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1Welding RobotChapter 1 IntroductionThe gas tungsten arc welding (GTAW) process is based on the electric arc established between a non-consumable electrode of tungsten and the work-pieces to be joined. Part of the heat generated by the electric arc is added to the workpieces, promoting the formation of a weld pool. The weld pool is protected from air contamination by a stream of an inert gas (Ar or He) or a mixture of gases.1．1．1 IntroductionThis process is also known as tungsten inert gas (TIG), although small amounts of non-inert gases may be used in the shielding mixture, such as hydrogen or nitrogen. Autogenous GTAW welding (without filler metal) is used in thin square edged sections (2mm), while V and X type edge preparations are needed in thicker sections. In this case, the addition of filler metal is necessary. This process is extensively used for welding thin components of stainless steel, aluminum, magnesium or titanium alloys as well pieces of carbon and low alloy steels. Heat input in GTAW does not depend on the filler material rate. Consequently, the process allows a precise control of heat addiction and the production of superior quality welds, with low distortion and free of spatter. It is less economical than other consumable electrode arc welding processes, due to its lower deposition rate, and it is sensitive to windy environment because of the difficulty in shielding the weld pool. Besides it shows low tolerance to contaminants on filler or base metals.1．1．2 Welding EquipmentIn this section the relevant aspects related to the welding equipment used with the GATW process will be reviewed, with the objective of exploring the implications for automatic robotic welding.1．1．3 Power SourcesPower sources for GTAW are generally of the constant current type with drooping volt-ampere static curves, Light weight transistorized direct current power sources are currently used, being more stable and versatile than the old thyristor-controlled units . In rectifier-inverter power sources the incoming AC current is rectified and then converted into AC current at a higher frequency than that of the mains supply, in the inverter. Afterwards high voltage AC current is transformed into low voltage AC current suitable for welding, in the transformer, and then rectified, The aim to increase the current frequency is to reduce the weight of the transformer and other components of the source such as 2inductors and capacitors. 1．1．4 Welding TorchThe welding torch holds the non-consumable electrode, assures the transfer of current to the electrode and the flow of shielding gas to the weld pool. Torches with welding regimes up to 200 A are generally gas-cooled and those with continuous operation between 200 and 500 A are water-cooled. 1．1．5 Non-consumable ElectrodesNon-consumable electrodes are composed of pure tungsten or of tungsten alloys. Pure tungsten electrodes can be used with DC but are more sensitive to contamination, have lower service life-cycle and exhibit higher tip deterioration than alloyed electrodes. These electrodes can be used in welding of aluminum and magnesium alloys on AC. Thoriated tungsten (2% ThO2) electrodes are widely used in industrial applications due to its excellent resistance to contamination, easy arc starting and stable electric arc. Concerns about safety, because thorium oxide is radioactive, led to the development of other electrodes containing small proportions (around 2%) of simple earth rare elements such as lanthanum, yttrium and cerium or even mixtures of several elements . These electrodes have better operational characteristics than thoriated electrodes and can be used in welding carbon and stainless steels,nickel and titanium alloys. Zirconiated tungsten electrodes are excellent for AC due to its good arc starting, high resistance to contamination and small tip shape deterioration.1．1．6 Arc Striking TechniquesArc initiation by touch striking was used formerly in manual GTAW, but this technique is very sensitive to tungsten contamination, adversely affecting the service life of the electrode. High-frequency-high-voltage (e.g. 3 kV at 5 MHz) supplies are currently used in arc striking and AC arc stabilization in manual GTAW systems. This arc starting technique usually produces interference in electronic equipment in the vicinity of the power source. Programmed touch striking is an alternative technique developed for automatic systems. In this technique current and voltage are limited when electrode touches in the work-piece, in order to prevent electrode contamination. A pilot arc starting can also be used to initiate the main electric arc, though a more complex torch is needed.1．1．7 Shielding Gas RegulatorThe regulator is a device that reduces source gas pressure to a constant working pressure, independently of source pressure variations. Pressure reduction can be made in one or two stages. Regulators in two stages give in general more stable output flow.1．2 Process Parameters3In this section the relevant parameters for the GTAW process will be reviewed with the double objective of presenting them and showing that they can certainly be used for automatic robotic welding.1． 2． 1 CurrentCurrent has direct influence on weld bead shape, on welding speed and quality of the weld. Most GTAW welds employ direct current on electrode negative (DCEN) (straight polarity) because it produces higher weld penetration depth and higher travel speed than on electrode positive (DCEP) (reverse polarity). Besides, reverse polarity produces rapid heating and degradation of the electrode tip, because anode is more heated than cathode in gas tungsten electric arc. Reverse polarity may be of interest in welding aluminum alloys because of the catholic cleaning action of negative pole in the work-piece, that is the removal of the refractory aluminum oxide layer. However alternating current is better adapted to welding of aluminum and magnesium alloys, because it allows balancing electrode heating and work-piece cleaning effects. Weld penetration depth obtained with AC is between depth obtained with DCEN and DCEP.1．2．2 Welding SpeedThe effect of increasing the welding speed for the same current and voltage is to reduce the heat input. The welding speed does not influence the electromagnetic force and the arc pressure because they are dependent on the current. The weld speed increase produces a decrease in the weld cross section area, and consequently penetration depth (D) and weld width (W) also decrease, but the D/W ratio has a weak dependence on travel speed. These results suggest that the travel speed does not influence the mechanisms involved in the weld pool formation, it only influences the volume of melted material. Normal welding speeds are from 100 to 500 mm/min depending on current, material type and plate thickness.1．2．3 Arc LengthThe arc length is the distance between the electrode tip and the work-piece. The arc length in GTAW is usually from 2 to 5 mm. If the arc length increases, the voltage to maintain the arc stability must increase, but the heat input to work-piece decreases due to radiation losses from the column of the arc. Consequently, weld penetration and cross section area of melted material decrease with increasing arc length.1．2．4 Shielding GasesShielding gases are used in GTAW in order to prevent atmospheric contamination of the weld metal. This contamination can produce porosity, weld cracking, scaling and 4even change in the chemical composition of melted material. Besides shielding gas also has a large influence on the stability of the electric arc. Gases with low ionization potential facilitate the ignition of the electric arc and those with low thermal conductivity tend to increase the arc stability. Argon is the most used GTAW shielding gas. It has low ionization potential and is heavier than air, providing an excellent shielding of the molten weld pool. Furthermore it is less expensive than helium, the other inert shielding gas used in the process. Argon is used in welding of carbon and stainless steels and low thickness aluminum alloys components. For welding thick aluminum work-pieces and other high-conductive materials, such as copper alloys, helium is recommended because it has higher ionization potential than argon, needing higher voltage for arc initiation and maintenance, but producing higher heat-input. Helium or helium/argon (30-80% He) mixtures allow increased welding speed and improved process tolerance. Mixtures of argon with up to 5% of hydrogen are frequently used in welding of austenitic stainless steels. Hydrogen increases arc-voltage and consequently heat input, increasing weld penetration and weld travel speed, as well improving weld appearance. Argon/hydrogen mixtures are also used in welding of copper nickel alloys. Argon is also used as back side shielding gas, mainly in welding of stainless steels, aluminum alloys and reactive metals.Flow rates of shielding gases depend on weld thickness, being 4-10 l/min for argon and 10-15 l/min for helium, because it is lighter than argon, and consequently less effective in shielding. Gases with a purity of 99.995% are used in welding most of the metals, though reactive materials such as titanium need contaminant level less than 50 ppm.1．2．5 Filler MetalsFiller metals are generally used for plate thickness above 2 mm, having chemical composition similar to that of the parent material. Filler metal diameter is between 1.6 and 3.2 mm and in automatic systems is normally added cold from a roll or a coil.1．2．6 Electrode Vertex AngleThe non-consumable electrode angle influences the weld penetration depth and the weld shape. Electrode angles between 30o and 120o are used. Small angles increase arc pressure and penetration depth but have high tip shape deterioration. Electrode angles from 60o to 120o maintain tip shape for longer periods and give welds with adequate penetration depth-to-width ratio.1．2．7 Cast-to-cast VariationCast-to-cast variation refers to variation observed in penetration of welds produced in the same welding conditions in several batches of austenitic stainless steel with 5nominally identical composition. These changes in the weld bead shape are attributed to variation in proportion of trace elements in the material, such as sculpture, calcium and oxygen. Variations in trace elements seem to affect surface tension and metal flow into the pool . Weld pool shape is also affected by electromagnetic forces, arc pressure and thermo capillarity forces. To minimize this problem several strategies have been adopted such as the use of higher currents or of pulsed current, the application of adequate shielding gases or the application on plate surface of flux coatings containing active ingredients.1．3 Process VariantsGTAW is regarded as a high quality process for welding thin metals using low travel speed and low electrode deposition rate, requiring highly skilled personnel in manual welding. Variants developed seek to improve productivity, mainly deposition rate, penetration depth and welding speed. These variants are implemented in automatic or robotic systems. Hot-wire GTAW is a variant where a heated filler wire is fed to the rear of the melted weld pool at a constant rate.Filler wire is resistance heated close to melting point using mainly AC power sources, in order to minimize magnetic disturbance of the electric arc. Deposition rates up to 14 kg/h can be attained with this process. It has been used in heavy wall fabrication, maintaining high joint integrity .The use of a dual-shielding GTAW technique, where an additional concentric gas shield gives an increase in constriction and stiffness of the electric arc, may be used to increase welding speed and penetration depth .Constriction of the arc is produced by the external cold gas flow which decreases temperature of the outer part of the arc, decreasing the arc cross section where current flow occurs, consequently increasing current density and temperature. Electrode gas and annular gas may be of the same or of different compositions, such as Argon plus 5% hydrogen for internal gas and argon for external gas when welding austenitic stainless steels. This technique also tends to increase the risk of undercut. Very high currents (I > 300 A) may also be used in a conventional automated GTAW process to increase the penetration depth, but defects may form and the process becomes unstable above 500 A. The key hole mode gas tungsten arc welding process, which was developed a few years ago, seems to be suitable for ferrous and non-ferrous materials in the range from 3 to 12 mm. However, this key whole technique is extremely sensitive to arc voltage, and loss of material may occur through the keyhole vent.