Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2017"

Mechanical behavior of selective laser melted 316L stainless steel

Abstract: The tensile, fracture, and fatigue crack growth properties of 316L stainless steel (SS) produced using the selective laser melting (SLM) technique were evaluated and compared with those of conventionally manufactured (CM) austenitic SSs. For SLM, both single melt (SM) and checker board (CB) laser scanning strategies were employed, so as to examine the effect of scanning strategy on the mechanical properties. The experimental results show that the SLM alloys' yield strength is significantly higher than that of CM 316L SS, a result of the substantial refinement in the microstructure. In contrast, only a marginal improvement in the ultimate tensile strength and a marked reduction ductility, which are a result of the loss of work hardening ability, are attributed to the absence of stress induced martensitic transformation common in CM austenitic SSs. In spite of these, the fracture toughness, which ranges between 63 and 87 MPa m 0.5 , of the SLM alloys is good, which is a result of the mesostructure induced crack tortuousity. The SLM process was found to marginally reduce the threshold stress intensity factor range for fatigue crack growth initiation and enhance the Paris exponent within the steady state crack growth regime. Both tensile and toughness properties were found to be anisotropic in nature. SLM with CB scanning strategy improves both these properties. All these observations on the mechanical properties are rationalized by recourse to micro- and meso-structures seen these alloys.

Microstructure and hardness studies of Inconel 718 manufactured by selective laser melting before and after solution heat treatment

Abstract: The microstructure of Additive Manufactured (AM) Inconel 718 in general and Selective Laser Melting (SLM), in particular is different from the material produced by conventional methods due to the rapid solidification process associated with the former. As a result, the widely adapted standard solution heat treatment temperature (

Microstructure and mechanical properties of Inconel 718 produced by selective laser melting: Sample orientation dependence and effects of post heat treatments

Abstract: Inconel 718 produced by selective laser melting (SLM) has been characterized with focus on the microstructure, the dependence of sample orientation on the mechanical properties and the effects of post heat treatments. The as-manufactured IN718 has a very fine cellular-dendritic structure with fine Laves phases precipitating in the interdendritic region, and electron backscatter diffraction (EBSD) analysis shows that both the vertically and horizontally built samples have relatively weak texture. The vertically built samples show lower tensile strength but higher ductility than the horizontally built samples, and the mechanism is shown to be partly due to the crystallographic feature but more importantly due to the different amount of residual stress and dislocations accumulated in these two kinds of samples. Applying heat treatments can significantly increase the strength while decrease the ductility correspondingly, and difference in yield strength between the vertically and horizontally built samples decreases with increasing the heat treatment temperatures, mainly due to the removal of residual stress and dislocations.

Change in microstructure of selectively laser melted AlSi10Mg alloy with heat treatments

Abstract: In the present study, we examined changes in the microstructure and mechanical properties of AlSi10Mg alloy, initially fabricated using selective laser melting (SLM) combined with a powder-bed system, by applying heat treatments at temperatures of either 300 or 530 °C. The as-fabricated samples exhibited a characteristic microstructural morphology and {001} texture. Melt pools corresponding to the locally melted and rapidly solidified regions were found to be composed of several columnar α-Al grains surrounded by fine eutectic Si particles. A fine dislocation substructure consisting of low-angle boundaries is present within the columnar α-Al grains. At elevated temperatures, fine Si phase precipitates within the columnar α-Al phase and coarsening of the eutectic Si particles occurs. These fine Si particles inhibit grain growth in the α-Al matrix, resulting in the microstructural morphology and [001] texture observed in the heat-treated samples. The dislocation substructure disappears in the columnar α-Al grains. Furthermore, the formation of a stable intermetallic phase occurs, reaching microstructural equilibrium after long-term exposure. The as-fabricated specimen exhibits a high tensile strength of approximately 480 MPa. The strength is independent of the tensile direction, that is, normal and parallel to the building direction. In contrast, the tensile ductility is found to be direction-dependent, and is therefore responsible for a fracture preferentially occurring at a melt pool boundary. The direction-dependence of the tensile ductility was not found in the specimen that had been heat-treated at 530 °C. The present results provide new insights into the control of the direction-dependence of the tensile properties of AlSi10Mg alloys fabricated by SLM.

A comparison on metallurgical behaviors of 316L stainless steel by selective laser melting and laser cladding deposition

Abstract: Selective laser melting (SLM) and laser cladding deposition (LCD) are two typical kinds of laser additive manufacturing techniques that have been developed for many years independently. Although they are based on the same principle of laser cladding, there are little comparison on the fundamental studies for metallurgical behavior (including melting and solidification behaviors) and the mechanical properties of these two techniques up to now. In this paper, the single-track formation and the deposition of block sample from 316L stainless steel powders have been carried out by both SLM and LCD techniques. A comparison on pool shape, cooling rate, columnar grain size and mechanical properties under different processing conditions by LCD and SLM respectively has been studied. It is found that, due to the increase of energy input and the decrease of depth-to-width ratio of melting pool (MP) from SLM to LCD, the primary cellular arm spacing (PCAS) of the sample increases from less than 1.0 µm to more than 15.0 µm, and thus the cooling rate of MP decreases from about 106 K/s in SLM to about 102 K/s in LCD. Furthermore, due to the decrease of cooling rate from SLM to LCD, the columnar grains of the as forming alloy are getting coarser. Especially, the relationship between gain size (λ) and the reciprocal of square root of cooling rate ( T ) in LCD significantly meets the classical linear function of λ = a + b / T (a and b are constants), while a new relationship of a cubic function is found in SLM, showing the different solidification characteristics between LCD and SLM. Lastly, the samples of 316L stainless steel by SLM have much stronger tensile strength but lower elongation than those by LCD, and the main reason is due to that the solidification behavior of the MPs by SLM can form much finer columnar grains than those by LCD.

Anisotropic tensile behavior of in situ precipitation strengthened Inconel 718 fabricated by additive manufacturing

Abstract: This study investigated the microstructure and anisotropic mechanical properties of selective laser melting (SLM) processed Inconel 718 (IN718) component. In as-fabricated alloys, ultrafine columnar grained microstructure with highly dispersed precipitates γ" phases at grains boundary and even-distributed γ' phases inside the grains were observed. It was demonstrated that the as-fabricated longitudinal samples showed lower ultimate tensile strength (UTS) of 1101 MPa but higher elongation of 24.5% compared to the transverse samples which showed UTS of 1167 MPa and elongation of 21.5%. The excellent mechanical properties of both the longitudinal and transverse samples can be ascribed to the refined microstructure of the SLM material resulting from the high cooling rate imposed by laser processing. The anisotropy in strength and ductility was attributed to the {100} fiber texture and columnar grain morphology. The {100} fiber texture of columnar grains leads to high strength in transverse direction, while the columnar grain boundaries also served as a path along which damage can preferentially accumulate, leading to fracture.

Effects of heat treatments on microstructure and properties of Ti-6Al-4V ELI alloy fabricated by electron beam melting (EBM)

Abstract: Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that is used to fabricate three-dimensional near-net-shaped parts directly from computer models. Ti-6Al-4V is the most widely used and studied alloy for this technology and is the focus of this work in its ELI (Extra Low Interstitial) variation. Microstructure evolution and its influence on the mechanical properties of the alloy in the as-fabricated condition have been documented by various researchers. In the present work, different heat treatments were performed based on three approaches in order to study the effects of heat treatments on the unique microstructure formed during the EBM fabrication process. In the first approach, the effect of various cooling rates after the solutionizing process was studied. In the second approach, a correlation between the variation of α lath thickness during aging and the subsequent effect on mechanical properties was established. Lastly, several combined solutionizing and aging experiments were conducted; the results will be systematically discussed in the context of structural performance and design.

In-situ residual stress reduction, martensitic decomposition and mechanical properties enhancement through high temperature powder bed pre-heating of Selective Laser Melted Ti6Al4V

Abstract: During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. The use of high temperature pre-heating during an SLM build can assist in reducing thermal gradients, enable a more controlled cooling with the possibility to control/tailor as-built mechanical properties. In this work a high temperature SLM powder bed capable of pre-heating to 800 °C is used during processing of Ti6Al4V feedstock. The effect of powder bed temperature on residual stress formation, microstructure and mechanical properties was investigated. It was found that increasing the bed temperature to 570 °C significantly reduced residual stress formation within components and enhanced yield strength and ductility. This pre-heating temperature enabled the decomposition of α ′ martensitic microstructure into an equilibrium α+β microstructure. At 570 °C the yield strength and elongation of components was improved by 3.2% and 66.2% respectively.

SLM-processed Sc- and Zr- modified Al-Mg alloy: Mechanical properties and microstructural effects of heat treatment

Abstract: Traditionally 4xxx casting alloys are used for the additive manufacturing of structurally optimised lightweight parts in space, aerospace and automotive. However, for such applications there is a need for hardenable high-strength Al-alloys exceeding the properties of the 4xxx alloys family. The study analyses the hardness response of different heat treatment temperatures and hold durations applied to a Sc- and Zr-modified Al-Mg (5xxx-) alloy (Scalmalloy®) processed by Selective Laser Melting, and compares the mechanical properties and microstructure in the as-processed and annealed condition, and these properties are clearly related to the very fine grained microstructure. The results show that the static mechanical properties are exceptionally good with R m -values exceeding 500 MPa along with almost no build-orientation related anisotropic effects, and a high ductility even in the heat treated condition. These properties are clearly related to the very fine grained material, along with the good hardenability of the alloy. The stress-strain curves show the typical Portevin-Le-Chatelier (PLC) effect as known for other 5xxx alloys. Due to significant grain boundary pinning by different particles the very fine-grained bi-modal microstructure originating from the SLM-process can be maintained even in the heat treated condition, and only a HIP treatment leads to local grain growth only in coarser grained regions.

Nanoindentation and wear properties of Ti and Ti-TiB composite materials produced by selective laser melting

Abstract: Ti and Ti-TiB composite materials were produced by selective laser melting (SLM). Ti showed an α΄ microstructure, whereas the Ti-TiB composite revealed a distribution of needle-like TiB particles across an α-Ti matrix. Hardness (H) and reduced elastic modulus (Er) were investigated by nanoindentation using loads of 2, 5 and 10 mN. The results showed higher H and Er values for the Ti-TiB than Ti due to the hardening and stiffening effects of the TiB reinforcements. On increasing the nanoindentation load, H and Er were decreased. Comparison of the nanoindentation results with those derived from conventional hardness and compression tests indicated that 5 mN is the most suitable nanoindentation load to assess the elastic modulus and hardness properties. The wear resistance of the samples was related to their corresponding H/Er and H3/Er2 ratios obtained by nanoindentation. These investigations showed that there is a high degree of consistency between the characterization using nanoindentation and the wear evaluation from conventional wear tests.

Influence mechanism of parameters process and mechanical properties evolution mechanism of maraging steel 300 by selective laser melting

Abstract: Selective laser melting (SLM) is one kind of additive manufacturing process to fabricate metal parts through laser melting. A maraging steel 300 was manufactured by SLM. And the influence of process parameters (laser powder, scanning speed and scanning space) on the relative density of maraging steel 300 was investigated first. Then a series of block and plate specimens were manufactured. Some specimens were taken as control groups, and others underwent heat treatment by solution treatment (ST) and solution treatment +aging treatment (ST+AT) respectively. The investigation involved microstructure, microhardness, tensile strength and impact toughness. It is shown that: low or high laser power, scanning speed or scanning space all reduced the relative density, and the optimized process parameters could be obtained by orthogonal experiment. After ST, the cellular structure and microscopic segregation disappeared, and the new smaller particles precipitated out after AT. The Ni, Mo and Ti dissolved in the matrix during ST separated out again forming tiny Ni 3 Mo, Fe 2 Mo and Ni 3 Ti particles during AT. The microhardness and tensile strength dropped a little with elongation increasing after ST. While they increased significantly with elongation decreasing after AT. The impact toughness increased little after ST, but decreased sharply after AT.

Twinning induced plasticity in austenitic stainless steel 316L made by additive manufacturing

Abstract: Additively manufactured (AM) 316L steel exhibits extraordinary high yield strength, and surprisingly good ductility despite the high level of porosity in the material. This detailed study sheds light on the origins of the observed high yield strength and good ductility. The extremely fine cells which are formed because of rapid cooling and dense dislocations are responsible for the macroscopically high yield strength of the AM 316L (almost double of that seen in annealed 316L steel). Most interestingly, twinning is dominant in deformed samples of the AM316. It is believed that twinning-induced plasticity (TWIP) behaviour to be responsible for the excellent ductility of the steel despite the high level of porosity. The dominant twinning activity is attributed to Nitrogen gas used in 3D printing. Nitrogen can lower the stacking fault energy of the steel, leading to the disassociation of dislocations, promoting the deformation twinning. Twinning induces large plasticity during deformation that can compensate the negative effect of porosity in AM steel. However, twinning does not induce significant hardening because (1) the porosity causes a negative effect on hardening and (2) twinning spacing is still larger than extremely fine solidification cells.

Microstructure and mechanical properties of the austenitic stainless steel 316L fabricated by gas metal arc additive manufacturing

Abstract: The austenitic stainless steel 316L was fabricated by gas metal arc additive manufacturing (GMA-AM) and its microstructure and room temperature tensile properties were investigated. Results show that in the GMA-AM 316L plate, a large number of well-aligned austenitic dendrites vertically orient, forming large columnar grains in the middle and some dendrites bent toward the plate surfaces, forming small columnar grains near the edges. The microstructure of GMA-AM 316L consists of δ, γ and σ phases. After one layer was deposited, the δ phase exhibited reticular morphology within austenitic dendrites. The δ phase redissolved in austenite with the intermetallic σ phases forming at γ/δ interfaces under the thermal cycles influence of subsequent three deposition layers. And under the thermal influence after the fourth layers, both δ and σ phases turned into fine vermicular morphologies within austenitic dendrites. The tensile properties of GMA-AM 316L steel are comparable to wrought 316L and exceed the industry requirements for 316L. Its fracture type is ductile fracture due to the obvious fracture surface dimples. The microcracks initiate at the interior of σ phases and grow into large cracks leading to materials failure.

Effect of heat treatment and hot isostatic pressing on the microstructure and mechanical properties of Inconel 625 alloy processed by laser powder bed fusion

Abstract: The effect of different heat treatments and hot isostatic pressing on the microstructure and mechanical properties of laser powder bed fusion IN625 alloy was studied. The heat treatments were: stress relief annealing, recrystallization annealing and low-temperature solution treatment. The resulting microstructure and crystallographic textures were studied using optical and scanning electron microscopy. The mechanical properties of the as-built and post-treated IN625 alloy were obtained after tensile testing at room temperature and at 760 °C (1400 °F), and compared to those of an annealed wrought alloy of the same composition.

Strengthening mechanism in graphene nanoplatelets reinforced aluminum composite fabricated through spark plasma sintering

Abstract: Graphene nanoplatelets (GNP) reinforced aluminum matrix composites, with ≤5 wt% GNP content, were synthesized by spark plasma sintering (SPS). GNPs were found to withstand severe conditions of high pressure and temperature during processing. Strength of composite was observed to be depending on the content and uniform dispersion of GNP in aluminum matrix, as verified by scanning electron micrographs. X-ray diffraction analysis confirmed that no reaction products exist at Al-GNP interface in significant amount. Instrumented indentation studies revealed improvement in hardness by 21.4% with 1 wt% GNP. This is due to the presence of stronger reinforcement, which provides high resistance to matrix against deformation. Improvement in yield strength and tensile strength was 84.5% and 54.8%, respectively, with 1 wt% GNP reinforcement. Properties deteriorated at higher concentration due to agglomeration of GNP. Reinforcing effect of GNPs, in terms of strengthening of composite, is found to be dominated by Orowan strengthening mechanism. Pinning of grains boundaries by GNPs led to uniform grain size distribution in the composites structure. Overall, graphene reinforcement has offered 86% improvement in specific strength of aluminum matrix.

Fatigue of AlSi10Mg specimens fabricated by additive manufacturing selective laser melting (AM-SLM)

Abstract: The fatigue resistance of AM-SLM AlSi10Mg samples built in the Z direction after various heat treatments was investigated. Specimens were tested in the as-built (AB) condition, after stress relief (SR) treatment and after SR and hot isostatic pressing (HIP) at either 250 °C or 500 °C. The AB machined and polished specimens displayed the highest fatigue limit ( S f = 125 MPa, N f = 10 7 cycles). SR and HIP cycles decrease the yield strength, hardness and fatigue limit. The SR and HIP treatment at 500 °C resulted in the lowest fatigue resistance due to significant microstructural changes. A relation between the yield stress and fatigue resistance was established. Linear elastic fracture mechanics were employed for evaluating fracture surface morphology. Based on the results of fracture surface characterization, values of the critical stress intensity factor ( K cr ) for AM-SLM AlSi10Mg specimens after various heat treatments were estimated.

Effect of heat treatment on microstructure, mechanical and corrosion properties of austenitic stainless steel 316 L using arc additive manufacturing

Abstract: The mechanical and corrosion properties of gas metal arc additive manufacturing (GMA-AM) 316L could be optimized by modifying the volume fractions of sigma (σ) and delta-ferrite (δ) phases through heat treatment. Results show that the heat treatment at 1000 °C to 1200 °C for one hour will not obvious influence the morphology of grains in steel but largely influence the contents of σ and δ phases. The heat treatment at 1000 °C effectively increases the amount of σ phase in steel, causing both increase of UTS and YS but decrease of El and RA. The heat treatment at 1100 °C to 1200 °C completely eliminates σ phase, leading to the decrease of UTS and YS but increase of El and RA. The σ phase has better strengthening effect than δ phase, but which may degrade ductility and increase the possibility for cracks generation in steel. Meanwhile, limiting the number of both σ and δ phases through heat treatment can improve the corrosion resistance of steel. And σ phase appears more detrimental impact on degradation the corrosion resistance of steel than δ phase.

Structure and mechanical properties of B2 ordered refractory AlNbTiVZrx (x = 0–1.5) high-entropy alloys

Abstract: Structure and mechanical properties of the AlNbTiVZrx (x = 0; 0.1; 0.25; 0.5; 1; 1.5) refractory high-entropy alloys were investigated after arc melting and annealing at 1200 °C for 24 h. The AlNbTiV alloy had a B2 ordered single phase structure. Alloying with Zr resulted in (i) change of the degree of order of the B2 phase; and (ii) precipitation of the Zr5Al3 and C14 Laves ZrAlV phases. The density of the AlNbTiVZrx alloys varied from 5590 kg m−3 for the AlNbTiV alloy to 5870 kg m−3 for the AlNbTiVZr1.5 alloy. The compression yield strength at 22 °C increased with an increase in the Zr content from 1000 MPa for the AlNbTiV alloy to 1535 MPa for the AlNbTiVZr1.5 alloy. The plasticity raised from 6% for the AlNbTiV alloy to >50% for the AlNbTiVZr0.5 alloy and then dropped to 0.4% for the AlNbTiVZr1.5 alloy. At 600 °C, the strongest alloy was also the AlNbTiVZr1.5, whereas, at 800 °C, the AlNbTiVZr0.1 alloy demonstrated the maximum strength. The plasticity of the AlNbTiV alloy at 600 °C increased up to 14.3%, while the Zr-containing alloys had lower plasticity. At 800 °C, all the AlNbTiVZrx alloys could be plastically deformed up to 50% of strain without fracture. Ordering in the alloys and the reasons of a complicated dependence of mechanical properties of the AlNbTiVZrx alloys on the Zr content and temperature were discussed.

Comparative study of commercially pure titanium produced by laser engineered net shaping, selective laser melting and casting processes

Abstract: Commercially pure titanium was produced using laser engineered net shaping (LENS) and selective laser melting (SLM) processes. The SLM and LENS processing parameters as well as critical aspects including densification and balling effect were investigated. The resulting properties were studied and compared with those from traditional casting. Investigation of the processing parameters showed that significantly higher laser power and energy density is required in LENS compared to SLM in order to obtain near full density (99.5%). The microstructural investigations revealed an α microstructure with mixed morphologies including plate-like and widmanstatten for LENS somewhat similar to the serrated and fine acicular α obtained from casting. In contrast, the SLM samples showed only martensitic α′ phase mainly with a lath-type morphology. The difference between SLM and LENS microstructures was discussed based on interrelated aspects including energy density, solidification rate and specific point energy. Differences in their microstructures are mainly associated with differing rates of cooling and differing energy densities during SLM and LENS processing. Compression and hardness tests indicated that SLM titanium possesses better mechanical properties due to a fine grain size and martensitic phase composition, whereas LENS and cast titanium with α microstructures show similar mechanical properties.

The influence of Laves phases on the high-cycle fatigue behavior of laser additive manufactured Inconel 718

Abstract: In this paper, a comparative study of high-cycle fatigue tests (T=650 °C, f=110 Hz, R=0.1, Kt=1) were carried out with wrought Inconel 718 and LAMed Inconel 718. The results show that the influences of the Laves phases on high-cycle fatigue properties are based on the applied stress amplitudes. At a low stress amplitude, most of the Laves phases held their original morphologies. The fatigue cracks stopped in front or detoured around them, which means that the unbroken Laves phases play an important role in hindering crack propagation. In this way, the high-cycle fatigue life of LAMed Inconel 718 was superior to that of wrought Inconel 718. However, at a high stress amplitude almost all of the Laves phases in the crack propagation region splintered into smaller fragments, parts of which separated from the austenite matrix. Microscopic holes or cracks were formed at the interface, which provided passages for the fatigue cracks to propagate. In this case, the Laves phases were harmful, leading to the degradation of fatigue performance in LAMed Inconel 718 compared with wrought Inconel 718.

Strain rate sensitivity and fracture mechanism of AlSi10Mg parts produced by Selective Laser Melting

Abstract: Implementation of Additive Manufacturing (AM) parts in the growing applications within the automotive and aerospace industries encourages further investigations of the material behavior under various strain rates, spanning from quasi-static to the high strain rate regimes. Although mechanical properties of AM-Selective Laser Melting (SLM) AlSi10Mg parts under a static regime have been investigated, the strain rate sensitivity of these materials, to the best of our knowledge, has not been discussed in the literature. In this work, the properties of AM-SLM AlSi10Mg material were systematically investigated under a wide range of strain rates, spanning from 2.77×10−6 to 2.77×10−1 S−1. The AM-SLM AlSi10Mg alloy, as opposed to Al alloys fabricated by conventional methods, was found to be strain rate sensitive with significant changes to the flow stress and strain hardening exponents with an increase in strain rate. The fracture mechanisms of these specimens, built in different orientations, are discussed.

Tensile deformation behavior and deformation twinning of an equimolar CoCrFeMnNi high-entropy alloy

Abstract: The tensile deformation and strain hardening behaviors of an equimolar CoCrFeMnNi high-entropy alloy (HEA) were investigated and compared with low and medium entropy equiatomic alloys (LEA and MEA). The HEA had a lower yield strength than the MEA because the addition of Mn weakens solid solution hardening in the HEA. However, deformation twinning induced the multiple stage strain hardening behavior of the HEA and enhanced strength and elongation. Using tensile-interrupted electron backscatter diffraction analysis, geometrically necessary dislocations were observed as plume-shaped features in grain interior, and a considerable texture was characterized, which is typical of face centered cubic metals. Moreover, the relationship between favorably oriented grains and twinning in the HEA bore a clear resemblance to the same tendency in TWIP steels. The thickness of the twin bundles was less than 100 nm. A high density of stacking defects was found in the nanotwins. Nano twinning and stacking faults were found to contribute to the remarkable mechanical properties. Deformation induced twinning not only demonstrated the dynamic Hall-Petch effect but also changed dislocation cell substructures into microband structures.

Microstructure evolution characteristics of Inconel 625 alloy from selective laser melting to heat treatment

Abstract: Superalloy Inconel 625 has been widely used in selective laser melting (SLM). Since SLM-induced microstructure with columnar grains, strong texture, porosity, and undesired properties, heat treatment is often used to tune the microstructure and mechanical properties. However, the microstructure evolution of IN 625 from SLM to heat treatment is poorly understood. In this study, IN 625 samples were SLMed and then heat treated at elevated temperatures. Microstructure evolution characteristics of the processed IN 625 alloy have been characterized using optical metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction, and micro indentation. Fine dendrite microstructures with strong texture parallel to layer build-up direction was observed in the as-SLM samples due to rapid cooling and epitaxial growth. High dislocation density and high microhardness were found in the γ matrix which also contains high Z-contrast precipitates. After annealing at high temperatures, random grain growth accompanied by dislocation annihilation and twinning occurs. The decrease in lattice parameter and the prevalence of large grain boundary misorientation in the γ matrix suggests that SLM-induced residual stress be significantly reduced. In addition, the uncertainty of mechanical properties due to the process variations was quantified.

δ Phase precipitation in Inconel 718 and associated mechanical properties

Abstract: Microstructures generated by δ phase precipitation in Inconel 718 (IN718) and related mechanical properties were investigated in this paper. Several heat treatments enabling the precipitation of the δ phase in various proportions and morphologies were conducted. Heat treatments were performed in a temperature range between 875 °C and 975 °C with time durations from 0.5 to 24 h. For each test, the microstructures were characterized and the volume fraction of the δ precipitates quantified. In parallel, uniaxial tensile tests were conducted in order to determine the mechanical properties of the alloy, namely the elongation at necking (N%), the yield strength (YS), the Vickers hardness (HV) and strain hardening coefficients (K, n). The results revealed that when γ″ and δ precipitates coexist, the material remains hardened regardless of the amount of δ phase. However, when only δ phase were presented in the matrix, its volume fraction did not significantly affect the formability of the material. It was observed that the ratio between intragranular and intergranular precipitates could be a critical parameter. Intergranular precipitates, when in sufficient amount, led to a better formability of the material. On the contrary, when the intragranular precipitates were maximized, they tend to harden the material. A solution treatment at 975 °C for 2 h was finally suggested as the best compromise to improve the formability of IN718.

High-temperature tensile deformation behavior and microstructure evolution of Ti55 titanium alloy

Abstract: High temperature tensile and electron backscatter diffraction (EBSD) techniques were combined to perform a systematic investigation for the hot deformation behavior and microstructure evolution of the Ti55 alloy Under temperatures ranging from 885 to 935 °C and stain rates of 83×10−4 s−1−133×10−2 s−1, all the flow curves of the Ti55 alloy exhibit similar behaviors: after reaching the peak flow stress, the curves enter into a softening stage and then remain a steady level, where the maximum superplastic elongation of 987% indicates good superplasticity Detailed microstructure characterizations under different deformation stages show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries (>15°) increase rapidly at the softening stage These observations are attributed to the dynamic recrystallization, in which low angle grain boundaries (

Experimental and numerical investigation of microstructure and mechanical behavior of titanium/steel interfaces prepared by explosive welding

Abstract: This paper presents a systematic study of structure and mechanical behavior of Ti/Fe explosive-bonded interfaces. The transient fluid-like behavior at the bonding zone is simulated using Smoothed Particle Hydrodynamic (SPH) numerical method. The interface is featured by a wave structure, resulted from heavy plastic deformation during the explosive welding. Melted zone resulted from the trapped jetting is surrounded by strongly deformed bulk materials. Fe2Ti intermetallic compounds with a mixture of FeTi+Fe phases are observed in the melted zone. A reaction layer (~700nm) consisted of nano-sized FeTi grains is formed at Ti/Fe material boundary. Nanoindentation tests and fracture observation confirm the brittle nature of Fe-Ti intermetallics formed in the explosive-bonded joint. Extremely temperature accumulated near the interface leads to recovery and recrystallization in deformed grains, which can accommodate relatively large strain near the interface.

On microstructure and mechanical properties of additively manufactured AlSi10Mg_200C using recycled powder

Abstract: Additive manufacturing of reactive metals has opened door to consider using metal 3D printing to manufacture aluminum alloys for different applications from automotive and aerospace to defense. However, one of the major milestones of adopting the technology is still the high price of metal powder in comparison to casting methods. Using recycled powder to additively manufacture parts can be considered one way to decrease the final price in this technology. In the present paper, the microstructural evolution and mechanical properties of the DMLS-AlSi10Mg_200C alloy manufactured by recycled powder were investigated. As the first step of this research, the powder characteristics, i.e. morphology, average particle size, microstructure and composition, of virgin, condensate and recycled AlSi10Mg powder were studied. It was inferred that virgin and recycled powders had comparable powder characteristics, which are very different in comparison to the condensate powder. In the second step of this research, microstructural and mechanical characterization of the as-built DMLS-AlSi10Mg_200C alloy using recycled powder revealed that elongation of horizontally built DMLS-AlSi10Mg_200C was higher compared to that of vertically built. In addition, the fracture surfaces of vertically and horizontally built samples were investigated and possible fracture modes discussed. It is confirmed that, the microstructure and mechanical behavior of the as-built as-built DMLS-AlSi10Mg_200C manufactured by recycled powder were similar to that of virgin powder.

Microstructure evolution and superior tensile properties of low content graphene nanoplatelets reinforced pure Ti matrix composites

Abstract: Titanium matrix composites with the discontinuous reinforcement of graphene nanoplatelets (GNPs) were produced by powder metallurgy and subsequent hot-rolling. In the process of spark plasma sintering (SPS), the GNPs were well preserved at low temperature and high compressive pressure. Hot-rolling process was applied to improve the microstructure and properties of the GNPs-Ti matrix composites. The GNPs were uniformly distributed and arranged along with the rolling direction (RD). Also, the GNPs blocked slipping so that the matrix generated {10 1 1} 1 2 > compressive twining to be compatible with deformation in the rolling process with the increase of GNPs content. Tensile strength test demonstrated an excellent ultimate tensile strength that was 54.2% higher than pure titanium with merely 0.1 wt% GNPs addition. The strengthening mechanism of composites was discussed by three main strengthening factors combined with a modified load transfer model and it was thought that the composites were strengthen by grain refinement, load transfer from Ti matrix to GNPs and texture strengthening.

Microstructure and anisotropic mechanical properties of EBM manufactured Inconel 718 and effects of post heat treatments

Abstract: Materials manufactured with electron beam melting (EBM) have different microstructures and properties to those manufactured using conventional manufacturing methods. A detailed study of the microst ...

Thermal and mechanical stability of retained austenite surrounded by martensite with different degrees of tempering

Abstract: The mechanical and thermal stability of austenite in multiphase advanced high strength steels are influenced by the surrounding microstructure. The mechanisms underlying and the relations between thermal and mechanical stability are still dubious due to the difficulty of isolating other factors influencing austenite stability. In this work, martensite/austenite microstructures were created with the only significant difference being the degree of tempering of the martensite matrix. Hence, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite. The thermal stability during heating of retained austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements. The mechanical stability was studied by performing interrupted tensile tests and determining the remaining austenite fraction at different levels of strain. The thermal stability of this remaining austenite after interrupted tests was studied by subsequent reheating of strained specimens. The results are evidence for the first time that thermal recovery of martensite during reheating assists austenite decomposition through shrinkage and softening of martensite caused by a reduction of dislocation density and carbon content in solid solution. This softening of martensite also leads to a subsequent reduction of austenite mechanical stability. Additionally, remaining austenite after pre-straining at room temperature was thermally less stable than before pre-straining, demonstrating that plastic deformation has opposing effects on thermal and mechanical stability.