Lessons from HILDA : a large-scale experimental investigation of steel friction stir welding for shipbuilding

Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. Preliminary studies have identified many positive effects on the properties of welded steel components. However, the fundamental knowledge of the process in relation to structural steel remains relatively limited, hence industrial uptake has been essentially non-existent to this date. Wider introduction of friction stir welding of steel in industry will require that the process becomes economically and technically competitive to traditional fusion welding methods, a condition primarily expressed as high speed welding of acceptable quality within specifications. The European-funded research project HILDA (High Integrity Low Distortion Assembly), the first of its kind in terms of breadth and depth, is concerned with enhancing the understanding of the process on low alloy steel and establishing its limits in terms of the two more significant parameters which can be directly controlled, tool traverse and rotational speed. For this purpose, a large-scale microstructure and property evaluation of friction stir welded low alloy steel grade DH36 plates commonly used in shipbuilding and marine applications has been undertaken. In this comprehensive study, steel plates of 2000 x 200 x 6 mm were butt welded together at gradually increasing tool traverse and rotational speeds trialling the outer boundaries of the process envelope and generating an extensive data set to account for a wide range of typical and atypical process parameters. A detailed microstructural characterisation study has investigated the effect of varying process parameters on the formed microstructure, and assessed the quality of each weld. In parallel, transverse tensile tests were performed on samples from each set of weld parameters to determine their tensile properties. This work was complemented by Charpy impact testing and micro-hardness testing in various weld regions. An in-depth fatigue performance assessment of steel joints has been implemented by employing a novel set of experimental procedures specific to friction stir welding drafted in collaboration with classification societies. The relevant study correlated the weldments’ fatigue behaviour to microstructural observations, hardness measurements and fracture surface analysis. The testing programme has examined a wide range of welding parameters and developed a preliminary process parameter envelope based on the outcomes of the microstructural evaluation and mechanical testing. Initial process parameter sets have been identified which may produce fast (in the region of 400-500 mm/min) welds of acceptable quality; this is a step change improvement to the currently employed welding traverse speeds for this process, thus promoting its technical competitiveness to conventional welding methods. Moreover, this step change in the technical viability of steel friction stir welding is seen to improve the impact toughness of the weld without compromising strength and hardness, as demonstrated by the Charpy impact testing results and micro-hardness measurements. The typical fatigue performance of friction stir welded steel plates has been established, exhibiting fatigue lives well above the weld detail class of the International Institute of Welding for fusion welding even for tests at 90% of yield strength, irrespective of minor instances of surface breaking flaws which have been identified. Analysis of the manner in which these flaws impact on the fatigue performance has concluded that surface breaking irregularities such as these produced by the tool shoulder’s features on the weld top surface can be the dominant factor for crack initiation under fatigue loading