Friction stir welding of steel

Within the EU-funded project 'High integrity low distortion assembly' (HILDA), there has been groundbreaking progress in developing this welding technique for steel. Project partners have shown that FSW is capable of welding all grades of steel used in the shipbuilding industry.

Significant efforts have been devoted to mechanically testing friction stir welds in steel grade DH36. Microstructural characterisation was the first experimental step towards establishing the limits of the FSW process.

A secondary outcome of the microstructural evaluation was information on undesirable process-induced defects that could compromise the integrity of the weld. The absence of flaws provides reassurance that the selected welding process parameters led to an acceptable level of quality.

Selection of the welding parameters is a complex process with many interdependent variables that are poorly understood.To fill the knowledge gap, microstructural and global modelling were combined to provide insights into the most important welding parameters as well as for predictions of post-weld conditions.

The aim of the project is to develop the scientific foundation for a reliable friction stir welding process applicable to steel. HILDA will improve the metallurgical knowledge to predict phase changes and stress regimes in the welded condition. This will deliver a cost effective, low distortion welding process for EU shipyards.

The main challenges to be solved for a wider application of FSW to steel include improvements of productivity (welding speed, tooling endurance and cost), an extended scope of application (weld geometry, steel grades and thicknesses) as well as the development of stable and predictable processes and a quality assurance strategy.

HILDA has demonstrated that butt welding of steel is possible, and process considerations indicate that the ability to weld in other configurations, for example lap and T geometries, already possible in materials such as aluminium, should also be possible in steel if an appropriate tool material and tool design can be achieved.

Comparison has been made of the costs and benefits of steel FSW in its current immature state with the costs of existing fusion welding techniques in steel, primarily MIG, SAW and laser hybrid welding. The opportunities for cost reduction, safety improvement and enhanced weld properties that friction stir welding ought to provide are identified and potential disadvantages noted.