The automotive industry has to deal with a variety of pressing issues, including labor and trade issues as well as changing consumer demands. In the midst of these, lightweighting continues to be a focus, which can be seen as more and more aluminum finds its way into vehicle design.
Although aluminum has been used in cars since 1899 when a sports car body made of aluminum premiered at the Berlin International Motor Show, steel has long been the go-to material for automobiles for its durability and flexibility. However, over the next eight or nine years, unprecedented growth is forecast for aluminum usage in automobiles.
A 2017 survey by Ducker Worldwide, a market research organization, said automakers are expected to continue to increase the adoption of low-weight and high-strength aluminum “at a faster pace than any time in history.” The question is: how will the resistance spot welding processes change as more and more applications weld aluminum instead of steel?
Another issue to address revolves around the manufacturer’s inclination to pack more robots into a single workcell, which has been a growing trend for some time. With manufacturing square footage at a premium, making the most of any footprint is essential.
Resistance spot welding has long held a big place in the hearts of automobile manufacturers. It’s an economical process that is adaptable for use on a variety of materials, including high-strength, press-hardened steels and aluminum alloys.
But as more manufacturers bring aluminum into the production facility, they’re going to have to accommodate the differences in how aluminum reacts to resistance spot welding versus steel. For example, the resistance spot welding process for aluminum requires a weld current in the range of 15 kA to 30 kA. For steel, it’s only 8 kA to 10 kA.
Josh Leath, product manager at Yaskawa America Inc.’s Motoman Robotics division, says there are tradeoffs when working resistance spot welding aluminum. With aluminum, manufacturers get a lightweight, strong piece of material to work with, but it requires a larger machine to make the spot weld, which doesn’t suit manufacturers that strive to get more robots into a confined space to reduce their overall footprint and achieve higher robot density.
There are several methods for reducing a workcell’s footprint. One solution is to tuck the cables and wiring that control the arm and end-of-arm tooling into the arm of the robot – a point of expertise for Yaskawa.
“We pioneered [the hollow arm technology] with arc welding,” Leath says of the technology used to hide wiring and cables. “The cables went through the arm, reducing interference. The technology has come back around to the spot welding side, knowing that aluminum is becoming more and more important.”
Leath explains that the advantage of the hollow arm technology is that it makes it easier to place numerous robots within the same workcell, all working on the same part.
“If I had four robots in very close quarters and all of them have external routed cables,” he says, “those cables are going to interfere with each other.”
Another solution for reducing a workcell’s footprint is to use smaller robotic equipment, such as the SP80, a popular Yaskawa robot specifically developed for resistance spot welding applications. The SP80 is an 80-kg robot, which is often fitted with a 150-kg ultra-light spot welding gun.
“They want the smallest robot they can fit into a space,” Leath says of automakers and their adoption of the SP80. “Before it was ‘the bigger the better.’ Now, they’re looking at density.”
A step up from the SP80 is the SP110, which is a 110-kg robot that can handle larger resistance spot welders. The next step up is the SP180 – a 180-kg robot. Yaskawa also offers other payload capacities and reaches of these robots, all tuned for maximum speed based on the physics of the arm and payload. Several will be highlighted at the company’s booth at Fabtech in Chicago in November.
Step in with laser
One of the limitations of a resistance spot welder is that the electrodes have to make a connection on either side of the materials being joined, which means larger and deep and/or deep-well materials will require a relatively massive welding gun to reach all the way around. This adds more cost and complexity to the system because there is more to program and more to work around.
When a resistance spot welding solution becomes too big, Yaskawa recommends bringing in a laser to assist, which is a hybrid approach combining robots using spot welders and robots equipped with laser welding technology. Specifically, the company recommends what is called a Laser Stepper, a device created by IPG Photonics that are no larger than a spot welder and can be used on the same Yaskawa robots that work with spot welders. The weld it produces can be customized and have the same properties as a spot weld. The advantages are that the stepper head only has to make contact with one side of the materials being joined, and the seam can be designed in such a way that it is more aesthetic than a typical spot weld.
“If I have a floor pan that is 6 ft. by 4 ft.,” Leath says of a part being manufactured with a Laser Stepper, “I don’t have to have a spot jaw that is 2 ft. long to get around the thinner point. It’s an advancement that works well with our robots.”
The same advantage resistance spot welding has over arc welding is also part of what the Laser Stepper system brings to the table. For example, arc welders use many consumables, such as wire, gas, torch nozzles and tips, and the process emits damaging ultraviolet rays that require safety precautions. Spot welding with the Laser Stepper, however, does not come with same consumable concerns. There is also less spatter contaminating the workcell, and harmful fumes and smoke are drastically reduced.
And even though there is a laser involved with the stepper system, the pressure of the laser head on the material creates a light-tight enclosure, which means there are no expensive and cumbersome light curtains required in the workcell.
So, the question becomes – if a Laser Stepper is so compact and performs essentially the same weld as a resistance spot welder – why not fill a workcell full of them instead of taking the hybrid approach? According to Leath, a big part of it comes down to cost.
“If you think about a typical automotive assembly line,” he explains, adding that automotive OEMs are famous for keeping a close eye on the bottom line, “there are dozens – if not hundreds – of spot/laser robots. The cost of the Laser Stepper is about four times that of the spot welding gun.”
However, Leath says the Laser Stepper is still cheaper than a gas metal arc welder or a remote fiber laser, which run around eight times the price of a resistance spot welding system. But even if the Laser Stepper is more costly than a spot welder, manufacturers are willing to make the investment for improved cycle times and higher robot density.
“Larger robots have to be spaced far apart, which means you’re losing precious floor space,” Leath says. “And that amounts to fewer processes that can happen simultaneously and, therefore, slower cycle times.”