Possibility Of Grain Refinement Research Articles

Investigation on the Dynamic Behavior of Weld Pool and Weld Microstructure during DP-GMAW for Austenitic Stainless Steel

The influence of heat and droplet transfer into weld pool dynamic behavior and weld metal microstructure in double-pulsed gas metal arc welding (DP-GMAW) was investigated by the self-designed high-speed welding photography system. The heat input, the arc pressure, the droplet momentum and impingement pressure were measured and calculated. It was found that the arc pressure is far less than the droplet impingement pressure. The heat input and droplet impingement pressure per unit time acting on weld pool were proportional to the current pulse frequency, which fluctuated with thermal pulse. The size and oscillation amplitude of the weld pool had noticeable periodic changes synchronized with the process of heat input and droplet impingement. Compared to the microstructure of pulsed gas metal arc welding (P-GMAW) weld metal, that of DP-GMAW weld metal was significantly refined. High oscillation amplitude assisted the enhancement of weld pool convection, which leads to more constitutional supercooling. The heat input and shear force during the peak of thermal pulse causing dendrite fragmentation which provided sufficient crystal nucleus for the growth of equiaxed grains and the possibility of grain refinement. The effects of current parameters on welding behavior and weld metal grain size are investigated for further understanding of DP-GMAW.

Advanced High Strength Steels: Improved Properties by Design of Textures and Microstructures

Texture and microstructure formation during annealing of a 75% cold rolled AHS steel, reheated at a rate of 10°C/s to ∼3000°C/s were studied with the purpose to investigate the interaction between the recrystallization and the austenite formation as well as the possibility of grain refinement. It was found that at ultra-high heating rates, the bcc-fcc phase transformation starts before the completion of recrystallization and the deformation texture is partially retained in the samples after annealing. The crystallographic texture observed in the martensitic phase, which is the product of intercritical austenite transformation in as-quenched samples, i.e., after the double α-γ-α’ transformation, is similar to the initial cold rolling texture, which is an indication for the texture memory effect. After ultra-fast reheating with average reheating rates of 1000°C/s and higher, a significant grain refinement was observed with an average calculated ferrite grain diameter of ∼2 μm. The obtained final ferrite grain size depends significantly on both the reheating temperature and the heating rate. The same effect is observed in 75% cold rolled quenching and partitioning steels for heating rates of 400°C/s, 800°C/s and 1000°C/s. In both cases the combination of fine grains, and weak RD/ND-fiber textures produce an increase of strength and ductility compared to material annealed using a conventional heating rate of ∼10°C/s.

Effect of ECAP process on structure and hardness of AlMg3 aluminium alloy

Purpose: In the present study, the effect of ECAP die and number of ECAP pressings on the structure evolution and hardness of AlMg3 aluminium alloy was investigated. Design/methodology/approach: Commercial AlMg3 aluminium alloy in the as-cast condition was processed by Equal Channel Angular Pressing method through route A using two different ECAP dies – conventional and modified with additional twist angle. Samples were processed at ambient temperature up to four passes. The investigation was carried out at ambient temperature. Two different ECAP dies were used to investigate the effect of design modification on the possibility of grain refinement to sub-micrometer size. Findings: The experimental results showed that the modification of ECAP die provides to additional grain refinement and introduces a greater amount of plastic strain into the material which results in greater increase in the properties of the investigated material. Research limitations/implications: The presented investigation results were carried out on samples, not on final products. Practical implications: Current research is moving towards to develop high strength materials with increased mechanical properties and refined microstructure that are known as ultra-fine-grained materials, compared to well-known with coarse-grained microstructure. Originality/value: The paper focuses on the investigation of microstructure evolution using mainly polarised light microscopy that reveals shear, micro-shear and slip bands that refine the microstructure of aluminium alloys. In addition, to evaluate the grain size of as ECAPed specimen, EBSD investigation was carried out.

The Effect of Low Deformation Asymmetric Rolling on Microstructure and Texture of the Polycrystalline Copper

Abstract In the most recent years the asymmetric rolling (AR) attracts attention of researchers and technologists. This process can improve some technological parameters (e.g. modification of rolling torque and load, power requirements, etc.) as well as provide the possibility of grain refinement in a relatively inexpensive way. Most of the reports concerning microstructural changes produced by AR refer to high deformations imposed in highly asymmetric conditions. However, such rolling conditions are difficult to control, so there are no prospects to their quick industrial implementation. The present paper refers to relatively low deformation and low asymmetry rate, that is much more interesting for the industry. It was shown that bending of the rolled band (important disadvantage of the AR technology) can be controlled by adjusting of the amount of deformation and asymmetry. It was also shown that ca. 30% reduction in thickness during cold rolling, together with a relatively low asymmetry, reduces significantly the grain size and produces a more fragmented microstructure inside grains of the polycrystalline copper comparing to the symmetric rolling (SR). The material hardness after AR is higher than after the SR. Moreover, the crystallographic texture asymmetry, expressed by its rotation around the transverse direction, is observed in the AR material.

Accumulative Roll Bonding and Post-Deformation Annealing of Cu-Al-Mn Shape Memory Alloy

Accumulative roll bonding is a severe plastic deformation process used for Cu-Al-Mn shape memory alloy. The main purpose of this study is to investigate the possibility of grain refinement of Cu-9.5Al-8.2Mn (in wt.%) shape memory alloy using accumulative roll bonding and post-deformation annealing. The alloy was successfully subjected to 5 passes of accumulative roll bonding at 600 °C. The microstructure, properties as well as post-deformation annealing of this alloy were investigated by optical microscopy, scanning electron microscopy, x-ray diffraction, differential scanning calorimeter, and bend and tensile testing. The results showed that after 5 passes of ARB at 600 °C, specimens possessed α + β microstructure with the refined grains, but martensite phases and consequently shape memory effect completely disappeared. Post-deformation annealing was carried out at 700 °C, and the martensite phase with the smallest grain size (less than 40 μm) was obtained after 150 s of annealing at 700 °C. It was found that after 5 passes of ARB and post-deformation annealing, the stability of SME during thermal cycling improved. Also, tensile properties of alloys significantly improved after post-deformation annealing.

Texture Formation in High Strength Low Alloy Steel Reheated with Ultrafast Heating Rates

Texture formation during annealing of a 95% cold rolled HSLA steel with 10°C/s and ~3000°C/s was studied with the purpose to investigate the interaction between the recrystallization and the austenite formation as well as the possibility of grain refinement. The recrystallization and austenite formation were monitored by means of optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). It was found that at extreme reheating rates of 1000°C/s and higher, the α-γ phase transformation starts before the completion of recrystallization and the deformation texture is partially retained in the samples after annealing. The crystallographic texture observed in the martensitic phase which is the product of intercritical austenite transformation in as-quenched samples, i.e. after the double α-γ-α’ transformation, is the same as the initial cold rolling texture, which is an indication for the texture memory effect. After ultra fast reheating with average reheating rates of 1000°C/s and higher a significant grain refinement was observed with an average ferrite grain size of ~1µm. The obtained final ferrite grain size depends significantly on both the reheating temperature and the reheating rate.

Nano grained AZ31 alloy achieved by equal channel angular rolling process

Equal channel angular rolling (ECAR) is a severe plastic deformation process which is carried out on large, thin sheets. The grain size could be significantly decreased by this process. The main purpose of this study is to investigate the possibility of grain refinement of AZ31 magnesium alloy sheet by this process to nanometer. The effect of the number of ECAR passes on texture evolution of AZ31 magnesium alloy was investigated. ECAR temperature was controlled to maximize the grain refinement efficiency along with preventing cracking. The initial microstructure of as-received AZ31 sheet showed an average grain size of about 21 μm. The amount of grain refinement increased with increasing the pass number. After 10 passes of the process, significant grain refinement occurred and the field emission scanning electron microscopic (FESEM) micrographs showed that the size of grains were decreased significantly to about 14–70 nm. These grains were formed at the grain boundaries and inside some of the previous larger micrometer grains. Observation of optical microstructures and X-ray diffraction patterns (XRD) showed the formation of twins after ECAR process. Micro-hardness of material was studied at room temperature. There was a continuous enhancement of hardness by increasing the pass number of ECAR process. At the 8th pass, hardness values increased by 53%. At final passes hardness reduced slightly, which was attributed to saturation of strain in high number of passes.

Fabrication of high strength nanostructured aluminium alloys by hydrostatic extrusion

Abstract In the present work, the possibility of obtaining high strength nanostructured aluminium alloys by hydrostatic extrusion has been shown. Billets of 2017 aluminium alloy were subjected to hydrostatic extrusion either immediately after water quenching or after subsequent ageing. The results have shown that processing by hydrostatic extrusion offers a possibility of grain refinement down to the nanometre scale in the age-hardenable aluminium alloy. However, the effectiveness of the process depends on its initial microstructure. The most promising results are obtained if the alloy is deformed in the as-quenched condition. The hydrostatically extruded aluminium alloy exhibits very high tensile strength with reasonable ductility. The stability of mechanical properties over a wide range of temperatures (from cryogenic to well above the ambient) is discussed. The results obtained are compared to similar 2XXX aluminium alloys processed by other severe plastic deformation methods.

アルミニウムの結晶粒微細化に及ぼす圧縮ねじり複合負荷の効果

A compressive torsion process that is one of the severe plastic deformation processes was applied to a commercial pure aluminum in order to obtain fine grain. In the compressive torsion processing the material was subjected to simultaneous compressive and torsional loading without change in its shape. The possibility of grain refinement and the effects of process conditions on the refinement were investigated by changing deformation temperature, compressive pressure, torsional cycle and torsional speed. Very fine grains around several microns were obtained by the compressive torsion process. However the grain refinement was not uniform in the specimen, the fine-grained region was concentrated in peripheral parts of the operating die. Decreasing processing temperature and increasing torsional cycle are effective to decrease grain size and extend finer grain area. The compressive pressure does not influence grain size and finer grain area. Increasing torsional speed enlarges grain size because of rise in temperature due to deformation heat generated in processing.

Grain refinement by utilizing grain-boundary reaction in heat-resistant alloys

The possibility of grain refinement by thermal cycling is examined on commercial heatresistant alloys, namely, an austenitic 21Cr-4Ni-9Mn steel and a cobalt-base HS-21 alloy, in which the grain-boundary reaction occurs. A thermal cycle is composed of a high-temperature ageing which causes the grain-boundary reaction, and a subsequent short-term heating at the resolution temperature, which ensures complete dissolution of the grain-boundary reaction precipitates into the matrix. The grain diameter is finally reduced to about one-half or one-third of the original grain size after four thermal cycles, while a larger grain-size reduction is observed in the specimens with initially larger grain size. The effects of the amount of the grain-boundary reaction and the heat-treatment conditions on grain refinement are also experimentally discussed.

Grain Refinement by Thermal Cycling in High-nitrogen Austenitic Heat-resistant Steels

A possibility of grain refinement utilizing the grainboundary reaction (GBR) by thermal cycling was investigated on high-nitrogen austenitic 21Cr-4Ni-9Mn steels. A thermal cycle employed in this study was composed of the ageing at 973 K or 1173 K, which caused about 50% of GBR in area fraction, and the short-time resolution heating (180 or 600 s) at 1473 K. The pearlitic nodules containing Cr 2 N nitride were formed by GBR during ageing in these steels. The number of the GBR nodules formed during ageing at 973 K was larger than that of the nodules formed at 1173 K when the amount of GBR was about 50%, and this led to a little smaller grain size produced by thermal cycling which involved the ageing at 973 K

Possibility of grain refinement for superplasticity of a Mg-AI-Zn alloy by pre-deformation

For a commercial Mg-Al-Zn alloy sheet, tensile tests are carried out under various strains, strain rates and temperatures to investigate the possibility of grain refinement by dynamic recrystallization during pre-deformation. It is found from the microstructural observations that relatively fine grains of 10 μm or so, in diameter are attained under the conditions of 250°C and 8.3 × 10−4 s−1. The specimen pre-strained under this condition also exhibits a fairly good superplasticity of total elongation beyond 300%.