**7.2 Surface roughness**

The machined surface typically contains irregularities and deviations from the desired form due to the machining operations, cutting parameters, and cutting conditions used. These deviations are normally assessed as roughness. The surface roughness acts as an indicator to determine the quality of the holes and surface finish. Cutting parameters are seen to be the most critical factor that could affect the surface roughness. The authors conducted a drilling experiment on Al7075-T6 in two different conditions (dry and with chilled air) and different cutting parameters to determine their effects on the machined surface roughness (Ra). The pressure and flow rate of the chilled air that was used during the drilling were 14 m/s and 6.9 bar, respectively. The surface roughness results, which were measured in terms of Ra are shown in **Figure 9**.

#### *Aluminium Alloys - Design and Development of Innovative Alloys, Manufacturing Processes...*

#### **Figure 9.**

*The effect of cutting speed and condition on surface roughness (Ra) in drilling Al 7075-T6 (heat-treated) using cutting speeds of 80, 120 and 160 m/min at a constant feed rate of 0.05 mm/rev and 6.5 mm diameter carbide drills.*

Increasing cutting speed from 80 to 120 and to 160 m/min resulted in increasing surface roughness by the values as shown in **Figure 9**. This is supported by a previous study [12], which also conducted drilling of Al 7075 alloy with three parameters which are cutting speeds (within range of 40 to 120 m/min), feed rates (within range of 0.05 to 0.15 mm/rev), and point angle (within range of 120 to 140°). It was found that an increase in cutting speeds from 40 to 120 m/min at a constant feed rate of 0.05 mm/ rev and point angle of 140° resulted in increased surface roughness value, Ra from 0.5 to 1.16 μm. During drilling operation at higher cutting speeds, the ductility properties of aluminum alloy will increase because of the higher heat generated that is caused by, the higher spindle speed and energy. The higher cutting temperature causes thermal

#### **Figure 10.**

*Thermal softening and material adhesion on the machined surface of Al 7075-T6 caused by drilling operations at high cutting speed (a) 5x magnification (b, c) 2000x magnification.*

### *Drilling of 7075 Aluminum Alloys DOI: http://dx.doi.org/10.5772/intechopen.102864*

softening of the workpiece, consequently, more material adhesion on the machined surface, as shown in **Figure 10**, hence higher surface roughness (Ra).

Furthermore, the increasing feed rate when drilling Al 7075-T6 was also found to result in increased surface roughness, as shown in **Figure 11**. The material removal rate increases when the feed rate increases, which leads to higher cutting forces hence poor surface finish of drilled holes and higher Ra value. Whereas, using chilled air (10°C) during drilling resulted in 25–60% higher Ra than dry drilling, as can be seen in **Figures 6** and **8**. Therefore, using chilled air in drilling Al 7075 is not favorable in terms of surface finish. This is likely due to chilled air causing which works hardening of the chip, which causes poor machined surface finish as the chip evacuates from the holes. The higher Ra when using a higher feed rate is typically due to the feed mark on the machined surface as shown in **Figure 12**. The application of high-pressure internal water-based cutting fluid has been recommended in drilling aluminum alloy to result in lower surface roughness [27]. The assistance of high-pressure cutting fluid (i.e., with a pressure of 50 bar) can improve chip evacuation hence less tendency of the chip scratching the machined surface. However,

#### **Figure 11.**

*The effect of feed rate and condition on surface roughness (Ra) in drilling Al 7075-T6 (heat-treated) using feed rates of 0.01, 0.05 and 0.1 mm/rev at a constant cutting speed of 120 m/min and 6 mm diameter carbide drills.*

#### **Figure 12.**

*Feed mark on the machined surface of Al 7075-T6 (heat-treated) caused by drilling at a high feed rate of 0.1 mm/rev; (a) the surface of the drilled hole that was sectioned into a half (b) the hole exit.*

careful handling and appropriate fluid disposal must be practiced to avoid environmental pollution and health issues to operators.
