DirDirect Laser Deposition f ect Laser Deposition for T or Tailorailored Structur ed Structuree

to combine the abovementioned advantages to develop tailored structures in order to accomplish complex and functional products. For this purpose, a specific case study was investigated, starting with the study of the appropriate powders to use and ending with the printing process using the DMG Mori Lasertec65. Microstructural and mechanical analyses were carried out to evaluate the products and to validate the process. The final results show the properties and performances of products obtained using this technology


Intr Introduction oduction
There is an increasing interest towards the hybrid structures, related to the possibility to obtain high performances in combination with tailored properties. In this field of research, additive manufacturing technologies play a key role due to several advantages, such as create complex-shaped components and lower prototyping costs. The present work can be contextualized in this scenario. Direct Laser Deposition (DLD) is the most suitable technology for these applications, considering that direct energy into a narrow, focused region to heat a substrate, melting the substrate and simultaneously melting material that is being deposited into the substrate's melt pool. Unlike powder bed fusion techniques, DLD processes are not used to melt a material that is pre-laid in a powder bed but is used to melt materials as they are being deposited [1]- [3]. The capability to produce fully dense, as well as gradient or hybrid objects, makes the DLD more attractive, comparing to powder bed systems, in the manufacturing of large and/or functionally graded components [4]. Aiming to create a multi-material component with tailored properties, a hybrid system machine is used that allows combining additive manufacturing and five-axis CNC machining [5]. Moreover, due to the influence of the feedstock material on the final result of the powder-based laser additive manufacturing process, the metal powders were analyzed [6]. For this purpose, a case study is analyzed, i.e. a flange for the motive sector. The flange shows a central threaded bushing that must resist to heavy loads, while the base of the flange must be tenacious and weldable to be easily bonded to the chassis of the car. In a previous work, the authors have also described the redesign of the flange in order to obtain the desired properties but with an optimizing shape [7]. With hybrid multi-material components, the presented experimental work has the aim to meet the needs of the industry that, nowadays, requires tailored structures with high performances and specific properties in different parts using. The feedstock material was chosen to achieve a successful printing process in combination with the desired properties of the final product. For these reasons, the central bushing is made of Fe55 TM , which is a tool steel with high strength and hardness for the desired application, and the gussets in 316L stainless steel and both are deposited on a laser-cut C40 plate-base ( Fig.1).
The design of the multi-mat Fig. 1. The design of the multi-material flange f erial flange for the aut or the automoti omotiv ve sect e sector or, print , printed in multi-mat ed in multi-material on a laser-cut plat erial on a laser-cut plate e base [7]. base [7].
Near spherical shaped gas-atomized 316L and Fe55 TM powders were supplied by Voestalpine Böhler Welding GmbH.
Powders have been characterized according to the ASTM Standard F3049 [8]. The particle size of the powders involved in this study ranges between 55 and 145 µm, and the chemical composition of each feedstock is given in Table 1.
T Table 1. Chemical composition of the used 316L and F able 1. Chemical composition of the used 316L and Ferr erro55 po o55 pow wders used in the additi ders used in the additiv ve manuf e manufacturing pr acturing process. ocess.
The AM process is also influenced by the flowability of metal powder [9], which affects its behaviour in the AM process. Indeed, the spherical-shaped particles facilitate easy flow of the powder through the deposition nozzle. For this reason, the morphological, as well as microstructural and chemical analyses were carried out by using a scanning Swift ED3000). In addition, powders were crosssectioned, mounted, lapped, and polished with a custom-made metallographic preparation for a better investigation.
As mentioned before, a laser-cut C40 steel plate is used as a build platform for the DLD printing process. Before experiments, the substrate was cleaned with ethanol. The DLD machine is a Lasertec 65 3D by Sauer GmbH/ DMG MORI AG, a hybrid additive and subtractive machine. The system was equipped with a Coax 14 powder nozzle and a diode laser with a maximum output of 2500W. The spot diameter was 3mm, with a tophat beam profile.
Using optimized parameter based on preliminary experiments, in terms of laser-power, scanning speed and powder feed rate, tensile specimens were printed in 316L and Fe55 TM in the x-y direction, three for each material to assess the repeatability.
Similarly, three flanges were printed according to the following procedure. First, the central bushing was deposited in Fe55 TM , followed by CNC machining to prepare the part for the subsequent deposition of the gussets. Then, the four gussets were deposited in 316L, and finally CNC machining operations were performed to achieve the desired dimensions and surface finishing. To study the adhesion between the different materials at their interfaces, and to check for probable internal defects, metallographic samples were extracted from the flange. Cross-sections orthogonal to the printing height were mounted in a conductive thermoplastic resin (Lucite supplied by Struers) and, then, lapped with grinding discs with final polishing using diamond of 1 µm particle size.

Mechanical pr Mechanical properties operties
To determine the mechanical properties, uniaxial tensile tests were carried out on 316L and Fe55 TM samples, according to ASTM E8M, and uniaxial static mechanical tests on the printed flange. Tests were carried out using a Galdabini QUASAR 50 testing machine, equipped with a 50kN load cell and at room temperature. The software associated with the Galdabini machine, LabTest, allows programming the tests according to the International Standards. Since the base part in C40 is the weakest material of the part and its deformation might influence the test, a custom-made experimental set-up as shown in Fig. 2 4 shows the SEM micrographs of 316L powders at different magnifications in order to appreciate both morphology and defects. In this case, most of the powders are spherical, although some irregular particles are depicted (Fig. 4b)).
In particular, it is possible to note that dimensions are strongly different from one particle to another, but for the additive process it represents an advantage, promoting a good powder flowability. The defects are similar to the ones previously described and are highlighted by red arrows: agglomeration of particles and voids are visible in the powders (Fig. 4b),c), d)). For completeness and consistency of the presentation of the results, EDS analysis was carried out also for stainless steel powders, but the results were consistent with the chemical data present in Table 1 and have shown that powders were free of contaminations. The near-spherical shaped geometry is also evident in Fig. 5, in which the cross-sections of the Fe55 TM and 316L single particle are shown. The microstructure is made of equiaxed grains, typical of gas-atomized steel powders, and no internal defects can be observed.

Mechanical char Mechanical charact acterization erization
Although at the beginning Fe55 TM and 316L are printed according to the standard size and according to above-

Flange char Flange charact acterization erization
To assess the adhesion between the Fe55 TM bushing, the 316L gussets and the C40 plate, the specimens were sectioned and observed, and the result is shown in Fig. 7. Although the powders showed several defects, during the process these disappear and it is possible to see that the final product are free of defects, and no porosities are detected specifically in the interfaces. is important that no discontinuity or voids were observed. It is also noteworthy that in some locations partial blending of materials was observed at the interfaces. This could be attributed to the Marangoni effect at the bottom of the melt pool [11], which may lead to irregularities in hardness profile, as it can be seen in this image.  The adhesion between materials and the effect of hardness were studied in the next stages of the research by performing a static mechanical test. Microhardness profiles are presented by the authors in a previous study [5]. In Fig. 8 it is possible to note the raise of the hardness from 316L to Fe55 TM , passing through a blended zone. In addition, for what concerns the uniaxial static mechanical tests, all the three samples reached a load of 47 kN without breaking the specimen, as reported by the LoadDisplacement curve in Fig. 9. The results of the tests are more than satisfactory.
Precisely, for the application of the car flange, the maximum load achieved is over the necessary requirements, for this reason and to preserve the load cell of the tensile test equipment, the test has been stopped.

Conclusions Conclusions
In this work Direct Laser Deposition was used to create tailored components. The first step is represented by the morphological study of the feedstock powders and the investigation of the mechanical properties of samples printed with the characterized powders, in order to realize a hybrid component. In this context, a case-study is presented: a flange for the automotive sector with tailored properties. It is successfully printed, at first depositing Fe55 TM for the central bushing to resist heavy load, then gussets in 316L to increase the stiffness and use a C40 plate-base to ensure the weldability. More than one advantage of the DLD process has been highlighted and, hence, the following conclusions can be drawn: • This process has ensured a metallurgical bonding between layer and substrate, and between the different materials. The continuity at the thee interfaces is demonstrated, indeed no distinct discontinuity or voids are detected in the multi-material component and a partial blending of the materials is provided. Indeed, after the analysis and the characterization of the particles, it is demonstrated that the use of feedstock powders with irregular shapes and sizes in the DLD additive process doesn't entail defects, such as voids, in the final component.
• The mechanical properties of both the single-material and multi-material components are provided, demonstrating that with the use of the DLD additive process, it is possible to achieve excellent mechanical properties, and optimize the properties according to custom requirements.