All manufacturing engineers know the score when it comes to body-in-white welding. Emerging technologies such as laser welding, remote laser welding, laser-arc hybrid welding and laser brazing are all knocking on the door. The only barrier thwarting their progress to even wider adoption is that spot welding is so downright efficient and cost effective.
As a consequence of this realisation, spot welding has enjoyed something of renaissance in terms of process development. At TWI (The Welding Institute) in the UK, the backbone of the organisation’s knowledge in automotive manufacture is built around the core joining technologies, at the forefront of which is SmartDress – a current project focussing on improving the performance of resistance spot welding.
SmartDress (www.smartdress-project.eu) is a consortiumbased project funded by the EC under Framework 7 and supported by a team of leading research/training institutions and European-based SMEs, all seeking to improve the reliability and efficiency of resistance spot welding. On automotive production lines a major cause of line stoppages are spot weld quality issues, and these usually arise from poor maintenance of consumable electrode tips on RSW guns.
Dressers are commonly utilised in an attempt to maintain tip quality, however, existing systems require manual set-up, optimisation and continued monitoring during production. With a vehicle typically rolling off a production line every 1-2 minutes, a tip dress issue causing even a 5 minute line stoppage once a day can result in a loss of 10 vehicles per week, equating to around €7 million of lost revenue per annum for an average production line, says TWI. SmartDress aims to develop an automated adaptive control system that will optimise, monitor and maintain electrode tip quality. The system will increase the production life of electrodes by a factor of two.
Another key feature of SmartDress will be to allow costcompetitive spot welding of aluminium, which OEMs presently consider to be commercially unviable. Dr Srinivas Nidamarthi, Technology Manager for Robots Systems at ABB Robotics, agrees that spot welding will retain its place as the premier body-in-white joining technology, in the near future at least.
“Initially, new and alternative joining techniques tend to be niche applications. I don’t see them taking the place of spot welding very soon, for example, as car designs are already in place for the next 10 years, through 2020,” he says. He adds that in modern cars, the body-in-white features between 15 and 20 different joining technologies, but the leading automated process remains spot welding, commanding anywhere between 30-60% of all joins.In a spinPerhaps unsurprisingly, many of the latest spot welding developments hinge around automation, such as Kuka’s patentedRoboSpin, for example. Unlike in conventional spot welding, here the robot can continue to move during welding – the weld gun is simultaneously rotated about the electrode axis. This transforms the traditional spot process into a continuous-path operation with uninterrupted robot motion. One of RoboSpin’s main advantages is repositioning times – in three reference cases, the maximum time saving demonstrated was up to 12%. Furthermore, among the positive effects of using a large rotational angle before the actual welding process is that coatings, such as zinc, paint and oxides are opened up. This results in significantly more constant contact between electrodes and the base material. As a consequence, weld nugget diameters are less scattered and the required weld quality is more reproducible than conventional RSW. A software package is currently being developed in co-operation with Kuka Belgium for userfriendly programming of the rotational motion. According to Kuka, the development of RoboSpin makes it possible for the first time to spot weld aluminium in an economical way. ‘RoboSpin Aluminium’ is scheduled to start in premium car body mass production in 2013.
Retaining the theme of automated spot welding, among Kawasaki’s latest body-in-white welding technologies is the development of next-generation BX robots. According to Kawasaki, the new BX range offers more welds per cycle – up to 20% faster on average, with up to 50% less installation space requirement.
“We’ve had strong interest and initial orders totalling in the hundreds of units from OEMs and Tier One suppliers,” says Yanik Van Coppenolle, Marketing Specialist at Kawasaki Robotics (USA) Inc. Available in two models, offering 100 and 200kg payload capacity, these optimised body shop robots ar said to excel at high speed shor movements. The improved seq by the servo welding guns, alon calibration, optimise acceleration gun speed, leading to cycle tim The BX-Series is manufactur reduction units for the upper a dressing of the robot from the mounting plate. With Kawasak package, the manufacturing lin engineering time is reduced.
By coupling all axes of the scanner with those of the robot as a guiding machine, the system commands a total of nine selectable axes. Regardless of whether the application is spot, circular, wave or stitching seams – the seam geometry is freely programmable and can therefore be adjusted to strength and component requirements. Rapid arrival at the next welding position is supplemented by quick rotating mirrors in the scanner head that enable fast positioning of the laser beam within the working field. Robot movement and speed can be optimised by optional offline programming, while software functions also calculate robot path inaccuracies as the current position is reported back within the operational cycle. Thus, repeat accuracies in the range of <0.2mm are possible.
Last but not least, the new scanner head doesn’t need an expensive flat field lens, making it an even more appealing solution for scanner welding in the harsh environment of the auto plant.
Despite the ongoing use of spot welding, there is no doubting the influence of laser technology on current bodyin- white developments. Laserline GmbH says the advantages of laser brazing with filler wire, for example, are most apparent when used in Class A show surfaces. Low warp, a stiff joint ready for the paint shop and almost no postprocessing makes it an attractive proposition. According to Laserline, compared with laser or spot welding, laser brazing improves the aesthetics of a joint so substantially that the usual plastic strip needed to cover it can be omitted. Using fibre-coupled, lamp pumped, Nd:YAG laser technology, laser brazing can achieve a typical process speed of around 2.4 m/min, but the advent of the latest fibre coupled diode lasers, fibre lasers, or disk lasers (all diodebased) have led to a step change in this area.Among the recent beneficiaries is the VW Touran production line in Wolfsburg, Germany. A process comparison was performed between a 4kW YAG laser and Laserline’s LDF 6000-100 6kW diode laser on the Touran’s zinc-coated steel sheets. While the existing YAG laser used a de-focussed beam, the new diode laser, which featured a tactile brazing head with CuSi3 filler wire, was set up with the 3.2mm diameter beam in the focus position. Both the brazing speed and joint quality were increased. By working in the focus of the top hat laser beam, better distribution of power density into the join boundaries was achieved. With the 6kW diode laser, edge notches were reduced by around 75% and maximum process speed increased to 4.4 m/min. VW decided to replace all YAG lasers on the Touran production line with 6kW diode lasers.
Fused with advantagesTrumpf is another technology supplier with a vested interest in laser-based joining technologies. The company has an extensive list of auto OEM users for its laser welding products, the motivation for which, says the company, includes smaller flanges – typically 8mm wide against 14.5mm needed for spot welding. Less weight, higher stiffness and lower heat input are also influencing factors. According to Trumpf, the big driver behind the growing trend of remote welding is the minimisation of nonproductive travelling time although, of course, the success of such a process is very much material dependent. With new materials being approved for automotive use with unerring frequency, process evolution is essential. In one current example, Trumpf has been working with Novelis on the remote laser welding (without additional wire) of ‘Fusion’ – an aluminium sheet innovation composed of multiple alloy layers. Featuring a clean and oxide-free interface between the layers, the material has been developed to serve customers who increasingly require alloys that combine the properties of strength, lightweight characteristics and formability. Conventional 6000 series aluminium alloys are known to exhibit hot cracking sensitivity during remote laser welding. This is due to the columnar grain structure in the welded zone linked with the slight amount of magnesium, silicon and/or copper in the material. As a result, welding can only be successfully performed with filler material and/or low welding speed.
However, Novelis Fusion Alloy 8840 features a specific cladding that makes it possible to weld without filler metal, and without any hot cracking. The material maintains all the advantages of 6000 series monolithic alloys, is compatible with remote laser welding (no gas protection) and is fully approved for automotive applications. Trials were successful in various edge preparations, including butt, overlap and T-joint, even with a minimum weld flange distance as small as 5mm.
Trumpf is also involved in case studies of laser-arc hybrid welding of aluminium body parts such as axle carriers. Here, the combination of laser and MIG offer better quality (laser introduces process stability) and gap behaviour (MIG is effective at bridging gaps), along with up to three times higher speed compared with MIG alone. As material technologies advance, there is a clear decision for OEMs to make: either adapt existing joining technologies or seek out new methodologies and processes intended to accommodate these trends. Spot welding is showing resilience, but few can doubt the continued advance of laserbased alternatives in the joining mix.