Showing papers on "Fractography published in 2019"

Fracture toughness of additively manufactured carbon fiber reinforced composites

Abstract: The fracture properties (stress intensity factor and energy release rate) of additively manufactured (AM) polylactic acid (PLA) and its short carbon fiber (CF) reinforced composites have been studied. The effects of CF reinforcement, nozzle geometry and bead lay-up orientations in fracture properties, void contents, and interfacial bonding were investigated. The fused filament fabrication (FFF)-based AM specimens using both circular and square shaped nozzle were printed and compared with the conventional compression molded (CM) samples. Compact tension (CT) specimens with different CF concentrations (0 wt.%, 3 wt. %, 5 wt.%, 7 wt.% and 10 wt.%) were printed with two bead lay-up orientations ( 45 0 / - 45 0 and 0 0 / 90 0 ) using PLA and CF/PLA composite filaments. The results show significant improvement in fracture toughness and fracture energy for CF/PLA composites in comparison to neat PLA. The fracture toughness was increased by 42% for 0 0 / 90 0 and 38% for 45 0 / - 45 0 bead orientations, respectively with 5% CF loading. The increase in fracture energy was observed to be about 77% for 0 0 / 90 0 and 88% for 45 0 / - 45 0 bead orientations, respectively for the same fiber reinforcement (5 wt. %). Such improvement in fracture properties is expected to be higher for all 90 0 bead orientations. The samples printed by square-shaped nozzle showed enhanced fracture toughness with less inter-bead voids and larger bonded areas in comparison to the circular-shaped nozzle. Although the fracture toughness showed very negligible differences between 0 0 / 90 0 and 45 0 / - 45 0 specimens, distinguishable variation may be seen in the case of 0 0 and 90 0 bead orientations. The crack propagation path and fracture mechanisms were studied using optical microscopy (OM) and scanning electron microscopy (SEM) examinations. Fractography revealed different modes of failure with a very high fiber orientation along the printing direction and a relatively higher void contents for 7 and 10 wt. % fiber reinforcement.

Investigating the effects of hybrid reinforcement particles on the microstructural, mechanical and tribological properties of friction stir processed copper surface composites

Abstract: Friction Stir Processing (FSP) technique was employed in this research to fabricate copper surface composites through incorporating a hybrid mixture of reinforcement particles. Hybrid combination of advanced nitride based ceramic particles (25% AlN + 75% BN) was dispersed onto the surface of copper matrix at varying volume fractions through FSP technique. Results demonstrated an increase in mechanical properties with respect to increase in the amount of particle dispersion. Mechanism of fracture of developed set of surface composites was studied with the aid of the fractography. A tremendous increase in wear resistance was observed with respect to increase in the hybrid particle dispersion owing the hardness and the self-lubricating characteristics of dispersed ceramic particles.

Mechanisms driving high-cycle fatigue life of as-built Inconel 718 processed by laser powder bed fusion

Abstract: This study investigates the relationships among the high-cycle fatigue life, surface roughness, and additive manufacturing processing parameters in laser powder bed fusion Inconel 718 in the as-built condition. Standardized fatigue specimens were manufactured using 25 different sets of processing parameters by varying laser power, scan speed, layer thickness, and build orientation, with three repeat specimens per parameter set. Surface roughness measurements were conducted using white light interferometry; high-cycle fatigue life was measured; and fractography analysis was performed using scanning electron microscopy. Two processing-parameter metrics were observed to dominate high-cycle fatigue life: build orientation and laser-energy density. Build orientation affected fatigue life due to the relationship between build orientation and surface roughness. Increasing surface roughness decreased the fatigue life due to increasing number of surface-crack initiation sites. For a fixed build orientation, the laser-energy density, outside of the optimal range, decreased the fatigue life due to sub-surface defects. Specifically, fractography analysis showed that sub-surface defects consisted of lack-of-fusion pores at low laser-energy densities and secondary cracking and pores (possibly related to keyholing) at high laser-energy densities. While variability in residual stresses among the specimens could also play a role, this work focuses on geometrical surface and sub-surface defects caused by different processing parameters and their corresponding impact on total fatigue life. Based on these findings, guidelines are offered to improve fatigue life of additively manufactured Inconel 718 in the as-built, non-heat-treated condition.

Mechanical properties and fracture of in-situ Al3Ti particulate reinforced A356 composites

Abstract: The microstructure and mechanical properties of in-situ 5 vol. % Al3Ti/A356 composites were studied either taking account of the effects of T6 heat treatment and strontium (Sr) addition or not. For both as-cast and T6 treated composites, the in-situ Al3Ti particulates are in a blocky morphology with average size of 5–6 μm. Sr modified the Si particulates by reducing their size and aspect ratio, without a noticeable influence on the Al3Ti particulates. Energy dispersive X-Ray spectroscopy (EDS) revealed the existence of Si in the blocky Al3Ti particulates in the as-cast composites likely to form an (Al,Si)3Ti phase. Solutionization at 540 °C for 4 h resulted in the enrichment of Si inside the Al3Ti particulates, raising a question about the stability of (Al,Si)3Ti as reported by others. For both as-cast and T6 treated A356 alloy, the yield strengths were enhanced by in-situ Al3Ti particles, but the elongation was adversely affected. In comparison, the tensile strength and ductility of the Al3Ti/A356 were simultaneous improved with the modification of Sr due mainly to the modification of eutectic Si particles. Microstructure-based representative volume element (RVE) simulations revealed the microscale damage evolution of the studied materials and provided a good prediction of their fracture behavior, substantiated by fractography.

Effects of different surface modifications on the fatigue life of selective laser melted 15–5 PH stainless steel

Abstract: Effects of machining, electro-polishing and laser surface re-melting (LRM) on the fatigue life of Selective Laser Melted (SLM) 15–5 precipitation hardening (PH) stainless steel are reported. Electro-polished surface showed ~97.4% reduction of surface roughness (Ra) and hence improved the fatigue life drastically (~100%) as compared to as-built specimens. Synergic effect of both compressive residual stress and reduction in surface roughness caused drastic improvement (~138%) in fatigue life of machined SLM specimens when compared to its as-built counterpart. Laser Surface Re-melting is found to be an effective technique to reduce the surface roughness (~91.2%) as well as sub-surface defects of geometrically complex as-built SLM specimen and thus improved its fatigue life by ~119%. Fractography analysis showed surface roughness, sub-surface defects and micro notches as the primary crack initiating factors for SLM specimens. High surface roughness in as-built part causes multiple crack initiation sites for specimens failed under both low and high stress amplitude. However, for machined, electro-polished and LRM SLM specimens distinct and multiple crack initiation sites could be observed when specimen failed under high cycle and low cycle regime respectively. However, for all the cases, fatigue life is found to be less compared to its wrought counterpart. Present study could be used as a guideline to select proper surface modification methods for SLM 15–5 PH stainless steel for a desired fatigue life.

Microstructure and mechanical properties of medium carbon steel deposits obtained via Wire and Arc Additive Manufacturing using metal-cored wire

Abstract: Wire and arc additive manufacturing (WAAM) is a 3D metal printing technique based on the arc welding process. WAAM is considered to be suitable to produce large-scale metallic components by combining high deposition rate and low cost. WAAM uses conventional welding consumable wires as feedstock. In some applications of steel components, one-off spare parts need to be made on demand from steel grades that do not exist as commercial welding wire. In this research, a specifically produced medium carbon steel (Grade XC-45), metal-cored wire, equivalent to a composition of XC-45 forged material, was deposited with WAAM to produce a thin wall. The specific composition was chosen because it is of particular interest for the on-demand production of heavily loaded aerospace components. The microstructure, hardness, and tensile strength of the deposited part were studied. Fractography studies were conducted on the tested specimens. Due to the multiple thermal cycles during the building process, local variations in microstructural features were evident. Nevertheless, the hardness of the part was relatively uniform from the top to the bottom of the construct. The mean yield/ultimate tensile strength was 620 MPa/817 MPa in the horizontal (deposition) direction and 580 MPa/615 MPa in the vertical (build) direction, respectively. The elongation in both directions showed a significant difference, i.e., 6.4% in the horizontal direction and 11% in the vertical direction. Finally, from the dimple-like structures observed in the fractography study, a ductile fracture mode was determined. Furthermore, a comparison of mechanical properties between WAAM and traditionally processed XC-45, such as casting, forging, and cold rolling was conducted. The results show a more uniform hardness distribution and higher tensile strength of the WAAM deposit using the designed metal-cored wires.

Microstructure analysis and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate

Abstract: Microstructure and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate were investigated in this work. The spray-formed alloy after hot extrusion experiment was treated with solid solution treatment and artificial aging. Microstructure analysis indicated the aged plate was dominated by elongated unrecrystallized grains, and had a rolling-type texture along extrusion direction with the highest intensity at Brass component. The existence of T1 phase strengthened the alloy crucially, but δ′ phase was basically absent. Then, the fully-reversed strain-controlled low-cycle fatigue tests were conducted at total strain amplitudes ranging from 0.4% to 1.0% for samples along two orthogonal directions. The stress-strain hysteresis loops were acquired, and the cyclic stress response curves were derived. At low strain amplitudes (0.4–0.5%), the initial cyclic hardening was slight and followed by a cyclic stability, while at higher strain amplitudes (0.6–1.0%), the alloy merely presented a continuously increasing cycle hardening behavior. Moreover, the fatigue life model based on the total strain energy was built and found to be suitable to predict life. Finally, the fatigue fractography observation showed that the fatigue source is relatively concentrated and the fracture surface had typical fatigue striations at 0.5% strain amplitude, while multiple cracks originated on the sample surface and the final fracture zone showed a ductile characteristic at 1.0%. The deformed microstructure near fracture surfaces were observed, and it was found that the cyclic hardening and stability were closely associated with the interaction between moving dislocations and obstacles including (sub)grain boundaries and secondary phase particles against them.

Plastic anisotropy and deformation-induced phase transformation of additive manufactured stainless steel

Abstract: Plastic anisotropy and deformation-induced phase transformation of additively manufactured (AM) stainless steels were investigated via in-situ neutron diffraction, electron backscatter diffraction, metallography, and fractography. Two types of tensile specimens were manufactured: (1) One sample was vertically fabricated with its tensile axis parallel to the z-direction (AM-V), (2) The other sample was horizontally fabricated with its tensile axis perpendicular to the z-direction (AM-H). A commercial 15-5PH stainless steel (CA) was used for comparison. AM steel revealed enhanced yield strength, tensile strength, and uniform elongation over CA, which was mainly due to grain refinement and transformation induced plasticity (TRIP). Different onsets of strain nonlinearity between AM-V and AM-H were closely related to martensitic phase transformation. Stresses estimated from lattice strains measured by neutron diffraction matched well with the applied stress-strain curves. After plastic deformation, voids were formed and congregated near the solidified line where fine grains were populated. Higher dislocation density was observed in the fine grain zone, and lower density was shown in the relatively coarse grain zone. AM steels exhibited significant anisotropic fracture behavior in terms of loading direction. In contrast to isotropic failure for CA and AM-V, AM-H revealed anisotropic failure with elliptical formation of the fracture feature. The fracture surface of AM-H possessed many secondary cracks propagating perpendicular to the building direction. The occurrence of secondary cracks in AM-H resulted in rapid load drop during tensile loading after necking.

Lithium disilicate glass-ceramic vs translucent zirconia polycrystals bonded to distinct substrates: Fatigue failure load, number of cycles for failure, survival rates, and stress distribution.

Abstract: The present study evaluated the fatigue behavior of monolithic translucent zirconia polycrystals (TZ) and lithium disilicate glass-ceramic (LD) bonded to different substrates. Disc-shaped specimens of ceramic materials TZ and LD were bonded to three substrates with different elastic modulus (E) (fiber-reinforced composite (FRC) – softest material, E = 14.9 GPa; titanium alloy (Ti) – intermediary properties, E = 115 GPa; and zirconia (Yz) – stiffest material, E = 210 GPa). The surfaces were treated and bonded with resin cement (disc-disc set-up). Fatigue testing followed a step-stress approach (initial maximum load = 200 N for 5000 cycles, incremental step load = 200 N for 10,000 cycles/step). The fatigue failure load and number of cycles until failure were recorded and statistically analyzed. Fractographic and finite element (FEA) analyzes were conducted as well. TZ ceramic depicted higher fatigue failure load, number of cycles until failure, and survival probabilities than LD, irrespective of the substrate. Moreover, TZ and LD presented better fatigue behaviors when bonded to substrates Ti and Yz in comparison to FRC. FEA revealed lower tensile stresses at restorative material when bonded to stiffer substrates. Fractography showed that the fracture origin started at bottom surface of restorative material (except for TZ bonded to Yz, in which crack initiated at load contact point). Translucent zirconia polycrystals present superior mechanical behavior than lithium disilicate glass-ceramic. The substrate type influences the mechanical performance of monolithic dental ceramics (stiffer substrates lead to better fatigue behavior).

Microstructure Characteristics and Comparative Analysis of Constitutive Models for Flow Stress Prediction of Inconel 718 Alloy

Abstract: An accurate constitutive model is essential for analyzing deformation behavior of material and reliable numerical simulations in metal forming processes. In this study, hot tensile tests of Inconel 718 alloy have been conducted over a wide range of temperatures (300-973 K at an interval of 100 K), strains (0.01-0.3 at an interval of 0.01) and quasi-static strain rates (0.0001, 0.001, 0.01 s−1). Flow stress behavior is significantly affected by test temperatures and strain rates. Microstructure characteristics of deformed test specimens have been examined using scanning electron microscope and electron backscatter diffraction (EBSD). The fractography study revealed that fracture is mix-mode type, i.e., ductile and brittle. Subsequently, EBSD analysis shown that dynamic recrystallization mechanism is more pronounced at a higher temperature. Furthermore, four constitutive models, namely modified Cowper–Symonds, modified Johnson Cook, modified Zerillie-Armstrong and integrated Johnson Cook–Zerillie-Armstrong (JC-ZA) models have been investigated for flow stress prediction. Capability of models has been evaluated based on the correlation coefficient (R), average absolute error (Δ) and its standard deviation (δ). Accurate prediction of flow stress behavior is found by integrated JC-ZA model with R = 0.9873, Δ = 2.44 and δ = 4.08%.

The effects of microstructure evolution on the fracture toughness of BT-25 titanium alloy during isothermal forging and subsequent heat treatment

Abstract: Isothermal compression with four different height reductions (0, 30%, 50% and 80%) and subsequent heat treatment was applied to BT25 alloy. Microstructure evolution following deformation and heat treatment was studied, meanwhile, the effect of various microstructure on fracture toughness of BT-25 alloy was analyzed in this paper. The microstructure observation and chemical analysis shows that volume fraction of globularized α phase increases with the increasing deformation, and the amount of Mo in α phase decreases with globularization. The fracture toughness exhibits a decreasing trend as the increasing volume fraction of globularized α phases. It is noteworthy that the fracture toughness of basket-weave structure has a significant decrease even though a small amount of α phases are globularized. The mechanism of this phenomenon could be explained by the different crack propagation mode between the microstructure with weaving tightly lamellar α phases and that with partially or fully globularized α phases. Fractography shows that fracture surface of undeformed material is characterized by deep secondary cracks and big fracture steps, and the surface becomes flatter with increasing volume fraction of globularized α phases. In addition, a model that can predict KIC using tensile properties is utilized. It can provide a relatively reliable prediction for fracture toughness of BT-25 alloy with globularized α phase. The model has a little error for prediction of fracture toughness with basket-weave structure due to underestimation of the secondary cracks.

Fatigue damage behavior in carbon fiber polymer composites under biaxial loading

Abstract: An investigation into the damage accumulation and propagation behavior in carbon fiber reinforced polymer (CFRP) composites under complex in-phase biaxial fatigue loading has been conducted. The goal is to capture early stage damage and obtain an improved understanding of damage propagation and associated degradation in material properties. Both cross ply and quasi isotropic laminate configurations have been studied and the tests were conducted under constant amplitude in-phase biaxial loading. An optimization technique was used to design the cruciform specimens for each stacking sequence. To understand the propagation of damage from the micro-to the macroscale, the fractured surfaces were analyzed, during various stages of fatigue, using electron microscope assisted fractography and a high-resolution camera. Material property degradation was determined by measuring the change in specimen stiffness to analyze the progression of fatigue damage and is correlated to the micro- and macroscale damage mechanisms and the biaxial fatigue loading parameters. The results provide insight into the initiation and propagation of damage mechanisms in CFRP composites which is essential to understanding the fatigue behavior of composite materials under complex multiaxial loadings.

Quantitative assessment of second phase particles characteristics and its role on the deformation response of a Mg-8Al-0.5Zn alloy

Abstract: The deformation and fracture characteristics of magnesium alloys are strongly influenced by the second phase particles. This study comprehensively investigates the three-dimensional microstructure of second phase particles, mechanical properties of individual particles and their effect on the bulk deformation behavior of the AZ80 magnesium alloy. X-ray microtomography has been used to quantify the 3D microstructure of second phase particles (Mg17Al12 and Al8Mn5) in terms of volume, morphology, and size. Further, to elucidate the contribution of second phase particles on the bulk deformation of the alloy, mechanical properties of the individual particle were obtained using nanoindentation. Both second phase particles were found to exhibit higher elastic modulus and hardness than the matrix. Post-indentation analysis using a combination of scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed the presence of pile-up in the vicinity of indents made on both the particles. Further, 3D microstructure-based finite element simulations predicted that the damage starts with the fracture of the particles at the sites of high stress concentration which is corroborated with fractography analysis.

Effect of Power Input on Metallurgical and Mechanical Characteristics of Inconel-625 Welded Joints Processed Through Microwave Hybrid Heating

Abstract: In the present study, welding of Inconel-625 through the use of microwave hybrid heating (MHH) has been achieved at two power levels 600 W and 900 W in a low-cost home microwave oven. Nickel-based powder EWAC was used as filler interface between faying surfaces. Effect of power variation on the metallurgical and mechanical characteristics of the microwave welded joints has been investigated. Developed joints were characterized through XRD, optical microscope, SEM, universal testing machine and Vickers microhardness tester. XRD study of the weld zone indicated the formation of various carbides and intermetallics. Joint microstructures witnessed a completely fused weld interface without any interfacial cracks. EDS analysis of the joint microstructure revealed lesser amount of segregation of niobium and molybdenum with the specimens developed at 600 W which could be attributed to the lower heat input associated with 600 W power that also resulted in fine grain structure. Further, the specimens processed at 600 W exhibited better tensile and flexural properties when compared to their counterparts produced at 900 W power. Fractography study of the specimens revealed a combined ductile and brittle fracture.

Investigation on material mixing during FSW of AA7475 to AISI304

Abstract: AA7xxx and AISI304 stainless steel (SS) are employed in promising applications. Al alloy-to-SS dissimilar joining is difficult and challenging. Major challenge in the joining of these alloys is the...

Microstructure anisotropy and its implication in mechanical properties of biomedical titanium alloy processed by electron beam melting

Abstract: Titanium alloy Ti-6Al-4V processed by electron beam melting (EBM) has a great potential for orthopedic and aerospace applications. However, the process induced porosity and microstructure anisotropy will have a significant impact on the material properties. This work has found that spherical and elongated pores with strong size effect are common characteristics for the as-EBM samples made with horizontal, diagonal, and vertical orientations w.r.t. the substrate. Furthermore, the major axis of the elongated pores is perpendicular to build direction for samples with different build orientations. The microstructure consists of columnar prior β grains delineated by grain boundary α and transformed α/β structures with α’ marteniste and basket weave morphology. Of note is that a high fraction of twin boundaries are prevalent in α (α’) phase. The configuration of the applied load w.r.t. the major axis of the elongated pores is the most significant influencing factor to mechanical properties, while the columnar prior β grain structure is secondary. Fractography reveals that microcracks tend to originate from elongated pores for cleavage fracture. In addition, the co-existing local terrace-like and shallow dimples are attributed to the intergranular crack propagation from the lamella α grain boundaries. Thus, the anisotropy of porosity and microstructure is of significance to enhance mechanical properties in process development.

Influences of Horizontal and Vertical Build Orientations and Post-Fabrication Processes on the Fatigue Behavior of Stainless Steel 316L Produced by Selective Laser Melting.

Abstract: In this paper, the influences of build orientation and post-fabrication processes, including stress-relief, machining, and shot-peening, on the fatigue behavior of stainless steel (SS) 316L manufactured using selective laser melting (SLM) are studied. It was found that horizontally-built (XY) and machined (M) test pieces, which had not been previously studied in the literature, in both stress-relieved (SR) or non-stress-relieved (NSR) conditions show superior fatigue behavior compared to vertically-built (ZX) and conventionally-manufactured SS 316L. The XY, M, and SR (XY-M-SR) test pieces displayed fatigue behavior similar to the XY-M-NSR test pieces, implying that SR does not have a considerable effect on the fatigue behavior of XY and M test pieces. ZX-M-SR test pieces, due to their considerably lower ductility, exhibited significantly larger scatter and a lower fatigue strength compared to ZX-M-NSR samples. Shot-peening (SP) displayed a positive effect on improving the fatigue behavior of the ZX-NSR test pieces due to a compressive stress of 58 MPa induced on the surface of the test pieces. Fractography of the tensile and fatigue test pieces revealed a deeper understanding of the relationships between the process parameters, microstructure, and mechanical properties for SS 316L produced by laser systems. For example, fish-eye fracture pattern or spherical stair features were not previously observed or explained for cyclically-loaded SLM-printed parts in the literature. This study provides comprehensive insight into the anisotropy of the static and fatigue properties of SLM-printed parts, as well as the pre- and post-fabrication parameters that can be employed to improve the fatigue behavior of steel alloys manufactured using laser systems.