This study focuses on hot and warm forming properties of aluminum alloy AA5182 sheet, with attention toward warm forming, by using gas pressure to form sheet material. A temperature range of 300°C to 450°C and a pressure range of 690 kPa (100 psi) to 2410 kPa (350 psi) were used in a test matrix of twenty one different test conditions for gas-pressure forming of a sheet into hemispherical dome in a gas-pressure bulge test. Multiple sets of tensile data were used to develop a material model that predicts the dome height and shape of an axisymmetric bulge specimen at any given time during forming. In simulations of the forming process, 17 simulations of the total 21 experimental conditions showed good agreement with the experimentally measured dome heights throughout forming tests. The four cases that did not show good agreement between simulation and experiment are a result of strain-hardening in the material during forming. Strain hardening was not significant in tension testing of specimens and was not accounted for in the material model, which considered only strain rates slower than for these experimental bulge testing. This demonstrates an effect which must be considered in future simulations to predict forming approaching warm conditions. Two experimental bulge specimens were cross-sectioned post forming and grain sizes were measured to determine if grain growth occurred during the forming process. Experimental bulge specimens show no grain growth during the forming process. The tensile specimens from which the material model data were taken were measured to determine if plastic anisotropy was a possible issue. All specimens measured were proved to have deformed nearly isotropically. The results of this study show that predicting warm and hot forming of aluminum alloy AA5182 using gas pressure is possible, but that a more complex material model will be required for accurate predictions of warm forming. This is a very important step toward making hot and warm forming commercially viable mass production techniques. / text

Magnesium alloy tubes have good formability at elevated temperatures. In this paper, firstly, uniaxial tensile tests are conducted to evaluate the flow stress of AZ61 magnesium alloy at different temperatures and strain rates. Secondly, a hydraulic warm forming machine with axial feeding, counter punch and internal pressure is designed and manufactured. Using this testing machine with the FEM results, experiments of hydraulic forming of AZ61 magnesium alloy tubes at different temperatures are carried out. The effect of loading paths on the product shape and formability at different temperature are discussed.

Electromagnetic (EM) forming is a high-speed forming process that uses the forces induced on a conductive workpiece by a transient high frequency current to form the workpiece into a desired shape. This thesis presents the results of an experimental and numerical study carried out to determine whether an EM forming process could be used to sharpen the radius of part pre-formed using a stamping process. Two processes were studied; a single step EM forming operation and a “hybrid forming” operation consisting of a conventional pre-forming step and an EM corner fill, both considering aluminum alloy AA 5754. The single step EM process proved unable to form acceptable samples due to excessive sample distortion, but was used to gain insight into the EM forming process. The hybrid operation consisted of pre-forming 1 mm AA 5754 sheet into a v-shape with a 20 mm outer radius using a conventional stamping operation and then reducing or “sharpening” the radius to 5 mm using EM forming. Sharpening the radius to 5 mm using conventional stamping was not achievable. The hybrid operation proved successful in forming the 5 mm radius, thus demonstrating that the material could be formed beyond its conventional formability limit using the hybrid operation. Numerical models were used to gain insight into the processes and the challenges involved in their numerical simulation. The numerical simulations showed that EM corner fill operation produces very high strain rates (10,000- 100,000 s-1) and complex three dimensional stress and strain states. The effect of the high strain rates could not be properly assessed, since no constitutive data was available for such high strain rates. The predicted stress states show that the process was not plane stress and that large through-thickness compressive stresses are produced that are favorable to damage suppression and through-thickness shear strains that increase ductility. The high strain rates and the complex stress and strain states are considered the likely causes for the observed increase in formability. The models provided valuable insight, but did not predict the final shape exactly and the possible reasons behind this are analyzed. The research indicates that features that are not achievable using traditional stamping techniques can be obtained with the hybrid EM forming process.

Electromagnetic (EM) forming is a high-speed forming process that uses the forces induced on a conductive workpiece by a transient high frequency current to form the workpiece into a desired shape. This thesis presents the results of an experimental and numerical study carried out to determine whether an EM forming process could be used to sharpen the radius of part pre-formed using a stamping process. Two processes were studied; a single step EM forming operation and a “hybrid forming” operation consisting of a conventional pre-forming step and an EM corner fill, both considering aluminum alloy AA 5754. The single step EM process proved unable to form acceptable samples due to excessive sample distortion, but was used to gain insight into the EM forming process. The hybrid operation consisted of pre-forming 1 mm AA 5754 sheet into a v-shape with a 20 mm outer radius using a conventional stamping operation and then reducing or “sharpening” the radius to 5 mm using EM forming. Sharpening the radius to 5 mm using conventional stamping was not achievable. The hybrid operation proved successful in forming the 5 mm radius, thus demonstrating that the material could be formed beyond its conventional formability limit using the hybrid operation. Numerical models were used to gain insight into the processes and the challenges involved in their numerical simulation. The numerical simulations showed that EM corner fill operation produces very high strain rates (10,000- 100,000 s-1) and complex three dimensional stress and strain states. The effect of the high strain rates could not be properly assessed, since no constitutive data was available for such high strain rates. The predicted stress states show that the process was not plane stress and that large through-thickness compressive stresses are produced that are favorable to damage suppression and through-thickness shear strains that increase ductility. The high strain rates and the complex stress and strain states are considered the likely causes for the observed increase in formability. The models provided valuable insight, but did not predict the final shape exactly and the possible reasons behind this are analyzed. The research indicates that features that are not achievable using traditional stamping techniques can be obtained with the hybrid EM forming process.

One of the first questions to consider when designing a new roll forming line is the number of forming steps required to produce a profile. The number depends on material properties, the cross-section geometry and tolerance requirements, but the tool designer also wants to minimize the number of forming steps in order to reduce the investment costs for the customer. There are several computer aided engineering systems on the market that can assist the tool designing process. These include more or less simple formulas to predict deformation during forming as well as the number of forming steps. In recent years it has also become possible to use finite element analysis for the design of roll forming processes. The objective of the work presented in this thesis was to answer the following question: How should the roll forming process be designed for complex geometries and/or high strength steels? The work approach included both literature studies as well as experimental and modelling work. The experimental part gave direct insight into the process and was also used to develop and validate models of the process. Starting with simple geometries and standard steels the work progressed to more complex profiles of variable depth and width, made of high strength steels. The results obtained are published in seven papers appended to this thesis. In the first study (see paper 1) a finite element model for investigating the roll forming of a U-profile was built. It was used to investigate the effect on longitudinal peak membrane strain and deformation length when yield strength increases, see paper 2 and 3. The simulations showed that the peak strain decreases whereas the deformation length increases when the yield strength increases. The studies described in paper 4 and 5 measured roll load, roll torque, springback and strain history during the U-profile forming process. The measurement results were used to validate the finite element model in paper 1. The results presented in paper 6 shows that the formability of stainless steel (e.g. AISI 301), that in the cold rolled condition has a large martensite fraction, can be substantially increased by heating the bending zone. The heated area will then become austenitic and ductile before the roll forming. Thanks to the phenomenon of strain induced martensite formation, the steel will regain the martensite content and its strength during the subsequent plastic straining. Finally, a new tooling concept for profiles with variable cross-sections is presented in paper 7. The overall conclusions of the present work are that today, it is possible to successfully develop profiles of complex geometries (3D roll forming) in high strength steels and that finite element simulation can be a useful tool in the design of the roll forming process.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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