I recently found some more great sites where I could read about piston alloys and aluminum materials with unusually high hot strength. In the 1960s, the very old and much used piston alloy RR58 (F1 only gave this stuff up in 1996 and Ducati still used it at least 10 years later) was chosen as structural material for the Concorde supersonic commercial airliner (now retired). Why? Conventional high-strength aluminum alloys begin to lose strength at temperatures as low as that of boiling water (212F). In cruise at Mach 2.04, the Concorde’s nose heated to 260F from air friction, while the rest of the structure reached 212F. A material able to withstand stress at somewhat elevated temperature was required. The original chemistry of RR58 is described in a 1928 British patent but the material entered service in the form of pistons in the Rolls-Royce Merlin V-12 piston engines in the Spitfire and Hurricane fighter aircraft of 1939 and subsequent. For use in the Concorde, the material was again studied and researched because Britain and France did not want to find one day that Concorde’s structure had aged to a strength level that was no longer safe. Metal, aging? The outstanding hot strength of this family of materials comes from the precipitation in them of hard intermetallic phases made up of aluminum plus copper, iron, and nickel. When the heat-treatment and aging are just right, a maximum number of extremely small precipitated particles forms in the aluminum, acting as “pins” to mechanically prevent planes of aluminum atoms from sliding across one another under stress—bump, bump, bump. The more a metal resists such deformation, the harder it is said to be. The problem that arises is this: When you operate the material in service at close to or above its heat-treatment temperature, the larger precipitated particles tend to grow at the expense of the smaller ones—a process called “Ostwald ripening.” At temperature, atoms migrate from smaller to larger particles, seeking a lower energy state. The example of this familiar to all of us is the disagreeable coarsening of the texture of ice cream that is kept in a freezer that’s not quite cold enough. By vigorously stirring the ice cream mixture during freezing, we limit the size of the ice crystals that form in it, thereby preserving its creamy texture. But if, like last year’s pistons in this year’s uprated F1 engine, we let the resulting ice cream get a bit too warm, there is enough thermal energy to let the big crystals grow and the small crystals disappear. In a piston that has operated for some hours, the measurable result of this gradual coarsening is a loss of hardness. If we section this piston and hardness-test it in many places with an indenter, we can get a good idea of how hot its various parts operated in service. Not only that—this process of coarsening of the precipitated intermetallic particles in the piston’s material leads eventually to reduced strength, then to slow yielding (creep), the appearance of internal voids, and finally the formation of cracks. In this way, if the temperature of such a part can be kept below, say, 400F, its life may be many thousands of hours. But if it rises just a few degrees its life may be only a few hours. The required life of MotoGP pistons under the 5 engines per rider, per season, rule is of the order of 12 to 15 hours at the very high acceleration stress they experience. Reaching that life from the previous 3 to 5 hours required much work and expense. We all carry with us questions we want answered, and it’s pleasant when such an answer is found. I had heard years ago that Harley-Davidson had employed an exotic aluminum alloy containing silver for the heads of its XR750 dirt-track engine. It turns out that in attempts to develop high-hot-strength alloys better than RR58 (the US equivalent is 2618), silver was sometimes substituted for nickel, in amounts of less than a percent (even this little silver boosted price considerably). This research was being driven primarily by the (then) perceived need for an improved structural material for future (and faster) supersonic transports. When the materials proved tricky and no commercial applications stepped up to pay the R&D bills, such programs were dropped. Go read all about it in “Creep Resistant Aluminum Alloys and their Applications,” by J.S. Robinson et al. It is wonderful reading, summed up in this quote: “Ingot and powder metallurgy manufacturing routes have all demonstrated alloys with superior short-term creep performance compared with 2618 but none have gone on to successful commercialization.” What has replaced 2618 in F1 pistons? It could either be one of those alloys “with superior short-term creep performance” or it could be one of the new metal matrix composites—aluminum filled with high-strength ceramic particles.Gadget Reviews: mamaktalk.com
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