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Augmenting automotive production

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Keely Portway looks at some of the latest ways laser surface treatment is being used to optimise automotive production

Coating a brake disc via EHLA (Credit: Fraunhofer ILT/Volker Lannert)

While laser processing is already well-established in automotive production, each year new techniques are being developed to bring further benefit to this prominent market sector. Some of these techniques, such as those in metal 3D printing and lightweight materials joining, are enabling new structural designs to be considered by automotive manufacturers for the parts they produce.

What is equally important, however, is the optimisation of the current production processes and parts used in the automotive industry, which is what a number of new surface treatment techniques currently under development are looking to address.

 

 

Making wear-resistant hot stamping tools

For example, at the recent ICALEO 2019 conference held in Orlando, Florida, Felix Spranger and his colleagues at the department for welding technology at the German Federal Institute for Materials Research and Testing (BAM), were reporting on their use of laser surface treatment to address an ongoing issue regarding the usage of hot stamping tools in automotive production.

Hot stamping enables components with high rigidity and low weight to be formed from high-strength steel sheets – typically boron-manganese steel 22MnB5. The technique has been increasingly used by automotive OEMs in recent years to produce lightweight components in the body structures of their vehicles.

The laser implantation process principle (Credit: BAM)

The steel sheets involved are coated with an Al-Si layer in order to avoid oxide scale formation and ensure corrosion protection. However, according to Spranger, this coating, in addition the 800ᵒC temperatures that are required to form the steel effectively, lead to ‘massive adhesive wear on the forming tools used, which not only significantly reduces their service life, but also reduces the quality of the parts made using them.’

‘Therefore, a time and cost consuming rework of the hot stamping tools is required,’ he continued. ‘This has to be prevented, as in conventional press shops this is a major problem.’

The researchers plan to address this issue using a technique known as laser implantation, which involves the local dispersion of hard ceramic particles – made from titanium compounds such as TiN, TiC, TiB2 – within a surface using a pulsed laser, in order to create microstructures exhibiting wear-resistant properties. With it they are able to generate dome-shaped microstructures on the surface of stamping tools, which provide a hardness value up to 1,600 Vickers when TiB2 particles are used. According to Spranger, these microstructures have proven to be very effective at reducing the wear of forming tools.

Comparison of implanted tools and conventional tools prior and after testing under hot-stamping conditions. (Credit: BAM)

‘With suitable laser implanted surface textures we are able to improve the tribological behaviour in stamping, since we steer the material contact between tools and sheets onto these discrete points,’ he commented. ‘Abrasive wear is reduced thanks to the high hardness of the tool surfaces, resulting in a more economic forming process.’

Laser implantation is particularly innovative as it combines two distinct approaches that are normally used in surface treatment – surface texturing and material optimisation – into a single processing step.

‘That’s the new thing we are doing here – creating both texture and wear-resistant properties,’ remarked Spranger. ‘At BAM we have been developing this technique for four years, and are currently the only institute working in this field. With our colleagues from the Institute of Manufacturing Technology of the Friedrich-Alexander-Universität Erlangen-Nürnberg, we are developing this technique extensively for usage on hot stamping tools, since there is a strong need to improve their tribological behaviour. We see a large motivation for this application for users of forming tools in the automotive industry.’

To carry out the process, the researchers spray the hard ceramic particles – which are commonly below 10μm in size – onto the tool surface. They then use a pulsed fibre laser from IPG to disperse the particles locally, working with pulse durations between 1.5-15ms, pule powers between 60-180W and spot diameters around 100µm. ‘With these laser parameters we are able to create a wide range of surface textures, for example dome-shaped or ring-shaped structures,’ Spranger said.

A cross-section of one dome-shaped TiB2 implant manufactured on AISI D2 tool steel (Credit: BAM)

The variety of surface textures that can be created using laser implantation also makes the technique applicable to applications other than improving forming tools. For example, the ring-shaped structures that can be created could be used in mechanical applications involving lubricant, as the lubricant could be stored in the indents of the rings and then used to remove any wear debris from between the points of contact.   

‘We have a wide range of applications that we can cover with this technique,’ confirmed Spranger. ‘In addition to forming tools, we are also looking to target a range of other complex tools, such as cutting tools, as well as gears.’

Spranger and his colleagues will be looking to extend their project into industry in the coming years. ‘At the moment we don’t have any industrial partners, but we will certainly be looking for some to work with in the future,’ he concluded. ‘We are looking for both laser companies who can get this process industry-ready, and also end-users, especially in the automotive industry, who can begin implementing this process in real situations.’

Putting the brakes on abrasion

More than many other automotive parts, brake discs are subject to repeated mechanical loads. As a result, they produce fine particulate matter, which can pose an environmental concern. In an effort to help significantly reduce the generation and impact of such particles, the Fraunhofer Institute for Laser Technology ILT and RWTH Aachen University have together been working on a new coating process for brake discs.

Traditional brake discs are made of grey cast iron containing lamellar graphite phases. The virtue of this material lies in its good thermal conductivity and high thermal capacity, all for a relatively low price. However, it also has a strong tendency to corrode and experiences high material wear during service, which results in the substantial emissions of fine particulate matter.

The new coating process, known as ‘extreme high-speed laser material deposition’ (or ‘EHLA’ from its German acronym), is capable of providing brake discs with an effective protection against wear and corrosion, while at the same time being both a fast and economic process.

A finished brake disc coated with the EHLA process (Credit: Buderus Schleiftechnik/HPL Technologies)

‘With EHLA, we apply coatings that form a metallurgical bond with the base material of the disc and therefore adhere very strongly, and do not flake and chip.’ said Thomas Schopphoven, team leader at Fraunhofer ILT, and one of the creators of the new process.

Within the process, the powder particles of the coating material are melted directly in the laser beam, rather than in a melt pool on the surface of the component – as is standard for laser material deposition (LMD). Since the melt pool is now fed by liquid drops of material rather than solid particles of powder, the coating process can be faster, rising from the 0.5–2m per minute with conventional LMD to as much as 500m per minute with EHLA. ‘This also reduces the exposure to heat of the material being coated,’ added Schopphoven. ‘Unlike conventional LMD, where the heat affected zone can have a depth of one or more millimetres, thermal exposure with the EHLA process remains in the micrometre range. This enables the use of new material combinations such as coatings for aluminium or – as with the brake discs – cast-iron alloys.’

The low heat input prevents the carbon dissolving from the brake disc into the melt, which avoids the resulting brittle phases, pores, joining defects and cracks in the coating and bonding zone. ‘In other words,’ said Schopphoven, ‘it is now possible, for the first time ever, to provide brake discs made of grey cast iron with an effective coating that is firmly bonded with the base material. Moreover, the EHLA process uses as much as 90 per cent of the fed powder material. It is therefore extremely resource-efficient and more economic. The basic requirements for the use in an industrial, mass-production setting are within reach.’

In fact, Schopphoven believes that industrial application could soon be a reality. ‘Initial investigations have demonstrated that the EHLA process is capable of reliably producing coatings – with different material combinations – for conventional brake discs made of grey cast iron,’ he said. ‘A system that is ready for use in mass production, including a modified grinding process for finishing the components, is currently under construction in Aachen.’

Improving power device production

Power devices are a type of component responsible for the precise controlling of electrical energy from a power source to a load according the load demand. They are used everywhere, from the electric engine control of modern machines and electric vehicles, to converters or ultra-compact mobile power supply units.

With the increasing prominence of e-mobility in the automotive market – manufacturers such as Volkswagen have now started producing electric cars with plans to make them more accessible – the demand for power devices used for electric engine control is expected to increase going forward.

Such power devices are made using silicon carbide (SiC) wafers, and are designed to work at high currents, high voltages and high temperatures. To facilitate this, they usually have a metal layer applied to their back side – typically nickel (Ni) – which is currently joined via a process called ohmic contact formation (OCF) to ensure a connection with low electrical resistance between the metal layer and the semiconductor material.

The challenge with this is that when OCF takes place, the front side of the SiC wafer has already been completely structured, and as a result is sensitive to high temperatures. During OCF, temperatures of around 1,000°C are required to join the nickel to the SiC – forming nickel silicide – therefore to avoid damage to the front side of the wafer, flash lamps are typically used. With a single flash they can transmit a sufficient amount of energy to the backside of the wafer – typically in the range of 0.5J/cm2-4J/cm2 – before critical temperatures are reached on the front side.

However, there is currently a trend towards using ever-thinner wafers in order to further reduce the electrical resistance and improve the heat conduction of power devices, which reduces the distance between the back side of the wafer and the thermally sensitive front side. As a result, the energy pulses of a flash lamp could potentially become too long, leading to thermal damage on the front side of the wafer.

3D Micromac's microPRO RTP system for OCF.

According to Dr Hans-Ulrich Zuehlke, 3D-Micromac’s product manager for the semiconductor industry, one solution to this problem is to replace the flash lamp with a selective laser annealing process, where the rear side of the wafer would instead be irradiated using a pulsed nanosecond UV laser. ‘Laser pulses with a duration of less than 100ns are long enough for a sufficient silicide formation, yet short enough to minimise the heat conduction towards the front side of the wafer,’ he explained.

A further benefit of using laser annealing is that it avoids the generation of large carbon clusters directly beneath the nickel silicide layer during OCF – as is the case when using flash lamps – which are known to cause deterioration of the adhesion of the metal layer on the wafer.

3D-Micromac has developed a laser system specifically for the OCF process under production conditions. Named the ‘microPRO RTP’, the system is based on the firm’s continued development of its ‘microDICE’ wafer processing platform, and is equipped with a diode-pumped solid-state (DPSS) UV laser. ‘The beam path is optimised for UV lasers and, combined with focus tracking, provides a sufficient depth of focus,’ said Zuehlke.

The system is available with two different beam paths. ‘The correct amount of energy is achieved in the Gaussian beam profile due to a high pulse overlap,’ Zuehlke continued. ‘Several pulses then merge into a virtual plateau in the temperature-time curve. Alternatively, a top hat profile with a low overlap is also available.’ By using a DPSS laser, high uptime of the system is ensured through a low maintenance effort, while also generating low operating costs. The system also has an included interface for coupling with the IT infrastructure of the semiconductor factory.

‘The microPRO RTP is already designed for the next generation of SiC wafers,’ Zuehlke concluded.

Exploring the limits of laser cladding

Laser cladding technology is well established in industry for increasing the performance of high-value components and protecting them from wear and corrosion.

Currently, however, the main challenge associated with laser cladding in comparison to other coating techniques, such as plasma transferred arc (PTA), is its low deposition rate. This is particularly significant considering the benefit the technique could have in the construction, mining, and oil and gas industries for coating large components.

Cladding using the Laserline OTS-5 optic and a coaxial rectangular nozzle COAX 11 from Fraunhofer IWS. (Credit: Laserline)

At the recent ICALEO 2019 conference held in Orlando, Florida, Dr Oleg Raykis, sales manager at diode laser manufacturer Laserline, shared with attendees the firm’s work on using laser powers of 20kW and above to ramp up the throughput of laser cladding.

Using such powers, which were delivered via a Laserline OTS-5 optic and the COAX 11 coaxial rectangular nozzle from Fraunhofer IWS, Raykis and his colleagues were able to achieve material deposition rates of up to 35kg/h using Inconel 625 powder, covering an area at a rate of 3.5m²/h, as shown in figure 1.

Figure 1: A linear correlation exists between between laser power and deposition rate.

‘A single layer thickness of approximately 1mm was achieved and maintained at a track width of 45mm,’ said Raykis. ‘Investigations on cladding overlapping tracks and multilayer coatings showed the smoothness of the cladded area and the almost non-existent porosity (see figure 2). Due to the high deposition volume, the dilution rate can be reduced to less than 5 per cent at a powder efficiency of more than 95 per cent.’

Figure 2: Cross section of a single cladded layer of showing low dilution and dense structure.

The achieved deposition rates outperformed the current state of laser cladding technology by far, and even exceed typical values of the PTA process, Raykis explained at ICALEO. He confirmed that the shown potential of upscaling the laser cladding process would be beneficial for coating large-scale components, such as agricultural parts, hydraulic cylinders and power plant components.

In order to bring the process further to successful commercialisation, however, a revised version of the processing head will first be required, according to Raykis, as well as a parameter study involving a variety of different materials.

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