Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2018"

Effect of scanning strategies on residual stress and mechanical properties 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. Currently post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. This study examined the effect of scanning strategy (scan vector lengths and scan vector rotation) and rescanning strategy on residual stress formation and mechanical properties of SLM Ti6Al4V parts. 90° alternating scanning strategy resulted in the lowest residual stress build-up for SLM Ti6Al4V parts built on both the standard and modified Renishaw platforms using a modulated Nd-YAG fiber laser. Scanning strategy did not show any direct correlation with mechanical properties. Re-scanning with 150% energy density resulted in 33.6% reduction in residual stress but the effect on mechanical properties was detrimental and samples failed prematurely. The study was based on detailed experimental analysis along with Finite Element simulation of the process using ABAQUS to understand the underlying physics of the process.

Correlation between process parameters, microstructure and properties of 316 L stainless steel processed by selective laser melting

Abstract: High-density 316 L specimens were fabricated by selective laser melting (SLM). Different processing parameters, including laser power (100, 200 W) and scanning strategies (alternating stripes without and with re-melting after every layer) were employed to evaluate their impact on microstructure and texture of the specimens. Microstructures of the specimens in as-built condition were characterised by columnar grains of austenite with intercellular segregation of Mo, Cr and Si, resulting in creation of non-equilibrium eutectic ferrite. It was found that laser energy density and scanning strategy strongly affect cellular substructure of austenite and amount of ferrite, as well as kind and degree of texture. Specific microstructure of austenite in as-built condition is the cause of almost double increase of yield strength accompanied by much smaller improvement of ultimate tensile strength and 1.4 times reduction of elongation at fracture in comparison of properties of hot-rolled SS316L sheet. Moreover, features of this substructure determine kind of the changes occurring during stress relieving at 800 °C for 5 h (among others, precipitation of sigma-phase strongly activated by presence of ferrite and residual stresses), demonstrated by decreased yield strength value with no significant changes of ultimate tensile strength and elongation. At the same time, an attempt was made to explain some unclearly interpreted observations in the literature related to a correlation between process parameters, microstructure and properties of SLM-processed steel 316 L.

Deformation behaviors and cyclic strength assessment of AZ31B magnesium alloy based on steady ratcheting effect

Abstract: In this paper, deformation behaviors and microstructure evolution of a hot-rolled AZ31B magnesium alloy under cyclic loadings are investigated. The relationship between plastic deformation and microstructure evolution and the crack formation mechanisms are discussed. Under a high cyclic stress (90–140 MPa), steady ratcheting effect occurred in the material and the development of ratcheting strain went through three stages: 1) Stage I - initial rapid increase stage; 2) Stage II - steady stage; and 3) Stage III - final abrupt increase stage. Under a low cyclic stress (≤ 90 MPa), inconspicuous ratcheting effect was found in the material, indicating a light damage in the material. When the cyclic stress is below 30 MPa, no ratcheting effect is found and only elastic deformation occurs in the material. The formation of cracks in Stages I & II is mainly due to the activation of the basal slip system. The mean geometrically necessary dislocations (GND) are calculated to analyze the relationship between the basal slip and the ratcheting effect during the cyclic loading. Finally, a new approach is proposed to estimate the AZ31B magnesium alloy’s cyclic strength (at 107 cycles) according to the cyclic stress at which steady ratcheting effect starts to occur in the material.

Effects of Selective Laser Melting additive manufacturing parameters of Inconel 718 on porosity, microstructure and mechanical properties

Abstract: The effect of SLM parameters on porosity, microstructure and mechanical properties is studied. To this purpose, the Selective Laser Melting (SLM) technology is applied to manufacture Inconel 718 specimens. The material, the manufacturing process, the Hot Isostatic Pressure (HIP), heat treatment, observation procedures and characterisation of mechanical properties are presented. A columnar-dendritic microstructure was observed on all the SLM specimens and a Volumetric Energy Density (VED) effect on the latter was also noted. The rate of porosity varies in relation to the VED and is considerably reduced after HIP. The heat treatment erases the dendritic microstructure, significantly enhances microhardness and confers on the alloy tensile mechanical properties comparable to forged Inconel 718.

Transition of twinning behavior in CoCrFeMnNi high entropy alloy with grain refinement

Abstract: Fully recrystallized CoCrFeMnNi high entropy alloys (HEAs) with different grain sizes ranging from 503 nm to 88.9 µm were fabricated by cold rolling and controlled annealing. Tensile tests were conducted at ambient temperature, and deformation microstructures were investigated using electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) techniques. It is found that strain-hardening curves changed dramatically with grain refinement. Deformation twinning prevails when the grain size is coarse, but it is absent when the grain size falls in the ultrafine-grained (UFG) regime. The transition of twinning behavior is supposed to be determined by the increased twinning stress with grain refinement. It is indicated that the twinning behavior of the HEAs is strongly dependent on the competition between the flow stress and critical twinning stress with grain refinement.

Wire arc additive manufacturing of Al-6Mg alloy using variable polarity cold metal transfer arc as power source

Abstract: A variable polarity cold metal transfer (VP-CMT) arc power source with different arc modes was employed in additive manufacturing Al-6Mg alloy parts. The microstructures were characterized using scanning electron microscopy with electron back-scattered diffraction. Even equiaxed grains in size of 20.6–28.5 µm with random orientation were obtained under VP-CMT mode, while a large number of columnar grains in bigger size exist in samples under other arc modes. Tensile strength of the VP-CMT sample with a maximum of 333 MPa is higher than that of the Al-6Mg wrought alloys due to fine-grain strengthening. However, the tensile strength of the VP-CMT sample in different tensile direction was anisotropic, with a percentage of 8–27%. The comprehensive analysis of defects and grain orientation showed that the micro pores in interlayer pore region lead to the anisotropy.

Microstructures and mechanical properties of TixNbMoTaW refractory high-entropy alloys

Abstract: Refractory high-entropy alloys (RHEAs) are newly developed candidate materials for high-temperature applications. Among the existing RHEAs, NbMoTaW RHEA possesses the best mechanical properties with combined high strength, excellent thermal stability and softening resistance at elevated temperatures. However, the NbMoTaW RHEA is quite brittle at room temperature, which would restrict its application as structural material. Here, TixNbMoTaW RHEAs were developed by alloying Ti in the NbMoTaW RHEA. It shows that the room temperature ductility of the RHEAs increases from 1.9% of the NbMoTaW RHEA to 11.5% of the TiNbMoTaW RHEA, and the yield strength increases from 996 MPa of the NbMoTaW RHEA to 1455 MPa of the TiNbMoTaW RHEA. In addition, the TixNbMoTaW RHEAs keep stable single BCC structure up to their melt points. The present result indicates that Ti addition could effectively enhance both the ductility and strength of the NbMoTaW RHEA. The combined performance of superior mechanical properties and high thermal stability of the TixNbMoTaW RHEAs promises them an important role in engineering applications.

Porosity formation mechanisms and fatigue response in Al-Si-Mg alloys made by selective laser melting

Abstract: Defects in Al-7Si-Mg and Al-10Si-Mg alloys produced by selective laser melting are categorised into three types. The first type are large irregular-shaped defects with unmelted powder particles, formed due to a lack of fusion as a result of insufficient volumetric energy density. The second type are small round gas pores below 5 µm in diameter, associated with high area energy density. These pores enlarge during solution heat treatment, but the enlargement is reduced significantly when the powder is pre-dried at 200 °C for 16 h under an argon atmosphere immediately before the build. The last type are large round keyhole type pores located at the base of melt pools. They can either form in contour scan regions, at the edges of core scans, or at island boundary overlap regions due to an excessive local energy density compared with the nominal energy density. Sub-surface porosity due to contour and core edge keyhole type defects can be more detrimental to the fatigue performance than net-shaped rough surfaces, but such sub-surface porosity can be minimised by either lowering the laser energy input for the contour scan and/or changing the way the laser turns between scan tracks.

Ultra-high strength WNbMoTaV high-entropy alloys with fine grain structure fabricated by powder metallurgical process

Abstract: An equi-atomic WNbMoTaV high entropy alloy (HEA) with a single body-centered cubic structure (BCC) was firstly fabricated by the powder metallurgical process of mechanical alloying (MA) and spark plasma sintering (SPS). Mechanical alloying behavior, microstructure and mechanical properties of the WNbMoTaV HEA were studied systematically. During MA, a single BCC phase was formed and the average particle size and crystallite size was refined to 1.83 µm and 66.1 nm, respectively, after 6 h of MA. Afterward, the as-milled powders were subsequently sintered in the temperature range of 1500–1700 °C. The microstructure of the sintered sample exhibits a few micrometer-scale grain size and a homogeneous BCC matrix with a small amount of oxide inclusion originated from oxidation during the powder metallurgical process. The bulk sample of the WNbMoTaV HEA sintered at 1500 °C shows an ultra-high compressive yield strength of 2612 MPa with a failure strain of 8.8% at room temperature, respectively. These mechanical properties of the WNbMoTaV HEA fabricated by the powder metallurgical process were attributed to the combined effects of grain boundary strengthening, substitutional solid solution strengthening, interstitial solid solution strengthening and Orowan strengthening by the oxide inclusions. Through a Hall-Petch analysis, the Hall-Petch coefficient of the WNbMoTaV HEA was derived. The WNbMoTaV HEA fabricated via the powder metallurgical process showed the best compressive yield strength when compared with the other reported refractory HEAs processed with arc-melting and casting.

Microstructural characterization and mechanical properties of TiB 2 reinforced Al6061 matrix composites produced using stir casting process

Abstract: Particulate reinforced aluminum matrix composites are the most promising alternative for applications where the combination of high strength and ductility is essential. In the present study, Al-TiB2 composites with different amounts of reinforcement (3, 6 and 9 wt%) were fabricated using stir casting method. after optimizing the process parameters such as preheating temperature, stirring speed and duration. Microstructure, mechanical properties and the fracture surfaces of tensile specimens were studied. Optical microstructure of prepared composites showed a uniform distribution of reinforcements in matrix and also SEM analysis demonstrated the strong bonding between matrix and reinforcements which is the consequence of improved wettability due to K2TiF6 addition and preheating of TiB2 powder before adding to the melt. In addition to microstructure uniformity, the tensile strength of composites was improved with increasing the volume fraction of TiB2 reinforcement particles without any significant decrease in elongation. SEM micrographs from fractured surfaces of tensile specimens approved that the ductile fracture is occurred in Al6061 alloy in all of prepared composites through nucleation and coalescence of micro-voids.

Laser powder bed fusion of Hastelloy X: Effects of hot isostatic pressing and the hot cracking mechanism

Abstract: Hastelloy X is the trademark for a nickel-based, high-temperature superalloy that is increasingly applied in gas turbine engines because of its exceptional combination of oxidation resistance and high-temperature strength The superalloy suffers from hot cracking susceptibility, however, particularly when processed using additive manufacturing and laser powder bed fusion (LPBF) This paper systematically studies for the first time the effect of post-treatment hot isostatic processing (HIP) on the microstructure and mechanical properties of LPBF-fabricated Hastelloy X, with an emphasis on fatigue performance The experimental results demonstrate that despite the very small number of remaining gas-filled micropores due to pressure counteraction, the high temperature and high pressure during the HIP process promote recrystallisation and closing of the internal microcracks and gas-free pores The HIP-processed specimens are shown to be roughly 130 MPa and 60 MPa weaker than the non-processed specimens in yield strength and ultimate tensile strength, respectively The HIP-processed Hastelloy X exhibits significant improvements in fatigue life, however: the effect of the HIP processing is apparent once the applied stress decreases This improvement in fatigue performance is attributable to the reduction in stress concentration and residual stress release caused by the HIP process The paper also studies the hot cracking mechanism and finds that intergranular microcracks generally occur along high angle grain boundaries; the interdendritic liquid pressure drop between dendrite tip and root is found to be a significant factor in the hot crack mechanism The significance of this research is in developing a comprehensive understanding of HIP processing on the fatigue behaviour of the LPBF-fabricated Hastelloy X The insights on the cracking mechanism, which presents a significant step towards using additive manufacturing to produce complex crack-free parts from this superalloy

Influence of heat treatments on microstructure evolution and mechanical properties of Inconel 625 processed by laser powder bed fusion

Abstract: This study investigated the mechanical behaviour and microstructure of as-built and heat-treated Inconel 625 (IN625) samples processed by laser powder bed fusion (LPBF). This process offers freedom in design to build complex IN625 components in order to overcome extensive machining. However, post heat treatments must be performed to obtain specific mechanical properties to match industrial requirements. For this purpose, different heat treatments were performed on IN625 samples, and through hardness measurements, three different heat treatments were selected, as optimised conditions. A direct ageing, a solutioning and a solutioning followed by ageing treatments were chosen to study the effects of these specific heat treatments on the microstructure and tensile properties, comparing them to those of as-built condition. The tensile properties of as-built and selected heat-treated IN625 samples showed superior values to minimum requirements for wrought IN625 alloys, whereas the investigation on the microstructures and fracture surfaces of as-built and heat-treated IN625 contributed to an understanding of the tensile properties evolution. The high tensile strength of as-built samples essentially derived from very fine dendritic structures mainly below 1 µm with high dislocation density and nanometric MC carbides. The high tensile properties of ageing treatments performed at 700 °C for 24 h, whether directly aged or post-solutioning, were found to be primarily dependent on γ" phases (10–30 nm) and M23C6 carbides formation. By contrast, the tensile properties of solution-treated IN625 samples at 1150 °C for 2 h showed higher ductility coupled to lower strength than other conditions, due to the grain growth.

Comparison of microstructures and mechanical properties of Inconel 718 alloy processed by selective laser melting and casting

Abstract: The paper comparatively investigates the microstructures and mechanical properties of Inconel 718 superalloy manufactured by selective laser melting (SLM) and casting. The finite element analysis (FEA) method is used to simulate the temperature fields during SLM and casting processes. Driven by ultra-high temperature gradient and ultra-fast cooling rate during SLM process, the fine grains (average grain size of 48 µm) and dispersed fine precipitation in SLM-ed sample even after HSA (homogenization + solution + aging) and HA (homogenization + aging) heat treatment significantly enhance its mechanical properties, which far exceeds that of casting with average grain size of 1300 µm, and is comparable to that of forging. The microstructure of casting with coarse irregular Laves phases, acicular δ precipitates and globular carbides in the interdendritic zones after HSA heat treatment and some defects existed possibly result in premature failure of tensile samples. The microstructure without δ phases but only some globular carbides in the grain boundary of SLM-ed sample after HA heat treatment possesses higher mechanical properties than that after HSA heat treatment, in which there is only some finer needle-like δ phase and few carbides are precipitated in the grain boundaries. The analysis shows the large amounts of δ phase precipitated in the matrix will deteriorate the plasticity of SLM-ed IN718 superalloy, the appropriate reduction of the δ phase will improve the strength and plasticity of material simultaneously.

Strengthening of stainless steel by titanium carbide addition and grain refinement during selective laser melting

Abstract: This study clarifies the role of micro- and nano-TiC added to 316L stainless steel fabricated by the selective laser melting (SLM) process, an emerging additive manufacturing technology, in the microstructural evolution and mechanical properties. Directionally fine cellular dendrites and columnar grains formed during the fast solidification in SLM-processed stainless steel. Interestingly, the addition of TiC particles in the steel matrix significantly reduced the cellular and grain sizes after solidification and also disrupted the established directional structures, particularly for nanoscale TiC. The composite, particularly with nanoscale TiC, also exhibited greater room- and high-temperature compressive yield strengths than unreinforced steel, mainly because of the combined effects of grain-boundary strengthening and Orowan strengthening. The strengthening effect was well described by the Zener pinning model. The compressed surfaces suggest that TiC particles hinder crack propagation, and the TiC distribution was critical in improving the mechanical properties. The SLM process can tailor the microstructure across a rather limited length scale; hence, to better control the mechanical properties of the resulting products, compositing the relevant feedstock powder is a highly attractive strategy for developing components with novel structures and unique properties.

Strain rate effects of dynamic compressive deformation on mechanical properties and microstructure of CoCrFeMnNi high-entropy alloy

Abstract: In this work, the effects of strain rate on mechanical deformation and microstructural evolution of CoCrFeMnNi high-entropy alloy (HEA) under quasi-static and dynamic compression were investigated. The HEA exhibited high strain-rate sensitivity values (m = 0.028) of yield strength under quasi-static conditions. In particular, due to the viscous drag effect, the variation of yield strength with strain rate under dynamic compression was much larger than that under quasi-static compression. Microstructural analysis using electron backscatter diffraction shows profuse twinning under both conditions. The dynamically deformed specimens exhibited strongly localized deformation regions (i.e., adiabatic shear bands). The process of dynamic compressive behavior in this HEA is competitive between hardening by dislocation and twinning, and thermal softening. To analyze numerically the flow behavior of the HEA under dynamic conditions, the modified Johnson-Cook model considering adiabatic temperature rise was employed. The modified Johnson-Cook model offered good agreement with experimental results regarding dynamic flow curves of this HEA.

A new grain orientation spread approach to analyze the dynamic recrystallization behavior of a cast-homogenized Mg-Zn-Zr alloy using electron backscattered diffraction

Abstract: A new grain orientation spread approach (GOS ≤ 5°) was proposed to study DRX of a Mg-Zn-Zr alloy during hot deformation. DRXed grains possessed random texture while the deformed grains contributed to basal texture. The alloy exhibited rapid DRX at low deformation strains followed by near-saturated behavior as strain increased.

Heavy carbon alloyed FCC-structured high entropy alloy with excellent combination of strength and ductility

Abstract: The effects of carbon content on the microstructure and room-temperature mechanical properties of Fe40Mn40Co10Cr10 high-entropy alloy (HEA) were systematically investigated. The results showed that heavy carbon alloyed HEA could possess supreme combination of high tensile strength (935 MPa) and high ductility (~ 74%). The excellent mechanical properties were ascribed to as follows: the high content interstitial carbon atoms strengthens the matrix greatly through suppressing dislocation motion and promoting the deformation-induced twinning at room temperature, which enhance the strength and ductility. Simultaneously, the ductility is further secured for single FCC structure maintained due to appropriate carbon alloying. Our findings provide a novel strategy for developing HEAs with excellent mechanical properties.

Deformation microstructures and strengthening mechanisms for the wire+arc additively manufactured Al-Mg4.5Mn alloy with inter-layer rolling

Abstract: Applying inter-layer rolling to the wire+arc additively manufacturing (WAAM) process with increasing loads of 15 kN, 30 kN and 45 kN, achieves excellent mechanical properties for 5087 (Al-Mg4.5-Mn) alloys. Compared with the as-deposited alloy, the average micro hardness, yield stress and ultimate tensile strength of 45 kN rolled alloys reached to 107.2 HV, 240 MPa and 344 MPa, which were enhanced by 40%, 69% and 18.2%, respectively. Primary coarse grain structures were found to become greatly refined with an evident rolling texture after deformation. The strengthening mechanisms mainly are deformation strengthening, grain refinement, and solution strengthening. Meanwhile, the elongation of rolled alloys stays over 20%. The plasticity was not obviously diminished compared with the as-deposited alloy. This is two times greater than the commercial wrought Al-Mg alloy with similar composition. The excellent plasticity may be chiefly due to grain refinement, pores closure and reduction, and grain recrystallization during the WAAM re-heating process. The combination process of rolling deformation with WAAM deposition is an effective technique in refining microstructure and improving mechanical properties for AM aluminum alloys.

Ultra-high strengthening efficiency of graphene nanoplatelets reinforced magnesium matrix composites

Abstract: Homogeneous magnesium alloy (ZK60) reinforced by low content of graphene nanoplatelets (GNPs) was fabricated by facile melt stirring and hot extrusion processes with cost effectiveness. GNPs were pre-dispersed with Mg powder and extruded into rods used as precursor for melting, which effectively guaranteed the integrity and dispersion of GNPs. In composites, GNPs closely combined with the magnesium matrix in nanoscale. Compared with ZK60 alloy, the composite with only 0.05 wt% GNPs can perform 62% enhancement in yield strength up to 256 MPa, exhibiting an ultra-high strengthening efficiency of 1550. Based on theoretical analysis, load transfer of GNPs contributed most (~ 72%) to the strength improvement of GNP/ZK60 composite, due to the 2D interfacial contacting and continuous combination of GNPs with matrix. This study explored the strengthening potential and mechanism of GNPs in metal matrix composites with insight of scale-up fabrication.

Anisotropic mechanical properties of graphene/copper composites with aligned graphene

Abstract: The isotropic mechanical properties of graphene/metal composites with randomly distributed graphene have been extensively studied. However, the anisotropic mechanical properties of aligned graphene/metal composites in both in-plane and through-plane directions have not yet been reported. Herein, we attempted to align graphene nanoplatelets (GNPs) in the Cu matrix via a vacuum filtration method followed by spark plasma sintering. It was demonstrated that a fairly good GNP alignment was achieved in the composites, leading to the prominent anisotropic mechanical properties with in-plane tensile strength and elongation significantly outperforming through-plane ones. Nevertheless, only moderate in-plane strength enhancement (26% at 10 vol% GNPs) was obtained in the composites, and this enhancement was further diminished to −7.1% with increasing GNP fraction to 20 vol%, which was attributed primarily to the weak GNP-Cu interface that is bonded by mechanical interlocking. Furthermore, the anisotropic mechanical behavior of aligned GNP/Cu composites was proposed to originate from the different interface failure modes of 'GNP slippage' and 'GNP peer-off' with the load parallel and perpendicular to the alignment direction, respectively. Therefore, further improvement of interfacial bonding strength will be an important step towards the optimization of the anisotropic mechanical properties of aligned graphene/metal composites.

Heat treatment effect on the mechanical properties and fracture mechanism in AlSi10Mg fabricated by additive manufacturing selective laser melting process

Abstract: Many researches have been conducted on the topic of the AlSi10Mg alloy, covering different aspects of the selective laser melting fabrication process. However, a database is still lacking much information and understanding regarding the properties of the material under different conditions, which will allow a tailoring of suitable properties to the required application. This work aims to provide a correlation between the mechanical properties of vertically built (Z samples) AlSi10Mg specimens subjected to different post-processing conditions and the change in properties in relation to these conditions and the fracture mechanism. Among these is the accepted T5 stress relief treatment, a modified T5 treatment, the as-built condition and a high temperature Hot Isostatic Pressing treatment. A more in-depth analysis of the fracture mode for the vertical build direction is provided with emphasis on the mechanism for each treatment as well as a quantitative analysis of the Full Width at Half Maximum via X-Ray diffraction measurements. The modified T5 treatment suggested was found to result in an increase in strength values beyond those of the as-built condition and a 64% increase in yield stress compared to the typical T5 treatment with a concurrent decrease in elongation values. It is suggested that at 200 °C nano-scale precipitation of Silicon particles occurs, responsible for the strength elevation. Charpy impact test results are provided for each condition and their fracture mode is compared to the tensile tests and discussed.

Effect of heat-treatment temperature on microstructures and mechanical properties of Co–Cr–Mo alloys fabricated by selective laser melting

Abstract: Selective laser melting (SLM) has attracted considerable attention as an advanced method for the fabrication of biomedical devices. However, SLM-manufactured parts easily accumulate large amounts of residual stress due to rapid heating and cooling, which negatively affects their mechanical properties. In this study, Co–Cr–Mo alloy specimens were fabricated by SLM and then heat-treated at various temperatures (750, 900, 1050, or 1150 °C) to relieve the residual stress and improve their mechanical properties. The alloy microstructure was analyzed via confocal laser scanning microscopy, scanning electron microscopy combined with energy dispersive X-ray spectroscopy, electron backscattered diffraction, and X-ray diffraction techniques, whereas the mechanical properties of the produced specimens were evaluated by tensile and Vickers hardness tests. The results showed that increasing the heat-treatment temperature from 750 °C to 1150 °C increased the ductility of the alloy and decreased its 0.2% offset yield strength and Vickers hardness. Both γ and e phases formed in all heat-treated specimens, and the volume fraction of the e phase decreased with increasing heat-treatment temperature. After the specimens were heated to 750–1050 °C, a recovery process was initiated, which proceeded as the temperature increased; however, the residual stress in the studied specimens was not sufficiently relieved. In contrast, after heating to 1150 °C, the formation of equiaxed grains and the drastic relief of the residual stress were observed simultaneously, accompanied by an increase in the elongation of the specimen and a decrease in its strength (as compared to those of the other heat-treated specimens), indicating that the specimen completely recrystallized and that the residual stress was the driving force of this recrystallization. Thus, heat-treating at 1150 °C for 6 h is an effective method for eliminating the residual stress, leading to a homogenized microstructure and satisfactory ductility.

Microstructure and mechanical properties of stainless steel 316L vertical struts manufactured by laser powder bed fusion process

Abstract: Stainless steel 316L (SS316L) vertical struts with various diameters ranging from 0.25 mm to 5 mm were manufactured by laser powder bed fusion (LPBF) process. A systematic investigation was conducted on the microstructure and mechanical properties of the produced struts. The struts possessed hierarchical microstructures consisting of cellular sub-grain structures inside columnar grains. The primary dendrite arm spacing (PDAS) of the cellular sub-grains decreased monotonically with increasing strut diameter until reaching a plateau after 1 mm. In contrast, the columnar grain width did not show a clear relationship with respect to the variation in the strut diameter. A to texture transition along the building direction (BD) of the struts was observed as the strut diameter decreased from 5 mm to 0.25 mm, which was attributed to the change of the heat extraction direction. Microstructure-property relations were established via Hall-Petch type correlations between the PDAS and the microhardness as well as the PDAS and the strengths of the struts, suggesting the importance of the role played by the cellular sub-grain structures in the strengthening of LPBF manufactured SS316L. Electron backscatter diffraction (EBSD) analysis confirmed that the strong texture within the thicker struts promoted the twinning-induced plasticity, and thus resulted in a better strength-ductility combination compared with that of the thinner struts with texture or weak texture.

Effect of platform temperature on the porosity, microstructure and mechanical properties of an Al–Mg–Sc–Zr alloy fabricated by selective laser melting

Abstract: In this work, an Al–Mg–Sc–Zr alloy was manufactured using selective laser melting (SLM) at platform temperatures of both 35 °C and 200 °C. The effects of platform temperature and applied energy density (E) on porosity characteristics were studied by image analysis. The results show that 60–70 J/mm3 is the minimum applied energy density threshold to build high density parts ( > 99.7%), and that the number density and size of pores follow similar trends for both platform temperatures even though the number density of pores is consistently lower at the 200 °C platform temperature. The optimum processing condition of E = 77 J/mm3 was selected for building thin plates to evaluate the tensile properties based on the lowest porosity and highest hardness results. Tensile results indicate that 35 °C fabricated specimens have a low anisotropy, while 200 °C fabricated specimens have higher as-fabricated strengths but inhomogeneous properties from bottom to top. After peak aging, all samples achieve very similar tensile properties with yield strengths of close to 460 MPa, though the elongation for the 200 °C fabricated specimens still presents a gradual decrease from top to bottom of the plate. The microstructure and property evolution was explained in terms of thermal history effects on the different types of grains and precipitates in this alloy.

Improved back stress and synergetic strain hardening in coarse-grain/nanostructure laminates

Abstract: The effect of back stress on the mechanical behaviors of heterogeneous material is studied in two modeled heterogeneous laminates, i.e. laminated structure with a nanostructured (NS) Cu-Zn alloy layer sandwiched between two coarse-grained (CG) pure Cu layers. The improved tensile ductility of NS layer is revealed and attributed to the constraint from the stable CG layers. It is found that the elastic/plastic interaction between NS and CG layers is capable of significantly improving the back stress, which makes a significant contribution to the synergetic strain hardening in low strain stage. Furthermore, a higher mechanical incompatibility permits stronger and longer mutual interaction between layers, i.e. coupling effect, which contributes to a higher back stress. These results improve our understanding about the role of back stress on mechanical behaviors of heterogeneous laminate materials.

Effect of morphologies of martensite–austenite constituents on impact toughness in intercritically reheated coarse-grained heat-affected zone of HSLA steel

Abstract: By using the gleeble-3500 simulator, three different two-pass weld thermal cycles were applied to base metal of a high strength low alloy (HSLA) steel with the purpose of obtaining martensite-austenite (MA) constituents of different morphologies in intercritically reheated coarse-grained heat-affected zones (ICGHAZ). Morphology of each MA constituent was characterized by maximum length, maximum width and aspect ratio (maximum length/ maximum width). Toughness of thermal simulated specimens was examined by using an instrumented Charpy impact tester. Behaviour of cracks was present by Charpy impact curve and typical data. Correlation between behaviour of cracks and morphologies of MA constituents was further analysed by observing crack propagation and microstructure of MA constituent. Fracture modes of MA constituents with slender and massive shape were proposed. Results show that slender MA constituents are more harmful to toughness compared with massive ones.

Effects of nano-graphite content on the characteristics of spark plasma sintered ZrB2–SiC composites

Abstract: In this study, ZrB2–25 vol% SiC composite containing 0, 2.5, 5, 7.5 and 10 wt% graphite nano-flakes were prepared by spark plasma sintering (SPS) process at 1900 °C for 7 min under 40 MPa. The fabricated composite samples were compared to examine the influences of nano-graphite content on the densification, microstructure and mechanical properties of ZrB2–SiC-based ultrahigh temperature ceramics. Fully dense composites were obtained by adding 0–5 wt% nano-graphite, but higher amounts of additive led to a small drop in the sintered density. The growth of ZrB2 grains was moderately hindered by adding nano-graphite but independent of its content. The hardness linearly decreased from 19.5 for the graphite-free ceramic to 12.1 GPa for the sample doped with 10 wt% nano-graphite. Addition of graphite nano-flakes increased the fracture toughness of composites as a value of 8.2 MPa m½ was achieved by adding 7.5 wt% nano-graphite, twice higher than that measured for the graphite-free sample (4.3 MPa m½). The in-situ formation of ZrC and B4C nano-particles as well as the presence of unreacted graphite nano-flakes led to a remarkable enhancement in fracture toughness through activating several toughening mechanisms such as crack deflection, crack bridging, crack branching and graphite pullout.

Microstructure and mechanical properties of a heat-treatable Al-3.5Cu-1.5Mg-1Si alloy produced by selective laser melting

Abstract: A heat-treatable Al-3.5Cu-1.5Mg-1Si alloy is successfully fabricated by selective laser melting and is investigated concerning microstructures and mechanical properties. The as-prepared samples show a fine-granular microstructure in the individual melt pool of the tracks and a coarse-granular microstructure in the areas between the tracks. After T6 heat treatment, the grain size of the specimens increases slightly and the Q phase formed in the as-prepared specimens transforms to Al2Cu(Mg), Mg2Si, and AlxMny. All Al-Cu-Mg-Si specimens before and after heat treatment fracture around the defects that were generated during processing and show intergranular fracture along columnar grains upon tensile quasi-static loading. The as-fabricated samples exhibit a yield strength (YS) of 223 ± 4 MPa and an ultimate tensile strength (UTS) of 366 ± 7 MPa with an elongation of 5.3 ± 0.3%. After T6 heat treatment, the YS and UTS increase dramatically to 368 ± 6 MPa and 455 ± 10 MPa due to the formation of nano-sized Al2Cu(Mg) precipitates, while the ductility remains fairly similar.

Effect of Sn content on strain hardening behavior of as-extruded Mg-Sn alloys

Abstract: The effects of Sn content on strain hardening behavior of as-extruded Mg-xSn (x = 1.3, 2.4, 3.6 and 4.7 wt%) binary alloys were investigated by uniaxial tensile tests at room temperature. Strain hardening rate, strain hardening exponent and hardening capacity were obtained from the true plastic stress-strain curves. After hot extrusion, the as-extruded Mg-Sn alloys are mainly composed of α-Mg matrix and second phase Mg2Sn, which only exists in Mg-3Sn and Mg-4Sn. Average grain size decreases from 15.6 μm to 3.6 µm with Sn content increases from 1.3 to 4.7 wt%. The experimental results show that Sn content decreases strain hardening ability of as-extruded Mg-Sn alloys, but gives rise to an obvious elevation in tensile strength, yield strength and elongation of them. With increasing Sn content, strain hardening rate decreases from 3527 MPa to 1211 MPa at (σ-σ0.2) = 50 MPa, strain hardening exponent decreases from 0.21 to 0.13 and hardening capacity decreases from 1.66 to 0.63. The variation in strain hardening behavior of Mg-Sn alloys with Sn content is discussed in terms of the influences of grain size and distribution of grain orientation.

Anisotropic tensile and actuation properties of NiTi fabricated with selective laser melting

Abstract: This study evaluates the anisotropic tensile properties of Ni50.1Ti49.9 (in at%) components fabricated using an additive manufacturing (AM) process of selective laser melting (SLM). Dog-bone shaped tensile specimens were fabricated in three orthogonal building orientations (i.e., horizontal, edge, and vertical) with two different scanning strategies (i.e., alternating x/y and alternating in ± 45° to the x-axis). Next, the samples were subjected to tensile testing until failure, shape memory effect tests and thermal cycling under constant tensile stresses up to 500 MPa. Their failure surfaces were analyzed for possible microstructural defects. It was revealed that the build orientation and scanning strategy affect the texture/microstructure, and hence the failure stress, ductility, shape memory effect, and functional stability. Samples fabricated in the horizontal orientation with alternating x/y scanning strategy had the highest ultimate tensile strength (606 MPa) and elongation (6.8%) with the strain recovery of 3.54% after 4 shape memory effect cycles. At stress levels less than or equal to 200 MPa, these samples had the actuation strain greater than 3.8% without accumulation of noticeable residual strain. It was observed that the scanning strategy of alternating in ± 45° result in degraded mechanical and shape memory response, particularly in horizontal and edge samples.