Production of Bulk Nanostructured Al-6082 Materials and Investigation of their Behaviors in Given Technological Process

Muamar Benisa and Galal Senussi

Abstract

Equal-channel angular pressing (ECAP) is one of the methods of severe plastic deformation for producing ultra fine-grained (UFG) material. A commercial Al-Mg-Si alloy (6082) was deformed by equal-channel angular pressing (ECAP) to produce bulk ultra fine-grained structure using route C technique up to eight passes with a high length to diameter ratio as 15-16. The products were investigated after one, four and eight passes. Upsetting test was used to determine the true stress true strain curves in all the three conditions. During upsetting the anisotropy induced by the ECAP technology led to asymmetrical bulging/buckling. This phenomenon was reduced by lowering the height to diameter ratio of the upsetting specimens. Backward extrusion tests were performed in all the three conditions to collect information using the ECAP produced raw materials in a real technology. The induced anisotropy was monitored also on the backward extruded pieces. This phenomenon is limiting the formability especially after eight passed.

Keywords: ECAP technique, Upsetting test, Flow curves, Extrusion process.

1. IntroductionNanostructured materials have been the subject of widespread research over the past

couple of decades with significant advancement in their understanding especially in the last few years [1,2,3]. The technique resulting in fabrication of nano- and submicrocrystalline metals and alloys which was called severe plastic deformation (SPD) assumes intense straining under high applied pressure at low temperatures (usually less than 0.4 Tm). Severe plastic deformation methods should meet a number of requirements which cannot be realized with traditional methods of plastic deformation, such as rolling, drawing or extrusion.

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First of all it is important to obtain ultra fine-grained structure with high angle boundaries to get change in properties. The second is formation of nanostructure in the whole volume of a sample providing stable material properties [4,5]. Finally because the work piece exposed to large plastic deformation, it should not has any mechanical damage or crack before and after the process. While there are different techniques to obtain nanostructure material, two of them are the most commonly used methods, the severe plastic torsion staining under high pressure (SPTS) and the equal channel angular pressing (ECAP),(Figure1) [6,7].

The process of extrusion consists of forcing a billet of metal through a die to produce a continuous length of constant cross section corresponding to the shape of the orifice. There are four basic types of extrusion: direct, indirect, hydrostatic, and impact. Indirect (reverse, inverted or backward) extrusion [8,9]: The die moves toward the billet, thus, except at the die, there is no relative motion at the billet-container interface. As a consequence, the frictional forces are lower and the power required for extrusion is less than for direct extrusion. In practice, a hollow ram carries a die, while the other end of the container is closed with a plate. In this type, the ram containing the die is kept stationary, and the container with the billet is made to move.

2. Experimental work and resultsIn this work we produced (ECAP) parts in three types of passes ( one , four and eight ) C

route. The material used in this study was a commercial Al-Mg-Si alloy (Al-6082). The main components of the alloy are showed in table (1). Before the ECAP deformation the material was annealed on 420ºC for 40 minutes. The annealed specimens were regarded as the as-received material. Cylindrical billets of 15 mm in diameter and 230 mm in length were pressed through an ECAP die set having 90º inter-sectioning channels with identical cross section. The ECAP equipment and the machine used are illustrated in Fig 2.

(a)(b)

Fig 1. Principles of SPD methods: (a) torsion under high pressure, (b) ECAP method

Table 1 .Chemical composition of Al-6082 alloyElement Al Mg Si MnPercentage 97 % (0.6-1.2 ) % (0.7-1.3 ) % (0.4-1 ) %

All Dimension in (mm)Fig 2. (a) Pressing machine with the ECAP die set,

Fig 2. (b) ECAP Die

Upsetting testA common method for the determination of flow curves is the application of the compression

test (or upsetting test.). Using this method, friction at the interface between the die and the specimen leads to the bulging of the sample and thereby to an inhomogeneous stress and strain rate. The specimens all had the same diameter of 10 mm and were machined from the pressed material along the longitudinal axis. Three sets of three specimens were prepared with the initial height dimensions of 15, 10 and 5 mm respectively. These specimen were machined from nanostructured Al-6082 alloy which produced by ECAP technique. The die was fixed in the center of the TIRA test 2300 machine (Fig 2). Compression loading was applied on the specimen up to 500 N then the specimen was unloaded. After cleaning the end surfaces of the specimen the height and the maximum and minimum diameter were accurately measured by a Mitutoyo equipment having 1/100 mm resolution. Then the specimen was reloaded by a load increment of 0.5 KN over the initial load and the procedure was repeated until the equivalent strain achieved 0.7 then the load increment was increased to 1 KN. The procedure continued up to the agreed load limit. The true stress values were calculated for each load value using the area obtained from the constant volume relationship as.

A0 h0=A f h f (1)

Where A0 , h0 are initial area and initial high of specimens and A f ,h f are final area and final high of specimens.

σ=PiA i (2)

Where σ is true stress, Pi is an instant load and Ai is an instant area The true strain was calculated by using the equation.

ε=lnhoh f (3)

The figures. 3, 4 and 5 present the results of the specimens with (ho/do=1.5), (ho/do=1) and (ho/do=0.5) ratio. True stress– true strain curves of normal structure and nanostructure true in: row material, one, four and eight passes, route C, from figure 6 we can show the different between the three flow curves of ECAP part after one pass with three different values of (h/d) ratio.

The nanostructure specimens have different curve shape and show higher stress than the conventional alloy. The shape of the curves in all three cases was typical, non-monotonous, and needed further study. The cross section of the specimens tended to change from circle to oval after ECAP passes that shows the presence of the induced anisotropy caused by this operation, and the maximum of this ovalizing was obtained after pass one. This can be explained by the fact that the cell shape returns back to original after double passes when route C is used.

0

50

100

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0 0.5 1 1.5 2equivalent true strain

true

str

ess

MPa

PASS 0

PASS 1

PASS 4

PASS 8

Fig 3 True stress–true strain curves of Al6082 after different passes,initial specimen size 10x15mm (ho/do=1.5)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.60

50

100

150

200

250

300

350

400

PASS 0

PASS 1

PASS 4

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Equivalent true strain

true

stre

ss M

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Fig 4 True stress–true strain curves of Al6082 after different passes with initial specimen geometry of 10x10 (ho/do=1)

050

100150200250300350400

0 0.5 1 1.5 2equivalent true strain

true

stre

sss

MPa

PASS 0

PASS 1

PASS 4

PASS 8

Fig 5 True stress–true strain curves of Al6082 after different passes with initial specimen geometry of 10x5 (ho/do=0.5)

0 0.5 1 1.5150

200

250

300

350

h/d=1.5

h/d=1

h/d=0.5

Equvalent true strain

True

str

ess

MPa

pass one

Fig 6 True stress–true strain curves for ECAP part after one pass at different height/diameter ratio

Backward Extrusion:

The mean task was to investigate the effect of deformation, which was performed by ECAP for the properties of the backward extruded products. The Fig 7 shows the equipment used for backward extrusion The applied ram speed was 2 mm/min.

The backward extrusion technology was used to test the further workability of the prepared nanostructure materials on pieces having 15 mm diameter and 10 mm initial height. A stroke of 8 mm was applied to produce cups with 15 mm outer diameter and 2 mm wall thickness. The basic of comparison was the as-received material (zero pass). From each material condition three parallel tests were performed. Fig 8 shows the typical extrusion forces as function of ram travel. The ECAP specimens needed higher force than the conventional one, and the forming force increased as the number of ECAP passes increased and showed a significant peak point, while it was absent in the case of conventional material (zero pass). Some characteristic features were observed on the pieces which were produced in ECAP materials. As it can be seen on Fig 9, the zero pass material did not show up anisotropic behavior, at least not parallel to the symmetry axis. It means, the cup height along the perimeter of the cup was identical. On the pieces after ECAP passes significant change was observed in the cup height (earring phenomenon) with two maximum and two minimum values (see Fig 9).

This earring is the most significant in the case of one pass and a bit less in the case of more passes. At pass one, as it was discussed before, the deformation cell has ellipsoid shape instead of spherical one that returns back when the sample is after four and eight passes

Fig. 7 .Backward extrusion Die : a Punch ,b :Inner ring

a b

During backward extrusion experiments the limitation in formability of Al 6082 alloy was monitored in cups produced after eight ECAP passes. Well visible crack appeared at the top edge of all these cups and their position was always the same, in between of the minimum and maximum point of earing, at about the middle in angle. In all cases just one crack was developed on one side.

On the other hand none of the other cups (specimens after one and four ECAP passes) showed this phenomenon. Fig 10 shows the appearance of this typical crack and its typical position.

Fig 8. Equipment for backward extrusion

0 2 4 6 8 100

10

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pass 0pass 1pass 4pass 8

Ram travel mm

Forc

e K

N

Fig 9 Backward extrusion force as a function of ram travel for conventional Al6082 and after ECAP (one, four and eight passes)

Fig 10 View of backward extruded cups, from left, after zero, one, four and eight passes

Fig 11 Typical crack on a cup produced by backward extrusion (after eight ECAP passes by route C, and Al6282 alloy

3. Conclusion.This study presented the behavior of nanostructured Al-6082 alloy. under upsetting and

backward extrusion processes. The results showed that in the upsetting processes. Different results were achieved based on the different ho/do ratios and for different ECAP passes. These processes demonstrated the effect of geometry on the non-isotropic specimens. The induced anisotropy of specimens was caused by the ECAP technique and its maximum was observed after pass one. The backward extruded appeared in the form of earring in two defined direction at the upper edge of the cup products, the maximum of earring was observed again after the first pass. The formability was reduced by the increasing number of

passes. Cracks appeared in defined position on cups produced after eight passes. The forming force increased with the increasing number of ECAP passes.

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