# Cutting Speed and Feed Influence on Surface Microhardness of Dry-Turned UNS A97075-T6 Alloy
## 1 Introduction  
Pure aluminum or aluminum with a very low weight percentage of alloying elements (1000 series) has, among its mechanical properties, a low wear resistance and a low hardness. Therefore, the addition of alloying elements implies an improvement of the aluminum surface characteristics and mechanical properties, making these alloys one of the most used materials in the industrial activity [1]. One of the industrial sectors in which aluminum alloys are of great interest is the aeronautical industry. Their excellent density and mechanical properties make this type of material suitable for the manufacturing of aircraft structural elements. In particular, the 2000 (Al-Cu) and 7000 (Al-Zn) series are widely used for structural elements in the wings, the fuselage, or the tail of the aircraft [2]. These structural components are usually high-rigidity elements and, frequently, they need to be manufactured in one piece. In these cases, the machining operations are usually quite competitive from a productive point of view. In particular, milling operations are used for large pocketing operations in rib-shaped elements, drilling operations are frequent in the machining of holes for rivets, and turning operations are used to manufacture coupled parts with cylindrical shapes [3]. Machining operations result in alterations in the surface of the stock material. These alterations not only affect their geometrical characteristics (at macro and micro scale) but also modify the physical-chemical and mechanical properties of the surface (residual stresses, microstructural alterations, cracks and microcracks, microhardness, corrosion resistance, etc.) [4; 5; 6]. All these alterations (both material properties and geometry) are part of the surface integrity concept [7; 8; 9], which is oneof the most valued aspects from the point of view of quality requirements, especially in structural elements for aircraft, where these requirements are very high due to functionality and reliability reasons [10]. Lubricants or coolants, applied during the machining process (cutting fluids), are traditionally used to increase the productivity of these processes and the machined part functionality. However, the current trend regarding the use of sustainable manufacturing technologies led to a reduction in the use of these highly polluting substances. This resulted in the application of more efficient techniques, such as MQL (minimum quantity lubricant), or the total suppression of these substances (dry machining) [11; 12; 13; 14; 15]. In addition, aluminum alloys are frequently used hybridized with other materials, such as carbon fiber-reinforced polymers (CFRP) or titanium, to form structural elements, such as fiber metal laminates (FML) [16]. In those cases, the chip recycling (mixture of metal, carbon fiber, and cutting fluids) becomes complex and expensive. In addition, it should be pointed that CFRPs do not exhibit good behavior under the cutting fluid action [17]. However, in dry machining, the thermal effects become more relevant compared to the effects of plastic deformation due to mechanical actions on the surface. This results in greater surface alterations at both microstructural and geometric levels [18; 19; 20]. Due to the high importance of the surface state in phenomena such as crack and microcrack initiation, which can affect significant properties such as fatigue resistance or corrosion [21], it is necessary to analyze the effect of the machining input variables on the machined part surface integrity. Thus, the surface microhardness of aircraft structural components is particularly relevant, given their influence on the bearing capacity and the wear resistance of these elements [22; 23]. In this sense, the cutting parameters are some of the most important machining input variables to keep in mind regarding this output variable.