Heat Affected Zone

Heat Affected Zone

The Heat Affected Zone (HAZ) is the area of base material which is not melted but has nevertheless had its microstructure and properties changed as a result of the welding.

The strength in the HAZ is reduced for many metallic alloys, but in particular for aluminium. The extent to which the material properties change is dependent on the following:

For temper state O, where the material is annealed, there is no change in strength in the HAZ. However, for heat treated alloys (designation of T), the reduction in strength varies, from 10% in plate T4 alloys, to 60% in drawn tube T6 alloys. It can be even more, like in the alloy 8011A-H16, where the reduction in strength in the HAZ is over 70% for welded plates.

In FEA, the reduced strength of the HAZ is often not taken into account. It can however be included if the HAZ is modelled as a separate part, joined to the part representing the base material.

The strength of the aluminium alloys in the unwelded zones and the HAZ can be found in EN1999-1-1 (Eurocode 9) table 3.2.

The exact size of the HAZ cannot be easily determined, but is, for aluminium alloys, taken to be 25mm or 3 times the thickness of the plate from the weld centreline as per US Aluminum Association, or taken from EN1999-1-1 (Eurocode 9) section 6.1.6.3, paragraph (3).

It should be mentioned that in addition to the reduced strength in the HAZ, residual stresses due to the welding process might further locally weaken a welded component. These stresses can however be determined by FEA.

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