Showing papers on "Fractography published in 2018"

On the effect of shot-peening on fatigue resistance of AlSi10Mg specimens fabricated by additive manufacturing using selective laser melting (AM-SLM)

Abstract: The effect on fatigue resistance of additively manufactured (AM) AlSi10Mg specimens fabricated by selective laser melting (SLM) following surface treatment by shot-peening was investigated. Specimen surface was shot-peened with either steel or ceramic balls. Nano-indentation measurements revealed that shot-peening caused surface hardening, with the hardness profile from the surface to the interior of the bulk disappearing 50 μm below the surface. Surfaces polished before shot-peening or following removal of about 25–30 μm from the surface after shot-peening by either mechanical or electrolytic polishing showed improved fatigue resistance and fatigue limit. Fractography of broken specimens demonstrated that for shot-peened specimens, the site of fatigue crack initiation was deeper than that for specimens that had not undergone shot-peening. The fracture area of AM-SLM AlSi10Mg specimens before and after shot-peening displayed a ductile fracture with relatively deep dimples. In contrast to AM specimens, the final fracture area of die-cast samples exhibited a brittle fracture surface, containing numerous cleavage facets and micro-cracks.

Influence of defects and as-built surface roughness on fatigue properties of additively manufactured Alloy 718

Abstract: Electron beam melting (EBM) and Selective Laser Melting (SLM) are powder bed based additive manufacturing (AM) processes. These, relatively new, processes offer advantages such as near net shaping, manufacturing complex geometries with a design space that was previously not accessible with conventional manufacturing processes, part consolidation to reduce number of assemblies, shorter time to market etc. The aerospace and gas turbine industries have shown interest in the EBM and the SLM processes to enable topology-optimized designs, parts with lattice structures and part consolidation. However, to realize such advantages, factors affecting the mechanical properties must be well understood – especially the fatigue properties. In the context of fatigue performance, apart from the effect of different phases in the material, the effect of defects in terms of both the amount and distribution and the effect of “rough” as-built surface must be studied in detail. Fatigue properties of Alloy 718, a Ni-Fe based superalloy widely used in the aerospace engines is investigated in this study. Four point bending fatigue tests have been performed at 20 Hz in room temperature at different stress ranges to compare the performance of the EBM and the SLM material to the wrought material. The experiment aims to assess the differences in fatigue properties between the two powder bed AM processes as well as assess the effect of two post-treatment methods namely – machining and hot isostatic pressing (HIP). Fractography and metallography have been performed to explain the observed properties. Both HIPing and machining improve the fatigue performance; however, a large scatter is observed for machined specimens. Fatigue properties of SLM material approach that of wrought material while in EBM material defects severely affect the fatigue life.

Microstructural tailoring and mechanical properties of a multi-alloyed near β titanium alloy Ti-5321 with various heat treatment

Abstract: A new near β-Ti alloy Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe (Ti-5321) with a unique combination of high strength and good fracture toughness was designed. The microstructure was tailored by changing the solution and ageing conditions, and the influences of microstructural evolution on tensile properties and fracture toughness of the alloy were investigated. The results showed that the volume fraction and size of primary α phase were decreased with increasing the solution temperature, while the morphology of secondary α precipitates was related to ageing temperature. The ultimate tensile strength (UTS), total elongation (EL) and fracture toughness can be achieved in a range of 1147–1439 MPa, 3–26% and 57–76 MPa m1/2, respectively, depending on the heat treatment parameters. An excellent balance of high strength and good ductility was realized after the solution treatment at 830 °C and ageing at 620 °C for 480 min, in which the UTS, EL and fracture toughness were 1238 MPa, 20% and 73 MPa m1/2, respectively. Morphological features of the fractography were discussed against the different microstructural morphologies, and this provided further information on the fracture behavior of the alloy.

Influence of process parameters and heat treatments on the microstructures and dynamic mechanical behaviors of Inconel 718 superalloy manufactured by laser metal deposition

Abstract: To achieve high-performance nickel-based superalloys by laser metal deposition (LMD) technology for applications in aeroengines, we prepared Inconel 718 superalloys by LMD with three groups of process parameters and then heat treated them by two different protocols. We carried out compressive experiments for these Inconel 718 samples over a wide range of strain rate (0.001–5000/s) to evaluate the effects of process parameters and heat treatments on their microstructures and dynamic mechanical properties. We observed both the initial microstructures and the failure characteristics of the samples using the optical microscope and the scanning electron microscope. We found that a higher energy input density during laser additive manufacturing led to a wider range of primary dendrite spacing. The plastic flow stress of the alloy decreased near-linearly with increase in primary dendrite spacing. The anisotropy of the compressive properties of the sample resulted from the anisotropy of the as-deposited and the direct aged structures, while the microstructural and mechanical anisotropy almost vanished after full heat treatment. We further carried out compressive experiments over a wide range of strain rate (0.001–5300/s) and temperature (298–1193 K) to understand the mechanical properties of Inconel 718 by LMD in an extremely high-strain rate and high-temperature loading environment. We noticed an anomalous high-temperature peak in the flow stress, in the flow stress vs. temperature relation, under different strain rates, and we proved that it is attributed to the third type of strain aging effect. Finally, by observing the compressive failure characteristics, we found that the propagation path of a crack is dependent on the loading direction. The compressive fractography morphology could reflect the effect of heat treatment on the ductility of the samples. Furthermore, it was evident that the initial defects (gas and shrinkage porosities) in Inconel 718 samples caused by LMD can contribute to the generation, deflection, and branching of cracks.

Friction-stir lap-joining of aluminium-magnesium/poly-methyl-methacrylate hybrid structures: thermo-mechanical modelling and experimental feasibility study

Abstract: In this research, the feasibility of friction-stir welding (FSW) for dissimilar lap-joining of an aluminium-magnesium alloy (AA5058) and poly-methyl-methacrylate sheets to attain sound and defect-free joints was examined. The inter-mixing flow patterns between the metal and polymer counterparts during FSW were predicted by employing three-dimensional finite element models. It is shown that the bonding mechanism between the dissimilar materials is mechanical interlocking at the interface which controls the joint strength depending on the processing parameters. The most suitable dissimilar lap-joining regarding microstructural soundness is attained at w= 1600 rev min−1 and v = 25 mm min−1. Under this condition, the maximum joint strength, which is about ∼60% of the weakest base material, is attained. Fractography indicates that the rupture occurs from the aluminium side.

Interlayer fracture toughness of additively manufactured unreinforced and carbon-fiber-reinforced acrylonitrile butadiene styrene

Abstract: This study presents development of a test method for characterization of interlayer, mode-I fracture toughness of fused filament fabrication (FFF) materials using a modified double cantilever beam (DCB) test. This test consists of DCB specimen fabricated from using unidirectional FFF layers, an 8 μm Kapton starter crack inserted in the midplane during the printing process, and reinforcing glass/epoxy doublers to prevent DCB arm failure during loading. DCB specimens are manufactured with a commercially available 3D printer using unreinforced Acrylonitrile Butadiene Styrene (ABS) and chopped carbon-fiber-reinforced ABS (CF-ABS) filaments. To examine the effect of the FFF printing process on fracture toughness, additional ABS and CF-ABS specimens are hot-press molded using the filament material, and tested with the single end notch bend (SENB) specimen configuration. The fracture toughness data from DCB and SENB tests reveal that the FFF process significantly lowers the mode-I fracture toughness of ABS and CF-ABS. For both materials, in situ thermal imaging and post-mortem fractography shows, respectively, rapid cool-down of the rasters during filament deposition and presence of voids between adjacent raster roads; both of which serve to reduce fracture toughness. For CF-ABS specimens, fracture toughness is further reduced by inclusion of poorly wetted chopped carbon fibers. Although this study did not attempt to optimize the fracture performance of FFF specimens, the results demonstrate that the proposed methodology is suitable for design and optimization of FFF processes for improved interlayer fracture performance.

Effect of Hot Isostatic Pressing and Powder Feedstock on Porosity, Microstructure, and Mechanical Properties of Selective Laser Melted AlSi10Mg

Abstract: AlSi10Mg tensile bars were additively manufactured using the powder-bed selective laser melting process. Samples were subjected to stress relief annealing and hot isostatic pressing. Tensile samples built using fresh, stored, and reused powder feedstock were characterized for microstructure, porosity, and mechanical properties. Fresh powder exhibited the best mechanical properties and lowest porosity while stored and reused powder exhibited inferior mechanical properties and higher porosity. The microstructure of stress relieved samples was fine and exhibited (001) texture in the z-build direction. Microstructure for hot isostatic pressed samples was coarsened with fainter (001) texture. To investigate surface and interior defects, scanning electron microscopy, optical fractography, and laser scanning microscopy techniques were employed. Hot isostatic pressing eliminated internal pores and reduced the size of surface porosity associated with the selective laser melting process. Hot isostatic pressing tended to increase ductility at the expense of decreasing strength. However, scatter in ductility of hot isostatic pressed parts suggests that the presence of unclosed surface porosity facilitated fracture with crack propagation inward from the surface of the part.

Evaluation of component repair using direct metal deposition from scanned data

Abstract: In this work, the repair volume of AISI H13 tool steel samples with hemisphere-shaped defects was reconstructed through reverse engineering and the samples were repaired by laser-aided direct metal deposition (DMD) using Co-based alloys powder as the filler material. Microstructure characterization and elemental distribution of deposits were analyzed using optical microscope (OM), scanning electron microscope (SEM), and energy dispersive spectrometry (EDS). Mechanical properties of repaired samples were evaluated via tensile test and microhardness measurement. The experiment showed that a gap between deposits and substrate exists if only employing the tool path generated from the reconstructed repair volume but the gap can be removed by depositing an extra layer covering that region. Microstructure and tensile test confirmed strong metallurgical bond in the interface. Defect-free columnar structure dominated the deposits near the interface while other regions of deposits consisted of dendrite structure with interdendritic eutectics. The tensile test showed that the repaired samples have a higher ultimate tensile strength (UTS) and lower ductility compared with those of base metal. Fractography from tensile test showed repaired samples fractured brittlely at the deposits section with cracking propagating along the grain boundaries. The hardness measurement showed that the deposited layers have a much higher hardness in comparison to the substrate.

Effect of temperature on the mechanical properties of 3D-printed PLA tensile specimens

Abstract: Purpose Recent advancements of 3D printing technology have brought forward the interest for this technique in many engineering fields. This study aims to focus on mechanical properties of the polylactic acid (PLA) feeding material under different thermal conditions for a typical fusion deposition of 3D printer system. Design/methodology/approach Specimens were tested under static loading within the range 20oC to 60oC considering different infill orientations. The combined effect of temperature and filament orientation is investigated in terms of constitutive material parameters and final failure mechanisms. The difference between feeding system before and post-3D printing was also assessed by mechanical test on feeding filament to verify the thermal profile during the deposition phase. Findings The results in terms of Young’s modulus, ultimate tensile strength (UTS), strain at failure (ef) and stress at failure (σf) are presented and discussed to study the influence of process settings over the final deposited material. Fracture surfaces have been investigated using an optical microscope to link the phenomenological interpretation of the failure with the micro-mechanical behaviour. Experimental results show a strong correlation between stiffness and strength with the infill orientation and the temperature values. Moreover, a relevant effect is related to deformed geometry of the filament approaching glass transition region of the polymer according to the deposition orientation. Research limitations/implications The developed method can be applied to optimise the stiffness and strength of any 3D-printed composite according to the infill orientation. Practical implications To avoid the failure of specimens outside the gauge length, a previously proposed modification to the geometry was adopted. The geometry has a parabolic profile with a curvature of 1,000 mm tangent to the middle part of the specimen. Originality/value Several authors have reported the stiffness and strength of 3D-printed parts under static and ambient temperature for different build parameters. However, there is a lack of literature on the combination of the latter with the temperature effects on the mechanical properties which this paper covers.

Split Hopkinson pressure bar tests for investigating dynamic properties of additively manufactured AlSi10Mg alloy by selective laser melting

Abstract: Dynamic properties of additively manufactured AlSi10Mg alloy by selective laser melting (AM–SLM-processed) have been investigated using the split Hopkinson pressure bar (SHPB) system. Additive manufacturing (AM) processes have attracted increased attention over the past three decades, and AlSi10Mg is an alloy commonly used in AM processes. AlSi10Mg is a widely used material, and has been a subject of extensive investigations concerning its microstructure, quasi-static properties, and post-processing. Nonetheless, dynamic mechanical properties of this alloy are yet to be explored over a wide range of strain rates. Dynamic properties of X- and Z-oriented AlSi10Mg alloy samples in the as-built and T5 heat-treated (T5-HT) states were investigated using SHPB under strain rates varying over a range of 700–7900 s−1. The investigation revealed an important dependence of dynamic properties of the said alloy on build orientation when subjected to strain rates range of the order of 1000–3000 s−1. At values of strain rate above and below this range, the observed dependency no longer existed. In addition, dependency of dynamic properties of the alloy on its thermal state (as-built versus T5-HT state) was investigated for the first time along with detection of no-strain-rate sensitivity of the AM-SLM-processed AlSi10Mg alloy. A pronounced ellipticity was observed in most samples, thereby reflecting the anisotropic nature of the alloy. Fractography and optical microscopy analyses revealed differences between fracture morphologies observed in the as-built and T5-HT samples. Cracks observed were predominantly of the radial type (with minor circumferential cracks) under the brittle fracture mode in as-built samples. In contrast, T5-HT samples mainly demonstrated the ductile mode of deformation.

Evolution of microstructure and tensile properties during the three-stage heat treatment of TA19 titanium alloy

Abstract: This study reported the development of a three-stage heat treatment for TA19 titanium alloy to obtain a ternary microstructure consisting of equiaxed, lamellar and acicular α, which were successively acquired in the first stage (I-stage), second stage (II-stage) and third stage (III-stage) treatments. The content ratio among the equiaxed, lamellar and acicular α was tailored by controlling the I-stage and II-stage temperatures. Tensile test revealed that an increase of I-stage temperature improved both ultimate strength (UTS) and elongation (EL) due to the increased content of lamellar α. However, an excessively high I-stage temperature led to a very low content of equiaxed α, coarsening of β grain and precipitation of grain boundary α, which reduced the EL. An increase of II-stage temperature resulted in an increase of UTS and a decrease of EL, because the increased content of acicular α produced a number of α/β interfaces, which strengthened the alloy but was detrimental to plasticity. The fractography analysis indicated that majority of heat treated specimens exhibited a completely ductile fracture mode, while a mixed mode of brittle and ductile fracture was observed in specimens, which was subjected to an excessively I-stage temperature.

Creep, fatigue, and fracture behavior of high-entropy alloys

Abstract: As high-entropy alloys (HEAs) are being actively explored for next-generation structural materials, gaining a comprehensive understanding of their creep, fatigue, and fracture behaviors is indispensable. These three aspects of mechanical properties are particularly important because (i) creep resistance dictates an alloy’s high-temperature applications; (ii) fatigue failure is the most frequently encountered failure mode in the service life of a material; (iii) fracture is the very last step that a material loses its load-carrying capability. In consideration of their importance in designing HEAs toward applicable structural materials, this article offers a comprehensive review on what has been accomplished so far in these three topics. The sub-topics covered include a comparison of different creep testing methods, creep-parameter extraction, creep mechanism, high-cycle fatigue S–N relation, fatigue-crack-growth behavior, fracture toughness, fracture under different loading conditions, and fractography. Directions for future efforts are suggested in the end.

Hot Corrosion and Low Cycle Fatigue of a Cr 2 AlC-Coated Superalloy

Abstract: Low temperature Type II hot corrosion is a serious problem for low cycle fatigue (LCF) failure of advanced turbine disk alloys operating at increased temperatures. Accordingly, the present effort studied 15–20 µm corrosion resistant Cr2AlC sputter coatings on Low Solvus High Refractory (LSHR) disk alloy LCF test specimens. These were cycled to failure at 840/−430 MPa and 0.33 Hz, after 500 h oxidation and 50 h of Mg-Na2SO4 hot salt corrosion, all at 760 °C. The coating successfully prevented hot corrosion pitting that was responsible for a 90% decrease in uncoated LCF specimens. However, fractography identified unintentional 15–30 µm deep defects produced by grit blast surface preparation of coated samples. These acted as failure origins and introduced anomalous life reduction for some coated test specimens. Furthermore, the presence and growth of an undesirable Cr7C3 second phase diminished protectiveness by promoting internal oxidation and embrittlement of the coating.

Studies on the weldability, mechanical properties and microstructural characterization of activated flux TIG welding of AISI 321 austenitic stainless steel

Abstract: The presence of titanium in AISI 321 austenitic stainless steel, mitigates the corrosion when deployed in specific applications such as boiler shells, aircraft exhaust manifolds and process equipment. Using Commercial flux purchased from Edison Welding Institute (EWI), studies were conducted to note the effect of Activated Tungsten Inert gas (A-TIG) welding on the surface morphology of type 321 austenitic stainless steel welds and compared with conventional TIG welding. A thin flux layer was applied to the surface of the 6 mm thick plate, followed by a conventional TIG welding process. The bead on trial results shows that compared with the TIG welding process, the flux causes the weld depth to increase, weld bead width to decrease and weld area to increase. This incredible depth of penetration (DOP) has been accomplished by the mechanisms of reversal of Marangoni flow and Arc constriction. From various experimental trials, arc length of 3 mm, welding current of 220 Amps and welding speed of 120 mm min−1 were found to be optimal and subsequently used as input parameters to produce a good quality A-TIG welded butt joint. The welded joint is subjected to transverse and longitudinal tensile and bend tests, charpy impact toughness tests, microhardness, optical microscope, x-ray diffraction analysis, ferrite number measurement and Scanned electron fractography. The welded joint exhibited improved tensile strength, bend and charpy impact toughness and hardness. The weld metal microstructure was observed to be austenite, delta-ferrite and TiC intermetallic compounds (Titanium carbides). The XRD pattern indicates that austenite and ferrite phases are present in both base and weld metal. The results of Fischer Feritscope FMP30 (ferrite measurement) show that the content of delta-ferrite in the weld metal (5.9 FN) is much higher than the parent metal (1.2 FN) and shows excellent mechanical properties in the A-TIG welded joint. Scanned electron fractography indicates that the failure of weld metal and base metal occurs in the ductile mode of fracture.

Effect of post processing on the creep performance of laser powder bed fused Inconel 718

Abstract: In this study, the creep performance of laser powder bed fusion manufactured Inconel 718 specimens is studied in detail and compared with conventional hot-rolled specimens alongside as-built then heat-treated and as-built then hot-isostatic pressed specimens. Hot-rolled specimens showed the best creep resistance, while the hot-isostatic pressed specimens yielded the worst performance, inferior to the as-built condition. Creep testing of all samples showed increased secondary creep rate was consistently correlated with a reduced life. Fractography revealed intergranular fracture was the primary failure mode for all as-built samples. Preferential intergranular precipitation in the case of the hot-isostatic pressed specimens during hot-isostatic pressing extensive intergranular cracking as the primary failure mechanism. Heat-treated specimens possessed only sparse intergranular precipitates, thereby explaining an improved creep lifetime. The hot-rolled specimens, having smallest grain size, showed the least extensive cracking, particularly in locations of finest grains, explaining avoidance of intergranular fracture as a key creep mechanism, thereby explaining the ductile creep fracture surfaces in the case of the hot-rolled samples.

Mechanical Characterization of Ceramic Nano B4C- Al2618 Alloy Composites Synthesized by Semi Solid State Processing

Abstract: Al2618 alloy composites containing 6 wt% nano B4C were prepared by liquid metallurgy two stage stir casting process Characterization was performed using scanning electron microscope to validate the homogenous distribution of nano B4C particles in the Al2618 alloy matrix The presence of B and C elements was confirmed by EDS analysis in the nano B4C reinforced metal composites The mechanical behavior of the prepared nano composites were evaluated as per the ASTM standards, and it was noted that the tensile strength (ultimate strength and yield strength) and compression strength increased with the addition of 6 wt% nano B4C particles Finally, to understand the failure mechanisms the fractography analysis was conducted on the broken tensile specimens by using SEM images