The premachining line consolidates all of the working steps downstream from forging under one roof. This has not only increased the in-house production depth in the Neckarsulm plant but has also cemented its status as a worldwide leading production facility for steel pistons. One particular highlight of the line is its spin welding cell.
Frictional heat instead of electric arcs
Now to explain how the process works or, more precisely, the flywheel friction welding variant used here: firstly, the top and bottom sections of the steel piston are machined. The bottom section is tightly mounted in a tailstock; the upper section is held from the chuck of a spindle possessing a flywheel. The flywheel is accelerated until it reaches a predetermined rotational speed. The power is then switched off and both parts are pressed together through a hydraulic cylinder exerting up to 100 metric tons of pressure.
The resulting friction decelerates the flywheel and simultaneously converts the kinetic energy into frictional heat, which is sufficient to melt both the adjacent surfaces of the two parts. In this state, the two component parts can be bonded. Besides the automotive industry, the flywheel friction welding process also for example finds application in aerospace technology.
The facility in Neckarsulm is now exclusively used for manufacturing steel pistons, having also been especially designed for this purpose. The friction welding cell is flexible in terms of the size and design of pistons it can accommodate: welding is possible for pistons with diameters ranging from 80 to 200 mm. Currently manufactured steel pistons are composed of the heat-treatable steel 42CrMo4 or the micro-alloyed steel 38MnVS6. However, one can also use alternative steel material grades, such as Inconel steel. The procedure offers many advantages over conventional welding with regard to both production flow and quality. For example, welding times are exceedingly short. “In the new friction welding cell, a commercial vehicle piston can be welded every 60 seconds,” explains Project Leader Christian Schaller. Furthermore, because the material is only heated and not melted during the process, thermal stress on the material is minimal, which bears out in the outstanding quality of the resulting bond. Schaller can also list further advantages: “The base material isn’t melted, so that prevents any danger of pore formation, solidification defects (like gas porosities) or slag inclusions. Even liquations – meaning undesired separations – can’t occur. The quality is on the same level as forging.”
Employees also appreciate flywheel friction welding when they are working near the largely automated welding cell: the process normally requires neither additional welding material or gas nor emits harmful radiation, splatters, or fumes. “No suction is necessary because workers face absolutely zero danger from hazardous substances,” explains Schaller. “Friction welding does without electric arcs or laser beams, which thereby eliminates any risk from optical radiation. This not only makes it easier to view the process but also removes any possible qualms or uneasiness about getting hands-on.”