Date post: | 14-Feb-2017 |
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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 --