Global Journal of Engineering Sciences (GJES)
A357
Aluminum Alloy Cutting Performance Finite Element Simulation
Authored by Cheng-Ming Huang
Abstract
Aiming
at the problem that deformation caused by cutting force in the cutting process
of A357 aluminum alloy, a two-dimension cutting simulation was established
based on ABAQUS. The curves of chip, cutting force and stress under different
cutting conditions were obtained by using Johnson-Cook constitutive equation.
The finite element simulation was applied to further analyze the effects of
different cutting conditions on chip, cutting force and stress. The simulation
results show that the cutting force increases with the increase of cutting
speed and increases with cutting depth in the experimental parameters range, it
also leads to poor cutting quality. As the cutting length changes, the
fluctuation of cutting force also increases and affects the overall quality of
the workpiece.
Keywords: Finite element simulation; Cutting
state; Cutting force
Introduction
Aluminum
is very soft, and it has low strength and low density. When mixed with other
metals, high strength aluminium alloys are formed, which can even surpass some
high-quality carbon steels. Because of its good plasticity, electrical
conductivity, thermal conductivity and resistance to metal corrosion, it is
widely used in industry. Aluminum alloys are second only to steel in usage and
range. It can be divided into deformed aluminium alloy and cast aluminium alloy
by machining method. The comprehensive mechanical properties of deformed
aluminum alloys are better than casting aluminum alloys. It can be processed
into various shapes and specifications of profiles and mainly used in the
manufacture of aviation equipment, automobile structures and doors and windows
for the construction industry. Casting aluminum alloy can obtain the complex
parts directly by casting process, so the processing cost can be reduced.
Over
the past few decades, it has been found in machining processes that surface
integrity includes surface roughness, residual stress, and microstructure,
because it significantly affects the fa tigue life and corrosion resistance of
the machined parts [1,2]. Machining parameters such as cutting conditions and
tool geometry greatly affect the surface integrity characteristics [3]. The
materials, tools, and equipment needed to conduct experimental tests are
expensive, and the process of setting up tests is often time-consuming.
Residual stress induced by cutting has been studied by many researchers through
finite element modeling.
The Establishment of Finite Element Model
A357 constitutive model was established
The
material constitutive model is used to describe the mechanical properties of
materials and to characterize the dynamic response of materials during
deformation. When the microstructure of the material is certain, the flow
stress is significantly affected by deformation degree, deformation velocity,
deformation temperature and other factors. Any change of these factors will
cause a great change of flow stress. Therefore, the material constitutive model
is generally expressed as the mathematical function relationship between flow
stress and deformation parameters such as strain, strain rate and temperature.
It is very important to establish the constitutive model of materials, not only
in formulating reasonable processing technology, but also in studying the
theory of metal plastic deformation. In modern plastic machining mechanics
represented by plastic finite element, the flow stress of material is an
important parameter when input and its accuracy is also the key to improve the
reliability of theoretical analysis. In this research, the material constitutive
model is a necessary prerequisite for numerical simulation of machining and an
important basis for predicting the deformation of part milling. Only by
establishing the stress-strain relationship with the change of strain rate and
temperature under the condition of large deformation can the plastic
deformation law of the material in the cutting process be accurately described
and then the deformation size and trend of the parts can be predicted under the
determined boundary conditions and the cutting load.
In the
process of cutting, the workpiece deforms under high temperature and high
strain, the time for the material being cut to become chips under the action of
a tool is very short, and the strain, strain rate and temperature of the
cutting layer are not uniformly distributed and the gradient varies greatly.
Therefore, the constitutive equation which can reflect the influence of strain,
strain rate and temperature on the flow stress of material is very important in
cutting simulation. At present, the constitutive models of plastic materials
commonly used are: Bodner-Paton, Follansbee-Kocks, Johnson-Cook,
Zerrilli-Armstrong, etc. Only The Johnson-Cook model describes the
thermoviscoplastic deformation behavior of materials at high strain rates. Johnson-cook
model holds that materials exhibit strain hardening, strain rate hardening and
thermal softening effects at high strain rates. Johnson-cook model is shown as
follows [4]:
In the formula, the
first item describes the strain strengthening effect of materials, the second
item reflects the relationship between the flow stress and the increase of
logarithmic strain rate, and the third item reflects the relationship between
the flow stress and the exponential decrease of temperature Trrepresent the reference strain rate and
the reference temperature respectively, Tm Is the melting point of the
material. In the formula, A, B, n, C and m are five undetermined parameters; A,
B and n represent the strain hardening term coefficients of materials; C represents
the strain rate strengthening term coefficient of the material; m represents
the thermal softening coefficient of material.
Material damage initiation criterion
In the finite element
simulation process, chip separation is manifested as unit failure and
separation problems, a rule is required for the degree to which the unit
deforms before it separates. There are two kinds of separation criteria at
present, one is to establish the geometric separation criterion of unit
separation according to some geometric quantities and the other is based on
some physical quantity, such as the stress and strain of the element in front
of the tip during cutting. If the specified physical quantity exceeds the given
value, unit separation is considered necessary.
In the previous work, it is
found that this method has a disadvantage, it needs to define the shape of the
cutter in advance to take it as the material flow boundary, but it is difficult
to determine its parameters. As shown in Figure 1 below, due to the shape
problem of the tool in the early stage, there was no chip, and all the failure
elements were deleted.
The
Shear Damage model is adopted here to define the failure strain.
The establishment of geometric
model
The two-dimension cutting
finite element simulation model is shown in Figure 2, the approximate size is
10 and the size of the model workpiece is 2mm × 0.6mm , The front Angle of the
prop is 70° and the back Angle is 5°. In this article, we define the plane
cutting- edge swept as cutting plane, the plane cross cutting-edge and
perpendicular to motion direction as base plane. The front Angle is dihedral
angle between the front cutting surface and the base surface, the back Angle is
dihedral angle between the back cutting surface and the In modeling, the
workpiece is of two-dimensional plane deformable type and the tool is of
analytic rigid body. Shell elements are used in Zhang Wei’s [6] model, A
separate line is defined in the cutting process, that is the deformation
cutting and the billet connection part. This model is not that complicated. It
also can be used to simulate the temperature distribution in cutting process,
but the separate line is not defined. Therefore, the analytical steps are dynamic,
temperature-displacement and explicit. A split surface is set 0.11mm away from
the upper surface of the workpiece to refine the mesh. The stable quadrilateral
plane strain grid is adopted in mesh division of workpiece, the element type is
the second - order precision four - node reduction integral thermal coupling
element (CPE4RT), adaptive grid technology is used to partition and set the
reference point in the upper right corner of the cutter. The cutting tool as
the first surface, the cutting layer and adjacent parts as the second surface
adopt the motion contact method, set the friction coefficient as 0.3. It is
also different from Zhang Wei’s model. Define the lower surface of the
workpiece and the side surface away from the tool as fully fixed. The cutting
speed and boundary conditions of the defined tool reference point are uniformly
distributed. The initial temperature of the cutting area is defined as room
temperature 300K.
The simulation scheme
In this paper, single factor
control method is used for simulation, only a single factor in cutting depth
and cutting speed can be changed. The stress distribution curves of the cutting
area under different working conditions are studied, the specific scheme is
shown in Table 6.
Simulation Results and Analysis
Simulation results of different cutting speeds under the same
cutting depth and contact length
When the cutting depth is
0.1mm, the contact length between the tool and the workpiece is 0.2mm, the two curves
in the change curve represent the force on the X-axis and Y-axis of the tool
respectively, it can be seen by comparing the curve of cutting force under
different cutting speeds, The force on the tool in the vertical direction is
slightly greater than that in the horizontal direction, The horizontal
direction fluctuates less than the vertical direction, Therefore, this paper
will discuss the change of force in the vertical direction. Because there is a
sudden fluctuation in velocity at the initial moment, And the force on the end
of the curve decreases, So the effective interval for comparing these curves is
some time in the middle. The specific force changes in each case are discussed
below.
• When the cutting speed was
7850mm/s, the Y-axis force was between 200N-400N, with small curve fluctuation
and good surface quality, as shown in Figure 3, Figure 4.
• When the cutting speed was
8400mm/s, the Y-axis force was between 250N-600N, with small curve fluctuation
and good surface quality, as shown in Figure 5, Figure 6.
• When the cutting speed is
8900mm/s, the force on the Y-axis is between 300N-600N, the curve fluctuates
greatly, and the cut surface quality is poor, as shown in Figure 7, Figure 8.
• When the cutting speed is
9450mm/s, the Y-axis force is between 300N-600N, the curve fluctuates greatly,
and the cut surface quality is poor, as shown in Figure 9, Figure 10.
• When the cutting speed is
9950mm/s, the Y-axis force is between 300N-700N, the curve fluctuates greatly,
and the cut surface quality is poor, as shown in Figure 11, Figure 12.
• When the cutting speed is
10450mm/s, the force on the Y-axis is between 400N-750N, the curve fluctuates
gently, and the cut surface quality is poor, as shown in Figure 13, Figure 14;
It can be seen from the
force analysis of each curve that the force decreases with the change of
velocity, then increases from small to large, and then decreases again. On the
whole, the force fluctuation is relatively stable, and the smaller speed is at
7850mm/s and 8400mm/s cutting speed. The final velocity is set at 7850mm/s
according to the machined surface quality.
Simulation results of different cutting depth at the same
cutting speed and contact length
At a cutting speed of
7850mm/s, the contact length was 0.2mm, it can be seen that the cutting force
is increasing in the simulation of the cutting process at different cutting
depths and the curve of cutting force changing with time. The contents are as
follow:
• When the depth is 0.08mm,
the force on the Y-axis is between 150N-500N, the curve fluctuates greatly, and
the surface quality after the resection is good, as shown in Figure 15, Figure
16;
• When the cutting depth is
0.1mm, the force on the Y-axis is between 200N-400N, the curve fluctuates
gently, and the surface quality after cutting is good, as shown in Figure 3,
Figure 4;
• When the cutting depth is
0.12mm, the Y-axis forces between 250N-550N, the curve fluctuates greatly and
the surface quality is good, as shown in Figure 17, Figure 18;
• When the cutting depth is
0.14mm, the Y-axis forces between 300N-600N, the curve fluctuates greatly, and
the surface quality is general, as shown in Figure 19, Figure 20;
• When the cutting depth is
0.16mm, the Y-axis forces around 300N-8000N, the curve fluctuates greatly, and
the surface quality is general, as shown in Figure 21, Figure 22;
It can be seen from the force
analysis of each curve that the force decreases with the change of cutting
depths, then increases from small to large, and then decreases again. On the
whole, Under the cutting depth of 0.1mm and 0.12mm, the force fluctuation is
relatively stable and small. The final cutting depths is set at 0.1mm according
to the machined surface quality .
Simulation results of
different contact lengths at the same cutting speed and cutting depth
At a cutting speed of 7850mm/s
and a cutting depth of 0.1mm, the drawing of cutting process and the curve of
cutting force changing with time are simulated for different cutting contact
lengths. It can be seen from the curve that the cutting force is increasing
with the increase of contact length, and the details are as follows:
• When the contact length is
0.15mm, the Y-axis force is between 100N-500N, the curve fluctuates gently, and
the surface quality after the resection is poor, as shown in Figure 23, Figure
24;
• When the contact length is
0.2mm, the Y-axis force is between 200N-400N, the curve fluctuates gently, and
the surface quality after the resection is good, as shown in Figure 3, Figure
4;
• When the contact length is
0.25mm, the force on the Y-axis is between 150N-400N, the curve fluctuates
greatly, and the surface mass after the resection is poor, as shown in Figure
25, Figure 26;
• When the contact length is
0.30mm, the Y-axis forces around 150N-400N, the curve fluctuates greatly, and
the surface quality after resection is poor, as shown in Figure 27, Figure 28;
From the perspective of
cutting force fluctuation and machining quality surface, the longer the cutting
length is, the greater the influence of force on the overall quality of the
workpiece is. Appropriate reduction of cutting length can ensure the overall
quality of the workpiece.
Conclusion
In
this paper, the high speed cutting of aluminum alloy is studied and analyzed by
finite element two-dimensional cutting simulation model. The simulation was
carried out with different cutting speeds, cutting depth and cutting length.
The distribution of the workpiece in the elastic-plastic stress field is known,
and the variation of the cutting force and the workpiece surface quality with
the cutting speed, cutting length and cutting depth is obtained, conclusions
are as follows:
• The
cutting speed has significant influence on the cutting force and cutting
quality. When the cutting speed is kept at a low level, the cutting force
fluctuation is not very great. But with the increase of speed, the fluctuation
degree of cutting force will increase, and the cutting quality will also become
worse.
• The
influence of cutting depth on cutting force is also very obvious, With the
increase of cutting depth, the cutting force increases and high degree of
fluctuation; The impact on the quality of the cutting, from the cutting
surface, or more rough.
• The
cutting length has obvious influence on the cutting force. As the cutting
length increases, the cutting force increases slightly. From the cutting
quality, the cutting surface is relatively smooth, the impact is not very big.
In
general, in order to ensure good cutting quality, the cutting speed can be
reduced appropriately, try to control within 8m / s , the overall impact on the
workpiece is not big
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