The present study investigates the effect of coupled thermo-mechanical phenomena, referred to thermal evolution and plastic deformation, during hot stamping of precipitation hardenable aluminum alloy AA7075 by using an in-situ approach of differential cooling in combination with electron microscopy. To explore the intrinsic geometry-dependent local strain distribution of the tailored microstructure, digital image correlation coupled with tensile tests is used. The precipitates size distribution, morphology and types are investigated using electron channeling contrast imaging and differential scanning calorimetry. Results indicate that the forming tool temperature and contact time within closed forming tools are influential parameters for tailoring the microstructure distribution. A sigmoidal curve-like hardness distribution is found for the forming tool temperature difference of 24 degrees C-300 degrees C and 24 degrees C-350 degrees C, showing a steep decrease from 180 HV5 to 123 HV5. This is explained by the detected coarse and lath-shaped quench-induced precipitates in the soft zone and formation of fine strengthening eta '-phase type in the hard zone in areas with higher supersaturation. Transition zones contain a mixture of precipitates structures of varying morphology and types, with a lower aspect ratio on the heated section. This fact leads to an inhomogeneous plastic deformation mainly located in the heated zones and consequently a high strain hardening rate due to the increased inter-particle-spacing and higher dislocation mean free path. Post-cooling after differential cooling generates higher mechanical properties for water-quench compared to air-cooling, as it prevents the nucleation of quench-induced precipitates and segregation of solute atoms to grain boundaries.

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In the past decade, aluminum alloys have become important structural materials in the automotive industry, thanks to their low density, high strength, high fracture toughness, and good fatigue performance. However, an important limitation of aluminum alloys is their poor formability at room temperature; as a result, numerous studies have been conducted with the aim of developing forming techniques to overcome this and facilitate the forming of more complex‐shaped components. Following an overview on the metallurgical background of aluminum alloys, this article reviews recent developments in forming processes for aluminum alloys. The focus is on process variants at room temperature and at higher temperatures and on a new hot forming technique promising considerable improvements in formability. This review summarizes the influence of different process parameters on microstructures and mechanical properties. Particular emphasis is given to process design and to the underlying microstructural phenomena governing the strengthening mechanisms.

The recently increasing demand for hot stamped aluminum components in the automotive and aerospace industries explains the necessity of designing efficient and resource-conserving thermo-mechanical processes. Within the thermo-mechanical process, the simultaneous effect of deformation and temperature accelerate the precipitation kinetics. Therefore, this study focuses on the combined effect of forming and aging processes on the mechanical properties of high-strength aluminum alloys AA6082 and AA7075. For this aim, two different thermo-mechanical aging process strategies after solution heat treatment and quenching in a water-dilutable polymer quenchant are proposed. The superpositioning of the forming step is either performed at the beginning or continuously during the aging treatment. The resulting mechanical properties are characterized using tensile tests. With increasing the plastic elongation, there is an increase in yield and tensile strength, which is accompanied by a significant decrease in strain after failure. Both thermo-mechanical aging strategies reveal mechanical properties similar to the conventional T6 peak aged condition with a significant reduction in process time from 24 h to 5 h.

The present work focuses on the prediction of the hot deformation behavior of thermo-mechanically processed precipitation hardenable aluminum alloy AA7075. The data considered focus on a novel hot forming process at different tool temperatures ranging from 24∘C to 350∘C to set different cooling rates after solution heat-treatment. Isothermal uniaxial tensile tests in the temperature range of 200∘C to 400∘C and at strain rates ranging from 0.001 s−1 to 0.1 s−1 were carried out on four different material conditions. The present paper mainly focuses on a comparative study of modeling techniques based on Machine Learning (ML) and the Zerilli–Armstrong model (Z–A) as reference. Related work focuses on predicting single data points of the curves that the model was trained on. Due to the way data were split with respect to training and testing data, it is possible to predict entire stress–strain curves. The model allows to decrease the number of required laboratory experiments, eventually saving costs and time in future experiments. While all investigated ML methods showed a higher performance than the Z–A model, the extreme Gradient Boosting model (XGB) showed superior results, i.e., the highest error reduction of 91% with respect to the Mean Squared Error.

The present study investigates the effect of two different microstructural conditions on the hot deformation behavior of precipitation-hardenable AA7075 by compression tests ranging from 200 °C to 350 °C and strain rates from 0.1 s−1 to 10 s−1. The first condition is solution heat-treated and quenched in water, whereas the second condition is achieved by subsequent artificial aging and stabilization for 24 h at the respective intended deformation temperature. Both conditions indicate an increase in flow stress with increasing strain rate and decreasing deformation temperature. Moreover, with increasing deformation temperature and decreasing strain rate, the flow behavior gradually changes as dynamic recrystallization becomes the dominant factor for the flow curve appearance. At the same deformation temperature, higher flow stresses are obtained for the as-quenched condition due to the dynamic precipitation and growth of very small precipitates (r < 20 nm) during hot deformation. For the deformation temperature of 200 °C and the strain rate of 10 s−1, higher peak stresses of 110 MPa are obtained for the as-quenched condition. This is confirmed by the transmission electron microscopy investigations, which show the formation of very fine precipitates for the as-quenched condition, while coarse precipitates can be found in the stabilized microstructure. Despite this observation, the work hardening analysis reveals lower strain-hardening rates for the as-quenched condition and higher critical stresses for the onset of dynamic recrystallization compared to the thermally stabilized microstructure.

The present study focused on the influence of different aging conditions on the strain-dependent damping of the high-strength aluminum alloy AA7075. For this purpose, different artificial aging strategies were carried out after solution heat treatment with subsequent water quenching to identify correlations between microstructural evolution, hardness development, and individual material damping. The resulting material damping was measured using an experimental setup based on the principle of electromagnetic feedback. Scanning transmission electron microscopy (STEM) investigations were carried out using a scanning electron microscope (SEM) to characterize the material’s microstructure. Depending on the aging conditions, the damping investigations revealed specific characteristic behaviors in the strain-dependent range from 1 × 10−7 to 0.002. Peak aging conditions showed lower damping than the overaged conditions but resulted in the highest hardness. The hardness decreased with increasing aging time or temperature.

Inspired by steel forming strategies, this study focuses on the effect of differential cooling on mechanical properties and precipitation kinetics during hot stamping of high strength AA7075 alloy. For this aim, different forming strategies were performed using segmented and differentially heated forming tools to provide locally tailored microstructures. Upon processing, uniaxial tensile tests and hardness measurements were used to characterize the mechanical properties after the aging treatment. Microstructure investigations were conducted to examine the strengthening mechanisms using the electron channeling contrast imaging (ECCI) technique in a scanning electron microscope (SEM). Based on the obtained results, it can be deduced that the tool temperatures play a key role in influencing the mechanical properties. Lower tool temperatures result in higher material strength and higher tool temperatures in lower mechanical properties. By changing the cooling rate with the use of differently heated forming tools, the mechanical properties can be controlled. Microstructure investigations revealed the formation of very fine and homogeneously distributed particles at cooled zones, which were associated with elevated mechanical properties due to the suppression of second phase particle formation during cooling. In contrast, coarse particles were observed at lower cooling rates, explaining the lower material strength found in these zones.

The aim of this work was to investigate the effect of hot deformation on the aging behavior of precipitation-hardenable aluminum alloy AA7075 within a novel thermo-mechanical forming process, in order to gain insight into its precipitation kinetics. For this purpose, the material was formed at 420 °C after undergoing solution treatment to different strain levels ranging from 2% to 10% to obtain different dislocation densities. After undergoing hot deformation, aging at 120 °C with different parameters was carried out to improve the material hardness. The resulting material properties and microstructure evolution were characterized afterward using hardness measurements and a transmission electron microscope (TEM). TEM investigations revealed the formation of very fine particles for the material formed at 2%, as well as at 10%, of formed material, which act as effective barriers to dislocation motion. It was found that the response of artificial aging on the deformation degree in hot forming was less than expected due to the thermally activated mechanisms, leading to a decrease in dislocation density. Therefore, a dramatic increase in material hardness with the increase in hot deformation was not observed.

The focus of this study is on the characterization of the local plastic deformation and failure of functionally graded 22MnB5 plates after hot stamping using an in situ approach. Various forming strategies at elevated temperatures are performed using differentially tempered forming tools to locally tailor the microstructure. By controlling the cooling rate, graded as well as martensitic and ferritic–pearlitic microstructures are obtained. Experimental analysis includes evaluation of the material‐ and geometry‐dependent local strain evolution by tensile tests coupled with digital image correlation conducted on two different sample geometries, tapered and nontapered. Experimental results reveal a much more complex deformation behavior for the functionally graded than for homogenous samples. As expected, the distribution of plastic strain of the graded sample is depending on the mixture of its constituting microstructures; however, different types of localized failure after necking are found. Obviously, failure evolution also depends on the sample geometry. Failure patterns observed include shear bands, crossed bands, or homogenous localization of strain, i.e., traditional necking. Eventually, in the case of the nontapered geometry, a lower level of local plastic strain with multiple necking sites is observed. Based on findings presented, detailed process‐microstructure‐property‐damage relationships are established.

Herein, the effects of a very recently introduced novel thermomechanical process route on the microstructural evolution and dynamic tensile deformation behavior of two different precipitation hardenable aluminum alloys, i.e., AA6082 and AA7075, are studied. The investigated materials are hot formed and quenched in differently tempered tools to reveal the influence of cooling rate. Microstructure analysis is conducted to study the influences of different cooling strategies on the microstructure evolution and prevailing strengthening mechanisms in the investigated conditions. Dynamic tensile tests at strain rates of 40, 200, and 400 s−1 coupled with digital image correlation are further conducted to study the mechanical performance and the local deformation behavior. Experimental results show a decrease in yield and tensile strength for the material quenched at higher tool temperature. With increasing strain rate, strength and elongation to failure of the investigated alloys increase for all conditions. The obtained mechanical properties can be rationalized based on the prevailing microstructural features. Both alloys show fine and well‐distributed precipitates upon fast quenching, whereas coarse precipitates prevail upon slow cooling.

This study focuses on the high temperature characteristics of thermo-mechanically processed AA7075 alloy. An integrated die forming process that combines solution heat treatment and hot forming at different temperatures was employed to process the AA7075 alloy. Low die temperature resulted in the fabrication of parts with higher strength, similar to that of T6 condition, while forming this alloy in the hot die led to the fabrication of more ductile parts. Isothermal uniaxial tensile tests in the temperature range of 200–400 °C and at strain rates ranging from 0.001–0.1 s−1 were performed on the as-received material, and on both the solution heat-treated and the thermo-mechanically processed parts to explore the impacts of deformation parameters on the mechanical behavior at elevated temperatures. Flow stress levels of AA7075 alloy in all processing states were shown to be strongly temperature- and strain-rate dependent. Results imply that thermo-mechanical parameters are very influential on the mechanical properties of the AA7075 alloy formed at elevated temperatures. Microstructural studies were conducted by utilizing optical microscopy and a scanning electron microscope to reveal the dominant softening mechanism and the level of grain growth at elevated temperatures.

One of the most important prerequisites to meet the increasing demand for efficient technologies for micro-part production is constituted by the ability to overcome existing process limitations by new innovative technological approaches. By the introduction of a new process variant based on a hybrid material condition between solid and liquid state, such an approach is presented. This micro semi-solid manufacturing technology, so-called Micro-Thixoforming, was initiated, on the one hand, by being aware of the technological limits of existing microforming and micro-casting processes and, on the other hand, by a comprehensive understanding of the special rheological mechanisms of metallic materials in semi-solid state finally establishing the desired potential to follow the trend towards miniaturisation with drastically reduced process restrictions. However, this promising potential can only be successfully exploited when the initial idea, which is based on phenomenological considerations, can be transferred to a process technology with sufficient practical relevance. Therefore, the presented new integrated process concept for Micro-Thixoforming is particularly characterised by the application of unconventional solutions for the main process steps: raw material conditioning, thermal pre-processing, semi-solid forming and thermal post-processing. To give an indication of the innovative character of the chosen practical solutions liquid metal jet technology, LASER·induced plasma shockwaves and high pressure water jet should be mentioned. However, what is even more important in this context is the ability not only to realise a process concept but, beyond that, to recognise the further potential regarding new strategies for material design arising from the availability of this process. Such a strategy e.g. consists of utilising the well-known segregation effect, which notably often is negatively associated with semi-solid forming. However, for the envisaged technological approach, controlled segregation aims at a defined adjustment of functionally graded properties for the produced micro part.

Micrometric surface topologies are required for a wide range of technical applications. While lowered surface features have been used for many years to improve the tribological behavior of contacting surfaces, there are also other fields of application, where the potential of elevated surface features is known, e. g. for metal forming tools. However, the demand for a high wear resistance of these structures often inhibits an industrial application. A solution is offered by structuring techniques that use additional material. A promising approach is the localized dispersing of hard ceramic particles by pulsed laser radiation, the so-called laser implantation. This paper describes the potential to adjust the geometry as well as the mechanical properties of laser implanted surfaces by means of microstructural and topological investigations. Afterwards, results of a wear test are given and different applications for this structuring technique are discussed. It can be shown that dome-shaped or ring-shaped structures on a micrometric scale can be produced with high hardness and wear resistance.

Tool wear is one of the main concerns in hot stamping tools. Tool life, part complexity and quality and tool maintenance schedules are all greatly affected by wear mechanisms changing the tool surface. This is particularly true in the case of the commonly used Aluminium-Silicon coated Boron steel, where material transfer from the sheet metal coating to the tool surface generates macroscopic damages in a relatively low number of cycles.In this work, two non-destructive techniques (direct measurement through optical profilometry and indirect measurement through surface replication) have been applied to characterize adhesive wear on tool steel inserts obtained from two different high temperature strip drawing based setups for simulation of the process of hot stamping of Al Si coated boron steel. The obtained measurements have been validated by analysing cross sectional samples of the studied inserts.Results show the applicability of these non-destructive techniques to the characterization of adhesive wear on industrial tools, which can be used to optimise tool maintenance methods and schedules.

Proceedings : 4th International Conference Hot Sheet Metal Forming of High Performance Steel June 9 - 12, 2013, Lulea, Sweden

Due to an increasing amount of hot stamped high strength components in several automotive applications with different challenges for corrosion protection, weldability and surface quality, an increase in the application of coatings with particular thermal sensitivity and, finally, due to dramatically decreasing cycle times in heating and cooling, requirements on time-temperature course in press hardening are becoming more stringent. Therefore, the necessity of having access to detailed information on thermo-physical phenomena, e.g. melting points and presence of melt phases of applied coatings, phase transformations or exact tribological consistency of surface layers becomes evident. The presented paper shows results of thermal measurements and tribological investigations revealing such information for different types of Aluminum and Zinc based coatings.

Within the recent years press hardened ultra-high-strength steel components developed a key role concerning the complex crash management in automotive applications. Due to the need for the integration of at the same time high intrusion resistance and improved absorption of crash energy by controlled plastic deformation, the use of tailored components with graded properties became increasingly important. Nowadays established process variants produce such functionally graded components for body-in-white application by the utilisation of previously graded starting material or by differential thermo-mechanical process variants but are still limited to only two dimensional gradation.In this paper a new approach is presented which enlarges the spectrum of property tailoring to a real three dimensional gradation in the field of hot sheet metal forming. Multi-material composites in sheet metal sizes are created in-situ within the hot stamping process chain. Mechanical testing and metallographic inspection as well as hardness measurement are used to describe the special properties. Their multifunctional features and failure behavior under different loading situations is the key to a further increase in energy absorption and crash performance and thus increased passenger safety.

This work describes the mechanical behavior under monotonic and cyclic loads of 22MnB5-AlSi blanks heat treated and cooled down using four different strategies permitting to obtain fully hardened and soft microstructures. Fatigue samples have been trimmed using different parameters to evaluate the effect of cut edge properties through the comparison with trimmed + polished cut edge samples. Results show that only marginal improvements of as-trimmed fully hardened samples are obtained despite their increased tensile strength with respect to the softer samples. However, when defects in the cut edge or the AlSi coating are removed, their fatigue resistance is significantly increased. As-trimmed soft ferritic-perlitic samples show the lowest tensile strength but reasonable fatigue resistance due to very good damage tolerance capabilities. In contrast, as-trimmed bainitic samples, even if tensile properties are better than the ferritic-perlitic, their increased fatigue sensitivity leads to the lowest results under cyclic loading amongst the studied materials. Thus, attention must be paid in transition and hard zones of tailored parts, trimming processes must be optimized to enhance fatigue performance by reducing cut edge defects.

In the development of optimal forming processes FE-based process simulation is more and more integrated. For these process simulations it is crucial to provide appropriate models and the according parameters of mechanisms relevant for forming processes. Friction between the tool and the workpiece is one of those mechanisms. Based on the basic friction models extended models have been developed to introduce the influence of local contact conditions. It would thus be beneficial if existing friction testing methods could allow the determination of the required parameters. The ring compression test is a commonly used standard friction test. In this work standard and treated samples of a modified ring geometry made of AA1050 are investigated with confocal microscopy after applying the ring compression test supported by FE analysis. The experimental and further theoretical results reveal the limits of applicability of this test on the determination of parameters for extended friction models.