Greg Blackman on how titanium laser welding can be used to replace rivet joints in aircraft
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 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.
Pulse shaping to reduce weld defects
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 high-carbon steels, dissimilar materials, or crack-sensitive materials.
An enhanced spike shape is good for welding copper alloys and other reflective materials with a high conductivity, where there’s an initial high peak to couple into the material. Once the initial spike of energy starts melting the surface then absorption increases about 20 times, so the rest of the laser pulse energy can be much lower, Naeem explained. The spike is in the order of 0.5 to 5ms in duration.
Another beam shape is spike removed, which is useful for low penetration welds that require good cosmetic appearance. Ramp-up shape, which slowly increases the intensity in the welding pulse, is useful for materials with low melting points or low reflectivity. Allen said during his ILAS presentation: ‘You do need to think about the process before and while you are applying laser welding: how you do it and whether it’s economical to apply, and the performance is satisfactory.’
In the Oliver project Queen’s University Belfast carried out offline simulation and cost modelling of the processes, to make sure the fixtures were compatible with the welding equipment and the laser could access the areas it needed to access. The laser joining process was developed by TWI using IPG Photonics equipment, and the Manufacturing Technology Centre (MTC) designed fixtures and carried out non-destructive testing. Along with welding thin sheets, the group at TWI investigated joining thicker (4-6mm) pieces of titanium with titanium metal matrix composites (MMC). This meant a strut structure could be fabricated that would weigh significantly less than if the strut was made entirely out of titanium, Allen said.
The aerospace firms wanted to use near-net-shape manufacturing and assembly methods to cut down on the weight of these cylindrical struts. The Oliver project demonstrated that this can be achieved using laser welding. Furthermore, the low heat input of laser welding meant carbon-fibre reinforced Ti-MMCs could be incorporated in these assemblies, which, again, leads to weight savings.
A final demonstrator produced as part of the project were wing leading edge structures. These are also made from sheets of titanium, but the maximum width of the sheet is limited which, in turn, limits the maximum length of leading edge structure. The project developed high-quality laser butt welding procedures, which, with minimal additional processing, are then suitable for air flow surfaces.
Naeem, in his talk, commented that Prima Power Laserdyne is now looking to develop pulse shapes that will improve laser welding of additive manufactured components, including those made from titanium alloys.
Primus Aerospace invests in titanium additive manufacturing
Primus Aerospace, a contract manufacturing partner to aerospace, defence & space OEMs, has joined additive manufacturing (AM) firm Velo3D’s partner network by investing in a Ti6Al4V Sapphire metal AM system.
This is the first titanium-dedicated metal 3D printer from Velo3D that will be used solely for aerospace & defence applications. Primus has identified the system as a solution for many applications it currently serves in the production of cube satellites, hypersonics and turbine engines. The company is a top-tier supplier for the leading defence primes and the majority of aerospace OEMs, including Lockheed, Boeing, Northrop Grumman, General Dynamics, and Raytheon.
Equipped with two 1kW lasers and capable of printing 60cm3 of material per hour, the Ti6Al4V Sapphire can print low angles and overhangs down to zero degrees, as well as large diameters and inner tubes up to 100mm without the need for supports. This not only reduces the need for post-processing, but also overcomes the ‘45 degree rule’ for conventional AM, which recommends supports for any surface less than 45 degrees. This increases the wide range of designs that can already be developed with additive technology. The system offers a cylindrical 315mm diameter build chamber, available in 400mm and 1,000mm high configurations. In-situ metrology sensors are used to enable visibility into the quality of every layer of the build.
Engineers examine a titanium fuel tank printed with no internal supports. Such tanks/pressure vessels are designed for use in aerospace and defence applications. (Image: Velo3D)
The partnership with Velo3D will enable Primus Aerospace to deliver unique design freedom and high-quality AM services to its customers. Using the new manufacturing solution, Primus is looking to unlock powerful design and manufacturing capabilities that will enable the realisation of previously unattainable geometries and optimised solutions as well as the exploration of novel aerospace applications. ‘Primus is proud to be a leader in this manufacturing category,’ said Gary Vaillancourt, vice president of engineering and technical sales.
‘Our customers require maximum performance of their aerospace-related systems and, together with Velo3D, we can redefine what is possible in manufacturing through advanced AM technology.’
‘Primus Aerospace is an excellent partner for us with their customer focus, commitment to innovation, and adoption of leading-edge technology,’ added Benny Buller, founder and CEO of Velo3D. ‘Our capabilities will help them deliver to engineers and supply chain managers the part designs they want, not the limited part geometries that other commodity-AM suppliers say they can have. The synergies between our two companies will support developers of new products to optimise their designs without compromise or restraint.’
Primus Aerospace has now taken delivery of the titanium Sapphire System and will begin offering titanium printing at its facility located in Golden, Colorado.