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This study provides a quantitative way to tailor the grain structure in laser powder bed fusion (LPBF). Square-bottomed columnar grains (SCGs) were developed with a certain width roughly equal to the hatch spacing. The development of SCGs relied on different distinguishable regions, which were identified based on the differences in microstructural features between the melt-pool side and centreline. High lattice rotation accumulated at the melt-pool centreline, leading to grain boundaries forming at the centreline regions. The ultrasonic attenuation measurements and microhardness tests further validated the controllable properties. The findings indicated a novel approach to customise the material property. Agency for Science, Technology and Research (A*STAR) Published version This work was supported by A∗STAR Science and Engineering Research Council [Grant Number A20F9a0045]; Shun Hing Institute of Advanced Engineering, The Chinese University of Hong Kong [Grant Number RNE-p2-21].

As additive techniques such as laser powder bed fusion find increasing adoption industry, the ability to adapt these processes to industrially relevant materials is paramount. This adaptation can represent a significant challenge when working with wrought alloy feedstocks, which often result in brittle or porous parts lacking the mechanical properties of their conventionally wrought counterparts. One such alloy, aluminium 6061, is a highly used alloy in the aerospace, automotive, and semiconductor manufacturing industries. The conventionally manufactured components can have complex morphologies and may be assemblies of multiple individual components. As such, the ability to use an additive approach, and produce these as single parts can lead to significant benefits.In this work, we examine laser powder bed fusion of aluminium alloy 6061. The effects of process parameters such as laser power, beam scan speed, hatching distance, spot size was examined with a view towards developing an optimised process for this traditionally wrought alloy. Parts were examined for porosity and microstructure, with an aim to develop greater than 95% relative densities. To aid in process optimisation, in-situ pyrometry was deployed to understand the effects of the process parameters and develop a robust and repeatable process for producing 6061 components.

This paper investigates the effects on the material microstructure of varying the Inter-Layer Cooling Time (ILCT) during the printing process in laser powder bed fusion (L-PBF) multi-laser machines. Despite these machines allowing higher productivity rates compared to single laser machines, they are affected by lower ILCT values, which could be critical for material printability and microstructure. The ILCT values depend both on the process parameter sets and design choices for the parts and play an important role in the Design for Additive Manufacturing approach in L-PBF process. In order to identify the critical range of ILCT for this working condition, an experimental campaign is presented on the nickel-based superalloy Inconel 718, which is widely used for the printing of turbomachinery components. The effect of ILCT on the microstructure of the material is evaluated in terms of porosity and melt pool analysis on printed cylinder specimens, considering ILCT decreasing and increasing in the range of 22 to 2 s. The experimental campaign shows that an ILCT of less than 6 s introduces criticality in the material microstructure. In particular, at an ILCT value of 2 s, widespread keyhole porosity (close to 1‰) and critical and deeper melt pool (about 200 microns depth) are measured. This variation in melt pool shape indicates a change in the powder melting regime and, consequently, modifications of the printability window promoting the expansion of the keyhole region. In addition, specimens with geometry obstructing the heat flow have been studied using the critical ILCT value (2 s) to evaluate the effect of the surface-to-volume ratio. The results show an enhancement of the porosity value (about 3‰), while this effect is limited for the depth of the melt pool.

The demand for high-performance aluminum components drives research into the design of novel alloys that can be processed by laser-based additive manufacturing. In recent years, the addition of grain refiners proved to be an effective strategy to reduce the hot-cracking of high-strength Al alloys. In this study, the solidification and aging behavior of an Al2139 alloy doped with additions of Zr and Ti for L-PBF was investigated. These elements favored the formation of a fine-grained structure free of cracks. The formation of Al3(Zr,Ti) inoculants was predicted by Scheil simulations and observed as cuboidal particles in the center of α-Al grains. The microstructure of the as-built material featured fine and fully equiaxed grains, which appeared comparatively finer at the edge (300–600 nm) and coarser (0.8–2.0 μm) at the center of the molten pools. In both cases, there was evidence of Cu and Mg micro-segregations at the grain boundaries. The microhardness of 109.7 HV0.5 in the as-built state was increased to 186.1 HV0.5 after optimized T4 heat treatment, responsible for the precipitation of many rod-shaped Zr- and Ti-based second phases and quasi-spherical Cu-, Mn-, and Fe-rich particles. Prolonged exposure carried out to simulate high-temperature service caused a drop in microhardness and marked modification of the microstructure, evidenced by the rearrangement and subsequent spheroidization of Cu- and Mg-rich particles at the grain boundaries.

Additive manufacturing is rapidly evolving and revolutionizing the fabrication of complex metal components with tunable properties. Machine learning and neural networks have emerged as powerful tools for process–property optimization in additive manufacturing. These techniques work well for the prediction of a single property but their applicability in optimizing multiple properties is limited. In the present work, an exclusive neural network is developed to demonstrate the potential of a single neural network in optimizing multiple part properties. The model is used to identify the optimal process parameter values for laser power, scan speed, and hatch spacing for the required surface roughness, relative density, microhardness, and dimensional accuracy in stainless steel parts. In-house-generated experimental data are used to train the model. The model has seven neurons in the hidden layer, which are selected using hyperparameter optimization. K-fold cross-validation is performed to ensure the robustness of the model, which results in a mean squared error of 0.0578 and R2 score of 0.59. The developed model is then used to predict the optimal process parameters corresponding to the user-required part properties. The model serves as a significant pre-processing step to identify the best parameters before printing, thus saving time and costs for repeated part fabrication. The study provides more insights into the usage of a single artificial neural network for the optimization of multiple properties of printed metal parts.

Metal additive manufacturing technologies have great potential for future use in load-bearing aerospace applications, requiring a deeper understanding of mechanical performance and influencing factors. The objective of this study was to investigate the influence of contour scan variation on surface quality, tensile and fatigue strength for laser powder bed fusion samples made of AlSi7Mg0.6 material and to create high-quality as-built surfaces. The samples were produced with identical bulk and different contour scan parameters to accommodate the investigation of the impact of as-built surface texture on mechanical properties. The bulk quality was evaluated by density measurements according to Archimedes’ principle and tensile testing. The surfaces were investigated using the optical fringe projection method, and surface quality was assessed by the areal surface texture parameters Sa (arithmetic mean height) and Sk (core height, derived from material ratio curve). Fatigue life was tested at different load levels, and the endurance limit was estimated based on a logarithmic-linear relation between number of cycles and stress. All samples were found to have a relative density of more than 99%. Surface conditions distinctive in Sa and Sk were successfully created. The resulting mean values of the ultimate tensile strength σult are between 375 and 405 MPa for 7 different surface conditions. It was confirmed that the influence of contour scan variation on bulk quality is insignificant for the assessed samples. Regarding fatigue, one as-built condition was found to perform as well as surface post-processed parts and better than the as-cast material (compared to literature values). The fatigue strength at the endurance limit for 106 cycles is between 45 and 84 MPa for the three considered surface conditions.

The introduction of Sc/Zr inoculates to aluminium (Al) alloys for laser powder bed fusion (LPBF) provides numerous benefits, including laser processability improvement, solidification microstructure control and mechanical property enhancement. Though great efforts have been put into tailoring the microstructure and room temperature mechanical properties via process parameter optimisations, the potential roles of Sc/Zr inoculate modified Al alloys for high-temperature applications were still underexplored. In this study, the microstructural stability and the elevated temperature tensile behaviours of LPBF-processed Al-Mn-Sc alloy were systematically evaluated. The alloy demonstrated high microstructural stability after both heat treatment and high-temperature tensile testing for up to 573 K. The applied tensile testing temperature and strain rate played significant influences on the elevated temperature tensile properties and deformation behaviours. Unusual intermediate temperature embrittlement (also known as ductility dip) and yield drop behaviours were observed under certain testing temperature and strain rate regimes, and the underlying deformation mechanisms were elucidated in detail. The present study is expected to shed light on future high-performance, high-temperature Al alloy development for the LPBF process.

Stress corrosion cracking (SCC) behaviour of laser powder bed fused stainless steel 316L was evaluated with U-bend testing in oxygenated high-temperature water at 288 °C. The effects of solution annealing temperature (1066, 1150 and 1200 °C), surface finish (as-built and wire cut) and the sample orientations on the SCC behaviour were studied. The results show that a higher annealing temperature introduces further recrystallization, coarsening of the oxide inclusions and a decrease in resistance to SCC. A wire cut surface leads to a higher susceptibility to SCC than the as-built surface. Sample orientation was found not to affect the SCC susceptibility.

The mechanical properties of laser powder bed fused (LPBFed) Invar 36 alloy have been limited by the presence of manufacturing defects. It is imperative to investigate the influence of these defects on the mechanical behavior of LPBFed Invar 36 alloy. In this study, in-situ X-ray computed tomography (XCT) tests were conducted on LPBFed Invar 36 alloy fabricated at different scanning speeds to examine the relationship between manufacturing defects and mechanical behavior. For LPBFed Invar 36 alloy fabricated at a scanning speed of 400 mm/s, the manufacturing defects were randomly distributed and tended to be elliptical in shape. Plastic deformation behavior was observed, and failure initiated from defects inside the material resulting in ductile failure. Conversely, for LPBFed Invar 36 alloy fabricated at a scanning speed of 1000 mm/s, numerous lamellar defects were observed mainly located between deposition layers, and their quantity was significantly increased. Little plastic deformation behavior was observed, and failure initiated from defects on the shallow surface of the material resulting in brittle failure. The differences in manufacturing defects and mechanical behavior are attributed to changes in input energy during the laser powder bed fusion process.

In-situ pre-decorated different content of TiB2 particle reinforced 2024 Al matrix composites (xTiB2/2024Al composite, x = 0, 1, 2, 4, 6, 8 wt.%) were printed by laser powder bed fusion (LPBF). A quantitative equation was established to quantify heterogeneous nucleating potency by TiB2 nanoparticles on the grain size of the LPBFed samples. Optimized mechanical properties were obtained in the as-printed 4TiB2/2024Al alloy which proved the feasibility of replacing the wrought part with the LPBFed one. This manuscript provides a method to determine the optimized content of TiB2 nanoparticles to change the noncastable material to the printable one. This manuscript provides a method to determine the optimized TiB2 content to change the noncastable material to the printable one, and can potentially guide the introduction of many other nanoparticles.

The fatigue crack initiation of as built (AB) laser powder bed fusion (LPBF) AlSi10Mg is highly affected by the presence of large porosities inherent to the process. Hot isostatic pressing (HIP) is a potential means of reduction of this detrimental porosity. Classic high temperature HIP treatments at about 500 °C and 100 MPa pressure lead to significant strength loss that is partially recovered after additional solution and ageing heat treatments, i.e. overall a three-step treatment. In this work, a 350 °C HIP treatment performed under a higher pressure of 300 MPa for 2 h has been identified as a promising post-treatment for LPBF AlSi10Mg using finite element method simulations of the HIP process. This treatment is shown to suppress these large porosities and lead to higher fracture strain and total fatigue life than after the classic three-step treatment (HIP ? two steps of heat treatment). Both HIP treatments also present higher fracture strain compared to the AB material. Thus, this new 350 °C HIP treatment efficiently removes the need for post-treatments after HIP. The 350 °C at 300 MPa HIP treatment improves the yield strength weighted fatigue resistance compared to the AB material but not the absolute fatigue life.

Due to their light weight and outstanding mechanical properties at high temperatures, Ti3Al-based intermetallic alloys have driven increasing interest from both academia and industry; however, when additive manufacturing (AM) is applied to them, the outcome is hardly satisfying. In this work, we report a crack-free Ti3Al-based alloy fabrication by laser powder bed fusion (LPBF) using a mixture of a commercial Ti-48Al-2Cr-2Nb powder and a pure Ti powder. With the aid of a high cooling rate during LPBF, the as-built sample shows a ductile β phase with some partially-melted particles. After the heat treatment, partially-melted particles were dissolved, and the sample showed equiaxed α2 precipitates in the β matrix. The hardness was 515 ± 38 HV in the as-built sample and 475 ± 37 HV in the heat-treated sample. This study shows a novel strategy to fabricate crack-free Ti3Al-based alloy using LPBF from powder blends.

Ti-based metal matrix composites (MMCs) represent a material class with highly desired performance for the aerospace industry, but implementation has been hindered by poor processability. It is now being realized that MMCs can be formed in-situ through exploitation of the rapid cooling rates of laser pow- der bed fusion, and targeting invariant reaction compositions to minimize the propensity for solidification cracking. We perform laser line scans and surface remelts of arc-melted Ti-0.38C wt.%, Ti-1.67B wt.%, Ti-8.5Si wt.%, and Ti- 32.5Fe wt.% eutectic compositions to assess potential material amenability to the laser powder bed fusion (LPBF) process at low-cost as well as determining the nanostructure and hardness properties. The results indicate that MMCs are amenable to LPBF and may outperform conventional alloys.

There is limited research on metastable pitting corrosion in an acidic environment, and acid is a major challenge for material corrosion. Therefore, this work investigated the metastable pitting corrosion of laser powder bed fusion (LPBF)-produced Ti–6Al–4V, in Hank’s solution, at different pH values (pH = 3, 5, and 7). This work investigated the effect of acid on the characteristics of passive films, as well as the change in metastable pitting behavior. Based on the results of electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS), the passive film will be inhibited and dissolved under the influence of H+. The higher the concentration of H+, the thinner the passive film. Potentiodynamic polarization tests reveal that LPBFed Ti–6Al–4V in Hank’s solution, at pH 3, has more obvious metastable pitting corrosion. This is because the higher the H+ concentration, the more Cl- is adsorbed on the surface of the passive film, which is prone to generate soluble chlorides by competitive adsorption with oxygen atoms and thus develop into metastable pitting corrosion.

Surface roughness quality is still a significant problem in the laser powder bed fusion (LPBF) process. This study proposes a wobble-based scanning strategy to improve the insufficiencies of the traditional scanning strategy with regard to surface roughness. A laboratory LPBF system with a self-developed controller was used to fabricate Permalloy (Fe-79Ni-4Mo) with two scanning methods: traditional line scanning (LS) and the proposed wobble-based scanning (WBS). This study investigates the influences of these two scanning strategies on porosity and surface roughness. The results imply that WBS can maintain higher surface accuracy than LS, and the surface roughness can be reduced by about 45%. Furthermore, WBS can produce periodic surface structures arranged in fish scales or parallelograms with appropriate parameters.

Lightweight structures with an internal lattice infill and a closed shell have received a lot of attention in the last 20 years for satellites, due to their improved stiffness, buckling strength, multifunctional design, and energy absorption. The geometrical freedom typical of Additive Manufacturing allows lighter, stiffer, and more effective structures to be designed for aerospace applications. The Laser Powder Bed Fusion technology, in particular, enables the fabrication of metal parts with complex geometries, altering the way the mechanical components are designed and manufactured. This study proposed a method to re-design the original satellite structures consisting of walls and ribs with an enclosed lattice design. The proposed new structures must comply with restricted requirements in terms of mechanical properties, dimensional accuracy, and weight. The most challenging is the first frequency request which the original satellite design, based on traditional fabrication, does not satisfy. To overcome this problem a particular framework was developed for locally thickening the critical zones of the lattice. The use of the new design permitted complying with the dynamic behavior and to obtain a weight saving maintaining the mechanical properties. The Additive Manufacturing fabrication of this primary structure demonstrated the feasibility of this new technology to satisfy challenging requests in the aerospace field.

Positron annihilation lifetime spectroscopy (PALS) has been used for the first time to investigate the microstructure of additively manufactured molybdenum. Despite the wide applicability of positron annihilation spectroscopy techniques to the defect analysis of metals, they have only been used sparingly to monitor the microstructural evolution of additively manufactured metals. Molybdenum and molybdenum with a dilute addition (0.1 wt%) of nano-sized silicon carbide, prepared via laser powder bed fusion (LPBF) at four different scan speeds: 100, 200, 400, and 800 mm/s, were studied by PALS and compared with electron backscatter diffraction analysis. The aim of this study was to clarify the extent to which PALS can be used to identify microstructural changes resulting from varying LPBF process parameters. Grain sizes and misorientation results do not correlate with positron lifetimes indicating the positrons are sampling regions within the grains. Positron annihilation spectroscopy identified the presence of dislocations and nano-voids not revealed through electron microscopy techniques and correlated with the findings of SiO2 nanoparticles in the samples prepared with silicon carbide. The comparison of results indicates the usefulness of positron techniques to characterize nano-structure in additively manufactured metals due to the significant increase in atomic-level information.

Laser powder bed fusion (L-PBF) has attracted great interest in recent years due to its ability to produce intricate parts beyond the capabilities of traditional manufacturing processes. L-PBF processed biomedical implants are usually made of commercial pure titanium (CP-Ti) or its alloys. However, both alloys are naturally bio-inert, and thus reduce the formation of apatite as implants are put into the human body. Accordingly, in an attempt to improve the bioactivity of the materials used for making orthopedic implants, the present study decomposed fluorapatite material (FA, (Ca10(PO4)6F2)) into the form of nano-powder and mixed this powder with CP-Ti powder in two different ratios (99%Ti + 1%FA (Ti-1%FA) and 98%Ti + 2%FA (Ti-2%FA)) to form powder material for the L-PBF process. Experimental trials were conducted to establish the optimal processing conditions (i.e., laser power, scanning speed and hatching space) of the L-PBF process for the two powder mixtures and the original CP-Ti powder with no FA addition. The optimal parameters were then used to produce tensile test specimens in order to evaluate the mechanical properties of the different samples. The hardness of the various samples was also examined by micro-Vickers hardness tests. The tensile strength of the Ti-1%FA sample (850 MPa) was found to be far higher than that of the CP-Ti sample (513 MPa). Furthermore, the yield strength of the Ti-1%FA sample (785 MPa) was also much higher than that of the CP-Ti sample (472 MPa). However, the elongation of the Ti-1%FA sample (6.27 %) was significantly lower than that of the CP-Ti sample (16.17%). Finally, the hardness values of the Ti-1%FA and Ti-2%FA samples were around 63.8% and 109.4%, respectively, higher than that of the CP-Ti sample.

Ultrasonic Joining (U-Joining) is a novel friction-based joining technique that produces through-the-thickness reinforced hybrid joints between surface-structured metals and thermoplastics. The process feasibility has been successfully demonstrated to join metals and unreinforced or fiber-reinforced polymer parts by applying horizontal vibration. However, intense tool wear was observed for the explored combinations of materials, which could diminish the mechanical performance of the produced joints and hinder the process application. These investigations left an unexplored field regarding the application of different vibration modes, which could represent good solutions to minimize the intense tool wear reported. Therefore, the present study aims to explore the application of vertical vibration and to identify possible advantages and disadvantages of this variation. The case-study combination of additively manufactured 316L stainless steel and 20%-short-carbon-fiber reinforced poly-ether-ether-ketone was selected for this purpose. Initially, a set of optimized joining parameters was obtained for the vertical variation following a one-factor-at-a-time approach. In a previous study, the joining parameters were already optimized for the horizontal mode, and the results were used for comparison purposes. Single-lap shear joints were produced using both optimized modes, and the process monitoring indicated that joints produced using vertical vibration reached a lower joining energy input for a given joining time. The produced joints were tested, and joints produced with the horizontal variation achieved higher ultimate lap shear forces than the ones achieved by the vertical ones: 3.6 ± 0.3 kN and 1.6 ± 0.3 kN, respectively. Microstructural investigations at the fractured surfaces showed that this difference is due to insufficient frictional heat generation at the metal-composite interface when vertical vibration is applied. Therefore, the temperatures reached during the joining cycle are not enough to melt the polymer completely at the interface, preventing a complete surface wetting of the metal and reducing the micromechanical interlocking and adhesion bond between the parts, thereby diminishing the mechanical performance of the produced joints.

In the context of the use of AM, particularly in the L-PBF technique, the printability characterization of material occurs through the identification of its printability map as a function of printing process parameters. The printability map identifies the region where the powder melting is optimal and ensures a dense and defect-free material. Identifying the zones affected by physical phenomena that occur during the printing process which lead to material defects such as keyhole, lack of fusion and balling mode is also possible. Classical methods for the characterization of material and the identification of its printability map require the printing of a large number of specimens. The analysis of the specimens is currently time-consuming and costly. This paper proposed a methodology to identify optimal process parameters in L-PBF using an integrated single and multi-tracks analyses embedded in an overall algorithm with detailed metrics and specific factors. The main scope is to speed up the identification of printability window and, consequently, material characterization, reducing the number of micrographic analyses. The method is validated through an experimental campaign assessing the material microstructure in terms of porosity and melt pool evaluation. The case study on IN718 superalloy shows how the application of the proposed method allows an important reduction of micrographic analysis. The results obtained in the case study are a reduction of 25% for the complete definition of the printability map and more than 90% for identifying the zone with a high productivity rate.

Magnesium alloys are perspective materials for use in transportation, aerospace and medical industries, mainly because of their good load-to-weight ratio, biocompatibility and biodegradability. For the effective production of magnesium components by the laser powder bed fusion (LPBF) process, the process parameters with verified mechanical properties need to be determined. In this paper, we prepared bulk samples with a high relative density of AZ91D magnesium alloy. Tensile tests were then performed on LPBF samples to evaluate the mechanical properties. Our results show that the bulk samples achieved a relative density >99%, in multiple planes over the full sample height, while the mechanical properties reached values of YS = 181 MPa, UTS = 305 MPa and A5.65 = 5.2%. The analysis by scanning electron microscope revealed fine β-Mg17Al12 particles in the microstructure, which have a positive effect on the mechanical properties. The chemical composition of magnesium alloy AZ91D changed slightly during processing by LPBF due to the evaporation of the Mg content. However, the resulting composition still corresponds to the range specified by the ASTM standard for the AZ91D alloy.

This work aims to investigate the processability of René 80 via laser powder–bed fusion (L-PBF). René 80 is a poorly weldable Ni-superalloy, currently processed via investment casting to fabricate turbine blades working at an operating temperature of about 850 °C. The L-PBF parameters optimization aims to increase part integrity and enhance processing repeatability. This part was tackled by creating a complete design of experiments (DOE) in which laser power, scan speed and hatching distance were varied accordingly. Optimizing the abovementioned parameters minimized the crack density and pore area fraction. Hence, five parameter sets leading to a crack density lower than 100 µm/mm2 and a pore fraction between 0.045% and 0.085% were selected. Furthermore, the intra-print repeatability was studied by producing three specimens’ repetitions for each optimal set of parameters in the same build. The porosity value obtained was constant among repetitions, and the crack density (around 75 µm/mm2) had a slight standard deviation. The third step of the research assessed the inter-prints repeatability by producing a replica of the five selected parameter sets in a different build and by comparing the results with those studied previously. According to this latter study, the porosity fraction (ca. 0.06%) was constant in intra- and inter-print conditions. Conversely, crack density was lower than 100 µm/mm2 only in three sets of parameters, regardless of the intra- or inter-build cross-check. Finally, the best parameter set was chosen, emphasizing the average flaw fraction (least possible value) and repeatability. Once the optimal densification of the samples was achieved, the alloy’s microstructural features were also investigated.

TiC/graphene/Ti6Al4V composites were prepared by laser powder bed fusion using graphene and Ti6Al4V powder. The differences in microstructure and high-temperature mechanical properties between the Ti6Al4V alloy and the TiC/graphene/Ti6Al4V composite were studied. The tensile and microhardness of the two materials were tested at 400 °C, 500 °C, and 600 °C; the results of the TiC/graphene/Ti6Al4V composite were 126 MPa, 162 MPa, and 76 MPa and 70 HV, 59 HV, and 61HV, respectively, higher than those of the Ti6Al4V alloy. These results happened because graphene reacted with Ti to form TiC particles, which were homogeneously distributed amongst α’ acicular martensite. The addition of graphene refined the size of the acicular α’ martensite. At the same time, the graphene and TiC particles showed a dispersion-strengthening effect. The mechanical properties of the TiC/graphene/Ti6Al4V composite were improved by the combination of fine-grain strengthening and dispersion strengthening mechanisms.

Hypereutectic Al-high Si alloys are of immense interest for applications in the automotive, space or electronic industries, due their low weight, low thermal expansion, and excellent mechanical and tribological properties. Additionally, their production by laser powder bed fusion (LPBF) technology provides high flexibility in geometrical design and alloy composition. Since, most of the alloy properties could be improved by increasing the Si content, there is much interest in discovering the maximum that could be realized in LBPF Al-high Si alloys, without the appearance of any material failure. For this reason, in this work the production of Al-high Si alloys with extremely high silicon content of up to 70 wt.% was fundamentally investigated with respect to microstructure and mechanical properties. Highly dense (99.3%) and crack-free AlSi50 samples (5 × 5 × 5 mm3), with excellent hardness (225 HV5) and compressive strength (742 MPa), were successfully produced. Further, for the first time, AlSi70 LBPF samples of high density (98.8%) without cracks were demonstrated, using moderate scanning velocities. Simultaneously, the hardness and the compressive strength in the AlSi70 alloys were significantly improved to 350 HV5 and 935 MPa, as a result of the formation of a continuous Si network in the microstructure of the alloy. With respect to the powder source, it was found that the application of powder blends resulted in similar alloy properties as if pre-alloyed powders were used, enabling higher flexibility in prospective application-oriented alloy development.

Duplex stainless steel powders for laser additive manufacturing have not been developed extensively. In this study, the melts of a super duplex stainless steel X2CrNiMoCuWN25-7-4 (AISI F55, 1.4501) were atomized with different process gases (Ar or N2) at different atomization gas temperatures. The process gas N2 in the melting chamber leads to a higher nitrogen dissolution in the steel and a higher nitrogen content of the atomized powders. The argon-atomized powders have more gas porosity inside the particles than the nitrogen-atomized powders. In addition, the higher the atomization gas temperature, the finer the powder particles. The duplex stainless steel powders showed good processability during PBF-LB/M (Laser powder bed fusion). The gas entrapment in the powder particles, regardless of the gas chemistry and the gas content, appears to have a negligible effect on the porosity of the as-built parts.

The objective of this work is to study the possibility of obtaining dense parts using water atomized AISI 316L steel powder in the L-PBF process. Despite its irregular, non-spherical, particle morphology, it has a significantly lower cost. 25 samples were produced varying the laser power and the scanning speeds to determine the optimal processing conditions. Additionally, hot isostatic pressing (HIP) was performed after the L-PBF process to further increase densification. Selected samples were subjected to microstructural characterization. The best densification results obtained were for the sample produced with the laser power of 173 W and scanning speed of 600 mm/s, where densifications close to 98% were obtained. HIP post-processing promoted increased densification of samples with closed porosity, allowing samples with densification above 95% to reach values close to 100%. HIP did not promote the closure of open pores. The results indicate that the use of water atomized AISI 316L in the L-PBF process combined with post-processing by HIP can produce dense engineering components and at the same time reduce the production costs of the manufactured components, mainly because it is a lower cost raw material when compared to the commonly used feedstock obtained by gas atomization.

Superelastic metallic materials possessing large recoverable strains are widely used in automotive, aerospace and energy conversion industries. Superelastic materials working at high temperatures and with a wide temperature range are increasingly required for demanding applications. Until recently, high-temperature superelasticity has only been achievable with multicomponent alloys fabricated by complex processes. In this study, a novel framework of multi-scale models enabling texture and microstructure design is proposed for high-performance NiTi fabrication via laser powder bed fusion. Based on the developed framework, a Ni-lean Ni(49.4 at.%)-Ti alloy is, for the first time, endowed with a 4% high-temperature compressive superelasticity. A 001 texture, unfavorable for plastic slip, is created to realize enhanced functionality. The unprecedented superelasticity can be maintained up to 453 K, which is comparable with but has a wider superelastic temperature range (∼110 K) than rare earth alloyed NiTi alloys, previously only realizable with grain refinement, and other complicated post-processing operations. At the same time, its shape memory stability is also improved due to existing textured 100 martensite and intergranular precipitation of Ti2NiOx. This discovery reframes the way that we design superior performance NiTi based alloys through directly tailoring crystallographic orientations during additive manufacturing.

Hadfield steel (HS) containing ∼0.83 wt% Carbon has been manufactured using the laser powder bed fusion (LPBF) of mixed Fe-Mn, pure-Fe and Fe-C powders. Results show that the as-fabricated alloy is fully austenitic with the absence of carbides—a typical feature in the cast alloy that necessitates its solutionizing and quenching. The strength (σy = 595.5 ± 18.1 MPa, σu = 950.2 ± 28.6 MPa) and hardness (318.6 ± 7.1 HV) of the LPBF HS are superior to those of the conventionally fabricated HS, while the impact toughness is similar, and ductility is inferior. The yield strength enhancement is mainly due to the refinement in the grain size and increase in the dislocation density, which occurs due to the rapid solidification conditions that prevail during LPBF. This rapid solidification also prevents carbide formation and retain the main alloying elements (C and Mn) in solution. Thus conventional water quenching process can be completely eliminated. Agency for Science, Technology and Research (A*STAR) Published version This work was supported under the Structural Metal Alloy Program (SMAP), grant No. A18b1B0061, in Agency for Science, Technology and Research in Singapore.

In this work, the effectiveness of residual stress relief annealing on a laser powder bed fusion (L-PBF) manufactured austenitic stainless steel, alloy 21-6-9 was investigated. Residual stress levels were gauged using geometrical distortion and relaxation testing results. In the investigated temperature interval (600–850 °C), shape stability was reached after subjecting the as-built material to an annealing temperature of 850 °C for 1 h. Microstructural characterization and tensile testing were also performed for each annealing temperature to evaluate the alloy's thermal stability and the resulting tensile properties. In the as-built state, a yield strength (YS) of 640 MPa, ultimate tensile strength (UTS) of 810 MPa and 4D elongation of 47% were measured. Annealing at 850 °C for 1 h had little measurable effect on ductility (48% 4D elongation) while still having a softening effect (UTS = 775 MPa, YS = 540 MPa). From the microstructural characterization, cell-like features were observed sporadically in the annealed condition and appeared stable up until 800 °C after which gradual dissolution began, with the last remnants disappearing after subjecting the material to 900 °C for 1 h. Validerad;2023;Nivå 2;2023-03-01 (joosat);Funder: Space for Innovation and Growth; The Swedish National Space Agency; GKN Aerospace Sweden AB. RISE, Research Institutes of Sweden AB; Västra Götalandsregionen; Tillväxtverket; European Regional Development Fund; Spacelab project (Grant number 20201639)This article has previously appeared as a manuscript in a thesis.Licens fulltext: CC BY-NC-ND License

Nanomaterials : open access journal 13(8), 1341 (2023). doi:10.3390/nano13081341, Published by MDPI, Basel

This study investigated the in-situ gaseous (under 150 bar) hydrogen embrittlement behaviour of additively manufactured (AM) Inconel 718 produced from sustainable feedstock. Here, sustainable feedstock refers to the Inconel 718 powder produced by vacuum induction melting inert gas atomisation of failed printed parts or waste from CNC machining. All Inconel 718 samples, namely AM-as-processed, AM-heat-treated and conventional samples showed severe hydrogen embrittlement. Additionally, it was found that despite its higher yield strength (1462 ± 8 MPa) and the presence of δ phase, heat-treated AM Inconel 718 demonstrates 64% lower degree of hydrogen embrittlement compared to the wrought counterpart (Y.S. 1069 ± 4 MPa). This was linked to the anisotropic microstructure induced by the AM process, which was found to cause directional embrittlement unlike the wrought samples showing isotropic embrittlement. In conclusion, this study shows that AM Inconel 718 produced from recycled feedstock shows better hydrogen embrittlement resistance compared to the wrought sample. Furthermore, the unique anisotropic properties, seen in this study for Inconel 718 manufactured by laser powder bed fusion, could be considered further in component design to help minimise the degree of hydrogen embrittlement.

Materials 16(11), 4014 (2023). doi:10.3390/ma16114014 special issue: "Special Issue "Hot Isostatic Pressing (HIP), Heat Treatment and Additive Manufacturing of Metal Alloys" / Special Issue Editors: Prof. Dr. Daniele Ugues, Guest Editor; Dr. Emilio Bassini, Guest Editor", Published by MDPI, Basel

Much of the latent promise of metal additive manufacturing (AM) rests in the potential for controlled creation of spatially tailored microstructures, designed to optimize key build-scale properties through systematic variation across a build. Component optimization possibilities and performance potential expand enormously when this becomes possible. However, the extreme conditions created by AM energy sources and the nature of alloy solidification under such conditions are not adequately understood. Modeling and simulation tools capable of quantitatively predicting AM microstructural outputs would enable a major leap forward. We demonstrate through experimental validation that a multiscale (nm/ns to mm/ms) simulation framework coupling CALPHAD thermodynamic models, microstructure scale phase field simulations, and laser track scale multiphysics simulations can quantitatively predict tailored microstructure formation in laser processed Ti-Nb. Extensive simulations and analysis of detailed microstructural predictions over a broad range of conditions reveal scaling laws for characteristic microstructural features that should generalize to a wide range of materials. Several of our findings highlight the central importance of the alloy freezing range ΔTf, which provides the basis of a generalized strategy for optimizing spatial control of microstructure during AM. This proposed strategy integrates alloy design with process design & control to identify optimal material and process condition combinations. We have also identified conditions and phenomena that appear to require a more complete treatment of far from equilibrium solidification kinetics, highlighting a fundamental need in the field. We anticipate that further development of such methodologies will contribute centrally to realizing the potential of materials with spatially tailored microstructure. keywords: Additive manufacturing, Laser powder bed fusion, Multiscale modeling, Phase field modeling, Solidification, Tailored microstructure ispartof: MATERIALS TODAY vol:51 pages:65-86 status: published

Journal of materials science 2021, 1-15 (2021). doi:10.1007/s10853-021-06577-8, Laser powder bed fusion is an additive manufacturing process that employshighly focused laser radiation for selective melting of a metal powder bed. Thisprocess entails a complex heat flow and thermal management that results incharacteristic, often highly textured microstructures, which lead to mechanicalanisotropy. In this study, high-energy X-ray diffraction experiments were carriedout to illuminate the formation and evolution of microstructural featuresduring LPBF. The nickel-base alloy Inconel 625 was used for in situ experimentsusing a custom LPBF system designed for these investigations. The diffractionpatterns yielded results regarding texture, lattice defects, recrystallization, andchemical segregation. A combination of high laser power and scanning speedresults in a strong preferred crystallographic orientation, while low laser powerand scanning speed showed no clear texture. The observation of a constantgauge volume revealed solid-state texture changes without remelting. Theywere related to in situ recrystallization processes caused by the repeated laserscanning. After recrystallization, the formation and growth of segregations werededuced from an increasing diffraction peak asymmetry and confirmed by exsitu scanning transmission electron microscopy., Published by Springer Science + Business Media B.V, Dordrecht [u.a.]

Over the last decade, the enormous potential of metal additive manufacturing (AM) processes has led these technologies to establish their position in many industries. Much effort is being made toward their widespread application; however, much remains to be done to achieve full industrialization of these processes. Therefore, many companies, research centers and universities are investing in comprehensive research and development activities in order to further promote the industrialization of metal AM. This review traces the progress of metal AM technologies through an investigation of patents. In the present study, beam-based metal AM patents were searched through the Orbit Intelligence database. First, the number of patents per year was studied, indicating that, as expected, there is strong growth in AM patenting activities. The patents were afterward examined in order to highlight the key players in the field, and it was found that the main players investing in this market are: multidisciplinary companies, AM machine producers, end users working, especially in the aerospace sector, universities and research centers. The patents were then analyzed to understand the technology domains covered by each key player and their trend of investments. Finally, the patents in the field of Materials and Metallurgy were studied individually to identify the main topics faced by the most used alloy classes: Al-, Ni- and Ti-based alloys and steels. The extensive study of these patents clearly indicated that the main gaps to fill in metal AM are strongly material dependent and that it is possible to find correlations between the alloy classes, their main industrial applications and their specific AM processability issues. The current study provides insights into global trends that can help industrial markets to identify the right investment direction and research to identify topics for future investigation.

Hierarchically porous-structured materials show tremendous potential for catalytic applications. In this work, a facile method through the combination of three-dimensional (3D) printing and chemical dealloying was employed to synthesize a nanoporous-copper-encapsulating microporous-diamond-cellular-structure (NPC@DCS) catalyst. The developed NPC@DCS catalyst was utilized as a heterogeneous photo-Fenton-like catalyst where its catalytic applications in the remediation of organic wastewater were exemplified. The experimental results demonstrated that the NPC@DCS catalyst possessed a remarkable degradation efficiency in the removal of rhodamine B with a reaction rate of 8.24 × 10-2 min-1 and displayed attractive stability, durability, mineralization capability, and versatility. This work not only manifests the applicability of the proposed NPC@DCS catalyst for wastewater purification in practical applications but also is anticipated to inspire the incorporation of the 3D printing technology and chemical synthesis to design high-performance metal catalysts with tunable hierarchical micro- and nanopores for functional applications. National Research Foundation (NRF) The authors acknowledge the financial support from the National Natural Science Foundation of China (nos. 51775207 and 51905192), Fundamental Research Funds for the Central Universities (no. 2020kfyXJJS088), and National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme through the Marine and Offshore Program.

A coupled heat source model that combined a Gauss surface heat source with a Gauss cylindrical volumetric heat source was introduced to simulate temperature field distribution and melt pool characteristics using a finite element simulation (FEM) method for the deep and narrow melt pools formed in laser powder bed fusion (L-PBF) aiming at commercial pure titanium (CP-Ti). For comparison, the same simulations using the Gauss surface heat source model and the double ellipsoid heat source model were also performed. The simulated melt pool geometries using the coupled heat source model match well with the measurements, with an average error of 1% for the melt pool depth and 7% for the width. Based on the single-track experimental results, it was found by comparing the simulated results from the three heat source models that the coupled heat source model had better accuracy than the other two. Then, the temperature field and the melt pool geometries of CP-Ti fabricated at different laser power levels from 300 W to 500 W and scanning speeds from 600 mm/s to 4000 mm/s were simulated. According to the simulated maximum temperature and geometries of the melt pool, a suitable process parameters map for CP-Ti was obtained. The reported experimental results agree well with the simulated map. The coupled heat source model is more accurate and applicable for the deep and narrow melt pools formed during L-PBF.

A fully transient discrete-source 3D Additive Manufacturing (AM) process model was coupled with a 3D stochastic solidification structure model to simulate the grain structure evolution quickly and efficiently in metallic alloys processed through Electron Beam Powder Bed Fusion (EBPBF) and Laser Powder Bed Fusion (LPBF) processes. The stochastic model was adapted to rapid solidification conditions of multicomponent alloys processed via multi-layer multi-track AM processes. The capabilities of the coupled model include studying the effects of process parameters (power input, speed, beam shape) and part geometry on solidification conditions and their impact on the resulting solidification structure and on the formation of inter layer/track voids. The multi-scale model assumes that the complex combination of the crystallographic requirements, isomorphism, epitaxy, changing direction of the melt pool motion and thermal gradient direction will produce the observed texture and grain morphology. Thus, grain size, morphology, and crystallographic orientation can be assessed, and the model can assist in achieving better control of the solidification microstructures and to establish trends in the solidification behavior in AM components. The coupled model was previously validated against single-layer laser remelting IN625 experiments performed and analyzed at National Institute of Standards and Technology (NIST) using LPBF systems. In this study, the model was applied to predict the solidification structure and inter layer/track voids formation in IN718 alloys processed by LPBF processes. This 3D modeling approach can also be used to predict the solidification structure of Ti-based alloys processes by EBPBF.

A laser powder bed fusion process operating at elevated temperatures is introduced capable of fabricating crack-free and dense intermetallic titanium aluminide alloy specimens as well as demonstrator components using a base plate heating up to 900 °C and a unique heating system of the uppermost powder bed layer up to 1200 °C. Two so-called 4th generation alloys, TNM and TNM+, were used for this study. The microstructure and its evolution during subsequent heat treatments were investigated and explained by employing scanning electron microscopy, hardness testing, X-ray diffraction, differential scanning calorimetry and thermodynamic equilibrium calculation. Selected specimens were subjected to creep tests at 750 °C. The microstructures after processing consist of extraordinarily fine lamellar γ-TiAl/α2-Ti3Al-colonies with globular γ and βo-TiAl grains for both the TNM and TNM+ alloy, exhibiting a microstructure gradient from the last consolidated powder layer down to the starting layer due to cellular reaction, which increases the amount of globular γ and βo at the boundaries of the γ/α2-colonies. During annealing in proximity to the γ-solvus temperature, banded microstructures might form, as the α-grain size is only partially controlled by heterogeneously distributed γ/β-phase, which stems from the process-related Al loss. Additionally, the occurrence of thermally-induced porosity is investigated. Optimizing the microstructure to a homogenized, almost fully lamellar microstructure, involved annealing in the β-single phase field region and led to improved creep properties. Finally, TNM demonstrator components with complex geometries, such as aero engine blades and turbocharger turbine wheels, are fabricated by employing the novel laser powder bed fusion process.

In this study, a physics-based analytical method was proposed for the prediction of upper surface roughness in laser powder bed fusion (LPBF). The temperature distribution and molten pool shape in the melting process were first predicted by an analytical thermal model. The cap area of the solidified molten pool was assumed to be half-elliptical. Based on this assumption and the principle of mass conservation, the cap height and the specific profile of the cap area were obtained. The transverse overlapping pattern of adjacent molten pools of upper layer was then obtained, with given hatch space. The analytical expression of the top surface profile was obtained after putting this overlapping pattern into a 2D coordinate system. The expression of surface roughness was then derived as an explicit function of the process parameters and material properties, based on the definition of surface roughness (Ra) in the sense of an arithmetic average. The predictions of surface roughness were then compared with experimental measurements of 316L stainless steel for validation and show acceptable agreement. In addition, the proposed model does not rely on numerical iterations, which ensures its low computational cost. Thus, the proposed analytical method can help understand the causes for roughness in LPBF and guide the optimization of process conditions to fabricate products with good quality. The sensitivity of surface roughness to process conditions was also investigated in this study.

Materialia 26, 101615 (2022). doi:10.1016/j.mtla.2022.101615, Laser powder bed fusion (LPBF) was used to manufacture two high entropy alloys (HEAs) within the Al-Co-Cr-Fe-Ni-Zr system. The selected compositional ranges were similar to alumina forming austenitic (AFA) steels but omitting interstitials and replacing Nb with Zr as the Laves forming element. The Fe-rich austenitic HEAs were prepared with varying Al and Zr content to evaluate the influence on the presence, size, and distribution of the intermetallic (IM) precipitates. The LPBF process combined with a single (950 °C 6 h) heat treatment formed large, elongated grains with a fine dispersion of multiple different nano-sized IM phases. Synchrotron high energy x-ray diffraction (HEXRD) revealed the cubic M$_{23}$Zr$_6$ as the main Zr-rich IM phase, stabilized by the multi-element mixture (M=Co, Fe, Ni) and the high cooling rates of the LPBF process. Further HEXRD in-situ compression was performed from room temperature to 900 °C to evaluate the phase stability, thermal expansion, and the strength contribution of the austenitic matrix and the M$_{23}$Zr$_6$ and NiAl B2 IM phases. The evolution of lattice strain and full width at half maximum of the reflexes was tracked using a statistical model, enabling quantitative analysis along the deformation., Published by Elsevier, Amsterdam

We developed and applied a model-driven feedforward control approach to mitigate thermal-induced flaw formation in laser powder bed fusion (LPBF) additive manufacturing process. The key idea was to avert heat buildup in a LPBF part before it is printed by adapting process parameters layer-by-layer based on insights from a physics-based thermal simulation model. The motivation being to replace cumbersome empirical build-and-test parameter optimization with a physics-guided strategy. The approach consisted of three steps: prediction, analysis, and correction. First, the temperature distribution of a part was predicted rapidly using a graph theory-based computational thermal model. Second, the model-derived thermal trends were analyzed to isolate layers of potential heat buildup. Third, heat buildup in affected layers was corrected before printing by adjusting process parameters optimized through iterative simulations. The effectiveness of the approach was demonstrated experimentally on two separate build plates. In the first build plate, termed fxed processing, ten different nickel alloy 718 parts were produced under constant processing conditions. On a second identical build plate, called con-trolled processing, the laser power and dwell time for each part was adjusted before printing based on thermal simulations to avoid heat buildup. To validate the thermal model predictions, the surface tem-perature of each part was tracked with a calibrated infrared thermal camera. Post-process the parts were examined with non-destructive and destructive materials characterization techniques. Compared to fixed processing, parts produced under controlled processing showed superior geometric accuracy and resolu-tion, finer grain size, increased microhardness, and reduced surface roughness.(c) 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). National Science Foundation (NSF); Department of Energy (DoE) [OIA-1929172, CMMI-1920245, CMMI-1739696, ECCS-2020246, PFI-TT 2044710, CMMI-1752069, CMMI-1719388, DE-SC0021136]; NSF INTERN program [CMMI-1752069]; CMMI Data Science Activities [CMMI-1752069]; Major Research instrumentation grant [CMMI-1920245]; DoE [DE-SC0021136]; National Nanotechnology Coordinated Infrastructure [ECCS: 2025298]; Nebraska Research Initiative through the Nebraska Center for Materials and Nanoscience; Nanoengineering Research Core Facility at the University of Nebraska Lincoln; NSF Research Experience for Teachers grant [RET: EEC1953382] Published version This work was supported by the National Science Foundation (NSF) and Department of Energy (DoE) under awards OIA-1929172, CMMI-1920245, CMMI-1739696, ECCS-2020246, PFI-TT 2044710,CMMI-1752069, CMMI-1719388, and DE-SC0021136. Understanding the causal influence of process parameters and thermal history on part quality and detection of defect formation using in-situsensing was the major aspect of CMMI-1752069 (Program Officer:Kevin Chou). The use of graph theory for feedforward control in additive manufacturing was proposed in OIA-1929172 (Program Officer: Jose Colom-Ustariz). The experiments for this work were carried out at Edison Welding Institute (EWI) by Alex Riensche , Reza Yavari, and Ben Bevans through OIA-1929172 with guidance from Ajay Krishnan. Supplemental funding for CMMI-1752069 was obtained through the NSF INTERN program (Program Officer: Prakash Balan) and CMMI Data Science Activities (Program Officer: Martha Dodson). These supplements funded Reza Yavari's andZiyad Smoqi's research. Commercialization of the graph theory thermal approach for ultrafast simulation of metal additive manufacturing processes is being pursued under PFI-TT (Program Officer: Samir Iqbal).The X-ray CT analysis was conducted on the instrument partially funded through the Major Research instrumentation grant(CMMI-1920245, program officer: Wendy C. Crone). Ben Bevans'work was funded partially through the DoE Grant DE-SC0021136.The materials characterization research was performed in part in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure under award no. ECCS: 2025298, and with support from the Nebraska Research Initiative through the Nebraska Center for Materials and Nanoscience and the Nanoengineering Research Core Facility at the University of Nebraska Lincoln. Josie Gilligan was supported through NSF Research Experience for Teachers grant RET: EEC1953382.All builds and data collection efforts were executed at EWI, and Ajay Krishnan's efforts was sponsored by the EWI Additive Manu-facturing. The authors thank Dr. Brandon Lane of National Institute of Standards and Technology, and Dr. Abdalla Nassar formerly of Penn State, Applied research Labs for valuable insights and discussions.

This study investigated the application of an in envelope additive/subtractive (LPBF) manufacturing method (Matsuura LUMEX-Avance-25) to fabricate IN718 benchmarking coupons. The coupons were then examined comprehensively for surface finish both with and without high-speed micro-machining. The microstructure of the manufactured IN718 coupons was investigated thoroughly in the as-fabricated condition and following three different standard and one non-standard post-processing heat treatments. As built coupons revealed columnar grain morphology mainly along the direction with a cellular dendritic sub-grain structure and without any strengthening precipitates. Grain size, aspect ratio, and texture were maintained after each of the applied four heat treatments. Only one of the standard heat treatments resulted in the δ phase formation. The other three heat treatments effectively dissolved the Laves phase preventing the δ formation while promoting the formation of γ′/γ″ precipitates. Despite the observed differences in their microstructures, all of the heat treatments resulted in similar yield and ultimate tensile strength values that ranged between 1103–1205 MPa and 1347–1387 MPa, respectively. These values are above the minimum requirements of 1034 MPa and 1241 MPa for the wrought material. The non-standard heat treatment provided the highest elongation of 24.0 ± 0.1% amongst all the heat-treated specimens without a significant loss in strength, while the standard heat treatment for the wrought parts resulted in the lowest elongation of 18.3 ± 0.7% due to the presence of δ phase.

In this paper, thin layers of NiTi shape memory alloy (SMA) triply periodic minimal surface lattices (TPMS) are fabricated using laser powder bed fusion (LPBF), considering different laser scanning strategies and relative densities. The obtained architected samples are studied using experimental methods to characterize their microstructural features, including the formation of cracks and balling imperfections. It is observed that balling is not only affected by the parameters of the fabrication process but also by structural characteristics, including the effective densities of the fabricated samples. In particular, it is reported here that higher densities of the TPMS geometries considered are generally associated with increased dimensions of balling imperfections. Moreover, scanning strategies at 45° angle with respect to the principal axes of the samples resulted in increased balling.

Additively manufactured components by laser powder bed fusion (LPBF) often suffer from stress-induced cracks (e.g. delamination), especially at the build-substrate interfaces where stiff mechanical constraints and large thermal gradients coexist. To reduce the probability of cracking, this work proposes an innovative strategy to optimize the geometry of the substrate by reducing its mechanical stiffness and, consequently, the stress accumulation during LPBF. To assess the feasibility of the strategy, a coupled thermo-mechanical finite element model, calibrated with the experimental evidence obtained from the LPBF metal deposition of a bridge-type structure, is used to predict the thermo-mechanical behavior of two T-shape AM parts built on (i) a typical solid substrate and (ii) a groove patterned substrate, respectively. The results show that several visible cracks appear at the interface between the build and the typical solid substrate due to stress concentration (up to 1600 MPa), while a crack-free component can be manufactured by adding grooves through the thickness of the substrate, without compromising the resulting microstructure and microhardness of the metallic materials with high crack sensitivity. The difference between the groove patterned substrate design with respect to the use of support structures used for printing cantilever structures is clarified to further justify the novelty of the proposed approach. This work was funded by the Enterprise Ireland project (No. CF-2020–1564-A/B), the National Key R&D Program of China (No. 2016YFB1100100), the European KYKLOS 4.0 project (No. 872570) and the China Scholarship Council (No. 201906290011). The authors also acknowledge the AR-Lab and the AML in TCD.

Amano H., Yamaguchi Y., Ishimoto T., et al. Reduction of spatter generation using atmospheric gas in laser powder bed fusion of Ti-6Al-4V. Materials Transactions 62, 1225 (2021); https://doi.org/10.2320/matertrans.MT-M2021059., Laser powder bed fusion (LPBF), a typical additive manufacturing (AM) process, is a promising approach that enables high-accuracy manufacturing of arbitrary structures; therefore, it has been utilized in the aerospace and medical fields. However, several unexplained phenomena significantly affect the quality of fabricated components. In particular, it has been reported that the generation of spatters adversely affects the stability of fabrication process and degrades the performance of the fabricated components. To realize high-quality components, it is essential to suppress the generation of spatters. Thus far, the suppression of spatter generation has been attempted based on the process parameters; however, this has not been adequately discussed in terms of the fabrication atmosphere. Therefore, in this study, we focused on the fabrication atmosphere and investigated spatter generation using gas with different physical properties rather than conventionally used argon. It was observed that the spatter generation during the fabrication of the Ti6Al4V alloy could be significantly suppressed by changing the atmospheric gas, even under constant LPBF process parameters. We proved that the fabrication atmosphere is an important factor to be considered, apart from the process parameters, in AM technology.

Efficient and power-dense electrical machines are critical in driving the next generation of green energy technologies for many industries including automotive, aerospace and energy. However, one of the primary requirements to enable this is the fabrication of compact custom windings with optimised materials and geometries. Electrical machine windings rely on highly electrically conductive materials, and therefore, the Additive Manufacturing (AM) of custom copper (Cu) and silver (Ag) windings offers opportunities to simultaneously improve efficiency through optimised materials, custom geometries and topology and thermal management through integrated cooling strategies. Laser Powder Bed Fusion (L-PBF) is the most mature AM technology for metals, however, laser processing highly reflective and conductive metals such as Cu and Ag is highly challenging due to insufficient energy absorption. In this regard, this study details the 400 W L-PBF processing of high-purity Cu, Ag and Cu–Ag alloys and the resultant electrical conductivity performance. Six Cu and Ag material variants are investigated in four comparative studies characterising the influence of material composition, powder recoating, laser exposure and electropolishing. The highest density and electrical conductivity achieved was 88% and 73% IACS, respectively. To aid in the application of electrical insulation coatings, electropolishing parameters are established to improve surface roughness. Finally, proof-of-concept electrical machine coils are fabricated, highlighting the potential for 400 W L-PBF processing of Cu and Ag, extending the current state of the art.

Stress corrosion cracking (SCC) of laser powder bed fusion-fabricated 316L was studied under the variation in energy input density to emulate the existence of distinctive types of defects. Various electrochemical polarization measurements were performed in as-received polished and ground states, to elucidate the effect of defect type on corrosion and SCC behaviour in marine solution. The results revealed severe localized corrosion attack and SCC initiation for specimens with a lack of fusion pores (LOF). Moreover, the morphology of SCC was different, highlighting a more dominant effect of selective dissolution of the subgrain matrix for gas porosities and a more pronounced effect of brittle fracture at laser track boundaries for the specimens with LOF pores.