Cracking Susceptibility Research Articles

Design high-strength Al–Mg–Si alloy fabricated by laser powder bed fusion: Cracking suppression and strengthening mechanism

Considerable researches on laser powder bed fusion (LPBF) have proved the high crack susceptibility of aluminum alloys and the necessity to increase the processability of LPBF for broader applications. To eliminate the hot cracking and achieve high performance, the present work devised a novel component with the advantages of superior processability, refined microstructure, and high strength based on theoretical calculation of crack susceptibility. The addition of 4.4 wt% Mg and 3.1 wt% Si was capable of obtaining the relative density of 99.6% and eliminating the hot cracks under the processing parameters of 300 W-1000mm/s-0.10 mm. This LPBF fabricated alloy formed granular Mg2Si and rod-shaped Si phases, and an ultimate tensile strength of 460 MPa, yield strength of 385 MPa, and elongation of 3.2% were obtained. In this work, hot cracks could be solved by composition adjustment without adding any nano nucleating agent. Detailed strengthening mechanism analysis revealed the source of high strength corresponding to refined microstructure, strengthening phase, and composition design. This work guides for defects suppression and composition design in additively manufactured high-strength Al–Mg–Si alloys and other aluminum alloys.

Investigating zinc-assisted liquid metal embrittlement in ferritic and austenitic steels: Correlation between crack susceptibility and failure mechanism

Investigating zinc-assisted liquid metal embrittlement in ferritic and austenitic steels: Correlation between crack susceptibility and failure mechanism

Shear properties of engineered cementitious composites using polyvinyl alcohol fiber

The utilization of Polyvinyl Alcohol (PVA) fibers in Engineering Cementitious Composites (ECC) has garnered significant attention in the realm of construction materials. This composite, formed from cement and ultra-ductile fibers, demonstrates exceptional properties, including high strength, resistance to elevated temperatures and corrosion, making it indispensable in structural engineering. Notably effective in seismic-resistant infrastructure and enduring structures subjected to harsh environmental conditions, ECC with PVA fibers stands out for its ductility and strainhardening capabilities. This study delves into ECC by incorporating various materials like cement, fly ash, sand, PVA fibers, and superplasticizer. PVA fibers, despite being costly, significantly augment ECC's ductility, strain-hardening behavior, and energy absorption properties. The inclusion of these fibers bolsters both shear and compression strength, elevating overall structural performance. Unlike conventional concrete, ECC showcases a remarkable tensile strain capacity of 3- 7%, contributing to its distinct characteristics. The literature review consolidates studies investigating PVA fiber's impact on ECC's mechanical properties, emphasizing enhancements in strength, toughness, and crack control. Factors such as fiber types, density, and additives are explored, showcasing how ECC with PVA fibers can augment performance and sustainability in construction materials. Findings reveal an increase in compressive strength at 7 days with added PVA fibers, albeit with some variations at 28 days. Similarly, shear strength escalates with increased PVA fiber content up to 1.5%, enhancing bonding and load-bearing capacity. However, higher fiber content at 2% causes increased water consumption, reducing load-carrying capacity. ECC fortified with PVA fibers demonstrates superior strength and durability compared to traditional concrete, overcoming brittleness and crack susceptibility. The research highlights the influence of fiber content on enhancing bonding and shear strength, establishing the potential for ECC with PVA fibers to revolutionize construction materials and practices.

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

Accelerated material development for laser powder-bed fusion using the arc melting process

Metal additive manufacturing has in recent years gained an increasing amount of attention, especially the subgroup of laser powder-bed fusion and aluminium alloys. However, established alloys are designed for casting and forging and often require alterations to make them eligible for the challenging processing conditions. The material selection is limited and calls for new alloys tailored specifically for additive manufacturing. In this work, an analysis suite is proposed as a tool to investigate material systems quickly and in-expensively for use in additive manufacturing. The selected material system is the Al7075 aluminium alloy, which is susceptible to cracking caused by hot tearing. To resolve this issue, it is mixed with varying quantities of silicon. The effect of silicon on solidification, grain refinement, and the resulting crack susceptibility is investigated with thermodynamical calculations considering the columnar to equiaxed transition, optical microscopy, and scanning electron microscopy after being processed by arc melting. The thermodynamical calculations of the compositions indicated a trend between the decreased columnar to equiaxed transition point at elevated temperature gradients to the silicon concentration. The experimental results reflected a similar trend by observing the reduction of the average grain size in the material system from 2470 μm2 to 323 μm2 for a composition with 0 wt.% and 10.5 wt.% silicon respectively. A composition of interest from the result was further mixed with zirconium hydride to investigate its grain refining properties on the alloy. The average grain size was reduced from 1055 μm2 to 453 μm2 by the inclusion of 0.24 wt.% zirconium. As such, this work provides a new approach to investigating a material system for use in additive manufacturing.

Characteristics of Cold Cracking in the Heat-Affected Zone of Carbon Steel

The effects of hydrogen and microstructure on the cold cracking susceptibility of the heat-affected zone (HAZ) of carbon steel were investigated. Various carbon steel plates with tensile strengths of 400- and 500-MPa grade were employed, and the cold cracking susceptibilities of the HAZs were evaluated using implant tests. To investigate the effects of hydrogen and microstructure, implant test assemblies were produced with two types of welding electrodes (low-hydrogen and cellulose) and two different welding conditions (low and high heat inputs). The results indicate that the microstructure was more important than hydrogen in determining the cold cracking susceptibility of the HAZ of carbon steel.

Influence of the process speed in laser melt injection for reinforcing skin-pass rolls

Laser melt injection is a technology for producing metal matrix composite (MMC) layers on tools such as skin-pass rolls by injecting hard particles into a laser-induced weld pool. However, low process speeds prevent the application of laser melt injection on a large scale. To overcome this drawback, a new approach is presented: High-speed laser melt injection (HSLMI) is a promising method for generating highly wear-resistant MMC layers on tools with high productivity. For the first time, high process speeds of up to 100 m/min were reached with HSLMI of spherical fused tungsten carbide (SFTC) particles into the steel 1.2362 that is used for skin-pass rolls. In this paper, the influence of the process speed on the microstructure and on the wear resistance of the MMC layer is investigated. The microstructure of the steel matrix changes from a dendritic to a needle-shaped structure when process speeds of 60 m/min or higher are applied. Furthermore, the steel matrix often features cracks. The SFTC particles show a dissolution seam. It was found that both the crack susceptibility and the SFTC dissolution can be reduced significantly by increasing the process speed. The wear behavior of the MMC layers was studied in a pin-on-plate test. It was found that the SFTC reinforcement leads to a significant improvement in wear resistance over the nonreinforced steel substrate. The wear volume was reduced from 3.6 to 0.1 to 0.3 mm3 by an SFTC particle-reinforcement. Abrasion was the substantial wear mechanism.

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

Laser Additive Manufacturing of TC4/AlSi12 Bimetallic Structure via Nb Interlayer.

The TC4/AlSi12 bimetallic structures (BS) with Nb interlayer transition were fabricated by laser additive manufacturing (LAM). The results showed that the TC4/AlSi12 BS with Nb interlayer prepared with optimized process parameters can be divided into three regions (the TC4 region, Nb region and the AlSi12 region) and two interfaces (the TC4/Nb interface and the Nb/AlSi12 interface). The high melting point (Ti, Nb) solid solution formed in the Nb region acted as a diffusion barrier between the TC4 alloy and the AlSi12 alloy, thereby effectively inhibiting the formation of Ti-Al intermetallic compounds (IMCs). With the decrease of the laser output power for AlSi12 deposition, the NbAl3 IMC changed from layered to dispersed distribution, while γ-TiAl and Ti5Si3 IMC disappeared, thus significantly reducing the crack susceptibility of the BS deposited layer. The tensile strength of TC4/AlSi12 BS with Nb interlayer was about 128MPa, and the fracture was located near the Nb/AlSi12 interface.

Microstructure and thermal cracking susceptibility of dissimilar resistance spot welded austenitic and mild steels

Microstructure and thermal cracking susceptibility of dissimilar resistance spot welded austenitic and mild steels

Processing of Aluminum Alloy 6182 with High Scanning Speed in LPBF by In-Situ Alloying with Zr and Ti Powder

The demand for high-strength aluminum alloys for the laser powder bed fusion (LPBF) process is still growing. However, to date, the crack susceptibility of conventional alloys as well as the high prices for specially developed alloys are the main obstacles for the use of high-strength aluminum alloys for LPBF. In this paper, crack-free LPBF samples with a relative density >99.9% were processed from AlMgSi1Zr (6182 series alloy) powder, to which 0.5 wt.-% Zr and 0.5 wt.-% Ti were added via mechanical mixing. No hot cracks were found in the µCT scans. Moreover, a fully equiaxed microstructure with a mean size of the α-Al grains of 1.2 µm was observed in the as-built parts. Al3(Zr,Ti) particles were observed, acting as efficient heterogeneous grain refiners for α-Al by building a semi-coherent interface. Unmolten Ti and Zr particles with sizes up to 80 µm were found in the α-Al phase. The resulting fine-grained microstructure led to a tensile strength of 329 ± 4 MPa and a total elongation at a break of 11.4 ± 0.9% after solution heat treatment, quenching in water, and subsequent artificial ageing.

Insights into the stress corrosion cracking resistance of a selective laser melted 304L stainless steel in high-temperature hydrogenated water

Selective laser melting (SLM) offers unprecedented advantages in manufacturing complex components used in nuclear reactors, and thus has promising application in the nuclear power industry. The compatibility of printed materials with typical reactor coolant should be carefully investigated as the printed materials possess unique microstructure features (e.g. columnar grain, dislocation cell and inclusion) which would inevitably affect the performance. In this work, the stress corrosion cracking (SCC) susceptibility of selective laser melted (SLMed) 304L stainless steel (SS) was evaluated in high-temperature hydrogenated water through slow strain rate tensile test and compared with two wrought 304 SSs. Interestingly, the SLMed 304L SS showed much lower SCC susceptibility than the two wrought 304 SSs, and this finding was mainly ascribed to its optimized composition. From the aspect of corrosion, the intergranular oxidation resistance of SLMed 304L SS is significantly improved due to the reduced contents of Si and Mn. In addition, the high dislocation density in SLMed 304L SS can accelerate the diffusion of solute atoms, thus further enhancing the resistance to intergranular oxidation and promoting the precipitation of oxide inside the crack which barricades the ingress of corrosive media. As for the mechanical aspect, under the combined effects of dislocation cells and nano-oxide inclusions, the SCC initiation can be alleviated as the dislocation motion in the grain matrix is hindered and planar slip is suppressed. Therefore, the optimized chemical composition and printed microstructure render this SLMed 304L SS more resistant to SCC initiation.

Creep microindentation of low-density oil well cement and the implication on radial cracking risk of cement sheath

To better provide zonal isolation for the production of oil and gas, low-density oil well cement (LD-OWC) filling a deep oil well was developed to reduce the high hydrostatic pressure caused by the cement slurry. The key mechanical properties of LD-OWC and related cracking susceptibility, nevertheless, have not been fully understood. This work was to characterize the elastic and creep properties of LD-OWC using microindentation and assess the radial cracking risk of the cement sheath. With two types of LD-OWC cured at different temperatures, our measurement through mercury intrusion porosimetry (MIP) showed that their porosities depended on the slurry densities level, and the basic creep exhibited a logarithmic increase in the long term. Mechanical properties such as Young's modulus (E), indentation hardness (H), and creep modulus (C) of LD-OWC were statistically characterized, in particular, E and C were in line with those obtained by macroscopic tests. Moreover, a viscoelastic sheath model was developed to evaluate the stress redistribution in the steel casing, the cement sheath, and the formation in the long term. We found that lower slurry densities of OWC reduced the risk of radial cracking. Taking advantage of parametric analysis, the power-function correlation between the safety factor of radial cracking and the mechanical and geometrical sheath properties was also demonstrated.

Correlation of Lack of Fusion Pores with Stress Corrosion Cracking Susceptibility of L-PBF 316L: Effect of Surface Residual Stresses.

Stress corrosion cracking (SCC) of laser powder bed fusion-fabricated 316L was studied under the variation in energy input density to emulate the existence of distinctive types of defects. Various electrochemical polarization measurements were performed in as-received polished and ground states, to elucidate the effect of defect type on corrosion and SCC behaviour in marine solution. The results revealed severe localized corrosion attack and SCC initiation for specimens with a lack of fusion pores (LOF). Moreover, the morphology of SCC was different, highlighting a more dominant effect of selective dissolution of the subgrain matrix for gas porosities and a more pronounced effect of brittle fracture at laser track boundaries for the specimens with LOF pores.

SCC fracture location shifting affected by stress-controlled fatigue damage of NiCrMoV steel welded joints

The influence of stress-controlled fatigue (SCF) damage on the fracture location shifting of welded joints for NiCrMoV steels in the corrosive solution of 3.5wt.% NaCl is systematically investigated by electrochemical measurements, immersion tests, slow strain rate tensile (SSRT) tests, and fractural morphology observations in the present work. The results show that both the yield strength (YS) and the ultimate tensile strength (UTS) of welded joints are increased after the SCF tests. The susceptibility of stress corrosion cracking (SCC) and galvanic corrosion of the SCF damaged welded joints are higher than that of the as-received specimens. In addition, the fracture locations of welded joints after SSRT tests in the corrosive solution are shifted with the degree of SCF damage. Different degrees of fatigue damage are accumulated in the three zones of welded joints during the cyclic deformation, which could change the competitive relation of the most susceptive SCC zone between the fusion zone and the middle of WM.

In-Service Corrosion Performance of Automotive AA7075 Sheet Alloy

Aluminum alloys offer higher specific strength than advanced high strength steels, making them preferred material choice for automotive light weighting. Among them, AA7075 sheet alloy offers significantly higher strength than 5xxx and 6xxx alloys and has been developed to provide high in-service strength for automotive structural applications. However, there is a need to understand the stress corrosion cracking (SCC) susceptibility of AA7075 under “in-service” conditions for the automotive market. Currently there are no commonly accepted corrosion testing specifications or evaluation criteria for AA7075 sheets among material suppliers and automotive original equipment manufacturers. Furthermore, there is a lack of in-service data to validate the accelerated laboratory exposure testing. In this study, in-service SCC performance of the AA7075 test coupons was evaluated by road exposure for a two-year period under harsh Canadian winters. The SCC specimens were loaded using four-point bending frames, and were investigated in various tempers and surface conditions (bare vs. e-coated). Overall, the road exposure results were consistent with the lab accelerated testing such as slow strain rate tensile testing in specific environments. Coating provided sufficient corrosion protection such that the stressed coupons survived after 2-year road exposure without reduction in strength. For bare metals, overaged T73 and in-service (T6+paint bake) tempers outperformed T6 temper. Localized corrosion of AA7075 sheets with various tempers was also studied using potentiodynamic polarization technique.Acknowledgements: Dr. Danick Gallant, Aluminum Technology Center at National Research Council Canada; Kennesaw lab support in Novelis Global Research and Technology Center.

Stress corrosion cracking susceptibility and surface characterization of nanocrystalline Cu–Cr alloys in dilute ammonia solution

A series of nanocrystalline Cu–Cr alloys was high-throughput synthesized onto micromachined cantilever arrays by magnetron co-sputtering. Based on the consistent relationship between film-induced stress and stress corrosion cracking (SCC) susceptibility, the effects of Cr content on the SCC susceptibility of Cu–Cr alloys in 0.2 M NH3·H2O solution were determined via film-induced stress measurement. The results show that the film-induced tensile stress gradually decreased with the increasing Cr content, that is, the SCC susceptibility decreases. Chemical composition and phase composition of the corrosion product film formed on Cu-11.46 at.% Cr alloy immersed in 0.2 M NH3·H2O solution for 1 h were investigated.

Laser powder bed fusion of an Al-Cr-Fe-Ni high-entropy alloy produced by blending of prealloyed and elemental powder: Process parameters, microstructures and mechanical properties

High-entropy alloys in the Al-(Co)-Cr-Fe-Ni system have been designed along several pathways to obtain tailored microstructures with remarkable mechanical properties, such as high strength and ductility. Additive manufacturing techniques, such as laser powder bed fusion, have been utilized and aim towards further optimization of their mechanical properties. However, processing challenges exist for these promising alloys, mainly due to their crack susceptibility as recently shown in the Co-free Al-Cr-Fe-Ni alloy. In this study, gas atomized prealloyed powder of a baseline Al-Cr-Fe-Ni high-entropy alloy was mixed with Fe and Ni elemental fine powder particles, to produce a powder blend with a modified composition. It was aimed to improve the alloy processability via designing the solidification path by reducing the Al content. The synthesized novel alloy composition was processed by laser powder bed fusion and crack-free material with a metastable, near single-phase face-centered cubic microstructure was obtained. Process parameters for minimum porosity were identified and net-shape tensile specimens were produced. Heat treatment led to a refined dual-phase microstructure consisting of face-centered cubic and body-centered cubic phases with promising mechanical properties, such as high tensile strength, reasonable elongation and notable strain hardening.