Hot Forming Research Articles

Retracted: Numerical Simulation of Hot Stamping Forming of AZ Series Magnesium Alloys and Optimization of Die Process

Retracted: Numerical Simulation of Hot Stamping Forming of AZ Series Magnesium Alloys and Optimization of Die Process

Advancements in processing of Ni-based superalloys by microstructure engineering via discontinuous γ′ break-down

Wrought γ′-strengthened Ni-based superalloys are critical materials for gas turbine disks in aircraft and power-generation applications. However, their high inherent strength makes their hot formability challenging, and this is often related to intergranular cracking. We propose that this type of crack formation may be mitigated by grain boundary serrations which has not yet been demonstrated in superalloys. Here, we introduce engineered grain boundary serrations in a Ni-based superalloy via discontinuous γ′ precipitation during slow cooling after super-solvus solutionising. We compare the formability of this microstructure to a counterpart with globular γ′ and straight grain boundaries, during sub-solvus compression at 950–1130 °C. We demonstrate that the microstructure with discontinuous γ′ shows excellent cracking resistance, while the one with globular γ′ develops severe intergranular cracks during compression ≤1100 °C. The discontinuous γ′ microstructure exhibits up to 26% lower peak flow stresses and recrystallises at temperatures ≤1100 °C, compared to the globular γ′ microstructure where recrystallisation does not start until 1130 °C. Compression of the discontinuous γ′ microstructure at 1000–1050 °C yields recrystallised fractions of >75 vol.% and results in a duplex γ-γ′ microstructure with grain diameters <4 μm. We argue that its significantly improved formability results from these engineered serrated grain boundaries, the coarsened γ′ morphology, energy absorption by the break-down of discontinuous γ′, and the resulting development of a fine-grained duplex γ-γ′ microstructure. Integrating such discontinuous γ′ precipitation into advanced manufacturing routes will facilitate or, in some cases, even enable manufacturing of parts for aircraft turbines and gas engines at lower temperatures and forces.

A comparative study of limit diagram of ultrasonic-assisted and conventional hot incremental forming of Ti-6AL-4V alloy

Electric hot incremental forming (EHIF) technique is a novel sheet metal forming method for low-formability materials like Mg and Ti-6Al-4V alloys. There are some defects in EHIF including low surface quality, unexpected sheet tear, and oxidation of the fabricated workpieces. These defects limit its widespread use. In this article, longitudinal ultrasonic vibrations of forming tools are used for Ti-6Al-4V sheets in the EHIF process. This technique led to a significant reduction of friction coefficient and improvement of workpieces’ surface quality. Also, a combination of mechanical vibrations and heat reduced the phase transformation temperature of α phase (HCP structure) to the more formable β phase (BBC structure) at 600 °C. It improved Ti-6Al-4V sheets’ formability. In this study, various tensile and compressive strains were created by manufacturing different geometries such as cone, frustum, torispherical, and five-lobe. These strains are used to compare the forming limit diagram of the ultrasonic-assisted and conventional hot incremental forming. The design of the experiment and analysis of variance were employed to identify the optimum condition for creating the largest strains. Also, effective parameters were optimized by response surface methodology. Then major and minor strains for both techniques were compared. Achieved strains in all types of geometries for ultrasonic-assisted technique were higher than those achieved by a conventional process.

HOT-DEFORMATION BEHAVIOR AND PROCESSING MAP OF 22MnB5 HIGH-STRENGTH STEEL

In order to explore the hot formability of 22MnB5, a hot-tensile test of 22MnB5 high-strength steel plate was carried out using a Gleeble-1500 hot-tensile-testing machine. The stress-strain curves at 850 °C to 950 °C and 0.01 s–1 to 10 s–1 were obtained. The results show that 22MnB5 has an obvious positive-strain-rate sensitivity, and its peak stress decreases with the increase of temperature. The mechanical constitutive equation of 22MnB5 was established by using the improved Johnson-Cook model, and the fitting value was compared with the experimental value. Based on the dynamic material model, the hot-processing map of 22MnB5 was established and analyzed. The hot-processing process was determined as follows: deformation temperature 880 °C to 930 °C, strain rate 0.01 s–1 to 1 s–1.

Neural network-based ductile fracture model for 5182-O aluminum alloy considering electroplastic effect in electrically-assisted processing

Complex components made of 5182-O aluminum alloy are usually formed at high temperatures due to their low ductility at room temperature, and the advanced current assisted processing can achieve energy-saving, high-efficiency, and green production compared with traditional hot forming. In this study, the coupled effects of electrical pulse, temperature, strain rate, and strain on flow behavior and ductile fracture are comprehensively investigated by experiments and constitutive modeling. In order to investigate the influence of electroplastic effects in different stress states, shear, hole and notched specimens are tested by isothermal tensile test and electrically-assisted isothermal tensile test in temperature ranges from 300 to 423 K and strain rates from 0.001 to 0.1/s. The evolution of the microstructure is characterized by electron backscatter diffraction to reveal the mechanism of the experimental phenomenon observed. An artificial neural network model is developed to characterize the dynamic hardening behavior and ductile fracture, and further embedded in ABAQUS/Explicit for numerical simulation. The results show that the experimental results are highly non-linear and coupled for the flow curves. At the same temperature, electric pulses have an independent effect on suppressing the Portevin–Le Chatelier effect and can reduce the deformation resistance, thereby forming products at low energy. The non-monotonicity of the fracture loading paths obtained from the hybrid experimental–numerical analysis is affected by strain rate hardening, thermal softening and electroplasticity. The neural network-based plasticity model is calibrated and validated to describe fracture initiation considering stress state, current density, temperature and strain rate.

Nonlinear unloading-reloading behavior and macro/micro models during multi-pass cyclic hot compression of aluminum alloys

Unloading-reloading behavior is of crucial importance in multi-pass forming of metal materials, the nonlinear of which creates challenges for the analysis of springback and residual stress of the precision manufacture components. Especially for the hot bulk-formed material at elevated temperature, the thermal-mechanical coupling effect makes the unloading-reloading process more complicated and thus more difficult to model and control, which remains to be solved. In this paper, the unloading-reloading behavior dependent on strain rate and deformation temperature and its model of hot compressive deformed aluminum alloys at elevated temperature were studied from the macroscopic and microscopic perspectives. The effect of deformation temperature, strain rate and strain on the unloading-reloading behavior of hot compressive deformed aluminum alloys were revealed, and corresponding micro-mechanism was elucidated and then discussed. Macroscopically, a strain-, strain rate- and temperature-dependent nonlinear macroscopic model describing unloading–reloading process was established, which can be used to describe the unloading stress relaxation and predict springback during multi-pass hot bulk forming. Microscopically, it is suggested that the relaxations of back stress and forest dislocation hardening stress collectively affect the unloading stress relaxation, due to the decrease of mobile and immobile (forest) dislocation density in unloading, while the dislocation-related behavior before yield, i.e. the dislocations’ bowing-out under pinning, affects the reloading behavior. Based on the micro-mechanism, a physically-based micro-model for describing unloading–reloading was constructed which further is used to clarify the physical mechanism of internal stress relaxation and inelastic strain recovery under multi-pass hot compression deformation. The comparison of the calculation results of the models with those of experiments indicated that the established unloading–reloading macroscopic and microscopic models are able to predict the stress–strain response upon the unloading–reloading loop of multi-pass cyclic hot compression deformation.

Contact Temperature Measurements on Hybrid Aluminum–Steel Workpieces in a Cross-Wedge Rolling Process

The Collaborative Research Center 1153 is investigating a novel process chain for manufacturing high-performance hybrid components. The combination of aluminum and steel can reduce the weight of components and lead to lower fuel consumption. During the welding of aluminum and steel, a brittle intermetallic phase is formed that reduces the service life of the component. After welding, the workpiece is heated inhomogeneously and hot-formed in a cross-wedge rolling process. Since the intermetallic phase grows depending on the temperature during hot forming, temperature control is of great importance. In this paper, the possibility of process-integrated contact temperature measurement with thin-film sensors is investigated. For this purpose, the initial temperature distribution after induction heating of the workpiece is determined. Subsequently, cross-wedge rolling is carried out, and the data of the thin-film sensors are compared to the temperature measurements after heating. It is shown that thin-film sensors inserted into the tool are capable of measuring surface temperatures even at a contact time of 0.041 s. The new process monitoring of the temperature makes it possible to develop a better understanding of the process as well as to further optimize the temperature distribution. In the long term, knowledge of the temperatures in the different materials also makes it possible to derive quality characteristics as well as insights into the causes of possible process errors (e.g., fracture of the joining zone).

The Effect of Hot Forming–Quenching Integrated Process Conditions on Friction and Wear Behavior for AA7075 Alloy

The Effect of Hot Forming–Quenching Integrated Process Conditions on Friction and Wear Behavior for AA7075 Alloy

An Inverse Optimization Method for the Parameter Determination of the High-Temperature Damage Model and High-Temperature Damage Graph of Ti6Al4V Alloy.

Ti6AL4V alloy is widely used in the biomedical and energy vehicle industries, among others. Ti6Al4V alloy cannot be fabricated at ambient temperatures; hence, it requires hot forming. However, this method is susceptible to crack defects. The crack defect problem of Ti6AL4V alloy in the hot-forming process cannot be ignored, so we must develop a precise hot-forming damage prediction model. In this study, three high-temperature damage models of Ti6Al4V alloy were developed, considering the temperature and strain rate. These models were derived from the normalized Cockcroft and Latham (NCL), Oyane, and Rice and Tracey (RT) damage models. The damage parameters of the models were identified using a genetic algorithm combined with finite element simulation. The force accumulation error of the Ti6AL4V alloy specimen, which was obtained from a simulated thermal tensile test and an actual test, was used as an optimization target function. Then, the damage parameters were optimized using the genetic algorithm until the target function reached the minimum value. Finally, the optimal damage model parameter was obtained. Through program development, the three high-temperature damage models established in this paper were embedded into Forge® NxT 2.1 finite element software. The simulated thermal tensile test of Ti6AL4V alloy was performed at a temperature of 800-1000 °C and a strain rate of 0.01-5 s-1. The simulated and actual fracture displacements of the tensile specimens were compared. The correlation coefficients (R) were calculated, which were 0.997, 0.951, and 0.912. Of the high-temperature damage models, the normalized Cockcroft and Latham high-temperature damage model had higher accuracy in predicting crack defects of Ti6Al4V alloy during the hot-forming process. Finally, a fracture strain graph and a high-temperature damage graph of Ti6Al4V alloy were constructed. The Ti6Al4V alloy damage evolution and thermal formability were analyzed in relation to the temperature and strain rate.

Research on asynchronous quenching behavior of boron steel tubular parts with variable cross-sections in hot gas forming-quenching integrated process

The hot gas forming of complex-shaped boron steel tubular parts has an asynchronous quenching characteristic, making it difficult to control the microstructure and properties of formed tubular parts. To investigate the asynchronous quenching rule during hot gas forming, a tubular part with variable cross-sections was designed. The asynchronous quenching behavior and microstructure evolution under different process parameters were analyzed via phase transformation-based finite element simulations. Furthermore, the formed tubular parts were obtained under corresponding experimental conditions, the microstructure, hardness, and tensile strength of the tubular parts were tested and compared with simulation results. The results confirmed that the initial forming temperature and bulging pressure determine the area and cooling rate in the non-contacted zones, thereby achieving the quenching strengthening during the cooling of the tubular parts. With the increase in bulging pressure, the cooling rate of the corner zone will be accelerated, and the martensitic volume fraction will be significantly improved. Additionally, the pressure holding time affects the quenching effect in the non-contacted zones. With an initial forming temperature of 850 °C and bulging pressure of 15 MPa, when the pressure holding time increased from 15 s to 20 s, the quenched tubular part was mainly composed of martensite, as a result, the hardness and tensile strength of the part could reach 494 HV and 1486 MPa, respectively. Therefore, in hot gas forming-quenching integrated process of complex-shaped boron steel tubular parts, the cooperative regulation of microstructure and mechanical properties could be achieved with appropriate process parameter adjustment.

Micropore Evolution and Damage Behavior of Rapid‐Solidified Al–Zn–Mg–Cu Alloy during Hot Plastic Forming

Rapid‐solidified Al–Zn–Mg–Cu alloys possess widespread application prospects owing to their excellent properties, particularly high specific strength. Nevertheless, their further development for use as advanced structural parts is significantly limited by their intrinsic porosity. Herein, the flow stress subroutine and micropore evolution model are combined to predict the cracking and damage behavior of a rapidly solidified Al–Zn–Mg–Cu disk‐shaped part during hot forging. The results reveal that the damage to the part during plastic forming is inversely proportional to the relative density. Reasonable matching between the height‐to‐diameter ratio (H/D) and the initial relative density is the key factor in avoiding cracking and damage to the part. The closure sequence of the micropores is from the center to the outside of the billet. A billet with an H/D of 1 and initial relative density of 0.95 can reach full density after forming, and a damage‐free part can be obtained. These simulation results are verified by analyzing the microstructural characteristics and mechanical properties of the actual forged part.

Numerical simulation of workpiece temperature field during radial shear rolling of biomedical Co-Cr-Mo alloy

This paper describes the analysis of temperature field of workpiece during radial shear rolling (RSR) of biomedical Co-28Cr-6Mo alloy. In the process of hot metal forming, the temperature distribution in the deformation zone has a significant influence on the quality and properties of the final product. In some cases, the deformation within a narrow temperature range is required to provide the desired properties. To obtain data on change in temperature, the computer simulation of RSR was carried out using finite element method. According to simulation results, the temperature distribution of bar depending on the variable process parameters such as temperature of workpiece, rotary velocity of rolls, elongation ratio, diameter of workpiece, and temperature of tool was evaluated. It was determined that the most significant parameters influencing on the distribution of the temperature field are the workpiece heating temperature and the rolling speed. The RSR process makes it possible to effectively manage the distribution of temperature field due to selecting technological parameters and, thereby, control the formation of the alloy structure. The data obtained can be useful for developing the modes of RSR of Co–Cr–Mo alloy.

Eigenstrain‐based analysis of why uniformly shot peened aluminium plates bend more in the rolling direction

AbstractShot peen forming is a process widely used to shape aircraft components such as wing skins, yet its fundamental working is still crudely understood. It is understood that a light conventional peen forming treatment applied uniformly over an initially flat plate will induce isotropic in‐plane stretching of the surface layer and will thus lead to a panel curving with identical curvatures in all directions. However, [1] made the startling observation that uniformly peen formed aluminium plates of different aspect ratios all bent along their laminating direction irrespective of the peening direction. This experimental result is counterintuitive because the residual stresses due to the lamination process are 1 order to 2 orders of magnitude smaller than those induced by the shot peen forming treatment. In the present study, we apply the eigenstrain theory to estimate the effect of the different sources of anisotropy on uniformly peen formed aluminium plates. Potential sources of anisotropy included the plastic anisotropy of rolled aluminium, nonequibiaxial initial stresses that redistribute when their equilibrium is disturbed by peening, the geometry of the specimens and externally applied prestress. For the alloy and peening conditions considered, we show that plastic anisotropy had no discernible influence on the resulting shape of the peen formed specimens. Initial residual stresses, on the other hand, caused slightly larger bending loads in the rolling direction of the alloy. Although the magnitude of these loads was approximately 30 times smaller than peening‐induced loads, it was sufficient to overcome the geometric preference for rectangular sheets to bend along their long side and cause all unconstrained specimens to bend along the rolling direction instead. Our analysis highlights the importance of the history of the material that is being peened. Residual stresses already present in the part before peening must be considered to ensure good simulation predictions.

A novel quench-form and in-die creep age process for hot forming of 2219 thin aluminum sheets with high precision and efficiency

A novel quench-form and in-die creep age process for hot forming of 2219 thin aluminum sheets with high precision and efficiency

Hot Deformation Behavior and Processing Maps of an As-Cast Al-5Mg-3Zn-1Cu (wt%) Alloy.

One of the key issues limiting the application of Al-Mg-Zn-Cu alloys in the automotive industry is forming at a low cost. Isothermal uniaxial compression was accomplished in the range of 300-450 °C, 0.001-10 s-1 to study the hot deformation behavior of an as-cast Al-5.07Mg-3.01Zn-1.11Cu-0.01Ti alloy. Its rheological behavior presented characteristics of work-hardening followed by dynamic softening and its flow stress was accurately described by the proposed strain-compensated Arrhenius-type constitutive model. Three-dimensional processing maps were established. The instability was mainly concentrated in regions with high strain rates or low temperatures, with cracking being the main instability. A workable domain was determined as 385-450 °C, 0.001-0.26 s-1, in which dynamic recovery (DRV) and dynamic recrystallization (DRX) occurred. As the temperature rose, the dominant dynamic softening mechanism shifted from DRV to DRX. The DRX mechanisms transformed from continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX), and particle-stimulated nucleation (PSN) at 350 °C, 0.1 s-1 to CDRX and DDRX at 450 °C, 0.01 s-1, and eventually to DDRX at 450 °C, 0.001 s-1. The eutectic T-Mg32(AlZnCu)49 phase facilitated DRX nucleation and did not trigger instability in the workable domain. This work demonstrates that the workability of as-cast Al-Mg-Zn-Cu alloys with low Zn/Mg ratios is sufficient for hot forming.

Simulation of Cross Wedge Rolling and Hot Extrusion-Combined Forming Process for Axle Sleeve

In this paper, an axle sleeve is formed through a combined cross wedge rolling (CWR) and hot extrusion process, and the combined forming process is simulated via finite element analysis software Deform-3D. The forming mechanism is revealed by analyzing the stress and strain distribution, the temperature variation and the metal flow law of the workpiece during CWR and hot extrusion. Combined with CWR and hot extrusion forming experiments, the feasibility of a combined rolling and extrusion process to produce an axle sleeve is verified. It has been proven that the outer steps of the axle sleeve produced through the rolling extrusion composite process are well formed, the flange extrusion cavity is full, the metal streamline is continuous, the axis of the inner hole does not easily deviate, the product quality is good and the production efficiency is high.

Development of steel for hot stamping of 22mnb5 steel by joule effect heating

With the increase in demand and development of the automobile sector, where it becomes important to improve the efficiency of energy consumption of vehicles, one can observe the need for materials that are both light and have high mechanical strengths. The Ultra High Strength Steel (UHSS), which include TWIP (Twinning-Induced-Plasticity) and Hot Stamping are characterized by unique microstructures and metallurgical properties that allow car manufacturers to meet the diverse functional requirements of vehicles. Boron steel is suitable for mechanical hot forming. In the last few years, hot stamped parts have occupied a prominent place in the bodies of bodies due to being in line with the demands mentioned above. There is a lot of research under development for this technology, be it in materials, means of production, coatings and applications. Currently, there is a patent that generates a technological restriction for the processing of hot stamping; in which it is to use boron steel with Al-Si metallic coating and heating the blank to be hot stamped through a radiation furnace. The objective of this work is to use a new processing alternative through another metallic diffusion coating of Fe-Zn and heating of the blank to be hot stamped by Joule effect; generating an industrial experiment and prototyping for characterization. The results showed the feasibility of using boron steel with Fe-Zn diffusion coating in hot forming applications by heating by Joule effect, with good surface quality, meets the mechanical properties and microstructure, without cracks in the steel, excellent adhesion of the paint layer and good resistance to corrosion.

Grain boundary serration tuning and its effect on hot workability of a wrought superalloy

This study aims to elucidate the underlying mechanism of grain boundary serration and investigate its effects on hot workability of U720Li superalloy. Industrial scale as-cast ingots were subjected to heat treatments at 1180 °C followed by different cooling rates (15 °C/s, 0.1 °C/s, 0.02 °C/s and 0.003 °C/s) prior hot deformation. With 15 °C/s fast cooling, samples possessed straight boundaries and fine L12 structured γ’ particles dispersion. When the cooling rate was decreased to 0.1 °C/s and below, continuous and discontinuous precipitation of the γ' phase led to the formations of Type-I and Type-II grain boundary serrations, respectively. Lamellar γ/γ’ behind the mobile grain boundary was observed for Type-II boundary, which exhibited larger degree of undulation than that of Type-I boundary. With increasing fractions of Type-II boundary, the hot formability of U720Li was significantly improved during compression tests at 1000 °C; 70 % reduction ratio (true strain=1.2) could be achieved without crack formation. This work presents a simple process route to improve the hot deformability of U720Li fabricated on an industrial scale.

Multi-Objective Optimization of Dimensional Accuracy in Electric Hot Incremental Sheet Forming

The forming defects of parts mainly include fracture, springback, thermal expansion, and rough surface during electric hot incremental forming, in which the springback and thermal expansion directly affects the dimensional accuracy of parts. In this paper, the combined optimization of process parameters and unsupported distances was proposed to control the dimensional accuracy of forming parts in the electric hot incremental sheet forming process. The predictive model of major errors was established through the response surface methodology, and then the multi-objective optimal model was obtained using Non-dominated Sorting Genetic Algorithms-II (NSGA-II). Meanwhile, a multi-objective optimal result was determined according to the error compensation feature of the forming process. On this basis, the effect of unsupported distances on dimensional errors was analyzed in detail, and a valid unsupported distance was proposed to further improve the forming accuracy of the whole forming region. Finally, the experimental result demonstrated that the combined optimal method proposed was accurate and feasible.

DRX mechanisms of a Ni-Co-W type superalloy with typical columnar grains during hot compression

Columnar evolution and DRX mechanisms of a Ni-Co-W type superalloy with typical columnar grains during hot deformation were investigated processed at the γ′ sub/super-solvus temperatures. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) was employed to characterize the nucleation at columnar boundary. The results show that columnar grains gradually replaced by new dynamic recrystallization (DRX) grains and nucleation was prioritized at original columnar boundary or/and folding position. Meanwhile, DRX mechanisms of the columnar structure associated with γ′ phase were identified deeply. Discontinuous dynamic recrystallization (DDRX) was the main mechanism when deformation processed at γ′ sub/super-solvus temperatures. Bulging columnar grain boundaries are first as the nucleation sites of the new DDRX grains under various conditions. High angle grain boundaries (HAGBs: θ > 15°) bends to the side with γ′ phase, and then the DRX grains formed at the bulging grain boundary. In addition, high dislocation density zone occurred around the large size γ′ phases (more than 1 µm), leading to the misorientation generated rapidly and then the HAGBs formed. The formation of DRX grains by this way is also called particle stimulated nucleation (PSN) mechanism. Continuous dynamic recrystallization (CDRX) occurs when hot deformation processed at γ′ sub-solvus temperature with high strain level, which is accompanied by dislocation accumulation and an obviously increase in the fraction of grain boundaries with 10–15° misorientation angle. The findings help to understand different DRX mechanisms of Ni-based superalloy with columnar structure during microstructure evolution under hot forming, and are also beneficial for optimizing the microstructure to control the required properties of the Ni-based superalloys for manufacturing turbine disc.