Alloy Specimens Research Articles

Fatigue and fracture in dual-material specimens of nickel-based alloys fabricated by hybrid additive manufacturing

The integration of additive manufacturing with traditional processes, termed hybrid additive manufacturing, has expanded its application domain, particularly in the repair of gas turbine blade tips. However, process-related defects in additively manufactured materials, interface formation, and material property mismatches in dual-material structures can significantly impact the fatigue performance of components. This investigation examines the low cycle fatigue and fatigue crack growth behaviors in dual-material specimens of nickel-based alloys, specifically the additively manufactured STAL15 and the cast alloy 247DS, at elevated temperatures. Low cycle fatigue experiments were conducted at temperatures of 950 °C and 1000 °C under a range of strain levels (0.3%–0.8%) and fatigue crack growth tests were conducted at 950 °C with stress ratios of 0.1 and −1. Fractographic and microscopic analyses were performed to comprehend fatigue crack initiation and crack growth mechanisms in the dual-material structure. The results consistently indicated crack initiation and fatigue fracture in the additively manufactured STAL15 material. Notably, fatigue crack growth retardation was observed near the interface when the crack extended from the additively manufactured STAL15 material to the perpendicularly positioned interface. This study highlights the importance of considering yield strength mismatch, as well as the potential effects of residual stresses and grain structure differences, in the interpretation of fatigue crack growth behavior at the interface.

Laser ultrasonic characterization of grains in titanium alloy by the inversion method

Abstract The polycrystalline materials have been usually described by the average grain size (AGS). This paper provides a nondestructive method for determination of the AGS in titanium alloy from an inversion of experimental ultrasonic attenuation spectrum (UAS). The detailed analyses of the inverse algorithm and sensitivity of UAS to the AGS are presented, and the sensitivity is identified as a criterion for selecting the experimental data of UAS used in inversion. An example of the inverse method and experimental measurement for UAS comparison was given for the titanium alloy specimen with the AGS of 26 μm. The relative error between the AGS determined by the inversion and scanning electron microscope (SEM) observation was found to be about 1.5 %. The provided method demonstrates the feasibility of laser ultrasonic technology used for the non-destructive evaluation of AGS in polycrystalline materials.

Design, analytical and computational analysis, and development of a high-precision CNC spindle for a vertical machining center

This research focuses on design, simulation and development of a high precision Computer Numerically Controlled (CNC) spindle for vertical machining center. High speed spindle is designed and analyzed under the varying load conditions. SolidWorks 2020 is used for designing of CNC spindle (based on ISO BT40 standards) and ANSYS Workbench 2020 is used for analysis. Moreover, analytical calculations for deformation are also done. Three different materials i.e. AISI 4340 steel, Grey cast iron, and Ti6Al4V alloy are chosen for the analysis. Effect of varying load on deformation and stress on CNC spindle is discussed. AISI 4340 steel showed better mechanical characteristics as compared to other materials. Moreover, the results showed that steel gives lesser values of deformation as compared to other materials, when applied under different load conditions. Although Grey cast iron can be economical and weight of the spindle can also be reduced but Grey Cast iron has greater value of deformation when load is applied. On the other hand, Ti6Al4V alloy specimen exhibited less stress as compared to other materials but mechanical properties in terms of modulus of elasticity are not that good as AISI 4340 steel. So, AISI 4340 steel could be a good option for spindle materials that require desired properties such as strong corrosion resistance, tensile strength, yield strength, and propagation. This will boost the accuracy and efficiency of spindle. Based on these results, high-precision CNC spindle was developed.

Evaluation of friction and wear depth during the cyclic loading on partly melted LPBF particles of LPBF Cu alloy

In this work, the resistance against nanoindentation of the Laser Powder Bed Fusion (LPBF) CuCrZr alloy specimen was studied at the ramp load of 2000–3000 μN on 100 points. It was found that the triangular shape indentation cavity and the pile-up material increased along the diagonal direction. The LPBF CuCrZr alloy contained few partly melted LPBF particles which have three different structures namely columnar, cellular and equiaxed. The nanowear test was conducted on the above three structures. The tribological property of partly melted LPBF particles was analyzed at 2000 μN cyclic load. The wear depth in the partly melted LPBF particles at the columnar structure for cycle-1, cycle-2 and cycle-3 were 836 nm, 918 nm and 980 nm, respectively. The wear depth in the partly melted LPBF particles at the cellular structure for cycle-1, cycle-2 and cycle-3 were 650 nm, 780 nm and 810 nm, respectively. The wear depth in the partly melted LPBF particles at the equiaxed structure for cycle-1, cycle-2 and cycle-3 were 650 nm, 720 nm and 780 nm, respectively. The obtained outcomes were correlated with the tribological behavior of non-defective part. The non-defective part exhibited higher wear resistance than the partly melted LPBF particles. The percentage increase in wear resistance at the columnar non-defective part over partly melted LPBF particles for cycle-1, cycle-2 and cycle-3 were 22.24 %, 20.5 % and 20.4 %, respectively. Similarly, the cellular and equiaxed non-defective parts have higher wear resistance when compared to the partly melted LPBF particles. Moreover, the decrease in Coefficient of friction was observed from one cycle to the next cycle both in the partly melted LPBF particles and non-defective part in all three structures.

Predicting fatigue life and crack growth rate of TC4 titanium alloy based on PINN before and after ultrasonic impact treatment

This study conducted fatigue crack growth rate (FCGR) experiments on TC4 titanium alloy specimens under different stress ratios (R=0.06, 0.1, 0.5) before and after ultrasonic impact treatment (UIT) conditions. Based on this, a Physics-Informed Neural Network (PINN) model was employed to predict the FCGR of TC4 titanium alloy. As a control group, the traditional Walker FCGR formula was fitted using experimental data. The results indicate that the stress ratio R does not significantly affect the FCGR of TC4 titanium alloy. Under the same range of stress intensity factor (ΔK), the FCGR (da/dN) decreases with increasing R. The FCGR prediction established by the PINN model can well reflect the nonlinear characteristics of the crack growth process rate. The coefficient of determination R2 for fitting the experimental data reached above 0.9900, generally higher than the 0.9545 of the Walker formula. Although the degree of dispersion increased after UIT, the PINN model demonstrated stable prediction accuracy compared to the Walker formula. In addition, when calculating the fatigue crack growth life using the cycle-by-cycle method, the PINN model also exhibits stable prediction accuracy compared to the Walker formula, and this advantage becomes more apparent with increased data dispersion.

Multi-criteria decision making for better quality indicators in µ-EDM using TiN-coated WC electrode

ABSTRACT It is highly essential to enhance the micro EDM process while machining titanium specimens. The utilization of coated electrode and adaptation of optimization algorithms in the process can enhance the material removal mechanism. In the present work, an attempt was made to find the influence of TiN coated tungsten carbide electrode in micro EDM process while machining titanium alloy specimens. It was also attempted to adopt DEAR-based optimization in the process. The TiN coating can produce lower TWR with better surface quality than uncoated tungsten carbide electrode. The capacitance has higher influential nature in µ-EDM since it controls the charging and discharging process during the spark formation across discharge gap. It was found that voltage (Level 3), capacitance (Level 3), and the tool rotation (Level 2) can create better quality measures under the prediction error of 2.3%. The optimal combination can enhance the machining accuracy with tiny craters formation.

Surface Roughness Evaluation of AZ31B Magnesium Alloy After Rough Milling Using Tools with Different Geometries

This paper presents the experimental results of a study investigating the impact of machining parameters on 3D surface roughness after rough, dry milling. The following 3D roughness parameters were analysed: Sa (arithmetic mean height), Sq (root mean square height), Sz (maximum height), Sku (kurtosis), Ssk (skewness), Sp (maximum peak height), and Sv (maximum pit height). Roughness measurements were made on the end face of the specimens. Additionally, 3D surface topography maps and Abbot-Firestone material ratio curves were generated. Carbide end mills with variable rake and helix angles were used in the study. Experiments were conducted on AZ31B magnesium alloy specimens using a contact-type profilometer. The machining process was conducted using the parameters of so-called high-speed machining. Three variable technological parameters were analysed: cutting speed vc, feed per tooth fz, and axial depth of cut ap. The results showed that the surface roughness of the rough-milled specimens depended to a great extent on the tool geometry and applied machining parameters. Feed per tooth was found to have the greatest impact on surface roughness parameters. Lower values of the analysed surface roughness parameters (and therefore higher surface quality) were obtained (in most cases) for the tools with a rake angle γ of 5° and a helix angle λs of 50°. The results provided both theoretical and practical knowledge about the achievable surface roughness after rough milling using tools with different tool blade geometry. It was shown that rough milling is an effective and efficient type of machining for the AZ31B alloy.

Effect of the countersunk rivet head dimensions on fatigue behavior of riveted specimens of 2024-T3 alloy

PurposeThis study aims to investigate the effect of countersunk rivet head dimensions on the fatigue performance of the riveted specimens of 2024-T3 alloy.Design/methodology/approachThe relationship between rivet head dimensions and fatigue behavior was investigated by finite element method and fatigue test. The fatigue fracture of the specimens was analyzed by scanning electron microscopy.FindingsA change of the rivet head dimensions will cause a change in the stress concentration and residual normal stress, the stress concentration near the rivet hole causes the fatigue crack source to be located on the straight section of the countersunk rivet hole and the residual normal stress can effectively restrain the initiation and expansion of fatigue cracks. The fatigue cycle will cause the rivet holes to produce different degrees of surface wear.Originality/valueThe fatigue life of the specimens with the height of the rivet head of 2.28 mm and 2.00 mm are similar, but the specimens with the height of the rivet head of 1.72 mm were far higher than the other specimens.

Influence of surface pre-deformation on the Portevin-Le Chatelier effect and the related multiscale complexity of plastic flow in an Al-Mg alloy

The influence of the surface pre-deformation on jerky flow caused by the Portevin-Le Chatelier (PLC) effect was investigated using flat tensile specimens of an Al-Mg alloy. Although jerky flow represents a macroscopic plastic instability, the underlying mechanisms stem from self-organization of dislocations, which pertains to deformation processes at mesoscopic scales. To provide a comprehensive approach, the investigation was carried out by coupling tensile tests, digital image correlation and acoustic emission techniques, each targeting a particular range of scales. Thin superficial layers were pre-deformed using surface mechanical attrition technique (SMAT). It was found that the observed effects depend on which surfaces are processed. Overall, the treated samples exhibited an enhanced yield strength without deterioration of ductility in comparison with the initial material. These changes in the general mechanical behavior are likely to be correlated with the changes in jerky flow. Using digital image correlation, a tendency to delocalization of bursts of plastic flow was found for both the PLC bands and smaller-scale strain heterogeneities outside the bands, the latter having been little studied so far. Unexpectedly, SMAT occurred to modify plastic flow drastically at this scale. In addition to clarification of the role played by the surface in the PLC effect, these findings provide new insights into relationships between deformation processes at distinct scales.

Influence of melt parameters on the microstructure of electron beam melted Ti–6Al–4V

Additively manufactured, Electron Beam Melted (EBM) specimens of the titanium alloy, Ti–6Al–4V, have been produced using a process window determined through a normalised energy density method. Two batches were manufactured and compared using identical energy density values with differing beam current, power, and beam velocity. A stable process window has been demonstrated with a Vickers hardness range of 360–395 VHN resulting from α lath coarsening from 0.7 μm up to 3 μm. A range of macro morphologies have been reported and relate to the hatch overlap and beam velocity parameters. Base plate position does not appear to influence microstructure or micro-hardness. Prior-β columnar and colony size increases with α lath width resulting from increased energy input; however, each grain type appears to respond differently to either beam velocity or hatch space variation. Average α lath width values show greater correlation to energy density, which demonstrates the dependence of grain formation on hatch overlap.

Impact of Aging Time on the Morphological, Mechanical, and Tribological Behavior of Al-6.6Si-0.2Mg-0.2La Hypoeutectic Alloy

Abstract The present work aimed to examine the influence of Lanthanum (La) on the morphological, mechanical, and tribological characteristics of Al-6.6Si-0.2Mg alloy. The alloy specimens were exposed to an aging process at 180°C for 8h and 12h. The morphological transformation due to La addition was analysed with an emphasis on altering primary and eutectic silicon morphology, grain refinement, and intermetallic phase distribution. The experimental investigations were carried out using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) whereas the hardness and wear characteristics were analyzed by performing Vickers microhardness and pin-on-disc tests. The obtained results demonstrated that lanthanum inclusion causes significant microstructure refinement, resulting in enhancing the microhardness by up to 60 %, proliferating the tensile characteristics by 70%, and aiding in improving the tribological characteristics of the Al-Si-Mg alloy. The results provide a clearer understanding of the alloy modification process and offer valuable insights for enhancing the performance of Al-Si-Mg aluminium alloys in automotive and aerospace applications.

Influence of scanning strategies on microstructure and mechanical properties of laser powder bed fusion 2195 Al-Li alloy

Optimizing the scanning strategy is considered as an effective approach to address the challenges of laser powder bed fusion(LPBF). However, the impact of the scanning strategy on lightweight Al-Li alloys remains unclear. In this study, various scanning strategies LPBF were employed to process 2195 Al-Li alloy specimens, and a systematic investigation was conducted on the effects of the laser path on the defects, microstructure, and mechanical properties of 2195 Al-Li alloy. The results revealed that the specimens processed with the linear strategy exhibited the lowest gas pores and crack density, achieving a densification rate of 98.6 %. Furthermore, adjusting the scanning strategy led to changes in grain morphology among specimens; those processed with linear scanning demonstrated smaller average grain size (19.7 μm), a lower percentage of high-angle grain boundaries (HAGBs) (57.4 %), and a higher texture index compared to other strategies tested. When employing linear strategy for processing, tensile performance reached 315.03 MPa with 5.03 % elongation due to concise thermal history and fast cooling conditions associated with this particular scanning method. This study provides valuable insights into optimizing scanning strategies for LPBF 2195 Al-Li alloy.

Influence of femtosecond laser textured shapes on the friction characteristics of 7075 aluminum alloy

Different surface dimples (circular, square, and triangular) were fabricated on 7075 aluminum alloy specimens using a femtosecond laser to enhance their wear resistance. The influence of these surface dimples on the friction performance of the aluminum alloy under dry sliding conditions was investigated by comparing the coefficient of friction (COF), wear track morphology, and wear rate. All the textured samples exhibit varying degrees of improvement in the wear resistance compared to the untextured sample, and the circular texture is the most effective. Wear track analysis reveals that abrasive and adhesive wear are the primarily wear mechanisms. Thus, introducing surface dimples significantly enhances the wear resistance of the 7075 aluminum alloy, providing a novel strategy for optimizing its friction performance through design.

Material Characterization and Electrochemical Properties of Titanium Alloy 5553 Prepared by Selective Laser Melting as Processed and after Abrading and Polishing.

The microstructure, Vickers microhardness, and electrochemical properties of an additive manufactured titanium alloy, Ti-5553 (Ti-5Al-5Mo-5V-3Cr wt %), are reported on. The alloy specimens were fabricated by selective laser melt processing. The surface morphology and electrochemical properties of the as-processed and surface-pretreated (abraded and polished) Ti-5553 specimens were investigated. The as-processed specimens had a nominal density of 4.62 ± 0.04 g/cm3. Based on comparison with the reported density for the die-cast alloy, the specimens were 99-100% dense with ca. 1% porosity. Optical microscopy and scanning electron microscopy revealed some micropores, balling features, and fusion pore defects across the surface of the alloy (XZ plane-orthogonal to the build direction). The nominal Vickers microhardness was 292 ± 2 HV. Detailed electrochemical characterization of the as-processed and surface-pretreated alloys revealed reproducible open-circuit potentials (OCPs), linear polarization resistances (R p), and potentiodynamic polarization curves for both specimen types in naturally aerated 3.5 wt % NaCl at room temperature. For the surface-pretreated alloys, the OCP was 225 mV more noble, the anodic current in the potentiodynamic polarization curves was 72× lower, the cathodic current was 8× lower, and R p was larger by 426× than the values for the as-processed specimens. The collective electrochemical data revealed that the surface-pretreated alloys exhibit greater corrosion resistance than the as-processed alloys due to a reduction of the real area and the formation of a more passivating oxide layer.

Development of Chitosan/Mg−Zn−HAP/ZrO2 Bilayer Coating on Ti Alloy for Biomedical Applications

AbstractTitanium alloys (Ti−6Al−4V) are being used in many biomedical applications due to their unique properties of bioactivity, excellent mechanical properties, low toxicity, biocompatibility, and long‐term implant application for their satisfactory corrosion resistance. We have developed a new two‐step fabrication process of zirconium oxide (ZrO2) and Chitosan/Mg−Zn−HAP (Chitosan/M−HAP) bilayer layer coating on Ti alloy for biomedical applications. Flower‐structured bilayers were coated by the electrochemical deposition method. Electrochemically deposited bilayer‐coated Ti alloy specimens were characterized by various analytical techniques like field‐emission scanning electron microscope (FE‐SEM), X‐ray diffraction (XRD), and EDS mapping analysis. Furthermore, the developed bilayer (Chitosan/Mg−Zn−HAP/ZrO2) coated Ti alloy samples improved mechanical properties such as adhesion strength (20.1±0.4 MPa) and hardness (446±4 Hv), compared to other coatings. An investigation of polarization curves showed a positive shift in the polarization values (Ecorr, and Icorr) of the Chitosan/M−HAP/ZrO2 bilayer coatings on Ti alloy. Bilayer‐modified Ti alloy samples show good antimicrobial response toward Gram‐negative and Gram‐positive bacteria. Meanwhile, the cell viability and proliferation of the bilayer‐coated samples were evaluated and the exhibited result improved cell proliferation, and bioactivity compared to other coated materials. From the results, it can be evident that the developed Chitosan/Mg−Zn−HAP/ZrO2 bilayer‐coated Ti alloy could be a good potential candidate for biomedical applications.

Microstructure and flame-retardant properties of the TF550 titanium alloy by ultrasonic vibration assisted laser solid forming

More and more attention has been paid to the application of flame retardancy and surface strengthening of titanium alloys in the aerospace field. In this paper, ultrasonic vibration assisted the laser solid formed (LSFed) TF550 flame-retardant titanium alloy was studied, and the influence of ultrasonic vibration on the microstructure and flame-retardant properties of the LSFed TF550 alloy was analyzed. The results showed that the LSFed TF550 alloy after ultrasonic vibration treatment effectively reduced the generation of pores. When the ultrasonic amplitude was 60 μm, the porosity was the lowest. The porosity of the sample was reduced from 3.84 % to 0.53 %, with the reduction of 86 %. The ultrasonic vibration results in the CET transformation of LSFed TF550 alloy specimen, and the average grain length-width ratio decreases from 5.3 to 1.2, and the microstructure of the specimen changes from the average width of 214 μm to the average grain size of 144±12 μm. Especially when the ultrasonic amplitude is 90 μm, the equiaxial grain size and the grain length-width ratio are the lowest. The effect of ultrasonic vibration at different positions was different. The higher the deposition height, the more columnar crystals in the microstructure and the less equiaxed crystals. When the deposition height was 43 mm, equiaxed crystals and columnar crystals began to coexist. Due to the grain refinement effect of ultrasonic vibration, the number of grain boundaries was increased, and the flame-retardant properties of the LSFed TF550 alloy was improved.

Low-cycle fatigue behavior of coated and uncoated single crystal Ni-based superalloy at various temperatures

Within this work, the effect of different oxidation-resistant coatings on the low-cycle fatigue (LCF) behavior and damage mechanisms of third-generation single crystal (SC) Ni-based superalloy was investigated. Stress-controlled LCF tests of bare alloy, MCrAlY-coated and PtAl-coated thin-walled specimens were carried out at 760 °C, 850 °C and 980 °C. A unique clamping system was meticulously designed to overcome the clamping challenge in high-temperature furnaces. The cracking behaviors and failure modes were determined by advanced microstructure characterization. The results show that the coatings are detrimental to the LCF lifetime of thin-walled specimens, especially at low-medium temperatures of 760 °C and 850 °C. For bare alloy, the tip cracks nucleate from the surface oxide layer, whereas cracks in the coating-substrate system nucleate prematurely from the coating surface and propagate to the substrate. The oxygen diffusion and the frontward propagation of cracks are synergistically enhanced in the SC substrate, resulting in oxidation-assisted crack propagation and ultimate failure. Finally, A fatigue life prediction model is proposed by introducing coating-induced crack initiation and oxidation effects, where the predictive results fit well with experimental results. This study provides a comprehensive insight into the damage mechanisms and coating-induced LCF life degradation of the coating-substrate system.

Constitutive model of die-cast light-alloy thin-walled parts considering geometric imperfection

Replacing steel with aluminum and shifting from forging to casting have become important means of automotive lightweight. Large integrated die-cast parts are gaining popularity among original equipment manufacturers. Engineers often ignore the influence of geometric imperfection on mechanical properties when simulating strength of thin-walled parts, therefore causing a bias. To investigate the relation between geometric imperfections and mechanical properties of die-cast light alloy, quasi-static tensile tests are conducted on the specimens of JDA1b aluminum alloy and JDM1 magnesium alloy. The specimens exhibit varying geometric imperfection factors (from 0 to 90 %) achieved by introducing circular holes with different diameters. The results show that the strength and elongation of JDA1b and JDM1 alloys decrease significantly as the geometric defect factor increases. Even small holes can significantly affect tensile strength. A constitutive model that incorporates the stress limit value and geometric imperfection factors is proposed, which has higher accuracy than the J–C model. Experiments and simulations on a die-cast thin-walled part validated the idea and proposed constitutive model. These findings provide essential insights into the influence of structural holes on the mechanical properties of die-cast light-alloy materials. The proposed constitutive model offers high-precision computational support for simulating the mechanical performance of parts.

Numerical Simulation of Electromagnetic-Thermal-Fluid Coupling for the Deformation Behavior of Titanium-Aluminum Alloy under Electromagnetic Levitation.

Electromagnetic levitation (EML) is a good method for high-temperature processing of reactive materials such as titanium-aluminum (Ti-Al) alloys. In this study, the oscillation and deformation processes of Ti-48Al-2Cr alloy specimens at different high-frequency currents during the EML process were simulated using the Finite Element Method and Arbitrary Lagrangian-Eulerian (ALE) methods. The data of oscillation, stabilization time, deformation, and distribution of electromagnetic-thermal-fluid fields were finally obtained. The accuracy of the simulation results was verified by EML experiments. The results show the following: the strength and distribution of the induced magnetic field inside the molten droplet are determined by the high-frequency current; under the coupling effect of the electromagnetic field, thermal field, and fluid field, the temperature rise of electromagnetic heating is rapid, and accompanied by strong stirring, resulting in a uniform distribution of the internal temperature and a small temperature difference. Under the joint action of gravity and Lorentz force, the molten droplets are first within a damped oscillation and then tend to stabilize with time, and finally maintain the "near rhombus" shape.

DBTT and recrystallization behavior analyses for rolled and forged potassium-doped tungsten alloys

Tungsten-potassium (WK) alloys are regarded as one of the candidates for plasma-facing materials (PFMs) intended for the divertor and first wall of future fusion reactors. Determining the range of the ductile-to-brittle transition temperature (DBTT) and the recrystallization temperature (RCT) for the WK alloy is critical as it is closely related to the safe operational temperature window of the fusion reactor. Variable temperature tensile tests were conducted on rolled and high energy rate forging (HERF) WK alloy specimens to elucidate their DBTT. Additionally, gradient high-temperature isochronal annealing was performed on the HERF samples, coupled with Vickers hardness characterization, to ascertain their distinct RCT for various orientation planes within the same alloy. Subsequently, the DBTT of the as-rolled WK alloy is determined to be approximately between 100 °C and 150 °C, while that of the HERF WK alloy is found to be around lower than 200 °C. Furthermore, the RCT of the ND plane of the HERF WK alloy is approximately 1600 °C, whereas that of the RD counterpart exceeded 1700 °C. The internal mechanisms underlying these phenomena are comprehensively analyzed in this study. This work has identified the safe service temperature window for future fusion reactors with this kind of advanced WK alloy and provided a new perspective for determining the RCT of many other W-based materials.