Wednesday
Effect of Building Orientation and Specimen Position on the Properties Ti-6Al-4V Alloys Manufactured via L-PBF
P. Davies, Sandvik Additive Manufacturing
Great Britain
The Ti-6Al-4V alloy is of great importance for the medical, aerospace and automotive industries due to its biocompatibility, corrosion resistance and high specific strength. As the production of complex-shaped materials via additive manufacturing technologies develop rapidly, a special attention is given towards understanding the effect of the microstructure evolution on the mechanical properties of Ti-6Al-4 parts. In this work, the assessment of atomized powders is thoroughly carried out, followed by manufacturing via laser powder bed fusion (L-PBF) and post-processing heat treatments, including hot isostatic pressing (HIP). Fundamental knowledge is built through the analysis of the resultant microstructure and mechanical properties. Room temperature tensile strength and impact toughness properties are reviewed. The presence of defects and their influence on the performance of heat-treated and HIPed parts, as well as the influence of processing parameters and specimen orientation are also discussed to evaluate building parameters and optimized heat treatments to achieve superior properties.
Influence of Pulsed Wave Laser Emission on the Surface Morphology of Parts Made of Alloy Ti-6AL-V4 Processed By Laser Powder Bed Fusion
T. Laag, Fraunhofer ILT - Institute for Laser Technology
Germany
Titanium alloy TiAl6V4 is one of the most common alloys comercially processed by the additive manufacturing (AM) technology Laser Powder Bed Fusion (LPBF), especially within aeronautics industries. As a result of the specific material properties of titanium, e.g. a comparatively low density of 4,4 g/cm3 and a low thermal conductivity of approx. 6,6 W/mK, specific challenges occur when processing alloy TiAl6V4 by powder bed based AM fabrication processes such as LPBF. Firstly, significant amounts of powder particles usually agglomerate to the part surface due to the low powder particle mass in conventional LPBF processing strategies using continous wave (cw) laser beam sources. The degree of powder agglomeration is dependent of both the melt pool fluctuations and the capillary induced gas suction within the processing zone. Based on literature, both mechanisms are expected to be significant when using cw laser beam sources as liquid melt pool volumes are comparatively large and a continously moving gas capillary that induces a continuously gas suction is formed when applying a cw exposure. Secondly, due to the large melt pool volumes generated by cw exposure and the low thermal conductivity of titanium, heat accumulation especially in filligree structures is a commonly observed phenomenon when processing TiAl6V4 by LPBF. The heat accumulation results in excessive melting phenomena and significant melt pool enlargement. As a consequence, geometric deviations in sharp angled geometric features and filligree structures is often observed for alloy TiAl6V4. The correction of these deviations, e.g. by subtractive post processing, is challenging for filigree structures and fine geometric details, as the part contour can be damaged by the post processing tool. Hence, a high as-built geometric accuracy and surface quality would reduce cost of post-processing and, thus, cost of high accuracy build parts manufactured by LPBF.
Within this work, the contour vectors of part geometries are exposed in pulsed wave emission mode and the hatching vectors in cw emission mode, respectively. Experimental work is performed on a comercially TruPrint 3000 LPBF machine setup. A constant nominal layer thickness of 60 µm and a focal spot diameter of 100 µm is used throughout the experiments. The effect of pw contour exposure parameters, e.g. laser power, scannings speed, pulse duration, pulse frequency on the surface morphology and geometric accuracy is investigated. By discretizing the emitted laser energy and intermittent solidification of the melt pools in between pulses, smaller melt volumes compared to cw exposure are generated. It can be shown, that minimum melt pool size is observed, when discrete solidification of adjacent contour melt pools is established. Abrasive sandblasting is applied on selected specimen as post-processing. The effect of the smaller melt pool size on sintered powder agglomerates and geometric accuracy is characterized by optical surface roughness measurement and scanning electron microscopy in as-built and sandblasted condition and compared to specimen manufactured with conventional cw contour exposure parameters.
Improving Densities of Titanium Alloys Using Constrained Hydrogenation Assisted Densification (CHAD) Method
C. Zhou, Central South University
China
Full density of sintered Ti alloys is difficult to achieve through conventional pressureless sintering techniques. We introduce a novel approach for densifying powder metallurgical (PM) additive manufactured (AM) and Ti alloys through constrained hydrogenation assisted densification (CHAD). This method utilizes hydrogenation-induced volumetric expansion and constrains a sintered Ti body in a rigid mold to achieve densification. The results demonstrate that the residual porosity in CP-Ti and Ti-6Al-4V alloys can be effectively reduced or removed by CHAD, leading to improved tensile properties. The described method can be used as an efficient and cost-effective technique to make Ti and Ti alloys with high density and improved mechanical properties.
Fatigue Resistance of Laser Powder Bed Fused Ti64 Components with Intentionally-Seeded Porosity
E. Moquin, École de technologie supérieure
Canada
To study the influence of processing-induced porosity of the fatigue life of laser powder bed fused Ti-6Al-4V components, specimens were printed using low, optimal and high energy densities (LED, OED, HED) to intentionally seed the porosity of different levels, morphologies and distributions. Before fatigue testing, all the specimens were inspected using X-ray microcomputed tomography. The OED printing resulted in specimens with 0.003% of porosity, while both the LED and HED printings resulted in specimens with a tenfold increased porosity (~0.04 and ~0.03%, respectively), but different pore size and morphology distributions: LED-printed specimens with elongated and aligned pores (aspect ratio 0.55) and HED-printed specimens with more spherical and random pores (aspect ratio 0.74). Results of the force-controlled fatigue testing (R=0.1) showed that the runout (107) was obtained at a stress range of 495 MPa for OED specimens, 450 MPa for HED specimens and 360 MPa, for LED specimens. Fractography analysis has also been conducted on all the specimens. The tests results were correlated to the scan data to find how one of the pore characteristics (the square root of the projected area of the defect) is related to the number of cycles to failure. This analysis was conducted for each set of printing parameters and each sample of a specific group in order to build the Kitagawa-Takahashi diagram linking the size of defect to the allowable stress range for the fatigue life of N˃107.
Part Specific Testing of Aircraft Primary Structure Additively Manufactured by Electron Beam Powder Bed Fusion Technology
S.E. Atabay, National Research Council
Canada
Additive manufacturing (AM) involves layer by layer fabrication of a part, from a CAD design, using a powder or wire feedstock. Electron beam powder bed fusion (EB-PBF), also known as electron beam melting (EBM), is a metal AM method that uses an electron beam heat source for successive melting and solidification of a thin layer of powder in a vacuum environment, which is especially useful for the processing of reactive materials, such as titanium alloys. Ti-6Al-4V is an important α + β titanium alloy for the aerospace industry and, thus, the influence of processing parameters and post-processing heat treatments on the microstructure and resulting mechanical properties of EB-PBF fabricated Ti-6Al-4V has been extensively investigated using standard coupon geometries and methods. However, the complexity of the AM process creates unique challenges in terms of characterization and qualification of the produced parts. The existing challenges include inconsistency in the results when scaling from coupon to part level, due to defects, anisotropy, and unintentional microstructural variations.
This study aimed to better understand these challenges and develop guidelines to increase the efficiency of certification testing for aerospace primary structural components. For this purpose, microstructural and mechanical characterization of a corner fitting bracket from the Lockheed CP-140/P-3, currently in use with multiple Air Forces was conducted. An Arcam Q20+ (GE Additive) EB-PBF system was utilized to fabricate the bracket from Grade 5 Ti-6Al-4V powder. The fabricated parts were surface machined before mechanical testing and one bracket was subjected to hot isostatic pressing (HIP) before final machining. Microstructural investigation of the components was done both in the as-fabricated and HIPed conditions. The effect of the post-processing treatment on the mechanical properties was also studied by micro-hardness mapping and part-specific tensile testing. Standard sub-sized specimens were extracted from different locations within the bracket and subjected to uniaxial tensile testing. Finally, fractography was performed on the tested specimens. The mechanical performance results point to interesting differences between the printed benchmarking coupons and specimens extracted from printed parts, which will be discussed with the goal of linking processing, microstructure and mechanical properties.
Investigation of the Influence of Powder Oxygen Homogeneity on the Processability and Properties of the L-PBF Processed Ti-6Al-4V
M. Habibnejad-Korayem, AP&C, A GE Additive Company
Canada
Current powder specifications for additive assume homogenous chemical composition of powder feedstock to ensure the final properties of printed components. For the case of Ti-6Al-4V composition, one of the main elements influencing the final properties of printed components, and which is evolving as powder is recycled, is the oxygen content. Oxygen can be distributed at either the interstitial free positions or surface. Such variations allow the differentiations between grade 23 and 5. In general, the main criterion to select among the above material grades is the fact that the higher oxygen content of the grade 5 enhances the strength while the lower oxygen content of the grade 23 results in superior ductility. The mixture of the above grades to control the average chemical composition could minimize the disposal of powder after recycling thus maximizing the usage of the powder and lowering the overall cost. Some may expect that such mixtures could bring inconsistency and heterogeneity in the localized oxygen distribution and thus the mechanical properties. This talk presents the experimental data to clarify the uncertainties introduced into the L-PBF processability by comparing a uniform chemical composition with a mixed one, both meeting the grade 23 nominal chemical composition, and comment on the final mechanical and physical properties of L-PBF printed components, particularly the resulting microstructure. Optical metallography and electron backscattered diffraction methods were employed to investigate the impact of oxygen distribution on the microstructure indicating the promotion of alpha phases in the printed microstructures in non-uniformly distributed oxygen samples. Also, the correlation between the microstructure and tensile properties have been discussed by an emphasis on the influence of promoted alpha phase formation.
Advances in Powder Production Technology Enable the Use and Progress of Additive Manufacturing Methods like Selective Laser Melting
L.P. Motibane, Council for Scientific and Industrial Research
South Africa
Nitinol (NiTi)’s shape memory and superelasticity as well as its biocompatibility make it suitable for biomedical applications. Porous NiTi has a lower elastic modulus than solid NiTi and behaves most like bone, making it most appropriate to bone implant purposes. Advances in powder production technology enable the use and progress of additive manufacturing methods like Selective Laser Melting (SLM). Selective Laser Melting produces complex, near net-shape and thin structures such as porous lattices that are difficult and almost impossible to manufacture using conventional methods. While the stage is well set to produce complex parts for biomedical applications using SLM, there is still a need for further understanding and development of optimal process parameters. The study seeks to find the optimal Selective Laser Melting processing parameters and their effect on the mechanical and shape memory response of porous NiTi lattice structures. Commercially available pre-alloyed titanium rich Ni49.7Ti50.3 powder was used to manufacture porous lattice structures using different Selective Laser Melting processing parameters. The lattice shape recovery and mechanical response to cyclic loading is evaluated. The variation in scanning speed and laser power is expected to influence the shape memory recovery. The effect of total energy density input will also be reported.
Production Routes for Specialized, Spherical, Biocompatible Ti-Based Powders
K. Ratschbacher, GfE Metalle und Materialien GmbH
Germany
There are few titanium alloys, certified for medical applications, but lots more under investigation. To make those commercially available, industrial scale production routes are required.
The production routes for spherical, biocompatible Ti-powders involve feedstock production for EIGA (electrode inert gas atomization). EIGA atomization parameters must be tailored to maximize the yield for the particle size distribution, required for the processing through additive manufacturing or MiM (metal injection moulding) [3]. Inert gas screening of the raw powder ensures flowability and controlled grain size distribution within the powder. The metallurgical EIGA-feedstock production of the alloys, Ti 13Nb 13 Zr and Ti 12Mo 6Zr 2Fe as well as a titanium containing, biocompatible HEA (high entropy alloy) from the TiZrTaNbHf system [4] are introduced. Depending on the alloying elements and the amount present in the alloy, the feedstock production routes include compacting the alloying elements into consumable electrodes, which are molten into an ingot via Vacuum Arc Melting (VAR) or Electron Beam Melting (EBM). To obtain homogeneous feedstock for the Electrode Induction Melting Inert Gas Atomization (EIGA), the ingot is remolten in an Induction Skull Melter (ISM) and gravity cast into a crucible. For materials with high melting points and big differences of vapour pressures between the alloying elements, a powder metallurgical feedstock production route is available. Different screening techniques through sieving, air-classification and sifting are available. The results of comparative screening trials are introduced.
Closing Remarks
L.P. Lefebvre, M. Brochu, V. Brailovski
Canada