Three types of clad sheets, Cu/Al, Cu/AA5052, and Cu/AA5083, were produced by cold roll bonding with the rolling reduction of 50% and 75%. Tensile shear tests which give tensile shear strength were performed in order to assess the bond strength. Scanning electron microscopy was performed on the fractured interface produced by the tensile shear tests, which suggests that the fracture occurs within the Al alloy layer. The tensile shear strengths considering the area fraction of deposit of Al alloy on Cu side were compared with the shear stress converting from the ultimate tensile strengths. As a result, the tensile shear strength of the clad sheets is attributed to the shear strength of Al alloy layer close to the well bonded interface. A simple model was proposed that explains the effects of the rolling reduction and area fraction of deposit of Al alloy.
Composite materials which have superior characteristics compared with each composed material are widely used, and metals can be used as the composed materials. For instance, Cu/Al composites have smaller density than Cu and higher thermal and electrical conductivities than Al [
There are two types of roll bonding: one is hot roll bonding and the other is cold roll bonding, of which definition is that the former and the latter are carried out above and below recrystallization temperature, respectively. During hot rolling, intermetallic layers are often formed at the interfaces of clad sheets. Such intermetallic layers would reduce the bond strength for Cu/Al composites [
Li et al. reviewed the bond strength of clad sheets formed by cold roll bonding for several combinations of fcc composed layers and concluded that the sufficient bonding could be achieved rather easily between fcc metallic layers [
There are a few related models and theories for the bonding and the bond strength of clad sheets. Jamaati and Toroghinejad suggest that the faced and contaminated surface layers of metals are destroyed during roll bonding and virgin surfaces appear from the crack at the contaminated surfaces [
In the present study, three types of Al alloys are chosen with strength lower than, nearly equal to, and higher than that of Cu. Using these alloy combinations, we will investigate the effect of rolling reduction and strength of composed metallic layers on bond strength of Cu/Al alloy clad sheets.
Materials used in the present study are pure Cu (purity of 99.99%: 4N-Cu) and three types of Al alloys (4N-Al with 99.99% purity, AA5052, and AA5083). The chemical compositions of AA5052 and AA5083 are shown in Table
The chemical composition of AA5052 and AA5083.
Element (mass %) | Si | Fe | Cu | Mn | Mg | Cr | Zn | Al |
---|---|---|---|---|---|---|---|---|
AA5052 | 0.10 | 0.26 | 0.02 | 0.04 | 2.50 | 0.20 | 0.03 | Bal. |
AA5083 | 0.14 | 0.20 | 0.02 | 0.68 | 4.40 | 0.12 | 0.00 | Bal. |
Although there are many parameters affecting cold roll bonding such as rolling conditions, shape factor in rolling, surface conditions, adsorbed contaminants, oxide films, surface preparation methods, lubricant conditions, and postheat treatments [
The ARB process is one of the severe plastic deformation processes which allows giving large amount of plastic strain for metallic sheets in order to fabricate ultrafine grains (UFGs) [
The effective prevention of heat generation is very important since the temperature increase may create intermetallic layers between the metal sheets which would deteriorate the bond strength. Here, the sample coordination was defined as rolling direction (RD), normal direction (ND), and transverse direction (TD).
Judging from the fact that the length of the plates after 50% and 75% of rolling reduction became almost twice and four times, respectively. Therefore, the plastic deformation during the cold rolling can be approximated as the plane strain compression along ND and the length change along TD can be neglected.
The combinations of the metal sheets were 4N-Cu/4N-Al, 4N-Cu/AA5053, and 4N-Cu/AA5083. Thus, there are six types of samples with regard to the combination of the metals and the thickness reduction. Hereby, the samples are denoted as combination of metals and the thickness reduction, such as Cu/Al-50% and Cu/AA5052-75%. Here, it should be noted that the thickness reduction values have a scatter of about a few percent, but this amount of difference in the thickness reduction did not affect the Vickers hardness
Vickers hardness tests were carried out using micro-Vickers hardness machine MXT-
Two types of mechanical tests, uniaxial tensile tests and tensile shear tests, were performed using a tensile test machine NMB TG-50 kN (Minebea) at room temperature (R.T.). Specimens for the uniaxial tensile tests were cut using the wire-electric discharge machine, and the surface of the specimens was mechanically polished using SiC paper up to #4000. As shown in Figure
Details of the specimens for tensile shear tests.
Sample name | Width, |
width of slits, |
Area of bonding, |
---|---|---|---|
Cu/Al-50% | 4.73 | 0.35 | 1.66 |
Cu/Al-75% | 4.73 | 0.34 | 1.61 |
Cu/AA5052-50% | 4.73 | 0.75 | 3.55 |
Cu/AA5052-75% | 4.73 | 0.55 | 2.60 |
Cu/AA5083-50% | 4.73 | 0.75 | 3.55 |
Cu/AA5083-75% | 4.73 | 0.55 | 2.60 |
Schematic illustrations of (a) a specimen for the uniaxial tensile test and (b) a specimen for tensile shear test. (c) The magnified schematic illustration of (b) indicated as square dot lines in (b).
After the tensile shear tests, the surfaces of destroyed roll bonded interface were observed by a digital microscope VHX-500 (Keyence). Backscattered electron image (BEI) of some delaminated surfaces were obtained using a scanning electron microscope (SEM) JSM-7001F (JEOL) with a field emission gun and an acceleration voltage of 15 kV. It was also confirmed by energy dispersive X-ray spectrometry (EDS) JED-2300 equipped in another SEM JSM-7100F (JEOL) that the difference of contrast on BEIs was due to the difference of metals. Two-dimensional EDS mappings were performed with the acceleration voltage and the illumination current of 15 keV and 1.8 nA. The data analysis was performed using a software analysis station (JEOL). As a result, the bright and the dark regions on the fracture interfaces were attributed to Cu and Al alloy regions, respectively. Image thresholding was applied for obtained BEIs, and the area fractions of Cu and Al alloy regions were evaluated.
As shown in Figure
s-s curves of the annealed sheets.
As can be seen, AA5083 has the highest yield stress followed in order by AA5052, 4N-Cu, and 4N-Al. After work hardening, ultimate tensile strength (UTS) of 4N-Cu becomes comparable to that of AA5052 and total elongation is the largest in 4N-Cu.
AA5000 series is a well-known Al-Mg solid solution strengthening type alloy, and the difference of the concentration of Mg affects the mechanical properties. Actually, the UTS of AA5083 is higher than that of AA5052. A part of the s-s curves of AA5052 and AA5083 shows serration which is associated with dynamic strain aging [
Figure
s-s curves of the clad sheets and the composed layers. (a) Cu/Al-50%, (b) Cu/Al-75%, (c) Cu/AA5052-50%, (d) Cu/AA5052-75%, (e) Cu/AA5083-50%, and (f) Cu/AA5083-75%.
Rolling reduction dependence on the 0.2% proof stress of (a) the Cu/Al clad sheet and the composed layer, (b) the Cu/AA5052 clad sheet and the composed layers, and (c) the Cu/AA5083 clad sheet and the composed layers.
Rolling reduction dependence on the ultimate tensile stress of (a) the Cu/Al clad sheet and the composed layer, (b) the Cu/AA5052 clad sheet and the composed layers, and (c) the Cu/AA5083 clad sheet and the composed layers.
All Cu layers and Al layers show the ductile s-s curves, whereas AA5052 and AA5083 layers fracture suddenly after they reach the plastic instability region. It is similar to the case of the s-s curves of the annealed AA5052 and AA5083 sheets which also fracture suddenly as shown in Figure
From the deformation stress point of view, the s-s curves of Cu/Al and Cu/AA5052 clad sheets seem to be almost the average of the s-s curves of composed layers. On the other hand, it is not clear to discuss the deformation stress of Cu/AA5083-50% and Cu/AA5083-75% since the deformation stresses of the clad sheets are comparable with either the Cu or the AA5083 layers.
Nevertheless, the clad sheets can be considered to be composite consisting of two layers. If the fractions of the cross-sectional area which are being normal to the tensile direction of composed Cu and Al alloy layers are
Figure
Results of tensile shear tests, (a) and (b) are Cu/Al-50% and Cu/Al-75%, (c) and (d) are Cu/AA5052-50% and Cu/AA5052-75%, and (e) and (f) are Cu/AA5083-50% and Cu/AA5083-75%. Left and right column represent load-displacement curves and shear stress displacement curves, respectively.
The
The trends after the elastic region are classified to be two types: one is Cu/Al and the others are Cu/AA5052 and Cu/AA5083. In the former case,
The
The fractured interface was observed using SEM after the tensile shear tests. It is worthwhile to emphasize that the BEIs of Cu side seem to be partly covered by Al alloy, whereas BEIs of the Al alloy side seem to be only Al alloy. Although the morphology of Cu side is different among all the samples, the morphology of Al alloy side is almost the same as shown in Figure
Fractured interface of Al alloy side after a tensile shear test.
Thus, only the BEIs of Cu side are shown in Figure
Fractured interface after tensile shear tests, (a) Cu/Al-50%, (b) Cu/Al-75%, (c) Cu/AA5052-50%, (d) Cu/AA5052-75%, (e) Cu/AA5083-50%, and (f) Cu/AA5083-75%.
Image thresholding was applied to the BEIs and the area fraction of Al deposit on Cu side
(a) Area fraction of deposit and (b) apparent tensile shear strengths obtained from tensile shear tests, corrected tensile shear strength with considering the area fraction of deposit on the fracture interface, and tensile shear strengths converted from the ultimate tensile strength of the Al alloy layers for Cu/Al-50%, Cu/Al-75%, Cu/AA5052-50%, Cu/AA5052-75%, Cu/AA5083-50%, and Cu/AA5083-75%.
As shown in Figure
The shear strength of Al alloy layers can be converted from the UTS of Al alloy layers using the following assumption. Von Mises’s equation allows to convert three-dimensional stress condition to uniaxial tress condition [
In order to understand the above results about
Figure
A schematic illustration of interface before and after cold roll bonding. Three types of interface can be assumed: type A, B, and C interfaces. They are indicated by arrows. Only Type B becomes a well-bonded interface.
Area fraction of deposit versus rolling reduction. Dashed line represents the estimated values following the proposed model.
Strictly speaking, the separation of deposit also occurs along TD if the BEIs are carefully checked, which can be also understood by the proposed model since there is small elongation along TD about 1.5 % after roll bonding.
Experimental results show the higher
Let us consider the plastic deformation of one composed layer before and after rolling reduction. Here, we assume that the layer satisfies constant volume condition and ideal plane strain compression. Thus, (
If the virgin surface appears completely randomly after roll bonding, the expected probability of well bonded type-B interface
Figure
In this study, three types of clad sheets: Cu/Al, Cu/AA5052, and Cu/5083 were fabricated by cold roll bonding with the rolling reduction of 50% and 75%. The tensile shear tests were performed in order to measure the bond strength. SEM observations revealed that the fracture occurs within the Al alloy layer close to the interface. Thus, the part of the fracture interface on the Cu side was covered by the Al alloy after the tensile shear tests. The area fraction of Al alloy deposit on the Cu side was measured. If the rolling reduction is the same, the area fraction of the deposit increases with decreasing strength of the Al alloy layer. All the clad sheets show larger area fraction of deposit at 75% of rolling reduction compared with that at 50% of rolling reduction. The difference in tensile shear strength, which is considered to be the bond strength of clad sheets, can be understood by considering both the strength of composed layers and the area fraction of deposit. The relationship between the area fraction of deposit and the rolling reduction can be explained by a simple model proposed in this study.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was financially supported by a Grant-in-aid for Scientific Research on Innovative Area “Bulk Nanostructured Metals” no. 22102006 through the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. The authors are most grateful to Professor Eiichi Sato, Japan Aerospace Exploration Agency (JAXA), for the use of the rolling mill in his laboratory.