SLM process control - pre-processing of reactive aluminium powders to minimise porosity

A few months ago, we were wondering about process control in powder bed fusion of reactive powders. What are the impacts of particles’ surface contamination on the fabrication of metal components? And what are the best ways to minimise it during the complete manufacturing cycle?
 
Then, very few studies were trying to assess the impact of powder particles surface chemistry on the process (powder spreading, melt wettability, pores formation, etc…) and on the final product characteristics (relative density, etc).
 
As more data get publicly available, we can present the results of a detailed investigation aiming to 1) understand the effects of powder surface chemistry, 2) minimise particles surface contamination on the finished products and 3) improve SLM process control.

Aluminium particles’ surface chemistry (as-received)

Reactive materials are naturally sensitive to oxidation and moisture pickup. As a result a moisture skin can develop after fabrication during handling, inappropriate storage and manual sieving/recycling.
 
Aluminium surface chemistry is typically complex, with oxygen present in three common forms:

• sesquioxide (Al2O3),
• carbonated hydroxide (Al-CHxO)
• and moisture (H2O) [1].

The relative density of AlSi cubes fabricated by SLM at different energy densities using as-received and dried powder [7].

Influence of aluminium particles’ surface chemistry during SLM

As it disintegrates during SLM, this skin can drive the formation of deleterious oxide and hydroxide and impede full densification of components. [see our article on formation of hydrogen pores in AlSi10Mg alloy caused by dissolved hydrogen].
​
More precisely, research shows that aluminium can react with moisture at high temperatures [2]:

• At a temperature from 500–800C, Al reacts with moisture to produce Al oxide.
• At a temperature above 800C, Al reacts with moisture to produce Al hydroxide [Al(OH)3].

 
Given the high temperature rise in the melt pool during SLM ([3-5] up to 1000C), the surface moisture present on AlS12 particles is likely to react with the aluminium to produce both Al oxide and hydroxide. Both are known to contribute to defects formation and significantly reduce relative density in the final parts [6].

Pre-processing aluminium powder – impact of drying step on particles’ surface chemistry

​In-situ analysis conducted on as-received powder shows that typical particles’ surfaces contain Al metal and Al–O–CHxO [7].
 
During drying, measurements suggest that the relative content variation on the surface of the Al metal and Al–O–CHxO is due to a reduction in moisture H2O rather than disappearance of Al2O3 (which is very stable at low temperature ~100C) or CHxO (no change measured).
 
This shows that the moisture skin can be removed by low temperature drying.

Variation of−O–CHxO- and moisture from the powder surface upon in-situ drying for different periods of time [7].

Pre-process drying of aluminium powder – impact on pores formation

​Compared to cubes SLMed using dried powder, AlSi12 cubes fabricated using as-received powder contain higher contents of oxide and hydroxide. This is most likely caused by the reaction with the moisture during processing.
 
In general Al oxide and Al hydroxide are known to decrease the wettability of the molten AlSi and possibly also trigger balling during SLM (tends to promote pores formation [8]).
The hydrogen produced as a result of the reaction between moisture and Al can also generate the nucleation and growth of hydrogen pores [9] and influence the relative density of components.
It has also been shown that the enhanced convection in the melt pool during SLM can help transfer the gaseous pores into the bottom of the melt pool where they get trapped during solidification [10].
 
Drying as-received powder before processing would effectively remove the moisture skin and the amount of oxides to minimise defects formation during processing. 

Take away

• Removing surface moisture from the AlSi powder particles prevents the formation of Al oxides and hydroxides during processing, reduce pores formation mechanisms and enhance the relative density of final components.
• Adding a pre-process drying step of the feedstock (1h at 100C) before SLM of aluminium powders increases relative density of components. 

References:
[1] N. Phambu, Mater. Lett. 57 (2003) 2907–2913.
[2] J.E. Hatch, Aluminum Properties and Physical Metallurgy, American Societyfor Metals Metals Park Ohio, Metals Park, Ohio, 1984.
[3] X.P. Li, C.W. Kang, H. Huang, T.B. Sercombe, Mater. Des. 63 (2014) 407–411.
[4] X.P. Li, M. Roberts, Y.J. Liu, C.W. Kang, H. Huang, T.B. Sercombe, Mater. Des. 65(2015) 1–6.
[5] S. Katakam, J.Y. Hwang, H. Vora, S.P. Harimkar, R. Banerjee, N.B. Dahotre,Scripta Mater. 66 (2012) 538–541.
[6] S. Das, Adv. Eng. Mater. 5 (2003) 701–711.
[8] X.P. Li et al. / Additive Manufacturing 10 (2016) 10–14
 [8] D.D. Gu, W. Meiners, K. Wissenbach, R. Poprawe, Int. Mater. Rev. 57 (2012)133–164.
 [9] C. Weingarten, D. Buchbinder, N. Pirch, W. Meiners, K. Wissenbach, R.Poprawe, J. Mater. Process. Technol. 221 (2015) 112–120.
[10] D. Dai, D. Gu, Mater. Des. 55 (2014) 482–491.