Yuji Kume

Microstructure refinement of Al-Si alloy using compressive torsion processing

A Compressive Torsion Processing (CTP) is a unique severe plastic deformation process which can easily apply very large strain without shape change to a work piece. Hypereutectic Al-Si alloys have good properties such as low thermal expansion and high wear resistance. It is important for the alloys to control the size of second phase particles (primary and eutectic silicon, intermetallic compounds) as well as grain size of aluminum matrix. In the present work, the CTP was applied to hypereutectic Al-Si alloy (AA390) to investigate the possibility of microstructure refinement of the alloy and the mechanical property of processed alloy was also investigated by tensile test.

Homogeneity of Grain Refinement of Aluminum Alloy with Compressive Torsion Processing

Compressive torsion combined loading that uses relatively low compressive pressure has a great advantage of microstructure refinement of cylindrical metal blocks without changing their shape while processing. In the present work, effects of processing temperature and rotation times on homogeneity of the refined microstructure were investigated for Al-5%Mg alloy. Although lower processing temperature was effective to obtain fine grains, it was difficult to obtain homogeneous refinement at lower temperature. Higher processing temperature was favorable to obtain homogeneous microstructure, for instance, in the cylindrical specimen of φ25×10 mm the homogeneous refinement could be obtained at higher temperatures than 373K. Increasing rotation times was also effective to obtain homogeneous refined microstructure for thicker specimens.

Influence of Compressive Torsion Processing Temperature on Microstructure Refinement and Property of Aluminum Alloy

A compressive torsion processing (CTP) was applied to hypereutectic Al-Si alloy in order to raise ductility and formability by microstructure refinement of the alloy. The CTP is a unique severe plastic deformation process and it can easily apply large strain to a work piece without change in shape. In the present work, influence of compressive torsion processing temperature on microstructure refinement and tensile property of hypereutectic Al-Si alloy is dealt with. When the CTP was applied on the Al-Si alloy, primary and eutectic Si particles were refined more effectively at lower processing temperature. Total tensile elongation of CTPed alloy was four times as large as that of non CTPed one. Distribution of the total elongation was quite uniform in the whole CTPed specimen.

Upgrading Property of A-Fe Alloys through the Microstructure Refinement by Compressive Torsion Processing

Cast AlFe alloys containing several percent iron have low ductility because of their brittle precipitates. Therefore, precipitate refinement is very important for improving their mechanical properties. In recent decades, severe plastic deformation processes have been developed to achieve this grain refinement. For example, our previously proposed severe plastic deformation process, called compressive torsion, is quite effective for not only grain refinement but also precipitate refinement even in brittle materials. In the present work, precipitate refinement of cast Al—Fe alloys by compressive torsion and the resulting improvements in their tensile properties were investigated. Compressive torsion with various numbers of revolutions was applied to Al—Fe alloys at 373 K. Then, the alloys were subjected to tensile testing at room temperature, 473 K, and 573 K. The obtained experimental results indicated that the initial eutectic microstructure of the alloys disappeared after the compressive torsion processing. All large precipitates with sizes of more than 200 μm were refined, and their sizes were reduced to several tens of micrometers. Furthermore, these refined precipitates were dispersed homogenously in the alloy microstructure. In result, the tensile properties of the alloys, namely, their strength and elongation, were improved remarkably. In particular, the elongation reached more than 30% at room temperature.

Effect of Number of Revolutions on Shear Strain Distribution in Specimen Processed by Compressive Torsion Processing

Compressive torsion processing (CTP) can cause huge shear strain in a cylindrical workpiece by simultaneous compressive and torsional loading without changing the cylindrical shape. In the present work, the distribution of shear strain in a specimen subjected to CTP was measured in a model experiment using two kinds of aluminum alloys, and the effect of the number of revolutions on the strain distribution was investigated. Internal shear strain of the worked specimen can be quantified by measuring the displacement of the interface between the two kinds of alloys in the cross section. The shear strain in the worked specimen has a gradient distribution not only in the radial direction owing to the geometric feature, but also in the axial direction because of the frictional constraint of the container. However, the measured shear strain was almost in agreement with value predicted by geometric calculation. The shear strain distributions were proportional to the radius and number of revolutions in the part of less than 15 mm in radius of a cylindrical specimen 41 mm in diameter and 10 mm in height.

Visualization and Quantification of Severe Internal Deformation on Compressive Torsion Process

Compressive torsion process (CTP) which was developed by authors is effective process for grain and precipitates refinement of metallic materials with a severe plastic deformation. In the CTP, a cylindrical specimen is subjected to simultaneous compressive and torsional loading without change in its shape. However, metal flow and strain distribution in the processed specimen are not cleared, because the deformation is very large and complicated. In the present work, visualization of internal deformation of specimen processed by CTP was investigated using dual alloy etching technique. Two kinds of aluminum alloy were prepared by cutting on fan-like shape and alternately placed to a cylindrical shape. After CTPing, contrasts in the specimen were observed by polishing and etching. The internal distribution of shear strain was quantified by measuring the displacement of interface between the alloys. As a result, the visualization and quantification of internal deformation was successfully carried out using the technique. The internal strain distribution was varied not only in radial direction but also in longitudinal direction because of frictional constraint on the lateral face. A laminate contrast of the alloys observed on the vertical cross section was well related with the strain distribution in the specimen.