Formability investigation of 6061-T6 aluminum alloy sheet at elevated temperatures

doi:10.21203/rs.3.rs-1281040/v1

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Formability investigation of 6061-T6 aluminum alloy sheet at elevated temperatures

https://doi.org/10.21203/rs.3.rs-1281040/v1

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Aluminum alloy sheets are widely used in different industries that the main reason is the high strength to weight ratio of these sheets. Among these sheets, the 6061-aluminum alloy (AA 6061) is more useable in automotive and aircraft industries, because of its high strength and more formability properties which have rather than most aluminum alloy sheets. Although AA 6061 sheet has more formability compare to most other aluminum alloy sheets, still it has low formability at room temperature which is one of its drawbacks. Therefore, in the present study, the formability of AA 6061-T6 sheets with thickness of 2 mm is investigated at different temperatures in the range of room temperature (RT) to 300℃. For this purpose, the experimental tests of forming limit diagram (FLD) are done at different temperatures by using the non-isothermal Nakajima standard die based on the ISO 12004-2 standard. The FLD tests are done with no lubrication in the sheet and punch contact at different temperatures. The results indicated that the FLD level and formability increased by temperature increase from RT to 100℃ and then decreased by temperature increase from 200℃ to 300℃. Results showed that happening of adhesive wear between the punch and the sheet surface is the main factor that restricted the material flow and deformation of sheet when the temperature is more than 100℃. The higher temperature which is used in the forming process, the stronger adhesive wear will happen.

AA 6061-T6 sheet

elevated temperature

forming limit diagram (FLD)

formability

Limit dome height (LDH)

adhesive wear

Aluminum alloy sheets are widely used in different industries such as automotive and aircraft, because of the high strength to weight ratio. The 6061-aluminum alloy is one of these alloys with low density, high strength and corrosion resistance which are used in the automotive, aircraft, highway and marine applications. Besides these useful properties of AA6061, the sheet metal of this alloy has low formability at the room temperature (RT). Necking and fracture are the main defects which limit the formability of AA6061 sheet in the RT. The forming limit diagram (FLD) is used as a criterion for formability prediction of sheet metal in the forming processes. It displays in principal strain space (major and minor strains) at the onset of local necking. Therefore, many studies were done on the formability and FLD of aluminum alloys at elevated temperatures. These studies have indicated that the formability of aluminum alloys can be increased at elevated temperatures. Ju et al. [1] investigated the formability of AA 5182-O at room temperature and elevated temperatures. They used a non-isothermal finite element model (FEM) to investigate the effect of sheet temperature and punch speed on the formability of AA 5182-O at elevated temperatures. Safdarian [2] used numerical criteria of ductile fracture and forming limit diagram for FLD and forming limit stress diagram (FLSD) prediction of AA6061. The numerical results were compared with the experimental ones. The results showed that ductile fracture criterion has good accuracy for FLD and FLSD prediction of AA6061 sheet. Chen, Fang and Zhao [3] investigated the effect of temperature and forming speed on the formability of AA 6061 sheet at elevated temperature using isothermal FLD die. Their results showed that the FLD of AA6061 increased by the temperature increase and forming speed decrease. Motamedi and Hashemi [4] studied the FLD of AA6061 sheet at room and elevated temperature based on the Marciniak and Kuczynski (M-K) model using numerical and experimental methods. The numerical and experimental results indicated that FLD level increased by temperature increase. Abedrabbo, Pourboghrat, and Carsley [5] used FEM and experimental tests to investigate the formability of AA3003-H111 aluminum alloy. They used M-K model for FLD and failure location prediction. They used user material subroutine (UMAT) in the LS-Dyna to model the temperature-dependent Barlat YLD96 anisotropic yield function. Tokia et al. [6] studied the stretch formability of high strength steel sheets at elevated temperatures from 200℃ to 600℃. The experimental results of this study showed the maximum dome height in the warm forming was lower than that in cold forming. Li et al. [7] used experimental tests to study the FLD of magnesium alloy (AZ31B) sheet at different temperatures of 150 ℃, 200 ℃, and 300 ℃. The results indicated that at temperature below 150 ℃ the brittle fracture happened and ductile fracture at temperatures above 200 ℃. The FLD level of AZ31B magnesium alloy sheet increased by the temperature increase. Tisza et al. [8] investigated the formability of AA 5754 and AA 6081 at elevated temperatures. Their results indicated that the FLD level increased by temperature increase till 200 ℃ and it decreased by temperature increasing more than 200 ℃. Zhang et al. [9] studied the effect of temperature and strain rate on the FLD of AA 5086 sheet using finite element method. Their results indicated that the sheet formability decreased by punch speed (strain rate) increasing and the FLD level increased by the temperature increase. Hsu et al. [10] investigated the formability of magnesium alloy AZ31B-O at 300℃ using out-of-plane (limit dome height (LDH)) and in-plane (Marciniak) tests. Their results showed that bending strain, friction and normal pressure on the sheet in the LDH test led to the higher FLD level than the Marcinik test. Kaya, Spampinato, and Altan [11] investigated the formability of aluminum and magnesium alloy sheets at elevated temperature in the nonisothermal deep drawing process. They studied the effect of constant and variable punch speed on the formability of these sheets. Their results indicated that the thickness distribution of the formed part improved by using variable punch speed. Kim et al. [12] used thermomechanically coupled finite element analysis (FEA) to investigate the formability of aluminum alloy sheet at elevated temperatures. The FLD test was done at different temperatures of 250℃, 300℃ and 350℃. The numerical and experimental results indicated that limiting strain increased by increasing of forming temperature. Abedrabbo, Pourboghrat, and Carsley [13] investigated the formability of AA5182-O and AA5754-O aluminum alloys at elevated temperature by using a coupled thermomechanical FEA for temperature range 25-260℃. They used temperature-dependent anisotropic yield function of Barlat’s YLD2000-2d by user material subroutine (UMAT) in the FEM. Their results indicated that the punch should be maintained at the lowest temperature possible for maximum formability. Saxena, Drotleff and Mukhopadhyay [14] studied the FLD of Al 6014 and DP 600 alloy sheet at elevated temperature in range of 200℃ and 250℃. The strains in the sheet surface were measured using digital image correlation method. The results indicated that the FLD level of Al 6014 increased by temperature increase, but this trend was vice versa for DP 600. Bagheriasl and Worswick [15] investigated the formability and FLD of AA3003 aluminum alloy sheet at elevated temperature. Their results showed that the formability at 250℃ improved more than 200% compared to room temperature. Tagata et al. [16] investigated the superplastic blow forming limit of 5083 aluminum alloy at temperature of 773K and strain rate of 1×10^-3s^-1.

In the present study, the forming limit diagram and formability of AA6061-T6 aluminum alloy sheet with thickness of 2mm are investigated at elevated temperatures. The non-isothermal limit dome height (LDH) die is used in the experimental tests to identify the FLD of AA6061-T6 at different temperatures in the range of room temperature to 300℃. The aluminum blanks are grid marked with circles of 2.5 mm by an electrochemical etching method to measure major and minor strains after deformation. The effect of friction condition in the punch and sheet contact is investigated on the formability of AA6061 sheet at room temperature.

2.1 Material properties

Whereas AA6061 aluminum alloy sheet is used in different industries such as automotive and aircraft, this sheet with thickness of 2 mm is used to investigate the formability and FLD at RT and elevated temperatures. The chemical composition of AA6061-T6 aluminum alloy is shown in Table 1. The uniaxial tensile test based on the ASTM-E8 standard [17] was used to evaluate the mechanical properties of AA6061-T6 sheet at RT. Fig. 1 shows engineering stress-strain curve of this aluminum alloy.

Table 1

Chemical composition of aluminum alloys 6061-T6
Alloy	Mg	Si	Fe	Mn	Cr	Zn	Cu	Ti	V	Al
Wt (%)	2.77	0.597	0.441	0.0329	0.252	< 0.04	0.339	0.0466	0.0347	95.38

In the present study, the FLD test based on the ISO 12004-2 standard [18] was used for formability and FLD investigation of the AA6061-T6 sheet. Therefore, seven standard samples were cut based on the Hasek method [19] for FLD calculation in the experimental tests. The geometry and dimensions of four samples are shown in Fig. 2. As Fig. 2 shows the rolling direction of the blanks is parallel to the length of the blank. This direction is based on the ISO 12004-2 standard. Three other samples are rectangular samples with length of 175 mm and width of 125 mm, 150 mm and 175 mm. The specimens were grid marked with circles of 2.5 mm by an electrochemical etching method to measure the major and minor strains after sample deformation.

2.2 Experimental set up for FLD

The forming limit diagram (FLD) test is a standard test for the formability investigation of sheet metals in the forming processes. As mentioned before, the FLD test based on the ISO 12004-2 was used for formability investigation of AA6061 aluminum alloy at room and elevated temperatures. The FLD is drawn using minor and major principal strains which are extracted from the sheet surface after the FLD test. When the strain state of any region of sheet metal in the forming process is under the FLD, that region is safe. In the present study, the non-isothermal Nakajima standard die was used for FLD calculation based on the ISO 12004-2 standard test. This die consists of a hemispherical punch of 101.6 mm diameter, a die, and blank holder. Fig. 3 (a) shows the schematic of this standard die with its dimensions. As Fig. 3 (b) shows the punch was heated with four cartridge heaters up to 300 ℃. The experimental setup (punch, die, blank holder and heating elements) is shown in this figure. This setup was installed on a 60-ton tensile and compression test machine to form the AA6061 blanks until fracture. The machine was equipped with load and displacement sensors and experimental tests were stopped when the forming load decreased suddenly.

whereas the FLD tests are done at elevated temperatures until 300℃ and the properties of lubricant like oil changed at this temperature, tests were done in the dry condition at room and elevated temperatures. Punch speed was 15 mm/min. An optimum blank holding force in the range of 60–100 kN was applied on the upper die to prevent the material movement from the flange to the die opening area.

3.1 Effect of temperature on the formability of AA6061-T6

The load-displacement curves were extracted from experimental FLD tests by the data acquisition system at various temperatures. These curves are compared in Fig. 4 for samples with different widths. As this figure shows, for all samples with different widths, the maximum load endurance is for samples that were formed at room temperature. The load is decreasing by the temperature increase from RT to 300℃. The punch displacement or forming height of all samples with various widths is increasing by temperature increase from RT to 100℃ and is decreasing by temperature increase from 100℃ to 300℃. Therefore, the maximum formability of AA6061 samples happen at the temperature of 100℃ and it is diminishing by temperature increase. As Fig. 4 shows, the load-displacement curve decreases suddenly for samples which formed in the RT and 100℃, but this curve had variant trend for temperature of 200℃ and 300℃ in the fracture point. Indeed, samples which were formed at temperatures of 200℃ and 300℃ had load endurance capacity after the fracture point. This difference of curve trend at fracture point maybe affected by two factors of fracture type and adhesive wear. The fracture type of aluminum sheets at various temperatures can influence on the curve trend. The fracture of AA6061 sheet is brittle at the RT and 100℃, but it is ductile fracture at elevated temperatures such as 200℃ or 300℃. Adhesive wear between the punch and aluminum sheet at higher temperatures (more than 100℃) is the second factor which influence on the load-displacement curve at fracture point. This phenomenon restricts the punch movement and the load doesn’t decrease suddenly at fracture point because of adhesive wear between punch and sheet surface.

The temperature effect on the forming height of experimental samples with various widths is shown in Fig. 5. As this figure shows, samples which were formed at 100℃ had the maximum forming height. The forming height is increasing by temperature enhancement from RT to 100℃ and then is decreasing by temperature increase to 300℃. Furthermore, the forming height is increasing by sample width enhancement at different temperatures.

Figure 6 shows the comparison of samples after the FLD test at various temperatures for samples with width of 25 mm, 50 mm and 75 mm. As Figure 6 (a) shows, the brittle fracture happened for samples with width of 25 mm at RT and 100℃ and these samples separated into two parts in the fracture point, but the ductile fracture with necking happened for two samples at 200℃ and 300℃. The fracture position of samples was compared to the sample pole (dashed line) in Figure 6. The fracture position moved toward the sample pole by temperature increase. As Figure 6 (b) and (c) shows, the fracture is also brittle and far from the sample pole for those with width of 50mm and 75mm at RT and 100℃, but it is ductile and near the pole for these samples at 200℃ and 300℃.

Figure 7 shows the comparison of samples with 150 mm width after the FLD test at different temperatures. As this figure shows, the fracture position is far from the sample pole at the room temperature, but it moved toward the pole by temperature increasing from 100℃ to 300℃. The fracture happened in one region without necking for sample that formed at RT, but for other three samples necking happened along the fracture in a circular path.

Figure 8 shows the comparison of samples with of 175 mm width after the FLD test at various temperatures. This figure also indicated that fracture position moves toward the sample pole by temperature increase.

3.2 Effect of temperature on the FLD of AA6061 sheet

The forming limit diagram is a two-dimensional curve in the minor and major strains diagram. As mentioned before, the samples were grid marked with circles of 2.5 mm by an electrochemical etching method to measure the major and minor strains after sample deformation. These circles are converted to the ellipse in the deformed region and the minor and major strains are measured by the comparison of small diameter (d_min) and large diameter (d_max) of ellipse with the circle diameter, respectively (Fig. 9). As Fig. 9 shows an optical microscope was used to measure the ellipse diameters. The major and minor strains were measured near the fracture line and from the safe ellipses for all of the samples with different width. The true major and minor strains were calculated by measuring change of the principal directions, d_max and d_min, of the ellipse with reference to the initial diameter, d, of the circle:

$${\epsilon }_{major}=\text{ln}\left(\frac{{d}_{max}}{d}\right) , {\epsilon }_{minor}=\text{ln}\left(\frac{{d}_{min}}{d}\right)$$

where d_max and d_min are the ellipse diameters and d is the initial diameter of the circle.

Figure 10 shows the forming limit diagram of the AA6061-T6 sheet at different temperatures. As this figure shows, the FLD level increased extremely by the temperature increase from room temperature to 100℃ and then decreased by the temperature increase from 200℃ to 300℃. This figure indicates that the formability of AA6061 sheet increased by temperature increases from room temperature to 100℃ and then decreased by temperature increase from 200℃ to 300℃.

This trend happened in the punch’s load-displacement curves which were presented in Fig. 4 for different temperatures. The forming load and forming height are increasing by temperature increase from RT to 100℃ and then are decreasing by temperature increase to 300℃. In order to understand the reason of this phenomenon, the sheet and punch contact area in the FLD tests are compared in Fig. 11 for various temperatures. As this figure shows, the sheet surface which is in the punch contact has different colors for samples that formed at temperatures of 200℃ and 300℃ compared with two other samples that formed at RT and 100℃. The increase of punch temperature more than 100℃ causes adhesive wear between the punch and sheet surface. This phenomenon restricts the material flow and sheet deformation which is in the punch contact. This phenomenon was also reported by Hardell and Prakash [20]. The adhesive wear tendency increases by temperature increase from 100℃ to 300℃. Therefore, the formability and FLD level is decreasing by the punch temperature increase from 100℃ to 300℃ when there was no lubricant between the punch and sheet surface.

3.3 Effect of lubrication on the formability of AA6061 sheet

The effect of lubrication on the formability of the AA6061 sheet was investigated in the FLD tests at room temperature. For this purpose, two contact conditions with lubricant and without lubricant (dry) were considered in the blank and punch contact. The oil was used as a lubricant in the blank and punch contact. The effect of lubrication on the load-displacement of punch is shown in Fig. 12 for samples with various widths. As this figure shows, forming height and load endurance are increasing in the lubrication condition.

The effect of friction condition between the punch and AA6061 blanks on the FLD at room temperature is shown in Fig. 13. As this figure shows, the FLD level is increasing by using oil as lubricant in the sheet and punch contact area. The FLD and load-displacement curves indicate that the formability are increasing by using lubricant in the AA6061 blanks and punch contact area. The friction force between the blank and punch in the FLD tests is against the punch movement direction and restricts the material flow in a part of AA6061 blank which is in the punch contact. Therefore, the formability and the FLD level increase by the elimination of this friction force.

In the present study, the effect of punch temperature was investigated on the formability and FLD of AA6061-T6 sheet. The FLD tests were done based on the ISO 12004-2 standard at different temperatures in the range of room temperature to 300℃.

The forming height and FLD level is increasing by temperature increase from room temperature to 100℃. The increase of punch temperature more than 100℃ causes adhesive wear between the punch and sheet surface and this phenomenon restricts the material flow and deformation of sheet region which is in the punch contact. Therefore, the formability and FLD level diminished by the punch temperature increase from 100℃ to 300℃ when there was no lubricant between the punch and sheet surface.

The effect of lubricant existence in the punch and aluminum sheet contact was investigated on the FLD level by using oil. The forming height and FLD level increased by using oil as lubricant in the punch and sheet contact at room temperature.

Availability of data and materials

All data generated or analysed during this study are included in this published article.

Competing interests

The authors declare no competing financial interests.

Funding

No funding was received.

Authors' contributions

The experimental work of present study was designed and performed by R. Safdarian. Evaluation of obtained results as well as writing of article was done by author.

Acknowledgements

Not applicable.

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Formability investigation of 6061-T6 aluminum alloy sheet at elevated temperatures

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Abstract

Figures

1 Introduction

2. Methodology

2.1 Material properties

2.2 Experimental set up for FLD

3. Results

3.1 Effect of temperature on the formability of AA6061-T6

3.2 Effect of temperature on the FLD of AA6061 sheet

3.3 Effect of lubrication on the formability of AA6061 sheet

4. Conclusion

Declarations

References

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