STRENGTHENED METAL TUBES

Field of the Invention

The present invention relates to metal tubes and methods of strengthening metal tubes.

Background of the Invention

A number of techniques have been proposed for mechanically strengthening metal work-pieces. Some of these techniques have been referred to in the art as "severe plastic deformation" or "SPD". Severe plastic deformation involves applying a severe shear strain within the thickness of the metal work-piece to result in grain refinement of the metal work-piece. One such technique is equal channel angular pressing (ECAP) , which involves forcing the metal work-piece through a channel which has an angle in it to produce a high strain in the work-piece. The channel has a constant cross-section, enabling the work-piece to be grain refined without a change in its cross-sectional dimensions. Furthermore, the work-piece can be passed multiple times through the channel, with each pass introducing additional strain and therefore additional grain refinement.

However, ECAP has a number of drawbacks which makes it commercially unfavourable in that it is a batch process and requires a high amount of energy to apply the strain.

Other severe plastic deformation techniques have been proposed in particular for work-pieces that have a thickness which is much less than its diameter, such as "high pressure torsion" or "HPT". High pressure torsion involves a disc-shaped work-piece being deformed under high hydrostatic pressure between two anvils that are rotating with respect to each other. For high pressure torsion, a severe shear strain is imposed within the thickness of the sample due to the difference in magnitude of the material flow velocities at two large surfaces, rather than by a change in the velocity direction (as is the case for ECAP) . HPT, however, like ECAP is typically operated as a batch process and is limited to only certain work-piece shapes and in particular is not suitable for metal tubes.

Accordingly, there is an ongoing desire to develop a commercially useful method and apparatus for improving the strength of metal tubes.

Summary of the Invention

In one aspect, the present invention provides a method for producing a strengthened metal tube comprising drawing a metal tube between a mandrel and a die whilst applying shear stress and hydrostatic pressure to the tube to produce the strengthened metal tube.

Applying shear stress may comprise rotating one or more of the mandrel, the die or the tube as the tube is drawn between the mandrel and the die.

Applying shear stress may comprise counter rotating the mandrel and the die.

Applying shear stress may comprise rotating the mandrel and the die at different rotational velocities.

Applying shear stress may comprise rotating the mandrel or the die at a relative angular velocity (ω) of (0.01 - 6.28) s ^-1 to the other. It is noted that the relative circular velocity Vi is determined from the angular velocity by Vi= ωϋ ₀ /2, where D ₀ is the outer diameter of the tube.

The metal tube may be drawn at a drawing velocity V ₂ of 0.001 - 1 m/s.

Applying hydrostatic pressure may comprise reducing the thickness of the tube using the mandrel and the die.

The tube's thickness may be reduced by 10-90%.

Applying hydrostatic pressure may comprise contacting a tube wall thinning portion of the mandrel with the inner surface of the tube and reducing the tube wall thickness by deforming the tube between the tube wall thinning portion of the mandrel and the die. The method may comprise rotating the mandrel or the die at a relative circular velocity and drawing the metal tube between the die and mandrel at a drawing velocity such that the ratio (V ₂ /V ₁ ) of drawing velocity (V ₂ ) to circular velocity (Vi) is less than the ratio of the length of the tube wall thinning portion of the mandrel to the outer circumference of the tube.

In some embodiments, the method may result in through-thickness grain refinement of the metal within the tube wall .

In other embodiments, the method may result in grain refinement of a portion of the metal.

The grain refined portion in this embodiment may be a near surface region of the strengthened metal tube.

According to another aspect, the present invention also provides an apparatus for producing strengthened metal tubes, the apparatus comprising:

a mandrel;

a die positioned around and spaced from the mandrel; and

a drawing mechanism for drawing a tube through a space between the mandrel and the die in a drawing direction;

wherein the apparatus is operable to apply shear stress and hydrostatic pressure to the tube as it is being drawn .

One or more of the die, the mandrel or the drawing mechanism may be rotatable about an axis parallel to the drawing direction.

The die and the mandrel may be counter-rotatable .

The die may be a cylindrical die.

A portion of the space between the die and the mandrel may reduce in width in the drawing direction.

The mandrel may have a tapered portion which

increases in diameter in the drawing direction. The mandrel may have a tube wall thinning portion which contacts the inner surface of the tube and in use reduces the wall thickness.

The tapered portion may have a tapered profile defining an outer surface which is at an angle, a, to the drawing direction, wherein a = 1-75° .

The tube wall thinning portion may have a length, L, ranging from 0.03to to 51.6to, where to is the thickness of the tube wall fed to the apparatus .

In another aspect, the present invention provides a method of producing a strengthened metal tube, the method comprising drawing a metal billet between a mandrel and a die whilst applying shear stress and hydrostatic pressure to produce the strengthened metal tube.

In a further aspect, the present invention also provides a strengthened metal tube produced by the above described methods or produced using the above described apparatuses .

Brief Description of the Figures

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures, in which:

Figure 1 is a schematic view of an apparatus for strengthening metal tubes according to an embodiment of the present invention;

Figure 2 is (a) an optical micrograph and (b) scanning electron micrographs of a copper tube which has been strengthened according to an embodiment of the present invention;

Figure 3 is transmission electron micrographs of an aluminium alloy 6060 tube (a) before any treatment and

(b)-(d) after being strengthened according to an

embodiment of the present invention; and

Figure 4 is a sequence of transmission electron micrographs showing the microstructure across the wall tube thickness for a steel tube that has been strengthened according to an embodiment of the present invention. The numbers beneath the micrographs refer to the distance from the inner surface of the tube wall.

Detailed Description of Embodiments

Referring firstly to Figure 1, an apparatus 10 for strengthening metal tubes is shown schematically. The apparatus 10 comprises a die 2, a mandrel 3 and a drawing mechanism in the form of a drawing bench 4. The die 2 is positioned around and spaced from the mandrel 3. A metal tube 1 is drawn through the space 5 between the die 2 and the mandrel 3 by operation of the drawing bench 4 in a drawing direction as indicated. The apparatus 10 is operated to apply shear stress and hydrostatic pressure to the tube 1 as the tube is drawn between the die 2 and the mandrel 3. This results in grain refinement of at least a part of the metal tube and accordingly strengthening that part of the tube or the entire tube. The apparatus 10 is of particular advantage in that it allows for continuous processing of metal tube. Tubes of any length can be drawn between the die and the mandrel as required. Depending on the material, the tube dimensions and the required strengthening, the metal tube can be processed by the apparatus in a single pass or may undergo multiple passes between the die and the mandrel. The apparatus 10 may also be utilised to draw metal tubes from metal billets such that the tubes are of higher strength than if drawn using conventional means .

In the particular embodiment shown in Figure 1, the shear stress is applied by rotating the die 2 about an axis which is parallel to the drawing direction whilst the mandrel remains stationary or floating. However, in other embodiments, the mandrel 3 may rotate as well as or instead of the die 2. If both are rotated, they may rotate in the same direction or may counter rotate, usually about a common axis. A further variation involves rotating the drawing bench 4 so as to rotate the tube 1 as it is being drawn between the die and the mandrel. Rotation of the drawing bench may occur alone or in combination with rotation of the die and/or mandrel.

The amount of shear strain which is produced in the metal tube as it is drawn through the apparatus 10 is determined by the difference in magnitude of the material flow velocities at the inner and outer surfaces of the tube, which results from the difference in rotational speed of the mandrel 3 and the die 2. Typically the relative angular velocity (ω) of the die 2 with respect to the mandrel is 0.01-6.28 s ^-1 . However, the level of shear strain and thus the strengthening of the tube material achieved are controlled by the ratio between the

(translational ) drawing velocity (V ₂ ) and the circular (circumferential) velocity (Vi) . The drawing velocity (V ₂ ) itself is typically 0.001-1 m/s . If the drawing velocity is too high relative to the circular velocity,

insufficient grain refinement not able to enhance the strength of the metal tube will occur. Conversely, if the circular velocity is much greater than the drawing velocity, the process may not be fast enough to ensure a high throughput of material. This in turn may render the process not viable for large-scale manufacturing. This ratio will be discussed in further detail below.

Depending on the wall thickness, the grain refinement will occur throughout the tube wall or only in near surface regions of the tube. Rotation of the die will typically result in efficient grain refinement and strengthening near the inner surface of the tube, while rotation of the mandrel will lead to the predominant occurrence of the effect near the outer surface, which is in contact with the die. Accordingly, rotation of the die or the mandrel can be selected depending on the end use of the metal tube. Advantageously, it has been found that grain refinement in only these near surface regions provides the tube with significant strength and hardness. This means that grain refinement of the metal bulk may be unnecessary, reducing the amount of shear load that is required to be applied and consequently the amount of energy that is used.

Hydrostatic pressure is applied to the tube 1 by reducing its wall thickness as it is drawn between the mandrel 3 and the die 2. This is achieved without reducing the external diameter of the tube 1, although the internal diameter is increased.

To reduce the wall thickness, the mandrel 3 is provided with a tapered portion 6, having a generally conical shape. The tapered portion 6 is positioned within the die 2 and its tapered profile increases in diameter in the drawing direction (thus reducing the width of the space 5 between the mandrel and the die in the drawing direction) . At least a part of the tapered portion defines a tube wall thinning portion 7 of the mandrel, which increases from a diameter (Di) corresponding to the internal diameter of the metal tube being fed to the apparatus, to a diameter (D _f ) corresponding to the internal diameter of the strengthened metal tube produced. The tube wall thinning portion 7 engages the inner surface of the tube 1. The cylindrical die 2 is of constant internal cross-section. Thus, as the tube 1 is drawn over the mandrel 3, a reduction (At) in the tube wall thickness occurs about the tube wall thinning portion of the mandrel from an initial thickness tc to a final thickness t _f .

Typically, the wall thickness reduction (At) of the metal tube ranges from 10-90%.

It is to be appreciated that this wall thinning could be achieved in another embodiment where the mandrel is of constant cross-section and the internal diameter of the die is reduced in the drawing direction as this would also have the same effect of reducing the space between the die and the mandrel in the drawing direction.

Because of its tapered profile, the outer surface of the mandrel at the tapered portion 6 is accordingly at an angle, a, relative to the drawing direction. Typically, a is in the range from 1-75° The length, L, of the tube wall thinning portion is subsequently derived from this angle, a, and the reduction in tube wall thickness (At) that is to be achieved by the apparatus as shown below:

With the angle a varying between 1 - 75° (and At varying between 10-90%) as referred to above, L can take values within a range of 0.03to to 51.6to. The tapered portion's overall length can be anything provided that it is greater than L. Thus, the mandrel 3, in particular the tapered portion 6, is designed and constructed based on the internal diameter and wall thickness of the metal tube being fed to the apparatus as well as on the desired final dimensions of the strengthened metal tube and the

ductility of the material from which the tube is formed.

The ratio of the drawing and circular velocities ratio (V ₂ /V ₁ ) is controlled with respect to the geometries of the mandrel and tube. In particular this ratio is controlled to be less than the ratio of the length (L) of the tube wall thinning portion of the mandrel to the outer circumference of the tube, nominally nD ₃ . The reason for this limitation is to ensure that the tube material undergoes torsional deformation (shear) corresponding to a full rotation of the tube with respect to the mandrel or the die before it moves out of the process zone by translation movement of the tube. This relation can also be expressed, based on the previous equation, as:

V _{2 ;} L _(0.1-0.9)t„

V _l πϋ ₀ π(ίΆϊΐ a)D ₀

Operation of the apparatus 10 can be carried out with the metal at room temperature or at elevated temperatures, depending on the material. All materials for which conventional tube drawing is possible can be processed by the technique proposed, including pure metals and alloys such as copper, steel, aluminium alloys and magnesium alloys . Examples

Example 1

Copper and aluminium alloy tubes were drawn using an apparatus as shown schematically in Figure 1. The tubes fed to the apparatus had the following dimensions

- Tube Length: 50mm

- Tube OD, D ₀ : 10mm

- Initial tube wall thickness, to: 1mm

The die had an internal diameter corresponding to the outer diameter of the tube and the mandrel had a tapered portion with the following dimensions:

- wide diameter, D _f : 9.2mm

- mandrel angle, a: 33°

The drawing process occurred with the mandrel held stationary and the die being rotated at an angular velocity (ω) of Is ^-1 . The drawing bench drew the metal tubes between the rotating die and the stationary mandrel at a drawing velocity (V ₂ ) of 2mm/ s . The tubes were reduced to a final wall thickness of 0.8mm.

Figure 2 provides an optical micrograph and scanning electron micrographs of the copper tube before and after being subjected to the deformation by the apparatus. As seen in particular in Figure 2 (b) , there was significant grain refinement from ~20 μπι to less than 1 μπι near the inner (mandrel-facing) surface of the tube wall.

Figure 3 provides a series of transmission electron micrographs of the AA6060 tube before and after being subjected to the deformation by the apparatus. Figure 3 (a) shows the initial alloy microstructure before deformation, which has a grain size of 15 to 90 μπι.

Figure 3(b) shows the microstructure near the inner surface of the tube and Figures 3 (c) and (d) show the microstructure of the tube near the outer surface after the tube has been processed in the apparatus. As can be seen from these micrographs, there has been significant grain refinement near both the inner and outer surfaces of the tube wall to grain size below 1 μηι.

Microhardness data was also obtained for the copper tube before and after it had been subjected to the strengthening treatment. The results of these tests are present below:

- Initial Vickers hardness: 99 HV

- Vickers hardness near the outer surface of the wall after deformation: 110 HV

- Vickers hardness in the middle of the wall after

deformation: 145 HV

- Vickers hardness near the inner surface of the wall after deformation: 172 HV

As expected (because the mandrel was stationary and the die was rotated) the highest hardening effect was achieved near the stationary part of the tooling (the mandrel) . An increase of local hardness of over 70% was achieved, demonstrating the efficacy of the strengthening treatment .

Example 2

Steel tubes (having a length of 50 mm, an outer diameter of 20 mm and an initial wall thickness of 1.7 - 2.0 mm) of three different levels of carbon content were processed using the same apparatus as used in Example 1.

Each steel tube type was processed using different ratios of rotational to drawing velocities in accordance with Table 1 below:

Table 1

Figure 4 shows the microstructure of Steel C after processing at the rotational to drawing velocity ratio of 385 across the tube wall with radius, r=0.00 mm indicating the tube wall at the mandrel and r=0.8 mm indicating the tube wall at the outer wall. The numbers beneath the micrographs refer to the distance from the inner surface of the tube wall.

Similar microstructures were observed for all samples (from Table 1) . It was noted that greater grain refinement occurred at higher ratios of rotational to drawing velocity for all three steels.

As exemplified in Figure 4, it was apparent that all samples underwent substantial grain refinement. A minimum grain size of ~100nm close to the mandrel was attained.

It was also observed that an apparent grain size gradient exists, in which the size of lamellar grains changes from the order of 100 nm at the inner wall of the tube to ~1 μπι towards the outer wall of the processed tubes.

As a result of grain refinement the ultimate tensile strength was improved over that of the as received steel grades .

It is to be understood that, if any prior art publication is referred to herein, such reference does not - re constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.