| Summary: | 碩士 === 國立高雄第一科技大學 === 機械與自動化工程研究所 === 100 === Inconel 718, a nickel-based alloy, is used extensively in the manufacture of aircraft engines. A sound welding process for Inconel 718 is a critical capability for all engine manufacturers in the aerospace industry. Welding parameters and the heat treatment state of the material have a significant effect on the quality and cost of the welding process. The Inconel 718 material used in this study was either solution heat-treated or solution heat-treated and then precipitation-hardened. The purpose of this study was to evaluate the effect of pre-weld heat-treat and key Electron-Beam (EBW) weld process parameters on visual characteristics and mechanical properties of EB-welded Inconel 718 groove welds.
First, Inconel 718 sheets 3.175mm (1/8 in) thick were cut to suitable size coupons and heat treated in a vacuum furnace to either solution or precipitation-hardened heat treat states. A Taguchi Design of Experiments was conducted using the L18 Orthogonal Array in order to identify significant factors from variables that included: pre-weld heat treat state, electron beam current, travel speed, beam focus, beam deflection in X and Y axes, and deflection frequency. The heat treated coupons were then EB-welded, and samples for metallographic and mechanical testing were cut from each weld specimen in order to evaluate the effect of each variable on visual characteristics and mechanical properties of the welds.
Next, four visual weld characteristics – the depth of underfill, the height of drop-thru, the minimum weld width, and the difference between face and root weld widths - were optimized through Principal Component Analysis (PCA), and a subsequent test run confirmed these results.
Then, three weld mechanical properties – tensile strength, microhardness, and bending strength – were optimized through Principal Component Analysis and through PCA coupled with Fuzzy Theory. These results were also replicated with a confirmation run.
It can be concluded from these experiments that: (1) pre-weld heat treat state – whether solution or solution followed by precipitation-hardening, had no significant effect on visual weld characteristics or on hardness; (2) welded specimens that had been solution heat-treated exhibited straining at parent material during mechanical testing, whereas those that were solution and then precipitation-hardened exhibited no strain before sudden fracture at the weld; (3) welds on precipitation-hardened specimens exhibited higher tensile and bending strength.
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