An experimental and numerical study on aluminum alloy tailor heat treated blanks

Information is presented on the conceptualization, experimental study, and numerical process simulation of tailor heat treated aluminum alloy blanks. This concept is intended to improve the forming behavior of aluminum parts in challenging conditions. The implementation requires precise control of laser heat treatment parameters within a suitable industrial framework. The study details material properties, heat treatment parameters, and experimental results for the strength and elongation properties of an AA6063-T6 aluminum alloy. Constitutive modeling is applied using the Hocket–Sherby equation, which allowed us to establish a correlation between laser heat treatment maximum temperature and the corresponding material softening degree. Based on the generated flow stress–strain curves, a numerical simulation of a representative case study was performed with Abaqus finite element software highlighting potential improvements of tailor heat treated blanks (THTB). The influence and effectiveness of heat-affected zone (HAZ) dimensions and material softening were analyzed.This research was funded by Projects I&DT SIT—Softening in Tool, grant number CENTRO02-0853-FEDER-045419 and METRICS (UID/EMS/04077/2020)

Aluminum Alloys Behavior during Forming

Industrial revolution toward weight reduction and fuel efficiency of the automotive and aerospace vehicles is the major concern to replace heavy metals with light weight metals without affecting much strength. For this, aluminum alloys are the major contributors to those industries. Moreover, aluminum alloys are majorly categorized as 1xxx, 2xxx, 3xxx, 4xxx, 5xxx, 6xxx, 7xxx, and 8xxx based on major alloying elements. Among all, 2xxx, 5xxx, 6xxx, and 7xxx are having majority of applications in the abovementioned industries. For manufacturing any engineering deformable components, forming characteristics are must. Forming behavior of aluminum alloys has been evaluated through different processes including deep drawing, stretching, incremental forming, bending, hydro forming etc., under different process conditions (cold, warm, and hot conditions) and process parameters. Each process has its own process feasibility to evaluate the formability without any forming defects in products. The present chapter discusses a few important processes and their parameter effect on the aluminum alloys through the experimentations and simulation works

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

Modelling of phase transformation in hot stamping of boron steel

Knowledge of phase transformations in a hot stamping and cold die quenching process (HSCDQ) is critical for determining physical and mechanical properties of formed parts. Currently, no modelling technique is available to describe the entire process. The research work described in this thesis deals with the modelling of phase transformation in HSCDQ of boron steel, providing a scientific understanding of the process. Material models in a form of unified constitutive equations are presented. Heat treatment tests were performed to study the austenitization of boron steel. Strain-temperature curves, measured using a dilatometer, were used to analyse the evolution of austenite. It was found that the evolution of austenite is controlled by: diffusion coefficient, temperature, heating rate and current volume proportion of austenite. An austenitization model is proposed to describe the relationship between time, temperature, heating rate and austenitization, in continuous heating processes. It can predict the start and completion temperatures, evolution of strain and the amount of austenite during austenitization. Bainite transformation with strain effect was studied by introducing pre-deformation in the austenite state. The start and finish temperatures of bainite transformation at different cooling rates were measured from strain-temperature curves, obtained using a dilatometer. It was found that pre-deformation promotes bainite transformation. A bainite transformation model is proposed to describe the effects of strain and strain rate, of pre-deformation, on the evolution of bainite transformation. An energy factor, as a function of normalised dislocation density, is introduced into the model to rationalise the strain effect. Viscoplastic behaviour of boron steel was studied by analyzing stress-strain curves obtained from uni-axial tensile tests. A viscoplastic-damage model has been developed to describe the evolution of plastic strain, isotropic hardening, normalised dislocation density and damage factor of the steel, when forming in a temperature range of 600°C to 800°C. Formability tests were conducted and the results were used to validate the viscoplastic-damage model and bainite transformation model. Finite element analysis was carried out to simulate the formability tests using the commercial software, ABAQUS. The material models were integrated with ABAQUS using VUMAT. A good agreement was obtained between the experimental and FE results for: deformation degree, thickness distribution, and microstructural evolution

Hydroforming of locally heat treated tubes

In tube hydroforming, the process chain can be very long as it may involve several pre-forming operations (e.g. bending, crushing, end forming, etc.) which are usually followed by an intermediate annealing stage. Conventional annealing is performed in batches and it is often perceived as a long, relatively expensive and non-environmentally friendly operation. For this reason, in this paper local intermediate heat treatment is proposed as a promising alternative solution, in order to reduce the throughput process time. The study has been carried out on a real tubular motorcycle part, by performing both experiments and numerical simulations, in order to verify whether local annealing can be an effective substitute of conventional global annealing. Several alternative ways of locally heat treating an Al6060 tube right before hydroforming have been investigated. The results demonstrate that a feasible solution can be found, with local heat treatment of relatively small portions of the tube

Hybrid Bulk Metal Components

In recent years, the requirements for technical components have steadily been increasing. This development is intensified by the desire for products with a lower weight, smaller size, and extended functionality, but also with a higher resistance against specific stresses. Mono-material components, which are produced by established processes, feature limited properties according to their respective material characteristics. Thus, a significant increase in production quality and efficiency can only be reached by combining different materials in a hybrid metal component. In this way, components with tailored properties can be manufactured that meet the locally varying requirements. Through the local use of different materials within a component, for example, the weight or the use of expensive alloying elements can be reduced. The aim of this Special Issue is to cover the recent progress and new developments regarding all aspects of hybrid bulk metal components. This includes fundamental questions regarding the joining, forming, finishing, simulation, and testing of hybrid metal parts

Application of control theory to dynamic systems simulation

The application of control theory is applied to dynamic systems simulation. Theory and methodology applicable to controlled ecological life support systems are considered. Spatial effects on system stability, design of control systems with uncertain parameters, and an interactive computing language (PARASOL-II) designed for dynamic system simulation, report quality graphics, data acquisition, and simple real time control are discussed

Tribological behaviour of high thermal conductivity tool steels for hot stamping

In the last years, the use of High Strength Steels (HSS) as structural parts in car manufacturing, has rapidly increased thanks mainly to their favourable strength to weight ratios and stiffness, which allow a reduction of the fuel consumption to accommodate the new restricted regulations for CO2 emissions control, but still preserving or even enhancing the passengers’ safety. However, the formability at room temperature of HSS is poor, and for this reason, complex-shaped HSS components are produced applying the plastic deformation of the sheet metal at high temperature. The use of hot stamping technology, which was developed during the 70’s in Sweden, has become increasingly used for the production of HSS for the car body-in-white. By using this technology, several improvements have been made, if compared with the forming at room temperature, such as the reduction of spring back and the forming forces, the production of more complex shapes, a more accurate microstructure control of the final piece and the achievement of components with high mechanical properties. The hot stamping process of HSS parts consists mainly in heating a metal sheet up to austenitization temperature and then a simultaneous forming and hardening phase in closed dies, water-cooled, to obtain a fully martensitic microstructure on the final components; in this way, ultimate tensile strength passes from 600 MPa up to 1500-1600 MPa. Anyway, several tribological issues arise when the die and metal sheet interact during the forming process at elevated temperatures; the absence of any types of lubricant due to elevate process temperature and in order to preserve the quality of the part for the later stages of the process chain, leads to high friction forces at interface; moreover, and the severe wear mechanisms together with surface damage of forming dies, can alter the quality of the component and can also have an high impact on the process economy due to frequent windows-maintenance or reground of tools. Furthermore, considering that the thermal conductivity of the die material influences the cooling performance, obtained during the quenching phase, and being the quenching time the predominant part of the cycle time, the productivity of the process is influenced too. On this base tool steels play a capital role in this process, as they strongly influence the properties of the obtained final product and have a strong impact to investment and maintenance costs. The survey of the technical and scientific literature shows a large interest in the development of different coatings for the blanks from the traditional Al-Si up to new Zn-based coating and on the analysis of hard PVD, CVD coatings and plasma nitriding, applied on dies. By contrast, fewer investigations have been focused on the development and test of new tools steels grades capable to improve the wear resistance and the thermal properties that are required for the in-die quenching during forming. The research works reported are focused on conventional testing configurations, which are able to achieve fundamental knowledge on friction behaviour, wear mechanisms and heat transfer evaluation, with both a high accuracy for the process parameters and less information about situations that replicate the thermal-mechanical conditions to which the forming dies are subject during the industrial process. Alternatively, the tribological performance have been studied through costly and time-consuming industrial trials but with a lower control on process parameters. Starting from this point of view, the main goal of this PhD thesis is to analyse the tribological performance in terms of wear, friction and heat transfer of two new steel grades for dies, developed for high-temperature applications, characterized by a High Thermal Conductivity with the purpose to decrease the quenching time during the hot stamping process chain and overcome the limits in terms of process speed. Their performances are compared with a common die steel grade for hot stamping applications. To this aim, a novel simulative testing apparatus, based on a pin on disk test, specifically designed to replicate the thermo-mechanical cycles of the hot stamping dies, was used to evaluate the influence of different process parameters on the friction coefficient, wear mechanisms and heat transfer at interface die-metal sheet. Unlike other research works reported in the literature, which individually analyse the friction, the wear mechanisms and thermal aspects, by means of the methodology used in this thesis, the tribological characterization as a whole is obtained by means of a single approach, in order to analyse the simultaneous global evolution of the tribological system

Developing a Thermometallurgical Model and Furnace Optimization for Austenitization of Al-Si Coated 22MnB5 Steel in a Roller Hearth Furnace

Lightweighting of vehicles while preserving crash-worthiness, in order to satisfy stringent restrictions imposed by the government on the automotive industry, has become a sought after solution which can be realized via hot-forming die quenching (HFDQ). HFDQ is a process where boron-manganese steel blanks, a grade of ultra-high strength steels with a thin eutectic Al-Si coating, are heated beyond TAc3 to achieve a fully austenitic microstructure, a precursor for martensite. Heat treatment is performed using 30 to 40 meter long roller hearth furnaces, comprised of multiple heating zones, with two key objectives: (1) ensure complete austenitization of blanks and (2) transformation of the Al-Si coating into a protective Al-Si-Fe intermetallic coating. Blank heating rates are controlled by the roller speed and zone set-point temperatures, which are currently set by trial-and-error procedures. Therefore, a thorough understanding of the furnace parameters and the industrial objectives are essential. Patched blanks, with spatially varying thickness, leads to inhomogenous heating, making this relationship elusive. Previous furnace-based energy models only focused on simulating the sensible energy of the load with no explicit information about the latent energy associated with austenitization. Consequentially, the latent term had been incorporated into the sensible energy term thereby defining an effective specific heat. In order to realize how blank heating rate influences microstructural and Al-Si layer evolution, a model coupling heating and austenite kinetics is necessary. This integrated model serves as means for optimizing the heating process. In this work a thermometallurgical model is developed, combining a heat transfer submodel with two austenite kinetic submodels, an empirical first-order kinetics model and a constitutive kinetics model, via the latent heat of austenitization. The models simultaneously predict the heating and austenitization curves, for unpatched/patched blanks heated within a roller hearth furnace. Validation studies showed that the first-order kinetics model reliably estimated heating and transformation kinetics compared to the constitutive model. The validated models are then used to optimize the zone set-point temperatures, roller speed, and cycle length for a 12-zone roller hearth furnace whilst minimizing the cycle time in a deterministic setting. A gradient-based interior point method and hybrid scheme were used to assess the constrained multivariate minimization problem with two alternative austenitization constraints imposed: a soak-time based and explicitly modeled requirement. In both cases, the most savings in cycle time were achieved using the explicitly modeled phase fraction austenite constraint, with reductions of approximately 2 to 3 times from the nominal settings

Investigation of Resistance Spot Weld Failure in Tailored Hot Stamped Assemblies

This thesis presents the results from the mechanical characterization of resistance spot welds within hot stamped USIBOR® 1500-AS steel sheet (1.2 and 1.6 mm thickness) with tailored properties. Three parent metal conditions, ranging from a fully martensitic (495 HV) to a mixed ferritic-bainitic microstructure (211 HV), were obtained using in-die heating (IDH) to control the cooling (quench) rate during the hot stamping process. Flat sheets and hat channel geometries were produced through die quenching in which the die was maintained at 25, 400, and 700°C with strengths of 1,548, 817, and 671 MPa, respectively. The as-quenched sheets were resistance spot welded and mechanically tested in lap shear, cross tension, V-bend, and JIS tensile coupon geometries to characterize the mechanical response of single welds under various loading orientations. Hardness testing was conducted on the welds to investigate the hardness (strength) distribution in the welded region, heat affected zone (HAZ) and parent metal. The drop in hardness within the HAZ increased as the parent metal strength increased, such that the hardness of the HAZ was similar for all three spot welded parent metals (211-318 HV). In the mechanical testing, strain localized in the HAZ (or nugget) with the result that the strength of the welds was relatively constant for all die quench conditions. For the range of material conditions considered, the lowest weld strength 4.0-4.5 kN was measured in the cross-tension tests compared to 12.1-15.0 kN for the lap shear tests (data for 1.2 mm sheet). A new mechanical test, termed the “Caiman”, was developed to study groups of welds under both static and dynamic Mode I structural loading. Channel sections comprising of fully quenched material as well as hot stamped components with tailored, lower strength flanges were joined via spot welding. The experiments subjected the welded regions to Mode I tensile loading to investigate how failure propagated along the spot weld line. It was found that the energy absorption within the welded connections was higher for the lower strength (tailored) parent metal conditions, largely due to activation of plastic deformation within the parent metal as opposed to the fully martensitic condition for which deformation was confined to the weld nugget or HAZ. High speed thermal imaging was shown to be an effective method to detect failure of individual welds and track failure propagation within the welded assembly due to local adiabatic heating associated with weld fracture or pullout. Temperature increases of 7 °C were typical of quasi-static loading whereas increases of 80 °C were observed under dynamic loading. It was determined that initiation of the first weld failure was delayed slightly under dynamic loading (relative to quasi-static); however, failure propagation after initiation was more rapid for dynamic loading for a given load point displacement. Numerical simulations of the lap shear and cross tension single weld experiments were used to calibrate weld failure models within the commercial finite element software LS DYNA, with relatively good correlation to the experimental data. The calibration exercise revealed the importance of simulation of the post failure response, in particular the use of the “fade energy” numerical parameter, to more accurately capture the energy released during the weld fracture event. The weld failure models calibrated from the single weld tests were applied to simulate the static and dynamic Caiman experiments without additional “tuning”. For the quasi-static Caiman simulations, the predicted load-displacement response and failure propagation were relatively accurate, with higher errors for the fully martensitic case since the HAZ was not modelled. The dynamic simulations aligned relatively well with the experimental results