Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2019"

Additive manufacturing of functionally graded materials: A review

Abstract: Functionally graded materials (FGMs) represent a class of novel materials in which compositions/constituents and/or microstructures gradually change along single or multiple spatial directions, resulting in a gradual change in properties and functions which can be tailored for enhanced performance. FGMs can be fabricated using a variety of well-established processing methods; however, it is also known that there are inherent drawbacks to existing synthesis methods. As an emerging technology that provides a high degree of control over spatial resolution, additive manufacturing (AM) provides an intriguing pathway to circumvent the drawbacks of currently available methods. AM involves the selective deposition of individual layers of single or multiple materials, and as such it offers the potential of local control of composition and microstructure in multiple dimensions; such process conditions, in principle, can be tailored to construct complex FGMs with multi-dimensional and directional gradient structures. In this review paper, our current understanding of important issues, such as modeling, processing, microstructures and mechanical properties, as related to FGMs produced via AM, are described and discussed in an effort to assess the state of the art in this field as well as to provide insight into future research directions.

Correlation between arc mode, microstructure, and mechanical properties during wire arc additive manufacturing of 316L stainless steel

Abstract: Wire arc additive manufacturing (WAAM) features advantages such as low cost and high disposition rate, and thus WAAM is a feasible additive manufacturing process. Although some characteristics of WAAM have been documented in the literature, the process stability, structural integrity, component morphology, microstructure, and mechanical properties during WAAM under different arc modes are not comprehensively demonstrated and understood. Here, we performed WAAM experiments with 316L stainless steel under different arc modes and a constant deposition rate, and then we discussed the mechanism and impact of the arc mode on the manufacturing process stability, structural integrity, microstructures, and mechanical properties. The results indicate that the SpeedPulse and SpeedArc additive manufacturing processes are relatively stable, significantly efficient, and structurally sound. Although the deposition rate and scanning speed of SpeedPulse WAAM and SpeedArc WAAM are the same, SpeedArc WAAM has a lower heat input and a higher cooling rate. Therefore, SpeedArc WAAM produces a finer solidification structure than SpeedPulse WAAM. The ultimate tensile strengths of the SpeedPulse and SpeedArc additive manufactured specimens along the horizontal direction are greater than 540 MPa and slightly greater than previously reported results. Due to the lower heat input and finer solidification structure, a component produced by SpeedArc WAAM has greater tensile strength and hardness than a component produced by SpeedPulse WAAM.

Role of melt pool boundary condition in determining the mechanical properties of selective laser melting AlSi10Mg alloy

Abstract: We describe here a comprehensive study on the effect of cellular structure and melt pool boundary (MPB) condition on the mechanical properties, deformation and failure behavior of AlSi10Mg alloy processed by selective laser melting (SLM). The morphology of melt pool (MP) on the load bearing face of tensile samples was significantly different with build directions. It resulted in different mechanical properties of the samples with different build directions. Furthermore, the microstructure analysis revealed that the MP in the SLM AlSi10Mg alloy mainly consisted of columnar α-Al grains which were made of ultra-fine elongated cellular structure. Electron back-scatter diffraction (EBSD) analysis revealed that the long axis of cellular structure and columnar grains were parallel to , which resulted in fiber texture in SLM AlSi10Mg alloy. However, Schmid factor calculation demonstrated that the anisotropy of mechanical properties of the SLM AlSi10Mg alloy built with different direction was mainly dependent on the distribution of MPB on the load bearing face, and not texture. The defects including pores, residual stress and heat affected zone (HAZ) located at MPB made it the weakest part in the SLM AlSi10Mg. The sample built along horizontal direction exhibited good combination of strength and plasticity and is attributed to the lowest fraction of MPBs that withstand load during tensile. MPB had strong influence on the mechanical properties and failure behavior of SLM AlSi10Mg built with different directions.

Effect of heat treatment on microstructure and mechanical properties of 316L steel synthesized by selective laser melting

Abstract: The influence of annealing at different temperatures (573, 873, 1273, 1373 and 1673 K) on the stability of phases, composition and microstructure of 316L stainless steel fabricated by SLM has been investigated and the changes induced by the heat treatment have been used to understand the corresponding variations of the mechanical properties of the specimens under tensile loading. Annealing has no effect on phase formation: a single-phase austenite is observed in all specimens investigated here. In addition, annealing does not change the random crystallographic orientation observed in the as-synthesized material. The complex cellular microstructure with fine subgrain structures characteristic of the as-SLM specimens is stable up to 873 K. The cell size increases with increasing annealing temperature until the cellular microstructure can no longer be observed at high temperatures (T ≥ 1273 K). The strength of the specimens decreases with increasing annealing temperature as a result of the microstructural coarsening. The excellent combination of strength and ductility exhibited by the as-synthesized material can be ascribed to the complex cellular microstructure and subgrains along with the misorientation between grains, cells, cell walls and subgrains.

Study of heat treatment parameters for additively manufactured AlSi10Mg in comparison with corresponding cast alloy

Abstract: A solution, quenching and ageing heat treatment is often performed on additive manufactured AlSi10Mg parts to dissolve the anisotropy due to the layer-by-layer building. This study investigates the influence of temperature and duration of solution and ageing treatment on microstructure, hardness and density of AlSi10Mg alloy produced by direct metal laser sintering. A parallel investigation is carried out on AlSi10Mg samples produced by gravity casting to analyse the different response to the same heat treatment conditions. The highest hardness, combined with an acceptable increase of porosity, is reached after selected heat treatment parameters. It was also found that, compared to the as-produced condition, this treatment leads to a decrease of ultimate tensile strength, without affecting the yield strength of additive manufactured samples, and reduces the difference in properties along the two building directions. The high properties of the as-produced samples are related to the finer microstructure and, as proved by the differential scanning calorimetric measurements, to the self-quenching phenomenon.

Effects of annealing on the microstructure and mechanical properties of selective laser melted AlSi7Mg alloy

Abstract: Selective laser melting, due to the high energy density input and the small interaction time (106 K/s), can result in an ultrafine microstructure and excellent mechanical properties. However, due to the nonuniform nature of the temperature distribution and the transition from liquid to solid, high residual stresses exist. High residual stresses in the parts can increase the risk of material distortion and cause many problems, such as dimensional inaccuracy or cracks. This work systematically investigated the influence of annealing on the microstructure and mechanical properties of SLM-processed AlSi7Mg alloy parts. The residual stresses relaxed significantly after annealing. The Vickers hardness and the tensile stress greatly reduced, while the elongation increased. The fracture mode of the as-fabricated sample was ductile and brittle mixed fracture, whereas the fracture morphology of the annealed sample presented many more dimples, and the elongation also increased, which indicated the ductile mode.

Microstructure and enhanced strength of laser aided additive manufactured CoCrFeNiMn high entropy alloy

Abstract: The CoCrFeNiMn high entropy alloy (HEA) bulk alloy was successfully manufactured using laser aided additive manufacturing (LAAM). The microstructure and mechanical behaviors of the as-fabricated HEAs were investigated. The as-built HEAs exhibit directional solidification at regions close to the melt-pool boundaries, forming dendritic columnar grains and transiting to equiaxed grains further away from the boundaries. Compared with the conventionally cast HEAs, the CoCrFeNiMn fabricated using LAAM possesses significantly higher yield strength (518 MPa) and ultimate tensile strength (660 MPa). The strengthening effect is attributed to finer grains and could be explained quantitatively through grain boundary strengthening. The as-built HEA shows a simultaneous enhancement in yield strength and ductility with decreasing testing temperature. The improved low temperature tensile properties could be ascribed to the formation of deformation twins at low temperature, which results in a steady strain hardening behavior. This work demonstrates the potential of using LAAM technology to extend the application of HEAs with fabrication of larger and more complex parts with good mechanical properties.

Heat treatment of Inconel 718 produced by selective laser melting: Microstructure and mechanical properties

Abstract: Inconel 718 (IN718) samples have been produced by selective laser melting (SLM). The effects of solution temperature, time and cooling rate as well as aging hardening on the microstructure and mechanical properties of SLMed IN718 have been studied. It is found that the as-fabricated IN718 is characterized with fine cellular dendrites with Laves phase precipitating in the subgrain boundaries, which is profoundly different from cast and wrought materials and needs different heat treatment schedules. The relationship between the minimum solution time and solution temperature is established and it provides a basis for the selection of solution treatment parameters. In addition, decreasing the cooling rate of solution treatment will contribute to the precipitation of strengthening phases. The precipitation temperatures of γ′ and γ″ are about the same for SLMed and wrought IN718, but the former has a faster aging response. The tensile properties of SLMed IN718 can be tuned in a large range by properly varying the microstructure. The highest elongation of 39.1% can be obtained after solution treatment (water quenching) without aging treatment and the highest yield and tensile strength (1374/1545 MPa) can be obtained after the direct aging treatment. The match of strength and ductility is able to be tailored by controlling the amount of strengthening phases, which can be realized by adjusting the cooling rate of solution treatment and aging time.

Mechanism of high yield strength and yield ratio of 316 L stainless steel by additive manufacturing

Abstract: 316 L stainless steel, high density dislocations (~1.14 × 1015 m−2) obtained by selective laser melting process that play an important role in high yield strength. Dislocation slip and twinning during entire plastic deformation process, which maintained strain hardening rate at an ideal level and obtained outstanding ductility and resulting high yield ratio.

Compression behavior of the graded metallic auxetic reentrant honeycomb: Experiment and finite element analysis

Abstract: In this paper, the quasi-static compression behavior of graded metallic auxetic reentrant honeycomb was firstly studied. Using the selective laser melting method, the unidirectionally-graded auxetic honeycomb (UGAH) and bidirectionally-graded auxetic honeycomb (BGAH) with the equal mass were fabricated to conduct the compression tests. The deformation mode, crushing stress and Poisson's ratio distribution of graded auxetic honeycombs were presented. Experiment results showed that the compression behavior of graded auxetic honeycomb was different from that of graded honeycomb with positive Poisson's ratio owing to the horizontal shrinkage deformation and the gradient distribution had effects on the crushing stress and Poisson's ratio. Then, a finite element (FE) model of the graded auxetic honeycomb was established and the calculation results were systematically compared with the experimental results. The effects of gradient distribution on the energy dissipation characteristics of the graded auxetic honeycomb were studied. The results showed that the energy dissipation of the UGAH was lower than that of the BGAH before the graded layer with the maximum cell-wall thickness was densified.

Tailoring the microstructure and mechanical properties of AISI 316L austenitic stainless steel via cold rolling and reversion annealing

Abstract: Tensile properties of cold rolled AISI 316L stainless steel after full reversion of martensite to austenite, recrystallization of retained austenite, and grain growth were studied at 850, 950, and 1050 °C. At higher temperatures, it was found that the kinetics of the reversion and recrystallization processes enhance but coarser grain sizes will be obtained at the end of recrystallization. At 1050 °C, appreciable grain growth was observed after the completion of the recrystallization process, which was not the case for a low temperature of 850 °C. At the stage of full recrystallization, by decreasing the annealing temperature, the yield stress (YS) and the ultimate tensile strength (UTS) values increased and total elongation decreased, which was related to the grain size strengthening by the Hall-Petch law. However, the Hall-Petch slope for the UTS was found to be much smaller than that of YS, which reveals that YS has greater grain size dependency. The latter was ascribed to the improved work-hardening behavior and enhanced transformation-induced plasticity (TRIP) effect by coarsening of grain size. To obtain high-strength and ductile steel with tensile toughness higher than 300 MJ/m3 and yield ratio of ∼0.5, the average grain size of ∼3 μm was found to be desirable.

Effects of SiC particle size on mechanical properties of SiC particle reinforced aluminum metal matrix composite

Abstract: In this paper, effects of SiC particle size on the mechanical properties and the failure mechanisms of Al/SiC composites under compression with strain rates ranging from 0.001 to 5200 s−1 were investigated. Al/SiC composites consisting of 65 wt% SiC particles with the average size of 10 μm and 50 μm were studied respectively. The quasi-static (strain rate of 0.001s−1) and dynamic compression tests (strain rates of 2200s−1-5200s−1) were performed separately for both materials with different SiC particle size. And optical microscope (OM) was used to observe the failure characteristics of the reclaimed specimens. The results show that as the increasing strain rate, results in the improvement of yield strength of both composites. Moreover, the composites with the SiC particle size of 10 μm have a larger yield strength than that 50 μm one. The microstructure analyses reveal that separation of SiC particles from aluminum matrix and fragmentation of SiC particles are the main failure mechanisms of Al/SiC composites subjected to compressive loading. The adhesive force between the SiC particles and aluminum matrix has a significant effect on the compressive resistance of the composites. The Al/SiC composites with the small SiC particles size are superior to the larger one under compressive loading due to the more compact interface and larger tension for the movement of dislocations. The perforation test was carried out for the material with the SiC particle average size of 10 μm, and its anti-penetration performance is about 1.50 times higher than 10CrNiMo steel.

Selective laser melting of Ti–35Nb composite from elemental powder mixture: Microstructure, mechanical behavior and corrosion behavior

Abstract: The availability of alloyed powder feedstock and chemical inhomogeneity, which often occur when using elemental mixed powder, have been long-term concerns of selective laser melting (SLM) of metallic materials. In this work, a Ti–35Nb alloy (in wt.%) was manufactured using SLM from elemental mixed powder to study the microstructure, mechanical behavior, and corrosion properties of the resultant parts. Microstructural characterizations show that the SLM-produced Ti–35Nb is composed of fine near β phase dendrites and undissolved Nb particles, which produces in a relatively low Young's modulus (84.7 ± 1.2 GPa). The chemical homogeneity and microstructural homogeneity are improved by heat treatment, resulting in a more homogeneous microstructure and smaller Nb particles. The undissolved large Nb particles play an important role in the overall performance of the SLM-produced materials, because the boundaries of undissolved large Nb particles in the as-SLMed part act as initiation sites for slip bands. The compressive fracture mechanism illustrates the propagation, arrest and merge of shear bands, thereby revealing the effects on the yield strength and plasticity. The electrochemical experiments show the stable corrosion resistance of as-SLMed sample and the improved corrosion resistance of the heat-treated counterparts. This work sheds insight into the SLM of Ti–Nb powder mixtures for biomedical applications. In particular, the relatively low cost and easy manufacture of elemental powder as feedstock offer significant advantages to the additive manufacturing industry.

Ultrahigh-performance TiNi shape memory alloy by 4D printing

Abstract: For additively manufactured components, it's widely accepted to have high enough energy input to facilitate nearly full density and low enough energy input to avoid cracking tendency. In this work, ultrahigh-performance Ti 50.6 Ni 49.4 (at.%) shape memory alloy (SMA) was manufactured by selective laser melting (SLM) under high enough energy inputs (155–292 J/mm 3 ). The microstructure, phase transformation behaviors, mechanical and shape memory properties of the SLM-manufactured SMA were investigated by various characterization methods of X-ray diffraction, scanning and transmission electron microscopies, differential scanning calorimetry, room temperature and stress-controlled cyclic tensile tests, etc. Results show that the martensite content and the austenite and martensitic transformation temperatures decrease with the decrease of laser energy input (the increase of laser scanning speed). Interestingly, the SLM-manufactured SMA exhibits ultrahigh tensile strength of 776 MPa and elongation of 7.2% under room-temperature tensile condition. In addition, stress-controlled cyclic tensile tests under 400 MPa indicate that the SLM-manufactured SMA has ultrahigh shape memory effect of 98.7% recovery ratio and 4.99% recoverable strain after ten times loading-unloading cycle. The ultrahigh mechanical and shape memory properties are associated to the combined effects of dispersedly distributed nano-sized Ti 2 Ni precipitates, ultrafine grains and profuse dislocations in the SLM-manufactured SMA. This work substantiates, for the first time, high enough energy input in SLM can be applied to manufacture ultrahigh-performance TiNi SMAs.

Contribution of Mg2Si precipitates to the strength of direct metal laser sintered AlSi10Mg

Abstract: Microstructure of deformed DMLS-AlSi10Mg under quasi-static loading was studied using TEM to elaborate the strengthening mechanisms. In addition to Orowan (due to presence of Si precipitates), Hall-Petch (due to eutectic Si walls), and dislocation hardening (due to pre-existing entangled dislocations) mechanisms, Mg2Si precipitates (colonies) contributed to the strength of alloy by impeding dislocation motion. The level of this contribution was evaluated as ~13 MPa by comparing the modeled and measured yield strength.

Interfacial microstructure and mechanical properties of 316L /CuSn10 multi-material bimetallic structure fabricated by selective laser melting

Abstract: Selective laser melting (SLM) is a metal additive manufacturing (AM) technique that can fabricate complex parts of any shape. In this paper, the interfacial microstructure and mechanical properties of 316L/CuSn10 bimetallic structure were studied, and the ability of self-developed multi-material SLM equipment to form multi-material bimetallic structures was described. The investigations of 316L/CuSn10 bimetallic structure involved microscopic features, phase analysis, microhardness, tensile properties and bending properties. Moreover, the mechanical properties of multi-material specimens were compared with that of single material samples. In addition, scanning electron micrograph shows that the width of the bimetallic fusion zone is about 550 μm, and dendritic crack sources was found on the boundary between the bimetallic fusion zone and the steel region. In the direction perpendicular to the interface, the Vickers microhardness value gradually changed from 233.1 ± 8.1 HV in the steel zone to 154.7 ± 6.0 HV in the bronze zone. The non-standard tensile samples were printed and tested for evaluating tensile properties, the ultimate strength of 316L/CuSn10 joint was 423.3 ± 30.2 MPa comparing with the 316L stainless steel of 673.1 ± 4.2 MPa and the CuSn10 Tin-bronze of 578.7 ± 30.6 MPa. Tensile stress-strain curves and fracture characteristics show that the fusion zone of steel and bronze exhibits brittle fracture mechanism. Furthermore, three-point bending test was used to evaluate the interfacial bonding strength of the bimetallic structure, and results show that the maximum flexural strength of 316L/CuSn10 bimetallic structure isn't in middle of but below that of 316L stainless steel and CuSn10 Tin-bronze. The research founding that SLM can obtain 316L/CuSn10 bimetallic structure with good joint strength by adopting island scanning strategy and inter-layer stagger scanning strategy in the interfacial layers.

Heat treatment and properties of a hot work tool steel fabricated by additive manufacturing

Abstract: Additive manufacturing (AM) is increasingly used for the manufacturing of tools and dies; in this respect, apart from the optimization of processing parameters, it is important to establish the most proper heat treatment conditions for the fabricated parts. In this paper, the microstructure, and some properties of H13 hot work tool steel fabricated by selective laser melting (SLM) have been evaluated after direct tempering and in quenched and tempered condition. The as-built microstructure consists of a partially tempered martensite and a much higher amount (up to 19%vol) of retained austenite (RA) compared to the quenched steel (RA

Effect of second phase particles on mechanical properties and grain growth in a CoCrFeMnNi high entropy alloy

Abstract: Effect of annealing of a cold-worked CoCrFeMnNi alloy at temperatures of 500–900 °C for 1–50 h on the structure and mechanical properties was studied in the present work. Annealing for an hour resulted in: i) recrystallization of the face-centered cubic (fcc) matrix at 600–900 °C; ii) precipitation of a Cr-rich body-centered cubic (bcc) phase at 500–700 °C or a sigma phase particles at 600–800 °C. Moreover, an increase in the annealing time to 50 h at 600 °C resulted in a continuous growth of both the fcc grans and bcc/sigma particles and in an increase in the fraction of the sigma phase at the expense of the bcc phase particles. The fcc grains growth was found to be controlled by the pinning effect of the second phase particles. Soaking for an hour at 500–600 °C resulted in a substantial increase in strength of the alloy due to the second phases precipitation. Meanwhile annealing at the higher temperatures as well as an increase in the annealing time at 600 °C resulted in softening; however, even after 50 h annealing, the alloy demonstrated reasonably high strength. In the latter case fine fcc grains, preserved due to the pinning effect by the second phases particles, contributed to strength mainly.

Crystallographic-orientation-dependent tensile behaviours of stainless steel 316L fabricated by laser powder bed fusion

Abstract: In the present study, single-crystalline-like bulk stainless steel (SS316L) specimens with a {110} Goss texture were produced by laser powder bed fusion (LPBF). The tensile behaviours of the LPBF-fabricated SS316L along the , and crystallographic directions were systematically investigated. The samples along the three crystallographic directions enabled a broader strength-ductility paradigm of LPBF-fabricated SS316L and exhibited a superior strength-ductility synergy over their traditionally manufactured counterparts. The tensile responses of the SS316L samples were highly dependent on their crystallographic orientations. The orientated samples exhibited higher yield strength (YS) than those of the and orientated samples, which was mainly attributed to the lower Schmid factors of the grains along their tensile axes (TAs). The dominant deformation mechanisms were found to be dislocation slip and deformation twinning for the and orientated samples, respectively. For orientated samples, significant deformation twinning as well as evident lattice rotation were observed simultaneously. The higher tendency towards deformation twinning of the and orientated samples arose from the larger separations between the partial dislocations in these samples, which reduced the effective stacking fault energies as well as the critical stresses for deformation twinning significantly. Due to the higher propensity towards deformation twinning, the and orientated samples showed better ductility over the orientated samples by facilitating twinning-induced plasticity (TWIP) effect. Furthermore, the lattice rotation of the samples during tension featured a modest TWIP effect which enabled a more prolonged strain hardening rate uphill than that of the samples, resulting in a superior ductility with a total elongation (TE) of ~100 %.

Assessment of mechanical properties and fatigue performance of a selective laser melted nickel-base superalloy Inconel 718

Abstract: In the present work a nickel-base superalloy Inconel 718 manufactured by selective laser melting (SLM) was investigated with focus on microstructure, orientation-dependent mechanical property and fatigue performance. Comparative material testing and characterization of the SLM and the forged Inconel 718 revealed significant differences in both microstructures and mechanical properties. The columnar grain structure of SLM alloy leads to the orientation-dependent mechanical properties, which matches the Hall-Petch relation. The inhomogeneous microstructures and slit shaped lack-of-fusion (LoF) defects from the SLM process result in worse fatigue performance and deteriorate the fatigue crack growth behavior.

Direct laser metal deposition of Inconel 738

Abstract: Inconel 738 is one of the widely used nickel-based superalloys in high-temperature applications, especially in land-based and aerospace gas turbine engines. This paper reports the feasibility of direct laser metal deposition (LMD) of Inconel 738. Cracks evolved during deposition at the substrate/deposit interface and within the deposit along high angle grain boundary for scanning speed of 6 and 12 mm/s due to the intense residual stress and incipient melting. Results showed liquation cracking due to low melting crack boundary γ′ and significant micro-segregation of Al and Ti along the crack boundaries. By maximizing the energy density and by reducing the scanning speed to 3 mm/s, crack-free single wall specimens were successfully manufactured. Microstructural evolution of primary, secondary, grain boundary γ′, MC carbides, and M2B borides in the as-deposited and heat-treat specimens are discussed. Mechanical properties and microstructural development were investigated using tensile testing and scanning electron microscopy. Energy dispersive spectroscopy confirmed significant micro-segregation on various elements along the interdendritic and grain boundaries. X-ray diffraction validated the presence of the observed carbides and boride in the as-deposited and heat-treated samples.

Joint contribution of transformation and twinning to the high strength-ductility combination of a FeMnCoCr high entropy alloy at cryogenic temperatures

Abstract: The microstructure-mechanical property relationships of a non-equiatomic FeMnCoCr high entropy alloy (HEA), which shows a single face-centered cubic (fcc) structure in the undeformed state, have been systematically investigated at room and cryogenic temperatures. Both strength and ductility increase significantly when reducing the probing temperature from 293 K to 77 K. During tensile deformation at 293 K, dislocation slip and mechanical twinning prevail. At 173 K deformation-driven athermal transformation from the fcc phase to the hexagonal close-packed (hcp) martensite is the dominant mechanism while mechanical twinning occurs in grains with high Schmid factors. At 77 K athermal martensitic transformation continues to prevail in addition to dislocation slip and twinning. The reduction in the mean free path for dislocation slip through the fine martensite bundles and deformation twins leads to the further increased strength. The joint activation of transformation and twinning under cryogenic conditions is attributed to the decreased stacking fault energy and the enhanced flow stress of the fcc matrix with decreasing temperature. These mechanisms lead to an elevated strain hardening capacity and an enhanced strength-ductility combination. The temperature-dependent synergy effects of martensite formation, twinning and dislocation plasticity originate from the metastability alloy design concept. This is realized by relaxing the equiatomic HEA constraints towards reduced Ni and increased Mn contents, enabling a non-equiatomic material with low stacking fault energy. These insights are important for designing strong and ductile Ni-saving alloys for cryogenic applications.

Influence of post treatment on microstructure, porosity and mechanical properties of additive manufactured H13 tool steel

Abstract: Additive manufacturing (AM) is an attractive manufacturing technology in tooling applications. It provides unique opportunities to manufacture tools with complex shapes, containing inner channels for conformal cooling. In this investigation, H13, a widely used tool steel, was manufactured using a laser powder bed fusion method. Microstructure, tensile mechanical properties, hardness, and porosity of the AM H13 after stress relieve (SR), standard hardening and tempering (SR + HT), and hot isostatic pressing (SR + HIP + HT) were investigated. It was found that the microstructure of directly solidified colonies of prior austenite, which is typical for AM, disappeared after austenitizing at the hardening heat treatment. In specimens SR + HT and SR + HIP + HT, a microstructure similar to the conventional but finer was observed. Electron microscopy showed that SR and SR + HT specimens contained lack of fusion, and spherical gas porosity, which resulted in remarkable scatter in the observed elongation to break values. Application of HIP resulted in the highest strength values, higher than those observed for conventional H13 heat treated in the same way. The conclusion is that HIP promotes reduction of porosity and lack of fusion defects and can be efficiently used to improve the mechanical properties of AM H13 tool steel.

Reducing arc heat input and obtaining equiaxed grains by hot-wire method during arc additive manufacturing titanium alloy

Abstract: Arc additive manufacturing technology has high deposition efficiencies and material utilization rates. However, the large heat input and high temperature gradient during arc additive manufacturing process lead to the formation of the coarse columnar grains. Due to the presence of the coarse columnar grains, the mechanical properties of the part have great anisotropy which limits the application of it. In this experiment, a method named hot-wire arc additive manufacturing was adopted to reduce the arc heat input and refine the columnar grains, and four thin-walled samples with the material of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si were manufactured. It is interesting to find that the coarse columnar grains have been greatly refined, and finally a part consisting of equiaxed grains and short columnar grains was obtained. At the same time, the width of the α-lath has also been refined. The mechanical properties are in accordance with the grain changes and the anisotropy almost disappeared. And it seems that a part with comprehensive mechanical properties can be obtained by the hot-wire arc additive manufacturing.

An experimental study of residual stress and direction-dependence of fatigue crack growth behaviour in as-built and stress-relieved selective-laser-melted Ti6Al4V

Abstract: Selective-laser-melting (SLM) is a powder-bed fusion additive-manufacturing process that has the potential to deliver three-dimensional complex parts with mechanical properties comparable or superior to parts produced via traditional manufacturing using cast and wrought alloys. Concerns for metallic parts built via SLM are the process-induced residual stresses, and anisotropic mechanical properties. This paper investigates the effect of residual stresses on the fatigue crack growth rate of SLM Ti6Al4V in as-built and stress-relieved conditions. Neutron diffraction and the contour method are employed to measure residual stresses in compact-tension samples. Neutron diffraction results are in good agreement with the contour method. It was found that tensile stresses are present at the notch root and the free edge areas, and compressive stress is seen in the middle of the sample. The tensile stresses in the as-built condition resulted in a higher fatigue crack growth rate. After stress relieving by heat treatment, the tensile residual stress diminished by around 90%, resulting in decreased crack growth rate. The build direction was seen to affect the crack growth rate, although the trend was different between the as-built and stress-relieved conditions.

Al0.5FeCoCrNi high entropy alloy prepared by selective laser melting with gas-atomized pre-alloy powders

Abstract: The Al0.5FeCoCrNi HEA was fabricated by selective laser melting (SLM) with gas-atomized pre-alloy powders. The BCC phase in the powders transforms to FCC phase in the SLM-processed sample. The SLM-processed sample has excellent tensile properties with the yield strength and ultimate tensile strength of 579 MPa and 721 MPa, respectively.

Effect of heat treatment on the microstructure and mechanical properties of maraging steel by selective laser melting

Abstract: This work investigates the solution phenomenon, aging behavior and room-temperature mechanical properties of maraging steel manufactured by selective laser melting (SLM). Different heat treatment experiments, including solution treatment (ST), direct aging treatment (DAT) and solution + aging treatment (SAT) are designed. Microstructure analysis indicates that ST and SAT will eliminate the cellular and lath structures, but DAT has little effect on these. The content of austenite increases with the addition of DAT temperature and holding time. While austenite is almost undetectable in ST and SAT samples. Meanwhile, both the elongation and toughness of the samples with DAT gain a slight improvement with the temperature increasing. Importantly, DAT yields similar microhardness, tensile strength and impact toughness to SAT, although the resultant microstructures are completely different. The results demonstrate that DAT can achieve the similar mechanical properties to SAT samples. Samples with high mechanical properties (microhardness of 653.93 HV and ultimate strength of 2126.30 MPa) have been obtained by DAT at 520 °C for 6 h as well as solution treatment at 900 °C for 1 h and aging treatment at 520 °C for 6 h. This investigation reveals the evolution regularity of microstructure, microhardness, tensile performance and impact toughness of maraging steel manufactured by SLM after different heat treatments.

Effect of temperature on the stacking fault energy and deformation behaviour in 316L austenitic stainless steel

Abstract: The stacking fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial disloc ...

An investigation on the effect of powder recycling on the microstructure and mechanical properties of AISI 316L produced by Directed Energy Deposition

Abstract: Directed energy deposition (DED) has been employed to produce AISI 316L samples. Microstructure and primary cellular arm spacing (PCAS) are studied analysing the relationship with the cooling rate at the different heights of DED processed 316L stainless steel sample. It is found that, by increasing the deposition distance from the substrate, the PCAS of the sample increases from 2.9 to 4.5 μm, as a consequence of the decreased cooling rate and thermal gradient. On the other hand, in the last deposited layers, the PCAS of the sample decreases from 4.5 to 3.3 μm, because of the changes in cooling mechanisms. The phase composition of samples after deposition is revealed and compared with the predictions based on the Schaeffler and Pseudo-binary diagrams. It is revealed that the final microstructure is characterized by austenitic dendrites together with some residual delta ferrite located at dendritic arms location. Lastly, the effect of using fresh or recycled powders, on the microstructure and mechanical properties of DED 316L stainless steel parts is investigated. It is found that the samples fabricated using recycled powders have rather similar tensile strength levels, but much lower elongation than those produced using fresh powder due to a lower inclusions content and of their average lower size. The nature of these inclusions is discussed as well as the reason for their increase both in numbers and size.

Microstructure and properties of Cu-Cr-Nb alloy with high strength, high electrical conductivity and good softening resistance performance at elevated temperature

Abstract: Cu-0.47Cr-0.16 Nb (wt%) alloys were designed and prepared, and the microstructures of the Cu-Cr-Nb alloy were investigated using transmission electron microscopy and three-dimension atom probe tomography technique. After homogenizing at 950 °C for 4 h, cold rolling by 80% reduction, then aging at 450 °C for 30 min, the micro-hardness, electrical conductivity, tensile strength, yield strength and elongation of the alloy were up to 150 HV, 89.1%IACS, 453 MPa, 443 MPa and 11.4%, respectively. The tensile strength / elongation of the alloy approached 399 MPa / 22.9% as test at 200 ℃, 334 MPa / 14.8% for 300 ℃, and 282 MPa / 12.3% for 400 ℃, respectively. The Cr2Nb phase with an average size of 700 nm and Nb phase with an average size of 500 nm formed during the solidification process, while the nano-scale Cr-rich phases precipitated from the solid solution during the aging process. The sizes of Cr2Nb and Cr-rich phases were close to each other during aging. These precipitates located in the grain and sub-grain boundary could effectively pin the movement of boundary, resulting in a high strength of the alloy at the elevated temperature. The addition of Nb can promote the precipitation of Cr from the solid solution during aging, thus both strength and electrical conductivity of the alloy were improved.