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Driving deposition

A high-speed laser material deposition process developed by Fraunhofer ILT has the potential to capture a portion of the chrome plating market worth €2 billion, according to researchers at the institute. Known by its German acronym EHLA, the new laser cladding technology can reach a deposition rate of up to 500 metres a minute, compared to 0.5 to 2 metres per minute for standard laser metal deposition (LMD).

The technology was developed at Fraunhofer ILT in Aachen and RWTH Aachen University to overcome some of the drawbacks of both conventional cladding methods and laser cladding in an economical way.

While standard LMD injects a powdered filler material into a laser-induced melt pool on the surface of the component, in EHLA the powder is injected directly into the laser beam, causing it to melt before it reaches the pool. By not having to wait for the powder to liquefy in the melt pool, the processing speed is dramatically increased to 500 metres a minute. Such speeds mean the process can be used for large-scale component coating, including those that aren’t currently coated because of their size.

One common cladding technique is hard chrome plating, where chromium from a chromic acid solution is deposited on components in an electrochemical bath. The process is far from optimal, as the hard chrome layers are known to delaminate easily, and microcracks can form. The process also consumes a lot of energy and the chromium (VI) material used has such a negative impact on the environment that, as of September 2017, the substance has been added to an EU directive, meaning it can be only used with special authorisation or a permit.

Alternative coating methods such as thermal spraying, where only about half of the material used ultimately forms a coating, and conventional deposition welding, where multiple layers are required to counteract any mixing between base and cladding layers, also have drawbacks. Even LMD, while enabling thinner layers to be used, has proven too slow for use on large components, as the low surface deposition rates make it suited only for certain anti-corrosion and wear applications.

In addition to addressing the speed issues of both conventional and laser cladding methods, the EHLA process also solves the inefficiencies of spray technologies by using approximately 90 per cent of the cladding material, making it far more resource-effective and economical. Also, the layer formed by the new method is particularly dense, meaning only one layer gives sufficient protection. Complex pre-treatment of the substrate is no longer needed either, as EHLA is able to form a firm bond between the coating and substrate. Further benefits include being able to form smoother, thinner layers than standard LMD, 10 times less rough and measuring 25 to 250µm thick – previously, layers less than 500µm were not possible at high deposition speeds.

‘A significant advantage [also] lies in the heat input,’ explained Thomas Schopphoven, head of productivity and system technology at the LMD group at Fraunhofer ILT, and one of the creators of the new process. ‘Using EHLA shrinks the heat-affected zone by a factor of 100, from between 500µm and 1,000µm in conventional LMD down to just 5-10µm.’ EHLA thus makes it possible to coat heat-sensitive components, which excessive heat input had made impossible up until now. ‘This new process can also be used for entirely new material combinations, such as coatings on aluminium base alloys or cast iron,’ said Schopphoven.

As well as the mechanics offered by EHLA, the process also has a new powder feed nozzle that can operate up to 10 times faster than those used in conventional LMD.

According to Schopphoven, this new nozzle had to meet certain requirements in order for EHLA to be viable for industry: an adjustable powder gas beam caustic for optimal injection of powder into the laser beam; the nozzle had to generate a dense powder gas jet to maximise powder efficiency; consistent powder gas jet quality to give flow rates exceeding 500kg; and the nozzle components had to be replaced easily.

EHLA is already being used by certain companies, such as Dutch firm IHC Vremac Cylinders, which uses it to coat hydraulic cylinders 50cm in diameter and up to 10 metres in length for off-shore applications. Additionally, in close cooperation with Acunity from Aachen, a spin-off of Fraunhofer ILT, Hornet Laser Cladding, another Dutch company, delivered the first EHLA system to China for use in both research and industrial applications at the Advanced Manufacture Technology Center of CAMTC in Beijing. The augmented capability of the new feed nozzle has also generated interest at UK organisation TWI, which wants to develop it further.

‘EHLA’s application potential is enormous,’ concluded Schopphoven. ‘In 2015, the worldwide market for hard chrome plating was estimated at $13.64 billion, while the market for thermal spraying amounted to $7.56 billion. If EHLA could capture a 10 per cent share of the surface refining market this new process could account for an annual market volume of €2 billion.’ EHLA also has the potential to ensure that coating jobs remain in Europe, according to Fraunhofer ILT, countering the trend of such jobs being outsourced to low-wage countries. 

Competitive cladding

The types of laser used in cladding have evolved in recent years, according to Carl Hauser, an additive manufacturing consultant at TWI, making the transition from CO2 to fibre lasers, then to disk lasers, which are currently favoured by TWI, and now to diode lasers, which are increasingly being adopted in cladding because of their low cost and flexibility.

‘The beam quality [of a diode laser], however, hasn’t traditionally been as good as a fibre laser, but it has been improving,’ said Hauser, who would like to see a full study on the advantages of diode lasers for laser metal deposition. ‘There hasn’t been a solid systematic study on the merits or the difference between a fibre and a diode laser. At the moment their particular benefits are unclear, as some lasers are better for processing certain materials than others.’

While there is some competition between laser technologies in the cladding market, Frank Gaebler, director of marketing at Coherent, observed that the real battle taking place is actually between laser technologies as a whole and the conventional cladding methods still used, such as arc and spray-based technologies.

‘The costs of these traditional cladding methods are comparably smaller than using lasers. However, with the decreasing investment prices of fibre and diode laser technologies, the lasers could capture market share,’ Gaebler said.

Traditional cladding methods, while being more cost effective, often result in mixtures between the original base material and the cladding material, which can prevent a pristine surface being created and lead to more layers being needed.

EHLA can be used to coat piston rods with a length of up to 10 metres. Credit: Fraunhofer ILT

‘With laser cladding it’s not necessary to add these extra layers, as very little mixture occurs with the base material; this saves a lot of time in the process,’ said Gaebler. ‘The other advantage is the reduced thermal impact offered by lasers, which provides a higher quality cladding and avoids the distortions seen when using arc-based technologies.’ 

Cost over quality

Fibre lasers and disk lasers, while offering a better beam quality than diode lasers, do not necessarily make the best tools for cladding, according to Gaebler. ‘Laser cladding doesn’t necessarily require a high-quality gaussian beam; you can get away with a top hat profile, which may actually be preferred for some applications,’ he said.

Surface treatments can range from processing in the millimetre range, to applying coatings to very large surfaces found on oil and gas shafts and hydraulics. ‘As a laser manufacturer you have to be able to deal with both types of application: the large area cladding with high deposition rates at very high speeds, plus the fine detail cladding,’ said Gaebler.

Coherent has therefore chosen to follow the transition to diode lasers and supply both fibre-coupled and free space sources – rather than fibre or disk lasers – for cladding, emphasising the benefits of a lower cost of ownership and simpler design over higher beam quality. ‘Most cladding applications can be done with diode lasers,’ commented Gaebler. ‘We think they’re the best choice for this application.’

The lack of fibre coupling in Coherent’s free space diode offering, the HighLight D-series, enables it to clad large areas at high speeds, making it suitable for applications in the oil and gas or mining industries. Alternatively the company’s fibre-coupled DF-series is better suited to performing finer-detailed cladding applications in multiple directions, making it ideal for repairing tools.

Engineering manager Andres Veldman from IHC Vremac Cylinders and Fraunhofer ILT's Thomas Schopphoven (left) prepare the way for EHLA in series application. Credit: Fraunhofer-Gessellschaft

‘You cannot have this variety with fibre lasers,’ said Gaebler. ‘You would have to design an optic that takes the smaller beam of 100 to 400µm and spread it out over a beam profile of around 6 x 36mm, which is expensive. With direct diode lasers you can simply get this beam profile out of the array; it’s much simpler to implement.’ Coherent is now considering higher power fibre-coupled diode lasers for cladding, improving on its current 6 to 8kW offerings.

Despite their processing capability already being sufficient for most cladding applications, certain optics are currently being explored in industry in order to enhance the beam quality of diode lasers, according to Gaebler. The market is yet to determine, however, whether these optics could be more beneficial than the fibre technologies already in use. These developments are being made on top of the general ongoing improvements in lasers in terms of footprint, cost of ownership, serviceability and lifetime. 

Improving all round

The laser source is only one part of a cladding system. Gaebler commented: ‘Looking at the cladding process itself, most things are happening in the head, with more sensors [being added].’

With the upcoming demand for smart manufacturing in Industry 4.0 environments, there will be an increasing need for laser companies to offer additional sensors that monitor laser parameters – such as beam quality and power measurement – and convey them to machines that offer feedback about the status of the equipment. Automated predictive maintenance could ensure that end users are achieving the maximum uptime with their products.

Improvements are also having to be made in cladding software, according to Hauser of TWI, as the complexity of surfaces in need of processing is increasing, which has proven particularly challenging with the current programmes available.

‘One of the issues is that the software used isn’t advancing as much as it perhaps should at the moment,’ he said. ‘There are key software houses working to produce software for laser cladding, but because of the diversity of laser metal deposition – it’s used for 3D printing, coatings, repair, etc – there’s no software package that’s available for everything.’

The challenge, according to Hauser, is that a lot of the software currently in use has stemmed from traditional laser marking and cutting software. While this makes it well suited to working with single layers of material – marking and cutting are often only done on a single plane – the multiple and often differing layers of surfacing and 3D printing can be very challenging to prepare tool paths for. ‘That’s where some of the current available software falls down,’ Hauser confirmed. ‘It’s very time-consuming and isn’t very automated.’

TWI has therefore taken the decision to write its own software in order to address the increasing variety of applications arising.

‘We’re looking at improving the automation of our software, putting our know-how into the technology and into the software as well, so it’s a more automated environment to generate tool paths for the process,’ Hauser explained. ‘This is more of a research tool for us at the moment rather than a commercial product that we’re offering.’

While TWI’s software is not currently available on the market, Hauser suspects that in the future, after having worked with clients who then want to take up the firm’s additive technology offerings – including its software – TWI could end up supplying the software or at least supporting it for the clients it’s worked with. 

Lasers for repair

The complex surfaces mentioned by Hauser are occurring at an increasing rate in tool repair – one of the growing applications of laser cladding that extends tool lifetime and reduces overall process costs.

‘We’re getting quite a lot of interest now in a range of industries, such as aerospace, oil and gas, and power generation, who are all interested in the repair of high value components, such as turbines or tooling,’ said Hauser. ‘We’ve also developed methods to deposit mixed-material systems onto the edges of box cutter blades, which are high volume, low value products.’ Hauser believes that the number of repair applications involving these lower value, higher volume products will increase in the future, aided by the generally decreasing cost of laser hardware.

The type of laser used for repair can vary depending on the tool being processed. For small tools, very low power pulsed lasers, YAG lasers or QCW fibre lasers in the order of 150 to 300W are favoured, according to Gaebler of Coherent, whereas for the very large tools used in automotive, fibre coupled diode lasers are more suited to addressing the multi-directional cladding that’s required.’

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