Properties and microstructure of double-sided friction stir-welded microalloyed steel

Abstract

This study investigated the properties and microstructures of EH46 microalloyed steel following double-sided friction stir welding. The partially welded material was reversed, completing the weld. Yield strength, elongation and toughness of the weld were similar to those of the base material. During welding, the ferrite-pearlite in the base material transformed to mainly upper bainite, but the overlap or interference zone in the centre of the weld consisted predominantly of 0.5-3 µm recrystallised ferrite grains. The microhardness increased from 240 HV in the base material to 350 HV in bainite but decreased 200-240 HV in the overlap zone. These substantially differing microstructures suggest that the overlap or interference zone was processed at a lower temperature than the remainder of the weld, probably in ferrite.

Evolution of Microstructure During Double-Sided Friction Stir Welding of Microalloyed Steel

“…These include the possible retention of some transition metal carbides-25 to 100 nm in size, identified in BM, in the IZ, and an understanding of the role they have in developing the microstructure of IZ. Also, if, as was proposed, [10,15] the FSW temperature associated with IZ was below Ac 1 , and below Ac 3 during TMAZ formation, then it is important to understand the dissolution of pearlite in the banded BM during the processing, and in what form that carbon assumes in the weld zone.…”

“…[11][12][13] Several publications describe the results of this double-sided FSW technique for steels. [10,14,15] The study reported in Reference 10 investigated the dominant deformation mechanisms by looking at the evolution of microstructure and crystallographic texture in different regions of the weld, but has not considered in detail the evolution of microstructure at submicron scales.…”

“…From the published studies, [1,5,9,10,15,17] it is evident that a complex microstructure develops during the single-sided FSW process, and this appears to be exacerbated in the double-sided FSW. Most of the previous microstructural observations on FSW steels have been carried out with optical and scanning electron microscopy (SEM), supplemented with EDX and EBSD information.…”

“…Recently, [10,15] an attempt has been made to explain differences in the hardness levels, by considering the variations in microstructure in these regions. In addition to the above zones in TMAZ, in the double-sided FSW investigated in the current study, there exists a zone in the center of the stir zone in which the material experiences the welding process twice, where the second weld pass is partially (i.e., depending on the probe length) overlapping with the root of the first weld pass.…”

“…The number of ferrite grains in the TMAZs increased as the weld approached the IZ, while the IZs comprised mainly of recrystallized equiaxed ferrite grains, in the range of 0.5 to 3 lm, [10] containing subgrains and cells, together with dislocations interacting with fine-scale precipitates. [15] This preliminary microstructural investigation raised a number of questions, which necessitate a more comprehensive exploration of the microstructure. These include the possible retention of some transition metal carbides-25 to 100 nm in size, identified in BM, in the IZ, and an understanding of the role they have in developing the microstructure of IZ.…”