EFFECTS OF POSITIONING CONDITIONS ON MATERIAL PROPERTIES IN POWDER BED FUSION ADDITIVE MANUFACTURING

Among additive manufacturing technologies, Powder Bed Fusion (PBF) is considered the most common process. Although the PBF has many advantages, some issues need to be clarified, such as positioning. In this study, the effect of positioning on the microstructures in the PBF method was investigated. Ti6Al4V samples were manufacutred in different positions on the building platform and investigated by means of temperature, porosity, microstructure and hardness. In this study, martensitic needles were detected on the microstructure samples. Some twins were noticed on primary martensitic lines and the agglomeration of β precipitates was observed in vanadium-rich areas. The positioning of samples were revealed to have an effect on temperature gradients and the average martensitic line dimensions. Besides, different hardness values were attained depending on sample positioning conditions. As a major result, cooling rates were found related to the positions of samples and the location of points on the samples.


INTRODUCTION
Additive manufacturing (AM) processes are considered game-changer in the industrial field due to their ability to produce complex geometries without waste of feedstock material [1].In particular, Powder Bed Fusion additive manufacturing technologies, which includes selective laser melting (SLM) and electron beam melting, are used to manufacture prototypes and small series of metallic complex parts [2].In the SLM process a wide range of materials is available for many applications and novel materials can be developed for specific applications [3].SLM technique comprised many advantages such as complex-shaped, light weighted and custom-made part manufacturing; nevertheless, some issues like hot cracks, residual stresses, porosities and lack-of-fusion defects can be present in SLM parts [4].Lack of fusion defects occurred due to inadequate laser energy density.To prevent this defect, the energy concentration must be tuned by changing the manufacturing parameters [5].The heating of the building platform is generally performed to decrease thermal stresses and reduce the formation of hot cracks within the material microstructure [6].The relation between thermal gradient and residual stress was revealed by experimental methods and finite element analysis in ref. [7,8].Different thermal histories can lead to substantial differences in terms of microstructural and mechanical properties of the final component [9].Estimating and controlling the microstructure of the processed material would be beneficial to achieve enhanced mechanical properties [10].Many parameters such as laser speed, energy density, layer thickness and hatch distance affect the microstructure of materials during SLM process.Furthermore, some extra parameters that affect material microstructure should be considered, including orientation and position of components on the building platform and temperature distribution [11].Ti6Al4V is widely used for additive manufacturing of components that are employed in medical and aerospace industries due to its lightness, biocompatibility and good compromised of mechanical properties [13].Several studies regarding the effect of manufacturing parameters on Ti6Al4V microstructure were published [14,15].Parameter optimization was accomplished to obtain small grains and homogenous microstructures [16].Numerous studies addressed the investigation of microstructural defects such as cracks, and lack of fusion pores [17].Grains of α'-phase and acicular martensite needles were found within the microstructure of T6Al4V produced by SLM [18].As the cooling rate increases, the microstructure shows a higher quantity of martensite, leading to an embrittlement of the material [12].In addition to that, some of the thermal situations could cause differences in the microstructures.In particular, higher grades of additional martensitic lines, dislocations, and twins emerged [19].Prior researches have emphasized that twins, which were, restrained the elongation of martensitic lines and lead to precipitation of fine grains, lead to increased mechanical properties.Furthermore, secondary martensite lines affected the mechanical properties of materials as well.Some authors have driven the further development of the additive manufacturing of Ti6Al4V considering the effect of process temperatures and cooling rates on the final microstructure.The complexity of the microstructure consisted of a large martensitic lines, arising of secondary, ternary and quaternary martensitic lines, dislocations and twins [20,21].
In the literature, besides microstructure of SLM additive manufactured Ti6Al4V material was examined many times, there was no study about manufacturig of multiple samples at the same time.Positioning-related microstructures were not focused on before in the articles.In this study, samples of Ti6Al4V were produced by SLM placed in different positions on the building platform in order to evaluate the effect of temperature distribution and cooling rate on the microstructure and porosity of the material.Moreover, the effect of different microstructure on the final hardness of samples was evaluated.

MATERIALS AND METHODS
The chemical composition of the gas-atomized Ti6Al4V powder is reported in Table 1.A EOS M280 DMLS system was used to produce 5 cubic samples using the gas-atomized Ti6Al4V powder.In the study, results were interpreted clearly with the production of 5 samples representing different positions.With the manufacturing of samples larger than 5 mm, the inadequacy of mechanical properties and problems in the microstructure have been largely avoided.The mechanical properties increase with increasing sample size [22,23].The majority of prior researchers have found that the optimum parameters were defined as 1250 mm/s scanning speed, 170 W laser power, 0.7 mm hatching distance, 0.03 mm layer thickness, 67° rotational scanning, and 50.7 J/mm3 energy density [24].Ti6Al4V gas atomized powder from EOS GmbH was used in the present study.The morphology of the powder particles was spherical in shape.However, several satellites are attached to the powder particles.The average diameter of the powder particles is observed to be 36 ± 5 μm [25].Purity of argon gas was 99.999% to guarantee a controlled atmosphere during the process.The cubic samples were sized as 10x10x10 mm 3 .The distance between cubes was 5 mm.The distribution of the samples on the building platform is depicted in Fig. 1.Besides, 2 mm block type support structures were implemented.Manufactured samples were cut from the build platform by wire electrical discharge machine.
The temperature of molten materials was found by the experimentally validated thermal camera measurements.Curves of temperature were achieved by Optris PI160 thermal camera system with 120 Hz frequency.The sections of samples parallel to the building platform were investigated by optical microscope in order to perform relative density analysis.The relative density values of samples were reported in Fig. 3 and range between 99.98% and 99.95%.These results were similar to the literature   Results were reported in Fig. 5. Martensitic laths with different thicknesses are visible within the microstructure of the as-built material.Thin martensitic laths were recognized on the top surface of the samples (Fig. 5A-B), while thick martensitic laths were observed in the middle section (Fig. 5C-D).It was thought that the higher maximum temperature values achieved in the top section as seen (Fig. 2).This could lead to lower cooling rates due to lower temperature gradient, which was related to cooling through the melting temperature to mean layer temperature.
Mean layer temperature increased during manufacturing from 35 o C to 75 o C. It was occurred owing to more effectual repetitive laser heating and cooling cycle in the middle section than the top section.Similar to this study, it was found that primary and secondary α′ martensites were a little coarsening, while the finer ternary and quartic martensite laths were forming  SEM images reported in Fig. 7 shows the evidence of white spots within the microstructure of the as-built material.These spots were ascribed to the β phase, related to agglomerations of vanadium atoms as observed with EDX analysis.Vanadium content was 4.9% in the material and 0.7% more than the standard.The β phase can appear in form of white spots due to the higher vanadium content that results in a brighter contrast [37,38].The zone of Fig. 7-A showed the more brittle and harder mechanical properties.Finally, the widths of primary martensitic laths were measured in order to understand the influence of the building position and maximum temperature of samples on the final microstructure.Results were reported in Fig 8.
The widths of martensitic lines were measured on 7 different laths with SEM images.Mean standard deviation of the measurements was calculated as ±0.135 µm.Thinner martensitic laths are visible on top sections of both center and back samples.The samples, which are placed nearby the argon gas inlet, were cooled faster according to temperature values of samples (Fig. 2).Thus the maximum temperature results observed higher far samples from the gas inlet.This phenomenon led to emerging thinner martensitic lines on neighboring fields to argon gas flow due to lower temperature gradient.While coarser primary martensite laths were formed, finer grains were existed due to forming secondary, tertiary and quartic martensite laths.Besides, in nearby areas such as edges and corners of two adjacent samples, secondary heat effects were noticed by analyzing the width of martensitic lines according to the observations in Fig 5.Although the primary martensitic lath width of the top section was finer, additional martensitic lines (finer than the primary martensitic lath) on the middle section led microstructure to more complex than the top section.Though the primary martensitic laths of the middle areas were larger than the top areas, middle areas were observed more complex than the top areas in the light of figures 5 and 6.Significantly, lower hardness values were noticed on sample 3 than sample 5 due to coarser laths.In the literature, similar results were taken that The width of the acicular α′ martensite increased and the length decreased with an increasing number of melting steps from single to triple melting.Since the position of the back sample is closer to the gas the material, leading to higher microhardness values, can experience inlet, higher cooling rate.Accordingly, the hardness on edges and corners were ranged between 365-400 HV and micro hardness values at the center points varied between 390-400 HV.The results were similar with previous studies wherein the hardness of the Ti6Al4V material in PBF additive manufacturing was measured between 330-407 HV.With the repetitive and high energy density melting process, the structure of the α′ martensite phases changed and the hardness increased [39,40].Nevertheless, in case of the reaching excessive hardness, outer surfaces would be brittle.Therefore, unexpected failure mechanisms could be occurred.

CONCLUSIONS
In this study, the effects of positioning of samples on the manufacturing platform were investigated by various methods.Remarkable results were identified as seen below.
• Some residues and exhaust gasses came in sight during the melting powders by the laser.Some additional constituents in the powders were burned and the residues which came up by the melting process were relocated to the sample 1 owing to the argon gas flow.Hence, the sample 1 was affected negatively and had a high porosity rate more than the others.• Temperatures of all samples were linearly increased during manufacturing.The temperature gradient on the samples was not homogenous due to their positions on the building platform.Remarkably, the temperature on sample 3, located at the center of the platform, had higher temperature values and lower temperature gradient than the others.Sample 3 had boundary conditions that made heat transfer difficult due to being surrounded by other samples.• The top section of samples showed thinner primary martensitic lines than the middle (inner) section but also the minimum primary martensitic line widths of the middle section were thinner than top section.The middle section of samples included secondary and ternary numerous sub-martensitic lines more than the top section.Furthermore, the center of the samples included thicker martensitic lines than side areas.As the laser melting the upper layers, previous layers were influenced.Further, heat performs like heat treatment upon the material.Upper layers of samples had fewer complex microstructures.
• The most significant differences in temperature and hardness were noticed between the sample 3 and 5 since the higher cooling rate was observed on the back sample due to its closer position to gas flow hole.Higher cooling rates prompted to acquire harder samples because of martensitic transformation.Middle sections of samples had higher hardness values than the top section due to the complexity of α' phases and it was associated with multiple quenching of the material.

Figure 3 .
Figure 3. 1 A. Schematic of position of samples on building platform; B. Photo of manufactured samples; C. Schematic of analyzed sections

Figure 2 .
Figure 2. Peak temperatures on manufactured samples [30].Mean Standard deviation of the results was found as 0.005%.Higher maximum temperature values can promote the formation of a larger quantity of sputters, exhaust gasses and residues of burnt materials during the laser scan [31-34].These phenomena can also cause increased porosity.

Figure 3 .
Figure 3. Porosity ratio of samples with regard to positions

Figure 4 .
Figure 4. A) Low-magnification and B) high-magnification optical images of the section parallel to the building platform of as-built samples

[ 21 ]
. The High magnification SEM images are shown in Fig 6.Similar in the literature, twins were seen on the primary martensitic laths.Besides, primary, secondary and ternary α' martensitic laths were ascertained in the as-built microstructure.Thermal history induced severe internal stresses caused dislocations such as twins.

Figure 8 .
Figure 8.Primary martensitic lath width for different sections of samples

Figure 9 .
Figure 9. A) Micro hardness values of center point of samples, B) Micro hardness values of edge of samples, C) Micro hardness values of corner of samples