Laves phase in superalloy 718 weld metals
01 Jan 1995-Journal of Materials Science Letters (Kluwer Academic Publishers)-Vol. 14, Iss: 24, pp 1810-1812
TL;DR: In this article, the formation of laves phase in high heat input gas tungsten arc (GTA) and low heat input electron beam (EB) welds of 2 mm thick superalloy 718 in as-welded and postweld heat treated (PWHT) conditions is evaluated.
Abstract: The important consequence of the solidification in cast or welded superalloy 718 is the segregation of Nb and the formation of laves phase. Laves phase is a brittle intermetallic topologically close-packed phase with hexagonal structure, known for its detrimental effect on mechanical properties at room temperature [1]. Although data available with regard to wrought materials is somewhat elaborate it is not true for welds in general and for electron beam welds in particular. In this letter the formation of laves phase in high heat input gas tungsten arc (GTA) and low heat input electron beam (EB) welds of 2 mm thick superalloy 718 in as-welded and post-weld heat treated (PWHT) conditions is evaluated. The results have a bearing on the tensile ductility and other properties of welds. Sheets of superalloy 718 of thickness 2 mm in solution treated condition (chemical composition in Table I) were autogenously welded by automatic GTA and EB welding processes, resulting in full penetration using the weld process parameters listed in Table II. The as-welded samples were subjected to two PWHT schedules: direct duplex ageing and solution treatment followed by ageing. Solution treatment was carried out at 980 °C for 20 rain with air cooling and the duplex ageing was carried out at 720 °C for 8 h with furnace cooling to 620 °C for 8 h with air cooling. The as-welded and heat treated samples were then subjected to scanning electron microscopic (SEM) examination and quantitative electron probe micro-analysis (EPMA) for the analysis of microsegregation of elements and determination of the formation of laves phase. Figs 1 and 2 show SEM micrographs of the aswelded microstructures of EB and GTA weld metals, respectively. Essentially, the solidified structure is of dendritic type. EB weld metal showed relatively finer
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TL;DR: In this article, a microstructures and a columnar architecture as well as mechanical behavior of as-fabricated and processed INCONEL alloy components produced by additive manufacturing using electron beam melting (EBM) of prealloyed precursor powder are examined.
Abstract: Microstructures and a microstructural, columnar architecture as well as mechanical behavior of as-fabricated and processed INCONEL alloy 625 components produced by additive manufacturing using electron beam melting (EBM) of prealloyed precursor powder are examined in this study. As-fabricated and hot-isostatically pressed (“hipped”) [at 1393 K (1120 °C)] cylinders examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive (X-ray) spectrometry (EDS), and X-ray diffraction (XRD) exhibited an initial EBM-developed γ″ (bct) Ni3Nb precipitate platelet columnar architecture within columnar [200] textured γ (fcc) Ni-Cr grains aligned in the cylinder axis, parallel to the EBM build direction. Upon annealing at 1393 K (1120 °C) (hot-isostatic press (HIP)), these precipitate columns dissolve and the columnar, γ, grains recrystallized forming generally equiaxed grains (with coherent {111} annealing twins), containing NbCr2 laves precipitates. Microindentation hardnesses decreased from ~2.7 to ~2.2 GPa following hot-isostatic pressing (“hipping”), and the corresponding engineering (0.2 pct) offset yield stress decreased from 0.41 to 0.33 GPa, while the UTS increased from 0.75 to 0.77 GPa. However, the corresponding elongation increased from 44 to 69 pct for the hipped components.
241 citations
TL;DR: In this article, the authors compared the microstructure and the mechanical properties of the produced specimens, directly after the manufacturing process and additionally after two diverse heat treatments subsequent to manufacturing process.
Abstract: Selective laser melting, a quite new layer-wise manufacturing process for metals, is used for processing the nickel-based superalloy IN718 The objective of this work is to compare the microstructure and the mechanical properties of the produced specimens, directly after the manufacturing process and additionally after two diverse heat treatments subsequent to the manufacturing process As the resulting microstructure and properties for specimens manufactured by selective laser melting are directional, all investigations are made for specimens oriented vertically and horizontally Optical, scanning, and transmission electron microscopy are carried out in order to characterize the microstructure explicitly For investigating the texture of the material, additional EBSD measurements are undertaken Mechanical tests include tensile testing at room temperature and at elevated temperatures and hardness measurements The investigations reveal a very good quality of the SLM-produced specimens Nonetheless, differences in the grain sizes, the orientation, and especially in the precipitation behavior could be found
221 citations
30 Sep 2016-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the creep properties of a polycrystalline nickel-based superalloy produced via selective laser melting were investigated, and it was shown that the additively manufactured material showed superior creep strength compared to conventional cast and wrought material.
Abstract: The creep properties of a polycrystalline nickel-based superalloy produced via selective laser melting were investigated in this study. All heat treatment conditions of the additively manufactured material show superior creep strength compared to conventional cast and wrought material. The process leads to a microstructure with fine subgrains. In comparison to conventional wrought material no Niobium rich δ phase is necessary to control the grain size and thus more Niobium is available for precipitation hardening and solid solution strengthening resulting in improved creep strength.
160 citations
TL;DR: Inconel 718 (2mm thick) was welded using argon and helium gas shielded tungsten arc welding process with a filler metal and the cooling rates calculated as mentioned in this paper.
Abstract: Inconel 718 (2 mm thick) was welded using argon and helium gas shielded tungsten arc welding process with a filler metal. Both constant current and compound current pulse modes were applied and the cooling rates calculated. The dependence of Laves phase formation, dendrite arm spacing and niobium segregation ratios in fusion zone on the nature of shielding gases and current was studied. The maximum instantaneous weld cooling rate was achieved for the combination of Helium shielding gas and compound current pulse mode. This ultimately resulted in reduction of laves phase, segregation of niobium and dendrite arm spacing in the fusion zone.
121 citations
TL;DR: In this article, the microstructural evolution of the IN718 samples and the effects of energy input (Ev) on micro-structural architectures, dendritic morphology, precipitated phases and thus microhardness were investigated in detail.
Abstract: In this study IN718 samples were deposited by direct laser fabrication (DLF) technology in argon atmosphere from pre-alloyed powders. The microstructural evolution of the samples and the effects of energy input (Ev) on microstructural architectures, dendritic morphology, precipitated phases and thus microhardness were investigated in detail. For microstructure of the as-DLFed samples, at a lower Ev, the columnar grains were very continuous and uniform, while at a higher Ev, the columnar grains were no longer continuous and a layer banded structure was present. With increasing Ev, there was a dendrite to cell transition (DCT) in dendritic morphology evolution of the as-DLFed IN718 samples. Also, two critical points of Ev to determine whether the dendritic morphology of the as-DLFed IN718 sample is dendrites, cells or both were found to be about 220 J/mm3 and 550 J/mm3 respectively in this study. For precipitated phases, the size and the amount of the Laves phase within interdendritic boundaries were increased with increasing Ev, while the volume fraction of precipitated γ″ and γ′ phases in matrix γ of the as-deposited IN718 samples was getting small. As a consequence, the microhardness of the sample increases by decreasing Ev for a constant overlap rate between two neighbor cladding tracks, similarly, the microhardness also increases by decreasing Ev for a constant laser scanning velocity.
110 citations
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TL;DR: In this article, the effects of microstructural variations on the fracture toughness properties of Alloy 718 base metal and welds at 24 to 538°C were reviewed and the results were analyzed statistically to establish minimum-expected toughness values for use in fracture control analyses.
Abstract: This paper reviews the effects of microstructural variations on the fracture toughness properties of Alloy 718 base metal and welds at 24 to 538°C. Seven different base metal lots, including five base metal heats and three different product forms from one of the heats, were tested in both the conventional (ASTM B637) precipitation treatment condition and a modified treatment condition that was developed to improve the fracture resistance for this superalloy. A gas-tungsten-arc weld was tested in both heat treatment conditions and the as-welded condition. Significant heat treatment and heatto-heat variations in fracture toughness were found and the results were analyzed statistically to establish minimum-expected toughness values for use in fracture control analyses. In the conventional heat treatment (CHT) condition, the presence of coarse second phase precipitates, 6 phase in the base metal and 6 plus Lavesphase in the weld, controlled the fracture behavior by causing premature microvoid nucleation and growth. The higher annealing temperature used during the modified heat treatment (MHT) dissolved these coarse particles and suppressed premature microvoid coalescence. This accounted for the improved fracture resistance exhibited by MHT materials. Heat-to-heat variations in fracture toughness behavior were attributed to differences in precipitate morphology and alternate secondary fracture mechanisms. INTRODUCTION Alloy 718 is a high strength nickel-base superalloy that possesses excellent corrosion and oxidation resistance, coupled with good tensile and creep properties. As a result, this alloy is used extensively in structural applications in the aerospace, nuclear, cryogenic and petrochemical industries. In addition, Alloy 718 has been selected for several welded applications because it exhibits superior weldability relative to most superalloys. The enhanced weldability characteristics are associated with the sluggish precipitation kinetics of the primary strengthening y" (bodycentered-tetragonal Ni,Nb) phase[l,2]. This sluggish age hardening behavior results in a relatively high ductility heat-affected-zone and fusion zone during cooling and aging. This permits relaxation of residual stresses and thereby improves the strain-age cracking resistance. Superalloy 718-Metallurgy and Applications Edited by E.A. Loria The Minerals, Metals 8 Materials Society, 1989 517 For most structural components, this superalloy is given a conventional heat treatment (CHT) per ASTM 8637: annealed 1 h at 954°C and air cooled to 24"C, aged 8 h at 718°C and furnace cooled to 621"C, aged at 621°C for a total aging time of 18 h and air cooled. This heat treatment, however, has contributed to a series of failures in welded structures by severely reducing the ductility and impact toughness of the weld fusion zone[3-51. The inferior toughness was attributed to the presence of Laves phase in the CHT weld metal. To increase ductility and fracture properties, the Idaho National Engineering Laboratory[5,6] developed a modified heat treatment (MHT): annealed 1 h at 1093°C and cooled to 718°C at 55"C/h, aged 4 h at 718°C and cooled to 621°C at 55"C/h, aged 16 h at 621°C and air cooled. The slower cooling rate reduced thermal stresses and the high annealing temperature dissolved the Laves phase, which restored adequate impact toughness to the fusion zone. Many Alloy 718 components are highly loaded during service, so fracture control is a primary design consideration. Structural integrity assessments of such high strength structures require a comprehensive understanding of the fracture toughness characteristics for this superalloy. Since this alloy is metallurgically complex, involving precipitation of several phases, its fracture resistance is expected to be strongly influenced by heat treatment, processing history, melt practice and alloy composition. A series of investigations[7-111 were conducted at Westinghouse Hanford Company to evaluate the influence of microstructural variations on the fracture properties for Alloy 718 base metal and welds and the results were reviewed in this paper. The parameters examined in these studies included the effects of heat treatment, product-form and heat-to-heat variations. Fracture toughness tests were performed using linear-elastic and elastic-plastic (J ) test techniques. (K c) Toughness values for the wrough I material were statistica f F y analyzed to establish minimum-expected toughness levels that account for material variability. Metallographic and fractographic examinations were also performed to relate key microstructural features and operative fracture mechanisms to macroscopic properties. EXPERIMENTAL MATERIALS AND PROCEDURES Five heats of Alloy 718 were examined in the CHT and MHT conditions. In addition, three product forms (plate, round bar and upset forging) from a single heat were studied. Details of the material supplier, product form and melt practice for the seven material lots are summarized in Table I. A material lot is defined as a two letter code that designates a particular heat/product form combination: the first letter denotes the heat: the second letter indicates the product form (i.e., forging). P for plate, B for bar and F for A gas-tungsten-arc (GTA) weld was also tested in the as-welded, CHT and MHT conditions. The welding procedures used to manufacture this weld were detailed in Reference 7. Chemical analyses and tensile properties for the test materials were reported in References 9-11. The CHT wrought metal exhibited lo-20% higher yield strength and O-10% higher ultimate strength levels relative to its MHT counterpart. Ductility values were generally found to be insensitive to heat treatment. In contrast to the base metal response, yield strength levels for the age-hardened welds were relatively insensitive to heat treatment, while the ultimate strength for the CHT weld was slightly lower than that for the MHT weld. The two heat treated welds exhibited comparable uniform and total elongation values, but the reduction in area for the MHT weld was much greater than that for its CHT counterpart. The strength in the as-welded condition was approximately half of that for the aged materials.
16 citations