APEM jowatal Advances in Production Engineering & Management Volume 11 | Number 4 | December 2016 | pp 345-354 http://dx.doi.Org/10.14743/apem2016.4.232 ISSN 1854-6250 Journal home: apem-journal.org Original scientific paper Experimental modeling of fluid pressure during hydroforming of welded plates Karabegovic, E.a*, Poljak, J.a aFaculty of Technical Engineering, University of Bihac, Bihac, Bosnia and Herzegovina A B S T R A C T A R T I C L E I N F O The procedure of hydroforming belongs to one of the modern methods of sheet and tube design, usually of complex configuration. Research in the field of plastic forming using fluids usually relates to the analysis of important parameters that would enable high-quality design of elements and execution of the process in stable conditions. The hydroforming process of welded sheets found its application in manufacturing of tanks and other sheet parts in automotive industry, where, in addition to technical and technological characteristics of the obtained piece, it is necessary to achieve stability of the process and its economic feasibility. Experimental research in this paper had been aimed at the analysis of results and modeling of working fluid pressure during hydroforming of welded sheets of two kinds of material (St 37 and Al 99.5) for two sheet thicknesses (1.5 mm and 2.0 mm). Modeling was done by regression method, whose analysis is the determination of functional relationships between a dependent variable and two independent variables. Application of mathematical modeling method enabled working fluid pressure which confirmed the impact of input variables of hydroforming process (yield strength and sheets thickness) onto the values of working fluid pressure. Experimental results obtained for working fluid pressure enabled easier planning and projection of hydroforming process. © 2016 PEI, University of Maribor. All rights reserved. Keywords: Forming Hydroforming Welding sheet metal Fluid pressure Modelling Regression *Corresponding author: edina-karabeg@hotmail.com (Karabegovic, E.) Article history: Received 15 September 2016 Revised 20 November 2016 Accepted 24 November 2016 1. Introduction The development of the automotive industry is based on the emergence of new materials and technologies for their processing. New methods of processing materials enable the achievement of technical and technological characteristics required by the market of the finished product. Hydroforming is the process of forming sheets and tubes during which, by function of fluids underpressure, pieces, most often of complex shapes, are formed for the automotive industry [1-4]. The process is quick, inexpensive and meets the quality of shaped elements. So far, a number of studies have been conducted that included analytical, experimental, numerical, mathematical and other analyses of various treatment processes [5-10]. Hein and Vollertsen (1999) have conducted experimental and numerical research in order to establish the technological and economic characteristics of the process during hydroforming of double sheets [11]. The analysis and comparison of conventional deep drawing and process of sheet element hydroforming by using the finite element method [12] is the area of research of Chang et al. (2001). By application of CCD (Charge Coupled Device) camera, Groche, P. et al. (2007) conducted the control of sealing system during the execution of sheet hydroforming process. 345 Karabegovic, Poljak The control of fluid pressure upon hydroforming of double sheets is an area of research of Assempour and Emami (2009) [13]. In their paper Ertugrul et al. (2009) analyzed hydroforming of laser welded sheets [14]. In addition, Liewald and Bolay (2010) in their paper state the analysis of hydroforming process of double sheets [15]. By application of finite element method (FEM) Zhang et al. (2015) analyzed the impact of stress or pressure of fluids on the improvement of quality of the double sheets hydroforming [16]. As can be seen, there is a constant need to improve the technical conditions and methods for execution of the hydroforming process. This paper gives the analysis of the experimental execution of hydroforming process of welded sheets of two types of materials with different thicknesses. During hydroforming of pieces with defined size and shape, we have measured the working fluid pressure and displacement (expansion) of welded sheets. The experimental results were used in the process of mathematical modeling. Mathematical model for working fluid pressure was obtained for defined conditions of process execution. 2. The hydroforming process Hydroforming of connected sheets is a technique of forming by effect of fluid pressure in the interior of the welded sheets. Thereby, the sheets are deformed (spread) to the interior shape of the die which defines the shape and dimensions of the finished piece. The hydroforming process of welded sheets is used for making fuel tanks, the car doors, etc. Execution process scheme is given in four phases, as shown in Fig. 1: a) Placement of welded sheets into a die (matrix), b) Activation of pressure and pre-forming, c) Calibration with final pressure of the fluid, d) The finished piece is removed from the die. With this process it is possible to form welded sheets of the same or different material and thickness. For the analysis in this paper, we conducted hydroforming of work element of the defined shape. (a) (b) final of forming m i i finished piece M fd) Fig. 1 Scheme of welded sheet hydroforming process 3. Experimental measurement of working fluid pressure The following parameters were defined for the execution of the experiment: • Geometrical shape and dimensions of finished pieces, • Material (sheet) to produce beginning piece, • Method of sheet welding, • Tools for hydroforming and fluid, 346 Advances in Production Engineering & Management 11(4) 2016 Experimental modeling of fluid pressure during hydroforming of welded plates • Pressure system or measuring amplifier device (pump), • System for control and measurement of process parameters: computer and measurement equipment for information gathering. Fig. 2 gives geometric shape and dimensions of the finished piece to be formed [17]. v/ Fig. 2 Geometric shape and dimension of finished piece 3.1 Beginning piece and tools for hydroforming When producing pieces (beginning piece), two types of materials (sheets) were applied, aluminum (99.5 % ) and steel (St 37). Raw parts were produced from cold rolled sheets with thickness: s = 1.5 mm and s = 2.0 mm. Mechanical properties of the material for raw parts are given in Table 1. Table 1 Mechanical properties of materials Types of materials Yield strength, N/mm2 Mechanical strength, N/mm2 Modulus of elasticity, N/mm2 Steel St 37 235 410 2.1106 Aluminium 99.5 100 120 0.7 ■ 104 MIG welding procedure (protective argon gas) was selected for welding of the two materials. Fig. 3 depicts the position of raw part in the tool for hydroforming of welded sheets. Tool for hydroforming of welded sheets was produced from structural steel St 37 and consists of two parts (upper and lower matrix) and connection bolts. Fig. 3 The position of beginning piece in the tool for hydroforming of welded sheets 3.2. Pump for achieving working fluid pressure Hydraulic high-pressure pump pmax = 3-107 Pa was used for the execution of the experiment Working fluid for hydroforming is oil „Inol hidrol-X46", of density 0.884 g/cm3 (20 °C). Advances in Production Engineering & Management 11(4) 2016 347 Karabegovic, Poljak 3.3 Measuring equipment Measuring amplifier device "Spider 8" with eight independent measuring channels was used for the measurement of working fluid pressure and displacement (expansion) of sheets, as shown in Fig. 4. measuring amplifier device "Spider 8" pressure sensor Fig. 4 Measuring equipment sensor feed Applied sensor for the measurement of the working pressure-P8AP, given in Fig. 5, operates on the principle of strain gauges. The nominal sensitivity of the sensor is 2 mV/5-107 Pa. Sensor measuring range is 0-5-107 Pa. Displacement sensor-WA20, shown in Fig. 4, operates on inductive principle. Nominal sensitivity of the sensor is 80 mV/20 mm. Measuring range of the sensor is 0-20 mm. Fig. 5 depicts the position of the sensor for measuring fluid pressure and displacement (expansion) of sheets during hydroforming of welded sheets. Î sensor feed pressure sensor beginning piece ' H CI Fig. 5 The position of sensors during hydroforming of welded sheets and shaped piece 3.4 Number of tests in the experiment Experimental measurements of the defined parameters are aimed at practical application of the results obtained in the planning, design and implementation process. Research and analysis of processes are important for achieving greater process stability in the given circumstances. Number of probes in this experiment was determined by the rotatable plan of the experiment and expression: N= 2li~p + 2k + n0 =nk +na +n0 = 22 + 2-2 + 5 = 13 (1) tool 101 Advances in Production Engineering & Management 11(4) 2016 Experimental modeling of fluid pressure during hydroforming of welded plates where N is total number of experiments, nk is number of change of variables, no is number of repetitions in plan center, and na is number of symmetrically positioned points at plan center. Experiment plan matrix has the shape presented in Table 2. Experiment was conducted in the laboratory of the Faculty of Technical Engineering of the University of Bihac. Table 2 Experiment plan matrix Input variables of the process Physical values Coded values Trial No. Yield strength, N/mm2 (0D.2) Thickness, mm ( si) Xo Xi X2 Xi X2 Xi2 X22 Vector output Yi 1 100 1.5 1 -1 -1 1 1 1 Yi 2 235 1.5 1 1 -1 -1 1 1 Y2 3 100 2.0 1 -1 1 -1 1 1 Y3 4 235 2.0 1 1 1 1 1 1 Y4 5 168 1.75 1 0 0 0 0 0 Y5 6 168 1.75 1 0 0 0 0 0 Y6 7 168 1.75 1 0 0 0 0 0 Y7 8 168 1.75 1 0 0 0 0 0 Ys 9 168 1.75 1 0 0 0 0 0 Y9 10 263 1.75 1 1.414 0 0 2.0 0 Y10 11 72 1.75 1 -1.414 0 0 2.0 0 Y11 12 168 2.10 1 0 1.414 0 0 2.0 Y12 13 168 1.39 1 0 -1.414 0 0 2.0 Y13 4. Measurement results Experimental results of working fluid pressure and displacement (expansion) of sheets during hydroforming of welded aluminum and steel sheets, with thickness of 1.5 mm and 2.0 mm, are given in Fig. 6. Experimental results of working fluid pressure for hydroforming of welded aluminum sheets are given in Fig. 7. Experimental results of working fluid pressure for hydroforming of welded steel St 37 sheets are given in Fig. 8. Comparative results of the working fluid pressure for hydroforming of welded sheets of aluminum and steel with 1.5 mm thickness are given in Fig. 9. Comparative results of the working fluid pressure for hydroforming of welded sheets of aluminum and steel with 2.0 mm thickness are given in Fig. 10. Fig. 6 Experimental results of working fluid pressure and displacement during hydroforming of welded sheets Advances in Production Engineering & Management 11(4) 2016 349 Karabegovic, Poljak Working fluid pressure for hydroforming of welded Al sheets Number of probe Fig. 7 Working fluid pressure for hydroforming of welded aluminum sheets Working fluid pressure for hydroforming of welded St 37 sheets CN ^ 100 2 « 80 o tf 60 S- i*-000000^ Steel 5=1.5 mm Steel 5=2.0 mm £ 40 i-, a. 12 3 Number of probe Fig. 8 Working fluid pressure for hydroforming of welded steel sheets Working fluid pressure for hydroforming Al and St 37 5=1.5 mm Number of probe Fig. 9 Comparative results of working fluid pressure for aluminum and steel sheets with 1.5 mm thickness N 100 B 2 80 S 60 Č 40 m P 20 Working fluid pressure for hydroforming Al and St 37 5= 2.0 mm -Al -Steel Number of probe 103 Fig. 10 Comparative results of working fluid pressure for aluminum and steel with 2.0 mm thickness Advances in Production Engineering & Management 11(4) 2016 Experimental modeling of fluid pressure during hydroforming of welded plates Analysis of the experiment results The analysis of the obtained experimental values of the working fluid pressure for 13 samples (welded sheet 1.5 mm and 2.0 mm thickness, aluminum Al 99.5 % and steel St 37) provides the following conclusions: • Average value of the working fluid pressure to form 2.0 mm thick aluminum sheet compared to the sheet thickness of 1.5 mm is higher for around 10.38 %, • Average value of the working fluid pressure to form 2.0 mm thick steel sheet compared to the sheet thickness of 1.5 mm is higher for around 7.67 %, • Increase in the working fluid pressure in both materials is the result of an increase in the thickness of welded sheets, • The design of welded 1.5 mm thick steel sheet requires higher values of the working fluid pressure for around 42.84 % compared to welded aluminum sheets of the same thickness, • The design of welded 2.0 mm thick steel sheet requires higher values of the working fluid pressure for around 41.12 % compared to welded aluminum sheets of the same thickness, • Increase in the working fluid pressure during hydroforming of welded steel sheets is justified due to the differences in the mechanical properties of steel and aluminum, • Average value of displacement deviations for 13 samples is about 7 %, which is caused by changes in the structure and quality of the sheet. 5. Modeling of working fluid pressure during hydroforming od welded sheets The measured values of the working fluid pressure and displacement after conducted 13 experiments will be used to define a mathematical model that, in the appropriate level of accuracy, adequately describes the hydroforming process of welded sheets with defined shape and conditions of the process execution. The model would be used in the design phase of the process, in analyzing and forecasting the state of the process [18]. The analysis of the stochastic process starts from the general functional relationship between the dependent variable size (7,) and independent variables (x,), which can be presented with the model as follows: Yi f(Xi) f{x1,x2) i.e. Y = p = f(ao.2,s) Coded values of physical quantities are obtained using the following expressions: (2) (3) = 1 + 2 ln< 0.2; In a 0.2; lnah ? —ln