A

Seminar on

Narrow Gap GMAW and Flux Core GMAW

Narrow Gap Gas Metal Arc Welding

Process characteristic:

Narrow, almost parallel weld edges

The small preparation angle has the function to compensate the distortion of the joining members

Multi pass technique where the weld build-up is a constant one or two bead per pass

Usually very small HAZ caused by low energy input

Why Narrow Gap?

• With increasing thickness of the members-complexity of the fabrication increases

• There is increased volume and cost of the consumables,

• Increased probability of the discontinuities & defects,

• Increase in residual stresses and distortion,

• Increased heat input giving rise to greater damage to microstructure and mechanical properties, loss of toughness.

Definition

Narrow Gap Welding (NGW), also called narrow groove welding, is a term

applied to arc weld made in thick plates using a square groove weld joint or

a V groove weld joint with a groove angle less than 10˚ and utilizing it for 9

to 12 mm wide gap.

NG has been used for thick plates 50-300 mm. Below 50 mm, not advisable

to use NG process.

NGW is not welding process, it is a special bead deposition technique.

NGW requires square groove only.

Various Narrow Gap Welding Processes

Narrow Gap Gas Metal Arc Welding (NG-GMAW)

Narrow Gap Gas Tungsten Arc Welding (NG-GTAW)

Narrow Gap Submerged Arc Welding (NG-SAW)

Narrow Gap Flux Core Arc Welding (NG-FCAW)

Typical joint configuration

Various Aspects of NG-GMAW Process

Classification of NG-GMAW

Edge preparation and backing techniques

Torch design

Gas feeding system

Electrode Wire – Feeding Systems

Automation and mechanization- Proximity, Seam track

Wire feed

A B

A: Contact tube is inserted

into the gap

B: Contact tube is above

work piece

Welding Torch

Work Piece

Electrode Wire

Backing strip

Joint Gap

Classification of NG-GMAW

Edge preparation and backing techniques

A B C

A. Steel strip

B. Rigid ceramic strip

C. Flexible heat resistant

strip

Torch design

It should reach up to the bottom of the narrow gap It should travel along the narrow gap without touching sidewalls and thus

without causing short circuit It should ensure effective and reliable gas shielding to the weld pool and

also prevent aspiration of air into the shielding zone It should not get overheated and it should able to carry the required welding

current To avoid problem of overheating, torch can be internally water cooled, by

constructing water channels Torch body can be made from highly thermal conductive materials such as

copper Contact tube should accept replaceable contact tip

Basic components of NG-GMAW torch

Contact tube: Smooth feeding of electrode, proper transfer of currentContact tip: Directing the wire towards weld pool, wear resistanceShielding gas nozzles: Providing laminar flow of the gasWater cooling tubes/channels: Effective cooling.

Gas Feeding Techniques

Cooling water

Cooling water

Nozzle hole

Shielding gas

Cooling water

Cooling water

Nozzle hole

Shielding gas

Shielding Box

Electrode Wire – Feeding Systems for NG-GMAW

(A)Tandem electrode, (B) Oscillation electrode (C) Weaving electrode (D) Waved electrode (E) Twisted electrode.

A B C D E

Cont….

A: Two wires with controlled by two contact tubes are used in tandem, The arcs directed towards each sidewall, producing a series of overlapping fillet welds.

B: Oscillation can be created mechanically by moving the contact tube across the groove.

C: Along with a forward motion during welding, the contact tube twists to the right and left, which gives the arc a weaving motion.

D: This electrode is formed into a waved shape by the bending action of a “flapper plate” and feed rollers as they rotate.

E: The twist electrode consists of two intertwined wires which, when fed into the groove, generates arc from the tip of the two wires. Due to the twist, the arcs describe a continuous rotational movement which increases penetration into the sidewall without any special weaving device.

Challenges of NG-GMAW

Most common defects encountered during welding NG-GMAW and need to be overcome are:

• Porosity

• Residual Stresses and Distortion

• Magnetic Arc Blow

• Lack of Sidewall Fusion

Magnetic Arc Blow in NG-GMAW

• A welding arc is like a cylindrical conductor carrying current and therefore it carries a magnetic field around it.

• This magnetic field magnetizes the mild steel work piece- tends to deflect arc from its vertical line in the joint gap-leads to loss of arc stability.

• Magnetic field around the arc exerts a force on the electrons and ions-arc to be, deflect away from the normal arc path, this is called Arc Blow.

• Arc instability produces spatters, lack of sidewall fusion and porosity.

Advantage

• Profitable because low consumption quantities of filler material, gas and/or powder due to the narrow gaps.

• Excellent quality values of the weld metal and the HAZ due to low heat input.

• Decrease tendency to shrink.

• Minimum joint preparation, minimum joint volume.

• Good mechanical properties.

• Low parent metal dilution, small heat affected zone.

• Low distortion.

Disadvantage

• Higher apparatus expenditure, especially for the control of the weld head and the wire feed device.

• Increase risk of imperfections at large wall thickness due to more difficult accessibility during process control.

• Repair possibility more difficult.

Applications

• Pressure vessels and boilers, Carbon steel and alloy steel used in pressure vessels have been the object materials for NGW.

• Large industrial components: hollow shaft used in large compressor, a generator and lifting equipments. Longitudinal and circumferential joint in the hull and heavy sections of a cement kiln and thick wall frame of a large press welded using NGW.

• Offshore structure and shipbuilding: In the recent years, with the rapid increasing demand of offshore structure for production of oil and natural gas, the use of thick plates over 100mm has increased.

• Railway Tracks

• Building and bridges: In Japan, NGW has been applied widely in the construction of buildings.

Effect of Electromagnetic Arc Oscillation in NG

• Magnetic arc oscillation resulted in uniform and sufficient penetration to both groove faces

Cont…

• A magnetic field externally applied to a welding arc deflects the arc by electromagnetic force (Lorentz force) in the plane normal to the field lines.

• The welding arc can be deflected forward, backward, or sideways with respect to electrode and welding direction, depending upon the direction of the external magnetic field.

• If a unidirectional magnetic field is applied to an AC arc, or an alternating field is applied to a DC arc, then the arc can be oscillated in the position normal to the direction of welding, and this has been used to improve the arcs with both gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW)

Cont…

•Subjecting the welding arc to transverse magnetic fields increases the welding speed several times, at which rate undercut-free and no-porosity welds can be made. 

•An alternating parallel magnetic field causes the arc to oscillate in a position normal to the direction of welding, which has an effect like mechanical weaving.

Welding Characteristics of Electromagnetic Arc Oscillation

Fig: Arc column images for various oscillations widths during narrow groove welding.

Fig: Arc column images for various root opening widths during narrow groove welding

Arc Images

• The welding experiments in a square-groove butt joint with a 10-mm root opening under the following conditions:

280 A, 31 V, travel speed 22 cm/min,

Contact tip-to-work piece distance 15 mm,

Arc oscillation frequency 30 Hz.

• The oscillation width increases with increasing flux density.

• The oscillation width was small at a density of 25 gauss, and oscillation effect could not be expected.

• At 75 gauss, the width was too large and undercut at the groove face could be expected.

Cont.…

• Second images of the arc column for root openings of 8 and 10 mm.

• Despite operating under the same welding conditions, same magnetic flux density, the arc shape and oscillation width are very different from each other if the root opening decreases from 10 to 8 mm.

• In the 8-mm root opening, the deflected arc occurred between wire tip and groove wall.

Gas Metal-Arc Welding

A wire is used as an electrode and a welding arc is struck between the electrode and the base metal.

Usually operates on direct current electrode positive (DCEP) or direct current reverse polarity (DCEN)

The electrode melts and becomes part of the weld.

The process is controlled by a constant voltage, which is set by the operator.

Schematic illustration of the gas metal-arc welding process.

FCAW (Flux Core Arc Welding)

Electrode is in the form of a hollow wire which contains the flux

The electrode becomes the filler metal and the flux in the electrode provides the shielding for the molten puddle; additional shielding may be provided by an auxiliary shielding gas

The electrode is provided in coils and therefore the process becomes continuous as compared to SMAW

Self shielded Gas shielded

22 - 29

Self shielded

Comparison

Self shielded:• Does not require external shielding gas. • The weld pool is protected by gas generated when flux from the wire is

burned. • Self-shielded wire is different from gas shielded wire in that it must

operate on DCEN.• Self shielding flux cored wire is very portable because it does not require

an external shielding gas.• Self shielded wire is often used in the construction industry where

welding outdoors is very common.• It is optimal for outdoor procedures, since flux is build into the wire for

positive shielding even in windy condition.• No use of gas, much cheaper and quicker to start than gas shielded.

Gas shielded:

Cont…

• Require external shielding gas and the slag is easy to remove. • The shielding gases most commonly used are very similar to GMAW usually

straight Co2 or an argon/Co2 blend.• Suitable when welding on thicker metals or in out-of-position applications. • Just like GMAW, gas shielding flux cored should be run on DCEP• FCAW with CO2 or Ar-CO2 mixture shielding offers higher efficiency,

smoother bead appearance, better slag removal, and lower spatter, thereby cutting welding costs.

• The slag is very fast freezing which helps provide higher deposition rates

especially in the vertical-up position.

Reference

1:”Gas Metal Arc Welding”, AWS-Handbook volume: 1

2: Characteristics of Welding and Arc Signal in Narrow Groove Gas Metal Arc Welding Using Electromagnetic Arc Oscillation

Experiments produce optimum parameters for obtaining uniform and sufficient groove face penetration BY Y. H. KANG AND S. J. NA