Mechanical behaviour and texture of annealed AZ31 Mg alloy deformed by ECAP

Abstract

AZ31 Mg alloy samples were processed by equal channel angular pressing (ECAP) at 220°C for four passes. An average grain size of ～1·9 μm with reasonable homogeneity was obtained. The ECAP process imparted large plastic shear strains and strong deformation textures to the material. Subsequent annealing of the equal channel angular pressed samples produced interesting mechanical behaviours. While yield strength increased and ductility decreased immediately after undergoing ECAP, annealing at temperatures <250°C restored ductility significantly at a small decrease in of yield strength. Annealing at temperatures >250°C reduced yield strength without additional improvement in ductility. It is believed that the combination of stress relief via dislocation elimination, refined microstructure and the retention of a strong ECAP texture at low annealing temperatures enhance ductility. High temperature annealing breaks down the ECAP texture resulting in no further improvement in ductility. The results show that the mechanical properties of the alloy can be positively influenced by annealing after ECAP to achieve a combination of strength and ductility.     

Producing High‐Strength Metals by I‐ECAP

Incremental equal channel angular pressing (I‐ECAP) is used in this work to produce ultrafine‐grained (UFG) pure iron, aluminum alloy 5083, commercial purity titanium (grade 4), and magnesium alloy AZ31B. Pure iron is processed at room temperature, aluminum alloy at 200 °C, titanium at 320 °C, and magnesium alloy at 150 °C. Strength improvement, attributed to the grain refinement below 1 μm, is reported for all processed materials. The yield strength increase is the most apparent in pure iron, reaching almost 500 MPa after one pass of I‐ECAP, comparing to 180 MPa in the as‐forged conditions. UFG titanium, aluminum, and magnesium alloys obtained in this study reached yield stress of 800, 350, and 300 MPa, respectively, in each case exhibiting the yield strength increase by at least 30%, comparing to the alloys processed by conventional metal forming operations such as forging and rolling.     

Mechanical,machining and corrosion properties of Al5083-carbon nanotubes composite produced by friction stir processing

In the current work, composites of Al5083 aluminum alloy and carbon nanotubes were developed by friction stir processing. Grain size reduction was observed in the composite from a starting size of 115 μm±4.6 μm to 11 μm±3.3 μm. Higher hardness, yield strength and ultimate tensile strength were measured for the composite at the cost of losing ductility compared with friction stir processed Al5083 and base alloy. This behavior can be understood by considering the influence of grain size and carbon nanotubes. Machining studies carried out by conducting drilling experiments demonstrate decreasing cutting forces for the composite compared with friction stir processed Al5083. However, compared with base alloy, composite exhibited higher cutting forces at all of the cutting parameters. Corrosion behavior of the materials assessed by electrochemical tests demonstrates the promising effect of grain refinement on enhancing the corrosion resistance of friction stir processed Al5083. However, presence of carbon nanotubes marginally decreased the corrosion resistance of composite compared with friction stir processed Al5083. From the results, it can be understood that the addition of carbon nanotubes significantly enhance the mechanical properties and machinability. However, addition of carbon nanotubes on decreasing the corrosion performance is a noteworthy observation.     

Improve stress corrosion cracking resistance and mechanical properties of Al‐Zn‐Mg‐Cu‐Zr alloy with heterogeneous lamellae structure produced by high pressure torsion

High pressure torsion experiment followed with heat treatment were carried out on as‐cast Al‐Zn‐Mg‐Cu‐Zr alloy at 400 °C under the pressure of 1 GPa. The microstructure, mechanical property and stress corrosion resistance properties for the as‐cast and high pressure torsion processed samples were studied. The results show that high pressure torsion processing can improve the mechanical property by the refinement of grains and grain boundary precipitates, as well as the homogeneous distribution of fine matrix precipitates. On the other side, the grain refinement, broken of coarse grain boundary precipitates and narrowed precipitates free zone caused by the high pressure torsion result in the improvement of stress corrosion cracking resistance. And due to the influence of heterogeneous lamellae structure, the sample after 0.5 high pressure torsion turn shows preferable mechanical property and stress corrosion cracking resistance than the sample after 2 high pressure torsion turns.     

Grain refinement in as cast Al–Mg–Mn alloy during high temperature equal channel angular pressing

Abstract

An as cast Al–Mg–Mn alloy with coarse equiaxed grain structure was processed by equal channel angular pressing (ECAP) at 350°C up to eight passes. Systematic studies were made on the microstructural evolution during ECAP by optical microscopy, electron backscattered diffraction and TEM. Equal channel angular pressing led to a considerable grain refinement, resulting in an average cell size of about 1 μm and a fraction of high angle boundaries of 75% after eight pressing passes. Deformation bands were not developed during the ECAP process, and a reasonably equiaxed substructure was obtained even after one pass. The main mechanism of grain refinement was attributed to the continuous dynamic recrystallisation based on the motion of deformation induced dislocations. Discontinuous recrystallisation at grain boundaries and triple junctions also contributed to the refinement, which played an important role especially at high strain of eight passes.     

Microstructure,mechanical and degradation properties of equal channel angular pressed pure magnesium for biomedical application

Abstract

Magnesium is a biocompatible and biodegradable metal, which has attracted much interest in biomedical engineering. Pure magnesium shows the low strength and plasticity at ambient temperature. Microstructure, mechanical properties and degradation properties of the equal channel angular pressed pure magnesium have been investigated for biomedical application in detail by optical microscopes, mechanical properties testing and corrosion testing. The results have revealed that the processing temperature and routes are important factors that affect the properties of pure Mg by equal channel angular pressing. The two-step equal channel angular pressing processing (one pass at 360°C and three passes at 200°C) has been successfully applied to control the microstructure, mechanical and degradation properties of the pure Mg. Optical microscopy observation has indicated that the grain size of the as cast pure magnesium has been significantly decreased after equal channel angular extrusion, which has mainly contributed to the high tensile strength and good elongation. Equal channel angular pressed pure magnesium has provided moderate corrosion resistance, which has opened a new window for materials design, especially for biomedical.     

The effect of SMAT-induced grain refinement and dislocations on the corrosion behavior of Ti–25Nb–3Mo–3Zr–2Sn alloy

A nanocrystalline layer, which consists of pure β phase with high density of dislocations on Ti–25Nb–3Mo–3Zr–2Sn alloy, was fabricated by surface mechanical attrition treatment (SMAT). The corrosion behavior of the as-SMATed sample, together with the solution-treated coarse-grained and 200 °C annealed SMATed samples, was investigated by potentiodynamic polarization and electrochemical impedance spectroscope (EIS) techniques in physiological saline and simulated body fluid (SBF) solutions. The results demonstrate that the corrosion resistance of the studied alloy in both of the solutions considerably increased as the grain size decreased from microscale to nanoscale, which is ascribed to the dilution of segregated alloying elements at grain boundaries and the formation of more stable and much thicker passive protection films on the nanograined samples. Although the SMAT-induced grain refinement and dislocations both have positive effects on the corrosion behavior of the studied alloy, our post annealing experimental results indicate that the improved corrosion resistance is mainly due to the grain refinement.     

Effect of integrated extrusion–equal channel angular pressing temperature on microstructural characteristics of low carbon steel

Abstract

In the present research, a combined forward extrusion–equal channel angular pressing was developed and executed for the deformation of a plain carbon steel. In this method, two different deformation steps, including forward extrusion and equal channel angular pressing, take place successively in a single die. The deformation process was performed at different deformation start temperatures (800, 930 and 1100°C). Three-dimensional finite element simulation was used to predict the strain and temperature variations within the samples during deformation. With microstructural observations and the results of finite element simulation, the main grain refinement mechanisms were studied at different deformation temperatures. The results show that the forward extrusion–equal channel angular pressing is effective in refining the ferrite grains from an initial size of 32 μm to a final size of ～0·9 μm. The main mechanisms of grain refinement were considered to be strain assisted transformation, dynamic strain induced transformation and continuous dynamic recrystallisation, depending on the deformation temperature.     

Effects of aluminum and copper cover tube casing on the ECAP process of AM30 magnesium alloy

Aluminum (Al) and copper (Cu) thin tube casings were employed to facilitate the equal channel angular pressing (ECAP) process of wrought AM30 magnesium (Mg) alloy. We covered the Mg rods (diameter, 14?mm) with pure copper (Mg/Cu) and Al-1050 alloy (Mg/Al) sheaths in the form of tubes (thickness, 2.5?mm). We then performed the ECAP processes at 200, 250, and 275°C using the conventional ECAP of AM30 alloy and covered tube casing (CTC) methods with a single pass. We assessed the effect of CTC on ECAP temperature, grain refinement, uniformity of structure, hardness distribution, and strength. The results of Mg/Cu and Mg/Al bimetal sample tests showed that there was a significant decrease in the process temperature when compared with the conventional ECAP of AM30. The sheath resolved the segmented material flow that occurred in a conventional ECAP of material at 250°C, and transformed it to a uniform flow in CTC bimetal samples. Grain sizes of the CTC samples decreased by 20% compared with the conventional ECAP of AM30 samples. Furthermore, grain uniformity, strain distribution homogeneity, tensile yield strength (TYS), and elongation increased. However, we observed a similar trend of deformation twinning in compression test results for the ECAP and CTC methods, and there was no significant variation in material yield asymmetry between the ECAP and CTC samples.     

Role of biomineralization on the degradation of fine grained AZ31 magnesium alloy processed by groove pressing

Groove pressing (GP) has been successfully adopted to achieve fine grain size up to 7 μm in AZ31 magnesium alloy with an initial grain size of 55 μm. The effect of microstructural evolution and surface features on wettability, corrosion resistance, bioactivity and cell adhesion were investigated with an emphasis to study the influence of deposited phases when the samples were immersed in simulated body fluid (SBF 5 ×). The role of microstructure was also evaluated without any surface treatments or coatings on the material. GPed samples exhibit improved hydrophilicity compared to the annealed sample. After immersion in SBF, specimens were characterized using scanning electron microscopy (SEM), energy dispersive X-ray (EDAX) analysis and X-ray diffraction (XRD) methods. More amount of white precipitates composed of hydroxyapatite and magnesium phosphate along with magnesium hydroxide was observed on the surfaces of groove pressed specimens as compared to the annealed specimens with an increase in immersion time in SBF. Corrosion behavior of the samples estimated using potentiodynamic polarization curves indicate good corrosion resistance for GPed samples before and after immersion in SBF. The MTT assay using rat skeletal muscle (L6) cells revealed that both the processed and unprocessed samples are nontoxic and cell adhesion was promising for GPed sample.     

Role of particles on microstructure and mechanical properties of the severely processed 7075 aluminium alloy

The 7075-T6 Al alloy was processed by accumulative roll bonding (ARB) up to five passes by a 3:1 thickness reduction per pass at 300, 350 and 400 °C. Microscopical examinations revealed that the particle distribution varies with the processing parameters. As a consequence, the particle pinning effect decreases with increasing processing temperature and the number of ARB passes. For this reason, the evolution of grain structure with the number of passes is different for each processing temperature, and the superplastic properties are determined not only by the microstructural features before deformation, but also by the alloy thermal stability. The best superplastic properties are attained by the sample processed up to three passes at 300 °C, obtaining elongations to failure in excess of 200 %. Finally, ARB processing at high temperature leads to a decrease of hardness with respect to the peak-aged as-received alloy, especially with increasing number of passes and with decreasing processing temperature, due to the variation of the interparticle mean spacing and the amount of solute atoms.     

Strain path effects on microstructural evolution and mechanical behaviour of constrained groove pressed aluminium sheets

Change in strain path during severe plastic deformation (SPD) of metallic materials has shown significant influence on the microstructural evolution, grain boundary characteristics and mechanical behaviour. In the present work high purity aluminium sheets are severe plastically deformed at room temperature by conventional constrained groove pressing (CGP) technique and cross-CGP technique up to 2,4, and 6 passes thereby imparting total effective plastic strain of 2.32, 4.64 and 6.96 respectively. Change in strain path is imposed during cross-CGP by rotating the sheets by ± 90° along thickness axis between each pass. Microstructural evolution of processed sheets studied by electron back scattered diffraction (EBSD) analysis revealed ultra-fine grains (~ 1 μm) irrespective of change in strain path. Analysis of grain boundary characteristics showed significant influence of strain path change on the evolution of relative fraction of low angle and high angle boundaries. The grain refinement mechanism during deformation processing in both conventional CGP and cross-CGP is corroborated to the evolution of misorientation distribution. Though considerable improvement in room temperature tensile characteristics is observed in both cases, cross-CGP processed Al sheets exhibited superior tensile properties.     

Influence of heated forming tools on corrosion behavior of high strength aluminum alloys

In this study forming tools temperated at 24 °C and 350 °C were used to systematically investigate the influence of different cooling rates on the mechanical and corrosion properties of a high strength aluminum alloy AA7075 within a novel thermo-mechanical process that combines forming and quenching simultaneously. The samples formed within heated tools reveal higher ductility and lower material strength compared to the parts processed in cold tools. In addition, the corrosion behavior changed between samples formed with 24 °C forming tools and 350 °C forming tools, respectively. Through cyclic polarization in chloride containing aqueous media a change in the hysteresis and shift of open circuit potential was observed. Metallographic investigation revealed that there was also a very different corrosion morphology for the samples formed within the heated tools. No change in average grain size could be detected but changes of the microstructure in subgrain scale that occur during the forming within the heated tools are responsible for this effect. In further research, the effect of various cooling rates on mechanical and corrosion behavior and the microstructure will be investigated by variation of the forming tool temperature.     

Enhanced mechanical properties and electrical conductivity in ultrafine-grained Al alloy processed via ECAP-PC

The objective of this work is to study the effect of grain refinement using equal channel angular pressing with parallel channels (ECAP-PC) on microstructure, mechanical properties, and electrical conductivity of an Al–Mg–Si alloy. The coarse grained (CG) material is subjected to ECAP-PC processing at 100 °C for 1, 2, and 6 passes. Mechanical behavior of the Al–Mg–Si alloy after ECAP-PC processing and its electrical conductivity are analyzed with respect to the microstructure developed during ECAP-PC processing. The effect of artificial aging (AA) on the microstructure, mechanical properties, and electrical conductivity of the ECAP-PC processed Al–Mg–Si alloy is investigated. It is shown that the microstructure developed during ECAP-PC processing affects the kinetics of the aging process that, in turn, affects the mechanical properties and electrical conductivity of the material. It is demonstrated that both mechanical properties and electrical conductivity of the Al–Mg–Si alloy can be simultaneously enhanced via intelligent microstructural design through optimization of the thermo-mechanical processing applied to this material.     

Microstructure and mechanical properties of Mg–Zn–Ca alloy processed by equal channel angular pressing

An ultrafine-grained (UFG) Mg–5.12 wt.% Zn–0.32 wt.% Ca alloy with an average grain size of 0.7 μm was produced by subjecting the as-extruded alloy to equal channel angular pressing (ECAP) for 4 passes at 250 °C. The fine secondary phase restricted the dynamic recrystallized (DRXed) grain growth during the ECAP processing, resulting in a remarkable grain refinement. A new texture was formed in the ECAPed Mg alloy with the {0 0 0 2} plane inclined at an angle of 58° relative to the extrusion direction. The yield stress (YS) was decreased in the as-ECAPed alloy with finer grains, indicating that the texture softening effect was dominant over the strengthening from grain refinement. The ductility of the as-ECAPed alloy was increased to 18.2%. The grain refinement caused an obvious decrease in work hardening rate in the as-ECAPed alloy during tensile deformation at room temperature.     

Microstructural and mechanical properties evolution of magnesium AZ61 alloy processed through a combination of extrusion and thermomechanical processes

In this work, the microstructures and tensile properties of a commercial magnesium alloy “AZ61” processed by a combination of hot extrusion and thermomechanical processing (TMP) were investigated. The TMP was consisting of two or three hot rolling steps with large reductions per pass, thus allowing significant grain refinement. The microstructural evolution has been studied by means of optical and scanning electron microscopes, as well as X-ray diffraction analysis. The as-cast material is extruded in the form of a cylinder with initial diameter of 250 mm to a final diameter of 110 mm (80% reduction in cross-sectional area). Then hot rolling regimes were performed at 300 °C with different percentage of strain per pass. Tensile and hardness tests were performed in the samples (as-cast, extruded, and rolled) at room temperature in order to evaluate the mechanical properties of the material. The results of experiments demonstrated that fine grain size might be achieved in magnesium alloy AZ61 by using a two-step processing route involving an initial extrusion step followed by thermomechanical processing with large reduction in thickness per pass. This two-step process, designed to achieve average grain sizes of 10–20 μm.     

Effect of grain refinement and crystallographic texture produced by friction stir processing on the biodegradation behavior of a Mg-Nd-Zn alloy

The application of a single pass of friction stir processing(FSP) to Mg-Nd-Zn alloy resulted in grain refinement, texture evolution and redistribution of second phases, which improved corrosion resistance.In this work, an as-rolled Mg-Nd-Zn alloy was subjected to FSP. The microstructure in the processed zone of the FS-400 rpm alloy exhibited refined grains, a more homogenous grain size distribution, less second phases, and stronger basal plane texture. The corrosion behavior assessed using immersion tests and electrochemical tests in Hank's solution indicated that the FS-400 rpm alloy had a lower corrosion rate, which was attributed to the increase of basal plane intensity and grain refinement. The hardness was lowered slightly and the elongation was increased, which might be attributed to the redistribution of the crushed second phases.     

Effect of heat treatment on the corrosion resistance and mechanical properties of selective laser melting Ti6Al4V alloy

The present work shows that the effect of several heat treatments on the corrosion resistance and mechanical properties of Ti6Al4V processed by selective laser melting (SLM). The microstructure of Ti6Al4V alloy produced by selective laser melting exhibited bulky prior β columnar grains, and a large amount of fine acicular martensites α′ were observed inside the prior β columnar grains. The acicular martensitic α′ were transformed to a mixture of α and β after heat treatment, and the grain size increases with the increase of heat-treated temperature. The results of 3.5 wt% NaCl solution electrochemical corrosion test showed that the heat-treated samples possess a higher corrosion resistance than the as-received sample. Among of them, the sample after heat-treated at 730 °C exhibited best corrosion resistance and excellent fracture strain. The sample heat treated at 1015 °C showed worst mechanical properties due to the formation of Widmanstätten structure.     

The effect of microstructure on the morphology of fatigue cracks in UDIMET® 720

The low cycle fatigue (LCF) behaviour of four variants of UDIMET^® 720 was investigated. The materials comprised a fine grained (approximately 10 μm), powder processed material with a fine bimodal distribution (～20 and 80 nm) of secondary γ′; the same material, but with enlarged secondary γ′ (～480 nm); a coarse grained powder processed material (grain size ～62 μm) and finally a cast and wrought material with a similar microstructural scale to the fine grained powder processed alloy, but with reduced interstitial element content. LCF testing was undertaken on corner notched square section specimens at 20, 300 and 600 °C with a frequency of 0.25 Hz, a cyclic stress range of 500 MPa and an R ratio of +0.1. At 20 and 600 °C fracture was found to be macroscopically flat for all materials. However, at 300 °C, significant shear fracture was observed in the two materials that had a fine grain size and a fine secondary γ′ size, leading to a characteristic ‘tear‐drop’ appearance. Only minor shear fracture was observed in the coarse grained and enlarged secondary γ′ materials. Tensile tests indicated that weak dynamic strain ageing occurred in all materials at 300 °C. The fine grained powder processed U720 also exhibited dynamic strain ageing at 600 °C, but this was not the case for the coarse grained or cast and wrought materials. The origin of the shear fracture are discussed and related to the microstructure.     

The role of low-angle grain boundaries in multi-temperature equal channel angular pressing of Mg–3Al–1Zn alloy

Equal channel angular pressing was used to process an AZ31B magnesium alloy (nominally Mg–3Al–1Zn in wt%) at temperatures decreasing from 200 to 150 °C. The resulting microstructure was characterized by electron backscattered diffraction to reveal the role of low-angle grain boundaries in grain refinement. It was found that low-angle grain boundaries with misorientation angles lower than 5° are surrounded by regions of increased strain gradients, which can stimulate the generation of non-basal slip dislocations during the equal channel angular pressing at temperatures of approximately 150 °C. The strain gradients in the vicinity of the grain boundaries with misorientation angles in the range of 5°–10° were less frequent or were completely absent for high-angle grain boundaries with misorientation angles higher than 10°. This article also discusses the importance of low-angle grain boundaries for the generation of non-basal 〈c+a〉 dislocations needed for successful equal channel angular pressing of AZ31B at temperature of 150 °C.