Ductility-dip Cracking Research Articles

Strength and ductility of additively manufactured 310 austenitic stainless steel via wire-arc directed energy deposition: The role of columnar grain growth and ductility-dip cracking

This study investigates the underlying factors governing the mechanical properties of single-wall additively manufactured SS 310 austenitic stainless steel via wire-arc directed energy deposition. It demonstrates the predominant microstructural feature in the printed SS 310 stainless steel is the formation of epitaxial large columnar austenite grains, which promotes the occurrence of sub-solidus solid-state ductility-dip cracking (DDC) during the multi-layer additive manufacturing process. While the yield strength and tensile strength of wire-arc additively manufactured SS 310 are comparable to those of wrought annealed AISI 310, the short micro-cracks and the presence of δ-ferrite hinder the work hardening rate and uniform elongation. Additionally, micro-cracks promote void nucleation during the ductile fracture process, resulting in a noteworthy reduction in post-necking elongation and energy absorption capability. The stress-strain behavior of the manufactured part exhibits anisotropy due to the growth of columnar grains, the heterogeneous periodic microstructure, and the orientation of the ductility-dip cracks. To fully harness the potential of wire-arc additive manufacturing as a cost-effective and sustainable manufacturing process, it is imperative to optimize the grain structure and minimize residual stress to eliminate the occurrence of DDC in the production of SS 310 austenitic stainless steel.

Effect of Inconel 718 Filler on the Microstructure and Mechanical Properties of Inconel 690 Joint by Ultrasonic Frequency Pulse Assisted TIG Welding.

Ultrasonic frequency pulse assisted TIG welding (UFP-TIG) experiments were conducted to join Inconel 690 alloy (IN690) by adding Inconel 718 alloy (IN718) as the filler. The effect of the filler on the microstructure, mechanical properties, and ductility dip cracking (DDC) susceptibility of IN690 joints were investigated. The results show that a variety of precipitates, including MC-type carbide and Laves phases, are formed in the weld zone (WZ), which are uniformly dispersed in the interdendritic region and grain boundaries (GBs). The increase in the thickness of the IN718 filler facilitates the precipitation and growth of Laves phases and MC carbides. However, the formation of Laves phases in the WZ exhibits a lower bonding force with the matrix and deteriorates the tensile strength of IN690 joints. Due to the moderate content of Laves phases in the WZ, the IN690 joint with 1.0 mm filler reaches the maximum tensile strength (627 MPa), which is about 96.5% of that of the base metal (BM). The joint with 1.0 mm filler also achieves the highest elongation (35.4%). In addition, the strain-to-fracture tests indicate that the total length of cracks in the joint with the IN718 filler decreases by 66.49% under a 3.8% strain. As a result, the addition of the IN718 filler significantly improves the mechanical properties and DDC resistance of IN690 joints.

Weld Cracking Susceptibility of High-Cr Ni-Base Fe Alloys and Its Improvement—Development of Novel Test Method for Ductility-Dip Cracking and New Alloy

Weld Cracking Susceptibility of High-Cr Ni-Base Fe Alloys and Its Improvement—Development of Novel Test Method for Ductility-Dip Cracking and New Alloy

Solute segregation in a rapidly solidified Hastelloy-X Ni-based superalloy during laser powder bed fusion investigated by phase-field and computational thermal-fluid dynamics simulations

Solute segregation significantly affects material properties and is a critical issue in the laser powder-bed fusion (LPBF) additive manufacturing (AM) of Ni-based superalloys. To the best of our knowledge, this is the first study to demonstrate a computational thermal-fluid dynamics (CtFD) simulation coupled multi-phase-field (MPF) simulation with a multicomponent-composition model of Ni-based superalloy to predict solute segregation under solidification conditions in LPBF. The MPF simulation of the Hastelloy-X superalloy reproduced the experimentally observed submicron-sized cell structure. Significant solute segregations were formed within interdendritic regions during solidification at high cooling rates of up to 1.6 × 106 K s−1, a characteristic feature of LPBF. Solute segregation caused a decrease in the solidus temperature (TS), with a reduction of up to 38.4 K, which increases the risk of liquation cracks during LPBF. In addition, the segregation triggers the formation of carbide phases, which increases the susceptibility to ductility dip cracking. Conversely, we found that the decrease in TS is suppressed at the melt-pool boundary regions, where re-remelting occurs during the stacking of the layer above. Controlling the re-remelting behavior is deemed to be crucial for designing crack-free alloys. Thus, we demonstrated that solute segregation at the various interfacial regions of Ni-based multicomponent alloys can be predicted by the conventional MPF simulation. The design of crack-free Ni-based superalloys can be expedited by MPF simulations of a broad range of element combinations and their concentrations in multicomponent Ni-based superalloys.

Thermal faceting on the ductility-dip cracking fracture surfaces of nickel-based alloys – Occurrence, characterization, and implications for the cracking mechanism

Ductility-dip cracking (DDC) fracture surfaces containing thermal faceting (TF) are documented in this study. Research began with the discovery of TF on the DDC fracture surface in the weld metal of high-chromium nickel (Ni)-based filler metal 52M, a valuable alloy to the nuclear industry. Fracture surfaces were generated using strain-to-fracture (STF) and simulated strain ratcheting (SSR) testing on a Gleeble® thermomechanical simulator. Initial fractography performed by scanning electron microscopy (SEM) revealed faceting with preferred grain orientations and faceting interacting with void nucleation near grain boundary (GB) triple points. Internal voids were exposed via metallography, where TF was also observed on the free surface. A thin orientation- and site-specific foil was extracted from the faceted fracture surface using focused ion beam (FIB) nanofabrication. Subsequent scanning transmission electron microscopy (STEM) characterization down to atomic-resolution revealed the facets have perfect crystallographic orientations parallel to close-packed directions. Sharp edges and transitions between individual facets on the scale of individual atomic columns and atomic planes were observed. An analysis of the deformation microstructure and the chemical composition underneath the faceted surface showed a low dislocation density and no evidence of localized segregation or diffusion, respectively. These findings have led to the development of a hypothesis related to the interdependence of TF and DDC and support initial claims regarding TF formation in DDC voids at elevated temperatures.

Welding hot cracking characteristics and fracture mechanisms of nickel‐based alloy NS1402 under different constraint conditions

AbstractThe influence of trace elements variation and constraint conditions on welding hot cracking was investigated to reveal the cracking characteristics of nickel‐based alloy NS1402. The fishbone tests and full restraint welding tests have been conducted to determine the cracking ratio. The mechanical and metallurgical fracture mechanisms were elaborated simultaneously. And the low‐melting point liquid membrane, which was a key metallurgical factor, was found in cracking fracture surface. Numerical simulations were conducted to illustrate the effects of transverse tensile stress/strain during welding processing. Results show that the liquid membranes were composed of FeNi, FeP, Co3Fe7, etc., indicating that liquid membranes were developed to higher melting point by Co3Fe7 eutectics due to the addition of Co, and it contributes to reducing cracking susceptibility of NS1402. The cracking was ductility‐dip cracking (DDC), which propagated in the forms of intergranular or transgranular fracture. Thus, adjusting element content and degrees of constraint could control the cracking tendency.

Cracking behaviour and its suppression mechanisms with TiB2 additions in the laser additive manufacturing of solid-solution-strengthened Ni-based alloys

This study systematically investigated the cracking mechanisms in the laser powder bed fusion (LPBF) of GH3230 solid-solution-strengthened Ni-based alloy (GH0). The results show that the micro-cracks that formed in GH0 specimens included both solidification and solid-state cracks. The initiation of solidification cracks was associated with the formation of continuous liquid films on high-angle grain boundaries at the final stage of solidification, while the solid-state cracks were found to be ductility-dip cracks, associated with a reduction in the material's plasticity within the heat-affected zone. The study also found that the residual stresses decreased with increasing LSS values, leading to reductions in crack length. The introduction of 1 wt% TiB2 particles to GH3230 (GH1-composite) was found to suppress cracking by promoting grain refinement and generating special high-angle grain boundaries, although residual thermal stresses increased. The ultimate tensile strength values of the GH0 and GH1-composite specimens at 900 °C were found to be 213 and 352 MPa, respectively. These findings provide significant insights into the LPBF of high-performance crack-free Ni-based superalloys.

Laser powder bed fusion of a Ni3Al-based intermetallic alloy with tailored microstructure and superior mechanical performance

Ni3Al-based alloys are excellent candidates for the structural materials used for turbine engines due to their excellent high-temperature properties. This study aims at laser powder bed fusion and post-hot isostatic pressing (HIP) treatment of Ni3Al-based IC-221 ​M alloy with a high γ′ volume fraction. The as-built samples exhibits unavoidable solidification cracking and ductility dip cracking, and the laser parameter optimization can reduce the crack density to 1.34 ​mm/mm2. Transmission electron microscope (TEM) analysis reveals ultra-fine nanoscale γ′ phases in the as-built samples due to the high cooling rate during rapid solidification. After HIP treatment, a fully dense structure without cracking defects is achieved, which exhibits an equiaxed structure with grain size ∼120–180 ​μm and irregularly shaped γ′ precipitates ∼1–3 ​μm with a prominently high fraction of 86%. The room-temperature tensile test of as-built samples shows a high ultimate tensile strength (σUTS) of 1039.7 ​MPa and low fracture elongation of 6.4%. After HIP treatment, a significant improvement in ductility (15.7%) and a slight loss of strength (σUTS of 831.7 ​MPa) are obtained by eliminating the crack defects. Both the as-built and HIP samples exhibit retained high σUTS values of 589.8 ​MPa and 786.2 ​MPa, respectively, at 900 ​°C. The HIP samples exhibita slight decrease in ductility to ∼12.9%, indicating excellent high-temperature mechanical performance. Moreover, the abnormal increase in strength and decrease in ductility suggest the critical role of a high γ′ fraction in cracking formation. The intrinsic heat treatment during repeating thermal cycles can induce brittleness and trigger cracking initiation in the heat-affected zone with notable deteriorating ductility. The results indicate that the combination of LPBF and HIP can effectively reduce the crack density and enhance the mechanical properties of Ni3Al-based alloy, making it a promising material for high-temperature applications.

Directed energy deposition of Invar 36 alloy using cold wire pulsed gas tungsten arc welding: Effect of heat input on the microstructure and functional behaviour

Invar alloys exhibit low thermal expansion and are useful in applications requiring dimensional stability when subject to temperature changes. Conventional production of Invar faces certain challenges that can be offset by exploiting additive manufacturing processes. This study employed pulsed gas tungsten arc welding (GTAW) to deposit Invar 36 alloy blocks at five heat inputs (HI) ranging from 200 to 550 J mm−1. The results show that the microstructure comprised of columnar grains and remained in the austenitic phase regardless of the HI. Ductility dip cracking was found to prevail in all the blocks except the block deposited at the lowest HI. The decreased susceptibility to cracking with a reduction in the HI was due to the preservation of the grain boundary area, consequently leading to an improved partitioning of strain among the grain boundaries. On lowering the HI from 550 to 200 J mm−1 the average yield strength, tensile strength and elongation improved by 16%, 23% and 38%, respectively. The HI had a negligible effect on the mean linear coefficient of thermal expansion (CTE) in different temperature ranges as the CTE values were nearly identical between the blocks deposited at 200 and 550 J mm−1. In general, the CTE in the building direction was slightly higher than the travel direction, with a maximum difference between the CTE of the two directions being 15%. In summary, this work demonstrates the application of the cold wire GTAW process as an alternative to conventional/laser based methods for realizing the functional properties of Invar.

Analysis of the ductility dip cracking in the nickel-base alloy welding overlay on heat exchanger tube sheet

In this paper, the cracking failure of the weld overlay of the heat exchanger tube sheet was analyzed. The weld overlay, welded by ErNiCrMo-3, cracked several times within a short period after the heat exchanger was put into operation, resulting in internal leakage from the heat exchanger. The failure causes are investigated by means of macroscopic observation, metallographic analysis, scanning electron microscope (SEM) coupled with energy dispersive spectroscopy (EDS) analysis, and hardness test. The synthetical analysis shows that a sudden drop in ductility of the weld overlay material occurs at the time of microcracking, and a phenomenon similar to grain boundary sliding is observed in the fracture morphology image at high magnification. Therefore, the microcrack on the weld overlay is a kind of ductility dip crack at elevated temperatures. Improper welding strategy and inappropriate choice of welding material is the root cause of ductility-dip microcrack during the welding process. The microcracks on the weld overlay propagate during the operation of the heat exchanger, causing it to leak several times in a short period.

Influence of Grain Boundary Morphology on DDC Sensitivity of Nickel Base Alloy Deposited Metal

Effect of grain boundary morphology on ductility dip cracking (DDC) sensitivity of nickel base alloy inconel52 deposited metal was researched by welding thermal simulation method and high temperature tensile test. The sample was hold at 1300 °C for 2S ~ 10s and then stretched at its DDC sensitive temperature 1050 °C at different tensile rates. The DDC sensitivity was compared by reduction of area (VoA) of tensile test sample. The results show that straight grain boundary reduces VoA, precipitates in grain boundaries increases VoA, and VoA increases with the increase of tensile rate. Straight grain boundary causes stress concentration and strain localization at the trigeminal grain boundary, curved grain boundary decreases the maximum Mises stress which make more uniform stress distribution. Precipitates on the grain boundary can play a role of locking the grain boundary migration and disperse the strain concentration at the trigeminal grain boundary. The lower the strain rate, the longer the deformation time, which will lead to decrease of dislocation movement rate. The smaller the critical shear stress of grain boundary sliding, the smaller the deformation resistance, and the full progress of dislocation movement and climbing. Effect of strain rate on DDC needs more research.

Crack-Free 30% Chromium-Nickel Alloy Welding Products for Nuclear Service

Prior research in the development of 30% chromium-nickel alloy nuclear welding wires has resulted in the resolution of primary water stress corrosion cracking (PWSCC) and ductility dip cracking (DDC) as well as improvement in solidification cracking (SC) resistance. The resolution of DDC exhibits some Laves phase, which has a negative effect on SC resistance. In this study, the use of an alternate carbide former, tantalum (Ta), in combination with niobium (Nb) was researched. Three heats of recently designed Filler Metal 52MSS-Ta (i.e., HV1648, HV1673A, and VX131WXW) were melted, fabricated, and systematically studied. DDC and SC were evaluated with thermodynamic modeling using the Scheil solidification simulation model, two types of varestraint tests, and strain-to-fracture (STF) testing. The varestraint and STF test results showed an improved SC resistance with reduced Laves phase and concurrent excellent DDC resistance. Optimized compositions with low Laves phase also exhibited high threshold strain values (TSVs) in the STF test. VX131WXW — which contains 2.81 wt-% Ta, 0.6 wt-% Nb, and 6 wt-% iron (Fe) - exhibited a TSV of 24%. Thermo-Calc computed the Laves phase to be 0.24% for VX131WXW compared to 0.06% in HV1673A. This difference in Laves phase resulted in the lower SC resistance of VX131WXW compared to HV1673A when measured with longitudinal varestraint testing. The maximum crack distance for HV1673A was about 0.6 mm while that of heat VX131WXW was about 1.0 mm. The typical diluted weld deposit made with VX131WXW was also resistant to PWSCC due to the chromium content exceeding 24%. These simultaneous results mark progress toward crack-free welds and provide direction for further optimization of Ta-containing filler metals.

Novel design of weld vector route for dissimilar nonferrous plates laser welding in battery manufacturing for electric vehicles

In this research, Al/Cu thicker plates are overlap-joined based on an experimental simulation to improve the metallurgical quality of laser welding the busbar tab to interconnect plate in manufacturing production for electric vehicle battery. The influence of weld vector route on the weld formation, microstructure and mechanical properties has been investigated via stereological microscope, metallographic microscope, material tensile testing machine, scanning electron microscope and energy dispersive spectroscope. Ductility-dip cracking (DDC) easily takes place along fusion zone migrated grain boundaries (MGBs) in local zones of brittle medium-concentrated IMC phases (AlCu+Al3Cu4) of the plain straight weld produced by conventional process, which leads to a maximum load force of only 915 N in tensile shear test. The designed Al/Cu laser welding with novel weld vector route could reduce the level of restraint in the weld metal and the degree of stress relaxation during reheating, which in turn prevents the occurrence of cracks. In tensile shear test, the dummy specimens have achieved an excellent performance with a maximum load force of 1380 N and a prominent plasticity owning a certain degree of dimple-like ductile fracture composition during deformation to break. This novel design of laser welding can successfully improve both the product quality and production efficiency for a green low-carbon manufacturing process, and concurrently reduce the time cost and energy consumption in rework process for nonconforming battery modules. Furthermore, from aspect of failure mode and effects analysis (FMEA), the novel process could enhance the safety and reliability of battery pack in physics and to refrain from potential failures resulting from thermal runaway, vibration, or impact due to pressure and dynamic mechanical loads when electric vehicles are running on the roads.

Correlation of imposed mechanical energy with ductility-dip cracking in a highly restrained weld of Alloy 52

Ductility-dip cracking (DDC) is a complex weldability issue affecting industries using high-chromium nickel-based filler metals required for superior resistance to stress corrosion cracking, most notably in the nuclear power generation industry. DDC is a solid-state cracking mechanism which occurs in the temperature range of 0.5–0.8TM and is primarily attributed to grain boundary sliding. Above 0.8TM, recrystallization inhibits initiation of DDC and has been observed to arrest propagation. A computational model of thermo-mechanical behavior in a narrow groove multipass weld of DDC-susceptible filler metal was developed in past research and revisited here to calculate and analyze the mechanical energy imposed during the welding process. It was found that areas in the weld with high imposed mechanical energy (IME) in the recrystallization temperature range coincided with regions devoid of cracking but rich in evidence of recrystallization. Weld areas with low IME in the recrystallization temperature range but with higher IME in lower temperature ranges coincided with regions where DDC is observed. This agrees with past research suggesting recrystallization acts to prevent cracking. This paper presents an integrated computational materials engineering (ICME) approach that includes the IME to better predict and quantify DDC susceptibility in multipass welds.

Study on the ductility-dip cracking and its formation mechanism of additive manufactured NiCrFe-7A alloy: Effect of the carbon content

The effect of carbon content on the ductility-dip cracking susceptibility of the additive manufactured NiCrFe-7A alloys was investigated. The temperature-strain curves were measured by using strain-to-fracture test via Gleeble-3500 thermal simulator. Results show that the minimum threshold strains of NiCrFe-7A alloys with low carbon and high carbon are 3.61% and 4.72%, respectively. Higher carbon additions promoted more carbides to precipitate at grain boundaries, then improved the resistance to ductility-dip cracking. Moreover, a higher proportion of twins and recrystallization occurred in the NiCrFe-7A alloy with high carbon was beneficial to hinder ductility-dip cracking propagation.

On the Influence of Alloy Composition on the Additive Manufacturability of Ni-Based Superalloys

The susceptibility of nickel-based superalloys to processing-induced crack formation during laser powder-bed additive manufacturing is studied. Twelve different alloys—some of existing (heritage) type but also other newly-designed ones—are considered. A strong inter-dependence of alloy composition and processability is demonstrated. Stereological procedures are developed to enable the two dominant defect types found—solidification cracks and solid-state ductility dip cracks—to be distinguished and quantified. Differential scanning calorimetry, creep stress relaxation tests at 1000 °C and measurements of tensile ductility at 800 °C are used to interpret the effects of alloy composition. A model for solid-state cracking is proposed, based on an incapacity to relax the thermal stress arising from constrained differential thermal contraction; its development is supported by experimental measurements using a constrained bar cooling test. A modified solidification cracking criterion is proposed based upon solidification range but including also a contribution from the stress relaxation effect. This work provides fundamental insights into the role of composition on the additive manufacturability of these materials.

Study on the Hot Cracking Law of Inconel 690/52M Welding Material on F304LN Base Metal by Multi-Layer Cladding

During the welding of 690 nickel-based alloy, solidification cracking (SFC) and ductility-dip cracking (DDC) easily forms, which has a negative effect on the quality of welded joints and service life. The present study examined the effects of welding heat input and cladding layers on the SFC and DDC, as well as their formation mechanism. The microstructure observation, elemental distribution, and Varestraint test were carried out. The results show that SFC and DDC were formed for the Inconel filler metal 52M, and SFC is more prone to form than DDC. The alloy elements such as Fe, Si, C, and P from base metal can expand the solidification temperature range, such that the SFC sensitivity increases. With the increase of welding heat input, the grain size of cladding metal is increased with a great SFC sensitivity. The increasing welding heat input also makes DDC possible due to the formation of a large angle grain boundary.

Resistance of Austenitic Stainless Steels to Ductility-Dip Cracking: Mechanisms

In Ni-based alloys, precipitates that form along grain boundaries (GBs) during terminal solidification have been shown to pin GBs and resist GB sliding, which can cause ductility-dip cracking (DDC). As a result, it is often suggested that the stainless steel skeletal/lacy  in a  matrix resists DDC because it pins GBs. In the present study, austenitic stainless steels 304, 316, 310, and 321 were quenched with liquid Wood’s metal (75˚C) during welding. Quenching captured the elevated-temperature micro-structure and simultaneously induced cracking, thus revealing the mechanisms of the resistance to DDC. In addition, DDC was much higher in 310 than 304, 316, and 321, which is consistent with results of conventional tests. Both 304 and 316 solidified as columnar  grains, with continuous  formed along GBs soon after solidification to resist DDC along the GBs. 321 solidified as equiaxed grains of  instead of columnar, and the tortuous GBs associated with equiaxed grains resisted DDC. 310, however, solidified as coarse, straight  grains with little  along the GBs, and solidification GBs migrated to become locally straight. The resulting GBs were long, straight, and naked, which is ideal for DDC. In 304, 316, or 321, skeletal/lacy  in a  matrix did not exist in the fusion zone near the mushy zone, where DDC occurs. This proved skeletal/lacy  cannot resist DDC as often suggested. Instead, the present study identified two new mechanisms of resistance to DDC: 1) formation of continuous or nearly continuous  along boundaries of columnar  grains and 2) solidification as equiaxed  grains.

Experimental investigation into microstructure, mechanical properties, and cracking mechanism of IN713LC processed by laser powder bed fusion

IN713LC is a Ni-based superalloy with extensive uses in the aerospace industry. However, fabricating IN713LC components using additive manufacturing technologies such as laser powder bed fusion (LPBF) is extremely challenging due to its high composition of alloying elements (e.g., Al and Ti), which increase its susceptibility to cracking. Accordingly, the present study conducts an experimental investigation into the LPBF processing parameters (i.e., laser power and scanning speed) which enable the fabrication of LPBF IN713LC parts with a high relative density and minimal cracking. The effects of heat treatment on the microstructure and mechanical properties of the as-built parts are then examined. The results show that IN713LC is prone to the two micro-cracking mechanisms, namely solidification cracking due to the formation of low-melting-point elements at the grain boundaries and ductility dip cracking (DDC) as a result of grain boundary sliding. However, given an appropriate selection of the LPBF processing conditions, as-built IN713LC components with an average yield strength of 800 MPa, an ultimate tensile strength of 998 MPa, and ultimate elongation of 12.5% at room temperature are better than those of as-cast IN713LC specimens. Notably, the solid solution heat treatment process yields a significant improvement in the yield strength compared to that of the as-built sample. Moreover, all the mechanical properties of the sample processed by solid solution heat treatment are comparable to those of the as-cast IN713LC sample.

Effect of TiC particles on ductility dip cracking susceptibility of Ni-base superalloy

Ni-base superalloy FM-52M is mainly used in welded structures of key equipment of nuclear power plants, such as pressure vessels and heat dissipation evaporators. It is prone to form coarse columnar microstructures during welding and has significant ductility dip cracking (DDC) susceptibility. This work attempted to introduce micrometre-sized TiC particles into FM-52M welds to refine the microstructure and alleviate sensitivity to DDC. A Ni–TiC coating with good dispersion and uniformity of TiC particles was first prepared on the substrate by composite electroplating. The TiC particles were then directly introduced into the molten pool during tungsten–inert gas welding. Scanning electron microscopy and electron backscatter diffraction were used to analyse the microstructure of the weld and strain-to-fracture testing was performed to evaluate its DDC sensitivity. The experimental results showed that solidification grains of the weld were obviously refined and its high-temperature mechanical properties were reinforced. The minimum critical strain for DDC increased to approximately 3.5%, and the crack morphology was small and tortuous. Compared with the original material, the microstructure and DDC resistance of the weld were improved by introducing TiC particles.