Control of the weld pool is essential for a quality weld. Key parameters that influence weld pool control include heat input, material thickness, thermal conductivity, preheating temperature, and electrode manipulation. Proper edge preparation is also important, with the required thickness depending on factors like material thickness, welding process, and joint type. Position also impacts the maximum current that can be used.

1. Essential parameters in fusion welding
Control of weld pool is the key to a good quality weld.
There should be good bonding between weld material and plate
material.
For this to happen, the parent material must be melted before filler
metal is allowed to flow into the joint. Otherwise there is lack of
fusion at the boundary.
The melting must be achieved as quickly as possible and its extent
must be limited so that welder can control the weld pool.

2. Metal
Relative thermal
conductivity
Copper 100
Aluminium 62
Steel 14
Lead 8
Heat Input
The parameters involved in effective melting of the parent metal during
welding are:
•Heat input
• Metal thickness, and joint type
• Thermal conductivity
• Temperature of the parent metal before welding (preheating)
• Melting point
• Electrode angles and manipulation

3. Heat input (joules/mm) = current (A) x arc voltage (V) x 60
travel speed (mm/min)
Penetration: Depth to which parent metal is fused.
Dilution:Change in composition of weld metal because of
melting of parent metal

4. Edge preparation
3 mm for MMA, TIG and MIG at low current (200A)
6 mm with MIG for high current (say 400 A)
5 mm
The thickness of the material which can be butt-welded using square
edges for the joint depends on the amount of penetration available.
Current is the limiting factor in Arc welding

5. Single bevel edge preparation for thicker material to provide
access to arc. Groove is filled by depositing a number of runs.

6. Depending on:
•Type of process
• Type of work
• Position of welding
• Access for arc and electrode
• Volume of deposited weld metal
• Dilution
• Cost of preparing edges
• Shrinkage and distortion
Typical butt joint Edge preparation

8. Positional welding
Maximum current is lower in
positional welding. 150 A against
350 A for flat joint positions.
Weld pool easiest to control in flat
position. High currents can be
used leading to faster welding.
Lower heat input to give small pool.
Direction of arc varied.
Arc force helps to keep the pool in place.

10. Sequence of weld deposit for a
given thickness depends on
position.

11. Material
Atmosphere in arc gap
Arc length has minor effect
Control of welding parameters

12. TIG welding: Arc length control

14. Drooping Characteristics
Constant Current

17. MMA welding: Arc length control

18. MIG welding : Self adjusting arc
Burn-off curves for
low C steel wires in
CO2 welding

19. Heat input control in arc welding
Arc length, voltage and current
Electrode feed rate
Travel Speed
Constant potential supply
for self-adjustment arc.

20. Operating characteristics of MMA welding
Functions of flux covering
Weld metal protection: Protection by a slag layer, Formation of gases
Arc stabilisation: Includes ease of establishing the arc-quick ionisation of
gas in the arc gap and at a low voltage
Inclusion of TiO2, K-silicate and CaCO3 facilitates ionisation.
Silicates and oxides also help stabilise the arc.
Control of weld metal in position: Slag acts as mold and hold weld metal in
position during position welding
Quick freezing slag

21. Control of surface profile: Profile depends on surface tension
Surface tension depends on O content of flux

22. Control of weld-metal composition:
•Alloying
•Deoxidation: Weld pool contains large amount of oxygen
FeO + C Fe + CO
2FeO + Si (from ferro-silicon) 2 Fe + SiO2
P or Zn as deoxidiser for Cu welding (may be added in core)
•Contamination: Hydrogen from absorbed or combined moisture
Ca-containing electrodes give lowest hydrogen
Iron-powder addition: High deposition rate, easy to detach slag,
high electrode efficiency (75-90% without Fe powder)

23. Constituent Primary function Secondary function
Iron oxide Slag former Arc stabiliser
Titanium oxide Slag former Arc stabilizer
Magnesium oxide Fluxing agent --
Calcium fluoride Slag former Fluxing agent
Potassium silicate Arc stabiliser Binder
Other silicates Slag formers and binders Fluxing agents
Calcium carbonates Gas former Arc stabilizer
Other carbonates Gas former --
Cellulose Gas former --
Ferro-manganese Alloying Deoxidiser
Ferro-chrome Alloying --
Ferro-silicon Deoxidiser --
MMA electrode constituents and their functions
Note: Binders are used to give the flux covering mechanical strength and to help it adhere to the core
wire. Fluxing agents are used to adjust surface tension and wetting characteristics.

24. Four major Groups of electrode in steel welding (MMA)
• Acid covering: Contains oxides and silicates
High oxygen content
Porous and easy to detach slag
Low strength but good ductility
• Cellulose covering: Organic material containing cellulose (wood pulp)
Thin slag layer need not be removed in some cases
Poor surface profile, good mechanical properties
High hydrogen content, not suitable for high
strength steel
• Rutile covering: Based on Ti-oxides
Good slag forming; stable, easy to use arc
Medium oxygen content good surface profile
Good slag detachibility
High hydrogen content
High tensile strength not possible

25. • Basic covering: Calcium compound (CaF2 and CaCo3)
Low hydrogen (baked at 480O
C, stored at 150O
C)
Used for welding high strength steels
Low oxygen content
Slag difficult to detach
Gives more fume
Current range for MMA electrodes (450 mm long, 2.5 – 6.3 mm dia)

26. Choice of electrode
• Composition of metal
• Risk of weld metal cracking
• Required mechanical properties
• AC or DC power supply
• Position of welding
• Thickness of parent metal
• Type of joint
Electrode specification system : E 43 22 R 160 27
BS 639 (based on ISO 2560)

27. Operating characteristics of TIG welding
Welding Current: DC with electrode as negative for all metals except Al.
HF arc starter or high voltage spark to ignite the arc

28. Welding Electrode: Pure tungsten but thoria (DC) or zirconia (AC)
containing tungsten is preferred to improve arc striking and stability.
Shielding gas: Argon
Higher arc voltage and heat input with helium. Gives
improved welding of high thermal conductivity metals.
Greater depth of penetration  higher travel speed
75%He+25%Ar for Al and Cu plates and sections
30%He+70%Ar for Al and Cu sheet
1-5% H + Ar for high speed welding of SS and Ni alloys
With (pure) He shielded arc DC supply can be used for
welding Al plates. Arc strong enough to break oxide film

29. TIG welding for sheet < 4 mm; above this travel speed is slow.
At currents < 10 A arc unstable. Pulsed current at 1-10 Hz can be used.

30. Operating characteristics of MIG welding
MIG process is comparable with MMA welding. Faster than MMA
but more expensive. Freedom from frequent electrode change.
Attractive for sheet metal industry (car bodies).
Electrode wires: Diameter range from 0.8 to 1.6 mm.
Alloying additions and deoxidiser in wire. Or else
Flux cored electrode (1-3 mm dia) is used.
FCAW
Suitable for steels, Cr-Mo steels, SS steels
and abrasion resistant steel.
Shielding gas: CO2 or Ar + 20% CO2 or self
shielding electrode (no gas shielding needed).

31. Shielding gases for MIG/MAG welding
Metal Shielding gas
Aluminium and alloys Pure argon
Nickel and alloys
Argon + helium
Pure argon
Copper Argon + helium
Stainless steel Argon + 3% oxygen
Low-carbon steel and carbon-
manganese steel
Argon + helium + CO2 or
CO2 or Argon + O2 or
Argon + 5 to 20% CO2
Steels with 1 to 2% chromium Argon + 5 to 20% CO2 or
Argon + 5% O2
Steels with more than 2%
chromium
Argon + 5% O2

32. Welding Current: DC with electrode as positive.
High current (300 A) for flat plate welding
Low current (50 to 180 A) for sheet and positional welding
Spray transfer

33. Dip transfer

34. Pulsed transfer

35. Constituent Primary function Secondary function
Iron oxide Slag former Arc stabiliser
Titanium oxide Slag former Arc stabilizer
Magnesium oxide Fluxing agent --
Calcium fluoride Slag former Fluxing agent
Potassium silicate Arc stabiliser Binder
Other silicates Slag formers and binders Fluxing agents
Calcium
carbonates
Gas former Arc stabilizer
Other carbonates Gas former --
Cellulose Gas former --
Ferro-manganese Alloying Deoxidiser
Ferro-chrome Alloying --
Ferro-silicon Deoxidiser --