Study on Weldability of Dispersion Hardening Alloy (1st report)

Behavior of ThO₂ particle under the melting-solidification of dispersion hardening alloy TD nickel

Abstract

Dispersion hardening alloys posses superior tensile properties at elevated temperature. On welding of dispersion hardening alloys, some problems regarding brittleness at the joint caused by agglomeration and decomposion of dispersed particles can be pointed out.
This study is investigated on weldability of TD nickel, especially on behavior of dispersing particles in welded zone at the first step of the study on weldability of dispersion hardening alloys.
Ths results obtained from this study are summarized as follows.
(1) In case of fusion welding of TD nickel, fusion zone is micrographically separated into three different regions. That is Region I, Region II and Region III. Even if melting-solidification time is about 1/6 sec, its separation is occured.
(2) By transmission electron microstructures of solidified TD nickel, it became evident that the previous three regions are separated by distribution of ThO₂ particles. Namely, in Region I, ThO₂ particles make agglomeration and growth to some degree and form the honeycomb structure. In Region II, ThO₂ particles make agglomeration and growth larger than in Region I. In Region III, ThO₂ particles are excluded when matrix nickel is melted and solidified, and so it had the solidified structure of pure nickel.
(3) It is experimentally evident that ThO₂ particles are excluded out of the surface of molten nickel, in spite of the difference of density between molten nickel and ThO₂.