Functionally graded Inconel 718 processed by additive manufacturing: Crystallographic texture, anisotropy of microstructure and mechanical properties

There are several physical reasons for anisotropic mechanical properties in additively manufactured metals. These include but are not limited to directionally dependent grain and phase morphology; crystallographic texture; directional porosity/defects; and heterogeneity associated with the melt pool, layer wise microstructure. All of these are prevalent in most additive manufacturing processes, and it is difficult to separate out the role that each play in the mechanical anisotropy. This review focuses on studies that have attempted to or reasonably isolate one or two of these sources rather than simply report on trends in mechanical properties. This is not an exhaustive review covering all additive process or mechanical properties; the main assessment is on laser powder bed fusion (LPBF) metals and tensile test results (modulus, yield strength, ultimate tensile strength (UTS), elongation, and fracture surface analysis). In summary, the primary sources of anisotropic tensile properties for LPBF alloys are crystallographic texture, anisotropic microstructure morphologies, lack of fusion defects, and the melt pool macrostructure. Within anisotropic microstructures, elongated grains appear to be secondary compared to the preferential distribution of phases and features (e.g., grain boundary alpha, precipitates, etc.). Anisotropic modulus and yield strength are primarily caused by crystallographic texture. This is supported by crystal plasticity simulations. Anisotropic elongation is primarily caused by anisotropic microstructure morphologies, lack of fusion defects, and melt pool macrostructure. The evidence to support this comes from fracture surfaces that follow these features. Melt pool macrostructure is the most challenging to experimentally isolate from the list of other sources of anisotropy. Strategies to characterize and manipulate crystallographic texture, porosity, grain and phase morphology, and melt pool macrostructures are required to better understand and control mechanical anisotropy in AM metals.

Revealing anisotropic mechanisms in mechanical and degradation properties of zinc fabricated by laser powder bed fusion additive manufacturing

Metal additive manufacturing (AM) facilitates the production of complex geometries with enhanced functionality. Among various AM techniques, laser powder bed fusion (LPBF) is distinguished by its precision and exceptional mechanical properties achieved via laser fusion deposition. Recent advancements in AM have focused on combining LPBF with post-processing methods such as cold rolling, high-pressure torsion, and forming processes. Therefore, understanding the forming behavior of LPBF-processed materials is essential for industrial adoption. This study investigates the stretch-flangeability of LPBF-fabricated 316L stainless steel, emphasizing its anisotropic microstructure and mechanical properties. Hole expansion tests were employed to assess stretch-flangeability in comparison to wrought 316L stainless steel. The results demonstrate that LPBF-processed samples exhibit significant anisotropic behavior, demonstrating the influence of microstructural evolution on formability. These findings contribute valuable insights into optimizing LPBF materials for industrial forming applications.

Synergetic strengthening of additively manufactured (CoCrFeMnNi)99C1 high-entropy alloy by heterogeneous anisotropic microstructure

Advancements in understanding the microstructure and properties of additive manufacturing Ti-6Al-4V alloy: A comprehensive review

Effects of heat treatment on microstructure, mechanical and corrosion properties of 15-5 PH stainless steel parts built by selective laser melting process

Mapping the roles of scan strategy and build orientation in predicting the crystallographic texture and yield strength of 316L stainless steel produced by laser powder bed fusion

Fiber laser beam welding of additive manufactured 316L austenitic stainless steel with wrought 2507 super duplex and wrought 904L super austenitic stainless steels: Crystallographic texture, microstructure, and mechanical properties

Additive manufacturing (AM) technology can potentially disrupt offshore and marine industry by effectively reducing manufacturing processes and leadtime, and enabling new product designs. A review of existing literature has revealed limited knowledge in using AM to process shipbuilding materials, which can otherwise enhance existing shipbuilding manufacturing workflow. This has led to the research motivation to establish the technical feasibility of using selective laser melting (SLM) to process ASTM A131 EH36 shipbuilding steel. SLM process is a widely understood AM technology that has been used extensively to process other type of steels, for example stainless steel 316L. EH36 belongs to a class of high tensile low alloy steel which has traditionally been processed through casting. However, knowledge on using lasers to process EH36 is largely unknown, especially in AM applications. Using SLM to process EH36 is novel and will solve the problem of lack of knowledge in this field. The study thus aims to further the understanding of the mechanical properties and microstructure of SLM processed EH36. A preliminary investigation was first carried out to establish the technical feasibility of using SLM to process EH36. The process parameters obtained were then used to further the investigations. Heat treatment process was applied as a possible post processing technique. Mechanical testing was carried out to characterise its mechanical properties. The fracture surfaces and microstructures were then studied to characterise the material. The results showed that EH36 can be processed using SLM without any visible cracks. The mechanical properties of as built SLM processed EH36 exhibit very high tensile strength, but low ductility. The ductility can be improved through tempering heat treatment process, but at the partial sacrifice of tensile strength. Fractography analysis also affirmed the lack of ductility in the SLM processed EH36 samples. Finally, the microstructure showed that fine grain size and martensitic microstructure were the primary drivers behind its high tensile strength. Tempered samples experience grain coarsening and phase transformation to a mainly ferritic structure, which led to recovery in its ductility but a decrease in the tensile strength. A scaled model was fabricated to validate the results from the microstructure studies, and the findings indicate similar microstructures formed. This demonstrates the repeatability of the developed process parameters on fabrication of complex joints. The study contributes to the scientific knowledge with regard to material and mechanical property characterisation of SLM processed EH36. The findings from the fractography and microstructural analysis will contribute towards building the knowledge and facilitate future work on AM of EH36.

The anisotropy of microstructures and mechanical properties of FeCoNiCr 0.5 high‐entropy alloys (HEAs) prepared by selective laser melting (SLM) were investigated in this study. An anisotropic microstructure was obtained for the as‐built FeCoNiCr 0.5 HEA sample. Specifically, in the XOY plane (perpendicular to the building direction), the {110} texture is dominant, and the grain size is mostly between 40 and 60 μm with equiaxial morphology. In the XOZ plane (parallel to the building direction), columnar grains have a length of 80–100 μm, and there is no obvious preferred orientation in the structure. This structure leads to anisotropic tensile behavior in the sample where the yield strength, tensile strength, and elongation of FeCoNiCr 0.5 HEA in the XOY plane are 528 MPa, 546 MPa, and 49%, respectively, whereas those in the XOZ plane are 547 MPa, 642 MPa, and 28%, respectively. The effect of deformation twins on the evolution of the work‐hardening rate is also discussed, considering the anisotropy condition.

Research status of laser powder bed fusion Al–Li alloys and its improvement measures

Effect of scanning strategy on the anisotropy in microstructure and properties of Cu-Cr-Zr alloy manufactured by laser powder bed fusion

Additive manufacturing (AM) has shown advantageous aspects over conventional manufacturing methods, e.g., additional design freedom, ability to fabricate complex shapes without the need for post-processing, and enhanced mechanical performance of the fabricated parts. The development and application of AM techniques, such as laser powder bed fusion (LPBF), however, are hampered by the relatively limited types of alloys that are amenable to AM, e.g., 316L stainless steel, Inconel 718, and Ti64. Therefore, metal AM is still in its nascent stage, with the exploration of new alloy chemistries and/or fabrication of components with unique functional and/or mechanical properties being relatively unexplored. The combination of in-situ alloying and compositionally graded alloy is promising to accelerate the composition screening/alloy designing for AM, as it can rapidly examine the printability, micro-/meso-structure, and properties of the AM-produced parts and therefore shorten the time span between conceptualization of new alloys and their deployment in service. In this study, among the several available AM processes, the LPBF technique is selected to fabricate compositionally graded alloys, as it has higher spatial resolution and lower cost, compared to directed energy deposition (DED) and electron beam powder bed fusion (EBPBF). With three different LPBF setups and four alloy systems examined in this study, the following objectives are achieved: (i) screening out, from a large range of compositions, the alloys that are amenable to LPBF, (ii) examining the effects of the LPBF characteristics, e.g., rapid cooling rate, on the micro-/meso-structure and properties of the fabricated parts with varying chemical compositions, and (iii) evaluating the fabrication methods in terms of types of achieved gradation (stepwise or smooth), efficiency, and level of chemical segregation. Through pre-packing the mixed powders in the powder supply bin, compositionally graded Fe-Al (Al contents in range of 9.8–40.8 at.%), and AlxCrCoFeNi (x = 0.07–0.88 molar ratio) were fabricated and studied. The chemical compositions, microstructures, and mechanical properties were examined to investigate the effect of Al contents. In Fe-Al alloys, the critical Al content for crack-free printing was examined to be 35 at.% and a columnar to equiaxed grain structure transition with increasing Al content is seen along the gradient/building direction, due to the varying levels of constitutional undercooling (CUC). Regarding the AlxCrCoFeNi alloy, the maximum Al that can enable crack-free printing was observed to be x (molar ratio) = 0.5, and the phase variations in the graded alloys were found to be closely associated with x. A CoCrMo-Ni graded alloy (26.8–9.8 wt.% Ni) with smooth compositional gradient was successfully fabricated using a customized LPBF system, where the gradient was created along the transverse direction instead of building direction. Two types of grain morphologies and chemical segregation bands, which are either rich in Ni or in CoCrMo alloy, were observed. Incomplete mixing of the powders combined with the high aspect ratio of the melt pool and the 67° scan rotation between successive layers are the reasons behind the formation of these bands. Detailed mechanical property characterization shows that the microscale chemical segregation can not only strengthen the matrix but also improve the work hardening ability of the bulk material through kinematic hardening mechanism. Compositionally graded Cu-Ni alloys with 0–9.8 wt.% Ni were fabricated using a novel and simple powder supply system in LPBF. With no need to alter the mechanical setup of the LPBF machine or its powder feeding system, this strategy can efficiently create smooth compositional gradient without introducing significant chemical segregations. 7.6 wt.% Ni was proved to be the lower threshold for fabricating nearly full-density Cu-Ni binary alloy. Cu-7.6Ni-3Al (wt.%) was further designed and fabricated. After aging till the peak condition, the Cu-Ni-Al alloy shows significantly higher strength and electrical conductivity than in the as-printed (AP) condition, due to the formation of the Ni3Al precipitates during aging. In this study, three different in-situ alloying methods are utilized on four alloy systems to explore and potential of utilizing compositionally graded alloy as a high-throughput method to study the composition-microstructure-property relationship of the LPBF produced alloys. On the basis of the findings, it is believed that such method can contribute greatly to the alloy designing and further development of AM.

Improvement of corrosion resistance of austenitic 316L stainless steel via laser powder bed fusion (LPBF) is currently a prominent research topic; however, the effects of crystallographic texture and the related grain boundary density on the corrosion resistance of LPBF-fabricated parts have not been elucidated. For biomedical applications, crystallographic texture control from a single crystalline-like to randomly oriented polycrystalline microstructure is highly attractive for optimizing the mechanical properties (particularly the Young’s modulus) of implants. An investigation of the impacts of crystallographic planes and grain boundaries exposed to the biological environment on corrosion behavior is necessary. 316L stainless steels with different crystallographic textures and grain boundary densities were successfully fabricated via LPBF. The corrosion resistances of the LPBF-fabricated specimens were comprehensively assessed by anodic polarization, dissolution, and crevice corrosion repassivation tests. The LPBF-fabricated specimens showed extremely high pitting potentials in the physiological saline compared with the commercially available counterparts, and importantly, excellent pitting corrosion resistance was observed irrespective of the crystallographic planes and grain boundary density exposed. Moreover, the LPBF-fabricated specimens did not show metastable pitting corrosion even in an accelerated test using an acid solution. The repassivation behavior of the specimens was not affected by LPBF. Such a drastic improvement in the corrosion resistances of the LPBF-fabricated specimens might be attributed to suppression of inclusion coarsening owing to the rapid cooling rate during solidification in LPBF. By using LPBF, the desired crystallographic texture can be introduced based on the desired mechanical properties without concern for corrosiveness. • LPBF dramatically improved localized corrosion resistance of 316L stainless steel. • Corrosion resistance of the LPBF specimens was independent of the exposed plane. • Precipitation and growth of inclusion was suppressed within nanometer-size by LPBF.

Effect of nano-TiB2 particles on the anisotropy in an AlSi10Mg alloy processed by selective laser melting