Stress concentrations in weld heat affected zones in aluminium-zinc-magnesium alloy

by T.K. Chan

The motivation for this investigation came from the Defence Research Agency (DRA) - Military Division (formerly known as the Royal Armament Research and Development Establishment). Cracks had occurred during testing of the Launching Nose Heavy units of the Medium Girder Bridge at load levels less than expected. The Medium Girder Bridge is fabricated from the 7019 Al-Zn-Mg heat-treatable alloy.

The cracks were located in the tensile bottom chord of the units, at a short distance from the toe of the fillet welds of the jaw connection. It was suggested that the cracks were probably located within the heat-affected zone (HAZ) of the welds where the strength of the material is severely reduced.

The objective of the project was to clarify the mechanism of failure in load-diffusing welded joints and to propose design guidance for this class of structural detail.

The experimental investigation into the effects on 7019 alloy of a range of thermal histories has produced new data on the mechanical properties of the material. The HAZ for a single linear weld pass can now be more accurately defined due to additional experimental data on its extent and severity of the strength degradation. The effect of two parallel welds laid consecutively has also been investigated. The results show that elevated plate temperatures at the start of welding may severely reduce material strength in the HAZ. There are indications that some of the recommendations in the recently revised British Standard BS 8118 : Structural use of aluminium are not conservative.

A finite element heat transfer model has been developed for comparison with a classical heat transfer solution to extend results obtained from single weld runs to multiple weld runs and to more complex welding configurations.

Tensile tests on cruciform specimens with different welding configurations, finger geometry and layout have shown that the load carrying capacity depends more on welding configuration than on finger profile. Failure was often characterised by a 'dish-like' crack initiation within the HAZ. The full plastic-strength of the connection was not achieved in some cases.

Finite element analyses of the cruciform joints indicated that the presence of a weak zone in the load path tended to concentrate strain in this zone and to initiate failure there. This strain concentration was located in the HAZ at the finger ends, with significant thinning of the material predicted before failure.

A simple model for the behaviour of welded cruciform connections with a reduced-strength zone has been developed and a notional critical load based on a limiting strain has been introduced to predict the ultimate loads. A parametric study shows that the single most important factor affecting the ultimate strength of these connections is the material strength in the OA zone.

The experimental and analytical results have been collated and presented in the form of guidelines and recommendations for the design and analysis of load diffusion joints in the Al-Zn-Mg alloy.