Surface Characterization in Milling of Aluminum-Lithium Alloy
Authors: Matsumura Takashi , Morii Daisuke , Tamura Shoichi 
Published: August 2024

Abstract
Aluminum-lithium (Al-Li) alloy having superior properties in the mechanical strength with the lightweight is expected to be used for structural parts in aircrafts. However, its machining process has some issues to finish a high-quality surface of part. Adhesion of material deteriorates the surface finish especially in cutting of Al-Li alloy. Residual stress should also be controlled in manufacturing of parts applied to aircrafts. The cutting parameters and the tool, therefore, should be determined considering the surface qualities.
This study investigates the cutting process of Al-Li alloy in terms of the surface characterization. In this paper, the cutting characteristics of Al-Li alloy are compared to those of aluminum alloy (A7075) in milling. The cutting forces of Al-Li and A7075 alloys are discussed for the cutting speeds and the feed rates. In milling of Al-Li alloy, the decreasing rate of cutting force with the cutting speed is larger than that of A7075 alloys up to a cutting speed of 300 m/min. The cutting forces of Al-Li and A7075 alloys does not change above 300 m/min.
Adhesion of material on the finished surface of Al-Li alloy is more severe compared to that of A7075 alloy at low cutting speeds. The cutter marks are formed clearly above a cutting speed of 180 m/min. Adhesion is quantified by the deviation of height in the surface profile from the geometrically calculated profile, in which the tool inclination induced by tool distortion is taken into account. More adhesion is measured on the finished surface of Al-Li alloy than that of A7075 alloy.
Residual stresses were measured around the center of machined groove. Regarding the characteristics for the cutting speed at a feed rate of 0.04 mm/tooth, residual stresses of Al-Li alloy are distributed in mostly tensile; while residual stress of A7075 alloy change from compressive to tensile with increasing the cutting speed. In milling of Al-Li alloy, when the feed rate is small, residual stress becomes compressive. Then, residual stress shifts to tensile with increasing the feed rate. Residual stress is also associated with the resultant force applied to surface around the center of groove and its direction angle. The direction angle is defined as the angle of upward inclination with respect to the cutting direction. Residual stress is compressive at a cutting speed of 300 m/min when the resultant force is small. Residual stress, then, changes to tensile with increasing the resultant force. In terms of the characteristics for the cutting speed at a feed rate of 0.04 mm/tooth, residual stress becomes compressive with large resultant forces at low cutting speeds. When the cutting speed is low, adhesion of material at the tips of cutting edge promotes compressive stress. Regarding the direction angle, a good correlation between the direction angles and residual stresses though the direction angle is distributed in the narrow range for the testing samples. The inclination of resultant force becomes upward with increasing the direction angle, which acts to tensile stress applied to a surface.
According to the results in this paper, in milling of Al-Li alloy, the cutting speed is taken to disappear adhesion above 180 m/min at a feed rate of 0.04 mm/tooth. Residual stress-free surface could also be achieved at a cutting speed 180 m/min. Tensile stress promotes with increasing the cutting speed.