Understanding the densification behaviours and formation mechanisms of defects are essential to fabricate high quality and high strength aluminium components using selective laser melting (SLM) ...technology. In this work, the effects of laser power and scanning speed on the densification, defects evolution and their formation mechanisms in a SLMed 2024 aluminium (Al) alloy were investigated in consideration of the corresponding laser energy input, melting mode transition and microstructural evolution. The results showed that optimizing the processing parameters effectively reduced the porosity level below 1% by avoiding the lack of fusion and keyhole melting mode, and minimizing the gas pores. However, optimization of the processing parameters could not eliminate the columnar structure associated with the SLMed 2024 Al alloy, which contributed to the hot-tearing cracks in the SLMed parts. It was found that the dependence of porosity formation on SLM processing parameters was contrary to the crack density. Hence, to further improve the SLM-processability of the 2024 Al alloy it is necessary to develop SLM methods in order avoid the hot-cracking within the optimized processing parameter window associated with the minimum porosity formation.
Abstract
Additive manufacturing (AM) creates digitally designed parts by successive addition of material. However, owing to intrinsic thermal cycling, metallic parts produced by AM almost inevitably ...suffer from spatially dependent heterogeneities in phases and mechanical properties, which may cause unpredictable service failures. Here, we demonstrate a synergistic alloy design approach to overcome this issue in titanium alloys manufactured by laser powder bed fusion. The key to our approach is in-situ alloying of Ti−6Al−4V (in weight per cent) with combined additions of pure titanium powders and iron oxide (Fe
2
O
3
) nanoparticles. This not only enables in-situ elimination of phase heterogeneity through diluting V concentration whilst introducing small amounts of Fe, but also compensates for the strength loss via oxygen solute strengthening. Our alloys achieve spatially uniform microstructures and mechanical properties which are superior to those of Ti−6Al−4V. This study may help to guide the design of other alloys, which not only overcomes the challenge inherent to the AM processes, but also takes advantage of the alloy design opportunities offered by AM.
The advanced electron backscatter diffraction (EBSD) technique was used to examine the microstructure of a widely used A517GrQ low-carbon low-alloy steel after different heat treatments. Three ...distinguishable microstructures were studied. Slow cooling in the furnace after austenitization led to the formation of a granular structure that consisted of massive ferrite and randomly distributed M–A constituents. Medium rate cooling in air produced granular bainite that was composed of lath ferrite, and M–A constituents were distributed between the laths. Lath martensite was formed by fast cooling into ice brine. EBSD analysis revealed that, in one austenite grain, the massive ferrite in the granular structure and the lath ferrite in the granular bainite were predominately separated by high-angle boundaries, whilst the ferrite laths in the martensite were separated by low-angle boundaries. The specimens with granular bainite formed by medium rate cooling had higher strength (both yield strength and tensile strength), and also almost 5 times higher Charpy impact energy than that of the specimens containing granular structure obtained at the slow cooling. The strength of the specimens with lath martensite after quenching into ice brine was slightly higher than the granular bainite but were associated with much lower Charpy impact energy. The present work indicates that it is critical to control the cooling rate after austenitization in order to simultaneously achieve high strength and high toughness of low-carbon low-alloy steels.
Additive manufacturing (AM), known as 3D printing, enables rapid fabrication of geometrically complex copper (Cu) components for electrical conduction and heat management applications. However, pure ...Cu or Cu alloys produced by 3D printing often suffer from either low strength or low conductivity at room and elevated temperatures. Here, we demonstrate a design strategy for 3D printing of high strength, high conductivity Cu by uniformly dispersing a minor portion of lanthanum hexaboride (LaB
) nanoparticles in pure Cu through laser powder bed fusion (L-PBF). We show that trace additions of LaB
to pure Cu results in an improved L-PBF processability, an enhanced strength, an improved thermal stability, all whilst maintaining a high conductivity. The presented strategy could expand the applicability of 3D printed Cu components to more demanding conditions where high strength, high conductivity and thermal stability are required.
Nanocrystallization mechanism of beta phase in the bulk coarse-grained Ti-6Al-4V by high energy shot peening was investigated via high-resolution transmission electron microscopy. The results ...suggested that dislocation gliding was first initiated in beta phase at and around the intersections of the phase boundary with the high-density dislocation structures in alpha phase followed by the formation of dislocation tangles and dislocation walls. As the short axis sizes of alpha phase approached that of beta phase, the dislocation tangles and dislocation walls gradually evolved into the high-angle grain boundaries and subdivided the original grains into the equiaxed ultrafine grains. Under the ultrahigh strain and strain rate, the equiaxed ultrafine grains were eventually refined to randomly oriented nanograins via dynamic recrystallization. In addition, the nanograins would be further refined via dislocation motion upon further straining.
The thermal cycling of additive manufacturing can act as an in-situ intrinsic heat treatment (IHT), thereby producing spatially dependent microstructures and mechanical properties. This work ...demonstrates how to minimise the IHT effect to achieve uniform and enhanced tensile ductility of Ti−5Al−5Mo−5V−3Cr produced by laser powder bed fusion (L-PBF). It is found that the thermal cycling, in conjunction with substrate heating, can trigger the formation of isothermal ω and/or α phases in the β matrix, which leads to non-uniform and inferior tensile ductility. Ceasing substrate heating and/or increasing interlayer deposition time from 15 to 30 s enable substantial ductility improvement but fail to eliminate the ductility variation. Through designing a gradient interlayer deposition time (30 s – 45 s – 30 s), the tensile ductility increases 4–5 fold to ∼19% without any notable variation. The design strategy may help to tailor the microstructures and mechanical properties of other alloys which suffer from the same issue.
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•The gradient nanocrystalline structure was induced in treated layer of TC17.•The thickness of nanograin layer with an average grain size of 10.5nm was 20μm.•The composition of the treated layer of ...TC17 was discussed.•The gradient variation of the microhardness was obtained in treated layer of TC17.
The gradient nanocrystalline structure from the topmost surface to the matrix of a bulk coarse-grained TC17 was attained by using high energy shot peening treatment at an air pressure of 0.35MPa and a processing duration of 30min. The thickness from the topmost surface with a grain size of about 10.5nm to the matrix with a micrometer structure was about 120μm, and the thickness in the nanocrystalline layer was about 20μm. The microscopic and nanocrystalline structure characteristic in the treated layer were investigated via X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The nanograins layer, the nanometer-thick laminated structure layer, the refined grains layer and the low-strain matrix layer occurred in sequence from the topmost surface to the matrix, and therefore the gradient nanocrystalline structure in the treated layer was produced by using high energy shot peening. TEM investigation confirmed that the dislocation activity with very high stacking fault energy induced by surface severe plastic deformation mainly controlled the grain refinement. The microhardness (HV0.02) from the topmost surface to the matrix gradually increased by 43% from 440 to 629 and the gradient variation of the microhardness with the depths from the topmost surface to the matrix of treated TC17 was obtained.
Grain refinement of additively manufactured titanium and titanium alloys can be promoted via adding foreign elements or particles, but it may lead to a reduction in ductility due to the formation of ...brittle intermetallic compounds. The present study shows that in-situ grain refinement of commercially pure titanium (CP-Ti) can be achieved through properly controlling the selective laser melting (SLM) parameters. It was found that higher input energy density worked in favour of grain refinement. Detailed microstructural characterization coupled with multi-physics simulation were performed to reveal the grain refinement mechanism. This was attributed to the intrinsic heat treatment (IHT) effect which resulted from the cyclic reheating inherent to the SLM process. As a result, the refined CP-Ti exhibited an exceptionally high ductility of 34.3 ± 0.5% without notable mechanical anisotropy. This work demonstrates the feasibility of utilizing thermal cycling of additive manufacturing (AM) to refine grains of metals without changing the composition.
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•Isothermal compression causes the flow and fragmentation of 3D plate-shaped α phase in TC17 alloy.•Increasing the α/α/β and α/β/β triple junctions induces the nucleation and growth of globular α ...grains and the static globularization is promoted.•Asynchronous and uneven formation of globular α grains forms a 3D irregular α plate and the β evolution at α/α/α triple junctions transforms it to α rods.•β/β/β triple junctions induces the nucleation and growth of α phase along the β/β interface to form α lamellae and the static globularization is inhibited.
Lamellar globularization in the dual-phase titanium alloy is the key to improving plasticity and strength. However, the mechanism has not been fully elucidated so far. In this work, the role of phase/grain boundary in the static globularization of TC17 alloy was systematically studied by setting different α phase content before annealing through low- and high-temperature deformation. Isothermal compression causes the parallel distribution and fragmentation of 3D α plates and few globular α particles are formed at a strain rate of 1 s–1. Post-deformation annealing promotes the static globularization of α phase while it is affected by initial α phase content. After 730 °C deformation, the development of α/α interface by absorbing dislocations promotes the formation of globular α grains based on the nucleation of separated α particles and pre-recovery α subgrain during subsequent annealing. The α/α/β and α/β/β triple junctions formed due to high α content with about 36% volume fraction are favorable for the further nucleation and growth of globular α grains by reducing interface energy, forming a 3D irregular α plate. Then nucleation and growth of the β phase dominate the microstructure evolution during subsequent annealing, resulting in the local dissolution of the plate and formation of α rods. After 850 °C deformation, the α phase tends to nucleate at the β/β/β triple junctions and grow into a lamellar shape along the high energy β/β grain boundary due to low α content with about 7% volume fraction. The α nucleation that maintains the Burgers orientation relationship (BOR) with the surrounding β phase grows along the habit plane and thickens slowly, resulting in the formation of a precipitated α plate with a flat surface and the suppression of static globularization. The comprehensive investigation of lamellar globularization provides guidance for optimizing the 3D microstructure and properties of dual-phase titanium alloy.
In most previously constructed lattice inspired structures, atoms were not included. Thus, their mechanical behaviours might not well simulate the lattice structures. In the present work, a ...body-centred cubic (BCC) inspired structure with atoms as reinforced nodes and atomic bonds as connected struts, was additively manufactured using Ti-6Al-4V alloy via a selective laser melting. Results of mechanical compression test and finite element analysis revealed the higher yield strength, collapse strength and energy absorption capability of the BCC-inspired structure with reinforced nodes than the ones without reinforcement.
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