原田 泰典

Long cups of magnesium alloy were formed at ambient temperatures by multi-stage deep drawing processes. In general, typical uses of magnesium alloys are for automobile and aircraft components, portable device, luggage, and care goods. However, magnesium alloys are difficult to plastically deform at room temperature. This is due to the hexagonal close-packed structure restricts slip to the basal planes. On the other hand, Mg-Li alloys, which have the BCC crystal structure, can be formed at room temperature. In multi-stage deep drawing, the Mg-Li alloy sheets were employed and a flat sheet blank is formed into a cylindrical by a punch. Various cups were drawn by exchanging the punch and ringed die. The die was flat in the first stage, and was taper without a blankholder in the subsequent stages. The effects of the ratio of blank diameter to punch diameter and blank thickness on the deep drawability were examined. It was confirmed that the Mg-Li alloy long-cups were successfully formed by a multi-stage deep drawing operation in cold.

Shot peening is a surface treatment that improves the performance of engineering components. In conventional shot peening, the medium consists of small spheres, which are usually made of high-carbon cast steel; the diameter of the spheres is in the range from 0.3 to 1.2mm. More recently, however, a new type of microshot has been developed to enhance the peening effect. The diameter of the spheres in the new medium is in the range from 0.03 to 0.15mm. In the present study, the effect of microshot peening on the surface characteristics and fatigue strength of spring steel was investigated. The injection method of the microshot was of the compressed air type. The microshots of 0.1mm diameter were high-carbon cast steel and cemented carbide, and the workpiece used was the commercially spring steel JIS-SUP10. The surface roughness, hardness, compressive residual stress, and fatigue strength of the peened workpieces were measured. The surface layer of the workpieces was sufficiently deformed by microshot peening. A high hardness or residual stress was observed near the surface. The shot peening process improved the fatigue life in the high cycle range. The use of hard microshots such as cemented carbide was found to cause a significantly enhanced peening effect for spring steel.

The workability of cast pure chromium was investigated through the study of rolling and isothermal upsetting. The present cast block produced by means of a high energy induction slag melting (HESM) process was 99.99 % mass purity and this material had large crystal grains elongated along the solidification direction. The ductile-brittle transition temperature (DBTT) of small specimens taken from the block was found to be about 500 K in a tension strained at 0.024 s(-1). Considerable workability appeared in upsetting at 400 K, at which the small workpiece could be deformed to 25 % in height reduction without cracking on the piece wall. The upsetting workability increased rapidly with increasing temperature, that is, the piece was deformed up to 80 % at 773 K and more than 90 % at higher temperatures than 873 K. At all temperature, cracks occurred in the as-east specimen deformed by a conventional rolling. In addition, cracks occurred slightly in the specimen even in a sandwich rolling by which sintered chromium was successfully deformed. Cracking in the as-cast specimen was mainly due to the coarse structure with large crystal gains. Once the coarse gram structure in the cast material was changed to a finer one by upsetting and successive annealing treatment, the present chromium could be successfully rolled to a thin sheet at temperatures above 673 K.

To improve the surface properties of magnesium alloy, a lining process using shot peening was carried out. The lining of magnesium alloy with metals such as titanium and nickel is useful in heightening the corrosion and wear properties for the components. In the shot peening experiment, the foil set on the magnesium workpiece is pelted with many shots at a high velocity. The foil is bonded to the surface of the workpiece due to plastic deformation induced by the collision of the shots. A pure aluminum foil is inserted between the hard foil and magnesium workpiece to assist the bonding. To heighten the bondability, the foil and workpiece are heated and the flow stresses of the metals are reduced. In this experiment, a centrifugal shot peening machine with an electrical heater was employed. The workpieces were commercially magnesium alloys AZ31B and AZ91D, and the foils were commercially pure aluminum, pure titanium and pure nickel. The effects of collision speed and the processing temperature on the bondability were examined. It was found that the foils were successfully bonded to the surface of workpieces by the shot peening process.

For net shape forming using pressure welding without machining after joining, shapes of workpieces were optimized by simulations of metal flow using a finite element method. Round bars having taper cut and concave arc cut shapes were dealt with for the simulation of pressure welding. It was found from the simulations that arc cut shape makes an initial joining surface expand into a whole joining area and that the joining surface expansion is controlled by means of a diameter of the initial joining surface. Aluminum round bars provided with a concave arc cut edge were experimentally pressure welded at room temperature and it was found that a joint formed at the joining surface expansion more than 5 times has a bond strength more than a tensile strength of the base metal. It was concluded that the concave arc cut shape is a suitable edge shape for the pressure welding.

Experiments were performed with high purity chromium to make a search for some method for improving the ductility at room temperature. Single crystal and bicrystal, both prepared from one-directionally solidified ingot, were found to be ductile by the tensile test at a relatively low strain rate. Also, it was shown that there was a possibility of improving the ductility of polycrystalline chromium by controlling the grain boundary structures in a proper manner, for example, by increasing a population of the low energy boundaries such as the Sigma3 coincidence boundary in the specimen using thermomechanical treatments.

A lining process using shot peening was carried out to apply the partial lining. The position of the collision of shots on the surface of the workpiece was limited by a masking plate. The foil was slightly larger than the masking plate, and was fixed for the collision by the plate. A centrifugal type shot peening machine having an electrical heater was employed. The workpieces were aluminum alloys and magnesium alloys, and the foils were commercially pure nickel, commercially pure titanium and stainless steel SUS304. To assist the bonding, a pure aluminum foil was inserted between the foil and workpiece. The accuracy of the lined shapes for the masking shapes was investigated. The partial lining of circles and triangles on the surface of the workpiece succeeded. The bonding of the lined workpiece was confirmed to be sufficient from a bending test of lined workpieces.