Predicting Susceptibility to Solidification Cracking and Liquation Cracking by CALPHAD

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.

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.

Liquation Cracking in a Row 1 Turbine Vane

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.

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.

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.