Malvern Q&A on metal powders for additive manufacturing

Jul. 24, 2017 – In recent years, additive manufacturing (AM) has transitioned successfully from a prototyping tool to a still new, but established and economically viable choice for routine manufacturing.

AM offers certain specific advantages relative to alternative powder metallurgy methods, ranging from high material use efficiency to design flexibility, and is particularly suitable for the production of small to medium volumes of relatively small components, as well as enabling the creation of totally new complex parts that were previously unachievable.

Below is an exclusive Q&A with Dr. Cathryn Langley, associate product manager, Analytical Imaging, Malvern PANalytical.
 
Q: AM gets a lot of coverage in the media these days. Can you summarize why there has been so much interest?

CL: AM, or 3D printing, rightly garnered a huge amount of interest when it first emerged back in the mid-to-late 90s. For many, it was a technology of the future that was heralded to become the next household gadget. However, that vision hasn’t yet materialized and the sensationalism overlooked its true potential – to transform our manufacturing industries. AM can be used to produce complex parts without the design constraints of traditional manufacturing routes and offers the potential for high material efficiency — less waste — than subtractive manufacturing techniques, such as machining. Its ability to perform rapid prototyping is revolutionizing innovation and accelerating speed to market in diverse industries.  

The term AM covers a range of different manufacturing routes, some of which are polymer-based, and others powder-based — primarily using metal powders. AM is still in its infancy and has potential use across a wide range of applications and material types. Though it is unlikely to ever become a viable route for the mass manufacture of simple parts, scaling it up so that it becomes the norm for producing complex and replacement parts is the next step. To achieve this, the technology, methods, materials and expertise to handle AM must be all brought in line.

Q: What are the applications for which it’s proving most useful?

CL: Polymer-based AM techniques based on extrusion are commonly used for rapid prototyping and domestic use AM. Metal powder-based techniques are being used to fabricate end-use products for use in biomedical, aerospace and automotive applications. The most celebrated uses of AM are within biomedical, with high profile cases of 3D-printed prosthetics, cardiovascular stents and models to assist surgeons — for example, prior to separating conjoined twins. In aerospace, Boeing produces environmental control ducting (ECD) for its 787 aircraft using AM, transforming it from a 20-part piece requiring complex assembly to a single piece. Even further afield, the Mars Rover comprises 70 3D-printed custom parts that boast exceptional structural strength and scientists aboard the International Space Station are exploring its use to make spare parts.

Q: Why are the properties of the powders so important?

CL: Poor powder quality can produce defects in the end part including pores, cracks, inclusions, residual stresses and sub-optimal surface roughness, as well as compromising throughput.

It is the physical characteristics of a metal powder that define AM performance. These characteristics include both bulk properties of the powder and properties of the individual metal particles. Key bulk properties are packing density and flowability. Powders that pack consistently well to give a high density are associated with the production of components with fewer flaws and consistent quality. The ability to spread evenly and smoothly across a bed and to form a uniform layer with no air voids is essential. For this, the flowability of the powder is also critical. Both bulk density and flowability are directly, though not exclusively, influenced by particle size and shape.

Q: What properties are most important when it comes to AM metal powders and why?

CL: Size and shape because they influence flow and packing, as mentioned above, but chemistry is also paramount. A powder needs to comply with the alloy composition of the material specified, and grade must be carefully selected so as to control the interstitial elements present — such as oxygen or nitrogen — which can impact the properties of the finished part. In addition, AM powders must be free from foreign particulate contamination — from other material batches at the powder production plant, the AM facility, or debris in processing/recycling equipment. Contaminant levels of just a few parts per million can be significant in terms of component quality.

Q: What are the best techniques for measurement?

CL: For powder bed processes, properties such as particle size and shape are most critical. Depending on which AM process and machine is used to create a part, the powder used will be subjected to different flow, stress and processing regimes. Ensuring the raw material can stand up to the job is the difference between metal AM success and failure.

Particle size distribution and morphology data to help determine powder packing density and flowability can be measured using laser diffraction and automated imaging.

Laser diffraction measures particle size distributions by measuring the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample. Large particles scatter light at small angles relative to the laser beam and small particles scatter light at large angles. The angular scattering intensity data is then analyzed to calculate the size of the particles responsible for creating the scattering pattern, using the Mie theory of light scattering. The particle size is reported as a volume equivalent sphere diameter.

Q: What challenges are people currently using this technology to solve?

CL: Up to one third of the production cost of an AM component is the cost of the powder used, with commercial viability resting on establishing a robust supply chain and effective powder recycling strategies. Validating the supply chain is also of critical importance, especially in industries such as aerospace and biomedical, which have stringent quality requirements.

Q: Where do you think AM will be in the next five years?

CL: We are sure that AM will continue to develop as quickly as it has been doing for the last five years and soon, it will become a well-accepted technology in diverse manufacturing settings. Upcoming developments include the use of multiple autonomous robots for collaborative printing (“Are we ready for the attack of the spider robots?”) and the emergence of factory floor hybrids that combine milling machines with AM. This latter development will mean an even greater connection between powder requirements, AM technology and end applications, which is likely to advance knowledge and understanding of their interdependence. More broadly, much innovation is taking place among the AM technology suppliers to increase speed and, in turn, open up the market further. Patented here in the U.K., high speed sintering (HSS) is a new AM technique that uses inkjet print heads and infra-red heating technology that makes products layer by layer from polymer powder materials. This process is starting to gain momentum, so watch this space.