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Oxidation is a common issue in directed energy deposition (DED) based additive manufacturing (AM) processes. Severe oxidation is usually considered to be detrimental, which should be avoided. However, it is found that the surface oxidation of the deposited materials can also lower the laser reflectance, thereby increasing the utilization rate of laser energy and improving deposition efficiency. In this study, stainless steel 316 L (SS 316 L) was deposited using laser aided additive manufacturing (LAAM) process in the localized argon gas shielding environment and controlled argon environment, respectively, to investigate the evolution of oxidation on the surface and in the deposited material, as well as the effect of oxidation on the deposition efficiency and material properties. Results showed that severe surface oxidation of the materials deposited in the localized argon gas shielding environment effectively lowered the laser reflectance (14.5 %), which is much lower than that deposited in argon environment (71 %). Subsequently, with the same process parameters, the deposition efficiency in the localized argon gas shielding environment can be 1.57 times of that obtained in the controlled argon environment. Meanwhile, compared with the tensile properties of SS 316 L deposited in the controlled argon environment (YS 560 MPa, UTS 717 MPa, elongation 64 %), the tensile properties of SS 316 L deposited in the localized argon gas shielding environment (YS 456 MPa, UTS 631 MPa and elongation 57 %) are lower，whereas still show a good combination of strength and ductility.

Current heterostructured materials have significant potential for improving the strength-ductility trade-off, but the interfacial bonding capabilities are limited. In this research, composites with TiN/Ti6Al4V interlayer structure were in-situ synthesized by atmospheric remelting using laser directed energy deposition (LDED). The elemental diffusion, microstructure, and mechanical properties of TiN/Ti6Al4V interlayer structure composites were investigated. The mechanical properties of the composites were improved after nitrogen remelting due to the strengthening of internal TiN ceramic particles. The microhardness of TiN/Ti6Al4V composites increased by up to 209.8% to 1141 HV0.2, ultimate tensile strength by 6% and elongation by 34.5% to 1071.4 MPa and 15.6%, respectively. This research demonstrates that a flexible combination of the LDED process and reaction atmosphere can synthesize new periodic layered ceramic/metal composites with excellent mechanical properties. This research offers the groundwork for the fabrication of in-situ strengthening continuous gradient materials.

•Steel/copper bimetallic structure with good metallurgical bonding using LPBF.•Obtains interfacial heterostructures in steel/copper bimetallic structures.•The heterostructures generate nano hardness imbalance to induce cutting defects.•Stress concentration makes the cutting force suddenly drop at the interface.

Additive manufacturing enables processing of functionally graded materials (FGMs) with flexible spatial design and high bonding strength. A steel-copper FGM with high interfacial strength was developed using laser powder bed fusion (LPBF). The effect of laser process parameters on interfacial defects was evaluated by X-ray tomography, which indicates a low porosity level of 0.042 % therein. Gradient/fine dendritic grains in the interface are incited by high cooling rates, which facilitates interface strengthening. Multiple mechanical tests evaluate the bonding reliability of interface; and the fatigue tests further substantiate the ultrahigh bonding strength in FGMs, which is superior to traditional manufacturing methods. Mechanisms of the high interfacial bond strength were also discussed. [ABSTRACT FROM AUTHOR]

ABSTRACTA two-step double-side friction stir welding (DS-FSW) technique was proposed to join 20 mm thick Mg–Y–Nd magnesium alloy plates. FSW was first carried out on one side of the butt joint, and then the plates were left-right reversed for the other side welding. The DS-FSW joint was free of defects and the weld nugget zone (NZ) showed fine equiaxial grains. An overlap zone with ultrafine grain (∼2.19 µm) and dispersive particles existed between the two welding tracks, beneficial to the integrity of the joint. The ultimate tensile strength (σUTS), yield strength (σ0.2), and elongation rate (δ) of the joint were 277. 6, 204.1 MPa, and 7.27%. A high joint efficiency (σUTSratio between the joint and parent material) was achieved at ∼82.6%.

A two-step double-side friction stir welding (DS-FSW) technique was proposed to join 20?mm thick Mg–Y–Nd magnesium alloy plates. FSW was first carried out on one side of the butt joint, and then the plates were left-right reversed for the other side welding. The DS-FSW joint was free of defects and the weld nugget zone (NZ) showed fine equiaxial grains. An overlap zone with ultrafine grain (~2.19?µm) and dispersive particles existed between the two welding tracks, beneficial to the integrity of the joint. The ultimate tensile strength (sUTS), yield strength (s0.2), and elongation rate (d) of the joint were 277. 6, 204.1?MPa, and 7.27%. A high joint efficiency (sUTSratio between the joint and parent material) was achieved at ~82.6%.

The demand for micro-joining of aluminium (Al) alloy thin sheets has increased for electronics and energy industry. Micro laser welding is preferred over other conventional joining techniques, such as resistance spot welding and soldering due to the advantages of high precision, design flexibility and throughput. Nonetheless, this process could be associated with lack of fusion, formation of weld defects, serious weld distortion and poor weld strength. Laser welding of Al alloy has been known to form solidification cracks. This study aims to investigate the role of process parameters on the crack formation of Al alloy. A pulse wave Nd:YAG laser source was used for laser spot welding of 250 μm thick Al alloy sheets. Process parameters namely pulse duration and peak power were varied and the weld quality obtained was investigated. Weld quality in terms of weld geometry, weld microstructure, formation of weld defect, shear strength of welds, fracture mechanism of welds and micro-segregation in welds were systematically investigated. Results obtained were indicative that formation of crack is strongly determined by the level of pulse energy applied in welding and the degree of micro-segregation developing within welds. Fracture analysis of the welds reveals that the penetration depth of welds as well as the direction of pull force applied during the shear strength tests affects the fracture mechanism. In summary, defect-free welds were achieved by micro laser welding process in this study.

Support structures are critical to the successful printing of the overhang structures in selective laser melting. The heat transfer performance of support structures has significant influence on the temperature distribution and cooling rate within the overhang structures which in turn determine the microstructure and residual stress. In the present study, functionally graded support structures have been proposed and their thermal performance has been numerically investigated, with the consideration of different materials, cooling times and gradedness values. It has been found that functionally graded support structures can maintain a higher temperature level than the conventional uniform support structure at the bottom of overhang, which is equivalent to an extra pre-heating effect. The temperature fluctuation and cooling rate at the bottom of overhang can also be reduced by adopting the functionally graded support structures.

According to the increasing needs of three-dimensional printing technologies to satisfy high-level requirements, customization, and complicity, the quality of three-dimensional printed part becomes an important issue due to the layer-wise nature of additive manufacturing process. The objective of this study is to propose a methodology to identify the quality of three-dimensional printed parts with circular holes in the laser aided additive manufacturing process. We utilize a response surface methodology to represent the relationship between input variables (chord height tolerance and diameter of a hole) and response (geometric error) for evaluating the geometric accuracy of the three-dimensional printed parts with the diameter of holes. From the calculated response surface methodology, we conclude that the proposed methodology can be utilized as a process design guide to guarantee the quality of a part printed from the laser aided additive manufacturing process. The efficiency and limitations of the proposed methodology are verified by conducting a case study.

Additive manufacturing (AM) techniques are ideal for producing customized products due to their high design flexibility. Despite the previous studies on specific additive manufactured customized products such as biomedical implants and prostheses, the simultaneous optimization of components, materials, AM processes, and dimensions remains a challenge. Multidisciplinary design optimization (MDO) is a research area of solving complex design problems involving multiple disciplines which usually interact with each other. The objective of this research is to formulate and solve an MDO problem in the development of additive manufactured products customized for various customers in different market segments. Three disciplines, i.e. the customer preference modeling, AM production costing, and structural mechanics are incorporated in the MDO problem. The optimal selections of components, materials, AM processes, and dimensional parameters are searched with the objectives to maximize the functionality utility, match individual customers’ personal performance requirements, and minimize the total cost. A multi-objective genetic algorithm with the proposed chromosome encoding pattern is applied to solve the MDO problem. A case study of designing customized trans-tibial prostheses with additive manufactured components is presented to illustrate the proposed MDO method. Clusters of multi-dimensional Pareto-optimal design solutions are obtained from the MDO, showing trade-offs among the objectives. Appropriate design decision can be chosen from the clusters based on the manufacturer׳s market strategy.

With the capability of fabricating parts directly from digital models without tooling, additive manufacturing (AM) technologies have great potentials for customised products with complex shapes and superior performances. This paper develops commonality and performance metrics that can be used to evaluate the design of product families implemented with additive manufactured modules. The complex commonality index (CCI) is proposed to measure parametric, modular, and process sharing within a product family. Design and production costs of additive manufactured modules are incorporated in the formulation of the CCI. The market share (MS) is formulated to measure a product family's performance based on customer-perceived utilities. A product family design optimisation problem with the CCI and MS as objective functions is proposed. A case study on an R/C racing car family design demonstrates the proposed methodologies, and the result provides designers with Pareto-optimal solutions for additive manufactured module selection and design parameter identification. [Received 18 April 2016; Revised 2 September 2016; Accepted 9 November 2016]

It is well-known that grain refiners can tailor the microstructure and enhance the mechanical properties of titanium alloys fabricated by additive manufacturing (AM). However, the intrinsic mechanisms of Ni addition on AM-built Ti–6Al–4V alloy is not well established. This limits its industrial applications. This work systematically investigated the influence of Ni additive on Ti–6Al–4V alloy fabricated by laser aided additive manufacturing (LAAM). The results showed that Ni addition yields three key effects on the microstructural evolution of LAAM-built Ti–6Al–4V alloy. (a) Ni additive remarkably refines the prior-βgrains, which is due to the widened solidification range. As the Ni addition increased from 0 to 2.5 wt. %, the major-axis length and aspect ratio of the prior-βgrains reduced from over 1500 μm and 7 to 97.7 μm and 1.46, respectively. (b) Ni additive can discernibly induce the formation of globular αphase, which is attributed to the enhanced concentration gradient between the βand αphases. This is the driving force of globularization according to the termination mass transfer theory. The aspect ratio of the αlaths decreased from 4.14 to 2.79 as the Ni addition increased from 0 to 2.5 wt. %. (c) Ni as a well-known β-stabilizer and it can remarkably increase the volume fraction of βphase. Room-temperature tensile results demonstrated an increase in mechanical strength and an almost linearly decreasing elongation with increasing Ni addition. A modified mathematical model was used to quantitatively analyze the strengthening mechanism. It was evident from the results that the αlath phase and the solid solutes contribute the most to the overall yield strength of the LAAM-built Ti–6Al–4V–xNi alloys in this work. Furthermore, the decrease in elongation with increasing Ni addition is due to the deterioration in deformability of the βphase caused by a large amount of solid-solution Ni atoms. These findings can accelerate the development of additively manufactured titanium alloys.

Globular α phases can significantly improve the ductility of titanium alloys. Cyclic heat treatment (CHT) has been proved to be an effective way to induce the formation of globular α phases in α+β titanium alloy Ti–6Al–4V fabricated by laser aided additive manufacturing (LAAM). However, there is no prior research reporting methods for obtaining globular α phases in LAAM-built near-α titanium alloys. This work investigated the cyclic heat treatment (CHT) procedures suitable for the LAAM-built near-α titanium alloy Ti–6Al–2Sn–4Zr–2Mo (Ti6242) to attain the globular α phases. The results show that 980 ​°C is the most suitable upper temperature limit for CHT. However, it is difficult to achieve a high volume fraction of the globular α phases in the LAAM-built Ti6242 alloys through CHT, which is ascribed to the low composition gradient caused by more α-stabilizing elements and fewer β-stabilizing elements. The as-built sample demonstrated elongation of 6.3%, which is lower than the AMS 4919J standard (elongation ≥10%). After 980 ​°C CHT and 980 ​°C CHT with solution heat-treatment, the formation of the globular α phases significantly increased the elongation to 13.5% and 12.9%, respectively. Although the mechanical strength is reduced after heat-treatment, the room-temperature tensile properties still exceed the AMS 4919J standard. Fractography examination showed that the as-built sample exhibited a mixed brittle and ductile fracture behavior, while the 980 ​°C CHT and 980 ​°C CHT with solution heat-treated samples displayed ductile fracture.

•High-mass-proportion TiCp/Ti6Al4V composites with fully dense prepared by directed energy deposition.•The changes in microstructure and orientation relationship was discussed in detail.•Some primary TiC and α-Ti existed a semi-coherent interface and a key relationship.•Hardness, wear resistance, and thermal conductivity increased, while tensile performance decreased with increasing TiCp content.•Knowledge of microstructure-property relationships was studied in detail.

The purpose of this paper is to investigate the effects of TiC nanoparticle content on microstructures and tensile properties of the IN718/TiC nanocomposites fabricated by selective laser melting (SLM). 0.5wt%, 1.0wt%, and 2.0wt% of TiC nanoparticles are added to the IN718 powders. The bulk-form IN718/TiC nanocomposites with different TiC contents are fabricated in-situ by SLM using the same process settings. The evolution of microstructures and tensile properties as the effect of changing the TiC content is studied using the optical microscopy, scanning electron microscopy, X-ray diffraction analysis, and tensile testing. The increase of TiC content refines the microstructure, promotes the formation of the cellular morphology, and reduces the size and continuity of Laves precipitates. Increasing the TiC content improves the yield strength and ultimate tensile strength but decreases the ductility. The grain refinement, dislocation bowing, dislocation punching, and the reduction in Laves precipitate contribute to the strengthening effect in the IN718/TiC nanocomposites.