Papers by Author: J.R. Cho

Abstract: Considering the exhaust valve spindle consists of a valve head and a stem comprising the body, the use of different materials, that is, Nimonic 80A for the head and SNCrW for the stem, can reduce the manufacturing cost dramatically. The inertia welding was conducted to make the large exhaust valve for low speed marine diesel engines, superalloy Nimonic 80A for valve head of 540mm diameter and high alloy SNCrW for valve stem of 115mm diameter. Due to different properties of material like thermal conductivity and flow stress on the two sides of the weld interface, modeling is crucial in determining the optimal weld geometry and parameters. FE simulation was performed by the commercial code DEFORM-2D. A Good agreement between the predicted and actual welded shape was observed. It was expected that simulation will significantly reduce the number of experimental trials needed to determine the weld parameters, especially for welds of very expensive materials or large shaft. A variety of tests, including microstructure observation, tensile, hardness and fatigue test, are conducted to evaluate the quality of welded joints.

Abstract: The manufacturing process of the exhaust valve in large marine diesel engines consists of an upset forging and final forming process. In the past, the exhaust valves in large marine diesel engines have been made through free forging by using the stretch forming method. This method has used the ingot and the billet as a preliminary forming process. Nimonic 80A, a superalloy, is presently used for the material of the exhaust valve. For the forming method of the valve, the electric upset method is used. Solid bar is raised up to the forming temperature by using electric energy and is continually deformed by upset pressure. The electric upsetting processing is a useful method for the high quality of exhaust valves in large marine diesel engines. It can keep the continuous grain flow, excellent mechanical property, and corrosion resistance because of the elaborate macrostructure of the valve face. The purpose of this paper is to predict the optimum process condition through simulation of the exhaust valve with a diameter of 73mm. The experimental result of an exhaust valve with a diameter of 19mm is in good agreement with the simulation result using the “QForm” that can solve electric upsetting problems. Finally, the optimal manufacturing process of the electric current and the upset load are surveyed.

Abstract: The numerical prediction of the effective mechanical properties of the reinforced braid inserted in automobile power steering hose is addressed. The key role of the reinforced braid layer is to suppress the excessive radial expansion of the hose subject to high pressure and temperature. The reinforced braid layer is in the structure composed of wrap and fill tows inclined to each with the specific helix angle. In order to predict the effective mechanical properties, we construct a 3-D finite element model of the unit cell (or RVE) of the reinforced braid in a periodic pattern, in which the detailed geometry of individual fiber tows is fully modeled. By making use of the superposition method and the 3-D finite element analysis, the effective mechanical properties are predicted. Numerical experiments illustrating the theoretical work are also presented.