## 1 Introduction  
Aluminum and its alloys are considered one of the easiest machining materials. However, these materials are difficult to drill with conventional drilling (CD) due to their ductility and softness causing persistent elongated contact with cutting edges and flutes. Several methods such as using a special tool geometry, coating tool, or even reducing the material removal rate (MRR), etc., have been employed to deal with such problems. Recently, a promising technique well known as vibration-assisted machining (VAM) has been reported to overcome problems of drilling difficult-to-cut materials, including aluminum and its alloys. Compared with CD, there have been significant improvements shown in thrust force, burr size, tool wear, noise reduction, and surface finish, etc., from ultrasonic-assisted drilling (UAD). However, other advantages of UAD, such as reducing torque and cutting temperature--the most two important factors which may break the process or shorten the tool life--have not been reported so far. Overall, the improved cutting machinability of aluminum in UAD and the causes behind this phenomenon have also been rather limited in the literature. The purpose of this paper is to deal with these problems. Recently, ultrasonic-assisted drilling of aluminum and its alloys has been an attractive field of study. In a remarkable study by [PERSON] et al. [1], an experimental comparison between CD and UAD processes for Al-2024-T6 was carried out. The result shows that all investigated terms includingthrust force, morphology chip, built-up edge, exit burr, and hole oversize from UAD were reported as significantly improved compared with those from CD. In research by [PERSON] and [PERSON] [2], the authors predicted and compared thrust force induced in UAD and CD of Al-6061-T6. The conclusion shows that there is a vibration frequency that minimizes thrust force in different drills and cutting conditions. Burr height reduction of up to 85%, tool life increase (in terms of number of holes drilled), and an up to 20% reduction in the thrust force of UAD compared with those of CD have also been revealed in another study by these authors [3]. In other investigation, [PERSON] et al. [4] found that the thrust force of UAD is much smaller than that of CD, with a larger vibration amplitude resulting in the smaller thrust force of the cutting edge. A study by [PERSON] et al. [5] verified several improvements in machining characteristics due to UAD in drilling, including faster penetration rates, reduction of tool wear, surface finish, roundness and straightness of holes, and the reduction or complete elimination of burrs when drilling ductile plates. Nevertheless, the effectiveness of UAD in deep hole drilling of aluminum alloys, where the depth-to-diameter ratios are larger than 5, have rarely been found in the literature. Investigations considering the reduction of torque and temperature as a primary factor which leads to major improvements in UAD compared with CD of other materials have also been limited in the literature. An experimental study from [PERSON] and [PERSON] [6] drilling Inconel 738-LC considered on improvement in average surface roughness and circularity, hole oversize, and drill skidding, causing discontinuous and finer chips in UAD in contrast to in CD. Advantages of UAD such as reducing the thrust force and the cutting torque, causing less burning to the cutter and prolonging the tool life, have been suggested and need to be further validated. [PERSON] et al. [7] showed that producing smaller chips was a main effect of UAD for Inconel 738 LC superalloy. A study by [PERSON] et al. [8] on drilling carbon fibre reinforced plastics (CFRP) revealed that UAD resulted in lower thrust forces (up to 30%) and lower surface roughness than did CD, especially for low feeds and cutting velocities. Recently, [PERSON] et al. [9] found that a 40% reduction in thrust force can be achieved with UAD Inconel 738 LC as opposed to CD. In research by [PERSON] et al.