Metal powder re-use in additive manufacturing (AM) has been investigated using a Renishaw AM250 system and titanium alloy Ti6Al4V ELI (extra low interstitial) powder. This study is a follow-on from the encouraging results obtained in a previous study in which the chemical and physical effects of re-using metal powder through the AM250 twenty times was investigated
Renishaw additive manufacturing systems use metal powder bed fusion to build complex components directly from CAD data. Objects are built layer-by-layer by spreading out fine metal powders and selectively melting areas with a high-power laser. Any un-melted powder remaining after a component has been built can then be re-used for another build. There is a question mark over whether there is a limited number of times that metal powders can be cycled around an AM process because chemical and physical properties are bound be changed, but to what extent?
Multiple factors
There are multiple factors which contribute to consistent, repeatable, and reliable AM builds, such as laser focus, filter condition, and z-axis movement – for this study we looked at feedstock (metal powder). The properties of the powder and the machine parameters that are used to process it are closely related, so the chemical and physical properties of the powder are critical. Manufacturers are naturally concerned that the condition of the powder they are processing is predictable and stable in order to have confidence in the quality of the manufactured components.
1. powder capture capsule, 2. tensile bars (×6) and 3. density block
The importance of economics
Economics is also an issue; the fine metal powder used in laser melting can be costly, so waste should be avoided. In most cases, only a small proportion of the powder that is laid down in a build process is actually melted into a component – most is left un-melted and is therefore available for re-use.
Some of the benefits of near-net-shape manufacturing depend on such recycling. If we are forced to consider un-melted powder as contaminated and therefore unfit for re-use, then the cost of additive manufactured parts is likely to be prohibitive for many industries.
In order to increase the robustness of the study, more tensile bars were built for each run; three to be tested with an ‘as built’ surface and three machined. Powder capture capsules and density blocks were also built.
A single batch of powder was cycled through Renishaw’s additive manufacturing process for a total of 38 builds until there was no longer enough powder remaining for a test sample height. Powder was analysed for oxygen and nitrogen content, Hall flow, particle size distribution (PSD) and density. There was found to be a small but steady increase of both oxygen and nitrogen concentrations in the powder as the build number increased. At around 16 builds the oxygen level started to fluctuate around the upper limit for the specification.
Nitrogen stayed within the material specification maximum limit as well as the more stringent lower allowable 300 ppm stated for some other grades. It is likely that under normal running conditions with regular topping up of the silo with virgin powder these levels will remain within the boundaries of the ELI specification.
Both levels stayed well within the boundaries of the grade 5 specification. PSD values shifted very slightly upwards due to gradual loss of smaller particles resulting in an increase in Hall flow speed. Upper tensile strength was found to be higher for the machined, compared to the rougher ‘as built’ tensile bars.
General conclusions
The changes observed over the period of the two studies do not seem to affect the way that the AM system functions with the powder, therefore the material parameters are still valid for the powder from start to end.
There was a general increase in oxygen and nitrogen levels observed over both studies. These impurities are most likely picked up by particles close to the weld pool which are heated but not melted. The levels of both impurities do eventually reach the maximum allowable for ELI grade Ti6Al4V, however they stay well within the specifications for the more widely used grade 5 Ti6Al4V.
It is likely that under a normal running scenario in which the silo is topped up with virgin powder at regular intervals the levels of oxygen and nitrogen would stay within acceptable boundaries as the new powder blends with the used. This however will require backing up with data.
Flow improves as the number of builds increases, this is most likely due to narrowing of the PSD by gradual removal of smaller particles. Powder morphology does not significantly change over the entirety of the study. Occasional misshapen and agglomerated particles occur within the bulk but at a very low occurrence, and the effect does not seem to be cumulative.
Observations
The observations gained from this study suggest that there is no requirement to dispose of un-melted Ti6Al4V after it has been cycled around the AM250 system after a limited number of times. This does however depend on the specific requirements of the component material properties, in which case a stringent blending regime may be required for traceability.
The effects of powder re-use will potentially be different for other materials in terms of physical property effects, however because titanium has a high propensity to pick up interstitial impurities, the pickup rates observed can be thought of as a ‘worst case’ for commonly used metal AM powders.
