Analysis of machining-induced residual stresses of milled aluminum workpieces, their repeatability, and their resulting distortion

The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-021-07171-7 ORIGINAL ARTICLE Analysis of machining-induced residual stresses of milled aluminum workpieces, their repeatability, and their resulting distortion Daniel Weber 1 & Benjamin Kirsch 1 & Christopher R. Chighizola 2 & Christopher R. D’Elia 2 & Barbara S. Linke 2 & Michael R. Hill 2 & Jan C. Aurich 1 Received: 15 January 2021 / Accepted: 27 April 2021 # The Author(s) 2021 Abstract Machining-induced residual stresses (MIRS) are a main driver for distortion of thin-walled monolithic aluminum workpieces. Before one can develop compensation techniques to minimize distortion, the effect of machining on the MIRS has to be fully understood. This means that not only an investigation of the effect of different process parameters on the MIRS is important. In addition, the repeatability of the MIRS resulting from the same machining condition has to be considered. In past research, statistical confidence of MIRS of machined samples was not focused on. In this paper, the repeatability of the MIRS for different machining modes, consisting of a variation in feed per tooth and cutting speed, is investigated. Multiple hole-drilling measurements within one sample and on different samples, machined with the same parameter set, were part of the investigations. Besides, the effect of two different clamping strategies on the MIRS was investigated. The results show that an overall repeatability for MIRS is given for stable machining (between 16 and 34% repeatability standard deviation of maximum normal MIRS), whereas instable machining, detected by vibrations in the force signal, has worse repeatability (54%) independent of the used clamping strategy. Further experiments, where a 1-mm-thick wafer was removed at the milled surface, show the connection between MIRS and their distortion. A numerical stress analysis reveals that the measured stress data is consistent with machining-induced distortion across and within different machining modes. It was found that more and/or deeper MIRS cause more distortion. Keywords Machining-induced residual stresses . Distortion . Milling aluminum . Finite element method 1 Introduction The surface integrity is an important domain of part quality, especially the part quality of milled monolithic thin-walled aluminum components in the aerospace industries [1]. There is a constant need for improved surface integrity and enhanced functional performance of machined components [2]. Residual stresses (RS) are one attribute of the surface integrity and have a major influence on in-service failures such as corrosion and fatigue life of parts [3]. RS are defined as the internal * Daniel Weber 1 Institute for Manufacturing Technology and Production Systems, Technische Universität Kaiserslautern, Gottlieb-Daimler-Str, D-67663 Kaiserslautern, Germany 2 Department of Mechanical and Aerospace Engineering, University of California, One Shields Avenue, Davis, CA 95616, USA stresses locked in a body, where force and torque equilibrium prevail and no thermal gradients appear [4]. It is known that RS in thin-walled monolithic aluminum parts, where up to 90% of the initial material is removed, cause distortions. These distortions lead to high costs due to remanufacturing and part rejection [4]. In this context, one must distinguish between two sorts of RS [5]. One sort are the initial bulk residual stresses (IBRS), which exist previous to machining in the blank material. They are caused by processes like heat treatments (e.g., quenching) and appear throughout the entire part-thickness [5]. The second sort are the machining-induced residual stresses (MIRS), which are driven into the material during the machining process. In terms of surface integrity, the MIRS are from greater interest, because their penetration depth is limited to a shallow layer just under the part surface. A typical MIRS profile in a milled aluminum material looks square root shaped (-√-) with compressive RS near the surface [5]. It is known that different machining parameters and different tool geometries cause different MIRS [5]. Especially the high mechanical loads which occur during machining induce Int J Adv Manuf Technol a non-uniform plastic deformation on the surface layers of the materials [6]. Typically, those deformations lead to a square rooted shaped compressive residual stress profile, which varies in the maximum residual stress (MaxRS), the depth of it (tm), and the penetration depth (tp) depending on the machining parameters. The penetration depth is hereby defined as the thickness of the layer containing MIRS. Research investigating the effect of the feed per tooth, cutting speed, width of cut, depth of cut, and tool geometry was conducted in the past. It was found that an increase of the feed per tooth leads to higher MaxRS at greater depths (tm) when milling Al7449-T7651 samples with end mills and cutters with indexable inserts parallel to feed direction [7, 8]. The depth tm increased from 0 μm to a maximum of approx. 45 μm, while the MaxRS increased from approx. −325 MPa to −400 MPa for milling with a cemented carbide helical cutter (d= 20 mm) for an increased feed per tooth fz from 0.05 mm to 0.30 mm (ap= 4 mm, ae= 20 mm) [7]. The same trend of an increased MaxRS was investigated for milling Al7050-T6 with inserts in cutting direction [9] and Al7050-T7451 with end mills in feed and cutting direction [10]. In contrast, the investigations of Tang et al. showed no systematic trend of MaxRS and tm parallel or perpendicular to the feed direction when milling Al7050-T7451 with end mills [11]. The depth tm, e.g., lays between 15 and 20 μm and the MaxRS in feed direction does not change significantly (approx. −80 MPa) for an increased feed per tooth from 0.1 to 0.2 mm when milling with a three fluted cemented carbide end mill (d= 20 mm, R= 3 mm, ap= 2 mm, ae= 10 mm, n= 16000 rpm) [11]. Past research showed that for the variation of cutting speed there is no common effect on the MIRS. Denkena et al. [8] and Perez et al. [3], e.g., observed that increased cutting speeds for milling Al7449-T7651 and Al7050-T7451 respectively, with indexable inserts, lead to an increase in maximum compressive stresses. But in other studies by Denkena et al., the use of helical cutters for different cutting speeds did not influence MaxRS and tm at all [7]. MaxRS of approx. −300 MPa was measured at a depth between 30 and 45 μm for different vc between 250 and 1500 m/min [7]. Furthermore, decreased compressive RS for an increase of the cutting speed were observed by Tang et al. [11] and Rao et al. [9]. The MaxRS in feed direction was measured to −120 MPa (n= 4000 rpm) and −90 MPa (n= 16000 rpm) [11]. Different researchers found that the MaxRS increased with increasing depth of cut [7, 12]. In contrary for the variation of the width of cut, the highest MaxRS were found for the lowest width of cut [7]. In term (...truncated)