 |
Plasma-welding-tips:How to exploit Unique Capabilities.SOLUTIONS with Effective, Powerful Advice
Plasma-welding-tips are useful reminders intended to stress the main characteristics of Plasma Arc Welding (PAW) for the correct exploitation of its specific advantages. The first of the Plasma-welding-tips is a correct definition of PAW, useful for understanding the differences that make the process unique. Plasma Arc Welding is defined as a gas shielded arc welding process where the heat for welding is generated by an arc producing a collimated plasma by the passage of the shielding gas through a constricting nozzle. The copper alloy nozzle is called constricting because it presents a limited diameter orifice, through which the shielding gas, highly ionized by its transit through the arc, must pass. The concentrated and collimated jet stream of ionized gas (composed of nearly equal number of electrons and ions of gas atoms and molecules) that exits at very high temperature (about 17,000 0C or 30,000 0F) from the constricting nozzle, is called a plasma column. Plasma Arc Welding can be performed with or without additional filler metal. In manual applications, the filler metal as added as needed by hand. In the mechanized version, the filler metal is added from the side by a wire feeder. Plasma-welding-tips concerns understanding the specific features of the process detailed below, and using its capabilities for obtaining successful welding results.
Arc Modes Plasma-welding-tips The arc is produced between a nonconsumable tungsten electrode and either the workpiece (transferred arc mode) or the constricting nozzle (nontransferred arc mode). In the transferred arc mode the electric current is discharged directly from the torch to the workpiece through the conducting plasma. The workpiece is included in the electric circuit but the nozzle is not. Heat is obtained by a combination of anodic heating, where electrons from the negative tungsten electrode impinge on the workpiece, and from the plasma jet radiating its heat to the surroundings. Plasma-welding-tips: In certain cases, where the current is too high for a given orifice size, a secondary arc may be produced between the nozzle and the workpiece, a case called double arcing. This condition is a faulty one and must be avoided as it causes damage to the nozzle while impairing weld quality. It can be corrected by reducing the current or by selecting a larger orifice nozzle. In the non transferred arc mode the workpiece is excluded from the electric circuit. This mode is useful for cutting and joining non conductive materials or where low energy concentration is needed. The arc is struck and maintained between the electrode and the constricting nozzle. Heat comes only from the plasma jet. In practice a non transferred low power pilot arc is always ignited at start for arc initiation (even in transferred mode applications), by using a superposed high frequency power supply that is then disconnected. Shielding gas Plasma-welding-tips The plasma jet must be limited in flow to avoid turbulence. Therefore for most operations an additional stream of the same or of a different shielding gas is provided through an outer gas nozzle to protect the weld pool from air contamination. Orifice gas is usually argon. Shielding gas can be argon or a mixture of argon and 2-5% of H2. For high conductivity metals like aluminum or copper argon, helium or mixtures of 75%He-25%Ar may be used. Operating modes Plasma-welding-tips Melt-in mode Microplasma includes processes performed with current from 0.1 to 15 A and is most useful for delicate and precision joints (performed under a microscope) to be welded with minimum heat input. Medium current plasma uses current levels from 15 to about 100 A. Keyhole mode This mode, with higher currents, permits welding of relatively thick material, without beveling of the edges, in one single pass, at higher welding speed than other processes would allow. There is some danger of keyhole closure porosity if the termination sequence is not perfectly correct. Welding Current Plasma-welding-tips Direct Current with Electrode Negative (DCEN) is the most used, except for aluminum and magnesium. For maximum heat input control, the direct current can be pulsed between two levels, a low current level permitting partial solidification and a higher level for melting and welding. For aluminum and magnesium it is advantageous to use the cleaning feature provided by cathodic etching, obtained by reverse polarity with Electrode Positive. However a constant current (DCEP) would heat excessively the electrode. Therefore either an alternating current is used or a pulsed square wave current with alternating pulses of opposing polarity: a long pulse of DCEN (to maximize heat input to the workpiece) followed by a very short pulse of DCEP (for oxide removal). The square wave alternating current with individually adjustable current polarity, current levels and duration times is also called Variable Polarity Plasma Arc (VPPA). Power sources are of inverter type or of Silicon Controlled Rectifiers (SCR). The power source is generally of constant current type. Minimum open circuit of 80V is required to permit easy arc initiation. Besides control of welding current and voltage it should also allow controlled adjustment of upslope for the welding start and of downslope for weld termination. Torch Plasma-welding-tips Torch size is designed as a function of the nominal power rating. Concentricity of electrode and nozzle must be assured for consistent results. Cables are needed for power supply, while hoses convey the gases (inner plasma or orifice gas and outer or shielding gas) and liquid coolant. Lighter torches are available for manual welding. Sturdy construction permits continued use at elevate current levels for mechanized welding. Plasma-welding-tips: Advantages In the plasma arc torch the nonconsumable tungsten electrode is inside, at a certain distance, called electrode set-back, from the constricting nozzle face. Therefore no touch starting is possible, there is no danger of electrode contamination of the workpiece, a major advantage of PAW and a considerable source of downtime in GMAW. Arc Length Another advantage of PAW as compared to GTAW, is the directional stability (stiffness) of the plasma column. While arc blow may be a serious hindrance in GTAW, the plasma jet is much stiffer and therefore much less affected by magnetic fields. It is remarked that this characteristic requires less skill on the part of the welder for producing acceptable welds. High current density and high energy concentration characterize PAW, permitting lower heat input for plasma welding, with low speed plasma jet obtained by large orifices, low gas flow rate and low currents. Note: For plasma cutting, on the contrary, high jet speed, high current, high orifice gas flow rate would be required. But the most important advantage of PAW at high current levels is the capability of operating in keyhole mode, mostly without filler metal, a unique feature present only in more complex processes like Electron Beam Welding (EBW) and Laser Beam Welding (LBW). As already indicated the keyhole mode permits reduced amount of joint preparation, very deep penetration for a single pass without filler metal, high welding speed and economic performance. Plasma-welding-tips: Limitations Cost of equipment is higher than that of GMAW. The PAW process has less tolerance for poorly fitted joints. Higher complexity of equipment requires more attention in scheduled maintenance of the torch. More parameters to control may also confuse less prepared welders and operators. Concluding Plasma-welding-tips Applications requiring either precise low heat input with controllable low current (microplasma) or deep penetration in thick joints with minimum preparation (keyhole mode) and high quality economic performance are those that best exploit the unique advantages of PAW. AWS C5.1-73 AWS A5.12/A5.12M-98
Any questions? Write them down and send them to us by e-mail, by a click on Your Questions and Feedback.
To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics. Back HOME
Tig Welding Tips
POWERED BY:
Click on this Logo NOW! Copyright (c) 2006, 2007, 2008
Plasma-welding-tips are useful reminders of typical and advantageous applications of PAW and of the main differences that provide to the PAW process its unique and most sought for characteristics. -
3 Minute Shortcut. Click on SBI! TV Show! |
