WO2012161594A1

WO2012161594A1 - Bimetallic connections for heavy current applications

Info

Publication number: WO2012161594A1
Application number: PCT/NZ2012/000072
Authority: WO
Inventors: Mark Tarrant; Zhan Chen; Song Cui
Original assignee: NATIONAL ALUMINIUM Ltd
Current assignee: NATIONAL ALUMINIUM Ltd
Priority date: 2011-05-23
Filing date: 2012-05-23
Publication date: 2012-11-29
Anticipated expiration: 2013-11-23
Also published as: NZ593011A; AU2012259533A1; AU2012259533B2

Abstract

The present invention relates to electrical connections between aluminium and a dissimilar metal contact such as copper for use in an electrolytic cell for the refining of zinc or in a reduction cell for the smelting of aluminium. A cathode flexible (6) comprises a plurality of aluminium flexible sheets or tails (8) for electrically connecting a reduction cell rail to a buss bar of an aluminium smelter reduction cell via an intermediate aluminium head (7) and copper contact (9). The copper contact (9) is electromechanically connected and combined by a series of friction stir welds (10) across the width of the overlap of the aluminium head (7) and the copper contact (9) and between the overlap between the aluminium head (7) and the cathode flexible tails (8). Such a joint provides for improved electrical conductivity and mechanical joint strength resulting in improved durability and decreased a cost of manufacture.

Description

BIMETALLIC CONNECTIONS FOR HEAVY CURRENT APPLICATIONS

STATEMENT OF CORRESPONDING APPLICATIONS

The present invention is based on the provisional specification filed in relation to New Zealand patent application number 593011 , the entire contents of which are incorporated herein.

TECHNICAL FIELD

The present invention relates generally to bimetallic connections for heavy current applications. Particularly, although not exclusively, the present invention relates to electrical aluminium to dissimilar metal contact for use in refining zinc, aluminium or other non-ferrous metals in a electrolytic cell.

BACKGROUND ART

Purification of non-ferrous metals such as zinc is achieved through use of an electrolytic cell comprising a series of alternating positive electrodes (anodes) and negative (cathode) plate(s) which are immersed in an electrolyte solution (such as zinc sulphate). The cathode and anodes are connected to an electrical current to cause a reduction and an oxidation reaction. Positive ions then migrate from the anode and are deposited on the cathode sheet(s) to form a pure metal sheet(s). When the reaction has finished the pure metal deposit is then recovered by stripping from the cathode sheet(s).

The cathode sheet(s) are manufactured commonly of aluminium and are connected to an aluminium head welded to the top of the cathode sheet(s) for current transmission to the cathode sheet(s). For example, the aluminium head in the form of a headbar as used in zinc smelters is connected to a single aluminium cathode sheet and is also provided with a pair of hooks for transportation of the plate to and from the electrolytic cell.

In aluminium smelters cathode flexible tails connect from the reduction cell rail to the buss bar providing an electrical connection via an intermediate aluminium head and copper contact. The flexible tails permit expansion and contraction during heavy current transfer.

It is the quality of the copper connection to the aluminium head in the headbar of the zinc smelter and the aluminium flexible tails that directly influences the electric resistance of the connection and thus affects the energy consumption of the smelting process. Traditional fusion welding does not satisfactorily weld copper to aluminium and therefore explosive bonding is used for making the connection.

Alternatively, the copper contact is provided with a threaded stud and screwed onto the end of the aluminium head. Although providing a strong mechanical joint with a relatively low contact resistance, this method of connection suffers from the disadvantage that the threads deteriorate over time due the electrolysis corrosion and the electrical resistance increases. In addition, after mechanical stripping of the deposited material the threaded joint loosens during use.

Alternatively, the copper contact is silver plated prior to MIG welding the copper contact to the aluminium head. The weld quality and strength are difficult to control.

Electricity is a major cost component in the production process of electrolysis. There are several areas where power is lost and converted to heat between busbar and the cathode sheet(s). In some designs the aluminium head uses a cast copper contact that has 1 or 2 aluminium straps explosively bonded to a copper face. The aluminium straps are then MIG welded to the aluminium head. There are 4 areas where power loss can occur:

1. The connection between bus bar and copper contact due to wider

dimensional tolerances of the copper casting;

2. The explosive copper to aluminium bond has increased electrical

resistance;

3. The MIG welds of the aluminium straps to the aluminium head has

increased electrical resistance.

4. The explosive weld is susceptible to electrolytic corrosion and chemical attack and requires protection from the corrosive cell house atmosphere and requires silver solder sealing to be used at the explosive join edges.

It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice. All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.

It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided a cathode for use in an electrolytic/reduction cell, the cathode comprising:

• An aluminium head configured for connection to at least one cathode sheet within the electrolytic/reduction cell, the aluminium head comprising:

o a first connection portion; and

• a dissimilar metal electrical contact configured for connection to a busbar, the dissimilar metal electrical contact comprising a second connection portion configured for connection to the first connection portion wherein the first connection portion and the second connection portion are electromechanically connected and combined by friction stir welding.

Preferably, the aluminium head is selected from the group consisting of: an aluminium headbar and an aluminium cap for connection to a plurality of aluminium flexible sheets.

Preferably, the first connection portion is a slot. More preferably, the slot is a square-end slot.

Preferably, the dissimilar metal electrical contact is made from copper. Preferably, the first connection portion is a projection. More preferably, the projection has a tapered profile.

Preferably, the at least one cathode sheet is a plurality of aluminium flexible tails.

According to another aspect of the present invention there is provided a method of manufacture of a cathode for use in an electrolytic/reduction cell, the method comprising the steps of: a. Providing an aluminium head configured for connection to at least one

cathode sheet, the aluminium head comprising a first connection portion; b. Providing a dissimilar metal electrical contact configured for connection to a busbar, the dissimilar metal electrical contact comprising a second connection portion; and

c. Friction fitting the first connection portion to the second connection portion; and

d. Electromechanically connecting and combining the first connection portion to the second connection portion by friction stir welding.

Preferably, the method of manufacture of a cathode for use in an electrolytic/reduction cell also comprises the step of:

• Abrading the exterior surface of the second connection portion after step b but before step c.

Preferably, the aluminium head is selected from the group consisting of: an aluminium cap for connection to a plurality of aluminium flexible tails.

More preferably, the method of manufacture of a cathode for use in an electrolytic/reduction cell also comprises the steps after step b but before step c of:

• Electromechanically connecting and combining aluminium to the second connection portion by friction stir welding; and

• Machining the aluminium friction stir welded to the second connection

portion to a substantially square end.

More preferably, the method of manufacture of a cathode for use in an electrolytic/reduction cell also comprises the pre-steps of: a¹. Clamping the plurality of aluminium flexible sheets between a pair of rigid aluminium plates; a². Electromechanically connecting and combining the pair of rigid aluminium plates and the plurality of aluminium flexible sheets by friction stir welding.

Preferably, the method of manufacture of a cathode for use in an electrolytic/reduction cell also comprises the step: e. Connecting the aluminium head to the at least cathode sheet from step a by butt welding.

A method of purifying a non-ferrous metal in an electrolytic/reduction cell, comprising the steps: a. providing an aluminium head configured for connection to at least one

cathode sheet within the electrolytic/reduction cell, the aluminium head comprising:

o a first connection portion;

o a dissimilar metal electrical contact configured for connection to a busbar, the dissimilar metal electrical contact comprising a second connection portion; and wherein the first connection portion and the second connection portion are electromechanically connected and combined by friction stir welding b. passing an electric current between an anode and the cathode sheet(s). BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described by way of example only and with reference to any one of the accompanying drawings in which:

Figure 1a shows a side view of a preferred embodiment of the invention in the form of a headbar for a cathode sheet for an electrolysis reduction cell;

Figure 1b shows an end cross sectional view of the preferred embodiment shown in Figure 1a with electrical contact connected; Figure 2a shows a perspective view of the electrical contact shown in Figure 1b;

Figure 2b shows a top view of the slot in an end of the preferred embodiment shown in Figure 1a;

Figure 3 shows a side end view of the preferred embodiment shown in Figure

1 with a known headbar;

Figure 4a shows a perspective view of another preferred embodiment of the present invention in the form of a cathode flexible tail;

Figure 4b shows a perspective view of the electrical contact shown in Figure

4a;

Figure 5a shows a sectional top view of the connection between the body of the preferred embodiment shown in Figure 4a and the electrical contact;

Figure 5b shows a side view of the connection shown in Figure 5a;

Figure 6a shows a photograph of the preferred embodiment shown in Figure

4a;

Figure 6b shows a photograph of a known cathode flexible tail;

Figure 7a shows a perspective view of a friction stir weld (FSW) conducted using a tool with a narrow shoulder;

Figure 7b shows a side section view of the FSW formed with the tool shown in

Figure 7a;

Figure 8 shows a schematic sectional view of the passes made by the tool shown in Figure 7a to form a FSW;

Figure 9a shows tensile strength testing samples of FSWs with weld number;

Figure 9b shows an electrical conductivity sample used in testing the FSWs shown in Figure 10a;

Figure 10a shows a narrow shoulder FSW formed with the tool shown in Figure Figure 10b shows a wide shoulder FSW formed with the tool shown in Figure 7a;

Figure 11 shows a graph of load versus elongation curve of a tensile strength testing experiment of the FSWs;

Figure 12 shows a graph of load versus linear speed of the tool used to form narrow shoulder FSWs such as that shown in Figure 10a; and

Figure 13 shows a graph of load versus linear speed of the tool used to form wide shoulder FSWs such as that shown in Figure 10b.

BEST MODES FOR CARRYING OUT THE INVENTION

Headbar for use in a zinc smelter

Referring to Figures 1a and 1b a first embodiment of the invention in the form of a headbar for a cathode sheet for use in an electrolytic cell is generally indicated by arrow 1.

The headbar 1 is made of aluminium and welded to the top of an aluminium cathode sheet (not shown) to provide transportation of the cathode sheet via lugs or hooks 2 on the headbar 1. A copper contact 3 to at one end of the headbar 1 provides electrical contact between the cathode of one cathode sheet and anode of an adjacent like plate. One end of the headbar 1 comprises a first connection portion in the form of a slot 4 (best seen in Figure 2a) which is configured to receive a corresponding second connection portion in the form of a tongue 5 of the copper contact 3 (best seen in Figure 2b). A solid state electrical connection of the headbar 1 to the electrical connection 3 is formed at the seam of the slot 4 and tongue 5 by friction stir welding (FSW). A known headbar 100 which has an electrical connection 200 formed by an explosive bond weld 300 as shown in Figure 3.

The friction stir weld is a solid state full contact weld originally developed by TWI (UK) in 1991. During formation of a FSW butt joint, a non-consumable rotating tool, with a wider shoulder and a narrow threaded pin, is inserted into the seaming line of two adjacent metal plates and transverses along the seam. Heat is generated through friction and plastic deformation of work piece, which softens (not melts) the work piece so that the tool will move the material ahead of it to the back, thus making a joint. Based on the same thermo mechanical principle, Friction Stir Welding (FSW) involves joining two overlapping metal plates. A person skilled in the art will appreciate that the shape and configuration of the headbar 1 and electrical contact 3 may differ without departing from the scope of the present invention depending on the specification needed for the particular type of smelter the headbar is to be used with. For example, the slot 4 and tongue 5 could be a different shape and configuration such as a taper to increase the bimetallic contact area between the headbar 1 and electrical contact 3. The slot 4 could be machined in the headbar 100 (such as by sawing, lathing or drilling) or fabricated (such as using a laser or CNC cutting process). Similarly, the dimensions of the FSW will vary without departing from the scope of the present invention, depending on the specification required for the particular application. In addition the lugs 2 could also be joined to the body of the headbar 1 by FSW to provide a stronger bond than existing MIG shrink welds.

The slot 4 is machined 100 m shorter than the tongue 5. Press fit connection creates friction and surface disturbance between the copper tongue 5 and the aluminium slot 4 to improve metallic connection and improve electrical conductivity at the contact regions where FSW has not been done. Optionally, the surface of the copper contact tail 9a is abraded by sanding (for example with 60 grit sandpaper) or grinding to increase the surface area and therefore provide a stronger contact with the aluminium body 7.

During use, once the cathode sheet is immersed in electrolyte (such as zinc sulphate) within an electrolytic cell, current transferred to the cathode which results in deposition of pure zinc from the anode to the portion of the cathode sheet which is immersed in the electrolyte. After the electrolysis process is complete, the pure deposited zinc is stripped from the cathode sheet via insertion of a stripping knife between the deposited zinc layer and the cathode sheet.

Cathode flexible tails for use in aluminium smelters

A second preferred embodiment of the present invention is shown in Figures 4 and 5. A cathode flexible 6 comprises an aluminium head 7 and an end tail of flexible aluminium sheets 8. The flexible sheets 8 are connected to the busbar (not shown) through which current is supplied to the aluminium smelter reduction cell. The cathode flexible 6 also comprises a copper electrical contact 9 for electrical connection to a current collector rail. A slot 7a in the body 7 and a tongue 9a of the electrical contact 9 are in a tapered configuration to provide greater surface contact area on connection of the slot 7a and tongue 9a. Multiple FSW 10 are formed across the width of the overlap between the body 7 and the contact 9 and between the overlap between the body 7 and the cathode flexible tails 8. The FSW 10 penetration is approximately 15 mm and is 300-600 mm in length.

The cathode flexible 6 of the present invention is shown in Figure 6a together with a known cathode flexible 400 (Figure 6b). The cathode flexible 6 comprises an aluminium head 7, an end tail of flexible aluminium sheets 8 and a copper contact 9 which has been electromechanically connected to the aluminium head 7 by FSW according to the present invention. The known cathode flexible 400 has the copper contact 9 connected to the aluminium head 7 with a standard explosive bond weld 500.

Example 1: A first series ofFSWs performed on full-scale a head bar

Referring to Figures 7a and b, FSW on one side of a cathode headbar for a zinc smelter were simulated with FSW on a 4 mm thick aluminium plate 11 overlapping a 4 mm thick copper plate 12 by 25 mm (the overlap shown by arrow 13). The mechanical and electrical properties of samples taken from those welds were then conducted so that the optimum FSW condition could be determined. FSW was then performed on a full scale aluminium-copper headbar under the previously determined optimum conditions; so that the overall electrical conductivities of the FSW and explosive bonded headbars can be compared to each other.

Variation of Key Welding Parameters

During the first series of FSW studies, linear speed (v) and shoulder diameter (Dshouider) were varied. Given the fact that the aluminium-copper interface is 50 mm in length, which can be considered as a short welding distance, only relatively slow linear speeds were chosen. This is because there will be more time for the welds to stabilize over the welding distance. The parameters used and designated weld number is summarized in Table 1 below. Weld No. v (mm/min) D_Shouider (mm) Weld No. v (mm/min) Dshouider (mm)

N-1 20 15 W-1 20 22

N-2 80 15 W-2 80 22

N-3 56 15 W-3 56 22

Table 1. Summary of linear speed and shoulder diameter used in FSWs.

The letters "N" and "W" stands for welds conducted with a narrow and a wide tool shoulder respectively.

Machine, Tool, and Materials

A normal milling machine was used to perform FSW. Rotation speed (ω) and tilt angle (Θ) were kept constant at 1000 rpm (clockwise) and 2.5° respectively. These values have been widely used in commercial FSW applications, and thus can be considered as standard values. Tool pin 14A was 3.5 mm long, 6 mm in diameter with left hand M6 threads. Tool 14 (made of H13 tool steel) was CNC machined and then heat treated. Heat treatment included firstly heating tools up to 1050 °C for one hour following by oil quenching; and then tempering them at 600 °C for one hour.

During the first series of FSWs, aluminium AA6061 (11 ) and copper (12) plates were machined to 200 mm long and 4 mm thick, with width of 60 and 100 mm respectively.

When a wider shoulder-tool 14 is used, the same weld set up was used but with the overlapped plate distance increased to 32 mm. During all welds, the tool 14 was plunged into the aluminium plate 11 in such a way that the tool shoulder 14 is 0.3 mm below the surface of the aluminium plate 11 , leaving a gap of 0.2 mm between the bottom of the pin 14A and the copper plate 12 (provided the pin 14A is 3.5 mm long) as shown in Figure 6B. Three welds were conducted on a set of aluminium 11 and copper 12 plates, each with a length of 65 mm. Four jaws were clamped at the outer corners of the aluminium plate 11 and copper plate 12 during FSW.

Example 2: A second series of FSWs performed on full-scale a headbar

The headbar (1 ) was clamped using a bench vice with steel sheets inserted alongside the copper contact (3) in order to avoid the aluminium bar being distorted laterally. The detailed weld passes are shown in Figure 8. In the first weld (15), the tool pin 14A covers the outmost 6 mm (in width) of the aluminium-copper interface; whereas during the second pass (16), of the tool 14 was moved towards the copper cathode for 6 mm. Hence, the two passes in total cover two thirds of the total aluminium-copper interface (12 mm in width). The same tool plunging method (stated above in Example 1 ) was used.

Mechanical and Electrical Testing

Mechanical strength testing samples (~ 10 mm wide) and electrical conductivity testing samples (~ 6 mm wide) were taken from the ends of the FSWs using a wire cutting machine. A tensile strength testing machine was used, and the testing speed was set at 3 mm/min. Tensile testing samples are shown in Figure 9a, with the weld number indicated (W1-3 being control MIG weld samples and N1-3 being FSW samples).

During conductivity tests, a constant current of 30 A was supplied. Two holes (tapped with M4 threads-not shown) were drilled on both ends of the samples and the centre-to-centre distance is kept constant at 120 mm (Figure 9b). M4 bronze bolts 17 attached with wires were tightly screwed onto the holes in the samples to ensure current supply is strong. A voltage meter was attached to these bolts 17 so that the voltage drop can be measured across the weld sample (a discussion of the conductivity results are discussed below).

Appearance and Weld Structures

Results of FSWs performed on aluminium plates overlapping copper plates, using a narrow and a wide shoulder are shown in Figures 10a and 10b respectively. The linear speed used in each weld has been given in Table 1. It can be clearly observed that the outer boundary of the aluminium plate was severely deformed when a wide shoulder (Figure 10b) was used. This because the amount of heat input during FSW is significantly enhanced as the shoulder diameter increases, which further causes an intensive material softening effect, thus promoting distortion of the aluminium plate. Overall, welds conducted with a narrow shoulder (Figure 10a) display better finishing appearance.

Tensile Strength and Conductivity Testing The load verses elongation curve of a tensile testing experiment (of sample N-1 as shown in Figure 9a) is shown in Figure 11. As can be seen, the load applied (in Newtins, N) on the sample increases linearly as the sample was being elongated. The maximum load before fracture was detected as 1767 N. Maximum loads achieved during the tensile testing of all samples, and the corresponding sample width, are summarized in Table 2 below. Loads which can cause a weld of a unit width (1 mm) to fracture can be calculated by dividing maximum loads by sample width. These results are given in Table 2 under the "unit width load" column.

Ί— "

Weld Voltage drop Resistance Sample Width Unit Width No. (mV) (mQ) (mm) Resistance (mQ)

N-1 2.92 0.097 6.4 0.0152

N-2 2.88 0.096 5.8 0.0166

N-3 3.61 0.120 5.3 0.0227

W-1 3.28 0.109 6 0.0182

W-2 2.86 0.095 5.7 0.0167

W-3 3.65 0.122 5.2 0.0234

Table 2. Summary of voltage drop of all weld samples and the corresponding width of sample.

As shown in Figure 12, the unit width load (N/mm) is plotted against linear speed (mm/min). As can be clearly observed, under the same linear speed, the weld conducted with a smaller shoulder (15 mm) is always stronger than the weld conducted with a wider shoulder (22 mm). It might be suggested that the overall weaker mechanical strength is related to the excessive heat input when a wide shoulder is used. Larger heat input tends to promote the formation of fragile intermetallics at the aluminium-copper interface; also the formation of "hooks" at aluminium-copper interface because material flow is more intensive as it is excessively softened. There is a near-linear positive relationship between unit width load and linear speed. Similar to increasing shoulder diameter, reducing v also increases the heat input during FSW, thus resulting weaker mechanical strength due to the reasons stated above. The voltage drop across two points (with a constant distance of 110 mm) on the FSW samples (6 mm in width) was measured. The voltage drops are summarized in Table 2. With a constant current supply of 30A, the total resistance of the weld (110 mm long) can be determined by dividing the voltage by current. Furthermore, the resistance of the weld per unit width can be calculated by dividing the total resistance by sample width. These results are also given in Table 2.

In Figure 13, unit length resistance (Ω/min) is plotted against linear speed (mm/min). The data suggests that under the same speed, welds conducted with a smaller shoulder have smaller resistance comparing to welds conducted with a larger shoulder, although the difference seems quite small under relatively high linear speed. Furthermore, regardless of the shoulder diameter, a moderate linear speed promotes the highest resistance and thus the lowest conductivity.

Combining the observations made from Figure 12 and Figure 13, it can be suggested that, at the same linear speed, the welds conducted using a narrow shoulder have better mechanical strength and lower electrical resistance comparing to welds obtained using a wide shoulder. If a narrow shoulder is chosen, using the fastest linear speed (80 mm/min) yields the best mechanical strength and the second lowest electrical resistance. Thus, it is reasonable to suggest that using a narrow shoulder (15 mm) and linear speed = 80 mm/min gives overall the most desirable welds, hence it can be considered as the optimum welding condition for the current application.

The following conclusions can be drawn:

1. Mechanical strength of the weld (Unit Width Load) has a linear positive relationship with linear speed.

2. Electrical resistance of the weld (Unit Width Resistance) is highest at a moderate linear speed (56 mm/min), while lower at 20 and 80 mm/min.

3. Increase in shoulder diameter yields lower mechanical strength and higher electrical resistance.

4. For current application, the optimum condition is: tool rotation speed = 1000 rpm, linear speed = 80 mm/min, tool pin angle = 2.5° and plunge depth = 3.8 mm.

Example 3: Volt drop measurements FSW joined cathode headbar

A 200 amp electrical load was applied across a sample of a cathode headbar 1 (as illustrated in Figures 1 b and 3) joined by a series of FSW according to the present invention. A settling time of 1 minute was allowed before the volt drop was measured. An identical method was used with a known cathode headbar 100 (as shown in Figure 3) with a standard explosive bond weld 300.

The volt drop of the FSW sample was 9.254E-05 volts with a measurement uncertainty of 1.33E-06 volts. A volt drop across the standard MIG weld cathode flexible tail was 1.3094E-04 volts with a measurement uncertainty of 1.42E-06 volts.

These results show a lower volt drop across the FSW joint as compared to the MIG weld control joint. The efficiency of power transfer across the FSW joint is therefore improved.

FSW joined cathode flexible tail

A 500 amp electrical load was applied across a sample of a cathode flexible tail 6 (as illustrated in Figures 4 and 5) joined by a series of FSW 10 according to the present invention to simulate the 350-450 °C temperature attained by the cathode during normal use in an electrolytic cell. At these temperatures the cathode leaves 8 expand and contract significantly. A settling time of 1 minute was allowed before the volt drop was measured. An identical method was used with a known cathode flexible tail with a standard MIG weld.

The volt drop of the FSW sample was 2.4791 E-04 volts with a measurement uncertainty of 1.33E-06 volts. A volt drop across the standard MIG weld cathode flexible tail was 3.2443E-04 volts with a measurement uncertainty of 1.34E-06 volts. A current stability uncertainty of 150 mA was calculated during testing of both samples.

The present invention offers notable advantages over the prior art including: • improved electrical conductivity and lower electrical resistance via a straighter current path and greater surface area between the aluminium head and dissimilar metal electrical contact which results in power cost savings and a resultant environmental benefit;

• elimination of MIG welds and improved precision alignment of the parts;

• improved mechanical joint strength and rigidity which results in improved durability and reduced stress: cathode sheet assemblies are subjected to harsh operating conditions and mechanical handling. Aside from the cathode sheet which wears out faster than the aluminium head, the aluminium head itself can get damaged in service so need to be as strong as possible. The electrical connection of the present invention gives increased structural strength and potential for longer service life through higher weld strength and reduced annealed area in the aluminium head compared with MIG welds;

• decreased susceptibility to corrosion between the electrical contact and the aluminium head which results in improved reliability and decreased maintenance costs;

• decreased cost of manufacture of cathodes; and

• improved environmental impact through the electrical connections of the present invention only using a small amount of electrical energy in their fabrication. They do not consume explosive materials, MIG weld shielding gases and wire or high MIG welding electrical energy.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims.

Claims

WHAT WE CLAIM IS:

1. A cathode for use in an electrolytic/reduction cell, the cathode comprising:

o a first connection portion; and

2. The cathode for use in an electrolytic/reduction cell as claimed in claim 1 wherein the aluminium head is selected from the group consisting of: an aluminium headbar and an aluminium cap for connection to a plurality of aluminium flexible sheets.

3. The cathode for use in an electrolytic/reduction cell as claimed in claim 1 or claim 2 wherein the first connection portion is a slot.

4. The cathode for use in an electrolytic/reduction cell as claimed in claim 3 wherein the slot is a square-end slot.

5. The cathode for use in an electrolytic/reduction cell as claimed in any one of claims 1 to 4 wherein the dissimilar metal electrical contact is made from copper.

6. The cathode for use in an electrolytic/reduction cell as claimed in any one of claims 1 to 5 wherein the first connection portion is a projection.

7. The cathode for use in an electrolytic/reduction cell as claimed in claim 6 wherein the projection has a tapered profile.

8. The cathode for use in an electrolytic/reduction cell as claimed in claim 1 wherein the at least one cathode sheet is a plurality of aluminium flexible tails.

9. A method of manufacture of a cathode for use in an electrolytic/reduction cell, the method comprising the steps of: a. Providing an aluminium head configured for connection to at least one cathode sheet, the aluminium head comprising a first connection portion; b. Providing a dissimilar metal electrical contact configured for connection to a busbar, the dissimilar metal electrical contact comprising a second connection portion; and

10. The method of manufacture of a cathode for use in an electrolytic/reduction cell as claimed in claim 9 wherein the method also comprises the step of:

11. The method of manufacture of a cathode for use in an electrolytic/reduction cell as claimed in claim 9 or claim 10 wherein the aluminium head is selected from the group consisting of: an aluminium cap for connection to a plurality of aluminium flexible tails.

12. The method of manufacture of a cathode for use in an electrolytic/reduction cell as claimed in claim 11 wherein the method of manufacture of a cathode for use in an electrolytic/reduction cell also comprises the steps after step b but before step c of:

• Machining the aluminium friction stir welded to the second connection

portion to a substantially square end.

13. The method of manufacture of a cathode for use in an electrolytic/reduction cell as claimed in claim 12 wherein the method of manufacture of a cathode for use in an electrolytic/reduction cell also comprises the pre-steps of. a¹. Clamping the plurality of aluminium flexible sheets between a pair of rigid aluminium plates; a². Electromechanically connecting and combining the pair of rigid aluminium plates and the plurality of aluminium flexible sheets by friction stir welding.

14. The method of manufacture of a cathode for use in an electrolytic/reduction cell as claimed in any one of claims 9 to 13 wherein the method of manufacture of a cathode for use in an electrolytic/reduction cell also comprises the step:

• Connecting the aluminium head to the at least cathode sheet from step a by butt welding.

15. A method of purifying a non-ferrous metal in an electrolytic/reduction cell, comprising the steps: c. providing an aluminium head configured for connection to at least one

o a first connection portion;

o a dissimilar metal electrical contact configured for connection to a busbar, the dissimilar metal electrical contact comprising a second connection portion; and wherein the first connection portion and the second connection portion are electromechanically connected and combined by friction stir welding d. passing an electric current between an anode and the cathode sheet(s).