This is part 2 about solid versus cored wire for MAG welding. Part 1 is about the different types of wire and their properties. This part looks in more detail at melting speeds and application possibilities.
Current density is the determining factor for how much of a wire can melt per time unit, usually gr/min or kg/h. Current density refers to the number of amperes per mm2. The calculation in case of a solid wire is as follows; current divided by the surface area of the wire. Suppose welding is carried out with 280 amperes and different wire diameters, the following current densities apply.
- 280 A : 0.785 mm2 (surface area of 1.0 mm wire) = 357 amperes/mm2
- 280 A : 1.13 mm2 (surface area of 1.2 mm wire) = 248 ampere/mm2
- 280 A : 1.54 mm2 (surface area of 1.4 mm wire) = 181 ampere/mm2
To get similar melting rates with all wire diameters, the current has to be increased considerably when using 1.2 mm or 1.4 mm.
This is very different with cored wire! Now the current only goes through a very thin-walled tube and not the filling. As a result, the current density will increase enormously and so will the precipitation. When you look closely into the welding arc when using wire filled with a metal powder, you will see in the heart of the arc a kind of dark colored rod that seems to be pushed into the puddle. That is the metal powder filling that will only melt in the puddle. So it also matters whether you weld with a folded or pulled cored wire. With the latter, the wall is thicker and therefore the current density is lower, with the same current setting.
Current load capacity
Solid and cored wires have different current carrying capacities. The wire/gas combination ultimately determines the maximum current carrying capacity.
A solid 0.8 mm wire welded in an 80/20 gas has a maximum current load of 240 amperes. A 1.0 mm wire in the same gas has a maximum current load of 280 amperes and a 1.2 mm 320 amperes. Of course you can weld with a higher current, but due to the extra heat, many alloying elements will evaporate, reducing the quality of the weld.
In case of pulsed arc welding with these wires in 80/20 the current is even lower. Respectively 200 amperes for 0.8 mm, 240 amperes for 1.0 mm and 260 amperes for 1.2 mm. This is because a pulsed arc is much warmer than a spray arc and so alloying elements will evaporate even faster.
If you set a current of 300 amperes on the power source, that doesn’t mean that you are actually going to weld 300 amperes. How much power eventually reaches the wire depends on a number of factors.
First, the surface of the wire is very important. Whether the wire is copper plated or blank, as with folded wires, can cause a big difference in current transfer. Also grease residues can severely hamper the current transfer. If a felt is fitted in the wire feed case to degreasing the wire, replace it in time. In case of prolonged use, the wire will only become greasier.
The quality of the welding gun and the wear parts in it also play an important role. Think for example of the quality of the copper used in the wear parts and the power cable and whether the power cable has the correct number of mm2 litz for the welding current. If welding with higher currents, good water cooling is also important.
In the range of contact tips it is important to choose the right quality. The correct mounting (read: fastening) of the contact tip is of crucial importance. Finally, it is important to check whether the workpiece cable is properly fastened. It often happens that this clamp becomes red hot, due to incorrect fastening. It is better to put the heat that is lost here into the wire.
Gases also affect the melting rate, the weld arc and the puddle. The chosen gas will react almost equally in case of a solid or a cored wire. A mixed gas with 20% CO2 will be very active in the puddle, while a mixed gas with 8% CO2 will give a much nicer separation of the droplets and less splash losses. It is however much less active in the puddle. If a part oxygen is added to gas mix, for example a 90-5-5 (CO2 90% – Ar 5% – O2 5%), the precipitation will increase significantly. Oxygen is very active at the wire tip and ensures a finer droplet transition and thus a higher precipitation. On the other hand, oxygen is not active in the puddle. When welding without pulse, a fusion error is easily made. The gas composition to be chosen will depend strongly on the work to be performed and the type of arc that is used.
A cored wire has a higher melting rate than a solid wire. However, this does not always mean that the use of cored wire is better. That is a lot more nuance to it.
First of all, the welder; can he put the offered amount of material in the seam in such a way that no welding errors occur when working with high currents? And, in that case, can the welder still achieve a reasonable duty cycle? Since a cored wire is a lot more expensive than a solid wire, it must be carefully considered whether a cored wire will offer sufficient advantages in terms of cost price and technology. Especially in manual welding, where the higher melting rates are limited in use.
On the other hand, a filled wire has a nicer arc distribution, which, if material with a mill scale is welded, results in a nice flat weld with usually less finishing to be done.
Perhaps you could say that the use of cored wires is best suited to automated or robotic welding.
Cored wire is therefore not a miracle product as sometimes manufacturers would have you believe, but very useful for a number of applications. Especially because the composition of the powder can influence the weld quality. There is a wide range of wire types available for this purpose.