*3.2. Tool Wear in Conventional Drilling UNS A97075*

During conventional drilling of the aluminium alloy UNS A97075, the interaction between the workpiece and the tool caused a large helical chip (Figure 8), which makes machining difficult [2]. The zinc (Zn) contained in the UNS A97075 alloy provides a high degree of plasticity, making breakage difficult due to its ductility [3].

**Figure 8.** Chip evacuation during dry drilling of alloy UNS A97075.

The secondary adhesion of the aluminium alloy on the tool was the main wear mechanism detected. This was favoured by the absence of refrigeration and lubrication, as happened in this case. Thus, in the first stage, the aluminium matrix was melted down and welded onto the tool surface, forming a pure Al layer and giving rise to the primary BUL (Figure 9a). This circumstance provided the conditions for the mechanical adhesion of the alloy, mainly in the areas of the edge, resulting in a built-up edge (BUE) that grew to a critical size at which it began to extrude, giving rise to a secondary BUL that was deposited as a second layer over the primary BUL [4–6,17].

The location of the BUE is usually over boundary edge zones, but the BUL, during the drilling, may extend to areas outside the rake face, such as the chip evacuation channel, seriously hampering the quality of the process (Figure 9b).

**Figure 9.** Wear by bonding mechanisms (BUL–BUE) on the tool after conventional drilling of the aluminium alloy UNS A97075: (**a**) Main cutting edge; (**b**) evacuation channel.

The raised friction in the piece–tool–chip interactions during cutting resulted in an elevation of the process temperature, increasing the affinity between the tool material and the aluminium, and intensifying the plasticity of the alloy, which facilitated this type of wear [7–9]. This process caused higher adhesion of the aluminium over the rake face, as in the regrowth of the cutting edges (Figure 10).

**Figure 10.** Increased bond wear with the number of holes.

The analysis of the cutting edges showed irregularities. These were produced by the dynamic phenomenon of primary adhesion, which, with the detachment of the BUE and its new generation, slowly removes material from the tool (Figure 11a). At the same time, this action can cause abrasive wear on the tool surface or affect the quality of the surface generated on the workpiece itself (Figure 11b).

**Figure 11.** (**a**) Primary adhesion mechanism; (**b**) geometric irregularities observed in the cutting edges.

#### *3.3. Tool Wear in Conventional Stack Drilling CFRP/UNS A97075*

When drilling stacks of CFRP/UNS A97075, the drill bit comes into contact with two materials at the same time and acts on them with similar parameters.

Observation of the tools showed how adhesion was the main type of wear in the conventional drilling of this stack. During machining, the rake face was subjected to increased pressure and temperature [18,19], hence helping in the formation of an adhesive layer in the contact zone between the tool and the workpiece [14,20]. Adhesion wear was caused by the mechanical removal of the tool material when the adhesive junctions were broken. The effects of the abrasion wear mechanism were attenuated by the adhered material (Figure 12).

**Figure 12.** Wear located on the tool after conventional drilling of the CFRP/UNS A97075 stack: (**a**) Flute (BUL); (**b**) cutting lip (BUE y BUL); (**c**) tool tip; (**d**) joining of primary and secondary cutting edges (BUL).

It was seen how the adhered material was composed of both aluminium alloy and carbon fibre particles. This caused a geometric change in the tool, affecting the final quality of the hole [20–22] but also its mechanical characteristics, since the incorporation of the Al–CFRP mixture was abrasive to the material that had to be machined (Figure 13).

This blend of aluminium and CF seemed to facilitate chip adhesion along the tool during dry drilling of CFRP/ UNS A97075, as shown in Figure 14.

One method to decrease wear during the drilling process is to reduce the temperature. This can be done by the use of lower cutting speeds and higher feed rates and the use of advanced techniques (vibration-assisted drilling, strategies with minimum quantity lubricant (MQL) and cryogenic machining, for instance).

**Figure 13.** (**a**) Composition analysis of the BUE and release side after dry drilling of the stack CFRP/UNS A97075; (**b**) details of the BUE with carbon nanotube adhered to aluminium.

**Figure 14.** Chip removal during stacking drilling CFRP/UNS A97075 using conventional technology.
