ALUMINIUM ALLOYS
CHARACTERISTICS THAT INFLUENCE WELDABILITY
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Formation of refractory oxide
High thermal conductivity
High thermal expansion
Hydrogen solubility:
High in liquid state
Low in solid state
• Sensitivity to Hot cracking
TWO MAIN GROUPS
• Heat treatable alloys
Strengthening by precipitation hardening
• Non heat-treatable alloys
Strengthening by cold working only
Heat treatable alloys
• 2XXX
Al-Cu, Al-Cu-Mg, Al-Cu-Li
• 6XXX
Al-Mg-Si
• 7XXX
Al-Zn, Al-Zn-Mg, Al-Zn-Mg-Cu
• 8XXX
Al-Li-Cu-Mg
Non heat-treatable alloys
• 1XXX
Pure Al
• 3XXX
Al-Mn
• 4XXX
Al-Si
• 5XXX
Al-Mg
• 8XXX
Al-Fe, Al-Fe-Ni
HEAT TREATABLE ALLOYS
• Heat treatment:
Precipitation / Age
hardening treatment
• Steps in HT:
Solutionising
Quenching
Aging
Al-Cu Alloy system
Problems in welding of
Heat treatable alloys
• Hot cracking
• Porosity
• HAZ degradation
• Oxide formation
• High thermal conductivity
HOT CRACKING
• Greater amounts of alloying –
Wider solidification temperature range
• Large change in volume upon solidification
• High thermal expansion
SOLIDIFICATION CRACKING
• Weld zone
• Along centre of weld / termination centres
• Factors:
• Degree of restraint
• Weld metal composition – especially though filler metal
• Heat input - high heat input promotes cracking
Relative cracking sensitivity
LIQUATION CRACKING
• At PMZ, due to large amount of alloying
• Formation of eutectics / low melting constituents at grain
boundaries
• Higher heat input widens PMZ
• Low melting fillers provide less susceptibility
• Solidification shrinkage stresses at low temperature
• Solidification of PMZ to occur prior to solidification shrinkage
stresses
Porosity
Hydrogen solubility in Al
• Excess hydrogen forms pores.
• Pore’s buoyancy velocity should be more than
velocity of solidifying front.
• Lower welding speeds create slower
solidification fronts.
PRECIPITATION HARDENED ALLOY WELDED IN FULL HARD CONDITION
HAZ Degradation
OXIDE FORMATION ON SURFACE
• Strong chemical affinity for oxygen
• Accelerated by thermal treatment and moist
atmosphere
• Melting point of Al2O3: 20500C
• Improper fusion
• Prevents arc initiation
OXIDE REMOVAL
• Thicker oxides by chemical / mechanical
means
• Self cleaning by appropriate polarity
Problems in welding of
Non-heat treatable Al alloys
• Hot cracking – some times occurs.
But not so severe.
• Porosity
• HAZ degradation
HAZ degradation
• Limited to recovery, recrystallisation and grain
growth
• Weld metal is the weakest zone
• First HAZ is fully annealed.
Work hardening effect will be removed.
Selection of filler metal for Al alloys
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The aluminum-silicon alloys (4xxx series) are seen predominantly as filler
alloys and commonly contain 4.5 percent to 13 percent silicon. Exercise
care when welding a 1xxx series (pure aluminum) base alloy with a 4xxx
series filler alloy to prevent a weld metal chemistry mixture within this
crack-sensitive range.
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The aluminum-copper alloys (2xxx series) exhibit a wide range of cracksensitive characteristics. Some of these base alloys are not suitable for arc
welding because of their sensitivity to hot cracking, but others are welded
easily using the correct filler alloy and welding procedure.
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The aluminum-magnesium alloys (5xxx series) have the highest
strengths of the non-heat-treatable aluminum alloys and, for this
reason, are very important for structural applications. magnesium
content of 0.5 to 3.0 percent in aluminum produces a cracksensitive composition.
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Selection of filler metal for Al alloys
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As a rule, the aluminum-magnesium base alloys with less than 2.5
percent magnesium content can be welded with either the 4xxx
series or the 5xxx series filler alloys, depending on your weld
performance requirements. The aluminum-magnesium base alloys
with more than 2.5 percent magnesium typically cannot be welded
successfully with the 4xxx series filler alloys because of problems
associated with decreased ductility and increased crack sensitivity.
• The aluminum-magnesium-silicon alloys (6xxx series) are heattreatable alloys and contain about 1.0 percent Mg2Si. These alloys
cannot be arc welded successfully without filler alloy. They can be
welded with 4xxx series or 5xxx series filler alloys depending on
weld performance requirements. It is very important to dilute the
base material with sufficient filler alloy to reduce weld metal crack
sensitivity and hot cracking problems
Nickel Alloys
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FCC
No phase transformation
Similar in many respects to the austenitic stainless steels
Coeff. of Thermal. Expn. < Stainless steel
Distortion control measures are similar to Carbon steel
Applications
– high temperature oxidation and creep resistance
– aggressive corrosive environments
– very low temperature cryogenic applications.
• 2 families of alloys
– solid solution strengthened alloys
– precipitation hardened alloys.
Common Ni alloys
Alloy 200
Monel/Alloy 400 SS
Monel/Alloy K500PH
Alloy 600
Alloy 617
Alloy 625
Alloy 718
Alloy 800
Alloy 825
Nimonic
CP 99.2Ni
68Ni-33Cu
65Ni-3Al-0.5Ti-32Cu
SS 75Ni-15Cr-8Fe
SS 46Ni-22Cr-9Mo-12Co
SS 64Ni-22Cr-8Mo-3Fe-4Nb
PH 52Ni-19Cr-3Mo-5Nb
SS
SS
PH
32Ni-22Cr-42Fe42Ni-22Cr-28Fe-2Cu
44Ni-16Cr-3Mo-29Fe
METALLURGICAL DIFFICULTIES
•Hot cracking in weld metal or PMZ
Source of problem: S
P, Pb, Bi, B also contribute.
Contamination by grease, oil, dirt, etc.
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Machining / stainless steel wire brushing followed by
degreasing is necessary prior to welding.
Any heat treatment must be carried out using S - free
fuel or by using electric furnaces.
METALLURGICAL DIFFICULTIES
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Porosity due to Nitrogen.
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As little as 0.025% nitrogen can form pores in the solidifying WM
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Quite light draughts can disrupt the gas shield and cause porosity.
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Use good shield, particularly in site welding applications.
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Use good gas purity and efficiency of the gas shield
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Oxygen can also cause of porosity, when it combines with carbon in
the weld pool to form carbon monoxide. Use sufficient deoxidants.
METALLURGICAL DIFFICULTIES
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Sluggish weld pool - does not flow freely
Lumpy and convex weld bead
•In materials susceptible to
Ductility Dip Cracking, a steep
drop in ductility occurs in the
temperature range between the
solidus (T S ) and approximately
0.5 T S
•occurs in the weld metal, HAZ,
or reheated weld metal.
•problem in applications where
high - Cr ( ∼ 30 wt%)
Remedies
• sufficient Nb to ensure that NbC forms at the end of solidification, thus
promoting weld metal microstructures with tortuous grain boundaries that
are resistant to DDC
• precautions should be taken to minimize weld restraint through either joint
design or process variables.
Strain-Age Cracking
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Strain-age cracking occurs during PWHT in the Ni-base
alloys strengthened by precipitation hardening with the
intermetallic phase Ni3(Ti,Al).
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This form of cracking has also been known more
generically as ‘PWHT cracking
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Generally, higher (Ti and Al) content in the alloy will
promote strain-age cracking since rapid precipitation of the
Ni3(Ti,Al) intermetallic leads to embrittlement before stress
relief.
Titanium Alloys
GENERAL CHARACTERISTICS
• Low density – SG:4.51
• Good corrosion resistance
• MP: 1670 C
• Rapid oxidation, above 650 C
• Dissolution of oxide
• Embrittlement by O, N, H
CLASSIFICATION
• Commercially pure
• Alpha and Near Alpha
• Alpha – Beta
• Meta stable beta
Difficulties in welding
WELDABILITY TEST
Y-GROOVE TEKKEN TEST
60º
2 mm
COLD CRACKING TEST
IMPLANT TESTING
implant specimens
Implant testing machine
IMPLANT TEST
Base plate for implant test
Implant specimen
IMPLANT TEST
HOULDCROFT TEST - SELF RESTRAINT TEST
VARESTRAINT TEST
VARESTRAINT TEST
VARESTRAINT TEST
12.5
25
125
Thickness: 3 mm
All dimensions are in mm
Varestraint specimen dimensions
VARESTRAINT TEST RESULT
0
