Multiple direct extrusion: A new technique in grain refinement

Materials Science and Engineering A 550 (2012) 293–299

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Multiple direct extrusion: A new technique in grain refinement L. Zaharia, R. Chelariu, R. Comaneci ∗ “Gheorghe Asachi” Technical University of Iasi, Faculty of Materials Science and Engineering, D. Mangeron 61A, 700050 Iasi, Romania

a r t i c l e

i n f o

Article history: Received 24 November 2011 Received in revised form 20 April 2012 Accepted 24 April 2012 Available online 30 April 2012 Keywords: Extrusion Bulk deformation Grain refinement

a b s t r a c t A novel high-straining bulk deformation technique based on repeating conventional direct extrusion is presented. This technique, named multiple direct extrusion (MDE), uses a square container with a rectangular die aperture that can achieve a minimum 50% reduction/pass in the cross section of the billet. After extrusion, the new billet is cut perpendicular to the longitudinal axis. The resulting halves are then joined to obtain a square shape again so that the direct extrusion process can be repeated. Two processing routes are possible before reintroducing the billet into the container: no rotation and 90◦ rotation around the longitudinal axis. During each cycle, the billets change their geometrical shape and as a result, the cross section area gets smaller. A mechanism of grain fragmentation during MDE based on the analysis of velocity discontinuities along slip lines in the deformation zone is suggested. Four cycles of MDE were applied to commercial copper and the potential for grain refinement, and the improvement in mechanical properties were evaluated. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent decades, so-called “top-down” techniques have been used to realize the direct conversion of bulk metallic materials from their conventional grain size to an ultra-fine grain (UFG) (100 nm–1 ␮m). This “top-down” approach includes severe plastic deformation (SPD) techniques [1], which are generally repetitive deforming processes. In these processes, a specimen is deformed several times below its recrystallization temperature without any change in its cross-section. There are some well-known SPD techniques frequently used for grain refinement such as equal channel angular pressing (ECAP) [2], accumulative roll-bonding (ARB) [3], high pressure torsion (HPT) [4], cyclic extrusion–compression (CEC) [5], repetitive corrugation straightening (RCS) [6], and multiaxial compressions/forgings (MAC/F) [7]. The SPD processes are attractive due to changes in microstructural refinement and the increase in mechanical properties that occurs when the grain reaches submicron size [8]. Direct extrusion produces a large strain, and therefore this process has been investigated for grain refinement. Using high ratio extrusion (HRE) in one/two steps [9,10] or hydrostatic extrusion with an extra high ratio [11], favorable results in grain refinement were reported. Specifically, in the HRE process, the grain, which started at an initial size of 100 ␮m, reached 5 ␮m after one step and 2 ␮m after two steps. However, some practical issues such as the large deformation force, high pressure in dies, and heating due to

∗ Corresponding author. Tel.: +40 232 27 86 83; fax: +40 232 23 00 09. E-mail address: [email protected] (R. Comaneci). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.04.074

the thermal effect of deformation are common phenomena during HRE, making this technique difficult to use. A new technique for grain refinement, named multiple direct extrusion (MDE), using repeated cycles of conventional direct extrusion is proposed. This paper describes the MDE technique, the grain fragmentation mechanism and experiments that were undertaken to demonstrate the validity of the process. Furthermore, the potential for grain refinement and mechanical properties improvement are evaluated. 2. Principle of the multiple direct extrusion technique The MDE process is a repetitive procedure, but unlike wellknown SPD processes, the work piece changes its shape during deformation. The initial billet (square shaped in cross-section) is extruded several times through a rectangular die aperture. One side of the aperture is a submultiple (half, third, fourth and so on) of the other side, so the extrusion ratio per pass is 2, 3, and 4 (50%, 66.6%, and 75% reduction, respectively). The principle of MDE is presented in Fig. 1 for a 50% cross-section area reduction. MDE starts by introducing a billet into a container and direct extruding it until the punch arrives near the deformation zone. The extrusion process is stopped, the punch is lifted and a new billet is introduced into the container to push out the previous billet that remained in the deformation zone. For 50% reduction, the length of the extruded product becomes double compared with the initial billet according to constancy of volume. After the complete extrusion, the product is transversely cut at the output ends (to remove the nonconforming zone) and then at the middle of its length. The two half-billets are

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Fig. 2. Typical regions in direct extrusion.

Fig. 1. Principle of multiple direct extrusion.

longitudinally joined to again obtain a square shape in crosssection, and thus a new billet is obtained. For the next pass, two processing routes can be used: - Route A: no rotation; - Route B: 90◦ rotation around z axis. Using route A, the extruded billet will result in a rectangular shape after each pass. Only strips can be obtained by this approach. The choice of route B will give extruded billets with rectangular or square shapes after even and odd passes, respectively. This procedure can be repeated several times until the imposed strain is achieved. From this point of view, the number of passes depends of the initial cross-section area size. Table 1 shows the evolution of the cross-section of billets after each pass for the two processing routes and corresponding strains that are usually used in extrusion. The effective (von Mises) strain was calculated. For comparison, the maximum effective strain/pass is 1.15 for ECAP with a 90◦ die angle [2], and 0.8 for ARB [12].

velocity. Region II, called the deformation zone (DZ), is where the material undergoes continuous plastic deformation. In region III, the material moves to the exit of the die with constant velocity vIII without any further deformation. Due to volume constancy, vIII = (H/h) vI . Region II is separated from regions I and III by entry and exit surfaces i and e . They belong to the slip line field of DZ and correspond to the beginning and end of deformation. In the past, much attention has been devoted to the study of the extrusion mechanism through wedge-shaped die using the slip line field solution [13–16]. The effects of friction and die geometry on the slip line field configuration have also been analyzed. Generally, assuming slipping friction during direct extrusion, four slip line field configurations for wedge-shaped die can be drawn, designated as type I–IV configurations [17]. Each of them addresses a specific extrusion ratio for a given die angle (␣) and friction factor (m). Among the proposed slip line models, the type I solution

3. Deformation analysis: mechanism of grain fragmentation In the proposed model for deformation analysis during the MDE process, the work material is divided in three regions as shown in Fig. 2. In region I, the material, assumed to be rigid, moves downward with a constant velocity vI , which is equal to the punch

Fig. 3. Slip line field in DZ (type I) for direct extrusion through wedge-shaped die.

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Table 1 The evaluation of strain in MDE process. Pass

Route A

Route B

er

εvM

rA [%]

0

–

–

–

1

2

50

√2 3

2

4

75

2 √2 ln 2 = 2 × 0.8

ln 2 = 0.8

3

. . .

2n

n

1 − (1/2n )

0.8n

er , extrusion ratio; rA , cross sectional area reduction; εvM , effective (von Mises) strain.

(shown in Fig. 3) is the most appropriate for MDE analysis and therefore will be used in the following approach. The geometry of this slip line configuration is defined by the field angles , ϕ, and  and the semi angle ˛ of the die. The value of  is dependent on the friction condition at the interface between the die and the work material (for a frictionless case,  = /4). The range of possible values of ϕ corresponds to the range of reductions for which the field is valid. The slip line field AECFB tends to its limit when the extrusion ratio tends to its maximum value (for which  = 0). When  → 0, the point E→D, and the point C → F so that the slip line field AECFB can be approximated with a triangular one ABC (Fig. 4), in which AC ≡ i and BC ≡ e (Figs. 2 and 4). The kinematically admissible velocity field associated with the slip line field from DZ becomes a triangular velocity field and takes the configuration shown in Fig. 4a. In this field, the material moves parallel to the inclined wall with velocity vw . In each point on the surfaces i and e the velocities vI and vw suddenly change the direction. Let us take an arbitrary point on AC and the corresponding point on BC. Because of the continuity condition in material flow, the normal components of velocity across the boundary of the two regions must be equal, so (vIn = vwn )|AC and (vIIIn = vwn )|BC .

Fig. 5. The tool set used for MDE.

Parallel to the surfaces AC and BC, the tangential components may have different values: (vwt = / vIt )|AC and (vwt = / vIIIt )|BC . The differences (vwt − vIt )|AC and (vIIIt − vwt )|BC define the velocity discontinuities, giving rise to tangential stress  [18].

Fig. 4. Kinematically admissible flow field: (a) components of velocities on slip lines; (b) schematic grain fragmentation mechanism during MDE.

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Fig. 6. Copper specimens processed by MDE, route B: (a) initial; (b) after first pass; (c–e) after 2, 3 and 4 passes.

When an elongated and thin enough grain (obtained from a previous MDE pass) crosses one velocity discontinuity surface along which tangential stress exceeds strength in pure shear ( > k), the grain strength will be overcome and the fragmentation process starts (Fig. 4b). Therefore, the velocity discontinuity surfaces play an important role in grain fragmentation. The higher the number of discontinuity surfaces, the more efficient the fragmentation is.

4. Experimental procedure A series of experiments were undertaken to demonstrate the validity of this novel technique and evaluate the potential for grain refinement.

Commercial copper (purity 99.98%) was used in this study. The specimens were machined from a rolled bar with 30 mm diameter in a billet with dimensions 20 × 20 × 50 mm3 . A subsequent annealing at 800 ◦ C for 30 min followed by water quenching was performed to eliminate the strain hardening from previous metalworking processes and achieve a good workability of the material. After this treatment, the grain size reached approximately 145 ␮m. The specimens were extruded using a 750 kN hydraulic press with 10 mm/s ram speed at room temperature. Between cycles, the joined billets were rotated 90◦ (route B). Zinc stearate was used as a lubricant to reduce friction at the metal–tool interface. Fig. 5 shows the equipment and tool assembly used for the MDE experiments. For microstructural observations, the extruded specimens were prepared on both lateral surfaces and in cross-section

Fig. 7. Optical micrographs of copper after the first pass MDE processing.

L. Zaharia et al. / Materials Science and Engineering A 550 (2012) 293–299

Fig. 8. Microstructure evolution during MDE: (a) initial state; (b–d) after 2, 3 and 4 passes.

using standard procedures. These surfaces were subsequently etched with a solution of 40% nitric acid. To see the grain size evolution after four passes, preliminary microstructure investigations using an electronic microscope (QUANTA 200 3D) were performed. Tensile testing was carried out to evaluate the mechanical properties after each MDE cycle. The tensile tests were conducted at room temperature using a computer-controlled testing machine (Instron 3382) with a constant strain rate of 6.7 × 10−3 s−1 applied throughout the entire test according to ISO 6892-1: 2009 recommendations. For each pass, many specimens were tensile tested to determine the tensile strength (Rm ) and elongation at fracture (A). All of the specimens were machined to the size ␾3 mm × 15 mm to realize proportional test pieces. After each pass, microhardness measurements were performed. Special attention was paid to studying the effect of non-uniform strain distribution on each sample after four cycles based on the microhardness–strain correlation. Vickers microhardness testing was performed from 10 to 10 mm along the longitudinal axis, using 100 gf weight and a 25 s loading time. Fig. 9. SEM micrograph after four passes of MDE.

Fig. 10. Tensile testing results for MDE processed copper: (a) true stress vs. true strain; (b) tensile strength (Rm ) and elongation at fracture (A) evolution.

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5. Results and discussion

5.2. Tensile properties

Fig. 6 shows the image of specimens processed by MDE with 90◦ rotation between cycles (route B). After 4 passes, 8 thin billets with 5 mm × 5 mm in cross-section were obtained (Fig. 6e).

Fig. 10a shows the tensile testing results. The evolution of tensile strength (Rm ) and elongation at fracture (A) are revealed in Fig. 10b (pass “0” corresponds to the specimens in fully annealed condition, i.e. the initial state). Experimental data show that 0.2% proof stress is approximately 40 MPa for annealed copper, but it reaches approximately 80 MPa and 110 MPa after the first and the 4th passes, respectively, without an obvious saturation. This is because of the inherent heterogeneous structure after MDE with elongated grains that still have a large number of mobile dislocations during a subsequent postdeformation process. If the microstructure becomes homogeneous with small equiaxed grains having a constant average dislocation density, the saturation of 0.2% proof stress could occur, as was observed, for instance, for copper processed by ECAP [20,21]. The tensile test after MDE revealed a significant work hardening (Fig. 10b) related to microstructural characteristics of the material such as grain size and/or dislocation density. A major increase of tensile strength can be observed at the first and the last cycle. After four cycles of the MDE process, the tensile strength of copper is more than two times higher than that of the initial state. The elongation at fracture in the MDE process decreased from approximately 60% to approximately 25% after the first pass and then remained almost constant. Generally, in high strain processes, elongation does not exceed 10%. This surprising result obtained in MDE process can be explained by the fact that in all microstructures resulted after ultra-high straining processes, there is a mixture of submicronic and micronic grains even at very large strain. The submicron grains are responsible for the high strength while the micron-sized grains impart a good ductility [22].

5.1. Microstructure In MDE, the grain fragmentation occurs along shear planes, which are surfaces of velocity discontinuities. These surfaces are schematically presented in Fig. 7. The fragmentation process begins when the elongated grains become sufficiently thin so that the tangential stress (which acts on the surfaces of velocity discontinuities) can start the shearing process at the sub-grain boundary level. During the passing of material through discontinuity zones, the misorientation of the new grains, resulting in the fragmentation process, is taking place as described by Gholinia et al. [19]. By repeating the direct extrusion process, the elongated grains after the first pass are again elongated during subsequent passes. The aspect ratios were estimated at 6.80 after the first pass and 7.40 after the second pass. The elongation of grain is a characteristic of the direct extrusion process. This aspect suggests that a combination of MDE and ECAP may be an interesting topic for future studies. The intense elongated grains after 2–3 passes by MDE will be more efficiently fragmented during the intense shearing that occurs in ECAP process. The shearing in MDE is not significant after the first pass because the grain must be elongated enough so the fragmentation process becomes more intense after several passes, as seen in Fig. 8. The elongated grains during first pass will be more easily fragmented after the second pass and so on. During each pass, grain fragmentation occurs twice because there are two discontinuity surfaces (Figs. 4b and 7). A notable decrease in grain size after the second and third passes can be observed. After the second pass, the initial square shape cross section of billet becomes square again, and the deformation process along the directions x and y (Fig. 1) produces a uniform grain refinement after even passes when route B is used. An SEM micrograph of copper after four MDE passes is shown in Fig. 9. Fine grains having 0.25–0.8 ␮m perpendicular to the extrusion direction can be observed. However, the grains still remain elongated (1 ␮m) along the extrusion direction. The microstructure consists of micron- and submicron-sized grains. It is expected that the aspect ratio of elongated grains will decrease if the MDE process continues. Further detailed microstructure investigations are necessary to confirm this hypothesis.

5.3. Microhardness The method used to determine experimental strain distributions across extruded bars by the MDE process was the well-known method of microhardness–strain correlation [23,24] based on the Vickers microhardness tests. According to this method, the measured microhardness is converted into corresponding effective strain. Therefore, the evolution of (micro)hardness describes the (in)homogeneity of strain along a specific direction. To quantify the strain inhomogeneity, a coefficient of variation of microhardness was defined as a normalized measure of data dispersion: CvHV (%) =

St. DevHV HVav

Table 2 Microhardness values and the coefficient of variation of microhardness. Microhardness 1

2

3

4

5

6

E1

109.3

115.7

114.9

114.4

115.3

117.3

E2

114.7

120.3

120.8

119.1

116.7

114.6

E3

113.3

118.3

118.5

112.7

109.1

107.6

E4

109.2

110.1

107.9

109.7

102.1

98.3

HVav

111.6

116.1

115.5

113.9

110.8

109.4

St. DevHV

2.42

3.82

4.87

3.40

5.78

7.34

CvHV (%)

2.16

3.29

4.22

2.98

5.21

6.71

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Fig. 11. Microhardness evolution: (a) during MDE process; (b) after four passes (E1–E4 are revealed in Table 2).

where St. DevHV =



(HVi − HVav )2 /n is the standard devia-

tion, and HVav = HV1 + HV2 + HV3 + · · · + HVn /n is the average value of Vickers microhardness. Due to symmetry (Fig. 6e), only four processed bars obtained after the last MDE pass were subjected to the microhardness investigation. Measurements were performed at 6 equidistant points, and the average values are presented in Table 2. The microhardness evolution during the MDE process and along extruded bars after four passes is shown in Fig. 11. The results from Table 2 confirm the tendency towards uniformity of mechanical properties after the last MDE pass as described at the end of Section 5.2. The coefficient of variation of microhardness does not exceed 7% in similar points of the four bars. This indicates that the strain is almost the same in all extruded bars after the last pass. As seen in Fig. 10a, microhardness of copper during the MDE process increases from approximately 50 HV (in non-deforming state) to approximately 120 HV after four passes. The evolution is similar to that described for SPD processes [25,26] and consists of doubling of the microhardness after the first pass, a large increase after the second pass, and an insignificant increase after subsequent passes. In the MDE process, the deformation histories are obviously different for the head and tail parts. Before the beginning of the extrusion process, the heads of billet(s) are in an incompressible state (due to the contact between work piece and punch), while the rest of the material is subjected to free upsetting until contact with the die walls takes place. Therefore, the heads of the pieces are less deformed and this leads to a lower microhardness in this region (Fig. 11b). 6. Conclusions A new grain refinement technique based on MDE was described, and a grain fragmentation mechanism during the process involving velocity discontinuities surfaces was suggested. Copper specimens were processed by MDE, and thin square bars with refined grains were obtained after four passes. Experimental investigations including microstructure evolution, tensile tests and microhardness measurements were carried out to support the validity of the proposed technique.

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