Durabowl is multinational technology-provider Federal-Mogul’s name for a new production method for pistons which gives greater resistance to high thermal and mechanical loadings. This is obviously a very relevant area of research, as demands for higher thermodynamic efficiency and lower component masses make increasing demands on the alloys we use. F-M had current and future generations of turbodiesels in mind when the Durabowl was developed; the Company claims that its technology improves piston durability by more than four times under severe operating conditions compared with a conventional cast piston.
Like virtually all pistons used in car engines, the Durabowls are cast from aluminium alloy; as usual, the alloying material is silicon.
Federal-Mogul Durabowl piston Al-Si microstructure: normal (l) and after re-melting (r).
After analysing failed parts, Federal-Modul identified that both thermal and mechanical failures of the piston bowl can be traced to the presence of free primary silicon particles distributed throughout the aluminium. The presence of these silicon particles isn’t a ‘fault’: it is quite normal, but the distribution and size of the silicon particles in the aluminium structure is very hard to control or predict. At least, this is true with conventional casting techniques.
Under thermal loads, aluminium expands four times as more than silicon. This creates stresses within the structure of the alloy as the temperature fluctuates. Additonally, the repeated mechanical shock loads delivered each time the cylinder fires can potentially lead to fatigue failure from the corners of the silicon particles: it is a truism that, so far as metals are concerned, curves are strong and corners are weak. But silicon is a necessary constituent of the aluminium alloy, controlling the overall thermal expansion properties of the metal and offering good castability, so it can’t be eliminated.
The only potential solution to this problem until now has been fibre-reinforced pistons. Clearly, though, these are complex and expensive to manufacture: the molten alloy has to infiltrate the fibres during casting. Also, there is not yet a reliable, non-destructive way to test the integrity of the finished part. But Federal-Mogul’s Durabowl pistons can be tested using the same techniques that are used for conventional castings: F-M uses an eddy current test.
Federal-Mogul Durabowl piston.
So what is a Durabowl piston? A part is cast conventionally and pre-machined. The alloy around the rim of the piston bowl is then re-melted; the re-melted alloy cools a thousand times faster than it did when originally cast, which leads to much smaller silicon particles in the micro-structure: only one tenth of the previous size.
The process is physically simple. The sophistication is in the control of key parameters to ensure consistent quality. The result is a high-performing and very cost-effective product compared with both fibre-reinforced and steel pistons.