25 mm) are given in Fig. 8, where line 00/01 and a/b/c represent engineering stress-strain relationships of blank trails and Al/ Cu couples, respectively. Their corresponding compressive behaviour are shown in Fig. 8(b–d). As shown in Fig. 8(a), Al/Cu couples fabricated through this special process exhibit a slightly higher yield strength and yield strain:
Fig. 4. Calculated value of this study in “the weld-ability window” of Steel/Cu: (a) and Al/Cu: (b). 247
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Fig. 5. Welding results of Steel/Cu and Al/Cu couples (a: original experimental results; b: transversal profile; c: longitudinal profile).
yield strength (120 MPa, copper cylinders: 116 MPa), but quite a different yield strain (4.5%, copper cylinders: 3.2%). And the transversal engineering stress of copper cylinders is always higher than that of Al/ Cu specimens at the same engineering strain. Specifically, 228.2 MPa to 207.7 MPa (+9.87%), 406.5 MPa to 373.7 MPa (+8.78%), and 724.5 MPa to 637.5 MPa (+13.6%) when the axial engineering strain reaches 20%, 40% and 60%, respectively. Macroscopically, Al/Cu specimens display a rather good ability to resist lateral loads (remain bonded when the transversal engineering strain reaches 23.5%, Fig. 9(b)), and failed until the strain reaches 47.7% (σ47.7% ≈ 450 MPa), Fig. 9(c). After the transversal compression test (εlast ≈ 70.0%), Al/Cu specimens remain bonded above and below but detached and cracked at the centre, too, as shown in Fig. 9(d), with a lateral expansion rate of 0.92 (12 mm /13 mm; copper cylinders: 0.85 (11 mm/13 mm)). Shear strength- engineering strain relationships of samples from different parts, the schematic diagram for compression-shear tests, and samples before/after the tests are shown in Fig. 10. According to Sun et al. [22], the shear strength of joint can be calculated by Eq. (3), where τ is the shear strength, F is the peak value of the force loading, d is the diameter of the interface, h is the height of samples and π is the ratio of circumference.
340 MPa/4.8%, compared to those of copper cylinders: 330 MPa/3.9%. After the axial engineering strain exceeds 18.8%, the engineering stress of copper cylinders gradually becomes higher than that of Al/Cu specimens at the same engineering strain. Specifically, 413.0 MPa to 407.4 MPa (+1.37%), 600.2 MPa to 554.5 MPa (+8.24%), and 961.8 MPa to 827.6 MPa (+16.2%) when the axial engineering strain reaches 20%, 40% and 60%, respectively. Macroscopically, Al/Cu specimens display a typical bulging waist at the axial engineering strain of 11.3% (Fig. 8(b)), and when the strain reaches 41.5% (σ41.5% ≈ 570 MPa), the outer aluminum tube fails with an angle of 83° with the RD direction, as shown in Fig. 8(c). After the longitudinal compression test (εlast ≈ 64.5%), Al/Cu specimens remain bonded above and below but detached and cracked at the centre, Fig. 8(d), with a lateral expansion rate of 1.13 (17 mm/15 mm, with local detachment and crackings; copper cylinders: 0.87 (13 mm/ 15 mm)). Transversal compression (perpendicular to RD direction) properties of Al/Cu specimens and blank trails (pure copper cylinders: Φ25 mm × 13 mm) are given in Fig. 9, where line 00/01 and a/b/c represent engineering stress-strain relationships of blank trails and Al/ Cu couples, respectively. Their corresponding compressive behaviour are shown in Fig. 9(b–d). As shown in Fig. 9(a), Al/Cu specimens exhibit a slightly higher
Fig. 6. BSE images of Al/Cu welded interface from (a) the first third; (c) the middle third; (e) the last third and selected points for EDS analysis. 248
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Fig. 7. EDS results of selected points (a: EDS image of point *, b: detailed testing values).
Fig. 8. Longitudinal compression of Al/Cu specimens and blank trails.
Fig. 9. Transversal compression of Al/Cu specimens and blank trails.
τ = F / (π × d × h)
third display an obviously lower failure strain (7.82%, descent rate: -19.8%) as well as a significantly lower strength (28.1 MPa, descent rate: -34.5%). We contribute the great difference above to the waveform changes of the interface and those unusual microcracks near the interface (for samples from the first third). Just like we mentioned above, the interface of Al/Cu couple changes from an unstable one with microwaves (beginning), via a regular one with microwaves (middle), to a final flat one (last), which means line a-1/2/3 are meant to continue to rise and
(3)
As shown in Fig. 10(d), all samples after the compression-shear test failed at the welding interface in 360°, but their shear strength-engineering strain relationships are rather different: samples from the middle part exhibit the highest shear strength (42.9 MPa) and failure strain (9.75%). Samples from the first third show a slightly lower strength (39.2 MPa, descent rate: -8.62%), but a remarkably lower failure strain (5.63%, descent rate: -42.3%). And samples from the last
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Fig. 10. Compression-shear tests of Al/Cu specimens (a-1/2/3: from the first third, b-1/2/3: from the middle third, c-1/2/3: from the last third).
changes remarkably along the RD direction. With several waves opposite the RD direction (circled in red, Fig. 11(a and b)), wavelength and amplitude from the first third vary from 28 μm to 182 μm and 14 μm to 50 μm, respectively. When it comes to the middle third, wavelength/amplitude of the bonding interface gradually increase to stable ones: 211 μm/64 μm, despite that there are occasionally some microwaves (circled in yellow) between two regular ones. Finally before the interface eventually becoming a flat one, about 250 μm in wavelength and 36 μm in amplitude, as shown in Fig. 11(f), some continuous microwaves with about 43.2 μm/18.5 μm in wavelength/ amplitude are observed (circled in blue, Fig. 11(e)). Around the bonding interface of Steel/Cu couples, we also pick six different points in Fig. 11 for the EDS analysis. As seen in Fig. 12, results indicate that atomic ratios of steel and copper around the interface varies from 47/53 to 75/25. Since the Steel-Cu phase diagram points out that the Fe and Cu elements are miscible without limit when they are both in liquid conditions [23], and the matrix metals around the bonding interface during the explosive welding process are considered to be melted due to the remarkable energy coming from both explosives and the impact [24], here we also hold the view that there is no intermetallic compounds but only supersaturated solid solutions with high ratios of both Fe and Cu (detailed atom ratios are shown in Fig. 12(b)) around the Steel/Cu bonding interface [25–27].
3.4. Mechanical properties of the Steel/Cu couple With respect to the longitudinal compression of Steel/Cu specimens, on the one hand, the axial quasi-static force to compress a whole Q235 steel pipe with 83 mm in outer diameter and 9.5 mm in wall thickness longitudinally (along RD direction) to axial strain of 70% can exceed 5800 kN (by calculation), there are few MTS instruments with such high quasi-static pressure. On the other hand, according to Andrews et al. [28], when the wall thickness/diameter (t/D) equals 0.1875 and the length/diameter (L/D) is between 1.0–1.5, this specific longitudinal compression fall into the “Single Folds” category, indicating bulking modes such as diamond mode, concertina mode or Eular (beam) mode will not appear during the longitudinal compression. Thus, here we choose to cut small piers, symmetrical to the diameter with 5 mm on each side in width and 10 mm in length, from the Q235 steel pipe (83 mm/64 mm in outer/inner diameter) and welded Steel/ Cu couple (about 88 mm/64 mm in outer/inner diameter) for the longitudinal compression, rather than compressing the whole pipes, as shown in Fig. 13(b and d). Engineering stress-strain relationships of blank trails (Q235 piers, line 00/01) and Steel/Cu specimens (line a/b/ c) are given in Fig. 13(a). Their corresponding compressive results are shown in Fig. 13(c and e).
Fig. 11. OM images of the Steel/Cu welded interface from the first third: (a, b), the middle third: (c, d), the last third: (e, f) and selected points for EDS analysis.
exceed middle parts’ shear strength, similar to the tendency shown in Fig. 15(a), but they actually fail far too early at a quite low shear strain (with considerable microcracks near the interface).
3.3. Microstructural observation of the Steel/Cu couple With respect to the Steel (no.1, pipe)/Cu (no.2, pipe) couple, a Leica DM4M optical microscope is used to evaluate its welding quality, rather than the Gemini SEM 500 (since the atomic numbers of Steel/Cu are so close (26/29) that images under SEM/BSE are quite indistinct). As shown in Fig. 11, the bonding interface of Steel/Cu couple also 250
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Fig. 12. EDS results of selected points (a: EDS image of point ×, b: detailed testing values).
Fig. 13. Longitudinal compression of Steel/Cu specimens and blank trails.
Fig. 14. Transversal compression of Steel/Cu specimens and blank trails.
4 mm; Q235 piers: 0.95 (about from 9.5 mm × 10 mm × 10 mm to 15 mm × 15 mm × 4 mm)). Transversal compression (perpendicular to RD direction) properties of Steel/Cu specimens and blank trails (Q235-steel pipes: Φ83 mm (outer diameter, with wall thickness: 9.5 mm)×43 mm) are shown in Fig. 14, where line 00/01 and a/b/c represent engineering stress-strain relationships of blank trails and Steel/Cu couples, respectively. Their corresponding compressive results are shown in Fig. 14(b–e). As shown in Fig. 14(a), the transversal engineering stress of steel pipes is always higher than that of Steel/Cu specimens at the same engineering strain. Specifically, 86.9 MPa to 80.2 MPa (+8.35%), 97.4 MPa to 86.3 MPa (+12.9%), and 108 MPa to 90.7 MPa (+19.1%) when the axial engineering strain reaches 5%, 10% and 20%,
As shown in Fig. 13(a), Steel/Cu couples fabricated by this unique method exhibit a lower yield strength/strain: 598 MPa/5.8%, compared to these of Q235 piers: 607 MPa/8.1%. When the axial engineering strain exceeds 14.5%, the engineering stress of steel pier gradually becomes higher than that of Steel/Cu specimens at the same engineering strain. Specifically, 904.7 MPa to 833.2 MPa (+8.58%), 1333.2 MPa to 1160.6 MPa (+14.9%), and 2045.5 MPa to 1703.3 MPa (+20.1%) when the axial engineering strain reaches 20%, 40% and 60%, respectively. Macroscopically, after the longitudinal compression test (εlast ≈ 70.0%), Steel/Cu specimens remain bonded at the centre but detached near the ends, cycled in red in Fig. 13(e), with a volume compressibility of 0.84 (about from 12 mm × 10 mm × 10 mm to 18 mm × 14 mm × 251
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Fig. 15. Compression-shear tests of Steel/Cu specimens (line a-1/2: from the first third, line b-1/2: from the middle third, line c-1/2: from the last third, line d: from the last third & with several initial defects).
4.5%, respectively. Samples from the middle part exhibit the highest shear strength and failure strain: 42.9 MPa and 9.75%. (4) Bonding interface of Steel(pipe)/Cu(pipe) couple also changes from a wavy, irregular one to a flat one along the RD direction. Atomic ratios of steel and copper around the interface varies from 47/53 to 75/25. (5) Steel/Cu specimens’ yield strength/strain in longitudinal compression and compressive strength/failure strain in transversal compression are 598 MPa/5.8% and 90.97 MPa/21.0%, respectively. Samples from the first third exhibit the highest shear strength, failure strain and minimum failure angle: 76.1 MPa, 19.9%, and 130°.
respectively. And Steel/Cu specimens exhibit a slightly lower yield strength (80.2 MPa, Q235 pipes: 76.0 MPa), a close yield strain (3.7%, Q235 pipes: 3.3%), but a very different compressive strength/failure strain (90.97 MPa/21.0%, Q235 pipes:132.2 MPa/40.1%). Macroscopically, all samples display a typical collapse mechanism of monolithic tubes (with six plastic hinges, white cycles in Fig. 14(b and d)), discussed at length by Shen et al. [29]. And when the transversal engineering strain researches 40.1% and 21.0%, Q235-steel pipes and Steel/Cu specimens failed at both sides and at bottom (the other parts remain bonded), respectively, as shown in Fig. 14(c and e). Shear strength-engineering strain relationships of samples from different parts and their corresponding results after tests are given in Fig. 15(a–e), respectively. Loaded by the same mode shown in Fig. 10(b), Steel/Cu bonding interfaces’ shear strength are also calculated by Eq. (3). As shown in Fig. 15(a), samples’ shear strength, failure strain and last failure angles from different parts varies remarkably: 76.1 MPa, 19.9%, and 130° from the first third (line a-1/2); 50.9 MPa, 18.9%, and 196° from the middle third (line b-1/2); 23.2 MPa, 10.4%, and 260° from the last third (line c-1/2, without initial defects); and 14.7 MPa, 8.03%, and 360° from the last third (line d, with initial defects). We contribute the difference above to the changes of the bonding interface, too, namely, gradually changes from a wavy, irregular one to a flat one (larger wavelength) along the RD direction.
Declaration of Competing Interest None. Acknowledgments We would like to acknowledge the financial support received from the National Natural Science Foundation of China (Contract Grant number: 51674229 and 51874267). References
4. Conclusions
[1] Findik F. Recent developments in explosive welding. Mater Des 2011;32:1081–93. [2] Wronka B. Testing of explosive welding and welded joints: joint mechanism and properties of explosive welded joints. J Mater Sci 2010;45:4078–83. [3] Aizawa Y, Nishiwaki J, Harada Y, Muraishi S, Kumai S. Experimental and numerical analysis of the formation behavior of intermediate layers at explosive welded Al/Fe joint interfaces. J Manuf Process 2016;24:100–6. [4] Zhou G, Ma H, Shen Z, Li L. Application of a new cleaner emulsion-explosive formula: Cu/Al parallel plates explosive welding. Propellants Explos Pyrotech 2018;43:1041–7. [5] Gharahshiran MR, Khoshakhlagh A, Khalaj G, Bakhtiari H, Banihashemi AR. Effect of postweld heat treatment on interface microstructure and metallurgical properties of explosively welded bronze-carbon steel. J Cent South Univ 2018;25:1849–61. [6] Bataev IA, Bataev AA, Mali VI, Pavliukova DV. Structural and mechanical properties of metallic-intermetallic laminate composites produced by explosive welding and annealing. Mater Des 2012;35:225–34. [7] Shiran MKG, Khalaj G, Pouraliakbar H, Jandaghi MR, Dehnavi AS, Bakhtiari H. Multilayer Cu/Al/Cu explosive welded joints: characterizing heat treatment effect on the interface microstructure and mechanical properties. J Manuf Process 2018;35:657–63. [8] Yang M, Ma HH, Shen ZW. Study on self-restrained explosive welding with high energy efficiency. Int J Adv Manuf Technol 2018;99:3123–32. [9] Satyanarayan A, Mori M, Nishi K, Hokamoto. Underwater shock wave weldability window for Sn-Cu plates. J Mater Process Technol 2019;267:152–8. [10] Parchuri P, Kotegawa S, Yamamoto H, Ito K, Mori A, Hokamoto K. Benefits of intermediate-layer formation at the interface of Nb/Cu and Ta/Cu explosive clads. Mater Des 2019;166.
An unique manufacturing process which we call it “the Russian-dolllike experimental arrangement” is used to simultaneously weld Steel/ Cu pipes and Al/Cu pipe/rod. Analysis of microstructures and mechanical properties of samples facilitate following conclusions: (1) With the benefits of remarkably improving energy efficiency of explosives and waiving the necessity of high-strength external constraints, doomed to fail at last but still required in the explosively-welded pipe/rod area, this specific arrangement can be well used to simultaneously weld two couples in a single explosive welding experiment. (2) Bonding interface of Al(pipe)/Cu(rod) couple changes from an unstable one with considerable cracks (more than those from the middle part), via a regular/wavy one(middle), to a final flat one. Intermetallic compounds such as AlCu and Al2Cu are identified around the welding interface via EDS analysis. (3) Al/Cu specimens’ yield strength/strain in longitudinal compression and transversal compression are 340 MPa/4.8% and 120 MPa/ 252
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[11] Shiran M, Bakhtiari H, Mousavi S, Khalaj G, Mirhashemi SM. Effect of stand-off distance on the mechanical and metallurgical properties of explosively bonded 321 austenitic stainless steel-1230 aluminum alloy tubes. Mater Res 2017;20:291–302. [12] Shiran MKG, Khalaj G, Pouraliakbar H, Jandaghi M, Bakhtiari H, Shirazi M. Effects of heat treatment on the intermetallic compounds and mechanical properties of the stainless steel 321-aluminum 1230 explosive-welding interface. Int J Miner Metall Mater 2017;24:1267–77. [13] Mendes R, Ribeiro JB, Loureiro A. Effect of explosive characteristics on the explosive welding of stainless steel to carbon steel in cylindrical configuration. Mater Des 2013;51:182–92. [14] Hokamoto K, Shimomiya K, Nishi M, Krstulovic-Opara L, Vesenjak M, Ren Z. Fabrication of unidirectional porous-structured aluminum through explosive compaction using cylindrical geometry. J Mater Process Technol 2018;251:262–6. [15] Crossland B, Williams J. Explosive welding. Metallurg Rev 1970;15:79–100. [16] Loureiro A, Mendes R, Ribeiro JB, Leal RM, Galvao I. Effect of explosive mixture on quality of explosive welds of copper to aluminium. Mater Des 2016;95:256–67. [17] Zhang H, Jiao KX, Zhang JL, Li JP. Microstructure and mechanical properties investigations of copper-steel composite fabricated by explosive welding. Mater Sci Eng 2018;731:278–87. [18] Zhou G, Ma H, Shen Z, Chen P, Yu Y. Study on a new cleaner emulsion explosive containing common clay. Propellants Explos Pyrotech 2018;43:789–98. [19] Yu Y, Ma HH, Zhao K, Shen ZW, Cheng YF. Study on underwater explosive welding of Al-Steel coaxial pipes. Cent Eur J Energ Mater 2017;14:251–65. [20] Wang YX, Li XJ, Wang XH, Yan HH. Fabrication of a thick copper-stainless steel clad
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plate for nuclear fusion equipment by explosive welding. Fusion Eng Des 2018;137:91–6. Wang JM, Zhang Y, Wang YF, Zhu X. Explosive mass-bonding property in aluminum alloy-steel explosive welding. J Mater Eng 2009;S2:443–7. Sun XJ, Tao J, Guo XZ. Bonding properties of interface in Fe/Al clad tube prepared by explosive welding. Trans Nonferrous Met Soc China 2011;21:2175–80. Zhang H, Jiao KX, Zhang JL, Li JP. Microstructure and mechanical properties investigations of copper-steel composite fabricated by explosive welding. Mater Sci Eng 2018;731:278–87. Bataev IA, Lazurenko DV, Tanaka S, Hokamoto K, Bataev AA, Guo Y, et al. High cooling rates and metastable phases at the interfaces of explosively welded materials. Acta Mater 2017;135:277–89. Zheng YX, Lv JF, Lai ZN, Lan ZY, Wang H. Innovative methodology for separating copper and iron from Fe-Cu alloy residues by selective oxidation smelting. J Clean Prod 2019;231:110–20. Bina MH, Dehghani F, Salimi M. Effect of heat treatment on bonding interface in explosive welded copper/stainless steel. Mater Des 2013;45:504–9. Gladkovsky SV, Kuteneva SV, Sergeev SN. Microstructure and mechanical properties of sandwich copper/steel composites produced by explosive welding. Mater Charact 2019;154:294–303. Andrews KRF, England GL, Ghani E. Classification of the axial collapse of cylindrical-tubes under quasi-static loading. Int J Mech Sci 1983;25:687–96. Shen JH, Lu GX, Ruan D, Seah CC. Lateral plastic collapse of sandwich tubes with metal foam core. Int J Mech Sci 2015;91:99–109.
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Microstructure and mechanical properties of simultaneously explosivelywelded Steel/Cu pipes and Al/Cu pipe/rod Guoan Zhoua,b, Junfeng Xua, Zhaowu Shena, Honghao Maa,c,
T
⁎
a
CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230027, PR China Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong 999077, PR China c State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Russian-doll-like experimental arrangement Explosive welding Microstructure Mechanical properties
Through an unique manufacturing process which we call it “the Russian-doll-like experimental arrangement”, Steel/Cu pipes and Al/Cu pipe/rod are simultaneously explosively welded in a single experiment. After that, optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), longitudinal compression, transversal compression and compression-shear tests are conducted to analyze samples’ microstructure and mechanical properties. Results show that bonding interfaces of Al/Cu and Steel/Cu couples change from unstable ones, via regular/wavy ones, to final flat ones along the detonation direction. AlCu and Al2Cu are identified around the Al/Cu interface via EDS analysis, and atomic ratios of steel and copper around the Steel/Cu interface varies from 47/53 to 75/25. Samples’ yield strength/strain in longitudinal compression and transversal compression for Al/Cu couple are respectively 340 MPa/4.8% and 120 MPa/4.5%. Steel/Cu specimens’ yield strength/strain in longitudinal compression and compressive strength/failure strain in transversal compression are 598 MPa/5.8% and 90.97 MPa/21.0%, respectively.
1. Introduction Bimetallic composites are expected to be used widely at urban construction, petroleum transportation, aerospace field etc., since they can perfectly combine parent metals’ mechanical -chemical advantages [1,2]. I.e., Steel/Cu bimetallic pipes can used in the blast furnace as cooling staves and Al/Cu bimetallic composites are excellent candidates for air conditioners and cathode conductive heads. Normally considered as a solid-state process, explosive welding is one of the advanced manufacturing processes that are able to fabricate bimetallic composites. It works by detonating an explosive coating and further accelerating the flying plate/pipe to a quite high velocity towards the base plate/pipe/rod [3–5]. The whole process would eventually cause severe, but localized, plastic flow and lead to a linear or wavy interface between two parts. Based on the collision velocity Vc and collision angle β (or the impact velocity Vp), the “weld-ability window” is widely used to predict possible explosively-welded results of metals. Under the condition of Vp=Vc×sin β, two of these three parameters are dependent on detonation velocity of explosives, thickness of flying plate/pipe, and the stand-off distance. It has been decades since the concept of explosive welding is
proposed, but there are still quite a lot of researchers who focus on improving traditional explosive welding methods. I.e., Bataev et al. welded 21-layer Al-Ti laminate composites at a time with 4200 m/s ammonite 6 GV (commercially available in Russia) and 4600 m/s emulsion explosive [6]; Shiran et al. investigated the effect of time and temperature on the explosively-welded multilayer Cu/Al/Cu sample and proved that both layer thickness and types of intermetallic compounds at interfaces altered during the hear treatment process [7]; Yang et al. used the aluminum honeycomb configuration to hold the weight of all parts and proposed a “self-restrained setup” to fabricate explosively-welded plates with higher energy efficiency [8]. Just in 2019, Satyanarayan et al. discussed the effects of heat input and depth of water on the welding interface of tin-copper via underwater explosive welding technique [9]; Parchuri et al. investigated the bending strength of explosively-welded Nb/Cu and Ta/Cu couples with and without an intermediate layer [10]. Compared with papers mentioned above, there seems to be fewer works focused on the explosively-welded pipes/rod and less innovation in this area. Specifically, Shiran et al. used different compounds of ammonium nitrate/T.N.T to weld 321 austenitic stainless steel and 1230 aluminum tubes. They further investigated the effects of stand-off
⁎ Corresponding author at: CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230027, PR China. E-mail address: [email protected] (H. Ma).
https://doi.org/10.1016/j.jmapro.2019.10.004 Received 24 August 2019; Received in revised form 26 September 2019; Accepted 1 October 2019 Available online 11 October 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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distance and heat treatment on the microstructure and mechanical properties of welded samples [11,12]. Mendes et al. studied the influence of explosive characteristics on the weld interfaces of stainless steel AISL 304L to low alloy steel 51CrV4 in a cylindrical configuration [13]. Based on Mendes et al.’s work, Hokamoto et al. used the similar hollow cylindrical emulsion-explosive configuration to fabricate unidirectional porous-structured aluminum [14]. Different from the traditional way to explosively weld pipes, namely, accelerating an inner pipe to a proper velocity to impact outer pipes, Mendes et al. discussed the method of using hollow cylindrical emulsion-explosive configuration to compress outer pipes to impact the inside rod [13]. Nevertheless, this specific processing technology requires a high-strength external constraint outside of explosives to reduce the weakening effect of sparse wave on explosives’ propagation. And these external constraints are doomed to fail after several experiments. In Hokamoto et al.’s work, they waived the high-strength external constraint, but the thickness of core emulsion-explosive setup exceeds 25 mm, much beyond the critical detonation diameter (dcr) of the chosen explosives [14]. In other words, the weakening effect of sparse wave on explosives is still rather serious, and they have to add additional explosives (increasing thickness) to ensure the propagation of explosives. Therefore, to further improve the energy efficiency of explosives and waive the necessity of high-strength external constraints, doomed to fail at last but still required in the explosively-welded pipe/rod area, here we introduce an unique manufacturing process which we call it “the Russian-doll-like experimental arrangement” to weld Steel/Cu pipes and Al/Cu pipes/rod at a time. With properly matched diameters of pipes/rod, this novel experimental arrangement can help us obtain two explosively-welded couples simultaneously in a single experiment.
Table 1 Chemical composition of the chosen metals. Materials
Steel-Q235 Copper-T2 Aluminum1060
Chemical composition (wt.%) Fe
Mg
C
Mn
Si
S
Cu
Al
Balanced 0.005 0.35
– – 0.03
0.2 – –
0.5 – 0.03
– – 0.25
0.05 0.005 –
– Balanced 0.05
– – Balanced
5 g FeCl3+20 ml HCl+100 ml alcohol, and 2 ml HF + 3 ml HCl+5 ml HNO3+190 ml H2O, respectively. Since Crossland et al. has stated in detail that detonation velocity of explosives used in the explosive welding area should not exceed 1.2 times the minimum bulk sound speed of all metal parts [15], and explosives with detonation velocity between 2000 m/s–3000 m/s are frequently used to weld Steel/Cu and Al/Cu couples [16,17], here we choose HGMs-sensitized emulsion explosives whose detonation velocity is about 2815 m/s to fill the tubular emulsion-explosive configuration (part no.3, shown in Fig. 3) and the hollow truncated cone emulsionexplosive setup (part no.6, shown in Fig. 3). Chemical compositions and detonation properties of the chosen explosives are given in Table 2, and detailed parameters of this kind of HGMs-sensitized emulsion explosives are discussed by Zhou et al. at length [18].
2.2. Experimental layout and size design As shown in Fig. 2, the proposed arrangement to weld Steel/Cu pipes and Al/Cu pipe/rod at a time requires that all pipes/rod should be placed concentrically and the tubular emulsion-explosive configuration (no.3) is placed exactly between the T2 copper pipe (no.2) and the 1060 aluminum pipe (no.4). Designed with sufficient thickness to hold the high pressure coming from explosives, part no.1 is a Q235 steel pipe, and no.5 is a T2 copper rod. Once the tubular emulsion-explosive configuration (no.3 with thickness of 10 mm, while that value in Hokamoto et al.’s work exceeds 25 mm [14]) is detonated through a hollow truncated cone emulsion-explosive setup (no. 6) and a detonator (no. 7), as shown in Fig. 3, high-pressure detonation products coming from part no.3 shall accelerate the T2 copper pipe (no.2) to impact the outer Q235 steel pipe (no.1) and compress the 1060 aluminum pipe
2. Experimental section 2.1. Preparation of materials A Q235 steel pipe (outer) coupled with a T2 copper pipe (inner), and a 1060 aluminum pipe (outer) coupled with a T2 copper rod (inner) are chosen to weld. Their corresponding optical microscope images before the experiment and chemical composition are respectively given in Fig. 1 and Table 1. To be specific, the etchant solutions for SteelQ235, copper-T2 and aluminum-1060 are 4 ml HNO3+100 ml alcohol,
Fig. 1. OM images of pipes/rod before experiment: (a: steel-Q235, pipe; b: copper-T2, pipe; c: aluminum-1060, pipe; d: copper-T2, rod). 245
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continuous melted layer among two parts, and high enough impact pressure at the collision point to exceed the yield stress of materials preparing for welding (equations of these four limit lines are discussed by Mendes et al. at length) [13], calculated points of this study are respectively inside the Steel/Cu and Al/Cu weld-ability window (as shown in Fig. 4).
Table 2 Details of the chosen HGMs-sensitized emulsion explosives (HGMs: hollow glass micro-spheres). Chemical compositions (wt.%) Emulsion matrix
59.1
HGMs C18H38 3.2
NaNO3 7.9
C14H44O6 1.6
H2O 7.0
21.2
2.4. Characterization methods
Others −3
Bulk density / (g·cm
0.74
)
Detonation velocity / (m·s−1)
Explosive brisance /mm
2815
7.5
After the welding experiment, a scanning electron microscope operated at 15 kV (SEM, machine model: Gemini SEM 500) equipped with an energy dispersive X-ray spectrometer (EDS), and an optical microscope (OM, machine model: Leica DM4M) are firstly used to examine samples’ welding quality along the detonation direction (the RD direction). On the other hand, longitudinal compressive tests, transversal compressive tests and compression-shear tests of samples are performed on MTS 810 under the condition of quasi-static loading with strain rate control (10−3 s-1) at room temperature. To be specific, the heights of samples (Al-Cu/Steel-Cu) for longitudinal compression, transversal compression and compression-shear tests are 25/× mm (nearly equals to the external diameter), 13/43 mm (about half of the external diameter) and 5/5 mm, respectively, where × means that we seek an alternative way to evaluate the longitudinal compression properties of Steel/Cu samples (stated at length in Section 3.4).
(no.4) to impact the inner T2 copper rod (no.5) at the same time. Apart from a suitable detonation velocity of emulsion explosives, this unique fabrication method also requires a proper control of all pipes/rod’s thicknesses and the distance between each part. As shown in Table 3, in order to popularize this fabrication method in the explosive welding area, here we only choose from those mass-produced pipes/rod with standard diameters and thickness (no additional mechanical processing is required). With respect to the stand-off distance between each part, Yu et al. proposed Eq.(1) in the explosively-welded Al-Steel coaxial pipes situation [19], where S and δ represent the ideal stand-off distance and the thickness of flying pipe, respectively. S ≈ (0.5-1.0)× δ
3. Results and discussion
(1)
Macroscopically explosively-welded results and their profiles are shown in Fig. 5. Due to the jet produced by part no. 6 (geometric reason), there is a spherical groove (about 3 mm in diameter, marked in Fig. 5(a)) on top of the Al/Cu explosively-welded rod. Because of this, the section 8.5 mm from the top (Al/Cu couple) is not welded, while the Steel/Cu couple is not affected.
2.3. Weld-ability windows of Steel/Cu and Al/Cu couples As mentioned in the introduction part, explosively-welded quality is highly affected by the impact velocity Vp (or the collision angle β, calculated by Eq. (2) [13]) and the collision point velocity Vc (numerically equals to explosives’ detonation velocity), so here we calculate and present these three values of this study in Table 4.
3.1. Microstructural observation of the Al/Cu couple
1/4
sin
β 1 (Tm C0)1/2 ⎛ kCp C0 ⎞ = ⎜ ⎟ 2 N 2Vc2 ⎝ ρhf ⎠
Along the RD direction, SEM/BSE images of the Al(no.4, pipe)/Cu (no.5, rod) couple’s bonding interface are given in Fig. 6, where (b), (d) and (f) are enlarged images from (a), (c) and (e), respectively. As shown in Fig. 6(a, c and e), the bonding interface of Al/Cu couple changes remarkably: quite unstable at the beginning, having a wavelength varies from 125 μm (circled in yellow, two rather close peaks) to 425 μm at a relative constant amplitude of about 100 μm, and even a wave is found opposite the RD direction (circled in red); relatively, the metal/metal interface observed from the middle part is much more regular, with a period of about 400 μm and amplitude of about 120 μm; nevertheless, this regular wavy interface gradually becomes a flat one in the last third of the sample, with more than 450 μm in wavelength but less than 80 μm in amplitude. Microcracks shown in Fig. 6(b, d and f), caused by rapid
(2)
where N is a constant: 0.11; Tm is the melting temperature of flying pipe (℃); C0 is the bulk sound speed of flying pipe (cm·s−1); Vc is the collision point velocity (cm·s−1); k is the thermal conductivity of flying pipe (erg· cm−1·℃ −1); Cp is the thermal capacity of flying pipe (erg·g−1·℃ −1); ρ is the flying pipe’s density in (g·cm-3) and hf is the thickness of flying pipe (cm). Compared with “the weld-ability window” of Steel/Cu (proposed by Wang et al. [20]) and Al/Cu (proposed by Loureiro et al. [16]), where the leftmost, rightmost, upper and lower limit lines of “the weld-ability window” are respectively linked to the formation of a metal/metal wavy interface, a jet at the collision point, purpose of avoiding a
Fig. 2. Layout of the Russian-doll-like experimental arrangement:(a, b) and five-storey Russian dolls: (c). 246
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Fig. 3. Profiles of all parts: (a, with no. 7: a detonator) and size design of part no.3 and no.6: (b).
We attribute these unusual microcracks to the part no. 6. Due to its specific geometric shape (hollow truncated cone), there shall be a jet immediately after its detonation (discussed above in Fig. 5(a)). After that, all detonation products coming from no. 6 will continue to move downwards, axially compress part no. 5 and further make it radially expand, which means in this case part no.5 (the copper rod) has an opposite velocity moving towards part no.4 (the aluminum pipe). As the distance increases (along the RD direction), this compression effect caused by the high pressure of detonation products decreases significantly. This makes the relative impact pressure at the first third higher than that at the middle part. In order to identify the intermetallic compounds around the bonding interface, we picked six different points in Fig. 6 for the EDS analysis. As seen in Fig. 7, results of selected points indicate that atomic ratios of aluminum and copper around the interface are about 54/46 and 65/35. These chemical compositions can be referred to the Al-Cu phase diagram and point out the presence of AlCu and Al2Cu phases.
Table 3 Size design of metal pipes/rod for the welding experiment. Part
Material
Function
External diameter (thickness), length / mm
S/δ (mm)
no. no. no. no.
Steel Q235 Copper T2 Aluminum1060 Copper T2
Parent pipe Flying pipe Flying pipe Parent rod
83 60 30 22
2/3
1 2 4 5
(9.5), 105 (3), 105 (2), 105 (11), 105
2/2
Table 4 Calculated values of this experiment. Parameters
hf /mm
Vc =Vd /(m·s−1)
Vp/(m·s−1)
β/°
Steel/Cu pipes Al/Cu pipe/rod
3 2
2815 2815
527.5 747.5
10.8 15.4
solidification and further cooling of the dissimilar partners, are also found rather unusual. Previous researchers have proved that the impact pressure between a parent pipe and a flying pipe increases along the RD direction [19], and micro-cracks are more likely to form around the bonding interface if the impact pressure is huger [21]. But here, for these Al/Cu couples fabricated through this experimental arrangement, we find that there are more microcreaks near the bonding interface at the first third than these at the middle third, and even no fewer than these at the last third, indicating that the impact pressure between aluminum pipe (no.4) and copper rod (no.5) at the first third is quite likely higher than that at the middle part.
3.2. Mechanical properties of the Al/Cu couple Longitudinal compression (along the RD direction) properties of Al/ Cu specimens and blank trails (pure copper cylinders: Φ25 mm × 25 mm) are given in Fig. 8, where line 00/01 and a/b/c represent engineering stress-strain relationships of blank trails and Al/ Cu couples, respectively. Their corresponding compressive behaviour are shown in Fig. 8(b–d). As shown in Fig. 8(a), Al/Cu couples fabricated through this special process exhibit a slightly higher yield strength and yield strain:
Fig. 4. Calculated value of this study in “the weld-ability window” of Steel/Cu: (a) and Al/Cu: (b). 247
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Fig. 5. Welding results of Steel/Cu and Al/Cu couples (a: original experimental results; b: transversal profile; c: longitudinal profile).
yield strength (120 MPa, copper cylinders: 116 MPa), but quite a different yield strain (4.5%, copper cylinders: 3.2%). And the transversal engineering stress of copper cylinders is always higher than that of Al/ Cu specimens at the same engineering strain. Specifically, 228.2 MPa to 207.7 MPa (+9.87%), 406.5 MPa to 373.7 MPa (+8.78%), and 724.5 MPa to 637.5 MPa (+13.6%) when the axial engineering strain reaches 20%, 40% and 60%, respectively. Macroscopically, Al/Cu specimens display a rather good ability to resist lateral loads (remain bonded when the transversal engineering strain reaches 23.5%, Fig. 9(b)), and failed until the strain reaches 47.7% (σ47.7% ≈ 450 MPa), Fig. 9(c). After the transversal compression test (εlast ≈ 70.0%), Al/Cu specimens remain bonded above and below but detached and cracked at the centre, too, as shown in Fig. 9(d), with a lateral expansion rate of 0.92 (12 mm /13 mm; copper cylinders: 0.85 (11 mm/13 mm)). Shear strength- engineering strain relationships of samples from different parts, the schematic diagram for compression-shear tests, and samples before/after the tests are shown in Fig. 10. According to Sun et al. [22], the shear strength of joint can be calculated by Eq. (3), where τ is the shear strength, F is the peak value of the force loading, d is the diameter of the interface, h is the height of samples and π is the ratio of circumference.
340 MPa/4.8%, compared to those of copper cylinders: 330 MPa/3.9%. After the axial engineering strain exceeds 18.8%, the engineering stress of copper cylinders gradually becomes higher than that of Al/Cu specimens at the same engineering strain. Specifically, 413.0 MPa to 407.4 MPa (+1.37%), 600.2 MPa to 554.5 MPa (+8.24%), and 961.8 MPa to 827.6 MPa (+16.2%) when the axial engineering strain reaches 20%, 40% and 60%, respectively. Macroscopically, Al/Cu specimens display a typical bulging waist at the axial engineering strain of 11.3% (Fig. 8(b)), and when the strain reaches 41.5% (σ41.5% ≈ 570 MPa), the outer aluminum tube fails with an angle of 83° with the RD direction, as shown in Fig. 8(c). After the longitudinal compression test (εlast ≈ 64.5%), Al/Cu specimens remain bonded above and below but detached and cracked at the centre, Fig. 8(d), with a lateral expansion rate of 1.13 (17 mm/15 mm, with local detachment and crackings; copper cylinders: 0.87 (13 mm/ 15 mm)). Transversal compression (perpendicular to RD direction) properties of Al/Cu specimens and blank trails (pure copper cylinders: Φ25 mm × 13 mm) are given in Fig. 9, where line 00/01 and a/b/c represent engineering stress-strain relationships of blank trails and Al/ Cu couples, respectively. Their corresponding compressive behaviour are shown in Fig. 9(b–d). As shown in Fig. 9(a), Al/Cu specimens exhibit a slightly higher
Fig. 6. BSE images of Al/Cu welded interface from (a) the first third; (c) the middle third; (e) the last third and selected points for EDS analysis. 248
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Fig. 7. EDS results of selected points (a: EDS image of point *, b: detailed testing values).
Fig. 8. Longitudinal compression of Al/Cu specimens and blank trails.
Fig. 9. Transversal compression of Al/Cu specimens and blank trails.
τ = F / (π × d × h)
third display an obviously lower failure strain (7.82%, descent rate: -19.8%) as well as a significantly lower strength (28.1 MPa, descent rate: -34.5%). We contribute the great difference above to the waveform changes of the interface and those unusual microcracks near the interface (for samples from the first third). Just like we mentioned above, the interface of Al/Cu couple changes from an unstable one with microwaves (beginning), via a regular one with microwaves (middle), to a final flat one (last), which means line a-1/2/3 are meant to continue to rise and
(3)
As shown in Fig. 10(d), all samples after the compression-shear test failed at the welding interface in 360°, but their shear strength-engineering strain relationships are rather different: samples from the middle part exhibit the highest shear strength (42.9 MPa) and failure strain (9.75%). Samples from the first third show a slightly lower strength (39.2 MPa, descent rate: -8.62%), but a remarkably lower failure strain (5.63%, descent rate: -42.3%). And samples from the last
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Fig. 10. Compression-shear tests of Al/Cu specimens (a-1/2/3: from the first third, b-1/2/3: from the middle third, c-1/2/3: from the last third).
changes remarkably along the RD direction. With several waves opposite the RD direction (circled in red, Fig. 11(a and b)), wavelength and amplitude from the first third vary from 28 μm to 182 μm and 14 μm to 50 μm, respectively. When it comes to the middle third, wavelength/amplitude of the bonding interface gradually increase to stable ones: 211 μm/64 μm, despite that there are occasionally some microwaves (circled in yellow) between two regular ones. Finally before the interface eventually becoming a flat one, about 250 μm in wavelength and 36 μm in amplitude, as shown in Fig. 11(f), some continuous microwaves with about 43.2 μm/18.5 μm in wavelength/ amplitude are observed (circled in blue, Fig. 11(e)). Around the bonding interface of Steel/Cu couples, we also pick six different points in Fig. 11 for the EDS analysis. As seen in Fig. 12, results indicate that atomic ratios of steel and copper around the interface varies from 47/53 to 75/25. Since the Steel-Cu phase diagram points out that the Fe and Cu elements are miscible without limit when they are both in liquid conditions [23], and the matrix metals around the bonding interface during the explosive welding process are considered to be melted due to the remarkable energy coming from both explosives and the impact [24], here we also hold the view that there is no intermetallic compounds but only supersaturated solid solutions with high ratios of both Fe and Cu (detailed atom ratios are shown in Fig. 12(b)) around the Steel/Cu bonding interface [25–27].
3.4. Mechanical properties of the Steel/Cu couple With respect to the longitudinal compression of Steel/Cu specimens, on the one hand, the axial quasi-static force to compress a whole Q235 steel pipe with 83 mm in outer diameter and 9.5 mm in wall thickness longitudinally (along RD direction) to axial strain of 70% can exceed 5800 kN (by calculation), there are few MTS instruments with such high quasi-static pressure. On the other hand, according to Andrews et al. [28], when the wall thickness/diameter (t/D) equals 0.1875 and the length/diameter (L/D) is between 1.0–1.5, this specific longitudinal compression fall into the “Single Folds” category, indicating bulking modes such as diamond mode, concertina mode or Eular (beam) mode will not appear during the longitudinal compression. Thus, here we choose to cut small piers, symmetrical to the diameter with 5 mm on each side in width and 10 mm in length, from the Q235 steel pipe (83 mm/64 mm in outer/inner diameter) and welded Steel/ Cu couple (about 88 mm/64 mm in outer/inner diameter) for the longitudinal compression, rather than compressing the whole pipes, as shown in Fig. 13(b and d). Engineering stress-strain relationships of blank trails (Q235 piers, line 00/01) and Steel/Cu specimens (line a/b/ c) are given in Fig. 13(a). Their corresponding compressive results are shown in Fig. 13(c and e).
Fig. 11. OM images of the Steel/Cu welded interface from the first third: (a, b), the middle third: (c, d), the last third: (e, f) and selected points for EDS analysis.
exceed middle parts’ shear strength, similar to the tendency shown in Fig. 15(a), but they actually fail far too early at a quite low shear strain (with considerable microcracks near the interface).
3.3. Microstructural observation of the Steel/Cu couple With respect to the Steel (no.1, pipe)/Cu (no.2, pipe) couple, a Leica DM4M optical microscope is used to evaluate its welding quality, rather than the Gemini SEM 500 (since the atomic numbers of Steel/Cu are so close (26/29) that images under SEM/BSE are quite indistinct). As shown in Fig. 11, the bonding interface of Steel/Cu couple also 250
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Fig. 12. EDS results of selected points (a: EDS image of point ×, b: detailed testing values).
Fig. 13. Longitudinal compression of Steel/Cu specimens and blank trails.
Fig. 14. Transversal compression of Steel/Cu specimens and blank trails.
4 mm; Q235 piers: 0.95 (about from 9.5 mm × 10 mm × 10 mm to 15 mm × 15 mm × 4 mm)). Transversal compression (perpendicular to RD direction) properties of Steel/Cu specimens and blank trails (Q235-steel pipes: Φ83 mm (outer diameter, with wall thickness: 9.5 mm)×43 mm) are shown in Fig. 14, where line 00/01 and a/b/c represent engineering stress-strain relationships of blank trails and Steel/Cu couples, respectively. Their corresponding compressive results are shown in Fig. 14(b–e). As shown in Fig. 14(a), the transversal engineering stress of steel pipes is always higher than that of Steel/Cu specimens at the same engineering strain. Specifically, 86.9 MPa to 80.2 MPa (+8.35%), 97.4 MPa to 86.3 MPa (+12.9%), and 108 MPa to 90.7 MPa (+19.1%) when the axial engineering strain reaches 5%, 10% and 20%,
As shown in Fig. 13(a), Steel/Cu couples fabricated by this unique method exhibit a lower yield strength/strain: 598 MPa/5.8%, compared to these of Q235 piers: 607 MPa/8.1%. When the axial engineering strain exceeds 14.5%, the engineering stress of steel pier gradually becomes higher than that of Steel/Cu specimens at the same engineering strain. Specifically, 904.7 MPa to 833.2 MPa (+8.58%), 1333.2 MPa to 1160.6 MPa (+14.9%), and 2045.5 MPa to 1703.3 MPa (+20.1%) when the axial engineering strain reaches 20%, 40% and 60%, respectively. Macroscopically, after the longitudinal compression test (εlast ≈ 70.0%), Steel/Cu specimens remain bonded at the centre but detached near the ends, cycled in red in Fig. 13(e), with a volume compressibility of 0.84 (about from 12 mm × 10 mm × 10 mm to 18 mm × 14 mm × 251
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Fig. 15. Compression-shear tests of Steel/Cu specimens (line a-1/2: from the first third, line b-1/2: from the middle third, line c-1/2: from the last third, line d: from the last third & with several initial defects).
4.5%, respectively. Samples from the middle part exhibit the highest shear strength and failure strain: 42.9 MPa and 9.75%. (4) Bonding interface of Steel(pipe)/Cu(pipe) couple also changes from a wavy, irregular one to a flat one along the RD direction. Atomic ratios of steel and copper around the interface varies from 47/53 to 75/25. (5) Steel/Cu specimens’ yield strength/strain in longitudinal compression and compressive strength/failure strain in transversal compression are 598 MPa/5.8% and 90.97 MPa/21.0%, respectively. Samples from the first third exhibit the highest shear strength, failure strain and minimum failure angle: 76.1 MPa, 19.9%, and 130°.
respectively. And Steel/Cu specimens exhibit a slightly lower yield strength (80.2 MPa, Q235 pipes: 76.0 MPa), a close yield strain (3.7%, Q235 pipes: 3.3%), but a very different compressive strength/failure strain (90.97 MPa/21.0%, Q235 pipes:132.2 MPa/40.1%). Macroscopically, all samples display a typical collapse mechanism of monolithic tubes (with six plastic hinges, white cycles in Fig. 14(b and d)), discussed at length by Shen et al. [29]. And when the transversal engineering strain researches 40.1% and 21.0%, Q235-steel pipes and Steel/Cu specimens failed at both sides and at bottom (the other parts remain bonded), respectively, as shown in Fig. 14(c and e). Shear strength-engineering strain relationships of samples from different parts and their corresponding results after tests are given in Fig. 15(a–e), respectively. Loaded by the same mode shown in Fig. 10(b), Steel/Cu bonding interfaces’ shear strength are also calculated by Eq. (3). As shown in Fig. 15(a), samples’ shear strength, failure strain and last failure angles from different parts varies remarkably: 76.1 MPa, 19.9%, and 130° from the first third (line a-1/2); 50.9 MPa, 18.9%, and 196° from the middle third (line b-1/2); 23.2 MPa, 10.4%, and 260° from the last third (line c-1/2, without initial defects); and 14.7 MPa, 8.03%, and 360° from the last third (line d, with initial defects). We contribute the difference above to the changes of the bonding interface, too, namely, gradually changes from a wavy, irregular one to a flat one (larger wavelength) along the RD direction.
Declaration of Competing Interest None. Acknowledgments We would like to acknowledge the financial support received from the National Natural Science Foundation of China (Contract Grant number: 51674229 and 51874267). References
4. Conclusions
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An unique manufacturing process which we call it “the Russian-dolllike experimental arrangement” is used to simultaneously weld Steel/ Cu pipes and Al/Cu pipe/rod. Analysis of microstructures and mechanical properties of samples facilitate following conclusions: (1) With the benefits of remarkably improving energy efficiency of explosives and waiving the necessity of high-strength external constraints, doomed to fail at last but still required in the explosively-welded pipe/rod area, this specific arrangement can be well used to simultaneously weld two couples in a single explosive welding experiment. (2) Bonding interface of Al(pipe)/Cu(rod) couple changes from an unstable one with considerable cracks (more than those from the middle part), via a regular/wavy one(middle), to a final flat one. Intermetallic compounds such as AlCu and Al2Cu are identified around the welding interface via EDS analysis. (3) Al/Cu specimens’ yield strength/strain in longitudinal compression and transversal compression are 340 MPa/4.8% and 120 MPa/ 252
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