Dissimilar material microwelding gets ready for industry uptake

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Spiral welds can be made at the interface between transparent material and metal to bond the two together. (Image: Heriot-Watt University)

Matthew Dale learns that ultrafast lasers can now be used to bond transparent materials to metals, unlocking a plethora of applications

The ability to bond metals to transparent materials such as glass, quartz and sapphire using ultrafast lasers is now ready for industry uptake, and is generating interest in a number of different sectors. 

This was the message for delegates of the recent Industrial Laser Application Symposium (ILAS). 

Bonding metals to transparent materials is frequently needed in modern manufacturing, for example in the assembly of electro-optics, and for hermetic sealing. 

It is currently achieved using a number of conventional methods, each of which introduce their own undesirable issues while having limited material combinations that they can join. 

From January 2018 to September 2020, the partners of the Innovate UK project ‘Ultraweld’ – Oxford Lasers, Heriot-Watt University, Coherent, Leonardo, CPI, Gooch & Housego and Glass Technology Services – were working to develop a process known as ultrafast laser microwelding for joining such dissimilar materials. They also aimed to design and build an ultrafast laser microwelding prototype machine and demonstrate it on real devices in key selected advanced applications. 

‘All of these objectives have been achieved,’ stated Dimitris Karnakis, technical manager of R&D projects at laser systems integrator Oxford Lasers, who noted that there are only a few research groups around the world currently working on this process. ‘There’s a lot of work being done on glass-to-glass microwelding with ultrafast lasers, but very few groups have managed to do it on glass to metal.’ 

Why is it needed? 

Dissimilar materials such as glass and metal are typically difficult to weld together due to them having different thermal properties – the high temperatures and highly different thermal expansions involved can cause the glass to shatter. 

Several well established methods already exist for joining dissimilar materials in production, for example: adhesive, diffusion, glass frit, anodic and arc bonding; mechanical fastening; and soldering. However, each of these methods comes with its own drawbacks. While some are operator-skill dependent, leading them to being error-prone, others might not be truly hermetic, or require large heat input, the use of interlayers, or lengthy, multi-step processes (including post processing). 

Adhesives in particular cause numerous issues that make them undesirable for use in electro-optics assembly. They are messy to apply and shrink around two to five per cent in volume when curing, which can cause stress and deformation. They are also sensitive to the environment: softening and losing strength at high temperatures; becoming stiff and brittle at low temperatures; losing strength in corrosive environments; and swelling in the presence of moisture. Outgassing – where organic chemicals from the adhesive are gradually released – is also a major issue, as this can contaminate any delicate electro-optics components they are used on, reducing their lifetime or even optically damage them. 

The process 

Electro-optics assembly therefore still requires high-yield, repeatable and reliable bonding technologies that have long lifetimes and can resist harsh operating environments involving vibrations and large temperature changes. Ultrafast laser microwelding has the potential to offer such a process. 

The technique operates by focussing the beam of an ultrafast (picosecond/femtosecond) laser through a transparent material to its interface with metal. The ultrashort pulses create a very small and highly intense spot at the interface between the two materials – megawatt peak power is achieved over an area just a few microns across. This creates a microplasma – like a tiny ball of lightning – inside the material, surrounded by a highly confined melt region which then cools to create a strong bond without cracking the transparent material. The pulsed beam is translated across the interface in the desired toolpath, such as a spiral, until the required weld is achieved.

The process of ultrafast laser microwelding. (Image: Heriot-Watt University)

Ultrafast laser microwelding offers the benefits of being a non-contact, high-precision and high-speed digital process that requires no intermediate layers or post processing. It can also bond a wide range of materials in a single machine, minimising the floor space required on a factory floor. What surprises Karnakis about ultrashort laser microwelding in particular, is that the laser intensity window for the process is relatively wide, meaning a wide range of laser intensities can be used without damaging the transparent materials. ‘I was very surprised to find out that with such a large intensity window we can actually weld a lot of different types of glass with different nonlinear absorption properties,’ he said. ‘This means that, from an industrial perspective, this process is very reproducable.’ 

High application potential 

The process has a plethora of applications in numerous sectors. The ability to bond glass to metal can be used in the manufacture of lasers and sensors in instrumentation for sectors such as aerospace, defence, satellite communications, and surveillance, or in the packaging of laser diodes, photonic integrated circuits and micro-electromechanical systems. 

The technique can also be used to achieve hermetic sealing, which opens up numerous applications in, for example: creating vacuum insulated glazing windows (Karnakis noted that skyscraper builders have expressed interest in this); moisture ingress prevention in avionic systems, photovoltaic panels, watches and inorganic LEDs (for harsh environments such as vehicle headlights and streetlights); medical implant assembly (devices encapsulated with special quality glass that go inside the body); sealing pharmaceutical vials; achieving pressure capability for underwater systems; and manufacturing OLED devices. 

Quartz wave plates welded to a stainless steel base. (Image: Oxford Lasers and Gooch & Housego)

For this last application, the interest comes from the fact that manufactures of OLED devices have to go to extreme lengths to encapsulate them – which, according to Karnakis, is a source of great expense that contributes considerably to the overall cost of producing OLED devices. ‘So if we had a technique to encapsulate these using glass, which is a natural encapsulant, then that would go a long way,’ he remarked. 

The technique has also generated a lot of interest in the space sector in the assembly of instrumentation, satellite communication systems and camera sensors. The Ultraweld partners have even had discussions with NASA regarding the adhesives it currently uses. According to Karnakis, due to the extreme conditions of space, NASA is only able to use a handful of specific adhesives based on the amount of outgassing they exhibit. As a result, NASA is investigating laser microwelding as an alternative to using adhesives. 

Putting it to the test 

As part of the Ultraweld project, Oxford Lasers has developed a Class-1 laser safe prototype microwelding machine at Technology Readiness Level 6 (demonstrated in a relevant environment). The system is based on Oxford Lasers’ C-Series Tool and uses an industrial-grade ultrafast infrared laser with adjustable pulse duration between 300fs and 10ps. The system uses a flexible beam delivery system with variable focus and high-resolution optical scanning, a custom software interface and various beam diagnostics to tailor the process as needed. The prototype is now available to industry for proof-of-concept feasibility and pre-production trials.

Using the machine, the researchers were able to bond together a range of different material combinations including, among others: crystal quartz to stainless steel, NBK7 glass to aluminium or stainless steel, fused silica to stainless steel, sapphire to stainless steel, and calcium fluoride to stainless steel. 

A range of demonstrator components made from different materials were produced to demonstrate the bonding, with the researchers using a range of tests to characterise the bonds. Samples faced tests such as repeated thermal cycling between -65°C and +95°C, prolonged vibration tests, shear tests at varying forces, hermeticity tests and stress birefringence tests.

Left: an N-BK7 lens welded to an aluminium ring mount. Right: A BK7 wedge prism welded to an aluminium base. (Image: Oxford Lasers and Gooch & Housego)

Karnakis showed attendees some of these components: A BK7 wedge prism (maximum thickness 18mm) welded to an aluminium base, which demonstrated an average shear bond strength exceeding 75N; a 9mm thick, 10 x 10mm sapphire cube welded to stainless steel, which withstood 200N of shear force; a 75mm diameter, 9mm thick fused silica window welded to stainless steel that also withstood 200N of shear force; a 9mm thick, 10 x 10mm calcium fluoride cube welded to stainless steel that withstood 50N of shear force; two 2 x 1mm-thick quartz waveplates welded to a stainless steel metal base; and a 40mm diameter, 4mm thick N-BK7 lens welded to an aluminium metal ring mount. 

‘We’ve had great characterisation results so far that show promise in terms of bond strength, hermeticity and other performance metrics,’ said Karnakis. ‘There’s a lot of interest in this process and definitely a great opportunity for industrial uptake.

The future of ultrafast laser microwelding 

While ultrafast laser microwelding has been proven and is now ready for industry uptake, the physics of this technique are still not yet fully understood, according to Karnakis. ‘A lot of work has happened so far to understand this process, but we are still in early days at the moment and don’t understand it very well,’ he said. ‘This is a little bit problematic because we cannot expand on the design rules until we understand the physics further. Process design criteria are mostly based on empirical data so far.’ 

Oxford lasers plans to expand its inhouse bond characterisation capabilities to fine-tune ultrafast laser microwelding across different applications. It also plans to expand the matrix of material combinations it can weld, including other glass-metal combinations, glass-ceramics, glass-semiconductors and semiconductors-metals. The firm will also be working to optimise toolpaths for performing large area welding, and for welding intricate shapes.

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