arc welding arc theory ctu in prague faculty of mechanical...

CTU in Prague

Faculty of Mechanical Engineering

Ing. Petr Vondrouš, PhD., IWE

Arc welding

Arc theory

1st semester 2015/2016

FSW – Friction stir welding

ČVUT, Fakulta strojní, Ústav strojírenské technologie, skupina svařování

An arc is an electric current flowing

between two electrodes through an

ionized column of gas. A negatively

charged cathode and a positively

charged anode create the intense heat

of the welding arc. Negative and

positive ions are bounced off of each

other in the plasma column at an

accelerated rate.

Needs energy to ionize and to sustain

plasma.

Electric arc



Electric arc is connecting places with the lowest voltage –

plasma – high speed of movement (100 m/s)

Energy is released at the arc edges and is practically used.

High temperature

Intesive glow

Diameter 1 - 10 mm

At current over 0,3 A

At Higher pressures

Current 10-2000 A, voltage 10-50 V.

Electrones emitted from cathode passes through arc collumn

and are removed by anode.

Characteristics



Power, shape of electric arc can be varied setting different welding

paremeters. By this wide variety of welding methods can exist.

SMAW

GMAW

GTAW

PAW



• Description is done on GTAW arc, because the arc is stricken btween

tungsten non melting electrode and BM without changes in length,

width, and is stable, with no change in U, I.

• Parts of arc:

Cathode area

Arc collumn

Anode area

Theory of electric arc



Electric arc

On the heated cathode, where the emission energy is the lowest, the

electrones are emitted. Emission of electrones causes decrease of

potential, voltage (10-16 V).

Thermically emitted electrones in electrical field increase speed, kinetic energy

high enough to ionize gas atoms.

Electrically conductive high temperature collumn (PLASMA), where U, I are

stable. Thermodynamic processes of Disociation, ionization, excitation

and recombination takes place.

Anode is current in device, accepts electrones. Energy is transformed into

heat and el.-mag. radiation.

Recombination of electrones and iones, energy release – higher

temperature than at the cathode.

Decrease of voltage 4-8 V.



• Arc is low temperature plasma, its properties depend on

• Electrode material

• Welded material

• Gas

• Geometry

• Parameters – I, U

Arc properties



Disociation, Ionization

At high temperatures molecules disintegrate to separate atoms– energy

needed is called DISSOCIATION energy.

E.g. 02, H2, N2, CO2

Atoms recieving enough energy (ionization energy) release electron, so that free

electron and positive ion are created.

E.g. H + energy H + + e -

easy to ionize elements – easy to start the arc – stability

Lot of energy to ionize elements release more energy at the anode – hotter arc.

H H + 4,48 eV H H+



Low ionization energy is good for arc stability – Cs, K, Na

additions

Disociation, ionization energies



Excitation, Recombination

In the plasma, energy transfer is done by excitation and relaxation, i.e.

emission.

Due to much energy levels, wide spectra of energy is emitted – UV,

visible, IR radiation.

At the outer edges of the arc recombination takes place – energy

example H+ + H+ H2 + 4,48eV



Mixture of neutral and charged particles.

Overall – electrically negative.

Plazma is:

- Conductive

- High thermal, electrical conductivity

- Influenced by electric and magnetic field

Gas x Plasma – main difference is electrical conductivity

Plazma



Arc

Direct polarity, DC -, DCEN In direct polarity, DC+, DCEP



Thermal bilancy

Arc has temperature between 3000-15 000 ͦC.

Created heat Q is heating up cathode, anode and environment.

Q = Qa + Qc + Qe

Depending on type of electrode the heating of electrode can be positive

(SMAW, GMAW) o negative (GTAW).

For every method there is different efficiency: k = η = (Qa + Qc) / Q

Efficiency factor: norm EN 1011-1

Heat input into WM: q = k (U.I) / v [J/mm]

Method SAW SMAW,

GMAW

GTA,

PAW

Efficiency k 1,0 0,8 0,6



Material transfer

Very important is molten metal transfer into the weld puddle. It depeds on:

- method

- material

- electrode size

- Current

- voltage

- Gas

Heat of arc melts end of electrode, so that drop of molten metal is created.

Forces present:

Gravitation

Surface tension – molten metal

Electromagnetic forces – Lorenz forces „PINCH EFEFCT“ radial el.-mag.

force



Forces

1- electrode,

2 – molten area,

3 – molten filler wire,

4 – cathode area,

5 – arc column,

6 – anode area,

7- radial pressure,

a –gravity,

b – radial el.-mag. force pressure (pinch efekt),

c – surface tension,

d – electro-static forces,

e – evaporation,

f – surface tension,

g – plasma, gas flow



Acting forces change material transfer:

Short circuiting vs Spray transferSurface tension (Fs) el-mag. force (Fm)

Evaporation force (Fp) hydrodynamic force (Fh)

Gravitation force (Fg)

Lower I, U Lower I, U



Starting the arc

Environment is the highest at the beginning, more energy must be

supplied.

Arc starting voltage U = 60-70 V

Electrode strike (SMAW, SAW, MIG/MAG)

High frequency ionization (GTAW)

Starting current (GTAW)

Standard arc voltage 10 – 50 V, current 10 – 2 000 A.



Electric arc parameters

Voltage – influences arc length

Current – power, arc diameter

Material, gas – influence

After starting the arc, temperature increases,

so voltage can decrease.

Arc resistence in non linear, it is influence by

1) Catode - non linear resistor

2) anode – non linear resistor

3) arc column – linear resistor, U=R.Ilinear

U = Uc + R.I + Ua



Static arc characteristic

Arc static characteristic expresses influence of voltage on current for

constant arc length, for specific method and environment ...

It is often simplified

to line –practically used

range of current .

U2 = 20 + 0,04 I2

Valid for SMAW upto 600 A



GTAW (WIG, TIG) :

U2 = 10 + 0,04 I2 (V) upto 600 A,

over 600 A U2 = const. = 34 V

GMAW (MIG, MAG) :

U2 = 14 + 0,05 I2 (V) upto 600 A,


SAW:

U2 = 20 + 0,04 I2 (V)

U2 = 14 + 0,05 I2 (V) upto 600 A,


NORMALIZES arc characteristics



Welding source characteristic Arc characteristics

Source setting changes the curve slope 1-4

arc welding arc theory ctu in prague faculty of mechanical...

Documents