FUSION WELDING.ppt

Welding

1. Welding Processes and
Weld Defects
Dr. G.D. Janaki Ram
IIT Madras

2. Welding Processes
•Fusion welding
–Base materials melt
–Filler material may be added
–90% welding is fusion welding
•Solid-state welding
–No base material melting
–Plastic deformation of base materials
–Material can get red hot
–Several metallurgical advantages
–Numerous established applications

3. Fusion Welding Processes
•Heat sources
–Fuel burning (Oxy-acetylene gas welding)
–Electric arc (MMAW, SAW, GTAW, GMAW, FCAW,
PAW)
–Laser beam (LBW)
–Electron beam (EBW)
•Electric arc processes dominate

4. Thermal Diffusivity
•A measure of the energy input required
to create and sustain a molten puddle
•= k/C
p
–
Cu = 1.14 cm
2
/s, 
Fe = 0.028 cm
2
/s

5. Heat Input and Energy Density
•Heat input = heat utilized for melting + heat conducted
sideways (wasted)
•Peak temperature decreases as a function of distance away
from weld center line
•HAZ shows inferior properties
•HAZ width increases with heat input

6. Heat Input and Energy Density
•Concentrated heat sources higher
melting efficiency
•Energy density
–Fuel burning < electric arc < laser/electron
beam
–Energy density within arc welding processes:
•MMAW < GTAW < PAW
•High energy density processes produce
narrow HAZs
•Always: Use lowest heat input consistent
with penetration

7. Need for Edge Preparation
•Beyond a certain thickness, through-
thickness melting not possible with electric
arcs
–Prepare plate edges to facilitate access
–Use filler metal to fill the groove
–Weld progressively in multiple passes

8. Joint Types and Welding Positions
Joint Types
Welding Positions

9. Arc Welding Processes
•Consumable electrode processes (MMAW,
SAW, FCAW, GMAW)
–With flux shielding (MMAW, SAW, FCAW)
–With inert gas (GMAW) or CO
2 shielding (CO
2
Welding)
•Non-consumable electrode processes (GTAW,
PAW)
–Inert gas shielding

10. MMAW
•Arc striking
•Polarity
–DCEN (straight)
–DCEP (reverse)
•Most heat generates at
anode
•DCEN good for
penetration
•DCSP good for filling
Arc forces due to interaction between magnetic field and electric current
Spray-type metal transfer due to arc forces (important for out-of-position
welding)

11. MMAW
•Flux coating on electrodes
–Slag formers to protect the weld metal
–CO
2 formers for additional protection
–Ionizing elements for greater arc stability
–Alloying elements
–Iron powder to increase deposition rate
•Slag removal is important
•Simple, inexpensive, and portable equipment
–Field applications, out-of-position welding
•Best used for steels
•Problems
–Flux protection won’t work for most non-ferrous metals
–Unavoidable interruptions (for changing electrodes)
–Low energy density
–Manual welding
–Time consuming for thick sections
–Large current electrode overheating flux disintegration

12. SAW
•Search for: Continuous automatic welding, Better
protection, Thick section welding SAW
•Slow cooling rates (soft and ductile welds)
•Large currents deep penetration
•Large heat inputs
•Restricted to horizontal position

13. FCAW
•Idea
–Tubular “flux-cored” wire
Versatile and
continuous process,
but many
disadvantages of
MMAW remain

14. GTAW
•Non-consumable electrode
•Inert gas shielding (Ar or He)
•Good for welding NFMs
•Precise process –good
for root passes and
thinner gauges
•Filler addition possible,
but cumbersome
•Manual or automatic
•Slow process
•Out-of-position welding
difficult

15. GMAW
•Consumable electrode
•Inert gas shielding
•Good for welding NFMs
•Fast process
•Out-of-position welding
•Semi-automatic or automatic
•For ferrous materials, CO
2
shielding is adequate

16. PAW
•Arc constriction
•Strong directional arc
•Plasma gas and shielding gas
•Automatic
•Keyhole welding
•Variable polarity

17. Laser Beam Welding
•High energy density
•Rapid cooling rates
•Precision welding/micro-
joining
•Thick section welding
•Several metallurgical
advantages

18. Electron Beam Welding
•Electrons are accelerated to
very high velocities (up to
2/3rd of the speed of light)
•Electrons impart their kinetic
energy to the work piece
upon impact, generating heat
•A keyhole forms due to
intense heating
•The keyhole travels with the
beam, which is traversed
along the joint line
•As the beam moves away,
surrounding liquid metal
flows in closing the cavity,
where it solidifies forming the
joint

19. Comparison of GTA and EB Welds

20. Weld failures
•Failures occur due
–Design
–Materials
–Execution
Defective weld
Ignorance
Negligence
Human error

21. Weld Defects
•Defects and discontinuities
–Defects cannot be tolerated
–Discontinuities can be tolerated
•Defects arise due
–Inherent process limitations
–Material behavior
–Faulty practice
–Operator error or negligence
•Defects
–Cracks
–Cavities/pores
–Solid inclusions
–Imperfect fusion
–Imperfect shape

22. Weld Cracking
Cause: Embrittlement + residual stresses

23. Solidification cracks
Steel
Al Al

24. HAZ Liquation Cracking
•Occurs in PMZ/HAZ
–Grain boundary
melting
–Stresses
Al-Cu alloy 2219
•Low melting fillers
–PMZ solidification precedes WM
solidification
•Fine grained base metals
•Low heat inputs and high energy density
processes
–Minimize HAZ/PMZ width

25. Cold Cracking/Hydrogen-Induced
Cracking/Delayed Cracking
•H in weld metal
•Stresses
•Susceptible
microstructure
•Low temperature
•High strength steels are
more susceptible
•Control measures
–Minimize stresses
–Clean surfaces
–Bake electrodes
–Stress relieve

26. Reheat Cracking
•Cr-Mo-V steels
•Cause: Carbide precipitation near dislocations in
HAZ during stress relieving (550-650˚C)
•Remedies: Low HI, Low restraint, rapid heating
through critical temperature range

27. Lamellar tearing
Susceptible steel with
elongated inclusions

28. Gas Porosity
•Gas porosity –CO boil in steels and H porosity in Al
alloys
–Causes: Porous oxide layers, wet or contaminated (oil,
grease, etc.) plate surfaces, insufficient shielding gas, etc.
–Remedies: Remove oxide/anodized layers, check shielding
gas, and always clean part surfaces just prior to welding
H Gas Porosity in an Al
alloy weld Vapor porosity

29. Wormhole porosity

30. Solid inclusions
•Slag inclusions
•Oxide inclusions
•W inclusions
Slag inclusions

31. Incomplete Fusion and Lack of
Penetration
Fillet weld
Butt weld

32. Imperfect Shape

33. Are these really dangerous?
Effect of undercut on
fatigue life of EB welded
carbon steel

34. Backing strip and misalignment

35. You can have many defects in a
given weld!
Lack of fusion (A), lamellar tearing (B), poor profile (C),
Slag inclusions (D), and undercut (E)

36. Weld Defects -Summary
•Defects arise due
–Inherent process limitations
–Material behavior
–Faulty practice
–Operator error or negligence
•Defects
–Cracks
–Cavities/pores
–Solid inclusions
–Imperfect fusion
–Imperfect shape
•Always: know the cause, take corrective measures,
watch