IMPACT OF MACHINING PARAMETERS ON FATIGUE BEHAVIOUR OF
15%VOL.-SiCP REINFORCED ALUMINIUM MATRIX COMPOSITES
A. Mkaddem, P. Ghidossi, S. Crequy and M. El Mansori
Ecole Nationale Supérieure d’Arts et Métiers
ENSAM, LMPF-EA4106
Châlons-en-Champagne, France
ABSTRACT
The proposed paper aims to investigate the impact of the machining conditions on the machinability and fatigue variance
of aluminium matrix reinforced with silicon carbide (SiC) particulates. An experimental approach has been conducted
through a Design Of Experiment (DOE) including three machining speeds and two feed rates. Firstly, tensile test is used to
characterise the mechanical characteristics of the material. Secondly, specimens were prepared for each combination of
the DOE and the chip shape change is detailed. The surface quality of machined specimens is considered for analysing
the influence of machining parameters on the fatigue behaviour of the considered composite. Additionally, the morphology
of uncoated tungsten carbide tools used for turning operations is inspected to show how turning operation can affect the
flank cutter. Finally, the fatigue life of the considered composite is studied under two stress amplitudes, using four-point
reversed bending loads. Different experiments have been performed at room temperature up to failure in order to
investigate the behaviour of material in each case of chosen domain. Particularly, the fatigue life under stress-controlled
conditions is discussed by report to machining parameters.
Introduction
High quality product becomes more and more required. Particulates reinforced composites have been established as
competitive materials for naval, automotive and aerospace manufactory. However, the fatigue behaviour of such materials
is so complex and diverse that much more investigations still required for completing knowledge in these fields.
It has been demonstrated also in the research of Dermakar [1] that the main parameter which governs the mechanical
properties of MMC is the reinforcement volume fraction. In particular, it seems that the modulus of elasticity has no
sensitivity to the matrix material choice and the elaboration mode. Metal matrix composites (MMC) have been increasingly
used for critical structural applications in industrial sectors. This is essentially due to their excellent stiffness to density and
strength to ratios [2]. Metal matrix composites have the advantage to be reinforced with different type of fibres or
particulates and different reinforcement percents.
Nowadays, the MMC reach the potential to replace conventional metals in some components. In recent years, great
progress has been noted in the field of these materials that they become so known and used for production in different
aerospace sectors. In spite of their popularity, their elaboration causes yet many questions that must be resolved for a
better use in manufacturing. The specific behaviour of metal matrix composites enlarges their application types and
provides potential energy savings. Since some previous years, a great deal of research has been conducted to improve
the properties of composite materials [3-4]. Composite materials are also considered for many high temperature
applications in advanced aerospace, vehicles and gas turbine engine components.
In a pioneer work [5], it has been demonstrated that aluminium alloys discontinuously reinforced with ceramic have
significant potential for structural applications due to their outstanding combination of high specific strength and stiffness
as well as density. These properties have made metal matrix composites an attractive candidate for use in weightsensitive and stiffness-critical components in aerospace, transportation and industrial sectors. The effects of the silicon
carbide particles on the fatigue behaviour of matrix composites have been investigated by Kaynack et al. [6] whereas
works that treats the effects of the silicon carbide particulates on the fatigue behaviour are until our days poor. It is
between the main reasons that let thinks to make on this investigation.
Although the number of studies on the fatigue of MMC, many information still yet needed for understanding the interaction
between machining process and the subsequent behaviour. This work aims to reveal some details about the impact of
machining factors on the fatigue limit of 15% vol.-silicon carbide composite. Especially, three cutting speeds and two feeds
were retained for experiments. Additionally, roughness is measured for different combination factors in order to display the
correlation between cutting conditions and resistance limit when the composite is loaded using four point bending tests. In
spite of the importance of the obtained results, this work remains far to be complete.
Experimental procedure
The Al/SiCp-MMC composite studied in this paper deals with 16mm diameter bar of aluminium matrix reinforced with
discontinuous silicon carbide particulates (SiCp). The average dimension of the SiCp particulates is about 5 to 8µm. A
typical microstructure of the Al/SiCp-MMC is shown in figure 1. It has been to note that reinforcement is distributed almost
homogenously into the longitudinal cross section of the material and there is no detected preferred orientation contrary to
materials reinforced with discontinuous or long fibres for which orientation is easily detected.
Figure 1. Microstructure of 15%vol.-SiCp reinforced MMC.
The different sets of experiments were performed by turning operation on a combination using 30, 60 and 90m/min cutting
speeds and 0.1mm/rev and 0.3mm/rev feed rates. The depth of cut is retained constant at 1.25mm for all cases in this
study. Table 1 shows the details of the silicon carbide reinforcement particulates.
Table 1. Properties of the reinforcement particulates (SiCp).
Volume fraction (%)
Specific gravity
Tensile strength (MPa)
Specific strength (MPa)
Modulus of elasticity(GPa)
Specific modulus (GPa)
15
3
3900
1172
425
143
Particularly, uncoated tungsten carbide (WC) tools were used for preparing specimens as recommended in literature [7-9].
The specifications of used tool are given in Table 2. In each parameters combination, a new tool is used for finishing the
job surface.
Table 2. Specifications of the cutting tool used in experiment.
Tool material and grade
Specification
Clearance angle (°)
Cutting edge angle (°)
Nose radius rε (mm)
Uncoated tungsten carbide tool (WC)
VCGX 16-04-04-AL-H10
5
35
0.4
Diameter iC (mm)
Tool width s (mm)
Length edge l (mm)
15.875
4.76
9.525
Analysis of results
The material that is investigated concerns 15%vol.-SiCp reinforced composite using an aluminium alloy matrix. Tensile test
has been performed for the considered material by the reason of describing its behaviour in a realistic way.
Tensile test
Standard specimen with a length of 55mm and a diameter of 6.105mm is used for this purpose. The composite response
is plotted in figure 2.
The composite material reacts according two phases. In the first stage of lading, deformations are low and tensile force
seems to increase linearly with the rise of displacement, the behaviour is then elastic. After the elastic limit, the second
stage begin, deformations increase on and ensile force increases in a non linear way with the displacement. The material
flow occurred and the behaviour is then considered plastic.
Referring to the tensile data, the yield stress of the studied material is about 300MPa, whereas the maximal tensile stress
is reached for 496MPa. The non linear response is modelled by the Ludwick law in such a manner that the equivalent
stress of the composite might be calculated for each strain value as given bellow:
σ c = σ yc + k c (ε Pl ) n
Where
σ yc , is the yield stress of the composite, k c
and
n
(1)
are respectively the hardening modulus and the hardening
component of the power law. Tensile test has lead to deduce the values of the hardening parameters;
n = 0.24 .
k c = 900 MPa and
600
Linear
Non linear
Stress (MPa)
500
400
300
200
100
0
0
0,01
0,02 0,03 0,04
0,05 0,06 0,07 0,08
0,09
0,1
Strain
Figure 2. Tensile test response of 15%vol.-SiCp reinforced MMC.
The experimental procedure consists of turning operations during which chip form, cutting tools flank wear and roughness
are investigated. The cutting operations are carried out using a CNC machine. Uncoated tungsten carbide (WC) tools are
chosen to execute the turning operations. The required conditions of cutting have been defined using three machining
speeds and two feed rates.
Machining tests: chip formation
A continuing problem with the Al/SiCp-MMC is that they are difficult to machine due to the hardness and the abrasive
nature of the SiC or other reinforcing particles [10]. The particles used in MMC can be harder than tungsten carbide (WC),
the main constituent of hard metal and even than most of the cutting tool materials.
Figure 3 shows the chip formation at different stages of the use of an uncoated tungsten carbide tool at constant speed,
constant depth of cut and constant feed rates. The chip shape seems to be very sensitive to the cutting time of the tool.
The length of chip increases more and more with the rise of cutting operations number. It’s already known that
discontinuous chips are much desired during cutting because they prevent damage at finish surfaces. In machining of
Al/SiCp-MMC, the successive use of the tool leads to the formation of flank build-up and formation of continuous chip as
shown in figure 3 after high number of cutting operations.
From the microphotes appearance, it can be observed that the discontinuity of the chip is obtained frequent at the first
stage of the tool life. After 10 operations of cutting, a wavy shape is noted. Then, the use of carbide tool evolves in such a
manner to give segmental chip that is called also semi-continuous chip as can be seen in (c) and later continuous one as
observed in (d) and (e).
It has been noted during machining of the considered material that the tungsten carbide tool loses rapidly its performance
which is due essentially to the presence of the hard particles of the silicon carbide. The carbide tools life depends highly in
the volume fraction of SiCp-reinforcement in such composite.
One cutting
operation
(a)
10 cutting
operations
(b)
20 cutting
operations
(c)
40 cutting
operations
50 cutting
operations
(d)
(e)
Figure 3. Chip formation of 15%vol.-SiCp reinforced MMC at different stages of cutting.
The addition of SiCp reinforcements into the aluminium matrix reduces the ductility and makes the material ideal to
produce segmental type chips during machining. Manna et al. [11] studied the chip formation and explained that during
machining, the propagation of crack under the tool effect is accelerated by the upward and side curling action of the chip,
which helps to produce a small or discontinuous chips as seen in (a).
They noted also that during machining, when the material undergone its shear limit by the cutting tool, the initiation of
cracks forms the outside free surface of the chip and separation of SiC particles and Al-matrix within the chip forms some
small voids. Once, this material was sheared further, the coalescence of the voids caused the crack growth and
propagation in a specific manner along the shear plane through the thickness of this chip. As a result, fracture takes place
and sliding of material formed wavy or toothed chips as given in (b).
Tool wear
Rapid damage of cutting tool persuades the deterioration of the work surface, increases machining times by reduction of
tool life, increases the down time for exchanging and resetting of cutting tools and ultimately increase the cost of
production. For the present application, the morphology of the tools has been examined in order to understand the wear
mechanism for this type of tools. The morphology of tools after machining is provided in figure 4.
Cutting speed: 30m/min
Feed rate:
0.1mm/rev
Depth of cut: 1.25mm
(a) Adhesion
(b) Erosion
Cutting speed: 30m/min
Feed rate:
0.3mm/rev
Depth of cut: 1.25mm
Figure 4. Cutting edges of uncoated tungsten carbide tools after turning operations.
It has been observed for each operation that work material adhered to the edges of tools as seen in figure 4a. The
adhesion phenomenon is well known in machining of hard material ultimately for the most important cutting speed as the
consequence of the increase in the degree of thermal softening of the chip material [11, 12]. When the Al/SiCp-MMC slides
over the edge of the hard cutting tool (WC) during turning, it always presents a newly formed surface to the same portion
of the cutting edge. Consequently, due to friction, the temperature and pressure level conduct the particles of the Al/SiCpMMC to adhere to the cutting tool edge.
Furthermore, the turning tests showed that the tungsten carbide (WC) tools are inadequate for machining Al/SiCp-MMC
because the tool wear is observed very early in such machining material that causes a very poor wok finish surfaces.
The hard particles (SiC) of the composite that come into contact to the hard surface, act as small cutting edges to enhance
the deterioration of the cutting tool edge and result an erosion phenomenon characterised by the lost of the surface layers
of the active zone of tool, figure 4b.
Fatigue test
As many industrial components undergo cyclic loading during working, the fatigue tests using four-point reversed bending
are widely adopted in researches. It is for this reason that fatigue tests were performed in SIMPLEX bending machine
equipped by a counter and four-point fixture devices as can be seen in figure 5a.
Specimen is fixed at their limit sections and loaded at the standard zone using a constant bending load. The required load
is adjusted using the GL weight that varies from 0 to 30kg. When the load to apply for test is higher than the maximum
value obtained by the adjustment of GL, regulation might be done by using an overweight Gz. The counter has to ensure
the registration of cycle number once test is started. Then, specimen has to be solicited to failure in order to investigate
the composite resistance.
l0
l
SPECIMEN
l0
(a)
Engine
Counter
(b)
Figure 5. (a) Four-points bending machine for fatigue test, (b) test specimen.
Figure 5b shows the typical design which is widely adopted by researchers as a standard shape for cylinder specimens.
2
The material is delivered as beams with square section of 22×22mm which have been served to cut the required
specimens. For each combination of cutting parameters, two specimens are prepared for testing on bending machine. The
working section, where material is subjected to the quite load, is a 9.48mm-diameter and 96mm-length.
For testing, two stress magnitudes have been retained for each combination. Specially, fatigue tests were performed for
specimen cut using the considered speed ranges and 0.3mm/rev-feed rate under 280MPa and 300MPa applied stresses.
All specimens are located according to the turning parameters that are used during preparation.
The high cycle fatigue (HCF) property resistance of particulates reinforced metal matrix composites depends on several
factors including particulates type, size and volume fraction, matrix microstructure, particulates-matrix interfaces
characteristics and ultimately conditions of cutting such as tools type as mentioned above. Herein, matrix is used as pure
aluminium alloy which avoids the complex phenomena in matrix microstructure.
Generally, processing conditions which are used for producing Al/SiCp-MMC enhances bonding at particulates-matrix
interfaces. Thus, it has been assumed that failure under fatigue load is essentially resulted as a consequence of global
fatigue mechanism of the composite.
Figure 6 shows the influence of cutting speed on fatigue life of the studied material under stress-controlled conditions.
Results are given for 0.3mm/rev feed rate. Initially, it can be noted that curves evolves according same manners when
cutting speed varies from 30 to 90m/min. Globally, fatigue life decreases with the increase of cutting speed notably for [30,
60]m/min speed ranges. After that, the number of cycle at failure seems to be less sensitive to the variation of cutting
speed.
Specially, when cutting speed increases from 30 to 60m/min, the cycles number at failure decreases about 0.57 times less
when the applied stress is equal to yield stress and about 0.78 times less when the applied stress is equal to 280MPa.
Contrary when speed varies from 60 to 90m/min, the fatigue life of loaded material increases sensitively and slightly.
Under high stress, materials undergoes more severe working conditions so the working life have to be logically lower than
the one when material is subjected to low stress magnitudes. The relative position of the curves seems to be evident.
120000
σ
110000
fatigue
100000
= 280 MPa < σ yc
Number of cycles
90000
80000
70000
60000
50000
40000
30000
σ
20000
fatigue
= 300 MPa = σ yc
10
20
10000
0
0
30
40
50
60
70
80
90
100
Cutting speed (m/min)
Figure 6. Number of cycles at failure vs. cutting speed (feed rate = 0.3mm/rev).
Furthermore, the average variation of working life when applied fatigue stress increases by about 7%, is about 54% for the
full cutting speed domain. Then, for such reinforced matrix, it is advised to use the less speed ranges in order to improve
the working life of the components. Moreover, fatigue life of the material evolves non-linearly with the speed variation
which can be explain, in first time, by the plastic flow aspect of material behaviour at failure under such stress magnitudes.
Microstructure properties
For investigating the fatigued failure surface, scanning electron microscope (SEM) has been used. Interesting details were
revealed when microstructure is examined. Figure 7a showed typical mode of crack propagation for fatigued surface.
(a)
(b)
SiC
Crack
propagation
(c)
(d)
SiC blocks
Figure 7. SEM fractographs of the Al/SiCp-MMC cycled up to failure at stress amplitude of 300MPa.
(a) Failure mode, (b) SiC particulates distribution (c) brittle feature of SiC blocks and (d) crack initiation at failure edge.
Immediately, it can be noted that failure started at the machined surface and propagate to the internal zones of the loaded
section. In addition, it can be seen locally that micro cracks initiated at the interface between SiC particulates and
aluminium alloy matrix. The debonding mechanism at interfaces is considered as the main feature that is observed in
silicon carbide reinforced aluminium matrix composites [13].
Figure 7b show the fractographs of Al/SiCp metal matrix composite deformed up to failure using four-point reversed
bending test under stress amplitude of 300MPa and cutting speed of 60m/min. Globally, fracture might be considered
ductile as can be observed referring to the average magnification.
The metallurgical powder technology is the main process for elaboration of composite materials. The distribution of
reinforcement within the matrix is not usually perfect which lead in several time to the presence of reinforcement blocks
that play hardly on the fatigue mechanism and ultimately on the fatigue life of components. These blocks can induce some
stress concentration as a result of the large difference between the matrix strength and the SiC particulates strength. A
typical block sample of silicon carbide particulates is shown in figure 7c. It is evident that the presence of SiC blocks
conduces to decohesion of the blocks from aluminium matrix under cycling which will decrease the fatigue life of
components. The fracture of such reinforcement blocks has to be predominant and contributes to premature fatigue life
because of the brittle nature of the SiC by report to the ductile nature of aluminium alloy matrix.
Figure 7d presents an example of crack propagation within the fatigued surface under above mentioned test conditions. It
can be evoked that micro cracks initiates at the free edge and evolves into internal zone of failure section.
Conclusions
The aim of this study was to contribute to the machining and fatigue literature through an experimental testing data and
observations on the behaviour of SiC particulates reinforced aluminium alloy matrix. Tensile test, machining tests and
fatigue tests using four-point reversed bending loads were performed. Then, the following conclusions can be revealed:
•
•
•
Chip formation is very sensitive to the cutting time of tool. Especially, Al/SiCp metal matrix composite seems to be
machined hardly by using uncoated tungsten carbide (WC) tool. It has been noted, particularly, that adhesion
phenomenon appears rapidly when such tool is used for turning operations.
Bending tests let said that application of high stress amplitudes by the way of a reversed loading device
decreases ultimately the fatigue life of composite components. Contrary, the use of low levels of cutting speed
enhances the fatigue life of products subjected to cyclic loading.
The scanning electron microscope observations deal with understanding the micro mechanisms of failure when
specimens made by composite are loaded cyclically up to failure. Particularly, decohesion mechanism, presence
of brittle SiC blocks and propagation mode of failure have been discussed in details.
Acknowledgment
Authors are grateful to ’FORGES de BOLOGNE’ group for its technical support and especially to ’J. Tschofen’ for its
assistance during this work.
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