Commercial electrical conductor wires are currently produced from aluminum alloys by multi-step deformation processing involving rolling and drawing. These processes typically require 10 to 20 steps of deformation, since the plastic strain or reduction that can be imposed in a single step is limited by material workability and process mechanics. Here, we demonstrate a fundamentally different, single-step approach to produce flat wire aluminum products using machining-based deformation that also ensures adequate material workability in the formed product. Two process routes are proposed: (1) chip formation by free-machining (FM), with a post-machining, light drawing reduction (<20%) to achieve desired finish and (2) constrained chip formation by large strain extrusion machining (LSEM). Using commercially pure aluminum conductor alloys (Al 1100 and EC1350) as representative material systems, we demonstrate key features of the machining-based processing, including (a) single-step processing to achieve flat wire geometries, (b) surface finish (Ra = 0.2 to 1.0 μm) comparable to that of commercial wire products made by drawing/rolling, (c) deformation control independent of wire size, and (d) hardness increases of 50–150% over that of annealed wires, while retaining high electrical conductivity (>56% IACS). The wire microstructure, which can also be varied via the large-strain deformation parameters, is correlated with mechanical and electrical properties. Implications for commercial manufacture of flat wire products are discussed.
Aluminum 1100 (99%) and electrical conductor (EC) grade 1350 (99.5%) are the main commercially pure aluminum alloys used in electrical conductor applications because of their high electrical conductivity and inherent capability to be strengthened by cold-working. In the annealed (O-temper) condition, these alloys have very low strength (UTS < 90 MPa), but high ductility (elongation (EL) > 40% in 50 mm) and high electrical conductivity (57–62% IACS, depending on purity) . This low strength precludes their direct use in the annealed (or as-cast) condition in many electrical applications. Therefore, various strain-hardened tempers, ranging from H12 (quarter-hard) to H18 (full-hard) and H19 (extra-hard), to improve strength have been developed utilizing plastic deformation techniques such as wire drawing and rolling [2,3]. Strain hardening is the preferred mode of strengthening, as it does not significantly reduce the electrical conductivity, in contrast to solid solution strengthening.
Typical conductor wire forms are round and flat wire. Flat wire is produced from rolled or drawn round wire by additional flat rolling steps. The H18 condition corresponds to a maximum cold rolling reduction of 75%, with residual ductility of only 1–4% uniform EL depending on the form and process. The H19 temper, the highest commercial strain-hardened temper, corresponds to the maximum possible plastic reduction that leaves a minimum ductility of 1.2% EL  in round wire forms. Stranded conductors such as the Aluminum Conductor Steel Reinforced Cable (ACSR) are hard-drawn to the H19 condition to meet the high strength requirements of outdoor power transmission. Bus conductors and transformer strips, among many other conductor forms, are also produced in different tempers to meet mechanical and physical property needs desired in the applications.
Before the advent of continuous wirebar casting over 50 years ago, delivering the wire products in the various temper conditions was achieved through a long and expensive, three-stage processing route—casting ingots, rod rolling, and wire drawing . Still a standard today, coilable wirerod commonly called redraw rod is produced in standard sizes of 9.5 and 12.7 mm diameter [5–7]. From the rod rolling stage, the redraw rod is then processed through a series of incremental wire drawing or cold rolling passes/steps (typically 10 to 20) to the final round or flat wire form. Standardized flat wire dimensions range between 1.6 mm and 12.5 mm in width and 0.6 mm to 7 mm in thickness . Typical final round-wire diameter ranges between 0.2 mm and 6.5 mm . To reduce the number of production steps and cost of producing wire-rod, almost all of the recent industrial casting practices have fused the casting and rod rolling stages into a single continuous in-line process. Wheel-belt continuous casting machines feed the cast bar directly into a multi-stand rolling mill (Fig. 1). Properzi and Southwire continuous casting systems are examples of commercial processes used in the production of continuously cast and rolled Al wirerods . Nevertheless, multiple reduction steps (typically 10 to 20) by hot rolling are still an intrinsic feature of wire-rod production by these in-line processes.
While there have been the aforementioned advances in wirerod production, the cold-working stage in the process stream, which provides the preferred means of strengthening due to its small effect on electrical conductivity, has not seen any revolutionary measures that can reduce the number of deformation steps and yet deliver wire with the required properties. To avoid defects such as cracks and wrinkles, the typical area reduction per pass in cold rolling and wire drawing is seldom greater than 30% . This low reduction per pass necessitates the use of many deformation passes, typically 10 to 20, to achieve the large shape change and desired mechanical properties in the wire products, especially hard-drawn wires (see stages enclosed in a dotted rectangle in Fig. 1). The overall wire process efficiency can be greatly improved if the number of passes in the plastic deformation processing stage can be reduced; this is the focus of the present study.
The mechanism of strengthening through cold-work is primarily by generation and interaction of dislocations. At high plastic strains, strengthening by grain refinement also becomes a key contributing factor. This latter mechanism has motivated the exploration of severe plastic deformation (SPD) techniques, which can impose large strains well beyond what is achievable with conventional cold rolling and drawing methods. Equal Chanel Angular Pressing (ECAP) and some of its variants are prime examples of SPD techniques, which have shown promise in enhancing mechanical strength through grain refinement. But ECAP processes also involve multiple passes of deformation, with as many as seven to nine passes typically reported to reach the desired large strains [11–14]. Another drawback of ECAP is its inability to produce long continuous product forms of small cross section due to the discrete (small length) billet workpiece. This has been partially rectified by the development of ECAP-Conform, which combines ECAP and the continuous extrusion process known as Conform, to produce continuous wire forms [15,16]. In as much as ECAP-Conform can also affect grain refinement and improve mechanical properties, it is a complex process to setup and requires a workpiece that is already available in long lengths. And, equally importantly, achieving a high degree of strain hardening still requires multiple passes of deformation [15,17–19].
In this study, we demonstrate a fundamentally different, single-pass (deformation processing) approach to producing flat wire Al products. The approach uses shear-based deformation processing structured around chip (wire) formation by machining processes—Free Machining (FM) and Large Strain Extrusion Machining (LSEM) (Fig. 2). The processing has the potential to replace the many steps in the current hot/cold rolling processing with a single processing step, thus making flat-wire products manufacturing less complex. The work builds on the prior success of LSEM in producing bulk foil and strip forms, with fine-grained microstructures and improved mechanical properties (strength and formability) from structural alloys such as magnesium, copper, aluminum, and iron [20–22].
2 Shear-Based Deformation Processes
Unlike conventional cold rolling and wire drawing, the strain imparted in FM and LSEM can be controlled independently of the final wire thickness via α and λ. In FM, λ increases with decreasing α, but is otherwise governed by the material deformation response; the more ductile alloys tend to give larger λ and thus a higher strain at a given α. However, in the LSEM, both α and λ (and thus wire thickness and strain) can be set a priori. At high deformation speeds (rates) of 1–5 m/s, the PDZ deformation is near-adiabatic with high strain rates and high temperatures. Another special feature of the PDZ in the LSEM is the high hydrostatic pressure due to the “compression coupling” of the cutting tool with the constraining tool. This hydrostatic pressure, normalized by the shear yield stress, k, is typically ∼2 for LSEM . In contrast, in the deformation zone of (frictionless) conventional extrusion, it is only ∼0.85 for die half angle of 30 deg . The combination of high hydrostatic pressure and adiabatic heating in LSEM is favorable for alloy workability and hence beneficial for flat wire or rod production [21,26].
3 Experimental Details
The alloys used in the experiments are two of the main commercially pure aluminum alloys, Al 1100 and EC1350, used in electrical conductor applications. Table 1 gives their nominal chemical composition. The main difference between the alloys is in their impurity content (most importantly, Fe and Si).
For the wire production experiments, disk-shaped workpieces, 6 mm thick × 140 mm diameter, were machined from annealed plates of the two alloys. The annealed (initial) workpiece condition was selected as it resembles an as-cast condition in terms of the initial (large) grain size. The Al 1100 workpiece disks were machined from a ¼-in thick plate in the half-hard (H14) condition (McMaster-Carr, Elmhurst, IL). The EC1350 was not available in the plate form. Hence, plates were produced by melting, casting, and cold rolling certified wirerod obtained from a leading flat wire manufacturer, Prysmian Group, Indianapolis, IN. The disks were then annealed at 300 °C for 2 h prior to the LSEM and FM.
The fully annealed microstructures in the workpieces are shown in Fig. 3. This initial workpiece condition was chosen intentionally, so that process capability to make wire could be demonstrated even for alloys of low workability arising from the gummy behavior typical of annealed Al. Both alloys exhibit an equiaxed grain structure with average grain size of ∼50 µm in the Al 1100 (Fig. 3(a)) and ∼200 µm in the EC1350 alloy (Fig. 3(b)). The variation in the grain size of the alloys after the same annealing treatment (300 °C/2 h) can be attributed mainly to their impurity differences. The higher concentrations of Al-Fe particles and impurity solute in solution in the Al 1100 alloy (Table 1) inhibit recrystallization and grain growth during annealing.
In the plane-strain deformation configuration of Fig. 2, the width of the wire produced is equal to the workpiece disk thickness; therefore, any desirable wire width can be produced by adjusting the workpiece disk thickness. The deformation conditions were selected based on preliminary FM experiments as rake angle, α = 20 deg, undeformed chip thickness, t0 = 0.13 mm, and cutting velocity, V0 = 6 m/s. Under similar cutting conditions, the thickness of the wire produced in the (unconstrained) FM sets the upper limit for achievable LSEM wire thickness. Hence, any specific wire thickness smaller than that of the FM wire can be obtained in the LSEM by adjusting the constraint (λ value). This capability is demonstrated in the study by performing LSEM under different levels of constraint (λ) in the range of 1–5.
The microstructure of the workpiece and wire samples was characterized by optical metallography. For this purpose, specimens were mounted in epoxy to avoid heating, ground with various grit sizes of SiC paper, and finally polished with diamond and colloidal silica (0.06 µm). The specimens were etched by immersion in NaOH etchant (10 g in 100 mL H2O) for 5 min. The microstructure was then observed under an optical microscope.
Surface quality (topography, roughness) of the wires was measured with a 3D optical profilometer (Zygo NewView 8000). Surface profiles were generated from an area, 3 mm by 3 mm, at a lateral resolution of 2.9 µm. Both surfaces of the flat wire were profiled. Following ASME (B46.1-2019) standards  and Rank Taylor Hobson  recommendations, the Ra values were determined by applying a Gaussian filter with a cutoff wavelength of 250 µm. This filter cutoff was deemed to be best suited for the topographical features observed on the wire surfaces. A minimum of 20 line scans across the wire surface was considered. The direction of the line scans was selected to be parallel to the wire length-direction since this is perpendicular to the lay of the FM wire-free surface.
Two key property performance attributes for the wire are mechanical strength and electrical conductivity (=1/resistivity). The strengths of the workpiece and wires were characterized by Vickers hardness measurements using a Wilson microhardness tester (Tukon 1202). At least 10 indents at 50 gf load were made on each sample. The indent separation distance was kept sufficiently large, so as to avoid any errors due to overlapping of plastically deformed regions of neighboring indents. The mean +/- one standard deviation values are reported for the hardness.
Electrical resistivity was determined from resistance measurements made on 0.6 m, 0.8 m, and 0.9 m gauge lengths of the wires inline with ASTM standard B193 . The 4-point probe measurement technique, which eliminates contact and leads wire resistances, was used to ensure high accuracy in these measurements. Due to the low resistivity of the alloys, the resistance was sensitive to small variations in wire dimensions. Therefore, wire cross-sectional areas were computed from density and measured mass of the samples for the given wire lengths. The electrical conductivity in %IACS (International Annealed Copper Standard) was calculated by taking the ratio of the Cu wire standard resistivity of 1/58 µΩmm2/m  to the measured resistivity of the wires. The wires were also annealed, and the microhardness and electrical conductivity measurements were repeated, to determine the properties of the wires in the O-temper condition.
4 Results and Discussion
4.1 FM and LSEM Wires.
Long, continuous flat-wires, 6 mm wide and 0.2 to 0.9 mm in thickness (rectangular cross sections), were produced from the disk workpiece using the FM and LSEM (Fig. 4). The uniformity in the thickness along the wire length in the LSEM and FM + Drawing (D) wires was determined from caliper measurements. These variations were found to be quite small, <0.2% of wire thickness. The FM wires exhibit a smooth and shiny appearance on one face—the side in contact with the tool rake face—and a matte (dull) surface on the other side (chip free surface). The latter is a consequence of the unconstrained flow of the material (Fig. 4(a)). The LSEM wires, on the other hand, show a similar, smooth shiny surface finish on both of the wire faces—the rake and constraint face sides (Fig. 4(b)). The LSEM constraint prevents material flow normal to the surface on the backside of the wire and this restraint is the primary reason for the improved smooth appearance.
It is useful to examine the nature of the flow on the free surface of the wire in FM in order to understand the dull surface appearance. The material flow behavior in machining is characteristic of the alloy and its initial deformation state . In conventional cutting or FM, ductile alloys such as the commercially pure aluminum of the present study exhibit a highly unsteady mode of material flow during chip formation—sinuous flow—characterized by extensive redundant deformation and vortex-like flow components . The unsteady nature of this flow is responsible for the poor machinability of these alloys (workability), a characteristic often referred to as “gummy.” This results in unusually thick wires in FM, with thickness ratios of λ > 5 in both alloys (Table 2), and fine-scale roughness on the wire free-surface with the dull appearance. The extensive thickening also implies that large strains are imposed in the wires (Eq. (1)). Based on dimension measurements on the wire, as well as direct in situ analysis of material flow via high-speed imaging, the effective strains in the FM wires were estimated, by the upper bound model, to be in the range of three to four for the two alloys (Table 2). These estimates are necessarily conservative, given the redundant deformation resulting from the sinuous flow. The Al 1100 LSEM wire shown in Fig. 4(b) was produced at λ = 5, corresponding to an effective strain of 2.8 (Eq. (1)). Keeping the undeformed chip thickness (to) constant, thinner wires, with smaller strain ∼0.9, could be produced by setting λ = 1.5 (Table 2). It should be noted that LSEM offers independent control of strain and thickness via the tw and λ parameters.
4.2 Surface Roughness.
Optical profilometry (Figs. 5(a) and 5(b)) revealed that the unconstrained sinuous flow occurs across the entire width of the free (back) surface of the FM wires, resulting in ridges running perpendicular to the wire flow direction (WFD). This sinuous flow is triggered by plastic buckling of grains followed by material folding in the deformation zone . The higher frequency of these ridges in the Al 1100 alloy, as shown by the height profiles in the WFD-RFN plane along the length, correlates with the smaller starting grain size of the Al 1100 alloy (∼50 µm), as against a smaller frequency in the larger grain size (∼200 µm) EC1350 alloy. Interestingly, with fine-grained/pre-worked alloys, with less ductility, the flow is usually uniform with much smaller ridges on the wire back surface . Additionally, fine tool marks can be seen along the WFD arising from the nonuniformity of the tool cutting edge.
The surface roughness along the wire back surface, expressed in terms of the arithmetic mean height (Ra), is 4.5 µm and 8.9 µm for the Al 1100 and EC1350 alloy, respectively. These represent somewhat rough surfaces for wire products, beyond the typical finish limits (Ra = 0.8 to 3.3 µm) for commercially drawn wires , as well as of rolled products (Ra = 0.4 to 0.8 µm) [34,35]. It arises mainly from the sinuous flow mode coupled with the large initial workpiece grain size (50–200 µm) and necessitates the use of a follow-on, light-cold drawing step to improve the surface finish. As will be seen, this roughness is minimized in the LSEM by use of the constraint, which suppresses the sinuous flow. In contrast, the shiny wire surfaces that arise from contact with the tool rake face (Fig. 4) have much smaller Ra values, between 0.24 and 0.31 µm (Figs. 5(c) and 5(d)), with both the Al 1100 and EC1350 alloy. This is more than an order of magnitude smaller than the Ra on the FM wire back surface, and far superior to that of drawn or rolled wire products.
By applying a sufficient constraint to the chip back surface, the degree-of-freedom available for sinuous flow development within the deformation zone is removed . This is what is done in the LSEM by adjusting the constraint level (λ) in the deformation zone. Furthermore, under these conditions, the wire thickness (tw) is now pre-determined (Fig. 2(b)) and even independent of the strain. The LSEM constraint greatly reduces the surface roughness (Ra = 0.36 µm) on the wire back surface, see Fig. 6, compared to that in the FM. In fact, this roughness is now close to that recorded (Ra = 0.25 µm) on the wire rake-face side. It also shows that the constraint contact condition is very similar to that on the rake face. Overall, the LSEM wire surface roughness is superior to that of wires produced by cold rolling or drawing . The very good surface finishes resulting from the LSEM offer a promising pathway to producing flat wires directly from annealed/as-cast workpieces, with significantly reduced processing steps compared to the traditional wire processing routes.
4.3 Mechanical Properties.
The high strain levels (∼1 to 4) in the FM and LSEM wires (Table 2), and associated hardening, are similar to those occurring in SPD processes like ECAP. An important distinction is that in the machining-based processes, unlike the SPD processes and conventional rolling/ drawing, the shape change and large plastic strains are accomplished in a single deformation pass.
Table 3 compares Vickers hardness and electrical conductivity of the machined wires with those of cold-rolled and drawn wires in the H18 and H19 conditions. The utility of the shear-based processes in increasing the hardness (strength) of the wires is evident. The hardness of these wires is 1.5 to 2.5 times that of the annealed workpieces (∼25 HV), this increase being a consequence of the large strains imposed. The hardness of the FM and LSEM wires compare favorably with those processed by multi-step cold rolling and wire drawing (H18 and H19 hard-rolled/drawn temper hardness), see Table 3. They are also close (within 15%) to that of commercially pure Al wires produced by the ECAP-Conform process after four passes . The flexibility in controlling the deformation in the wires, via the shear-based processes, adds to the versatility of these processes.
|Alloy||Condition||HV||Electrical conductivity (%IACS)|
|Al 1100||Annealed||27 ± 2||57.9 ± 0.2|
|FM||63 ± 1||56.0 ± 0.2|
|LSEM||48 ± 1||58.2 ± 0.1|
|EC1350||Annealed||25 ± 2||63.5 ± 0.5|
|FM||56 ± 1||60.2 ± 0.7|
|LSEM||45 ± 3||62.3 ± 0.4|
|Alloy||Condition||HV||Electrical conductivity (%IACS)|
|Al 1100||Annealed||27 ± 2||57.9 ± 0.2|
|FM||63 ± 1||56.0 ± 0.2|
|LSEM||48 ± 1||58.2 ± 0.1|
|EC1350||Annealed||25 ± 2||63.5 ± 0.5|
|FM||56 ± 1||60.2 ± 0.7|
|LSEM||45 ± 3||62.3 ± 0.4|
Another mechanical property attribute, i.e., formability, of the FM and LSEM wires is illustrated in Fig. 7. Despite the very large level of plastic strain imposed during the FM (ɛ = 3.2), the wires are seen to exhibit surprisingly high formability in 0 T bend testing under plane strain conditions. At the bend radius of “zero,” where the wire is flattened onto itself, the maximum circumferential (bend) strain is at least ɛ ∼ 1.0 on the tension side of the bend, as determined from the bend radius and wire thickness . And there is no visible evidence of cracking on this surface despite the highly strained outer fibers (Fig. 7). Based on this attribute seen in the bend test, the wires should also have a higher ductility in tension compared to the H19 temper at similar strength . Further work is needed to establish, more quantitatively, the origins of this high ductility. It is hypothesized that this high ductility is a consequence of strong crystallographic shear textures imposed in the wire by the machining , unlike the case of rolling or drawing [39,40].
4.4 Electrical Properties.
Plastic deformation generally has little effect on the electrical conductivity of metals, in contrast to even small impurity concentrations. This is due primarily to the low potency of dislocations to cause scattering of conducting electrons [41,42]. Hence, only a very small decrease (<5%) in electrical conductivity, with respect to the annealed condition, occurs in wires produced both by the single-step, shear-based processes, and the multi-step, conventional processes (Table 3). On the other hand, the effect of the metallurgy on the conductivity is much greater. Due to its lower impurity content, the EC1350 shows a much higher electrical conductivity (63.5 IACS) in the annealed condition than the Al 1100 (57.9 IACS). Thus, for purposes of strengthening without compromising on electrical conductivity through the application of high plastic strains, the efficacy of the single-step FM and LSEM techniques cannot be overemphasized.
The microstructures of the FM and LSEM wires were analyzed by optical microscopy. Figures 8(a) and 8(b) show microstructures of the Al 1100 and EC1350 wires (longitudinal cross sections) produced by the FM process. Based on the flow line pattern, two regions of intense deformation can be identified. The first is due to the primary shear plane deformation, which gives rise to the nearly straight flow lines in the bulk of the wire that are oriented nearly perpendicular to the wire face (see PDZ in Fig. 8(b)). These flow lines are lines of maximum elongation and not the direction of maximum shear (shear plane), as is sometimes mistakenly assumed . The second is due to frictional deformation at the tool-wire contact along the rake face, region SDZ (Secondary Deformation Zone) in Fig. 8(b). Because of the frictional drag along the rake face, these lines are inclined at an acute angle, almost parallel, to the wire face in contact with this face. The large deformation within these zones causes the grains and second phase particles to be reoriented along the flow lines. The angle between the flow lines and the shear plane is directly related to the shear strain imposed in the PDZ. In contrast to the primary deformation, which is imposed throughout the wire, the SDZ arising from the friction is quite shallow, extending only about 5–10 µm deep into the wire from the rake face.
Figures 8(a) and 8(b) also show that the free (back) surface of the two alloy wires has some mushroom-like structures, all along this surface. These structures arise from the unconstrained flow of the material as noted earlier and constitute the roughness that gives rise to the matte appearance for this surface in Fig. 4(a). The average thickness of these structures is a small fraction of the total thickness of the wires; hence, their effect on the wire electrical properties is small especially for applications that require high packing density of the wires. Under the same cutting conditions, the EC1350 wire has a greater thickness, and thus higher effective strain, than the Al 1100 wire; this is due to its greater ductility arising from lower impurity levels and larger grain size.
The application of the constraint via LSEM suppresses the formation of the mushroom-like structures (unsteady flow/instability) on the backside of the wire (Fig. 9). Hence in contrast to the FM wire, the LSEM wire shows negligible surface waviness along the WFD, besides a smooth shiny surface. The Ra values for this wire on both the constraint and rake faces are 0.36 and 0.25 µm (Fig. 6), which compare closely with the rake face values for the FM wires.
Unlike FM, the LSEM wire microstructure contains an additional SDZ due to the wire drag-interaction with the constraining tool (Fig. 9). The SDZs are highly deformed regions, as revealed by the flow line structure, and the depth to which these lines extend into the wire thickness shows the extent of the surface strain. The SDZ on the rake face side of the wire due to the main cutting tool extends to a greater depth than the one due to the constraining tool, as shown in Fig. 9(a) for λ = 5. It is a consequence of the larger stresses imposed by the cutting tool. These differences in the secondary shear zone extent are enhanced with decreasing λ, as shown in Fig. 9(b).
Commercially pure aluminum alloys in a strain hardened condition are highly susceptible to recovery and recrystallization, even at temperatures as low as 80 °C . The calculated adiabatic temperature rise, based on measured cutting forces for λ = 5, is 100 °C to 125 °C and increases with decreasing λ . Due to greater deformation-induced adiabatic heating, the LSEM wires undergo dynamic recovery or recrystallization depending upon the extent of the constraint used. Figure 9(c) shows a recrystallized microstructure in LSEM wire with λ = 1.5 for the EC1350 alloy. This is reflected in the substantially smaller grain size of ∼5 µm in the wire, as against a starting grain size of ∼200 µm. This example of deformation control in LSEM illustrates how finished wire with an annealed microstructure can also be produced in a single step, by making use of in situ dynamic recrystallization, without the need for a separate annealing heat treatment.
4.6 Drawing of FM Wires.
Since one (back) surface of the FM wires is quite rough relative to drawn/rolled products, the use of small drawing reductions to the wires, post FM, was investigated as a means of improving the surface finish. This combined process is termed the FM + D process. Figure 10 shows FM Al 1100 alloy wire after a small 20% cold-drawing reduction using a tool steel drawplate with rectangular die openings. The starting structure was similar to that shown in Fig. 8(a). The mushroom-like structures on the original wire-free surface are nearly eliminated. Surface profilometry showed a reduction in the free surface roughness from ∼4.5 µm Ra (initial FM condition) to 0.93 µm Ra in the FM + D condition (Fig. 10(b)). This is well within the range of commercial wire products , as discussed earlier. Interestingly, the mushroom-like features at the free surface do not bend or fold due to the drawing. Likewise, no significant changes in the FM flow line profiles occur after drawing, nor are any fractures observed. These observations show that the FM wires, despite the high degree of strain-hardening by the cutting shear-deformation, still have significant residual workability and can be drawn to substantial reductions if needed. This workability is most likely a consequence of favorable shear textures imposed by the cutting process . Thus, applying a secondary process such as drawing, with a small reduction (<20%), provides a practical route to producing FM wire with a surface similar to commercial wire products. However, this finish is still inferior to LSEM wire products. Although the FM + D process uses an additional processing stage beyond FM for improving surface finish, it still offers a substantial reduction in the number of processing steps vis-a-vis rolling/drawing. The latter processes require at least 10 steps, to reduce a standard wire rod, 9.8 mm initial diameter, at a typical reduction of 35% per pass, to a wire product of equivalent dimensions as the FM + D wire.
Two machining-based processes, large strain extrusion machining (LSEM) and free machining (FM), are demonstrated for single-step, shear-based deformation processing of aluminum electrical conductor wires from the annealed large-grained feedstock. Long, continuous flat wires, 6 mm wide and thickness ranging between 0.2 and 0.9 mm, and with smooth surface finish, Ra ∼ 0.2 to 0.4 µm, are produced in a single step by LSEM. The LSEM wire finish is superior to that of cold-rolled or drawn wires. With the FM process, while one of the wire surfaces is again quite smooth as in the LSEM, the other back surface of the wire has roughness higher than that of commercial rolled/drawn wire products. However, a light cold-drawing/rolling step (∼20% thickness reduction), post the cutting, is sufficient to improve the surface finish into the range typical of commercial wire products. The processing of wires by FM + D is still much simplified compared to the commercial multistage flat-wire production.
The wires produced by the shear-based processes exhibit better or comparable combinations of strength and conductivity compared to H18 or H19 commercial wires of the alloys. This is due to the imposition of deformation strains of up to 4 in the shear-based processing, which increases strength without compromising on electrical conductivity. LSEM, as a process, is more flexible than FM or multistage cold rolling, since the wire thickness and strain can be independently controlled; and a range of microstructures, from highly cold-worked to partially annealed, can be produced by varying the deformation-induced heating. The wires even in the highly strain-hardened condition are found to possess sufficient formability, thus making them amenable to secondary processing through wire drawing or rolling.
The results thus show that shear-based processing can be used to produce wire products with high strength, conductivity, and formability, all in a single step of deformation. The wires can also be used as half-products for further processing by conventional forming methods. The shear-based processing framework likely uses less energy and has a smaller production cost.
This work was supported in part by NSF grants CMMI-1562479 and DMR-1610094 and the US DOE-EERE program via Award DE-EE0007868. We would like to thank Prysmian Group (General Cable), Indianapolis, for providing us with commercial samples of the 1350 alloy and for discussions.