STICKING AND GALLING PHENOMENA IN EJECTION PROCESS OF COATED CORE PINS FROM Al-Si-Cu ALLOY CASTING

Wear of high pressure die casting (HPDC) tools used for processing of aluminum alloys is nowadays successfully reduced by application of physical vapor deposited (PVD) coatings. Although the coated tool parts are inert to cast alloy and resist erosion, their performance and endurance are dependent on cast alloy sticking and galling phenomena which occur during casting ejection. Considering that knowledge about these specific wear mechanisms is the driving force for development of coatings for HPDC tools, further research is required on this topic. To study wear phenomena of duplex CrN and TiAlN coatings in contact with Al-Si-Cu alloy ejection test is employed. The coatings were produced with different roughness. Before and after the tests, samples were evaluated by profilometry and different microscopy techniques. Cross sectional analyses of pin-casting assemblies revealed that pin and casting form a completely interlocked contact. Intermetallic phases of Al-Si-Cu alloy more intensively precipitate on the pin surfaces than the aluminum matrix. For both coatings it was found that the shape of the ejection curve and values of ejection force depend on the roughness of pin samples. After the ejection, as a result of sticking and galling, a thin layer of cast alloy remained on all investigated samples. Its morphology reveals the pin release mechanisms, sticking effects and type of sliding wear. For rough samples, as a result of ploughing and adhesion the cast alloy remained inside the grooves on the surface. Such release process requires less load but induce a stick slip effect that promotes adhesion. On smooth samples random islands of sticking layer are observed which mostly consist of Al-Si-Cu alloy intermetallic phases. On these samples, cast alloy agglomerates on nodular coating defects which promotes thickening of a built-up layer and wrenching of defects. The post polished samples exhibited the thinnest built-up but required the highest forces for ejection.