原田 泰典

ばね鋼にマイクロショットピーニングを施し，硬さや表面粗さなどの表面特性について調べたのち，疲労限度や破面観察などの疲労特性に及ぼす投射条件の影響について調べた．ショットピーニングは空気式機械を用い，投射材は鋳鋼製および超硬合金製の微粒子を用いた．ショットピーニングを施した試験片に対して，硬さ試験や疲労試験などを行った．マイクロショットピーニングはばね鋼の表面粗さや表面硬さなどの表面特性および疲労強度の改善に有効であり，とくに超硬合金製投射材によるピーニング効果が高いことを明らかにした．

This paper deals with the effect of shot peening on the bending strength of an AZ31 magnesium alloy pipe. Magnesium alloy has a wide range of application prospects in the automobile and electronic industries. AZ31 alloy is the most widely used commercial magnesium alloy. In our experiment, extruded pipes of 22 mm O.D., 18 mm I.D., and 2 mm wall thickness were used. Shot peening treatments were applied to the surface of the workpiece using an impeller-type or an air-type peening machine. Tensile and compressive tests were carried out under axial load at a crosshead speed of 10 mm/min, and bending strength tests were performed at a crosshead speed of 100 mm/min under lateral load using an Instron-type testing machine. In the bending tests, the peened workpiece could withstand higher bending yield load. The surface layer of the peened workpieces was also observed by electron backscatter diffractometry (EBSD). It was found that multiple deformation twins were formed during shot peening. The results of the present study revealed that the compressive yield stress of shot-peened pipes is strongly related to their bending strength.

The effect of functionally graded TiN coating on the wear and fretting fatigue behavior of titanium alloy was investigated. The two kinds of monolayer TiN coatings and three kinds of functionally graded TiN coatings were deposited on Ti-6Al-4V alloy substrate using an ion beam assisted deposition technique. Monolayer TiN coating was fabricated by an electron beam evaporation of titanium and simultaneous nitrogen ion bombardment at the acceleration voltage of 0.2 or 2.0 keV. The later consisted of top TiN layer and functionally graded layer. During the fabrication of functionally graded layer, the mixing ratio of nitrogen and argon gas that were used for the generation of ion beam increased stepwisely. The mixed layer formed by ion beam bombardment became thicker by an increase in acceleration voltage. The adhesion and wear resistance improved with an increase in the acceleration voltage. Moreover, inserting the functionally graded layer with the layer thickness of 250 nm improved them significantly. However, further increase in graded layer thickness deteriorated the adhesion and wear resistance. The fretting fatigue life was evaluated by a four-point bending fatigues test. The fretting fatigue life of specimen coated with monolayer TiN film was longer than that of uncoated specimen. The TiN coating with functionally graded layer was the most effective to improve the fretting fatigue life. It was concluded that the insert of functionally graded layer with appropriate layer thickness is effective way for improvement of wear and fretting fatigue behavior.

近年，金型などの大きな負荷のかかる工業製品に対する長寿命化の要求が高まっている．低い製作コストで長寿命化を図るためには必要部分にのみ高強度な合金被膜を形成する方法が有効と考えられ，本研究ではレーザ熱処理を用いて炭素鋼表面に局所的なFe-Al系合金被膜を作成する手法を提案する．ショットライニングにより炭層鋼に貼り付けたアルミ箔にデフォーカスしたCO<sub>2</sub>レーザ照射することで境界面での鉄とアルミの相互拡散を促進しFe-Al系合金を成長させる．生成されるFe-Al系合金の組成は熱処理温度に依存するため，いろいろな熱処理条件でレーザを照射した試験片の断面を観察した結果，高速度レーザ走査によって表面を加熱することで母材の性質を変えずに局所的にFe-Al系合金被膜を生成できることを示した．

Shot peening is widely utilized to improve the fatigue property of mechanical parts for transportation equipment such as cars and airplanes. Also, this technology is being applied as a film-forming technology in order to improve surface quality. The authors have recently proposed new joining methods using shot peening, shot lining. In this method, the metals are bonded with the dissimilar metal by applying plastic deformation and the pressure. The thin foil can be joined to the substrate surface by the pressure generated by the hit of the shots. In this study, the formation of an Fe-Al intermetallic compound film on high-speed tool steel by shot lining and heat treatment was investigated. In the experiment, a centrifugal-type peening machine with an electrical heater was employed. The shot medium was high-carbon cast steel. The substrate was a commercial high-speed tool steel JIS-SKH51, and the foil was commercially available pure aluminium. The shot lining process of tool steel with an aluminium foil was carried out at 573K in air using a peening machine. Heat treatment was performed at diffusion temperatures from 923 to 1573K in vacuum. The lined substrates exhibited a harder layer of aluminium-rich intermetallics in the diffusion temperature range of 923 to 1173K. When the temperature of the lined substrates was more than 1273K, the surface was covered with thicker and highly anticorrosive layers of iron-rich intermetallics. We found that the present method could be used for the formation of functional films on high-speed tool steel. ? (2014) Trans Tech Publications, Switzerland.

The Mechanical property of free-standing Cu/Al_2O_3 multilayer thin film was investigated by a method of micro-bending test. The Cu and Al_2O_3 monolayer thin film and Cu/Al_2O_3 multilayer thin film were deposited on Si substrate by an electron beam deposition. Free-standing micro-beam specimens were fabricated using a focused ion beam system. The micro-bending tests were conducted using a nanoindenter. Young's moduli of free-standing thin films were estimated by FE analysis including indentation displacement. Young's moduli of Cu and Al_2O_3 films were relatively close to those of bulk material, and that of Cu/Al_2O_3 film with 20 layers was between Cu and Al_2O_3 films. On the other hand, the fracture strength of Cu/Al_2O_3 film with 20 layers was larger than that of Al_2O_3 film.

The Mg-Li alloy which is a beta type alloy was subjected to cold deep drawing, and the formability of Mg-Li alloy sheet was investigated by the square cup deep drawing process. This alloy is about 20 % lighter than a practical alloy. The square cups of magnesium alloy were formed at ambient temperatures. The material was the beta type alloy, Mg-14%Li-1%Al. The initial thickness of the blank was 0.5 to 1 mm in thickness. In the deep drawing, the Mg-Li alloy sheets were employed and a flat sheet blank is formed into a square by a punch. Also, the formability of the Mg-Li alloy was examined by multi-stage deep drawing. Various cups were drawn by exchanging the punch and die. The die was flat in the first stage, and was taper without a blankholder in the subsequent stages. It was confirmed that the Mg-Li alloy square cups were successfully formed by cold deep drawing.

This paper deals with effect of shot peening on bending strength of cylindrical pipe of AZ31(Mg-3%Al-1%Zn) magnesium alloy. Specimen shape of the pipe is 240mm in gauge length and 330mm in total length, 22mm and 18mm in outer and inner diameters. Shot peening treatments were given on the surface of the pipe using an impeller-type peening machine and/or an air-type peening machine. Tensile and compressive tests were carried out under axial load at a crosshead speed of 10mm/min. Bending strength test was also performed at a crosshead speed of 100mm/min under lateral load using an Instron-type testing machine. From the experimental result, the bending yield load of the pipe was strengthened after shot peening treatment. It was found that the compressive yield stresses of the shot-peened pipe were strongly related to their bending strengths.

The formation of an Fe-Al intermetallic compound film on hot work tool steel by shot lining and heat treatment was investigated. In the shot lining method, the metals are bonded with the dissimilar metal by applying plastic deformation and the pressure. The effect of heating temperature on intermetallic formation of aluminum and base material bonded by the lining method was studied. In the experiment, a centrifugal-type peening machine with an electrical heater was employed. The shot lining process of hot work tool steel with an aluminum sheet was carried out. Heat treatment was performed at diffusion temperatures from 973 to 1373K in vacuum. When the temperature of the lined substrates was more than 1173K, the surface was covered with highly anticorrosive layers of Fe-rich intermetallics. It was confirmed that the present method could be used for the formation of functional films on hot work tool steel.