fusion welding parameters

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.

Metal Relative thermal conductivity

Copper 100Aluminium 62

Steel 14Lead 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

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

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

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

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

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.

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

Material

Atmosphere in arc gap

Arc length has minor effect

Control of welding parameters

TIG welding: Arc length control

Drooping Characteristics

Constant Current

MMA welding: Arc length control

MIG welding : Self adjusting arc

Burn-off curves for low C steel wires in CO2 welding

Heat input control in arc welding

Arc length, voltage and currentElectrode feed rateTravel Speed

Constant potential supply for self-adjustment arc.

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

Control of surface profile: Profile depends on surface tension

Surface tension depends on O content of flux

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)

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.

Four major Groups of electrode in steel welding (MMA)

• Acid covering: Contains oxides and silicatesHigh oxygen contentPorous and easy to detach slagLow strength but good ductility

• Cellulose covering: Organic material containing cellulose (wood pulp)Thin slag layer need not be removed in some casesPoor surface profile, good mechanical properties High hydrogen content, not suitable for high strength steel

• Rutile covering: Based on Ti-oxidesGood slag forming; stable, easy to use arcMedium oxygen content good surface profileGood slag detachibilityHigh hydrogen contentHigh tensile strength not possible

• Basic covering: Calcium compound (CaF2 and CaCo3)Low hydrogen (baked at 480OC, stored at 150OC)Used for welding high strength steelsLow oxygen contentSlag difficult to detachGives more fume

Current range for MMA electrodes (450 mm long, 2.5 – 6.3 mm dia)

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 27BS 639 (based on ISO 2560)

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

Welding Electrode: Pure tungsten but thoria (DC) or zirconia (AC) containing tungsten is preferred to improve arc striking and stability.

Shielding gas: ArgonHigher 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 sections30%He+70%Ar for Al and Cu sheet1-5% 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

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.

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).

Shielding gases for MIG/MAG weldingMetal Shielding gas

Aluminium and alloys Pure argon

Nickel and alloysArgon + heliumPure argon

Copper Argon + heliumStainless 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

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

Dip transfer

Pulsed transfer

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 agentsCalcium carbonates Gas former Arc stabilizer

Other carbonates Gas former --

Cellulose Gas former --

Ferro-manganese Alloying Deoxidiser

Ferro-chrome Alloying --

Ferro-silicon Deoxidiser --