TECHNICAL FIELD

Copper to aluminium electrical connections are commonly used to connect copper contacts to aluminium cables in for example battery cable connections. However, problems arise when welding metal parts of dissimilar material in that brittle intermetallics may form and weaken the joint. There is also a risk for contact corrosion due to the different electrical potential between the metals.

Aluminum-based electrical cables are desirable due to weight savings and consequent reduction of fuel consumption, and are increasingly replacing the relatively heavy cables made of copper.

The present invention relates to a flat metal cable provided with a means for connection of a second metal cable, such as an aluminium conductor. The invention also relates to a connected structure comprising the flat metal cable, wherein a second cable is connected to the flat metal cable.

BACKGROUND ART

A method for electrically connecting a copper cable to a stranded aluminium cable is known from EP2735397. The connection is made by rotating a tool placed with sustained pressure in an area of the bottom part of the contact until the material of the conductor is softened due to increased temperature of the material of the conductor caused by the friction heat generated. The process is limited to round cable connections.

Connections between flat cables and car batteries are known from US2016250984, in which the flat part of an aluminium cable is connected to a vehicle motor battery via a connection bolt by means of ultrasonic or friction welding. The many interfaces of such a joint cause the electrical resistance of the connection to increase.

SUMMARY OF THE INVENTION

The motor space of vehicles is increasingly limited due to requirements of more

functionality of cars and a wish to keep the weight of the car as low as possible. The use of a flat cable takes up less space than a round cable and can more easily be bent to fit into different confined spaces. Flat cables however cannot be joined to other cables in an easy manner. Aluminium very quickly forms an oxide on its surface when exposed to air and the oxide is difficult to break through in welding processes unless special methods are applied.

There is thus a need to find a flat cable with a connection that may be used to join the flat cable to a second cable in a way that limits the number of interfaces in the connection and the number of process steps for joining and at the same time gives a high conductivity across the joint.

The present disclosure relates to a flat cable configured to accommodate a second metal cable, where the flat cable has a hollow riser extending from the flat upper or lower side of the cable. The riser is preferably located at one or both ends of the cable.

The riser may be welded onto the cable, or attached in other suitable manner, but the preferred method for making the flat cable with a hollow riser extending from one of the flat surfaces is reversed extrusion. By integrating the riser into the material of the cable by reversed extrusion the conductivity and the mechanical properties of the cable connection are not deteriorated by a weld.

A part of the cable may be covered by a polymer coating, such as a polyamide coating, in order to isolate the cable from other metal parts of the engine.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention is received from the detailed description when read in conjunction with the accompanying drawings.

Figure 1 shows a drawing of the flat metal cable according to the invention.

Figure 2 shows a coated flat metal cable according to one embodiment the invention.

Figure 3 shows a connected structure comprising the flat metal cable according to the invention.

Figure 4 shows the process of reverse extrusion used to form the riser on the flat metal cable of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a first metal cable (here forth called the flat metal cable) having a flat upper and a lower surface, located opposite each other, comprising at least one hollow riser extending approximately 90 degrees from one of the flat surfaces of the cable, the riser being configured to house a second metal cable. The cable is preferably made from commercially pure aluminium or an aluminium alloy, more preferably from one of the alloys AA1370, AA8176 or AA1350.

Preferred dimensions of the flat metal cable are 1-30 mm height and 10-80 mm width, preferably 1-5 mm height and 10-30 mm width, but other dimensions are of course possible.

There may be one or several risers extending from the flat surface of the cable, preferably the riser is located at the one or both ends of the cable. The riser and the cable is made as one part, without interconnecting welds or joints.

The present invention also provides connected structure according to figure 3 comprising the flat metal cable, wherein a second metal cable (3), such as a stranded aluminium cable, is connected to the hollow riser by inserting the second cable into the hollow riser and joining the cables e.g. by friction welding of the second metal cable with the lower surface of the flat cable in the area underneath the riser. The second metal cable is preferably a cable of rounded shape. A round cable is easier to bend in all directions so that the shape can be adapted to various requirements. A flat cable is useful when a higher heat emission is desired due to the larger surface/ cross section ratio of flat cables in comparison to round cables. A flat cable ratio also has the advantage of an easy connection to other parts by welding, e g friction stir welding, or similar known processes. Moreover, a flat cable is advantageous when a small bending radius is required (it can be bent to a radius of almost 0°) or when the cable needs to be hidden in the compartment. A combination of the flat metal cable and a round or rounded cable provides a combination of the above advantages.

The friction welding may be performed by inserting the second metal cable into the hollow riser and contacting a rotating tool with the lower surface (B) of the flat cable in the area underneath the riser or by rotating the second metal cable inside the hollow riser until a partial melting of the second metal cable and/or the riser bottom surface (C) takes place.

In figure 1 an embodiment of the flat cable (1 ) with the extruded hollow riser (2) is shown. The cable is for example made by extruding a flat solid body from an aluminium billet in a conventional manner. The material of the first and second metal cables are preferably commercially pure aluminium alloy or an alloy with good conductivity and mechanical properties such as AA1370, AA8176 or AA1350. The riser is then drawn from the solid body by reverse extrusion directly from the flat cable material, as exemplified in figure 4. The reverse extrusion is conducted by forcing a tool, such as a steel block, against the upper flat surface of the cable (A). A moving central ram forces the material to extrude metal from the said block up into the cavity of the tool, resulting in a tubular riser extending from said flat surface. Material from the cable is used for forming the riser, so that the resulting thickness of the flat cable may be somewhat reduced under the riser. If needed excess material on the riser is removed by a cutting operation. Before extrusion of the riser, a part of the cable may be coated with a polymer, e g a polyamide, to isolate the cable electrically from the surrounding parts. Battery cables are exposed to harsh environmental conditions such as high temperatures, oil, dirt, salt water and wear against which the polymer coating protects. The coating (5) is preferably applied by co-extrusion, as exemplified in

W020141071 12, but also other methods, such as powder coating may be used to coat the cable. A part of the cable may be left uncoated or part of the coating removed so that the metal of the flat cable is exposed. The part of the flat cable not coated with a polymer is inserted into the reverse extruder and material is drawn into a cylinder hollow riser. The method comprises the steps of: providing a flat metallic profile (1 ), forming from said flat metallic profile a riser body (2) having a substantially solid cross-section, by subjecting the metallic profile to reverse impact extrusion by means of a cylindrical extrusion tool (D) comprising a housing mandrel (M). The riser body is converted into a thin-walled hollow riser by penetration of the mandrel (M) into the riser body and the resulting material flow between the mandrel and the walls of the cylindrical tool. The process may be conducted in one step or by successive formation of the riser body and the riser walls. In such a way a coated cable with the back-extruded riser according to figure 2 is formed.

The riser is preferably located at one or both ends of the first metal cable, so that the riser occupies a part of the upper surface (A) and that a few millimeters of the first cable remains flat outside the riser.

A second metal cable may then be attached to the hollow riser, e g by inserting an end of the second metal cable (3), such as a stranded aluminium cable, into the hollow riser and joining the second metal cable to the first metal cable by contacting the tip of a rotating tool (4), such as a steel cylindrical tool with the lower surface (B) of the flat cable in the area underneath the riser. The material of the riser and the stranded cable is partially melted due to the friction heat generated and when the tool is retracted the materials are joined to form the structure shown in figure 3. A connection between the second metal cable and the flat metal cable may alternatively be achieved by rotating the second metal cable after insertion into the hollow riser until the metal surfaces partially melt and join on solidification.

The opening of the riser connection of the invention may be circular or oval or any other shape that can accommodate an end of a second metal cable for attachment thereto.

The invention shall not be considered limited to the illustrated embodiments, but can be modified and altered in many ways, as realized by a person skilled in the art, without departing from the scope defined in the appended claims.