Optimization of Cutting Parameters for Pocket Milling on the Skin Plate in Al and Al-Li Materials

L'objectif de cette étude est d'optimiser des paramètres de coupes pour l'usinage de poche sur une plaque mince en alliage d'aluminium et en alliage d'aluminium-lithium. Ces plaques minces sont utilisées dans l'industrie aéronautique pour fabriquer le fuselage d'un avion. Présentement, ces poches sur les plaques minces sont fabriquées par usinage chimique. Cette méthode chimique est dite nocive pour l'environnement. La méthode chimique pourrait être remplacée par une méthode mécanique comme l'usinage. En plus, les paramètres de coupes seront optimisés pour l'alliage d'aluminium-lithium. L'effet des paramètres de coupes a été étudié par des expériences utilisant la méthode de Taguchi. L'analyse de rapport signal sur bruit (Signal to Noise ratio) a été menée sur les données recueillies pour illustrer la significativité des facteurs des plans d'expériences et de leur contribution. La rugosité de la surface sur les pièces a été aussi étudiée et des paramètres optimaux ont été définis. Des vérifications ont été accomplies et la poche sur la plaque a été usinée à la satisfaction des exigences de l'ingénierie de l'industrie.

Abstract

In the present work the pocket machining (milling) of the thin skin components made of aluminium and aluminium-lithium (Al-Li) alloys is studied. These milled components are known as principle parts of commercial airplanes. They have significant impacts on the airplane body weight and fuel consumption. Chemical milling is the main method used for pockets machining on these components. However, this method is not considered as an environmentally friendly operation due to severe contamination problems. To remedy these difficulties, this study intends to replace the chemical milling by an alternative machining method capable to do pocket machining. To that end, pocket milling was selected as machining method. Furthermore, in order to reduce the weight of airplane, an alternative material such as Al-Li alloys is proposed to replace the aluminum alloys. In the first phase of this study, a comprehensive literature review was conducted on milling and pocket milling of aluminum and aluminum-lithium alloys. The sample parts required for cutting operations were prepared in accordance with in specified dimensional geometries of the real parts used in industry. A milling fixture was then designed and manufactured in order to perform machining operations on the sample parts. The experimental tests were planned according to the Taguchi method design of experiment. The cutting parameters studied included: RPM, chip thickness (feed rate), depth of cut and lubricant. The one way and profile contouring milling operations were selected as machining strategies. A process failure mode and effect analysis (FMEA) was executed to determine the main failure modes during pocket milling operations and the surface roughness was used as performance criteria. The experimental results were analyzed using Signal to Noise ratio (S/N) strategy though Taguchi method. According to the experimental results, the optimal setting levels of cutting parameters are RPM (10000 rev/min), chip thickness (0.0508 mm), depth of cut (0.45 mm) and lubricant (MQL, 40 ml/min). Finally, the experimental verification tests were performed. According to the literature, a similar machining specification can be applied for conventional aluminium alloys and the Al-Li alloys. Consequently, in order to reduce the experimental cost and time, the optimum setting levels of process parameters proposed in this work could be applied in the machining of Al-Li work pieces.