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Friction stir welding the next generation of lightweight vehicles

TWI is working with a consortium of several organisations to develop new strategies of manufacturing aluminium alloy pressed parts for the transport industry. Funded by Innovate UK, project LightBlank is showing how to integrate friction stir welding (FSW) with advanced stamping techniques like hot form quenching (HFQ). The main objective is to reduce the cost of producing structurally optimised pressed aluminium alloy parts for the automotive, rail and aerospace sectors.

Manufacturers are being challenged by progressively stricter carbon dioxide emission regulations and an overall demand for more efficient and lower consumption vehicles. One of the cornerstone measures to tackle climate change was announced by the EU in 2009, committing car manufacturers to cut the average carbon dioxide emissions of new models from 130g/km in 2015 to 95g/km by 2020. Even lower targets are anticipated.

One of the most immediate ways to decrease fuel consumption and emissions is to reduce the weight of the vehicle. Designers increasingly rely on numerical modelling to determine the part load-bearing topology and identify potential weight savings.

Tailor welded blank (TWB) fabrication is commonly used to shape stamped parts according to the predicted stress distribution. Tailor welded blanks are semi-finished parts typically produced by joining sheets with different thicknesses or different alloys that are subsequently formed to the final structural shape. Their use in the automotive industry has increased significantly since the 1990s as a method of minimising part weight without compromising the structural integrity and crashworthiness of the vehicle.

Aluminium alloys are often the commercially viable choice to replace formed steel parts in car body structures and reduce overall weight. However, joining aluminium alloys sheets of dissimilar grades or thicknesses to produce TWB can involve numerous challenges. Designers typically expect reduced mechanical strength in the joint and heat-affected zone when fusion welding aluminium alloys. This decrease in strength hinders the weight reduction exercise, as the components are typically manufactured thicker to compensate for any local reductions in strength caused by the welding process.

Hot form quenching is a pioneering technology developed by the Imperial College of London which allows the formation of aluminium alloy TWB parts of higher strength and complexity. Hot form quenching combines solution heat treatment, forming, quenching and artificial ageing. Deeper draw depths can be achieved while restoring the parent material properties. However, early research has shown that fusion welds often struggle to withstand the elongation and strain rates required for some of the more ambitious geometries.

Fig. 1: Cross section of an FSW joint between AA6082-T6 plates with thicknesses of 3mm and 5mm.
Fig. 1: Cross section of an FSW joint between AA6082-T6 plates with thicknesses of 3mm and 5mm.

Friction stir welding overcomes many of the problems associated with joining aluminium using traditional techniques. Invented at TWI in 1991, it is fast becoming the technique of choice for manufacturing lightweight transport structures. Preliminary findings of the project demonstrate that TWB panels joined by FSW exhibit the ductility required to press more complex geometries. As such, the combination of HFQ and FSW provides new opportunities for both technologies. Currently, complex parts are superplastic formed and then joined. The LightBlank consortium is demonstrating how such parts can be joined as flat blanks and then formed, removing the costs associated with complex welding fixtures.

Initial findings indicate that replacing a conventional cold-formed aluminium alloy front door inner panel with an optimised friction stir tailor welded blanks formed by HFQ could reduce its weight by at least 200g. Considering a typical automotive yearly production rate, there is a potential to save 120 tonnes of material per year. For the average petrol car, this means 385 tonnes of carbon dioxide emissions will be prevented and 14,480 litres of fuel will be saved each year. Discussions with rail and aerospace partners have suggested similar opportunities.

LightBlank’s aim is to develop and fully implement a UK-based supply chain to manufacture aluminium alloy friction stir tailor welded blanks formed by HFQ. The following partners are involved to develop industrially relevant case studies:

  • Impression Technologies Ltd
  • PAB Coventry Ltd
  • Morgan Motor Company Manufacturing Ltd
  • Bombardier Aerospace UK Ltd
  • Bombardier Transportation UK Ltd
  • Imperial College, London

TWI is contributing by assisting the design-for-manufacture process, developing effective FSW procedures and designing flexible tooling systems for rapid production.

For more details on the LightBlank project visit www.lightblank.com or email contactus@twi.co.uk.

Fig. 2: Small-scale automotive-based prototype, produced by forming a friction stir tailor welded blank.
Fig. 2: Small-scale automotive-based prototype, produced by forming a friction stir tailor welded blank.
Avatar João Gandra Principal Project Leader – Friction and Forge Processes

João specialises in friction welding processes, including Friction Stir Welding. His current role is to support TWI Member companies seeking to adopt these technologies to manufacture new or existing products. He acts as a consultant during product development, design-for-manufacturing, prototyping, technology transfer and continuous improvement. Most of his experience was gathered in the aerospace, rail and automotive sectors. Before joining TWI, João completed a PhD in Manufacturing and Industrial Management at the Technical University of Lisbon, where he also worked as part-time lecturer and researcher. He has published over 20 peer-reviewed publications and conference papers, actively participating in international standards committees like the ISO 25239 for Friction Stir Welding.

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