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Investigation Of Microstructure And Stress Distribution Of The Super-Ni/NiCr Joint Zone By TIG Welding

Posted on:2011-02-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:C Z XiaFull Text:PDF
GTID:1101360305450562Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Laminated composite material is a kind of new materials that could meet the particular performance of modern aircraft engines. Due to its special high temperature performance, Super-Ni/NiCr laminated composite has a luciferous prospect in aviation, aerospace and other high-tech fields. However, Super-Ni/NiCr is difficult to be welded and easy to induce thermal stress due to its unique composition and the laminated composite structure. Therefore the welding problem of Super-Ni/NiCr is the key to block its wide application.The weldability of Super-Ni/NiCr laminated composite was studied via TIG with filler alloy. The microstructure, elements distribution, fine phase structure and stress distribution of the TIG joint were researched to reveal the relationship of welding parameter, microstructure, microcracks and stress distribution.The division of the characteristic zone of Super-Ni/NiCr TIG joint was put forward to include transition zone of Ni cover layer with weld, transition zone of Ni80Cr20 base layer with weld and weld center. The transition zone of Ni cover layer with weld included fusion zone(FZ) and heat affect zone(HAZ) of Ni cover layer. The transition zone of Ni80Cr20 base layer with weld included FZ and HAZ of Ni80Cr20 base layer. The weld center included columnar grain zone and equiaxed grain zone. With the welding heat input increasing, the equiaxed grains in weld center became gross. A sound joint could be formed with small heat input. However, its difficult to get a good joint with heat input above 11kJ/cm.The fine phase structure and element distribution of Super-Ni/NiCr TIG joint was analyzed by means of X-ray diffraction (XRD), electron probe microanalysis (EPM A) and transmission electron microscopy (TEM). The results indicated that the joint zone was constituted ofγ-Fe,δ-Fe,γ-Ni (Cr, Fe), }-Ni and FeNi intermetallic compound. Fusion zone of NiCr base layer was jagged and elements transition zone about 83μm was formed with Fe element transited from weld to the base metal, while Ni transited in the contrary and Cr not transited obviously. The element transition zone of Ni cover layer fusion zone was about 30μm. The Ni content in the weld near Super-Ni/NiCr side was about 40%, Creq/Nieq<1.52, with AF solidification mode. Directional columnar grains was formed, and the weld meat has tendency to hot cracking.Diffraction analysis indicated that the internal stress of weld metal was high. The main phase in the Super-Ni/NiCr TIG joint wasγaustenite with a small amount ofδferrite phase distributed in the y-phase grain boundaries, and the lattice orientation betweenγaustenite and 8 ferrite phases was (111)γ//(110)δ,[211]γ//[111]δ.The ordered phase Ni3Fe with FCC structure was formed in the weld. In the fusion of Super-Ni/NiCr, the lattice orientation between y austenite and a ferrite phases was (110)α//(111)γ,[111]α//[211]γ. Theγ-Fe andγ-Ni (Fe, Cr) coexisted in the fusion zone of the laminated composite.The stress fields of Super-Ni/NiCr and 1Cr18Ni9Ti steel TIG joint were studied based on the analysis of microstructure. The results indicated that the stress concentrated seriously in the fusion zone of Super-Ni/NiCr, mainly affected byσx andσy stress. The interface of NiCr base layer and Ni cover layer was the weakness region while the laminated composite being welded. The peak value ofσx tensile stress was 29.1 MPa andσx tensile stress was 40.1MPa, in the HAZ of the laminated composite. Along the welding direction, in first welding position of the fusion zone of the laminated composite was mainly affected byσx compression stress, in the behind position was mainly affected byσx tensile stress.The microstructure and stress distribution of Super-Ni/NiCr TIG joint was firstly studied in this paper, which provided experimental and theoretical basis for improving the application of Super-Ni laminated composite. The conclusions obtained from this paper have established an important foundation to study deeply the weldability of high temperature composites.
Keywords/Search Tags:Laminated composite, Tungsten Inert Gas welding, Division of characteristic zone, Fine phase structure, Stress distribution
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