Solidification Cracking Susceptibility Research Articles

Discovery of microsegregation-aided transformation and twinning-induced plasticity in low Mn steel through directed energy deposition of functionally graded materials

Functionally graded materials (FGMs) combining two dissimilar steels, stainless steel 316 L and high-strength low-alloy steel, were additively manufactured using directed energy deposition. High-throughput characterization of the dissimilar steel FGM led to the discovery of a low Manganese (Mn<1 wt.%) TRIP (transformation-induced plasticity) and TWIP (twinning-induced plasticity) steel with a metastable microstructure enabled by additive manufacturing. Microsegregation from non-equilibrium solidification caused phase stability and stacking fault energy heterogeneity, leading to TRIP and TWIP effects in the as-built condition without heat treatment. Tensile testing of the new as-built TRIP and TWIP steel resulted in ultimate tensile strength of 960 MPa, yield strength of 415 MPa, total elongation of 26 %, and a unique strain hardening rate that increases after yielding. We compare experimental measurements of microsegregation with thermodynamic modeling to discuss the impact of microsegregation on phase stability, stacking fault energy, and solidification cracking susceptibility in additively manufactured FGMs. The highlights of this work include the discovery of a novel pathway for achieving TRIP and TWIP effects in additively manufactured steels without heat treatment. This work also shows that the TWIP effect can be introduced without high Manganese content playing a critical role in adjusting stacking fault energy.

Effect of Single Crystal Growth and Solidification Grain Boundaries on Weld Solidification Cracking Behavior of CMSX-4 Superalloy

Single-crystal superalloys have been popularly employed in high-temperature parts of gas turbines, such as blades. However, the welds of such alloys are highly susceptible to solidification cracking, which limits their applicability to high-temperature turbine blades. In this study, the effects of characteristics of weld solidification on solidification cracking susceptibilities (solidification brittle temperature range, BTR) were fundamentally investigated for the CMSX-4 single-crystal superalloy. We applied a transverse-Varestraint test procedure for both the linear and oscillated arc welds by changing the weld solidification characteristics, such as the degree of single crystal growth and formation of solidification grain boundaries. The BTR for the CMSX-4 alloy is 336 K for linear welding condition, whereas the values are 434 K and 342 K for 0.6 and 1.5 Hz oscillated welds. Interestingly, the BTR continuously increases with the weld oscillation frequency. By contrast, almost no changes in the weld mushy-zone temperature range are theoretically calculated for each welding condition via the diffusion-controlled Scheil model. The mechanism underlying the increase in BTR under oscillation welding is clarified based on the relationship between the achievement ratio of the weld single crystal growth and fraction of high-angle (&gt;15&lt;sup&gt;o&lt;/sup&gt;) solidification boundaries, which affect severe dendrite coalescence undercooling. The lower fraction of the high-angle weld solidification grain boundaries attributed to the superior achievement ratio of weld single crystal growth, which reduces the dendrite coalescence undercooling and BTR. Consequently, it enhances the solidification crack propagation resistance.

Influence of the temperature gradient and the pulling velocity on solidification cracking susceptibility during welding: A phase field study

The influence of the temperature gradient and the pulling velocity on solidification cracking susceptibility (SCS) during welding is studied by simulating the liquid channel evolution with a quantitative phase field model. Increasing the pulling velocity or decreasing the temperature gradient increases the pressure drop from the dendrite tip to the coalescence point, leading to an increase in SCS. Decreasing the primary dendrite arm spacing (PDAS) decreases the permeability of the liquid channel and the liquid channel length at the same time, resulting in a decrease in the pressure drop and SCS when the PDAS is small. Consideration of the PDAS dependency on the temperature gradient and the pulling velocity influences the value of the pressure drop but does not change the tendency of SCS. The conclusions are also valid for an alloy with strong back diffusion. As the temperature gradient and the pulling velocity within the melt pool are controlled by process parameters, the findings from this work provide a theoretical basis to optimize process parameters to avoid solidification cracking.

Solidification crack elimination in TIG welding of Al7075: Experimental investigation and modified RDG modelling

Although solidification crack susceptibility modelling has been a point of interest for many years in fusion welding of aluminum alloys, the underlying mechanism of solidification crack elimination is not yet fully understood. To contribute in closing this knowledge gap, the present study investigates and proposes a new modified Rappaz-Drezet-Gremaud (RDG) physical model in autogenous and heterogeneous TIG welding of Al7075 sheets. The core novelty of the new model is that it includes the shrinkage strain rate, which allows to better capture the effects of capillary pressure, coherency point, as well as final dendrite or grain size in the fusion zone. In agreement with the experimental results and previous models, the new model shows that the defining mechanism of eliminating solidification cracks in heterogeneous joints is the capacity of nanoparticles to initiate heterogeneous grain nucleation, altering the fusion zone grain morphology from dendritic to fine equiaxed, with only little impact of the resulting grain size. This leads to two main effects causing reduced solidification crack susceptibility: (1) a reduction in coherency point by about 15 % from 902 K to 879 K, which reduces strain accumulation on the coherent solid network; (2) a considerable increase in capillary pressure across the entire mushy zone, allowing to maintain the total pressure at void/liquid interfaces in the positive range, which prevents the growth of potentially formed voids between grains at any temperature during solidification. In contrast, dendrite widths below 68 μm are required to prevent solidification cracks in autogenous joints without TiC nanoparticles, indicating a greater role of grain refinement. Overall, the proposed model highlights the more determinant role of the temperature-dependent solidification shrinkage strain rate in solidification crack susceptibility as it is an order of magnitude greater compared to the temperature-independent thermal contraction strain rate that has been the main or sole focus of previous models.

Effects of ferrite content and concentrations of carbon and silicon on weld solidification cracking susceptibility of stainless steels

In the welding of austenitic stainless steels, the formation of approximately 5% δ-ferrite phase is often used to decrease the susceptibility to solidification cracking. The partition coefficients of impurity elements such as phosphorous and sulfur of the ferrite phase are high, which expand the temperature range of solidification. At the same time, it is well known that the solidification cracking susceptibility increases with increasing δ-ferrite content above 20%. Thus, high δ-ferrite content increases the solidification cracking susceptibility, even though the δ-ferrite phase has high solubility of the impurity elements. The aim of this work was to investigate the effects of the δ-ferrite content and the concentration of carbon and silicon on the solidification crack susceptibility of Fe–18%Cr stainless steel with various nickel concentrations. The ferrite content was controlled by adding various amounts of nickel. The brittle temperature range (BTR) hardly changed with increasing δ-ferrite content, regardless of the type and concentrations of carbon and silicon. Therefore, the effect of the ferrite content on solidification cracking susceptibility must be small. In addition, the BTR increased with the concentrations of carbon and silicon. The specimens containing higher concentrations of carbon exhibited higher BTRs than those containing silicon. It is considered that the solidification segregation corresponding to the partition coefficient of carbon to the δ-ferrite phase induces an increase in cracking susceptibility. In addition, the formation of austenite during solidification reduces cracking susceptibility owing to the higher solubility of carbon.

Estimation of solidification cracking susceptibility in Al–Si–Cu alloy weld: effects of anisotropic permeability and deformation orientation

Weld solidification cracking susceptibility (SCS) was evaluated using an analytical model developed considering the anisotropic permeability (i.e., permeability which is different in directions parallel and perpendicular to the primary dendrite arms) and deviation of the imposed deformation from the normal direction with respect to the primary dendrite arm (ϕ). The model was investigated for Al–Si–Cu alloys in the solid fraction range from 0.87 to 0.94. Low Si- high Cu content alloys showed the maximum SCS due to variation in the physical properties of weld pool, i.e., viscosity, solidification range and solid fraction as a function of temperature. Decrease in deviation of the deformation direction (ϕ) led to increase in the SCS. Using the anisotropic permeability as a function of the primary dendrite arm spacing (λ1) led to a change in the peak position of shrinkage pressure depression towards the high Si-high Cu content alloys. Arc oscillation imposed a dual impact on the SCS of weld: (i) increased the SCS by enhancement of the base metal dilution, i.e., adding more Cu into the weld pool and (ii) decreased the SCS by refining the weld microstructure and twisting the grain boundaries which in turn changed the deformation direction on the grain boundaries. Results of the weldability test on Al–Cu sheets welded using Al–Si filler metal were in good agreement with the model prediction.

Evaluation of Solidification Cracking Susceptibility Factors in Carbon Steel and Availability of Ti Addition Based on Theoretical Analysis

Evaluation of Solidification Cracking Susceptibility Factors in Carbon Steel and Availability of Ti Addition Based on Theoretical Analysis

Characterisation of structural modifications on cold-formed AA2024 substrates by wire arc additive manufacturing

Wire arc additive manufacturing, employing conventional fusion welding processes, shows excellent versatility and can therefore be used for structural modifications of existing parts to create hybrid components. This study evaluates the modification of cold-formed AA2024 metal sheets with 2xxx filler metals to produce 2xxx hybrid structures. The alloys are investigated by thermodynamic simulations and the solidification crack susceptibility is assessed by calculating the index for hot cracking susceptibility. Using cold metal transfer, single-layer depositions are generated on profile sheets after which the specimens are characterised by three analysis procedures: metallographic analysis, chemical analysis, and hardness testing. Results indicate that AA2024 wire material, although generally considered difficult to weld, is more suitable for the additive modification of AA2024 sheets and profiles, as crack-free specimens can be deposited on cold-formed substrates.

Circumventing Solidification Cracking Susceptibility in Al-Cu Alloys Prepared by Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) of Al-Cu alloys shows high susceptibility to cracking due to a wide solidification temperature range. In this work, 2024 alloys were manufactured by LPBF at different laser processing parameters. The effect of processing parameters on the densification behavior and mechanical properties of the LPBF-processed 2024 alloys was investigated. The results show that the porosity increases significantly with increasing laser power, while the number of cracks and lack-of-fusion defects increase distinctly with increasing scan speed. The solidification cracking susceptibility of the LPBF-processed 2024 alloys prepared at different processing parameters was analyzed based on a finite element model, which was accurately predicted by theoretical calculations. Dense and crack-free 2024 samples with a high densification of over 98.1% were manufactured at a low laser power of 200 W combined with a low laser scan speed of 100 mm/s. The LPBF-processed 2024 alloys show a high hardness of 110 ± 4 HV0.2, an ultimate tensile strength of 300 ± 15 MPa, and an elongation of ∼3%. This work can serve as reference for obtaining crack-free and high-performance Al-Cu alloys by LPBF.

Solidification Cracking Susceptibility Predictions for Stainless Steels

Solidification Cracking Susceptibility Predictions for Stainless Steels

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.

Assessment of the Solidification Cracking Susceptibility of Welding Consumables in the Varestraint Test by Means of an Extended Evaluation Methodology

Various test methods are available for assessing the susceptibility of materials to solidification cracking during welding. In the widely used Varestraint test, the crack length is selected as a criterion as a function of the applied bending strain. Unfortunately, the crack length does not characterize the material behavior alone but depends to varying degrees on the individual test parameters used, which makes the interpretation of the results difficult. In addition, the crack length is not comparable under different test conditions. To overcome these disadvantages, we have developed a novel evaluation methodology that decouples the machine influence from the material behavior. The measured crack length is related to the maximum possible value specified by welding speed and deformation time. This relative crack length is calculated numerically, considering the orientation of the cracks. Experiments on two high‐alloy martensitic welding consumables show that, in contrast to the conventional evaluation, a comparison of different welding parameters becomes possible. Furthermore, the strain rate proved to be a suitable crack criterion in agreement with Prokhorov's hot cracking model.

Determination of Location-Specific Solidification Cracking Susceptibility for a Mixed Dissimilar Alloy Processed by Wire-Arc Additive Manufacturing

Solidification cracking is a major obstacle when joining dissimilar alloys using additive manufacturing. In this work, location-specific solidification cracking susceptibility has been investigated using an integrated computational materials engineering (ICME) approach for a graded alloy formed by mixing P91 steel and Inconel 740H superalloy. An alloy mixture of 26 wt.% P91 and 74 wt.% Inconel 740H, with high configurational and total entropy, was fabricated using wire arc additive manufacturing. Microstructure characterization revealed intergranular solidification cracks in the FCC matrix, which increased in length along with the enrichment of Nb (~27 to 56 wt.%) and Cu (~87 wt.%) in the middle and top regions. DICTRA simulations to model location-specific solidification cracking susceptibility showed that the top region with the highest cooling rate (270 K/s) has the highest solidification cracking susceptibility in comparison with the middle and bottom regions. This is in good agreement with the experimentally observed varying crack length. From Scheil simulations, it was deduced that enrichment of Nb and Cu affected the solidification range as high as ~77%, in comparison with the matrix composition. The overall solidification cracking susceptibility and freezing range was highest for the 26 wt.% P91 alloy amongst the mixed compositions between P91 steel and 740H superalloy, proving that solidification characteristics play a major role in alloy design for additive manufacturing.

Strain-based investigation on solidification crack susceptibility of 6005 aluminium using adjustable ring mode (ARM) laser welding

Solidification cracks are among the most deteriorating defects observed in Remote Laser Welding (RLW) of 6xxx aluminium alloys. This research has been designed to study the solidification crack susceptibility during RLW of AA6005 alloy using Adjustable Ring mode (ARM) laser technology. The experimental setup included a high-speed camera on the self-restraint test rig, and the strains near the root of the solidification front were identified with a Finite Element based Digital Image Correlation (DIC) technique. This DIC technique allows the precise measurement of displacements and evaluates the strains at the root of the melt pool, and further correlates the critical strains formed near the root with the power ratios of the core and ring beam. A process window relating the total laser power and the power ratio to the cracking susceptibility was identified. Although a linear dependency exists between power ratio and weld penetration depth, the same does not apply to the crack sensitivity since a parabola-like relationship was observed. Findings suggested that the transition from coarse to fine equiaxed grains at increasing power ratios reduces the crack sensitivity to the threshold power ratio of 1.5. It reveals that the cracking susceptibility is determined by the combined effect of the grain refinement (predominant at power ratios below 1.5) and the permeability in the fusion zone (predominant at power ratios above 1.5).

Solidification Cracking Susceptibility Predictions for Stainless Steels

Solidification Cracking Susceptibility Predictions for Stainless Steels

Influence Mechanism of Solidification Mode on Solidification Cracking Susceptibility of Stainless Steels

Influence Mechanism of Solidification Mode on Solidification Cracking Susceptibility of Stainless Steels

Multiphase-field simulation of grain coalescence behavior and its effects on solidification cracking susceptibility during welding of Al-Cu alloys

Solidification cracking (SC) is highly related to the grain coalescence behavior during welding of aluminum alloys. In this study, the grain coalescence behavior and its effects on solidification cracking susceptibility (SCS) were investigated using the multiphase-field approach. Why SCS is high at a certain value of Cu concentration and why SC often occurs at high misorientation angles are revealed. Firstly, nominal compositions of Cu affect the morphology of microstructure during solidification. The crystals morphology is cellular at the low concentration, while the crystals are dendritic at the high concentration in the columnar grain region. The SCS of cellular grains is higher than dendrites due to the high volume fraction of solid when the grains/subgrains bridge. Under the action of tensile stress, the scarce residual liquid phase cannot backfill in time. Secondly, high misorientation angles make grain boundary energy in the solid–solid interface (σSS) is high. It is found that σSS suppresses the grain coalescence and increases the SCS of alloys. This leads the emergence of SC at high misorientation angles during welding. In this study, the coalescence behavior of grains during solidification is visually presented by simulation and the coherency point at the last-stage solidification is achieved accurately.

Effect of ERNiFeCr-2 Filler Metal on Solidification Cracking Susceptibility of CM247LC Superalloy Welds

The metallurgical aspects of weld solidification cracking in Ni-based superalloys (with Ti+Al &gt; 5 mass%) have not been widely investigated thus far. Herein, the solidification cracking susceptibility of the CM247LC superalloy and its welds with ERNiFeCr-2 filler wire was quantitatively evaluated using a novel modified Varestraint testing method, for the successful manufacturing of CM247LC superalloy gas turbine blades. It was found that the solidification brittle temperature range (BTR) of the CM247LC superalloy was 400 K. This measurement was obtained with a high-speed thermo-vision camera. The BTR increased to 486 K for the CM247LC/ERNiFeCr-2 welds (dilution ratio: 74%). Theoretical calculations (i.e., the Scheil equation, performed using Thermo-Calc software) were conducted to determine the temperature range in which both solid and liquid phases coexist, together with the microstructural characterization of the solidification cracking surfaces. The greater increase in BTR for the CM247LC/ERNiFeCr-2 welds than that for CM247LC was attributed to the enlargement of the solid–liquid coexistence temperature range. This correlated with the formation of a low-temperature Laves phase during the terminal stage of solidification, and was affected by the diluted Nb and Fe components in the ERNiFeCr-2 filler metal. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to be an effective testing process for solidification cracking behavior in the manufacturing of high-soundness CM247LC superalloy welds.

Effect of ERNiFeCr-2 Filler Metal on Solidification Cracking Susceptibility of CM247LC Superalloy Welds

The metallurgical aspects of weld solidification cracking in Ni-based superalloys (with Ti+Al &gt; 5 mass%) have not been widely investigated thus far. Herein, the solidification cracking susceptibility of the CM247LC superalloy and its welds with ERNiFeCr-2 filler wire was quantitatively evaluated using a novel modified Varestraint testing method, for the successful manufacturing of CM247LC superalloy gas turbine blades. It was found that the solidification brittle temperature range (BTR) of the CM247LC superalloy was 400 K. This measurement was obtained with a high-speed thermo-vision camera. The BTR increased to 486 K for the CM247LC/ERNiFeCr-2 welds (dilution ratio: 74%). Theoretical calculations (i.e., the Scheil equation, performed using Thermo-Calc software) were conducted to determine the temperature range in which both solid and liquid phases coexist, together with the microstructural characterization of the solidification cracking surfaces. The greater increase in BTR for the CM247LC/ERNiFeCr-2 welds than that for CM247LC was attributed to the enlargement of the solid–liquid coexistence temperature range. This correlated with the formation of a low-temperature Laves phase during the terminal stage of solidification, and was affected by the diluted Nb and Fe components in the ERNiFeCr-2 filler metal. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to be an effective testing process for solidification cracking behavior in the manufacturing of high-soundness CM247LC superalloy welds.

Printability assessment with porosity and solidification cracking susceptibilities for a high strength aluminum alloy during laser powder bed fusion

The rapid development of additive manufacturing requires a large number of printable metallic materials for various engineering applications. In this work, the printability of a high strength aluminum alloy AA2024 was evaluated for laser powder bed fusion (LPBF) with comparisons to the widely used AlSi10Mg. Strikingly different solidification cracking networks were generated when using diverse scanning strategies and overlap rates of adjacent tracks. Depending on the overlap rates, the crack propagation patterns transited from the longitudinal-dominant to the transverse-dominant. The lengths and propagation angles of the transverse zigzag cracks were strongly interdependent and both increased with the overlap rate of adjacent tracks. The characterization results implied that these diverse crack propagation patterns originated from the columnar grain structure, which were produced in the localized solidification conditions determined by the track-wisely moving molten pool. The characteristics of the thermal cycles and solidification conditions of AlSi10Mg and AA2024 were further examined using a comprehensive phenomenological model for a better understanding of the printability of AA2024. The results showed that AA2024 generated smaller molten pool and that AA2024 was more susceptible to lack of fusion defects given the same LPBF process conditions. Supported by the evaluated printability of AA2024, a comprehensive scheme targeting the printing of crack-free LPBF builds was proposed considering the heat input, the overlap rate, and the scanning strategies of the laser beam.