Showing papers by "Taylan Altan published in 2018"

Improving Hole Expansion Ratio by Parameter Adjustment in Abrasive Water Jet Operations for DP800

Abstract: The use of Abrasive Water Jet (AWJ) cutting technology can improve the edge stretchability in sheet metal forming. The advances in technology have allowed significant increases in working speeds and pressures, reducing the AWJ operation cost. The main objective of this work was to determine the effect of selected AWJ cutting parameters on the Hole Expansion Ratio (HER) for a DP800 (Dual-Phase) Advanced High-Strength Steel (AHSS) with s0 = 1.2 mm by using a fractional factorial design of experiments for the Hole Expansion Tests (HET). Additionally, the surface roughness and residual stresses were measured on the holes looking for a possible relation between them and the measured HER. A deep drawing quality steel DC06 with s0 = 1.0 mm was used for reference. The fracture occurrence was captured by high-speed cameras and by Acoustic Emissions (AE) in order to compare both methods. Results indicated that using, regardless of the material, a small standoff distance, high water pressure, and slow traverse speed and cutting the sample underwater will delay the fracture in a hole expansion operation. Furthermore, the AE have proven to be adequate to measure cracks when optical methods are not feasible. In conclusion, based on the impact of the aforementioned parameters, it is possible to select, appropriately, the AWJ operation parameters to achieve the edge stretchability required for each forming process. Downloaded from SAE International by David Diaz-Infante, Tuesday, October 02, 2018

Design of Progressive Die Sequence by Considering the Effect of Friction, Temperature and Contact Pressure

Abstract: Progressive and transfer dies are used for forming of sheet metal parts in large quantities. For a given part, the design of progressive die sequence involves the selection of the number of forming stages as well as the determination of the punch and die dimensions at each stage. This design activity is largely experience-based and requires prototyping involving several trial and error operations. In some cases, empirical data and the experience based design procedure can be combined with Finite Element Method (FEM) based analysis to reduce time and cost. Often, when using FEM in progressive die design, friction and its effect upon temperatures is not adequately considered. However, at each forming station the plastic deformation and the tribological conditions influence the material flow as well as the temperatures and pressures at the tool/workpiece interface. The performance of the lubricant and coolant, used in progressive die forming, is affected significantly by interface pressure and temperatures. Therefore, a progressive process and die design methodology should include the consideration of metal flow as well as temperatures and pressures. Heat transfer coefficient, friction, plastic deformation, forming speed at each forming stage, time for part transfer from one stage to the next, and the ability of the used lubricant to cool the dies, have considerable effect upon a successful stamping. This paper describes a method for designing a progressive die sequence for forming axisymmetric sheet metal parts. The methodology for process sequence design combines experience based empirical data obtained through previous designs, design rules and numerical simulations including plastic deformation and friction. The initial experience-based design was refined using FEM and the thinning of the material in each successive drawing stage was calculated. The thermo-mechanical model was obtained using a constant friction coefficient along the tool/workpiece contact zone. Finally, the tool/workpiece interface temperature and the normal pressures were estimated in order that the lubricant can be selected based on these process conditions. The design predictions, made by using empirical data and FEM, were compared with experimental data.

Determination of variable E-modulus through wipe bending test: application to springback prediction

Abstract: Nonlinear elastic behavior and degradation of the E-modulus with increasing plastic strain in advanced high strength steels makes springback prediction more challenging. The conventional method for determining the E-modulus degradation with plastic strain is the loading-unloading-loading tensile test. This paper proposes a new methodology to determine E-modulus variation using a wipe bending operation. During wipe bending, the sheet material experiences simultaneous tension and compression loading through the sheet thickness, so the test conditions closely emulates actual metal forming conditions. Wipe bending tests for 1.2 mm MP980 steel sheet samples were conducted using different bending angles and springback was measured for each sample. A finite element model of the bending process was also developed. A constant apparent E-modulus was determined for each bending angle by comparing the springback predicted by the finite element model with the springback measured during the wipe bending test. Average effective strain was also calculated for each bending angle using FE simulations. A curve relating the E-modulus variation to effective strain was developed by correlating the apparent E-modulus and the average effective strain at each bending angle. Inputting this curve into the FE simulation revealed that springback prediction improved significantly compared to the case of using a constant E-modulus.

Effect of E-Modulus Variation on Springbackand a Practical Solution

Abstract: Springback affects the dimensional accuracy and final shape of stamped parts. Accurate prediction of springback is necessary to design dies that produce the desired part geometry and tolerances. Springback occurs after stamping and ejection of the part because the state of the stresses and strains in the deformed material has changed. To accurately predict springback through finite element analysis, the material model should be well defined for accurate simulation and prediction of stresses and strains after unloading. Despite the development of several advanced material models that comprehensively describe the Bauschinger effect, transient behavior, permanent softening of the blank material, and unloading elastic modulus degradation, the prediction of springback is still not satisfactory for production parts. Dies are often recut several times, after the first tryouts, to compensate for springback and achieve the required part geometry. In this study, the effect of Young’s modulus (E-modulus) on springback is investigated. Current challenges in determination of E-modulus through tensile test are discussed and a practical method is proposed which has the potential to improve springback prediction after the first die tryout. In this method, the unloading elastic modulus is adjusted by measuring the springback of the part produced during the first tryout and comparing it with finite element (FE) simulation results. The unloading elastic modulus obtained from this method is called the “apparent E-modulus”. This method is applied to three bending cases: a wipe bending, a U-drawing, and a 3-D crash forming of an actual production part. Results show that the springback can be predicted fairly accurate using the apparent E-modulus and a simple isotropic hardening model.