Besides BIW, many component welding operations are in need of improvements in productivity and quality
It is the archetypal image of automotive manufacturing: a huddle of robots working frantically around a body-in-white (BIW) on its conveyor track. However, every vehicle contains a myriad of sub-structures, closures and individual components that also require welding in some shape or form, and ensuring that these are completed as efficiently and cost-effectively as possible is no less demanding.
Linde India is currently in the process of improving the welding supply chain serving a major bus, truck and light commerical vehicle manufacturer. The OEM uses scores of vendors across India to supply individual chassis, undercarriage and wheel components, all of which necessitate welding. “Looking to instigate a general quality improvement drive, the manufacturer happened to see one of our demonstrations involving the welding of steel tipper bodies using Linde advanced shielding gas,” explains Udit Banerjee, customer segment manager – manufacturing industries at Linde India.
“In comparison with standard CO2, advanced shielding gases comprising two components or more provide both an aesthetic improvement and reduced cost per metre thanks to faster run rates, savings in wire consumption and reduced labour requirement," he continues. "In our demonstration on the tipper bodies, this could be proven using digital data logging.”
Improving welding results
It is well documented that selecting the right shielding gas can optimise welding results. Not only do shielding gases protect the finished weld from the effects of oxygen and nitrogen in the atmosphere, they also have a positive effect on weld-metal properties such as strength, corrosion resistance and toughness. In addition, they can optimise the weld bead shape and size, weld porosity and fusion, plus accelerate the welding process and minimising spatter.
The combinations of different gases are often tailored to specific materials to deliver tangible savings. For instance, adding oxygen and/or CO2 to a shielding gas for the MAG welding of carbon steel increases its oxidation potential. In general, for a given welding wire, the higher the oxidation potential of a shielding gas, the lower the strength and toughness of the weld. This occurs because the oxygen and CO2 in the shielding gas increase the number of oxide inclusions and reduce the level of materials such as manganese and silicon in the weld metal.
Shielding gas can also improve weld quality by reducing the level of weld reinforcement, which can be a problem because it increases the stresses at the toes of a weld. In severe cases, weld reinforcement can even lead to cracking at the edges of the weld, particularly under fatigue conditions. The normal method of removing or reducing the reinforcement is to grind off the surplus weld metal, but this is both costly and time consuming. A correctly balanced shielding gas will reduce the surface tension of the weld metal, allowing the solidifying weld pool to sink and offer lower reinforcement.
Another point of note is that, when MAG welding carbon steels, the use of pure CO2 can cause large amounts of spatter to be ejected from the weld pool. Removing spatter after welding is not only costly but can cause problems if the component is subsequently painted or coated, as the small marks left behind show up as surface imperfections. Spatter is also extremely wasteful. To indicate the extent of the waste, the MAG welding of carbon steel using CO2 shielding gas typically produces 17g of spatter for every metre of weld completed, which is the equivalent of two metres of 1.2mm diameter welding wire being thrown away.
“The process from loading the blanks to the finished component, for instance, could not take longer than 37 seconds at each station" – Randy Koch, KWD
Automation can significantly improve component welding processes, as can be witnessed at the Wolfsburg, Germany, factory of KWD Automobiltechnik, where eight welding cells equipped with ABB robots are producing parts such as wheel housings, sills and A-pillars from sheet metal blank to finished part in a little over 30 seconds.
In the case of a wheel housing, the process begins with an operator placing several sheet-metal blanks into the fixture of a turntable along with another part that has already had adhesive applied robotically in a previous step. The operator then closes the door, activating the welding process. A turntable inside the welding cell positions the blanks in front of two IRB 6640 robots, which make several spot welds to join the blanks together. Another robot removes the joined part and positions it under a stationary welding gun, which makes further spot welds. Finally, the component is positioned beneath a scriber for marking. The entire process takes just seconds.
Optimising cycle times
KWD also produces A-pillars and sills for the new model of a well-known German car manufacturer. Realising that a fresh solution was needed to meet the customer's demands, KWD had rather stringent requirements for cycle times between parts. “The necessary parts could not be produced with our existing equipment,” explains KWD production manager Randy Koch, describing why the company invested in eight new automated welding cells. “The process from loading the blanks to the finished component, for instance, could not take longer than 37 seconds at each station.”
Thanks to intensive simulations carried out in advance, ABB engineers were able to guarantee this cycle time. A total of 20 IRB 6640 robots, and one IRB 2600 robot, are now working in the cells. Safety features such as access control, enclosures, light barriers and roll-up doors were also installed, with the robot-maker taking responsibility for cell planning, design, manufacture, delivery, assembly and commissioning.