Related: Automotive Materials
As Tom Jones might croon, “It’s not unusual”—and my apologies to Mr. Jones for what follows—for engine blocks to be made from aluminum these days, as aluminum’s lesser weight is advantageous for meeting CAFE requirements. But it’s also no great revelation that if a manufacturer intends to make good on its powertrain warranty, it’s either pressing or casting iron cylinder liners into these blocks. That is, unless that manufacturer is DaimlerChrysler and the block in question is for a Mercedes-Benz V-engine. In this case, an aluminum-silicon alloy liner is cast into the aluminum block.
|Underneath the nice looking plastic cover in the engine compartment of this 2001 Mercedes-Benz SLK 320 is a 3.2-L V6. Like Mercedes’ other V-engines, it features cast-in aluminum-silicon alloy cylinder liners.|
So how is it that Mercedes is using an aluminum liner, while most of the rest of the world are using iron ones? Before you jump to the conclusion that this is due to superior German engineering, consider that it was actually a Welsh company, Osprey Metals Ltd., that developed the spray forming method for creating the very durable aluminum-silicon alloy material used in the Mercedes liners.
Osprey Metals is a subsidiary of Sandvik AB, Sweden, but its roots go back to 1974 when the company was formed by three researchers who had invented spray forming. The process that’s evolved over the years can be used to develop aluminum alloys, copper alloys, superalloys, and metal-matrix composites. It works by using nitrogen or argon gas to create a spray of molten metal that is precisely deposited on a rotating collector. As the disc rotates, the ultra-fine spray solidifies in a perfect, high-density pattern to create a solid billet. This entire process happens within a chamber, so it’s both highly controlled and safe.
While this sounds incredibly simple, it’s not. If it were, Osprey Metals probably wouldn’t be able to license the process to companies like Germany’s Peak Werkstoff GmbH, which supplies the Mercedes’ cylinder liners. Peak manufactures the cylinder liners from spray-formed billets by swaging the billets into tubes. Of course, once the liners are cast into the block, they are turned and honed as would be expected.
The key benefit of spray forming is that the rapid solidification of the sprayed droplets gives the resultant material a very fine and uniform grain. This is particularly advantageous for the aluminum-silicon cylinder liners, as it provides a strong and flaw-free structure with better machinability and reduced wear characteristics than could be had from a conventional aluminum casting. Powertrain engineers hoping to extract every last drop of performance from their engines will be happy to note that the aluminum-silicon liners actually have smoother contact surfaces, thereby reducing friction, which coincidentally helps to lower emissions and oil consumption attributable to ring blow-by.
Compared to a cast iron liner, the spray formed aluminum-silicon one does cost more, however its use carries other advantages. Most importantly, there is a weight savings of 500 g per cylinder liner (or approximately 1.1-lb. ea.). Machining cost is also lower, as the substitution of aluminum for iron in the liners removes the need for mixed material machining on the top of the block where it mates with the cylinder head. A further bonus of the all-aluminum construction is easier recycling.