HAZ in Aluminum Welds

Understanding HAZ in Aluminum Welds

Aluminum has been growing in the welding industry as an alternative to steel. As a result, there’s a need for those involved in these projects to understand this material, including the types of alloys, how they obtain their strength in the HAZ, and the potential for changes in strength when welding.

Types of Aluminum Alloys

There are seven series of wrought aluminum alloys. Each is different when it comes to its applications and characteristics.

Series Primary Alloying Element

1xxx Aluminum - 99.00% or Greater

2xxx Copper

3xxx Manganese

4xxx Silicon

5xxx Magnesium

6xxx Magnesium and Silicon

7xxx Zinc

Principle Effects of Primary Alloying Element

Pure Aluminum 1xxx

Although this series is the almost pure aluminum, it will respond to strain hardening, especially if it contains appreciable amounts of impurities, such as iron and silicon. However, even in the strain-hardened condition, it has very low strength when compared to other series. These alloys are also non-heat-treatable.

Most common applications: Aluminum foil, electrical busbars, metalizing wire, and some chemical tanks and piping systems.

Copper (Cu) 2xxx

The aluminum-copper alloys typically contain 2-6% 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.

Most common applications: Aerospace, military vehicles, and rocket fins.

Manganese (Mn) 3xxx

Manganese increases strength 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, they are rarely used for major structural applications.

Most common applications: Cooking utensils, radiators, air conditioning condensers, evaporators, heat exchangers, beverage containers, residential siding, and handling and storage equipment.

Silicon (Si) 4xxx

Silicon reduces melting temperature and improves fluidity. Silicon alone in aluminum produces a non-heat-treatable alloy; however, with magnesium, it produces a precipitation-hardening heat treatable alloy. Consequently, there are both heat treatable and non-heat-treatable alloys within this series.

Most common applications: Filler wires for fusion welding and brazing of aluminum.

Magnesium (Mg) 5xxx

Magnesium 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 are optimal and extensively used for structural applications.

Most common applications: 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 produces the compound magnesium-silicide (Mg₂Si). This provides the 6xxx series with its heat treat-ability. These alloys also extrude both easily and economically. For this reason, they are most often found in an extensive selection of extruded shapes. These alloys also 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.

Most common applications: Handrails, driveshafts, 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 (in conjunction with 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. However, some of these alloys can be susceptible to stress corrosion cracking and are not usually fusion welded. Other alloys within this series are often fusion welded with excellent results.

Most common applications: Aerospace, armored vehicles, baseball bats, and bicycle frames.

How Aluminum Alloys Obtain Their Strength

The main reason for adding the major alloying elements is to improve physical and/or mechanical characteristics. Typically, adding alloying elements promotes 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 – increases the strength of materials when heat treatment cannot. 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

Solution heat-treating is achieved by heating a material to a suitable temperature, holding it at that temperature for a long enough time to allow constituents to enter into a solid solution, then cooling rapidly to hold the constituents in the solution. Usually, this is followed by precipitation hardening, or what is also known as “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 a metallurgical structure that provides superior mechanical properties.

The Effect of Arc Welding on the Heat Affected Zone (HAZ)

To make a welded joint in an aluminum structure using the arc welding process, melting of the base material must occur. While melting, heat transfers through conduction into the base material adjacent to the weld.

Typically, the completed weldment is divided into:

• The weld metal
• The heat-affected zone adjacent to the weld
• The base material beyond the HAZ that has been unaffected by the welding operation

Because the HAZ in aluminum welds will experience cycles of heating and cooling, arc welding on materials that have been strengthened – by work hardening or precipitation hardening – will change its properties. They may be extremely different from the original base alloy and the unaffected area of the base material (see figures 1 and 2).

Non-Heat Treatable Alloys

Aluminum alloys – strengthened by strain hardening – can be restored to a full soft, ductile condition by annealing. Annealing eliminates the strain hardening and the microstructure that is developed because of cooled working. The heating of the HAZ in aluminum welds is sufficient to anneal the base material within the HAZ area. The minimum tensile strength requirements for as-welded, non-heat-treatable alloys are therefore based on the annealed strength of the base alloy. Typical tensile strengths for no-heat treatable alloys are shown in table 1.

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

Heat-Treatable Alloys

With heat-treatable alloys, the HAZ in aluminum welds will not be fully annealed. Typically, the HAZ is not maintained at an adequate temperature for a sufficient period to anneal fully. The effect on the HAZ of a heat treatable alloy that is welded in the solution, heat-treated, and artificially aged is typically one that is partially annealed and over-aged. This is created 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 are shown in Table 2.

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

Summary

Dependant on the particular aluminum alloy type and its temper, there can be a 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.

Fig 1

Fig 2