Additive Manufacturing Route Research Articles

Modeling of binary alloy solidification under conditions representative of Additive Manufacturing

Understanding the interplay among process parameters, microstructure, and presence of defects is critical to optimizing Additive Manufacturing (AM) routes for producing specialty alloy parts with unique geometry, properties, and compositional features. However, since it is not feasible to obtain experimental results for every possible set of AM process, process conditions, and alloy composition, computational modeling is becoming an increasingly important tool for understanding key physical processes, with the hope of developing a framework for AM materials design. One such physical process of interest is the dendritic solidification within grains; we implement a Cellular Automata (CA) model for coupled solute transport and growth of cellular and dendritic β-Ti alloy colonies under thermal conditions typically encountered in AM processes. Quantitative agreement with analytical models of the undercooling-solidification velocity relationship was achieved, as well as qualitative agreement with trends in primary dendrite arm spacing (PDAS), secondary arm development, and compositional profile with changes in solidification conditions. The roles of solute diffusivity, interfacial energy, and alloying addition are considered as well. Under rapid solidification conditions, extension to include local non-equilibrium for solute allowed for the modeling of solute trapping along dendrite stems as well as qualitative representation of the dendritic to banded morphology transition. To quantitatively reproduce non-equilibrium solidification at the dendritic colony scale and more accurately estimate rapid solidification microstructures, use of multiple grids or more complex CA rules would be in order.

4D printing of net shape parts made from Ni-Mn-Ga magnetic shape-memory alloys

This work investigates an additive manufacturing route of producing functional net shaped parts from pre-alloyed magnetic shape-memory Ni-Mn-Ga powders. Three types of Ni-Mn-Ga powders were used in this investigation: spark eroded in liquid nitrogen (LN2), spark eroded in liquid argon (LAr), and ball milled (BM). Additive manufacturing via powder bed binder jetting, also known as 3D printing (3DP), was used in this research due to both relatively easy control of part porosity and the possibility to obtain complex shaped parts from Ni-Mn-Ga alloys. The four-dimension (4D) is created by the predictable change in 3D printed part configuration over time as the result of shape-memory functionality. Binder jetting of Ni-Mn-Ga powders followed by curing and sintering proved successful in producing net shaped porous structures (spring-like, 3-D hierarchical lattice structures, etc.) with good mechanical strength. Parts with porosities between 24.08% and 73.43% have been obtained by using powders with distinct morphologies. The printed Ni-Mn-Ga parts undergo reversible martensitic transformation during heating and cooling, which is a prerequisite for the shape-memory behavior. Thermo-magneto-mechanical trained 3D printed parts obtained from ball milled Ni-Mn-Ga powders showed reversible magnetic-field-induced strains (MFISs) of up to 0.01%. The additive manufacturing is a viable technology in solving the design issues of functional parts made of Ni-Mn-Ga magnetic shape-memory alloys (MSMA).

Comparison Study on Additive Manufacturing (AM) and Powder Metallurgy (PM) AlSi10Mg Alloys

The microstructural and mechanical properties of AlSi10Mg alloys fabricated by additive manufacturing (AM) and powder metallurgy (PM) routes were investigated and compared. The microstructures were examined by scanning electron microscopy assisted with electron-dispersive spectroscopy. The crystalline features were studied by x-ray diffraction and electron backscatter diffraction. Room-temperature tensile tests and Vickers hardness measurements were performed to characterize the mechanical properties. It was found that the AM alloy had coarser Al grains but much finer Si precipitates compared with the PM alloy. Consequently, the AM alloy showed more than 100% increment in strength and hardness compared with the PM alloy due to the presence of ultrafine forms of Si, while exhibiting moderate ductility.

Polymers Selection for Harsh Environments to Be Processed Using Additive Manufacturing Techniques

In this paper, a methodology for selecting a combination of polymers and their associated additive manufacturing (AM) routes is presented. The binomial consisting of materials performance for the intended application and the most suitable AM process and its production strategy is solved by the application of a multicriteria approach along with a stringency level methodology. The case study has been the analysis of physical and radiation tolerance features of thermoplastic nature candidates for the additive manufacture of mechanical, electromechanical, and electrical components for harsh environments in a nuclear power plant. The obtained results allow select an AM route along with a production strategy based on large batches or small batches. Using a selective laser sintering additive manufacturing route, PP + EPDM can be a good option to manufacture mechanical heavily stressed components, whereas PA can be a versatile material to manufacture friction components or films and sheets for electrical applications. In addition, PE would be a good option for high voltage insulation. Finally, PS would be used in radio frequency and microwave applications. On the other hand, fused deposition modeling techniques are more suitable for several materials, such as PC for mechanical applications, PE for electromechanical applications, and IR for electrical/electronic applications.

Fabrication of Organic Photo Detectors Using Inkjet Technology and Its Comparison to Conventional Deposition Processes

The current upscaling of large area electronics is pushing the technological and cost limits of conventional processing and materials. Introduction of new materials, sophisticated device architectures and advanced deposition techniques has made organic photo detector (OPD) attractive for many applications. Spin coated Titanium Oxide (TiOx) has been successfully established as an electron transport layer (ETL) in many optoelectronic devices, including OPDs. In this paper, on one hand OPDs are fabricated with TiOx as ETL deposited by sputtering and electron beam techniques, where the dark current density is reduced by two orders of magnitude up to 10−7 A/cm2 while retaining high photo responsivity. On the other hand, inkjet printing is demonstrated as an alternative additive and digital manufacturing route for the vacuum-free deposition of TiOx and active layer composition of poly(3-hexylthiophene) and [6,6]-phenyl-C60-butyric acid methyl ester (P3HT:PC60BM). OPDs fabricated with inkjet printed TiOx layers showed comparable performance as the state of the art devices fabricated by spin coated TiOx. In addition, OPDs were manufactured using inkjet technology for both the active and ETL layers. Our research proves that there is a significant influence of the deposition technique to the corresponding device performance. The results obtained in terms of figure of merit for OPDs (dark current density of $2.7\times 10^{-5}$ A/cm2, signal to noise ratio of 320, external quantum efficiency of 45% and fabrication yield of 30%–60%) from the developed inkjet printing process shows that further investigation on the ink formulation and optimizations can result into better performing OPDs.

Mechanical and phase transformation behaviour of biomedical Co-Cr-Mo alloy fabricated by direct metal laser sintering

Biomedical cobalt-chromium-molybdenum (Co-Cr-Mo) alloys have been used in the cast or wrought form as prosthetic implant materials for more than five decades. Recently, additive manufacturing techniques are employed to produce parts directly from metal precursor powder with a composition suitable for biomedical applications. In the work present, we examine both mechanical behaviour and γ→ε phase transformation in the heat treated Co-Cr-Mo alloy fabricated via additive manufacturing route. Results suggest, that selection of appropriate heat treatment and control of martensitic transformation can significantly influence mechanical properties. The studied Co-Cr-Mo alloy can be considered as a potential material for biomedical applications.

A multiscale comparison of stochastic open-cell aluminum foam produced via conventional and additive-manufacturing routes

With the exciting potential of additive manufacturing of metals to produce geometrically complex structures come many unknowns and uncertainties regarding the process-microstructure-property relationships of the additively manufactured (AM) parts, especially in comparison to their conventionally manufactured counterparts. This work attempts to elucidate some key differences between AM and cast parts by conducting a multiscale comparison of samples that are intended to be identical, except for the route by which each was manufactured. The samples of interest are open-cell foams of an Al-Mg-Si alloy (Al 6061). The baseline open-cell foam is conventionally produced via investment casting. Copies are produced using laser powder bed fusion. Full-field deformation is characterized under compressive loading using in-situ X-ray computed tomography. The foams are compared in terms of global load versus displacement response, local failure mechanisms, and characteristics of the grain structure.

Informing additive manufacturing technology adoption: total cost and the impact of capacity utilisation

Informing Additive Manufacturing (AM) technology adoption decisions, this paper investigates the relationship between build volume capacity utilisation and efficient technology operation in an inter-process comparison of the costs of manufacturing a complex component used in the packaging industry. Confronting the reported costs of a conventional machining and welding pathway with an estimator of the costs incurred through an AM route utilising Direct Metal Laser Sintering (DMLS), we weave together four aspects: optimised capacity utilisation, ancillary process steps, the effect of build failure and design adaptation. Recognising that AM users can fill unused machine capacity with other, potentially unrelated, geometries, we posit a characteristic of ‘fungible’ build capacity. This aspect is integrated in the cost estimation framework through computational build volume packing, drawing on a basket of sample geometries. We show that the unit cost in mixed builds at full capacity is lower than in builds limited to a single type of geometry; in our study, this results in a mean unit cost overstatement of 157%. The estimated manufacturing cost savings from AM adoption range from 36 to 46%. Additionally, we indicate that operating cost savings resulting from design adaptation are likely to far outweigh the manufacturing cost advantage.

Impact response of additively manufactured metallic hybrid lattice materials

Additive manufacturing (AM) enables the design of new cellular materials for blast and impact mitigation by allowing novel material-geometry combinations to be realised and examined at a laboratory scale. However, design of these materials requires an understanding of the relationship between the AM process and material properties at different length scales: from the microstructure to geometric feature rendition to overall dynamic performance. To date, there remain significant uncertainties about both the potential benefits and pitfalls of using AM to design and optimise cellular materials for dynamic energy absorbing applications. This experimental investigation focuses on the out-of-plane compression of stainless steel cellular materials fabricated using selective laser melting (SLM), and makes two specific contributions. First, we demonstrate how the AM process itself influences the characteristics of these cellular materials across a range of length scales, and, crucially, how this influences the dynamic deformation. Secondly, we demonstrate how an AM route can be used to add geometric complexity to the cell structure, creating a versatile basis for future geometry optimisation. Starting with an AM square honeycomb (the reference case), we add porosity to the walls by replacing them with a lattice truss, while maintaining the same relative density. This geometry hybridisation is an approach uniquely suited to this manufacturing route. It is found that the hybrid lattice-walled honeycomb geometry significantly outperforms previously reported AM lattices in terms of specific strength, specific energy absorption, and energy absorption efficiency. It is also found that the hybrid geometry outperforms the benchmark metallic square honeycomb in terms of energy absorption efficiency in the intermediate impact velocity regime (i.e. between quasi-static loading and loading rates at which wave propagation effects begin to become pronounced), a regime in which the collapse is dominated by dynamic buckling effects.

TiAl alloys in commercial aircraft engines

The present article will describe aspects of the science and technology of titanium aluminide (TiAl) alloy system and summarise the low and high temperature mechanical and environmental properties exhibited by different alloy generations. In terms of processing developments, conventional gravity casting and near net shape casting would be discussed in detail. Also newer and non-conventional forging and additive manufacturing routes would be briefly highlighted. Extensive investigations of TiAl alloys have enabled their commercial implementation in aerospace and automotive industries. The GEnx™ engine is the first commercial aircraft engine that used TiAl (alloy 48–2–2) for their low pressure turbine blades. Among non GE engines, recently, new β-stabilised TiAl alloy (TNM) is being used to manufacture LPT blades for PW1100G™ engines. TiAl materials and design processes can reduce engine weight and improve engine performance.

Fatigue life of additively manufactured Ti–6Al–4V in the very high cycle fatigue regime

The present study reports on the impact of two different additive manufacturing routes, i.e. selective electron beam melting (EBM) and selective laser melting (SLM) on the fatigue life of the titanium alloy Ti–6Al–4V in the high cycle fatigue (HCF) and in the very high cycle fatigue (VHCF) regime. Cylindrical fatigue specimens were manufactured by EBM and SLM and tested in differently post-treated conditions. The EBM specimens were tested in the as-built condition, SLM manufactured specimens were heat treated at 800°C and hot isostatically pressed, respectively. The type, size and location of every crack initiating discontinuity was determined and thoroughly investigated by scanning electron microscopy. Three main findings were derived from the present investigation. Fatigue properties of as-built EBM and stress relieved SLM specimens in the HCF and VHCF regimes are very similar. Defect types and defect sizes of similar nature were observed in both conditions. In consequence, stress relieving heat treatment has not to be conducted in EBM processed Ti–6Al–4V. The shape of the discontinuities, however, currently is not considered by traditional approaches for estimation of fatigue strength. The fatigue performance of the SLM processed and subsequently hot isostatically pressed condition is superior to its two additively manufactured counterparts and similar to conventionally processed material. Duplex S–N behavior with a clear transition area was observed in case of the Ti–6Al–4V specimens tested.

On the mechanical and electrical properties of copper-silver and copper-silver-zirconium alloys deposits manufactured by cold spray

In this work, several copper alloy deposits were manufactured by cold spray with helium as accelerating and carrier gas. Electrical conductivity was measured to establish the potential of cold spray as a manufacturing process for high strength (>500MPa) and high conductivity (>90% IACS) copper alloys. The deposits which are characterized by a low oxygen content (<200ppm) and a low porosity level (<0.1%) present yield strength values up to about 700MPa and electrical conductivity values up to 58.2 MS/m (100.3% IACS). Results show that, even if a compromise has to be made between the properties according to the objectives of the application, this additive manufacturing route appears suitable for the production of large copper alloys parts with high mechanical properties and high electrical and thermal conductivity. The role of alloy composition and post heat treatments on the strength and conductivity of the deposits was especially considered in this work. Cold spray deposits properties were finally compared with those obtained with other manufacturing routes.

Additive manufacturing in prosthesis development – a case study

Purpose – The purpose of this paper was development of patient-specific femoral prosthesis using rapid prototyping (RP), a part of additive manufacturing (AM) technology, and comparison of its merits or demerits over CNC machining route. Design/methodology/approach – The customized femoral prosthesis was developed through computed tomography (CT)-3D CAD-RP-rapid tooling (RT)-investment casting (IC) route using a stereolithography apparatus (SLA-250) RP machine. A similar prosthesis was also developed through conventional CT-CAD-CAM-CNC, using RP models to check the fit before machining. The dimensional accuracy, surface finish, cost and time involvement were compared between these two routes. Findings – In both the routes, RP had an important role in checking the fit. Through the conventional machining route, higher-dimensional accuracies and surface finish were achieved. On the contrary, RP route involved lesser time and cost, with rougher surface finish on the prosthesis surface and less internal shrinkage porosity. The rougher surface finish of the prosthesis is favourable for bone ingrowths after implantation and porosity reduce the effective stiffness of the prosthesis, leading to reduced stress shielding effect after implantation. Research limitations/implications – As there is no AM machine for direct fabrication of metallic component like laser engineered net shaping and electron beam melting in our Institute, the metallic prosthesis was developed through RP-RT-IC route using the SLA-250 machine. Practical implications – The patient-specific prosthesis always provides better fit and favourable stress distribution, leading to longer life of the prosthesis. The described RP route can be followed to develop the customized prosthesis at lower price within the shortest time. Originality/value – The described methodology of customized prosthesis development through the AM route and its advantages are applicable for development of any metallic prostheses.

Microstructure and Tensile Strength of Rapid Manufacturing Parts

The purpose of this work is to investigate the microstructure and tensile strength of Ti6Al4V pre-alloyed powders produced by a direct metal laser sintering technique. Traditionally, Ti6Al4V products for biomedical applications were produced through hot working or machining of wrought semi-finished products. A change in the production route for manufacturing Ti6Al4V products, from the more traditional methods to an additive manufacturing route, requires an investigation of microstructure and mechanical properties because these are strongly influenced by the production route. The microstructure obtained through rapid solidification during laser sintering shows a very fine α+β lamellar morphology. There is also evidence of martensite which was expected due to high solidification rate of the liquid pool from a temperature above the β-transus during the laser sintering process. Structurally, good mechanical properties which are comparable to the bulk material were obtained.

Additive layer manufacture of Inconel 625 metal matrix composites, reinforcement material evaluation

While some high performing nickel based superalloys are difficult to process through additive layer manufacturing routes, it is considered that more dilute nickel superalloy (in this case Inconel 625) based metal matrix composite may improve high temperature properties, while retaining good weldability for use in laser melting deposition technologies. In this paper, the effect of 5wt% additions of 3 differing ceramic reinforcement particles to an Inconel 625 matrix was observed when processed by high powered laser melting, and compared to the processing characteristics of the pure matrix alloy. It was found that while additions of silicon carbide increased material hardness by 130%, levels of porosity and cracking also increased. The addition of aluminium oxide had no appreciable effect upon material hardness, while also compromising the integrity of the material. However additions of titanium carbide resulted in samples of similar density and consistency as the matrix alloy but with 30% increased hardness. However comparison of the width of single laser melted tracks of both Inconel 625 and Inconel 625 with titanium carbide composite indicated a need to further characterise the process window for this proposed MMC system.