## 2 Materials and Methods  
Aluminum A380 alloy, whose chemical composition is shown in Table 1, is widely used owing to its good castability, corrosion resistance and specific strength [48]. To produce the test specimen, an aluminum A380 block was first milled with a cutting speed, \(v_{c}\) of 120 m/min, feed rate, \(f_{n}\) of 0.1 mm/tooth and depth of cut, \(a_{p}\) of 1 mm. The mixture of a 1% weight ratio of TiH\({}_{2}\) and 70 g of aluminum A380 chips were first cold compressed in a circular mold. Subsequently, the cold compressed cylinder was then heated to 420 \({}^{\circ}\)C and hot pressed to produce the final precursor element with a diameter of 40 mm and height of 20 mm. Finally, two precursors were foamed in a preheated square mold at 620 \({}^{\circ}\)C for 20 min, producing an 80 mm \(\times\) 65 mm \(\times\) 40 mm-sized square foam. Drilling experiments were conducted on a vertical machining center Spinner VC 560 using a series 601 uncoated HSS drill produced by Maykestag, Ferlach, Austria. The chosen drill had a diameter of 5 mm and a tip angle of 118\({}^{\circ}\). Three cutting environments were tested: cold compressed air, minimum quantity lubrication and cutting fluid. Due to its suitability for machining aluminum alloys, a water-miscible Rhenius TU 30T Castrol, Shanghai Haipingyao Trading Co., Ltd., Shanghai, China coolant was used in the experiments implementing MQL and cutting fluid. Thrust forces were measured using a Kistler 9271 dynamometer, Kistler GmbH, Vienna, Austria. The dynamometer was paired with a Kistler 5070 charge amplifier, Kistler GmbH, Vienna, Austria. The amplified signal was then acquired using a BCMC USB-AD16f data acquisition card, BMC Messsysysteme GmbH, Maisach, Germany. Finally, the thrust force signal was analyzed using NextView 4 software. Using a DinoLite digital microscope, AnMo Electronics Corporation, New Taipei City, Taiwan, built-up edge (BUE) formation was analyzed after each tool pass. A new tool was used for each test. Test workflow is shown in Figure 2.  
Cold compressed air cooling is a relatively novel approach to reduce tool wear during machining operations. Tool cooling using cold compressed air was achieved using a Cold Air Gun System 610 BSP, Cincinnati, USA. This mechanical device, shown in Figure 3, has a maximum airflow capacity of 425 L/min and works by separating compressed air into a hot and cold flow. Flow separation is achieved by tangentially forcing compressed air into the vortex tube, which causes the air to circulate at speeds up to one million revolutions per minute. After separation, the hot stream exits out the back of the tube, where a regulation valve allows for hot stream temperature regulation. The cold stream, whose temperature can reach as low as \(-34\)\({}^{\circ}\)C, is shot out of the nozzle directed at the tool tip. Cold air guns are very robust as they have no moving parts and only require readily available compressed air. Due to the low temperatures that can be achieved with cold air guns, dimensional errors and tool wear can be greatly reduced. To determine the optimal valve settings, preliminary foam drilling experiments were conducted. To reduce workpiece expenditure, Taguchi's L9 orthogonal array was selected for these investigations as it requires a fairly low number of experiments to produce reliable results. The orthogonal array uses three controllable factors varied in three levels. Three cutting speeds, feed rates and valve positions were varied in accordance with Table 2. Selected valve values were 90%, 60% and 30% cold air mixture at the valve end. Measurements of the outlet temperature were conducted using a Uni-Trend UNI-T UT161D digital multimeter, UNI-TREND Technology EU GmbH, Augsburg, Germany and Uni-Trend UT-T 12, Augsburg, Germany K-type thermocouple.