Experimental and numerical studies of the effects of forming speed on the formability of tailor welded blanks (TWB)

نوع: Type: thesis

مقطع: Segment: PHD

عنوان: Title: Experimental and numerical studies of the effects of forming speed on the formability of tailor welded blanks (TWB)

ارائه دهنده: Provider: Ahmad Amini

اساتید راهنما: Supervisors: Dr Ali Alavi Nia

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Dr Hassan Moslemi Naeini, Dr Heshmatollah Haghighat, Dr Faramarz Fereshteh-Saniee

زمان و تاریخ ارائه: Time and date of presentation: 9/11/2023

مکان ارائه: Place of presentation: Amphitheater of the Faculty of Engineering

چکیده: Abstract: In recent years, the use of tailor-welded blank (TWB) in the automotive and aerospace industries is increasing due to its advantages, such as improving local strength, reducing weight, saving production costs, reducing fuel consumption, and polluting the environment. Many studies have been done to investigate the effects of various parameters on the formability of TWB at quasi-static conditions. But, despite the widespread use of TWBs in the construction of various parts of the car body and the need for forming them at different speeds, the forming speed effect as an influential factor in the forming processes of TWB consisting of steel sheets has not been considered. So, this study was conducted to investigate the effect of forming speed on the formability of TWB and its constituent sheets. For this aim, a fiber laser was used for welding TWB composed of St14 steel sheets with thicknesses of 0.7 and 1 mm. The weld zone was examined using metallographic and microhardness experiments. Uniaxial tensile tests were performed at five strain rates of 0.001, 0.01, 0.1, 1, and 10 s-1 at the room temperature to investigate the tensile properties of base metals and TWB. A forming die with a hemispherical punch was designed and manufactured to experimentally investigate the formability. Formability tests were performed to construct forming limit curves (FLC) for quasi-static and impact forming conditions. Finite element simulation was used to create the numerical FLC. In the simulation, the material anisotropy was considered. Moreover, the material model was defined in the simulation considering the strain rate effect by the VUHARD subroutine. The results showed that the presence of bainitic microstructure in the weld zone has increased the microhardness twice compared with the base metals. In the tensile tests, with the increase of the strain rate from 0.001 s-1 to 10.00 s-1, the yield strength (YS) of 0.7 mm base metal, 1 mm base metal, and TWB was increased 86.4%, 78.3%, and 81.3%, respectively. In this range of strain rates, the increase of ultimate tensile strength (UTS) of 0.7 mm base metal, 1 mm base metal, and TWB was 20.9%, 22.8%, and 22.9%, respectively. Additionally, at all the strain rates, the YS and UTS of TWB were higher than base metals and close to the thin sheet of TWB. As the strain rate raised, the total elongation (TE) of the TWB decreased. However, the TE of the base metals first had a downward trend with the increase of the strain rate up to 0.1 s-1 and then had an upward trend with the further enhancement of the strain rate. Furthermore, the post-uniform elongation (PUE) of the base metals and TWB increased with enhancing the strain rate. With the increase of the strain rate up to 10 s-1, some important strain-hardening indicators, including the uniform elongation (UE), the ratio of UTS to YS, and strain-hardening exponent (n), for 0.7 mm base metal, decreased by 33%, 35.2%, and 35.5% and for 1 mm base metal, these reductions were 33.6%, 31.1%, and 31.1%, respectively. In addition, the tensile behavior of base metals was predicted with a good accuracy using a material model for different strain rates. The indexes of limit thickness ratio (LTR) and difference of ratios (DR) were used to analyze the plastic deformation of thick and thin sheets of the TWB at various strain rates. As the strain rate increased, LTR and DR reduced. This event indicated the reduction of the plastic deformation of the thick part of TWB and, as a result, the decrease of UE of TWB when the strain rate increased. Furthermore, these results were consistent with the experimental findings. In the quasi-static forming of base metals, the fracture was away from the dome center. But, in impact forming, the fracture occurred near the center of the dome. However, in most deformed TWB samples at both forming speeds, strain concentration, and failure happened near the dome center on the thin sheet. The forming limit curve (FLC) of impact forming of the base metals was shifted downwards and towards more positive values of minor strain compared with quasi-static forming. The FLC of TWB in impact forming was lower in comparison with the FLC of quasi-static forming, especially on the left side of the forming limit diagram (FLD). Forming limit in plane strain state (FLC0) in impact forming of base metals and TWB was lower than quasi-static forming. The dome height of the base metals and TWB in impact forming was lower in comparison with quasi-static forming. These findings show the reduction of formability in impact forming at the intermediate strain rate. The simulation results implied that in the impact forming compared to quasi-static forming of base metals, the maximum major strain and thickness variation were situated in the smaller portion of the samples. The simulation of both forming speeds of TWB showed that the peak of major strain and the maximum thickness reduction took place in the thin sheet of TWB near the connection line of thin and thick sheets. Moreover, the plastic strain and thickness reduction of the TWB thin sheet occurred in a smaller area of the impact forming compared to the quasi-static forming. Overall, the simulation results, including fracture position, strain distribution, and FLC in both forming processes, were in good agreement with the experimental findings

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