Golden years of laser welding
Around 20 years ago, a technician clearing out a room in the laser laboratory at The Welding Institute (TWI) walked into Paul Hilton’s office and showed him a brown crumpled envelope. On the envelope was written: ‘World’s first laser cut’, and in it was a small piece of metal.
The first gas assisted laser cut was made in May 1967 at the Services Electronic Research Laboratory (SERL) in Harlow, near Cambridge, UK by the then deputy scientific director of TWI, Peter Houldcroft, with the assistance of ABJ Sullivan. TWI is also based near Cambridge in the UK. The work was published, but is hardly ever referenced. Houldcroft became aware that SERL was operating a 300W CO2 laser for military purposes in 1965, only two years after Patel demonstrated lasing action from CO2 gas. These early experiments may mark the start of laser material processing as it is known today.
Houldcroft and Sullivan used the slow flow CO2 laser to cut 1mm thick sheet steel with an oxygen assist gas. The laser could be pulsed at 100Hz with a maximum 300W of power from 10 metres of slow flow CO2 gas technology. The cutting head used an aluminised steel mirror, a common salt lens and a salt pressure window. The cutting nozzle tip was not unlike those in use today.
The observations made in the 1967 paper included: the kerf width depended on the focused spot size and not the gas jet size; the cut edges were free from microcracking; and negligible distortion was reported. It was also noted that the process induces no mechanical forces on the cut part. These early observations are the same features widely cited today as the benefits of laser cutting.
Keyhole laser welding, however, was the driver to develop the fast axial flow CO2 laser, as keyhole welding requires significantly higher laser powers than could be generated by slow flow technology. A team at TWI developed a fast axial flow test bed, employing a supersonic gas jet to homogenise the CO2 discharge, while at the same time cooling the gas. This resulted in powers of 1kW per metre of discharge – higher than expected.
A prototype 2kW laser was produced and, in 1970, the laser was used to demonstrate TWI’s first deep penetration keyhole laser weld. The work was never published. ‘We don’t know whether it was the first in the world,’ said Hilton, as scientists in the United States were also working on welding with cross-flow lasers at a similar time, but they didn’t publish either. Hilton, who is now technology fellow for laser materials processing at TWI, was speaking on 19 July at an event at the institute’s headquarters near Cambridge, UK.
‘The idea of the supersonic nozzle in the fast axial flow laser is fundamentally behind every fast axial flow laser ever manufactured,’ Hilton continued. ‘What’s significant about this is that, unfortunately, TWI didn’t patent either the fast axial flow technology or laser cutting. If they had, then I think the taps in the bathrooms [at TWI] would be gold-plated. So that’s a good message for everybody: if you’ve got a good idea, protect it.’
There are probably more than 10,000 fast axial flow CO2 lasers manufactured today. The original laser design was sold by TWI to the British Oxygen Company’s Industrial Laser Division, along with the manufacturing rights. Later, the fast axial flow design was taken up by companies including Electrox in the UK, Rofin-Sinar in Germany, Daihen in Japan, and PRC in the USA.
TWI then stopped developing lasers and by 1983 the institute had a 10kW cross-flow laser. With 10kW TWI could look at thick section keyhole welding. ‘It was fairly obvious that when you process with high power densities at 10µm wavelength then you generate very strong plasma and this can attenuate the beam. There are various absorption mechanisms going on,’ Hilton said. TWI used a directed jet of helium to reduce significantly the effects of the plasma, particularly at slow welding speeds.
Over the years TWI has continued to work in this area. By 1998 it combined three Nd:YAG lasers, each with 3.5kW power, along an optical fibre, which gave 9kW at the workpiece. At this time, no one had more than 3 to 4kW of power at 1µm wavelength, Hilton noted. What was seen at this kind of power intensity was something very different from what had been seen with CO2 lasers – no bright plasma ball but a fiery yellow plume, which still affected the weld quality. ‘We found that we could apply a gas jet to the side, blow the plume away, and you could get much better weld quality,’ explained Hilton. Heavy gases, such as argon, were much more effective than helium at 1µm and were able to produce good welds in 15mm thick steel.
TWI started to look more closely in 2002 at the interaction of a 1µm beam with steels using spectroscopy. It showed the advantages of plume attenuation in 2007 where it looked at the effects of weld quality in titanium and, by applying this technique, was able to get a much more stable weld with reduced occurrence of porosity. Plume attenuation allowed the institute to generate welds in 6mm titanium that were of aero engine quality standard. Later, in 2009, TWI refined the technique, using side and cross gas jets to remove the plume produced by a 4kW high brightness fibre laser, and achieved welds in 10mm steel that are almost indistinguishable from electron beam welds.
Welding takes off
In 1997, Airbus was looking at ways of making its aeroplanes more fuel efficient, by promoting lamina flow to reduce drag across the aerofoil of the wings and tail. To do this, Airbus needed to pump air out through laser drilled holes in various sections of the leading edge. The inside of the aerofoil was complex, and the inner sections needed to be laser welded into position to create leak tight enclosures for different air flow rates. TWI used a 2kW laser with beam delivery via a 1mm diameter optical fibre to a process head that was only 35mm in diameter, so that it could reach down into the depths of the component. ‘One of the welded sections was certainly flown in an aeroplane in 1998, an A320 used for trials,’ Hilton said. ‘It’s possible that this is the first time a laser weld had ever flown in an aeroplane.’
Around the same time, TWI was asked to investigate welding another complex geometry, a curved jet engine mixer designed to reduce noise. The constituent components needed to be trimmed and joined, which was achieved using a multi-axis CO2 laser gantry system to do both the cutting and the welding.
The engineers used wooden jigging and fixturing. The parts were arc-tack welded together and then laser tack welds were superimposed along all the joints. The 3D welding was accomplished using filler wire. ‘Even to date, this is one of the most complex structures I’ve ever seen laser welded,’ Hilton remarked.
Quite early on lasers were used for processing plastics; thin polyethylene welded very nicely and quickly with CO2 lasers at 10µm. Some plastics, however, transmit 1µm radiation and there are laser material processing applications that use the concept of transmission through one piece of plastic, and absorption in another, to make welds.
In 1999, TWI developed the ClearWeld technique for welding plastics. Here, a layer of infrared absorbing dye is applied between the two plastics being joined. The laser beam passes through one, is absorbed by the infrared dye, which heats up, melts the plastic and, with a degree of pressurethe joint is made. This technique has the advantage that welds don’t need to be made between clear and black polymers; any colour of plastic that sufficiently transmits 1µm radiation can be used. The technique is also applicable to welding technical textiles. TWI has produced a shirt in which all the seams and even the button holes are laser welded.
‘We can also do quite clever things with this technique,’ Hilton said. By coupling diode lasers, a beam comprised of two slightly different wavelengths can be produced. The dyes can be made selective enough that there can be one absorbing at one wavelength and transmitting at another. TWI put that idea into practice to make a triple bond in a tough technical textile. ‘When these [seams] were tested for helium tightness, we were told they were the best joint that they’d [the client] ever seen,’ Hilton said.
TWI has worked on several occasions using tailored energy distributions from diffractive optics, effectively to create a hologram of different beam patterns. TWI has designed diffractive optics to hit 16 soldered spots on an electronic circuit board simultaneously, with one laser pulse. ‘I don’t think anybody has done anything like that since,’ remarked Hilton. A similar technique was used to cut and weld thin plastic foils at high speed, simultaneously.
Work was also commissioned to find out whether it was feasible for a laser to weld an oil pipe 4.5km below the surface of the earth. The pressure can be enormous at these depths, and TWI was asked to find out if it was possible to weld at 500 bar. To do this, TWI designed a special pressurised system with a 15mm-thick sapphire window through which the laser beam entered. The team managed to melt metal at 500 bar, although didn’t produce keyhole welds. ‘At 500 bar, if you have a helium atmosphere, the change in refractive index alters the beam focus position by 5mm,’ explained Hilton. ‘If you put argon in there at 500 bar, where the density of argon is similar to petrol at room pressure, then that will change the focal length of the optical system by nearly 40mm. In addition, 15mm thick sapphire also has an effect on the focal position. So we had to get all that kind of thing right, otherwise we wouldn’t have any intensity at the workpiece.’ The team ended up with about 2.6kW of power in a 0.9mm spot.
Powering nuclear decommissioning
‘From 1967 onwards, most of the research done on laser cutting was to produce samples with a high quality cut,’ commented Hilton. This needs a close and small tolerance between the nozzle tip and the work surface. ‘If you throw away the requirement to produce a high quality edge, then you also throw away the requirement to have a high tolerance on the standoff distance. That is very interesting when it comes to aspects of nuclear decommissioning using laser cutting, when the cutting tool might not be able to get exactly to the right point on the component,’ explained Hilton.
In various projects from around 2010 onwards, TWI has been working on applying laser cutting for decommissioning. In 2014, this culminated in the size reduction of radioactive material, in the form of a Magnox fuel waste skip at Hinkley Point power station in Somerset, UK. The skips were divided into four sides and the base for further processing.
‘The material is 6mm thick on two sides, so it’s quite easy to cut, but we also have to cut what is called the furniture, on the other two sides, consisting of various welded strengthening sections and lifting points, which is more challenging,’ Hilton said.
In this work, the part to be cut was transported to the laser system in a purpose-built cell. For some radioactive components this is not possible and the alternative is to take the laser system to the part. TWI has a laser currently in operation at the Sellafield nuclear site in the UK, where it is cutting up a 1,300mm diameter dual-walled stainless steel dissolver vessel. The cutting head is attached to a snake arm robot from OC Robotics. The cutting is being performed inside a cell whose walls are 1m thick concrete. The dissolver has been in the cell for 30 years.