MICROSTRUCTURE OF LASER TREATED A1 ALLOYS

In this study the microstructures of laser treated ultra pure Al and two Al-Si alloys (Al-0.4 Si and Al-0.75 Si) were investigated. In ultra pure Al a large number of dislocation loops were found especially at higher laser scan velocities. During annealing only at laser scan velocities above 2 cm/s a large quantity of dislocation loops became visible. Both results indicate that at high laser velocities vacancies are frozen in, but at laser velocities around 1 cm/s there is still enough time at high temperature to reduce the vacancy concentration towards lower super-saturation. In Al-Si alloys the dislocation density rises with higher laser scan velocities probably caused by the smaller distances between the eutectic cell walls. In these alloys entangled dislocation structures were found in contrast to ultra pure Al. For solidification structures consisting of an eutectic material with a high hardness as for an eutectic structure in Al-Si alloys it was found that the hardness can be described by a pile up mechanism, which depends on the difficulty to exert stresses on neighbouring cells due to thick and hard walls The hardness has been described by a 1/d2 dependence, i.e. it is mainly determined by the small size d, of the solidification structure.