이종재료 마찰압접을 이용한 초내열합금 대형 배기밸브 스핀들 개발

Abstract

Inertia welding is a solid-state welding process in which butt welds in materials are made in bar and in ring form at the joint face, and energy required for welding is obtained from a rotating flywheel. The stored energy is converted to frictional heat at the interface under axial load. The quality of the welded joint depends on many parameters, including axial force, initial revolution speed and energy, amount of upset, working time, and residual stresses in the joint.

Inertia welding was conducted to make the large exhaust valve for low speed marine diesel engine, superalloy Nimonic 80A for valve head of 540mm (21.2") and high alloy SNCrW for valve stem of 115mm (4.53"). Due to different material characteristics, such as, 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 is observed. It is expected that modeling will significantly reduce the number of experimental trials needed to determine the weld parameters, especially for welds of very expensive materials or large shaft.

Many kinds of tests, including macro and microstructure observation, chemical composition, tensile, impact, hardness and fatigue test, are conducted to evaluate the quality of welded joints.

A variety of tests, including macro and microstructure observation, chemical composition, tensile, impact, hardness and fatigue test, are conducted to evaluate the quality of welded joints. Microstructure and chemical compositions for friction welded joints were carried out using an optical microscope and SEM. The recrystallized and transformed zone were observed from the center line to 3.2 mm of depth in the Nimonic 80A and to 3.5 mm in the SNCrW. The mixed layers of two base materials were observed in the range of 400μm for the welded joint. It has better tensile and yield strength compared to SNCrW and better elongation and reduction of area than that of Nimonic 80A

The hardness of the welded joint was measured using the micro Vickers hardness test at room temperature. The fatigue test was carried out with the rotary bending test for base metal and friction welding joints. The fatigue strength for welding joint is 341MPa at 1E+7 cycles. It is higher than that of the SNCrW in 260MPa.

Based on the results of the tests, it can be concluded that the inertia welding joints of the superalloy exhaust valve spindle has better properties than the material specification of SNCrW.