Experimental and Numerical Study of Electromagnetic Forming of AZ31B Magnesium Alloy Sheet

Wrought magnesium alloys are interesting materials for automotive and aeronautical industries due to their low density in comparison to steel and aluminium alloys, making them ideal candidates when designing a lower weight vehicle. However, due to their hexagonal close‐packed (hcp) crystal structure, magnesium alloys exhibit low formability at room temperature. For that reason, in this study a high velocity forming process, electromagnetic forming (EMF), was used to study the formability of AZ31B magnesium alloy sheet at high strain rates. In the first stage of this work, specimens of AZ31B magnesium alloy sheet have been characterised by uniaxial tensile tests at quasi‐static and dynamic strain rates at room temperature. The influence of the strain rate is outlined and the parameters of Johnson‐Cook constitutive material model were fit to experimental results. In the second stage, sheets of AZ31B magnesium alloy have been biaxially deformed by electromagnetic forming process using different coil and die configurations. Deformation values measured from electromagnetically formed parts are compared to the ones achieved by conventional forming technologies. Finally, numerical study using an alternative method for computing the electromagnetic fields in the EMF process simulation, a combination of Finite Element Method (FEM) for conductor parts and Boundary Element Method (BEM) for insulators, is shown.     

Gas‐pressure Forming of an AlMg‐Alloy Sheet at Elevated Temperatures

Forming of automotive leightweight parts using aluminium offers numerous advantages. Compared to other wrought aluminium alloys, in particular AlMg‐alloys generally show a good formability which is favourable for the production of complex parts. However, forming of Mg‐containing alloys at room temperature leads to yielding patterns preventing their implementation for class‐A‐surface applications. Furthermore, the formability of steel still exceeds that of AlMg‐alloys at room temperature. Thus, in the present study, sheet metal forming is applied at a temperature range that is typical for warm forming. It is supposed to profit from the advantages of warm forming like high achievable strains and improved surface quality of the formed part, while not having the disadvantages of long production times and high energy consumption, which is correlated with superplastic forming. Applying fluid‐based sheet metal forming in this paper, nitrogen is used as fluid working medium to satisfy the demand on high temperature resistance. Concerning the blank material used, formability of Mg‐containing aluminium alloys shows strong strain rate sensitivity at elevated temperatures. To figure out the optimal strain rates for this particular process, a control system for forming processes is developed within the scope of this paper. Additionally, FE‐simulations are carried out and adapted to the experiment, based on the generated process data. FE‐investigations include forming of domes (bulging) as well as shape‐defined forming, having the objective to increase formability in critical form elements by applying optimal strain rates. Here, a closed‐loop process control for gas‐pressure forming at elevated temperatures is to be developed in the next stages of the project.     

Formability of stainless steel

The forming behavior of austenitic stainless steels (types 201, 301, and 304) and ferritic stainless steels (types 437, 439, 444, and 468) was investigated. The tensile behavior and the forming-limit diagrams (FLDs) for these grades were determined. The ferritic alloys behave similarly to plain carbon steels and are relatively insensitive to small variations of strain rate and temperature. The formability of the austenitic alloys is influenced greatly by martensitic transformation during straining. The fraction of martensite transformed as a function of strain was found to be very sensitive to temperature, which, in turn, depends on the strain rate at typical testing rates (10⁻³ to 10⁻¹/s). At low rates (when the specimen remains near room temperature), the formability of the austenitic alloys is markedly improved by transformation strengthening. The enhancement of formability is largest on the biaxial side of the FLD, because the fraction martensite transformed was found to depend on the absolute thickness strain, which is maximized in the balanced biaxial strain state.     

Innovative Technology for Preparation of Seamless Nitinol Tubes Using SHS Without Forming

This paper presents innovative technology for the production of seamless Ni-Ti tubes using self-propagating high-temperature synthesis (SHS). The proposed production technology is a unique method which removes the need of forming operations, reduces machining processes, and at the same time it eliminates the negatives of production Ni-Ti alloys by conventional melting methods. The proposed process consists in SHS reaction in evacuated silica tube with the use of extremely high heating rate (over 300 K min^?1).     

Investigation on the Effect of Pre-heating with Butane Gas Torch in Formability of High Tension Steel (HTS) by Using Taguchi Experimental Method in Roll Forming Process

This study was carried out to evaluate a new roll forming process involving pre-heating using a gas torch. The temperature distribution for the formed sheet was observed using a 3D-IR graph generated from a thermal imaging camera. The appropriate distance between the formed sheet and the butane gas torch was also determined based on the results of the flame characteristics. The thermal effects of the formed sheet were confirmed by the temperature distribution. Spring-back analysis was applied to the Nominal the Best characteristic of Taguchi’s experimental method. In spring-back analysis, the forming speed is an influential variable. Bow analysis was applied to the Smaller the best characteristic of Taguchi’s experimental method. Lastly, at room temperature, the roll forming process was performed with pre-heating and formability was analyzed with respect to spring-back, bow and variance of bending angle (buckling). Spring-back, bow and buckling with the roll forming process involving pre-heating got improved by 0.97°, 0.17 mm and 0.20, respectively, compared to the same processes at room temperature. Forming speed appeared to have the most influence on the formability and pre-heating was found to improve the formability in the roll forming process.     

Production of Structure Components with Selective Properties by means of Action Media based Cold Forming

Minimising a metallic component's weight can be achieved by either using lightweight alloys or by improving the component's properties. In both cases, the material formability affects the utilisability for mass production processes. Most of the high‐strength materials show a material‐restricted formability and are difficult to forge. The formability of a material is described by its maximum forming limit. Large plastic strains can lead to mechanical damage within the material. A promising approach of handling low ductile, high‐strength alloys in a forming process is deformation under superimposed hydrostatic pressure by active media. In the present study, the influence of superimposed hydrostatic pressure on the flow stress is analysed as well as the forming ability for different sample geometries at different hydrostatic pressure and temperature levels. The experimental results show that the superimposed pressure has no influence on the plastic deformation, nor does a pressure dependent near‐surface material hardening occur. Nevertheless, the formability is improved with increasing hydrostatic pressure. The relative gain at room temperature and increase in the superimposed pressure from 0 bar to 600 bar for tested materials was at least 140 % and max. 220 %. Therefore, a cold forming process under superimposed pressure is developed to produce structure components with selective properties. For example, the gain in formability will be used to enlarge local plastic strains to higher limits resulting in higher local strain hardening and hardness. This offers new design possibilities with selectively adjusted local structure or structure component properties, especially adapted to their technical application. Additionally, by applying damage models, finite‐element analysis is used in order to predict damage occurring in the cold forming process under superimposed hydrostatic pressure for various sample geometries.     

Dynamic Strain Aging of Various Steels

The dynamic strain aging characteristics of two dual phase steels, a high strength low alloy (HSLA) steel, a 1008 steel and an interstitial free (IF) steel were determined from tensile properties at temperatures in the range 295 to 460 K (22 to 187 °C) and strain rates between 6 × 10^-6 to 10^-2s^-1. All except the IF steel were found to be susceptible to dynamic strain aging, as evidenced by increases in tensile strength. The largest positive change was observed in the 1008 steel while the dual phase and HSLA steels showed much smaller increases. Also, large decreases (up to 75 pct) in uniform elongation were noted for the 1008 steel while the decreases were minimal for the dual phase and HSLA steels. The IF steel did not strain age and showed a slight increase in uniform elongation with increasing temperature. Based upon uniform elongation as an indicator of formability, formability might be improved in dual phase or HSLA steels by reducing the concentration of free interstitials in the ferrites through chemistry control.     

Simulation of the Forming Behaviour of a Boron Modified P91 Ferritic Steel

The forming behaviour at high temperature of a modified 9%Cr‐1%Mo (P91) ferritic steel containing B and Ti for elevated temperature service was investigated. The microstructure of the as‐received material is mainly martensite at room temperature, but special etching revealed prior austenite grains of about 25 μm in size. Torsion tests were conducted at temperatures in the range 850 to 1250 °C to simulate the hot rolling process under comparable conditions of temperature, strain rate and strain. The deformation data obtained from these tests were correlated with the Garofalo equation with a stress exponent of 4.6 and an activation energy of 315 kJ/mol. This equation was used to predict the formability behaviour for the rolling process and also to determine the maximum forming efficiency and stability of the steel. A temperature of 1200 °C is recommended to conduct the forming process.     

Forming Limit Investigation of AA6061 Friction Stir Welded Blank in a Single Point Incremental Forming Process: RSM Approach

In the present work, an experimental study was made to identify formability characteristics of aluminum 6061 fiction stir welded blank in an incremental forming process. Forming limit diagram, bowl height, minimum thickness and thickness distribution were studied to find formability of tailored welded blank. Firstly, series of experiments were carried out to find which joining direction caused higher formability and desired forming limit curve. For this purpose, joints with three different directions (i.e. rolling direction, transverse direction and diagonal direction) were prepared and formability i.e. formed bowl height until failure along with forming limit curves were compared. After finding the best joining direction, effect of welding process parameters i.e. rotational speed, plunge depth and travel speed on formability of welded blanks were analyzed by using response surface methodology (RSM). The results were discussed according to microstructure and fractography analysis obtained by scanning electron microscopy. After finding the effects, welded blanks with optimal parameter combination were fabricated and effect of incremental forming parameters i.e. spindle speed, feeding rate and axial step on thickness distribution were analyzed. Here, RSM was also used to find parametric and optimum setting of parameters. From the results, it was obtained that joints with diagonal direction caused higher value of bowl height. Also, tool rotary speed of 1600 RPM, travel speed of 40 mm/min and plunge depth of 0.15 mm caused higher value of bowl height implying high formability. Furthermore, it was found that selection of 0 RPM spindle speed, 600 mm/min feeding rate and axial depth of 0.6 mm resulted in higher thickness distribution.     

Deformation Mechanisms in the Near-β Titanium Alloy Ti-55531

The hot formability of a near-β titanium alloy is studied near the β transus temperature to determine the mechanisms of deformation. Compression tests of Ti-5Al-5Mo-5V-3Cr-1Zr are carried out using a Gleeble^®1500 device between 1036 K and 1116 K (763 °C and 843 °C) and strain rates between 0.001 and 10 s^?1. The achieved flow data are used to calculate the efficiency of power dissipation, the strain rate sensitivity, and instability parameters derived from different models. Constitutive equations are built using the stress values at the strain of 0.4. Light optical microscopy and EBSD measurements are used to correlate the parameters that describe formability with the microstructure. It is found that hot deformation is achieved by dynamic recovery in the β phase by subgrain formation. Geometric dynamic recrystallization along the β grain boundaries takes place at large deformations, high temperatures, and low strain rates. On the other hand, for high strain rates, continuous dynamic recrystallization by lattice rotation already starts at a local strain of 1. Different phenomenological models are used to predict the flow instabilities, where the flow-softening parameter α _i provides the best correlation with microstructure as well as the physical understanding. The instabilities observed in this alloy are strongly related to flow localization by adiabatic heat.     

Superplastic Forming of Multipass Friction Stir Processed Aluminum-Magnesium Alloy

Multipass friction stir processing (FSP) of AA5086 Al-Mg alloy was carried out to obtain bulk fine grain material for superplastic forming. FSP produced inhomogeneous microstructure in the thickness direction. The aim of the present work was to understand superplastic forming behavior of distinct microstructural layers, i.e., nugget layer (NL) containing microstructure from nugget zone, thermo-mechanically affected/heat-affected layer (TL) containing microstructure from thermo-mechanically affected/heat-affected (TMAZ/HAZ) zone, and composite layer (CL) containing microstructure from both the above zones (nugget and TMAZ/HAZ). Superplastic forming of NL, TL, and CL blanks was carried out at constant gas pressure. Three different forming gas pressures of 0.75, 1.15, and 1.5 MPa corresponding to strain rates of 5 × 10^?4 s^?1, 1 × 10^?3 s^?1 , and 5 × 10^?3 s^?1, respectively, were used. Forming characteristics of CL were found to be comparable to that of NL and even better at higher forming pressures. Concomitant microstructural evolution during bulging of CL and NL plays an important role here.     

The influence of stacking fault energy on the mechanical behavior of Cu and Cu-Al alloys: Deformation twinning, work hardening, and dynamic recovery

The role of stacking fault energy (SFE) in deformation twinning and work hardening was systematically studied in Cu (SFE ∼78 ergs/cm²) and a series of Cu-Al solid-solution alloys (0.2, 2, 4, and 6 wt pct Al with SFE ∼75, 25, 13, and 6 ergs/cm², respectively). The materials were deformed under quasi-static compression and at strain rates of ∼1000/s in a Split-Hopkinson pressure bar (SHPB). The quasi-static flow curves of annealed 0.2 and 2 wt pct Al alloys were found to be representative of solid-solution strengthening and well described by the Hall-Petch relation. The quasi-static flow curves of annealed 4 and 6 wt pct Al alloys showed additional strengthening at strains greater than 0.10. This additional strengthening was attributed to deformation twins and the presence of twins was confirmed by optical microscopy. The strengthening contribution of deformation twins was incorporated in a modified Hall-Petch equation (using intertwin spacing as the “effective” grain size), and the calculated strength was in agreement with the observed quasi-static flow stresses. While the work-hardening rate of the low SFE Cu-Al alloys was found to be independent of the strain rate, the work-hardening rate of Cu and the high SFE Cu-Al alloys (low Al content) increased with increasing strain rate. The different trends in the dependence of work-hardening rate on strain rate was attributed to the difference in the ease of cross-slip (and, hence, the ease of dynamic recovery) in Cu and Cu-Al alloys.     

Coupling Experiment and Simulation in Electromagnetic Forming Using Photon Doppler Velocimetry

Modeling electromagnetic forming processes is in many ways simpler than modeling traditional metal forming processes. In electromagnetic forming the problem is often dominated by inertial acceleration by a magnetic field. This problem is better posed than the more common ones in metal forming that are often dominated by complex three‐dimensional constitutive behavior and frictional effects. However, important aspects of the problem are dominated by the constitutive properties of the material, and often electromagnetic forming is performed in a regime where there is little reliable material strength data. Strain rates are often high (10² to 10⁴ s^?1 is the typical range for electromagnetic forming). Also, heat is generated both by Joule heating as well as by plastic deformation, and peak temperatures can be quite high. Also, while high‐temperature, high‐strain‐rate data is scarce, there is very little data in cases where temperature rises significantly over very short time periods (tens of micro‐seconds) as in electromagnetic metal forming. This rapid temperature rise is very important to the material response because the short time scales largely preclude the material from recovery and recrystallization processes, and precipitates cannot dissolve as they normally would in an age‐hardening alloy in these time scales. This paper will show how advanced instrumentation, particularly the Photon Doppler Velocimeter (PDV) can be coupled with electromagnetic forming and provide both avenues to characterize the high strain rate strength of the material as well as to provide clear experimentally measured data that can be used to compare experiments with numerical models.     

Liquid Film Migration in Warm Formed Aluminum Brazing Sheet

Warm forming has previously proven to be a promising manufacturing route to improve formability of Al brazing sheets used in automotive heat exchanger production; however, the impact of warm forming on subsequent brazing has not previously been studied. In particular, the interaction between liquid clad and solid core alloys during brazing through the process of liquid film migration (LFM) requires further understanding. Al brazing sheet comprised of an AA3003 core and AA4045 clad alloy, supplied in O and H24 tempers, was stretched between 0 and 12 pct strain, at room temperature and 523K (250 °C), to simulate warm forming. Brazeability was predicted through thermal and microstructure analysis. The rate of solid–liquid interactions was quantified using thermal analysis, while microstructure analysis was used to investigate the opposing processes of LFM and core alloy recrystallization during brazing. In general, liquid clad was consumed relatively rapidly and LFM occurred in forming conditions where the core alloy did not recrystallize during brazing. The results showed that warm forming could potentially impair brazeability of O temper sheet by extending the regime over which LFM occurs during brazing. No change in microstructure or thermal data was found for H24 sheet when the forming temperature was increased, and thus warm forming was not predicted to adversely affect the brazing performance of H24 sheet.

     

Identification of Constitutive Model Parameters for Nimonic 80A Superalloy

Nimonic 80A is a nickel-chrome superalloy, commonly used due to its high resistance against creep, oxidation, and temperature corrosion. This paper presents the material constitutive models of Nimonic 80A superalloy. Johnson–Cook (JC) and modified JC model is preferred among the different material constitutive equations (Zerill Armstrong, Bodner Partom, Arrhenius type) due to its accuracy in the literature. Three different types of compression tests were applied to determine the equation parameters. Firstly, quasi-static tests were performed at room temperature. These tests were conducted at 10^?3, 10^?2, and 10^?1 s^?1 strain rates. Secondly, compression tests were performed at room temperature at high strain rates (370–954 s^?1) using the Split-Hopkinson pressure bar. Finally, compression tests were performed at a temperature level from 24 to 200 °C at the reference strain rate (10^?3 s^?1). Johnson–Cook and modified JC model parameters of Nimonic 80A were determined with the data obtained from these tests, and they were finally verified statistically.     

Flow Behavior of SUS304 Stainless Steel Under a Wide Range of Forming Temperatures and Strain Rates

To determine the flow behavior of SUS304 stainless steel under different conditions, axisymmetric compression tests were conducted over a wide range of forming temperatures (25 °C to 400 °C) and strain rates (10⁻³ to 10 s⁻¹). Flow curves were obtained for different forming conditions to study the influence of the forming temperature and strain rate on the flow behavior. Moreover, electron backscatter diffraction analysis, X-ray diffraction analysis, transmission electron microscopy, and Feritscope were used to study the microstructure evolution of SUS304 stainless steel under different conditions for determining the underlying reasons for the variations in flow behavior. The experimental results indicated that the flow stress decreased with increasing the forming temperature. With increasing strain rate at 25 °C to 200 °C, the flow stress first increased and then decreased; however, the strain rate had little effect on the flow stress at 300 °C and 400 °C. By analyzing the variation in the phase transformation inside compressed SUS304 stainless steel samples under different forming conditions, the key factors affecting the flow behavior of stainless steel were identified. Finally, by examining the variation in the martensite content and the dislocation density, the dominant deformation mechanism under different forming conditions was determined.