A Novel Ironing Process with Extra High Thickness Reduction: Constrained Ironing

A new ironing method entitled constrained ironing is proposed for producing thin-walled cans and components with uniform thickness. This method based on compressive stresses could reach a higher ironing limit ratio or thickness reduction ratio (TRR) without interruption for additional processing such as multi-stage ironing and annealing between different stages. When the constrained ironing starts, the punch pushes the constrained material to reduce the thickness from the outer surface of the cup. The state of the stresses is fully compressive in this new process while it is tensile in the conventional ironing method. The results showed that after constrained ironing process, the tensile strength and hardness increased to 204 MPa and 85 HV, respectively, from the initial values of 71 MPa and 25 HV. Thus, very high TRR is achievable in the constrained ironing process. Obtaining a higher TRR about 80% after only single stage ironing, removing the annealing stages, and obtaining higher strength and hardness of the ironed cup are several advantages of the proposed method. This novel and simple process could be very promising for the industrial applications to replace the conventional process and to reduce the final product cost.

Keywords: Compressive; Constrained; Experiment; Higher; Ironing; Ratio; Reduction; Stress; Thickness

Ironing is a conventional metal forming process for producing thin-walled cans having uniform thickness after deep drawing process with greater height to diameter ratio [[1][2][3]]. The first invention contained the basics of the ironing process published in the USA in 1904 [[4]] though the researchers tried to optimize this method for industrial applications [[5]]. One of the problems in the ironing process is the low thickness reduction ratio (TRR) in the conventional ironing process which is about 30%. A multi-stage ironing process is used for producing cups needing greater thickness reductions [[7]]. At each stage of the process, decreasing the diameter of the dies is needed to achieve the desired wall thickness. The work hardening resulted from plastic deformation in deep drawing or ironing stages decreases by annealing [[9]]. Also, the annealing process is used to increase the ductility and remove the residual stresses immediately after deep drawing or after different stages of multi-stage ironing [[11]]. However, the continuous deep drawing or ironing process is desired to avoid annealing between stages from the viewpoint of the productivity and decreasing the production cost [[12]]. Thus, many studies have been carried out to decrease the number of ironing stages and remove the annealing process between stages. The effect of the ultrasonic vibration on the workability of the copper cups in the ironing process was studied by Siddiq and Sayed [[14]]. Kampuš and Nardin [[15]] improved the formability by converting the tensile stress to the compressive stress in the deformation zone and increased the TRR about 15%. To prevent damage of the cup wall at one stage ironing, optimizing 35% of TRR was obtained by Courbon [[16]]. Finite element simulation and experimental investigations on the ironing limit ratio under warm condition were carried out by Singh et al. [[17]]. However, the method is applicable to a small reduction. Odell [[18]] used FEM simulation to examine the effects of the die geometry and friction on the maximum reduction ratio. Delarbre and Montmitonnet [[19]] determined the possible maximum reduction of 50% on stainless steel cup during only single pass. Tirosh et al. [[20]] used the hydrostatic ironing with a very high fluid pressure to achieve a higher TRR. However, their method needs the high fluid pressure about 600 MPa. Hydro-ironing method was introduced to solve the hydrostatic ironing problems by Shirazi et al. [[11]]. They used hydro-ironing process with a hydrostatic oil pressure of 15 MPa and a loader force of about 63 KN to achieve a TRR about 70%. Accurate control of the process parameters (fluid pressure, loader force, and punch force) and needing almost complicated apparatus are important drawbacks of hydro-ironing method. Although the hydro-ironing process resolved several drawbacks of the previous ironing methods, but the solutions of some problems have not yet been identified.

It is accordingly an object of the present study to provide a novel and simple ironing method for manufacturing of thin-walled cylindrical cups with a higher TRR during only single stage ironing. In this paper, constrained ironing is presented and applicability of this novel process is investigated using experimental tests.

In the conventional ironing process shown in Fig. 1, a punch is pressed to pull the deep drawn cup to a deformation zone to reduce the wall thickness. If the TRR is selected higher than a distinct value (about maximum 30%), necking will occur in the cup wall as a result of tensile stresses. So, in the conventional ironing method, higher TRR could not be achieved. The constrained ironing process was designed in which no tensile stress is applied to the cup wall. Schematic of constrained ironing process at the beginning and during the processes were shown in Fig. 2. The deep drawn cup at the starting of process is put into the gap between the punch and die as shown in Fig. 2(a). The ironing punch moves down and presses the deep drawn cup into the deformation zone to reduce the cup thickness as shown in Fig. 2(b). The cup thickness in the outlet is thinner than that in the inlet. So, the velocity of the thinned material is higher than that of the punch. Consequently, the distance between the cup and punch bottoms is increased with the process proceeding. In the other word, no tensile stress is applied to the cup wall, and very high TRR could be obtained.

Graph: FIGURE 1 —Conventional ironing process.

Graph: FIGURE 2 —A schematic illustration of constrained ironing at the (a) beginning of the process and (b) during the process.

A circular sheet blank of commercially pure Al (AA1050) with 2.5 mm thickness and 84 mm diameter annealed at 415°C for 3 hr was used [[11]]. Cup-shaped components of 50 mm in outer diameter and 15 mm in cup wall height were processed via deep drawing process. Two sets of deep drawing and constrained ironing dies were manufactured from tool steel and hardened to about 50 Hardness Rockwell C (HRC). The lubricant was used to reduce the friction between the aluminum cups and constrained ironing die. The experiments were carried out with a 60 ton hydraulic pressing machine at room temperature. In order to study mechanical properties of the ironed cup, tensile tests along the axial direction and microhardness tests were carried out. Tensile tests were done using a 30 ton INSTRON universal test machine at strain rate of 10−4 at room temperature. The HV microhardness testing was performed using an indenter load of 100 gr and a loading time of 10 sec.

Figure 3 shows deep drawn and constrained ironed cups. As shown, a significant thickness reduction was achieved after only a single stage of constrained ironing process. A large height ironed cup was made from a short deep drawn sample. The comparative cross sections of deep drawn and ironed cups together with the thickness distributions were shown in Fig. 4. Position "1" is the starting point of the cup wall and position "8" is the center of the cup bottom. The initial thickness of the deep drawn cup was about 2.35 mm that reached 0.48 mm after the ironing process. From the upper edge to the bottom of the ironed cup wall, uniform distribution with a tolerance of ±0.02 is observed. So, it is clear that in constrained ironing process, the TRR of 80% was obtained during a single stage. In this process, the forming limit depends on the stress state in the deformation zone. Compressive stress in the cup wall provides possibility of achieving higher TRR [[4]]. In constrained ironing, the cylindrical punch imposed axial force on the constrained cup wall. The higher compressive force improves workability and closes the cavities that appear during the deep drawing process [[15], [21]]. Punch has the effect of the promoting the material flows when the compressive force is applied to the material in the forming zone.

Graph: FIGURE 3 —Short deep drawn and large height constrained ironed cups.

Graph: FIGURE 4 —A comparative view of the cross sections of deep drawn and ironed cups and thickness distributions along the walls.

Figure 5 shows the engineering stress–strain curves obtained at room temperature from the annealed and constrained ironed aluminum samples. The obvious strain hardening, lower tensile strength (71 MPa), and larger elongation (∼63%) were observed in the annealed sample. As expected, the strength of the constrained ironed samples increases significantly over the annealed sample. It is clear that the little ductility and lower elongation is observed in the constrained ironed sample due to the work hardening. Table 1 shows the ultimate strength, yield strength, and elongation of the annealed and ironed samples. The annealed sample has the tensile strength of 71 MPa with 63% elongation due to the annealing heat treatment conducted before ironing process. In constrained ironed sample, the tensile strength increases to the value of 204 MPa while the elongation decreases to ∼7%. Increase in accumulated strain with decrease in hardening exponent reduces elongation after the ironing process. This higher value of strength has never been seen after other ironing processes. Shirazi et al. [[11]] achieved the tensile strength of about 145 MPa and elongation of about 7% for the same material used here. Compared to the hydro-ironing process [[11]], 27% higher strength and similar elongation of 7% were obtained in constrained ironing. This may be due to the higher compressive stresses applied to the material through the constrained ironing process.

Graph: FIGURE 5 —Engineering stress–strain curves of raw and constrained ironed materials.

TABLE 1.—Mechanical properties of annealed and constrained ironed samples.

The microhardness distribution along the deep drawn and constrained ironed aluminum cup walls was presented in Fig. 6. The microhardness was measured along the vertical wall of the cup from point "a" to "b." A significant increase in the microhardness of constrained ironed cup (∼85 Hv) was seen from the initial value of 25 Hv for the annealed cup. The hardness of the cup processed by constrained ironing is about 40% higher than that processed by hydro-ironing [[11]] method. This improvement may be related to not only higher value of strain applied to the material but also higher compressive stresses applied during this process. Also, hardness distribution is almost more uniform along the wall of constrained ironed cup compared to hydro-ironed sample.

Graph: FIGURE 6 —Hardness distribution in the deep drawn and ironed cup walls.

Higher TRR in constrained ironing (80%) compared to other methods (30% in conventional −70% in hydro-ironing) is the major advantage of this novel process. Another advantage is applying the pure compressive stresses to reach a higher TRR and to reach improved mechanical properties such as higher strength and hardness. The hardness in the ironed cup wall in hydro-ironing process is about 40.1 HV [[11]]. However, this value is about 85 HV for constrained ironing. Also, the hardness distribution in the axial direction along constrained ironed cup wall is more uniform than that in the hydro-ironed cup wall. The increase of the tensile strength of constrained ironed cup is larger than that in other ironing processes. Simplicity of the constrained ironing process is as well as another advantage in which a simple punch and die by a conventional press machine is required.

The ironing ratios (IR) for various ironing process in comparison with this work were shown in Fig. 7. In the conventional ironing process, the TRR of 30% for aluminum and 36% for steel cans during the single pass ironing were reported. Also, the maximum IR is 12% for one stage ironing in the multi-stage ironing process [[11]] which may be affected by lubrication condition [[22]]. Danckert [[23]] used FEM simulation with the slap method to obtain the critical TRR of thin-walled cans, and its ratio is close to 26%. Kampuš and Nardin [[15]] reached 35% reduction using an imposed axial force on the cup wall before the entrance to the forming zone. Singh et al. [[24]] determined the maximum TRR of 40% in the ironing process under warm condition. Tirosh et al. [[20]] achieved 56% reduction of the cup initial thickness made of 18 Ni 250 steel using the hydrostatic ironing with fluid pressure drive of 600 MPa. Shirazi et al. [[11]] achieved 66.9% reduction by using hydro-ironing process in which a loader force of 63 KN and hydrostatic oil pressure of 15 MPa were used. In above-mentioned processes with TRR about 50–79%, high fluid pressure is used which may need additional apparatus. An important advantage of the constrained ironing compared with other ironing techniques is the ability to achieve higher TRR (at least 80%) after only single stage ironing without using fluid pressure. This advantage could reduce the number of steps in the beverage cans manufacturing and industrial components. Low number of manufacturing stages is directly related to production costs and times. Also, because of the compressive nature of the process the process, it may be used for ironing of hard to deform materials.

Graph: FIGURE 7 —Ironing thickness reduction ratio (TRR) in the constrained ironing process in comparison with other techniques.

Constrained ironing was introduced as a novel technique suitable for producing cup-shaped thin-walled components. The constrained ironing process was successfully applied to an aluminum deep drawn cup and outstanding capabilities of the process are listed as follows

• Very high TRR of about 80% through only one ironing stage was achieved. This is desirable for industrial applications from the viewpoint of manufacturing time and the cost saving.
• Higher compressive stresses improved the workability of the material.
• Higher TRR removes the additional annealing process between the forming stages.
• Higher strength and hardness of the ironed cup were obtained.
• The tensile strength increases about threefold as compared to the annealed sample.
• The microhardness increased to about 85 HV from an initial of 25 HV.