Arc welding is a subset of fusion welding process where the heat is supplied from an electric arc for melting the faying surfaces of the base metals for coalescence formation. This arc is established between an electrode and the base metals under sufficient potential difference. This arc welding electrode can be of two types – consumable and non-consumable. A consumable electrode is allowed to melt due to arc heat, and subsequently deposit on the weld bead to supply filler metal. Thus a consumable electrode serves two purposes, namely arc formation and filler supply. Such electrodes are usually made of materials having similar composition to that of the base metals. SMAW, GMAW, SAW, etc. processes employ consumable electrode. On the other hand, a non-consumable electrode is not meant for melting during welding. It remains intact, and assists in arc formation only. Filler metal, if required, is supplied additionally by feeding a separate filler rod. Such electrodes are usually made of high-melting-temperature materials such as tungsten or graphite. CAW, AHW, TIG, etc. processes utilize non-consumable electrode.
Tungsten Inert Gas (TIG) welding, formally known as Gas Tungsten Arc Welding (GTAW), is one arc welding process where electric arc is established between a non-consumable electrode and the base metals. TIG welding electrode is made of tungsten (W) usually with one or more alloying elements like lanthanum (La), thorium (Th), cerium (Ce), zirconium (Zr), etc. Electrode material is independent of the composition of the base metals that are to be joined. Since the electrode is non-consumable, filler metal is required to supply additionally by feeding a separate filler rod. However, TIG welding is also frequently carried out in autogenous mode (i.e. joining without using filler metal). Inert gas such as argon, hydrogen, or helium is also supplied at the welding zone for shielding purpose. No flux is required here. TIG welding can be carried either manually or semi-automatically using a mechanized arrangement. The process is also free from spatter formation. If carried out properly by experienced welder, TIG welding can offer defect-free sound welding with good weld bead appearance requiring minimum effort.
Submerged Arc Welding (SAW) is also one fusion arc welding process where the electric arc is established between a consumable electrode and base metals. As the electrode is consumable, it melts down due to arc heat and subsequently deposits on the weld bead. Accordingly, no filler metal is required to supply separately. However, autogenous welding is not feasible as filler (electrode) is inherently supplied in this process. SAW electrode comes in the form of small diameter wire that is continuously fed from a spool. The electrode composition is usually similar to that of the base metals. Although the wire electrode is bare (no flux coating), it does not employ any shielding gas as well. Instead granular flux is deposited in large quantities ahead of the welding torch. Such flux gets melted owing to arc heat. The molten flux mixes with the oxides and other impurities of the molten metal pool to create slag. The liquid slag floats over the molten weld metal and creates a barrier to restrict atmospheric air to come in contact with the hot weld bead. This process is also free from spatter formation. SAW offers very high filler deposition rate and deeper penetration up to 10 mm in single pass. Thus this process is preferred for filling wide and thick gaps.
One crucial difference between TIG and SAW can be made based on the location and medium of arc formation. In TIG welding, the arc forms and sustains within gaseous medium (air). It remains visible from outside. It is relatively easier for the welder to continue welding as the welding zone is visible. However, loss of arc heat is relatively more. On the contrary, the SAW arc sustains within liquid slag layer. The welding zone remains submerged under thick layers of granular flux and slag layer. The entire welding zone remains hidden and thus it cannot be observed from outside. This makes manual welding very difficult. Accordingly, SAW is mostly carried out semi-automatically using mechanised system. As the arc remains submerged, the heat loss is also minimum. Several similarities and differences between TIG and SAW are given below in table format.
Similarities between TIG and SAW
- Both TIG and SAW are fusion welding processes as the coalescence is produced by melting the faying surfaces of the base plates. No external pressure is applied in either of these processes.
- Both are arc welding process as the heat required for melting the base plates is supplied by an electric arc constituted between an electrode and the components to be joined.
- Bare electrode is employed in both the processes. This is unlike a coated electrode used in shielded metal arc welding.
- Both the processes are free from weld spatter. Accordingly, loss of filler metal in the form of spatter does not happen in either of these processes.
- Both can fetch reliable joints with good weld bead appearance.
Differences between TIG and SAW
| TIG | SAW |
|---|---|
| A non-consumable electrode is used in TIG welding. | A consumable electrode is used in SAW. |
| TIG welding electrode is made of tungsten (with few alloying elements like lanthanum, thorium, cerium, zirconium). The electrode composition is independent of the components to be joined. | SAW electrode metal varies widely based on the composition of the components to be joined. Electrodes can be made of ferrous metals, copper alloys, nickel alloys, etc. |
| Filer metal, if essential, is required to supply externally by feeding a filler rod into the welding zone. | No separate supply of filler is needed as the electrode itself acts as filler metal. |
| Filler metal deposition rate is very low. | It offers significantly high filler metal deposition rate. |
| Autogenous welding (joining the components without the filler metal) is feasible by TIG. | Autogenous welding is not feasible by SAW as the filler is inherent to this process. |
| The electrode is of short length (around 10 – 20 cm). | It employs very long continuous wire-type electrode that is fed from a spool. |
| Suitable inert gas (helium, argon, nitrogen) is supplied from external cylinder for shielding the welding zone. No flux is desired. | Shielding is achieved through granular flux that is delivered ahead of the arc. No shielding gas is required. |
| No slag formation occurs on the weld metal owing to the absence of flux. | The flux gets melted due to arc heat and subsequently mixes with oxides and other impurities of the molten weld metal to generate slag. |
| TIG welding can be carried out either manually or automatically. | SAW is mostly carried out automatically through mechanised arrangements. |
| Arc length is not adjusted automatically. It must be set or fixed by the welder. | Arc length is adjusted automatically without human intervention. |
| The arc and molten metal pool remain exposed and visible from outside. | The arc and molten metal pool remain submerged under thick layers of flux and slag. Thus the arc remains hidden and cannot be seen from outside. |
| Equipment and accessories like gas cylinder, hose, etc. are required. | Accessories like flux hopper, roller guide, etc. are desired. |
| It has limited depth of penetration, typically a maximum of 3.0 mm per pass. | It offers significantly higher depth of penetration, up to 10 mm per pass. |
| Edge preparation is desired whenever the components have thickness greater than 3.0 mm. | No edge preparation is required for plate thickness of up to of 10 mm. |
| It is neither productive nor economic for joining thicker sections. Rather, it is preferred for defect-free reliable joining of thin plates with good appearance. | It is preferred for joining thicker sections in a productive and economic way. |
| TIG welding can be applied to any positions, including inclined, vertical, and overhead. | SAW is particularly suitable for flat horizontal welding position only. |
| The process is associated with the risk of tungsten inclusion defect. However, no chance of slag entrapment in this process. | Tungsten inclusion defect does not occur. However, slag entrapment defect may occur if welding is not carried out properly. |
References
- A Text-Book of Welding Technology by O. P. Khanna (Dhanpat Rai Publications).
- Manufacturing Technology: Foundry, Forming and Welding by P. N. Rao (Tata McGraw Hill Education Private Limited).
- Comprehensive Workshop Technology (Manufacturing Processes) by S. K. Garg (Laxmi Publications Private Limited).