Plasma arc welding technology

Plasma arc welding technology mainly includes joint form, parts assembly and clamping, selection of coal gas and welding process parameters.

Plasma arc welding joint form

The universal joint forms of plasma arc welding are :I, single V and U, double V and U groove. In addition to butt joints, plasma arc welding is also suitable for fillet welds and T-joints.

For welds with a thickness greater than 1.6mm but less than the thickness values shown in Table Plasma arc welding technology-1, i-shaped groove can be used for single-side welding using the small-hole method. For thick parts with large thickness, large blunt edge and small Angle groove butt can be used. The first weld shall be welded by penetration method, and the filling pass shall be completed by penetration method. When the thickness of the weldment is between 0.05 mm and 1.6mm, penetration welding is usually adopted. Figure Plasma arc welding technology-1 shows the joint.

(a) I-joint; (b) flanged butt joint; (c) crimped corner joint; (d) The end joint δ is one plate thick; h height of a roll,h=(2-5)δ

Assembly and clamping of plasma arc welding parts

The edge of the groove at the arc starting point of the low-current plasma arc welding must be in close contact, and the gap should not exceed 10% of the metal thickness. When it is difficult to meet this requirement, the filler metal must be added. For thin plate with thickness less than 0.8mm, see Table Plasma arc welding-2 , Figure Plasma arc welding-2, Figure Plasma arc welding-3 for the transfer and clamping of welded joints

（a）interval （b）misalignment （c）The clamping distance

Selection of welding gas in plasma arc welding

When plasma arc welding is carried out, plasma gas must be transmitted to the welding gun compression nozzle, and protective gas must be transmitted to the welding gun protective gas cover to protect the welding pool and the metal near the joint area.

Ar is usually used as ionic gas in welding, which is suitable for all metals. In order to increase the heat input to welding parts and improve welding productivity and joint quality, H₂, He and other gases can be added to Ar respectively. For example, when welding stainless steel or nickel alloy, 5%~7.5% H₂ can be added to Ar; When welding titanium and titanium alloys, 50%-75% He can be added to Ar. For copper welding, 100% He or 100% N₂ high-current plasma arc welding gas can be selected as shown in Table Plasma arc welding-3. The ionic gas and protective gas have the same composition. If not, the stability of plasma arc will be affected.

Gas selection for low current plasma arc welding is shown in Table Plasma arc welding-4. In this process, Ar is used as ionic gas, which makes non-transfer arc easy to ignite and stable combustion. The composition of protective gas may be the same or different from that of ionic gas.

Selection of welding parameters for plasma arc welding

In addition to the process parameters, plasma arc welding quality is directly related to the material properties of welding parts, the rationality of welding gun structure, and the processing accuracy of gun body. If the concentricity of the nozzle to the electrode can not be guaranteed to be less than 0.05mm, the arc is unstable and the weld is easy to produce defects such as edge biting. The influence of process parameters on weld forming is discussed below.

There are many technological parameters of plasma arc welding, among which the most important ones are welding current (I), welding speed (V) and ionic gas flow (Q). In addition, there are nozzle aperture (d_c) internal shrinkage length of electrode (L_y), distance between nozzle end face and weldment (H), protection gas flow (Q1), channel length (L_c).

See Figure Plasma arc welding-4. The following three important parameters I, V and Q are described.

Welding current (I)

I increases, and plasma arc penetration is strong. I Based on the thickness of the plate or penetration requirements. I too small to form a hole; If I is too large, the diameter of the small hole is too large, causing the molten pool metal to fall.

Welding speed (V)

When other plasma arc welding process parameters remain unchanged, the pore diameter decreases or even disappears when V increases. On the contrary,V is too low, resulting in metal overheating, back weld metal sink or molten pool leakage. V is determined in relation to I. In order to obtain stable keyhole effect and smooth weld forming, when Q is constant, it is necessary to keep increasing V while increasing I; If V is constant, I decreases as Q increases

Ionic gas flow rate (Q)

The plasma flow force and penetration capacity increase with the increase of Q. In order to form stable keyhole effect, sufficient ionic gas flow must be guaranteed under other conditions unchanged. When the nozzle aperture is determined,Q is determined with I and V

The reference values of the process parameters of general small-pass plasma arc welding and penetration plasma arc welding are listed in Table Plasma arc welding-5 & Table Plasma arc welding-6