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Tata Motors Company introduced flash butt welding of stranded aluminum cables replacing common copper battery cables in 1974. The changeover to aluminum cables was due to an acute shortage of copper cables. I was with Tata Motors as Quality Assurance Officer and this challenging task was entrusted to me.
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Aluminum stranded cable is easily joined to zinc alloy end connectors by the mechanical pressure crimp process. However, crimped joints have serious application problems. Aluminum is prone to surface oxidation. The excellent electrical conductivity of aluminum gets significantly reduced by the oxidized surface film. Contact electrical resistance is high, and as such the voltage drop increases radically. The performance tests were a total failure.
It is impossible to prevent surface oxidation of aluminum. Its electrical conductivity is 60% of copper. This was increased to 62% by a new invention by National Metallurgical Laboratory, Jamshedpur, India in 1973. Adding a small quantity of rare earth element in molten aluminum causes a radical desirable change in the microstructure. The oxides are contained within the grain boundary. Nevertheless, aluminum surface oxidation stays intact on the surface.
Flash butt welding of multi stranded aluminum cables was a challenge which was tackled quite easily. Zinc alloy (1% Copper max) pressure die cast hollow sleeve was inserted into the stranded cable. The hollow sleeve covers 25 mm (37 aluminum wires of 1.6mm diameter) bare wires and 20 mm is covered by the PVC sheath of the cable. Sleeve support is necessary to avoid distortion of the wires under axial compressive force in the flash butt welding process. PVC sheath cover protects the aluminum bare wires. 37 wires aluminum cable was used having same current carrying capacity of the prevailing copper cable.
The zinc alloy used is known as Zamak or Zilloy.The Zamak solid end connector diameter is the same as the hollow sleeve outer diameter. Please see the assembly configuration in the photos below (please click to enlarge).
We had a Mckay Federal Flash Butt Welding Machine in our plant for flash butt welding of gear shift levers steel forgings. We used this equipment for flash butt wielding stranded aluminum cables alternatively.
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A Tata truck engine starting current drawn by an Aluminum battery cable is around DC 200 amps. Welded joints are robust and free from oxides. Durability is ensured. It replaces costly Copper-stranded cables. Electrical performance is superior to Copper cables. The flash butt welding process is fast and is ideal for mass production.
Inline flash butt welding portable equipment is used for joining railway lines continuously in a short time. The bulge at the joint is leveled by online grinding. Simultaneous ultrasonic tests are done to ensure a sound weld joint. The joint is of forge quality. The old method of endothermic welding the rail joints is a localized molten steel casting process. Mechanical properties are inadequte for modern high speed rail traffic.
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Photos of flash butt welded Aluminum Battery Cable assembly
Photo A: The assembly configuration of the aluminum cable shows the sequence of flash butt welding process.
Photo B: Connector brass lugs are dip soldered.
Photo C: Cross sectional view of copper cable soldered joint. Please note the black spots in the gaps. The entire cross-section is occupied with high resistance solder alloy. The cross-sectional view shows the solid zinc alloy lug surround by a thin, peripheral layer of solder metal. The dip solder joint has a protective PVC cover against acid spill.
Photo D: Left- battery voltage drop in milli volts versus battery current in amperes. Right- battery current flow in amperes for 1 minute versus resulting temperature rise at the battery cable joint. The chart shows comparison between the electrical performance data of the copper and aluminum battery cables.
Please note: Lead tin solder alloy is banned. Lead free soldering alloy is now mandatory.
Photo E. Inline portable Flash Welding Equipment is used for joining high strengh I section rails for sustaining high dynamic loads by high speed train traffic.