Liquation Cracking Research Articles

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

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.

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

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.

Liquation and Crystallization Cracks in Aluminum Alloy AA2024 during Selective Laser Melting

In this work we studied the microstructure and microhardness of standard AA2024 alloy and AA2024 alloy with the addition of 1.5% Y after pulsed laser melting (PLM) and selective laser melting (SLM). The SLM process was carried out with a 300 W power and 0.1 m/s laser scanning speed. A dispersed microstructure without the formation of crystallization cracks and low liquation of alloying elements was obtained in Y-modified AA2024 aluminum alloy. Eutectic Al3Y and Al8Cu4Y phases were detected in Y-modified AA2024 aluminum alloy. It is led to a decrease in the formation of crystallization cracks The uniform distribution of alloying elements in the yttrium-modified alloy had a positive effect on the quality of the laser melting zone (LMZ) and microhardness.

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

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.

Mechanisms of Cracking in Laser Welding of Magnesium Alloy AZ91D

Considerable research has been carried out to study the laser welding of magnesium alloys. However, the studies are mainly devoted to butt welding, and there has been limited information in the published literature concerning the bead-on-plate laser welding of AZ91D, even though bead-on-plate welding is required for the repair of cast AZ91D parts with surface defects. In the present investigation, surface cracking of the weld metal was observed when an AZ91D magnesium alloy was bead-on-plate welded using the laser welding method. This paper presents the experimental results and analyses to show that the cracking is “solidification cracking” initiated from the weld surface under high thermal stresses. This is in contrast to the “liquation cracking” observed in heat affected zones in tungsten inert gas welding of the same magnesium alloy. Laser power was found to be one of the main factors affecting the distance of the crack propagation. The higher laser power resulted in longer crack propagation distance into the weld metal. It is demonstrated that hot cracking could be avoided by lowering the laser power and welding speed.

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

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.

Analysis of liquation and solidification cracks in the electron beam welding of GTD-111 nickel-base superalloy joint

The weldability of GTD-111 superalloy by electron beam welding was studied, and the cracking evolution of solidification and liquation, as well as microstructural characteristics, were also investigated. The weld metal microstructure consisted mainly of the inter-dendritic phases of MC, γ-γ′ and γ′ nanoparticles, primarily responsible for the solidification crack in the weld metal. In addition to solidification cracks, the formation of liquation cracks in HAZ was an important challenge with electron beam welding. It was found that M5B3, γ-γ′, MC and γ′ phases leave formation during the increase of non-equilibrium temperature in HAZ, with the formation of partial liquation in the phase-matrix interface and absorption of shrinkage stresses due to welding cooling. Both solidification and liquation cracks decreased as the heat input increased due to the increase in beam current. This was attributed to the reduction of cooling stress and the increase of backfilling capacity.

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

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.

Hot cracking in ZK60 magnesium alloy produced by laser powder bed fusion process

Additive manufacturing of ZK60 alloy by laser powder bed fusion (LPBF) has promising prospect for light-weight structures and biodegradable implants. Though hot cracking of ZK60 has been widely reported during welding processes, it is surprisingly that no literature so far mentioned it during the LPBF. This paper identified serious solidification and liquation cracking in ZK60 LPBF samples. Low-melting eutectic precipitation was characterized as Mg7Zn3 along the cracks. Ruptured by thermal stress, solidification cracking occurred in the depositing layers due to the wide solidification range, while liquation cracking occurred in the deposited layers due to repeated heating. The two types of cracks merged into a connected network, which hugely deteriorated the relative density of LPBF samples.

Hot cracking behavior of additively manufactured D2 steel

Additive manufacturing shows great potential for overcoming the limitations of traditional manufacturing processes, especially for tool steels exhibiting high hardness and wear resistance. However, hot cracking often limits the application of additively manufactured parts, and hence, clarifying the hot cracking mechanisms in engineering alloys is of great importance. In this work, the mechanism of hot cracking during direct laser deposition of D2 tool steels was studied. Thin-wall specimens were deposited under different laser powers (400 and 500 W) and the length of the cracks varied with the laser power and the position in the specimen. The cracks mainly formed in the inter-dendritic regions which comprised eutectic austenite and M7C3 carbides. The thermal history of the build was measured by pyrometry and the size of the cracks increased with the increasing number of reheating events above the solidus temperature. Based on these results, hot-cracking of the studied D2 steel could be ascribed to liquation cracking, induced and intensified by the repetitive liquation of the eutectics.

Influence of heat treatment on creep behavior of IN625 coating on a Ni3Al-base superalloy

IN625 coatings applied by selective laser melting show high resistance at high temperatures but the creep behavior of such coatings is not well understood. In this paper, the microstructure and creep behavior of this coating applied to Rene80 alloy under different heat treatment conditions, were investigated by a small punch creep test. The results show that coating application in the form of solution heat treatment + coating + aging heat treatment leads the maximum creep life of about 135 h. In cast-coated specimens, liquation cracks due to incomplete solution of γ-γ′ eutectic, γ′ phase, Cr-Mo boride, and MC carbide in heat affected zone reduced the creep life by about 63 h. In these samples, unlike the heat treatment sample, which experienced inter-granular creep failure, the failure occurred exactly from the coating-substrate interface.

Liquation Cracking in a Row 1 Turbine Vane

Abstract A first-stage turbine vane was received in the laboratory directly from fabrication, prior to its use in engine service. The part had not yet been covered with its customary coating system that protects these parts against hot corrosion. A first visual inspection revealed multiple cracks on the airfoil’s hot gas path side, fairly centered in the part. After cutting the part open, it soon became apparent that the cracking was even more severe inside, suggesting crack initiation from that cooled side. Fractography allowed to determine liquation cracking as the metallurgical failure mechanism. Since the part was received immediately after pre-heat before plasma coating, that process step was concluded to have caused the cracking.

Comparison of wire-arc and powder-laser additive manufacturing for IN718 superalloy: unified consideration for selecting process parameters based on volumetric energy density

Wire-arc additive manufacturing (WAAM) as well as powder-laser additive manufacturing (PLAM) are both promising cladding additive manufacturing (AM) techniques for fabricating large IN718 superalloy parts. Correct selection of initial process parameters is an important prerequisite to ensure the success of subsequent AM stage and its dynamic adjustment. For comparing the relationship between the process parameters and their influence on the quality of WAAM and PLAM cladding beads, the relationship between the energy density and the most controllable parameters was comparatively studied from the perspective of unified energy dissipation and the weldability. The equal effective volumetric energy density can be obtained under different combinations of WAAM and PLAM parameters. The defects such as porosity formation, centerline grain boundary, and liquation cracking are mainly affected by the cladding speed, rather than the effective volumetric energy density. During WAAM and PLAM, the corresponding effective volumetric energy density range which can avoid internal and external defects is in the theoretical weldable zone of IN718 superalloy. The key to obtain a defect-free cladding bead is to properly control the energy input and its distribution. The high material utilization under equal effective power and cladding speed is the main reason why WAAM is more efficient than PLAM. According to the energy dissipation hypothesis and related formulas, different energy beam-based AM processes can be further compared under the equal energy input, which provides a basis for the selection of initial process parameters and the dynamic adjustment of main parameters.

Processing window for laser metal deposition of Al 7075 powder with minimized defects

There is a relative lack of data on the processing parameters suitable for laser metal deposition (LMD) of Al alloys, particularly high strength alloys. The objective of this study is to outline a systematic investigation to define the processing window for LMD of Al alloy 7075. Single linear tracks were fabricated by varying the powder mass flow rate, laser power, and scan speed according to a 3 3 factorial experimental design, in order to determine conditions that produce deposits with suitable cross section geometries. Single layer multi-track deposits were fabricated using parameters selected from the linear track experiments, and with varying track overlap values. Low energy input and powder deposition conditions were found to produce single track and multi-track deposits with porosity less than 0.3 and 1 %, respectively. Cuboid samples were produced using parameters selected from the single layer multi-track processing window. However, porosity of at least 4 % was observed in the cross section for all cuboid samples. Liquation cracking was not observed in the samples, despite clear signs of grain boundary liquation. Microcracks with a width of ∼1 μm were observed in the deposited material, and the results indicate that these are solidification cracks due to insufficient eutectic liquid to backfill and ‘heal’ incipient cracks. It is concluded that low energy density and powder mass deposition conditions can produce crack-free single layer deposits of 7075 with low porosity, but the elimination of interlayer porosity and microcracks in multi-layer deposits requires further study.

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

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.

Correlative Synchrotron X-ray Imaging and Diffraction of Directed Energy Deposition Additive Manufacturing

The governing mechanistic behaviour of Directed Energy Deposition Additive Manufacturing (DED-AM) is revealed by a combined in situ and operando synchrotron X-ray imaging and diffraction study of a nickel-base superalloy, IN718. Using a unique DED-AM process replicator, real-space imaging enables quantification of the melt-pool boundary and flow dynamics during solidification. This imaging knowledge was also used to inform precise diffraction measurements of temporally resolved microstructural phases during transformation and stress development with a spatial resolution of 100 µm. The diffraction quantified thermal gradient enabled a dendritic solidification microstructure to be predicted and coupled to the stress state. The fast cooling rate entirely suppressed the formation of secondary phases or recrystallisation in the solid-state. Upon solidification, the stresses rapidly increase to the yield strength during cooling. This insight, combined with the large solidification range of IN718 suggests that the accumulated plasticity exhausts the ductility of the alloy, causing liquation cracking. This study has revealed the mechanisms that govern the formation of highly non-equilibrium microstructures during DED-AM.

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.

Microstructure, cracking behavior and mechanical properties of René 104 superalloy fabricated by selective laser melting

Selective laser melting (SLM) is a progressive forming technique for manufacturing complicated components, whereas the microcracks emerge in the preparation process of powder metallurgy (PM) nickel-based superalloy samples undesirably. In this study, René 104 PM nickel-based superalloy samples were prepared by SLM, and the microstructure, cracking behavior and mechanical properties of the as-printed samples were systematically investigated. The results demonstrate that the microstructure of the SLMed René 104 alloy is composed of sub-micron cellular structures on the XY plane, and columnar grains paralleled to building direction on the XZ plane, which is corresponding to the intense<001>fiber texture. Both the solidification cracking and liquation cracking propagate along the high angle grain boundaries (HAGBs) in the SLMed René 104 samples, among which thermal stress and low melting point γ-γ′ eutectics have significant effected on cracking behavior. Additionally, the tensile performance of as-fabricated René 104 samples is obviously anisotropic, which is ascribed to the directional growth of columnar grains with<001>fiber texture and the microcracks are perpendicular to the tensile test direction. This research provides the relationship among the microstructure, microcracks and mechanical properties for the René 104 PM nickel-based superalloy fabricated by SLM and exhibits the potential to fabricate similar alloys without cracks.

On the oxygen-induced hot cracking in a direct laser deposited Ni-based superalloy

We report on the observation of a novel hot cracking mechanism in a direct laser deposited nickel-based superalloy, using joint electron, X-ray, and atom microscopy. The majority of the observed cracks were related to Ni and Mo oxides formed in grain interiors as well as at grain boundaries. Using atom probe tomography, we detected an intermediate zone between oxide particles and the base alloy, where oxygen existed in solid solution and chemical compositions varied due to the oxide formation. The variation in composition lowered the melting point of the matrix while the oxides promoted stress concentration and crack nucleation, thus causing liquation cracking during reheating.

Prospects of pulsed current arc welding on aerospace grade Hastelloy X

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 develops the secondary carbide precipitates. It promotes the development of hot cracks in the weldment. In this study, joining of Hastelloy X plates was carried out by constant current gas tungsten arc welding (GTAW) and pulsed current gas tungsten arc welding (PCGTAW) with C263 filler wire. The result discovered that no hot cracks were formed in the weldment. In constant current mode, Cr-rich and Mo-rich Cr23C6 (M23C6), Fe2MoC, Fe3Mo3C (M6C), and Cr2Ti precipitates were observed. Whereas, in pulsed current mode, Ni3(Al, Ti), Ni3Ti, Co3Ti, Cr2Ti precipitates are found due to the segregation of Co, Al, and Ti. No Cr-rich and Mo-rich carbide phases identified in pulsed current weldment due to rapid cooling rate and higher thermal gradient observed during solidification. The tensile results revealed that 8.23% increase in the ultimate tensile strength and a 29.62% increase in elongation of pulsed current mode welding compared to constant current welding. Further, the microhardness and impact toughness of PCGTAW is 3.32% and 5.45% higher than GTAW, respectively. In pulsed current welding, better mechanical properties were identified compared to constant current welding. The nonappearance of Cr and Mo-rich phases and refined microstructure in the weldment are the main reason for better strength.