Formability Studies on Transverse Tailor Welded Blanks

Transcription

1 Formability Studies on Transverse Tailor Welded Blanks V.Vijay Bhaskar and K.Narasimhan * Dept of Metallurgical Engineering and Materials Science Indian Institute of Technology, Bombay, Powai, Bombay , India *Corresponding Author; nara@met.iitb.ac.in Abstract. Tailor Welded Blanks (TWB) technology is one of the several approaches that have been used to reduce the weight of the automobile body. TWBs are made up of two or more blanks having different/same properties (geometry, material etc.) prior to forming. The formability of these blanks depends on material and geometric parameters like strength ratio and thickness ratio. The study of these blanks can be classified on the basis of the weld orientation chosen viz. transverse weld or longitudinal weld with respect to the major straining direction. This paper studies the formability issues related to transverse TWB by FE simulation. The formability is assessed by analyzing tensile and Limit Dome Height (LDH) tests. The weld region is assumed to be a line in all the simulations. While modeling the tensile test, ultimate tensile strength (UTS) and elongation are monitored, and in LDH testing, pole height and maximum load (in near plane strain condition) are monitored. LDH testing shows that as thickness ratio increases, the load bearing capacity and the pole height decreases. There is a contribution from both the thicker and the thinner blank to the overall deforming volume. Failure location analysis shows that there is an abrupt change in the location of the failure from punch nose region to weld line region as the thickness ratio reaches a critical magnitude (1.08). The study of material properties shows that as the yield strength ratio (S ratio) and strain hardening exponent ratio (N ratio) between the blanks increases, the maximum load which the blank can sustain without failure (UTS) increases. This becomes constant and comparable to that of single sheet at higher N and S ratios. INTRODUCTION Tailor welded blank (TWB) comprises of two or more sheets welded together in a single plane prior to forming. This technology came into practice in mid 80 s and since then its application in automotive component industry is progressing at a rapid rate. Some of the advantages are weight reduction, reduction in processes and materials requirement due to presence of sheets of varying thickness, reduction in scrap generation, improved tolerances, increased stiffness, requirement of fewer parts and optimized use of steel properties. The suitability to tailor welded product is judged by its formability studies. The formability studies of the tailor welded blanks provide a validation for the applicability of this technology in the automobile industry. Broadly, the formability depends on two important types of parameters namely material parameters and process parameters. The important material parameters [1] are n (strain hardening exponent), m (strain rate sensitivity) and r (plastic strain ratio). The second factor on which formability depends is the process involved. Different types of process like stamping, deep drawing etc differ in the value of the blank holding force, friction coefficient, number of drawbeads etc. Formability studies of tailor welded blanks differ from the normal sheet metal, because here the variation in thickness and material of blanks welded and various other issues related to weld region like weld orientation, weld bead width, weld position etc, need to be considered. Formability of tailor welded blanks depends on type of the welding process employed, weld orientation to the major straining direction, welding parameters, weld location, material/thickness difference and forming processes. Scriven et.al [2] compared the forming limit curve by varying the rolling direction in both the longitudinal and transverse loading cases. It was found that the case of unwelded sheet with rolling direction as X- direction has relatively higher FLC as compared to longitudinally welded sheets with rolling directions as X and Y respectively. Heo et al [3] conducted a set of experiments on controlling the weld line movement by optimizing drawbeads. In order to investigate the effect of drawbead dimensions, single circular drawbeads of 0.0, 1.5, 2.0, 3.0, 4.0 and 5.0 mm radius were prepared and were installed on the thinner side parallel to the weld. 699

2 In all the cases, it has been found that the weld line moves in the thicker part of TWB in the punch head area, while it moves to the thinner parts in the flange area, due to the difference in the formability and the strength in the two parts. Choi et.al [4] conducted a set of experiments to study the effect of initial weld line position on the weld line movement during the deep drawing operation. From the results, it has been concluded that larger the difference of thickness and the longer the distance of the initial weld line, the more dislocated is the weld line from the initial position measured from the center of circular specimen. This paper deals with the study of formability of TWBs using Uniaxial Testing and LDH testing. progression curve for the first case is shown in Figure 1. As the thickness ratio is increased, there is a decrease in the strength as well as ductility of the tailor welded blank. Elongation shows a declining trend as the thickness ratio is increased. This can be attributed to the fact the greater the thickness ratio, more uneven is the distribution of the load between the blanks. METHODOLOGY For the Uniaxial testing, a model of the blank is developed using Solid works (CAD) and imported to FE based software OPTRIS. One part of the blank is fixed by creating a plane normal to it and node velocities are assigned to nodes on the opposite edge of the blank. Solver is used for simulation of this set up and results are obtained. In this tensile testing model of TWB, effects of parameters that can be simulated are: Rolling direction, Thickness ratio, Base material properties and Weld orientation. In LDH testing, a model of the blank, blank holder, punch and die are prepared in Solid works and imported to OPTRIS. Using the process setup, all the parts are aligned along one axis. Simulation is carried out using solver and the results are analyzed. In the LDH model of TWB, effect of following parameters can be simulated: Rolling direction, Thickness ratio, Base material properties and Weld orientation. FIGURE 1: Effect of thickness ratio on load progression curve of tailor-welded blanks in case of transverse weld when thicker blank thickness is kept constant Maximum load that the TWB can sustain also decreases when the thickness ratio is increased. This is to be expected since the inhomogenity in thickness increases. RESULTS AND DISCUSSION The TWBs are subjected to different conditions and are tested for Uniaxial Tensile Testing and Limited Dome Height Test (LDH). Two SPCC blanks of initial thickness 1.2 mm are taken and the variation of the thickness ratio is studied by gradually reducing the thickness of one of the blanks keeping other constant. Thickness ratio is varied from 1 to 1.5 in two ways. In first case the thicker blank is kept constant and the thickness of thinner blank is varied and in second case the thinner blank is constant and thicker blank is varied. The load FIGURE 2: Effect of thickness ratio on load progression curve of tailor welded blanks in case of transverse weld when the thinner blank thickness is kept constant. When the thickness of the thicker blank is varied as shown in Figure 2, similar trends are observed. In case of transverse weld, since the failure occurs in the thinner section, varying the thickness of thinner blank shows a change in 700

3 elongation. Since the thinner material is kept constant in this second approach, there is no significant change in the peak load. Another way of monitoring the effect of thickness ratio is by varying the weld line position. Since in a transverse weld, the thinner blank is the one where the failure occurs, by increasing its percentage in the overall TWB, its contribution to the overall deformation can be estimated. For this purpose, the weld position is varied from 10 mm length to 55 mm length, such that the contribution of the thinner blank to the overall TWB increases. The load progression curves in all the cases are compared in the Figure 3. As can be seen from Figure 3, the load bearing capacity decreases. Quite similar to our expectations, it is found that as the contribution of the thinner blank to the overall TWB increases, the behaviour becomes similar to that of a single sheet of thinner blank (0.8mm). OLI in 11 mn sheet 10 m -OJB2 - -fijm -CLOfl -QJ2 - Dl:a*a:r,c* aieiig Wm blank Qmg FIGURE 4: Thickness strain distribution along the blank perpendicular to the weld in TWBs for different weld line position. (1) TWB (2) TWB (3) TWB (4) TWB (5) TWB (6) 0.8 mm sheet (7) 1.2 mm sheet (8) 5-55 TWB The effect of material properties on TWBs can be studied based on the n-ratio and the S-ratio where n represents the strainhardening coefficient and S represents the yield strength. When two sheets of different 'n' and yield strength values are welded together to form a TWB, the one with lower yield strength will undergo more deformation than the other one. Thus, there is more chance for failure to occur in that blank. Transverse weld is considered in all the simulations. Material combinations for forming TWB are selected based on their strain hardening coefficient (n) value and strength (yield strength) value. TWB formed by those combinations are simulated and the load progression curve is compared for them. FIGURE 3: Load progression curves obtained by varying the weld line position across a TWB (1.2mm/0.8mm) For a better insight, thickness strain distribution is monitored along the cross-section of the TWB for each of the above mentioned cases (shown in Figure 4). As the contribution of thinner blank increases, a change in thickness strain is observed in the thinner blank only, while a minimal change is observed in the thicker blank. This shows that the thinner blank is dominant in the overall deformation behaviour. Thus, the effective length in a transverse TWB is the length of the thinner blank. Thinner blank is the one which will decide the properties of the whole TWB. Thicker blank is behaving as a rigid support to the thinner blank. i "I.: & m W Nrittep?p«ni2J S1 V SS Sfiitio: :! SOW S182 9i22 O.2?S 0a : ;«6:1S::- ;0;22 -:9J B: O',78": MSI '&82: : ft-s4; $38; ^'l^sa: f;4. FIGURE 5: (a.)the load progression curve for Albased alloys showing the effect of N and S ratio when TWB is formed by aluminum alloys combination (b) The properties of both the combinations taken. When TWB is formed by taking 6009 and 5182 combination and 6009 and 6016 combination, load progression shown in Figure 5 701

4 is obtained. In both the plots values are considered till UTS only. Thickness of both the blanks is 1.2 mm. Presently, it is observed that in case of aluminum alloys, when the N ratio and S ratio is increased, ductility as well as strength of TWB decreases. It is because 6016 has lower n and yield strength value than Lower n means less strain hardening or in other words less ductility. Consider the TWB formed by taking steel combinations as shown in Figure 6. There is not much difference in the N ratio but there is a significant difference in the strength values. Thus, there is a little difference in the strength values of the TWB so formed. There is no significant difference in the % elongation of both the combinations, which is quite expected since the n values of SPCE and st!4 are almost the same. increased, which causes the early failure of the TWB. TABLE 1: Comparison of pole height when the thickness ratio is varied from 1 to 1.5 Thickness Pole Height (mm) Ratio A clearer picture of the decreasing pole height with the increasing thickness ratio can be obtained when the load progression curves are plotted for all the above mentioned cases as in Figure 7. Pole height decreases as thickness ratio increases. TO n sheet 1.2 nmi FIGURE 6: (a.)the load progression curves for steels to show the effect of N and S ratio when TWB is formed by steel combination (b) The properties of both the combinations taken The effect of thickness ratio on the LDH properties of TWB is studied for transverse weld. The thicker section is kept constant and thinner sheet is varied with the materials of the sheet being the same. In a LDH testing, there are two strain paths which will be studied. One is biaxial stretching and the other is plane strain condition. As can be seen from Table 1, there is a decrease in pole height as the thickness ratio is increased. This is due to the uneven distribution of the load between the sheets as the ratio is FIGURE 7: Load progression curves for TWBs with different thickness ratios subjected to LDH testing (TR: Thickness Ratio) This occurs because effectively the deforming volume becomes smaller and smaller as the thickness ratio increases. It clearly indicates that the TWB has lesser pole height than the single sheets, which again supports the earlier observation that the formability of TWB is lower than that of the single unwelded sheets of base metals. As the thickness ratio is increased, there is an increasing tendency of failure near the weld region in the thinner blank. This is quite expected since at higher thickness ratio, more will be the inhomogenity in thickness and so load distribution will not be uniform. 702

5 n m m m m FIGURE 8: Variation of maximum load with the percentage length of thinner blank in TWB A similar study as was performed in tensile testing is conducted to see the contribution of the thinner blank to the overall deformation of TWB. Figure 8 shows the variation of the maximum load with position of weld line. As can be seen from the figure, maximum load becomes almost constant as the weld line moves towards the ends (consider TWB, TWB, TWB, and TWB) and is lowest for the TWB case, when the weld line is exactly at center. 1*5 3 at "5ft I IS-!«- & fill. * * ± * * * * A a 10 3D m m m m TO m m im Percentage iengtfi of thinner biiok h TWB f%j FIGURE 9: Variation of Pole Height with percentage length of thinner blank in TWB Pole height shows more or less the same trend as was seen in the maximum load case. Pole height is lowest in the TWB case out of all the cases considered as shown in Figure 9. Unlike the case in the tensile testing, in LDH test, the thinner blank as well as thicker blank contributes to the overall deformation. Failure generally occurs due to friction in a LDH test of an unwelded sheet. But when LDH test is performed on a TWB, then there are two possible reasons for failure, i.e. due to friction and the inhomogenity in the thickness between the blanks. The inhomogenity in thickness of the two blanks forces the crack to occur at weld line for higher thickness ratios. At lower thickness ratio, the TWB acts similar to a m Distance itafig the crd5s-s5cttofii;mmi FIGURE 10: Thickness distribution of TWBs with thickness ratio 1.07, 1.08 & 1.09 along the crosssection of the blank single blank and thus the failure is primarily due to friction between the. blank and the punch. Thus, there exists a critical thickness ratio above which the inhomogenity in thickness between the blanks is responsible for failure. The variation of thickness along the major strain direction for three different thickness ratios is presented in Figure 10. From this it can be observed that if the thickness ratio is below 1.08, then failure occurs in the punch nose region, but in other cases, the failure is seen at weld location. CONCLUSIONS In the present work, formability of Transverse Tailor welded blanks is studied using two tests, namely, uniaxial tensile testing and LDH testing. In uniaxial tensile testing, extensive study of effect of thickness ratio and material properties has been done. In LDH testing, simulations have been done to study the effect of thickness ratio on the formability of TWBs. The conclusions from the results obtained have been summarized below. Uniaxial tensile testing Effective length in transverse TWB is the length of the thinner blank. Thicker blank merely acts as a clamping support to the thinner blank. Thinner blank will be the one which will decide the properties and behaviour of the TWB. If the Y.S ratio and n-ratio are high, then the ductility is significantly affected. LDH testing As thickness ratio increases, load bearing capacity as well as pole height decreases. At high thickness ratios, these values become almost constant. 703

6 Thickness strain plot indicate that there is deformation in the thinner as well as the thicker blank unlike tensile testing, where effectively only thinner blank was contributing to the overall deformation. The pole height and maximum load are minimum, when the weld line is exactly at center. Failure location analysis shows that there exists a critical thickness ratio above which failure location changes from punch nose region to near weld line region. This critical thickness ratio is REFERENCES [1] Sheet formability testing, Brian Taylor, Staff Research Scientist, General Motors Cooperation. ASM Handbook, Mechanical Testing, 9th Edition, Vol 8, pp [2] P. J. Scriven, J. A. Brandon, N. T. Williams Relative influence of sheet rolling direction and weld orientation on formability of laser welded steel sheet, Ironmaking and steelmaking, Vol 24, No.1 (1997), pp [3] Y. M. Heo, S. H. Wong, H. Y. Kim, D. Seo The effect of the drawbead dimensions on the weld line movements in the deep drawing process of tailor welded blanks, Journal of Materials Processing Technology. Vol 113(2001); [4] Y. Choi, Y.M. Heo, H. Y. Kim, D. Seo Investigations of weld line movement for the deep drawing process of tailor welded blanks Journal of Materials Processing Technology, Vol 108 (2000) pp