Parameters Influencing PTA Welding Dilution Rate

Dilution rate is very important for welding process and it directly affects the deposit hardness. Electric current is well known for adjusting the dilution in a PTA welding, however there’re serval other parameters.

1. Movement Speed

The energy of the plasma arc is very strong, so the welding movement speed is very important. If the movement is too fast, although the bead shape is good, the heat of the arc cannot reach the workpiece, and the welding layer and the substrate cannot be combined. If the movement is too slow, the heat of the arc is transmitted to the workpiece too much, so that the dilution rate of the weld bead is too large, and the hardness of the welding layer will become low. The oscillation width is generally controlled at 10-15mm, and the dilution rate is controlled at about 5%.

2. Center Gas

The flow rate of the center gas directly affects the strength (blowing force) of the plasma arc. A bigger flow rate leads to a stronger arc strength, and the dilution rate of the welding layer is accordingly larger, vise verses. A small flow rate also has bad impact on life time of nozzle and electrode. The size of the flow should be referred to the gas flow table, and it should be adjusted based on actual experience.

3. Powder Feeding Gas

The function of powder feeding gas is to convey the powder preventing from clogging in the feeding pipeline and ensure that the powder is uniformly sprayed from the nozzle. A bigger feeding gas leads to larger dilution rate.

4. Powder Feeding Volume

The single pass welding layer is generally controlled at a thickness of 1.5-3mm, up to 6mm (generally not recommended). Hence the volume of powder feeding is set at about 20-25 g/min. If a larger weld bead is needed, multiple passes can be applied. A smaller feeding volume leads to smaller dilution rate.