Constitutional Liquation Research Articles

Weldability of new Ni-based superalloy G27: Effect of pre-weld solution annealing on the hot cracking susceptibility

The hot cracking susceptibility of a new superalloy G27 was investigated by Varestraint weldability testing. The influence of pre-weld solution annealing conditions recommended by the alloy manufacturer to obtain different grain sizes (1010 °C/1 h, 1066 °C/0.5 h, and 1110 °C/1 h) for specific applications was studied. The fusion zone solidification cracking was independent of pre-weld solution annealing, and they are associated with the γ/Laves eutectic constituents. The solution-annealed material at 1010 °C/1 h showed the lowest extent of liquation cracking, while the 1110 °C/1 h condition was the most susceptible. The increased level of liquation cracking was primarily attributed to an increased grain size of pre-weld solution-annealed material. The heat-affected zone liquation cracking was identified to be caused by constitutional liquation of NbC and supersolidus melting.

The role of the microstructural changes during induction preheating on the HAZ liquation cracking susceptibility of Ni-based superalloy

This work presents the influence of high-frequency induction preheating (900, 1000, 1100 °C) on liquation crack formation in the René 108 Ni-based superalloy. The investigation was divided into two parts: (1) characterization of the material's microstructure after preheating and (2) determining the influence of preheating on liquation cracking during autogenous gas tungsten arc welding. During preheating, the dissolution of γ′ precipitates showed accelerated progress with increase in temperatures. This dissolution involved the continuous thinning of each precipitate, as well as more intricate mechanisms, such as splitting. The mean size of the secondary γ′ decreased from 0.32 to 0.26 μm. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The formation of a thin non-equilibrium liquid film along high-angle grain boundaries led to the crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products were visible on the crack edges independently of preheat temperature. Preheating at 900 °C decreased the length and amount of liquation cracks, while preheating at 1100 °C allowed to prevent them due to the liquid-healing effect.

Weldability study of 304HCu stainless steel using varestraint and “Gleeble” based hot ductility tests

The susceptibilities of 304HCu SS to solidification and liquation cracking are assessed in the current work and the underlying mechanisms are clarified. The development of γ + Nb4C3 eutectic along the interdendritic areas in the fusion zone is attributed for solidification cracking in this stainless steel. To forecast and rank the susceptibility of an alloy to solidification cracking, a novel technique is put forth and experimentally confirmed. Using the measured temperature gradient, welding speed and Scheil's solidification module, a relationship between the liquid fraction and mushy zone width is derived. Liquation cracking susceptibility evaluated using “Gleeble™” based hot ductility test is observed to be high which is attributed to the constitutional liquation of primary Nb(C,N) and secondary M23C6 phases present in the base metal. Further, the influence of phosphorus on the local solidus temperature in the partially melted zone of the alloy is discussed in the context of liquation cracking.

Effect of friction stir welding parameters on microstructure and mechanical properties of the dissimilar alloys of AZ91D and AA7075

Friction stir welding is an efficient joining process for light metal alloys, offering benefits such as high joint efficiency and reduced grain sizes. This work is to investigate the process parameters that can improve the welding quality and reduce the brittle intermetallic compounds, including Al3Mg2 (β) and Al12Mg17 (γ) in the welded region. It was perceived that the process parameters influenced the morphological characteristics and input heat, thereby affecting the materialization of intermetallic compounds. The primary diffraction peaks of both Al and Mg were present in all the joints. Two specific intermetallic phases, Al3Mg2 and Al12Mg17, were formed as a result of the diffusion of Al and Mg atoms during welding. These intermetallic compounds were found in all the joints. The Al12Mg17 phase, when mixed with the Mg phase, tended to migrate towards the weld surface where the AZ91D plate was positioned. This migration might lead to constitutional liquation, which refers to the melting of specific phases during solidification, resulting in the materialization of eutectic microstructures. The highest ultimate tensile strength (UTS) value of 116.64 MPa was achieved with a rotational tool speed of 800 r/min and a traverse speed of 20 mm/min. On the other hand, the lowest UTS value of 68.32 MPa was observed with a rotational tool speed of 400 r/min and a traverse speed (TS) of 20 mm/min. It was noted that lower UTS values were associated with the materialization of dense layers of intermetallic compounds (Al12Mg17 and Mg + Al12Mg17) in the stir zone when using lower rotational tool speed and TS. It suggests that improper processing parameters can lead to the materialization of intermetallic compound in the stir zone, which subsequently decreases the mechanical properties of the weldments.

Influence of rotational tool speed on metallurgical characterization of friction stir welded joint of AZ91D and AA2024

This study investigated the dissimilar friction stir welding (FSW) of AA2024 and AZ91D and aimed to explore the effect of rotation tool speed (RTS) on welding quality and the formation of intermetallic compounds (IMCs) in the welded region. The optimal welding parameters for achieving a sound weld were determined to be a traverse speed (TS) of 35 mm/min and an RTS of 700 r/min. The presence of two specific intermetallic phases, Al3Mg2 and Al12Mg17, was observed in all joints, with Al12Mg17 showing a tendency to migrate toward the weld surface adjacent to the AZ91D plate, possibly resulting in constitutional liquation during solidification. The results revealed that the morphological characteristics and input heat significantly influenced the formation of IMCs. Moreover, the study highlighted that improper selection of processing parameters, particularly using lower RTS and TS, led to the development of dense layers of IMCs, such as Al12Mg17 and Mg + Al12Mg17, within the stir zone (SZ). Consequently, this led to a decrease in the ultimate tensile strength (UTS) of the FSWed joints. The highest UTS value of 147.26 MPa was achieved with an RTS of 700 r/min and a TS of 35 mm/min, while the lowest UTS value of 113.47 MPa was observed at an RTS of 500 r/min and a TS of 35 mm/min.

Evolution of microstructure in nickel-based C-HRA-2 alloy during welding thermal simulation

The welding heat-affected zone (HAZ) of C-HRA-2 nickel-based alloy at different welding peak temperatures (Tp) (850 to 1450 °C) was obtained through welding thermal simulation. The microstructure of the different simulated HAZs was characterized by an optical microscope (OM), a scanning electron microscope (SEM), and a transmission electron microscope (TEM). The microhardness of simulated HAZs was examined using a microhardness tester. The results indicated that fine and dispersed M23C6 carbides were identified along the grain boundaries in the simulated HAZs at 1050 and 1150 °C. In the simulated HAZs above 1250 °C, eutectic microstructure of γ matrix bonded with M23C6 carbides was identified close to the grain boundaries due to constitutional liquation. The liquefaction of γ matrix and M23C6 carbides primarily accounted for the simulated HAZ liquation of C-HRA-2 alloy during continuous welding heating. The content of the liquefied phase was increased as Tp was increased. The liquefied phase was converted into the eutectic microstructure of γ matrix and M23C6 carbides in the cooling process. Besides 1050, 1150, and 1450 °C, the microstructures under other Tp were similar to the microstructure of the Base Metal (BM). The microhardness distribution of simulated HAZs tended to be higher in the middle and lower at both ends with the increase of Tp. Furthermore, the result indicated that the microhardness of the simulated HAZ was the closest to BM at 1050 °C.

The role of the strengthening phases on the HAZ liquation cracking in a cast Ni-based superalloy used in industrial gas turbines

This work presents the influence of microstructural constituents on liquation crack formation in the cast Ni-based superalloy, René 108. The investigation was divided into three parts: characterisation of the material's microstructure in pre-weld condition, hot ductility studies and analysis of liquation cracking induced by the gas tungsten arc welding process. Using advanced electron microscopy techniques it is shown that the base material in pre-weld condition is characterised by a complex microstructure. The phases identified in René 108 include γ matrix, γ' precipitates, MC and M23C6 carbides, and M5B3 borides. Based on Gleeble testing, it was found that René 108 is characterised by high strength at elevated temperatures with a maximum of 1107 MPa at 975 °C. As a result of constitutional liquation, the superalloy’s strength and ductility were significantly reduced. The nil strength temperature was equal to 1292 °C, while the nil ductility temperature was 1225 °C. The low ductility recovery rate (32.1), ratio of ductility recovery (36.2) and hot cracking factor (Rf = 0.05) values confirmed the low weldability of Renѐ 108. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The thin non-equilibrium liquid film, which formed along high-angle grain boundaries, led to crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products are visible on the crack edges.

Liquation crack-free welding strategy for 247LC DS superalloy by control of pipeline diffusion via ultra-high-speed laser beam scanning

Under completely suppressed constitutional liquation of MC carbide, unexplored 247LC superalloy single-mode fibre laser welds experience unexpected liquation cracking. These cracks continuously propagate from a solidification crack, particularly at highly misoriented and epitaxially grown solidification grain boundaries, and dominated by pipeline diffusion of impurities. Two novel metallurgical preconditions were identified which, if satisfied, ensure complete suppression of liquation cracking of 247LC superalloy welds. These preconditions are (i) the constitutional liquation of MC carbide at the heat-affected zone and (ii) pipeline diffusion of impurities at the fusion zone. It is also highly recommended that the welding is carried out in a single directionally solidified grain under scanning conditions, avoiding the formation of highly misoriented solidification grain boundaries.

Liquation-induced loss of ductility in AA7075 aluminium alloy fusion welds

This paper investigates the effect of liquation in the partially melted zone (PMZ) on the stress–strain behaviour of the AA7075 arc fusion welds. It is identified that the presence of micro-sized Cu-rich second phases in the initial microstructure of the AA7075 is responsible for constitutional liquation in the PMZ. Although the liquation cracking can be avoided using the proper selection of filler metal, it is demonstrated that the formation of brittle Al2Cu-type intermetallic-containing eutectic structure resulting from non-equilibrium solidification of the liquated grain boundaries can induce a severe loss of ductility in the PMZ. This calls for proper strategies to minimise the grain boundary liquid film thickness to produce a more reliable fusion weld on AA7075 alloy.

Keyhole TIG welding of newly developed nickel-based superalloy VDM Alloy 780

The influence of keyhole TIG (K-TIG) welding parameters on the weld geometry and defects of a new nickel-based superalloy VDM Alloy 780 alloy were investigated using a statistical design of experiment, and the microstructures of the heat-affected zone (HAZ) and fusion zone (FZ) of the K-TIG-welded VDM Alloy 780 were characterized. No cracks are found in the FZ and HAZ. Travel speed significantly influences the minimum weld width (Wm), face and root excess weld metal, face underfill, and average pore diameter. Welding current and the interaction current*travel speed significantly influence the face and root excess weld metals, respectively. Interdendritic microconstituents in the FZ are identified as (Nb,Ti)C particles and γ/Laves eutectic constituents based on SEM–EDS analysis. In addition, plate-like precipitates likely to be δ/η phase are observed surrounding the interdendritic Laves eutectics, and γ′ precipitates are found to be inhomogeneously precipitated in the FZ. In the partially melted zone (PMZ), (Nb,Ti)C, Laves phase, and plate-like particles are found in the liquated and resolidified regions. Constitutional liquation of (Nb,Ti)C in HAZ is observed where the liquated (Nb,Ti)C appears to be associated with the γ/Laves eutectic as a resolidified product.

On the Rate Dependence of Precipitate Formation and Dissolution in a Nickel-Base Superalloy

The temporal dependence of gamma ' dissolution in the polycrystalline Ni-base superalloy RR1000 has been studied with implications to thermo-mechanical processing. A resistivity-based method using an electro-thermal mechanical testing (ETMT), which overcomes the drawbacks associated with other approaches, such as calorimetry, dilatometry, and diffraction, has been used to explore the effect of transient and isothermal thermal cycles. This is supplemented by DICTRA numerical models that simulate the diffusion within the gamma phase up to the gamma /gamma ' interface. It is demonstrated that dissolution is affected by heating rate as well as the precipitate size. Below a threshold heating rate of sim 0.1 {^circ }C s^{-1}, the dissolution kinetics are marginally affected, however, is sensitive to microstructure. The role of precipitate size during dissolution is governed by diffusion flux in the gamma phase at the gamma /gamma ' interface, which is inversely proportional to size. It is argued that numerical simulations that predict constitutional liquation during rapid heating by altering the width of the computation domain to match the average precipitate size of the gamma ' population will yield inaccurate predictions. The influence of the heating rate on the nucleation undercooling, during subsequent cooling, has also been addressed. With increasing heating rates, the local gamma ' solvus temperature is shifted to progressively higher temperatures. Unless complete dissolution of gamma ' occurs prior to subsequent cooling, erroneous interpretations of nucleation undercooling can arise.

The Effects of Postweld Processing on Friction Stir Welded, Additive Manufactured AlSi10Mg

Given the limited build volumes for most current additive manufacturing (AM) machines, a method for taking advantage of the unique capabilities offered by AM while combining it with traditional manufacturing methods is needed. Welding of AM-produced components is a solution to this challenge. The objective of this study was to determine the feasibility of using friction stir welding (FSW) to join AlSi10Mg melted with laser powder bed fusion (PBF-L) and examine the effects of postweld heat treatment (HT) and hot isostatic pressing (HIP) on overall joint quality and mechanical performance. Samples were examined using optical microscopy, scanning electron microscopy, hardness testing, and tensile testing. Examination of the samples that underwent a postweld annealing HT revealed cracking along the stir zone and the thermomechanically affected zone (TMAZ) boundary. Examination of the crack revealed evidence of liquation near single-phase silicon precipitates in the TMAZ despite the annealing temperature being 27°C (81°F) below the solidus temperature of the material according to the material specification. Using a calculated pseudobinary phase diagram of AlSi10Mg, the annealing HT was determined to be in the partial liquation regime for AlSi10Mg. The voids and crack formation mechanisms were determined to be caused by constitutional liquation coupled with the unique TMAZ microstructure and stress state. The as-welded and HIP coupons were void and defect free, and FSW was determined to be a feasible method of joining PBF-L aluminum alloys with minimal knockdown in tensile strength.

Contribution of constitutional liquation of the segregation phase to improve the electrochemical performance of Al–Sn based anodes

A liquid segregation phase was constructed in an aluminum anode to improve the electrochemical performance of Al–air batteries.

Influence of strain, temperature, and strain rate on interfacial structure and strength of AZ31BMg/6063Al formed by plastic deformation bonding

Strain, and temperature, and strain rate, are critical parameters in many joining and forming processes of Al/Mg alloys hybrid structures/components, but the relation between those parameters and interfacial bonding strength remains to be quantified. In the present study, influences of true strain (0.29−0.92), strain rate (10−3-10 s-1), and temperature (325–443 ℃) on microstructure and interfacial bonding strength of AZ31BMg/6063Al formed via plastic deformation were studied, and mechanism-based constitutive equations describing the relation between those parameters and interfacial bonding strength were first developed. Mg and Al alloys cannot be bonded at true strains 0.29 and 0.51 lower than theoretical threshold true strains; while metallurgical bonding appears at Al/Mg interfaces as the true strain is increased to 0.92. When the bonding interface is not fully bonded, a decreased strain rate or an increased temperature helps to improve the metallurgical bonding area, resulting in the enhancement of bonding strength. However, once the bonding interface has been fully bonded, a decrease in strain rate or an increase in temperature leads to an increase in intermetallic compounds (IMCs) thickness, thus almost linearly reducing bonding strength. The IMCs thickness has an exponential relation with time and an Arrhenius relation with temperature. The interfacial bonding strength is governed by both the bonding degree and IMCs thickness. The combination of a large strain rate and a high temperature (below constitutional liquation) helps to form a fully bonded interface with a small IMCs thickness, thus obtaining the maximum bonding strength. The present research findings and developed models provide guidance for parameters optimization in joining or forming processes of Al/Mg alloys hybrid components via plastic deformation.

Microstructural evolution and liquation cracking in the partially melted zone of deposited ERNiCrFe-13 filler metal subjected to TIG refusion

The microstructure of ERNiCrFe-13 multipass weld metal has been shown to contain Laves/γ or σ/γ eutectic constituents that can increase susceptibility to solidification and weld metal liquation cracking resulting from the low eutectic reaction temperature. Under poor heat dissipation conditions such as on the edge of large thickness welded components, a partially melted zone (PMZ) may form in the weld metal during multipass welding. The microstructural evolution and liquation cracking susceptibility of this PMZ in ERNiCrFe-13 multipass welds have received little attention. In the present study, a tungsten inert gas (TIG) refusion process is used to simulate a thermal cycle with a long elevated temperature dwell time in order to investigate the microstructural evolution and liquation cracking in the weld metal PMZ. The results show that the eutectic microstructures in the PMZ evolve into three eutectic morphologies after TIG refusion, including long linear chains extending perpendicular to the boundary between the refusion zone and PMZ, skeletal structures, and fine lamellar networks. This evolution contributes to constitutional liquation occurring at the γ/Laves and γ/σ interface. Nb and Mo play a leading role in the constitutional liquation of γ/Laves and γ/σ eutectic microstructures, respectively. Liquation cracking in the PMZ is shown to occur along the linear chain grain boundaries resulting from constitutional liquation.

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 variation in heat input. Advanced characterization techniques such as high-speed Nano-Indentation testing and micro X-ray diffraction techniques were utilized to analyze microstructural variations in correlating mechanical behavior of low-melting eutectics formed in partially melted zone (PMZ). Results indicated that the liquation cracking in PMZ was associated with the constitutional liquation reaction between intermetallic carbides and austenite matrix. With increase in heat input, its susceptibility reduced, although with enhancement in grain-boundary liquid film formation and was associated with the constitutional liquation of (Cr, Mo)-rich M23C6 carbides and γ matrix. Combined effect of induced thermal stress with presence of low-melting eutectics in partially melted zone was found to greatly influence liquation-cracking susceptibility in both the welding processes. Indeed, ternary eutectic comprising metallic carbides of Cr and Mo and austenite-matrix with low nano-hardness values compared to that of hard carbides present in base metal and weld fusion zone re-confirmed their influence on cracking susceptibility. Increase in heat input enhanced dissolution of Mo-rich M6C carbides that attributed to stress-relaxation and thereby reduce cracking propensity.

Evaluation of Liquation Cracking Behavior and Susceptibility in Heat-Affected Zone of CM247LC Superalloy Welds for Turbine Blade Application

In this study, the weldability of the as-cast CM247LC superalloy for turbine blade applications was metallurgically evaluated 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 (HAZ) of gas tungsten arc welds, and the cracks were characterized as liquation cracks, since a cobble or dropletshaped crack surface consistent with a liquid film was clearly confirmed. Microstructural analysis of the cracking surface and thermodynamic calculations helped elucidate the metallurgical mechanisms of the liquation cracking. In other words, the cracking was attributed to liquation of the γ-γ’ eutectic colony and the constitutional liquation of the MC-type carbides: these phases existed in the as-cast microstructure. In particular, it was calculated that liquation of the γ-γ’ eutectic colony during welding occurs at least at 1488 K and that constitutional liquation of MC-type carbides begins at 1411 K, while the equilibrium solidus temperature of the CM247LC alloy is 1530 K. Finally, the liquation cracking susceptibility was quantitatively evaluated through a spot-Varestraint test, and it was confirmed for the first time that the higher susceptibility of as-cast samples can be suppressed by employing a pre-weld heat treatment such as solution treatment.

The role of the pulsed-wave laser characteristics on restraining hot cracking in laser cladding non-weldable nickel-based superalloy

A paraxial powder-feeding system by employing a quasi-continuous fiber laser machine was established to repair non-weldable nickel-based K447A alloy. In the present study, the duty cycle (DC) of the pulsed-wave laser was chosen to reveal the influence on hot cracking. The results showed that the total length of hot cracking on the 10.5 mm-length longitudinal section of the cladding zone reduces from 3.263 mm to 0.092 mm with the decline of DC from 80% to 30%. The intermittent occurrence of the laser beam induced the ripple microstructure in the cladding layer (CL). It is the high cooling rate in laser extinguishment process that caused the fine dendrite zone which provides more resistance for hot cracking propagation in the CL. The liquid film in the heat-affected zone (HAZ) mainly results from the M5B3/γ, γ′/γ, and MC/γ constitutional liquation successively among 1200–1300 °C. Heat input is the main factor affecting hot cracking in the HAZ. With the decrease of DC, the dendrite spacing of the fine dendrite zone decreases in the CL, and the depth of liquefaction zone decreases in the HAZ, as a result, the length of hot cracking in the CL and HAZ both decreases.

Influence of Hot Isostatic Pressing on the Hot Ductility of Cast Alloy 718: The Effect of Niobium and Minor Elements on the Liquation Mechanism

The influence of two hot isostatic pressing (HIP) treatments on liquation behavior was investigated and compared with regard to the extent of heat-affected zone liquation cracking in cast Alloy 718. The extent of liquation was seen to increase after HIP treatment at 1190 °C due to solute changes caused by the homogenization of Nb, which contributed to extensive grain boundary melting. The HIP treatment at 1120 °C exhibited lower liquation with contributions from particle liquation of the Laves phase and constitutional liquation of NbC carbides. This was also reflected in a lower ductility recovery temperature, with slower recovery for the former due to the extensive liquation. Interestingly, the nil ductility temperatures were both below the predicted equilibrium solidus of the alloy, which suggests that the ductility drop is related to liquation caused by solute segregation at the grain boundaries.

Investigation of secondary phases and tensile strength of nitrogen-containing Alloy 718 weldment

The influence of nitrogen content on the precipitation of secondary phases and the tensile strength of Alloy 718 during gas tungsten arc welding was investigated. Various types of precipitates were characterized using scanning electron microscopy and transmission electron microscopy. The results showed that in the fusion zone, the volume fraction of Nb-rich phases such as Laves, (Nb,Ti)C, and δ phases, as well as Ti-rich phases such as (Ti,Nb)CN and (Ti,Nb)N, increased with increase in the nitrogen content due to the microsegregation of Nb and Ti within interdendritic areas. Nitrogen was also found to decrease the size of γ″ particles within γ dendrites. For precipitates in the partially melted zone, constitutional liquation was observed for both (Nb,Ti)C and (Ti,Nb)N particles. Based on the results of tensile tests, the weld containing 0.015wt% nitrogen exhibited the highest ultimate tensile strength (UTS), whereas more addition of nitrogen led to a decrease in both the UTS and yield strength due to the increased content of brittle Laves phases and decreased size of γ″.