Alloy 7 18 is a widely used superalloy which shows excellent performance at moderate temperatures. This desirable performance originates from the composition and processing conditions. Many manufacturers choose to investment cast Inconel 7 18 for their components because of investment casting's ability to produce complex parts at reasonable costs. However, improper casting conditions can cause deleterious defects to occur such as porosity. The prediction of the porosity distribution in shaped castings is a popular research topic at the present time. A number of semi-empirical criteria functions have been proposed to assist in such predictions, the functions being generally obtained from computer modeling of casting solidification. Although considerable attention has been given to the use of porosity criteria functions with aluminum based alloys, only limited consideration has been given to superalloy castings. Four sets of plate castings were investment cast under varying foundry conditions and the porosity distributions metallographically characterized. The castings were top poured and contained vertical 50.8 mm by 50.8 mm plates of the following thicknesses: 2.54, 12.7, and 25.4 mm. A computer solidification model was developed for the castings and utilized to examine the effectiveness of various porosity criteria functions for predicting porosity. The computer model was shown to be effective in predicting unfed centerline shrinkage in the 25.4 mm thick plates. In addition, the predicted porosity trends of two widely used porosity criteria functions were shown to be consistent with the measured amounts of porosity in three of the four 12.7 mm thick plates. However, significant process variations, e.g. inclusion content, can dominate the development of porosity in castings and make the prediction of porosity an inexact science. These effects and the effective application of porosity criteria functions in predicting process parameters for alloy 7 18 casting are also discussed.
The Friction Stir Welding(FSW) has mainly been used for making butt joints in Al alloys. Development of Friction Stir Lap Welding(FSLW) would expand the number of applications. Microstructure of FSLW in A5052-H112 alloy was investigated under varying rotation and welding speed.As the rotation speed was increased and the welding speed was decreased, a amount of heat was increased. As a result, bead interval was narrower, bead width are larger, and experimental bead interval was almost similar to theoretical bead interval.Typical microstructures of FSLW A5052-H112 alloy consist of three zones, including Stir Zone(SZ), Thermo-Mechanically Affected Zone(TMAZ) and Heat Affected Zone(HAZ). As a amount of heat was increased, average grain size was larger in three zones. Nevertheless, the aspect ratio was almost fixed for FSLW conditions.The misorientation of SZ, HAZ and TMAZ was examined. A large number of low angle grain boundaries, which were formed by severe plastic deformation, were showed in TMAZ as comparison with SZ and HAZ.Microhardness distribution was high in order of BM, SZ, TMAZ, and HAZ. The Micro-hardness distribution in HAZ, TMAZ of upper plate were lager than lower plate.Relationship between average grain size and microhardness was almost corresponded to Hall-Petch equation.
The as-cast alloy 718 shows a multiphasic structure which consists of dendrites, carbides and Laves phases. The amount, size, shape, and dispersion of these phases affect the properties and the processing which follows, This structure was characterized by measurements of volume fraction, size, dispersion, and shape factor of carbide and Laves phases. Secondary arm spacing and Vickers hardness were also measured i.n the as-cast condition. All the above measurements were carried out on two different sized castings which had different cooling conditions, Compositional analysis of the phases in the castings concerned are clearly linked to solidification behavior ofthe alloy. These values were statistically analyzed to distinguish the effects of the casting conditions.
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