Casting Requirements On Light Metal Crankcases For Thermally Sprayed Fe-Based Bore Coatings

Keywords: Aluminum casting, surface roughening, PTWA, TWAS, Al-crankcases, automotive

Since 10 years, cast iron-liners have been replaced in several engine projects by Fe-based thermally sprayed coatings in the bores of a light metal crankcase. In contrast to cast-in liners the linerless versions of these Al-crankcases are very demanding concerning the porosity and tensile strength in the areas around the bores. The casting porosity has to be diminished to maximum pores smaller than 1mm² due the roughening procedure, either mechanical roughening (MR) or water jet roughening (WR), in order to prevent either tool failure (MR) or widened pores (WR).

At Nemak Dillingen these challenges are met by the Core Package Process (CPS), offering the advantages of a highly flexible casting design and a nearly unlimited choice of the cast alloy. These boundaries enable the production of light weight crankcases made of the strong and creep resistant Al-Si-Cu based secondary alloy A319. The high quality of the cylinder bore surface is achieved by a carefully designed thermal household of the solidifying casting. The cylinder chill form a stable and sound shell in the very beginning of solidification, whereas feeding takes places from the sidewall structure of the crankcase. At the same time specially designed chills for the bearing seat enable a very short solidification time, the resulting stiffness is crucial for highly loaded diesel engines.

After casting and machining the crankcases have been mechanically roughened and coated with 0.8 % C-Steel. The coatings and the interface between the coating and the casted Al-substrate have been investigated by means of light microcopy regarding the interlock between coating and substrate and the near surface porosity of the casted Aluminum.