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Acta Metall Sin  2019, Vol. 55 Issue (9): 1221-1230    DOI: 10.11900/0412.1961.2019.00097
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Microstructure and High-Temperature Deformation Behavior of Dissimilar Superalloy Welded Joint of DD407/IN718
LIU Yang,WANG Lei(),SONG Xiu,LIANG Taosha
Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110819, China
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Abstract  

Welding is an important joining method to fabricate the dissimilar welding integral blisk structure of single crystal and polycrystalline superalloy. The microstructure and properties of the welded joint are the key factors to determine the reliability of the integral blisk structure of dissimilar superalloys. The single crystal superalloy of DD407 and polycrystalline superalloy of IN718 were butt welded by continuous fiber laser system. The evolution of microstructure and composition segregation of the welded joints fabricated under the optimized welding parameters as-welded (AW) and after post weld heat treatment (PWHT) were investigated. The high temperature tensile deformation behavior of the welded joint after PWHT was also examined. The results show that the microstructures of fusion zone (FZ) in the welded joint consist of planar crystal, cellular crystal, columnar crystal and equiax crystal. The difference of the dendrite microstructures between the two sides of the weld centerline is very obvious. In terms of the joint as-welded, the microhardness of the FZ is low and there exists obvious micro-segregation. After PWHT, the micro-segregation has been improved and the microhardness increases significantly in the FZ which is much more than those of both base metals (BMs) of DD407 and IN718 alloys. There exists local hardening zone in the heat-affected zone (HAZ) of DD407 single crystal alloy and narrow softening zone and grain boundary liquation phenomenon in the HAZ of IN718 polycrystalline alloy. The ultimate tensile strength and elongation of the welded joint after tensile test at 650 ℃ are 1111 MPa and 9.42%, respectively. And the tensile specimen of the welded joint fails in the BM of IN718 polycrystalline alloy. The main deformation mode of the laser welded joint at high temperature includes the multi-slips of dislocation in the BM and FZ of single crystal alloy, and the dislocation slip and grain-boundary sliding in the BM of polycrystalline alloy. The tensile fracture surface is characterized by multi-source cracking, and the dimple and crystal sugar shaped facture surface exist simultaneously in the crack source area, which is a mixed fracture of microvoid aggregation and intergranular fracture. So the tensile fracture mechanism contains micro-void accumulation fracture and inter-granular fracture. The grain boundary liquefaction in the HAZ of IN718 polycrystalline does not affect the short-time high temperature mechanical properties of the welded joints.

Key words:  fiber laser welding      dissimilar welded joint of single crystal and polycrystalline alloy      dissimilar welded joint of single crystal and polycrystalline alloymicro-segregation      high-temperature deformation      fracture behavior     
Received:  03 April 2019     
ZTFLH:  TG132.32  
  TG407  
Fund: Supported by National Natural Science Foundation of China(Nos.51571052、 U1708253,51874090);Fundamental Research Funds for the Central Universities of China(No.180213006)
Corresponding Authors:  Lei WANG     E-mail:  wanglei@mail.neu.edu.cn

Cite this article: 

LIU Yang,WANG Lei,SONG Xiu,LIANG Taosha. Microstructure and High-Temperature Deformation Behavior of Dissimilar Superalloy Welded Joint of DD407/IN718. Acta Metall Sin, 2019, 55(9): 1221-1230.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00097     OR     https://www.ams.org.cn/EN/Y2019/V55/I9/1221

MaterialCCrCoWMoAlTiNbTaFeNi
DD4070.0078.0505.5005.0002.2505.9502.000-3.500-Bal.
IN7180.04019.000--3.0500.5000.9005.250-Bal.52.500
Table 1  Chemical compositions of DD407 and IN718 alloys
Fig.1  Schematic of samplling positions (a) and geometry dimension (b) of specimens for the tensile test of the laser welded joint of DD407 and IN718 alloys (BMDD407—base metal of DD407, BMIN718—base metal of IN718, FZ—fusion zone)
Fig.2  Top and bottom surface morphologies (left) and overall views (right) of the laser welded joints of DD407 and IN718 alloys under different laser welding parameters (R—welding speed, P—laser power)(a) R=2.5 mm/min, P=1.8 kW (b) R=2.5 mm/min, P=2.0 kW (c) R=2.1 mm/min, P=1.4 kW(d) R=2.1 mm/min, P=1.6 kW (e) R=2.1 mm/min, P=1.8 kW
Fig.3  Microstructures of laser welded DD407/IN718 joint (Focus lens is 250 mm, defocus is -8 mm, welding power is 1600 W, welding speed is 2.1 mm/min. HAZDD407—heat-affected zone at DD407 side, HAZIN718—heat-affected zone at IN718 side)(a) microstructure of HAZDD407 (b) overall view (c) microstructure of HAZIN718 (d) γ' of BMDD407(e) microstructure of FZ (f) Laves phase on the boundary liquation in the HAZIN718(g) γ' of HAZDD407 near BMDD407 (h) dendrite structure of position for I zone in Fig.3b(i) γ' of HAZDD407 near FZ (j) dendrite structure of position for II zone in Fig.3b
Fig.4  SEM images showing the microstructure of the laser welded joint of DD407/IN718 after post weld heat treatment(PWHT) (The insets show the selected locations of Figs.4a and b)(a) microstructure of FZ (b) microstructure of HAZIN718
Fig.5  Microhardness distributions of the laser welded joint of DD407 and IN718 alloys as-welded (AW) and PWHT in the positions for I zone (a) and II zone (b) in Fig.3b
Fig.6  Element distributions of the welded joint of DD407 and IN718 alloys in the positions for I zone (a) and II zone (b) in Fig.3b
Fig.7  Stress-strain curves (a) and tensile properties (b) of the laser welded joint of DD407 and IN718 alloys and the corresponding BMs
Fig.8  SEM images showing tensile failure location and fracture morphology of laser welded joint of DD407/IN718 and corresponding base metals(a) tensile failure location (b) tensile fracture morphology of the laser welded joint of DD407/IN718 alloys(c) tensile fracture morphology of the BMIN718 (d) dimple fracture morphology of the welded joint(e) intergranular fracture morphlolgy of the welded joint (f) intergranular fracture morphlolgy of the BMIN718
Fig.9  SEM images of the laser welded joint of DD407 and IN718 alloys at the location of fusion line of DD407 (a, b), FZ of DD407 (c, d), FZ of IN718 (e, f) and fusion line of IN718 (g) in the positions for I zone (a, c, e, g) and II zone (b, d, f)
Fig.10  Low (a) and high (b) magnified SEM images showing micro-cracks near the fracture surface of the tensile specimen of the laser welded joint of DD407 and IN718 alloys
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