THE EFFECT OF THE SHAPE OF WELDING WIRE TIP ON ARC IGNITION IN

MAG/MIG WELDING

Marjan Suban, Janez Tušek

Institut za varilstvo (Welding Institute), Ljubljana, Slovenia

ABSTRACT: MAG and MIG welding processes can attain high efficiency as a low-cost

welding process and it is used widely all over the world. However, generation of spatters is

inevitable because of disturbances in metal transfer. For high quality welding it is thus

highly important to reduce spatter. The major causes of spatter during welding are bursting

of gas bubbles, explosive gas generation, electrodynamic arc force, and short-circuing

restriking of the arc. But spatter also occurs at the beginning of welding, when the arc is

ignited. Beside the workpiece surface condition, static and dynamic characteristics of the

power source, the process of arc ignition is also affected by the shape of welding wire tip.

The article describes the ignition process and the spatter formation. In the experimental

studies, the influence of differently cut-off welding wire tips and tips with different residual

drops on welding current and voltage was studied.

KEYWORDS: MIG/MAG welding process, arc ignition, spatter

1 INTRODUCTION

MAG and MIG welding processes have known wide applications ever since 1950 due to their high

productivity, possibility of automation, and low welding costs. Their development was

accompanied by continuous improvements of power sources, driving systems, control modes,

shielding gases etc. The main purpose of improvements was to improve the control of metal transfer

from the welding wire to the weld pool. Consequently, quality of welded joints improved as well.

Development laboratories all over the world have thus focused their attention on reduction of

spatter. From the point of view of quality of the welded joint, spatter is an undesired phenomenon in

metal transfer. Development of pulsed-arc welding in which metal transfer is strictly controlled, i.e.,

only one drop is detached per pulse and it always winds its transfer up in the weld pool, has come

closest to the idea of spatterless welding.

2 METAL TRANSFER AND SPATTER

In gas-shielded metal-arc welding, the filler material will melt in the arc, become liquid, take shape

of a droplet of the molten metal due to action of forces, and is then transferred through the arc to the

weld pool. Welding current is one of basic factors influencing arc forces and, consequently, the

mode of metal transfer. The mode of metal transfer affects metallurgical and chemical processes

between the molten metal and the shielding medium, weld shape, spatter and, indirectly, mechanical

properties of the welded joint. Depending on the balance of arc forces, various modes of metal

transfer may occur. Table 1 and Figure 1 show classification of the modes of metal transfer

according to IIW Commision XII-F recommendations [1, 2].

Table 1: Classification of metal transfer according to IIW recommendations [1]

Mode of metal transfer Welding processes (examples) Fig. 1

1 Free flight transfer

1.1 Globular transfer

1.1.1 Drop transfer MIG welding, low current density a

1.1.2 Repelled transfer MAG welding with CO2 b

1.2 Spray transfer MIG welding, MAG welding, high current

density

1.2.1 Projected transfer MIG, MAGM welding,

pulsed-arc welding

c

1.2.2 Streaming transfer MAG and MIG welding, higher current

density

d

1.2.3 Rotating transfer Plasma-MIG welding, MAG and MIG

welding with high current density

e

1.3 Explosive transfer Metal arc welding, MAG welding f

2 Bridging transfer

2.1 Short-circuiting transfer MAG welding, low current density g

2.2 Bridging transfer without

interruption

Welding with cold or hot wire additions

3 Slag-protected transfer

3.1 Flux-wall guided transfer Submerged arc welding h

3.2 Other modes Metal arc, electro slag, cored wire

Figure 1: Modes of metal transfer according to IIW classification [2].

Spatter is an unwanted phenomenon in metal transfer where a certain volume of the filler material

(sometimes also a portion of the parent metal), i.e., spatter, is diverted from the path expected, i.e.,

from the welding wire to the weld pool, and solidifies on the workpiece surface or sticks to the

contact tube or gas nozzle. Spatters on the workpiece impair its aesthetic appearance, and result in

additional costs of their elimination. On the other hand, spatters on a gas nozzle reduce flow of the

shielding gas, produce a turbulent gas outflow, which results, in turn, in a weaker gas shielding of

the arc and the weld metal. Cleaning of the gas nozzle and the contact tube results also in additional

costs.

The major factors influencing the generation of spatter are gas formation in the molten metal and

explosion of gas bubbles, electrodynamic arc forces, and repeated arc ignition in the short-circuiting

mode of metal transfer. A certain number of spatters is generated also at the beginning of welding,

i.e., in arc ignition. The average droplet temperature, i.e. spatter temperature, is equal to 2400 °C.

When a droplet with this high temperature falls on the workpiece surface or a gas nozzle, the

surface becomes locally melted; therefore, a joint (lack of fusion) between the droplet and the

workpiece occurs. The longer the droplet flight, the lower the droplet temperature. Droplets falling

on a more distant surface do not stick to it. In case the workpiece surface is galvanised (zinc has a

lower melting point than steel), even droplets of which temperature has already fallen below 1500 oC can stick to it.

3 ARC IGNITION

The process of arc ignition in MIG and MAG welding processes proceeds in the following steps.

First the welding device starts a flow of the shielding gas. After a time interval, which can be preset,

the wire-drive mechanism is started. The welding wire starts moving from the contact tube to the

workpiece. At the same time the power source is switched on, and in that moment no-load voltage

U0 generates between the contact tube and the workpiece.

The speed at which the wire is moving towards the workpiece can be different depending on the

wire-drive mechanism. Roughly we differ four different modes of wire-drive start:

• conventional start (the drive system tends to reach the final wire-feed rate set as quick as

possible);

• soft-start, in which attaining of the final wire-feed speed set is more slow;

• initial start with a lower wire feed and an increase in the wire feed speed to the final speed set

when the arc ignites;

• initial start with a lower speed; when the wire comes into contact with the workpiece (short

circuit), the wire starts moving back to the contact tube so that a gap of the size of the arc length

occurs between the welding wire and the workpiece, and it is only then that the wire is fed at the

speed set;

The purpose of the different types of starts of the wire feed system is primarily to reduce initial

spatter.

At the moment when the welding wire comes into contact with the workpiece surface, a short

circuit occurs. The voltage, being initially equal to the no-load voltage, decreases to zero in a

moment, the welding current, however, starts increasing. Because of a high contact resistance

between the wire tip and the workpiece and high welding current, the wire temperature starts

increasing fast. A result of the temperature increase is melting of the welding wire. A short-circuit

bridge between the wire is interrupted due to electromagnetic forces, and an arc ignites between the

wire and the workpiece (Figure 2). The process of interrupting the short-circuit bridge is

accompanied by a partial evaporation of the material, which makes ionisation easier. This is the

moment that the first spatters occur.

U [V]

I [A]

Time t [ms]

start of wire feed drive wire contact

and power source

Figure 2: Variations of welding voltage and welding current with normal (ideal) arc ignition.

The process of ignition shown as variations of welding voltage and welding current is characterised

by the following phases:

• start of the wire feed drive and power source,

• wire contact (short circuit),

• unstable part of welding characterised by short circuits with higher frequency of repetitions,

• a stable part of welding.

The time between the first wire contact and stable welding is defined as the stabilisation time of

welding.

The quality of arc ignition is affected by several factors such as no-load voltage, the mode of drive-

mechanism start, wire-tip shape, thickness of the oxide layer at the wire tip, welding-surface

condition, and dynamic characteristic of the power source (rate of increase of welding current).

U [V]

I [A]

Time t [ms]

start of wire feed drive a large part of wire melts second

and power source arc does not ignite wire contact

first wire contact

Figure 3: Sputtering arc ignition.

no-load voltage U0

short-circuiting metal transfer

short-circuiting

metal transfer

no-load voltage U0 no-load

voltage

U0

stabilisation time

of welding

stabilisation time of

welding

Not every arc ignition is so normal as shown in Figure 1. It often happens that a large part of wire

melts at the moment of short-circuit heating of the wire. Consequently, a too large gap occurs

between the unmolten welding wire and the workpiece, and the arc will not ignite. The process of

arc ignition is repeating as long as the arc ignites. Such an arc ignition is called sputtering-arc

ignition (Figure 3) [4]. This is followed by an increased spatter as in the case of the normal arc

ignition.

4 INFLUENCE OF THE SHAPE OF THE WIRE TIP ON ARC IGNITION

The influence of the wire-tip shape was to be determined experimentally. As measurement

quantities of the quality of arc ignition, the variations of welding voltage U [V] and welding current

I [A] were monitored.

A basic part of the experiment to be evaluated was arc ignition in surfacing on a steel plate having a

clean, machined surface. For welding we used the synergic power source, the welding wire SG2

with 1,2 mm in diameter, and a shielding gas composed of 18% CO2 and 82% Ar. The welding

parameters were the same with all the measurements. They were the following: a welding current I

of 100 A, a welding voltage U of 16,7 V, a wire feed speed of 2.2 m/min, a distance between the

contact tube and the workpiece L of 27 mm, a welding speed of 0.24 m/min. The only parameter

changing was the wire-tip shape. The wire tip was chamfered at different angles (Figure 4a). The

table accompanying Figure 4 states different experimental cases.

Case No. Figure Dimension

1 4a α= 0o

2 4a α= 45o

3 4a α= 60o

4 4b d = 1.25 mm

5 4b d = 1.4 mm

6 4b d = 1.6 mm

7 4b d = 1.8 mm

8 4b d = 1.9 mm

Figure 4: Shapes and dimensions of welding-wire tips.

A rough assessment of the time variations of welding voltage and welding current gives the results

stated in Table 2. With the wire chamfered at an angle of 60° (case 3), the arc ignition is very nice,

almost without spatters. Suitable variations of welding voltage and welding current are shown in

Figure 5. In this case the stabilisation time stated in Table 2 is very short, i.e., 169 ms.

The other four cases (cases 1, 2, 4, and 5) also show a normal ignition, but longer stabilisation times

can be noticed, i.e., 640 ms on the average. Thus it can be stated that there is no essential difference

if welding is started with the wire chamfered at a smaller angle or with a wire having a droplet at its

tip, which should not be too large.

The sputtering arc ignition occurs only after the droplet diameter has reached 1.6 mm or more. The

time variations of welding voltage and welding current in the case of arc ignition with a 1.9 mm

droplet are shown in Figure 6. With the sputtering arc ignition the stabilisation time of welding is

essentially longer, i.e., 1200 ms, which is almost twice as much as in the case of the normal arc

ignition. As already mentioned, with the sputtering arc ignition also a larger part of welding wire

will melt, and become a spatter. Figure 7 shows three spatters (cases 6, 7, and 8). In case 6, the

molten wire end became a spatter to stick to the gas nozzle, in the other two cases, spatters stuck to

the workpiece.

a) b)

wire SG2

α

d

Figure 5: Arc ignition in case 3.

Figure 6: Arc ignition in case 8.

Table 2: Results of the experimental work.

Case No. Mode of

ignition

Stabilisation time of the arc

[ms]

1 normal 643

2 normal 566

3 normal 168

4 normal 637

5 normal 703

6 sputtering 1086

7 sputtering 1283

8 sputtering 1239

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

t [ms]

I [A

]

0

10

20

30

40

50

60

70

80

90

100

110

120

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2

U [V

]

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

600

t [ms]

I [A

]

0

10

20

30

40

50

60

70

80

90

100

110

120

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2

U [V

]

Figure 7: Larger part of the wire as spatter in sputtering arc ignition.

5 CONCLUSIONS

The paper describes a method for evaluation of arc ignition in MAG and MIG welding processes.

The experimental results indicate that there are two types of arc ignition, i.e., the normal one and

the sputtering one. It is struggled for the normal arc ignition since this case the spatter formation is

reduced, the stabilisation time of welding shorter. In order to obtain the normal arc ignition, the

wire tip should be chamfered or there should be no droplet larger than 1.4 mm in diameter. A

disadvantage of the sputtering arc ignition is a large number of spatters, which were initially a part

of the molten welding wire. The latter stick readily to the gas nozzle or the workpiece.

With reference to the damage produced by spatters it should not be forgotten that spatters are as far

as protection and safety in welding are concerned a very dangerous thing. There has already been

many a fire started due to spatters produced during welding. It is therefore very much desired that

the arc ignites and welding is carried out with as little spatter as possible.

REFERENCES

1. N. N.,Welding in the World, 15 (1977) 5/6, 113-117.

2. M. Schellhase, Der Schweiβlichtbogen - ein technologisches Werkzeug, DVS, Düsseldorf,

1985.

3. N.N., The physics of spatter formation during dip transfer GMA welding, IIW/IIS Doc. 212-

738-89 (1989).

4. U. Dilthey, F Eichhorn, G. Groten, H. Matzner, Low-spatter ignition of the MIG-welding arc,

IIW/IIS Doc. XII-1181-90 (1990).

Case No. 6 Case No. 7 Case No. 8

1 cm