Soutenance mémoire
Criteria to evaluate the machinability of a material
Because of the complexity of evaluating machinability, various criteria were developed to transform the concept of machinability in an applicable way. The most important among these are [3, 4]:
tool life
cutting forces
chip formation, and
surface roughness.
In addition, other factors can be taken into consideration such as temperature rise, tool wear level, specific power consumed, dimensional tolerance and overall cost [14]. Although these four main criteria of evaluation simplify the study of machinability, the ease of measurability is not the same for the four criteria. While tool life can be easily evaluated by specific tool wear criteria, and cutting forces can be simply studied using dynamometry, chip formation includes chip breakability, chip morphology and built up edge. Moreover, the study of surface roughness extends to other aspects of surface integrity such as wear and stress concentration.
Tool Life
Tool life is one of the major economic issues in manufacturing processes. While the longer the tool goes, the cheaper the process becomes, however, the worse the work surface quality gets [15]. Three main kinds of failure can end the life of the tool: fracture failure, temperature failure and gradual wear. Fracture failure occurs by excessive applied forces over the cutting tool. Temperature failure takes place by excessive heat generation during the cutting process, which causes cutting point softening and deformation. Gradual wear failure occurs due to continuous interaction between the machined surface and cutting tool. Whereas the first two modes of failure are considered as premature failures, gradual wear failure is the preferred mode of failure because it represents the longest tool life for the cutting conditions [3].
While wear failure is a gradual process, the necessity to determine a specific limit to determine the end of tool life appears, especially with different areas and rates of wear taking place along the tool. In addition, tool life is not an absolute concept, but depends on the selected criteria according to the requirements of the operation [1]; so the criteria of tool life fulfill three main conditions; sustaining tolerances, maintaining surface quality and chip breakage efficiency [6]. Thus two main methods are used to evaluate tool-life capability: using tool life criteria such as burr height tolerance, or using tool life parameters such as tool life volume [16].
From a practical point of view, ISO 3685 mentions that the type of wear that is believed to contribute most shall be used as a guide of tool life criteria selection; otherwise, combined criteria can be used. Moreover, the same code mentions the specific values for flank and crater wear based on tool material, which can be summarized as follows.
Cutting Forces
Among the different criteria of machinability evaluation, cutting forces have a significant role in the response of the material to the cutting process, whereas thrust forces identify the required energy to form the chips as well as tool wear. Thrust force increases significantly with wear of the cutting tool [5]. Several researches [20, 21] were carried out to predict tool life and surface roughness. Valavan [22] developed an equation to predict tool life based on cutting forces and workpiece surface temperature, while Zedan concluded that the major problem with studying cutting forces is their strong dependence on cutting conditions [5], as cutting forces depend on work piece material features, the shape of undeformed chips, as well as cutting tool geometry [23]. Thus several studies and models were published to analyse the dependence between the cutting force and these factors, such as that of Wang et al. [24] who studied the cutting forces and their relationship with cutting parameters in the reaming process and affirmed that thrust force decreases significantly with increasing cutting speed.
In drilling, cutting forces become more complex to analyse, as there are three different cutting edges: the main cutting edge, the chisel edge, and the margin cutting edge. Thus different empirical, theoretical and computational models were developed in order to have an accurate prediction based on cutting conditions and workpiece-tool interaction. While Zedan gave approximate values of participation of each cutting edge in thrust force [5], Minukhin developed an improved model focusing on the primary cutting edge as the main generator for thrust force and torque.
Chip Formation
Although chip layer is disposable, it gives a significant indication on the quality of the machining process. Four main mechanisms of chip formation can be distinguished: continuous formation, lamellar formation, segmented formation and discontinuous chip formation [23] as shown in Figure 2-2. Continuous chips result from high speed machining of ductile materials with small feeds and depth, while chips tend to deform discontinuously because of high brittleness [3]. The main parameters that affect the dominant mechanism of chip formation are thermo-physical properties and metallurgical features of the material and the cutting process conditions [26, 27].
Moreover, measurements for chip morphology are extremely important to evaluate the quality of the machining process, such as chip volume ratio, which indicates the bulkiness of the chips. According to this measurement chips can be classified into eight classes [23]: ribbon chips, snarled chips, flat helical chips, long cylindrical helical chips, helical chip segments, spiral chips, spiral chip segments and discontinuous chips as shown in Figure 2-3. In drilling, a deeper investigation was carried out to classify six common shapes that may be generated in cast aluminum which are [28, 29]: conical chips, fan-shaped chips, chisel-edge chips, amorphous chips, needle chips and impacted chips. Elgallad [28] showed that conical chips and fan-shaped chips are in the desirable range of chip formation volume ratio, and their generation is caused by proper drilling process in aluminum cast alloys.
Built-up edge as well is an associated condition with chip formation mechanisms, in particular with continuous chip formation where chips adhere to the rake face and to cutting edges. Toenshoff summarized the conditions that cause built up edge, which are [23]:
If the material advocates strain-hardening
Stable and stationary chip formation
Stagnant zone exists in front of the cutting edge in the stream of material flow
Low temperature in chip formation zone, that does not allow recrystallization to take place.
Thus hardened particles of built-up edge can adhere to the cutting edge and change the geometry of the cutting tool, which causes poor surface finish and tool wear. Usually this adherence effect can manifest in two forms: built-up edge and built-up layer [14]. In aluminum alloys the built-up edge forms in a composition close to pure aluminum, because the melting point of intermetallic particles is much higher in comparison to aluminum [30, 31]. With higher metal removal rates, a transition takes place from the built-up edge to the flow zone which can be considered as a thermoplastic shear band [14].
Surface Integrity
The differentiation between surface roughness and surface integrity was developed in 1964 by Field and Kahles [32]. The concept was developed to cover the modifications that resulted in the machined surface and near surface region due to machining in topographical, physical, mechanical, chemical, metallurgical and biological features [33]. Figure 2-4 shows the main aspects focused upon and measured from the surface and subsurface layer. In the machining of aluminum alloys, the three aspects usually considered important are the topography, mechanical and metallurgical features.
For the surface conditions, two main aspects studied are surface topography and surface defects. For surface topography, according to El-Hofy, topographical features can be defined by roughness, waviness, lay and flaws [34]. As topographical features cannot be directly evaluated, different measuring procedures were designed to define surface characteristics based on three principles for testing [23]: tactile tests, optical tests and scanning probe microscopic tests. According to ISO 13565 four categories of parameters were selected for measurement [6, 32, 33]:
Amplitude parameters, such as arithmetic mean roughness and maximum peak to valley height
Spacing parameters, such as the mean spacing of the asperities at the level of the central line and peak count
hybrid parameters, such as the root mean square slope of the profile
waviness parameters, such as the mean value of the waviness
Surface defects include cracks, craters, scratches, inclusions and other features [35] which is not directly related to our point of research.
On the other hand, sub-surface alterations because of machining may include hardness changes, microstructure alterations, and residual stresses. The surface becomes exposed to strain hardening due to plastic deformation in the process of material removal. In addition, on the microstructural level, severe deformation can produce dislocations in the alloy matrix, and result in dynamic recrystallization and grain refinement [36, 37]. Several observations were recorded by Chen et al. [38] and Liu et al. [36] for phase formation in aluminum alloys due to machining. Internal residual stresses also can be locked in the alloy matrix after non-uniform plastic deformation because of misfit between grains due to dislocations or interface mismatching between grains [35, 39]. These residual stresses can have a positive effect if they are compressive type, or a negative effect in case of tensile type.
For surface integrity aspects in aluminum, proper cutting parameters have an advantageous effect on surface integrity. Cutting speed has a dominant effect on surface topography in comparison to the other machining factors [32]. Zedan et al. [40] investigated the drilling process in 6000 series of aluminum alloys and concluded that increasing cutting speed and feed rate also reduce the burr height significantly. Moreover, dry machining shows better surface roughness, while wet machining has a harmful effect on the diameter of the holes. But according to Shoemaker [35] in his investigation of 2024-T351 aluminum, the use of cutting fluid during machining does not have a significant effect on the residual stress profile, while other machining parameters such as cutting speed, feed rate and depth of cut have radical effect on the residual stress state [32, 41].
Factors affecting machinability
Based on the definition of the machinability process as the interaction between the workpiece, cutting tool and cutting medium for different removal sequences under different cutting conditions [2], the factors affecting machinability can be categorised into four categories, which are:
Workpiece aspects, such as alloy composition, microstructural features, physical and mechanical properties and thermal and mechanical treatments
Cutting tool aspects, which includes tool geometry, cutting tool material and tool wear studies
Removal sequence aspects, the cutting process itself such as drilling, tapping, milling, reaming, etc.
Cutting medium and cutting conditions to analyse the effect of cutting fluid on the process, as well as cutting parameters; cutting speed, feed rate and depth of cut.
The remainder of the literature review will follow this sequence in order to cover the main aspects influencing the machinability, to facilitate the interpretation of the results presented in Chapter 4.
Workpiece aspects
Introduction
In industry, it is rare to use aluminum in its pure form because of its softness, so different alloying elements and additives are added to aluminum to obtain alloys with suitable mechanical, physical and chemical properties. The major alloying elements in aluminum alloys are Silicon, Copper, Magnesium and Zinc [28, 42]. Silicon improves fluidity, castability and corrosion resistance but it reduces strength and machinability, so usually different modifiers are added to improve its properties. Copper on the other hand is used usually in both cast and wrought alloys to enhance strength and hardness at room temperature and elevated temperature, through precipitation hardening heat treatment. Magnesium is used in combination with copper to intensify age hardening process, or with silicon to improve characteristics after heat treatment in terms of strength and corrosion resistance. Zinc is usually paired with magnesium to ameliorate the response of the alloy to heat treatment.
Metallurgical parameters that affect machinability can significantly affect the machinability of aluminium alloys. The most important metallurgical factors are:
Alloying elements and additives
Morphology, grain size and volume fraction
Microstructure of the alloy
Different heat and mechanical treatments
Casting method and defects such as porosity
Thermal, mechanical and physical properties
In terms of alloying elements, some elements can provide a degree of lubricity, while other elements increase matrix hardness or form hard intermetallic phases [14]. While copper and magnesium improve the strength characteristics through precipitation, increasing their content generally ameliorates the hardness of the alloy matrix, thereby reducing the friction with the tool, so surface quality improves significantly and decreases the possibility of built-up edge formation in addition to smaller and better chip formation. Silicon, on the other hand, is used to improve the fluidity of aluminum, but it harmfully affects the cutting tool because of its abrasive effect, causing tool wear. Thus different modifiers such as Strontium are added in order to reduce the detrimental effects of eutectic silicon by changing its morphology from acicular to fibrous [45]. Adding heavy metals to alloying elements can reduce machinability significantly because of their tendencies to form complex intermetallic phases (usually with Fe and Si) which cause hard spots in the form of sludge particles, thus increasing built-up edge. The role of morphology and effect of different phases should also be considered in studying machinability of aluminium alloys. The phases be classified based on their solubility in the aluminium matrix through heat treatment as soluble and non-soluble phases.
Usually the soluble phases include the softer particles, while the insoluble phases are hard, brittle and abrasive particles, which are usually associated with large amounts of iron. So increase of iron concentration in the alloy reduces the machinability and increases tool wear. Different techniques are used to counter the effect of iron on machinability. In addition to lowering the iron concentration in the alloy, neutralizers and modifiers may be added to the alloy in order to change the morphology of the iron intermetallics formed from platelet-like β-Fe into α-Fe script like phase, which is less harmful for cutting tools.
From the morphological point of view, the role of metallurgical characteristics on the machinability of the alloy (workpiece) is as follows: the finer the size of grains, the better the overall machining characteristics. In case of the presence of hard particles in the alloy, it is best to have them as spheroidized and as dispersed as possible. From the alloying elements point of view, Colwell mentioned “The dominant variables governing tool life are the silicon content, the temperatures occurring at the contact surfaces of workpiece and tool, hard-spot or sludge inclusions, and non-metallic inclusions”.
Material properties such as strength, ductility and hardness also affect different machinability aspects as cutting force, chip formation and surface integrity. Generally, increase in strength of the alloy improves the surface finish of the machined part, but on the other hand, it can accelerate tool wear because of bad chip formation. An enormous amount of force is necessary in order to initiate formation of chips in a material with high strength. So for this kind of material, the design of the cutting tool should be taken into consideration a less positive cutting angle and a stronger tool material.
Hardness also affects the machining process significantly, where the cutting speed is basically selected based on the material hardness. Low hardness allows for increased cutting speed, and hence productivity in addition to improving tool life. Increase in material hardness causes significant increase in cutting forces and extra heat generation during friction. It was reported that there is a direct relation between the unit of cutting forces and the Brinell hardness number for cast iron, copper and carbon steel [47]. In addition to cutting forces, heat generation because of friction can promote element diffusion and chemical reaction during the cutting process, which can accelerate tool damage significantly. The following sections will focus on the Al-Cu and Al-Si cast alloys and the heat treatment regimes applied to them.
Metallurgical aspects of Al-Cu alloys
The use of aluminium castings in automobiles has increased from non-structural demands, as it is the case of cylinder heads and engine blocks, to structural parts, such as suspension struts due to the beneficial effects that arise by combining light weight and mechanical properties [48, 49]. The Al-Cu alloy is a high strength-ductility cast alloy. It is often used to cast large structures and bearing components to realize the integrated casting structure from assembly casting parts. Al-Cu alloys substituting for some forging blank may decrease production cost. Therefore, Al-Cu alloys have been widely used in aerospace, automobile, and airplane applications [50, 51]. Figure 2-5 shows the Al-rich portion of the Al-Cu phase diagram where the gap between the solidus line and liquidus line indicates the Cu content of α-Al is much lower than that of the liquid.
|
Table des matières
ACKNOWLEDGEMENTS
RÉSUMÉ
ABSTRACT
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
Chapter 1 DEFINITION OF THE PROBLEM
1.1 Introduction
1.2 Objectives
1.3 References
Chapter 2 REVIEW OF LITERATURE
2.1 Introduction
2.2 Machinability
2.2.1 Criteria to evaluate the machinability of a material
2.2.2 Factors affecting machinability
2.3 Workpiece aspects
2.3.1 Introduction
2.3.2 Metallurgical aspects of Al-Cu alloys
2.3.3 Metallurgical aspects of Al-Si alloys
2.4 Cutting tool aspects
2.4.1 Tool material and coating
2.4.2 Tool geometry
2.4.3 Tool wear
2.5 Machining process
2.5.1. Introduction
2.5.2. Cutting mechanics
2.5.3. Drilling process
2.5.4. Tapping process
2.6 Cutting parameters
2.6.1. Cutting fluids
2.6.2. Cutting kinematic factors
2.7 References
Chapter 3 EXPERIMENTAL PROCEDURES
3.1 Introduction
3.2 Preparation of Alloys and Casting
3.2.1. Materials preparation
3.2.2. Casting procedures
3.3 Heat treatment
3.4 Microstructural examination
3.5 Tensile testing
3.6 Machining procedures
3.6.1 Cutting force measurements
3.6.2 Output recording phase
3.6.3 Force calculation phase
3.6.4 Tool Life and Built-Up Edge
3.6.5 Chip characterization
3.7 Data analysis phase
Chapter 4 RESULTS AND DISCUSSION
4.1. Introduction
4.2. Drilling characteristics
4.2.1. Microstructure and tensile properties
4.2.2. Cutting forces
4.2.3. Chip shape
4.3. Tapping parameters
4.3.1. Introduction
4.3.2. Tapping forces
4.3.3. Tap wearing
4.4. References
Chapter 5 CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions
5.2 Recommendations for future work
Chapter 6 APPENDICES
6.1 Matlab code used in drilling force analysis
6.2 Matlab code used in tapping force analysis
Télécharger le rapport complet
