Forming Limit of Holed Dome in Two-Stage Stretch Forming

This paper deals with the accurate prediction of forming limit of a dome with a circular hole in two-stage stretch forming. Since this two-stage forming is a process of non-proportional deformation, a fracture criterion, which is capable to predict fracture limit of sheet metal under non-proportional strain path, is introduced. The limit dome height of steel blank is predicted by utilizing numerical optimization technique, in which the fracture criterion is used as a constraint function to avoid sheet breakage. The calculated result and the corresponding experimental result show that the fracture criterion can successfully predict the limit dome height in the two-stage forming while the classical forming limit diagram (FLD) overestimates it.

Improvement of Formability and Spring-Back of AA5052-H32 Sheets Based on Surface Friction Stir Method

A process to improve formability and spring-back was developed for AA5xxx-H temper sheets based on the surface friction stir (SFS) method. In the SFS method, a rotating probe stirs the sheet surface so that material flow and heat, which result from plastic deformation and friction, change the microstructure and macroscopic mechanical properties of the stirred zone and therefore, ultimately, the formability and spring-back performances of the whole sheet. When applied to AA5052-H32 sheets, the process improved formability and spring-back, as experimentally and numerically confirmed in the limit dome height and unconstrained bending tests.

Limit dome height and failure location of stainless steel tailor-welded blanks

Forming limits of stainless steel tailor-welded blanks (TWBs) are investigated through both testing and numerical simulation. Limit dome height (LDH) tests were performed for 1.2/1.0 mm TWBs with 0°, 90°, 45° weldment orientations and various blank widths. Numerical simulation of the LDH test was conducted with LSDYNA. Since TWB is, in reality, a structure, the forming limits of TWBs in terms of the LDH and failure location should be characterized rather than the conventional forming limit diagrams (FLDs). A localized necking criterion based on the vertex theory was employed to identify the failure sites of TWBs. The localized necking criterion was compiled into a computer program, which processed the output data from LSDYNA. The LDHs and failure locations were computed for various combinations of blank thickness and weldment orientation. The predicted LDH and failure locations were compared with the test results and found to be satisfactory.

Effects of Stress Relieving on Limit Dome Height of Titanium Tailor-Welded Blanks at Elevated Temperatures

This paper aims to study the effect of stress relieving on Limit Dome Height (LDH) of Ti-TWBs at elevated temperatures. This is achieved by developing a newly constructed heating system. The elevated temperature of the system can be varied and monitored by a separately control panel. All Ti-TWBs were prepared and used to examine the LDHs under elevated temperatures. Selected specimens were heat-treated at 600°C within an hour before being formed by HILLE machine. Meanwhile, the temperature of tool heating system was also adjusted from room temperature to 550°C. Specified tests were carried out to examine the stress relieving effects of Ti-TWBs on the LDHs with the temperature control panel. In addition, investigations were carried out to ascertain whether the elevated temperatures of the critical tooling components, i.e. the die and the blank holder, could result in any significant effects on LDHs of Ti-TWBs. The findings show that LDHs of Ti-TWBs can be improved by stress relieving. The stress relieving condition can be obtained by nearly isothermal forming of specimens at a range of 550°C to 600°C.

Study of Texture Effect on Sheet Failure in a Limit Dome Height Test by Using Elastic/Crystalline Viscoplastic Finite Element Analysis

By combining the crystalline orientation distribution with a hardening evolution equation, a new elastic/crystalline viscoplastic material model is established. We focus our discussion on looking primarily at the texture effects on the strain localization of limit dome height (LDH) tests which are simulated using our Dynamic-Explicit finite element code. Three crystalline models in addition to the classical plastic potential and associated flow law model (J2F) are employed. The results demonstrate that, according to our failure criterion, the random orientation model shows the earliest indication of failure. The better formability is obtained for aluminum alloy 6111-T4 and cube texture models than the random crystalline orientation model. The J2F model shows no signs of strain localization. A comparison between numerical results also confirms that the strain localization region and crystalline rotation are different, due to the crystalline orientation distribution, which is initially set.