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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 t...
Citations
... In our past research we investigated the effect of varying MIRS on distortion [17]. Furthermore, we developed a FEM model to predict the part distortion due to both MIRS and IBRS [18][19][20]. ...
... mm, vc=200 m/min, ae=4 mm, ap=3 mm) A cutter with indexable inserts (Sandvik 1 R590-110504H-NL H10) was used to face mill the top and backside surface (step 1, 4) and a regular end mill (Kennametal 1 F3AA1200AWL) to side mill the walls (step 2) and pockets (step 5). MIRS from the cutting operations face milling (step 1, 4) and pocket milling (step 5) were measured in previous research and plotted in Fig 2. For more information we refer to [17][18][19][20]. Since the total machined part is 20 mm thick, different z-offsets of the part in the 30 mm stock are possible: A symmetrical position, removing 5 mm each at top and bottom, and an asymmetrical one, removing 1.5 mm at the bottom and 7.5 at the top were investigated. ...
... The developed FEM distortion model was presented and validated in previous research [17][18][19][20] and is now used to investigate the effect of varying IBRS on distortion for different part geometries. It is summarized here as follows: A static, linear elastic FEM model was set up in ABAQUS 1 (E=71,700 MPa, ν=0.33). ...
Milled thin-walled monolithic aluminum structural parts are widely used in the aerospace industry due to their appropriate properties such as a high overall strength-to-weight ratio. The semifinished products made of aluminum alloy 7050 undergo a heat treatment to gain this increased strength and hardness: Typically, three steps including solution heat treatment, quenching, and age hardening are carried out. This also leads to high initial bulk residual stresses (IBRS) within the part in the range of ±200 MPa. In rolled aluminum plate, plastic stretch (T7451 heat treatment designation) provides very effective relief of residual stress, decreasing IBRS to a level of ±20 MPa. In large parts with low bending stiffness, even that low level of IBRS can cause appreciable distortion. Besides the IBRS, the machining-induced residual stresses (MIRS) contribute to the distortion. This study investigates how IBRS in different stress relieved 7050-T7451 semifinished products vary and how this variation affects the distortion of milled thin-walled monolithic structural parts. A linear elastic finite element distortion prediction model, which considers the IBRS as well as the MIRS as input, was used to analyze the effect of varying IBRS on the distortion for different part sizes and geometries. The model was validated by machining of those parts and measuring their distortion. IBRS were measured via slitting technique and MIRS via incremental hole-drilling.
... We observe such trend also on our data. In addition, milling the repair part influences the microstructure and the RS at the surface and subsurface [49,50]. As observed in Fig. 6, it is considered that milling mainly increases the compressive RS [51,52] and spatially redistributes the RS at the lower depths (<90 µm). ...
Large temperature gradients inherent to additive manufacturing (AM) processes induce large residual stress (RS) in the final part. Because RS can influence the tribological properties, this study focuses on the relationship between wear sliding properties and RS in IN718 coatings. Such coatings were deposited with a Laser metal deposition (LMD) machine using two different scanning strategies. The wear resistance and RS state were investigated after surface milling. RS were measured before and after wear tests on a reciprocating sliding test apparatus. Two different X-ray diffraction techniques were employed to measure the surface and subsurface state RS: Laboratory Energy Dispersive X-ray Diffraction (LEDXD) and Synchrotron X-ray Energy Dispersive Diffraction (SXEDD). Due to the milling process, the coatings show similar depth distributions of RS from 22 to 92 µm depth, but exhibit different magnitudes depending on the scanning strategy used. Reciprocating sliding wear tests induced high compressive residual stresses that erased the initial RS state, and a similar wear behavior was observed in the two samples. These samples possess similar texture and grain morphology. This demonstrates that the influence of RS on wear resistance is a second-order effect. Nevertheless, it was observed that RS can still impact the wear performance at the early testing stages of the repaired parts.
Graphical Abstract
... The authors' findings highlighted that the residual stress in these rings induces significant machining deformation up to 0.4 mm and induced an elliptical cross section with a maximum ellipticity of 0.52 mm. The workpiece thickness, pre-machining stress levels and their level of asymmetry, clamping setup, type of machining process, and removal strategy can also affect the way residual stress develops during machining and final part distortion [76,77]. ...
Aluminum alloys are widely used in many industries, including aerospace, automotive, civil, and electrical engineering. When compared to pure aluminum, most aluminum alloys have lower electrical and thermal conductivity, corrosion resistance, and weldability, as well as a low density and specific gravity. At the same time, the properties of aluminum alloys vary significantly depending on the group, which has a significant impact on their machinability. This review article is focused on the study of machining characteristics of aluminum alloys, such as machinability, surface integrity, tool wear and tool life, material removal rate (MRR), and chip morphology. The directions of increasing machinability by controlling cutting parameters, cutting environment, such as dry machining, conventional cooling systems, minimum quantity of lubricant (MQL), cryogenic lubrication (CL), with tool geometry, and textured tools, are also considered; tool materials include coating, vibration, thermally, and hybrid assisted machining. The article discusses the main types of machining, namely, turning, milling, drilling, and grinding. It shows ways to increase the machinability of machining on aluminum alloys, as well as the advantages and disadvantages. From the literature, it can be concluded that tool wear when machining aluminum alloys is 30–40% lower than when machining steel alloys due to their higher ductility and lower strength. Surface integrity, affected by the cutting parameters and cutting temperatures — which can reach between 200 and 400 °C — can vary by up to 15% in hardness and 20% in surface roughness. Cutting tool characteristics can enhance surface finish by up to 25% and extend tool life, reducing edge formation by up to 30%. Chip morphology, influenced by factors such as cutting parameters and tool material, can improve tool life by up to 35%. Vibration techniques can reduce thermal effects and improve surface finish by up to 40%, reducing cutting forces by around 30%.
... Pioneering studies have attempted to derive the RS distribution with depth (z) in the milled surface region using the electrolytic etching-deflection technique [9]. In recent years, researchers have experimentally confirmed that the maximum compressive RS arises at a z near the surface and that RS gradually decreases with increasing z [10][11][12]. Such a z-directed RS distribution is called a "root" (√) shape [1]. ...
... Such a z-directed RS distribution is called a "root" (√) shape [1]. Feng et al. used numerical analysis to obtain the RS distribution with z of milling and showed that the distribution is √-shaped [13,14], similar to prior experimental results [1,9,10,12]. The z range of the compressive RS and the z and magnitude of the maximum compressive RS in the near-surface RS distribution vary depending on milling parameters, such as cutting speed (v c ), feed per tooth (f z ), and z of the cut [15][16][17]. ...
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.
... 3.1 and 3.2) and the influence of the machining parameters, clamping strategy and tool type on the MIRS in the workpiece was identified (Sect. 3.2) [40]. This includes measuring the MIRS with various techniques [41] and a repeatability study [40]. ...
... 3.2) [40]. This includes measuring the MIRS with various techniques [41] and a repeatability study [40]. In the distortion investigations, different thin-walled geometries were manufactured, and their distortion was determined to investigate the influence of the IBRS, the MIRS (Sect. ...
... Down milling with cemented carbide end mills (Kennametal 1 F3AA1200AWL, d = 12 mm, z = 3, for more details see [40]), which represents a typical tool for machining of aerospace aluminum alloys, was carried out on a 5-axis DMG Mori 1 DMU 70 CNC machine to evaluate the effect of the machining on the MIRS. The above mentioned AA7050-T7451 samples with the dimensions 206 × 102 × 28.5 mm 3 were face milled. ...
Nowadays, aluminum components in aircraft are mainly found in the form of thin-walled monolithic structural parts of the internal fuselage and the wings as spars and ribs [1]. This is because these components have excellent material properties for lightweight applications, such as a high strength-to-weight ratio and good corrosion resistance [2]. A typical manufacturing process to produce such structural components is milling. For these weight-optimized, monolithic components, up to 95% of the material is removed by machining [3]. The challenge with these thin-walled structural components, which are up to 14 m long, is that part distortion can occur because of the manufacturing-specific process chain [4]. Residual stresses due to machining and upstream processes such as forming, and heat-treatments are known to be the key factor for causing those distortions [5].
In this research the effect of the residual stresses, the machining strategy, the part topology and the geometry, including the wall-thickness, on distortion were investigated experimentally, and simulatively by validated virtual models based on the finite-element method. Those models can then be used to predict the distortion. At the end distortion minimization techniques were derived.
... Yang et al. [21] studied the interactive effect of IRS and cutting force on machining distortion and found that the IRS is the main factor of distortion for aluminum alloys, but the cutting force is the main one for titanium alloy. Weber et al. [22] studied the repeatability of the machining-induced RS for different machining parameters and clamping strategies on distortion and revealed that more and deeper MIRS causes more distortion. Wan et al. [23] summarized the modeling to predict the MIRS, including empirical, analytical, and FEM models, and stated that MIRS is influenced by cutting parameters. ...
Monolithic aluminum alloy parts are highly required in the aeronautical industry, but they show significant geometrical distortion after the machining process. This work investigated the distortion attributed by the initial residual stress of the raw material and the machining-induced residual stress during the milling process, as well as exploring the effects of the machining toolpath strategy. Single-/multi-pocket parts were milled from 7050-T7451 aluminum blocks with different initial residual stresses, and an element deletion method was developed for the numerical study to simulate different sequences of material removal. It was revealed that the toolpath parallel to the long side of the block caused more distortion on the side surfaces of the final part. The value of distortion was positively correlated to the magnitude of the initial residual stress of raw material. The simulation results indicated that the distortion attributed by machining-induced residual stress accounted for about 15% of the final distortion. The finding promotes the design optimization of machining monolithic parts by minimizing distortion, thereby benefitting the application of large monolithic parts in industry.
... However, machining was not performed on the x-axis direction surface while performing machining on all other surfaces; this resulted in specimen K the transformation of the residual stress type to the tensile type in the x-axis at a depth of 50 μm and resulted in a very high tensile residual stress value at a depth of 100 μm. It is known from past work that machining is very effective in terms of its tendency to create compression-type residual stress in the 0-100 μm depths close to the surface [61][62][63][64]. In the y-axis, the residual stress type remains compressive type at all measured depths. ...
In this study; turning, milling, heat treatment and shot peening (SP) post-process operations alone or in different combinations were performed on Ti6Al4V specimens which were produced by selective laser melting (SLM); then the effects of these operations on the fatigue life, residual stress, microstructure, microhardness and surface roughness were investigated. SP processes were carried out in two different types, traditional and multiple, and it was observed that the multiple SP process, which was carried out with the same almen intensity as traditional, with a smaller ball diameter, was effective in reducing the surface roughness. As a result of the heat treatment, α + β transformation was observed in the microstructure. α phase was seen in both lamellar and globular forms. Traditional SP and multiple SP processes provided a significant increase in microhardness values in the 0–250 μm depth range. A significant decrease in the microhardness values of the heat-treated specimens was observed due to the α + β transformation in the microstructure. Traditional SP and multiple SP processes enabled the residual stress type to become compressive type in both the x-axis and the y-axis in the 0–150 μm depth which is a very critical region for fatigue cracks despite the x-axis direction surface unmachined. Fatigue test were performed on machined, multiple shot-peened and machined after heat-treated specimens. The best results in terms of fatigue strength were obtained in the specimens that were heat-treated. It was observed that the most important cause of fatigue crack initiation in multiple shot-peened specimens was an internal defect located deeper than 250 μm. Fisheye formation was observed on fracture surfaces of some multiple shot-peened fatigue specimens.
... Three cooling strategies -dry milling, milling with an emulsion as cooling lubricant (Emulcut 160 BW Petrofer 1 , volume flow of 17.25 l/min, measured) and cryogenic milling with CO2 snow, supplied externally by two nozzles with a total mass flow of 1.1 kg/min, measured), were investigated. In addition to the variation of the cooling strategy, the feed per tooth fz was varied (fz = 0.04 mm and 0.2 mm corresponding to a feed rate of vf = 637 mm/min, 3,183 mm/min respectively) according to previous studies to allow for different thermo-mechanical loading [12]. This resulted in six different cases (see Table 2), where the cutting speed vc = 200 m/min, width of cut ae = 4 mm and depth of cut ap = 3 mm were kept constant. ...
... A cemented carbide end mill (D = 1.6 mm, Richards MicroTool 1 , 874-T-0062 2FL SE STD CARB EM 1/16") was used to drill the hole in incremental steps. Their depths were chosen according to previous studies [12,17]. Six HD measurements were carried out per sample (see Fig. 4) and interpolated to the depth schedule to calculate the average stress and standard deviation at each depth. ...
Cryogenics, which are supplied at very low temperatures, can provide an eco-friendly alternative to conventional cooling lubricants in machining processes. In the past, cryogenic machining has mainly been investigated for machining difficult-to-cut materials, since thermally induced wear mechanisms usually dominate here; an improvement of the productivity and the part quality compared to machining with conventional cooling lubricants was found. Besides, some studies showed that cryogenic machining of aluminum alloys also led to benefits such as an increase in strength, a reduction in burr formation and an improvement of fatigue behavior and surface finish due to higher machining-induced compressive residual stresses in the boundary layer of the components. Especially the increase in strength and improvement of the fatigue behavior can lead to a weight-saving and energy-efficient component design, which plays a central role for components used in the aerospace industry. Here, thinwalled milled monolithic aluminum structural components are commonly used, because of their good material properties such as a high overall strength-to-weight ratio and good corrosion resistance. The distortion of those parts due to residual stresses is a common problem.
This research assesses the effects of the cryogenic machining on the quality of milled thin-walled monolithic aluminum structural parts relation to the thermal and mechanical load in the machining process.
The results showed that cryogenic machining induces higher and deeper compressive residual stresses into the part compared to dry milling due to higher forces and lower temperatures prevailing in the cutting zone. As a consequence, higher part distortions were found. However, cryogenic machining also led to an increased microhardness within the surface layer and a deeper hardening depth was achieved. In contrast, milling with an emulsion led to a better surface quality and less distortion.
... Depending on the amount magnitude of the mechanical, thermal, and metallurgical effects, which are linked to machining conditions, the penetration and shape of the stress profiles vary [83,84]. Even keeping constant the machining conditions, the repeatability of MIRS is a factor to be considered [85]. ...
... Contrary to the previous case, distortion prediction models considering only MIRS are very seldom. The bending moment caused due to induced superficial stresses leads to deformations, and depending on the magnitude of the introduced stresses and the stiffness of the part, this effect can become significant [15] and even predominant over others [82,85,170,176]. The main advantage is bypassing BIRS characterization which is often a challenge and involves uncertainty. ...
Machining precision components involves challenging distortion issues that entail high costs and material and energy waste to the industry. In parallel, advanced control of production processes is a rapidly growing field because of its unique capabilities to solve multi-agent nonlinear problems and develop control actions based on knowledge and experience. Despite the several studies carried out on the subject, research keeps fragmenting distortion issues in different niches of components, and comprehensive reviews considering distortion as a cross-cutting technical hitch have never been reported. In this paper, a study compiling recent advances in machining distortion control from a holistic perspective is presented. For the first time, distortion understanding is unified, offering a new perspective, more practical and comprehensive, which includes intelligent systems. This novel way of attaining the research on distortion distinguishes three interconnected pillars: distortion source identification and quantification, distortion simulation model development, and control strategies drafting and application. The paper guides the reader through several distortion investigations of different kinds and provides classifications never addressed in the field with which a profound understanding of the issue can be achieved. Finally, future trends and key enabling technologies to drive the advanced control and minimization of machining distortions are outlined.
... A random order of milling the samples was chosen to minimize the influence of the tool wear. The tool wear was monitored after milling each sample by a macroscope, so that worn tools were exchanged if wear in the form of corner break outs was detected qualitatively [10]. ...
Background
Distortion arises during machining of metallic parts from two main mechanisms: 1) release of bulk residual stress (BRS) in the pre-form, and 2) permanent deformation induced by cut tools. Interaction between these mechanisms is unexplored.
Objective
Assess this interaction using aluminum samples that have a flat surface with variations of BRS, where that surface is subsequently milled, and we observe milling-induced residual stress (MIRS) and distortion.
Methods
Plate samples are cut from two kinds of large blocks, one kind stress-relieved by stretching and a second kind solution heat treated, quenched and aged. The BRS field in the plates is known from a recent series of measurements, being small in the stress relieved plates (within ±20 MPa) and large (±100 MPa) in the quenched plates, varying from tension to compression over the surface that is milled. MIRS is measured following milling using hole-drilling. Distortions of thin wafers cut at the milled surfaces are used to elucidate BRS/MIRS interactions. A finite element (FE) model and a strength of materials model are each used to assess consistency between wafer distortion and measured MIRS.
Results
Milling in samples with high BRS magnitude changes the directions of MIRS and distortion relative to the milling direction, with the direction of maximum curvature rotating toward or away from the milling direction depending on the sign and direction of BRS. High magnitude BRS was also found to increase the wafer peak arc height, nearly doubling the amount found in low BRS samples.
Conclusion
Measured residual stress and observed wafer distortion both show interactions between MIRS and BRS. Stress analysis models show that the differences in measured MIRS are consistent with the differences in observed distortion.
