Characterising the effective material softening in ultrasonic forming of metals

[摘要] This thesis has presented experimental and finite element (FE) analyses of the static and ultrasonic forming of two metals; aluminium 1050 and magnesium AM50. Aluminium and magnesium are considered to be soft metals and can be easily shaped by any of the main industrial metalworking processes. Frequently aluminium and magnesium have been the subject of research studies. These two metals most commonly chosen in manufacturing industry because of their cost, mechanical properties and flexibility in processing. In this research, simple compression and forming tests were designed and the effects of superimposed ultrasonic excitation on workpiece and die, which is tuned to a longitudinal mode at 20.8 kHz, were studied via stress-strain measurements. Research through experiments and finite element simulations studies in the application of ultrasonic excitation has been carried out to gain quantitative understanding of the mechanisms of improvement in ultrasonic forming characteristics, such as a reduction in material flow stress and oscillatory stress. This research study has shown these mechanisms by applying ultrasonic vibration to the tool and die in the forming test and, similarly, effects were measured and predicted in the experimental and numerical analysis.The development and application of high power ultrasonic techniques in forming processes required the use of specifically designed ultrasonic components to correctly transmit the energy from the transducer to the workpiece and die interface. The application along with the ultrasonic vibration amplitude required for the process, were considered in order to design the most suitable horn profile. In this study, a 20 kHz transducer was used to provide up to 10 µm of peak-to-peak vibration amplitude, depending on the generator setting. Therefore, the booster and horn were designed to provide a range of ultrasonic vibration amplitudes between 5 to 20 µm and also used as a tool and die in the study of ultrasonic metal forming. The horn was designed using finite element modelling (FEM), and modal frequencies and associated mode shapes were subsequently confirmed using experimental modal analysis (EMA). The ultrasonic system has been measured and calculated as having a longitudinal mode of vibration at 20.8 kHz and to provide an amplitude gain of four. In this study, a generator uses mains electricity to generate a high frequency ultrasonic signal to drive the transducer, which is tuned to a specific frequency of 20 kHz. The booster and horn were designed to meet the criteria of transducer, which is to provide a longitudinal vibration at tuned frequency of 20 kHz. However, the profile of booster and horn have been measured and calculated as having a longitudinal mode of vibration at 20.8 kHz, which is considered close to the transducer tuned frequency.The review of previous studies of superimposed ultrasonic excitation on upsetting showed that the most experimental characterisations of the volume effects mainly depended on an interpretation of measurements of the mean flow stress, and have neglected the oscillatory stress. In this study, the characteristics of oscillatory stress and the material behaviour in plastic deformation when superimposed ultrasonic excitation is applied on a static compression test under dry friction were considered. The effects were explained in terms of flow stress reduction, oscillatory stress, mean flow stress, maximum and minimum path of oscillatory stress in the stress-strain diagram. The results showed that the static flow stress of compressive deformation was lowered by the ultrasonic vibration superimposed on the static load and this phenomenon has been referred to as the material softening mechanism which is influenced by volume and surface effects. The volume effect is defined as a reduction in flow stress of the material being formed and the surface effect is defined as a reduction in frictional conditions at the interface between the vibrating device and the workpiece. Finite element models were used to investigate numerically the volume and surface effects during ultrasonically assisted compression. The finite element models were developed using material model parameters which were identified from the experimental analysis. The influence of volume and surface effects were investigated separately in the FE model and it was shown that the volume effect dominated the effective material softening results during ultrasonic excitation. The application of ultrasonic excitation on metals under plastic deformation conditions has been investigated previously. Most researchers have reported that superimposing ultrasonic excitation on metal working processes reduced the material flow stress. A further study of superimposed ultrasonic excitation on a static load during elastic deformation in metal working was not investigated, so it is not possible to determine the effect of ultrasonic excitation on the material. In this study, the investigation of oscillatory stress behaviour in the ultrasonic compression test of cylinder metal specimens during elastic deformation was carried out. In the stress-strain diagram, the ultrasonic vibration was shown to have lowered the static flow stress during elastic deformation under dry contact conditions and it was found that the reduction in static flow stress linearly increased with ultrasonic vibration amplitude. The stress reduction was influenced by volume and surface effects which occurred during the superimposed ultrasonic excitation. The results also showed that the maximum path of oscillatory stress exceeded the static flow stress, however, the mean flow stress is lower than the static flow stress at the onset of ultrasonic excitation. To investigate the influence that volume and surface effects have on material softening during experimental compression tests, a series of FE models were developed. As mentioned previously, the FE models were developed using material model parameters which were identified from the experimental analysis in Figure A1, however, the mechanism of flow stress reduction which is related to acoustic softening and friction reduction which is labelled as (i) cannot be predicted in FE models. The FE models adopted the material softening effects in order to simulate realistic stress reduction compared with experimental results. The significant stress reduction in the FE analysis was obtained by adjusting the yield stress and contact conditions parameter. It was concluded that the surface effect dominated the stress reduction during metal upsetting test in elastic deformation.The study continued to a simple forming test where samples of flat sheet metal were forced into a shaped die by a shaped plunger on a test machine. The results of this study illustrated how ultrasonically assisted metal forming resulted in a lowering of the static forming force during ultrasonic excitation of the die. As a result, the static forming force was seen to be reduced by ultrasonic excitation of the die and the path of the maximum oscillatory force was observed to be parallel to or below the path of the static forming force. Force reduction was measured in these experiments using a high power ultrasonic transducer and also by tuning the die and then the punch during the metal forming test. It was found that a good coupling between punch, specimen and die allowed ultrasonic energy to be effectively transferred into the materials during superimposed ultrasonic excitation in the static forming test.This thesis has concluded that evaluation of the benefits of ultrasonic excitation not only relied on measurements of the mean flow stress alone but also on measurement of the oscillatory stress during superimposed ultrasonic excitation on forming tests. Findings on the effectiveness of ultrasonic excitation led the study to recognize that the lack of understanding of the effects of ultrasonic excitation on the forming process has resulted in difficulties in maximising the benefits and applications of this technology.

[发布机构] University:University of Glasgow;Department:School of Engineering

[关键词] Ultrasonic, metal forming, finite element, ultrasonic softening, ultrasonic die and horn