A look at modern methods of metal forming

Materials World magazine
1 Feb 2019
AFRC Rotary Friction Welding team

Modern day metal forming has come a long way from the traditional hitting with a hammer. Ellis Davies takes a look at modern methods at the centre of forming research and development in Scotland.

UK manufacturing is somewhat in flux. 2017 saw a 4.1% increase on manufacturing sales from 2016, but the final five months of 2018 posted a decrease, with the quarterly rate of GDP growth falling to 0.3% –  the weakest in six months. However, this shouldn’t be something to panic about just yet, particularly with the high value manufacturing catapult (HVMC) totalling £617m in assets in 2017-2018.
There is just one HVMC centre in Scotland – the Advanced Forming Research Centre (AFRC), which is part of the University of Strathclyde, UK. Based just outside Glasgow, the centre houses a range of forming equipment along with a series of testing labs and event spaces. Standing alone as Scotland’s only physical link to the HVMC, the centre approaches its work with a regional focus, helping to support Scottish manufacturing at all levels. Materials World paid a visit to the centre to see first-hand what is being developed.
Research on an industrial scale
In the manufacturing sector, downtime is a killer. Any period of time spent with the machines switched off costs money, so testing of new methods or equipment is a costly exercise that most, particularly SMEs, cannot afford or do not wish to stump up the cash for. 
This is an area in which the AFRC bridges a gap. The facility houses full industrial-size forming cells, with equipment such as a screw press that is the same size and model that you would find on a manufacturing floor. Therefore, work concerning new uses of equipment can be done at the level of real-world manufactures, reducing the usual time it takes to scale-up from laboratory research and testing. And, with the tasks being carried out at the AFRC, manufacturers are free to continue production using their current methods. 
Don’t stress
The ability to use full-scale equipment also helps the centre with one of its key focuses – residual stress. This is present in all materials and remains even after all external loading forces have been removed. These stresses can become an issue when machining a material, as the cutting process can exacerbate it and cause distortion in the final product. Typically, to compensate for distortion and residual stress, manufactures make parts that carry one-third unnecessary weight to prevent distortion during tooling and use. Saving this weight  is ideal for sectors such as aerospace, with one example coming in the form of an airplane rear wing, which when tooled by researchers at the AFRC following a prediction of the residual stresses in that part, was significantly smaller and lighter than those parts currently used. 
The team, headed by Salah Rahimi, has worked on predicting residual stress through testing and modelling using both destructive and non-destructive methods, including X-ray diffraction, electronic speckle pattern interferometry, hole drill and contouring using a CMM  machine. 
The most comprehensive technique/process/approach, to contour with CMM, requires fully destroying the part. But this gives the widest data set, so can be preferable when it is better to lose one part to testing than several to incorrect tooling. Contouring gives a spread of data regarding the residual stress of the part, which can then be fed into predictive models to inform the performance of the component in application conditions, such as high temperature environments, and show how best to tool it to make it lighter without causing distortion. The team says its models achieve 85-90% accuracy in residual stress predictions, which is enough to inform the tooling process. 
Ultimately, recognising that residual stress can be managed, rather than seen as a certainty in the tooling process, is something Rahimi and his team are keen to press. Predictions and taking steps to tool in ways that avoid distortion can save materials, time and money for manufactures. Going forward, AFRC is moving into ultrasonic testing for residual stress, which it hopes will provide detailed data without the need to destroy a part. 
Eric Galloway of MacTaggart Scott and a motorImproving manufacturing machinery 
As the AFRC is not in the business of manufacturing, its forming equipment does not need to continuously run, and can therefore be tinkered and experimented with. For example, the centre has done work on hybrid processing – fitting existing equipment with new functionalities to increase its productivity. In one case, a tooling machine at the centre was combined with metal additive manufacturing (AM) capabilities, to create a machine able to both tool down and add material using AM. So far, it has been used to repair worn dies for forging, but the team has also pegged it for possible applications in the oil and gas industry because of the need for components with long operational lives that avoid the cost and labour of replacement in hard to reach places, such as under water. 
The centre has also carried out work on its in-house forging cell. What started as a manually operated cell is now fully automated using a robotic arm. The arm is able to transfer material from the furnace to the 2,100t screw press, where it is shaped before being passed over to a clipping press to be trimmed of excess material. 
Further improvement can be seen at the multi-forge station. Originally, the forge was driven by a wheel mechanism, but has now been upgraded to run via two servomotors. This gives control of the press stroke and provides up to 5,000kN of grip load and 3,500kN upsetting force. 
Maximising material use 
It is apparent that a big focus for the AFRC is making forming methods and technologies more efficient and sustainable. To this end, much of the equipment on-site allows for the most efficient use of material when forming. Perhaps the best example of this is flow forming. This involves decreasing the wall thickness of a disk of material using a series of rollers, which stretch out the disk, vertically or horizontally, to become a pipe. It creates cylindrical components of varying sizes and thickness for a variety of applications in sectors such as aerospace. 
The big headline for this technique is its time and material savings. Other methods of forming similar products lose around 92% of the material, according to the AFRC, and can take days to fabricate – a part for an aeroplane turbine takes around 45 minutes to form. Also, flow forming allows a much higher material retention, of around 70%. 
The centre has applied this technology to real-world applications, including those afflicting Safran Landing Systems. The company, which produces landing and braking systems for civil and military aircraft, approached AFRC concerning the material wastage and long lead times in the manufacturing of their products. Trials to make methods more economical were conducted using the WF STR600 flow former machine at the AFRC site, testing a variety of materials and heat treatments. These trials found that if flow forming was implemented into the company’s manufacturing methods, costs would fall by 50% due to a greater use of the materials. 
New additions in welding
Since its inception in 2010, the AFRC has had a number of new additions to its forming workshop, and the facility has been shifted around frequently to make space for more equipment. During Materials World’s visit, a new piece of machinery was being unloaded and brought into a recently cleared space at the back of the workshop. It was a 300t rotary friction-welding machine – the larger sibling to the centre’s current 125t version. 
Rotary friction-welding is a solid state joining process that involves two parts, one being spun at high speed while the other remains stationary. The parts are brought together and the friction generated from the spinning heats both to the point where they begin to weld together. Once this happens, the spinning stops and the parts are pushed together and left to cool. 
This method of welding is particularly useful when attempting to connect two parts of different materials, and when welding super alloys that don’t usually form good bonds. It also creates a clean join because during the process the weld forms outwards, pushing any impurities on the surface away from the core. The process does not require any consumables and is quick and inexpensive to carry out. 
Although this is not a new technique, the AFRC has been looking into further uses for the technology. One such application could be for space shuttles and other exploratory vessels, where a certain area needs to break off at a given time. The welds can be engineered to fail under certain conditions, such as particular temperatures, therefore allowing a vessel to shed its excess weight without the need for extra mechanisms. 
Rotary Friction Welding machineForging forward
Lightweighting is also an area the centre is branching into, having recently begun work on opening a new facility specialising in this. The new building will house the AFRC’s lightweighting team, around one mile away from the main building, and focus on projects chiefly in the aerospace sector, as a number of relevant companies are also based in the surrounding location . 
Plus, the AFRC site itself is expanding with the looming addition of FutureForge, which is to be the ‘world’s most advanced hot forging research platform and will include a one-of-a-kind, industry 4.0-ready, demonstrator,’ according to the University of Strathclyde. The project will work with companies in the aerospace, automotive, oil and gas, energy, nuclear and rail industries to research and test new equipment and processes that will be immediately 4.0 compatible. 
Scotland’s Minister for Trade, Investment and Innovation, Ivan McKee, said on FutureForge, ‘The new facility will put Scotland at the forefront of the latest industrial revolution, helping some of the most traditional manufacturing businesses and their supply chains embrace the latest in digital technologies. [It] highlights, once again, the importance of Scotland as a centre for cutting-edge manufacturing technology, and demonstrates our world leadership ambitions.’
The project has received £16.5m from the UK Aerospace Research and Technology Programme, Scottish Enterprise and the HVMC, and will begin operations in 2020. It is predicted to generate around £40m of new collaborative R&D projects over 10 years. 
Overall, the AFRC is helping to keep Scotland in the conversation concerning high value manufacturing, and pushing the forming industry forward to be more competitive and compatible with future technologies.