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Investigation Of Microstructure And Stress Distribution Of The Fe3Al Joint Zone By Fusion Welding

Posted on:2010-09-12Degree:DoctorType:Dissertation
Country:ChinaCandidate:H J MaFull Text:PDF
GTID:1101360278474330Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
The particular performance of Fe3Al intermetallic makes it a luciferous prospect. However,Fe3Al is difficult to fusion welded due to the hard brittleness of this material.In this paper,the weldability of Fe3Al as well as Fe3Al/18-8 and Fe3Al/Q235 steels was studied via filler TIG and SMAW.The research methods of SEM,TEM,high-temperature DSC and ANSYS finite element software were used to reveal the relationship of microstructure with the performance.And the emphasis was put upon TIG joint zone.The results of series welding experiments indicated that a Fe3Al joint with good performance was obtained under the condition of welding heat input 5.5-6.9 kJ/cm, filler Cr23-Ni13 alloy via TIG and welding heat input 9.8-11.5kJ/cm,electrode E310-16 via SMAW.The shear strength of Fe3Al/Q235 joint was largest(591.1MPa), Fe3Al/18-8 joint the secondary and Fe3Al/Fe3Al joint the least(127.3MPa).The analysis of microstructure,cracks and fracture of Fe3Al joint were carried out based on the division of character zones.Fe3Al joint character zones included a homogeneous mixture zone(HMZ),a partial mixture zone(PMZ),a partially fused zone(PFZ) and a heat-affected zone(HAZ).The matrix of Fe3Al/18-8 and Fe3Al/Q235 HMZ wasγaustenite and pro-eutectoid ferrite(PF) precipitated along theγgrain boundary.The retainedδ,LM and Fe3Al isle existed in Fe3Al/18-8 PMZ. A 30μm wide austenite rich band distributed along PFZ with an angle of 50-70°.The Fe3Al/Fe3Al HMZ was composed of bulkα-Fe(Al) solid solutions and the fusion residence layer was formed in PMZ.Fe3Al welding cracks originated in the PFZ and extended along the PFZ and HAZ.The shear fracture of Fe3Al/18-8 and Fe3Al/Q235 joint were mostly transgranular cleavage,and the fracture of Fe3Al/Fe3Al joint was in intergranular mode.The phase constituents Fe3Al/18-8 joint were composed of Fe3Al,γ-(Fe,C), FeAl,α-Fe(Al),Ni3Al and(Cr,Fe)7C3.The phase structure of Fe3Al/Q235 joint constituted of Fe3Al,FeAl,Fe4Al13,α-Fe(Al),NiAl and(Fe,Cr).The phase constituents of Fe3Al/Fe3Al joint were relatively simple.The density of the dislocation inαphase was very high for upper bainite(Bu) in Fe3Al/18-8 HMZ,and theαplate was 0.3μm wide.The lattice orientation between a and y phases was(110) α//(111)γ,[001]α//[101]γ.The width of a plate in lower bainite(Bl) was 0.2-0.5μm. The lath martensite(ML) was 40nm in width in PMZ,and the width of retainedγmembrane was 10-20nm.The lattice orientation betweenα-Fe(Al) and Fe3C phases was(100)Fe3C//(110)α-Fe(Al),[100]Fe3C//[110]α-Fe(Al) in Fe3Al/Fe3Al PMZ.Two kinds of transformation models for Fe3Al B2 and DO3 ordered structure were revealed:the alternation model in the zones with higher dislocation density and precipitation transformation model in the zones with lower dislocation density in which globular DO3 domains precipitated from the B2 structure.And alternation model was the major model for the DO3-B2 transformation.The DO3-B2 transition temperature and enthalpy changes reduced in different degrees after welding.The less the welding heat input,the larger the reduction range was.The DO3-B2 transformation occurred gradually to lower temperature.On the basis of characterization and thermodynamics analysis,the mathematic models of DO3-B2 transformation were found.The Fe3Al joint containing different proportion of DO3 and B2 structure was obtained by controlling the welding heat input to satisfy various running conditions.The stress fields of Fe3Al/18-8 joint were studied based on the analysis of microstructure.The stress regions of Fe3Al/18-8 joint were divided into three character zones of stress jump zone,stress transition zone and stress stable zone to analysed the stress distribution of fusion zone,weld and HAZ in different cross-sections,respectively.The Fe3Al side fusion zone was the stress concentration zone.The stress jump zone was mainly affected byσy tensile stress,and the peak value ofσy was twiceσx.The stress stable zone was mostly affected byσx tensile stress.The weld was mainly affected byσy compression stress in stress jump zone andσx tensile stress in stress stable zone.The Fe3Al HAZ was mainly affected byσy tensile stress in stress jump zone andσy compression stress in stress stable zone.The welding cracks problem of Fe3Al brittle material was solved in this paper, especially without pre-heat treatment,which provided experimental and theoretical basis for improving the application of Fe3Al intermetallic.This paper presented the combination analysis of the microstructure and stress distribution,character zones division as the research main line and combination of characterization, thermodynamics and mathematics methods,which was a consideration to the welding research for high brittle materials.
Keywords/Search Tags:Fe3Al intermetallic, Fusion welding, Microstructure, Ordered structure transformation, Stress distribution
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