Feedability of welding wire is controlled by sliding contacts between contact tube and wire surface. It is very important for the stable arc discharge that feeding speed of wire does not deviate from the constant speed derived from average welding current. Temperature of contact tube, current branch, voltage drop, and contact resistance of the sliding contact were measured during welding of Cu-coated and non-Cu-coated solid wires. The sliding contacts of Cu-coated solid wire have both states of solid and liquid depending on welding current. On the other hand, the sliding contacts of non-Cu-coated solid wire have a state of liquid without depending on welding current. Discontinuity of solidification at the sliding contacts causes feeding resistance of welding wire. Shearing strength of bridge solidified at sliding contact is about 4.5N, and this force is amplified in liner tube to about ten times. The amplified force is feeding resistance of welding wire. Deviation of wire speed is 43% to average wire speed assuming spring constant of welding wire to be 6600N/m.
