Imed Zaghbani

Machining and Machinability of Aluminum Alloys

The use of materials with low specific weight is an effective way of reducing the weight of structures. Aluminum alloys are among the most commonly used lightweight metallic materials as they offer a number of different interesting mechanical and thermal properties. In addition, they are relatively easy to shape metals, especially in material removal processes, such as machining. In fact, aluminum alloys as a class are considered as the family of materials offering the highest levels of machinability, as compared to other families of lightweight metals such as titanium and magnesium alloys. This machinability quantifies the machining performance, and may be defined for a specific application by various criteria, such as tool life, surface finish, chip evacuation, material removal rate and machine-tool power. It has been shown that chemical composition, structural defects and alloying elements significantly influence machinability [W Konig et al., 1983]. Thus, with similar chemical compositions, the machinability of alloys can be improved by different treatments. Heat treatments, which increase hardness, will reduce the built-up edge (BUE) tendency during machining [M. Tash et al., 2006]. In the case of dry machining, the major problems encountered are the BUE at low cutting speeds and sticking at high cutting speeds, hence the need for special tool geometries [P. Roy et al., 2008]. It has been shown that high levels of Magnesium (Mg) increase the cutting forces at the same level of hardness [M. Tash et al., 2006], while a low percentage of Copper (Cu) in aluminum alloy 319 decreases the cutting force. Similarly, it has been found that heat treatment of 6061, especially aging, influences the forces only at low cutting speeds, while at high speeds, the influence is negligible because of the low temperature rise seen in the cutting zone [Demir H et al., 2008]. Cutting force is just one among several parameters to be considered for a full assessment of the machinability of metallic alloys, with the others being the tool life, the surface finish, the cutting energy and the chip formation mode. Aluminum alloys are classified under two classes: cast alloys and wrought alloys. Furthermore, they can be classified according to the specification of the alloying elements involved, such as strain-hardenable alloys and heat-treatable alloys. Most wrought aluminum alloys have excellent machinability. While cast alloys containing copper, magnesium or zinc as the main alloying elements can cause some machining difficulties, the use of small tool rake angles can however improve machinability. Alloys having silicon as the main alloying element involve larger tool rake angles, lower speeds and feeds, making

Robotic High Speed Machining of Aluminum Alloys

The robotic machining is one of the most versatile manufacturing technologies. Its emerging helped to reduce the machining cost of complex parts. However, its application is sometimes limited due to the low rigidity of the robot. This low stiffness leads to high level of vibrations that limit the quality and the precision of the machined parts. In the present study, the vibration response of a robotic machining system was investigated. To do so, a new method based on the variation of spindle speed was introduced for machining operation and a new process stability criterion (CS) based on acceleration energy distribution and force signal was proposed for analysis. With the proposed method the vibrations and the cutting force signals were collected and analyzed to find a reliable dynamic stability machining domain. The proposed criterion and method were validated using data obtained during high speed robotic machining of 7075-T6 blocks. It was found that the ratio of the periodic energy on the total energy (either vibrations or cutting forces) is a good indicator for defining the degree of stability of the machining process. Besides, it was observed that the spindle speed with the highest ratio stability criterion is the one that has the highest probability to generate the best surface finish. The proposed method is rapid and permits to avoid trial-error tests during robot programming.

A comprehensive analysis of cutting forces during routing of multilayer carbon fiber-reinforced polymer laminates:

Analyzing cutting forces during detouring of carbon fiber-reinforced polymer laminates at high cutting speeds is problematic as the recorded signal can be distorted due to resonance of the measuring system. In addition, excessive tool wear can render signal interpretation difficult. In the present study, a fully controlled experimental protocol is used to investigate the instantaneous cutting forces when milling carbon fiber-reinforced polymer laminates in a bid to avoid signal distortion and limit the tool wear effect. A polycrystalline diamond tool was selected for the experiments in order to limit the effect of tool wear on the recorded signals. The fiber orientation influences principally the cutting force amplitude, which varies nonlinearly with the feed. Based on this experimental data, a high-order mechanistic force model in terms of feed per tooth was proposed to predict the cutting forces. The tooth-to-tooth run-out was measured and included in the model, and the model was validated for different feeds, speeds, and number of plies. A good consistency between simulated and measured forces was observed. For the proposed model, the estimation error was approximately ±12.5%.

An experimental study on the vibration response of a robotic machining system

Robotic machining is one of the most versatile manufacturing technologies around, whose emergence helped reduce the machining cost of complex parts. However, its application is sometimes limited due to the low rigidity of the robot, whose stiffness leads to high vibration levels, which limit the quality and the precision of machined parts. In this study, the vibration response of a robotic machining system was investigated. To that end, a new method based on the variation of spindle speed was introduced for finishing the aluminum aerospace grade alloy (7075-T6) blocks. With the proposed method, the vibrations and the cutting force signal were collected and analyzed to find a reliable dynamic stability criterion, and the proposed criterion was validated using the machined surface roughness obtained. It was found that the directional root mean square (RMSdirectional) of the vibration signal is a good indicator for defining the degree of stability of the machining process. Moreover, it was observed that the spindle speed with the lowest RMSdirectional is the one that has the highest probability of generating the best surface finish. It was further demonstrated that the sensors are more efficient when positioned on the spindle. The proposed method is rapid and makes it possible to avoid trial and error tests during robot programming.

Milling burr size estimation using acoustic emission and cutting forces

Burr formation is one of the main concerns usually faced by machining industries. Its presence leads to additional part edge finishing operations that are costly and time consuming. Burrs must be removed as they are source of dimensional errors, jamming and misalignment during assembly. In many cases burrs may injure workers during handling of machined part. Due to burr effect on machined part quality, manufacturing costs and productivity, more focus has been given to burr measurement/estimation methods. Large number of burr measurement methods has been introduced according to various criteria. The selection of appropriate burr size estimation method depends on number of factors such as desired level of quality and requested measuring accuracy. Traditional burr measurement methods are very time consuming and costly. This article aims to present empirical models using acoustic emission (AE) and cutting forces signals to predict entrance and exit burrs size in slot milling operation. These models can help estimating the burrs size without having to measure them. The machining tests were carried on Al 7075-T6 aluminum alloy using 3 levels of cutting speed, 3 levels of feed rate, 3 levels of cutting tool coating and 2 levels of depth of cut. Mathematical models were developed based on most sensitive AE parameters following statistical analysis, cutting forces and their interaction on predicting the entrance and exit burrs size. The proposed models correlate very well with the measured burrs size data.© 2011 ASME

Evaluation of sustainability of mould steels based on machinability data

In this study, a new model was proposed for the Product Sustainability Index (PSI). The PSI estimation was based on the tool life, the cutting force, the surface finish, the energy consumption, the operation cost and the acoustic emissions. The PSI of four mould steels (SF-2312 [300 HB], SF-5 [300 HB], SF-2000 [341 HB] and SP-300 [341 HB]) was evaluated using the developed model and machinability data from dry and wet cutting tests. The model was able to predict the PSI of the considered mould steels at different cutting speeds, and the predicted results were in good agreement with the experimental measurements. The PSI was found to be dependent on lubrication mode, cutting speed and workpiece hardness. Steels with similar composition but different hardness exhibits different PSI.

Global Machinability of Al-Mg-Si Extrusions

Field performance, mechanical properties and workability usually sustain the develop‐ ment of new alloys. As far as the workability is concerned, most extrusions need not on‐ ly to meet good extrudability, but also good or acceptable machinability as some machining operations (eg. drilling or finishing machining) are usually required. Unfortu‐ nately, alloys with higher strength could have better machinability but lower extrudabili‐ ty; Aluminum alloys with excellent extrudability such as AA1060 or AA1100 (Figure 1) often exhibit low machinability, especially due to the chip formation and the workpiece material adhering to the cutting tool leading to the build-up-edge (BUE), modifying the cutting process, leading to tool breakage or deteriorating the surface finish when this BUE is broken. Al-Si-Mg alloys (6XXX series) usually exhibit good machinability and good extrudability; This is one of the reason why about 90% of most extruded alumi‐ num parts are in 6XXX family. The AA6262 which is recognized for its ease chip breaka‐ bility (a lot of second phase particles which help initiate fracture) generally leads to good machinability but poor extrudability because of its poor formability. Any new aluminum alloy with excellent machinability, extrudability and mechanical properties will therefore lead to considerable advantage compared to existing alloys.

Analysis and modelling of cutting forces during the trimming of unidirectional CFRP composite laminates

CFRPs are increasingly employed in the aircraft industry thanks to their high strength and high rigidity both properties that make them difficult-to-machine materials. Cutting parameters must therefore be selected with care; otherwise, damaged parts will ensue. The present study investigates the instantaneous cutting forces at play when trimming CFRP laminates. An experimental setup is proposed for changing the cutting parameters during trimming operations. The instantaneous cutting forces were recorded at high sampling frequencies, and then analysed and modelled. From the investigation, it was found that for the machined laminates, the fibre orientation does not significantly influence the profile of the tangential and radial forces; however, it influences their amplitude. To confirm that, machining tests were performed on aluminium alloy sheets. These tests allowed a comparison of the behaviour of an isotropic material with that of an orthotropic material. The comparison allowed the localisation of non-linearity sources. Employing these observations, it was demonstrated that the average thrust and feed forces vary non-linearly with the feed. Thus, a third-degree polynomial model was introduced to describe this variation. This mathematical description allowed a high-order mechanistic model to be built, simulating the instantaneous cutting forces for different feeds, speeds and fibre orientations.

A new sustainability model for machining processes

It is generally recognised that the sustainability concept integrates economic, environmental and social aspects. It would be interesting for manufacturers to be able to know which process, workpiece materials or machining condition is more sustainable than another. In this paper, a new model for the sustainability in the machining field is proposed with the goal to increase productivity, reduce power and energy used during cutting process and reduce the harmful impacts of the machining process on the environment and the occupational safety. Therefore, the evaluation method combines recyclability, energy, and particle and aerosol emission. Fuzzy logic helps to make decision on the evaluation of the sustainability of machining processes. The model is validated using test results from different cutting processes such as milling, drilling and turning.