GENERATION OF SHORT PROFILES FROM THIN PLATES BY PLASTIC DEFORMATION (SHEET-BULK METAL FORMING)
Sérgio José Ferreira Poças Furtado
Department of Mechanical Engineering, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001, Lisbon, Portugal, May 2016
Abstract
In this thesis, a new form to generate short “T” profiles, using a single metal sheet is analyzed, through a new technological process. The technological process being studied exploits the Sheet-Bulk metal forming technology which consists in applying simultaneous sheet and bulk forming operations, by applying small deformations on the sheet of metal in unconventional directions. In this case, applying small deformations in the orthogonal direction to the thickness of the plate and not along this direction, creating small local deformations.
This new alternative proposed has the potential to replace current solutions achieved by other processes and technologies. That’s where the name comes from Sheet-Bulk because it applies mass deformations in a metal plate.
The main objective will be to generate a short “T” profile using only one plate, not being necessary to join multiple plates using some type of joining process to connect them, without using methods that involve removing a lot of material from a block of raw material to achieve the final form. And it doesn´t require a machine that is only specialized for that process without the ability of customizing the final form. To achieve this, small plastic deformations will be applied on the plate in small increments of deformation.
The present work is supported by a study using numerical modelling and one experimentation study, that analyze the four variations that this method suffered and how it evolved, to be able to solve the problems that where found during the simulations and experiments, to be able to reach the final objective of generating a short “T” profile, verifying the applicability of the process.
Keywords: Sheet-bulk metal forming, Plastic Deformation, Experimental studies, Finite element method
1. Introduction
Due to the current situation where everything these days is done towards individual customization, and to obtain personalized components for several different applications, an idea emerged of generating a short metal “T” profile from a single metal plate. Using a new concept that is still in the development stage, the sheet-bulk metal forming, which simultaneously uses the plate deformation process (sheet) and mass deformation (bulk). Allowing it to generate these short profiles from a single
metal plate, using an economic process that spends little energy and doesn’t have high material removal rates, which is essential in these times where you want to cut costs to stay in the current competitive market. It also does not require large specialized equipment for this operation only.
2. Experimental background
2.1. Stress – strain curve
Throughout experimental work, the raw materials used were AA5754 H111 (Aluminum). The plates mechanical behavior is essential in what concerns numerical testing. Therefore the stress-strain behavior was determined by means of calculating an average that was obtain from three tests, two stack compression tests using 2 and 3 discs and a tensile test. Specimens used in the stack compression test were assembled by pilling up circular discs cut from the plate.
The compression test was carried out at room temperature on a universal testing machine with a crosshead speed of 100 mm/min, in order to reproduce quasi-static conditions, thus eliminating dynamic effects.
𝜎 = 348.6𝜀
0.22(1)
Figure 1 – Stress-Strain curve for aluminum 5754 H111
3. Finite element modeling
The experimental operating conditions managed to be numerically modelled with the finite element flow formulation allowing the author to use an in-house computer program (I-FORM). The implemented finite element flow formulation is based on the following variational statement,
∂Π =∫
𝑉𝜎̅𝛿𝜀̅̇ 𝒹𝑉 +𝛫 ∫
𝑉𝜀̇
𝑉𝛿𝜀̇
𝑉𝒹V - ∫
𝑠𝛾
𝑇
𝑖𝛿𝑢
𝑖𝒹S = 0 (2)
Where K is a large positive constant that enforces the constraint of incompressibility, and whose value can be directly related to the average stress by:
σ
m=
12
Κ ε̇
V= λ (3)
0 50 100 150 200 250 300 350
0 0.1 0.2 0.3 0.4 0.5
True Stress (MPa)
True Strain
TT SCT _2
3.1. First stage
It was simulated how the process using the first tool would interact with the workpiece and how it would deform the workpiece, for that we studied how the different combinations of parameters that can be controlled would affect the process as it is shown in figure 2.
h- the fisrt increment that is done and marks the thickness of shoulders of the profile d- displacement of the next increments
Figure 2 – 1st solution design
(a) (b)
Figure 3 – Material overlap along the length of the plate in the end of the 2º increment. (a)𝒍𝒈𝒂𝒑=10mm, h=5mm, d=2mm (b) 𝒍𝒈𝒂𝒑=10mm, h=5mm, d=5mm
As it is shown in figure 3, changing the parameters with this type of punch there is always an overlap of material between increments, due to a hardening that occurs. The material from the previous increment suffers a hardening which leads to the material below the shoulders and the hardened part to deform first and increasing the thickness in that section. Then the hardened material above will slide above and overlap it. Creating planes of overlapping material along the length of the plate. To solve this a new punch was designed and it is presented in the next subchapter.
Material overlap
𝑙𝑔𝑎𝑝=10mm, h=5mm, d=2mm 𝑙𝑔𝑎𝑝=10mm, h=5mm, d=5mm
Thickness of the shoulder Length of the shoulder
3.2. Second stage
In the second stage, we introduced a new tool to solve some of the problems of the first stage by adding an edge to the punch tool to generate some divided flow, so the material could flow better and more fluidly without overlapping itself in the following increments. The results are in Figure 5.
Figure 4 – 2nd solution design
(a) (b)
(c)
Figure 5 – Final mesh after three increments had been done to the plate using Punch2 and Die1 (a)𝒍𝒈𝒂𝒑=5mm, h=3mm, d=3mm. (b) 𝒍𝒈𝒂𝒑=10mm, h=3mm, d=2mm. (c) 𝒍𝒈𝒂𝒑=10mm, h=5mm, d=5mm
As it is shown in the previous figure, the divided flow edge can eliminate the material overlap, if used in small increments from 3mm below. But for bigger increments the overlap will still remain. We can also observe some small superficial defects left by the die during the deformation process. To reduce some of these superficial defects a new die was introduced and the results can be examined in the next subchapter.
𝑙𝑔𝑎𝑝=5mm, h=3mm, d=3mm
𝑙𝑔𝑎𝑝=10mm, h=3mm, d=2mm
Superficial defects Superficial defects
𝑙𝑔𝑎𝑝=10mm, h=5mm, d=5mm
Material overlap
Thickness of the shoulder Length of the shoulder
3.3. Third stage
In the third stage, we introduced a new die to solve the problems of superficial defects of the second stage by creating a fillet on the die, so the material could flow better and more continually without leaving marks and superficial defects on the workpiece. The results can be observed in Figure 7.
Figure 6 – 3rd solution design
(a) (b)
(c) (d)
Figure 7 -Final mesh after three increments had been done to the plate using Punch2 and Die2 (a) 𝒍𝒈𝒂𝒑=5mm, h=2mm, d=2mm. (b) 𝒍𝒈𝒂𝒑=10mm, h=3mm, d=3mm. (c) 𝒍𝒈𝒂𝒑=10mm, h=5mm, d=3mm.
(d) 𝒍𝒈𝒂𝒑=15mm, h=3mm, d=3mm
As it can be examined in Figure 7, we can see that the superficial defects were reduced by the introduction of the new die when the same conditions of the previous simulations are used, for small increments. But for bigger increments the overlap effect still occurs. Some problems were found in the experimental tests that weren’t predicted in the numerical simulation, and to solve those problems a final solution was created, which can be observed in the next subchapter.
Thickness of the shoulder
Material overlap
Superficial defects reduced
Length of the shoulder
3.4. Fourth stage
In the fourth stage, we introduced the combination of two different punches one to divide the flow of material and a second one to compress and give its shape. The reason was that the simulation couldn’t represent a problem which the experiment faced while doing the experimental study. In which the horizontal part of the punch next to the edge would still not let the material flow fluidly. This new solution was able to take care of that by changing the punch to separate the material first and then compress it one after the other instead of doing it simultaneously. The results are in Figure 9.
Figura 8 - 4th solution design
(a) (b)
Figure 9 –Final mesh using Punch3+Punch4 and Die2 (a) after three increments had been done to the plate, 𝒍𝒈𝒂𝒑=5mm, y=2mm, t=2mm. (b) after twelve increments had been done to the plate, 𝒍𝒈𝒂𝒑=10mm, y=2mm, t=2mm.
By examining the previous figure, we can see that using the final solution with the two independent punches alternately, we can reduce the superficial defects, create a more homogenized profile, without overlapping material along the length of the plate.
Thickness of the shoulder Length of the shoulder
4. Results and Discussion
This section of the paper exposes the experimental and numerical results in order to evaluate the process limitations and feasibility window in which it can operate.
4.1. Results
Figure 10 – Comparison between the curves of the first solution for 𝒍𝒈𝒂𝒑=10mm varying the initial increments and the ones after that
Figure 11 - Comparison between the curves of the second solution for 𝒍𝒈𝒂𝒑=5mm varying the initial increments and the ones after that and a curve for 𝒍𝒈𝒂𝒑=10mm in similar increment conditions as one of the previous 𝒍𝒈𝒂𝒑 (1stincrement of 2mm and the next of 2mm also).
0 200 400 600 800 1000
0 2 4 6 8 10 12
Load (kN)
Displacement (mm) Gap10 h3d3
Gap10 h3d5 Gap10 h8d3 Gap10 h5d5 Gap10 h5d3 Gap10 h5d2
0 200 400 600 800
0 2 4 6 8 10
Load (kN)
Displacement (mm) Gap05 h2d2
Gap05 h3d3 Gap05 h3d5 Gap10 h8d2
Figure 12 - Comparison between the curves of the third solution for 𝒍𝒈𝒂𝒑=5mm varying the initial increments and the ones after
Comparison between the numerical simulation and experimental results
Figure 13 – Comparison of the numerical simulation and experimental results for the final solution for 𝒍𝒈𝒂𝒑=5mm varying the first and next increments between values of 1mm to 3mm.
Figure 14 – Evolution of the results from Punch1, 2, 3+4 with the dies 1 and 2 0
200 400 600 800
0 2 4 6 8 10 12 14
Load (kN)
Displacement (mm) Gap05 h2d2
Gap05 h3d3
0 50 100 150 200 250
0 2 4 6 8 10 12
Load (KN)
Displacement (mm) Numérico
Experimental
Punch2 + Die2 Punch3,4 + Die2
Punch1 + Die1
4.2. Discussion
Analyzing Figure 13, the numerical-experimental comparison, it can be seen that there is a discrepancy in the load applied during the process material flow separation. After analysis, it was concluded that during the simulation, it was decided to eliminate small elements along the axis of symmetry to recreate the dunnage what happens in reality, but that this was not a good approximation of what happens experimentally. The elimination of elements along the axis causes the separation edge will not find direct contact along the axis, only having contact then between the side surfaces of the flow separation punch that opens the material of the plate, pulling the two streams sides. What it led to much lower forces in numerical simulation for phase flow separation.
In the final compression of the numerical simulation, there is a jump in the applied load, which does not happen in the experimental test, as in numerical simulation the flaps folded more than actually creating that power jump that can be seen in Figure 13 images. These images in Figure 13 show the moment before jumping to the load when the ends of the flaps not yet touched the upper surface of the matrix, the moment it touches the surface of the array and load the jump increasing, and then the continued compression of the profile tabs. While in reality the flaps are not folded and thus both the ends do not come into contact with the upper surface of the matrix in the same manner, making the load applied more continuous along the compression.
Some discrepancies found in the comparison are also due to the fact that the simulation modelling considered a perfectly rigid die assembly when in fact the die had some looseness and elasticity. And also the fact that it was difficult to align the workpiece and the punch, while the simulation would align it perfectly creating an identical material flow for both sides after the edge of the punch touches the workpiece, which doesn’t happen in reality.
5. Conclusion
The results of the numerical simulation showed that it is possible with the final solution to generate short profiles "T" from a single plate as it was intended. Being able to vary the thickness of the flange of the "T" and its length.
Through experimental tests some problems controlling the process were discovered due to the lack of rigidity of the machine, the gaps that existed in the modular die, and aligning the punch with the workpiece. Despite the difficulties encountered, it was verified by experimental tests that the innovative process studied in this work is applicable and can generate short "T" profiles as it was intended.
The final and main conclusion of this work is that this new technique Sheet-Bulk Metal Forming is feasible and applicable to generate profiles from a single plate, with even chances of more possible future experiences to further develop the process and extend its field of application.
All this using a non-specialized machine for the process as it would be an extruder, not using bonding processes between plates and using simple tools.
Acknowledgements
The author would like to acknowledge all the support and guidance of Professor Luís Alves, who always had new ideas and new advices to help fix some problems that this work faced. And also acknowledge the patience and the spirit of help that Professor Carlos Silva provided, by being always ready to help me during the experimental phase whenever it was necessary.
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