Prediction of Ductile Damage and Fracture in the Single and Multi-stage Incremental Hole-flanging Process Using a New Damage Accumulation Law

Incremental hole-flanging (IHF) is a process in which a sheet with a pre-cut hole is flanged by the single point incremental forming (SPIF) process. Fracture prediction in IHF, such as SPIF, is associated with many challenges due to the deformation mechanisms. The purpose of this paper is to overcome the existing limitations and challenges, and thus, to predict accurately failure in single, and multi-stage IHF processes. To this end, the modified Mohr-Coulomb (MMC) criterion was implemented using an appropriate user subroutine in a finite element method (FEM) model. The AA6061-T6 aluminum alloy sheet, which has low formability, and is fractured from its free edges in the IHF process, was examined as an example. Initially, a linear damage accumulation law, in which the prediction error is high due to the non-linear stress and strain states in the IHF, was used to predict the fracture. Therefore, in the next step, a non-linear damage accumulation function was utilized. While the non-linear accumulation accurately predicts the single-stage IHF fracture, it is not able to predict the fracture well in the multi-stage IHF. It was observed that in multi-stage IHF, the rate of damage accumulation decreases with increasing the number of forming stages. Accordingly, a new non-linear damage accumulation rule was developed. Experimental and numerical results indicated acceptable accuracy of the proposed non-linear accumulation in the fracture prediction in the single and multi-stage IHF process.

Forming a Flanged Hole When Quenching Press-Hardened Steel for Mechanical Fastening

The recent stringent regulations on vehicle safety and reducing CO2 emissions have led to a continuous increase in the application of press-hardened steel (PHS) in automobiles. Similar to other high-strength steels, assembling PHS components using the common welding techniques employed in automotive production lines is significantly difficult because of the surface coating layers and the additives within. This difficulty in post-processing, attributed to its high strength, also limits the mechanical fastening of PHS components. Therefore, this study aims to develop a process for forming a structure enabling mechanical fastening by sequentially applying piercing and hole-flanging operations during the hot stamping process. Our experimental apparatus was designed to perform the hole-flanging operation after the piercing operation within a single stroke at a specific temperature during the quenching process of PHS. At high temperatures of 440 °C or higher, the hole-flanging process was conducted in a direction opposite to that of the piercing operation for creating the pilot hole. An extruded collar with a height of 8.0 mm and a diameter of 17.5 mm was achieved, which is hole expansion ratio(HER) of 82.5%.

Investigation of Punch Shape and Loading Path Design in Hydro-Flanging Processes of Aluminum Alloy Tubes

Hydro-joining is composed of hydro-piercing, hole flanging and nut-inlaying processes. In this study, a new hydro-flanging process combining hydro-piercing and hydro-flanging is proposed. An internal pressured fluid is used as the supporting medium instead of a rigid die. Three kinds of punch head shapes are designed to explore the thickness distribution of the flanged tube and the fluid leakage effects between the punch head and the flanged tube in the hydro-flanging process. A finite element code DEFORM 3D is used to simulate the tube material deformation behavior and to investigate the formability of the hydro-flanging processes of aluminum alloy tubes. The effects of various forming parameters, such as punch shapes, internal pressure, die hole diameter, etc., on the hydro-flanged tube thickness distributions are discussed. Hydro-flanging experiments are also carried out. The die hole radius is designed to make the maximum internal forming pressure needed smaller than 70 MPa, so that a general hydraulic power unit can be used to implement the proposed hole flanging experiments. The flanged thickness distributions are compared with simulation results to verify the validity of the proposed models and the designed punch head shapes.

Revisiting flangeability in hole-flanging by single-stage incremental forming and conventional process

Recently, hole-flanging by single-stage incremental forming (SPIF) has been proposed as a suitable process to perform hole flanges for small- and medium-sized batches with high flexibility in shape. However, this incremental forming has many differences compared with the conventional press working operation in terms of strain and thickness distributions, failure mechanisms and flangeability measures. In fact, regarding the evaluation of the formability of the flanges, the classical Forming Limit Ratio (LFR) should be used with care to quantify this property in hole-flanging by SPIF. Additionally, the FLC (Forming Limit Curve for necking) and FFL (Fracture Forming Limit) curves, powerful tools for analysing sheet failure in practice, may also yield erroneous prediction of necking in conventional press working or fracture in SPIF. The aim of this work is to present a comparison and analysis of the formability of hole flanges performed by SPIF and press working in AA7075-O sheets. Two complementary parameters to the LFR to compare the flangebility in both operations are discussed, along with the influence of bending induced by the forming tool and the stress triaxiality in the evolution of the principal strains during the forming process. The results point out the limitations in the current practice.

Control-Oriented Characterization of Product Properties during Hot Hole-Flanging of X46Cr13 Sheet Material in a Progressive-Die

Robust and versatile production is enabled by a closed-loop control of product properties. This essentially relies on the characterization of the interaction between properties and available degrees of freedom to control the process. In particular, this work examines the setting of collar height, thinning, curvature, and hardness during hot hole-flanging of X46Cr13 sheet material with simultaneous heat treatment to identify approaches for a closed-loop property control in hot hole-flanging during multi-stage hot sheet metal forming. To scrutinize the adjustability of the hardness of X46Cr13 sheet material by heat treatment with rapid heating and short dwell times, quenching tests with austenitizing temperatures from 900 to 1100 °C and dwell times from 1 to 300 s were carried out. A hardness between 317 and 680 HV10 was measured. By analyzing the force-displacement curve and the contact situation between tools and blank during hot hole-flanging, an understanding for the process was established. To determine the adjustability of geometrical collar properties and the hardness of the collar, collars were formed at punch speeds between 5 and 100 mm/s and at different temperatures. Here, a dependency of the geometry of the collar on temperature and punch speed as well as setting of the hardness was demonstrated.