Simulation of High-Temperature AA5083 Bulge Forming with a Hardening/Softening Material Model

High-temperature bulge forming of AA5083 aluminum sheet was simulated with the commercial finite element (FE) code ABAQUS™. A material model that is strain rate sensitive and accounts for strain hardening and softening was used. Results were compared with data from AA5083 bulge forming experiments at 450 °C where the gas pressure was a prescribed constant value. The results show that the material model is capable of predicting the deformation and thinning behavior at different constant pressure levels. In ancillary simulations, time-varying pressure profiles were computed (rather than prescribed) with an internal ABAQUS™ routine that attempts to maintain the strain rate at the bulge dome pole within a specified range. The time-varying profiles, for which no experimental AA5083 bulge forming data exist, can be programmed into existing bulge testing instrumentation to validate the associated predictions of bulge dome height and thinning. The present effort represents a necessary step toward predicting gas pressure profiles by coupling the pressure profile with a desired sheet deformation rate.     

An investigation of enhanced formability in AA5182-O Al during high-rate free-forming at room-temperature: Quantification of deformation history

The goal of this work is to improve our understanding of formability enhancement in aluminum (Al) sheet alloys that has generally been observed during high-strain-rate forming. In the work presented here, experiments and numerical modeling were used to investigate the room-temperature formability of AA5182-O Al alloy sheet (1 mm thick) at high strain-rates using the electro-hydraulic forming (EHF) technique. A finite element model, using Johnson–Cook constitutive equation, was developed to simulate the high-rate forming behavior of Al under EHF and test samples were designed to obtain different strain paths at the apex of the EHF domes. The deformation history of Al sheets, under free-forming conditions and inside a conical die, was experimentally determined and compared to the model predictions. Experimental data shows that the high-rate formability of AA5182-O Al at minor strains of ∼−0.1 and ∼0.05, relative to its corresponding quasi-static formability, was enhanced locally by ∼2.5× and ∼6.5× under free-forming and when forming inside the conical die, respectively. The in-plane peak engineering strain-rate associated with the enhanced formability during free-forming was measured to be ∼3900/s while the pre-impact strain-rate during conical-die forming was estimated to be ∼4230/s. The strain-path associated with enhanced formability was experimentally determined under a free-forming case and was found to be in good agreement with that predicted by the numerical model. To the authors’ knowledge, these results are the first to experimentally quantify the deformation history associated with enhanced formability that has often been reported in the literature.     

Investigations on warm forming of AW-7020-T6 alloy sheet

7000 series aluminium alloys have greater strength than conventional aluminium alloys used in the automotive industry, but little has been reported on their formability. In this paper the strength and formability of age-hardenable AW-7020 alloy sheet in the T6 temper condition was investigated at temperatures between 150 and 250 °C by warm tensile, Swift-cupping and cross-die deep-drawing tests. Differential scanning calorimetry (DSC) investigations were carried out to study the precipitation state of AW-7020 sheet in as-received, warm cross-die deep-drawn and post-paint-baked conditions. Formability was found to improve at temperatures above 150 °C and was sensitive to temperature and strain rate. There was also an onset of dynamic recovery from 150 °C. DSC results showed the presence of η′ precipitates in T6 temper and that these coarsen during the warm cross-die deep-drawing and paint baking processes with ∼30% drop in ultimate tensile and yield strengths. Dynamic recovery and coarsening of η′ precipitates were found to contribute to the increase in formability at elevated temperatures.     

Reduction of a pre-formed radius in aluminium sheet using electromagnetic and conventional forming

Electromagnetic (EM) forming is a high-speed forming process that uses the forces induced on a conductive workpiece by a transient high frequency magnetic field to form the workpiece into a desired shape. This paper describes the results of a work undertaken to study the reduction of a 20 mm radius to 5 mm in 1 mm AA 5754 sheet by conventional metal forming process and by electromagnetic forming. The combination of conventional and EM forming will be referred to as “hybrid forming”. The 20 mm radius was pre-formed from flat sheet using a conventional die, punch and binder that allowed the material to draw in. The radius was then reduced to 5 mm, with no draw-in allowed for either process. Sheets were studied in the as-received condition and were also pre-strained to 5%, 10% and 15% to simulate strain path effects in a multiple stage forming operation. The process was modelled numerically to gain insight into the stress, strain and strain rate histories. The research indicates that features that are not achievable using traditional stamping techniques can be obtained with the aid of EM forming.     

Investigation on formability enhancement of 5A06 aluminium sheet by impact hydroforming

High strain rate (HSR) forming has been found to be able to enhance the formability of sheet metals like electro-magnetic forming. Impact hydroforming (IHF) is proposed, in which the sheet is formed with high-pressure pulse combining hydroforming and HSR forming. An IHF bulge test setup was designed, 5A06 aluminium sheet was tested with strain rate of 2 × 10³ s^?1 showing remarkable thickness strain increase compared with quasi-static condition. A new IHF equipment is designed, the IHF process was verified effective with the equipment, complicated aluminium aircraft sheet part with high drawing ratio was formed that cannot be formed with quasi-static hydroforming.     

High temperature micropillar compression of Al/SiC nanolaminates

The effect of the temperature on the compressive stress–strain behavior of Al/SiC nanoscale multilayers was studied by means of micropillar compression tests at 23 °C and 100 °C. The multilayers (composed of alternating layers of 60 nm in thickness of nanocrystalline Al and amorphous SiC) showed a very large hardening rate at 23 °C, which led to a flow stress of 3.1 ± 0.2 GPa at 8% strain. However, the flow stress (and the hardening rate) was reduced by 50% at 100 °C. Plastic deformation of the Al layers was the dominant deformation mechanism at both temperatures, but the Al layers were extruded out of the micropillar at 100 °C, while Al plastic flow was constrained by the SiC elastic layers at 23 °C. Finite element simulations of the micropillar compression test indicated the role played by different factors (flow stress of Al, interface strength and friction coefficient) on the mechanical behavior and were able to rationalize the differences in the stress–strain curves between 23 °C and 100 °C.     

Precipitation in ductile Fe–18Al–5Cr alloys with additions of Mo,W and C and effects on high-temperature strength

The strength of a Fe–Al-based alloy containing small additions of Mo, W and C has been determined from room temperature up to 800 °C, and the strain rate dependence of strength examined. Strength of the as-cast material is maintained at above 600 °C, but it is lost at higher temperatures, especially at slow strain rates. This behaviour is largely explained by solute hardening effects, with no sign of any precipitates forming. After a solution treatment, annealing material at 800 °C leads to the appearance of Fe–Mo–W carbides which provide better strength under conditions of high temperature and slow strain rate. The possibilities for improving high-temperature strength and creep behaviour by the formation of carbide or intermetallic precipitates are discussed.     

Temperature conditions during ‘cold’ sheet metal stamping

This paper investigates the friction and deformation-induced heating that occurs during the stamping of high strength sheet steels, under room temperature conditions. A thermo-mechanical finite element model of a typical plane strain stamping process was developed to understand the temperature conditions experienced within the die and blank material; and this was validated against experimental measurements. A high level of correlation was achieved between the finite element model and experimental data for a range of operating conditions and parameters. The model showed that the heat generated during realistic production conditions can result in high temperatures of up to 108 °C and 181 °C in the blank and die materials, respectively, for what was traditionally expected to be ‘cold’ forming conditions. It was identified that frictional heating was primarily responsible for the peak temperatures at the die surface, whilst the peak blank temperatures were caused by a combination of frictional and deformation induced heating. The results provide new insights into the local conditions within the blank and die, and are of direct relevance to sheet formability and tool wear performance during industrial stamping processes.     

Creep microstructures and creep behaviors of pure molybdenum sheet at 0.7 Tm

The creep behavior of high purity molybdenum (99.995 wt.%) is investigated at temperatures of 1600 to 2000 °C (0.61–0.77 T_m) by direct ohmic heating. The stress and temperature dependency of creep strain and rupture time are described through optical microstructure observations. Under low load and low temperature conditions, coarse secondary recrystallized grains caused by dynamic recrystallization are observed far from the crack tip. In contrast, under high load and high temperature conditions, coarse secondary recrystallized grains are only fully formed near the crack tip, while coarse secondary recrystallized grains and small primary recrystallized grains coexisted further away from the tip. The recrystallized grain size of the Mo-B sheet is smaller than that of the Mo-A sheet, and small primary and large secondary recrystallized grains are mixed throughout whole specimens of the Mo-B sheet. Mo-A sheet shows elongated ductile fracture, but Mo-B sheet shows irregular brittle fracture under the same conditions. The steady-state creep strain rate at 1800 °C is found to be 7.34 × 10^− 6, 2.83 × 10^− 5 and 1.53 × 10^− 4 s^− 1 under a constant stress of 5, 10 and 20 MPa, respectively. The stress exponent is estimated to be 3.85–3.98 and the strain activation energy for steady state creep is 362–413 kJ/mol.     

Deformation kinetics of nanocrystalline nickel

The work hardening and the strain rate sensitivity of flow stress were studied in the temperature range from 298 to 473 K for nanocrystalline (NC) nickel with average grain sizes of about 25 and 80 nm produced by pulsed electrodeposition. The rate of work hardening, the maximum flow stress and the strain rate sensitivity of flow stress increase with decreasing grain size. The data are compared to published data for NC Ni and found to be consistent. The common analysis of strain rate sensitivity in terms of thermal activation is critically discussed. It is proposed that the activation analysis gives information about thermally activated processes at grain boundaries which may be related with recovery of dislocations.     

Enhancing formability in hydromechanical deep drawing process adding a shallow drawbead to the blank holder

In this paper, a new method was proposed in order to enhance the limiting drawing ratio (LDR) of AA5754-O in the hydromechanical deep drawing process (HDD). In the proposed method, a shallow drawbead was added to the blank holder to increase LDR so as to provide strain hardening of a large region on the flange of the sheet material in addition to pre-bulging process which affects particularly only the initial stage but not the later ongoing process. So the LDR of the AA5754-O was increased from 2.65 to 2.787 by enlarging the region of strain hardening in the flange and partially reducing wrinkling tendency due to occurred tensile stresses using the convenient pressure and blank holder force profiles. The importance levels and their convenient values for height of drawbead, pre-bulge height and pressure, surface roughness of the punch were determined with analysis of variance (ANOVA) is a statistical method. ANOVA analysis illustrated that adding a shallow drawbead to the blank holder is the most effective factor between the investigated factors for the HDD process. While the effects of the pre-bulging pressure and pre-bulging height were determined as quite small, the surface roughness of the punch was found unimportant compared to the effect of the shallow drawbead. The highest LDR value was obtained with 1 mm drawbead height, 5 mm pre-bulging height, 10 MPa pre-bulging pressure and 2.8 μm surface roughness of the punch.     

An experimental and numerical study of prestrained AA5754 sheet in bending

This study focuses on the effects of prestrain magnitude on 3 mm AA5754 sheet in bending at nominal applied strain rates of 0.001/s and 0.1/s. The necessity of incorporating prestrain and strain rate effects into numerical simulations of bending is also evaluated. A series of experimental bend tests using axial compression in the longitudinal material direction were performed following plane strain prestrain in the transverse material direction. An inelastic buckling mode of deformation was produced with the peak buckling load increasing and the minimum load decreasing with larger magnitudes of prestrain. A semi-empirical material model, referred to as the Voce-MA model, was developed which incorporates strain rate-sensitivity of the flow stress, the prestrain magnitude and their interaction. Simulations of the bend tests using this material model were then performed in LS-DYNA at nominal applied strain rates of 0.001/s and 0.1/s for samples with 0, 3, 6 and 12% plane strain prestrain. It was shown that for AA5754 sheet in bending, prestrain effects must be considered in terms of current sheet thickness and material hardening. While the peak and minimum loads are not strain rate sensitive at the low rates used in this study, a rate-dependant material model is still necessary in order to account for the deviations in local strain rate from the applied strain rate. The Voce-MA material model was capable of representing prestrain and strain rate effects for all cases of AA5754 sheet in bending considered in this study.     

Microscale-calibrated modeling of the deformation response of low-carbon martensite

Micropillar compression tests were used to determine the uniaxial compressive stress–strain response of martensite blocks extracted from a low-carbon, fully lath martensitic sheet steel, M190, with the nominal composition C = 0.18, Mn = 0.47, P = 0.007, S = 0.006, Si = 0.18, Al = 0.06, Ti = 0.045, B = 0.0014 and balance Fe (all in wt.%). Specimens with a diameter exceeding ～1 μm and consisting of a single martensite block showed elastic–nearly perfectly plastic behavior with a yield stress of the order 1200 MPa. Similar specimens which contained multiple martensite blocks showed pronounced strain hardening, arising from the geometrical constraint produced by the interface(s). No size dependence of flow stress was observed in micropillars with diameters exceeding 1.0 μm, but a significant scatter in strength and hardening rate was observed in micropillars with smaller diameters. Flow data for micropillars in the size-independent regime were used to determine parameters in a crystal-plasticity-based model of martensite. Full three-dimensional crystal plasticity simulations, with material properties determined from micropillar tests, were then used to predict the macroscopic uniaxial stress–strain behavior of a representative volume element of martensite. The predicted stress–strain behavior was in excellent agreement with experimental measurements, and demonstrates the potential for micropillar tests to determine material parameters for individual phases of a complex microstructure.     

Influence of anisotropy of the magnesium alloy AZ31 sheets on warm negative incremental forming

Single point incremental forming of the magnesium alloy AZ31 sheets, which were fabricated by hot extrusion, slab + hot/cold rolling, strip-casting rolling and cross-rolling, respectively, was investigated at elevated temperatures. The results show that the anisotropy of the sheets fabricated by casting slab + hot/cold rolling and cross-rolling is not remarkable, and the formability is improved significantly. The circular, square and rotary cone parts were performed with satisfactory surface quality and without any microcracks successfully, and which is superior to those of the extruded sheet and the one-way rolled sheet. Therefore, anisotropy of the sheets has remarkable effects on the surface quality of the formed parts, and the effect becomes weakened with increasing temperature. It is proposed that cross-rolling sheet is much more suitable for warm SPIF process.     

Work hardening induced by martensite during transformation-induced plasticity in plain carbon steel

Transformation-induced plasticity (TRIP) steels are becoming increasingly exploited for industrial applications because they show high strength and high uniform elongation (ductility). Despite this interest, the relative contributions of the various strengthening and straining mechanisms are often poorly understood. In this study, neutron diffraction is employed to quantify the contribution of different mechanisms to ductility and work hardening for a 0.25 wt.% C steel. Differences in stress–strain response at different temperatures are related to the extent of the transformation of metastable austenite into martensite during deformation. At room temperature (RT) the transformation of austenite occurs gradually with straining, while at ?50 °C the transformation occurs almost from the onset of loading. The associated transformation strain is reduced, comprising nearly half the total strain, lowering the apparent elastic modulus and explaining the relatively low work hardening compared to RT straining. By contrast, deformation at RT after pre-straining at ?50 °C results in larger work hardening than for solely RT straining due to the higher martensite levels introduced at ?50 °C. This is due to composite load transfer to the strong constituent from the soft matrix. The extent of the transformation is quantified as a function of strain at both temperatures as well as its effect on the work hardening and elongation.     

Low-cycle fatigue and deformation substructures in an engineering TiAl alloy

The cyclic stress–strain (CSS) behaviour of a Ti–48Al–2Cr–2Nb (atomic%) cast alloy is investigated at 25 °C and 750 °C. Meanwhile post-mortem TEM observations are carried out in order to get insights into the deformation mechanisms governing this behaviour. It has been shown that, at 25 °C, the pronounced hardening observed on CSS curves at high total-strain-amplitude Δɛ_t values is related to a deformation mode mainly governed by twinning. Meanwhile the moderate hardening observed at intermediate Δɛ_t is associated to ordinary dislocation glide and the subsequent formation of a vein-like structure at intermediate Δɛ_t values. In contrast, at 750 °C the CSS behaviour is characterised by a rapid saturation of the stress amplitude, regardless of the applied strain amplitude, while the deformation structure analysis suggests a high activation of dislocation climb which is confirmed by in situ TEM ageing showing shrinkage of prismatic loops. Finally, the influence of parameters such as strain rate and strain ratio on CSS behaviour is also examined.     

Gas Pressure Profile Prediction from Variable Strain Rate Deformation Paths in AA5083 Bulge Forming

Two approaches to gas pressure profile prediction for bulge forming of AA5083 sheet under Quick Plastic Forming (QPF) conditions at 450?°C were investigated. The first was based on an algorithm internal to ABAQUS? wherein the gas pressure results from maintaining a constant effective target strain rate at the dome pole. In the second, the nonlinear long wavelength stability analysis was combined with a single creep mechanism material model that accounts for hardening/softening. A series of stability curves, which denote combinations of strain and strain rate for unmitigated thinning and, ultimately, rupture of an AA5083 bar, were computed. These are based on a parameter that characterizes an assumed geometric non-uniformity, ??. The associated uniaxial strains and strain rates were expressed in terms of von Mises effective strains and strains rates, and pressure profiles were computed. An ancillary approach to variable strain rate path prediction based on a thinning factor was used to suggest a suitable value of ?? in the stability analysis for a reasonable thinning level at the end of forming. Key advantages and disadvantages of both approaches to pressure profile prediction are examined relative to bulge forming time and thinning at a 50-mm dome displacement.     

Effect of specimen thickness on the creep response of a Ni-based single-crystal superalloy

Creep tests on Ni-based single-crystal superalloy sheet specimens typically show greater creep strain rates and/or reduced strain or time to creep rupture for thinner specimens than predicted by current theories, which predict a size-independent creep strain rate and creep rupture strain. This size-dependent creep response is termed the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal, constant nominal stress creep tests were performed on uncoated PWA1484 Ni-based single-crystal superalloy sheet specimens of thicknesses 3.18 and 0.51 mm under two test conditions: 760 °C/758 MPa and 982 °C/248 MPa. The specimens contained initial microvoids formed during the solidification and homogenization processes. The dependence of the creep response on specimen thickness differed under the two test conditions: at 760 °C/758 MPa there was a reduction in the creep strain and the time to rupture with decreasing section thickness, whereas at 982 °C/248 MPa a decreased thickness resulted in an increased creep rate even at low strain levels and a decreased time to rupture but with no systematic dependence of the creep strain to rupture on specimen thickness. For the specimens tested at 760 °C/758 MPa microscopic analyses revealed that the thick specimens exhibited a mixed failure mode of void growth and cleavage-like fracture while the predominant failure mode for the thin specimens was cleavage-like fracture. The creep specimens tested at 982 °C/248 MPa in air showed the development of surface oxides and a near-surface precipitate-free zone. Finite-element analysis revealed that the presence of the alumina layer at the free surface imposes a constraint that locally increases the stress triaxiality and changes the value of the Lode parameter (a measure of the third stress invariant). The surface cracks formed in the oxide scale were arrested by further oxidation; for a thickness of 3.18 mm the failure mode was void nucleation, growth and coalescence, whereas for a thickness of 0.51 mm there was a mixed mode of ductile and cleavage-like fracture.     

Microstructure–low cycle fatigue behaviour relationships in a PM γ-TiAl alloy

The low cycle fatigue (LCF) behaviour has been investigated in gas atomized powder compacts of a Ti–47.2Al–2.1Cr–2.1Nb alloy. The Cyclic Stress–Strain (CSS) behaviour at 20 °C and at 500 °C was found to be strongly dependent upon the microstructure. An increasing amount of lamellar colonies, a finer lamellar spacing and a smaller γ grain size were shown to markedly decrease the cyclic strain hardening rate. For the different microstructures involved, a correlation between the capability of developing a vein-like structure and the cyclic strain hardening rate has been established. The more prone the microstructure to develop a vein-like structure, the higher the cyclic strain hardening rate. Therefore, the present study helped determine a compromise heat treatment that can ensure an improved fatigue behaviour for such a γ-TiAl alloy.     

Characterization of Microstructure,Mechanical Properties and Formability of Cryorolled AA5083 Alloy Sheets

In this work, microstructure, mechanical properties and formability of cryorolled and annealed AA5083 alloy sheets have been characterized and a comparison has been made with cold rolled and annealed sheets. Five-millimeter-thick sheets of this alloy were cryorolled in multiple passes to a final thickness of 1 mm (80% reduction with a true strain of 1.6). Effect of annealing time and temperature on hardness has been studied, and it has been found that a short annealing at 275 °C for 15 min after cryorolling would yield a good combination of strength and ductility. Microstructural investigations showed that the cryorolled and short annealed samples possess bimodal grain structure which is responsible for better mechanical properties than cold rolled sheets. From the experimentally determined forming limit diagrams, the limit strains of cryorolled sheets have been found to be almost equal to conventional cold rolled and annealed sheets in all modes of deformation. No major differences have been found in strain distribution also. This work clearly demonstrates that cryorolling of AA5083 alloy sheets followed by a short annealing with bimodal grain structure can be used for sheet metal forming applications with higher strength and toughness than conventional sheets without any reduction in formability.