Additive manufacturing of metallic components – Process, structure and properties

Oxygen, one of the most abundant elements on Earth, often forms an undesired interstitial impurity or ceramic phase (such as an oxide particle) in metallic materials. Even when it adds strength, oxygen doping renders metals brittle1–3. Here we show that oxygen can take the form of ordered oxygen complexes, a state in between oxide particles and frequently occurring random interstitials. Unlike traditional interstitial strengthening4,5, such ordered interstitial complexes lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs)6–10. The tensile strength is enhanced (by 48.5 ± 1.8 per cent) and ductility is substantially improved (by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA with 2.0 atomic per cent oxygen, thus breaking the long-standing strength–ductility trade-off11. The oxygen complexes are ordered nanoscale regions within the HEA characterized by (O, Zr, Ti)-rich atomic complexes whose formation is promoted by the existence of chemical short-range ordering among some of the substitutional matrix elements in the HEAs. Carbon has been reported to improve strength and ductility simultaneously in face-centred cubic HEAs12, by lowering the stacking fault energy and increasing the lattice friction stress. By contrast, the ordered interstitial complexes described here change the dislocation shear mode from planar slip to wavy slip, and promote double cross-slip and thus dislocation multiplication through the formation of Frank–Read sources (a mechanism explaining the generation of multiple dislocations) during deformation. This ordered interstitial complex-mediated strain-hardening mechanism should be particularly useful in Ti-, Zr- and Hf-containing alloys, in which interstitial elements are highly undesirable owing to their embrittlement effects, and in alloys where tuning the stacking fault energy and exploiting athermal transformations13 do not lead to property enhancement. These results provide insight into the role of interstitial solid solutions and associated ordering strengthening mechanisms in metallic materials. Ordered oxygen complexes in high-entropy alloys enhance both strength and ductility in these compositionally complex solid solutions.

Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes

Laser-based additive manufacturing (LBAM) processes can be utilized to generate functional parts (or prototypes) from the ground-up via layer-wise cladding – providing an opportunity to generate complex-shaped, functionally graded or custom-tailored parts that can be utilized for a variety of engineering applications. Directed Energy Deposition (DED), utilizes a concentrated heat source, which may be a laser or electron beam, with in situ delivery of powder- or wire-shaped material for subsequent melting to accomplish layer-by-layer part fabrication or single-to-multi layer cladding/repair. Direct Laser Deposition (DLD), a form of DED, has been investigated heavily in the last several years as it provides the potential to (i) rapidly prototype metallic parts, (ii) produce complex and customized parts, (iii) clad/repair precious metallic components and (iv) manufacture/repair in remote or logistically weak locations. DLD and Powder Bed Fusion-Laser (PBF-L) are two common LBAM processes for additive metal part fabrication and are currently demonstrating their ability to revolutionize the manufacturing industry; breaking barriers imposed via traditional, ‘subtractive’ metalworking processes. This article provides an overview of the major advancements, challenges and physical attributes related to DLD, and is one of two Parts focused specifically on DLD. Part I (this article) focuses on describing the thermal/fluidic phenomena during the powder-fed DLD process, while Part II focuses on the mechanical properties and microstructure of parts manufactured via DLD. In this current article, a selection of recent research efforts – including methodology, models and experimental results – will be provided in order to educate the reader of the thermal/fluidic processes that occur during DLD, as well as providing important background information relevant to DLD as a whole. The thermal/fluid phenomena inherent to DLD directly influence the solidification heat transfer which thus impacts the part's microstructure and associated thermo-mechanical properties. A thorough understanding of the thermal/fluid aspects inherent to DLD is vital for optimizing the DLD process and ensuring consistent, high-quality parts.

An overview of Direct Laser Deposition for additive manufacturing; Part I: Transport phenomena, modeling and diagnostics

The present study contains a critical review of work on the formation of precipitates and intermetallic phases in dilute precipitation hardening Al–Cu–Mg based alloys with and without Li additions. Although many suggestions for the existence of pre-precipitates in Al–Cu–Mg alloys with a Cu/Mg atomic ratio close to 1 have been made, a critical review reveals that evidence exists for only two truly distinct ones. The precipitation sequence is best represented as: supersaturated solid solution→co-clusters→GPB2/S"→S where clusters are predominantly Cu–Mg co-clusters (also termed GPB or GPB I zones), GPB2/S" is an orthorhombic phase that is coherent with the matrix (probable composition Al10Cu3Mg3) for which both the term GPB2 and S" have been used, and S phase is the equilibrium Al2CuMg phase. GPB2/S" can co-exist with S phase before the completion of the formation of S phase. It is further mostly accepted that the crystal structure of S' (Al2CuMg) is identical to the equilibrium S phase (Al2CuMg). Th...

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Precipitates and intermetallic phases in precipitation hardening Al–Cu–Mg–(Li) based alloys

The mechanical and microstructural properties of 316L stainless steel (SS) fabricated via Direct Laser Deposition (DLD), a laser-based additive manufacturing method, are presented and compared with those of conventionally-built counterparts. Using a Laser Engineered Net Shaping (LENS ® ) DLD system, the time interval between successive layer deposits, or inter-layer/idle time, for fabricating cylindrical specimens vertically-upward was varied by building either one or nine samples per build plate – thus increasing total assembly volume per build. Subsequently, the effect of thermal history, as well as heat treatment, on microstructural (i.e. grain size and morphology) and mechanical (i.e. tensile, compression, and microhardness) properties of DLD parts were investigated. Results indicate that the DLD 316L SS samples produced herein have a higher yield and ultimate tensile strength relative to their cast and wrought forms. Furthermore, the thermal history, microstructural evolution, and mechanical properties of DLD 316L SS are shown to be dependent on the time interval between deposits. Longer local time intervals result in higher cooling rates, leading to finer microstructures, higher/uniform strength and lower elongation to failure. In addition, porosity and less integral metallurgical bonds are found to be more prevalent in locations further upward from the build plate due to reduced laser penetration depths (e.g. previous-layer remelting decreases). Conversely, parts manufactured with shorter time intervals were found to possess a coarser microstructure, lower strength and higher elongation to failure – attributable to lower cooling rates caused by an increased bulk temperature in the part. These results may aid in future design and control of more efficient, constant-power DLD processes – especially with regard to building multiple and/or larger parts; an approach desirable for minimizing small-to-medium lot production times.

Effects of process time interval and heat treatment on the mechanical and microstructural properties of direct laser deposited 316L stainless steel

Two aluminium alloys with nominal compositions of Al-8Fe-4Ce and Al-8Fe-1V-2Si (all compositions in wt %) were rapidly solidified by ultrasonic gas atomization. The atomized powders with an average particle size (d50) of 30 μm were vacuum hot pressed and subsequently hot extruded. The P/M extrusion exhibited similar microstructure and elevated temperature tensile properties. The tensile and stress rupture samples of both the alloys exhibited ductile dimple failure. However, the Al-Fe-V-Si extrusion samples exhibited significantly better creep and stress rupture properties. The Al-Fe-Ce alloy was found to be more susceptible to cavitation at elevated temperatures which resulted in poor stress rupture properties.

Microstructure and mechanical properties of RSP/M Al-Fe-V-Si and Al-Fe-Ce alloys

A commercial Al–Si–Mg composite was diluted to different SiC (mean size: 8 µm) contents, and was foamed using TiH2 by melt based technique at a fixed foaming temperature. Foams with high expansions were achieved and uniform cell morphology is obtained at and below 10 vol% SiC. Despite good expansions, certain amount of foam collapse occurs for 5 vol% SiC composite. This work brings out that presence of SiC alone can lead to good foaming but these foams are unstable prior to solidification. This instability can be correlated to in situ oxide content. Both compression and impact (perforation) tests show improved properties with the increase in foam density. Compressive strength is not influenced by SiC content, and energy absorption too seems to be insensitive to the rate of loading within six orders of magnitude.

Effect of SiC particle content on foaming and mechanical properties of remelted and diluted A356/SiC composite

Al–Si–Mg/SiCP foams were prepared by melt processing route with TiH2 as the blowing agent The effects of foaming temperature (640 and 670 °C), SiCP size (12 and 21 μm), and volume % (5–20) on liquid foam expansion and subsequent collapse were studied Irrespective of the particle size, significant collapse was observed in the liquid foam at 670 °C At the lower foaming temperature of 640 °C, the liquid foam collapse was low to moderate, with finer and higher (15/20%) contents of SiCp resulting in low collapse and the best overall quality Preferential segregation of particles to gas/metal interface, postulated by earlier investigators, was not observed in the present case The presence of SiC within the cell walls induced brittleness, and created irrecoverable damage during the initial phase of compressive deformation The compressive strength of the foams was lower than theoretical predictions Minor defects such as porosity, present within the cell walls, might be responsible in lowering of the compressive strength

Foaming experiments on LM25 alloy reinforced with SiC particulates

Among cellular metals, aluminium foams are the most commonly produced, and provide a unique combination of properties such as: very low density, high energy absorption under static and dynamic compressions, blast amelioration, sound absorption, and flame resistance. Applications in automotive and defence sectors have been reported. Foams based on high melting point metals such as nickel and its alloys are also under active development throughout the world for applications requiring corrosion and oxidation resistance coupled with high temperature strength and relatively high thermal conductivity. Ceramic foams were developed elsewhere in the world primarily for biomedical applications, but are also suitable for defence applications for high temperature insulation. These cellular materials will provide new materials options to designers of aerospace, transport, and other defence systems.Defence Science Journal, 2011, 61(6), pp.567-575, DOI:http://dx.doi.org/10.14429/dsj.61.640

https://www.publications.drdo.gov.in/ojs/index.php/dsj/article/download/640/556

Cellular Metals and Ceramics for Defence Applications (Review Paper)

A Fe-Cr-alloyed silicide coating has been formed on a commercially available Nb-alloy called C-103 by using a slurry-based technique followed by vacuum diffusion treatment at 1380 °C. The coating microstructure reveals a three-layer structure with the outer layer consisting of NbSi2 phase and the inner interdiffusion layer consisting of lower silicides such as Nb5Si3 and Nb3Si. The intermediate layer of the coating is comprised of a Fe–Cr alloyed Nb-silicide phase and NbSi2. The coating provides good short-term protection to the substrate against high temperature oxidation in air at 1100 and 1300 °C. It is found that the presence of the coating increases the tensile strength of the alloy.

Amol A. Gokhale

Papers

Microstructure and mechanical properties of RSP/M Al-Fe-V-Si and Al-Fe-Ce alloys

Effect of SiC particle content on foaming and mechanical properties of remelted and diluted A356/SiC composite

Foaming experiments on LM25 alloy reinforced with SiC particulates

Cellular Metals and Ceramics for Defence Applications (Review Paper)

Microstructure, oxidation resistance and tensile properties of silicide coated Nb-alloy C-103