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Joining technique could improve design optimisation

A new method of creating an extremely strong bond between lightweight aluminium and ultra-high-strength steel could help car manufacturers optimise vehicle design.

The new method from Brigham Young University’s (BYU) School of Technology may be helpful to vehicle manufacturers in achieving the 54.5 miles per gallon average the US Environmental Protection Agency is mandating for US fleets by 2025.

The technique, called friction bit joining, uses a small, consumable bit to create a solid-state joint between metals. The method was invented by manufacturing engineering technology professor Michael Miles and retired BYU professor Kent Kohkonen, in collaboration with Orem, Utah-based company MegaStir Technologies and Oak Ridge National Lab.

‘It’s all about making vehicles lighter and our process can help to combine steels and light metals in the same vehicle frame, which gives engineers more flexibility in designing an optimal structure,’ Miles said in a statement.

The latest development in the process successfully bonds lightweight aluminium with cast iron by inserting a thin layer of steel between the two metals, which facilitates the bonding process.

Findings of this research, carried out in collaboration with the University of Ulsan in South Korea, are published in the June issue of the International Journal of Precision Engineering and Manufacturing.

‘The motivation to make cars lighter was already there with a previous EPA mandate [34.5mpg by 2016], but that motivation has now increased with the latest mandate,’ Miles said. ‘Our process is a technical success in the effort to spot join dissimilar metals together; now we need to go forward with our partners to make it commercially viable.’

Currently, the automotive industry uses resistance spot welding to join steel stampings together into a completed body. In recent years, some aluminium parts have been introduced into the vehicle structure using a mechanical fastening method called self-piercing riveting. While this approach works to join lower strength steels with aluminium, it isn’t suitable for joining aluminium to ultra-high-strength steel.

BYU’s friction bit joining method has received funding from the US National Science Foundation, the US Department of Energy, the state of Utah and some auto suppliers in South Korea.

Applications for the process include areas of the vehicle frame where ultra-high-strength steel needs to be joined to a light metal.

For example, a vehicle manufacturer may want to use aluminium for the roof of a car while using ultra-high-strength steel for the A and B pillars of the frame that connect with the roof. Another example includes the incorporation of lighter weight metals on the interior parts of the car door.

Miles said the ability of friction bit joining to produce ‘incredible strength between two dissimilar metals’ will eventually benefit both automakers and other industries, like aerospace.

Readers' comments (2)

  • How do they deal with the aluminium oxide in the weld zone?

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  • We sent your query to BYU and this is the reply we received from Mike Miles: Aluminum oxide is not really an issue, but galvanic corrosion is. So the joint would need to be coated in order to prevent moisture from reaching it.

  • Galvanic corrosion problems would add argument to scrapping older vehicles. How would the failure be monitored? and at what cost?

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  • BYU's Mike Miles has provided this reply: Development work on dissimilar metal joints will need to include extensive corrosion testing, so that the life of the joint is known in advance. A similar issue exists for aluminum/steel joints where self piercing riveting is used. There are some vehicles using this technology, which functions quite well when the tensile strength of the steel is not above 800MPa. In these cases, there are coatings which protect the joint from moisture and limit the amount of corrosion over the life of the vehicle. So there is no effort to monitor the failure of these joints; they need to be sufficiently robust from a strength, fatigue, and corrosion perspective, so that monitoring isn't needed.

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