Recycling metal AM powder – a Titanium recycling study

SEM analysis of virgin powder and after builds 19 and 38 in the second experiment
SEM analysis of virgin powder and after builds 19 and 38 in the second experiment

In additive manufacturing (AM), metal powder is a critical contributor to both part performance and part cost. Lucy Grainger from Renishaw stresses that we therefore need to look after it carefully to maintain its quality and minimise waste. The complete article will be published in the upcoming edition of 3D fab+print magazine, but here’s already a sneak preview…

By Lucy Grainger, Product Marketing Engineer metal AM with Renishaw

Why recycle?

Any production process is only as reliable as the feedstock that it consumes.  For additive manufacturing (AM) using metal laser melting, 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 that they are processing is predictable and stable.

“If we are forced to consider unfused powder as contaminated and therefore unfit for re-use, then the cost of additively manufactured parts is likely to be prohibitive.”

Economics also come into this. The fine metal powder that we use for 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 welded into a component – most is left unfused and is therefore available for re-use.

The benefits of near-net-shape manufacturing depend on such recycling.  If we are forced to consider unfused powder as contaminated and therefore unfit for re-use, then the cost of additively manufactured parts is likely to be prohibitive.

How might powder deteriorate during laser melting?

The chemistry of the powder is our first consideration. We are seeking to produce solid structures comprised of the pure alloy. AM equipment providers go to great lengths to create an inert environment in which reactive powders are stored and in which the laser melting takes place, so that the metal alloy does not form oxides or nitrides as it is heated.

The next concern is the mechanical condition of the powder: we need it to flow consistently so that it can be distributed evenly across the bed. This ‘flowability’ characteristic is typically measured using a Hall flow test and is typically governed by the grain morphology, powder size distribution and packing density. We are therefore concerned about any chemical and physical changes to the unfused powder caused by laser melting over multiple re-use cycles, and subsequent powder management activities.

“Of the metal powders that are commonly used for AM, titanium alloys are the most susceptible to impurity pick-up from atmospheric gases and are also amongst the most costly.”

During laser melting the weld pool is heated very rapidly and some of the powder is converted into a gaseous condensate of the alloy – a mist of nano-particles.  AM machines provide a gas flow across the bed to carry these by-products away to prevent them from accumulating in the powder.

Rapid heating can also lead to some larger droplets of liquid metal being ejected from the weld pool, whilst trapped gas inside powder grains may also generate ‘splatter’. Powder grains near the edge of the weld pool may become fused together but not attached to the part, creating irregular shaped agglomerates and ‘satellites’ that may affect the powder properties.

Careful selection of processing parameters should limit ‘splatter’ effects, whilst the gas flow will also help to remove unwanted by-products from the processing zone.  Unfused powder is also sieved to remove larger particles, with the intention of retaining a constant particle size distribution.

Renishaw AM250 system
Renishaw AM250 system

Evaluating the impact of powder recycling

Given this potential for change in powder composition during AM processing, can we confidently recycle powders? Are the measures taken by the AM system providers effective in ensuring that the properties of the unfused powder are unaltered by the build process? There is only one way to find this out, and that is to test it.

Of the metal powders that are commonly used for AM, titanium alloys are the most susceptible to impurity pick-up from atmospheric gases and are also amongst the most costly. This makes titanium – particularly the commonly used Ti6Al4V alloy – the ideal choice for a recycling study.

The full article will appear in the May/June edition of 3D fab+print magazine.