Accumulative roll bonding (ARB) is a new and cost-effective process for the production of ultrafine-grained sheets by applying severe plastic deformation. Various mechanical properties of sheets improve after ARB process. However, the material formability severely decreases in the ARBed sheets restricting their usage in industrial applications. The main objective of this study is to investigate the effect of the high strain rates on the formability improvement of ultrafine-grained sheets manufactured by the ARB process. For this purpose, the electromagnetic forming (EMF) has been performed for applying high strain rates on ARBed pure copper sheets and its capability to resolve the low toughness issue of the ultrafine-grained products of the ARB process has been investigated. In the first step, several sheet samples have been subjected to one, two, and three ARB passes, and their mechanical properties such as microhardness, mechanical strength, and formability have been determined after each pass. The results have indicated a substantial increase in the sheets hardness and mechanical strength and a significant decrease in their formability especially after the first pass. Afterwards, the Nakazima test and the EMF have been used to form the ARBed sheet samples in the quasi-static and high strain rate conditions and then the forming limit diagram and formability of formed samples have been compared in these conditions. It has been found that the formability of the one-, two-, and three-pass ARBed sheets in plane strain conditions have been, respectively, increased 102%, 113%, and 99% in the high strain rate with respect to the quasi-static condition.
Research Article
Formability Enhancement of Ultra-Fine Grained Pure Copper Sheets Produced by Accumulative Roll Bonding Aided by Electromagnetic Forming
https://doi.org/10.21203/rs.3.rs-983388/v1
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Severe plastic deformation
Accumulative roll bonding process
Mechanical properties
High strain rate
Electromagnetic forming.
Production of metal parts with high strength and small weight to strength ratio is among the most important goals of the manufacturing processes. This goal can be achieved by creating ultrafine-grained microstructure in a number of metals as previous observations indicated [1]. Accordingly, many attempts have been made in recent years to develop new processes for the production of ultrafine-grained microstructure in metals and to investigate the mechanical properties of the resulting products [2].
Recently, applying severe plastic deformations during which intense extreme plastic strains are created in the metal part has been proposed as a promising approach to obtain ultrafine-grained structure, enhanced strength, and improved mechanical properties [3]. The resulting grain size and mechanical properties are dependent on different factors including the initial microstructure, the phase composition of the material, and most notably the method of applying the severe plastic deformation[4]. Several methods have been developed so far for creating severe plastic deformations in metals, all of which are capable of applying extreme plastic strains and fining the grains in single-crystalline bulk solids. The equal channel angular pressing (ECAP) method [5], the high-pressure torsion (HPT) process [6], the hydrostatic extrusion [7], the accumulative roll bonding (ARB) approach [8], the tubular channel angular pressing (TCAP) method [9], the parallel tubular channel angular pressing (PTCAP) method [10], the repetitive forging (RF) process [11], and the accumulative back extrusion (ABE) [12] are some of the most important techniques of the severe plastic deformation. Each of these techniques has different process specifications and is designed for specific applications. The ARB process considered in this paper is suited for the production of ultrafine-grained sheets with preferred mechanical properties. Many studies have been performed in the past on this process and the mechanical and metallurgical properties of its products made of different materials such as aluminum [13-15], copper [16, 17], and titanium [18, 19]. These studies indicated that the serious reduction in the sheet formability is the major drawback of this process limiting its application in the industry [20].
The formability behavior of sheet metals is dependent on multiple important factors including the material properties (such as strain hardening coefficient, anisotropy coefficient, and strain rate sensitivity) and the process parameters (such as temperature and strain rate). It has been observed in several studies that while increasing the strain rate leads to the increase of the material strength and the decrease of its formability in quasi-static conditions, the material behavior changes in very high strain rates, such that an increase in the strain rate improves the material formability in these conditions [21]. For instance, the investigation of Feng et. al [22] on the magnesium samples in different strain rates and temperatures demonstrated that the yield strength, the fracture elongation percentage, and consequently, the material toughness in high strain rates is higher than those in the quasi-static conditions. Furthermore, fractography of the test samples indicated that the fracture is more ductile in high strain rates compared with the quasi-static conditions.
In order to apply high strain rates and fast deformation in metals, several methods have been devised so far with promising potential for improving the metals formability with respect to traditional forming processes. Among these methods, explosive forming (EF) [23, 24], electrohydraulic forming (EHF) [25], and electromagnetic forming (EMF) [26] can be mentioned. In electromagnetic forming, the energy density of an electromagnetic field is employed for the fast forming of high electrical conductivity metals such as copper and aluminum [27, 28]. This process and its capability for improving the material formability have been the subject of some recent studies. Li et al. employed the EMF process to form Ti–6Al–4V titanium alloy sheets in high strain rate and increased the sheets forming limit by 24.37% [29]. Golovashchenko considered the formability of two aluminum alloys and the design of the coil in the pulse EMF process [30]. Their results indicated that forming in a conical die or into a V-shape die significantly improves the formability in comparison with the traditional process.
Despite these attempts, the effect of applying high strain rates on the properties of ultrafine-grained copper sheets has not been reported yet in the literature. This fact has motivated the present work to examine the effect of the high strain rate on the forming limit diagram and the formability of such sheets for the first time. For this purpose, the ARB process has been used to produce ultrafine-grained copper sheets in one, two, and three passes. Mechanical properties, microhardness, and fractography have been obtained for annealed and ultrafine-grained sheets. Furthermore, the Nakazima test has been used to predict the forming limit diagram of annealed and ARBed sheets in the quasi-static conditions. In the next step, EMF process has been performed in a circular open die (similar to that of the Nakazima test) to obtain the formability properties of these sheets in high strain rates. Finally, the results have been used to compare the properties of ultrafine-grained sheets formed under the quasi-static and high strain rate conditions. The details of investigations and results are presented in the following sections.
2.1 Metal samples
For performing the experimental tests, several belts of 0.5 mm thickness, 60 mm width, and 200 mm length were cut from a pure copper sheet. The chemical composition of the pure copper sheet obtained by the quantometery test is presented in table 1. To eliminate the effect of the production conditions, the samples were annealed for two hours at a temperature of 550° C and cooled down in the oven before performing the accumulative roll bonding process passes.
Table. 1 Chemical composition of the pure copper sheet
|
Element |
Wt% |
Element |
Wt% |
|
Zn |
0.0051 |
Al |
0.0019 |
|
Ni |
0.004 |
Si |
0.0014 |
|
Pb |
0.0027 |
S |
0.0011 |
|
P |
0.0024 |
Cu |
Rem |
|
Co |
0.0019 |
|
|
2.2 The accumulative roll bonding process
A schematic of the accumulative roll bonding process is shown in Figure 1. The most important part of the process is the sample preparation step. In this step, the potential contamination should be removed from the sheet surface to ensure a good metallurgical bond between the sheet layers. A suitable bond is crucial for accomplishing the accumulative roll bonding process. To prepare the samples for the Nakazima and other necessary tests in this work, samples were brushed by a stainless-steel wire brush and then were dipped for ten minutes in Acetone. This process is considered to be the best method for preparing the samples surface in the accumulative roll bonding process as suggested in previous studies [4, 31-33]. Next, two layers of sheet samples were stacked and fixed to each other tightly by fastening their corners with copper wires to avoid slipping during the accumulative roll bonding process [34]. Then the sheets were rolled to 50% of their initial thickness which is equal to the initial thickness of a single layer of the sheet. Again, the rolled sheets were cut into two similar pieces in the perpendicular direction to enable further passes on the ultrafine-grained sheets by the accumulative roll bonding process. The ARB process was accomplished in three cycles with a roll of 200 mm diameter.
2.3 Mechanical properties
To investigate the mechanical properties of samples, the tension test was performed according to ASTM E8/E8-m9 standard. Three standard tension test samples were prepared in the rolling direction by Wire Electro Discharge machine for annealed and ARBed samples. The tests were conducted with SANTAN testing machine at room temperature with a strain rate of 1*10-4 s-1.
2.4 Experimental determination of the forming limit diagram in static and high strain rate conditions
The Nakazima test was employed in this study to determine the forming limit diagram of ultrafine-grained sheets in the quasi-static condition. In this test, the sheet is clamped on an open circular die and is deformed by a hemisphere punch. Standard dimensions of FLD test samples are given in Figure 2 based on guidelines provided in [35]. Due to limitations in the production of ultrafine-grained sheet samples by the ARB process, performing the FLD test was not possible in standard dimensions. Therefore, the tests were carried out in ¼ size scales [4].
In order to perform the FLD test for each pass of the ARB process, six samples were prepared with ¼ scale of the standard dimensions, and circles of 2mm diameter were marked on their surface by an electrochemical marking machine. The SANTAN 50-Ton hydraulic press was employed for conducting the tests. The produced die was mounted on the press and the samples were clamped in the die one by one and deformed to the necking and cracking region by the punch. The initially marked sheet, the Nakazima test die, and samples after the FLD test are shown in Figure 3.
After Nakazima test, the circles marked initially on samples transform to ellipses due to applied strains. In the next step, the major (d1) and minor (d2) diameters of the ellipses were measured by a Myler ruler and the values of major and minor strains were determined by Eqs. (1) and (2). This way, a set of points was obtained and the FLD was determined by constructing a curve that passes through these points.
Since there is no standard for obtaining the FLD in the high strain rate condition and considering the limitation on the dimensions of the ultrafine-grained sheets produced by the ARB process, samples were prepared with a width of 25 mm and 50 mm. In order to determine the strains developed in the sheets, circles of 2 mm diameter were marked on their surface by the electrochemical marking process. Moreover, for high strain rate deforming of the samples in the electromagnetic forming, a circular coil was used along with an open circular die instead of punch employed in the standard FLD test. To eliminate the geometry effect in the comparison of these processes’ outcome, the size of the coil in the electromagnetic forming was set equal to that of the punch in the Nakazima standard test. The coil had an outer diameter of 100 mm and was wound in five turns from a copper belt of 8 mm thickness produced by a CNC milling machine. To avoid an electric discharge, Kapton adhesive tape was used as insulation between the coil turns, the coil and the die, and also the coil and the sheet. The employed die was composed of two upper and lower plates, of which the former held the coil, and the latter contained a hole of 140 mm diameter to allow passage of the electromagnetic field. The deformations were produced using a capacitor bank of 32 microfarads and by applying a voltage discharge of 10kV on the two ends of the coil. Figure 4 shows the schematic of electromagnetic forming, the coil, and the die used for electromagnetic forming.
3.1. Mechanical properties
Tensile tests were conducted to investigate the mechanical properties of the samples produced by the accumulative roll bonding process. The true stress-strain diagrams of the annealed and one, two, and three-pass ultrafine-grained copper sheets are shown in Figure 5. After each pass of the ARB process, the tensile stress of the sample increases while its elongation decreases. The most notable variation occurs in the first pass, and after that the slope of variation reduces in the later passes. The variation in the yield strength and elongation stems from two factors. The first factor is the strengthening due to the increase of the dislocations density resulted from the severe plastic deformation in the material during the ARB process [4]. The second factor is related to the grain size. Based on the Hall-Petch relation, the material strength is inversely proportional to the square of the grain size. Therefore, the material strength increases by decreasing the grain size [36]. The Formation of ultrafine grains in the range of some hundreds of nanometers after multiple passes of the ARB process leads to reduction of the dislocations effect on the increase of the material strength and hardness [37]. This factor reduces the contribution of the dislocations density in the material strengthening.
As can be seen in figure 5, the ultimate strength and elongation have, respectively, experienced significant increase and decrease during the first pass where the increase of the dislocations density is the dominant mechanism. While, in the next passes the rate of the increase in the ultimate strength and decrease in the elongation reduces owing to the saturation of the dislocations density and decrease of the strain hardening effect on the mechanical properties. The grain refinement is the dominant mechanism in these passes.
The results of the hardness test on the annealed and ARB processed samples are illustrated in Figure 6. According to this figure, the microhardness of the copper sheet increases significantly in the first pass of the ARB process, while the rate of increase reduces in the next passes. The hardness has raised from 65.8 VH for the annealed sample to 119.8 VH for the one-pass sample, the main reason of which is the considerable amount of strain hardening and increase of dislocations [38]. The hardness value has increased with a lower slope during passes two and three and reached 124.8 and 132 VH, respectively. Saturation of the dislocations density is the major reason for decrease of the hardness variation slope [39].
3.2. FLD determination for the Static and high strain rate conditions
Figure 7 shows the samples after Nakazima test. The principal diameters of the ellipses were measured on each sample to calculate the major and minor strains after the loading and draw the forming limit diagram.
The forming limit diagram for annealed and ARB processed copper samples after one, two, and three passes is presented in Figure 8. It can be seen that after performing the first pass of the ARB process on the sample, the area under the FLD severely decreased compared to that of the annealed sample. This mainly stems from the significant strain hardening that occurs in the first pass and its considerable effect on the forming limit diagram. By increasing the number of accumulative roll bonding passes, the area under the FLD curve increases gradually due to the reduction of the strain hardening effect, improvement of grain refinement in the microstructure, and homogenization of the dislocations density [4].
The variations of the elongation and the lowest major limit of FLD for copper samples after different passes of the accumulative roll bonding process are shown in Figure 9. It is observed that the lowest major limit of FLD and the elongation of the samples after the first pass are considerably lower than those of the annealed one. However, the elongationand especially the lowest major limit of FLD exhibit an increasing trend in the later passes. The cold work can be regarded as the main factor in the decrease of the elongation and the descend of the FLD curve area in the first pass of the accumulative roll bonding process [40, 41].
As previously described, the EMF test was performed on samples with widths of 25 mm and 50 mm. The mode of deformation in the sample of 25 mm width was close to the plane strain condition. The results for the forming limit diagram of ARBed samples in the quasi-static (Nakazima test) and high strain rate (EMF) conditions are presented in Figure 10 for the 0.5 mm thickness sheet. As shown in this figure, the value of the lowest major strain in on-pass samples increased about 102% from 9.2 to 18.6. This increase was about 113% for the two-pass sample and in this case, the lowest major strain increased from 9.8 in the static condition to 20.96 in the high strain rate condition. Finally, in the three-pass sample, the plane strain fracture limit in the high strain rate was approximately double of the limit in the static condition and increased from 10.7 to 21.29. This improvement in the samples formability can be attributed to the inertial effect and high-velocity impact between the die and metal sheet in the EMF process [30, 42-44]. The inertial effect was also introduced as the key factor for the improvement of the formability in the EMF of titanium sheets in [29, 45].
3.3. Fractography
The fracture surface morphology of samples after electromagnetic forming and the Nakazima test are shown in Figure 11 (a-e) for the one, two, and three-pass ARBed copper. Based on this figure, by increasing the number of passes, dimple size decreases due to high strain and severe grain refinement during the ARB process [46]. The scanning morphology of the fracture zone shows that in electromagnetic forming (high strain rate), the dimples are deeper than that in Nakazima test (quasi-static) counterpart, indicating that ultrafine-grained copper has better formability in high strain rate. Moreover, the dimples in electromagnetic forming are isotopically directed, while those produced in the Nakazima test are oriented in a certain direction as shown by red arrows.
In this paper, one, two, and three passes of the ARBprocess were employed to produce ultrafine-grained pure copper sheets. The strength and formability of these sheets were investigated by tensile test, microhardness test, Nakazima test, and electromagnetic forming.
It was observed that
- After each pass of the ARB process, the tensile strength of the samples increases while their elongation (formability) decreases. The most notable variation occurs in the first pass, and the slope of variation reduces in the later passes.
- The microhardness of the copper sheet increases significantly in the first pass of the ARB process, while the rate of increase reduces in the next passes.
- The formability of the ARBed sheets is much higher in the high strain rate condition (the electromagnetic forming) than the quasi-static condition (Nakazima test). Namely, the formability of the one-, two-, and three-pass ARB processed sheets increases 102%, 113%, and 99%, respectively, in the high strain rate with respect to the quasi-static condition.
- The dimples of the fracture surface in the electromagnetic formed samples are deeper compared to the Nakazima test. Furthermore, these dimples are isotopically directed in the electromagnetic forming, while they are oriented in a certain direction in the Nakazima test. These observations prove the superior formability of ultrafine-grained copper sheets in the high strain rate condition.
Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Conflicts of interest/Competing interests: The authors declare that they have no conflict of interest.
Availability of data and material: Not applicable.
Code availability: Not applicable.
Authors' contributions: All authors contributed equally to this work.
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