The aim of this study is to determine the sheet metal formability of AA-5083-O sheets processed by the Friction Stir Processing (FSP). The FSP process was studied and a FSP tool was built. Processing quality was verified by the metallography in the processing region, which established the voids presence. Tensile tests were carried out on FSP and non-FSP specimens, and the results showed that FSP specimens have 30% greater resistance than non-FSP ones. The formability of FSP sheets was produced in MSC-MARC and Abaqus and these software products were compared by using the nonlinear FEM code. The Forming Limit Diagram was built with the results from both software products. A device to process FSP sheet metals was developed and the sheets were processed to validate the results from the software. The tools made for the bulge tests were circular and ellipse-shaped. After the bulge tests, the commercial sheets showed close approximation to those obtained from the software. The FSP sheets broke when inferior pressure was applied because of the defects in the FSP process. The results of the FSP presented the same formability of commercial sheets, however, with 30% greater strength.
The Friction Stir Welding (FSW), patented in 1991 by the TWI Institute, is a method used to join two parts, defining a welding joint, with better mechanical properties than the original parts. Heat and friction are generated by the tool, which is inserted between the parts defining the plasticity condition. The tool is then removed from the parts and the regions welded are solidified and remain joined [
As the Friction Stir Processing (FSP) is an attractive way to refine grains, many studies on Superplasticity obtained from FSP joints were developed. The grain size obtained from a typical FSP joint is about 1 to 5
Since FSP refines the grains, it is important to apply this process to the formability of complex sheets. It is not possible to obtain complex sheets by using the conventional process of formability and, to obtain a complex part, the number of parts is increased, as for the emergency door of the BAE 125/800 airplane. When formed under superplastic condition, the number of parts was reduced to 76 allowing the use of 1000 bolts [
The Forming Limit Diagram (FLD) is used to forecast the forming limit of a sheet. The forming limit is defined as the state in which localized thinning on the sheet starts during the forming, leading it to rupture. The forming limit is conventionally defined by a curve drawn in a graph, in which the major deformation is in the
Practical tests are carried out to obtain the FLD diagram until the break occurs. These tests may be performed with the hydroforming, the bulge, or the Nakajima tests. After the tests, major and minor strains can be measured. Analytical models may be used to determine the FLD diagram and compare to the practical tests [
The bulge test consists in fixing a sheet blank into a circular or elliptical matrix and applying pressure with air or oil to deform it. When the matrix is circular, the sheet is obtained after its deformation. For this test, the sheet has circle graduation and the dimensional variations are measured to determine the FLD points [
Another way to study the formability of sheets is to form them at warm temperatures, as is the case of the AA-5083 aluminum alloy at 400°C [
The commercial software Abaqus was employed to simulate Nakajima tests in warm conditions by using the finite element method. For this case, a MTS universal tensile testing machine was used and adapted with a furnace to carry out the Nakajima tests at high temperatures. The comparison between the real and theoretical tests was made with the analytical model developed with Abaqus. This FEM software has the ability to solve thermocoupled problems at high temperatures, which allows reducing the number of practical tests [
Lee et al. [
Kim et al. [
Leitão et al. [
The objective of this study is to determine the formability of AA-5083-O sheets processed by FSP. The experimental procedure is illustrated in Figure
Schematic flowchart of the experimental procedure.
The FSP tool was designed and constructed, consisting of a body, a shoulder, which transmits the heat generated by friction to the sheets, and a threaded pin to transport the material. The body was made of SAE 1045 steel to maintain the temperature constant during the process. The shoulder and pin were made of S2 tool steel. The pin diameter is 8 mm and the shoulder is 14 mm. The length of the pin is 4 mm. The ratio of 1.75 between the pin and shoulder diameters was determined by experimental procedure. Ratios greater than 2 were used for preliminary tests, but the pin broke. There is a groove at the shoulder back to prevent the material from escaping during the process. Figure
FSP tool.
The AA-5083-O aluminum alloy used for this study is laminated alloy without heat or hardness treatment. The sheets are commercial, 6.35 mm thick. Table
Chemical composition of AA-5083-O.
Si | Fe | Cu | Mn | Mg | Cr | Zn | Ti | Bal. |
---|---|---|---|---|---|---|---|---|
(%) | (%) | (%) | (%) | (%) | (%) | (%) | (%) | (%) |
0.9 | 0.16 | 0.02 | 0.65 | 4.6 | 0.06 | 0.03 | 0.06 | 0.15 |
The FSP specimens processing was carried out by a conventional milling machine. The sheet was supported by high-speed steel grinding shims and fixed by clamps at the upper part, leaving just enough space to pass the tool between the shims. Just one side of the sheet was processed in the preliminary tests. Figure
Conditions to process preliminary specimens.
Condition | Rotational speed | Longitudinal speed |
---|---|---|
(rpm) | (mm/min) | |
1 | 500 | 65 |
2 | 500 | 88 |
3 | 500 | 104 |
4 | 500 | 125 |
5 | 400 | 65 |
6 | 400 | 88 |
7 | 400 | 104 |
8 | 400 | 125 |
9 | 328 | 65 |
10 | 328 | 88 |
11 | 328 | 104 |
12 | 328 | 125 |
Fixture assemblies for the FSP tests.
Figure
Specimen for the uniaxial test (mm).
Nesting the specimens on the blank (mm).
To process the specimen, it is necessary to process one side first and then the opposite side. A sequence was defined to avoid the tunnel defect, since during the process a small channel is left from the tool. First the raw side of the plate is processed. Then, because of the small channel, this side is milled before the opposite side is processed. This scheme is depicted in Figure
(a) Friction Stir Processing steps units (mm). (b) Tunnel defect after processing the opposite side.
Metallographic tests were performed with Keller attack. Figure
Metallography of FSP for CS1 condition.
Microhardness tests were performed for the CS1, CS2, and CS3 conditions. Two specimens without processing were studied for each condition; the specimens were embedded in Bakelite and sanded with 600-mesh granulation. The Shimadzu HMV2 microhardness tester was used with pyramidal tip with a square base angle of 136°. An indentation series was made with 2 mm spacing. Figure
Hardness tester.
(a) MTS tensile test machine. (b) Data acquisition system.
Two different approaches were used for the formability tests, bulge and Nakajima.
The bulge tests were carried out using 5 different surface sheets, with the CS1, CS2, and CS3 processing conditions. Figure
Blank holder used for the formability tests for MSC-MARC and Abaqus FEM commercial codes. (a) Circle. (b) 7 : 10 ellipse. (c) 6 : 10 ellipse. (d) 4 : 10 ellipse.
The thickness of the sheet for each test was 2 mm. The finite element mesh used for MSC-MARC was the membrane element. For Abaqus, the membrane element was not used because of convergence difficulties. Figure
FEM element mesh. (a) Abaqus. (b) Discrete rigid finite elements at the clamp. (c) MSC-MARC.
Figure
Boundary conditions of the models for MSC-MARC and Abaqus.
The pressure applied under the sheet represents the load in the models. The pressure lasts 60 seconds with single values for each model. The stop criterion was according to the stress on the model, when the tensile strength of the material was reached. The average pressure was 14 MPa.
For both Abaqus and MSC-MARC, the stress strain curve was used for the CS1, CS2, and CS3 conditions. These data were obtained from the uniaxial tests. The material is considered elastic plastic and the stress strain curve is inserted as a table. In MSC-MARC, it is possible to plot the stress strain curve. Figure
Stress strain curve plotted in MSC-MARC for condition 1.
The contact condition between the clamp and the sheet was established as rigid with a friction coefficient of 0.3 for both software products. The simulation time was 60 seconds. The number of steps was for circular models 600 steps and for the ellipse models 1200 steps.
The Nakajima test consists of forming sheets by using spherical punch, in which sheets with different sizes are formed. The sheets formed by Nakajima tests are 2 mm in thickness and are 100, 150, and 200 mm in size. The length of the sheet is constant at 200 mm. As for the bulge tests, the membrane element is used in MSC-MARC and the shell element in Abaqus. Figure
(a) Finite element mesh for punch and sheet in MSC-MARC. (b) Finite element mesh for the sheet in Abaqus. (c) Finite discrete rigid element mesh for the punch in Abaqus.
Figure
Boundary conditions for the Nakajima test.
The simulation time for both software products is 60 seconds, and the number of steps was 1200 for all models.
Special tools were used for the bulge tests to validate the FEM analysis in four geometries with the same dimensions as in the FEM analysis. A high pressure manual pump developed by Rudloff Industrial was used to expand the sheets with hydraulic oil. The pump is provided with a digital manometer to control the pressure applied during the test. Figure
(a) Tools to validate the FEM analysis of the bulge tests. (b) Manual hydraulic pump developed by Rudloff Industrial.
The sheets used for the bulge tests have larger sizes and because of this the processing showed cold conditions; therefore, CS1, CS2, and CS3 did not work for these cases. New tests with higher rotation speed were carried out and the new condition was applied. The rotation speed was 733 rpm and the longitudinal speed was 123 mm/min. A device to process the sheets was developed with a special characteristic that allows the sheet to move across. The distance between the tool and the bus machine is the limit to process one sheet. The step between the beads is 8 mm, which is the same as the pin diameter. Figure
Device to process larger sheets.
FSP of a large size sheet with a special device at 733 rpm; (a) special device in conventional milling, (b) 224 mm/min, (c) 168 mm/min, (d) 123 mm/min, (e) 88 mm/min, and (f) 65 mm/min.
A macroscopic study of defects was carried out during the preliminary processing according to Table
It is then possible to find the ideal conditions for the process by looking at Figure
Processing conditions obtained with AA-5083-O.
The microhardness tests showed an increase in the hardness for all FSP specimens. Figure
Microhardness test results for the FSP specimens.
A series of uniaxial tests was carried out, in which two were without processing and six were with processing conditions. In the stress strain curve for each condition, yield strength was plotted and the tensile stress increased for the processing conditions.
Figure
Numerical results of uniaxial tests.
Specimen |
|
|
|
|
|
|
---|---|---|---|---|---|---|
[MPa] | [MPa] | [%] | [%] | [MPa] | ||
Ox | 190 | 320.57 | 21.06 | 26.12 | 528.05 | 0.18 |
Ox1 | 191 | 319.07 | 21.1 | 25.77 | 527.95 | 0.19 |
CS1 | 200 | 325.34 | 21.44 | 26.24 | 584.88 | 0.23 |
CS1 | 203 | 334.02 | 18.7 | 26.92 | 609.23 | 0.24 |
CS2 | 235 | 341.92 | 20.85 | 27.39 | 568.44 | 0.19 |
CS3 | 225 | 339.23 | 17.74 | 24.24 | 616.22 | 0.23 |
CS3 | 227 | 336.03 | 15.29 | 26.3 | 549.28 | 0.19 |
Stress strain curves for specimens with and without processing.
Table
The results from the bulge and Nakajima tests are summarized by the figures below. Each figure shows the comparisons between MSC-MARC and Abaqus.
Figure
Applied pressure for the bulge tests using Abaqus and MSC-MARC.
Figure
Maximum domes heights for the bulge tests using Abaqus and MSC-MARC.
Figure
Maximum strain for the bulge tests using Abaqus and MSC-MARC.
Figure
Minimum strain for the bulge tests using Abaqus and MSC-MARC.
Figure
Von Mises stress distribution for the CS1 condition in Abaqus. (a) Circular. (b) 10 × 7 ellipse. (c) 10 × 6 ellipse. (d) 10 × 4 ellipse.
Von Mises stress distribution for the CS1 condition in MSC-MARC. (a) Circular. (b) 10 × 7 ellipse. (c) 10 × 6 ellipse. (d) 10 × 4 ellipse.
Figure
Maximum height for the Nakajima tests: comparison between MSC-MARC and Abaqus.
Figures
Major deformation for the Nakajima tests: comparison between MSC-MARC and Abaqus.
Minimum deformation for the Nakajima tests: comparison between MSC-MARC and Abaqus.
Figure
Von Mises stress distribution in Abaqus for the Nakajima tests in CS1 condition: (a) 100 mm, (b) 150 mm, and (c) 200 mm.
Von Mises stress distribution in MSC-MARC for the Nakajima tests in CS1 condition: (a) 100 mm, (b) 150 mm, and (c) 200 mm.
The Forming Limit Diagram is determined with the results from the FEM analysis. The right side is composed of the bulge tests and the left side is composed of Nakajima tests. Figure
FLD diagram results for MSC-MARC.
FLD diagram results for Abaqus.
A series of sheets was processed by FSP for the CS4 condition to validate the tests. Figure
AA-5083 sheets processed for the bulge tests. (a) Circular tool. (b) 10 × 4 ellipse. (c) 10 × 7 ellipse.
The wear of the tool was evaluated and no wear was observed after 25150 mm of processing. Figure
Wear of the tool pin after processing the sheets: (a) new tool, (b) 7550 mm, (c) 11710 mm, (d) 19070 mm, and (e) 25150 mm.
SEM images were made in CAM SCAN 3200 LN Microscope. Figure
SEM image of the FSP tool after 25150 mm of processing: (A) crack, (B) adhered aluminum, and (C) friction region.
The friction region (C) is indicated on the tool and is coated with aluminium but there is no significant wear on the tool after FSP.
Table
Comparison between the height of the domes and the strains in MSC-MARC and Abaqus in the sheets without processing.
Specimen | Pressure | Height |
|
|
---|---|---|---|---|
(MPa) | (mm) | |||
Ox circular, MARC | 9.228 | 24.117 | 0.118 | 0.118 |
Ox circular, Abaqus | 10.233 | 24.235 | 0.105 | 0.105 |
Ox circular, experimental | 9.228 | 19.4 | 0.055 | 0.055 |
Ox MARC, 10 × 7 ellipse | 8.4 | 36.642 | 0.113 | 0.082 |
Ox Abaqus, 10 × 7 ellipse | 7.5 | 33.364 | 0.131 | 0.076 |
Ox experimental, 10 × 7 ellipse | 8.4 | 34 | 0.105 | 0.065 |
The maximum strains occurred at the top of the sheet which is confirmed by the theoretical and software results. Figure
Comparison between maximum deformation in MSC-MARC and pratical results for an elliptical nonprocessed sheet.
The practical tests with processed sheets result in disruption before reaching the maximum pressure predicted by the software. This occurred because of the defects in the sheets caused by the lateral displacement of the tool. As the lateral displacement is 8 mm and the shoulder is 14 mm in diameter, a channel is formed in each pass, and when the next pass occurs, the material fills this void leaving a tunnel defect in each pass.
Figure
Crack on processed sheets after practical bulge tests. (a) Elliptical sheet and (b) circular sheet.
For this study, a FSP tool was developed with AA-5083-O aluminum alloy, so as to define suitable conditions to establish a friction stir process window: 500 rpm and 65 mm/min, 328 rpm and 88 mm/min, and 328 rpm and 65 mm/min. A series of uniaxial tests was carried out with friction stir processed specimens compared to those nonfriction stir processed. Their respective stress strain curves were obtained.
Nakajima and bulge tests were simulated using Abaqus and MSC-MARC commercial FEM software to determine the formability of the AA-5083-O aluminum alloy processed and nonprocessed by FSP. Their results were compared to those of the software.
The formability properties are concluded to be maintained after FSP, which is in agreement with [
Comparing Abaqus and MSC-MARC, the stability of the solution in relation to convergence is better for MSC-MARC when the contact property is added, while the Abaqus software shows convergence problems. On the other hand, the Abaqus interface is better than that of MSC-MARC. Abaqus also has parametric characteristics, which is an advantage to change the models. The results from the software products were very close, although Abaqus shows different results for the Nakajima studies.
Four tools were constructed and four AA-5083-O sheets were friction stir processed. The wear of the tool was evaluated and no wear was verified after 25150 mm of processing. A comparison between the FEM bulge test results was established and good approximation was obtained. Sheets with FSP broke before the target pressure was achieved which occurred because of defects in FSP.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge the support to the experimental tests carried out at Centro Universitario da FEI, Faculdade de Engenharia Industrial, São Bernardo do Campo, Brazil. They also thank Rudloff Industrial Ltda., São Paulo, Brazil, for the financial support and the manufacture of the tools and specimens and Instituto Mauá de Tecnologia, São Caetano do Sul, Brazil, for the support to the metallographic investigations.