Tool adhesion wear can be produced by two different ways. On the one hand, direct adhesion wear is caused by the incorporation of tool particles to the chips. On the other hand, indirect adhesion wear is caused by the incorporation of fragment of the workpiece material to the tool. When these fragments are removed, they can drag out tool particles causing tool wear. Indirect adhesion can be localised in the tool edge, giving rise to Built-Up Edge (BUE), or in the tool rake face, giving rise to Built-Up Layer (BUL). In this work microstructural differences between both effects in different machining processes of Al-Cu alloys have been analysed. From these microstructural differences, a model has been proposed for the mechanism of formation of both effects.