Showing papers by "Xinhua Wu published in 2018"

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.

Surface roughness of Selective Laser Melted Ti-6Al-4V alloy components

Abstract: A rational design of experiments was employed to evaluate the correlation between scan parameters and the resulting surface roughness of Selective Laser Melted Ti-6Al-4V components. There is a statistically significant difference in surface roughness values from specimens built with identical laser exposure parameters but located at different positions on the build platform. We hypothesise that this is a consequence of changing powder particle size distributions across the powder bed resulting from the combined actions of the recoater arm and gas flow. We further hypothesise that orientation of a part and the projected shape of the incident laser beam play a part in surface roughness variation at any given location. We found that during the powder re-coating process, fine particles tend to settle within a short distance from the re-coater starting position, accompanied by higher variability of local powder size distribution. Spatter material was found to be distributed across the powder bed by the gas flow. However, once at any given location the surface roughness of inclined surfaces is affected by the orientation of the surface to the centre of the build platform at which the laser beam originates. Each of these factors affects the surface roughness and has implications for the order in which parts are built in Selective Laser Melting.

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.

Mechanical Properties and In Vitro Behavior of Additively Manufactured and Functionally Graded Ti6Al4V Porous Scaffolds

Abstract: Functionally graded lattice structures produced by additive manufacturing are promising for bone tissue engineering. Spatial variations in their porosity are reported to vary the stiffness and make it comparable to cortical or trabecular bone. However, the interplay between the mechanical properties and biological response of functionally graded lattices is less clear. Here we show that by designing continuous gradient structures and studying their mechanical and biological properties simultaneously, orthopedic implant design can be improved and guidelines can be established. Our continuous gradient structures were generated by gradually changing the strut diameter of a body centered cubic (BCC) unit cell. This approach enables a smooth transition between unit cell layers and minimizes the effect of stress discontinuity within the scaffold. Scaffolds were fabricated using selective laser melting (SLM) and underwent mechanical and in vitro biological testing. Our results indicate that optimal gradient structures should possess small pores in their core (~900 µm) to increase their mechanical strength whilst large pores (~1100 µm) should be utilized in their outer surface to enhance cell penetration and proliferation. We suggest this approach could be widely used in the design of orthopedic implants to maximize both the mechanical and biological properties of the implant.

Effect of hot isostatic pressing on the microstructure and mechanical properties of additive manufactured AlxCoCrFeNi high entropy alloys

Abstract: Three high entropy alloys (HEAs), based on the AlxCoCrFeNi alloy system have been prepared by direct laser fabrication (DLF) with aluminium molar fractions (x) of 03, 06 and 085 These three alloys had FCC, duplex FCC + BCC, and BCC crystal structures, respectively The effect of hot isostatic pressing (HIP) on alloy density, microstructure and mechanical properties of these DLF bulk high entropy alloys was studied for the first time HIP was found to decrease the number of large pores (> 5 µm) in the as-deposited alloys, which equated to a marginal increase in density HIP also induced microstructural coarsening, chemical homogenisation and resulted in a general improvement in the mechanical properties of FCC HEA (x = 03) HIP improved the compressive properties of the dual phase HEA (x = 06), however, degraded the tensile properties as a result of the coarsening of hard BCC grain boundary precipitates The mechanical properties were compromised in the high aluminium (x = 085) HEA due to the formation of σ-phase at the phase and grain boundaries, which induced a brittle fracture in tension and compression A continuous cooling transformation (CCT) diagram for the σ-phase was determined by a dilatometric method and the critical cooling rate to inhibit σ-phase formation was found to be 1 K/s

Effects of preheating and carbon dilution on material characteristics of laser-cladded hypereutectoid rail steels

Abstract: The impacts of preheating conditions and carbon dilution on the microstructural and mechanical properties of laser cladded rails using single and double cladding layers have been investigated for a hypereutectoid steel grades typically used under heavy haul conditions. The microstructures in the HAZ showed that formation of martensite, which has a detrimental effect on behaviour in wheel-rail contact, was successfully inhibited by increasing the length of the preheated region using a preheating temperature of 350 °C. Dilution of carbon from the hypereutectoid substrate was observed and its effect on the microstructures of the 410L ferritic stainless-steel deposits was investigated. The formation of ferrite in the 410L cladding layers was attributed to the very low carbon content, and no carbide formation was observed on boundaries of the ferritic grains. The thickness of dilution band was determined to be approximately equal to the thickness of the first cladding layer. Texture measurement obtained by EBSD showed a random trend owing to the formation of martensite in diluted bands. Strong solidification fibre texture was developed for double deposition, particularly in the second deposit. Mechanical characterization of the 410L deposits undertaken in terms of Vickers microhardness, shear and tensile yield strengths, and ultimate tensile and shear strengths were correlated with the observed microstructural morphologies.

Novel implant for peri-prosthetic proximal tibia fractures.

Abstract: Background Repair of peri-prosthetic proximal tibia fractures is very challenging in patients with a total knee replacement or arthroplasty. The tibial component of the knee implant severely restricts the fixation points of the tibial implant to repair peri-prosthetic fractures. A novel implant has been designed with an extended flange over the anterior of tibial condyle to provide additional points of fixation, overcoming limitations of existing generic locking plates used for proximal tibia fractures. Furthermore, the screws fixed through the extended flange provide additional support to prevent the problem of subsidence of tibial component of knee implant. Methods The design methodology involved extraction of bone data from CT scans into a flexible CAD format, implant design and structural evaluation and optimisation using FEM as well as prototype development and manufacture by selective laser melting 3D printing technology with Ti6Al4 V powder. Results A prototype tibia implant was developed based on a patient-specific bone structure, which was regenerated from the CT images of patient’s tibia. The design is described in detail and being applied to fit up to 80% of patients, for both left and right sides based on the average dimensions and shape of the bone structure from a wide range of CT images. Conclusion A novel tibial implant has been developed to repair peri-prosthetic proximal tibia fractures which overcomes significant constraints from the tibial component of existing knee implant.

High strength aluminium alloy for rapid solidification manufacturing processes

Abstract: An aluminium based alloy, and a method for production of components by additive manufacturing (AM) or other rapid solidification process with the alloy, is based on the alloy having a composition with from 2.01 wt % to 15.0 wt % manganese, from 0.3 wt % to 2.0 wt % scandium, with a balance apart from minor alloy elements and incidental impurities of aluminium.