Technique For Grain Refinement Research Articles

Mechanical Properties and Microstructure of AZ31 Magnesium Alloy Processed by Intermittent Ultrasonic-Assisted Equal Channel Angular Pressing

An intermittent ultrasonic-assisted equal channel angular pressing (IU-ECAP) technique for grain refinement is proposed in this study. AZ31 Mg alloy samples were divided into two groups (IU-ECAP and ECAP) and processed using four passes along route BC, and the effects of ultrasonic vibration on the mechanical properties and microstructure were examined. Mechanical testing revealed that IU-ECAP significantly reduced the extrusion force relative to that for ECAP, whereas the microhardness and ultimate tensile strength were strengthened. The maximum true ultimate tensile strength and microhardness of the IU-ECAPed specimens were achieved using ultrasonic vibration with an amplitude of 35 μm and intermittent time of 2 s. In addition, the microstructures were the most homogeneous for the smallest grain sizes. The IU-ECAPed specimens also possessed less high-angle grain boundaries because of the lower temperature of the IU-ECAP die. Texture analysis revealed that after the 4th pass, the texture of the IU-ECAPed specimens along the (0001) crystal plane exhibited maximum reduction; furthermore, the textures along the (0001), ( $$01\bar{1}0$$ ), and ( $$\bar{1}2\bar{1}0$$ ) crystal planes were scattered homogeneously. These findings may be related to ultrasonic-induced grain boundary rotation.

Microstructure and texture evolution of complete Mg-3Al-0.2Ce alloy blanks upon multi-pass friction stir processing with spiral strategy

The objective of this study is to investigate the microstructure and texture evolution upon friction stir processing with multiple passes and spiral strategy, of Mg-3Al-0.2Ce alloy. Friction stir processing is a promising technique for grain refinement. Complete blanks of magnesium alloy are friction stir processed with all the combinations of two tool rotation speeds and two tool overlaps between passes. Optical microscopy, electron back scattered diffraction and texture analysis of the processed cross-sections are performed. Two distinct regions are observed in the processed blanks: banded region and non-banded region. Banded regions consist of much smaller grains as compared to the non-banded regions. The banded regions appear regularly at a distance corresponding to the tool overlap. The banded regions become more prominent at higher tool rotation speed and tool overlap. For the samples processed with higher tool overlap, the difference in grain sizes of banded and non-banded regions is significant, and interfaces between banded and non-banded regions are easily noticeable, which suggests bimodal microstructure. A tilted basal texture is revealed for all the processed samples. This is different from the oppositely tilted basal texture observed from the single pass friction stir processing of magnesium. The difference is attributed to the presence of resulting microstructure from both advancing and retreating side upon single pass friction stir processing. While for multi-pass friction stir processing with spiral strategy, the microstructure resulting from retreating side alone is present in the processed blanks. The close packing of similar microstructure from retreating sides of successive passes also led to higher texture index.

Evolution of microstructure and mechanical properties of Ti6Al4V alloy by multiple passes of constrained groove pressing at elevated temperature

Constrained groove pressing (CGP) is one of the best available sheet material processing technique for grain refinement and improving material properties without changing the original dimensions of the specimen. In this work, CGP is successfully performed on 10 mm thick mill-annealed Ti6Al4V plates at a temperature of 550 °C up to three passes. Microstructure studies reveal that grain size decreased from 18.81 μm in as received (AR) sample to 9.24 μm after third pass. XRD studies confirm the lattice strain and grain refinement phenomenon by peak shifting and broadening. Work hardening and grain refinement lead to increase in microhardness from 346 HV in AR to 392.1 HV, 365.9 HV and 371.4 HV in one pass, two pass and three pass CGP processed (CGPed) Ti6Al4V respectively. CGP leads to improvement of YS from 964.75 MPa in AR to 1085.01 MPa, 1068.11 MPa and 1078.63 MPa in one, two and three pass CGPed Ti6Al4V respectively. Similarly, UTS is observed to improve from 1014.12 MPa in AR to 1118.26 MPa, 1015.21 MPa, 1098.05 MPa in one, two and three pass CGPed Ti6Al4V respectively. SEM fractography reveals change in the mode of fracture of Ti6Al4V from ductile to ductile-brittle mode before and after the CGP process. Work hardening behavior of CGPed Ti6Al4V changes from stage III and IV to purely stage III along with initial instantaneous hardening rate increases after the first pass and decreases for the subsequent passes. CGP on Ti6Al4V alloy at 550 °C is found to have a significant effect on the stabilization of its microstructure and mechanical properties which can be useful for extending CGP for HCP alloys and their applications in aerospace and defense sector.

On the effects of electric current intensity and pulse frequency on the solidified structure of pure aluminum subjected to pulse magneto-oscillation treatment

Pulse magneto-oscillation (PMO) treatment has attracted growing interest as a technique for grain refinement during solidification. Attempts to clarify the effects of PMO treatment parameters have not yet yielded definitive information, because inconsistent results have been reported for the effects of current intensity and pulses frequency on the resulting crystal structures. The present study has investigated the effects of these parameters on the structure of solidified aluminum together with numerical simulations of the effects of electromagnetic field density and applied Lorentz force. It was found that the change in grain size resulting from increasing the electric current intensity, was greater than that resulting from increasing the pulses frequency. However, increasing the current intensity may also increase the induced heat within the charge, thus limiting its refinement efficiency. The simulation calculations showed that once solidification starts, it dramatically decreases the driving force, i.e. the Lorentz force, for grain refinement. This inhibition of the driving force should be taken into consideration in the construction of an efficient electrical system set-up for grain refinement in pure aluminum.

Fine-grained dual phase steel via intercritical annealing of cold-rolled martensite

The effect of intercritical annealing on the microstructure and mechanical properties of a low carbon steel with the chemical composition (wt%) of 0.12C–1.11Mn-0.16Si is studied. The cold rolled martensite and ferrite-pearlite microstructure are considered as the starting microstructures for producing dual phase (DP) steels. It is revealed that intercritical annealing of cold-rolled martensite is a viable technique for grain refinement of DP steels, where a microstructure consisting of ferrite grains with average grain size of less than 5 μm and fine martensite islands can be easily obtained. This DP steel exhibits much better strength-ductility balance compared with that obtained by intercritical annealing of the ferritic-pearlitic microstructure due to much finer microstructure and enhancement of work-hardening behavior in the former. The fine-grained microstructure is also obtained at higher intercritical annealing temperatures for the aim of increasing the volume fraction of martensite. Accordingly, the work-hardening response and the overall mechanical properties are enhanced.

Microstructural Characterization and Mechanical Properties of Cast Al-15Zn-2Mg Alloy Subjected to Severe Plastic Deformation

In the present work, Al-15Zn-2Mg alloy was processed by ECAP technique for grain refinement. The ECAP processing was conducted at 150 °C. Microstructural characterization was carried out in OM, SEM, TEM and XRD. To evaluate the mechanical properties, hardness measurement and tensile tests were conducted at room temperature. Microstructural characterization showed that, ECAP processing leads to decrease in the grain size of the alloy. Also, small amount of dislocations were also observed in the ECAP processed material. After ECAP processing, precipitates nucleation in the material was identified in the XRD analysis. The strength and the hardness values were increased after ECAP processing. After ECAP processing, microhardness of the material is increased from 173 Hv to 252 Hv and the UTS of the material is increased from 166 MPa to 362 MPa. After tensile testing, fracture surface of the cast material showed dendritic structure and the fracture surface of the ECAP processed material showed dimples

Microstructure Evolution and Mechanical Properties of AZ61 Mg Alloy Processed by Two-Pass Friction Stir Processing

Friction stir processing (FSP) has been considered as a promising technique for grain refinement. In this study, two-pass FSP was conducted under water to enhance the cooling rate during processing, and a fine-grained AZ61 magnesium alloy was prepared through this method. Compared with one-pass FSP, the microstructure of two-pass FSP alloy is more homogeneous and finer, with an average grain size of 1.8 μm. Both the tensile strength and ductility of the two-pass FSP sample are improved due to grain refinement. A tensile elongation of 31.8% was achieved in the two-pass FSP sample. The microstructure evolution mechanism during the processing was discussed based on the microstructure examination.

Optimization of process parameters during vibratory welding technique using Taguchi's analysis

Summary With an aim to improve the mechanical properties of a weld joint, a new concept of vibratory setup has been designed which is capable to stir the molten weld pool before it solidifies during shielded metal arc welding (SMAW) operation. Mechanical vibration having resonance frequency of 300 Hz and amplitude of 0.5 mm was transferred to the molten weld pool of 6 mm thick mild steel butt-welded joints during the welding operation. The experimental work was conducted at various ranges of frequencies, welding current and welding speed. Taguchi's analysis technique has been applied to optimize the process parameters; the response values for analysis are yield strength and micro-hardness. The test results showed that with the application of the vibratory treatment the values of hardness and tensile properties increased. The auxiliary vibrations induced into the weld pool resulted in increased micro-hardness of the weld metal which indicates the orientation of the crystal and refinement of grains took place. This study shows that vibration applied into the weld pool can be successfully improved the mechanical properties of welded joints. Thus this research attempt provided an alternative welding technique for grain refinement of weldments.

Ultra fine grained Ti prepared by severe plastic deformation

The positron annihilation spectroscopy was employed for characterisation of defects in pure Ti with ultra fine grained (UFG) structure. UFG Ti samples were prepared by two techniques based on severe plastic deformation (SPD): (i) high pressure torsion (HPT) and (ii) equal channel angular pressing (ECAP). Although HPT is the most efficient technique for grain refinement, the size of HPT-deformed specimens is limited. On the other hand, ECAP is less efficient in grain refinement but enables to produce larger samples more suitable for industrial applications. Characterisation of defects by positron annihilation spectroscopy was accompanied by hardness testing in order to monitor the development of mechanical properties of UFG Ti.

An upper bound solution for twist extrusion process

Twist extrusion, a promising severe plastic deformation technique for grain refinement down to ultrafine/nanocrystalline microstructures, was introduced as an attempt to provide large plastic deformation conditions similar to those in high pressure torsion while allowing large workpiece dimensions for industrial applications. As a relatively new severe plastic deformation technique, twist extrusion requires in-depth investigation of its plastic deformation characteristics. In this study, the twist extrusion process with a square shape die cavity has been analyzed using an upper bound solution to estimate the required power, deformation pattern, and optimum process condition. The analysis has been performed based on two kinematically admissible velocity fields while the effects of friction condition, die geometry, and mean equivalent strain have been considered. The results indicate that the die geometry and process parameters can dramatically change the deformation pattern and extrusion power.

An Analysis on the Deformation of Pure Copper during Multi-pass Equal Channel Angular Extrusion

Nanomaterials are produced by subjecting materials to severe plastic deformation (SPD) which has widely emerged as a technique for grain refinement in Al, Cu, Ti, Mg alloys with improved mechanical properties. Equal Channel Angular Pressing (ECAP) is one of the attractive SPD techniques because it offers potential for high strain rate super plasticity by effective grain refinement from macro gained structure to the level of sub-micron or nano-scale. The aim of this paper was to study the deformation homogeneity of pure copper during multi-pass equal channel angular extrusion. The Finite Element Modeling of ECAP is done in metal forming software FORGE. The experiment is performed in cold working conditions though route C with channel angle Φ = 90° and having different friction conditions. The evolution of equivalent strain is analysed for the deformed billet under different processing parameters upto 5 passes. Also, variation in extrusion force, energy and stresses are studied for the considered conditions. It is established through simulations that number of passes and friction plays significant role in strain homogeneity and changes in vonmises stresses. The study helps in developing ultra-fine grain materials for automobile and aerospace applications.

Finite element analysis of plastic deformation in twist extrusion

Twist extrusion (TE), a promising severe plastic deformation (SPD) technique for grain refinement down to ultrafine/nanocrystalline microstructures, was introduced as an attempt to provide large plastic deformation conditions similar to those in high pressure torsion while allowing large workpiece dimensions for industrial applications. As a relatively new SPD technique, TE requires in-depth investigation of its plastic deformation characteristics. The present study investigates the influence of process parameters such as backward pressure and friction on the loading history, the stress/strain states, and the final shape of processed workpieces using the finite element method. The results provide a basis for reasonable decision of processing conditions and also identify prerequisites for studies in formability and fracture of metals subjected to TE.

Simulation of Friction Stir Processing with Internally Cooled Tool

Friction stir processing (FSP) is considered to be a promising sustainable technique for grain refinement of metallic alloys. The heat generated during FSP promotes dynamic recrystallization in processed material which is essential for grain sub-division process. However, excessive heat generation can lead to high temperatures of >300°C that may cause abnormal grain growth in the processed material. On the other hand, repetitive high temperature heating cycles can reduce the lifetime of the FSP tool. Therefore, it is essential to manage the process heat not only to achieve homogeneity and finer grain sizes in the processed material but also to reduce tool wear. In this work, friction stir processing of AZ31B Mg with an internally cooled FSP tool is simulated by a three-dimensional CFD model. We have studied the effect of rapid tool cooling on temperature and flow stress distribution in processed material. Additionally, the grain size and hardness of the processed material is estimated by using Zener-Holloman and Hall-Petch based relationships. It was found that FSP with internally cooled tool is a promising approach that effectively controls temperature levels during processing. Therefore it enables the achievement of better mechanical properties by effective grain refinement and has a positive effect on tool life.

Simulation of Friction Stir Processing with Internally Cooled Tool

Friction stir processing (FSP) is considered to be a promising sustainable technique for grain refinement of metallic alloys. The heat generated during FSP promotes dynamic recrystallization in processed material which is essential for grain sub-division process. However, excessive heat generation can lead to high temperatures of >300°C that may cause abnormal grain growth in the processed material. On the other hand, repetitive high temperature heating cycles can reduce the lifetime of the FSP tool. Therefore, it is essential to manage the process heat not only to achieve homogeneity and finer grain sizes in the processed material but also to reduce tool wear. In this work, friction stir processing of AZ31B Mg with an internally cooled FSP tool is simulated by a three-dimensional CFD model. We have studied the effect of rapid tool cooling on temperature and flow stress distribution in processed material. Additionally, the grain size and hardness of the processed material is estimated by using Zener-Holloman and Hall-Petch based relationships. It was found that FSP with internally cooled tool is a promising approach that effectively controls temperature levels during processing. Therefore it enables the achievement of better mechanical properties by effective grain refinement and has a positive effect on tool life.

Processing and Characterization of Pure Nickel Sheets by Constrained Groove Pressing (CGP) Technique

Pure nickel sheets are severe plastically deformed by constrained groove pressing technique at room temperature up to three passes. A total strain magnitude of 3.48 is imparted to the sheets and further processing is limited by initiation of surface microcracks. The grain size evolution studied by optical microscopy reveals significant grain refinement at the end of third pass evidently illustrating the effectiveness of groove pressing technique for grain refinement in sheet materials. Vickers microhardness measured during different stages of groove pressing process clearly describes the deformation behaviour in different segments of slant and flat regions. The change in mechanical properties of constrained groove pressed sheet is evaluated by room temperature tensile and microhardness tests. Results showed considerable increase in strength and hardness during initial passes followed by slight drop during third pass.

Applicability of the groove pressing technique for grain refinement in commercial purity copper

Commercial purity copper sheets were subjected to a severe plastic deformation technique called groove pressing (GP) to a strain of 3.48 at both room and cryogenic temperatures. The mechanical properties and microstructure were studied as a function of the number of passes. A deformed microstructure with cell sizes approximately 0.5 μm in size were obtained from a starting annealed grain size of 78 μm.

Novel technique for grain refinement in aluminum casting by Al–Ti–B powder injection

This paper purposes a novel technique for grain refinement in Al casting by Al–Ti–B powder injection into molten Al. Al–Ti–B powders were made by soaking ribbons, obtained from drilling an Al–Ti–B grain refiner bar, in liquid nitrogen before grinding them into small powders. A simple device for this technique comprises a small hopper to store Al–Ti–B powders, a ball valve to control the release of the powders, and a hose to connect argon to purge all powders into the melt. This novel grain refinement technique provides higher efficiency and a shorter critical contact time, which are believed to be the reasons of smaller fractured TiAl 3 and TiB 2 particles and higher number of nucleation sites. In addition, the particles can be well distributed throughout the melt with the help of argon purging and act as heterogeneous nucleation sites.