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I am a PhD Researcher that focuses on characterising the microstructural behaviour of superalloys.

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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.

11 ITEMS PINNED

Evidence of back diffusion reducing cracking during solidification

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

Hot ductility and hot cracking susceptibility of Ti-modified austenitic high Mn steel weld HAZ

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

Liquation Microfissuring in the Weld Heat-Affected Zone of an Overaged Precipitation-Hardened Nickel-Base Superalloy

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

Heat-Affected Zone Liquation Cracking Resistance of Friction Stir Processed Aluminum-Copper Alloy AA 2219

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