Yamaha’s Fracture-Split Titanium Connecting Rods

Will Ti con-rods soon become commonplace in production motorcycles?

Yamaha YZF-R1 static 3/4 view

One of the first items on the "features" list of Yamaha's new YZF-R1/M is fracture-split titanium connecting rods. The advantage of using titanium rods is that, while they can display strength and fatigue properties equaling those of high-strength steels, their material has only 60 percent of the density of steel. Lighter weight means reduced bearing loads, lower vibration, and a potential for easier attainment of performance at higher rpm. In the immediate past, we have seen the adoption of forged aluminum pistons and forged connecting rods in high-performance engines, and Yamaha's move to titanium rods is a further step along the same road. As engines develop their power at higher revs, reciprocating parts must become lighter and stronger.

Titanium con-rods are not new. Jack Williams, father of former Norton engineer Peter Williams, knew in 1953 that the light metal was being adopted for aerospace and arranged to "acquire" enough to make test rods for the AMC single-cylinder racers he was then developing. When Superbike became the hot US roadracing class in the mid-1980s, lightweight rods from Jet Titanium got a lot of press. Honda later adopted titanium rods for its low-production RC30. Such rods were blanked by sawing or water-jet cutting from plate, then machined to dimension.

Yamaha’s fracture-split rods are something new. Fracture-splitting is a technique now used in the auto industry to speed production of steel rods by eliminating separate machining of the rod cap. Normally, there must be either serrations or pins to prevent relative sliding of the cap against the body of the rod. This is made all the more problematic by the extra difficulty of machining titanium. Each step costs money and time, both of which add to cost. By developing a way to apply this high-production fracture-splitting technology to titanium rods, Yamaha is telling the world that it plans to use enough of such rods to more than pay for the R&D. This implies titanium rods, not as expensive curiosities in homologation specials, but as routine parts for production streetbikes!

The inventors listed on Yamaha’s patent are heavy hitters. Tsuyoshi Kubota developed a resistance welding method of bonding valve seats to aluminum cylinder heads (rather than normal press fitting), enabling as much as a 50-degree Celsius reduction in valve temperature. Hiroshi Yamagata developed super-strong powder-metallurgy pistons, such as were used in Yamaha’s YZR500 Grand Prix bikes of the 1990s.

Fracture-splitting steel is relatively easy, but titanium’s high “elongation before failure” is a big problem. Titanium is tough. When a pistol bullet hits glass, fractures radiate at the speed of sound and the bullet blows on through. But when the bullet hits tough Lexan plastic, the plastic stretches into a “witch finger,” slowing the bullet to a stop. Having to stretch the tough plastic so far consumes the bullet’s energy. That stretching is elongation, and it makes titanium strongly resist fracture.

Yamaha YZF-R1 titanium connecting rod

Kubota and Yamagata therefore added together several techniques to reduce titanium's fracture toughness, just in the zone where the fracture split was desired. They knew the rest of the rod had to be in a condition that gives it the high strength and elongation so attractive as a con-rod material. Forged metal fractures more easily in a direction parallel with the grain flow lines produced in the forging process, and fracture is more difficult if it must cut through those flow lines.

By using a two-phase titanium alloy (one containing two different crystal habits, alpha and beta), Kubota and Yamagata had their choice of two manifestations of the lesser of the phases: 1) an “equiaxed” form that has high strength and elongation; and 2) an “acicular” form that is more brittle and less strong.

Like the welded steel of WWII US “Liberty” cargo ships, titanium has a brittle-to-ductile transition temperature. Some Liberty ships, in the cold of the North Atlantic, suffered catastrophic brittle failure of their welds, in some cases breaking in two. Despite the fact that this transition point for titanium is above room temperature, Yamaha’s development team found that immersion in in liquid nitrogen (-341 degrees Fahrenheit) assisted the fracture process.

Yamaha YZF-R1 crankshaft and connecting rods

Yamaha first extrudes a bar whose cross section is roughly that of a connecting rod. This causes the forging flow lines to be parallel with the future intended fracture plane. Heat treatment places the alpha phase of the alloy into acicular form (long radiating needles, with an effect something like that of flake graphite in cast iron; “break on dotted line”). This long bar is then sawn into individual rod blanks. When these blanks are then forged to their near-final shape, the forging process is made more vigorous in the main body of the rod, causing the acicular alpha structure to transform back into the equiaxed structure essential to high strength. But in the specific region where the fracture split is desired, the forging is less vigorous, such that the more brittle acicular alpha structure is retained there. Now the rod is finish machined. On the ID of the big end, small grooves are cut to initiate and guide the fracture in the next step.

Just before the rod is placed in a falling-weight fracture-splitting fixture, it is cooled by immersion in liquid nitrogen. A falling weight drops, striking a wedge, which rapidly applies tensile stress tending to separate the rod cap from the main body of the rod. Fracture begins at the cut grooves and moves outward. The resulting fracture surface has irregularities called “minute complementary features” in the patent, which interlock when reassembled, preventing relative sliding between cap and rod. Besides lowering production cost, fracture splitting also improves the roundness of the big end, thereby increasing the load-carrying ability of the bearing shells that will be assembled into it.

If, like myself, you find accounts of metals’ internal crystal structure both fascinating and mysterious, you can add to the experience by looking closely at the polished stone of monuments or old-time bank buildings. There, you will easily see large-scale crystalline structures, multiple phases, and other features that also have importance in metallurgy. Our material civilization depends on this knowledge.