Numerical Prediction of the Impact Response of Tailored 1500 Mpa and 2000 Mpa Press Hardening Boron Steel

This thesis investigates the response of 1500 MPa and 2000 MPa press hardening steel (PHS) in a dynamic crushing scenario. The work involves creating numerical models to simulate tailored channel sections. The zones of different hardness are created using the tailored hot stamping process (THS), where a heated die set is used to quench an austenitized blank to produce parts with zones of differentiating hardness. The 1500 MPa PHS was quenched using a die heated to different temperatures to produce a range of microstructures (225 HV to 473 HV) as well as the 2000 MPa PHS (297 HV to 574 HV). The as quenched specimens were characterized at strain rates of 0.003 s-1 (quasi-static), 0.1 s-1 (quasi-static), 100 s-1 (intermediate rate) and 1500 s-1 (high rate). The PHS exhibited strain rate-sensitive behavior that was logarithmic in nature. The fully quenched1500 MPa PHS exhibited an ultimate tensile strength that varied from 1502 MPa at a strain rate of 0.003 s-1to 2003 MPa at a strain rate of 1500 s-1. The 2000 MPa PHS exhibited a similar increase in strength with respect to strain rate. The materials were tested for their ability to be spot welded so that the numerical models had accurate spot weld parameters. Both materials were welded using the same weld settings and produced average weld nugget sizes of 5.8 mm (1500 MPa PHS) and 5.3 mm (2000 MPa PHS). Numerical models were developed using data from the uniaxial tensile testing and were simulated at different impact speeds. Axial crushing of tailored and fully hardened channels were simulated to compare the effects of tailoring PHS. The tailored and fully hardened channels absorbed similar amounts of energy. The tailored 1500 MPa PHS absorbed 4.84 kJ and 4.88 kJ for the tailored and fully hardened models. Similarly, the 2000 MPa PHS tailored and fully hardened models absorbed 4.92 kJ and 4.98 kJ, respectively. Although the absorbed energies are comparable, the tailored channels had a lower average impact force. The tailored channels also had more localized crushing, while the fully hardened channels deformed in several sections along their length

Lightweight design of a suspension arm by friction stir welding

The research seeks initially to investigate why a greater shift to lightweight technologies for suspension design has not occurred already over the mass market vehicle sector. It outlines the 'knock-on' benefits of lightweight design and identifies roadblocks which hinder progress. Recent annual metrics of vehicle performance related to mass are investigated. Focusing on individual areas of the suspension, benchmarking identifies the best practice amongst current designs. Manufacturing and process engineering strategies are proposed to support the development of lightweight products with considerably improved environmental acceptability.MIG (Metal Inert Gas) welding, universally accepted as the default joining technology in this field, was found to be restrictive to progress due primarily to detrimental effects on metallurgical, dimensional and process variation on both steel and aluminium products. The latest construction materials were reviewed for suspension application, but the focus remained on proposing light weighting solutions for material generically available in economic volumes today, but with new joining technologies to overcome current restrictions in using less of these materials for each component. Following a full review of the joining technologies available for automotive suspension construction, friction stir welding (FSW) was proposed as an alternative joining technology, with FSW replacing MIG in conjunction with extruded aluminium materials. This removed the barriers incumbent in the use of MIG, which demands a more conservative, heavier design to ensure adequate service lifetime. Design concepts were engineered to take maximum advantage of the strategy of aluminium, extrusions, assembled with friction stir welding. Several viable designs were conceived, from which two were developed and compared. The optimum design was then carried forward into a manufacturing feasibility stage. The extrusions were developed for ease of manufacture, and friction stir welding trials progressed on coupons (plain plates) to ensure that the process was viable. Aluminium in the soft and hardened conditions in different thicknesses and joint configurations were successfully friction stir welded during the trial. Future work would develop the extruded aluminium arm further, into the prototype phase, with sample extrusions being manufactured, FSW welded and assembled. Prototypes would then be rig tested to ensure mechanical and durability performance prior to vehicle trials. There are also possibilities in developing high strength thin wall multi-phase steel solutions, utilising Friction Stir Spot Welding (FSSW). This welding technology enhances the selection of high strength steels, as minimal strength is sacrificed during the joining operation

A state of the art review of hydroforming technology : its applications, research areas, history and future in manufacturing

Hydroforming is a relatively new metal forming process with many advantages over traditional cold forming processes including the ability to create more complicated components with fewer operations. For certain geometries, hydroforming technology permits the creation of parts that are lighter weight, have stiffer properties, are cheaper to produce and can be manufactured from fewer blanks which produces less material waste. This paper provides a detailed survey of the hydroforming literature of both established and emerging processes in a single taxonomy. Recently reported innovations in hydroforming processes (which are incorporated in the taxonomy) are also detailed and classified in terms of “technology readiness level”. The paper concludes with a discussion on the future of hydroforming including the current state of the art techniques, the research directions, and the process advantages to make predictions about emerging hydroforming technologies

Determination of the interfacial heat transfer coefficient in hot stamping of aluminium alloys

The characteristic properties of aluminium alloys, e.g. their high strength-weight ratio, high thermal conductance, excellent corrosion resistance and good recyclability, render them ideal materials to reduce air pollution and improve the fuel economy of vehicles. However, their low formability at room temperature limits their application in industry. In recent years, hot stamping was developed as a promising technology to form sheet metal components from aluminium alloys at elevated temperatures to increase their formability. The interfacial heat transfer coefficient (IHTC), an essential thermophysical parameter in hot stamping processes, should therefore be identified not only to retain the full mechanical strength of the formed components by achieving the critical quenching rates for different aluminium alloys, but also to optimise the production rate by controlling the quenching process. The present research aims to determine the IHTC values for 7075 and 6082 aluminium alloys under different experimental conditions. A dedicated IHTC test facility, IHTC-mate, was developed to precisely measure the temperature evolutions of the specimens and thus accurately determine their IHTC values with high stability and repeatability. Subsequently, the effects of the contact pressure, tool material, coating material, specimen thickness, lubricant and initial blank temperature on the IHTC were identified. It was found that the IHTC increased logarithmically with increasing contact pressure. In addition, the applications of tools, coatings and lubricants with higher thermal conductivities, as well as specimens with larger thickness and higher initial blank temperature could raise the IHTC values. Furthermore, a mechanism-based IHTC model was developed to predict the IHTC evolutions as a function of those influential factors, and enable the interaction between the IHTC evolutions and lubricant layer thickness diminution with sliding distance at different contact pressures and sliding speeds. Hemispherical dome and B-pillar forming tests were conducted to form 7075 and 6082 aluminium alloys. The good agreements between the experimental and simulated temperature evolutions of the components being formed validated the determined IHTC results and developed IHTC model. Consequently, the temperature evolutions and cooling rates of the components being formed in hot stamping processes could be predicted. Furthermore, the processing window and tool design could be optimised to achieve the critical cooling rates and thus retain the full mechanical strength of the formed components.Open Acces

Application of Surrogate Based Optimisation in the Design of Automotive Body Structures

The rapid development of automotive industry requires manufacturers to continuously reduce the development cost and time and to enhance the product quality. Thus, modern automotive design pays more attention to using CAE analysis based optimisation techniques to drive the entire design flow. This thesis focuses on the optimisation design to improve the automotive crashworthiness and fatigue performances, aiming to enhance the optimisation efficiency, accuracy, reliability, and robustness etc. The detailed contents are as follows: (1) To excavate the potential of crash energy absorbers, the concept of functionally graded structure was introduced and multiobjective designs were implemented to this novel type of structures. First, note that the severe deformation takes place in the tubal corners, multi-cell tubes with a lateral thickness gradient were proposed to better enhance the crashworthiness. The results of crashworthiness analyses and optimisation showed that these functionally graded multi-cell tubes are preferable to a uniform multi-cell tube. Then, functionally graded foam filled tubes with different gradient patterns were analyzed and optimized subject to lateral impact and the results demonstrated that these structures can still behave better than uniform foam filled structures under lateral loading, which will broaden the application scope of functionally graded structures. Finally, dual functionally graded structures, i.e. functionally graded foam filled tubes with functionally graded thickness walls, were proposed and different combinations of gradients were compared. The results indicated that placing more material to tubal corners and the maximum density to the outmost layer are beneficial to achieve the best performance. (2) To make full use of training data, multiple ensembles of surrogate models were proposed to maximize the fatigue life of a truck cab, while the panel thicknesses were taken as design variables and the structural mass the constraint. Meanwhile, particle swarm optimisation was integrated with sequential quadratic programming to avoid the premature convergence. The results illustrated that the hybrid particle swarm optimisation and ensembles of surrogates enable to attain a more competent solution for fatigue optimisation. (3) As the conventional surrogate based optimisation largely depends on the number of initial sample data, sequential surrogate modeling was proposed to practical applications in automotive industry. (a) To maximize the fatigue life of spot-welded joints, an expected improvement based sequential surrogate modeling method was utilized. The results showed that by using this method the performance can be significantly improved with only a relatively small number of finite element analyses. (c) A multiojective sequential surrogate modeling method was proposed to address a multiobjective optimisation of a foam-filled double cylindrical structure. By adding the sequential points and updating the Kriging model adaptively, more accurate Pareto solutions are generated. (4) While various uncertainties are inevitably present in real-life optimisations, conventional deterministic optimisations could probably lead to the violation of constraints and the instability of performances. Therefore, nondeterministic optimisation methods were introduced to solve the automotive design problems. (a) A multiobjective reliability-based optimisation for design of a door was investigated. Based on analysis and design responses surface models, the structural mass was minimized and the vertical sag stiffness was maximized subjected to the probabilistic constraint. The results revealed that the Pareto frontier is divided into the sensitive region and insensitive region with respect to uncertainties, and the decision maker is recommended to select a solution from the insensitive region. Furthermore, the reduction of uncertainties can help improve the reliability but will increase the manufacturing cost, and the tradeoff between the reliability target and performance should be made. (b) A multiobjective uncertain optimisation of the foam-filled double cylindrical structure was conducted by considering randomness in the foam density and wall thicknesses. Multiobjective particle swarm optimisation and Monte Carlo simulation were integrated into the optimisation. The results proved that while the performances of the objectives are sacrificed slightly, the nondeterministic optimisation can enhance the robustness of the objectives and maintain the reliability of the constraint. (c) A multiobjective robust optimisation of the truck cab was performed by considering the uncertainty in material properties. The general version of dual response surface model, namely dual surrogate model, was proposed to approximate the means and standard deviations of the performances. Then, the multiobjective particle optimisation was used to generate the well-distributed Pareto frontier. Finally, a hybrid multi-criteria decision making model was proposed to select the best compromise solution considering both the fatigue performance and its robustness. During this PhD study, the following ideas are considered innovative: (1) Surrogate modeling and multiobjective optimisation were integrated to address the design problems of novel functionally graded structures, aiming to develop more advanced automotive energy absorbers. (2) The ensembles of surrogates and hybrid particle swarm optimisation were proposed for the design of a truck cab, which could make full use of training points and has a strong searching capacity. (3) Sequential surrogate modeling methods were introduced to several optimisation problems in the automotive industry so that the optimisations are less dependent on the number of initial training points and both the efficiency and accuracy are improved. (4) The surrogate based optimisation method was implemented to address various uncertainties in real life applications. Furthermore, a hybrid multi-criteria decision making model was proposed to make the best compromise between the performance and robustness

Characterization of Spot Weld Failure within Weld Groups under Predominantly Shear Loading

This thesis examines the failure behaviour of spot welded connections in components using three different aluminium-silicon coated hot stamped steels, Ductibor® 500-AS, Ductibor® 1000-AS, and Usibor® 1500-AS, with 1.2 mm and 1.6 mm sheet thicknesses. Spot weld connections are first characterized using single spot weld experiments. A novel experiment is then developed to characterize weld failure propagation within groups of welds under predominantly shear loading conditions. The experiments are modelled to evaluate how a calibrated weld failure model from single spot weld test data performs in predicting spot weld group response. Tensile lap shear and cross tension single spot weld experiments were conducted under quasi-static conditions, according to the AWS D8.9M:2012 standard, for all materials and thicknesses considered in this work. Recorded force versus crosshead displacement curves and integrated absorbed energy versus crosshead displacement are reported for all single spot weld experiments. The higher strength materials, Ductibor® 1000-AS and Usibor® 1500-AS, exhibited brittle weld failure modes and thus absorbed almost no energy following failure initiation. The lap shear experiments showed similar levels of spot weld strength, around 15 kN, for all 1.2 mm specimens and approximately 20-26 kN for the 1.6 mm lap shear specimens. The cross tension experiments showed similar strength for the Ductibor® 500-AS and Ductibor® 1000-AS specimens, 7 kN for the 1.2 mm thickness and 12 kN for the 1.6 mm thickness. The peak loads for the Usibor® 1500-AS cross tension specimens were approximately 50% of the loads for the other two materials. The Ductibor® 500-AS specimens absorbed the most energy for the lap shear and the cross tension experiments which is attributed to increased parent metal deformation and more ductile weld failure characteristics. A new mechanical test, termed the Caiman Mode III, was developed to promote shear failure within a group of spot welds in a manner similar to a mode III fracture mechanics specimen. A custom rail design using U-channels, that is fabricated in stages, is selected. The Caiman Mode III experiments are tested under quasi-static and dynamic loading rate conditions to examine the mechanical properties of spot welds in a structure in which the applied load can be shared across multiple spot welds. The Caiman Mode III experiments further showed that the three materials have similar peak loads and that the Ductibor® 500-AS spot welds have the highest toughness and most absorbed energy of the three materials tested. Spot weld failure timing was recorded to characterize the extent and rate of spot weld failure propagation in the Caiman Mode III experiments. High speed thermal imagery was applied to determine precise spot weld failure times in both the quasi-static and the dynamic Caiman Mode III experiments since it was difficult to identify failure of specific weld from the force-displacement data. The Ductibor® 500-AS exhibited a slower rate of failure propagation through the weld group compared to the two higher strength alloys. Two different spot weld material models were considered in numerical simulations of the single spot weld and Caiman weld group experiments. The first weld material model, *MAT_100_DA, is used commonly by industry in car crash simulations, while the second weld material model, *MAT_240, used a cohesive zone approach that enables more direct control over the spot weld post-failure behaviour. Both weld material models were calibrated to the single spot weld experiments with respect to force versus displacement. The *MAT_100_DA model showed no post-failure unloading response for the normal-tensile loading condition, thus under predicting the total absorbed energy. The *MAT_240 model enabled more accurate post-failure unloading for all of the single spot weld conditions tested. Simulations of the Caiman Mode III experiments were performed to validate the calibrated single spot weld models. The *MAT_240 simulations and the *MAT_100_DA simulations show similar predictions for each material condition except for the Ductibor® 500-AS 1.6 mm model. The numerical simulations of the Caiman Mode III experiments were able to qualitatively predict the overall behaviour of the Caiman Mode III experiments, including aspects such as high initial load followed by load drops at each sequential weld failure and load drop off, as well as progressive failure propagation through the weld group. However, the simulations showed inconsistent peak force and energy absorption accuracy results when examining the results of different material and thickness simulations. The Caiman Mode III simulation inconsistencies for both material models suggests that the differences between the predictions and the experiments may be from physical inconsistencies between the design and the as-fabricated final specimens. Edge tear out failure was observed in the Caiman Mode III experiments due to 10 mm nugget-to-edge distance while the spot weld models were calibrated from single spot weld experiments with 20 mm weld-edge distance that exhibited button pull out failure instead

Two-Scale Thermomechanical Simulation of Hot Stamping

Hot stamping is a hot drawing process which takes advantage of the polymorphic steel behavior to produce parts with a good strength-to-weight ratio. For the simulation of the hot stamping process, a nonlinear two-scale thermomechanical model is suggested and implemented into the FE tool ABAQUS. Phase transformation and transformation induced plasticity effects are taken into account. The simulation results regarding the final shape and residual stresses are compared to experimental findings

Two-Scale Thermomechanical Simulation of Hot Stamping

Hot stamping is a hot drawing process which takes advantage of the polymorphic steel behavior to produce parts with a good strength-to-weight ratio. For the simulation of the hot stamping process, a nonlinear two-scale thermomechanical model is suggested and implemented into the FE tool ABAQUS. Phase transformation and transformation induced plasticity effects are taken into account. The simulation results regarding the final shape and residual stresses are compared to experimental findings

Numerical Modelling and Simulation of Metal Processing

This book deals with metal processing and its numerical modelling and simulation. In total, 21 papers from different distinguished authors have been compiled in this area. Various processes are addressed, including solidification, TIG welding, additive manufacturing, hot and cold rolling, deep drawing, pipe deformation, and galvanizing. Material models are developed at different length scales from atomistic simulation to finite element analysis in order to describe the evolution and behavior of materials during thermal and thermomechanical treatment. Materials under consideration are carbon, Q&T, DP, and stainless steels; ductile iron; and aluminum, nickel-based, and titanium alloys. The developed models and simulations shall help to predict structure evolution, damage, and service behavior of advanced materials