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)
Aluminium surface chemistry is typically complex, with oxygen present in three common forms:
- sesquioxide (Al2O3),
- carbonated hydroxide (Al-CHxO)
- and moisture (H2O) .
Influence of aluminium particles’ surface chemistry during SLM
More precisely, research shows that aluminium can react with moisture at high temperatures :
- 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 .
Pre-processing aluminium powder – impact of drying step on particles’ surface chemistry
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.
Pre-process drying of aluminium powder – impact on pores formation
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 ).
The hydrogen produced as a result of the reaction between moisture and Al can also generate the nucleation and growth of hydrogen pores  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 .
Drying as-received powder before processing would effectively remove the moisture skin and the amount of oxides to minimise defects formation during processing.
- 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.
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