African Fusion June 2015

AFRICAN FUSION

June 2015

Cover Story:

GMAW: cur ent progress

esearch and development of high productivity and/or

high quality welding processes have been conducted

for several years. The gas metal arc welding (GMAW)

process is widely used in various production fields because

it provides high deposition rate and/or it is suitable for auto-

matic/roboticwelding. In theGMAWprocess, theelectrodewire

melts due to arc heating and forms a molten metal droplet at

the wire tip. Then it detaches and transfers from the wire tip

to the weld pool. The series of wire melting and subsequent

molten drop transfer processes results in deposition of the

metal required to complete butt welds and/or fillet welds.

So the stability of the gas metal arc and the associated

metal transfer dominates weld quality and productivity. This

technical issue has been prompting welding machine mak-

ers, consumable makers and gases suppliers to improve and

develop high performance welding processes. In this paper,

current progress in the science and technology related to

GMAW processes is discussed based on the author’s personal

research history.

Introduction

In the Materials and Manufacturing Science division of the

Graduate School of Engineering at Osaka University, we have

been investigating welding phenomena associated with MIG/

MAG and TIG welding with both steady current and pulsed

current using analogue transistor controlled power sources

constructed by ourselves since around 1977[1][2]. Analogue

transistor power sources enable us to explore various current

waveforms. Then, in order to clarify the role of pulsed current

parameters such as pulse peak current, pulse width, pulse

frequency for rectangular pulse waveforms, metal transfer

and/or weld pool oscillation phenomena were analysed with

aid of high speed photography. In particular, we showed that

the detachment time of metal droplets from the wire tip is

significantly dependent on the magnitude of pulse peak cur-

rent, which is important for the ‘one droplet transfer per pulse’

control transfermodewidelyused invarious sectors of industry

at the present [3]. At the same time, our domestic welding

machine makers have confirmed that ‘one pulse-one droplet’

transfer is very effective for spatter reduction [4][5]. Following

this development, welding machines under transistor and/or

inverter control promptly spread, together with applications

that take best advantage of pulsed current welding processes.

Over the past few decades, current control using devices

such as power transistors, MOSFETs and IGBT have advanced

significantly. In addition, inverter circuitswith high-speeddigi-

tal control have been significantly improved. Separately from

gettinghighperformance frompower supplies, themechanical

control of wire feeding has been progressing. Accordingly, very

fast and precise control of current, voltage and wire feeding

are now available.

In recent years, commercial digitally controlledarcwelding

machines have been spreading for the practical applications

in various sectors of industries in all over the world.

On the other hand, recent progress in visualisation and

simulation of welding processes has been making it possible

to observe, measure and analyse welding phenomena at high

resolution in terms of both of time and space. The change from

qualitative understanding to quantitative is sure to result in an

evolution of welding processes towards better reliability and

higher productivity. Today, high performance digital cameras

have been spreading intoproduction sites and applied toprac-

tical fabrication: detection of electrode position and groove

shape,monitoring thearc/weldpool duringweldingoperation,

etc. The implementation of these systems and the resolution

of the challenges posed will be more actively pursued as the

availability of more explicit knowledge becomes available.

In this article, the science and technology of theGMAWpro-

cess are described in the light of current progress in relevant

and allied technologies.

Present state of GMAW

In the GMAW process, the filler wire is the electrode. The elec-

trodewire ismeltedby the arc heat and amoltenmetal droplet

forms at the wire tip. Then it detaches and transfers from the

wire tip to the weld pool, so the arc length and/or the arc root

locationare varied intermittentlywith eachmetal transfer. This

means that, in comparison with the TIG arc, the gas shielded

metal arc is less stable as a heat source in both time and space.

Also, the stability of metal transfer dominates weld quality

and productivity. These problems have been driving welding

machinemakers, welding consumablemakers, gas suppliers,

universities and institutes towards research and development

of welding processes for improved quality and productivity.

Various GMAW processes have been delivered to the market

corresponding to the needs of welding related products. In

order to highlight specific advantages of processes, these have

been taggedwith various catch phrases, such as

‘high stability

metal transfer andwelding arc’, ‘improved stiffness (directivity)

of welding arc’, ‘high deposition’, ‘deep penetration’

and

‘sound

welds’

, but the understanding of the specific or quantitative

effects of the newly developed processes is not always obvi-

ous, nor are the reasons for the improvement of weld quality

and/or productivity in practical use on fabrication sites. This

This paper by Yoshinori Hirata of the Graduate School of Engineering at Osaka University,

Japan, was presented at a plenary season of the IIW’s 67

International Assembly and

Conference in Seoul South Korea and highlights key current developments in quality and

productivity for the gas metal arc welding (GMAW) processes.

Current progress in the science and

technology of

GMAW processes

Yoshinori Hirata