Improvement Of Formability Research Articles

Improvement of formability and corrosion resistance of AZ31 magnesium alloy by pulsed current–assisted laser shock forming

This study adopted a novel pulse current–assisted laser shock AZ31B sheet micro-forming method (EP-LSF). The mechanism of improving the formability of AZ31B magnesium alloy by pulse current–assisted laser shock forming and the reason of improving the corrosion resistance were studied for the first time. Through laser shock–free bulging experiment, tensile test, optical microscope (OM), and X-ray diffraction, the change in formability was studied. After pulse current assisted–laser shock forming, the forming height of AZ31B magnesium alloy increases by 28.8%, the thinning gradient decreases by 6.7%, and the strain rate–sensitivity coefficient increases to 0.1452. The results show that the decrease of grain size and texture density is the reason why EP-LSF can further improve the formability of AZ31B magnesium alloy. The changes in corrosion resistance were studied by scanning electron microscopy and electrochemical tests. The results show that after EP-LSF, the corrosion current density of AZ31 magnesium alloy decreased, and the electrochemical impedance increased, indicating that this method further improved the corrosion resistance.

Improvement of deep drawing formability of Mg-6Al-1Zn magnesium alloy sheets with high strength utilizing aging precipitation

The effects of aging precipitation on microstructure, deep drawing formability and tensile properties, were investigated for Mg-6Al-1Zn (AZ61) alloy sheets. The alloy aged at 175 °C for 96 h with high strength exhibited a significant improvement in deep drawability with an increased limiting drawing ratio (LDR) from 1.5 to 1.9, when slightly increasing deformation temperature from room temperature to 100 °C. This LDR (1.9) was even larger than that (1.75) of the unaged alloy drawn at 100 °C. The improved deep drawability can be attributed to the prevention of cracking at the edge of drawn cup, which is closely related to the restrictions of growth of tensile twins and basal slip due to Mg17Al12 precipitates, resulting in the delayed texture strengthening during deformation. The proposed idea of utilizing aging precipitation provides a new strategy for the development of Mg alloy sheets with superior deep drawing formability and high strength.

Electroplasticity in electrically-assisted forming: Process phenomena, performances and modelling

Electrically-assisted forming has been proven to be an efficient process in terms of reducing forming flow stress, improving formability and supressing the springback for different hard-to-deform alloys. However, there is a controversial debate about whether the flow stress reduction with the introduction of electric current into a metallic specimen undergoing plastic deformation is attributed to Joule heat, additional athermal effect, namely the electroplasticity, or their combination thereof. There is a lack of thorough understanding of different mechanisms inducing the flow stress reduction and formability improvement, which would hinder the development and application of innovative electrically-assisted forming processes. Most prior works have examined only one or two materials with similar electroplastic behaviours, making it difficult to resolve this debate. In this study, three typical materials with diverse electroplastic behaviours, i.e. copper, SS304 and Ti6Al5V, were investigated and compared via electrically-assisted tensile tests and thermally-assisted tensile tests. The flow stress of the copper specimens in the electrically-assisted tensile test was lower than that in the thermally-assisted tensile test, which verifies the existence of electroplasticity. However, the flow stresses of the SS304 specimens in the electrically-assisted tensile test and thermally-assisted tensile test were similar at the same temperature, indicating that the athermal effect on the flow stress is not evident. For the Ti6Al4V specimens, the athermal stress reduction was not significant at lower current densities. However, the electrically-assisted and thermally-assisted tensile tests results began to differ when the current density exceeded 37.6 A/mm2. According to the further discussion on the microstructure evolution of different materials, the introduced current improves the dislocation motions and recovery and consequently reduces the flow stress in simple face-centered cubic structure materials such as copper. However, these effects can be suppressed by stronger effects such as the solution hardening in stainless steels. The current also affects the phase transition, which in turn influences the hardening behaviours of Ti6Al4V. By clarifying the cause of conflicting electroplastic effects, a uniform analytical method calculating the short- and long-range stresses that considers the correlation of electrical and thermal effects on dislocation motion, solution hardening and phase transformation was developed for electrically-assisted forming processes. The model was also experimentally verified.

Simulation-aided analysis on mechanical properties of dilute Mg-Zn-Ca alloy sheets

The relationship between Zn content and room temperature (RT) mechanical properties of dilute Mg-xZn-0.1Ca (x = 0.1, 0.2,0.3, 0.5, 1.5, 2.5 in wt%) alloy sheets was investigated. The index Erichsen (IE) value, an indicator of stretch formability, was found to increase significantly from 6.6 mm to 8.4 mm by increasing the Zn content from 0.2 wt% to 0.3 wt%. Visco-Plastic Self-Consistent (VPSC) simulation revealed there was no apparent difference in the relative activity of deformation modes in Mg-0.2Zn-0.1Ca and Mg-0.3Zn-0.1Ca alloys. On the other hand, electron back-scattered diffraction observation showed that a component having basal poles tilted towards the transverse direction (TD) began to appear when 0.3 wt% of Zn was added, and the typical TD-split texture was developed when the Zn content was increased to more than 1.5 wt%. Therefore, the rapid improvement of RT formability observed in this study can be attributed to the formation of TD-split texture. In addition to this, the efficient scheme for parameter fitting required to use VPSC simulation was also proposed in this study.

Improvement of stretch formability and weakening the basal texture of Mg alloy by corrugated wide limit alignment and annealing process

In this paper, a process of corrugated wide limit alignment (CWLA) was used to improve the stretch formability of large-dimension AZ31 Mg alloy sheets by inducing tensile twins. The effects of {10 1¯ 2} tensile twins on microstructure evolution, mechanical properties and stretch formability of AZ31 Mg alloy sheet was studied. The results indicated that the CWLA samples exhibited a weakened basal texture and a high fraction of twin boundaries due to the twinning and recrystallization behavior. After CWLA and annealing for 30min at 300 °C, the strain-hardening exponent value increased from 0.2 to 9.4, the Lankford value decreased from 2.85 to 0.37, and the Erishen value was increased by ∼95% compared to the as-received samples. The present study would provide a new insight into the development of Mg alloys having excellent stretch formability.

Contact-induced vibration tool in incremental sheet forming for formability improvement of aluminum sheets

Formability improvement is extremely crucial in incremental sheet forming (ISF) of low-ductility material and high wall angle 3-dimensional parts. The contact-induced vibration from the tool rotation provides an alternative route for optimizing formability; however, the underlying mechanism induced by the contact-induced vibration effect has not yet been fully elucidated. This study designed a contact-induced vibration tool (V-Tool) with a flat elliptical shape tip and applied it into the ISF of a 0.5 mm Al5052 sheet. Hyperbola shapes were manufactured by the ISF with V-Tool and conventional tool (C-Tool) with different rotation speeds to experimentally investigate the forming limit, temperature, hardness, fracture morphology, and material deformation. The main results show that the vibration effect and the resultant shear deformation by V-Tool result in the lower friction heat verified by the peak temperature from 180 °C to 80 °C and higher formability from the thinning rate of 54%–84% as compared to those of the conventional tool (C-Tool). Further, the deformation observation with the drilled holes on initial sheets indicates that the contact-induced vibration effect in the ISF with V-Tool may produce more shear deformation on the inner surface to delay the continuous enlargement of the drilled hole along the meridional direction.

Ultrasonic assisted incremental sheet forming: Constitutive modeling and deformation analysis

Ultrasonic vibration has been applied in the incremental sheet forming process and shows the potential to reduce the forming force and increase the formability, but the constitutive behavior and deformation behaviors of material under the high-frequency vibration are still not completely explained. In the present work, a hybrid constitutive model which combines the phenomenological, thermal activation and dislocation annihilation models is established and experimentally calibrated. Then, a finite element model is developed for the ultrasonic-assisted incremental sheet forming (UISF), in which the hybrid constitutive model and a user defined subroutine were incorporated to describe the ultrasonic effect and to simulate the high-frequency vibration, respectively. Compared with the experimental forming force, the FE analysis with the hybrid constitutive model shows good predictability with errors less than 10 %. Based on the simulation results, the improvement of formability was observed and the material deformation behavior during ultrasonic-assisted incremental sheet forming was analyzed in detail. Moreover, the material microstructural features were analyzed through Electron Back Scatter Diffraction (EBSD). The occurrence of relative larger grains and the increase of low angle grain boundaries (LAGBs) indicates the occurrence of ultrasonic-actuated dynamic recovery of the material.

Heterogenous Quenching and Partitioning from Manganese-Partitioned Pearlite: Retained Austenite Modification and Formability Improvement

Heterogenous Quenching and Partitioning from Manganese-Partitioned Pearlite: Retained Austenite Modification and Formability Improvement

Enhanced Grain Refinement, Texture Weakening and Formability Improvement in Al-Mg-Si-Zn Alloy Sheet Through Hot-Rolling Deformation

Enhanced Grain Refinement, Texture Weakening and Formability Improvement in Al-Mg-Si-Zn Alloy Sheet Through Hot-Rolling Deformation

Experimental and Finite Element Studies of Stretch Forming Process for ASS 316L at Elevated Temperature

Austenitic stainless steel 316 L grade is a material having extraordinary mechanical properties, low cost and easily available. This is the reason it was used in various industrial and nuclear applications. In the present work, ASS 316L nakazima specimens are stretched under hot forming conditions (750°C, 825°C and 900°C) at a constant strain rate (0.1s-1) along with three different orientations. These six types of nakazima specimens were used to know the formability behaviour of the material with the help of forming limit diagrams (FLD) obtained by the stretch forming process. A smaller change in Punch load and an increase in displacement were observed, which indicates the formability improvement of ASS 316L sheet metal with the increase in temperature. In addition, ABAQUS 6.13 computer code was applied for the prediction of formability from 750°C to 900°C.To improve the accuracy of the simulation, a number of integration points were accrued within the thickness direction, limiting dome height (LDH).The ductile fracture was observed from SEM images for all the temperatures. A close agreement was found between experimental and simulated results.

Heat treatment and temperature effects on formability of AA2014-T6 in incremental forming

ABSTRACT Aluminum alloys of various series are used extensively in the auto and aviation sectors because of their low weight to strength ratio. In the current article, one of the hard-to-form aluminum alloys, i.e., AA2014-T6 investigated for its formability and microstructural changes in incremental forming at room and elevated temperature of 250OC. The mechanical properties and formability of the material are evaluated using uniaxial tensile test and Varying Wall Angle Cone Frustums (VWACF), respectively. A novel experimental setup is designed with an induction coil to heat the sheet at elevated temperatures for incremental forming. As-received sheets formed at room and elevated temperature of 250OC, showed less improvement in material formability due to less number of slip planes. Thus, the samples are solutionized at 480OC for 3 hours followed by aging at 180OC for 8 hours. However, much improvement was not observed in room temperature formability, even though the ductility was improved significantly. But, forming at an elevated temperature of 250OC, the formability was enhanced by 956% compared to the as-received material. The formability of the material is correlated with the microstructural modification after solution treatment at room and elevated temperatures.

Substantial improvement in cold formability of concentrated Mg–Al–Zn–Ca alloy sheets by high temperature final rolling

Poor formability at room temperature (RT) has been the main bottleneck for a wider range of applications of concentrated Mg–Al–Ca(–Zn) sheet alloys. Here, we report that RT stretch formability of concentrated Mg–Al–Zn–Ca alloy sheets can be improved substantially by increasing the final rolling temperature. A Mg–6Al–1Zn–1Ca alloy sheet subjected to 510 °C final rolling shows an index Erichsen (I.E.) value of 7.9 mm, which is significantly higher than that of the same sheet subjected to 450 °C final rolling (4.1 mm). More excitingly, 510 °C final rolled Mg–6Al–1Zn–2Ca (wt%) alloy sheet exhibits a large I.E. value of 8.0 mm which has never been achieved in 2 wt% Ca-containing Mg alloys. A systematic microstructural investigation indicates that the substantial improvement in stretch formability of Mg–6Al–1Zn–xCa (x=1, 2) alloy sheets is mainly associated with weakened basal textures. To clarify the mechanisms responsible for the weak basal textures developed in the high temperature rolled sheets, electron backscatter diffraction (EBSD) assisted slip trace analysis and quasi-in-situ EBSD method were employed. EBSD-assisted slip trace analysis on the side surface of as-rolled Mg–6Al–1Zn–1Ca alloy sheets indicates that the activity of pyramidal <c+a> slip increases significantly by increasing the final rolling temperature from 450 to 510 °C. During subsequent annealing, grains deformed by pyramidal <c+a> slip evolve into new recrystallized grains with a wide spectrum of orientations, which plays a key role in the development of the weaker basal texture.

Cryogenic formability and deformation behavior of 2060 Al–Li alloys with water-quenched and T4 aged temper

In this study, the formability and deformation behavior of new third-generation aluminum-lithium alloys at temperatures ranging from 113 to 298 K were studied by uniaxial tension tests, Erichsen cupping tests, and microstructure observations. The results show that the water quenched 2060 alloy exhibits excellent formability at 113 K with a fracture elongation of 34.1%, and Erichsen cupping value of 10.1 mm that are 33.2% and 40.3% higher than those at 298 K. Homogeneous plastic deformation was obtained at the cryogenic temperature for the water quenched alloy, as was reflected by the following three observations: 1) homogeneous lattice rotation and fewer coarse slip bands occurred in the grains; 2) a large number of substructures (i.e., dislocation cells, subgrains) uniformly distributed in the interior grains; 3) serrated flow caused by the dynamic strain aging disappeared at the cryogenic temperature. The naturally aged 2060 alloy exhibited limited formability improvement. As temperature decreased from 298 K to 113 K, the fracture elongation increased slightly by 11.8% and the Erichsen cupping value decreased by 26.4% for the naturally aged Al–Li alloy. The δ' precipitation phase of the naturally aged Al–Li alloy gave rise to planar slip due to the low misfit with the Al matrix and induced stress concentration at the grain boundaries. Meanwhile, the planar slip resulted in a higher texture and anisotropy at the cryogenic temperature, triggering early fracture and decreasing the ductility.

炭素鋼・ステンレス鋼の加工熱処理による組織制御と強度・成形性の向上

炭素鋼・ステンレス鋼の加工熱処理による組織制御と強度・成形性の向上

Achieving single-pass high-reduction rolling and enhanced mechanical properties of AZ91 alloy by RD-ECAP pre-processing

It is widely recognized that rolling Mg alloys is difficult due to their poor ductility. Conventional rolling of Mg alloys often needs to be conducted at high temperatures with multi-pass small thickness reduction, and the strong basal texture often leads to poor ductility and plate cracking. Herein, we conducted a single-pass high-reduction rolling (SPHRR) on AZ91 alloy after pre-homogenization (PH) and RD-ECAP pre-processing (PH + ECAP). Results shows that the AZ91 rolled plate processed by SPHRR (PH + ECAP + R) exhibits a good combination of high tensile strength (ultimate strength of ~420 MPa, yield strength of ~335 MPa) and high ductility (elongation-to-failure of ~19%), together with significantly reduced edge cracks. Microstructure examinations revealed that the improved mechanical property is mainly attributed to a typical bimodal grain structure featured with relatively high Schmidt factors of basal slips (SF basal ). The fine-grain structure produced by the high-pass RD-ECAP pre-processing contributes to the improvement of formability, provides favourable conditions for dynamic recrystallization (DRX) in further rolling and also promotes the formation of the bimodal structure. Overall, the high-pass RD-ECAP is applicable to improve plastic formability of hard-to-deform materials like Mg alloys, and thus has a promising prospect for industrial applications.

Formability enhancement of 5052 aluminium alloy sheet in electromagnetic impaction forming

Electromagnetic impaction forming of 5052 aluminium alloy sheets using a V-shaped die and a conical die was performed. The conventional quasi-static forming limit curve (QS-FLC) is obtained according to the linear loading condition, which can only describe linear strain paths. However, it cannot accurately evaluate the formability of electromagnetic impaction forming, especially strain paths that have become highly non-linear. In this study, the formability of electromagnetic impaction forming, which considers a bilinear strain path and a continuous highly non-linear strain path, was investigated. The conventional QS-FLC is no longer suitable to evaluate the formability of electromagnetic impaction forming. Therefore, modified FLCs were established in strain space that follow bilinear strain paths and continuous non-linear strain paths. The actual formability improvement was compared with the modified FLC. The maximum safe strain of bilinear strain paths near the sidewall of V-shaped specimens increased in formability by approximately 30% at the discharge voltage of 20 kV compared with conventional QS forming. The maximum safe strains near the apex of the conical specimens increased in formability by approximately 100% at the same discharge energy. It should be noted that the increase in formability above the modified FLC is a conservative estimate, because the limit strains at the necking location were not known in the electromagnetic impaction forming. The influencing factors of the formability improvement of 5052 aluminium alloy sheet in electromagnetic impaction forming were also discussed.

On the interplay of friction and stress relaxation to improve stretch-flangeability of dual phase (DP600) steel

Industrial servo presses have been used to successfully demonstrate improved formability when deforming sheet metals. While the time dependent viscoplastic behavior of material is attributed to the observed formability improvement, much less effort has been devoted to understand and quantify the underlying mechanisms. In this context, the hole expansion test (HET) of a dual phase steel was interrupted at pre-defined punch travel heights to understand the time-dependent effects on stretch-flangeability. The effect of pre-strain, hold time and edge quality on hole expansion ratio (HER) improvement was studied. The present study shows that the HER improves significantly in interrupted HET. This improved HER is due to the combined effects of stress relaxation and friction on deformation behavior. The ductility improvement estimated from uniaxial stress relaxation tests was used to estimate the contribution of stress relaxation and friction, respectively, in HET. This study shows that friction plays a significant role in improving HER at high pre-strain. It was also demonstrated that frictional effects are largely influenced by edge quality.

Microstructure evolution and mechanical properties of (CoCrNi)90(AlTiZr)5(CuFeMo)5 multicomponent alloy: A pathway through multicomponent alloys toward new superalloys

Thermodynamic criteria of lattice distortion energy (ΔHel) and enthalpy of mixing (ΔHmix) were considered to design and estimate the initial phase formation and stability in (CoCrNi)100−x−y(AlTiZr)x(CuFeMo)y multicomponent alloy (MCA) system. This study aimed to chase a new alloy design pathway to develop superalloys by benefitting from the concept of multiphase MCAs. The non-equilibrium cast microstructure (CoCrNi)100−x−y(AlTiZr)x(CuFeMo)y displayed a metastable dendritic structure consisting of primary Co-Cr-Ni-rich FCC dendrites and an interdendritic eutectic mixture, comprising supersaturated cubic Laves-phase and FCC phase. The composition of the AB2-structure Laves-phase was identified as (Zr, Cr, Cu, Al, Ti)(Ni, Co)2. Following the short-term annealing, a tenuous network of Zr-rich phase with a cubic structure was formed by the decomposition of the Laves-phase structure. Phase fraction along with the morphology of the intermetallic phase was altered by treatment and eventually semi-circular islands emerged. Results of tensile and compression tests at different strain rates exhibited the detrimental effect of structural instability and dependency of rate-sensitivity to alloy microstructure. Strain-rate increment enhanced the contribution of ductile failure with slip-band indications; however, this was accompanied by activation of further deformation mechanisms and subsequent formability improvement at a higher rate.

Experimental study and modelling of the role of solutes, precipitates and temperature on the work-hardening of AA6xxx aluminium alloys

Attempts have been registered scientifically mostly for strained hardened Al alloys towards improvement of formability at warm temperatures. The varieties of possible precipitate populations add complexity in determining the mechanisms of work-hardening behaviour of the precipitation hardened alloys. Efforts were made, in the current work, to investigate the work-hardening response for EN AW-6016 and EN AW-6061 sheets of different temper (T4 and T6) at room and warm temperatures (150 and 250 °C) and at two different strain rates (0.1 and 0.01 s −1 ) through tensile experiments. Among these variants, the influence of temperature has been detected the most on yield strength and work-hardening rate. Lowering the efficiency of storing dislocations as well as activating dynamic recovery, both are responsible for the reduction in work hardening rate proportionally with temperature. The work-hardening model originally developed by Prof. E. Nes (Nes model) has been chosen to simulate the experimental stress-strain response of both alloys at room and warm temperature as the phenomenology of the model promised a better constitutive description of work hardening. Because Nes model was primarily designed for strained hardened Al alloys, the bottleneck, to apply this model for precipitation hardened alloys, was removed by adding contribution from the precipitates in terms of mean slip length of dislocations in the current version. The validity of the model was examined for 130 °C, 150 °C and 180 °C and at a strain rate of 0.1s −1 , once satisfied fitting was achieved for 6016 and 6061 alloys at RT and 250 °C at a strain rate of 0.01s −1 . The decrease of dynamic recovery during the change of temper state (T4 to T6) during RT deformation can be explained as a competition between solute depletion and precipitate growth. With minute quantitative differences, the influence of strain rate and temperature was reproduced well by the model.

Simultaneous improvements in strength and formability of modified Mg–2Zn alloys by alloying addition and twin-roll casting process

The limited applicability of magnesium alloy sheets is attributed to the absence of strength and poor formability at room temperature. Strength is generally correlated to formability. It is crucial to achieve simultaneous improvements in both strength and formability. In this study, a novel Mg–2Zn based-alloy sheet was designed with modifications by adding particular alloying elements and processing via twin-roll casting (TRC) process. Microstructure evolutions during fabrication process in three experimental alloys were compared. The effects of alloying addition and TRC process on microstructure and mechanical properties were analyzed. The results showed that simultaneous improvements in strength and formability of Mg–2Zn were achieved, accompanied with excellent trade-off relationship between strength and formability in magnesium composition systems. Refined grain size, weakened basal texture and altered second phase type were obtained by alloying addition. Formation of high-density nanoscale particles with uniform distribution was the main benefit of using the TRC process. Under the promoted activation of prismatic <a> slip due to massive second phase particles, modified Mg–2Zn alloys exhibited weaker basal texture during the rolling process. The main strengthening mechanisms of modified Mg–2Zn alloys included grain refinement strengthening, grain boundary strengthening, and precipitation strengthening. Improvement of formability was attributed to weakened basal texture.