INVESTIGATION OF SHAPE ACCURACY OF SELECTIVE LASER MELTED Ti6Al4V LATTICE STRUCTURE BY COMPUTER TOMOGRAPHY

Abstract

Selective laser melting is a widely applied additive manufacturing technology for metals, especially Ti6Al4V alloy today. Manufacturing of lattice structures stands in focus of scientific research, because of special technological advantages of those, like production of lightweight parts, opportunity of combination with other materials utilizing pores, biomedical applications, several scenarios of production resulting in tuned mechanical properties. In our experiments tensile test specimens were produced with periodic trabecular structure. Pore size of structure were varied. Shape accuracy of test specimens were measured by computer tomography. Size accuracy of the total specimen, size and shape accuracy of pores, places of material defects and excesses were identified and described quantitatively. Our experiments showed that most part of test specimens were precise enough. However special areas were identified with high inaccuracies. Moreover most dangerous errors, material absence and cracks also observed.

Headnote

Abstract: Selective laser melting is a widely applied additive manufacturing technology for metals, especially Ti6Al4V alloy today. Manufacturing of lattice structures stands in focus of scientific research, because of special technological advantages of those, like production of lightweight parts, opportunity of combination with other materials utilizing pores, biomedical applications, several scenarios of production resulting in tuned mechanical properties. In our experiments tensile test specimens were produced with periodic trabecular structure. Pore size of structure were varied. Shape accuracy of test specimens were measured by computer tomography. Size accuracy of the total specimen, size and shape accuracy of pores, places of material defects and excesses were identified and described quantitatively. Our experiments showed that most part of test specimens were precise enough. However special areas were identified with high inaccuracies. Moreover most dangerous errors, material absence and cracks also observed.

Keywords: lattice structure, Ti6Al4V, selective laser melting, shape accuracy, dimensional accuracy

INTRODUCTION

In 20th century manufacturing technologies went through the most extraordinary advancement in history of mankind [1-3]. Beside several areas of technological development additive manufacturing (AM) brought a revolutionary change opening up new doors of integration of production with other systems of economy like remanufacturing, informatics, robotics [4], logistics

[5], commerce, construction [6] and medicine [7-9]. Flexibility of AM strongly contributes to mass customization production emerging in the frame of industry 4.0 [10]. Number of research publications related to AM increase exponentially since 2000 [11]. AM with its new opportunities and challenges [12] also transforms even way of thinking in engineering design [13-15]. AM today is applicable for composing new materials with designed and tuned properties, like composites [16, 17] and special microscopic-level mixtures [18].

Standard ISO/ASTM 52900 introduces a nomenclature for AM technologies and concepts connected with AM [19]. This standard specifies seven class of AM technologies: binder jetting (BJT), directed energy deposition (DED), material extrusion (MEX), material jetting (MJT), powder bed fusion (PBF), sheet lamination (SHL) and vat photo polymerization (VPP).

Distinctive feature of PBF technology is that during the process thermal energy fuses a part of a powder bed. PBF technologies can be applied to many different materials, most frequent ones are plastics and metals. In the followings we deal with metals

Fusion comes true in two main ways, sintering and melting. The process is called sintering if original powder particles do not completely melt. When all powder particles are melted, it is called melting. Source of thermal energy is usually laser beam or electron beam.

Selective laser melting (SLM) is a PBF method which uses laser beam for full melting of material to be fused [20]. When it is applied for metals it is often called direct metal selective laser melting (DMSLM). This paper presents result of DMSLM applied on Ti6A?V material, and we refer to this briefly as SLM, because there is no risk of ambiguity.

Feedstock material of SLM is metal powder. Such kind of powders differ from base materials of powder metallurgy. Today a complete industrial background exists to supply AM market with powders of several metals and alloys. Most important properties of AM metal powders are particle size distribution, average size, particle shape, nominal chemical composition, and level of accuracy of chemical composition. Quality of metallic parts produced by AM depends of quality of metal powder which it is made of [21, 22].

SLM is a production method with several process parameters. Significant part of scientific literature treats how properties of parts produced depends of SLM process parameters [23-26].

Dimensional and shape accuracy are also dependent from SLM process parameters [27-32].

AM technologies in themselves are rarely able to produce ready to use parts. Commonly so-called postprocessing methods are used to achieve required quality of the product. Aims of postprocessing are surface modification in order to improve shape accuracy , wear resistance [33, 34], surface roughness [35-37].

Computer tomography first gained ground in biomedicine is a cost and expertise demanding method, at the same time it is increasingly applied also in other fields of industry for three-dimensional imaging of complex structures. This is especially true for inspection of additively manufactured lattice structures which form a new and promising class of parts [38, 39].

In this paper we introduce an experimental study on shape and size accuracy of a lattice structure produced by SLM from Ti6Al4V alloy. Since a lattice structure has numerous details inside, where those can not be observed by optical methods, computer tomography (CT) has been applied as imaging procedure. Aim of this study was to show out dimensional and shape deviations, material excess and lack, as well as cracks and fractures originated from manufacturing procedure.

MATERIAL AND METHODS

- Material Ti6AI4V

Samples were produced from Ti6Al4V ELI powder by SLM. Ti6Al4V is known as a widely applied metal alloy in industry, which is also intensively studied in science. In our experiments we used a special form of Ti6Al4V containing extra low interstitials (ELI). Chemical composition of Ti6Al4V ELI in weight% is 5.5-6.75% Al, 3.5-4.5% V, C < 0.08%, O < 0.2%, N < 0.05%, H < 0.015%, Fe < 0.4%. All other elements must be present in less than 0.1%, and total amount of other elements must be less than 0.4% [40]. This metal alloy has remarkable advantages like low mass density, good mass-strength ratio, chemical endurance and biocompatibility. It is applied in biomedical, pharmaceutical, vehicle, nuclear industries and several other fields [41, 42].

- Sample production

The shape of the sample is identical to a standard tensile test specimen with rectangular cross section expected that active zone (which part is used to break away) of it has lattice structure. Lattice is periodical. Unit cell is cubic with 2.5 mm edge length. Figure 1 shows overall shape of the test specimen and middle part with lattice structure magnified.

This sample was produced in horizontal orientation with its smallest faces down as left side of Figure 1 sows. Manufacturing machine was an EOS M290 400W. Production parameters were set to default values.

- Computer tomography

Computer tomography is often applied for inspection of additively manufactured metallic parts [38] [39].

Computer tomography may be known for most people from medical practice. Aim of such a measurement is to gain a 3D representation of internal structure of a body, which is not observable by optical methods. In industry aim is the same. Difference from medical machines is that specimen rotates and X-ray source stands, and usually it applies stronger X-ray for inspection.

A computer tomograph uses X-ray to radiograph the sample. By this way a projection of internal structure is captured like a shadowed plane image. As the sample rotates several such an image are taken. Spatial representation of internal structure is gained by numerical mathematical methods in digital form, usually as an STL (standard triangulation/tessellation language) model. Resolution is characterized with so-called voxel size, which depends on acceleration voltage, size of sample and distance of sample and source.

In our experiment a Werth industrial computer tomograph (CT) machine was applied with maximum 225 kV accelerating voltage.

In this study shape of the lattice was imaged, which is really the surface of the solid body. Now microstructure inside the material was not revealed.

RESULTS AND DISCUSSION

After CT measurement we had two electronic body representation of the specimen in STL format. First is the CAD body model which was the base of manufacturing. This is the ideal shape of the specimen. Second body model was resulted from CT inspection by processing software of the measurement system. This represents the realized shape of the body. The two STL file were compared with each other.

Figure 2 shows planes in which measured STL file was sliced and digital dimensional measurements and observations was taken.

Two STL files were fit to each other by least squares method. Figure 3 shows deviations of measured shape from designed (ideal) shape. This image represent absolute value of deviations with color code. It is observable that maximum deviation on the flags is +0.213 mm. Compared to classical manufacturing technologies this is a bad value. However we must not forget that there are biomedical applications where this accuracy is sufficient. For example a bone implant needs not higher accuracy since bone grows around it.

On Figure 4 we can see a highly important phenomenon. Original designed shape is rectangular with plane faces and sharp corners. In contrast to it, manufactured (realized) shape is quite rounded. This follows from principle of SLM. When particles of metal powder are melted those moves as fluid drops and their movement is determined by two factors: surface tension and wetting the underneath not melted metal part. Both effects makes the material arrange along rounded shapes and will never form either sharp corners or plane faces. Manufactured shape seems as it was designed from unit cells with spherical holes. SLM will never create sharp corners in micro scale.

On Figure 5 several material defects are demonstrated. This may come from contractile effect of surface tension. The higher is the temperature the stronger is this effect. It means that overheating is to be considered and corrected if we can see high number of such material defects.

Figure 6 shows a material excess. This may come from uneven powder spreading or splatter phenomena, which is similar to what is well known in case of welding.

SUMMARY AND CONCLUSIONS

In this study a tensile test specimen with lattice structure in the middle was investigated. The test specimen was produced by SLM in horizontal direction. Material of the specimen is Ti6Al4V. Shape and dimensional accuracy of the specimen was studied by CT inspection. Both designed and measured shape was available in STL files. Two body model were fit to each other and deviations were visualized and characterized numerically.

The following conclusions can be stated:

* Largest dimensional deviation of manufactured specimen from designed shape is in the magnitude of 0.2-0.3 mm. This is not sufficient in most mechanical engineering applications, but may be eligible for biomedical purposes.

* In macro scale the shape of the specimen stands close to the design.

* In micro scale shape of lattice details are rounded even if the original design was rectangular, brick-like with plane faces and sharp corners. Shape of pores is not cubic, indeed spherical or circular.

* Material defects can be observed at several locations of the model in the lattice structure. Thin rod-like fractions and fine details tend to be rounded by fluid behavior during the SLM process. This is valid for details with size under 1 mm.

* Material excess can also be observed. This may come from inaccuracies of powder spreading of splatter.

Acknowledgements

This work was supported by project entitled "Research of osteosynthesis of implants and development trabecular structure using additive manufacturing", and with identification number: GINOP-2.2.1-15-2017-00055. Test specimens were manufactured at the Biomechanical Laboratory at University of Debrecen, Hungary within the research work of the project. This project was supported by the Scientific Council of the University of Nyíregyháza.

Sidebar

Note: This paper was presented at IIZS 2022 - The XII International Conference on Industrial Engineering and Environmental Protection, organized by Department of Mechanical Engineering and Department of Environmental Protection of the Technical Faculty "Mihajlo Pupin" Zrenjanin, from the University of Novi Sad, in cooperation with partners - University Politehnica Timisoara, Faculty of Engineering, Hunedoara (ROMANIA), University "St. Kliment Ohridski", Technical Faculty, Bitola (MACEDONIA), "Aurel Vlaicu" University of Arad, Faculty Of Engineering, Arad (ROMANIA), University of East Sarajevo, Faculty of Mechanical Engineering East Sarajevo, Sarajevo (BOSNIA & HERZEGOVINA) and University of Giresun, Faculty of Engineering, Giresun (TURKEY) - in Zrenjanin, SERBIA, in 06-07 October, 2022.

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