Fast Anodizing of Aluminum Piston Heads

Hard anodizing of aluminum piston heads for internal combustion engines Is usually performed by immersion, with a time ranging from 40 to 50 minutes. This paper wii i present a new process, based on specially adapted hydrodynamics of the electrolyte, applying electrolysis parameter values outside the usual range and which greatly reduce the treatment time. In addition to applications to other components, it has been tested with Industrial partners on thermal fatigue test pieces and on pistons, with and without a precombustion chamber.The influence of anodizing parameters (pretreatment, electrolyte temperature, current density, bath composition) will be discussed in relationship with the practical properties of the coating layer such as thickness hardness and thermal fatigue resistance. Introduct ion Among the 37 million passenger cars and 14 million trucks manufactured in the world in 1994, a substantial part is equipped with diesel engines (23 YO of passenger cars in Europe, 50 Yo in France). The proportion is particularly high with the trucks (80 Yo in France for example). Compared with the gas engine, the piston of the diesel car is submitted to a higher working temperature, between 250 and 350 OC (480 and 660 OF), which is going to become higher, due to the increasing power developed by engines. The aluminum piston, generally in cast alloy AI Si 12 Cu Ni Mg, sustains important sollicitations in terms of thermal fatigue. For many years, the solution to minimize the effect of temperature has been to build a thick oxide layer on the piston head by hardcoat anodizing. Due to the low thermal conductivity of alumina (30 times less than the metal conductivity) this layer acts as a thermal barrier during the piston life. The process described in this paper enables the obtention of such layers with operating times significantly shorther than with classical treatments. Hard anodizing and its limits PrinciDle Hardcoat anodizing, which finds many applications in various industrial fields, provides a thick, abrasionresistant oxide film on the aluminum surface. Compared with standard anodic layers, hard films are obtained in specific conditions : lower temperature : between 0 and 10 "C (32 to higher current densities : between 2 and 3.5 Ndm2 special electrolytes, usually based on sulfuric acid. However, the growth mechanism of the oxide layer is quite similar to classical anodic films (Figure 1) : initially, a barrier layer is formed when the current is applied, an evolution to a porous and hexagonal structure then occurs with competition between the oxide formation by the current and its chemical dissolution by the electrolyte. 50 OF) against 20 to 25 "C (70 to 75 OF), (19 to 33 Nft2) against 1 to 1.2 Ndm2 (9 to 11 A/ft2), formation of the pores on the barrier laver surface initiation of the cells I growth of the hexagonal cells I Figure 7 : growth mechanism of a porous oxide layer The oxide structure itself will depend on the operating conditions. In particular, the pore-wall thickness will increase with the voltage. 269 1 In the case of hard anodizing, the low temperature of the electrolyte reduces chemical dissolution of the oxide. Coupled with a strong agitation it removes the heat produced by the electric current in the bottom of the pore (Joule's effect) and allows the use of high current densities. The resulting high voltage involves the formation of a more compact structure, a thicker, harder and more abrasion-resistant layer. Moreover, hard anodizing is less sensitive to alloy composition. The reason is the lower dissolution rate of intermetallic compounds which usually give some porosity in the oxides formed at room temperature. I ~imifs m r d A nodizing The use of high current densities accelerates oxide formation. However, the growth of the film will increase the voltage during the anodizing operation (Figure 2). 270 time + There are two limits : the power supply capacity, the "burning" phenomenon. The latter is due to a local heating in the pore, involving a rapid chemical dissolution of the oxide. The current lines distribution is then completely modified, with, as a consequence, a complete stop of the anodizing process. The control of the agitation plays a major role but, when the geometry and the operating conditons are fixed, there is a limit for the current density and the anodizing time. Concerning the treatment of aluminum piston head, anodic oxidation is usually performed by an immersion of 40-50 minutes for oxide layers 60 to 90 pm thick. That is why we developed a specific process where the aim was to retain the same properties for the oxide film while significantly reducing the operating time. Description of the process Characterization of the samples General orincble The process, as shown in Figure 3, is based on a concentric circulation of the electrolyte on the piston surface, allowing quick removal of the heat produced by the treatment. Electrolyte circulation is performed by a centrifugal pump. The acid is injected through a counter-electrode specially designed for that purpose. The jet strikes perpendicularly the piston head, then is recycled into the tank by a curved-shape baffle. A cooling system is used to maintain the electrolyte at low temperature. This system does not require a piston masking operation prior to anodizing treatment. Figure 2 : Voltage vs time for Hard Anodizing piston I counter-electrode & 9 , 11 .+ I, -----.AI 1 Figure 3 : fast Anodizing process &Several sets of samples have been tested on this pilot device : thermal fatigue test pieces, pistons with and without a precombustion chamber. (Figure 4)