New Mechanical Pretreatment Process for Thermally Sprayed Aluminum Crankcases

INTRODUCTION The increasing demand for vehicles with reduced fuel consumption and emission is the reason why the material aluminum is frequently used for crankcases in the automotive industry. A method to increase the wear resistance of the cylinder bore surface is to use hyper-eutectic cylinder liners, which are cast-in or are assembled by force fitting. Another possibility to improve the tribological behaviour is to coat the surface with a wear-resistant coating by thermal spraying. To guarantee functional and heavy duty coatings, it is necessary that there is a strong bond between the coating and the aluminum body. To achieve this aim, the surface of the cylinder bore has to be pretreated. Stateof-the-art are blast processes as well as employing chemical bond coats. These processes have some disadvantages like high investment and process costs, environmental pollution with chemical remainder or expensive grit reconditioning. Therefore, a new pretreatment process has been developed. This new machining process is a kind of modified fine boring process. During machining a defined profile is cut into the surface by using a PCD-tool with a special geometry. The tool rotates and moves axially with a defined feed rate through the cylinder bore. The resulting profile is a reproducible and defined structure which — regarded in a cross-section — reminds of a dovetail. The undercuts in the cylinder surface secure a strong bonding between the aluminum substrate and the coating. Adhesion tensile strengths up to 60 MPa were measured, which clearly exceed the values which can be achieved by high-pressure water-jet roughing. Another advantage of the mechanical pretreatment method lies in the fact that the machining can be accomplished on the fine boring machine. The fine boring process is a step in the process chain which cannot be substituted up to now. Therefore, there is no necessity for an extra pretreatment machine. This also means that additional time and therefore costs for the transport and a readjustment in the fixture of the crankcase can be avoided. Another advantage of this new machining process is the short cutting time with about 10s per cylinder bore, depending on material and cylinder length. Therefore, the process can be excellently integrated into the process chain. The significant advantages of this new pretreatment process seem to offer an interesting alternative to the state-of-the-art pretreatment processes for thermally sprayed light-metal crankcases.