3.1 factors in SLM
Enumerating all known factors in SLM has two advantages. Firstly, this is the
basis for the factor screening. Secondly, the theory on dimensionless numbers
asks to do this in a formal way, in order to avoid the omitting of influencing
factors. The influence of the factors will also be explained.
The list here is rather complete, although more factors might influence the
process. Factors like scan strategy or intensity distribution incorporate many
factors. And many factors are temperature-dependent: coefficients should be
included to describe this behaviour. At the moment, this would lead to far
and would not give better results.
3.1.1 Environment
The atmosphere in which the process takes place, is the first thing to control in
SLM. Non-inert particles, like oxygen, can react with the molten metal. This
will influence the final properties of the component and should be avoided.
Thus, a well-designed processing chamber is needed to be able to control the
environmental parameters in a proper way.
The atmosphere does not seem to contribute much to the heat balance,
meaning that the heat conductivity of the gas in the chamber, its heat capacity
and its heat exchange coefficient of convection can normally be neglected.
Inert gas Many gasses are known to be inert or having good properties for
some applications. H2 for example, causes the available oxygen to be reduced,
thus preventing metals to oxidise. For Ti-alloys however, H2 and N2 cause
embrittlement. Hence, Ar is used because He is 3 to 4 times more expensive.
For stainless steel, N2 is used, although some prefer using Ar1.