Cracking Susceptibility Index Research Articles

Stress Corrosion Behavior of MIG Welded Joints of 6082-T6 Aluminum Alloy at Different Temperatures

6082-T6 aluminum alloy is a commonly used aluminum alloy material in the field of rail transit because of its good molding properties, high mechanical properties, excellent corrosion resistance and weldability. In the high temperature and humid environment, the temperature change is bound to affect the stress corrosion resistance of the aluminum alloy and their welded joint. However, the influence mechanism of temperature on its stress corrosion resistance has not been explained in the existing research. In this paper, the mechanical properties and stress corrosion behaviors of melt-inert gas welded (MIG) 6082-T6 aluminum alloy welded joints were systematically studied under various temperatures condition. Results indicated the temperature scarcely affected stress corrosion cracking susceptibility index (PSCC) of base metal, while significantly affected the welded joint and higher temperature caused lower PSCC. After slow strain rate tensile test, a corrosion layer was formed, which was a typical brittle-toughness mixed failure, and the degree of brittleness increased with the increasing of temperature. Electrochemical analysis showed that corrosion resistance of the joint slightly decreased due to aluminum alloy accelerated dissolution caused by increasing of temperature. The proposed research will provide a theoretical basis for solving aluminum alloys used in rail transit, ship accessories and other industrial fields.

Achieving high mechanical and corrosion properties of AA2050 Al-Li alloy: The creep aging under plastic loading

The influences of elastic/plastic loading (100–220 MPa) on the creep behavior, mechanical properties, and corrosion behavior of creep-aged AA2050 alloys were investigated. The results show that the creep rate increased from 0.35 % to 0.61 % with the increase of stress from 100 MPa to 220 MPa. The creep rate was increased rapidly under plastic loading (220 MPa) due to the increased dislocation density. Meanwhile, the plastic loading shortened the peak-aged time of creep-aged alloys and achieved outstanding strength (UTS=534 MPa, YS=496 MPa, peak aged), which increased by 33 MPa and 32 MPa compared with elastic loading, respectively. The strength enhancement was attributed to the increase in dislocation density, weak oriented precipitation effect, and dense precipitation of T1 phases. Additionally, compared with elastic loading, GBPs under plastic loading coarsened and distributed discretely, their elements content distributed evenly, and the Cu content increased. Therefore, the intergranular corrosion (IGC) depth and stress corrosion cracking (SCC) susceptibility index (ISSRT) decreased from 174 μm, and 8.7 % to 121 μm, and 5.9 %, respectively. These findings pave a way in breaking curvature limit of creep aging technology.

Solidification cracking inhibition mechanism of 2024 Al alloy during oscillating laser-arc hybrid welding based on Zr-core-Al-shell wire

Solidification cracking (SC) of 2024 high-strength aluminium alloy during fusion welding or additive manufacturing has been a long-term issue. In this work, crack-free weld could be obtained using a Zr-core-Al-shell wire (ZCASW filler material, a novel filler) coupled with an oscillating laser-arc hybrid welding process, and we investigated the solidification cracking susceptibility (SCS) and cracking behavior of AA2024 weld fabricated with different filler materials. The cracking inhibition mechanism of the weld fabricated with ZCASW filler material was elucidated by combined experiments and phase-field simulation. The results show that the effectiveness of filler materials in reducing the SC gradually improves in the order of ER2319 filler material < ER4043 filler material < ZCASW filler material. The main cracking (when using the ER2319 filler material) branches and the micro cracking branches interact with each other to produce cracking coalescence, which aggravates the cracking propagation. The formation of the Al3Zr phase (when using the ZCASW filler material) promotes heterogeneous nucleation of α-Al, thereby resulting in finer and equiaxed non-dendrite structures, which shortens the liquid phase channels and decreases cracking susceptibility index |dT/d(fs)1/2| (T is temperature and fs is solidification fraction) at final solidification. A higher proportion (7.65 % area fraction) of inter-dendrite phase with spherical distribution state, a shorter (8.6 mm liquid channel length) inter-dendrite phase coupled with round non-dendrite structure (6 µm dendrite size) effectively inhibit the SC. The present study can be a useful database for welding and additive manufacturing of AA2024.

An efficient design of Al-Cu-Mg-Sc casting alloys by computational thermodynamics

A self-consistent thermodynamic description of the Al-rich corner Al-Cu-Mg-Sc quaternary system was established using the CALPHAD technique based on published experimental phase equilibria. Utilizing the crack susceptibility index, growth restriction factor, and thermodynamic calculations along the vertical section of Al-4Cu-1.5Mg-xSc (wt%), we developed a new Sc-modified Al-Cu-Mg casting alloy. The optimal addition of Sc was predicted to be 0.65 wt% by alloy design and confirmed through key experiments. This alloy achieves an ultimate tensile strength and elongation of 325.15 MPa and 7.63 %, respectively. The HRTEM technique determines that the primary/peritectic (Al)/Al2CuMg interface is partially coherent in three dimensions, while Al3Sc is uniformly distributed in the (Al) matrix, and the primary/peritectic (Al)/Al3Sc is fully coherent in three dimensions. In addition, the strengthening mechanism and CET mechanism were further analyzed and discussed.

Understanding the influence of boron in additively manufactured GammaPrint®-700 CoNi-based superalloy

Boron is commonly added to superalloys in small amounts to enhance creep resistance, but can lead to cracking at high concentrations, especially during the additive manufacturing process. Two variants of CoNi-based GammaPrint®-700 superalloy with different B contents (0.08 at% vs 0.16 at%) were printed via laser powder bed fusion (LPBF) with the same printing parameters, with only the high B alloy exhibiting solidification cracking. Atom probe tomography (APT) revealed stronger segregation behaviors in the high B alloy compared to the low B alloy at both the inter-dendritic regions and grain boundaries (GBs). The segregation behavior at inter-dendritic regions was well captured with Scheil simulation and can correlate with the existing cracking susceptibility index (CSI) on cracking tendencies, although high angle GBs are where cracking occurs according to electron backscatter diffraction (EBSD) measurements. Additionally, the extent of GB segregation was compared between the high B and low B alloy. Higher B additions led to significantly more GB B segregation in the high B alloy compared to the low B alloy. For the high B alloy, the cracked region of one GB exhibited higher levels of B compared to the uncracked region of the same GB. However, much higher B contents were also found in two other uncracked GBs in the high B alloy, which demonstrates that higher GB B concentrations are not fully responsible for the cracking. A much larger variance in GB B segregation content was found in the high B alloy compared to the low B alloy. These phenomena were explained with a solidification model with the GB segregation content expressed explicitly by a modified Langmuir-McLean equation. This model linked the GB segregation content with solidification undercooling, which can be used as quantitative cracking criteria for future builds.

From crack-prone to crack-free: Eliminating cracks in additively manufacturing of high-strength Mg2Si-modified Al-Mg-Si alloys

Large solidification ranges and coarse columnar grains in the additively manufacturing of Al-Mg-Si alloys are normally involved in hot cracks during solidification. In this work, we develop novel crack-free Al-Mg2Si alloys fabricated by laser powder-bed fusion (L-PBF). The results indicate that the eutectic Mg2Si phase possesses a strong ability to reduce crack susceptibility. It can enhance the grain growth restriction factor in the initial stage of solidification and promote eutectic filling in the terminal stage of solidification. The crack-free L-PBFed Al-xMg2Si alloys (x = 6 wt.%, 9 wt.%, and 12 wt.%) exhibit the combination of low crack susceptibility index (CSI), superior ability for liquid filling, and grain refinement. Particularly, the L-PBFed Al-9Mg2Si alloy shows improved mechanical properties (e.g. yield strength of 397 MPa and elongation of 7.3 %). However, the cracks are more likely to occur in the region near the columnar grain boundaries of the L-PBFed Al-3Mg2Si alloy with a large solidification range and low eutectic content for liquid filling. Correspondingly, the L-PBFed Al-3Mg2Si alloy shows poor bearing capacity of mechanical properties. The precise tuning of Mg2Si eutectic content can offer an innovative strategy for eliminating cracks in additively manufactured Al-Mg-Si alloy.

Assessment of process-induced cracks in hot-working operations using crack susceptibility index based on plastic instability criteria

A processing map representing the efficiency of power dissipation as a function of the temperature and strain rate has been used as a guide to depict the hot workability of metallic alloys. However, it has limitations in indicating the local positions where process-induced cracks may occur during hot-working operations. A process-induced crack is formed during hot deformation because the plastic instability associated with crack formation is more favorable than normal flow under a given set of process conditions. This paper proposes a novel approach to assess local cracks using the crack susceptibility index (CSI), which is based on Ziegler's plastic instability criteria. This solution provides useful information regarding the effect of each process parameter on the workability required for high-temperature processes. Lowering the CSI value by controlling process variables, such as the temperature and strain rate, is recommended for a design free from process-induced cracks. More importantly, a coded finite element subroutine coupled with the CSI can help visualize local positions where cracks may be produced during a hot-working operation. For experimental validation, a Gleeble test was performed on Ti6Al4V specimens containing four notches with different depths. The successful application of the CSI to the process design of shop-floor unmanned aircraft components helped confirm the applicability and validity of this study.

High strength Al0.7CoCrFeNi2.4 hypereutectic high entropy alloy fabricated by laser powder bed fusion via triple-nanoprecipitation

Eutectic high-entropy alloys, composed of FCC/B2 phases with a narrow solidification interval and excellent fluidity, have become a new hotspot in additive manufacturing. Nevertheless, their microstructures exhibit significant sensitivity to processing parameters, feedstocks, and composition, ultimately limiting the alloys’ engineering applications. Here, a hypereutectic Al0.7CoCrFeNi2.4 alloy with a low cracking susceptibility index was designed by Thermo-Calc calculation and fabricated by laser powder bed fusion. Results show that the as-printed Al0.7CoCrFeNi2.4 alloy manifests a stable cellular structure, coupled with appreciable ultimate tensile strength (≥1200 MPa) and ductility (≥20%) over a wide range of processing parameters. After aging at 800 °C for 30 min, outstanding strength (1500 MPa) and elongation (15%) were obtained. Considerable mechanical properties after aging stem from a triple strengthening mechanism, i.e., L12 nanoprecipitates and rod-shaped B2 particles within the FCC matrix, along with Cr-enriched spherical nanoparticles in the B2 phase. Meanwhile, hierarchical structure, i.e., FCC dominated matrix, a discontinuous B2 phase, a precipitation-free zone in the B2 phase, and a K-S orientation relationship between FCC and B2, facilitate to maintain excellent plasticity. These results guide designing HEAs by AM with controllable microstructures and outstanding mechanical properties for industrial applications.

CALPHAD-aided design of high-strength Al-Si-Mg alloys for sufficient laser powder bed fusion processability

This study proposes a design concept for an Al-Si-Mg ternary alloy to achieve both high strength and laser-based powder bed fusion (PBF-LB) processability using calculation of phase diagrams (CALPHAD). The thermodynamic calculations using the CALPHAD method evaluated the Al-10Si-4.5Mg (wt%) alloy composition for strengthening by both the Si and Mg2Si phases and high PBF processability by reducing the risk of hot tearing behind the small freezing range of 27 °C and crack susceptibility index (|dT/d(Fs1/2)|) of 77 °C. The PBF-LB Al-10Si-4.5Mg alloy sample (processed by laser power of 128 W and laser scanning velocity of 1000 mm/s) exhibited a fine three-phase microstructure and exceptional hardness of 199 HV. No hot tearing occurred, whereas many cracks preferentially propagated along the melt pool boundaries during the PBF-LB process. The severe lateral cracking resulted in a maximum relative density of 96 % in the Al-10Si-4.5Mg alloy samples. Preferential cracking could be responsible for the localization of the relatively coarse particles of the brittle Mg2Si phase in the cellular microstructures of the melt pool boundaries. Understanding the lateral cracking mechanism can provide new insights into alloy modification using the controlled solidification path to avoid a specific two-phase eutectic reaction (L→α-Al+Mg2Si) to improve the PBF processability. A modified alloy composition of Al-10Si-3Mg (wt%) was assessed using the CALPHAD approach. PBF-LB processing using the modified alloy powder alleviated cracking and improved the maximum relative density to 98 % for the PBF-LB manufactured samples. This result accentuated the validity of the design strategy for Al-Si-Mg ternary alloy utilizing the CALPHAD method.

Experimental investigation and kinetic analysis of Al–Zn–Mg alloy coating

The hot-dip 55 wt%Al–Zn-1.6 wt%Si-(0–3)wt.%Mg alloy coatings were experimentally investigated, and the solidification behaviors and hot cracking susceptibility were simulated by means of CALPHAD (CALculation of PHAse Diagrams) method. The scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA) and glow discharge spectrometer method (GDS) were utilized to determine the microstructures and the distribution of elements of the Al–Zn–Mg alloy coatings with different Mg contents. It is discovered that with the increase of Mg content, the percentage of the eutectic microstructure scales up, and the surface quality of the alloy coating is improved. Meanwhile, the bending properties of Al–Zn–Mg coatings with different Mg contents still requires further improvement according to the present bending test. Subsequently, the equilibrium solidification processes of the coatings were calculated using thermodynamic approach. In general, the calculated results reflect the solidified phases and the precipitated temperatures according to theory of equilibrium solidification. However, there still exist discrepancies between the thermodynamic calculation results and the observed experimental results during the practical galvanizing process, because the cooling rates were not taken fully into consideration. Consequently, the kinetic analysis was carried out to obtain the secondary dendrite arm spacing under different cooling rates. The cracking susceptibility index (CSI) was also calculated to predict the hot workability of the Al–Zn–Mg alloy coating. In summary, the appropriate increase of the cooling rate turns out be the effective approach to benefit the microstructure and the corrosion resistance of the Al–Zn–Mg coatings in the practical galvanizing process, and the coating is not suitable for the hot stamping process.

Laser powder bed fusion of immiscible steel and bronze: A compositional gradient approach for optimum constituent combination

The microstructures and the mechanical properties of a laser powder bed fusion (LPBF) manufactured alloy coupons that were compositionally graded with austenitic stainless steel 316 L and bronze (Cu10Sn), which are immiscible, were investigated. While the microstructure of the pure 316 L contains only columnar γ-Fe grains, the formation of the equiaxed α-Cu, with α-Fe particles embedded in them, at the grain boundaries of the γ-Fe grains was observed upon alloying with Cu10Sn, for up to 50 wt.%. In the graded alloys with > 50 wt.% Cu10Sn, the microstructure inverts and contains α-Fe particle embedded α-Cu matrix along with some scattered γ-Fe grains. The two-dimensional phase-field simulations were employed to reproduced the phase transition and microstructure evolution in different compositions, revealing that the nucleation of γ-Fe occurs synchronously with the spinodal decomposition of liquid and that with further cooling, Cu-rich liquid solidifies and nanoscale spherical γ-Fe precipitates appear. Cross-sections with 10–40 wt.% Cu10Sn were found structurally unreliable owing to the formation of liquation cracks and shrinkage pores, whereas the rest of the build contained few pores and was crack-free. The Scheil solidification simulations together with the crack susceptibility index calculations over the entire compositions range of the CGA, reveal that when with < 50 wt.% Cu10Sn, liquid feeding to interdendritic regions of γ-Fe is limited, which is otherwise necessary to mitigate shrinkage-induced stresses. The hardness, yield and tensile strengths of the compositionally graded alloy in the cross-section with 50 wt.% Cu10Sn are higher than those of the parent constituents and the one with 80 wt.% Cu10Sn has the best combination of strength and ductility. The variations in strength and ductility are attributed to the microstructural and compositional changes in α-Cu matrix that were influenced by the formation of α-Fe particles. Finally, the appropriate proportions and the influence of the LPBF method in successfully mixing Cu10Sn and SS316L was discussed.

Solidification cracking nature and sequence of different stainless steels

For magnesium, nickel, and stainless steel, solidification cracking is a critical failure mode that has been extensively studied for decades. The nature of solidification cracking can be investigated through experiments or theoretical calculations. Different models have been proposed for these calculations, including the cracking susceptibility index (CSI), which was developed in the last decade. Building a CSI map for a group of metals facilitates referencing or comparison when the metal is applied in casting, welding, or developing new alloys. In this study, the CSI of various types of stainless steel was examined, and CSI maps were constructed. For each type of alloy, the calculations include more than ten experimentally determined compositions from commercial alloys. The CSI maps of stainless steel were constructed and compared with those of the Cr and Ni equivalents, as well as their respective phase diagrams. The CSI sequence matched the cracking results of previous studies on austenitic steels. The ferritic type had a lower CSI but a higher cracking rate, which could be attributed to the ductility gap during the austenite-ferrite transformation, as reported in previous studies.

Prediction of the influence of welding metal composition on solidification cracking of laser welded aluminum alloy

The crack length of laser welding 6082 aluminum alloy with three types of 5000 series aluminum alloy welding materials was compared by fishbone test method. It was found that ER5A06 welding material corresponds to the shortest crack length, indicating that the welding material composition has an impact on the welding crack and the crack susceptibility could be controlled by adjusting the welding material composition. Combined with SEM and EBSD, the crack morphology was observed and determined to be solidification cracking. Through EDS, EPMA and thermodynamic calculation, it is found that the main elements affecting solidification cracking are Al, Mg, Si and Cu. The cooling rate of the fusion zone was calculated according to the secondary dendrite arm spacing and welding composition. Compared with casting and arc welding, the laser welding aluminum alloy fusion zone has rapid cooling rate. Due to the rapid cooling rate, the T-fs curves of Al-Mg, Al-Si and Al-Cu binary alloys were calculated by commercial thermodynamic software Thermo-Calc. Furthermore, the cracking susceptibility index (CSI) of binary aluminum alloys were also calculated to determine the element content range of the cracking. To understand the element interaction, both the machine learning and computational thermodynamic methods were employed to predict the influence of multi-component welding metal on solidification cracking susceptibility. Based on the calculation results of solidification cracking susceptibility for binary aluminum alloys, a data set containing 45 groups of aluminum alloy components was established and set as the input dataset. The CSI values corresponding to the compositions in the data set were set as the target and the relationship “welding metal composition - CSI” was founded. Based on the Kriging model, the influence of welding metal composition on solidification cracking susceptibility and the components of most crack susceptible were obtained, which provides a basis for welding materials design.

Effect of Composition on Crack Susceptibility Index of Al-Si and Al-Si-Mg Alloys

In order to reasonably design the composition of Al-Si based alloy and study the effect of alloying elements on hot crck of Al-Si based alloys, the effect of alloying elements on hot cracking of Al-Si based alloy was studied. The influence of different Si contents on the crack susceptibility index (CSI) of Al-Si alloy was calculated by using Pandat, and the influence of Mg addition on the CSI of the Al-Si-Mg alloy was also calculated. The solidification path affecting the CSI of the alloy was analyzed. The results show that when the Si content is below 10% (mass percentage), the CSI is almost non-existent and the Al-Si alloy will not show crack tendency; When the Si content reaches 11~12%, the CSI of the Al-Si alloy is very small. When the Si content reaches the hypereutectic region (Si ≥ 13%), the CSI of the Al-Si alloy begin to increase with increasing of Si content. The crack tendency of the Al-Si alloy in the hypereutectic region is larger. Adding Mg can effectively reduce the CSI of the alloy. Furthermore, with increasing of Mg content, the CSI of Al-Si-Mg alloy gradually decreases.

Microstructure characterization and mechanical properties of crack-free Al-Cu-Mg-Y alloy fabricated by laser powder bed fusion

Additive manufacturing of high-strength Al alloys (such as 2xxx, 6xxx and 7xxx series alloys) by laser powder bed fusion (LPBF) encounters a challenge because of their high susceptibility to solidification crack. The current work develops a crack-free and novel high-strength Al-Cu-Mg-Y alloy fabricated by LPBF using the rare earth yttrium (Y) modified 2024 alloy powders. In the LPBF-fabricated Al-Cu-Mg-Y alloy, the rare earth element Y is revealed as an effective alloying element for the solidification crack elimination, which is mainly attributed to the combination of the narrowed brittle temperature range, the decreased solidification crack susceptibility index and the refined grains. The element Y can also react with Al and Cu to form the Al 8 Cu 4 Y phases during the LPBF process. In the LPBF-fabricated Al-Cu-Mg-Y alloy, the larger number of Al 8 Cu 4 Y phases and the finer grains result in higher compressive yield strength than most LPBF-fabricated Al alloys. Interestingly, the LPBF-fabricated Al-Cu-Mg-Y alloy can be highly deformed without collapse up to a large compressive strain of ~70 %. After T6 heat treatment, the LPBF-fabricated Al-Cu-Mg-Y alloy exhibits a significant increase in the compressive stress after the yield point, corresponding to the homogeneously distributed Ω precipitate in the Al matrix. In addition, the tensile strength is lower than the compressive strength for the LPBF-fabricated and T6 heat-treated Al-Cu-Mg-Y alloys. This difference is related to the internal pores in the two alloys.

Weld Strength and Cracking Susceptibility Analysis of Pulsed TIG Welded Al-Mg-Si Alloy by Experimental Approach

Aluminium is a non ferrous corrosive resistance metal mainly used in automotive coolers, inter coolers and radiators. They are the part of automobile vehicles and made from aluminium alloys. The joints in this application are created by fusion welding process specifically Tungsten Inert Gas (TIG) and Pulsed current TIG (PCTIG) welding process. The working temperature range of different coolers, radiators are 20° C to 300° C and pressure ranges from 2.5 bar to 3.5 bar. During actual working of coolers, the weld joint experiences sudden high level and low level temperature changes. These will create high thermal stresses in the joints. It leads to cracking in the weld region and create failure of the weld joints. Various factors such as mechanical, metallurgical and thermal are responsible for cracking in the weld joints. Range of solidification temperature is one of metallurgical factor, stress generation is mechanical factor and cooling rate of weld metal is thermal factor for cracking. Houldcroft weldability test is employed to identify the cracking susceptibility (CS) of the weld. The current investigation is intended to discover the effect of pulse TIG welding process parameters on the mechanical properties and cracking susceptibility of precipitated aluminium Al-Mg-Si alloy. Diverse pulsed TIG welding process parameters such as peak current (Ip), base current (Ib) and frequency (f) were investigated with the objective of identify the tensile strength, yield strength and cracking susceptibility. The corresponding findings of optimum parameters are 180 peak current (Ip), 60 A base current (Ib) and at 6 Hz frequency (f) with tensile strength of 185.55 MP, yield strength 156.62 MPa. The significant of each parameter for tensile strength are Ip, Ib, Ip*Ib, Ip*f and Ib*f. The corresponding contributions in % are 14.56, 49.49, 12.26, 5.44, 12.15, 5.04, and 1.05 respectively. The statistical method such as Taguchi was employed for experiment design. Weldability test (Fishbone test) was performed on standard specimen and the cracking susceptibility index was identified. The cracking susceptibility index 5.26 % were observed with the value of 180 A of Ip, 80 A of Ib &amp; 2 Hz of frequency (f).The results give an idea about the effect of pulsed TIG welding parameters on mechanical properties and cracking susceptibility.

AZ-Mg Alaşımlarının Katılaşma Çatlama Duyarlılığına Karşı Dolgu Metal ve Alaşım Elamanlarının Etkilerinin Tahmin Edilmesi

Solidification cracking is a concern for welding magnesium (Mg) alloys. An index, the maximum │dT/d(fS)1/2│,was used with the thermodynamic software Pandat to make solidification cracking susceptibility predictions for AZ-Mg arc welds which have the main alloying elements of aluminum and zinc in the magnesium matrix. The effect of AZ101 magnesium filler on reducing crack susceptibility of commercially available AZ31, AZ61 and AZ91 Mg alloys was investigated with the crack susceptibility index. The filler metal AZ101 Mg alloy was found effective to reduce the susceptibility of all the three AZ-Mg alloys to solidification cracking. The influence of the amount of aluminum and zinc in the AZ-Mg alloys on the crack susceptibility was predicted using the cracking index and Pandat based on Scheil solidification model. The predictions based on the index were compared to the experimental crack susceptibility data of the AZ-Mg alloys, and it was seen that the general trend of both predictions and the reported data was consistent with each other. The predictions were explained in the light of the criterion proposed for solidification cracking.

Crack free metal printing using physics informed machine learning

Process parameters and thermophysical and mechanical properties of alloys affect cracking which remains a major challenge in metal printing. Cracks occur because of multiple mechanisms and currently, there is no unified mitigation strategy. Here we evaluate the effects of variables related to the physics of cracking computed by a mechanistic model and independent experimental data using machine learning to prevent cracking. The computed solidification stress, the ratio of the vulnerable and relaxation times, ratio of the temperature gradient to solidification growth rate, and cooling rates and experimental data are used to generate a cracking susceptibility index that predicts crack formation before printing. Computed values of these four variables when used in a decision tree, support vector machines, and logistic regression can predict crack formation with exceptional accuracy. Information gain, information gain ratio, and Gini index-based feature selection calculations provide the same comparative influence of these four variables. Results are presented as easy-to-use cracking susceptibility maps. Our approach can help in process optimization, designing new alloys, and solving problems in manufacturing beyond metal printing.

Back diffusion resisting solidification cracking in austenitic stainless steels

Austenitic stainless steels resist solidification cracking much better if ferrite (δ), not austenite (γ), is the primary solidification phase, but why? The curve of temperature T vs. fraction of solid fS was calculated for 304, which solidifies as primary δ-ferrite, and for 310, which solidifies as primary γ. Back diffusion, being much faster in bcc (δ) than in fcc (γ), flattens the curve much more in 304 than 310. Consistent with quenched 304 mushy-zone microstructure, back diffusion narrows the freezing temperature range, depletes the residual liquid, and lets δ dendrites bond together earlier to better resist cracking. It also reduces the crack susceptibility index |dT/d(fS )1/2| near fS = 1, consistent with the much less cracking observed in 304 than 310.

Study on hot cracking susceptibility of alloy 617M and an analysis on parameters used for its evaluation

Hot cracking study on nickel base superalloy, alloy 617M, a candidate material for Advanced Ultra Supercritical (AUSC) power plant applications, was carried out. The susceptibility of this alloy to solidification cracking was evaluated using varestraint (variable restraint) test set up and parameters like Brittleness Temperature Range (BTR) and Critical Strain rate per Temperature drop (CST) were estimated from the results. The BTR and CST for alloy 617M indicate higher susceptibility to solidification cracking than stabilized and un-stabilized austenitic stainless steels. However, mock-up welding of 400 mm diameter alloy 617M hollow forgings with weld thickness of 95 mm and alloy 617M boiler tubes of ~12 wall thickness made from this alloy did not reveal any significant cracking in the welds (except some isolated micro fissures) and hence, a detailed analysis of the results from varestraint tests was taken up. The study revealed the dual hot cracking behaviour of alloy 617M and the reasons behind such observation is elucidated. The microstructural characterisation of the hot crack tested specimens of alloy 617M revealed eutectics of austenite + carbides rich in Mo and Cr in the backfilled regions of the hot cracks in the fusion zone. The formation of significant fraction of eutectic phases along the interdendritic regions at the end of solidification is the primary reason for the high BTR exhibited by alloy 617M. Further, backfilling of the solidification cracks by this eutectic liquid is enabled under low augmented strain and the same is insufficient to aid backfilling in cracks at high restraints. The mechanism mentioned above was found to be the underlying reason for the dual behaviour revealed by alloy 617M. Hence, it was deduced that BTR and CST determined at saturated strain cannot be effectively used to predict the actual hot cracking behaviour of alloy 617M. A refined criterion-BTR at threshold strain is proposed to be an appropriate solidification cracking susceptibility index for this alloy.