Dynamic recrystallization (DRX) processes are widely used in industrial hot working operations, not only to keep the forming forces low, but also to control the microstructure and final properties of ...the workpiece. According to Poliak and Jonas, the onset of DRX can be detected from an inflection point in the strain hardening rate as a function of flow stress. Various models are available that predict the evolution of flow stress from incipient plastic flow to steady-state deformation in the presence of DRX, but their consistency with the criterion of Poliak and Jonas has not been investigated. This work analyzes the conditions that a flow stress model incorporating DRX has to fulfill to be consistent with the criterion of Poliak and Jonas. As the most important inconsistency, it is found that a model might suffer from insufficient differentiability at the critical point. For all models that use a classical JMAK equation for the DRX kinetics, it is shown that the Avrami exponent must exceed a value of 3. If the Avrami exponent is at most 3, a kink may develop in the strain hardening rate, and the second derivative criterion is violated. For DRX kinetics based on nucleation and growth rates that are functions of time, criteria are derived that ensure consistency with the criterion of Poliak and Jonas. DRX kinetics that are consistent with the second derivative criterion are put forward, drawing upon kinetics proposed by Cahn for transformations that originate at grain boundaries. Finally, a minimal model that is consistent with the second derivative criterion is formulated.
The sine law is a simple geometrical model for incremental sheet metal forming (ISF). It is based on the assumption that the deformation is a projection of the undeformed sheet onto the surface of ...the final part. The sine law provides approximations of sheet thinning for shear spinning and ISF at negligible computational cost, but as a plane strain model it can be applied only when plane strain deformation prevails.
In this paper, a new model for the process kinematics of ISF is presented that is more general than the sine law. The model treats ISF as an evolution of a surface from the undeformed sheet to the final shape. It computes trajectories of surface points based on idealized intermediate shapes, assuming that the deformation between intermediate shapes proceeds by displacements along the surface normal of the current shape. For 2D and axisymmetrical problems, an analytical solution of the model is developed, which is useful for visualizing and discussing the kinematics of ISF.
In order to use the new model with arbitrary parts, it was cast into a computer program that calculates membrane strains and the sheet thickness on a triangular mesh. For a benchmark shape, the model is compared to the sine law and experimental results. It is shown that the new model yields better thickness estimates than the sine law, especially in non-flat part areas where strains parallel to the direction of tool motion are significant.
The manufacturing of components from the titanium alloy Ti-6Al-4 V is of great significance for many industrial sectors. The production of high-performance Ti-6Al-4 V components typically requires ...multiple hot forging steps and leads to parts with tolerances that need extensive machining to create the final shape. For many applications, net-shape technologies such as additive manufacturing (AM) could enable a higher material yield. However, with the part size, cost and processing time in AM rise rapidly. Thus, the advantages of AM and forging operations could be exploited by combining both processes to new hybrid process chains. The present study investigates the use of Wire-Arc additive manufacturing (WAAM) for hybrid manufacturing of Ti-6Al-4 V aerospace components. Two process routes are investigated that combine forming and AM processes. In the first process route, a WAAM process is used to generate a pre-shaped semi-finished part. The semi-finished part will then be forged using a single forming tool to obtain the final part contour. The second process route utilizes a conventionally forged pre-form, onto which features of the final workpiece are added using WAAM. The results confirm that hybrid technologies combining WAAM and forging are very promising for Ti-6Al-4 V part production. A jet engine blade produced by WAAM and subsequent forging shows microstructures typically produced in conventional processing of Ti-6Al-4 V alloy and exhibits tensile properties, which exceed the specification level of cast and forged Ti-6Al-4 V material. Features created by WAAM on forged pre-forms are shown to reach the mechanical properties required to combine both technologies. The combination of WAAM and forging may hence be used to develop new manufacturing chains that allow for higher material yield and flexibility than conventional forging.
Cold spray (CS) has emerged as an appealing additive manufacturing (AM) technique for producing or repairing individual components or entire structures. Compared to fusion-based AM technologies, cold ...spray additive manufacturing (CSAM) offers distinct advantages in the fabrication of components, while avoiding some melting/solidification-related issues such as phase transformation and oxidation. It involves intricate processes that pose significant challenges for numerical modeling, particularly when simulating the entire process at a large scale. The smoothed particle hydrodynamics (SPH) method is highly suitable for handling large material deformations due to its Lagrangian and meshless nature. In this work, we develop an enhanced SPH method to conduct large-scale simulations of CSAM with different powder sizes, morphologies, and distributions. A modified material model has been incorporated to accurately capture the strain-rate hardening effects during the plastic stage. The computational scale is greatly improved by using a Message Passing Interface (MPI) based framework, enabling the simulation of approximately ten million SPH particles. To the authors' knowledge, this study marks the first attempt to numerically reproduce the entire process of CSAM with real powder sizes and distributions. Experimental data measured for a wide range of powder velocities are used to validate the simulation results and assess the prediction accuracy. Subsequently, we comparatively study the bonding mechanisms of powders with the same or different sizes, while also identifying a four-stage coating process. The effects of powder morphology on the bonding process are thoroughly investigated. A large-scale CSAM process is finally reproduced to demonstrate the capability of the present meshless scheme, and mechanisms of pore formation are analyzed, providing valuable insights for practical engineering applications.
A precise control of the Mg content in Al–Mg alloys is essential to obtain predictable mechanical properties but the processing of Al–Mg alloys often suffers Mg losses due to Mg evaporation and ...oxidation. A new high strength Al–Mg–Sc alloy designed for LPBF (laser powder bed fusion) processing has been developed here, where Mg losses are effectively prevented by the addition of a low amount of calcium. A LPBF processing window which results in built parts with a 99.7% relative density and no detectable loss of Mg has been identified. The as-built microstructure of the new Calciscal® alloy, studied by transmission electron microscopy, is found to comprise areas of fine equiaxed grains and areas of coarser grains, with many Al4Ca precipitates present at the grain boundaries and within the coarser grains. After a subsequent ageing of 1 h at 375 °C, the tensile strength of Calciscal® is increased by 44% compared to the as-built condition and reaches 522 ± 2 MPa. The increase in strength observed in the heat-treated condition comes from the additional precipitation of numerous finely dispersed Sc-rich precipitates. This high strength, combined with a good ductility, makes Calciscal® competitive to other Al alloys and suitable for structural applications. Moreover, Calciscal® shows very reproducible tensile properties thanks to the Ca addition which leads to a better control and less variations of the Mg content in the alloy.
► We present a case–control study of 6745 cyclist-motor vehicle collision casualties. ► We used linked police-reported road crash, hospital admission and mortality databases. ► Helmet use was ...associated with a reduced risk of head injury of up to 74%. ► Non-helmeted cyclists were more likely to display risky riding behaviour.
There has been an ongoing debate in Australia and internationally regarding the effectiveness of bicycle helmets in preventing head injury. This study aims to examine the effectiveness of bicycle helmets in preventing head injury amongst cyclists in crashes involving motor vehicles, and to assess the impact of ‘risky cycling behaviour’ among helmeted and unhelmeted cyclists. This analysis involved a retrospective, case–control study using linked police-reported road crash, hospital admission and mortality data in New South Wales (NSW), Australia during 2001–2009.
The study population was cyclist casualties who were involved in a collision with a motor vehicle. Cases were those that sustained a head injury and were admitted to hospital. Controls were those admitted to hospital who did not sustain a head injury, or those not admitted to hospital. Standard multiple variable logistic regression modelling was conducted, with multinomial outcomes of injury severity.
There were 6745 cyclist collisions with motor vehicles where helmet use was known. Helmet use was associated with reduced risk of head injury in bicycle collisions with motor vehicles of up to 74%, and the more severe the injury considered, the greater the reduction. This was also found to be true for particular head injuries such as skull fractures, intracranial injury and open head wounds. Around one half of children and adolescents less than 19 years were not wearing a helmet, an issue that needs to be addressed in light of the demonstrated effectiveness of helmets. Non-helmeted cyclists were more likely to display risky riding behaviour, however, were less likely to cycle in risky areas; the net result of which was that they were more likely to be involved in more severe crashes.
The behaviour of hollow and concrete filled thin-walled
steel tubular members subjected to transverse impact was previously investigated experimentally and theoretically. The members were restrained ...axially and rotationally at their ends. In this paper the study is extended to investigate nominally identically sized
stainless steel tubes, tested experimentally under the same conditions. Comparisons between the performance of the two materials are made. Both the steel and stainless steel tubular members, hollow and concrete filled, are then modelled numerically. The FE models are validated against the experiments, and subsequently extended to investigate the general behaviour of such members when used as columns or other axially load bearing structures. The influences of axial pre-load, rotational restraint at the member ends, axial restraint, metal material properties and concrete filling, are investigated. In particular, their effect on the capacity of the members to absorb transverse impact energy. A general design procedure for metal tubular members with or without concrete filling subjected to transverse impact is developed, in a format aligned with current static structural steel specifications.
► Concrete filling of tubular metal columns has little benefit for impact energy absorption. ► Stainless steel provides significant energy absorption benefits over steel. ► Axial restraint provides the most significant contribution to energy absorption capability. ► The transverse energy capacity may be determined from the design model presented.
•Rear-compared to front seat car passengers are sustaining injuries of a higher severity.•Rear seat car occupant protective mechanisms need examination.•An objective measure of injury severity ...available in ED records is needed.
The risk of serious injury or death has been found to be reduced for some front compared to rear seat car passengers in newer vehicles. However, differences in injury severity between car occupants by seating position has not been examined. This study examines the injury severity risk for rear compared to front seat car passengers.
A retrospective matched-cohort analysis was conducted of vehicle crashes involving injured rear vs front seat car passengers identified in linked police-reported, hospitalisation and emergency department (ED) presentation records during 2001–2011 in New South Wales (NSW), Australia. Odds ratios were estimated using an ordinal logistic mixed model and logistic mixed models.
There were 5419 front and 4588 rear seat passengers in 3681 vehicles. There was a higher odds of sustaining a higher injury severity as a rear-compared to a front seat car passenger, with a higher odds of rear seat passengers sustaining serious injuries compared to minimal injuries. Where the vehicle occupant was older, travelling in a vehicle manufactured between 1990 and 1996 or after 1997, where the airbag deployed, and where the vehicle was driven where the speed limit was ≥70km/h there was a higher odds of the rear passenger sustaining a higher injury severity then a front seated occupant.
Rear seat car passengers are sustaining injuries of a higher severity compared to front seat passengers travelling in the same vehicle, as well as when travelling in newer vehicles and where the front seat occupant is shielded by an airbag deployed in the crash. Rear seat occupant protective mechanisms should be examined. Pre-hospital trauma management policies could influence whether an individual is transported to a hospital ED, thus it would be beneficial to have an objective measure of injury severity routinely available in ED records. Further examination of injury severity between rear and front seat passengers is warranted to examine less severe non-fatal injuries by car seating position and vehicle intrusion.
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In this review the authors provide a comprehensive insight into additive manufacturing process mechanics which explore how thermodynamics give rise to characteristic microstructure ...and property. Discussion is then given over to how solidification microstructure is modified using processing conditions and utilisation of traditional casting methods which give rise to grain refinement and some cases ‘texture by design’.
The review then makes observation and provides analysis on the typical material types reported in powder bed fusion and directed energy deposition. These are organised to efficiently highlight the key observations in terms of alloy creation and adaption with a specific focus on the relevant additive manufacturing technique. The emergent alloy families are also explored alongside the rapidly expanding topic of functionally graded alloys produced by the breadth of additive manufacturing techniques.
The review concludes with a detailed critique of the state-of-the-art and an examination of where opportunities exist to advance the interdependency between process and emergent alloys exhibiting superior properties. The authors propose several areas worthy of consideration and attempt to provide inspiration for our community in the pursuit of new alloys and accompanying process designs for additive manufacturing.
Flexibility in metal forming Yang, D.Y.; Bambach, M.; Cao, J. ...
CIRP annals,
2018, 2018-00-00, Letnik:
67, Številka:
2
Journal Article
Recenzirano
Odprti dostop
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.
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