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Original Article
DOI: 10.1016/j.jmrt.2018.02.012
Enhancement of drawability of cryorolled AA5083 alloy sheets by hydroforming
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Fitsum Taye Feyissa
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fitsum.tf@gmail.com

Corresponding author.
, Digavalli Ravi Kumar
Indian Institute of Technology Delhi, Department of Mechanical Engineering, Hauz Khas, 110016 New Delhi, India
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Received 25 September 2017, Accepted 16 February 2018
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Tables (4)
Table 1. Mechanical Properties of cryorolled and annealed sheets of AA5083 alloy.
Table 2. Process parameters used in FE simulations and experimental work.
Table 3. Effect of coefficient of friction in FE simulation of hydroforming of square cups.
Table 4. Comparison of formability results in hydroforming and conventional deep drawing.
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Abstract

In this paper, an attempt has been made to enhance formability of cryorolled AA5083 alloy sheets (annealed at 275°C for 15min) in deep drawing of flat bottom square cup-shaped parts by hydroforming. Numerical simulations based on finite element method have been carried out to study the effect of important process parameters (fluid pressure and sealing force) on formability. The minimum corner radius and the maximum depth that can be achieved without failure and the maximum percentage thinning at the corners have been considered as measures of formability. The results have been compared with conventional deep drawing. The simulation results have also been validated with experimental work in both hydroforming and conventional forming. A process window with optimum combination of peak sealing force and peak pressure has been identified to form the cups up to full depth of the die without failure at die entry or bottom corners. Lubrication between the die and the blank reduced the minimum possible corner radius to nearly 16mm with thinning less than 6%. In conventional forming only 70% of the full depth could be obtained before failure with 16mm punch corner radius due to much higher thinning at the corners. This work demonstrates that, by hydroforming, formability of high strength cryorolled Al alloy sheets can be enhanced due to lower thinning and more uniform strain distribution and hence this process route (cryorolling followed by hydroforming) is a potential technique to produce complex parts from lightweight high strength Al alloy sheets due to enhanced formability.

Keywords:
Aluminum alloy
Cryorolling
Formability
FEM
Deep drawing
Hydroforming
Full Text
1Introduction

The demand for lightweight metals and alloys such as Al alloys has increased significantly in automotive industry in the recent decades [1,2]. But one of the main limitations of these alloys is lower strength and formability when compared to traditional low carbon steel sheets. Cryorolling is one of the important severe plastic deformation processes to produce ultrafine grained aluminum alloy sheets with high strength and hardness [3]. In cryorolling, sheets are rolled at liquid nitrogen temperature (−196°C) to obtain higher dislocation density when compared to conventional cold rolling by effectively suppressing the dynamic recovery. Many researchers focused their work on the effect of cryorolling on microstructural and mechanical properties of Al-alloys [4–7]. There have been some attempts to improve ductility of cryorolled Al alloys by post heat treatment at low temperature [8–14]. A homogeneous ultrafine grained structure with an average grains size of 300nm was achieved after a short-time annealing treatment at 300°C after 90% reduction [6]. Despite the advantages, all these studies clearly showed that the ductility of cryorolled sheets is poor in the as-rolled condition and it can be enhanced slightly by post heat treatment but even after short annealing treatment, they are not suitable for applications which require high formability in conventional forming. So, hydroforming could be an alternative to enhance formability of cryorolled Al alloy sheets.

Hydroforming is one of the emerging technologies that have large applications in automobile and aerospace industries [15–18]. Hydroforming processes can be further classified into hydro-mechanical deep drawing and pure hydroforming. In hydromechanical deep drawing, counter pressure (by fluid) is applied on the part during drawing to enhance formability and it has been investigated for laminated aluminum/steel sheets [19], rectangular cross-section components [20] and cups with stepped geometries [21].

In hydroforming process, a sheet metal is formed by fluid pressure in a die to obtain the desired shape unlike in conventional deep drawing where a rigid tool (punch) is used to apply the required drawing force. Required sealing force in hydroforming is applied on the upper die (blank holder or binder) to prevent leakage when the dies are closed. Both conventional and hydroforming processes are shown schematically in Fig. 1. Sheet metal hydroforming technology has many advantages over conventional forming process such as the ability to manufacture complex shapes due to enhanced formability, higher limiting draw ratio, better stiffness and rigidity of the parts, suitability for lightweight and thinner sheets and better surface quality [22–24].

Fig. 1.
(0.17MB).

(a) Conventional deep drawing and (b) sheet hydroforming.

With these advantages of hydroforming, it would be worthwhile to study formability of cryorolled aluminum alloys and compare with conventional forming to produce sheet metal parts with a combination of higher strength and improved formability thus combining two unconventional processes, namely cryorolling and hydroforming. Such studies have not been reported in the literature earlier. Therefore, in the present work, formability of cryorolled AA5083 alloy sheets of thickness 1mm has been studied in deep drawing of the flat bottom square cup-shaped parts by hydroforming by both numerical simulations and experimental work. Since the bottom corner is the most difficult region to form due to maximum thinning here [25], the minimum corner radius and the depth of draw that can be achieved without failure and the maximum percentage of thinning at the corner have been taken to be the parameters to assess formability in square cup hydroforming. The effect of important process parameters has been investigated and comparison with conventional forming is also presented.

2Material and processing

In this work, 5mm thick AA5083 (H116) alloy sheets have been solutionized and then cryorolled to a final thickness of 1mm in a number of passes on a 4 high rolling mill with a total reduction of 80%. The cryorolled sheets have been subjected to a short annealing at 275°C for 15min so as to enhance ductility without losing strength significantly. For EBSD analysis measurements, FEI Quanta 200F-FEG (field emission gun) SEM (scanning electron microscope) with TSL-OIM was used. The specimens were initially prepared as per standard optical metallographic procedures followed by electropolishing to obtain the required level of finish. The microstructure of the sheets obtained through EBSD is shown in Fig. 2 and it has been found to be a mixture of ultrafine recovered grains (0.95μm) and very fine recrystallized grains (2.1μm).

Fig. 2.
(0.33MB).

EBSD micrographs of cryorolled specimens after a short annealing at 275°C for 15min.

Uniaxial tensile tests were conducted using specimens prepared by laser cutting as per E8/E8M ASTM standard (Fig. 3). Tensile samples of cryorolled-annealed sheets were tested on an Instron machine at a constant crosshead speed of 2.5mm/min at room temperature. This corresponds to an initial strain rate of 0.8×10−3s−1. The mechanical properties of the final processed sheet are given in Table 1. To show the improvement in strength by cryorolling, mechanical properties of the same alloy in solutionized condition are also given in the table.

Fig. 3.
(0.07MB).

Dimensions of tensile test specimen.

Table 1.

Mechanical Properties of cryorolled and annealed sheets of AA5083 alloy.

Condition  YS (MPa)  UTS (MPa)  Elng. (%)  n  K (MPa)  Anisotropic parameters
            R0  R45  R90  R¯ 
Cryorolled-annealed  186.1  309.8  18.1  0.243  533  0.73  0.83  0.80  0.80 
Solutionized  111.8  286.1  23.4  0.341  642  –  –  –  – 

n and K are strain hardening exponent and strength coefficient respectively and R0, R45 and R90 are plastic strain ratios of the sheets determined at 0°, 45° and 90° to the rolling direction respectively.

3Numerical simulation of deep drawing of square cups3.1Hydroforming

Finite Element Analysis (FEA) has been carried out using commercially available software (Dynaform with LS-DYNA) to predict formability of cryorolled Al alloy sheets in hydroforming as well as in conventional forming. In this work, it was planned to draw a square cup of 100mm side with 35mm depth as shown in Fig. 4a. Using Blank Size Estimation (BSE) tool in the pre-processor, optimum blank size and shape have been predicted. Fig. 4b shows the geometry and dimensions of the blank used in FE simulations.

Fig. 4.
(0.24MB).

Dimensions of (a) targeted part and (b) initial blank.

In hydroforming simulation, tools (die and binder) were modeled as rigid bodies. In order to lower computational time Belytschko–Tsay triangular and rectangular thin shell elements were used [26]. The blank was meshed with an element size of 3mm and shell thickness was taken to be equal to sheet thickness. The number of elements was 2598 (blank), 5585 (die), 3050 (punch) and 2192 (blank holder). The process simulation consisted of a total number of 8.3×104 steps with a time step of 5.56×10−7s.

Barlat's 3-parameter plasticity model [27] has been used to model the yielding behavior of cryorolled AA5083 alloy sheets. It incorporates the effect of both normal and planar anisotropy in a polycrystalline sheet during plastic deformation. This represents the material behavior in close approximation to experimental behavior, especially in the case of Al-alloys [28]. In this model, the yield function (Φ) is given by:

where σ0 is yield stress in rolling direction.

K1 and K2 are stress tensor invariants.

σx, σy and τxy are plane stress components in orthotropic axes.

a, c, h and p are anisotropic material constants (coefficients in Barlat's yield model) and these are obtained from Lankford's parameters as given below:

p is normally calculated implicitly after knowing a, c and h[27].

M is Barlat's yield exponent. For face-centered cubic (FCC) materials like Aluminum alloys, the most suitable value of M is 8 [29].

The experimentally determined values of plastic strain ratio (R0, R45 and R90) were given as input using which the anisotropic coefficients in the model were calculated.

The stress–strain relation for strain hardening of the sheets during plastic deformation is represented by the following constitutive equation [30]:

where σ is flow stress, ¿yp is strain to cause yield, ε¯p is effective plastic strain and K is strength coefficient and n is strain hardening exponent. The elastic strain at yield can be found from:
The values of n and K (given in Table 1) were used as input to define the stress–strain curve.

The FE model for simulation of hydroforming of square cups is shown in Fig. 5a. The pressure boundary condition was defined by a pressure mask as shown in Fig. 5a. The pressure curve was defined for all the nodes within the area confined by the mask. Pressure acts along the vector normal to the blank elements. A typical pressure path in which pressure increases continuously from zero to a pre-determined peak pressure is shown in Fig. 5b. In this work, variable sealing force technique was used in which the sealing force was also increased continuously with fluid pressure to prevent oil leakage. A typical variation of sealing force with time (with peak sealing force in the range of 100–350kN) is also shown in Fig. 5b. The frictional condition was modeled using Coulomb's law of friction at the blank–die interface in which value of coefficient of friction was varied in the range 0.04–0.25 to model lubricated and dry conditions [31,32] during forming.

Fig. 5.
(0.21MB).

(a) FE model developed for simulation of hydroforming of square cups and (b) a typical variation of pressure and sealing force in FE simulations.

3.2Conventional forming

The FE model for simulation of conventional deep drawing of square cups is shown in Fig. 6a. The blank shape and size and the die geometry are the same as those explained in hydroforming simulation. The tools, namely, die, punch (with a corner radius of 16mm) and binder have been modeled as rigid bodies and meshed as explained earlier. In this simulation, the die and the binder were fixed while the punch was moved with a velocity of 3000mm/s in negative Z direction in order to minimize computational time (in actual experimental work, it was 20mm/min). The punch motion was defined using a trapezoidal profile as shown in Fig. 6b for a maximum punch displacement of 35mm. Coulomb's law friction was used to model the friction between the tools and the blank has been assumed to be 0.25 for dry condition and 0.04 for lubricated condition. A summary of process parameters used in both hydroforming and conventional deep drawing simulations is shown in Table 2.

Fig. 6.
(0.21MB).

(a) FE model used for simulation of conventional deep drawing of square cups and (b) velocity profile of the punch.

Table 2.

Process parameters used in FE simulations and experimental work.

Parameters  Range of values
  Hydroforming  Conventional forming 
Sealing force
Blank holding force 
100–350kN
– 

20–130kN 
Peak fluid pressure  10–30MPa  – 
Punch speed 
– 
20mm/min (Exp.)
3000mm/s (FEA) 
Friction and lubrication  μ=0.04–0.25 (FEA)
Dry and lubrication with teflon (Exp.) 
0.04 and 0.25 (FEA)
Dry and lubrication with teflon (Exp.) 

The Forming Limit Diagram, first proposed by Keeler and Goodwin [33], is an important tool to find the limit stains at which necking/failure occurs in a sheet metal. It is usually plotted as major principal engineering strain vs. minor principal engineering strain measured on the surface of blanks deformed under different load paths. To obtain limit strains in different states of strain, Hecker [34] proposed a standard method in which blanks of different widths are deformed using a cylindrical punch with a hemispherical bottom of 101.6mm diameter up to visible necking/failure on the blank. A circular draw bead is provided on the dies and hence clamping area decreases with decreasing blank width leading to change in mode of deformation. To measure the strain on the deformed sample, circular grid method is commonly used. Nakazima proposed [35] a modified blank geometry for evaluation of FLDs.

In both hydroforming and conventional forming simulations, failure has been predicted using Forming Limit Diagram (FLD) of the alloy. For this purpose, the FLD of cryorolled sheets of AA5083 alloy (annealed at 275°C for 15min) has been experimentally determined (shown in Fig. 7) and it has been used in the simulations for prediction of failure in the post-processor.

Fig. 7.
(0.17MB).

Forming Limit Diagram of cryorolled sheets annealed at 275°C for 15min.

4Experimental work

Experimental work has been conducted for validation of numerical simulation results and identification of optimum process parameters for square cup hydroforming using an existing hydroforming facility as shown in Fig. 8a[25]. The complete details of the experimental setup have already been reported by Modi and Kumar [25] and hence they are not elaborated here again. The experimental setup consists of drawing die, pressure plate and blank holder assembled on a single action hydraulic press. Data acquisition (DAQ) system and Programmable Logic Control (PLC) were used to give input and acquire data of sealing force and pressure as a function of time. Pressure has been increased up to the predetermined peak pressure linearly in a 60s time cycle and the required sealing force was varied along with fluid pressure in order to prevent oil leakage and prevent wrinkling during draw-in of the blank into the die.

Fig. 8.
(0.3MB).

Experimental setup used for (a) hydroforming and (b) conventional deep drawing of square cups.

For comparison of formability, the cryorolled Al-alloy sheets were also deep drawn using conventional forming technique. The tools (die, punch and binder) were designed and fabricated in order to draw the square cups with the same dimensions as in the case of hydroforming. The tools were assembled on a 100T double action hydraulic press (shown in Fig. 8b) for conducting the deep drawing experiments. The process parameters used in hydroforming and conventional forming experiments are also shown in Table 2. Experiments were conducted in both dry and lubricated (using a 0.1mm thick teflon sheet between the die and the blank) conditions.

Before performing the experiments, the blanks were marked with a 2.5mm diameter circular grid for the analysis of strain distribution. The major and minor diameters of deformed circles (ellipses) were measured using Grid Pattern Analyzer (GPA-100 model, ASAME) to obtain major and minor strains experimentally. The circular grid was marked with laser marking technique, which gives a very sharp and clear pattern with less than 5μm depth. Thickness variation in hydroformed and conventionally formed square cups was measured using an ultrasonic thickness gauge and depth and corner radius were measured by a Coordinate Measuring Machine (CMM).

5Results and discussion5.1Process window

Based on the large experimental data obtained from hydroforming of square cups under different combinations of peak fluid pressure and peak sealing force, an attempt has been made to determine a process window for successful forming of square cups by hydroforming. The resulting square cups from all the combinations of process parameters used in the experimental work have been categorized into incomplete forming, successful and failed as shown in Fig. 9.

Fig. 9.
(0.16MB).

Process window (region IV) for successful hydroforming of square cups using cryorolled aluminum alloy sheets.

For the combination of parameters in region-I, where both pressure and sealing force were low, the fluid pressure applied was insufficient for complete forming of the cup, especially in the corners. It only caused bulging of the sample in the center and full depth was not attained. Corners were not formed. An example is shown in Fig. 10a. Clearly, higher fluid pressures are required for improving the geometrical accuracy of the cup. Cups formed with combinations of pressure and sealing force in region-II failed at the die entry, as shown in Fig. 10b. It is due to the excessive sealing force which did not allow the blank from the flange region to flow into the die smoothly after initial bulging of the blank. In region-III, where a combination of high fluid pressure and high sealing force were applied, the cups failed at one of the bottom corners due to excessive thinning in the final stage of deformation (Fig. 10c). The maximum percentage of thinning in such cups has been found to be 24–25%. Apart from reducing fluid pressure, thinning in the corners can also be reduced by providing lubrication between the die and the workpiece which causes more uniform strain distribution as discussed later.

Fig. 10.
(0.19MB).

Experimentally formed square cups with different combinations of pressure and sealing force (a) incomplete forming, (b) failure at die entry, (c) failure at the bottom corner and (d) successfully formed cup.

Successful cups have been obtained with combinations of pressure and sealing force is shown in region IV. In this region, the cups were obtained with complete forming of the corners and full depth of the die. Though a very small radius at the bottom corners could not be achieved, all the other dimensions were obtained.

It can be clearly seen that excessive sealing force causes failure at the die entry and insufficient sealing force causes leakage and wrinkling of the drawn part. On the other hand, sufficiently high pressure is required to obtain the desired geometrical accuracy and the required sealing force also increases with increase in pressure. Otherwise, pressure build-up will not take place due to leakage. Hence the process window shown in region IV would be useful for choosing the right combination of processes parameters for successful drawing. A successfully formed cup is shown in Fig. 10d. The minimum corner radius and the maximum thinning in the successfully drawn cups varied with variation of combination of peak pressure and peak sealing force as discussed below.

5.2Influence of fluid pressure and sealing force on formability

FE simulation and experimental results show that the peak fluid pressure and the peak sealing force (due to variable sealing force technique used) are the most influential process parameters affecting formability of cryorolled AA5083 alloy sheets in hydroforming of square cups. Fig. 11 shows the influence of peak pressure and peak sealing force on maximum percent thinning and minimum corner radius in FE simulation as well as experimental results obtained from successful cups formed in dry condition.

Fig. 11.
(0.14MB).

Influence of peak pressure and peak sealing force on thinning and corner radius in successfully formed cups.

In successfully formed cups, maximum percentage thinning has always been obtained at one of the four bottom corners. As the peak pressure was increased, higher sealing force was necessary for sealing (to resist fluid pressure and prevent leakage). So, peak sealing was also increased with peak pressure. As the peak pressure was increased, more deformation occurred in the bottom corners in the final calibration stage and hence strains in these regions also increased. Due to biaxial stretching of the sheet in the corners in the final stage of deformation, thinning also increased and hence maximum percent thinning in the cup increased to nearly 20–22% for a maximum peak pressure of 24MPa. Due to higher deformation in the corners, the blank conformed to the die geometry more closely and hence the corner radius decreased with increase in peak pressure. A minimum corner radius of 19.8mm could be obtained without any fracture in dry condition.

In the case of minimum corner radius, predicted results from FEA agreed well with experimental results. But in the case of maximum percentage thinning, the predicted values are lower than that of the experimental results. It could be attributed to slight variation in sheet thickness after cryorolling and assumed value of coefficient of friction as 0.25 in simulations (to represent friction in dry condition, without lubrication).

5.3Effect of friction and lubrication

The effect of friction in hydroforming has been investigated by finite element simulation for a peak fluid pressure of 24MPa and a peak sealing force of 310kN which resulted in successful drawing. Simulations were done by varying the coefficient of friction between 0.04 (representing very well lubricated condition) to 0.25 (representing dry condition without any lubrication). The coefficient of friction between the binder and the blank in all the cases has been assumed to be μ=0.08 (a thin film of fluid is always present between the binder and the blank) [36]. The effect of friction coefficient on maximum percentage thinning, maximum major and minor true strains and the minimum bottom corner radius that could be achieved are presented in Table 3. As in the case when friction coefficient was 0.25, the maximum thinning has been observed in the bottom corners of the cup when simulations were done with reduced μ (0.04–0.2) to represent lower frictional (lubricated) conditions. It reduced with decrease in coefficient of friction from nearly 20% (μ=0.25) to 6% (μ=0.04).

Table 3.

Effect of coefficient of friction in FE simulation of hydroforming of square cups.

Friction coefficient (μMax. thinning at corner (%)  Max. true major strain at the corner (%)  Max. true minor strain at the corner (%)  Minimum corner radius (mm) 
0.04  5.6  8.5  −2.6  15.8 
0.08  6.2  8.7  −2.1  17.0 
0.125  7.1  7.9  0.0  17.6 
0.17  9.1  8.6  1.3  17.8 
0.20  10.6  9.3  2.3  19.0 
0.25  20.5  13.9  9.5  19.8 

The predicted formability and thinning from FE simulation of hydroforming for the highest and lowest cases of friction coefficient (μ=0.25 and μ=0.04) are shown in Fig. 12. A minimum bottom corner radius of 19.7mm has been predicted (Fig. 12a). The major and minor strain values (superimposed on the FLD) are lower than the limit strains indicating successful forming with μ=0.25, sealing force 310kN and peak fluid pressure 24MPa as shown in Fig. 12b. Thickness distribution in the cups shows a maximum thinning 20.5% (Fig. 12e). For lowest friction coefficient (μ=0.04), the predicted minimum bottom corner radius is 15.8mm (Fig. 12c) with maximum thinning of 5.6% as shown in Fig. 12f. More drawing-in has been observed when μ is less (0.04) and hence most of the strain data is on the left-hand side of the FLD (Fig. 12d) and biaxial stretching at the corners reduced significantly leading to lower thinning at the corners. In both cases, the cups could be formed with the maximum depth of 35mm.

Fig. 12.
(0.44MB).

Predicted formability and thinning in FE simulation of hydroforming for μ=0.25 (a, b and e) and μ=0.04 (c, d and f).

For the highest value of coefficient of friction (0.25), the frictional constraint at the interface between the die wall and the blank on both sides of the corner is also high and hence the material at the corner gets biaxially stretched due to increase in fluid pressure in the calibration stage as shown in Fig. 13.

Fig. 13.
(0.1MB).

Effect of friction between the die and the blank on thining and minimum corner radius in hydroforming.

Hence, thinning is high for high value of friction coefficient and the cups failed when maximum thinning at the corner is 22%. With decreasing friction coefficient, the constraint for sliding motion of the blank on the die surface reduced allowing more material to flow from the flange region and hence one of the true strains decreased and became negative due to drawing-in for μ value less than 0.1. With reduction in friction coefficient μ, the strain distribution is more uniform as more material is drawn into the die corner and thinning decreased allowing cup to be formed with lower corner radius without any failure. Hence, for a friction coefficient value of μ=0.04, cups with a minimum corner radius 15.8mm could be drawn successfully and maximum thinning in the corner is less than 6%. It clearly shows that lubrication between die and the blank is essential to form square cups with small corner radius without excessive thinning.

5.4Conventional deep drawing

Since a minimum corner radius of 15.8mm has been achieved in hydroforming of square cups with thinning of less than 6%, FE simulations of conventional deep drawing were carried out with a punch corner radius of 16mm. The maximum depth of the cup and maximum thinning were predicted from the simulations. A blank holding force of 80kN was applied to prevent wrinkling. Figure 14 shows failure in conventional deep drawing for two cases of friction (μ=0.25 for dry condition and μ=0.04 for lubricated condition). Strain values superimposed on the FLD show failure in-plane strain mode at the four bottom corners of the part which is the normal mode of failure in conventional deep drawing when the draw ratio exceeds the limiting draw ratio. Plane strain stretching causes maximum thinning and failure in the cup wall close to the punch corner. It is observed that the maximum depth that could be formed at failure was 19.9mm for μ=0.25 and 25.1mm for μ=0.04 (Fig. 14a and c). So it is predicted that the full depth of the part could not be obtained in conventional deep drawing. It is possible to obtain smaller corner radius in conventional deep drawing when compared to hydroforming but the possible draw depth decreases.

Fig. 14.
(0.4MB).

Predicted formability and thinning in FE simulation of conventional deep drawing for μ=0.25 (a, b and e) and μ=0.04 (c, d and f).

The thickness variations in the drawn cups are also shown in Fig. 14e and f for both cases of friction. Maximum thinning has always been observed at the corners and it decreases with decreasing coefficient of friction. The maximum thinning at the corner is 14.2% and 11.8% for the two cases of friction as shown in Fig. 14e and f, respectively.

5.5Comparison of hydroforming and conventional deep drawing and experimental validation

The results presented in the earlier sections clearly show that formability of cryorolled AA5083 alloy in deep drawing of square cups can be significantly enhanced by hydroforming process. Fig. 15a and c shows the failures in conventional deep drawing at the bottom corner with punch corner radius of 16mm in accordance with the failure predicted in FE simulations (presented earlier in Fig. 15a, b and e in dry condition, μ=0.25). As shown in Fig. 15b, the cup could be successfully formed in hydroforming with full depth but with a minimum corner radius of 20mm without lubrication. Even with teflon lubrication, only up to 50–70% of the maximum depth could be obtained in conventional deep drawing (Fig. 15a and b). However, in hydroforming with teflon lubrication, the total depth (35mm) could be formed with a minimum corner radius of 16mm as shown in Fig. 15d. The experimental findings corroborate the predictions of FE simulation in all these cases.

Fig. 15.
(0.18MB).

Experimentally formed cups (a) conventional (dry), (b) hydroforming (dry), (c) conventional (lubricated) and (d) hydroforming (lubricated).

The enhanced formability in hydroforming is mainly due to more uniform strain distribution leading to lower thinning at the corners. Hydrostatic strain suppresses the rate of void growth and hence delays onset of necking or failure during plastic deformation [37,38]. Variation of thinning in the formed cups obtained from FE analysis has already been discussed earlier and the maximum thinning has always been observed at the four corners. The comparisons of variation of percentage thinning along the XX and XY (diagonal) from the center of the bottom obtained from simulations of both conventional forming and hydroforming are shown in Fig. 16a and the same is shown in Fig. 16b for experimental results. In both figures, it can be clearly seen that the maximum thinning at the corner in hydroforming (5–6%) is much lower than in conventional deep drawing (12%). The thinning in the flat bottom region is higher in the case of hydroforming due to initial bulging. This makes the thickness distribution more uniform than in conventional deep drawing leading to larger overall depth before failure. In conventional deep drawing, failure occurs at the punch corner due to excessive thinning as shown in Fig. 17a and a successfully formed part from cryorolled and hydroformed sheet is shown in Fig. 17b.

Fig. 16.
(0.22MB).

Thickness distribution along XX and XY (diagonal) in (a) FE simulation and (b) experimental work.

Fig. 17.
(0.15MB).

Comparison of experimentally formed flat bottom square cups in (a) conventional forming and (b) hydroforming.

The summary of the results obtained from both simulations and experimental work on conventional deep drawing and hydroforming for lubricated condition is given in Table 4. The strength of Al alloys such as AA5083 has been enhanced through cryorolling by imparting large plastic strain. By combining this with hydroforming after a short annealing, formability of high strength cryorolled sheets has been enhanced such that they can be formed into useful components for automotive and aerospace applications. The present work demonstrates that this process route (cryorolling followed by hydroforming) is a potential technique to produce complex parts with high strength Al alloy sheets due to enhanced formability.

Table 4.

Comparison of formability results in hydroforming and conventional deep drawing.

  FE analysis (μ=0.04)Exp. (with teflon sheet)Remark 
  Depth (mm)  Min. corner radius (mm)  Max. thinning (%)  Depth (mm)  Min. corner radius (mm)  Max. thinning (%)   
CF  25.0  15.9  11.8  26.0  16.0  12.0  Failed 
HF  34.8  15.8  5.6  35.0  16.0  7.0  Successful 

CF, conventional forming; HF, hydroforming.

6Conclusions

In this work, formability of high strength cryorolled AA5083 Al alloy sheets in square cup deep drawing has been successfully enhanced by hydroforming. A process window with optimum combination of peak sealing force and peak pressure has been identified to form the cups with full depth of the die without failure at die entry or bottom corners. Maximum percentage thinning reduced with decrease in coefficient of friction from nearly 20% (μ=0.25) to 6% (μ=0.04). Experimental results also showed that with lubrication between the die and the blank, it is possible to achieve a minimum corner radius of nearly 16mm with thinning less than 6%. While it is possible to obtain smaller corner radius in conventional drawing than in hydroforming, the maximum possible draw depth is lower. In conventional forming, only 70% of the full depth could be obtained before failure with 16mm punch corner radius due to much higher thinning at the corners (around 12% with lubrication). Enhanced formability in hydroforming is mainly due to more uniform strain distribution leading to lower thinning at the corners and the hydrostatic stress suppresses the rate of void growth and hence delays onset of necking or failure during plastic deformation. It can be concluded that this process route (cryorolling followed by hydroforming) has a good potential to produce parts complex sheet metal parts from lightweight high strength Al alloy sheets due to enhanced formability.

Conflicts of interest

The authors declare that they have no conflicts of interest.

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Journal of Materials Research and Technology

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