Constrained Groove Pressing Passes Research Articles

Utilisation of fuzzy logic and genetic algorithm to seek optimal corrugated die design for CGP of AZ31 magnesium alloy

ABSTRACT Magnesium alloys are the first choice among other lightweight structural metals like aluminium, titanium and beryllium due to low density and excellent corrosion resistance. However, low slip and cold plastic processing ability limits its applications. Constrained groove pressing (CGP), one among the SPD techniques, is well suited for improving the material properties. Corrugated dies are designed to investigate the deformation behaviour of AZ31 Mg alloy samples. To minimise the number of numerical simulations, Taguchi’s L9 orthogonal array is selected for the grooved die dimensions (viz. groove angle, groove width and coefficient of friction). Elasto-plastic finite element analysis is performed by implementing a multi-criterion-based genetic algorithm optimisation tool for obtaining the optimal die geometry. Reduction in total deformation, increase in the equivalent stress and elastic strain of AZ31 sheet were possible with 50° groove angle, 3 mm groove width and 0.22 friction coefficient. Mamdani-based fuzzy logic soft computing tool is implemented for the model to examine the deformation behaviour of AZ31 Mg alloy. 7.86 % deviation is observed in the fuzzy logic model predictions in comparison with simulation results. The suggested die design is well suitable for multiple CGP passes, to improve the grain refinement in the processed sheet.

Effects of annealing on microstructure evolution and mechanical properties of constrained groove pressed pure titanium

Constrained groove pressing (CGP) is performed on pure Ti followed by intermediate annealing at 450–650°C to fabricate large-scale plate-shaped sheets with improved mechanical properties. The results reveal that intermediate annealing not only improves ductility, enabling the successful achievement of the three-pass CGP at room temperature, but also enhances the tensile strength during the subsequent passes. The process involving three CGP passes with intermediate annealing after the first pass (referred to as PAPP) is superior for strength improvement. Compared to that of pure Ti, the tensile strength of the PAPP specimen annealed at 550°C is improved by 67%. This is ascribed to the recovery of the secondary twins, 85° <11–20> during annealing at 550°C, resulting in the activated 29.5° <1–100> twin variant in the two subsequent CGP passes. Meanwhile, the dislocations within the shear bands recover and form dislocation networks or subgrains at 550°C, and thus the dislocations continue to entangle and pile up at these sub-grain boundaries in the subsequent pressing, which further contributes to the improvement in the tensile strength. However, recrystallisation occurs in the CGP-deformed pure Ti annealed at 650°C, leading to a decrease in the tensile strength.

Mechanical Behavior of Constrained Groove Pressed Stainless Steel and Pure Zinc

Constrained groove pressing (CGP), which is a severe plastic deformation technique, has been implemented on AISI 304 and commercially pure zinc samples in the present work. Four CGP passes were successfully applied to both materials providing a remarkable improvement in both strength and hardness values, despite reduced ductility. Average grain size values were diminished by around 45% for 304 stainless steel and 47% for commercially pure zinc. The highest levels of strengthening due to severe deformation were displayed after the initial passes for both materials. CGP was shown to be a viable method for strengthening based on deformation processing, whereby similar trends in microstructure and mechanical behavior were observed for both crystal structures.

Microstructural evolution and mechanical properties of 316 austenitic stainless steel by CGP

In the present work, 316 austenitic stainless steel was deformed employing constrained groove pressing (CGP). The processed samples of 316 austenitic steel were characterized using X-ray diffraction and electron backscattered diffraction. The analysis of XRD patterns confirms the transformation of the austenitic phase into the martensitic phase. The Rietveld refinement employing MAUD software on XRD patterns of deformed samples indicates the decrease in crystallite size with the increase in microstrain and dislocation density, which is also supported by electron backscatter diffraction results. CGP leads to an increase in strength and a decrease in ductility. The enhancement in yield strength, ultimate tensile strength and hardness after CGP is due to the combined effect of increased dislocation density, martensitic transformation, sub-structure formation and reduced crystallite size. The yield strength, ultimate tensile strength, and hardness values have increased from 428 to 1055 MPa, 702–1135 MPa, and 261 to 360 VHN in two passes of CGP, respectively. However, the toughness value is decreased from 566 to 205 J/m3. Modeled yield strength values in different conditions are in reasonable agreement with experimental results. The results of finite element analysis employing DEFORM-3D software are in good agreement with the theoretical analysis of CGP.

Effect of strain path on microstructure and mechanical properties of AZ31 magnesium alloy sheets processed by constrained groove pressing

Constrained groove pressing (CGP) is a novel severe plastic deformation (SPD) method suitable for fabricating ultra-fine grained (UFG) sheet metals. In this work, two passes of CGP were conducted to AZ31 magnesium alloy sheets at 523 K, and three routes including traditional CGP, 180° cross-CGP and 90° cross-CGP were employed to investigate the effects of pressing number and strain path on microstructure, texture and mechanical properties of the alloy sheets. The results show that, irrespective of CGP routes, a bimodal microstructure is developed in the deformed materials and the capability of CGP in fabricating UFG magnesium alloy sheets is confirmed by TEM observation. The superior grain refinement and microstructure homogenization is achieved in traditional CGP route from the as-annealed 9.85 μm–5.04 μm after one pass while no further improvement is observed after two passes. The average grain sizes are 5.69 μm and 5.18 μm after two passes of 180° cross-CGP and 90° cross-CGP, respectively. The average misorientation angle is mainly determined by the fraction of very high angle grain boundaries and texture and microstructure mechanisms dominate the generation of high angle grain boundaries in traditional and cross-CGP routes, respectively. The coexistence of strain free recrystallized grains and substructured/deformed grains is observed after one pass of traditional CGP due to incomplete dynamic recovery and dynamic recrystallization and promotes the subsequent activation of strain induced grain boundary migration, which may be postponed by employing cross-CGP routes. A double-peak basal texture with the basal poles inclined from ND to RD is developed in traditional CGP route while an inclination of the basal poles from ND to TD accompanied by a redistribution of the prismatic intensity toward RD is observed in cross-CGP routes due to the change of shear deformation direction. The texture softening plays a more important role in decreased strength than grain refinement in traditional CGP route while both grain refinement and texture modification contribute to the improvement of tensile properties in cross-CGP routes. Due to the inherent nature of CGP, the reverse loading condition between successive pressings cannot be changed even in cross-CGP routes. The relatively high processing efficiency in 90° cross-CGP route should be attributed to the unique strain accumulation sequence.

Microstructural Evolution and Mechanical Properties of Constrained Groove-Pressed 304 Austenitic Stainless Steel

The present work investigates the influence of constrained groove pressing (CGP) on the microstructural evolution and mechanical properties of 304 austenitic stainless steel. CGP shows a significant improvement in mechanical properties such as tensile strength and micro-hardness. XRD analysis shows the transformation of austenite phase into deformation-induced martensite phase which is also confirmed by vibrating sample magnetometer. Finite element analysis employing Deform 3D software shows the average induced strain of 2.29, after two passes of CGP. Rietveld refinements on XRD patterns affirm the decrease in crystallite size, increase in microstrain and dislocation density. EBSD also shows the formation of substructures due to CGP. The ultimate tensile strength of solution-treated specimens enhances from 729 to 1058 MPa, and the value of micro-hardness increases from 238 to 477 VHN after two passes of CGP. The augmentation in mechanical properties is attributed to the synergic effect of increased dislocation density, martensitic transformation, substructure formation and reduced crystallite size.

Warm Tensile Deformation and Fracture Behavior of AZ31 Magnesium Alloy Sheets Processed by Constrained Groove Pressing

Constrained groove pressing (CGP), as a promising severe plastic deformation method suitable for processing sheet metals, was applied to AZ31 magnesium alloy sheets, and ultra-fine grained structure was achieved after two passes of CGP at 473 K. A bimodal structure was also developed due to partial dynamic recrystallization (DRX) and deformation homogeneity. Tensile tests at temperatures from room temperature to 523 K and strain rates from 1 × 10−4 to 1 × 10−1 s−1 were conducted to the processed alloy sheets, and their tensile deformation and fracture behavior were investigated in this work. The maximum elongation to failure of 38.7% is achieved at 1 × 10−4 s−1 and 473 K, and the yield strength and tensile strength are 37.0 and 43.0 MPa, respectively. The strain hardening ability increases gradually with increasing strain rate at elevated temperatures, and its dependence on strain rate is more significant at lower temperatures. The strain rate sensitivity coefficient gradually increases with increasing temperature, and the relatively high values of ~ 0.17 and ~ 0.14 are obtained at 473 and 523 K, respectively, indicating the absence of superplastic behavior. The fracture morphology shows that with increasing temperature, the fracture mode changes from brittle fracture to ductile fracture, which is closely related to the microstructural evolution during tensile deformation. The grain coarsening at 523 K may result in the slight decreases of elongation to failure and strain rate sensitivity coefficient as well as the recovery of strain hardening exponent. The apparent activation energy at 423-523 K is estimated to be 68.8-105.5 kJ/mol. Conclusively, DRX and grain growth should be the dominant mechanism accounting for the warm tensile deformation of AZ31 magnesium alloy sheets processed by CGP, while grain boundary sliding mechanism may contribute little, due to the developed bimodal structure during CGP and the relatively low temperatures for the tensile tests.

The effect of microstructure parameters on the residual stresses in the ultrafine-grained sheets

In this research, the influence of microstructure parameters on the residual stresses of ultrafine-grained sheets was investigated. For this purpose, the constrained groove pressing (CGP) process was carried out on the copper sheets with 3 mm thickness, and residual stresses of the CGPed sheets was measured using the contour method. Microstructure of the CGPed specimens was evaluated by the optical microscopy, micro x-ray diffraction (micro-XRD), and transmission electron microscopy (TEM) experiments. Microstructure parameters including crystallites size, dislocations density, and lattice strain were calculated using Williamson-Hall and Williamson-Smallman equations, and the calculated results were validated by the TEM images. The influence of these parameters on the residual stresses was investigated by analysis of variance (ANOVA) method, and two approaches were considered in this way. According to the results, the CGP process can create nanostructures in the CGPed sheets, and with increasing number of CGP passes, grains size, crystallites size, lattice strain, and residual stresses decrease, and density of dislocations increases. Microstructure parameters have a significant effect on the macro-residual stresses, and strain is the most effective parameter. Also, in the ultrafine-grained sheets, micro-parameters have an undeniable contribution, which is the same as that of macro-parameters on the macro-residual stresses.

Microstructure and Mechanical Properties of 1050 Aluminum after the Combined Processes of Constrained Groove Pressing and Cold Rolling

The present work compares the microstructure and mechanical properties of 1050 aluminum subjected to constrained groove pressing (CGP) combined with cold rolling. Transmission electron microscopy observations showed that four passes of CGP refines the grain size to ~1.4 μm. Combining the CGP with cold rolling (rolling at a room temperature) brings about further reduction of the grain size and a significant improvement of the mechanical properties. It also changes the tensile deformation behavior of 1050 aluminum. The stress-strain curves of further cold rolled samples displayed a yield point phenomenon. A combination of high strength exceeding 300 MPa and good elongation to failure approaching 20% was obtained for 1050 aluminum after processing by 4 passes of CGP combined with subsequent 80% cold rolling.

An experimental investigation on the influence of elevated-temperature constrained groove pressing on the microstructure, mechanical properties, anisotropy and texture of 2024 Al sheets

The constrained groove pressing (CGP) is one of severe plastic deformation (SPD) methods for producing ultra-fine grained sheet metal. The present research work is concerned with CGP process of 2024 Al alloy, with a high strength to weight ratio, at 300 °C, and for three cycles. With this regard, several tensile tests were performed to study the mechanical properties of various samples in different directions. By computing the Lankford anisotropy coefficient, normal and planar anisotropy were determined for specimens produced via up to three CGP passes. Moreover, pole figures (PF) and inverse pole figures (IPF) of various samples were prepared for texture analyses and finding the causes of variations in their UTS, elongation and anisotropy parameters. On the basis of texture analysis, it was found that 1-pass CGP could result in the highest normal anisotropy which means the lowest risk of fracture in a subsequent metal forming process such as deep drawing. The highest texture intensity was also owned by the 1-passed specimen. On the other hand, the 2-passed workpiece demonstrated the lowest absolute planar anisotropy value which could be beneficial for an additional forming operation.

Numerical Analysis of Constrained Groove Pressing and Mechanical Behaviour of Processed 316L Stainless Steel

Among several existing and well established severe plastic deformation techniques, constrained groove pressing (CGP) is one of the prominent and trusted routes for producing ultrafine grained materials. In the present work medical grade 316L austenitic stainless steel sheet of 3 mm thickness was subjected to CGP up to two cycles. Samples obtained as a result of processing were investigated both experimentally and numerically through finite element (FE) analysis using DEFORM-3D software. XRD study revealed the transformation of austenitic phase to martensitic phase. Tensile and hardness tests were conducted to see the effect of processing on mechcanical properties. The ultimate tensile strength increases with the number of CGP passes from 767 (solution annealed) to 1162 MPa (after 2 cycles), similarly, yield strength increases from 269 (as received) to 328 MPa (after 2 cycles). Finite element analysis showed an imposed strain of 2.30 with standard deviation of 0.31 after two cycle of CGP is in coordination with experimental measured strain of 2.32.

Effect of stress relief annealing on microstructure, mechanical properties, and residual stress of a copper sheet in the constrained groove pressing process

In this research, the effect of stress relief annealing heat treatment on mechanical properties, microstructure, and residual stresses of a severe plastic deformation technique, known as constrained groove pressing (CGP), was investigated. For this purpose, the CGP process was performed on the commercial pure copper sheet with 3-mm thickness. The hardness, strength, homogeneity, microstructure, and residual stresses before and after stress relief annealing heat treatment were evaluated. Hardness and homogeneity on the surface and thickness were investigated and strength in both of groove direction and transverse groove direction were determined. Also, microstructure was studied along the thickness of CGPed samples, and the contour method was used for 2D residual stress measurement. The results indicate, with increasing number of CGP passes, hardness, strength, and homogeneity increase, while the mean grain size and residual stress decrease. Stress relief annealing reduces the mean grain size in the first pass but increases the mean grain size in the subsequent passes, causing a decrease of mechanical properties. Also, stress relief annealing increases residual stresses due to increasing microstructure heterogeneity.

Deep Drawing of constrained groove pressed EDD steel sheets

Constrained groove pressing (CGP) is a severe plastic deformation process for producing ultra-fine grained sheet metal without any change in the initial dimensions. CGP works on the principle of repetitive shear deformation of the material by alternate pressing by using asymmetrically grooved die and flat die constrained tightly by boundaries of the die. In this study, CGP, up to two passes has been performed on Extra deep drawing (EDD) steel specimens. To study the formability, the CGPed steel specimens have been deep drawn using a typical deep drawing setup and the limiting draw ratio (LDR) has been calculated. This LDR of CGPed material has been compared to the LDR of cups drawn using as-received EDD steel blanks. The formability of the CGPed material is almost the same as that of the as-received material. Apart from this, micro-hardness study of the cups has also been done on a similar comparative basis. These investigations show a slight improvement in the hardness values. It is found that by doing two passes of CGP on EDD steel, stronger cups can be produced without sacrificing much of the formability.

Microstructure, hardness homogeneity, and tensile properties of 1050 aluminum processed by constrained groove pressing

1050 commercial purity aluminum was subjected to severe plastic deformation through constrained groove pressing (CGP) at room temperature. Transmission electron microscope observations showed that after four CGP passes the majority of microstructure is composed of elongated grains/subgrains whose width/length average sizes are 506/1440 nm. This ultrafine-grained microstructure leads to a significant increase in yield strength of starting material from 93 to 182 MPa. At the same time, after four passes of CGP the material still displays a considerable ductility of 19%. Microhardness profiles reveal that average microhardness value in sample increases monotonically with increased straining during CGP. However, the degree of deformation homogeneity in samples remains almost unchanged at higher number passes. The latter was also confirmed by non-uniform distribution of imposed plastic strain in samples predicted by finite-element analysis.

Deformation Behavior of Severely Deformed Al and Related Mechanisms Through Warm Tensile Test

Flow stress and ductility behaviors of the annealed and severely deformed Al were investigated at warm deformation temperatures. Constrained groove pressing (CGP) method as a severe plastic deformation process was used. The tensile test was carried out at the temperature range of the 298-573 K and strain rate range of 0.001-0.1 s−1 to present the elevated temperature deformation behavior utilizing hyperbolic sine constitutive equation. The flow stress of the CGPed sample is increased with the number of CGP passes and decreased with temperature. Dynamic recovery and strain softening are found as main restoration mechanisms. Flow stress amounts are not remarkably affected by the strain rate. Values of the elongation are decreased with the number of CGP passes. Values of the calculated strain rate sensitivity are utilized to justify the elongation behavior. Shear bands created by CGP remarkably decrease the fracture elongation values. Temperature interval of 298-473 K cannot remarkably affect the flow stress and ductility. The interval of 473-573 K is chosen as critical temperature interval in which the values of flow stress and elongation are remarkably decreased and increased, respectively. Increasing the temperature up to 573 K causes recrystallization in shear bands. Scanning electron microscope was used to study fracture surface which can truly predict the elongation behavior. With increasing the temperature, the shear decohesion area is gradually replaced with fully dimpled structures. Finally, hot deformation activation energy for CGPed samples was calculated about 85 kJ/mol which is close to the grain boundary diffusion activation energy in pure Al.

Tensile Testing of Ultra-Fine Grained Extra Deep Drawing Steel Sheets

Mechanical properties of a material can be improved by refining the grain size of the material. Constrained Groove Pressing (CGP) is one such process which results in ultrafine grain size. In this study, as received Extra Deep Drawing (EDD) steel samples are subjected to multiple passes of CGP. The resulting microstructure and grain sizes are examined through an optical microscope and compared to the original microstructure. Results show that grains become finer with the increase in number of CGP passes(147μm in initial sample to 50μm after 3rdCGP pass). Further, tensile tests have been conducted on these CGPed samples and the tensile properties such as yield strength, ultimate tensile strength, percentage elongation, toughness, strain hardening exponent and hardening coefficient have been evaluated and discussed.

Differential scanning calorimetry study of constrained groove pressed low carbon steel: recovery, recrystallisation and ferrite to austenite phase transformation

Low carbon steel sheets are subjected to severe plastic deformation (SPD) via constrained groove pressing (CGP) up to five passes. As a result of this process, strain magnitude up to 5·8 is imposed to the sheets, which leads to grain size of 225 nm. These nanostructured steel sheets, due to their high dislocation density and ultrafine microstructure, are very sensitive to heating. In the present study, recovery, recrystallisation and ferrite to austenite phase transformation phenomena for the SPD steel are investigated using differential scanning calorimetry method. The results show that with increasing the strain in steel sheets, the deformed stored energy (released through recovery and recrystallisation) and enthalpy of ferrite to austenite phase transformation are significantly increased and varied in 38·5–85·8 and 109–156·1 MJ m−3 ranges respectively. In addition, transformation temperature is decreased from 761 to 750°C after five CGP passes. However, recovery stored energy, recovery and recrystallisation peak temperatures are not changed, considerably. Experimental data show that with increasing the hardness, the stored energy is increased. One empirical equation is developed for relationship between hardness and stored energy of severely deformed low carbon steel. In addition, using the dislocation model, this mentioned relationship is justified.

Effects of Friction on Constrained Groove Pressing of Pure Al Sheets

Constrained groove pressing (CGP) is a newly developed severe plastic deformation technique for producing ultra-fine grained sheet metals. In this study, the effects of friction on tensile properties, microstructure and strain distribution of commercially pure Al sheets were investigated using experimental and numerical methods. Results show that the improvement of contact condition between the dies and sheet suspends the occurrence of micro-cracks and induces more CGP passes. A finer and more homogeneous microstructure is obtained after 6 passes with Teflon layers as lubricant. The ductility of CGP sheets experiences apparent enhancement while the strength gradually decreases. In addition, the uniformity of strain distribution falls with the increase of friction coefficient.

Strain rate sensitivity and fracture behavior of severely deformed Al–Mn alloy sheets

► Aluminum alloy 3003 was processed by constrained groove pressing (CGP) technique. ► Strain rate sensitivity values increase with increasing imposed strain by CGP. ► Fracture behavior changes with imposed strain. ► Shear fracture angle increases with increasing of CGP pass number. Wrought sheets of an Al–Mn alloy are subjected to severe plastic deformation using constrained groove pressing (CGP) method. After applying CGP passes on the sheets, tensile tests at different strain rates are carried out and the strain rate sensitivity values are calculated. Results show that strain rate sensitivity is increased from 0.0081 in annealed sheets to 0.019 after three passes of constrained groove pressing. This arises from intrinsic decrease in grain size due to severe straining. Experimental results reveal that after three CGP passes, the rate of increasing in strain rate sensitivity is decreased. Fracture surfaces of the tensile samples are observed using scanning electron microscopy. It is found that with increasing pass number, shear fracture angle is increased from 45° for annealed sample to 60° after three passes of CGP. This is due to the change in the mechanism of fracture from tensile fracture to shear fracture which is detectable from the formation of parabolic dimple instead of hemispherical equiaxed ones.

Tensile Properties of 1060 Al Alloy Subjected to Constrained Groove Pressing

Three groups of commercial 1060 Al alloy sheets were subjected to constrained groove pressing (CGP) at room temperature using parallel CGP, 180° cross CGP and 90° cross CGP, respectively. Tensile properties and fracture modes of as-annealed and CGPed samples were investigated. The ultimate tensile strength (UTS) of 1060 Al increases significantly after CGP, while the elongation decreases. But they are strongly dependence on the number of CGP passes and the pressing modes. The UTS and elongation of the samples processed by 90° cross CGP are best, consequently, the static toughness of the 90° cross CGPed samples is enhanced. Besides, all CGPed specimens failed in a ductile manner. With increasing the number of CGP pass, the amount of small dimples increases, and the dimples become shallow and more uniform.