Arc Welding Continued

Arc Welding ProcessesLesson ObjectivesWhen you finish this lesson you will understand: The similarities and difference between the remaining arc welding processes Advantages and disadvantages of these arc welding processes Need to select between the processesLearning ActivitiesLook Up KeywordsView Slides; Read Notes, Listen to lectureView High Speed VideosDo demo on webDo on-line workbook

Keywords:Gas Tungsten Arc Welding (GTAW), Straight Polarity(SEN), Reverse Polarity (REP), Plasma Arc Welding (PAW), Keyhole, Electrogas (EGW), Electroslag Welding (ESW), Stud Welding,

Gas Tungsten Arc Welding (GTAW or TIG)

Effects of PolarityGas Tungsten Arc Welding

AdvantagesSuperior quality welds, generally free from spatter, porosity, or other defectsPrecise control of arc and fusion characteristicsWeld almost all metalsUsed with or without filler wireEasily automatedUsed in all positionsIntricate geometries weldableGas Tungsten Arc Welding

DisadvantagesLess economical than consumable electrode processes for sections thicker than 3/8 inch Lowest deposition rate of all arc processes Tungsten inclusionsHigher operator skill RequiredSensitive to draftsGas Tungsten Arc Welding

Turn to the person sitting next to you and discuss (1 min.): In all the previous arc welding processes metal wire or electrodes material melts and droplets of metal deposit on the weldment. Some of that metal as it goes through the arc ionized into metal ions and electrons which conduct the electricity through the arc. In GTAW there are no metal ions (or very few metal ions) in the arc since the tungsten electrode is non-consumable. What carries the current in the GTAW arc?

Linnert, Welding Metallurgy, AWS, 1994

AWS Welding HandbookEd 8 Vol 1 1987

Stinchcomb, Welding Technology Today, Prentice Hall, 1989

Advantages Greater Energy Concentration Improved arc stability over other processes Higher heat content, Higher travel speeds Greater penetration capabilities Finer sections can be welded with low current PAW than GTAW

Disadvantages Operator skill required is slightly greater than for GTAW Equipment more expensive Orifice replacement necessary

Take a moment this evening to view the High Speed Videos of Arc Welding Processes on the Video Page of the WE300 Website

Turn to the person sitting next to you and discuss (1 min.): The keyhole mode of welding is used because the weld speed can be very fast where molten metal flows around the hole and collides and solidifies in the rear of the hole. What might happen to the shape of the trailing weld pool if the speed was increased too fast?

AWS Welding HandbookEd 8 Vol 1 1987Flux cored Shielding

AWS Welding HandbookEd 8 Vol 1 1987External Shielding Gas

AWS Welding HandbookEd 8 Vol 1 1987

Advantages Used for thick weldments Produces sound welds Increased cost effectiveness as thickness increases

Disadvantages Once started, the weld must continue Coarse grained Heat Affected Zone

Turn to the person sitting next to you and discuss (1 min.): If insufficient cooling water is supplied to the shoes on electroslag welding what is likely to happen:a.) the shoe will warp and liquid steel will flow outb.) the shoe will melt and water will be absorbed by the weld metal later causing delayed or cold cracking.c.) an explosion will occur

Cary, Modern Welding Technology, 1989

Cary, Modern Welding Technology, 1989

Cary, Modern Welding Technology, 1989

Do the Arc Weld Demonstration from the Demonstration Web Page on the WE300 Website

In this presentation, we will continue our discussion of the arc welding process, looking at the gas tungsten arc process or GTAW, the plasma arc process or PAW, electroslag and electorgas welding processes and arc stud welding processes.In the gas tungsten arc welding (GTAW) process, the arc is established between the tip of a tungsten electrode and the workpiece to melt the base and filler material (if a filler metal is used). Because of its extremely high melting temperature, the tungsten electrode is considered to be non-consumable. An inert shielding gas protects the molten weld pool and the nonconsumable tungsten electrode. The process is often referred to as TIG, short for tungsten inert gas. Filler materials are specified through the same methods as for GMAW.The process may use direct current electrode positive, direct current electrode negative or alternating current. The chart above indicates the operating characteristics of each of these current types.DCEN or straight polarity is used for welding most materials other than aluminum. The electrode tip geometry is generally a sharp point with a small blunted end since most of heat balance is on melting of the base material.DCEP or reverse polarity is rarely used since it results in low penetration. Also the constant bombardment of the tungsten electrode by electrons in the DCEP mode degrades the electrode.Alternating current is used primarily to weld aluminum, which has a tenacious oxide surface layer. Although the diagram above states that there is a 50% cycle from DCEN to DCEP, it is possible through solid state electronics to vary the amount of time at each polarity and also the current at each polarity. GTAW produces precise and clean, nearly spatter free welds on almost all metals with superior quality in comparison to the other arc welding processes. It has found use in the aerospace, food processing, and nuclear industries. It is particularly useful on smaller sectioned parts and on reactive metals such as titanium.

It can be used with filler metal or without filler metal (autogenous). This process allows the heat source and filler metal additions to be controlled independently.It is easily automated and can produce welds in all positions, even with intricate geometries.Deposition rates are lower with GTAW than any other arc welding process. In general, the process is limited to thicknesses of 3/8-inch or less since productivity makes the process cost prohibitive. Tungsten inclusions or contamination of the weld pool may occur if the electrode touches the weld pool or proper gas shielding is not maintained.Manual GTAW requires more dexterity and welder coordination than with manual GMAW or SMAW. As with the other gas shielded processes, drafts can blow away the shielding gas, which limits the outdoor use of the process.

.The objective of the Plasma Arc Welding (PAW) process is to increase the energy level of the arc plasma in a controlled manner. This objective has been achieved by providing a special gas nozzle around a tungsten electrode operating on DCEN. The constricted plasma formed is highly ionized and concentrated.There are two variants of the Plasma Arc Welding (PAW) process. One is the transferred arc process and the second is the non-transferred arc process. In the transferred arc mode, an arc is struck between the electrode and the workpiece. In the non-transferred mode, the arc is struck between the electrode and the nozzle, thus eliminating the necessity to have the work as a part of the electrical system.A small flow of argon is supplied through the nozzle and its constricting orifice to form the arc plasma. Sometimes diatomic gases like nitrogen or hydrogen are used as plasma gases because of good heat transfer characteristics.

Shielding of the arc and weld zone is provided by a gas flowing through an encircling outer nozzle assembly. The shielding gas can be argon, helium, or mixtures. Two welding techniques are possible with PAW: melt-in, and keyhole. The Low current melt-in technique is normally used when welding thin sheet sections. Narrower welds can be made with PAW than with GTAW because of the constricted arc. High current Melt-in can be done on butt or lap joints up to about 1/8 inch thick. PAW can also be used on Titanium and other reactive metals.In the keyhole mode, a penetrating hole is formed at the leading edge of the weld pool. The molten weld metal flows around the hole and solidifies behind the keyhole to form the weld bead. Therefore, keyhole welds are complete penetration welds with high depth to width ratios. This results in low weld distortion. With operating currents up to 300 amperes, this mode can be used to weld materials up to about 3/4 inch thick, and to weld titanium and aluminum alloys. The major advantage of the plasma arc process is the greater energy concentration of the streaming plasma. This offers improved stability of the arc with higher heat content and thus faster travel speeds than other arc processes.

The stiffness of the plasma stream also promotes deeper penetration, and because of the heat concentration, finer sections can be welded.The penalties to be paid are that greater operator skill is required, the equipment is more expensive and orifice tips may require frequent replacement.Electrogas Welding (EGW) is a mechanized welding process that uses either flux-cored or solid electrodes. The process is normally performed in the vertical position, and is designed to weld a joint in a single pass by depositing weld metal into a cavity created by separated joint faces on two opposite sides and water-cooled molding dams or shoes on the other two sides. During welding, shielding of the massive molten weld pool is provided by a gas The gas may be produced by the flux cored electrode or from an external source or both.Electrogas welding utilizes machine welding equipment. The vertical movement of the welding head is usually automatic. Other mechanized oscillation movement or multiple wires may also be provided in particularly thick plate or large weldments. The Electroslag Welding process (ESW) is very similar to the Electrogas process in that a large weld pool is supported between the walls of a think plate and movable shoes which are moved up the plate as the weld progresses. Weld pool shielding is provided, however, by a molten slag bath. There are two variations of this process, the non-consumable guide method and the consumable guide method.

In the non-consumable guide method, the flux bath is refreshed with new flux occasionally. The consumable guide method uses a guide tube to guide the wire. It melts and adds additional weld metal. Some tube also contain flux coating so that flux in small quantities is continually added to the flux bath.The advantages of the Electrogas and electroslag processes are its ability to weld thick vertical sections in a single pass thus eliminating the need for multi-pass welding techniques. This results in increased cost effectiveness.Disadvantages of these processes are that once the arc is initiated, it is extremely difficult to stop the process as any termination will result in solidification defects occurring in the weld metal.

In addition, the high heat input with these processes cause heat affected zone grain growth which may deteriorate mechanical properties in this portion of the weld.In the arc stud welding process, the stud is initially in contact with the plate to which it will be welded with a ceramic ferrule sleeve in contact around it.. When current is initiated, a solenoid lifts the stud off the plate in order to establish an arc (illustration B in the above figure). After an appropriate amount of arcing, the current is terminated and the stud is driven into the workpiece (illustration C). After solidification, the ferrule is broken off and the weld is complete. The ferrule provides some shielding and support for the weld.

This diagram illustrates reduction in quality when to little or too much current is used, when the protective ferrule is not used, when weld time is too short and when proper attention has not been paid to alignment of the stud before welding.A variation of the arch stud welding process is the contact capacitor discharge process. In this weld, the stud has an initiation projection on the tip of the stud. Energy stored in a capacitor is discharged and current rapidly flows, melting the projection and initiating an arc and melting. The stud is then forced into the workpiece consummating the weld as current ceases.