Numerical simulation of solidification and liquation behavior during welding of low-expansion superalloys

Low-expansion superalloys are susceptible to weld solidification cracks and heat-affected zone (HAZ) microfissures. To predict solidification cracking, QBasic procedures were developed and solidification reaction sequence, type, and amount of eutectic product were calculated. As manifested, primary solidification is followed by L → (γ+NbC) and L → (γ+Laves) eutectic reaction sequentially for GH903 and GH907; hence, the terminal eutectic constituents are made up of γ/NbC and γ/Laves. While for GH909, only reaction L → (γ+Laves) occurs and more γ/Laves eutectic forms. Therefore, GH909 is more sensitive to solidification cracking. To predict HAZ liquation, cracking Visual FORTRAN procedures were developed, and constitutional liquation of NbC was simulated. As shown, solid dissolution of NbC prior to liquation decreases, and initial liquid film increases with the rate of thermal cycle. Higher rate of thermal cycle promotes the melting of the matrix adjacent to the liquid film and postpones the solidification of the liquid by the liquid-to-γ mode. Thus, more residual liquid film remains at the eutectic point, which will promote HAZ microfissuring. The increase in original grain size and peak temperature also promotes liquation. Finally, these conclusions were verified indirectly by hot ductility tests.     

Multisheet structure of Inconel 718 superalloy processed by LBW/SPF technology and its microstructure

Abstract

Multisheet structure of Inconel 718 superalloy will be widely used in vehicles as heat resisting and heat shielding structure due to its lightweight, high strength and stiffness. Multisheet structure of Inconel 718 superalloy was processed by laser beam welding and superplastic forming (LBW/SPF) technology in the present paper. Multisheet structure of Inconel718 superalloy processed by LBW/SPF technology exhibits good configuration and uniform thickness distribution. Laser beam welding parameters for multisheet structure were as follows: pulse frequency was 32 Hz; pulse duration 3 ms; peak power per pulse 4500 W; welding speed 180 mm min^–1; SPF parameters were as follows: temperature T_f=965°C; forming pressure P _f=4·2 MPa; forming time t _f=130 min. Microstructure of multisheet structure was studied carefully. Microstructure in weld fusion zone was constituted of austenite dendritics and Laves phase precipitated in interdendritics. After SPF process, austenite dendritics in the weld fusion became coarser and most of Laves phases were dissolved and turned into δ precipitated phase but a few of Laves phases were still reserved. And Nb concentration in dendritics increased to 5·42% compared to 2·82% under as welded condition. Weld metal hardness increased from 331·63 under as welded condition to 391·74 under post-SPF condition which was closed to the base material hardness of post-SPF. Grain size of base material grew slightly and an amount of precipitated phase appeared in the base material undergoing SPF process. The tensile test results of base material show that tensile strength increased obviously and the ductility decreased slightly after SPF process. Therefore, LBW/SPF technology is an appropriate forming technique for multisheet structure of Inconel 718 superalloy.     

Microstructural evolutions in dissimilar welds between AISI 310 austenitic stainless steel and Inconel 657

This investigation has been performed to characterize dissimilar metal welds between type 310 austenitic stainless steel (SS) and Inconel 657 superalloy. The welds were produced using four types of filler materials: Inconel 82, Inconel A, Inconel 617, and type 310 SS. The weldments were characterized in detail using optical metallography and scanning electron microscopy. It can be concluded that Inconel A weld metal does not promote severe hot cracking. Continuous NbC precipitates in the Inconel 82 weld metal can sensitize the weld metal to solidification cracking. The presence of high amounts of Mo in Inconel 617 weld metal led to the formation of brittle phases. In addition, continuous precipitates were observed in the 310 SS weld metal, which can lead to poor resistance of the weld metal to hot cracking. In the aged condition, Inconel 82 and Inconel A exhibited good thermal stability, whereas Inconel 617 and type 310 SS exhibited poor thermal stability. Also, after subjecting the heat-affected zone and interface between Inconel weld metal and base metals to aging treatment, unmixed zone of Inconel 657 base metal side has disappeared. Elimination of this region can be attributed to high-temperature interdiffusion of alloying elements. Finally, it is found that Inconel A and Inconel 82 weld metals are the best choices for the dissimilar welds performed here, respectively.     

热处理工艺对Inconel718合金多层夹芯板结构的微观组织与力学性能的影响

采用LBW+SPF组合技术制造Inconel718合金多层夹芯板结构.为了增强多层夹芯板结构使用时的安全性,研究其热处理技术.结果表明:Inconel718合金在焊接过程产生了Nb含量较高的Laves沉淀相;超塑成形后焊缝中的Nb元素的偏析问题得了缓解;经980℃固溶30min处理后,焊缝中的δ相完全回溶母体γ相,焊缝...     

Nanoindentation creep behavior of RPV’s weld joint at room temperature

SA508 Gr.3 steel has been widely used in nuclear reactor pressure vessels (RPV). Nuclear components are generally combined through arc welding processes, which always produces heterogeneous mechanical properties in heat affected zone (HAZ) of weld joint. In order to study mechanical heterogeneity of weld joint, HAZ was been divided into five small regions (HAZ1 to HAZ5) based on the distance from the weld center line. The elastic modulus, hardness, and creep deformations of five regions in HAZ were measured through nanoindentation, as well as base and weld metals. According to the experimental results, the HAZ2 region (belonging to the fine-grained HAZ) exhibited a significantly lower hardness and creep behavior. Strain rate sensitivities (SRS) in different regions were then estimated from the steady-state creep, and the HAZ2 region showed a relatively higher value. The influence of grain boundary fraction on the creep behavior of weld joints was discussed later. Furthermore, the results of SRS also indicated that the creep mechanism of tested regions could be dominated by dislocation activities.

     

Fracture toughness analysis of laser-beam-welded superalloys Inconel 718 and 625

In this study, two 3.2‐mm thick Ni‐base superalloys, Inconel 718 and 625, have been laser‐beam‐welded by a 6‐kW CO₂ laser and their room temperature fracture toughness properties have been investigated. Fracture toughness behaviour of the base metal (BM), fusion zone (FZ) and heat affected zone (HAZ) regions was determined in terms of crack tip opening displacement (CTOD) using compact tension‐type (C(T)) specimens. Laser‐beam‐weld regions showed no significant strength overmatching in both alloys. Ductile crack growth analysis (R‐curve) also showed that both materials exhibited similar behaviour. Compared to the BM there is a slight decrease in fracture toughness of the fusion and the HAZ.     

Microstructure and mechanical properties of Inconel 718 electron beam welds

Abstract

Bead on plate, full penetration electron beam welds were produced in 2 mm thickness sheets of Inconel 718 in the solution treated condition. Welds were subjected to an aging treatment with and without post-weld solution treatment. Weld microstructures, high temperature tensile properties and stress rupture properties were evaluated. The as welded fusion zone showed a considerable amount of interdendritic niobium segregation and brittle intermetallic Laves phase. The tensile and stress rupture properties of the welds after post-weld aging treatment were found to be inferior in relation to the base metal. Post-weld solution treatment at 980°C was found to result in partial dissolution of Laves phase, some reduction in niobium segregation and the formation of δ phase needles around the Laves particles. The use of 980°C solution treatment was found to improve the weld properties to some extent, although not to the level of the base metal. The reasons for this behaviour are discussed, correlating microstructures, fracture features and mechanical properties.     

Effect of flux addition on the microstructure and tensile strength of dissimilar weldments involving Inconel 718 and AISI 416

This research work articulates the microstructural features of the dissimilar weldments involving Ni based superalloy Inconel 718 and martensitic stainless steel, AISI 416 that is difficult to weld. Autogenous tungsten inert gas (TIG) welding was carried out with and without using flux to fabricate these bimetallic combinations. Microstructure at the fusion zones and the interfaces were characterized using optical and scanning electron microscopy. The results depicted the presence of fine martensite at the heat affected zone (HAZ) of AISI 416 and the formation of unmixed zone containing secondary phases at the HAZ of Inconel 718. Similarly, the formation of Nb rich eutectics along with sulfides was witnessed at the fusion zone of both the weldments. Owing to the low heat input witnessed during the flux assisted TIG welding, the Nb segregation was found to be minimal. Tensile studies reported that the fracture occurred at the fusion zone in both the cases. It was inferred from the tensile studies that the joint strength of the weldments with flux addition was greater than the ones without flux. This study demonstrated that dissimilar joints with complete penetration could be achieved in single pass using the TIG welding process with the aid of flux.     

Narrow-gap tungsten inert gas welding of 78-mm-thick Ti–6Al–4V alloy

Narrow-gap tungsten inert gas welding with in situ protective system is applied to 78-mm-thick Ti–6Al–4V alloy. The martensite basketweave microstructure is formed in fusion zone (FZ), and heat-affected zone (HAZ) near FZ consists of acicular martensite and a small portion of primary alpha phase. The HAZ near base metal (BM) consists of primary alpha and transformed beta phase. Microhardness values of FZ and HAZ are relatively higher, and peak values appear at HAZ near FZ. The mechanical heterogeneity is dependent on variation of beta phase and martensite, which in turn depend on specific thermal cycle(s). The average tensile strength reaches 783.3?MPa and is 91.2% of BM, which appears in the middle of the weld with orthogonal martensites.     

Effect of Welding Parameters on the Size of Heat Affected Zone of Submerged Arc Welding

This paper discusses the effect of welding parameters on the size of the heat affected zone (HAZ) and its relative size as compared to the weld bead of submerged arc welding. It is discovered that the welding parameters influences the size of weld bead and HAZ differently which can be relate to the effect of welding parameters on the various melting efficiencies. This difference in behavior of HAZ and weld bead can be explored to minimize the harmful effect of HAZ in future welds.     

HAZ microstructure simulation in welding of a ultra fine grain steel

In the present work the evolution of grain structure in the weld HAZ (heat affected zone) under welding thermal cycle was simulated. Especially the grain growth in the HAZ of a SS400 ultra fine grain steel was investigated. An integrated 3-D Monte Carlo (MC) simulation system for grain growth of the weld HAZ was developed based on Microsoft Windows. The results indicate that MC simulation is an effective way to investigate the grain growth in weld HAZ. The method not only simulates the non-isothermal dynamics process of the grain growth in the weld HAZ, but also visualizes the austenite grains realistically. Moreover, the thermal pinning effect can be easily included in the simulation process. The grain sizes of the CGHAZ (coarse grain heat affected zone) obtained from MC simulation are basically in agreement with the experimental measurement of the real welded joints under different heat input. Furthermore, the simulation indicates that the grain growth degree is higher for the SS400 ultra fine grain steel compared to conventional steel. With the increase in the heat input, the grain growth of the CGHAZ rapidly increases. Because the activation energy of the grain growth is lower for the SS400 ultra fine grain steel, austenite grains can grow at a relatively lower temperature, hence the range of the CGHAZ becomes wider.     

An investigation on thermal, metallurgical and mechanical states in weld cracking of Inconel 738LC superalloy

The influence of various conditions of Inconel 738 superalloy welding or deposition welding has been studied in order to shed light on the coupling between thermal, metallurgical and mechanical states in the heat affected zone (HAZ) in which cracking may occur particularly during welding and post-weld heat treatment. Predominant crack controlling factors have been highlighted thanks to different pre-weld and post-weld heat treatments, in addition to various welding rates and pre-heating prior to welding. These factors are mainly the material ductility related to the morphology and volume fraction of intermetallic precipitates and thermally induced residual stress. It has appeared that reducing thermally induced residual stress could be more effective for preventing cracking than controlling the material ductility in the related zones thanks to adjustment of pre-weld and post-weld heat treatments. With the objective of cracking remediation, welding on preheated parts leads to lower weld power, to reduce significantly thermal gradients, decreases thermally induced stress and impedes cracks formation despite some localized and temporary decrease in alloy ductility.     

车用TRIP590钢光纤激光焊接接头组织与性能研究

为了促进先进高强钢激光焊接技术的发展,采用光纤激光器对1.5mm厚的TRIP590钢板进行焊接,对焊接接头的微观组织、硬度以及拉伸性能进行了研究,分析了焊接速度对组织、性能的影响。结果表明:焊缝组织主要为板条状马氏体,热影响区可分为完全淬火区和不完全淬火区。焊接接头硬度分布不均匀,在热影响区或焊缝处硬度最高。随着焊接速度提高,热影响区马氏体含量增多,贝氏体含量减少,热影响区和焊缝组织变得细小。焊接速度为3~5m/min时,拉伸试样均断裂在母材,断后延伸率均超过30%,随着焊接速度提高,断后延伸率也有所提高,强塑积(PSE)均在20000MPa%以上,拉伸变形过程中相变诱发效应显著,大部分残余奥氏体转变为马氏体,在提高材料塑性的同时也提高了强度,实现了高强度和高塑性的统一。     

Effect of friction stir welding parameters on microstructural characteristics and mechanical properties of 2219-T6 aluminum alloy joints

A 2219-T6 aluminum alloy was friction stir welded in the present study. The results indicate that the recrystallized grains in the weld nugget zone (WNZ) of the joints exhibit the largest size in the middle part and the smallest size in the lower part. Furthermore, the void defect is formed in the joint when the rotation speed or welding speed is quite high. As the rotation speed or welding speed increases, the tensile strength of the joint firstly increases to a maximum value and then sharply decreases due to the occurrence of void defect. During tensile test, the defect-free joints welded at lower rotation speed are fractured in the WNZ, while those welded at relatively high rotation speed tend to be fractured in the heat affected zone (HAZ) adjacent to the thermal mechanically affected zone (TMAZ) on the retreating side.     

Microstructural analysis of fusion and heat affected zones in electron beam welded ALLVAC 718PLUS™ superalloy

The fusion zone and heat affected zone (HAZ) microstructures of electron beam welded superalloy 718PLUS™ (718 Plus) that has been newly developed by ATI ALLVAC were examined. The microsegregation pattern during solidification of the fusion zone indicated that while Fe, Co, W, and Cr segregated to the core of the gamma dendrites, Nb, Ti, and Al were extensively rejected into the interdendritic liquid. Electron diffraction and X-ray microanalysis using transmission electron microscopy (TEM) of the fusion zone showed that the major secondary phases that formed from the interdendritic liquid were gamma/MC type carbide eutectic and gamma/Laves eutectic constituents. HAZ microstructure showed partially melted zone immediately adjacent to the fusion zone and intergranular microfissuring associated with resolidified products which suggested that HAZ cracking in this alloy occurred by liquation cracking. Microstructural examination of the HAZ using analytical scanning electron microscope showed resolidified gamma/Laves eutectic on the cracked and backfilled grain boundaries. Fine resolidified MC type carbide particles were also observed in the HAZ. Causes of grain boundary liquation were identified and the solidification of intergranular liquid in the HAZ was discussed.     

Microstructure and mechanical properties of friction stir welded copper

The main objective of this investigation was to apply friction stir welding technique (FSW) for joining of 2 mm thick copper sheet. The defect free weld was obtained at a tool rotational and travel speed of 1,000 rpm and 30 mm/min, respectively. Mechanical and microstructural analysis has been performed to evaluate the characteristics of friction stir welded copper. The microstructure of the weld nugget (WN) consists of fine equiaxed grains. Similarly, the elongated grains in the thermomechanically affected zone (TMAZ) and coarse grains in the heat-affected zone (HAZ) were observed. The hardness values in the WN were higher than the base material. Eventually HAZ shows lowest hardness values because of few coarse grains presence. Friction stir welded copper joints passes 85% weld efficiency as compared to the parent metal.     

In-situ phase mapping and transformation study in fusion welds

Spatially resolved X-ray diffraction (SRXRD) technique has been applied using synchrotron radiation to map the phases present in fusion welds in situ with a sub-millimetre resolution. For titanium, exhibiting an allotropic transformation from a hcp α-phase to a bcc β-phase at ∼922°C, the following results have been obtained for the heat-affected zone (HAZ). (i) Co-existence of both α- and β-Ti phases in the HAZ as derived from the phase concentration profiles. (ii) The width of the HAZ was found to be 3.33±0.33 mm as defined by the existence of the high-temperature bcc phase in this zone. (iii) Peak profile and ex-situ post-weld ESCA analyses revealed additional hcp patterns attributable to two TiOx phases formed in an overlayer during the welding process by oxygen diffusion from the ambient at high temperature. The thickness of this TiOx overlayer varied from 50–85 nm, increasing towards the weld pool. The SRXRD technique provides real-time chemical dynamics and in situ phase mapping data for modelling of kinetics of phase transformation and microstructural evolution in allotropic and other more complex systems, under steep thermal gradients and non-isothermal materials processing conditions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of filler metals and welding techniques on the structure–property relationships of Inconel 718 and AISI 316L dissimilar weldments

In the present investigation, an attempt has been made to weld the precipitation hardened Nickel based super alloy Inconel 718 and austenitic stainless steel AISI 316L using Continuous Current Gas Tungsten Arc Welding (CCGTAW) and Pulsed Current Gas Tungsten Arc Welding (PCGTAW) process employing ER2553 and ERNiCu-7 fillers. Microstructure examination using optical and SEM analysis clearly witnessed the formation of unmixed zone at the Heat Affected Zone (HAZ) of Inconel 718 for all the joints. The studies showed the absence of deleterious phases in the CCGTA and PCGTA weldments employing ERNiCu-7. Tensile studies corroborated that the fracture occurred at the parent metal of AISI 316L in all the cases. It was inferred from the present study that PCGTA weldments employing ERNiCu-7 exhibited better metallurgical and mechanical properties. This study articulated the effect of filler metals on the structure–property relationships of the weldments.     

Single pass laser joining of Inconel 718 superalloy with filler

Abstract

Microstructure and mechanical property of CO₂ laser beam welded IN 718 superalloy were studied by electron microscopy and hardness testing. The use of a welding filler wire produced a sound fusion zone with no cracking but grain boundary microfissuring occurred in the heat affected zone (HAZ) and was observed to be significantly influenced by pre-weld heat treatment and laser welding speed. Crack-free weld was produced by a pre-weld heat treatment that minimised non-equilibirum grain boundary boron segregation and inhibited grain growth. While post-weld heat treatment (PWHT) reduced the difference between the hardness values of the base alloy, HAZ and the fusion zone, it resulted in increased HAZ cracking, which was likely aided by pre-existing cracks. The PWHT cracking was, however, avoided by subjecting pre-weld material to the heat treatment condition that produces crack-free weld during welding process.     

Effects of weld strength undermatch on fracture toughness of HAZ notched weldments in a HSLA steel

In the present work the effects of weld strength undermatch on fracture toughness of heat affected zone (HAZ) have been studied. In the investigation a high strength low alloyed steel (HSLA) with 800 MPa strength class was used, and the undermatched welded joints were made with two weld strength mismatch levels. Three-point bending test specimens with crack depth to specimen width ratio a/W ranging from 0.05 to 0.5 were extracted from the welded joints. The test results show that strength mismatching gives an obvious influence on the fracture toughness of coarse grained HAZ for the undermatched joints. The lower the weld strength mismatching, the higher the fracture toughness of the HAZ. In addition the tendency of fracture toughness change with crack depths is much the same as in previous studies on base metals or weld metals, that is, fracture toughness of the HAZ is increased with reduction of crack depths. From the measured results it shows that the macroscopically mechanical heterogeneity of the welds may have more important influence on the fracture toughness of the HAZ than the meso-heterogeneity in the reheated coarse grained HAZ. Furthermore, numerical verification indicates that the stress triaxiality at crack tip may be the essential reason for the change of fracture toughness of HAZ. It is also shown that the yield strength of HAZ determined by the limit load in the three-point bend test represents the combinative effects of HAZ and its surrounding materials. This revised version was published online in August 2006 with corrections to the Cover Date.