Machining induced residual stress in structural aluminum parts
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- Denkena, B., Boehnke, D. & de León, L. Prod. Eng. Res. Devel. (2008) 2: 247. doi:10.1007/s11740-008-0097-1
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Machining operations of aluminum structural parts are typically carried out under high feeds and high cutting speeds. Under these conditions, high thermomechanical loads are exerted on the workpiece, which may result in changes in the subsurface material. Residual stresses can be one of the machining induced changes and can lead to considerable rejection rates caused by part distortion. Due to their significant economic importance, it is essential to understand the influence of the machining process on the residual stresses in aluminum. This paper presents the influence of the machining parameters as well as the cutting edge geometry on residual stress of workpieces made out of a forged aluminum alloy.
KeywordsProduction processMillingResidual stress
An efficient and flexible manufacturing process is a requirement in the aerospace industry in order to cope with the market unpredictability. This requirement, together with the need to reduce fuel consumption, has led to the design of thin monolithic components. Generally, such components are produced out of prismatic aluminum blocks in order to avoid large stocks of preformed parts and additional weight due to fixtures like bolts, screws and pins. Therefore, up to 98% of the weight of the raw material has to be removed through machining processes [1, 2].
Developments in machines and cutting tools have enabled very high material removal rates through the technology known as high performance cutting (HPC). This technology offers an efficient solution for manufacturing of thin monolithic components. The advantages of high performance cutting in comparison to conventional milling processes are very high feed speeds by high cutting rates, which enable significantly reduced production times .
In the past few years, research work has focused on accomplishing these extremely high material removal rates. However, the influence of these production processes on the properties of these manufactured components has not been sufficiently investigated .
The residual stress state of a workpiece can significantly extend or shorten its lifetime . Furthermore, part distortion is a function of residual stresses and is caused by complex relationships between material processing, component design and manufacture. Residual stresses are especially important concerning part distortion for thin monolithic aerospace components .
Residual stresses are defined as mechanical stresses in a solid body, which is currently not exposed to forces or torques and which has no temperature gradient . It is well known that machining processes such as turning, milling and drilling, may create undesirable tensile residual stresses on the surface of workpieces leading to a reduction in the fatigue life of parts . During cutting the mechanical and thermal effects associated with chip formation cause inhomogeneous plastic deformation of the workpiece .
Residual stresses can be attributed to mechanical and thermal loads, which both occur during machining processes in an interdependent manner [9–11]. Their combination determines the final residual stress state of the workpiece [12, 13]. The superposition of the residual stresses induced during the material manufacture and machining operations results into the final residual stress distribution .
Different authors have carried out experimental investigations in order to study the influence of the machining parameters, tool geometry, cutting edge geometry, tool wear and cutting tool material on the residual stresses. Since the pioneering works in 1950s , a substantial amount of experimental work has accumulated regarding the development of residual stresses as a function of the machining process.
Plöger investigated the influence of high speed cutting on the residual stresses in turning of AISI 1045, while Gey  researched the influence of the cutting parameters on the residual stresses in end milling of TiAl6V4 . However, there is limited study on the effect of machining processes on residual stresses in aluminum alloys [5, 17]. Therefore, a comprehensive study of the influence of machining conditions on residual stresses in high strength aluminum alloys is much sought after in the manufacture of large and thin monolithic aerospace components.
Modifications of the machining process can significantly control the final residual stress state, due to their direct influence on the mechanical and thermal loads [9, 16, 18]. On the one hand, the mechanical loads exerted by the forces on the workpiece cause compressive residual stresses. On the other hand, rapid cooling of the very hot contact zones generates a shock on the superficial workpiece layers, which leads to tensile residual stresses .
A review of the literature available at the moment shows a consensus in the scientific community concerning the importance of an adequate design of the machining process in order to ensure the desired material properties in the workpiece subsurface. However, a prediction of the residual stress distribution is until now at most only qualitatively possible. A number of questions still persist about the causes and the mechanisms of residual stress generation in machining. It is very critical to find a fast and precise solution to predict residual stresses in a machined component given the process parameters and material properties . For this reason the selection of process parameters is currently performed without considering the residual stress problem due to the absence of predictive models of general validity. In fact, several empirical models are reported in the literature but, due to their experimental nature, their validity is limited to the materials investigated, and the application of these models to different materials requires the execution of a time-consuming experimental plan [13, 19, 20].
2 Experimental procedure
Two sets of milling experiments have been carried out in order to investigate the influence of the machining parameters as well as the cutting edge geometry on residual stress of workpieces made out of forged aluminum alloys. All machining tests were performed on a Heller MC16 milling centre. This milling machine has a spindle power of P = 25 kW, a maximal rotation speed of n = 24,000 rpm and a maximal feed speed of vf = 40 m/min.
The first set of experiments has been carried out with helical cutters made out of fine grain cemented carbide with a diameter of D = 20 mm. These cutting tools have an radial rake angle of γra = 14°, an radial clearance angle of αra = 12°, a helix angle of λ = 30° and a chamfer with a length of bf = 0.15 mm and an angle of γf = 1°. All tools used in this part of the research work exhibit the same geometry.
The aim of these investigations is the determination of the influence of the machining parameters on residual stress. For this reason, a broad variation of machining parameters has been chosen. Only one cutting parameter has been changed in each series of experiments, while all others have been kept constant. The standard parameters are vc = 1,250 m/min, fz = 0.20 mm, ap = 4 mm and ae = 20 mm.
A cutter with indexable inserts has been used in the second group of machining tests to allow a systematic preparation of the cutting edge. The microgeometry of the tools is characterised via the radius of the secondary cutting edge rß. Furthermore, the influence of the wear on the residual stress has also been subject of this study. The advance of the tool wear has been characterised via the width of the wear land mark at the clearance face VBB. The tool has a diameter of D = 40 mm and is fixed to a HSK63 tool holder. In all these experiments, the standard cutting parameters have been kept constant, i.e., vc = 1,250 m/min, fz = 0.20 mm, ap = 4 mm. The width of cut has been adapted to have a full diameter engagement and ae = 40 mm.
The machining tests have been performed on robust aluminum blocks with a thickness of 35 mm. This has been done to avoid any deformation, which could lead inaccurate results of residual stress measurements. The workpiece material is Al7449 T7651, which is used in structural aluminum parts due to the low residual stresses after forging and stretching by a high strength. The residual stresses throughout its whole depth lay within the range of σ = ±25 MPa.
The machining forces have been measured with a dynamometer of the type Kistler 9255B. Due to the high rotational speeds, a high sampling rate of 25,000 Hz and a low pass filter of Flp = 1,000 Hz have been used to record the force signals.
The residual stresses have been measured at the base of the milled workpieces with a 2 circle Bragg Brentano-Diffractometer of the type Seifert XRD 3000 P using Cr Kα radiation with 30 kV, 35 mA and a position sensitive detector. The analysis of the residual stresses was done according to the sin2-ψ method using the grid plane 311 with an angle of 2θ = 139.31°. Under these conditions, the maximal penetration depth of the radiation amounts to z = 11 μm. The residual stresses have been measured in the center of the milled slot.
Areas of 10 × 10 mm have been successively electropolished to measure the residual stress at different depths within the subsurface zone. These measurements were stopped after reaching a depth, at which the determined value crosses again the axis of neutral residual stress, i.e. σ = 0 MPa.
Past results have shown that it is necessary to analyze a residual stress depth profile within the subsurface zone, since the stress maxima during the machining process are usually present in a region below the surface. Therefore, the maximum residual stresses are induced within a layer underneath the surface .
3 Experimental results
3.1 Influence of the machining parameters on residual stress
The left chart of Fig. 2 shows that at an increase of the cutting speed from vc = 250 m/min to vc = 750 m/min cause a reduction of the compressive residual stresses at the surface. The maximum compressive residual stress in the subsurface zone is nearly constant in this range. Beyond this cutting speed, the residual stress values remain nearly constant. Furthermore, the right chart of Fig. 2 indicates that the variation of the cutting speed does not systematically influence the depth of the maximum of the residual stresses.
The constancy of the residual stresses for higher depths of cut is caused by identical engagement conditions between the tool end and the workpiece surface and the uniform force distribution throughout the whole depth of cut. This is caused by the uniform uncut chip thickness at all axial tool positions. Further, the right chart of Fig. 6 shows that the depth of cut does not present any influence on the depth of the layer with the maximum compressive residual stress.
3.2 Influence of the cutting edge geometry on residual stress
Moreover, the initial increase of the width of the wear land mark causes a deeper influence on the subsurface zone. For this reason, the surfaces produced with tools with VBB = 70 μm and VBB = 90 μm show compressive residual stresses up to a depth of around z = 120 μm. Further tool wear results into a considerable residual stress reduction. In this way, the workpiece produced with a tool with VBB = 125 μm is solely affected up to a depth of z = 25 μm.
4 Conclusions and outlook
Machining operations influence the final residual stress depth profile within the subsurface zone of workpieces out of forged aluminum alloys. The results presented in this paper show a clear influence the machining parameters, the cutting edge geometry and the tool wear on the residual stresses. It has been shown that an increase of the feed per tooth, a decrease of the width of cut or the application of higher radii at the secondary cutting edge lead to more pronounced compressive residual stresses at higher workpiece depths. To some extent, the analysis of the machining forces provides good explanations for the observed residual stress distributions. However, some of these phenomena may be caused by thermal effects of cutting operations. Therefore, future research activities will focus on thermal aspects.
The COMPACT project is a collaboration between Airbus UK (Project Co-ordinator), Alcan–Pechiney, Limerick University, University of Bristol, Enabling Process Technologies, Hannover University, EADS Germany, University of Patras, Alenia Aeronautica, Ultra RS, Institut National Polytechnique de Grenoble and the University of Sheffield. The project is jointly funded by the European Union Framework 6 initiative and the project partners.