Surface modification of Ti-6Al-4V powder during recycling in EBM process

Surface and Interface Analysis published by John Wiley & Sons Ltd Electron beam melting is an additive manufacturing technology in vacuum that suits Ti-6Al-4V parts, which has a high affinity with oxygen. Since the high cost of the feedstock powder, the un-melted powder is often recycled in the subsequent process. In this study, the influence of powder reuse on the surface characteristics of Ti-6Al-4V powder is examined using a variety of technology including scanning electron microscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The modification of surface morphology and chemistry either generally or locally are revealed and discussed, which helps the creation of powder recycling strategy. Compared with fresh virgin powder, “worn” and rougher powder particles are observed after recycling. Meanwhile, the average oxide thickness is slightly increased, and less Al enrichment was found at the surface. Locally varied chemistry/oxide thickness on different powder particles or different location on the same particle is observed.


Electron beam melting is an additive manufacturing technology in vacuum that suits
Ti-6Al-4V parts, which has a high affinity with oxygen. Since the high cost of the feedstock powder, the un-melted powder is often recycled in the subsequent process. In this study, the influence of powder reuse on the surface characteristics of Ti-6Al-4V powder is examined using a variety of technology including scanning electron microscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The modification of surface morphology and chemistry either generally or locally are revealed and discussed, which helps the creation of powder recycling strategy. Compared with fresh virgin powder, "worn" and rougher powder particles are observed after recycling. Meanwhile, the average oxide thickness is slightly increased, and less Al enrichment was found at the surface. Locally varied chemistry/oxide thickness on different powder particles or different location on the same particle is observed. manufacturing of complex products with near-net shape such as structural parts with internal features and complex internal cooling channels. AM has an ability to join the same materials or different materials together. The design freedom is very high. This enables faster product development, short development cycles, lighter products, and more efficient use of the material. There are many different additive manufacturing techniques available. Electron beam melting (EBM), developed by the Swedish company Arcam AB, is a core AM technology for building parts using high-energy electron beam, by which a large fraction of thermal energy is released to heat, sinter, and melt the powder material. EBM works in vacuum environment, which lowers the risk of oxidation, being a good choice for material with high affinity to oxygen. It involves the preheating of the powder with build temperature of 600-750 C before melting, which reduces the temperature gradient and thus decreases the residual stresses.
Titanium is a relatively light metal with density of 4.51 g/cm 3 , which is significantly lower than those of commonly utilized engineering metal Fe, Ni, and Cu (7.87-8.96 g/cm 3 ). Titanium alloys are well known as material for the aerospace industrial since 1940s.
The most popular Ti alloy is Ti-6Al-4V, a α + β alloy containing 6 wt% Al and 4 wt% V. More than 50% of all alloys in use today are of this composition. Titanium alloys have high strength to weight ratio, making them optimum materials for components such as airframes, blades, and disks in jet engine in the aerospace industry. Due to low density, good corrosion resistance, and biocompatibility, they are well suited for medical applications as body implants, e.g., artificial hip joints.
Titanium alloys have also found wide range of application in energy, automotive, sports, and customer goods industry. The conventional manufacturing relies on casting, rolling, forging, and machining. The EBM is a preferred AM technique for fabricating Ti alloy parts considering high affinity of Ti to oxygen. The microstructure of as-built Ti-6Al-4V is generally fine lamellar α + β with both colony and basketweave morphology. Anisotropic and graded microstructure 1 has been observed. In case of short builds, thin-wall structures, or net structures, the presence of α 0 martensite was also reported. Post heat treatments below the transus temperature induce coarser α lamellae when cooling slowly with some globular morphology. 2 In the powder-bed process, only a limited portion of powder is melted and solidified in an EBM process. Unmelted powder is intended to be reused due to the high powder cost. In the real industrial practice, hundreds of recycling steps are often performed, and building may have variable duration. Sieving operation is often conducted. Repeated heating and cooling in vacuum during an EBM process may modify the un-melted metal powder gradually in terms of shape, size distribution, surface morphology, and chemistry. The objective of this study is the influence of powder reuse on the surface chemistry and morphology.

| EXPERIMENTAL
Commercial Ti-6Al-4V powder produced by plasma atomization production was used in this investigation. More than 90 wt% has particle size between 45 and 106 μm. After each EBM building process using Q10+ system (Arcam EBM, Sweden), powder was sieved and reintroduced into a subsequent production line without any addition of virgin supply. Both the virgin powder and powders recycled for 5 and 10 times were analyzed.
PHI 5000 VersaProbe III scanning X-ray photoelectron spectroscopy (XPS) microprobe was used with monochromated Al Kα radiation (1486.6 eV) to investigate the chemistry including compositions and chemical states at powder surface. Spectra were recorded with a 100-μm X-ray beam size and pass energy of 140 and 26 eV for surveys and high-resolution measurements, respectively. Area mode was used with the selected size of 400 × 400 um covering more than 50 powder particles. The measurement was repeated at least twice on two specimens. This means XPS results shown in this paper provided the average chemical information of the powders. The F I G U R E 1 Scanning electron microscopy images of the virgin powder (A, B) and powders recycled for 5 (C, D) and 10 (E, F) times F I G U R E 2 X-ray photoelectron spectroscopy survey spectra from the powders (A); X-ray photoelectron spectroscopy depth profiles (B, C) and cation profiles (D, E) from virgin powder (B, D) and powders recycled for 5 times (C, E). The etch rate is 24.8 Å/min for Ta 2 O 5 with known oxide thickness compositions of local features on the powder surface were determined using a PHI 700 Scanning Auger electron spectroscopy (AES).
The electron accelerating voltage was 10 kV, and the beam current was 10 nA. Image was registered frequently in AES to ensure the data were acquired at the location intended. The oxide thickness is defined as the depth where the intensity of oxygen decreases half by means of successive XPS/AES analyses and argon ion etchings over an area  Figure 1 gives the SEM images from the powder studied. Virgin powder was highly spherical with some satellites and small particles ( Figure 1A). In general, it possesses relatively smooth surface with equiaxed structures ( Figure 1B). Sometimes acicular martensite was observed. Occasionally, elongated or broken particles could be detected. After recycling, it seems that there was fewer satellite. Most particles maintained spherical shape but less smooth. Roughening of the surface was often observed, as indicated by the arrows in Figures 1C and 1D. This might be related to the local oxidation or partial melting. Few aggregates could be encountered, as shown in the top of Figure 1C. Particles might be sintered due to melting ( Figure 1F). It should be mentioned that features similar to the virgin powder were also seen from both recycled powders. Theses powders were less affected by the EBM process, though the particles might be contaminated occasionally ( Figure 1E). XPS survey spectra from the powders studied are shown in  Figure 2D). The enrichment of Al was significantly reduced after recycling ( Figure 2E). Meanwhile, V at the surface might be slightly higher than the nominal concentration.

| RESULTS
Thanks to the high lateral resolution, AES is a wonderful tool to investigate the local surface chemistry. Figure 3 gives the oxide thickness at varied locations of different powders. For virgin powder ( Figure 3A), the oxide thickness was between 4.1 and 6.4 nm with an average value of 5.1 nm. It was slightly smaller than the value from XPS because the location was selected to avoid the shadowing effect in AES analysis. As a general trend, oxide became thicker after recycling ( Figures 3B and 3C). This was consistent with the results from XPS. However, surface oxide became less homogeneous and local thicker oxide was found. For example, points 3 and 7 in Figure 3B (recycled for 5 times) having local rough feature possessed significantly thicker oxide. Figure 4 exhibits the depth profiles of points 4 and 7 in Figure 3B. They differed in both oxide thickness and Al concentration. Compared with location 4 ( Figure 4A) which was smooth, the oxygen profile of point 7 became less steep ( Figure 4B), indicating increased oxide thickness. Meanwhile, the surface was more contaminated by carbon. Considerably more Al was detected in this local rougher place ( Figure 4D compared with Figure 4C).
The unevenness was even obvious for powder recycled for 10 times, as shown in Figure 3C. It should be mentioned that the oxide thickness obtained was dependent on the thermal history of the powder particles selected. Figure 5A is a local area found on the surface of the powder recycled for 10 times. Interestingly, AES analysis ( Figure 5B) indicated that point 2 was almost pure Al oxide, while point 1 also contained some Fe, which was a common impurity in Ti-6Al-4V. The temperature in the molten pool is estimated to be between 1900 and 2700 C. 4 Un-melted powders thus experience repeated heating and cooling. Aluminum has higher vapor pressure than Ti, and V has the lowest. The estimated vapor pressure for Al is 6.5 mbar at 2000 K and 0.01 mbar at 1500 K. The evaporation temperature is expected to be even lower in vacuum. 5 It is indeed a natural process that Al having high vapor pressure evaporates during the EBM process, leading to reduced Al concentration at the surface, as revealed in Figure 2C.

| DISCUSSION
Considering Al being a principal strengthening element of the  It is well known that O has large solubility in both α and β, especially in α. Oxygen pick-up and dissolution are also possible during powder recycling, especially at elevated temperatures.
Oxygen is a strong α stabilizer and in principle provides interstitial hardening with lowered ductility. Such chemical composition change is expected to affect the microstructure and consequently the properties locally.

| CONCLUSIONS
In this study, the influence of powder reuse on the surface characteristics of Ti-6Al-4V powder has been examined. Titanium oxide is dominant at the surface in general. Surface roughening, particle sintering, and partial melting are observed in the recycled powder.
Averagely, oxide layer becomes thicker. The overall Al detected at the surface of the reused powder is less, probably due to the evaporation.
On the other hand, localized oxidation with increased Al enrichment occurs owing to the high affinity of Al with oxygen. As a result, locally varied chemistry/oxide thickness on different powder particles or at different location on the same particle after recycling are found. This unevenness increases with recycling.