 micro-XCT reconstruction of Ti-6Al-4V powder with porosity trapped within powder particles shown in red [1] | Post-build hot iso-static pressing (HIP) has become a common practice for the closure of some types of defects in AM metal components. Assuming your products lend themselves to HIPing, can this post-processing method get rid of all porosity? |
Microstructural defects develop during metal AM processes and lower the mechanical properties of components. Hot iso-static pressing (HIP) is getting used as a post-processing technique in an effort to heal voids present in AM components. Its influence varies with the type of defects, such as cracks or types of pores.
It is important to categorise the types of pores present in metal AM. Porosity can arise from (a combination of) the following factors:
- Processing: parameters and/or and inhomogeneous powder feeding generate key hole effect and/or lack of consolidation;
- Solidification kinetics and post-heat treatment homogenisation: this depends on the feedstock composition;
- Feedstock contamination: oxidation and hydrogenation promote physi-adsorption and chemi-adsorption during phase change, where gases are released and trapped in the material;
- Feedstock properties: internal pores present in powder particles, particles features induce non-homogeneous feedstock behaviour, e.g. variable spreadability;
Key discriminating factors used to categorise pores is the presence of gas in voids and their formation mechanisms. In this article we are concerned with porosity passed on from particles’ pores and surviving AM processes. So how efficient is post-processing HIP in getting rid of this porosity? Gaseous pores are typically round with smooth internal surfaces typical of a certain internal gas pressure, in contrast to other voids that exhibit more rugged and irregular profiles.
Impact of HIP on gaseous pores present in virgin powder particles and surviving processing
These spherical pockets of inert gas in larger powders [link] are thought to originate from atomization gas entrapment during melt breakup by a high-energy mechanism [2]. Moreover, this powder-trapped porosity has been confirmed recently by through-penetration X ray visualization [1] to survive local melting and fusion zone turbulence in an AM process model study.
Ref[2] considers the fate of an entrapped gas pore during powder consolidation via hot isostatic pressing. Imagine a particle 50um in diameter with a gas-filled pore 25um in diameter. During HIPing, densification ceases when the gas pressure within the pore equals the applied pressure (~150MPa). At 1000 °C, the 50um gas pore shrinks to a diameter of about 2um under 150MPa applied pressure. Results [2] show that pores are observed in the hot iso-statically pressed 304 stainless steel.
Fraction of particles containing gas pores as a function of particle size [ref2]
However, if 10% of the particles 50um in diameter contain pores (check graph above), then after consolidation the gas pores will amount to only 0.03% porosity.
This amount and size of porosity is too low to be readily detected by density determinations made on the consolidated material and emphasizes the utility of performing density measurements on the starting powder to assess entrapped gas levels.
Image of 304 stainless steel powders consolidated by hot isostatic pressing, showing the presence of small gas pores [ref2]
It is important to point out that it is impossible to achieve actual full densification in powders provided that entrapped gas pores were present [2].
The amount of internal porosity in powder is a function of particle diameter for a fixed powder production setup and conditions. HIP is not entirely effective to heal the pores made from gas atomized powders with diameters greater than about 50–70 mm [1] as they tend to exhibit larger proportion of internal pores that survive EBM/PBF and by LENS/DED [3].
Yet, the final amount of porosity after HIPing may be small enough to present minimal issues in a given component for a given application. Additional factors to take into consideration are the distribution of these pores and their density and their location in the component.
References
[1] Cunningham, Narra, Montgomery, Beuth, and Rollett, 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 2017, DOI: 10.1007/s11837-016-2234-1
[2] B. H. RABIN et al., Characterization of Entrapped Gases in Rapidly Solidified Powders, Materials Science and Engineering, A 124 (1990) 1-7
[3] 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
[1] Cunningham, Narra, Montgomery, Beuth, and Rollett, 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 2017, DOI: 10.1007/s11837-016-2234-1
[2] B. H. RABIN et al., Characterization of Entrapped Gases in Rapidly Solidified Powders, Materials Science and Engineering, A 124 (1990) 1-7
[3] 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