Driving quality welds
Laser joining techniques must keep pace with the car industry’s requirements for lighter vehicles, increased automation, improved cost savings and faster, more accurate manufacturing processes.
Before the turn of the century, welds in mass-produced automotive body parts were almost exclusively fabricated from pressed steel sheets and joined using resistance spot welding. In the last 10 years, laser joining techniques have become a well-established tool for the automotive industry and continue to develop and be more widely applied.
Lasers are able to deliver a high-energy beam to a small area, which has big benefits when it comes to joining car parts. Jon Blackburn, manager of the lasers section at UK applied research and technology institute TWI, explained: ‘The very small local heated area [enables] highly accurate joining and, critically, low total heat input processing. This can be used to prevent damage to thermally sensitive components, such as electronics or battery applications, or to minimise thermal distortion during joining of components.’
There are many different applications of laser joining, from welding car keys to gear components. The laser is capable of delivering very rapid processing times, increased design flexibility, single-sided access, reduced flange widths, and increased torsional stiffness to improve vehicle structural performance and reduce material thicknesses.
Remote beam delivery
Remote laser joining is a relatively new technique that promises much for the automotive sector. Manipulating the beam at long stand-off distances for high-volume production is of particular interest to vehicle manufacturers. This method offers a more sophisticated approach with several benefits for car manufacturers.
Audi recently implemented a remote laser welding process to join aluminium door panels on its A8 series, which resulted in a 95 per cent reduction in production costs compared to traditional methods at its Neckarsulm facility. The cost savings predominantly came from the remote joining capabilities thanks to time savings of more than 50 per cent compared to tactile laser welding.
The process time was reduced from 25 seconds for tactile laser beam welds to just 12 seconds for remote welds. Such time savings came about, in part, thanks to the increase of the welding velocity and the reduction of laser power used, which produced a 24 per cent reduction in CO2 emissions.
During remote welding, a beam is moved around using a scanning mirror rather than shifting the entire processing head or workpiece from weld to weld. The technique also eliminates filler material and inert shielding gas to increase process efficiency.
Remote beam modulation enables controlled heat management, which reduces the focal diameters of the beam to sizes appropriate for joining thin sheets of material. The beam can also be positioned very accurately, which improves the integrity of the weld, and remote systems reduce hot crack susceptibility because the process needs less heat input.
But many challenges and limitations remain for laser joining techniques. Laser welding, in particular, has a limited ability to bridge gaps, ‘so component fit-up or geometrical tolerance must be very high compared with conventional joining techniques, although work is being done to address this challenge,’ Blackburn commented.
Narrow weld zones can also be relatively weak if loaded parallel to the weld direction, and sensitive to evaporating volatiles, which can destabilise the welding process and scatter laser energy, thereby further reducing efficiency.
Welding of lightweight materials has become increasingly important within the automotive arena, as Sullivan Smith, automotive programme manager at TWI, explained: ‘Inevitably material technology is being pushed to new bounds in pursuit of lighter car constructions. In particular, there is a focus on using higher strength steels, more aluminium alloys and introducing composites into auto bodies.’
Laser brazing, where a filler metal or alloy is heated to melting point and distributed between two or more close-fitting parts, offers several advantages to conventional laser welding techniques, when joining lightweight materials and making visible joints in parts such as roof joints, tailgates and C columns. It produces a smooth surface, avoids the undesirable effect of melting the zinc coating, and avoids corrosion and further treatments.
Audi recently worked with diode laser system firm Laserline to optimise aluminium welding of visible surfaces using diode lasers. The process used an aluminium and silicon alloy filler material to prevent hot cracking, where shrinkage cracks form during the solidification of the weld metal. The filler wire was added using tactile seam tracking optics. After three years, the process was optimised to create a high-spec joint without the need for further post-processing.
The move to lightweight materials brings a range of challenges for joining with a laser, such as the sheer range of alloys, as Marc Kirchhoff, industry manager automotive at Trumpf Laser, noted. ‘The car manufacturing industry currently works with 3,000 and 5,000 aluminium alloys and development efforts are aiming for 7,000 alloys, [but] it is not so easy to join such lightweight materials,’ he said. ‘Although there are some materials available that combine good mechanical properties and weldability, and could be welded without additional material, most of the alloys need some additional material.’
If such filler materials could be removed from laser joining methods for more aluminium alloys, this would bring further advantages to car manufacturers, in part because this would mean remote laser joining methods could now be used for lightweight materials. Kirchhoff added: ‘If we could introduce a new kind of process without the filler material, whether that is remote joining or something else, we would see increased productivity and increased cost effectiveness.’
Lightweight materials present a diverse range of challenges during the laser joining techniques currently employed in the car manufacturing industry, because of the diversity of materials being applied. Increasingly higher strength steels, for example, have microstructures that are damaged by heat from joining processes, and many high-performance aluminium alloys have problems with cracking susceptibility and weld porosity.
There is also growing interest in the use of non-metallic composite materials in the automotive industry. While the moulding and machining of composite-based parts is well established, the joining techniques still require some development. Smith added: ‘Newer composite-based materials do not yet have suitable joining solutions to allow them to be integrated into high-volume production without incurring excessive costs. The world of joining technology is working to solve this issue and laser-based solutions are under development.’
Lasers are the obvious choice to join these new materials, as it is the predominant technology for current automotive applications, so there would not be a big step-change to implement laser joining methods in the future. Kirchhoff said: ‘Laser technology is the common method, which is used across a range of applications for such new materials. For example, for composite materials, there is currently no productive laser technology for joining these materials. Cleaning is the main application area for lasers working with composites to increase the strength of the weld; furthermore there are processes to join composites and metal parts without any glue using the laser technology.’
Composite materials are polymers, and laser contour welding is one technique that could address the restrictions of traditional plastic welding techniques. A very precise application of energy is required for plastic welding, and contour welding – combined with remote joining techniques is being developed as a solution to the composite joining issue. For example, laser systems manufacturer LPKF and its partners developed a contour welding technique for the volume production of the large tail lights on Hyundai’s Equus vehicles, which also incorporated hybrid welding techniques.
Future of laser welding
The future direction for laser joining in car manufacture will see more mixed material structures used and improved production speeds. Smith said: ‘One of the fastest joining technologies that has the potential to allow this is laser technology, [but] in order to have an efficient low-cost production process, a single joining technology solution is required that is adaptable enough to be used across a very wide spectrum of materials and applications.’
He added: ‘Laser technology is moving towards a position where its flexibility will see it used in an increasingly broad sphere of application.’