Study on Formability Evaluation in Electromagnetic Forming Process of Aluminium Alloy Sheet

Abstract

Electromagnetic forming (EMF) process can effectively enhance the formability of aluminium alloy and fabricate complicated geometry which is difficult to be formed by conventional processes Since the duration of deformation in EMF is extremely short to get the stress or strain data by conventional measuring method it is also hard for engineers to obtain the FLD at high strain rate as reference when designing products Hence the main purpose of this dissertation is to establish a method that can be used for predicting formability of aluminium alloy sheet in EMF process This study conducted a series of EMF experiments of aluminium sheet A5052 from the macro scale to the micro scale to investigate the formability at high strain rates In the macro-scale forming free-impact EMF experiments and iterated simulation were designed to research the flow stress and the forming limit at high strain rate Aluminium sheet A5052-H32 with 0 5 mm thickness was propelled towards a punch using the electromagnetic force generated by a flat spiral electromagnetic coil Different values of the punch radius and the shapes of specimen used in the EMF experiments were chosen from the simulation results in order to generate various strain values and strain paths Furthermore the Johnson-Cook model was introduced using the flow stress data obtained by iteration analysis and the energy criterion was applied for the formability evaluation of aluminium sheet under high stain rates In order to understand the influences of grain size and strain rate on formability FLD tests and the limit dome height (LDH) tests were conducted at four different speeds ranging from 0 012m/s to 11 04m/s via stamping and electromagnetic forming process Aluminium sheets A5052 H32 with 0 5mm thickness were used for experiments and annealed at different conditions to obtain different microstructures (T/D ratios 6 99 ~27 17) The strain data of FLD were compared with the limit dome height measurement to clarify the influence of various grain sizes on formability In addition the fracture surface and the microstructure of the tested samples were observed by SEM and OM to understand the damage mode A modified criterion based on Zhuang’s model was proposed and applied for macro-micro meso scale forming in EMF process This study constructed Johnson-Cook model based on the iterated effective stress-strain curve Via the determined Johnson-Cook model the flow stresses under different strain rates were obtained and applied for process simulation which showed good agreement with the measured forming limits According to the results of LDH tests and the observation by SEM and OM it is concluded that the smaller the grain size is in high-speed forming the probability of the grain boundary damage increases and it could cause the poor formability For macro-micro meso scale forming the proposed criterion which based on the effects of grain size and strain path was verified by the experimental data The proposed modified criterion can be applied for formability prediction of A5052 aluminium sheet with T/D ratio 3 75~27 17 under high-speed forming In summary this study established a scheme that can be used for macro-scale and macro-micro meso scale formability evaluation of aluminium alloy sheet in EMF process The flow stress constructed by Johnson-Cook model can be used for predicting the deformation behaviour and the formability were evaluated by FLCs which were obtained by Zhunag's criterion and the proposed modified criterion for macro and macro-micro meso scale forming respectively

Date of Award	2018 Jun 22
Original language	English
Supervisor	Rong-Shean Lee (Supervisor)