THE IMPACT OF IRON CONCENTRATION ON THE PROPERTIES OF AN ADDITIVELY MANUFACTURED SOLID SOLUTION STRENGTHENED NICKEL BASE ALLOY

Open Access
Author:
Khayat, Zakariya Radwan
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 10, 2017
Committee Members:
  • Todd Palmer, Thesis Advisor
  • Allison Michelle Beese, Committee Member
  • Jingjing Li, Committee Member
Keywords:
  • Additive Manufacturing
  • Inconel 625
  • directed energy deposition
  • powder feedstock
  • Nickel base alloys
  • HIP
  • materials science
  • alloying elements
Abstract:
Additive manufacturing (AM) technology is capable of producing free-from geometries for large scale applications. Fabricating entire structures in a continuous layer-by-layer fashion can mitigate possible material property mismatch problems that have been noted in various welding operation studies. Inconel® 625 would be primarily used as the overlay material in a weld clad because of the high corrosion and wear resistance it exhibits. However, instead of only adding a few layers of material to a dissimilar base material, there is interest in using AM to process entire components made from Ni base alloys that will exhibit the material properties in bulk form. Alloying element compositions in Inconel® 625 can vary over rather wide ranges, with Fe, in particular, ranging from 0 to 5 wt%. The impact of changes in the Fe content on the properties of AM materials is investigated using a series of laser-based directed energy deposited (DED) builds with Fe contents of 1 wt% and 4 wt% in both the as deposited and post processed hot isostatically pressed (HIP) conditions. While similar solidification structures and microhardness values of the fabricated builds are observed with both powder feedstocks, the low Fe content feedstock samples displayed higher yield (520 MPa ± 12 MPa) and tensile strengths (860 MPa ± 27 MPa) and lower elongation values (36 % ± 5 %) in the as-deposited condition compared to yield (450 MPa ± 27 MPa) and tensile strengths (753 MPa ± 25 MPa) and elongation values (44 % ± 9 %) with the high Fe content feedstock samples. The differences in mechanical properties were highly dependent on the condition and were connected to differences in the strengthening mechanisms associated with the two different Inconel® 625 powder compositions. Low Fe content samples had an average grain size 10x smaller than that of high Fe content samples in the as deposited condition, which led to differences in mechanical properties. After HIP, Low Fe content samples displayed smaller grains, but compared to the as deposited condition, there was not substantial grain growth or recrystallization. The yield strength of both low and high Fe content builds decreased by 14%, while elongation increased by 15%. While on the other hand, post processed tensile strengths changed by only 3%. This small change in tensile strength can be traced to higher levels of strain hardening for high Fe content feedstocks. These differences in mechanical behavior can be attributed in part, to changes in precipitate formation after HIP. It is speculated that in low Fe content samples large Nb and Mo rich precipitates form that pin grain boundary motion while in the high Fe content samples, tiny Ti rich precipitates form that pin dislocation motion. Precipitate formation is denoted as secondary phase formation. In order to study the impact alloying elements had on the resulting secondary phase constituents, thermodynamic phase stability calculations were conducted. Calculations showed different phase formation tendencies for each powder feedstock material. In addition, phase stability calculations helped set guidelines in order to distinguish between secondary phase constituents based upon the Nb content. Coupled with a low C/Nb ratio and verified using an electron probe microanalysis (EPMA), secondary phase constituents were characterized as Laves phase