¤ Open Access
Closed-loop control of product properties in metal forming: A review and prospectus
Abstract Metal forming processes today operate with astounding productivity, repeatably creating precise parts in high volumes out of the stock sheet and bar products of the upstream metals industries. This achievement has come through decades of development of ever stiffer and more precise tooling used in fast-acting tightly controlled equipment, and yet in the wider context of manufacturing, metal forming processes seem to be less effective: tooling costs are high, and can only be justified by large batch production; the parts made by metal forming are usually not as required for assembly, and must be processed in further downstream machining operations; current processes do not respond well to process disturbances such as tool wear or unanticipated variation in material properties; twenty years of laboratory development of new flexible forming processes has led to little industrial take-up, due to a lack of precision. The missing ingredient in forming which gives rise to these problems is the absence of effective closed-loop control of product properties. The normal practice for blacksmiths and craft workers in former times – using their personal senses to adjust processing in response to evolving conditions – has been forgotten in the pursuit of process rigidity. This paper therefore aims to motivate a new wave of interest in applying closed-loop control of product properties to metal forming processes. A novel framework is developed to show metal forming processes at the heart of an outer control loop, and existing applications are reviewed. Surveys of sensors, actuators and modelling techniques reveal a rich seam of opportunities for new developments, and the paper concludes with some suggestions about near term opportunities for applying closed-loop control of properties to metal forming processes.
Deformation induced martensitic transformation in a 201 modified austenitic stainless steel
The production of low nickel austenitic stainless steels has increased considerably mainly due to nickel price evolution in the last years. In the present work, the susceptibility to deformation induced martensitic transformation of a 201 modified stainless steel was evaluated. The results were compared to existing results of traditional AISI 304 steel. The variation of martensite volume fraction against true strain was modeled by a sigmoidal equation and the transformation rate was also determined.
Sustainability issues in sheet metal forming processes: an overview
Abstract Environmental sustainability in manufacturing is nowadays an urgent and remarkable issue and the main concerns are related to more efficient use of materials and energy. In sheet metal forming processes there is still a lack of knowledge in this field mainly due to the need of a proper modelling of sustainability issues and factors to be taken into account. The aim of this paper is mainly to underline the state of the art from a forming point of view about the sustainability contributions offered in any phase of a product life cycle. Actually, a lack in terms of comprehensive contributions is present in the technical literature, thus, the authors try to give a sort of holistic vision aimed to provide basic guidelines in order to help in identifying the possible solutions with regard to all the phases of a forming product life cycle. The main attention was paid to sheet metal forming technologies. The paper gives an overview of the main topics concerning sheet metal forming problems related to energy and resource efficiency with the aim to stress the principal contributions which may derive from such processes to environmental performances of manufacturing.
Metal forming beyond shaping: Predicting and setting product properties
Abstract Metal forming is not only shaping the form of a product, it is also influencing its mechanical and physical properties over its entire volume. Advanced analysis methods recently enable accurate prediction of these properties and allow for setting these properties deterministically during the forming process. Effective measurement methods ensure the setting of these predicted properties. Several real examples demonstrate the impressive achievements and indicate the necessity of a paradigm change in designing products by including manufacturing-induced effects in the initial dimensioning. This paradigm change will lead to lightweight components and serve environmentally benign designs.
Modelling kinetics of strain-induced martensite transformation during plastic deformation of austenitic stainless steel
Abstract A model to simulate the kinetics of formation of strain-induced martensite (α ′ ) in austenitic stainless steel (SS) 304 is derived. The model predicts sigmoidal behaviour of the γ → α′ transformation. The model was successfully applied to various levels of α′ obtained after cold-rolling of type 304 SS and to many previously reported experimental data set with the volume fraction of α′ as high as 1.0. This model was compared with two other sigmoidal models and it was shown that the fitting parameters for the proposed model can be correlated to physical parameters associated with γ → α′ transformation. The fitting parameters of the model consist of the saturated level of α′ for a given condition, the strain value associated with the maximum rate of γ → α′ transformation, and a parameter that characterises the rate of γ → α′ transformation.
¤ Open Access
Precision Control of SMA Actuators with a Real Time Model-Based Controller and Extended VSC
Abstract: This paper presents a novel control strategy for precision position control of Shape Memory Alloy (SMA) actuators by using a combination of large and small signal controllers. The large signal controller, responsible for course positioning, is an NPID temperature controller that gets its desired temperature value online from a computationally efficient stress-strain and phase kinetics SMA model that accounts for time varying stresses. The small signal controller, responsible for fine positioning, is an extended variable structure controller that uses an asymmetrical boundary layer to prevent over-heating of the wire and whose control signals adapt to the dynamic load on the wire. The control strategy is tested on an SMA wire, first, with various constant loads and then with varying loads. The results show robust and precise tracking control within 0.03% strain with exceptional disturbance rejection.
¤ Open Access
A methodology for assessing the feasibility of producing components by flow forming
This paper describes a methodology for assessing the applicability of the flow forming process for the manufacture of specific components. The process starts by filtering potential candidates for flow forming from a component collection (e.g. company catalogue) and then carries out a detailed assessment of quantitative, technological and economic feasibility before determining a viable process plan. The process described uses analytical relationships and empirical criteria drawn from the literature.. A process time model (based on an analogy with CNC turning) is used to develop a hybrid cost model in order to evaluate economic feasibility. The paper concluded with a brief summary of the results of applying the process to an industrial case study.
Modification of Olson–Cohen model for predicting stress-state dependency of martensitic transformation
ABSTRACTThe well-known Olson–Cohen model is modified by incorporating the effect of stress state on the transformation kinetics in metastable austenitic stainless steels. By assuming isothermal condition, the relationships between the parameters of Olson–Cohen model, stress triaxiality and absolute value of Lode angle parameter have been established. The proposed model predicts that the saturation level and slope of transformation curve increase with increasing the stress triaxiality in addition to the absolute value of Lode angle parameter. Uniaxial and plane strain tension tests have been conducted on the two types of austenitic stainless steels AISI 304 and AISI 201 to evaluate the validity of the model. The results show that the predicted transformation curves are in good agreement with the experimental data.
Model-based precision position and force control of SMA actuators with a clamping application
Abstract This paper presents a novel control strategy for precision control of Shape Memory Alloy (SMA) actuators by using a combination of large and small-signal controllers. The large-signal controller, responsible for coarse positioning, is a nonlinear PID (NPID) temperature controller that gets its desired temperature value online from a computationally efficient stress-strain and phase kinetics SMA model. The small-signal controller, responsible for fine positioning, is variable structure controller that uses an asymmetrical boundary layer to prevent over-heating of the wire. Further, the control signals adapt to the dynamic load on the wire to prevent overshoots. The control results show precise tracking control with exceptional disturbance rejection for various loads. The control strategy is then used for force control in an SMA-based clamping vice to clamp a thin-walled glass test tube. While the large-signal controller remains unchanged, the small-signal controller is used as a force controller rather than a position controller. The control unit of the vice is an embedded processor, on which the control algorithms run with a sample time of 3 ms. The vice is used to clamp the test tube at various desired step and sinusoidal forces and shows accurate force tracking and good disturbance rejection.
¤ Open Access
Flexibility in metal forming
Abstract Flexibility in metal forming is needed more than ever before due to rapidly changing customer demands. It paves the way for a better control of uncertainties in development and application of metal forming processes. Although flexibility has been pursued from various viewpoints in terms of machines, material, process, working environment and properties, etc., a thorough study of the concept was undertaken in order to with problems of manufacturing competiveness and tackle new challenges of manufacturing surroundings. Therefore, in this paper, flexibility in forming is reviewed from the viewpoints of process, material, manufacturing environment, new process combinations and machine–system–software interactions.
¤ Open Access
Correlation between Martensitic Phase Transformation and Magnetic Barkhausen Noise of AISI 304 Steel
Abstract The movement of magnetic domain walls can be disturbed by material defects, also named pinning sites. When the magnetic field is changing, the domain walls move by “jumps” over these pinning sites. These jumps produce the so named magnetic Barkhausen noise (MBN). During walls movements also low amplitude and high frequency elastic waves are generated, they are called magnetic acoustic emission (MAE). Both MBN and MAE are technological and efficient alternatives of low cost in the study of ferromagnetic materials which suffer degradation by plastic deformation, presence of inclusions, different stresses configurations, etc., depending on their micro structural characteristics. For austenitic metastable stainless steel (no magnetic), plastic deformation induces martensitic phase transformation (ferromagnetic). The aim of this work is to correlate the MBN with the plastic deformation and martensitic phase transformation of AISI 304 stainless steel. Standard test samples were submitted to uniaxial tensile strength with different load schedules up to rupture. At every instant the MBN produced was registered. Then MBN was correlated with the steel microstructure when the martensitic phase was growing.