RESIDUAL STRESSES DUE TO DEEP-DRAWING OF PRE-COATED ALUMINUM-ALLOY SHEETS

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1 Materials Science Forum Online: ISSN: , Vols , pp doi:10.40/ 005 Trans Tech Publications, Switzerland RESIDUAL STRESSES DUE TO DEEP-DRAWING OF PRE-COATED ALUMINUM-ALLOY SHEETS Masatoshi Sudo 1, a, Tomohiko Iwase, Yoshiyuki Hattori and Madoka Nakajima 1 1 Kanazawa Institute of Technology Graduate students, Kanazawa Institute of Technology 7-1 Ohgigaoka Nonoichi, Ishikawa Japan a m-sudo@neptune.kanazawa-it.ac.jp Keywords: press formability, pre-coated aluminum sheets, press-forming condition, residual stresses Abstract. In this study, we examined how the press formability of A1 alloys sheets and the generation of residual stresses was influenced by pre-coating resins, lubricants, die shoulder radius, the punch shoulder radius, and so on. The use of a pre-coated layer offers advantages by preventing direct contact between sheet metal and the face of the die, and also by enabling the use of volatile oil lubricants that keep the factory environment cleaner. The circumferential residual stress (σ C ) of a cup becomes lower when the lubricant is less effective, and when the die shoulder radius is smaller. Introduction The use of pre-coated steel sheets has increased recently because the painting process can be omitted, which makes factories cleaner. It also omits the manufacturing process, and improves productivity. [1-3] So, the application of pre-coating to aluminum alloy sheets is attempted. Flaking of the pre-coat according to press forming and the formation of the residual stresses become problems in practical use. [4] The formation of the residual stresses in the product might cause something wrong, such as distortion and fracture. This paper deals with the effect of press forming conditions and pre-coating on the generation of circumferential residual stresses in pre-coated Al-alloy cup walls. Specimen and experimental procedures Material. Table 1 shows the specimens used. AISI1000 and 5000 series Al-alloy sheets were used with two types of pre-coating resins. The first type, a thick pre-coating composed of polyester and high-molecular polyester resins, was designed for painting. The second, a thin pre-coating composed of a urethane resin film, was designed to improve lubrication. Table shows experimental results of tensile properties. Each value is an average obtained from three directions. r is calculated according to the following equation: r =(r 0 +r 45 +r 90 )/4. Similarly, the anisotropy of mechanical properties is calculated by: r=( r 0 +r 90 -r 45 )/. The pre-coating treatment with polyester resin increased local elongationε l and decreased the 0.% proof stress σ y, tensile strength σ u, and uniform elongationε u. The urethane pre-coating produced different changes in mechanical properties, namely, a significant increase in proof stress and a minimal change in elongation. Deep drawing condition. The deep drawing condition and tooling geometry are shown in Table 3. Deep drawability was obtained as the limiting drawing ratio () using the Swift cup test. Tests were performed on blanks with different diameters, grouped into sets of five per each diameter tested. When five blanks of the same diameter were deep-drawn, the diameter was judged to be deep drawable if at least three of the blanks were not fractured. The largest deep-drawn blank diameter was then divided by punch diameter, and the value was taken as the. Three types of lubricants were used: machine oil (Mu-19) and a graphite grease (GG). The kinetic viscosity of Mu-19 (Mu) is.3 cst. [3] GG has the best lubricating properties. We also examined how All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-05/03/16,10:3:4)

2 Materials Science Forum Vols was influenced by the tooling geometry, i.e., the die and punch shoulder radii. The punch load/punch stroke curve was measured using an X-Y recorder. * Table.1 Specimens. Alloys Film composition Thickness/mm 1050-H4K H4 1.0 Polyester(P) 1100-H4(B) 1.0 * 505-O High molecular H34 Polyester H34(B) (HP) O 0. Urethane (U) 1050-O(B) 0. (B) Without pre-coating Table.3 Standard condition of deepdrawing test. Punch diameter dp φ40 Punch profile radius rp mm Clearance 0.0% Die profile radius rd 3mm Hold-down force 1kN Punch velocity 100mm/min Code Table. Mechanical properties of specimens. σ y [Mpa] σ u [Mpa] εu [%] 1100-H H4 (B) H H34 (B) O O (B) Εl [%] Circumferential residual stress measurement. The Crampton method was adopted for this experiment. This method makes the tube crack in the direction of its axis and gives circumferential stress σ C for the surface of the outside of the tube. This value is obtained by measuring external change. σ C is calculated according to the following equation: σ C =E/(1-ν ) t(1/d 0-1/D t ) (1) where, E : Young's modulus, ν: Poisson ratio, D 0 : initial outer diameter of the tube, D t : fractured outer diameter of the tube fractured. To apply this method, the length of the sample tube needed is twice the tube's diameter. A tube length of 0mm was used because of the shape limitation of the cup. In addition, the change in the deep drawing ratio along the longitudinal axis of the deep-drawn cup must be noted to interpret the results. Experimental Results The flaking appearance on the deep-drawn cups was visually examined. The adhesion of the pre-coated layer was so excellent that flake-off was observed only after second-step deep drawing or after warm deep drawing Fig. 1 shows the limiting drawing ratios () of the specimens used. The influences of types of pre-coat resins, lubricants, die shoulder radius, and drawing ratio on the circumferential residual stressesσ C were examined; the important results are as follows. Effects of pre-coating film and lubricant on deep-drawability. The effect of pre-coating film on the (using Mu as lubricant) is shown in Fig.. The urethane resin film improved the well; the polyester improved it slightly, but the high-polymer polyester did not. r r

3 360 Residual Stresses VII, ICRS7 The type of lubricant used also had a great effect on the, as shown in Fig. 3. The effectiveness of the lubricants in increasing value ( ) varied with the type of film. is calculated according to the following equation: = GG (value obtained using GG as a lubricant ) 0 (value obtained using machine oil/mu as lubricant). The urethane resin film improved the to such a great extent that the type of lubricant used could scarcely add to the improvement. As expected, high-performance lubricants such as graphite grease increased the significantly Alloy No. Figure 1 Effect of lubricants on of all specimens used Mu GG Specimens H4K H H4(B) O H H34(B) O O(B) H H34 material code O Figure Effect of pre-coating film on (Lubricaut used: Mu-19) coat bare H 1100H 505O 505H 1050O 1050H Material code Figure 3 Effects of kind of pre-coating film and lubricant on the increase in ( ) = GG (a value obtained using a graphite greese) - 0 (a value obtained using machine oil as lubricant) Residual stress / kn/mm Mu7-coat GG7-coat Mu76-coat Mu76-bare Uppe r Midd le L ower Position of cup wall Figure 4 Effect of lubricant and pre-coat on circumferential residual stress. (specimen:1100-h4) (7 or 76 means blank diameter) Effects of pre-coating film and lubricant on residual stresses. Fig. 4 shows some examples of the effect of pre-coating film and lubricant on residual stresses. The horizontal axis indicates the measuring positions of the cup wall. Theσ C of a cup formed using Mu (a less effective lubricant) is lower than that of a cup using GG as a lubricant. The pre-coated 1100-H4 cup has a lowerσ C value than the bare 1100-H4 cup has. Punch load / kn Mu7-coat GG7-coat Mu76-coat Mu76-bare Punch stroke / mm Figure 5 Punch load-stroke curves. (1100-H4)

4 Materials Science Forum Vols These change in pre-coating film and lubricant provoke change in punch load-stroke curves, as shown in Fig. 5. Effect of press forming tooling geometry on deep-drawability. Press forming tooling geometry, such as the radii of the die and punch shoulder, also affect the value. Fig. 6 shows the effect of the die shoulder radius (r d ) on. The value increased with increases inr d, though it should be noted that in some cases a large r d caused wrinkling in the specimen. A greater punch shoulder radiusr p improved also as shown in Fig. 7. The was about The clearance between the die and punch also affected the value. A smaller clearance improved the Die shoulder radius / mm H H O Figure 6 Effect of die shoulder radius(r d ) on (Lubricant used:mu-19) Punch shoulder radius / mm Figure 7 Effect of punch shoulder radius (r p ) on H H O Effect of die shoulder radius on residual stresses. Fig. shows that σ C increases largely with the increase in the die shoulder radius r d. The residual stress also increases with a larger punch shoulder radius as shown in Fig. 9. The smaller the clearance, the smaller the residual stress is. [5] Residual stress / kn/mm r d /mm Residual stress / kn/mm r p /mm Figure Effect of die shoulder radius(r d ) on circumferential residual stress. (1100-H4) Figure 9 Effect of punch shoulder radius (r p ) on circumferential residual stress. (1100-H4 Mu75) Discussion Effect of pre-coating on deep drawability. The deep drawability depended on the type of pre-coating. The polyester film decreased the punch load only minimally, while the urethane film decreased it substantially. This result clarifies why urethane film improves the while polyester

5 36 Residual Stresses VII, ICRS7 film does not. FEM simulation (ABAQUAS/Explicit) was performed to elucidate this phenomenon. The metal plasticity models use Hill s yield surface. Hill s anisotropic plasticity potential function, a simple extension of the von Mises function to allow anisotropic behavior, can be expressed as follows: f (σ)= -σ 33) + ( σ 33 σ 11) + H ( σ 11 -σ ) + Lσ 3 + Mσ 31 N F( σ G + σ () 1 The flow curve of specimens is expressed by the simple power curve relation, σ= K ε n (3) where n is the strain-hardening exponent and K is the strength coefficient. The effects of flow stress, the r-value, and the kinetic friction coefficient on deep drawing behavior are analyzed using FEM simulation. Fig. 10 shows one example of the calculated results. The thickness distribution from the center of the blank to the edge of a cup depends largely on the friction coefficient. Increases in the friction coefficient decreased the minimum thickness and thus decreased the. Effect of press forming tooling geometry on deep drawability. In simple plate bending tests, the bending becomes severe as the bending radius becomes small and the bending stress and strain increase at the outer surfaces. The plastic strain at the outer surface passes the strain calculated according to the simple model, and the decrease in plate thickness becomes much larger than expected. The bending strain and stress at the shoulder depend on the r d when a cup is deep-drawn. The tensile stress in the direction of the deep-drawing axis changes due to these phenomena. If the frictional force at the shoulder is neglected, the bending force P b can be expressed as follows. P b = σ y D d π t /ρ (4) Here, σ y : yield stress of the material, D d : inside diameter of die, t: sheet thickness, and ρ: r d +t/. P b for 3mm r d is calculated as three times greater than that for 10mm r d, when the sheet thickness is 0.mm. This means that the tension force in the direction of the drawing axis due to bending increases when the radius of the die shoulder becomes smaller than the sheet thickness. Fig. 11 shows that the maximum punch load increased as the punch shoulder radius became smaller, as expected. This may cause an approximately 7-10% difference in the reduction of sheet thickness in the cup wall. Therefore, the difference in the limiting drawing ratio can be attributed to the difference in r d. The effect of r p can be explained in the same way. Thickness / mm Friction coefficient Material : 1050O Thickness : 0.mm Blank diameter : 73mm Blank hold force : 1.0kN Punch velocity : 10m/s Punch load / [kn] 6 4 r d =3 r d = Distance from center of blank / mm Figure 10 Effect of friction coefficient on thickness distribution Punch stroke / mm Figure 11 Effect of die shoulder radius on punch load-stroke curves (1050-0)

6 Materials Science Forum Vols Effect of press forming conditions on circumferential residual stresses. The residual stress increases and deep-drawn cups become thicker when the deep drawing ratio increases and a lubricant with excellent lubrication is used. The residual stresses increase and punch loads decrease with the increase in radii of the die and punch shoulder. It is thought that formation of the circumferential tensile residual stress is at first due to the compression deformation in the flange. The bending and re-stretching at the die shoulder ultimately determine the circumferential tensile residual stresses at the outer surface and the compressive stresses at the inner surfaces. After deep drawing, the punch load is unloaded, but complete elastic recovery is not possible because of restraining as a container. This relationship can be explained by observing the calculation result of the stress change in the die shoulder due to the change inμ H,where μ is the coefficient of friction and H is the hold-down pressure. Fig. 10 shows an example of the results calculated, at constant hold-down pressure H = 10kN. An increase in μ increases the tensile stress in the direction of the blank radius or drawing axis, and decreases the compressive circumferential stresses. These changes in stresses result in these changes in strain in the direction of the blank radius, and cause reduction in the circumferential residual stresses. As for inferior lubrication, the friction resistance (μh) increases, where μ is the friction coefficient and H is the hold-down pressure. Therefore, it is thought that an increase in μ increases the tensile load which is applied axially during bending and unbending. This tensile load decreases wall thickness and also residual stress. On the other hand, graphite grease, which has excellent lubricant properties, makes the residual stress larger. The change in bending force Pb can also explain the effects of the radii of die and punch shoulders on the residual stresses. As already shown, the smaller the radius, the greater the tensile stresses along radial axis. The pre-coated layer decreases punch load and works as an excellent lubricant, which is thought to bring about larger circumferential residual stress. However, the residual stress obtained is smaller than that expected. This result suggests that factor other than lubricant plays a role in determining residual stress. The strength of the material after pre-coating is found to be smaller than that of base metal as shown in Table. This may explain the above-mentioned result. Summary (1) Pre-coating decreases tensile residual stress, even though it plays as excellent lubricant. () The press forming condition plays an important role in residual stress formation. For example, the improvement of lubrication and the increase in die shoulder radius increase residual stresses. References [1] T. Machida (1995), Press Technology (in Japanese) Vol. 33, No.1 pp.1-7 [] M. Sudo et al, (1997), Proceeding of the fifth international conference on residual stresses, Sweden, pp [3] M. Sudo, T. Oki, and Z. Shibata; Tetsu-to-Hagane (Journal of iron and steel institute of Japan), vol.7 (199), p.64 [4] Y. Osawa et al (1999), Data book on the press formability of pre-coated Aluminum alloy sheets, the Japan Institute of Light Metals, Tokyo, p.105 [5] Y. Shimazaki and K. Saito; Sosei-to-Kako (Journal of the Japan society for technology of plasticity) vol.1 (190), p.46

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