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The HAZ In Aluminum Welds

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To appreciate the affect of arc welding on the heat-affected zone of various aluminum alloys, it is necessary to evaluate the various types of aluminum alloys, how these alloys obtain their strength and the potential for changes in strength after welding.

The various types of aluminum alloys

Considering the seven aluminum alloy series used for wrought alloys, the main alloying elements used for producing each of the alloy series are immediately identifiable. Further examination of each of these elements’ effects on aluminum is possible. 

Series              Primary Alloying Element

1xxx                 Aluminum - 99.00% or Greater

2xxx                 Copper           

3xxx                 Manganese

4xxx                 Silicon

5xxx                 Magnesium

6xxx                 Magnesium and Silicon

7xxx                 Zinc

The principal effects of alloying elements in aluminum are as follows:

Pure Aluminum 1xxx - Although the 1xxx series are almost pure aluminum, they will respond to strain hardening, especially if they contain appreciable amounts of impurities such as iron and silicon.  However, even in the strain-hardened condition, the 1xxx series alloys have very low strength when compared to the other series of aluminum alloys. The most common applications for the 1xxx series alloys are aluminum foil, electrical buss bars, metalizing wire and some chemical tanks and piping systems. These alloys are non-heat treatable.

Copper (Cu) 2xxx – The aluminum-copper alloys typically contain between 2 to 6% of copper, with small additions of other elements.  The copper provides substantial increases in strength and facilitates precipitation hardening.  These alloys include some of the highest strength heat treatable aluminum alloys. The most common applications for the 2xxx series alloys are aerospace, military vehicles and rocket fins.

Manganese (Mn) 3xxx – The addition of manganese to aluminum increases strength to an extent through solution strengthening. It improves strain hardening and does not significantly reduce ductility or corrosion resistance. These are moderate strength non-heat treatable materials that retain strength at elevated temperatures. However, for major structural applications, they are rarely used. The most common applications for the 3xxx series alloys are cooking utensils, radiators, air conditioning condensers, evaporators, heat exchangers beverage containers, residential siding, and handling and storage equipment.

Silicon (Si) 4xxx – The addition of silicon to aluminum reduces melting temperature and improves fluidity. Silicon alone in aluminum produces a non-heat treatable alloy; however, in combination with magnesium, it produces a precipitation hardening heat treatable alloy.  Consequently, there are both heat treatable and non-heat treatable alloys within the 4xxx series, The most common application for silicon additions to aluminum is the manufacturing of aluminum castings.  The most common applications for the 4xxx series alloys are filler wires for fusion welding and brazing of aluminum.

Magnesium (Mg) 5xxx - The addition of magnesium to aluminum increases mechanical properties through solid solution strengthening. Additionally, it improves their strain hardening ability.  These alloys are the highest strength non-heat treatable aluminum alloys and they are optimal and extensively used for structural applications.  The 5xxx series alloys are produced mainly as sheet and plate and only occasionally as extrusions.  These alloys strain harden quickly, therefore, they are difficult and expensive to extrude.  Some common applications for the 5xxx series alloys are truck and train bodies, buildings, armored vehicles, ship and boat building, chemical tankers, pressure vessels and cryogenic tanks.

Magnesium and Silicon (Mg2Si) 6xxx – The addition of magnesium and silicon to aluminum produces the compound magnesium-silicide (Mg2Si).  The formation of this compound provides the 6xxx series their heat treat-ability.  The 6xxx series alloys extrude both easily and economically. For this reason, they are most often found in an extensive selection of extruded shapes.   These alloys form an important complementary system with the 5xxx series alloy.  The 5xxx series alloy used in the form of plate and the 6xxx series used in an extruded form are often joined to the plate.  Some of the common applications for the 6xxx series alloys are handrails, drive shafts, automotive frame sections, bicycle frames, tubular lawn furniture, scaffolding, stiffeners and braces used on trucks, boats and many other structural fabrications.

Zinc (Zn) 7xxx – The addition of zinc to aluminum (in conjunction with some other elements, primarily magnesium and/or copper) produces heat treatable aluminum alloys of the highest strength.  The zinc substantially increases strength and permits precipitation hardening.  Some of these alloys can be susceptible to stress corrosion cracking and for this reason are not usually fusion welded.  Other alloys within this series are often fusion welded with excellent results.  Some of the common applications of the 7xxx series alloys are aerospace, armored vehicles, baseball bats and bicycle frames.

How aluminum alloys obtain their strength:

As seen above, aluminum alloys consist of both heat treatable and non-heat treatable types.  The addition of alloying elements to aluminum is the principal method used to produce a selection of different materials used in a wide assortment of applications. The principle reason for adding the major alloying elements is to facilitate an improvement in the alloys physical and/or mechanical characteristics.  Typically, addition of primary alloying elements to aluminum is to provide improvement in work hardening and/or precipitation hardening characteristics.

Work Hardening

Work hardening, used extensively to produce the strain-hardened tempers in the non-heat treatable aluminum alloys, is an important process that increases the strength of materials that heat treatment cannot strengthen.  This process involves a change of shape brought about by the input of mechanical energy.  As deformation proceeds, the material becomes stronger but harder and less ductile.  For example, the strain hardened temper of H18, full-hard material is obtainable with a cold work equal to about a 75% reduction in area. The H16, H14 and H12 tempers obtained with lesser amounts of cold working represent three-quarter-hard, half-hard, and quarter-hard conditions, respectively.

Precipitation Hardening

Precipitation Heat treatment precedes solution heat-treating.  Solution heat-treating is achieved by heating a material to a suitable temperature, holding at that temperature for a long enough time to allow constituents to enter into solid solution, then cooling rapidly to hold the constituents in solution.  Usually this is followed by precipitation hardening, or what is also termed artificial aging. This is achieved by re-heating the alloy to a lower temperature and holding it at this temperature for a prescribed period.  The result is to produce a metallurgical structure within the material that provides superior mechanical properties.  If, during heat treatment, the material is held at temperature for too long or the temperature used is too high, the material will become over aged, resulting in a decrease in tensile strength.  It is important to recognize that the precipitation hardening process is both time and temperature controlled.

The Affect of Arc Welding on the Heat Affected Zone

In order to make a welded joint in an aluminum structure using the arc welding process melting of the base material must occur.  During the melting operation, heat transfers through conduction into the base material adjacent to the weld.  Typically, the completed weldment is divided into three distinct areas: the weld metal, the heat-affected zone adjacent to the weld, and the base material beyond the HAZ that has been unaffected by the welding operation.  Because the HAZ will experience cycles of heating and cooling during the welding operation, arc welding on materials which have been strengthened by work hardening or precipitation hardening,  will change  its properties and may be extremely different than that of the original base alloy and the unaffected area of the base material (see fig 1 and fig 2)

Non-Heat Treatable Alloys

What is important from a HAZ perspective is that aluminum alloys strengthened by strain hardening can be restored to a full soft, ductile condition by annealing.  Annealing eliminates the strain hardening, as well as the microstructure that is developed because of cooled working.  The heating of the HAZ, which takes place during the arc welding operation, is sufficient to anneal the base material within the HAZ area.  For this reason the minimum tensile strength requirements for as-welded non-heat treatable alloys is based on the annealed strength of the base alloy.  Typical tensile strengths of non-heat treatable alloys in their tempered condition and as-welded are shown in table 1

Heat Treatable Alloys

In the case of the heat-treatable alloys, the HAZ will not be fully annealed.  Typically, the HAZ is not maintained at an adequate temperature for a sufficient period to anneal fully the HAZ.  This does not suggest that experiences in a reduction in strength in the HAZ will not occur.  The affect on the HAZ of a heat treatable alloy that is welded in the solution heat-treated and artificially aged condition is typically one of partially annealed and over-aged.  This condition is affected by the heat input during the welding operation.  The general rule is, the higher the heat input, the lower the as-welded strength.  Typical tensile strengths of some of the heat treatable alloys in their temper condition and as- welded are shown in table 2.

Summary

Dependant on the particular aluminum alloy type and its temper, there are often significant difference between the tensile strength of the HAZ and the tensile strength of the unaffected area of the welded component.  The reduction in tensile strength of the HAZ under controlled conditions, particularly with the non-heat treatable alloys, can be somewhat predictable.  The reduction in tensile strength of the HAZ in the heat treatable alloys is more susceptible to welding conditions and can be reduced below the required minimum requirement if excessive heating occurs during the welding operation.

Table 1

Typical Tensile Strength Properties of Groove Welds

Non-Heat Treatable Alloys

 

Base Alloy & Temper

 

 

Base Alloy Tensile Strength - ksi

 

As welded Tensile Strength - ksi

1060-H18

19

10

5052-H32

33

27

5052-H39

42

27

5086-H34

47

38

5086-H38

53

38

5083-H116

46

43

3003-H34

35

16

3004-H38

41

24

 

Table 2

Typical Tensile Strength Properties of Groove Welds

Heat Treatable Alloys

 

Base Alloy & Temper

 

 

Base Alloy Tensile Strength - ksi

 

As welded Tensile Strength - ksi

6063-T6

31

19

6061-T6

45

27

6061-T4

35

27

2219-T81

66

35

2014-T6

70

34

7005-T53

57

43

 



Heat Affected Zone

Fig 1

 

Heat Treatable Alloys



Fig 2