1 + 1 = 1

The best connection is only as strong as the material itself. That is why the two halves of the MonoWeld® piston from MAHLE are joined by means of a special process: friction welding. This requires that the piston crown and skirt be rubbed together under massive pressure until they become inseparably joined. Quite simple really—if you can generate the necessary forces. But that requires particularly powerful equipment!

TWO PARTS ARE SIMPLY BETTER

How two halves become one.
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When things really heat up in the combustion chamber, the heat must be dissipated. It isn’t enough for the piston to be made of highly temperature-resistant material, the geometry of the cooling channel is just as important! The cooling channel, however, is located between the piston crown and skirt. If the piston is forged from a single block, incorporating the cooling channel is quite complex. So why not forge the piston in two parts, bore the cooling channel in both directions from the eventual center, and then join the two piston halves? A clever idea! The stability and load capacity of the joint were the only obstacles in the way of making it a reality.

  • Friction: one part of the piston is stationary, the other is rotated.
  • Bead formation: under the influence of the rotation and contact pressure of the two parts, the material begins to get hot and viscous at the joining surface—and the typical friction welding bead is formed.
  • Bead formation: under the influence of the rotation and contact pressure of the two parts, the material begins to get hot and viscous at the joining surface—and the typical friction welding bead is formed.

THE MORE POWER, THE BETTER

How do you form joints that are just as stable, or even more stable, than the base material itself? The answer is: with friction welding. But this requires elaborate equipment: a friction welding machine with a fixed and rotating workpiece holding fixture, including a heavy-duty power supply. In order to achieve a friction-welded joint from rubbing the piston parts together, the machine requires a driving force of 750 kW—as much power as a subway railcar!

MAHLE has been using friction welding for decades in valve manufacturing—to reliably connect the valve disk and stem. We are now applying this experience to the production of the new friction-welded MonoWeld® pistons. With great success: even leading commercial vehicle manufacturers are relying on our powerful technology in original equipment.

THE EVOLUTION OF COOLING CHANNELS

  • Ferrotherm
    FERROTHERM®—the articulated piston with a crown made of forged steel. Its cooling channel must be turned from solid metal. This requires a delicate, curved turning tool that has to be custom-made. The hardness and toughness of the steel generate very high cutting forces, and the manufacturing of the cooling channel is equally complex.
  • MONOTHERM® piston: the cooling channel is manufactured using an intricately manufactured, curved turning tool that must be delicate and stable at the same time. The cooling channel is subsequently sealed with two stamped sheet steel parts.
  • MonoWeld
    MonoWeld®—the friction-welded piston: manufacturing the cooling channel for this piston is considerably easier and faster with a conventional (and therefore more stable) turning tool. A half groove is turned in both the skirt and crown of the piston. Friction welding is used to create the complete, closed cooling channel.

TECHNICAL MESSENGER

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