Brief overview of aluminum weldng processes and related defects with possible remedies. Especially GMAW, GTAW and FSW.

1. WELDING OF ALUMINUM
ALLOYS
By
Chayon Mondal
M.Tech – I
Roll no. 16142006
Department of Metallurgical
Engineering
IIT(BHU), Varanasi

2. OVERVIEW
Importance of welding of Aluminum and it’s alloys
Classification of Aluminum alloys
Metallurgy of Aluminum alloys
Various methods used for welding
Welding limitations and remedial measures
Conclusion
References

3. IMPORTANCE OF WELDING OF
ALUMINUM AND ITS ALLOYS
Low relative density ( ~2.7 )
Reasonably high tensile strength and ductility
High strength to weight ratio
Excellent electrical and thermal conductivity
Corrosion resistance
Easy fabrication
Favourable economics

4. CLASSIFICATION OF ALUMINUM
ALLOYS
Most Aluminum alloys can be
broadly under 2 basic categories
Other than these 2, we can have
 Dispersion strengthened alloys using
powder metallurgy techniques – Rapid
Solidification or Mechanical Alloying
 Aluminum-Lithium alloys
 Metal-Matrix composites

5. PROPERTIES OF AL ALLOYS
Melting point < 660ºC, working temp. range < 250ºC
Long freezing range alloys
Strengthening mechanisms :
• Precipitation hardening – formation of coherent precipitates in Al-Cu alloys on
aging after quenching. [Cu + 2Al → θ’(GP zones) → θ (CuAl2)]
• Solid solution strengthening – substitutional solid solutions impede motion of
dislocations
• Dispersion strengthening – Dispersion of hard second phase particles in the matrix
• Cold working
High coefficient of thermal expansion (2x that of steel)
High thermal conductivity
Very high oxidising potential

6. EFFECT OF STRENGTHENING
MECHANISMS

7. COMPOSITION AND APPLICATIONS OF
SOME AL ALLOYS

8. WELDING PROCESSES USED
Fusion welding
Gas shielding, not flux shielding
High welding speed
Low welding current
High heat density
Processes : mainly GMAW, GTAW,
and EBW.
Filler material used is of similar
composition
Solid state welding
Almost all solid state welding
processes are suitable.
• Diffusion welding
• Low Temp./Cold welding
• Explosive welding
• Forge welding
• Friction Welding
• Friction Stir welding
• Ultrasonic welding
Limited by weld metal geometry – for
simple shapes only
Feasible only for bulk quantity
Special mention : Friction Stir
Welding

9. GAS METAL ARC WELDING
Modes of metal transfer:
 Globular mode – under influence of
gravity. Presents spatter and erratic
arc. Low currents.
 Spray transfer – Occurs at high
current levels. Low spatter. Metal
travels under influence of
electromagnetic force. High
deposition rate.
 Short circuit transfer – Low currents
and electrode diameters. Preferred
for Al welds of thin sections –
produces a fast freezing weld pool.
Shielding gases :
 He
 Ar
 CO

10. GAS TUNGSTEN ARC WELDING
Slow rate of deposition, multiple
passes reqd.
Thickness of weld limited to <6mm
Polarities : For Al-welding, DCEP or
AC is preferred.
AC has half cycle of surface oxide
cleaning followed by half-cycle of
metal penetration..

11. FRICTION STIR
WELDING
• Solid- state, hot shear joining
process
• Severe plastic deformation and
flow of this plasticised metal
occurs as the tool is translated
along the welding direction
• Parameters : tool rotation rate (in
rpm) and tool traverse speed
(mm/min). Additional parameters –
angle of tool and pressure.
• The spindle is made of tool steel
for Al alloys.

12. WELDABILITY OF AL ALLOYS
Weldability of Al alloys is defined by the resistance of the weld metal to solidification
cracking and porosity.
Effect of the Welding Process
Heat Effects
Dilution percentage
Effect of nature of base metals prior to welding
Surface condition
Chemistry
Mechanical properties
Effect of alloying elements
Hydrogen induced cracking ( HIC )

13. PROBLEMS WITH WELDING OF
ALUMINUM ALLOYS
Solidification cracking
Liquation cracking
Reduction of tensile strength in HAZ due to grain coarsening
Porosity, HIC, and inclusions of oxides.

14. SOLIDIFICATION CRACKING
Conditions:
Metal must lack ductility
Tensile stress developed as a result of contraction
must exceed the corresponding fracture stress
 Long freezing range alloys – long interval
between nil-ductility temp. and coherent
temp. Occurs near liquidus.
Occurs usually in TIG/MIG/EBW welding of
Al-Mg(5xxx) alloys (Mg – 0.5 to 2.5%)
Ref. Metallurgy of Welding, 6th ed. J. F. Lancaster

15. REMEDIES
Lowering the solidification
temperature range – bringing the
dilution (base metal + filler
metal) nearer to the the eutectic
composition i.e. using a higher
alloy content filler.
Eg. Al-5% Si → Al-12% Si
Low heat input
Fast welding speed.
Grain refinement - grain
boundary area inversely
proportional to grain dia d.
therefore if grain size is reduced,
residual intergranular liquid film
is also reduced, intergranular
cohesion is increased.
Methods :
• Addition of Ti, B or other alloying
elements
• Stirring of the weld pool –
ultrasonic/electromagnetic vibrator

16. LIQUATION CRACKING
Occurs when welding high
strength aluminium alloys of the
Duralumin or aluminium-
magnesium-zinc types
Associated with the presence of
low melting constiuents in the
structure
Occurs at relatively low heat
input rates
Remedied by
 Use of low melting point filler metal
 Increasing the welding speed.

17. POROSITY
Formation of Al2O3 which acts
as nuclei for gas pores
Sources of H2:
 Surface oxide film
 Lubricants on the filler wire
 Electrode coating
 From the shielding gas
From the use of excessive
currents in gas metal arc welding
which causes excessive
turbulence in weld pool resulting
in large continuous holes
(tunnelling)

18. REMEDIES
Hydrogen porosity :
 Surface cleaning
 Cleanliness of the filler wire
 Increasing the surface to volume ratio – using a larger diameter filler wire
Tunnelling :
 Limiting the current per pass
 Gravitational mode of metal transfer
 Improving inert gas shielding

19. THE HEAT AFFECTED ZONE
For precipitation hardening
alloys – there is dissolution or
coarsening of precipitates.
High heat input and preheating
- increase degree and width of
the HAZ.
Tensile failure occurs in the
HAZ.
PWHT required.
For Non-Heat treatable alloys -
HAZ degradation is limited to
recovery, recrystallization and
groin growth.
Weld metal is the weakest part,
due to residual stresses.
PWHT not required, as-welded
condition can be ued.

20. REMEDIES
HAZ degradation in Heat-
treatable Alloys
Multi-pass welding
Close control of inter-pass
temperature
Elimination of preheating
Fast welding at low currents
Post weld heat treatment –
Solution heating → quenching →
aging.

21. CONCLUSION
Fusion welding processes

23. REFERENCES
Metallurgy of Welding, 6th Edition, J. F. Lancaster
Welding Metallurgy, 2nd Edition, Sindo Kou
Modern Welding Technology, 6th Edition, Howard B. Cary and Scott
C. Helzer
Introduction to Physical Metallurgy of Welding, 2nd Edition, Kenneth
Easterling
ASM Handbook – Volume VI – Welding, Brazing and Soldering
Welding of Aluminum Alloys, R. R. Ambriz and V. Mayagoitia,
Instituto Politécnico Nacional CIITEC-IPN

24. THANK YOU.