Experimental investigation on the rectangular cup formability of Al-alloy sheet by superplastic forming technique

Transcription

1 Journal of Scientific & Industrial Research Vol. 73, January 2014, pp Experimental investigation on the rectangular cup formability of Al-alloy sheet by superplastic forming technique G Kumaresan 1* and K Kalaichelvan 2 1,2 Department of Production Technology, MIT Campus Anna University, Chennai , India Received 24 July 2012; revised 04 July 2013; accepted 03 September 2013 Superplastic behavior of certain metals and alloys having very fine grains, very large tensile elongations are obtained within certain temperature ranges at low strain rates. These alloys can be formed into complex shapes by superplastic forming, a process that employs coon metalworking techniques. This paper aims to study the formability characteristic for aluminium material (Al 7075) by considering variable forming pressure of 0.2MPa, 0.3MPa, 0.4MPa and 0.5MPa, remaining parameters were constant for all samples; a constant forming temperature of 530 C and the constant forming time of 120 minutes were selected for all samples. Keywords: Al-alloy sheet, Superplastic, Metal forming, Thermo-forming Introduction Superplastic alloys can be formed by such bulk deformation processes as compression molding, closed-die forging, coining, and extrusion. Sheet forming of these materials can also be carried out using such operations as thermoforming, vacuum forming, and blow forming. Superplastic sheet metal forming allows the production of complex parts that are not formable under normal conditions. Superplastic sheet metal forming processes normally are based on the same coon principle: the sheet metal is firmly clamped between the die halves and is blow-formed by means of gas pressure. Generally superplastic forming can only be achieved in a very narrow range of strain rates and temperature. Superplastic materials are relatively stable when deformed; this behavior is related to the observation that the flow stress of a superplastic material is very sensitive to the rate of deformation 1,2. The enormous stretchability of superplastic alloys at very low stress, and their high strain rate sensitivity which prevents localized deformation (thinning or necking) are the most important properties. The conditions for superplasticity are, Temperature equal to or above half the absolute melting point of the materials, Slow strain rates (usually 10-3 s -1 or slower) and Very fine grain size (grain diameters of a few micrometers or less). A long time is needed to form useful shapes at *Author for correspondence kumaresan@mitindia.edu low strain rates 3. With long times and high temperatures, grain growth may occur, negating an initial fine grain size 4, 5. For this reason, most superplastic alloys have two phase structures, or a very fine dispersion of insoluble particles. Both minimize grain growth. Both the low flow stress and high elongations can be useful in metal forming. The very low flow stresses permit slow forging of large and intricate parts with fine detail. This paper aims to study the formability characteristic for aluminium material (Al 7075) by considering variable forming pressure of 0.2MPa, 0.3MPa, 0.4MPa and 0.5MPa, remaining parameters were constant for all samples; a constant forming temperature of 530 C and the constant forming time of 120 minutes were selected. Experimental investigations Selection and processing of specimen material The 7075 Al alloy has been used in this study, and the analysed chemical composition was (in wt ) 5.6- Zn, 2.5-Mg, 1.6-Cu, 0.23-Cr, balance Al. The as received plates of 5 thickness were homogenized at 500 C for 1 hour. The fine grain microstructure in aluminium alloys required for superplasticity could be obtained through static or dynamic recrystallization. Static recrystallization forms a fine grain structure prior to superplastic deformation, where as dynamic recrystallization forms a fine grain structure during the early stages of superplastic deformation 6. A static

2 KUMARESAN & KALAICHELVAN: EXPTAL INVESTIGATION ON RECTANGULAR CUP OF AL-ALLOY SHEET 47 thermomechanical treatment process was carried out to produce a very fine grained microstructure, by using the modified Taharsahraoui method 7. The thermomechanical treatment process parameters used in this study are shown in Table1. Experimental device The experimental setup consists of a split furnace, an air compressor with tank, and a thermocouple to measure the die temperature. The forming chamber consists of top and bottom dies, and the recess is provided in the bottom die to hold the thermomechanically processed1.5 thick sheet blank. The top die is a rectangular shape. Experimental procedures The experimental work was carried out; there were four samples namely A, B, C and D. Only the forming pressure was changed, remaining parameters were constant for all samples; a constant forming temperature of 530 C and the constant forming time of 120 minutes were selected for all samples. The forming pressure of 0.2 MPa, 0.3MPa, 0.4MPa and 0.5MPa was chosen for sample A, sample B, sample C and sample D respectively. To determine the strain after the superplastic forming process, circle grids of diameter d 0 (5) marked on the sheets were employed to measure strain levels in each test. During forming the marked circles were distorted into ellipse and/or larger circles. Measurements of the major d 1 and minor d 2, diameters after deformation were made to determine the principal strains. Calculation of major and minor strain Laser marking method was used for marking the grid pattern on the sheet metal blank in order to study the surface strain analysis. The calculation of thickness strain from circular grid analysis, strain begins with the formula for the conservation of volume. A sample of material has a constant volume regardless of the forming forces applied in equation forms as given below. Table1 Thermomechanical treatment process parameters Stage Temperature Time Conditions Solution treatment 500 C 1h Furnace cooling to 380 C Overaging 380 C 2h Furnace cooling to 190 C Warm rolling 180 C Reduction of thickness Recrystallization 500 C 0.5 h Water quench Aging 180 C 0.5h Water quench Constant volume = length x width x thickness (Or) 1= (1+S 1 ) (1+S 2 ) (1+S 3 ) (1) Where, S 1 is minor strain, S 2 is major strain and S 3 is thickness strain. The surface strains S 1 and S 2 were measured by a flexible scale. The respective thickness strain was calculated by substituting the surface equation (1). In the present analysis, the following assumptions were made. 1 The material obeys Von mises effective stress and strain criteria. 2 The volume of the deforming material remains constant at any instant of forming. 3 The flow stress in the thickness direction is ignored for the deformation in a thin membrane. 4 The material is to be isotropic and incompressible, while flow stress depends on strain rate and temperature. A constitutive equation was employed in this study: σ = k έ m (2) Where σ is the flow stress, K is the strength coefficient, έ is the strain rate and m is the strain-rate sensitivity index of the material. σ ϴ, σ m and σ s are hoop, meridional and thickness stresses and Є ϴ, Є m and Є s are hoop, meridional and thickness strains. The volume of deforming material remains constant, which implies that, Є ϴ + Є m + Є s = 0 (3) The flow stress in the thickness direction is ignored. of major strain = [(major axis grid circle diameter)/ grid circle diameter] x 100 (4) of minor strain = [(minor axis grid circle diameter)/ grid circle diameter] x 100 (5) Results and discussion An important development in testing the formability of sheet metals is the forming limit diagram (FLD). A plot of the combinations of strains that lead to necking failure is called a FLD. The strain combinations below the curve are acceptable, whereas those above it will cause local necking. In order to develop the FLD, the major and minor engineering strains, as measured from the deformation of the original circles, that the original circle has deformed into an ellipse.

3 48 J SCI IND RES VOL 73 JANUARY 2014 The major axis of the ellipse represents the major direction and magnitude of stretching. The major strain is the engineering this direction, and is always positive, because of sheet- metal stretching. The minor strain can be either negative or positive. If, a circle is placed in the center of a tensile test specimen and then stretched, the specimen becomes narrower as it is stretched, and the minor strain is negative. If we place a circle on a spherical rubber balloon and inflate it, the minor and major strains are both positive and equal in magnitude. By comparing the surface areas of the original circle and the deformed circle on the formed sheet, also determined whether the thickness of the sheet has changed. Because the volume remains constant in plastic deformation, the area of the deformed circle is larger than the original circle, the sheet has become thinner. The data obtained from different locations in each of the samples are plotted in the form of curve. The curve represents the boundaries between failure and shape zone. The higher the curve, better the formability of the materials. The forming limit diagram for this alloy was plotted using the experimental data from the superplasic bulge test with the rectangular shape die. The shape of the forming limit curve in the case of superplastic forming is different from that of the conventional forming; Fig. 1 shows the formed component. In the first sample A was formed under forming pressure of 0.2MPa this pressure was not suitable for high formability of superplastic forming, so identified heavy fracture region, because the grains were high resist to the initial deformation. Sample B was formed under forming pressure of 0.3 MPa gives the good formability because of the grains slides easily in the forming pressure. Sample C was formed under forming pressure of 0.4MPa gives the very good formability because of the optimal strain rate for Fig. 1 Formed component Table 2 and strain for samples A and B Sample A axis in Sample B strain in superplastic forming. In the last sample D was formed under forming pressure of 0.5MPa this pressure was not suitable for high formability of superplastic forming, so identified heavy fracture region, because the grains slides rapidly, and it reduced the formability due to the instability of the grains during high forming pressure. Table 2 shows the and strain for samples A and B. Fig. 2 shows the Forming limit diagram for samples A and B. Table 3 shows the and strain for samples C and D. Fig. 3 shows the forming limit diagram for samples C and D. Initially, as the forming pressure increased the bulge height of the formed component also increased, because of an increase in the strain rate of forming. But, beyond a certain forming pressure (corresponding to the optimal strain rate for superplastic forming) the bulge height decreased with a further increase in the forming pressure. The thickness gradient is the ratio of the thickness at the circle to the neighbor circle. It was calculated for each sample. If it is less than 0.92, it should be the necking point. The red diamonds in the FLD diagram denote the occurrence of the necking point; where as green rectangles represent safe points.

4 KUMARESAN & KALAICHELVAN: EXPTAL INVESTIGATION ON RECTANGULAR CUP OF AL-ALLOY SHEET 49 Table 3 - and strain for samples C and D Sample C Sample D strain in Fig. 2 Forming limit diagram for samples A and B Fig. 3 Forming limit diagram for samples C and D

5 50 J SCI IND RES VOL 73 JANUARY 2014 Conclusion In this study, forming limit diagram are experimentally evaluated for the Al 7075 of thickness 1.5 by superplastic forming process. In the sample B gives the good formability because of the grains slides easily in the forming temperature of 530 C and forming pressure of 0.3MPa, but the sample C gives very good formability because of the optimal strain rate for superplastic forming of 0.4MPa. The formability decreased with a further increase in the forming pressure. Change in the forming pressure does not influence much the profile development during pressure thermo-forming. References 1 Horng-yu Wa, Jiin-her Hwang & Chui-hung Chiu, Deformation characteristics and cavitation during multiaxial blow forming in superplastic 8090 alloy, J Mater Process Tech, 209 B (2009) Chandra N, Rama S C & Chen Z, Critical Issues in the Industrial Application of SPF- Process Modeling and Design Practices, J Mater Trans, 40 B (1999) Xing H L, Wang C W, Zhang K F & Wang Z R, Recent development in the mechanics of superplasticity and its applications, J Mater Trans, 151 B (2004) Tomotake Hirata, Toshihiro Osa, Hiroyuki Hosokawa & Kenji Higashi, Effects of flow stress and grain size on the evolutoin of grain boundary microstructure in superplastic 5083 Aluminum alloy, J Mater Trans, 43 B (2002) Rhaipu S, Wise M L H & Bate P S, Microstructural gradients in the superplastic forming of Ti-6Al-4V, Metall Mater Trans A, 33 B (2002) Smolej A, Gnamus M & Slacek E, The Influence of the thermo mechanical processing and forming parameters on superplastic behaviour of the 7475Aluminum alloy, J Mater Process Tech, 118 B (2001) Taharsahraoui, Mohamedhadji, Nacerbacha & Riadbadji, Superplastic deformation behavior of 7075 aluminum alloy, J Mater Engg Perform, 12 B (2003)