Page 144 - Welding of Aluminium and its Alloys

MIG welding 129

7.12 Four roll MIG wire drive unit. Courtesy of TPS-Fronius Ltd.

7.2.2.4 Contact tip (tube)

The contact tip is a small but vital component in the welding power circuit.
The tip is formed from a tube made to be a sliding fit for the wire. It is
screwed into the torch head, ‘B’ in Fig. 7.10, and is the point at which the
welding current is picked up by the filler wire.The contact tip is made from
copper or brass and wears in use. It is therefore made to be replaceable.
The tip for aluminium welding may vary in length from 25mm to 100mm.
The longer contact tips provide the best current transfer conditions and
therefore the most stable welding conditions. Tips have been designed that
carry either a spring-loaded shoe to maintain a constant pressure on the
wire or with the hole offset in order to force the wire against one wall,
thereby improving and maintaining contact.
A worn contact tip may cause the wire to jam, resulting in a tangle at the
wire drive rolls. A perhaps more serious weld quality problem may also
arise from arc instability caused by the point at which the wire picks up the
current moving up and down the contact tip. This effectively changes the
wire stick-out length which in its turn affects the voltage, leading to arc
instability and lack of penetration defects. Poor contact between the tip and
the wire may cause arcing within the tip, giving rise to arc instability and
perhaps wire feed problems. Damage to the tip from spatter, accidental
touch-down or mechanical damage may cause similar problems.