*1.2. Tool Wear Mechanisms in Aluminium Drilling*

The main wear mechanism during the machining of aluminium alloys is adhesion. When the material that has to be machined comes into contact with the tool surface, it creates bonding forces much stronger than the mechanical strength of the materials in contact, resulting in the transfer of particles from one surface to another [14]. This type of wear can occur in two ways:

1. Primary or direct adhesion. The particles of the tool are adhered to the chip being welded by the action of the forces developed in the tool–material interchange. In this case, when the yield stress of the chip is higher than the breakage limit of the adhered particles, these are pulled out of the tool and transported by the chip. This event can also promote abrasion on the release side due to the friction caused by these particles [15–17].

	- Adhesion on the cutting edge or raised edge (BUE—built-up edge).
	- Adhesion on the release side (BUL—built-up layer).

This phenomenon involves modifications in the properties of the cutting edge, with successive layers of machined, welded and hardened material becoming part of the edge. As mentioned before, wear is a dynamic mechanism, so the filler edge or BUE can be detached and regenerated, slowly removing material from the tool and causing primary or direct adhesion [17].

The friction of the chip with the tool within a certain temperature range increases the affinity between the materials of the tool and the piece, which promotes wear by adhesion. This situation is usually mitigated by high cutting temperatures [16].

Continuous action of primary and secondary adhesion causes premature wear of the cutting tool, causing geometrical variations that affect both surface finishing and dimensional and geometrical tolerances (Figure 2).

**Figure 2.** Influence of adhesive wear on the surface, dimensional and geometric quality of the borehole.
