Initial Residual Stress Processing Strategy Processing Deformation Finite Element Block Processing Sequence In order to reveal the influence of different processing procedures on the machining deformation of the whole structure of aviation frame type, a prediction model for the deformation of frame type integral structure caused by initial residual stress was established based on finite element. According to the deformation law of frame parts caused by initial stress of blank, the deformation of whole structure parts based on aviation frame is minimized. The influence of processing process on deformation of frame structure is studied. The results show that when removing the frame material, the deformation of the workpiece can be slowed down by first removing the area with large deformation and relatively concentrated material. And symmetrical removal of the frame material can also slow the deformation of the workpiece.
## 1. INTRODUCTIONWith the rapid development of modern aviation industry, the
use of aircraft structural parts has put forward higher requirements, and more
and more integral structural parts have been adopted. However, the problem of
deformation of integral structural parts has been puzzling the aviation
manufacturing industry. European and American countries have long been
concerned about the processing deformation of aviation structural parts. Airbus
has been researching since the mid-1990s. At
the manufacturing level, a solution that requires only CNC personnel to do the
job [1][2]. The characteristics
of aviation integral structural parts mainly include large size, complex
structure and many thin walls. After NC machining, it is easy to cause
deformation [3] Airframe parts are an important part of the aviation
structure. The main features of the
frame parts are the following: the shape is relatively large, its bearing force
is also relatively large; the box is numerous and complex; thin-walled
structure, processing easy deformation; divided into single-sided frame and
double-sided frame, its structure is basically symmetrical up and down or left
and right. Although the shape of the frame structure is different, but in fact,
there are regular rules to follow, the box is mainly divided into one-sided and
two-sided, symmetrical and asymmetrical, in processing methods and processing
technology is basically the same[8]. The research object
of this paper is 7050-T7451 aluminum alloy single-sided frame whole structure.
Based on the finite element (finite element) Abaqus, a predictive model for the
deformation of frame-like structural parts caused by residual stress was
established. The deformation law of workpiece caused by initial stress of blank
is analyzed, and the influence of different frame processing technology on the
deformation of workpiece is studied. ## 2. MODEL OF FRAME CLASS STRUCTUREThe frame structure is complex in
structure, and most of the frames are irregular. Therefore, it is obviously
unrealistic to build a model exactly like the actual structure. It is necessary
to simplify the frame structure and ignore some structural features which have
little influence on the processing deformation. For example: process hole,
chamfer, etc. Figure 1 is a simplified model of a type of civil aircraft frame.
Its peripheral contour geometric size is 1000mm ×1000mm ×60mm, and the wall
thickness of the workpiece is 5mm.
## 3.
FINITE ELEMENT MODELING FOR MACHINING
DEFORMATION OF FRAME STRUCTURAL PARTS
1)
Aluminum alloy 7050-T7451 is selected as blank material, Young's
modulus is 71700MPa, Poisson's ratio is 0.33, and the overall size of blank is
1100mm ×1100mm ×60mm. Figure 2 shows the initial residual stress of the
7050-T7451 aluminum alloy pre-stretch plate produced by US Kaiser Aluminum
& Chemical Corp [10]. From figure 2, it can be seen that the
residual stress change in the rolling direction (the x-axis direction) is
obviously larger than that in the transverse direction (the y-axis direction).
With the neutral surface as the center, the residual stress in both directions
is symmetrical.
The method of
applying initial stress to finite element model is to discretize the initial
stress of aluminum alloy sheet. The discretized initial stress is then applied
layer by layer to the workpiece of the partitioned mesh. In order to improve
the accuracy and reduce the error caused by the discretization of the stress
field, the multi-peak Gaussian curve is fitted according to the initial stress
distribution characteristics of the blank 2)
Boundary
Conditions and Meshing Adopt the
solid-branch constraint, that is, the constraint of 6 degrees of freedom, after
processing, release the constraint. Unit type C3D8R is used in the calculation. In the process of
practical processing, the residual stress inside the workpiece is gradually
released. The redistribution of residual stress forms a new equilibrium state
inside the workpiece. After discretizing the initial stress of the blank, a
layer is applied to the finite element model. The blank is divided into 12
layers. 3)
Processing
simulation In the finite
element ABQUS, the material removal is realized by using the technology of life
and death unit [11] 。Select the material you want to remove and
calculate it as a load step in a finite element. With the removal of the
material, the residual stress within the workpiece is redistributed. ## 4. EFFECT OF FRAME MACHINING SEQUENCE ON PROCESSING DEFORMATION OF WORKPIECEFigure3 is a finite
element model that divides the area into A, B and C (). where A is the peripheral material to be removed, B is the
intermediate circular hole material to be removed, and C is the spacer material
to be removed. After processing, release its constraints and select point D as
the fixed reference point.
According to the
processing sequence of C-B-A, the material of the spacer frame is removed
first, then the material of the middle circular hole is removed, and the
peripheral material is finally removed. the material removal area and the
corresponding material removal rate are shown in table 1. On this basis, this
paper focuses on the effect of the removal of the bulkhead (area C) sequence on
the deformation of the workpiece.
## 4.1. THE EFFECT OF THE REMOVAL SEQUENCE OF "S" AND "T" ON THE DEFORMATION OF STRUCTURAL PARTSAs can be seen from
Figure 4, the spacer area is divided into the upper and the next two parts
(excluding A and B regions), of which 1,2,3,4.... represents the processing
order of the septum. Fig.4(a) is processed from the lower left corner Cg1 frame
to the Cn18 frame in the order in which the Arabic numerals are processed and,
in the direction, indicated by the arrow. That is, the lower area is removed
first and then the upper area is removed. Fig.4(b) is processed from the Ce1
frame to the Cj8 frame according to the sequence of processing of the marked
Arabic numerals and the direction indicated by the arrow. That is to remove the
upper area, and then remove the lower area. In order to facilitate the
following analysis, the processing sequence of the frame in Fig. 4(a)is defined
as``S'' , and the processing sequence of the frame in
Fig. 4(b)is defined as "T'' .
Figure 5 is the
corresponding workpiece stress diagram and deformation diagram obtained after
processing according to the processing sequence of Figure 4(a). As can be seen
from Figure 5(a), the maximum residual stress of the workpiece is 41.17 MPa. It
can be seen from Figure 5(b)
that the maximum deformation is located at the top of the workpiece and the
maximum deformation is 0.985mm.
Figure 6 is the
corresponding workpiece stress diagram and deformation diagram obtained after processing
according to the processing sequence of Figure 4(b). It can be seen from Figure
6(a) that the maximum residual stress of the workpiece is 41.17 MPa. It can be
seen from Figure 6(b) that the maximum deformation is at the top of the
workpiece and the maximum deformation is 1.210mm.
According to the
above analysis, there are two kinds of box removal methods:" S “. The
workpiece deformation obtained by the box removal of the "S ". That
is to remove the lower area first, and then remove the upper area, the
resulting workpiece deformation is relatively small. This is due to the
relatively large number of materials removed from the lower regions and their
concentration in the main deformed regions. Remove the lower area first, the
other areas of the workpiece can provide a relative support to slow the
deformation of the workpiece. Then remove the upper region with relatively
small deformation. ## 4.2. THE EFFECT OF THE REMOVAL SEQUENCE OF "U" AND "V" ON THE DEFORMATION OF STRUCTURAL PARTSFigure 7 is two different sequence of compartment
processing, where 1,2,3,4.... represents the processing order of the spacer.
Figure 7(a) is processed from the Cg1 frame to Ce18 in sequence according to
the sequence of processing of the marked Arabic numerals (Cg1-Cl2-Ch3-Ck4-Cf5-Cm6-Ci7-Cj8-Cn9-Co10-Cp11-Cq12-Cr13-Ca14-Cb15-Cc16-Cd17-Ce18).
Figure 7(b) is processed from the Cf1 frame to the Ce18 frame in the order in
which the marked Arabic numerals are processed and, in the direction, indicated
by the arrow. As in Figure 4
above, for the purposes of the following analysis, the sequence of frame
processing in Figure 7(a) is defined here as "U" and the sequence of
frame processing in Figure 7(b) is defined as "V".
Figure 8 is the
corresponding workpiece stress diagram and deformation diagram obtained after
processing according to the processing sequence of Figure 7(a). It can be seen
from Figure 8(a) that the maximum residual stress of the workpiece is 41.17
MPa. It can be seen from Figure 8(b) that the maximum deformation is located at
the top of the workpiece and the maximum deformation is 1.027 mm.
Figure 9 is the corresponding workpiece stress
diagram and deformation diagram obtained after processing according to the
processing sequence of Figure 7(b).
It can be seen from Figure 9(a)
that the maximum residual stress of the workpiece is 41.17 MPa. It can be seen
from Figure 9(b) that the maximum
deformation is located at the top of the workpiece and the maximum deformation
is 1.145 mm.
According to the
above analysis, there are two kinds of box removal methods:" S ".
"U " has a smaller deformation of the workpiece than" V ".
When removing the lower area first, the left and right symmetrical removal of
the spacer is less deformed than the sequential removal of the spacer. This is
because compared with the sequential removal of the spacer, the left and right
symmetrical removal of the spacer can make the change of residual stress in the
workpiece more stable, so the deformation is relatively small. ## 4.3. THE EFFECT OF THE REMOVAL SEQUENCE OF
"W" AND "X" ON THE DEFORMATION OF STRUCTURAL PARTS
Figure 10 shows the processing sequence of two
different removal materials. where Figure 10(a) is processed from the Ca1 frame, sequentially to the Ck18 frame
according to the processing order of the marked Arabic numerals. Figure 10(a) is processed from the Ca1 frame to the
Ck18 frame in order of processing of the marked Arabic numerals (Ca1-Cr2-Cb3-Ckq4-Ce5-Cp6-Cd7-Co8-Ce9-Cn10-Cf11-Cm12-Ci13-Cj14-Cg15-Cl16-Ch17-Ck18).
That is to remove the frame symmetrically from top to bottom. Figure 10(b) is processing from the Cg1 frame to the
Cr18 frame in sequence according to the processing order of the marked Arabic numerals
(Cg1-Cl2-Ch3-Ck4-Cf5-Cm6-Ci7-Cj8-Ce9-Cn10-Cd11-Co12-Cc13-Cp14-Cb15-Cq16-Ca17-Cr18).
That is, from the bottom to the left and right symmetrical way to remove the
box. As in figure 4 above, here the
sequence of box processing in Figure 10(a) is defined as “W” and the sequence of box processing in Figure 10(b) is defined as “X”.
Figure 11 is the corresponding workpiece stress
diagram and deformation diagram obtained after processing according to the
processing sequence of Figure 10(a).
It can be seen from Figure 11(a)
that the maximum residual stress of the workpiece is 41.17 MPa. It can be seen
from Figure 11(b) that the maximum
deformation is located at the top of the workpiece and the maximum deformation
is 1.215 mm.
Figure 12 is the corresponding workpiece stress
diagram and deformation diagram obtained after processing according to the
processing sequence of Figure 10(b).
It can be seen from Figure 12(a)
that the maximum residual stress of the workpiece is 41.17 MPa. It can be seen
from Figure 12(b) that the maximum
deformation is located at the top of the workpiece and the maximum deformation
is 0.953 mm.
According to the
above analysis, "W "(Fig.10(a)) and" X "(Fig.10(b)) are two types of box removal and the "X "(fig.10(b)) is less deformed than the" W
"(fig.10(a)). That is to say,
the workpiece deformation obtained by removing the frame from the bottom up is
relatively small. That is to remove the lower area with large deformation
first, the other areas of the workpiece can provide a relative support to slow down
the deformation of the workpiece; then remove the upper area with relatively
small deformation. To sum up, when
removing the frame material, first remove the area where the workpiece is
deformed and the material is relatively concentrated, then remove the other
areas. This will slow down the deformation of the workpiece. And symmetrical
removal of the frame material can also slow the deformation of the
workpiece. In order to more
accurately analyze the influence of the processing sequence of different
spacers on the deformation of the workpiece, the optimal processing sequence of
the spacers is found. On the basis of the C-B-A machining sequence, the
numerical analysis of the workpiece was carried out for 6 different frame
processing sequence. And the corresponding coordinate system is established.
This can more intuitively reflect the maximum deformation of each process and
easy to compare. Figure 13
shows the maximum deformation of the six different frame processing sequences.
Figure 13 shows that the "T" and
"W" removal methods are relatively large in the first step. whereas
the "S", "U", "V" and "X" four
compartment removal methods in the first step of the processing process
deformation is relatively small. This is because the "T" and
"W" frame are removed by removing the upper part of the box first,
while the "S "," U "," V" and "X"
frames are removed by removing the lower part of the frame first. The material
removed from the lower region is relatively large and concentrated in the main
deformation area. First remove the lower area, other areas of the workpiece can
provide a relative support to slow down the deformation of the workpiece. then
remove the upper region with relatively small deformation. ## 5. CONCLUSIONIn this study, the
paper analyzes the deformation problem of the structure parts of aviation
integral frame caused by different processing technology sequence of spacer
frame. Based on Abaqus, a finite element model of processing deformation caused
by initial residual stress in frame whole structure was established. The
deformation law of frame whole structure caused by initial residual stress of
blank is analyzed. The influence of the processing sequence of different frame
on the deformation of the whole structure of aviation frame was studied. The results show that when removing the frame material, the
deformation of the workpiece can be slowed down by first removing the area with
large deformation and relatively concentrated material. And symmetrical removal
of the frame material can also slow the deformation of the workpiece. ## SOURCES OF FUNDINGProjects supported by the National Natural Science Foundation of China (No. 51605037). ## CONFLICT OF INTERESTNone. ## ACKNOWLEDGMENTNone. ## REFERENCES[1] S Masoudi, S Amini, E Saeidi, H Eslami-Chalander. Effect of machining-induced residual stress on the distortion of thin-walled parts[J]. International Journal of Advanced Manufacturing Technology, 2015, 76(1-4): 597-608. [3] T Maeno, K Mori, R Yachi. Hot stamping of high-strength aluminium alloy aircraft parts using quick heating[J]. CIRP Annals-Manufacturing Technology, 2017, 66(1): 269-272. [4] RJH Wanhill, GH, Bray. Chapter 2-Aerostructural Design and Its Application to Aluminum-Lithium Alloys[J]. Aluminum-lithium Alloys, 2014,14(7): 27-58. [7] Yuan W J, Wu Y X. Mechanism of Residual Stress Elimination for Aluminum Alloy Thick Plate Based on Prestretching Process[J]. Journal of Central South University (Natural Science Edition),2011,42(8): 2303-2308. [9] A Singh, A Agrawal. Investigation of surface residual stress distribution in deformation machining process for aluminum alloy[J]. Journal of Materials Processing Technology, 2015, 225: 195-202.
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