ANALYSIS: AEROSPACE
Removing rivets for lighter flying
Greg Blackman on the benefits of laser welding titanium aircraft components
Aerospace manufacturers are looking to lighten the weight of aircraft, and laser welding has been shown to be a promising fabrication route when working with titanium. Chris Allen of TWI presented some results from the Innovate UK-supported Oliver project during the recent Industrial Laser Applications Symposium (ILAS). ‘Laser welding opens up
new design opportunities that are not necessarily available with other joining processes, because of some of the more novel joint configurations that can be considered,’ he said during his talk. Oliver – standing for ‘Optimised laser welding implementation via enabling research’ – brought together aerospace supply chain firms CAV, Leonardo and Tisics Titanium Composites with research partners, including
14 LASER SYSTEMS EUROPE SUMMER 2021
TWI, to investigate laser welding lightweight, near-net-shape titanium alloy and titanium metal matrix composite assemblies. One of the aims of the project
was to replace rivet joints with laser welds in thin-sheet structures that make up the nacelle bulkhead, a housing structure, and the firewall that is fitted around an aircraft engine. Both are currently made from overlapping titanium sheets and stiffeners, riveted together. The project demonstrated that rivets can be replaced with laser welds, which reduces the weight of the part. Testing showed that joint fatigue can be equalled or even bettered compared to rivets, and that manufacturing complexity and therefore cost can ultimately be reduced with laser welding because there are fewer processing steps. TWI applied lap joints to a four-part coupon structure
consisting of overlapping joints between four sheets of titanium. This structure was put forward by Leonardo to carry out fatigue tests. The joins were made at TWI using conventional laser welding equipment, along with IPG’s seam stepper welding head – both methods gave similar results. Allen reported: ‘The fatigue
life of welded joints was found to be an order of magnitude longer than riveted joints for a given loading scenario.’ He also said that modulating
a pulsed laser beam ‘was very successful in removing some of the porosity issues we had when not modulating’. For thin-sheet butt joining, modulated QCW welding of 0.9mm titanium at 1m/min gave lower porosity than CW welding with beam wobble at 5m/min. In an earlier ILAS
presentation, Dr Mohammed
Naeem at Prima Power Laserdyne, USA, spoke about the benefits of pulse shaping to reduce weld defects such as porosity. He said that when working with titanium components, one of the fundamental problems for aerospace firms is eliminating porosity for partial penetration welds. However, he said that porosity can be eliminated at the root of a partial weld by shaping the pulse. ‘Aerospace has very strict specifications, not only on porosity but the weld shape, the weld width, penetration [and others],’ he said. Naeem went through a number of different pulse shapes that can be employed, depending on the material and structure. He said that a ramp- down shape gives good control on the solidification process preventing internal defects, which is useful when welding
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