Showing papers on "Liquation published in 2021"

Effect of Pre- and Post-weld Heat Treatment on Microstructure and Mechanical Properties of GTD-111 Superalloy Welds

Abstract: In this paper, the liquation cracking and strain-age cracking behavior of nickel-based GTD-111 superalloy which is welded by Nd:YAG pulse laser through several heat treatment cycles, has been studied. The effort was to develop the most suitable microstructure concerning shape, morphology and γʹ phase volume fraction by several different heat treatment cycles in order to obtain a weld without any defects. The results revealed that γʹ, γ–γʹ eutectic, MC carbide, Boride enriched with Cr–Mo, and Ni–Zr intermetallic phase are the most important parameters in the formation of grain boundary melt and cracks in HAZ in casting conditions. Before welding, a full solution heat treatment resulted in the omission of liquation cracking, which attributed to full solution of boride and Ni–Zr intermetallic phases, and the solution treatment of a high percentage eutectic phases, and γʹ. Investigations on the hardness of the base metal showed that there was a direct relationship between hardness of the base metal and crack length. In such case, as the hardness of the base metal increases, it’s not only added to absorption of more welding stresses, but also stress release decreased, which resulted in the expansion of cracking in HAZ. Added to this, aging treatment of samples which were undergone aging treatment before welding, resulted in the formation of strain-age cracking due to γʹ phase precipitation.

Relationship between solidification and liquation cracks in the joining of GTD-111 nickel-based superalloy by Nd:YAG pulsed-laser welding

Abstract: Fusion welding of nickel-based superalloys with a total Al and Ti content of more than 6% has always been challenging. The reason for this is the occurrence of solidification and liquation cracks during welding. Investigating the mechanisms of formation of these two types of cracks can increase the theoretical and practical insight necessary to achieve a welded alloy without cracks. The objective of this research was to look at the link between solidification and liquidation fractures in Nd:YAG pulsed-laser welded GTD-111 nickel-base superalloy joints. The reason for choosing the pulsed-laser method is the mechanism of continuous welding of the pulses and the creation of separate microstructural zones due to the high liquation and solidification speed. Liquation cracks in the partially melted zone (PMZ) of base metal were shown to be significantly associated with solidification cracks in the weld metal, according to the findings. Liquation cracks in PMZ act as a strong place to create and extend solidification cracks. It was also observed that with increasing pulse frequency, the tendency to form both types of cracks decreases due to the increased tendency to back-filling.

Evolution of Y2O3 dispersoids during laser powder bed fusion of oxide dispersion strengthened Ni-Cr-Al-Ti γ/γ’ superalloy

Abstract: The successful synthesis of oxide-dispersion-strengthened (ODS) alloys via laser powder bed fusion ( L -PBF) requires a better understanding of the interaction of the oxide dispersoids with the metallic melt pool. Here, a γ/γ’-strengthened Ni-8Cr-5.5Al-1Ti (wt%) model alloy is studied, as a simplified version of the commercial CM247LC alloy, by melting pre-alloyed powders in which 0.5–1 wt% Y2O3 nanoparticles were added via mechanical alloying. The Y2O3 nanoparticles follow three distinct paths. First, the strong affinity between Y2O3 and Al leads to the formation of Y4Al2O9 slag which floats on the melt pool; if in excess, the slag leads to vertically aligned mm-size cavities, preventing complete consolidation of the alloy. Second, a high number density of oxide nanodispersoids is distributed within the alloy’s grain inducing a strong (100) texture and noticeably reduces grain size compared to the unmodified base alloy. Third, despite the high stability of Y2O3, the extreme temperatures achieved in the melt pool decompose some of the Y2O3 precipitates leading to the formation of Ni- and Y-rich particles (16 nm in radius) and Y segregation to the alloy’s grain boundaries. The local composition on cracked grain boundaries is consistent with Ni17Y2 having an embrittling and liquation effect. Based on these results, the critical role of Al in reacting with oxide nanodispersoids during L -PBF manufacturing is discussed, and various types of potentially more successful dispersoids are suggested.

Suppression of liquation cracking susceptibility via pre-weld heat treatment for manufacturing of CM247LC superalloy turbine blade welds

Abstract: In this study, the weldability of as-cast CM247LC superalloy was metallurgically evaluated for turbine blade applications in terms of its hot cracking behavior and susceptibility. For this purpose, a real blade was manufactured using a directional solidification casting process, and gas tungsten arc welding was performed at the tip and cavity of the upper blade. Hot cracking was confirmed in the heat-affected zone of the gas tungsten arc welds, and the cracks were characterized as liquation cracks. Metallurgical solutions to suppress the liquation cracking susceptibility were derived via visualization-based spot-Varestraint test. The alloy subjected to aging treatment exhibited the lowest liquation cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC welds were elucidated via microstructural analyses and thermodynamic calculations. The suppressed liquation cracking susceptibility of the aged CM247LC could be explained as follows: (i) reduced MC-type carbide, which lowered the local solidus temperature compared with the equilibrium solidus temperature of CM247LC and (ii) increased solidus temperature owing to γ’ precipitation γ’ within the γ matrix. To validate the suppressed liquation cracking susceptibility of the aged CM247LC specimen in real welding, it was subjected to gas tungsten arc welding. The results indicated considerable suppression of liquation cracking compared with as-cast welds; in particular, the reduced liquation cracking temperature range was well-reflected in the welding.

Predicting Susceptibility to Solidification Cracking and Liquation Cracking by CALPHAD

Abstract: In welding, liquation cracking can occur in the partially melted zone, leaving open cracks along the edge of the weld bead. Likewise, solidification cracking can occur in the mushy zone, leaving open cracks inside the weld bead (which is called the weld metal or fusion zone). The present study aims at demonstrating that CALPHAD-based modeling can help predict the susceptibility of alloys to both types of cracking. The basic relationship between temperature T and the fraction of solid fS of an alloy can be calculated using thermodynamic software and a database based on the alloy composition. For liquation cracking the T-fS curve of the weld metal can be compared with that of the workpiece to assess the susceptibility. For solidification cracking, on the other hand, the T-(fS)1/2 curve of the weld metal can be used to calculate the susceptibility. The composition of the weld metal depends on the compositions of the workpiece and the filler metal, and the percentage of the workpiece in the weld metal (called dilution). The susceptibility predictions based on these curves and comparison with welding experiments will be demonstrated using Al alloys, Mg alloys, and carbon steels as examples.

Effects of heat input on metallurgical behavior in HAZ of multi-pass and multi-layer welded IN-939 superalloy

Abstract: The hot components of a gas turbine are susceptible to damage in the high-temperature environment of turbine engine operation. Given that these components are relatively costly to manufacture, they are often repaired than replaced when damaged. Fusion welding is an economical method for repairing the damaged components of a gas turbine. This research examines the roles of heat input, pass number and layer number on the intergranular liquation cracking of the Inconel-939 (IN-939) precipitate nickel base superalloy during tungsten arc welding. Several specimens were welded with IN-625 filler alloy under argon gas by following the Taguchi method and an L8 array. These specimens were then characterized via metallographic investigations and hardness measurements. Results show that, during welding, the IN-939 heat-affected zone (HAZ) is acutely prone to cracks that propagate along the grain boundaries. Moreover, layer number, heat input and pass number significantly influence the microstructure and liquation cracking of HAZs with impact percentages of 72.37%, 22.17% and 4.84%, respectively.

Liquation cracking in Inconel 617 alloy by Laser and Laser-Arc Hybrid welding

Abstract: The present study investigates susceptibility of Inconel 617 to liquation cracking in laser-based welding techniques such as autogenous laser welding (ALW) and laser-MIG hybrid welding (LHW) with v...

The role of liquid feeding in nugget-edge cracking in resistance spot welding of dissimilar magnesium alloys

Abstract: In fusion welding, cast magnesium alloys typically present higher sensitivity to liquation cracking than wrought alloys. In this study, a fluid-mechanics-based analytical model was established to investigate an unusual phenomenon: a lack of cracking in cast magnesium alloy while cracking took place in wrought magnesium alloy. This was observed in resistance spot welding, in which the pressure within the molten nugget allows the liquid to feed through the interface between the fusion zone and the partial melting zone, an aspect which is not generally considered in the typical criteria for liquation cracking. The analytical model shows that the liquid is easy to feed into the inter-grain zone of a cast coarse-grained metal but difficult to feed into wrought metal. The liquid feeding effect changes the composition of the inter-grain region, causing the semi-solid grains to bond to each other tightly in the cast magnesium alloy. Electro-probe microanalysis illustrates the mass transfer pattern of liquid feeding at the interface between the fusion zone and the partial melting zone. Magnesium alloys AZ31, AZ91, and ZK61 were selected to carry out resistance spot welding tests of which results match the theory raised in this work well.

Influence of homogenisation treatments on the hot ductility of cast ATI® 718Plus®: Effect of niobium and minor elements on liquation characteristics

Abstract: The hot ductility of cast ATI® 718Plus® was assessed using the Gleeble thermo-mechanical simulator after being subjected to different homogenisation heat treatments. The hot ductility deteriorated significantly after long-dwell homogenisation heat treatments for 24 h at temperatures of 1120 and 1190 °C as compared with those treated at a short dwell time of 4 h at the same temperatures. The observed ductility deterioration was related to more extensive liquation along the grain boundaries caused by different mechanisms, e.g., liquation by solute segregation mechanism, Laves melting, constitutional liquation of MC carbides and supersolidus grain boundary melting, with the effect and extent depending on the solute changes after the homogenisation heat treatments. Furthermore, the role of Nb as the solute element and as the precipitate former, as well as the effect of minor alloying elements segregating along the grain boundaries, is discussed in connection to grain boundary liquation, which contributes to a better understanding of heat-affected zone liquation cracking susceptibility of cast ATI® 718Plus®.

Evaluation and Control of Liquation Cracking Susceptibility for CM247LC Superalloy Weld Heat-Affected Zone via Visualization-Based Varestraint Test

Abstract: The metallurgical aspects of weld cracking in Ni-based superalloys remain relatively unexplored in existing research. The present study performed comprehensive metallurgical and manufactural investigations into the weldability of an Ni-based superalloy, CM247LC, from the viewpoint of the liquation cracking behavior and its susceptibility. Metallurgical solutions to suppress the liquation-cracking susceptibility were derived via the visualization-based Varestraint test, and the possibility of liquation crack-free welding was explored by employing pre-weld heat treatments and laser beam welding. The alloy that was subjected to aging treatment exhibited the lowest liquation-cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest liquation-cracking susceptibility (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC weld were elucidated via microstructural analyses and thermodynamic calculations. The suppressed liquation cracking susceptibility of the aged CM247LC can be attributed to the MC-type carbide fraction and homogenized matrix phase, as compared with those of as-cast CM247LC. The aged CM247LC specimen was subjected to gas tungsten arc welding to validate its minimal liquation-cracking susceptibility. The results confirmed the suppression of liquation cracking, due to the low susceptibility of the specimen. However, crackfree welds could not be obtained. Finally, metallurgically sound welds without liquation cracks were successfully obtained via laser beam welding. The outcomes of the present study will facilitate the generation of electric power from fossil fuels via a clean and efficient gas turbine-based power generation cycle.

Prospects of pulsed current arc welding on aerospace grade Hastelloy X

Abstract: The conventional constant current arc welding of Hastelloy X (Ni-Cr-Fe-Mo) leads to the solidification and liquation cracks in the weldment. The higher heat supplied in constant current weldment de...

The role of shot peening on liquation cracking in laser cladding of K447A nickel superalloy powders over its non-weldable cast structure

Abstract: A novel strategy with shot peening (SP) pretreatment was proposed to prevent heat-affected zone liquation cracking during laser cladding of K447A nickel-based superalloy powder over its non-weldable cast structure. High dislocation density accumulated by the SP pretreatment drove the high-temperature zone of the heat-affected zone to recrystallize during the laser cladding process, as a result, the coarse columnar grain with an average dendrite stem and secondary dendrite arm width of 102.67 ± 4.24 μm was transformed into a fine equiaxed grain with an average size of 10 μm. SP could effectively inhibit liquation cracking. With the increase of SP duration, the tendency of the liquation cracking reduced. Compared with no SP BM, the total length of the liquation cracking in the longitudinal section of the laser cladding sample reduced from 1649 μm to 248.8 μm. In-situ experimental observation and process simulation were opted to study the evolution behavior of recrystallization. Recrystallization firstly developed in the region of the intergranular zone at ~1214 °C and gradually the base metal recrystallized between 1232 and 1242 °C except for the region with γ/γ′ eutectic structure. As the temperature rose further, a fine liquid film network developed in the range of 1260–1267 °C. Finally, the stress and strain-based criteria were exercised to evaluate the crack susceptibility of the liquid film. The fine liquid film network adequately constrained the liquation cracking by reducing the driving stress applied to the liquid film as well as by lowering the pressure drop of the liquid film caused by driving strain.

Metallurgical Assessment of Additive Manufactured Nickel Aluminum Bronze-316L Stainless Steel Bimetallic Structure: Effect of Deposit Geometry on the Interfacial Characteristics and Cracking

Abstract: Wire-arc additive manufacturing (WAAM) technique was adopted to fabricate bimetallic parts by depositing nickel aluminum bronze (NAB) in two different shapes (cylindrical rod and square bar) on a 316L stainless steel substrate. Crystallography and characteristics of the interfacial layer in both parts were investigated using energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and nanoindentation techniques. Within the first layer of the deposit, several rosette-like κI particles were formed in the cylinder-NAB, whereas it is not the case in the square-NAB. In both parts, a well metallurgically bonded interface without any pores and cracks with a thin (2 μm) intermetallic layer was formed along with the interface. This interface layer is a FeAl-based intermetallic in the cylinder-NAB part, whereas it is Fe3Al intermetallic phases in the square-NAB part, as confirmed by EDS and nanohardness analysis. Occasional liquation cracks and infiltration of NAB were observed on the austenitic grain boundaries in the heat-affected zone (HAZ) of 316L. The interface did not exhibit any strong texture orientation owing to the thermal gradient’s control, as NAB is more conductive than 316L. In this work, for both parts, the role of grain boundary misorientations across different grains of the interfacial layer and HAZ cracks and relative to the deposited NAB and stainless steel substrate was studied. The cylinder-NAB sample consisted of a higher fraction (75%) of low Σ3 coincident site lattice (CSL) boundaries compared to the square-NAB sample (55%). The findings were discussed in the context of the formation mechanism of the intermetallic layer.

Phase Relations in the Fe–S–C System at P = 0.5 GPa, T = 1100–1250°C: Fe–S–C Liquation and Its Role in the Formation of Magmatic Sulfide Deposits

Abstract: The liquid phases are represented by immiscible Fe–S and Fe–C melts under partial melting of the graphite-saturated Fe–S–C system at P = 0.5 GPa and T = 1150, 1200, and 1250°C. Fe, Ni, Pt, and Au are predominantly concentrated in the Fe–C melt, and Cu, Pd, and Ag occur in the Fe–S melt as a result of fractionation of elements between coexisting liquids. The features of the geological setting of the formation of magmatic sulfide deposits in the Norilsk region and the chemical composition of ores indicate the contamination of the Fe-sulfide melt with carbon with liquation into Fe–C and Fe–S liquids coexisting with the silicate melt. The enrichment of the ore-forming Fe-sulfide melt of Cu, Pd, and AgC is associated with the redistribution of ore-forming elements between them, thereby providing the Pd–Cu specialization of sulfide ores in the Norilsk region.

Nano-carbide strengthened as-welded joint for precipitation-hardened austenitic Fe–Mn–Al–C lightweight alloys

Abstract: To date, fusion welding the precipitation-hardened austenitic Fe–Mn–Al–C lightweight alloys still remains an insurmountable challenge. In the present study, the viability of using a Fe-28.3Mn-10.2Al-1.62C (in wt.% hereafter) as a welding filler wire for gas tungsten arc welding (GTAW) of as-hot-rolled Fe-29.2Mn-8.8Al-1.65C base material (BM) were systematically investigated. It was striking that in the as-welded condition, a high density of nano-sized κ-carbide precipitates was formed by spinodal decomposition within the significantly refined austenite dendrite cells in the fusion zone (FZ). This unique feature is critical to achieve a high weld strength in virtually all fusion welded precipitation-hardened alloys. In the heat-affected zone (HAZ), the strengthening nano-sized κ-carbides originally existed in the BM was also preserved with no sign of dissolution or noticeable coarsening. Consequently, the as-welded joint exhibited a fairly uniform microhardness across the FZ, HAZ and BM. More significantly, the entire as-welded joint is free of any solidification cracking and/or liquation cracking. The as-welded joint also exhibited an excellent combination of yield strength, ultimate tensile strength and ductility.

Heat-affected zone cracking in nickel-based superalloys and the role of minor elements

Abstract: The formation of heat affected zone cracking in fusion-welded materials is a major concern in the design and manufacture of nickel-based superalloy welded assemblies. It is a general weldability problem that affects a large number of advanced highly alloyed cast and wrought nickel base superalloys, particularly, those strengthened by ordered L12 intermetallic Ni3(Al, Ti or Ta) γ′ precipitates. While the problem of fusion zone cracking is also encountered in many of these alloys, it does not pose as great a challenge as HAZ liquation cracking because it can be essentially managed effectively by proper selection of filler materials and appropriate welding procedures. HAZ liquation cracking is, however, more insidious since the factors and phenomena contributing to its occurrence are often related to the composition of the material and its microstructure, both of which have been optimized to achieve desirable high temperature base metal properties. The HAZ cracking in the superalloys is generally intergranular and it usually associated with the formation of liquid film on HAZ grain boundaries during welding. The inability of this film to accommodate thermally and/or mechanically induced stresses experience during cooling results in grain boundary microfissuring through decohesion along one of the solid-liquid interfaces on the grain boundary and, thus, it is sometimes referred to as liquation cracking, hot cracking or hot tearing. Liquid film stage is the common element in various manifestations of hot tear near the complete solidification point of metals. The cooling cycle of HAZ intergranular liquid is somewhat similar to the final stages of solidification of castings and fusion zone in welds, hence, to a first approximation, the criteria that govern weld solidification cracking can be adopted to explain liquation cracking in the HAZ of weldments, and these are considered in this chapter.

Influence of Technological Factors and Long-term Operation on the Microstructure and Properties of Heat-resistant Materials

Abstract: In the manufacture of heat-resistant alloys, special melting methods are widely applied: in vacuum, in a protective atmosphere using electroslag and vacuum arc remelting, using various deoxidizers, small additives, including rare earth elements. For parts made by precision casting, the method of filling and crystallization is essential. Melting and casting methods affect the properties of the metal both before and after hot working (forging, rolling, heat treatment). It has been found that the methods of melting and casting affect the content of gases, various oxides in the form of films, non-metallic inclusions, harmful impurities (As, Pb, Bi), usually chemically undetectable in the metal, as well as the size of inclusions, their distribution within the grain and porosity. Vacuum remelting affects the anisotropy of the properties, the amount and nature of the distribution of non-metallic inclusions, hardenability, transition temperature of brittleness, and especially the liquation inhomogeneity of the metal.

An Experimental Study of the Melt Immiscibility and Distribution of Rare Earth Elements, Р2О5, and Nb2O5 between Immiscible Melts in the Monazite–SiO2–NaF–Nb2O5 System

Abstract: This study was made with the aim of developing a process for liquation fusion of rare earth–rare metal ocherous ores of carbonatite weathering crust deposits. The formation of two immiscible melts, silicate and phosphate–salt, in the monazite–SiO2–NaF–Nb2O5 system was studied. The melt liquation field in the isotherm occupies 50% of the diagram area at 1200°C, 39.0% at 1100°C, 20.0% at 1000°C, and <5% at 900°C. The silicate melt can be readily quenched into a glass containing fine spherical droplets of the phosphate–salt melt. The phosphate–salt melt is always in the crystalline state. The phosphate–salt melt and phosphate–salt spheres contain double phosphates and fluoride phosphates of rare earth elements (REEs) and sodium. Nb2O5 concentrates in the silicate melt. REE oxides, Nb2O5, and other components are distributed between the two coexisting melts in the contrast fashion. Rare earth oxalate and hydroxide concentrates were obtained from the phosphate–salt melt, and commercial niobium product containing ≤0.50% Р2О5, from the silicate melt.

The Impact of Casting Conditions on Edge Cracking of AA5182 Ingots During Hot Rolling

Abstract: The hot rolling of large ingots is the predominant process for producing plate, sheet, and foil. During the hot rolling of aluminum/magnesium alloys, edge cracking tends to dramatically increase with increasing magnesium concentration, often resulting in significant unplanned scrap generation. It has been observed that significant cracking most frequently occurs in the start-up region of the ingot, which prompted the current investigation into the impact of casting conditions on edge cracking. By taking samples from the start-up and steady-state regions of AA5182 ingots and subjecting them to different simulated pre-heat rates, we were able to observe a noticeable difference in liquation and void formation behavior on the ingot surface. For a given casting condition, there appears to be a threshold pre-heat rate beyond which significant void formation occurs. Our investigation indicates that the start-up region of DC cast ingots has a lower threshold temperature, thus leading to increased edge cracking.