Microstructural Investigations Research Articles

Investigation of melt pool dynamics and solidification microstructures of laser melted Ti-6Al-4V powder using X-ray synchrotron imaging

Titanium alloy (Ti-6Al-4V) is widely used in additive manufacturing (AM) industry. However, as laser powder-bed fusion (PBF-L) additive manufacturing (AM) advances towards reliable production of titanium parts, a thorough understanding of the process-structure-properties (PSP) relationships remain to be fully understood. A study of the laser melting was paired with high-speed X-ray synchrotron imaging at the 32-ID beamline of the Advanced Photon Source at Argonne National Laboratory. Simultaneous melting and imaging was carried out on a Ti-6Al-4V powder layer held in a custom device designed to mimic single-track scans of the PBF-L process at different laser power levels, powder size distributions, and cover gas environments (Ar and He) on top of AM Ti-6Al-4V base metal. It was found that the thickness of the powder layer significantly affected the melt behavior: too much powder led to the formation of molten droplets that wetted the surface of the titanium, yet did not contribute to a uniform melting profile. Residual gas pores in the atomized powder were also observed to contribute to the pores observed in the melt pool, with the porosity of the powder (defined as volume of pores divided by total material volume) constant with powder size distribution (i.e., larger particles contained more entrapped gas, which increased final part porosity). When varying Ar or He through the same gas flow meter settings and nozzle, the difference in flow rates likely contributed more to the resultant porosity of the solidified material than did the thermal conductivity of the gasses, with He being the greater contributor to porosity. The microstructure of the heat affected zone contained α′, α, and an increased β phase fraction relative to the base material. The crystallographic texture of the melt pool region adopted that of the base metal.

Short-term creep approach to redefining the role of 17-4PH stainless steel for high-temperature applications

The creep response of the 17-4PH martensitic age-hardening steel in H1150 state was investigated at 427 and 482 °C. Hardness measurements of the heads of the creep samples demonstrated that the material underwent additional age hardening during the high-temperature exposure. Microstructural investigations confirmed that the additional precipitation of carbides and the G-phase occurred at the lowest temperature. A set of constitutive equations previously developed to describe the creep response of particle-strengthened alloys was successfully used to obtain a comprehensive description of the experimental data. The value of the particle strengthening term was obtained from the hardness measurements and corresponded to the Orowan stress. The model accurately described the observed minimum creep rate dependence on the applied stress and explained the occurrence of lower values of the minimum strain rate observed during variable-load experiments.

Investigation of the effect of ball-burnishing treatment applied to Al 7075-T6 alloy on mechanical and crystallographic properties

ABSTRACT In this study, the mechanical, microstructural, and crystallographic effects of ball-burnishing applied to Al 7075-T6 alloy at 0.15–0.30 mm penetration depth, 0.15–0.30–0.60 mm/rev changing feed rates and 100–200 rpm range were investigated. After the ball-burnishing processes were carried out in different parameters, surface roughness measurements were applied to determine the surface qualities, and hardness and impact energy tests were applied to characterise the mechanical properties. Dislocation density and texture coefficient values were calculated by determining the crystallographic properties using X-ray diffraction. It has been determined that the ball-burnishing process directly affects the surface roughness, depending on the increasing rotational speed and feed rate. As a result of microstructural investigations, the average grain size decreased with increasing amount of deformation on the sample surface. At 200 rpm ball-burnishing parameter, the surface hardness of the sample increased by approximately 41% compared to the initial state and was measured as 265 HV2. It was determined that the ball-burnishing process did not positively affect the impact of the toughness of the alloy. The reason for this is that the roughness and scratches on the surface caused by the ball-burnishing process have a notch effect, increasing the crack formation and propagation speed.

Effect of cooling rate and Nb addition on the strengthening in eutectic CoCr1.3FeMnNi0.7Nbx (0 ≤ x ≤ 0.45) high entropy alloys comprising of low stacking fault energy FCC phase

The CoCr1.3FeNi0.7MnNbx (0.3 ≤ x ≤ 0.45) eutectic high entropy alloy (EHEA) comprising of ultrafine lamellae of low stacking fault energy (SFE = 11 mJ/m2) face centered cubic (FCC) and Laves phase are synthesized at cooling rates of 2.5–10 K/s (arc melted ingots, AMIs) and 4.2 × 102 K/s (suction cast bars, SCBs). The microstructural investigation suggests that secondary dendritic arm spacing (SDAS) and interlamellar spacing are refined in SCBs due to a higher cooling rate than the AMIs. Whereas, deformation twins evolve in FCC phase due to its low SFE. The EHEAs exhibit a high yield strength of 1327 MPa for x = 0.367 SCB and a large fracture strain of 20% for x = 0.3 AMI under compression. Systematic investigation on the influence of Nb addition, microstructure refinement, and evolution of deformation twins during straining on the strengthening mechanisms reveals that the phase interface strengthening, solid solution strengthening, and twin boundary strengthening play critical roles in providing the high flow stress. The post-deformation analysis and the presence of multiple positive slopes in work hardening curves confirms that the concurrent effect of the evolution of dislocations, deformation twins, and their mutual interactions are solely responsible for the severe strain hardening at multiple stages during plastic deformation. The low values of strain rate sensitivity (0.0106–0.0111) and the activation volume (28b3-41b3) suggest that the deformation kinetics is governed by the dislocation interactions with the ultrafine lamellae interface and twin boundaries.

Failure analysis of an AISI 310 stainless steel beam reinforcement fracture during service in a rolling beam furnace

This work investigated the premature failure of a beam reinforcement made of austenitic stainless steel AISI 310 used in the entrance of a rolling beam furnace working continuously at cycles from 650 to 850 °C. Fracture surface and microstructural investigations allowed to identify the fracture mechanism as fatigue failure. The fracture was probably initiated at the oxidized grain boundaries at an early stage of service life, when no microstructure degradation was present, while most of the fatigue crack propagation occurred after the precipitation of secondary phases. The microstructure investigation revealed severe microstructure degradation with the presence of intermetallic sigma (σ) phase (area fraction = 19 %) and M23C6 carbides (area fraction = 1 %), which were formed during the high-temperature exposure. Cracking of σ phases and decohesion at σ /austenite matrix interfaces were observed near the fracture and in the small punch test samples. Such discontinuities provide an easy path for crack propagation.

EVALUATION OF THE MAGNETIC PROPERTIES OF Ag-Au-Pd-Cu DENTAL ALLOYS

The paper discusses four Ag-Au-Pd-Cu dental alloys, with different chemical compositions, which are used to produce fixed dental-prosthetic structures. These dental alloys must have a low level of biological risk after being implanted in the oral cavity, which means minimal release of ions from their surface, or the absence of corrosion, and, recently, the absence of ferromagnetism has been introduced as an additional requirement. The latter requirement is particularly important when magnetic resonance (MR) is used in the medical diagnosis of a patient who has an implanted fixed dental-prosthetic structure. With this technique, the internal structure of the human body is imaged using a strong magnetic field, radio waves and computer technology. Therefore, the absence of ferromagnetic, embedded biomaterials is necessary in the part of the body where medical diagnostics are performed. Microstructural investigations of four Ag-Au-Pd-Cu dental alloys (Auropal S, Auropal SE, Midor S and Midor SE) and measurements of their magnetic properties were carried out as part of the research. The results showed that Ag-Au-Pd-Cu dental alloys have a stable microstructure, which allows them to be processed later into fixed prosthetic constructions. The measurements of the magnetic properties showed that the Ag-Au-Pd-Cu dental alloys are diamagnetic.

Microstructural investigation and performance of carbon matrix composites developed through facile pitch modification route

Increasing demand of high-performance materials lead to development of composites with affordable cost and processing route. Present work involves facile chemical modification (nitric acid and melamine) route to produce carbon matrix composites exhibiting progressive performance. Modified pitches and resultant carbon matrix composites were characterized by FTIR, optical microscopy, SEM, XRD, TGA, TMA, tribological study and compression strength. Chemical treatment given to pitch imparts active functional groups which help to improve affinity between pitch particles. Additionally, to evaluate influence of high temperature treatment, composites were also heated to 1400 °C. Optical images show that microstructure of carbonized (1000 °C) and further heat-treated (1400 °C) composites contain fine isotropic grains and flow domains, attributed to the effective influence of chemical modification. SEM images and Raman spectroscopy also confirmed the similar results of arrangement of carbon layers. TGA analysis revealed that heat-treated composites with acid modified pitch have better thermal stability at 800 °C in zero air. Coefficient of thermal expansion of all composites was measured and found very low i.e. 5 to 10 ppm/°C. Chemical treatment applied to the pitch yielded improved results for tribological performance, coupled with enhanced compressive strength. Notably, up until the present, it was difficult to find reports on fabrication of carbon matrix composites from chemically modified pitch.

Effect of Electrode Type and Weld Current on Service Life of Resistance Spot Weld Electrode

HSLA steels are widely preferred in the automotive industry due to their advantages of high strength, lightening of vehicle weight and formability. The fact that a lot of welding processes are carried out during the creation of the vehicle body has made the welding operations of these steels an important topic. In this study, zinc coated HX340LAD+Z sheet material was welded by RSW application in factory conditions. The welding opera-tion was carried out with different current intensities and 300 spot welds for three different electrodes, keeping the weld duration constant. After the welding trials, the shortening of the electrodes was examined and information about the electrode lifetime was obtained. The weld core diameter was measured on the sheet metal also microstructure and micro-hardness investigations were made. As a result of the findings, information was obtained about weld hardness and collapse. Determination of splashing in the HX340LAD+Z ma-terial was provided by SEM imaging and EDX studies. It was determined that F type Copper electrode was more efficient.

On the Possibility of Using Secondary Alloys in the Production of Aluminum-Based Metal Matrix Composite

Aluminum-based composites provide tribological performance and thermophysical properties that, combined with being lightweight, are suitable for their application in automotive brake discs. Aluminum alloys allow the use of secondary materials to produce composites, with the drawback of several elements, impurities, and oxides that can harm the mechanical and thermophysical properties. This preliminary study explored the mechanical and thermophysical performance of a composite material produced with a secondary matrix alloy. Overall, the results are promising, with a minimal decrease in mechanical and thermophysical properties despite clustered silicon carbide particles in the composite with the secondary matrix. The challenges in effectively dispersing carbides in the melt seem linked to aluminum oxides, and future microstructural investigations will aim to clarify this aspect.

Effect of Short Heat Treatments on the Microstructural Evolution and Hardness of Thixoformed Graphene Reinforced Aluminium Composites

Aluminium metal matrix composites are increasingly being used in numerous industries due to their lightweight nature and high strength. The incorporation of graphene into aluminium metal matrix composites has garnered significant interest owing to graphene's capacity to enhance numerous properties concurrently. Hence, this study aimed to fabricate a 0.3 wt.% graphene nanoplatelets reinforced A356 alloy (GNP-A356) composite through stir casting followed by thixoforming and short T6 heat treatment processes. It also evaluated the microstructure and mechanical properties of GNP-A356 after the thixoforming process and short T6 heat treatment. The microstructure of alloy and composite was confirmed by optical microstructure, field emission scanning electron microscopy images, and X-ray diffraction. Microstructure investigations demonstrate the impact of the stirring process on the structural transformation of the 𝛼-Al phase from the dendritic into a rosette-like and globular-like structure. The result also indicated that there was a transformation of eutectic silicon from the shape of a needle-like to spheroid structure after a short heat treatment, showing the efficiency of T6 heat treatment with a shorter time of solution treatment and ageing. Moreover, the increase in relative density and addition of reinforcement led to a significant increment in hardness. The result shows the hardness improved by 35.77% and 77.8% for as-cast and heat-treated composites, respectively, compared to A356 alloy.

Investigations of microstructure, impact toughness, and fatigue failure behavior of bulk UFG Al–Li 8090 alloy processed by multi-axial forging

Investigations of microstructure, impact toughness, and fatigue failure behavior of bulk UFG Al–Li 8090 alloy processed by multi-axial forging

CALPHAD-guided design and experimental study on a novel Al-Ti-Nb alloy

We have used a CALPHAD-guided design methodology to develop a novel lightweight Al-Ti-Nb alloy with nearly 50 vol% of in-situ formed trialuminide reinforcement. After compositional optimization, we selected Al87.5Ti6.25Nb6.25 (at%) as the experimental alloy composition and subjected it to a 24-hour homogenization heat-treatment at 475°C for attainment of equilibrium, guided by DSC and CALPHAD-derived melting point data. Phase and microstructure analysis revealed a microstructure with approximately 50 vol% of a FCC Al-rich matrix and 50 vol% of trialuminide phases for strengthening. The experimental onset melting point of the Al-rich matrix was determined to be 647°C, which significantly surpasses Al-Si based high-temperature alloys (577 ℃), showcasing the potential of this alloy as a high-temperature structural material. Nanoindentation tests demonstrated exceptional mechanical properties. The alloy was found to have a microhardness of 3350 MPa, which not only outperformed 7075 Al alloy, A390 Al-Si alloy, and commercially pure Ti but also exceeds the levels of Ti-6Al-4V alloy at nearly 28% lower density. This study's significance lies in its integration of CALPHAD predictions and meticulous microstructural investigation, offering an effective approach for the design and characterization of novel Al-based alloys.

Synergic investigation of microstructure, precipitation, and micro-segregation on Inconel 825 weldments: A comparative study between GTAW and EBW

Inconel 825, a Ni–Fe–Cr alloy highly regarded for its exceptional corrosion resistance and high-temperature strength. This investigation studies the effect of heat input, precipitation, and micro segregation of Inconel 825 weldments and compares the effectiveness of two welding techniques: gas tungsten arc welding (GTAW) and electron beam welding (EBW). The findings indicate that increasing heat input generally enhances weldment quality by reducing the lack of penetration. However excessive heat input in GTAW leads to root cracking and solidification cracking; while EBW demonstrates better control over undercut and maintains consistent weld quality even at higher heat inputs. Both GTAW and EBW samples exhibit dendritic grain morphologies with distinctive grain boundaries. Precipitates, such as Al4C3 and TiN, are observed in both processes, contributing to improved mechanical properties. While GTAW weldments show some degree of segregation for Mo, Cu, Ti, and Al, EBW weldments demonstrate negligible segregation for major alloying elements but micro-segregation of Ti and Al. EBW weldments additionally showcase higher average hardness, superior tensile strength, and ductility compared to GTAW weldments. This can be attributed to lower heat input, faster cooling rates, and a reduced rate of elemental segregation. Fractographic analysis reveals the presence of voids and micro-voids, indicating a ductile mode of failure for both GTAW and EBW samples. These void characteristics vary based on the heat input and microstructural features.

Influence of carbon nanotubes on printing quality and mechanical properties of 3D printed cementitious materials

This paper presents the impact of incorporating carbon nanotubes (CNTs) into the 3D printing of cementitious materials, along with the effective dispersion of CNTs. Compared to the control mix, adding CNTs with superplasticizer significantly enhanced the printing quality by reducing the error in height of two-layers from 38% to 30% and an 81% enhancement in the buildability. Moreover, rheology properties revealed shear-thinning behaviour with lower viscosity, resulting in improved flowability. The progressive increase in CNT concentrations up to 0.2% yielded a noteworthy improvement in the mechanical properties. At 28 days, the incorporation of 0.2% CNTs resulted in a significant increase in the flexural strength, compressive strength, and Young's modulus by 99%, 72%, and 43%, respectively, compared to the mix containing silica fume. Microstructural investigation of the CNT-cement matrix revealed nanoscale crack bridges formed by CNTs, reinforcing the cementitious material and improving its mechanical properties.

Microstructural investigation of ternary blended high-performance concrete using supplementary cementitious materials

Microstructural investigation of ternary blended high-performance concrete using supplementary cementitious materials

The effect of interlayer adhesion on stress distribution in 3D printed beam elements

Interlayer adhesion is one of the most pivotal aspects related to mechanical performance and durability of 3D printed concrete. However, due to continuous printing of multi-layered structures, large-scale buildings require long time intervals between layer deposition, with interlayer bond strength decreasing as time intervals between printed layers increase. This study evaluates comprehensively the effects of time intervals between layers: 1 h, 1.5 h, 2 h, 3 h, and 4.5 h as well as two different height to span ratios (hb/s). In order to assess the properties of interlayer regions microstructural examinations using techniques such as X-ray micro-computed tomography (micro-CT), scanning electron microscopy (SEM) were applied. In addition destructive testing of compressive, splitting tensile and flexural strengths along with crack opening analysis using the Digital Image Correlation (DIC) method was performed and supported with FEM analysis. The results show that interlayer adhesion strength decreased significantly for time intervals of up to 2 h, with a linearly decreasing trend. The reduction in strength the interlayer bond for a 2-h interval was 56.6%, compared to the reference specimen. Afterwards, the decrease in strength was minimal (up to 68.5% after 4.5 h time interval). However, microstructural investigations revealed that specimens with printing interval beyond 2 h exhibited noticeable continuous cracking in interfacial zone with widths reaching of up to 100 μm (SEM). At the same time micro-CT analysis confirmed creation of longitudinal pores which in turn was reflected in different cracking phenomena of specimens with longer printing intervals (>2 h). This phenomenon was further explained through FEM modeling. As an outcome it was proven through DIC that that increasing the time interval between the deposit of successive layers not only affects the strength of the entire beam, but also affects its cracking mode. The discontinuities in the interlayer contact zone leads to destruction characteristic of composite structures, thus the structure comes to be destroyed layer by layer, resulting from insufficient interlayer adhesion. The presented study is the first to fully evaluate the failure behavior of 3D printed specimens with long time intervals.

Synergistic effects of graphene oxide and fly ash on rheology, mechanical properties, and microstructure of highly-flowable cementitious grouts

Due to their excellent mechanical strength and low pric, cementitious grouts have emerged as the popular for grouting applications. However, excessive use of cement results in significant energy consumption and associated carbon emissions, which is a concern from a sustainability perspective. In this study, inexpensive industrial waste fly ash (FA) was used to enhance the performance of graphene oxide (GO)-based cementitious grouts. The influence of varying dosages of GO and FA on the rheology, mechanical strength, durability, and microstructure development of cementitious pastes was investigated. The results show that adding FA to GO-based cementitious grouts improves the workability of the paste. The high-flowability cementitious grouts with combined GO and FA exhibit Bingham's fluid behavior, indicating that the synergistic effect of GO and FA does not alter the flow behavior of the paste. Further, the relationship between the rheological parameters of the modified paste and time was developed. For 0.03 wt% GO and 20 wt% FA incorporation, the flexural and compressive strengths of the paste at 28 days increased by 28.5% and 30.4%, respectively, compared to the blank group. The chloride ion penetration coefficient at 180 days was 1.007, indicating that FA could effectively replace a portion of the cement while enhancing the durability of the grout. Microstructural investigations confirmed that the synergistic effect of GO and FA does not generate new hydration products. GO accelerates the hydration reaction and activates the secondary pozzolanic effect of FA, forming more hydration products and thereby enhancing the mechanical strength and durability of the grout.

Electrical conductivity upon sintering of pure Cu produced by material extrusion additive manufacturing: Effects of pore, grain size, and impurity

This study investigates the development of pores and microstructures at each processing step and sintering conditions in the material extrusion additive manufacturing process, examining their correlation with related physical properties. This is achieved by fabricating copper parts using material extrusion (MEX) and examining the primary factors that impact the electrical characteristics of the MEX parts. Initially, the application of solvent debinding to MEX-produced Cu parts effectively removes a portion of the binder, forming printing-pattern–induced pores and ineffectual pore channels. Subsequently, thermal debinding eliminates the remaining binder and promotes neck formation between the Cu particles. The process proceeds to a subsequent sintering step, where even after the final sintering, some pores remain. These pores are categorized as sintering-induced and printing-pattern–induced pores. The relatively large pore size due to the printing pattern is identified as the main reason for reduced density of MEX parts, especially those influenced by the contour pattern. As the sintering progresses, the grain size increases, with the overall grain size and shape becoming uniform during extended sintering periods. Impurities, specifically carbon and oxygen, are detected in the sintered part, and their concentrations rise with longer sintering times. After examining the variation in porosity, grain size, and impurity content with respect to sintering time, it becomes evident that porosity plays a pivotal role in influencing electrical conductivity. Consequently, the densest 48 h sintered sample, with a relative density of 95.5 %, achieves the highest electrical conductivity at 94.6 % IACS. This demonstrates that MEX can be effectively employed to craft complex-shaped Cu parts with outstanding electrical properties. In summary, densification is crucial for enhancing the electrical properties of Cu parts produced through MEX, achieved by minimizing printing-pattern–induced pores, the primary contributor to density reduction. This study is poised to advance the manufacturing of high-performance, intricate Cu components by providing insights into the development and locations of primary pores through detailed microstructural investigations.

Evaluation of mechanical properties and Fick’s diffusion behaviour of aluminum-DMEM reinforced with hemp/bamboo/basalt woven fiber metal laminates (WFML) under different stacking sequences

This work examines the mechanical properties and structural aspects of Al-Hybrid fibre metal laminates (HFMLs) reinforced with fibres from basalt, hemp, and bamboo, including commercial-grade Al6061 alloy mesh (Al-DMEM). Compression moulding of sustainable fibres into various stacking configurations, such as BAlB, BHAlHB, and BBbAlBbB, was required for fabrication. Using cutting-edge methods like SEM and EDX for microstructure investigation, extensive testing was carried out, including tensile, flexural, impact, interlaminar shear, hardness, and water absorption tests. Basalt/Bamboo/Aluminum/Basalt (BBbAlBbB) was the FML configuration with the best mechanical parameters, showing Vickers hardness (24.23 MPa), tensile strength (44.537 MPa), flexural strength (218.24 MPa), and impact resistance (2.01 J) (59.982 HV). In addition, BAlB showed 83.9 % Vickers hardness, 41.7 % flexural strength, and 90.4 % tensile strength of BBbAlBbB samples. BHAlHB displayed 92.9 % of the flexural strength, 65.3 % of the tensile strength, and 93.7 % of the Vickers hardness of BBbAlBbB. Bamboo is hydrophobic by nature, so adding bamboo fibres to BBbAlBbB significantly boosted its resistance to moisture. This study highlights the potential of hybrid FMLs as high-performance, eco friendly materials that are particularly well-suited for applications in interior and exterior panels for micro-mobility (E-vehicles), with AL-DMEM reinforcement offering significant benefits like improved fibre bonding and structural integrity.

Enhancement of mechanical and physical properties of Cu–Ni composites by various contents of Y2O3 reinforcement

The increasing demand for materials possessing enhanced mechanical strength, high thermal conductivity, and excellent electrical properties has grown significantly. Cu-matrix composites, especially Cu– Ni, present a promising candidate to fulfill these demands. In this study, Cu–Ni composites were successfully synthesized using powder metallurgy with various additions (0–1.5 wt%) of Yttria (Y2O3)-reinforcement aiming to enhance their mechanical, thermal, and electrical properties. The microstructural investigations demonstrated a uniform distribution of Y2O3 particles and a slight increase in porosity of the Cu–Ni matrix. The Cu–Ni composites with 1.5 wt% Y2O3 showed the presence of Cu2NiZn intermetallic compounds, potentially harming their physical and mechanical properties. Y2O3-reinforcement significantly increased the hardness and led to a moderate rise in the yield and ultimate compressive strengths. The results indicated that the Cu–Ni matrix without Y2O3-reinforcement had the highest coefficient of thermal expansion, which decreased with the addition of Y2O3, potentially leading to improved thermal properties of Cu–Ni composites. This study puts an emphasis on the importance of Y2O3 particles dispersion and on the extent of porosity in enhancing the thermal and mechanical properties of Cu–Ni composites.