Homogenization heat treatment of cast superalloy 718Plus - An investigation of the microstructural evolution upon heat treatment

Examensarbete för masterexamen
Master Thesis
Larsson, Josefin
Superalloys are commonly used in jet engines due to their high temperature stability and good endurance. The components found in the engines are of complex shape and therefore casting is utilized. One of the problems associated with casting of highly alloyed metals, such as superalloys, is that partitioning of elements tends to take place upon solidification. This might lead to formation of low temperature melting eutectics which further will have a negative effect on the weldability. Weldability is a major concern, and welding is in general needed for all casted components either in order to join structural parts or to repair cracks arising from the casting process. The lowered weldability is one of the reasons why secondary phases are not wanted in the material. In order to get rid of such hazardous phases, the material may be subjected to a heat treatment with the aim to homogenize the microstructure and thereby enhance the weldability, i.e. homogenization heat treatment. The aim of the work was to evaluate the homogenization response in cast superalloy 718Plus upon heat treatment. Alloy 718Plus is a newly developed superalloy in which the secondary phase Laves form upon casting. In order to examine the heat treatment response numerous of heat treatments were done and later analyzed. The methods used for analysis were manual point count, EDS and macro Vickers hardness. The work was done at Volvo Aero Corporation in Trollhättan. Regarding the results of the experiments it was found that there is a lower limit below which no dissolution of Laves phase takes place, even for long heat treatment times. Also it was found that melting of the eutectic phase decrease the time required for homogenization. It was further found that even tough dissolution of Laves takes place, further evening out of elements do not appear to take place in the samples. Also areas containing huge amounts of Laves phase were found in some samples and the reason for this need to be instigated further in order to evaluate the castability of alloy 718Plus.
Materialvetenskap , Produktion , Konstruktionsmaterial , Materials Science , Production , Construction materials
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