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How to Avoid Cracking
When Welding
Aluminum Alloys
By:
Tony Anderson CEng, FWeldI
Director of Aluminum Technology
ITW Welding North America
Welding Discontinuities in Aluminum
Weld Cracking
Porosity
Incomplete Fusion
Causes
&
Correction
Incomplete Penetration
(Hobart Aluminum Brochure - Page 21-33 – Problem Solving)
Welding Discontinuities in Aluminum
Characteristics of aluminum that can influence
weld quality
Incomplete Penetration and Fusion
Aluminum has a thermal conductivity 5 times that of
steel & an adherent oxide layer that has a melting
temp. around 3 times that of the aluminum alloy
Welding Discontinuities in Aluminum
Characteristics of aluminum that can influence
weld quality
Porosity
When aluminum is taken
to temp. above its melting
point (as it is when arc
welding) it becomes very
susceptible to hydrogen
absorption - which can
cause porosity
(Hobart Brochure - Page 23-25)
Weld Cracking in Aluminum
Unlike steel aluminum is not susceptible to
Hydrogen Cracking
•
Hot Cracking - which is sometimes referred to as
solidification cracking or liquation cracking is
the primary reason aluminum welds crack
•
Hot Cracking - is a high-temperature cracking
mechanism and is mainly a function of how
metal alloy systems solidify
(Hobart Aluminum Brochure - Page 26-28)
Weld Cracking in Aluminum
Understanding - Hot Cracking

Hot cracking is a welding problem that does not occur in
pure metals

The addition of alloying elements to a pure metal can
result in a number of different phases being formed during
solidification

One particular phase is the lowest melting point
composition of an alloy known as the eutectic composition
which freezes at one specific temperature, it is this
eutectic phase that can promote hot cracking

In most metals this effect is caused by impurities, sulphur
in steel and nickel is a good example where low melting
point sulphide eutectics are formed

In aluminum it is the deliberately added alloying elements
that form a range of eutectics with freezing points
substantially lower than the bulk metal
Problem Solving
Weld Cracking in Aluminum
Hot Cracking

If the difference in the melting point between the low melting
point eutectic and the bulk of the metal is sufficiently great then
the liquid film along the grain boundaries may part as the metal
cools and contracts – the result of this is hot cracking

Solidification strains are proportional to the temperature interval
over which solidification occurs (Coherence Range)
Weld Cracking in Aluminum
Hot Cracking

In general all aluminum alloys exhibit a peak in hot cracking sensitivity
and a high resistance to hot cracking at both low and high alloy content

On the low alloy content side there is only a small amount of eutectic
present which results in the liquid film on the grain boundaries being
discontinuous or very thin – Low crack sensitivity - 1

As the liquid film thickness increases the force required to tear the film
reduces and crack sensitivity increases – High crack sensitivity - 2

Once there is sufficient eutectic available it can begin to flow into and fill
any cracks that form – Low crack sensitivity - 3
2
1
3
Weld Cracking in Aluminum
Hot Cracking
 All aluminum alloys are susceptible
to some degree of hot cracking,
differing only in their degree of
susceptibility
 Cracking tests have been used to
determine the range of composition
within which an alloy has a high risk
of hot cracking
Problem Solving
Weld Cracking in Aluminum
The chemical composition of the weld which
is most often a mixture of base metal and
filler metal can greatly influence crack
sensitivity
“It’s all about the chemistry”
Weld Cracking in Aluminum
Crack Sensitivity Chart
Crack Sensitivity
0
Al - Cu
0
Al - Mg
0
Al - Mg2 Si
0
1
2
3
4
5
6
7
Percentage alloying element in the weld
(Hobart Aluminum Brochure - Page 26)
Weld Cracking in Aluminum
Crack in 6061-T6 – GTAW weld with no filler metal
Radiograph of cracked weld
Weld Cracking in Aluminum
Hot Cracking
Weld on the left
was welded
with the
addition of 4043
filler metal and
we see no
visible cracks
Weld on the
right is welded
without filler
metal
(autogenous)
and we see a
large centerline
crack
Visual Inspection of two welds made with the GTAW process on base metal 6061-T6
“It’s all about the chemistry”
Weld Cracking in Aluminum
Crack Sensitivity Chart
Crack Sensitivity
0
Al - Cu
0
Al - Mg
0
Al - Mg2 Si
0
1
2
3
4
5
6
7
Percentage alloying element in the weld
Area of interest – Al-Mg2Si
Weld Cracking in Aluminum
Crack Sensitivity Chart
Crack Sensitivity
6061 Base Metal has Mg2Si Content Around Maximum Crack Sensitivity
Al - Mg2 Si
0
1
2
3
4
5
6
7
Percentage Alloying Element In The Weld
Note: This is the reason that there are no 6xxx series
filler metals – they would crack when used
Weld Cracking in Aluminum
Weld A which was welded without filler metal has cracked during solidification
(Hot Crack) owing to the high crack sensitivity of the molten material (6061-T6)
Weld B on the same base material welded with 4043 filler metal has no cracking
WHY?
The 4043 filler metal provides a number of characteristics that help to prevent cracking
when welding the 6xxx series alloys:
 It combines with the base metal which lowers the percentage of Mg2Si in the
solidifying weld and thereby reduces crack sensitivity
 It has a lower solidification temperature which allows the base metal to solidify
prior to the weld
 The addition of higher percentages of silicon to the solidifying weld metal
decreases the total shrinkage during freezing
Weld Cracking in Aluminum
Dilution Effect on Crack Sensitivity
6061 Base Alloy – 1% Mg
5356 Filler Alloy – 5% Mg
Question: Which
of these two
welds will have
the highest
probability of
hot cracking?
Square-groove weld
Single-v-groove weld
20% Filler Metal
60% Filler Metal
80% Base Metal (Hobart Aluminum Brochure - Page 27) 40% Base Metal
Weld Cracking in Aluminum
Crack Sensitivity Chart
Crack Sensitivity
0
Al - Cu
0
Al - Mg
0
Al - Mg2 Si
0
1
2
3
4
5
6
7
Percentage alloying element in the weld
Area of interest – Al-Mg
Weld Cracking in Aluminum
Alloy Content Vs. Crack Sensitivity
Crack Sensitivity
1.8% Mg
3.4% Mg
Al - Mg
0
Composition Of Weld - Percent Alloying Element
Note: When welding the 6xxx series base metals, using a
single-v-groove weld rather than a square-groove weld can
significantly reduce the probability of cracking
Weld Cracking in Aluminum
Typical Dilution Ratios of Welded Joints
20%
Filler
Metal
80%
Base
Metal
60%
Filler
Metal
40%
Base
Metal
80% Filler Metal
20% Base Metal
Weld Cracking in Aluminum
Stress Crack Inside Pipe On The Opposite Side From Weld
Crack
6061-T6
Base Metal
Fillet Weld
Note: Excessive heat and shrinkage stress
Weld Cracking in Aluminum
Solidification Shrinkage Stress Crack
2014 Base metal
melting range
945-1180 Deg. F
4043 Filler metal
melting range
1065-1170 Deg. F
Note: Filler metal 4145 has a melting range of 970-1085 and
would provide the lowest crack sensitivity
Crater Cracking & Weld Termination
Crater Crack
AWS D1.2 2008 Structural Welding Code (4.17) Terminating a weld within a
joint may be done by any of the following methods or combination thereof:
• Reversing the direction of travel for a distance of at least 2 in [50 mm]
• Increasing travel speed
• Providing a build-up of metal and remolding the crater area flush with the weld
surface by mechanical means
• Using automated crater fill features in the welding equipment
• Rapidly triggering the gun manually as the weld is terminated
Weld Cracking in Aluminum
Plate Edge Cracking
When using thermal cutting
processes on the heat treatable
alloys such as the 6xxx series
there is a potential for cracking to
occur along the plate edge
AWS D1.2 – 4.11.2 - Plasma arc
and laser cut edges – For heat
treatable aluminum alloys, 1/8 in
(3mm) of material shall be
removed by machining from
plasma and laser cut edges that
will be welded
Aluminum Alloys For Marine Service
5xxx Series Alloys
•
Crack Sensitivity is not usually an issue (hot cracking)
• Mg alloys with less than 2.5%Mg (5052) can be welded with
a 4xxx or 5xxx series filler alloy
• Mg alloys with more than 3%Mg are usually welded with
filler alloy of very similar chemistry to that of the base alloy
• Alloys above 4% Mg can be weld autogenously
• No special precautions to be taken when plasma arc cutting
* However the manufacturing & testing procedures of the
5xxx alloys is important when it is used in marine
applications
Aluminum Alloys For Marine Service
5xxx Series Alloys – Corrosion Considerations
US Coast Guard (09/25/2003) – Aluminum Fracturing
“Exfoliation corrosion and fracturing problems have
recently been observed on several high-speed
passenger vessels built in the Puget Sound area”
• Between 1999 and 2001 there was around 750,000
pounds of 5083 H321 used in the marine industry that
had been produced by a manufacturer using a nonmarine process that made the material susceptible to
intergranular corrosion.
(As a result of this problem we now have ASTM B 928)
Aluminum Alloys For Marine Service
ASTM B 928
ASTM – Standard Specification for High Magnesium
Aluminum-Alloy Sheet and Plate for Marine Service
and Similar Environments – Designation: B 928/B
928M – 07
This specification has been developed to provide assurance to the
manufacturer and the end user that the material used for
manufacturing aluminum vessels has been correctly manufactured
and tested to ensue its suitability for marine applications
Note: what are the materials addressed and how does ASTM B 928 achieve its
objectives?
Aluminum Alloys For Marine Service
Chemical Composition Limits ASTM - B 928
Note: The Specification addresses these alloys in both the H116 & H321 tempers
Aluminum Alloys For Marine Service
5xxx Series Alloys – Corrosion Considerations
•
Sensitization – the development of a continuous grain
precipitate in 5xxx alloy temper material, that causes the
material to be susceptible to intergranular forms of
corrosion
• Intergranular Corrosion – corrosion that preferentially
occurs at, or adjacent to, the grain boundaries of a metal or
alloy.
• Exfoliation Corrosion – corrosion that proceeds laterally
from the sites of initiation along planes parallel to the
original rolling surface, generally at grain boundaries.
Aluminum Alloys For Marine Service
ASTM B 928
Examples of the corrosion morphology produced by test method G 67, for varying degrees
of sensitization, from pitting and general corrosion to intergranular corrosion.
Fig a and b
Show examples
of general
corrosion and
pitting corrosion
attack. These
samples are
examples that
would pass
Specification
ASTM B 928
Fig c and d
Show examples
of an
Intergranular
attack and are
examples of
material that
would fail
specification
ASTM B 928
Weld Cracking in Aluminum
The Prevention of Hot Cracking
• Avoid welding crack sensitive base alloys without filler
metal
• Use joint designs that help to ensure adequate additions of
filler metal
• Avoid weld profiles that are excessively concaved
• Avoid welds that are undersized
• Avoid excessive heat input
• Reduce excessive localized stresses
• Lower solidification temp. filler - relative to base metal
• Smaller solidification temp. range
• Filler metal selection – use the most appropriate filler metal
(use a filler metal selection chart)
Non-Weldable Aluminum Base Metals
WARNING
2xxx alloys with - Aluminum-copper-magnesium
Examples: 2017 and 2024
7xxx alloys with - Aluminum-zinc-copper-magnesium
Examples 7075, 7178, 7050, 7150
Note: Low melting point elements are preferentially precipitated into
grain boundaries – lowers and widens solidification temp. range at the
grain boundary. Easily crack during solidification shrinkage, difference
in galvanic potential at grain boundaries can make them susceptible to
stress corrosion cracking
Alloys used for machinability – Examples 6262 and 2011
Note: These alloy have lead, bismuth, and/or tin added to facilitate
machinability – these low melting additives seriously increase hot
cracking
(Hobart Aluminum Brochure - Page 10)
Aluminum Filler Metal Selection Chart
The Weld Meal Properties Used To Select The Most Appropriate Filler Metal
 Crack Sensitivity
 Elevated Temperature Service
 Strength
 Color Match After Anodizing
 Ductility
 Post Weld Heat Treatment
 Corrosion Resistance
 Toughness
(Hobart Aluminum Brochure – 3 Page fold-out at back of brochure)
Aluminum Alloys For Marine Service
Conclusion
• Be aware of the potential solidification cracking issues
associated with welding the 6xxx series aluminum alloys
• When procuring 5xxx series aluminum alloys for marine
applications ensure that they are ordered to the correct
temper and that they are manufactured and tested in
accordance with the requirements of ASTM B 928
• Make use of the filler metal selection chart for guidance
in choosing the most appropriate filler metal for your
particular application
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