An analysis of frictional effects in non-stationary contact problems for metal forming simulations

The finite element method (FEM) is widely used for the simulation of metal forming processes and has been successfully used in contact problems which arise in processes such as deep-drawing, punching, extrusion and rolling. All these processes involve friction between the contact surfaces: the sheet-metal workpiece and the toolpieces. The model of friction is thus an important part of any simulation of metal forming processes. Most FEM codes use a friction model that assumes that the contact surface is a plane. Attempts to address this problem have focused on the convective description of deformation, which has the advantage of being naturally extended to numerical methods like the FEM at the expense of additional computation and numerical complexity. The convective description is used in this work, which focuses on the numerical implementation of the objective measure. The effects of the rotation of the material contact point is taken into account by including objective time derivatives of the slipping (tangential) direction function. The objective rate of the direction function includes the surface spin induced by the rigid motion of a contact point sliding over the tool surface, and the material spin occurring during the elastic-plastic deformation of the blank. This is introduced by adapting the incremental relations of the friction slip. This thesis presents the results of numerical experiment to determine the influence that the rotation and convection of contact points has on the frictional stresses and slipping energy. Four different friction models are implemented within the finite element program ABAQUS and applied to simulations of standardmetal forming benchmark processes: the square-cup and s-rail deep drawing benchmarks of the Numisheet conferences, for which several experimental and numerical results are available to compare with the solution of a finite element simulation. The results for each metal-forming simulation are calculated for different friction models, and are compared and a choice made as to which is the “best” friction model for the process. Further, the reverse problem of determining the values of friction parameters by comparison of simulation and experimental results is performed for these benchmark problems. As there is yet no ideal friction model for all processes that are modelled, finding the most appropriate friction model by numerical means is proposed to improve the quality of a simulation

A novel method of detecting galling and other forms of catastrophic adhesion in tribotests

Tribotests are used to evaluate the performance of lubricants and surface treatments intended for use in industrial applications. They are invaluable tools for lubricant development since many lubricant parameters can be screened in the laboratory with only the best going on to production trials. Friction force or coefficient of friction is often used as an indicator of lubricant performance with sudden increases in friction coefficient indicating failure through catastrophic adhesion. Under some conditions the identification of the point of failure can be a subjective process. This raises the question: Are there better methods for identifying lubricant failure due to catastrophic adhesion that would be beneficial in the evaluation of lubricants? The hypothesis of this research states that a combination of data from various sensors measuring the real-time response of a tribotest provides better detection of adhesive wear than the coefficient of friction alone. In this investigation an industrial tribotester (the Twist Compression Test) was instrumented with a variety of sensors to record: vibrations along two axes, acoustic emissions, electrical resistance, as well as transmitted torsional force and normal force. The signals were collected at 10 kHz for the duration of the tests. In the main study D2 tool steel annular specimens were tested on coldrolled sheet steel at 100 MPa contact pressure in flat sliding at 0.01 m/s. The effects of lubricant viscosity and lubricant chemistry on the adhesive properties of the surface were examined. Tests results were analyzed to establish the apparent point of failure based on the traditional friction criteria. Extended tests of one condition were run to various points up to and after this point and the results analyzed to correlate sensor data with the test specimen surfaces. Sensor data features were used to identify adhesive wear as a continuous process. In particular an increase “friction amplitude” related to a form of stick-slip was used as a key indicator of the occurrence of galling. The findings of this research forms a knowledge base for the development of a decision support system (DSS) to identify lubricant failure based on industrial application requirements.Doctoral These

Development of Fast light Alloys Stamping Technology (FAST) for manufacturing panel components from Dissimilar Alloys – Tailor Welded Blanks (DA-TWBs)

The reduction of weight for car Body-in-White (BIW) structures through the use of high/ultra-high strength aluminium alloys is the most efficient way to achieve CO2 emissions and reduce fuel consumption. Hot and warm stamping are forming techniques commonly used in the automotive industry to form aluminium alloy sheets into structural components. However, it is challenging to improve the production rate and achieve further cost savings with these mature forming technologies. Moreover, there are significant challenges in current forming technologies to form dissimilar alloys, and the use of tailor welded blanks for BIW necessitates the development of novel forming technologies. The present work aims to develop a novel sheet metal forming technology – Fast light Alloys Stamping Technology (FAST) for manufacturing panel components from Dissimilar Alloys – Tailor Welded Blanks (DA-TWBs), whilst achieving desirable mechanical properties in a cost and time efficient manner. The dissimilar alloys in this study consist of two base materials of 6xxx series Al-Mg-Si and 7xxx series Al-Zn-Mg-Cu alloys, which were joined by friction stir welding. The feasibility of the FAST was initially studied on the aluminium alloys AA6082 and AA7075, then applied to the application of DA-TWBs by using the common processing window that was suitable for both AA6082 and AA7075. The optimisation of the processing window of the FAST process and a comprehensive understanding of the thermal-mechanical properties and a post-Paint Bake Cycle (PBC) strength investigation on various forming process condition were conducted. The implementation of the proposed FAST process was conducted by forming M and U-shaped panel components in lab scale. The FAST optimal process was successfully implemented to form a U-shaped component which was made from DA-TWBs at 300 °C and enabled a significant reduction of total cycle time from several hours to 10 seconds, which further improved the production rate to 12.5 spm (strokes per minute). In order to reduce experimental efforts, the present research described an efficient method to determine the critical processing parameters, i.e. the integration of the Finite Element (FE) simulated temperature evolutions with the Continuous Cooling Precipitation (CCP) diagrams of aluminium alloys. Through the optimisation of processing parameters, the temperature evolutions and CCP diagrams do not intersect, indicating that the post-PBC strength of the aluminium alloys could be fully retained after a proper artificial ageing process. A general aluminium alloy-independent model with one set of model constants was therefore developed to predict the Interfacial Heat Transfer Coefficient (IHTC) evolutions as a function of contact pressure, surface roughness, initial blank temperature, initial blank thickness, tool material, coating material and lubricant material. Subsequently, the predicted IHTC evolutions for AA6082 and AA7075 were used to simulate their temperature evolutions, which were then integrated with their CCP diagrams to identify the critical processing parameters in hot and warm stamping processes to meet the desired post-PBC strength of the AA6082 and AA7075, which were then experimentally verified by the results of the dissimilar alloy forming. A software agnostic platform ‘Smart Forming’, was developed to provide cloud Finite Element Analysis (FEA) of a hot and warm stamping process in three stages, namely pre-FE modelling, FE simulation and post-FE evaluation. When the desired materials and processing window were uploaded on the platform, the flow stress, material properties, IHTC and friction coefficient were predicted by the model-driven functional modulus and then generated in the form of compatible packages that could be implemented into the desired FE software. Subsequently, the FE simulation was performed either locally or remotely on the developed platform. When the simulated evolutionary thermomechanical characteristics of the formed component were uploaded, the formability, quenching efficiency and post-PBC strength could be predicted and then demonstrated on a dedicated visualiser on the developed platform. Cloud FEA of FAST was conducted to demonstrate the function of the developed platform, showing an error of less than 10 %. Open Acces

Application of evolutionary strategies to industrial forming simulations for the identification and validation of constitutive laws

In the last decade, the deployment of simulation systems in the automotive industry on the basis of the finite element method (FEM) became a standard for the evaluation of sheet metal forming processes. The technical and economic benefits of the FEM based simulation strongly depends on the accuracy of the computed prediction. The predictive capability of FEM based simulations is mainly determined by the chosen physical theory and its numerical solution. This thesis focuses on the application of optimization algorithms for the identification and validation of material models, which belong to the group of the constitutive laws. The objective of this thesis is threefold: Firstly, the identification of potentials regarding the material models in order to maximize the benefit of the FEM forming simulation and, secondly, the development of an identification and validation procedure for material models. Finally, the effect of the deviations between the measured data and the true values of the calibration experiments on the predictive capability of material models is investigatedUBL - phd migration 201

Study of HFQ forming process on lightweight alloy components

In order to reduce CO2 emissions and improve fuel efficiency for the aerospace industry, a leading edge sheet metal forming technology, namely solution heat treatment, forming and in-die quenching (HFQ) was utilised to form lightweight, complex-shaped components, efficiently and cost-effectively. The work performed in this research project contains two major achievements. The first achievement is successfully forming a complex AA2060 (Al-Li alloy) wing stiffener demonstrator part, and an L-shape AA7075 demonstrator part, without necking or fracture, using HFQ forming technology. The feasibility of forming the aluminium alloys was based on a series of fundamental experimental tests including uniaxial tensile test, isothermal forming limit test and artificial aging test. The second achievement is the development of a novel forming limit prediction model, namely the viscoplastic-Hosford-MK model. This model enables the forming limit prediction of AA2060 and AA7075 alloys under hot stamping conditions, featuring non-isothermal and complex loading conditions. This prediction model fills a significant need in industry for accurately predicting the forming limit of aluminium alloys under such complex forming conditions. The effectiveness of the developed model was analytically verified for AA2060, demonstrating accurate material responses to cold die quenching, strain rate and loading path changes. By applying the developed model to the hot stamping of an AA2060 component, its accuracy was successfully validated. Furthermore, the viscoplastic-Hosford-MK model was also demonstrated for use in industry by determining the optimum initial blank shape of an L-shape AA7075 component. An iterative simulation procedure implementing the forming limit prediction model was used to arrive at an optimum blank shape by the minimisation of the failure criterion. The optimised initial blank shape design was applied in the experimental hot stamping of a demonstrator AA7075 component. The accuracy of the developed model was validated by the successful forming of the component, without necking or fracture.Open Acces

Impulse-Based Manufacturing Technologies

In impulse-based manufacturing technologies, the energy required to form, join or cut components acts on the workpiece in a very short time and suddenly accelerates workpiece areas to very high velocities. The correspondingly high strain rates, together with inertia effects, affect the behavior of many materials, resulting in technological benefits such as improved formability, reduced localizing and springback, extended possibilities to produce high-quality multi material joints and burr-free cutting. This Special Issue of JMMP presents the current research findings, which focus on exploiting the full potential of these processes by providing a deeper understanding of the technology and the material behavior and detailed knowledge about the sophisticated process and equipment design. The range of processes that are considered covers electromagnetic forming, electrohydraulic forming, adiabatic cutting, forming by vaporizing foil actuators and other impulse-based manufacturing technologies. Papers show significant improvements in the aforementioned processes with regard to: Processes analysis; Measurement technique; Technology development; Materials and modelling; Tools and equipment; Industrial implementation

DEVELOPMENT OF RAPID DIE WEAR TEST METHOD FOR ASSESSMENT OF DIE LIFE AND PERFORMANCE IN STAMPING OF ADVANCED/ULTRA HIGH STRENGTH STEEL (A/UHSS) SHEET MATERIALS

Automotive companies are actively pursuing to increase the use of high-strength-lightweight alloys such as aluminum, magnesium, and advanced/ultra high-strength steels (A/UHSS) in body panel and structural part applications to achieve fuel efficiency while satisfying several environmental and safety concerns. A/UHSS sheet materials with higher strength and crashworthiness capabilities, in comparison to mild steel alloys, are considered as a near-term (i.e., ~5 years) choice of material for body and structural components due to their relatively low cost when compared with other lightweight materials such as aluminum and magnesium. However, A/UHSS materials present an increased level of die wear and springback in stamping operations when compared to the currently used mild steel alloys due to their higher surface hardness and high yield strength levels. In order to prevent the excessive wear effect in stamping dies, various countermeasures have been proposed such as alternative coatings, modified surface enhancements in addition to the use of newer die materials including cast, cold work tool, and powder metallurgical tool steels. In this study, a new die wear test method was developed and tested to provide a cost-effective solution for evaluating various combinations of newly developed die materials, coatings and surfaces accurately and rapidly. A new slider type of test system was developed to replicate the actual stamping conditions including the contact pressure state, sliding velocity level and continuous and fresh contact pairs (blank-die surfaces). Several alternative die materials in coated or uncoated conditions were tested against different AHSS sheet blanks under varying load, sliding velocity circumstances. Prior to and after wear tests, several measurements and tribological examinations were performed to obtain a quantified performance evaluation using commonly adapted wear models. Analyses showed that (1) the rapid wear method is feasible and results in reasonable wear assessments, (2) uncoated die materials are prone to expose severe form wear (galling, scoring, etc.) problems; (3) coated samples are unlikely to experience such excessive wear problems, as expected; (4) almost all of the the recently developed die materials (DC 53, Vancron 40, Vanadis 4) performed better when compared to conventional tool steel material AISI D2, and (5) in terms of coating type, die materials coated with thermal diffusion (TD) and chemical vapor deposition (CVD) coatings performed relatively better compared to other tested coating types; (6) It was seen that wear resistance correlated with substrate hardness

Closed-loop control of product properties in metal forming

Metal forming processes operate in conditions of uncertainty due to parameter variation and imperfect understanding. This uncertainty leads to a degradation of product properties from customer specifications, which can be reduced by the use of closed-loop control. A framework of analysis is presented for understanding closed-loop control in metal forming, allowing an assessment of current and future developments in actuators, sensors and models. This leads to a survey of current and emerging applications across a broad spectrum of metal forming processes, and a discussion of likely developments.Engineering and Physical Sciences Research Council (Grant ID: EP/K018108/1)This is the final version of the article. It first appeared from Elsevier via https://doi.org/10.1016/j.cirp.2016.06.00