Vacuum, anti-satellite technology help make metal powders
Advances in technology are helping create consistently-sized metal powders.
Achieving uniform consistency in metal powders is a challenge. Small particles can weld themselves to larger ones during production, leading to more irregular powders that do not flow effectively and affect the integrity of a component. Liberty Powder Metals Ltd (LPM), a powder metals enterprise at The Materials Processing Institute in Teesside, UK, uses vacuum and anti-satellite technology, developed by Atomising Systems Ltd (ASL) in Sheffield, UK, to manufacture more consistent and higher quality powders. LPM is opening a new factory in Middlesbrough and expects to start production by early 2020. Over the past four years the Advanced Manufacturing Supply Chain Initiative has funded the CASCADE project, which enabled the creation of LPM.
Gas and plasma atomisation are common production methods of metal powders for additive manufacturing. The distinction is a trade-off between price and quality – plasma atomised powders have the lowest oxygen and are highly spherical but expensive to make. Gas atomised powders are mostly spherical but several defects can occur that reduce sphericity, including splat caps, satellites and broken particles. Two common types of gas atomisers are air melt atomisers, which use nitrogen as the atomising gas, and vacuum melt inert gas atomisers (VIGA), which typically use argon as the atomising gas.
A VIGA system melts a given alloy in a vacuum induction furnace before pouring in the inerted atomiser, breaking apart the alloy using an inert gas such as argon or nitrogen. This molten spray then cools as it travels down the atomising tower, before being conveyed to a cyclone, where the powder is separated from the gas stream and falls into a hopper. The gas stream is split, with some gas exhausted and some recirculated into the main vessel. This can be used to create a variety of iron, nickel, cobalt and other alloys, and is designed to be easily cleaned between alloys.
The advantages of a VIGA system over an air melt gas atomiser are two-fold – first it allows a lower oxygen level in the powder, which can be critical for some parts. A typical air melt gas atomiser will produce powder with an oxygen content between 300–600ppm, whereas a VIGA system would expect less than 200ppm, dependant on the raw materials. The second advantage is that the lack of oxygen in the melting atmosphere can allow some materials to atomise more easily than in air melt furnaces – aluminium containing alloys such as Inconel 718 and 625 are examples of this, which can block nozzles with alumina in air.
Flow across a powder bed is key in additive manufacturing, as is the density of the powder – more spherical powder is denser in a bed. Flow is important for can filling in the hot isostatic pressing process and for feedstock flow in metal injection moulding.
There are two commonly accepted causes of satellites in gas atomised powder. The first is where eddy currents at the bottom of the vessel push the finest powder back into the atomising zone, sticking onto larger particles while still semi-molten. The second method is that, due to the relative difference in speeds in the atomising plume, smaller particles may be accelerated into collisions with slower moving, larger particles.
ASL’s proprietary anti-satellite system counteracts the eddy current issue – the system recirculates clean gas back into the top of the atomising vessel, stopping the eddy currents from pushing fine powder back into the atomising zone. This improves the powder shape as the larger semi molten particles don’t contact smaller particles to create an irregular large particle, instead solidifying into a spherical shape. The anti-satellite system does not affect the particle size produced by the atomiser, only the shape.
There are additional advantages of an anti-satellite system, for instance the high volume of colder gas entering around the atomising zone gives a lower temperature powder at the end of the process. The atomising gas used in modern gas atomisers is typically heated to between 200–500°C, and the heat from the molten metal only adds to this. Recirculating the cooler gas into the top of the vessel helps to bring the temperature of the powder down, resulting in a particulate that has lost enough heat to be processed immediately after atomising. One of the most common ways of testing the flow of a powder is using a hall flow test, which measures the time taken for 50g of the powder to flow through a 2.54mm orifice. Comparing powders is a tricky subject – particle size, chemistry and moisture all have dramatic effects on the flow properties of a powder and ideally should be kept constant to make meaningful comparisons.
In a hall flow test, comparing stainless steel grade 316L from an anti-satellite system and that from a regular gas atomiser, it was demonstrated that the anti-satellite powder flowed well, despite being slightly finer and theoretically having a worse flow.
The anti-satellite system helps develop highly spherical powder that contains few satellites and exhibits excellent flow rates. The anti-satellite technology could also represent an opportunity for existing users of plasma atomised powder to explore the more cost-effective option of using gas atomised powder.
Whole production chain
Parts that can be made with higher accuracy the first time will also reduce the need for finishing post production and in turn, increase the right first time (RFT) rate. In order to maximise the RFT and manufacturing efficiency, it is important to focus on process improvements in part manufacture. An example of such progress is where machine makers have increased the number of lasers in their machines to improve the build rate and control heat dissiptation within the build. It is also important to consider the manufacturing route of the powder, where processes are being improved incrementally over time to achieve powder that exhibits better flow rates and improved cleanliness.
Historically, the choice of powder for a particular process has been simple because each atomising method tends to produce distinct characteristics. Some of the most recognised methods include crushing from solids, water atomising, inert gas atomising, vacuum inert gas atomising and plasma atomising.
Crushed powder is angular and does not exhibit the flowability required for additive manufacturing processes, although plasma spheroidisation can be employed to improve the powder morphology, but at a higher cost. Water atomisation produces powder with very irregular morphology, which exhibits insufficient flow rate to be considered for additive manufacturing. Inert gas atomisation uses nitrogen or argon, and the gas will be selected according to the material requirements, for example nitrogen is a cheaper option when atomising ferrous alloys, whereas argon must be used when atomising nickel superalloys to prevent nitrogen pickup in the alloy, which can form deleterious titanium nitrides.
For ferrous and nickel-based alloys, vacuum induction gas atomisation is commonly employed to produce high-quality powder, such as for powder bed fusion and direct energy deposition processes that require low oxygen and nitrogen contents. For applications where very highly spherical powders are required, plasma atomisation is normally selected because historically there has been a noticeable difference in morphology compared with gas atomised powders.
When an end user becomes accustomed to a certain type of powder, often they will continue to choose that type even though technically it may not be offering any benefit. With the advent of anti-satellite technology, the lines are blurred in terms of powder morphology between gas atomised and plasma atomised powder.
Given that van der Waals forces between highly spherical powder particles can limit the flowability of plasma atomised powder, the cost of the powder versus the flow rate, subsequent manufacturing yield and RFT rate needs consideration.
A smoother process
Each additive manufacturing application offers a different set of challenges. However, the VIGA anti-satellite system offers additive manufacturing operators an opportunity to maximise build quality whilst not necessarily having to incur the cost of plasma.
*Gill Thornton is Research Development Manager and Tom Sellers is Commercial and Business Development Manager at Liberty Powder Metals Ltd, and Tom Williamson is Research and Development Manager at ASL. Liberty will be the first powder metal producer in Europe using ASL’s new process.