I am a PhD Researcher that focuses on characterising the microstructural behaviour of superalloys.
Follow this pinboard to obtain an understanding of liquation cracking and why it is so detrimental!
In 10 seconds? In the search for higher operating temperatures, the consequence is they become much more difficult to weld - none more so than nickel superalloy.
Don't believe it? Numerous research groups have carried out studies looking at various welding processes and analysed the dissolution kinetics of the secondary phases and how this effects the propensity of liquation cracking – see pinned articles to learn more.
What is meant by dissolution kinetics? The precipitates ability to dissolve back into the microstructure upon a fast heating rate. However, this is where the problem lies - more often than not, the precipitate does not fully dissolve back into the microstructure and this is what leads to liquation cracking.
Okay, so the incomplete dissolution causes the cracking? It is a combination of factors - liquation occurs during welding processes because of the extremely high heating rates, which prevents the completion of dissolution of the precipitates and therefore produces liquid films. However, the quantity of the liquid that forms as a consequence of constitutional liquation is dependent on as follows; heating rates, initial particle size and the dissolution kinetics of the particle.
Abstract: In the present study, liquation behavior including the constitutional liquation phenomena in the weld heataffected zone (HAZ) of nodular cast iron with 4% Si was investigated. The thermal cycle of the weld HAZ was simulated using a Gleeble simulator at various peak temperatures and a fast heating rate of 1000 °C/sec was selected to observe the constitutional liquation phenomenon. Differential thermal analysis has shown that the eutectic constituents of γ + Laves and γ + Fe3C begin to melt at 1120 °C and 1140 °C, respectively. Meanwhile, constitutional liquation by γ/graphite reaction occurred above 1140 °C, due to carbon enrichment around the graphite nodules.
Pub.: 27 Apr '12, Pinned: 11 Apr '17
Abstract: The behaviour of γ’ phase to thermal and mechanical effects during rapid heating of Astroloy, a powder metallurgy nickel-based superalloy has been investigated. The thermo-mechanical-affected zone (TMAZ) and heat-affected zone (HAZ) microstructures of an inertia friction welded (IFW) Astroloy were simulated using a Gleeble thermo-mechanical simulation system. Detailed microstructural examination of the simulated TMAZ and HAZ and those present in actual IFW specimens showed that γ’ particles persisted during rapid heating up to a temperature where the formation of liquid is thermodynamically favored and subsequently re-solidified eutectically. The result obtained showed that forging during the thermo-mechanical simulation significantly enhanced resistance to weld liquation cracking of the alloy. This is attributable to strain-induced rapid isothermal dissolution of the constitutional liquation products within 150 μm from the center of the forged sample. This was not observed in purely thermally simulated samples. The microstructure within the TMAZ of the as-welded alloy is similar to the microstructure in the forged Gleeble specimens.
Pub.: 23 May '14, Pinned: 11 Apr '17
Abstract: Authors: Z. Liang ; G. Qin ; H. Ma ; F. Yang ; Z. Ao Article URL: http://www.tandfonline.com/doi/full/10.1080/13621718.2016.1248648?ai=1xl5u&mi=kpptjb&af=R Citation: Science and Technology of Welding and Joining Publication Date: 2016-11-17T06:38:49Z Journal: Science and Technology of Welding and Joining
Pub.: 17 Nov '16, Pinned: 11 Apr '17
Abstract: The microstructure, phase composition, and mechanical characteristics of the structural constituents of an Al–Cu–Mg–Si alloy in which the liquation of grain boundaries occurred during heat treatment have been studied. Bands of the (Al + Al15(Fe, Mn)3Si2) eutectics have been observed at the grain boundaries. An algorithm for calculating the additional pressure, which results from mechanical impact on the metal containing these bands has been described. The microstructure, phase composition, and mechanical characteristics of the structural constituents of an Al–Cu–Mg–Si alloy in which the liquation of grain boundaries occurred during heat treatment have been studied. Bands of the (Al + Al15(Fe, Mn)3Si2) eutectics have been observed at the grain boundaries. An algorithm for calculating the additional pressure, which results from mechanical impact on the metal containing these bands has been described.1532
Pub.: 01 Dec '16, Pinned: 11 Apr '17
Abstract: Alloy 718 suffers from microfissuring in the weld heat-affected zone, and compositional, structural and mechanistic causes of this defect have been examined. Both bulk sulfur and bulk carbon increase microfissuring, and heat treatments typical ofhomogenization, solution annealing and age hardening reveal that microfissuring is sensitive to the microstructural and chemical distributions established during heat treatment. Increased grain size increases microfissuring more than heat treatment or bulk chemistry. The mechanistic cause of microfissuring, constitutional liquation of niobium carbide and Laves phases, produces intergranular liquid films with wetting angles that are dependent on the chemical composition of the grain boundary region prior to welding. Microfissuring also correlates with the temperature dependence of the intergranular liquid wetting angle in the heat-affected zone.
Pub.: 26 Oct '12, Pinned: 11 Apr '17
Abstract: Publication date: 1 January 2017 Source:Acta Materialia, Volume 122 Author(s): Jiangwei Liu, Henrique Pinho Duarte, Sindo Kou Al-Mg alloys, despite their wide freezing temperature range ΔT f , can have good resistance to cracking during solidification. To help understand why, the mushy zone of 5086 Al (∼Al-4.0 Mg) was quenched during arc welding and the cooling curve measured to locate the beginning of the original mushy zone (liquidus temperature T L ) and the end (eutectic temperature T E ). Since little eutectic was visible just slightly behind the beginning of the quenched mushy zone, little liquid was here in the original mushy zone, i.e., solidification already ended well above T E . Since no dendrites were visible, either, and since the highest Mg content measured was well below the maximum solubility in solid Al, C SM (17.5 wt% Mg), microsegregation was very mild here in the original mushy zone. These results suggest significant Mg back diffusion occurred during solidification (because of very high C SM ), causing: 1. fraction solid f S to increase much faster with decreasing temperature T, 2. ΔT f to narrow down, and 3. dendritic grains to bond together extensively (f S ≈ 1) to resist intergranular cracking earlier (well above T E ). Since d(f S )/dT increased, dT/d(f S ) 1/2 decreased to decrease the crack susceptibility index, i.e., the maximum dT/d(f S ) 1/2 . All these changes reduce the crack susceptibility. For comparison, 2014 Al (∼Al-4.4Cu) was also quenched during arc welding. At the end of the quenched 2014 Al mushy zone, continuous eutectic, dendrites and microsegregation were all very clear. Thus, solidification ended at T E and thin liquid films still separated grains at the end of the original mushy zone to allow intergranular cracking. Calculated T-(f S ) 1/2 curves showed the index is reduced significantly by back diffusion in Al-4.0 Mg (∼5086 Al) but not in Al-4.4Cu (∼2014 Al). Graphical abstract
Pub.: 05 Oct '16, Pinned: 11 Apr '17
Abstract: The effect of the alloying elements, the carbon, nickel, and the titanium in particular, on the hot ductility behavior of high manganese steel for cryogenic applications was investigated. A hot ductility test was carried out to clarify the mechanism of the heat affected zone (HAZ) liquation and ductility dip cracking that were observed at the HAZ of multi-pass welds. The brittle temperature range, critical strain temperature range, and overall ductility for each alloy were varied due to differences of the eutectic temperatures and the degree of dynamic recrystallization (DRX). The temperature of constitutional liquation between TiC and the austenitic matrix as well as the eutectic temperature of M3C/γ were determined based on the Ti/C ratio and Ti content in the Fe<img border="0" alt="single bond" src="http://cdn.els-cdn.com/sd/entities/sbnd" class="glyphImg">Ti<img border="0" alt="single bond" src="http://cdn.els-cdn.com/sd/entities/sbnd" class="glyphImg">C ternary system. The onset and degree of DRX depended on the grain size, the secondary particle distribution and the chemical composition that determines the stacking fault energy of each alloy. The HAZ cracking was consistent with aspects of liquation cracking including the constitutional liquation between the TiC and austenitic matrix, its reaction temperature, the final M3C/γ eutectic temperature, extent of liquation, followed by the ductility dip crack that was formed along with resolidified grain boundaries under the DRX temperature.
Pub.: 12 Sep '16, Pinned: 11 Apr '17
Abstract: The effect of preweld overaging heat treatment on the microstructural response in the heat-affected zone (HAZ) of a precipitation-hardened nickel-base superalloy INCONEL 738LC subjected to the welding thermal cycle (i.e., rapid) was investigated. The overaging heat treatment resulted in the formation of an interfacial microconstituent containing M23X6 particles and coarsening of primary and secondary γ′ precipitates. The HAZ microstructures around welds in the overaged alloy were simulated using the Gleeble thermomechanical simulation system. Microstructural examination of simulated HAZs and those present in tungsten inert gas (TIG) welded specimens showed the occurrence of extensive grain boundary liquation involving liquation reaction of the interfacial microconstituents containing M23X6 particles and MC-type carbides. In addition, the coarsened γ′ precipitate particles present in the overaged alloy persisted well above their solvus temperature to temperatures where they constitutionally liquated and contributed to considerable liquation of grain boundaries, during continuous rapid heating. Intergranular HAZ microfissuring, with resolidified product formed mostly on one side of the microfissures, was observed in welded specimens. This suggested that the HAZ microfissuring generally occurred by decohesion across one of the solid-liquid interfaces during the grain boundary liquation stage of the weld thermal cycle. Correlation of simulated HAZ microstructures with hot ductility properties of the alloy revealed that the temperature at which the alloy exhibited zero ductility during heating was within the temperature range at which grain boundary liquation was observed. The on-cooling ductility of the alloy was significantly damaged by the on-heating liquation reaction, as reflected by the considerably low ductility recovery temperature (DRT). Important characteristics of the intergranular liquid that could influence HAZ microfissuring of the alloy in overaged condition are also discussed.
Pub.: 28 Feb '07, Pinned: 11 Apr '17
Abstract: A detailed microstructural analysis was performed on a difficult-to-weld nickel-base superalloy, IN 738, subjected to linear friction welding and Gleeble thermomechanical simulation, to understand the microstructural changes induced in the material. Correlations between the microstructures of the welded and simulated materials revealed that, in contrast to a general assumption of linear friction welding being an exclusively solid-state joining process, intergranular liquation, caused by nonequilibrium phase reaction(s), occurred during joining. However, despite a significant occurrence of liquation in the alloy, no heat-affected zone (HAZ) cracking was observed. The study showed that the manufacturing of crack-free welds by linear friction welding is not due to preclusion of grain boundary liquation, as has been commonly assumed and reported. Instead, resistance to cracking can be related to the counter-crack-formation effect of the imposed compressive stress during linear friction welding and strain-induced rapid solidification. Moreover, adequate understanding of the microstructure of the joint requires proper consideration of the concepts of nonequilibrium liquation reaction and strain-induced rapid solidification, which are carefully elucidated in this work.
Pub.: 16 Aug '11, Pinned: 11 Apr '17
Abstract: As compared to Al alloys, which are known to be susceptible to liquation (i.e., liquid formation) and liquation-induced cracking, most Mg alloys have a lower eutectic temperature and thus are likely to be even more susceptible. The present study was conducted to study liquation and liquation cracking in Mg alloys during arc welding and friction-stir welding (FSW). Binary Mg-Zn alloys were selected as a model material in view of their very low eutectic temperature of 613 K (340 °C). Mg-Zn alloys with 2, 4, and 6 wt pct of Zn were cast and welded in the as-cast condition by both gas-tungsten arc welding (GTAW) and FSW. A simple test for liquation cracking was developed, which avoided interference by solidification cracking in the nearby fusion zone. Liquation and liquation cracking in GTAW were found to be in the decreasing order of Mg-6Zn, Mg-4Zn, and Mg-2Zn. Liquation cracking occurred in FSW of Mg-6Zn but not Mg-4Zn or Mg-2Zn. Instead of a continuous ribbon-like flash connected to the weld edge, small chips, and powder covered the weld surface of Mg-6Zn. The results from GTAW and FSW were discussed in light of the binary Mg-Zn phase diagram and the curves of temperature vs fraction solid during solidification.
Pub.: 15 Oct '14, Pinned: 11 Apr '17
Abstract: Abstract In the current work, the effect of friction stir processing on heat-affected zone (HAZ) liquation cracking resistance of aluminum-copper alloy AA 2219 was evaluated. In Gleeble hot-ductility tests and longitudinal Varestraint tests, the FSPed material, despite its very fine dynamically recrystallized equiaxed grain structure, showed considerably higher susceptibility to HAZ liquation cracking when compared to the base material. Detailed microstructural studies showed that the increased cracking susceptibility of the FSPed material is due to (i) increase in the amount of liquating θ phase (equilibrium Al2Cu) and (ii) increase in the population of grain boundary θ particles. An important learning from the current work is that, in certain materials like alloy 2219, the use of FSP as a pretreatment to fusion welding can be counterproductive.AbstractIn the current work, the effect of friction stir processing on heat-affected zone (HAZ) liquation cracking resistance of aluminum-copper alloy AA 2219 was evaluated. In Gleeble hot-ductility tests and longitudinal Varestraint tests, the FSPed material, despite its very fine dynamically recrystallized equiaxed grain structure, showed considerably higher susceptibility to HAZ liquation cracking when compared to the base material. Detailed microstructural studies showed that the increased cracking susceptibility of the FSPed material is due to (i) increase in the amount of liquating θ phase (equilibrium Al2Cu) and (ii) increase in the population of grain boundary θ particles. An important learning from the current work is that, in certain materials like alloy 2219, the use of FSP as a pretreatment to fusion welding can be counterproductive.θ2θ
Pub.: 27 Dec '16, Pinned: 11 Apr '17