Most of the additive manufacturing (AM) sector initially chose to work with just one specific alloy, that being the die-casting alloy AlSi10Mg. This is also considered to be equivalent to these other alloy designations: British casting alloy LM9, EN 43100, and similar to the US alloy A360.
Whilst this was not widely recognized as being a high strength casting alloy, for metal AM parts it has been shown to be capable of producing reasonably high strength after the appropriate heat treatment, although this fact has also been the subject of much contention.
Broadly speaking this alloy is age hardenable but there has been significant discussion on the appropriate heat treatment cycle required to achieve maximum strength. The standard heat treatment procedure for this alloy is solution treatment followed by artificial ageing, referred to as a T6 cycle. This has solution treatment above 500°C for 4-12 hours, but the temperatures should not exceed 550°C, followed by quenching in water or a polymer melt. The artificial ageing is then normally carried out between 155-165°C for 6-24 hours, with the exact time and temperature dictating the final properties. Hence the tensile strength can be anything from 220 MPa up to 340 MPa, with tensile yield occurring between 180 MPa and 280 MPa.
However, within the AM sector it has been very common to only stress relieve AM parts at temperatures between 300-400°C for various lengths of time, and following this single heat treatment much higher tensile strengths have been reported. UTS values higher than even 400 MPa have been published, and it is thought that this is a unique consequence of the way in which the parts are manufactured. However, it must be understood that still relatively little is fully understood about the strengthening mechanisms of this AM alloy. If, however, AM parts are heat treated to what are considered the industry standard T6 conditions, then it is more likely that the properties will be similar, if not the same as conventionally cast and heat treated parts.
The AlSi10Mg alloy has a number of very similar alloys. There has also been a fairly quick increase in the development of powders and laser parameters for these other alloys. In many cases, with the same parameters used to process them. These other alloys include L169 (A357), AlSi7Mg, and AlSi12Mg.
However, other traditional engineering aluminum alloys like 6061 are now routinely processed with metal laser powder bed fusion processes, and are able to match the performance of the conventionally produced materials. There has also been some recent success reported with 7075 but research is ongoing. Finally, there are proprietary alloys such as Scalmalloy developed by Airbus that will enable some exciting developments for usage in higher strength applications.
This information is excerpted from a Renishaw paper, The status quo of metal alloys for additive manufacturing.
