Aluminum alloys used in monolithic parts for aerospace applications are subjected to distortion and residual stress (RS) generated by milling, affecting the product fatigue life. Particularly, the change in RS with depth (z) has a characteristic distribution with a maximum compressive RS at a z several tens of micrometers from the surface; however, the RS value depends on the measurement method used. In this study, the RS distribution with z from the surface after milling was measured for the AA7050-T7451 aluminum alloy by two-dimensional X-ray diffraction (2D method). The results were compared with those of four prior measurement methods, and the validity of 2D method was verified. The changes in subsurface RS with z showed similar distributions under all measurement conditions except when cos(α)-XRD was employed. The 2D method provides high repeatability. The in-plane RS distribution was also measured using 2D method to investigate the effect of milling conditions on this distribution. The RS values varied markedly depending on the measurement position, particularly at a small collimator diameter of 0.146 mm, allowing detection of localized extreme RS values. The maximum RS at z = 0 mm was − 85.6 MPa at a cutting speed of vc = 200 m/s and feed per tooth of fz = 0.05 mm, while it was − 16 MPa for vc = 450 m/s and 6.8 MPa for fz = 0.2 mm, revealing that the compressive RS changes to tensile RS as vc and fz increase.

The performance of thinwalled, monolithic aluminum alloys used in the aerospace industry is significantly influenced by their surface morphology. This is composed of the mechanical and metallurgical boundary layer properties and the surface topography. The influence of the cutting parameters on surface roughness, microstructure, microhardness and machining induced residual stresses of milled AA 7050 is investigated in this research.

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.

Thin-walled milled monolithic aluminum workpieces are common in the aerospace industry, because of their advantages such as favorable material properties, established design methods, good machinability, known performance and reliable inspection techniques. Machining-induced residual stresses (MIRS) and initial bulk residual stresses (IBRS) are known to be the key factor for causing distortion of those workpieces. In this study the effect of both types, the milling path strategy and the wall thickness on the distortion is analyzed. A developed 3D linear elastic finite element (FEM) model, considering both types of RS as input, and predicting the resulting part distortion, is extended by using the true milling path as well as all RS contained in the entire workpiece. The simulation outcome is validated by experiments and distortion minimization techniques are derived. The results show that the model predicts the distortion shape and level for different wall thicknesses and milling strategies with a high accuracy. The experiments indicated that the distortion of low IBRS parts with 3 mm wall-thickness could be minimized by 42% by only changing the milling path. The simulation highlighted, that for smaller wall thicknesses, the distortion minimization potential is even higher (e.g. 2 mm wall thickness: up to 69% distortion minimization). For high IBRS parts, milling the inverse form of the predicted shape on the backside of the workpiece represents an alternative, which led up to a reduction of the distortion by 77%. In general, three main categories for precontrol distortion techniques can be identified as the process parameters, the part topology and the process strategy.