In situ tailoring microstructure in additively manufactured Ti-6Al-4V for superior mechanical performance (original) (raw)
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Acta Materialia, 2015
Novel ultrafine lamellar (a + b) microstructures comprising ultrafine ($200-300 nm) a-laths and retained b phases were created via promoting in situ decomposition of a near a 0 martensitic structure in Ti-6Al-4V additively manufactured by selective laser melting (SLM). As a consequence, the total tensile elongation to failure reached 11.4% while maintaining high yield strength above 1100 MPa, superior to both conventional SLM-fabricated Ti-6Al-4V containing non-equilibrium acicular a 0 martensite and conventional mill-annealed Ti-6Al-4V. The formation and decomposition of a 0 martensite in additively manufactured Ti-6Al-4V was studied via specially designed experiments including single-track deposition, multi-layer deposition and post-SLM heat treatment. The essential SLM additive manufacturing conditions for Ti-6Al-4V including layer thickness, focal offset distance and energy density, under which a near a 0 martensitic structure forms in each layer and then in situ transforms into ultrafine lamellar (a + b) structures, were determined. This is the first fundamental effort that has realized complete in situ martensite decomposition in SLM-fabricated Ti-6Al-4V for outstanding mechanical properties.
JOM, 2017
Recent progress has shown that Ti-6Al-4V fabricated by selective laser melting (SLM) can achieve a fully lamellar a + b microstructure using 60 lm layer thickness in the as-built state via in situ martensite decomposition by manipulating the processing parameters. The potential to broaden the processing window was explored in this study by increasing the layer thickness to the less commonly used 90 lm. Fully lamellar a + b microstructures were produced in the as-built state using inter-layer times in the range of 1-12 s. Microstructural features such as the a-lath thickness and morphology were sensitive to both build height and inter-layer time. The a-laths produced using the inter-layer time of 1 s were much coarser than those produced with the inter-layer time of 12 s. The fine fully lamellar a + b structure resulted in tensile ductility of 11% and yield strength of 980 MPa. The tensile properties can be further improved by minimizing the presence of process-induced defects.
Journal of Alloys and Compounds, 2012
The present work shows that optimization of mechanical properties via heat treatment of parts produced by Selective Laser Melting (SLM) is profoundly different compared to conventionally processed Ti6Al4V. In order to obtain optimal mechanical properties, specific treatments are necessary due to the specific microstructure resulting from the SLM process. SLM is an additive manufacturing technique through which components are built by selectively melting powder layers with a focused laser beam. The process is characterized by short laser-powder interaction times and localized high heat input, which leads to steep thermal gradients, rapid solidification and fast cooling. In this research, the effect of several heat treatments on the microstructure and mechanical properties of Ti6Al4V processed by SLM is studied. A comparison is made with the effect of these treatments on hot forged and subsequently mill annealed Ti6Al4V with an original equiaxed microstructure. For SLM produced parts, the original martensite α' phase is converted to a lamellar mixture of α and β for heat treating temperatures below the β-transus (995°C), but features of the original microstructure are maintained. Treated above the β-transus, extensive grain growth occurs and large β grains are formed which transform to lamellar α+β upon cooling. Post treating at 850°C for two hours, followed by furnace cooling increased the ductility of SLM parts to 12.84 ± 1.36 %, compared to 7.36 ± 1.32 % for as-built parts.
Additive manufacturing and postprocessing of Ti-6Al-4V for superior mechanical properties
MRS Bulletin
The capabilities of metal additive manufacturing (AM) are evolving rapidly thanks to both increasing industry demand and improved scientifi c understanding of the process. This article provides an overview of AM of the Ti-6Al-4V alloy, which has essentially been used as a yardstick to gauge the capability of each metal AM process developed to date. It begins by summarizing the metal AM processes existing today. This is followed by a discussion of the macro-and microstructural characteristics, defects, and tensile and fatigue properties of AM Ti-6Al-4V by selective laser melting, laser metal deposition (both powder and wire), and selective electron-beam melting compared to non-AM Ti-6Al-4V. The tensile and fatigue properties of as-built AM Ti-6Al-4V (with machined or polished surfaces) can be made comparable, or even superior, to those of Ti-6Al-4V in the most commonly used millannealed condition. However, these properties can exhibit a large degree of scatter and are often anisotropic, affected by AM build orientations. Post-AM surface treatments or both the post-AM surface and heat treatments are necessary to ensure the minimum required properties and performance consistency. Future directions to further unlock the potential of AM of Ti-6Al-4V for superior and consistent mechanical properties are also discussed.
The tensile properties, mode I fracture toughness (K Ic), fatigue crack growth behavior, and unnotched fatigue strength of additively manufactured Ti-6Al-4V (Ti64) alloy using selective laser melting (SLM) technique were investigated. Four different combinations of layer thickness (t)-scan rotation between successive layers (f), which resulted in mesostructures that range from through-thickness columnar prior b grains with square cross-sections, whose side lengths equal to the scan spacing, to near-equiaxed mesostructures in both build and transverse directions, were explored. Possible anisotropy in mechanical properties was investigated by conducting tests on samples whose loading axis is either parallel or perpendicular to the build directions. In all cases, the microstructure consisted of fine a/a 0 lath structure, where a 0 is the metastable martensitic Ti phase that is acicular in shape, within the prior b grains. Experimental results show that the process parameter combinations of t ¼ 60 mm and f ¼ 67 results in an alloy that exhibits high yield strength (>1100 MPa) and ductility (>12%) simultaneously, K Ic of 58 MPa ffiffiffiffiffiffi m p , and unnotched fatigue strength, which is similar to that of the same alloy but manufactured using conventional techniques. The anisotropy in properties, overall, was found to be not substantial, even in the case where columnar growth of prior b grains occurs in the build direction. The values of the Paris exponents for steady state fatigue crack growth (FCG) are much lower than those reported for conventionally manufactured Ti64, suggesting higher FCG resistance in SLM Ti64. Analysis of the effective microstructural length scale that controls the near-threshold FCG rate suggests that it is the colony size that dominates this behavior. Overall, the results of this study indicate directions for process parameter optimization that would lead to SLM Ti64 that is not only has high strength, but also is damage tolerant.
Acta Materialia 167 (2019) 136-148, 2019
The microstructure and phase composition in selective laser melted (SLM) Ti-6Al-4V plays a key role for its mechanical performance. The microstructure evolution in SLM Ti-6Al-4V was studied in the as-built condition and after sub-transus heat treatments between 400 C and 800 C focusing on elemental partitioning and the role of lattice defects on precipitation of the b phase. With SLM parameters corresponding to low volume energy density (E V ¼ 77 J/mm 3) the as-built microstructure consisted of acicular martensite and showed a higher density of lattice defects than that synthesized under high E V ¼ 145 J/mm 3 condition. High energy X-ray synchrotron diffraction indicated the presence of ~2 wt.% b-phase at this high E V. Moreover, atom-probe tomography revealed enrichments in b-stabilizers at one-and two-dimensional lattice defects. These fine enriched one-dimensional columnar and two-dimensional features are identified as precursors of b-phase, revealing the role of lattice defects for b-precipitation. Upon annealing at 400 C and 530 C, b-films began to fragment into bÀplatelets and nanoparticles, whereas annealing at 800 C led to a coarse-lamellar a/b-microstructure. Moreover, a 2-Ti 3 Al was found in the 400 C annealed condition. In line with the microstructure changes, Vickers hardness increased upon annealing at temperatures up to 530 C and dropped when coarsening occurred at higher temperatures. Substantial element partitioning occurred during thermally driven martensite decomposition, which was significantly stronger for Fe than for V.
Materials
An investigation of mechanical properties of Ti6Al4V produced by additive manufacturing (AM) in the as-printed condition have been conducted and compared with wrought alloys. The AM samples were built by Selective Laser Melting (SLM) and Electron Beam Melting (EBM) in 0°, 45° and 90°—relative to horizontal direction. Similarly, the wrought samples were also cut and tested in the same directions relative to the plate rolling direction. The microstructures of the samples were significantly different on all samples. α′ martensite was observed on the SLM, acicular α on EBM and combination of both on the wrought alloy. EBM samples had higher surface roughness (Ra) compared with both SLM and wrought alloy. SLM samples were comparatively harder than wrought alloy and EBM. Tensile strength of the wrought alloy was higher in all directions except for 45°, where SLM samples showed higher strength than both EBM and wrought alloy on that direction. The ductility of the wrought alloy was consist...
Journal of Alloys and Compounds, 2018
As a powder-bed-based additive manufacturing technology, selective laser melting (SLM) offers high-level flexibility and enables efficient fabrication of complex parts. In connection with complex thermal events occurring during dynamic sequential layer-by-layer deposition, the as-built material is usually hierarchical at different length scales and possesses anisotropy at each level. As a result of a moderate heating temperature of the baseplate and high cooling rates involved in the process, the as-built Ti-6Al-4V alloy has an α´ martensite microstructure. Microstructure evolution occurring during post-SLM heat treatment is strongly affected by the stability of the initial acicular martensite. The present study was aimed at developing an optimum post-SLM heat treatment scheme at a temperature below the β transus temperature, based on the understanding of microstructure evolution occurring during subtransus treatment and the resultant mechanical properties of the alloy.
Additive Manufacturing, 2014
The metal additive manufacturing industry is rising and so is the interest in new lattice structures with unique mechanical properties. Many studies have already investigated lattice structures with different geometries and their influence on mechanical properties, but little is known about the effect of specific processing characteristics that are inherent to metal additive manufacturing. Therefore this study investigates the effect of two crucial steps in the manufacturing process: the build orientation selection and heat treatment. In total the microstructure and static mechanical properties of five different orientations and three heat treatment conditions were evaluated using Ti6Al4V diamond like lattice structures. The results show a significant decrease in mechanical strength for samples that are built diagonally and a transformation of the microstructure after a HIP (hot isostatic pressing) treatment, resulting in a lower maximum strength, but higher ductility. In general, horizontal struts should be avoided during manufacturing, unless the applied load after manufacturing can be properly supported by other struts. Both a stress relief heat treatment and a HIP treatment can be used in statically loaded applications, whereas a HIP treatment is believed to be beneficial for dynamically loaded applications. This study enables an appropriate selection of build orientation and heat treatment of lattice structures for different applications.
Journal of Manufacturing Processes, Volume 35, Pages 538-546, October, 2018
Improving the metallurgical properties of the products fabricated by the Selective laser melting (SLM) process is still a challenge focusing on processing parameters and Energy Density (ED). In this study, the density, porosity and microstructure of cuboid Ti-6Al-4V alloy samples fabricated by the SLM process were investigated, paying particular attention to the manufacturing key factor ED. Seven different EDs between 39 J/mm 3 and 260 J/mm 3 were achieved by varying the scanning speed from 1000 mm/s to 150 mm/s. The results show the ED, as well as cooling conditions, have a great influence on the above mentioned metallurgical properties. Significantly different densities have been observed, with different Eds where the difference between the sample with the highest relative density and the sample with the lowest relative density was almost 2.45%. The highest relative product density of almost 99.45% was obtained at the ED value of 65 J/mm 3. The volume, shape and numbers of pores were different with different EDs. The resultant microstructure consisted of four typical hierarchical α′ martensites, namely, primary, secondary, tertiary and quartic α′ martensites within columnar prior β grains.