Making aluminium alloy 7075 weldable

Materials World magazine
,
27 Feb 2019

Aluminium alloy 7075 can now be made weldable to remove the need for joins and reduce weight. Rhiannon Garth Jones reports.

Lightweight materials are central to industrial manufacturing because they reduce fuel consumption and carbon emissions. From aeroplanes and rockets to cars and even the trusty bicycle, lightweight materials are a focus for innovation and research.

That’s why the latest approach from the UCLA Samueli School of Engineering, USA, is exciting. The strong, lightweight nature of aluminium alloy 7075 (AA7075) would make it ideal for many industrial applications but, since its discovery in the 1940s, it has been unweldable. Although still used, it is necessary to create joins using rivets and bolts, which reduces the overall strength of the material, as well as adding extra weight and manufacturing time.

The team at UCLA claims its new nanometallurgy approach has solved this problem and it is already in use. Comprising aluminum, zinc, magnesium and copper, the elements in AA7075 normally flow unevenly when it is heated during welding, creating cracks along the weld and making it ineffective.

UCLA’s approach infuses titanium carbide (TiC) nanoparticles into the AA7075 welding wires, which are sandwiched between the two pieces being joined. The TiC nanoparticles exhibit fine globular grains and a modified secondary phase, which eliminates the tendency of AA7075 to crack when being welded. They also show exceptional tensile strength in both as-welded and post-weld
heat-treated conditions.

The result is welded joints with a tensile strength up to 392MPa – twice as strong as other alloys routinely used in aircraft and automobile parts. Further improvements being worked on aim to bring the joints to 551MPa, which is comparable to steel.

‘This rather simple twist to the filler material of a fusion weld could be generally applied to a wide range of hot crack susceptible materials,’ according to the team. UCLA Raytheon Professor of Manufacturing and paper co-author, Professor Xiaochun Li, believes the study ‘just scratches the tip of an gigantic iceberg’ in the power of nanometallurgy to overcome some long-standing metallurgical barriers. He said they are ‘already working on developing nanotechnology-infused welding wires and materials to solve all other long-standing welding/joining problems’.

Li also believes ‘the technique will allow widespread use of AA7075 and other difficult-to-weld alloys in mainstream applications, such as cars or bicycles’ very soon. The team is currently working with a bicycle manufacturer on prototype bike frames, for instance.

Acknowledging the need for more practical testing and proof of the weld reliability during long-term services, Li said ‘it may take some time to fully certificate the welding products for long-term use. But we are very optimistic that this technology will have industrial impact and penetrate market rather quickly’.

Moreover, he said they have been conducting R&D to address concerns about the expense of implementation for the past 15 years. The key factor is the relatively high cost of nanoparticles. However, he stated, ‘Fortunately for welding wires, only about 1% or less of nanoparticles is needed in the metal wire, thus the added cost is minimal. Even at today’s financial analysis, the cost is not a limiting factor for the novel welding wires. At the beginning, the cost of the revolutionary welding wires could be about two to three times as expensive as the regular welding wires but it is well-justified, as it solves the problem that traditional wires cannot tackle.

‘It is expected that, in a few years, the cost will be in a similar range to traditional products since the new way of producing nanoparticles will become much more economical. I have every reason to believe that the new welding wires will prevail in the practical world very soon,’ Li said. If he’s right, it is very good news
for industrial manufacturing.

Read Nanoparticle-enabled phase control for arc welding of unweldable aluminum alloy 7075, published in Nature Communications, here: go.nature.com/2SDZXxk