Experimental implementation of the multipoint hydroforming process
Naceur Selmi1, Hedi BelHadjSalah2 1,2
Mechanical Engineering Laboratory (LGM), National Engineering School of Monastir (ENIM), University of Monastir,Avenue Ibn El Jazzar 5019 Monastir, Tunisia. 1
[email protected].
2
[email protected].
Abstract. Abstract The process of flexible hydroforming is a combination between the hydroforming and the multipoint flexible forming, which allows a synergy of the advantages of two processes. On one hand, the hydroforming process allows a contribution in the flexibility by replacing one of two shaping tools by a fluid, on the other hand, the multipoint flexible forming, allows modifying freely the final shape with its reconfigurable tool, constituted by a matrices of adjustable punch elements. This innovative process presents a potential interest by accumulating at once the advantages of hydro-forming and flexible multipoint hydroforming. The purpose of this paper is to present the process review and its experimental implementation to highlight the contribution in flexibility and to validate the feasibility of the multipoint flexible hydroforming and its ability to produce of complex metal sheet part with improved quality. Keywords: Hydroforming, Multipoint, Flexibility, Sheet metal forming.
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Introduction
Advanced industries are called to produce lighter and more complex sheet metal constituents with improved structural strength, and thinner profiles, with improved quality, as well as lower tooling costs, more polyvalent and resourceful forming tools, especially for industries with small or medium lot-size and with great varieties (aerospace industry, shipbuilding). To satisfy these requirements, several innovative and flexible processes had been developed these last decades, among these processes, the hydroforming and the multipoint flexible forming are the processes which carry most of interests. The hydroforming process is attractive compared with conventional solid die forming processes, the basic advantage consist to suppress one of two forming tools (punch or die), which is replaced by hydraulic pressure.
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An increasing interest was observed for the hydroforming process, various versions, were progressively proposed, for the production of lighter structures and complex forms The multi-point flexible forming (MPF) is another recent flexible technique for manufacturing three-dimensional sheet metal parts. In this process, the sheet metal can be formed between a pair of opposed matrices of punch elements instead of the conventional fixed shape die sets. The punches elements are controlled simply, by adjusting the height of the elements of both upper and the lower matrices, different curved surfaces can be created (figure1). By using this technology, production of many parts with different geometry will be possible just by using one same die set and the need to design and manufacturing of various die will be avoided that lead to great saving in time and manufacturing cost specially in the field of small batch or single production. Various versions of multipoint flexible forming were elaborated particularly in the naval and aerospace field, the investigations of Robert C. SCHWARZ [1], Ming-Zhe Li [2], [3], Yan [4], Zhong-Yi [5] and Hwang [6], concerned the flexible multipoint forming and its adequacy for a production of lighter structures and complex forms. However in multipoint processes, the direct contact between the blank and punch elements generates a severe dimpling on the final part. The insertion of elastomeric sheets (interpolator), between dies and blank, has been an efficient solution to attenuate dimpling severities [7, 8, 9, and 10]. Multipoint sandwich flexible forming (MPSF) (figure 2), has been another innovative version of the multipoint forming in this process, the movable multipoint die is substituted by a stack of elastomeric sheets, under the slide of the press, and one thick die sheet is inserted between lower multipoint die and interpolator to reduce dimples [7, 8, 9], but it is often necessary to adapt the shape of this stack to the depth of the part to be produced.
Fig. 1 Multipoint forming process.
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Fig. 2 Multipoint sandwich flexible forming (MPSF).
Presentation of the process
The multipoint flexible hydroforming (MPFH), object of this paper, is an original process which combines the hydroforming and the multipoint flexible forming, to obtain a synergy of the advantages of both processes. It allows to keep the
Experimental implementation of the multipoint hydroforming process
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whole flexibility of the basic multipoint flexible forming (with two multipoint discrete dies), by using uniquely one multipoint die to perform completely the final part shape, the second multipoint die is advantageously substituted by the fluid pressure, which can be applied via an elastomeric membrane (Figure 3). 2.1 Testing set up of the multipoint flexible hydroforming. To prove the feasibility and to carry out a valuable experimental investigation of the multipoint flexible hydroforming, an experimental prototype was designed and realized. The testing assembly of the process includes three basic parts. The hydroforming pressure, produced in the upper module, is applied, via the elastomeric medium, to the metal sheet workpiece, located at the middle module, to be conformed to the shape of the multipoint die (fig.4) in the lower module. The upper fluid cell module constitute the hydroforming tool; it performs the first aspect of process flexibility the second one is performed by the reconfigurable multipoint forming die in the lower module. During the forming phase of the metal sheet between the last two forming modules, upper’s or highest punch elements extremities, entering firstly in contact with formed sheet, produce extremely concentrated pressure that usually initiates forming dimples in the contacted sheet. To attenuate dimpling phenomena, the formed sheet is separated from multipoint tool by a medium sheets stack generally called interpolators. Through the thicknesses of sheet stack, localized high pressures are rearranged, the maximum of concentrated loading is moved from worked piece interface to that of interpolator stack, and local pressure loading is then better distributed at the worked sheet interface. The basic object of relocation of the concentrated contact loading is to moderate severities of the load boundary conditions that lead to better regularity and less dimpling of final part profile.
Fig.3: Multipoint flexible hydroforming process.
Fig.4: Multipoint die set up for (MPFH).
Considering the phenomena complexity, of multiple interactions between multipoint tools, interpolators and work piece for the multipoint flexible hydroforming process, analysis are focused, in recent investigations[12,13,14], on the most influent parameters on the quality of the final product. It emerges essentially, that an
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increase of the punch elements density, the thicknesses of the initial blank and the interpolator, improve the final part quality, with better thicknesses distribution, reduced residual stresses and dimples. The object of this paper consist to prove the feasibility and to carry out a valuable experimental investigations of the multipoint flexible hydroforming, and to highlight the ability of this process in flexibility and quality upgrading of final product. The attention is focused on a version using metallic die sheet medium instead of the elastomeric interpolator (conventionally inserted between multipoint die and blank side). The basic idea is to move, the boundary conditions severities, from soft elastomeric sheet to stiff metallic plate interpolator, this last one can be considered as a fast rigid die, formed (in situ) by the multipoint hydroforming tools, only in one step, it can be used for many thin shell formed parts, the shape of this fast rigid die can be corrected and readjusted to take in account the spring back, moreover this die sheet can be used for another final part shape.
3 Finite element analysis. ABAQUS/Explicit was used for Finite element analysis of the multipoint flexible hydroforming process presented in figure 4; the final part shape to be formed was a double curved shell with different depth, the shape function of desired final shape can be written in Cartesian coordinate system as follows: z (x,y) = ∑ i ,∑j aij.(xi.yj). (1) For examples: z = a.(x² + y²): Parabolic shape.
(2)
z = a.(x² - y²): Saddle shape. (3) The discrete die size was 100 mm x 100 mm, the density of punch elements setting was (11x11 and 21x21), many cases of blank and interpolator sheet thicknesses was used for analysis. The sheet stack (interpolator and blank) are simply positioned directly on the punch element matrix, as seen in figure 4, with free edges conditions.
Fig. 4: Geometry model of Multipoint flexible hydroforming process.
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The shape function (Analytical field) law can be settled before forming step of the process (fixed multipoint method) or progressively modified, throughout the forming process (progressive method). The fluid pressure was applied on the upper side of the blank in a linear smooth load path. The elasto-plastic material model was used for deformable steel sheets (Stainless steel material (E = 210 000 Mpa, v =0.3) with strain-stress curve presented in figure 5. The Mooney–Rivlin hyperelastic material model was used to elastomeric interpolators, this model was built from the uni-axial test data presented in figure 6 from the reference [7]. Dynamic/explicit method was used for the simulation, the deformable shell parts instances (blank and interpolator) are meshed in S4R elements, the rigid instances (punch element matrix etc.) are meshed in elements R3D4. The Coulomb law was used with friction coefficient of 0.1 for general contact interaction between constituents of the model.
Fig. 5: Stress–strain curve of Stainless steel.
Fig. 6: Uniaxial data of hyperelastic material.
3 Results discussions. 3.1 Effect of the process parameters. 3.1.1 Effect of the sheet thicknesses. The effect of blank thickness can be observed from the simulation results (Fig. 7), for lower thickness, all surface of final part is severely dimpled and the increase of thickness reduces dimpling and improves the profile regularity and the surface quality. This effect is confirmed by experimental way (fig. 8), by producing many doubly curved parabolic (shape part eq.(2)) for aluminum alloy sheet part produced, severe dimpling effect is observed for thin sheet (0.5 mm) with irregular profile and buckled edges, for relatively thicker sheet (2mm) successful sheet part is obtained with more regular profile with no significant dimples.
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Fig 7: Dimples: effect of blank thickness.
Fig 8: Dimples: effect of blank thickness (experimental results for 0.5mm and 2mm).
For a fixed blank thickness, the increase of the elastomeric interpolator thickness (fig.9) reduces slightly dimpling severities.
Fig. 9: Dimples: effect of interpolator thickness.
3.1.2 Punch elements density. Basic set up of multipoint die with 121 punch elements (11x11) was used for simulation and experimental investigations carried out in this work, buckling phenomena on the median edges of formed blank are occurred for the 1mm initial blank and 1mm elastomeric interpolator thicknesses, the increasing of punch density to 441 punch elements (21x21) (figure11) reduces buckling, residual stresses and improves profile regularity of final product, but increases largely the complexity of multipoint tool adjusting.
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Fig. 11 Dimples and edge buckling: effect of punches density.
4 Interest of the metallic die sheet The motivation here is to highlight ability of specific set up of the process, in flexibility and quality upgrading of final product, especially for thinner metal sheet. In this new version we will insert a metallic thick die sheet (fig.12), instead of the elastomeric interpolator conventionally inserted between multipoint die and blank side, the combination of both methods can be considered also. As illustrated in last results (figure 7 and 8), it was observed, more regular profiles and less sensitivity to edge buckling and dimpling, for thicker blanks. In other terms, for a given deep and punch density, thin blank is more sensitive for edge buckling and dimpling, it have tendency to flow around punch tip curvatures, and elastomeric interpolator have not sufficient stiffness to filter punch contact singularities. The basic idea is to move the boundary conditions severities, from soft elastomeric to thick and stiff metallic plate medium, that acts as a stiff interpolator filter towards contact singularities and improves ability to form a thin shell final part as seen in simulations results (fig.13 ). This method is validated by experimental way as illustrated in (fig.14), the aluminum alloy final product of 0.5mm is severely dimpled (without or with elastomeric interpolator), with 2 mm aluminum die sheet, better quality of outer edges and suppression of dimples are obtained fig . This confirm that a using of metallic die sheet medium is largely more relevant for thinner sheet product than increasing of punches density or elastomeric thickness The stainless steel final products of (without or with elastomeric interpolator of 4 mm thickness,) are significantly dimpled and buckled at the outer regions), better quality of outer edges and suppression of dimples are obtained with an aluminum alloy die sheet medium of 2mm (fig.15). Instead of request of costly increased number of punches for relatively thinner blank, the use of metallic sheet media is better and efficient way to eliminate significantly dimpling and edge buckling with reasonable density of punch elements. The solid metallic sheet medium made in elastoplastic material can be considered as a fast rigid die, formed (in situ) by the multipoint hydroforming tools, only in one step, it can be used for many thin shell formed parts, the shape of this fast rigid die can be corrected and readjusted to take in account the spring back or to re-
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use it another final part shape. The metallic medium die can be made in low cost steel or aluminum alloy for weightiness consideration.
Fig. 12 MPFH with metallic medium.
Fig. 13 a: elastomeric interpolator, b: metallic media.
Fig. 14 Aluminum alloy products: left: elastomeric interpolator, right: metallic media.
Fig. 15 Stainless steel product: left: With elastomeric interpolator, b: With metallic media.
Fig.14 Thickness strain reduction.
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Fig.15 Thickness distribution improvement.
Conclusions
The multipoint flexible hydroforming (MPFH) is an innovative sheet forming process which combining the hydroforming and the multipoint flexible forming, to obtain a synergy of the advantages of both processes. It allows keeping the whole
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flexibility of the basic multipoint flexible forming (with two multipoint discrete dies), by using only one multipoint die to perform the final part shape, the second die is substituted by the fluid pressure. This way allows a significant improvement in flexibility and quality to produce a diversity of shell product geometries with only one multipoint die. The feasibility of the multipoint flexible hydroforming is validated by the prototype of the process designed and realized by authors and the results of experimental investigations carried out are in good agreement with previous numerical analysis of the process, successful doubly curved shell product was successfully obtained by the process. This process is more accurate and low cost technology with great saving in time manufacturing especially in the field of fast prototyping, small batch or single production. Using of metallic sheet media is more efficient way to eliminate dimpling and edge buckling and to extend flexible multipoint hydroforming quality for thin sheet products with reasonable density of punch elements. The multipoint flexible hydroforming is an adequate mixing of the benefits of multipoint flexible forming and hydroforming processes, by using a metallic interpolator media, the versatility of the new process is extended to produce thin sheet product with improved quality. By this method the process can be quickly assorted for thin or thick shell parts; the result is an improved resourceful process with upgraded quality and reduced number of components.
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