 Images of gas atomized MAR-M-247 powder, sample1, size range: 45–106um [ref 15] | Some defects found in parts built using AM are related to processing parameters or to the composition of alloys optimised for more conventional, slow-cooling manufacturing process. These defects – such as porosity or cracking – can sometimes be healed by post-processing such as hot isostatic pressing (HIP). Yet, the formation of pores during AM is also linked to the quality of virgin powder. |
Degraded mechanical behaviour in AM specimens can be traced back to several types of microstructural defects that develop during the AM processes [1-8]. Pores and local voids are the most critical flaws. They can be avoided but they can also limit fatigue strength and fracture toughness to such an extent they impede the wider adoption of AM for critical parts [2, 4, 8, 9]. This is why, numerous studies are being carried out to identify optimum ‘‘minimum void” conditions [10], such as total void volume, void size distribution, and void shape.
Powder bed fusion, directed energy deposition and cold spray are the main AM technologies using metallic AM feedstock powders. They are most widely used to build complex shapes. Generally, a spherical powder shape is preferred for these technologies to enhance flowability, spreading and packing.
Internal porosity in powder particles
Nearly-spherical powder particles are typically produced by various inert gas atomization methods. Most techniques make use of an inert gas environment--typically argon or helium—to protect from oxidation and to promote convective cooling. During formation of the powder, liquid metal interacts with the inert gas environment, leading to the entrainment of significant levels of gas within the particles, irrespective of the alloy or the production technique [11].
Images of gas atomized MAR-M-247 powder, sample2, size range: 45–106um [ref 15]
Gas concentrations and porosities in particles both tend to increase in a non-linear fashion with increasing particle size. There is a large increase in gas content and porosity near a particle size of about 75um, after which the quantities remain relatively constant, irrespective of alloys systems and gas atomisation technique used. In addition, to gas entrapment, a secondary contribution from fine interdendritic solidification shrinkage may also explain some finer voids [12,1-4].
Examples of commercial gas atomized powders show typical trapped gas contents in internal pores are up to 28% of the powders (183/653um) from one vendor and 21% of the powders (121/575um) provided by a 2nd vendor [13].
Inert gases entrapped during powder production can survive local melting and fusion zone turbulence [9]. Voids are also retained during consolidation by hot extrusion [6]. When reheated to high temperatures in the absence of external pressure, the gas bubbles will coarsen, resulting the appearance of macroscopic gas porosity [6, 7].
Surface satellites on powder particles
Another type of internal porosity comes from satellites welded to particles that seem otherwise mostly spherical [14].
SEM images of gas atomized MAR-M-247 powder, sample1, size range: 45–106um [ref 15]
Unlike gas pores trapped inside spherical particles, particulates decorated with satellites impact the bulk behaviour of the feedstock. They impede continuous and homogeneous flowability when spread across a powder bed or when propelled by a carrier gas through the nozzle of a powder feeder. This variability in powder feeding can get in the way of homogeneous consolidation and promote the formation of porosity in AM structures.
Commercial gas atomized powders can appear reasonably spherical at low magnification but can show significant satellites at higher magnifications. Studies show this aspect is directly linked to the equipment design and parameters used in powder atomisation [13, 15].
Take-away
Rapidly solidified powders produced in inert gas environments generally contain internal pores. Optical metallography and porosity measurements can show significant amount (up to 28%) f trapped internal gas. In general, the gas content and porosity tend to increase with increasing particle size. This type of porosity tends to survive AM processing.
Satellites welded to nearly-spherical particles can prevent homogeneous powder feeding and impede homogeneous consolidation.
Satellites welded to nearly-spherical particles can prevent homogeneous powder feeding and impede homogeneous consolidation.
References
1 R. H. Vanstone, F. J. Rizzo and J. F. Radavich, in R. Mehrabian, B. H. Kear and M. Cohen (eds.), Proc. 2nd lnt. Conf. on Rapid Solidification Processing, Principles and Applications, Reston, VA, March 23-26, 1980, Claitor's Publishing Division, Baton Rouge, LA, 1980, pp. 260-272.
2 T. Tokarz and J. F. Radavich, Microstruct. Sci., 10 (1982) 51-56.
3 Y. F. Ternovoi et al., Sov. Powder Metall. Met. Cerarn., 24 (1) (1985) 10-13.
4 U. Backmark, N. Backstrom and L. Arnberg, Powder Metall. Int., 18 (6) (1986) 422-424.
5 J. E. Flinn, G. E. Korth and R. N. Wright, in P. W. Lee and J. H. Moll (eds.), Rapidly Solidified Materials: Properties and Processing, American Society for Metals, Metals Park, OH, 1988, pp. 153-162.
6 J. Prybylowski, R. M. Pelloux and P. Price, Powder Metall., 24(2) (1984) 107-111.
7 G. I. Parabina et al., Sov. Powder Metall. Met. Ceram., 16 (7) (1977) 560-561.
8 J. E. Flinn, G. E. Korth and R. N. Wright, in F. H. Froes and S.J. Savage (eds.), Processing of Structural Metals by Rapid Solidification, American Society for Metals, Metals Park, OH, 1987, pp. 459-467.
9 J.C. Bae et al., Scr. Metall., 22(1988)691-696.
10. R. Cunningham, S.P. Narra, C. Montgomery, J. Beuth, A.D. Rollet, Synchrotron based X-ray microtomography characterization of the effect of processing variables on porosity formation in laser power-bed additive manufacturing of Ti-6Al-4V, JOM 69 (3) (2017) 479–484.
11 B. H. RABIN, G. R. SMOLIK and G. E. KORTH, Characterization of Entrapped Gases in Rapidly Solidified Powders, Materials Science and Engineering, A 124 (1990) 1-7
12 H. E. Eaton and N. S. Bornstein, Metall. Trans. A, 9 (1978) 1341-1342.
13 I.E. Anderson et al., Feedstock powder processing research needs for additive manufacturing development, Curr. Opin. Solid State Mater. Sci. (2018), https://doi.org/10.1016/j.cossms.2018.01.002
14 N. Dombrowski, W. Johns, The aerodynamic instability and disintegration of viscous liquid sheets, Chem. Eng. Sci. 18 (1963) 203–214.
15 I.E. Anderson, E.M.H. White, J.A. Tiarks, T.R. Riedemann, J.D. Regele, D.J. Byrd, R. D. Anderson, Fundamental Progress Toward Increased Powder Yields from Gas Atomization for Additive Manufacturing, in: Ryuichiro Goto, J.T. Strauss (Eds.), Advances in Powder Metallurgy and Particulate Materials-2017, Metal Powder Industries Federation, Princeton, NJ, 2017, pp. 136–146.
1 R. H. Vanstone, F. J. Rizzo and J. F. Radavich, in R. Mehrabian, B. H. Kear and M. Cohen (eds.), Proc. 2nd lnt. Conf. on Rapid Solidification Processing, Principles and Applications, Reston, VA, March 23-26, 1980, Claitor's Publishing Division, Baton Rouge, LA, 1980, pp. 260-272.
2 T. Tokarz and J. F. Radavich, Microstruct. Sci., 10 (1982) 51-56.
3 Y. F. Ternovoi et al., Sov. Powder Metall. Met. Cerarn., 24 (1) (1985) 10-13.
4 U. Backmark, N. Backstrom and L. Arnberg, Powder Metall. Int., 18 (6) (1986) 422-424.
5 J. E. Flinn, G. E. Korth and R. N. Wright, in P. W. Lee and J. H. Moll (eds.), Rapidly Solidified Materials: Properties and Processing, American Society for Metals, Metals Park, OH, 1988, pp. 153-162.
6 J. Prybylowski, R. M. Pelloux and P. Price, Powder Metall., 24(2) (1984) 107-111.
7 G. I. Parabina et al., Sov. Powder Metall. Met. Ceram., 16 (7) (1977) 560-561.
8 J. E. Flinn, G. E. Korth and R. N. Wright, in F. H. Froes and S.J. Savage (eds.), Processing of Structural Metals by Rapid Solidification, American Society for Metals, Metals Park, OH, 1987, pp. 459-467.
9 J.C. Bae et al., Scr. Metall., 22(1988)691-696.
10. R. Cunningham, S.P. Narra, C. Montgomery, J. Beuth, A.D. Rollet, Synchrotron based X-ray microtomography characterization of the effect of processing variables on porosity formation in laser power-bed additive manufacturing of Ti-6Al-4V, JOM 69 (3) (2017) 479–484.
11 B. H. RABIN, G. R. SMOLIK and G. E. KORTH, Characterization of Entrapped Gases in Rapidly Solidified Powders, Materials Science and Engineering, A 124 (1990) 1-7
12 H. E. Eaton and N. S. Bornstein, Metall. Trans. A, 9 (1978) 1341-1342.
13 I.E. Anderson et al., Feedstock powder processing research needs for additive manufacturing development, Curr. Opin. Solid State Mater. Sci. (2018), https://doi.org/10.1016/j.cossms.2018.01.002
14 N. Dombrowski, W. Johns, The aerodynamic instability and disintegration of viscous liquid sheets, Chem. Eng. Sci. 18 (1963) 203–214.
15 I.E. Anderson, E.M.H. White, J.A. Tiarks, T.R. Riedemann, J.D. Regele, D.J. Byrd, R. D. Anderson, Fundamental Progress Toward Increased Powder Yields from Gas Atomization for Additive Manufacturing, in: Ryuichiro Goto, J.T. Strauss (Eds.), Advances in Powder Metallurgy and Particulate Materials-2017, Metal Powder Industries Federation, Princeton, NJ, 2017, pp. 136–146.