Evaluation of tool wear and machined surface integrity when drilling aluminium alloy 7075 under cold air and dry conditions

Abstract

Application of aluminium alloy 7075 (Al7075) in automotive components is in demand due to its high strength-to-weight ratio. For assembly purposes, drilling operations need to be performed on the components for joining using mechanical fasteners. Drilling in dry condition is typically conducted in industry to avoid environmental pollution caused by the usage and disposal of cutting fluid. However, dry drilling is challenging as it often results in high tool wear rates and poor machined surface finish. Using cold air in drilling operations is seen as an alternative to achieve a hygienic and clean process. This study investigates the tool wear mechanisms and machined surface integrity of Al7075 under dry and cold air (10°C) conditions. Drilling experiments were conducted using tungsten carbide cutting tools at cutting speeds of 82 - 163 m/min and feed rates of 0.01 - 0.1 mm/rev. The least tool wear was found to occur when drilling with the lowest cutting speed of 82 m/min and the highest feed rate of 0.1 mm/rev, which is 0.06 mm after 140 holes in dry condition, compared to 0.18 mm when using cold air. Whereas, the highest tool wear occurred when drilling with the highest cutting speed of 163 m/min at all feed rates in which the tool broke after only 10 holes in both dry and cold air conditions. The tool failure in dry drilling of Al7075 was found to be due to adhesive wear mechanisms as a result of chip adhesion on the cutting edges that led to cutting edge chipping. Whereas, drilling with cold air resulted in more fracture and edge chipping compared to dry drilling due to work hardening. Nevertheless, the application of cold air during drilling Al7075 at cutting speeds of 82 – 123 m/min and feed rates of 0.05 – 0.1 mm/rev particularly when the wear is minimum (hole 10) was found to improve the surface roughness (Sa) by 14% - 52% than dry drilling. This is due to less material adhesion on the machined surfaces when using cold air which caused 22 ºC lower temperature than dry drilling. As for burr height, the usage of cold air in drilling at cutting speeds of 82 – 123 m/min and feed rate of 0.1 mm/rev resulted in 38% - 68% lower burr height than dry drilling at the 10th holes. As more holes were drilled (hole 80), the use of cold air in drilling Al7075 still resulted in 78% lower burr than dry drilling at the cutting speed of 123 m/min and feed rate of 0.1 mm/rev. This study shows the usage of cold air when drilling Al7075 was not favourable to reduce tool wear, however, it can be beneficial in improving the quality of machined surface in terms of surface roughness and burr formation. Keywords: aluminium alloy 7075; drilling; cold air; tool wear; surface roughness; burr