Manufacturing of 6101 Aluminum Alloy Trapezoidal Wire for Overhead Conductors

Drawing of 6101 aluminum alloy trapezoidal wire requires close attention to die geometry and die surface quality, proper profile and alignment of forming rolls and adequate lubrication.  Precipitation heat treatment demands precise control of temperature in all phases of the process.

By Z. M. Kosek

6101 aluminum alloy trapezoidal wire in T81 temper is used for overhead self-damping conductors (Fig.1). This type of conductor has high wind-induced motion resistance, which protects the conductor from damaging strains caused by high frequency (5-50 Hz) vertical motion produced at relatively low wind speeds.  6101 aluminum alloy has superior mechanical properties compared to 1350 aluminum alloy, and can withstand much higher mechanical loads on the overhead lines.

Fig. 1  Typical cross-section of two layer self-damping conductor.

Physical Metallurgy and Properties

6101 alloy belongs to a group of heat treatable aluminum alloys, which significantly increase their mechanical properties by precipitation hardening.   It contains appreciable amounts of magnesium and silicon, which can precipitate as a fine dispersion of an intermetallic compound Mg2Si.  Chemical composition of 6101 alloy is given in Table 1 below.

Table 1.  Chemical Composition of 6101 Aluminum Alloy

Heat treating of this alloy consists of two steps: solution heat treatment, and precipitation heat treatment (also known as artificial aging).  During solution heat treatment at temperatures close to the metal's melting point, magnesium and silicon are dissolved in solid solution in the aluminum.  Then the metal is cooled rapidly, so that the magnesium and silicon remain is solid solution in a metastable state.   In this state the metal strength does not change, but its conductivity is significantly reduced. 

In a further heat treatment carried out at lower temperatures, magnesium and silicon precipitate as Mg2Si.  At this point, the strength and conductivity of the metal increase, as magnesium and silicon come out of solution.

Strengthening in this alloy occurs due to formation of Guinier-Preston zones with a structure corresponding to highly a ordered Mg2Si phase, and their ability to withstand the passage of dislocations.  Physical properties of the 6101 alloy are shown in Table 2.

Table 2.  Physical Properties of 6101 Aluminum Alloy

Drawing Process

A 9.5 mm diameter rod is used in the drawing process.   The rod is produced in a continuous casting and hot rolling process in T4 temper.   The drawing operation is carried out on a tandem slip type drawing machine.  A two die system is used to improve the wire alignment in the dies.  An example of die drafting is shown in Table 3.

Table 3.  Die Drafting for 6101 Trapezoidal Wire

* Diameter equivalent to trapezoidal segment

The following major areas have to be considered during the drawing operation:

Proper design and good surface finish of dies
Both tungsten carbide and polycrystalline diamond dies can be used in the drawing operation.  Due to economic reasons, polycrystalline diamond dies are used mainly for wire sizes below 2.5 mm.

The geometry of a tungsten carbide die should be as follows:
-  60 degrees entrance angle to allow an adequate supply of lubricant
- 16-18 degrees reduction angle
-  bearing length equal to 30% of die diameter
-  60 degrees back relief angle. 

The die surface should be highly polished, and meeting points of the reduction and back relief zones with the bearing should be well-blended with a minimum radius.

Proper wire alignment with the dies
Two sets of dies are used to improve the alignment between the wire and the drawing dies.  The first die (called a guide die) should have a diameter of 0.1 mm to 0.2 mm larger than the entry wire size.  This additional die provides a straight wire path to the drawing die.

Correct lubrication
6101 wire is much stiffer and harder than regular 1350 (EC) wire.   Therefore, it requires a good lubrication and cooling system to minimize friction between the wire and the dies, and provide proper cooling of the dies.  This system should be able to deliver oil between 60 and 100 l/min. per die at temperatures from 40°C and 50°C.  Extreme pressure additives should be used to improve lubrication.   Typical test parameters for the drawing oil are given in Table 4.

Table 4.  Parameters and Acceptable Limits for Drawing Oil

Wire Temperature
The wire temperature should not exceed 140°C at any stage of the drawing operation.  At this temperature partial heat treatment can take place, which changes the wire's mechanical and electrical properties.  This can result in inhomogeneous properties throughout the length of the wire.  As the wire temperature is directly related to the speed of the wire, the speed has to be adjusted so that the exit temperature is below 140°C.

Forming Process

Two horizontal forming rolls, made from Atlas "NN" steel hardened and tempered to 60 RC, with  highly polished working surfaces are used in the forming operation.  The design of the roll grooves is based on the required segment profile.

The rolls are installed on a rolling stand, which is located between a draw box and an outside draw block.  There is no finish die in this process.  The final die reduction is replaced by a forming roll reduction, which is between 20% to 23%.  This amount of area reduction is required to move the metal into the corners of the roll grooves and adequately fill the space between the rolls.  The rolls have to be perfectly aligned and installed in such a way that the cap of the trapezoidal wire faces the surface of the draw block, and is in the down position on the bottom.  The rolls are cooled by air to remove heat generated during the forming operation.

After forming, the wire profile is checked and verified using a template and a comparator.

Heat Treatment

After the drawing and forming operations, the wire is artificially aged to develop the required physical and electrical properties.  The time and temperature of the heat treatment are critical to the final properties and require very close control. 

As can be seen from Fig. 2, the tensile strength drops sharply when the temperature reaches more than 170°C.  Conductivity increases linearly with the temperature.

Fig. 3 shows the relations between aging time and tensile strength and conductivity at 170°C for wire drawn with 75% area reduction.   It can be seen that conductivity increases linearly with temperature, while tensile strength reaches a maximum value after 120 minutes, and then decreases.

Table 5 gives typical wire properties after drawing the wire with 75% area reduction and aging at 170°C for 5 hours, in a batch-type gas heated oven.

Table 5.  Typical Wire Properties After Drawing and Artificial Aging

Fig. 2  Aging temperature vs. tensile strength and conductivity;
area reduction 75%, aging time 120 min.

Fig. 3  Effect of aging time on tensile strength and conductivity;
area reduction 75%, aging temperature 170 °C

Conclusions

6101 trapezoidal wire can be successfully produced on a commercial scale using the manufacturing process described above. 

The drawing operation requires dies with mirror finished surfaces and proper geometry. 

Drawing lubricant with a high degree of film strength is needed to withstand the very high pressure and temperature, and to prevent metal to metal contact in the die. 

The profile of the forming rolls and their alignment are of the utmost importance.

Final heat treatment demands precise temperature control and an even distribution of heat inside the oven.

Bibliography

R. Brick, R. Gordon, A. Phillips, "Structure and Properties of Alloys", McGraw-Hill Book Company, (1965),
p. 155

Alcan Cable, "6101 and 6201 Redraw Rod User's Processing Guide" (1992)

Canadian Standards Association, CAN3-C49.7-M85 "Aluminum Round WIres for Use in Overhead Electrical Conductors" (1985)