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3D Printed Metal: the physics, the future and the engineering implications

Welcome to The Fircroft Report, the first in an occasional series of ‘long read’ blogs that examine the emerging trends and developments in engineering. In this first edition we take a look at the emerging concept of 3D Printed Metal. What impact will Metal Additive Manufacturing have upon engineering? Read on to find out more.

3D Printed Metal: the physics, the future and the engineering implications
The 3D printing of metal, also known as Metal Additive Manufacturing (AM), is one of the fastest growing sectors of the 3D printing world. It is a process of adding layers of material to create a complete 3D product, hence the name ‘additive’. This is in contrast to ‘subtractive manufacturing’ where products are cut out of a material.

The process of Metal AM involves elements of physics- more so than Plastic AM. Metals layers are fused with UV lasers, whereas plastics are extruded drop by drop. Both Physicists and Engineers have strong opinions on the usability and the future of Metal AM, and as expected these views sometimes differ.

What do engineers think of 3D Printing Metal?
Although confident in the general concept of Metal AM, Engineers are wary of its current usability.

One area of worry is the strain on the metal powder supplies. The more 3D printed components, the increased likelihood of poor-quality, defective metal powder in specific alloys for particular medical or aerospace components. Irregular particle size within the powder also causes concerns, as will be discussed later.

However, engineers cannot deny that Metal AM is ideal prototyping technology- something that it has been used for in the aerospace and automotive industries for years.

Jeff Luckett, the Director of Engineering at MBX Systems, is an advocate for this Metal AM prototypes. Using an outside company to fabricate metal prototypes would take months, but using their own 3D printer means they can reach ‘proof of concept’ stage far more quickly. This time-saving is reflected in the company revenues.

Luckett calls 3D printing ‘design driven manufacturing’ as opposed to the old concept of ‘manufacturing driven design’[i] which highlights the obvious benefit of less restrictive proposals in Metal AM.

Rolls Royce claim they have flight tested ‘the largest component ever built using ALM’[ii] in one of their jet engines- a component about the size of a tractor wheel. Meanwhile, in the car industry, using metal 3D printing for anything other than prototyping is unlikely in the near future. The process simply can’t compete with the speed at which cars currently roll off the production line. It’s in situations where speed isn’t key, such as in the building of personalised cars, that opportunities in the Metal AM production of cars might be closer than we think.

At present, engineers appear to be adamant that the process does not meet their criteria for quality.

What do physicists think of 3D Printing Metal?
Physicists have much more confidence in the current capabilities of Metal AM. Many are endeavouring to produce convincing content, models and high performance computing systems to determine the limitations of 3D printed metal and demonstrate its reliability to their naturally sceptical peers. Only when the complex physics behind the process is understood by both parties will the development of Metal AM methods pick up pace.

An article in January 2016’s edition of Applied Physics Reviews discusses how a scientific approach might persuade today’s Metal AM sceptics that 3D printed metal is reliable. The Director of Lawrence Livemore National Laboratory’s Accelerated Certification of Additively Manufactured Metals Initiative, Wayne King, and his team describe physics-based models for the Selective Laser Melting (SLM) option for Metal AM.

The first model looks into how the melt pool is formed and solidified, and ‘could be used to better understand how laser power, speed, beam size and shape affect different types of metals.’[iii] This will lead to improvements to the overall SLM method in the future.

The second looks at ‘calculating the effects of stress and heat arising from a given type of metal and laser process parameters.’[iv] This should improve the process of predicting defects during printing, which in turn should quickly improve quality and add a more scientific approach to what has previously been somewhat trial-and-error.

Both Engineers and Physicists look forward to the reduced timeframe of manufacturing that will come with reliable Metal AM. Accelerating the certification and qualification process of manufacturing will really add to the flexibility of 3D printing with metal.

Engineering Physicist, Mark Wendman theorizes that with the huge bandwidth of multibeam pattern architecture, ‘both speed and productivity could increase tenfold in 3D printing with metal.’ [v]
Wendman also goes on to speculate that alternative heat management could significantly benefit the development of 3D printing. ‘Part cooling engineering combined with…thermal imaging cameras or other temperature metrology for controlling thermal loads, and heating with specific algorithms designed for 3D printing with metal’[vi] could result in fewer distortions in Metal AM parts.

Physicists believe that the above methods of improvement and testing should assure Engineers that quality standards will not waver for Metal AM pieces.

How does 3D printed metal compare to ‘normal’ metal?
3D printed metals are possible in a number of forms, from steel to aluminium, cobalt to copper, and even precious metals, so there isn’t an issue with the variety of 3D printed metal on offer. That said, stainless steel has proven itself to be the strongest of the 3D printed metals. Because it can be plated with other materials, Stainless Steel has the advantage that the finished product doesn’t have to be its usual silver colour.

The advanced microstructure of AM metals means that their strength is thought to be increased and ductility decreased in comparison to usual casted or wrought alloys. For example, the manufacturers Renishaw 3D printed a metal bike frame which was lighter yet stronger than a conventional frame. This is because material could be concentrated on high stress areas, but reduced where it was not needed.

It has also been reported that the companies who produce powders for more traditional powder processes (HIP, MIM, PS) will have the opportunity to try and grab a market share of the powders developed for AM Metals. This suggests that the quality of metals produced by the AM methods should not be below par as they will be produced by experienced companies.

Finally, as the process is ‘additive’ rather than ‘subtractive’ this means that only the necessary material is used, resulting in less waste.

On the other hand, the size and shape of metal powder particles can vary so it is important that the machine manufacturers work closely with the powder suppliers to ensure consistency. This level of scrutiny could add to production timeframes.

An irregular shape can be problematic for powder particles because it causes a faster flow, potentially reducing the packing density. Nevertheless, it isn’t unheard of to produce spherical powers with varying particle sizes which is optimized for additive manufacturing.

However, ‘the particle size distribution of metallic powder particles impacts the density of AM parts...the particle size distribution does not only affect the density but also the mechanical properties and surface quality of the parts.’[vii] Different particle sizes also increases open porosity which reduces the toughness of the AM parts. Repetitive stress on said parts can cause cracking, which makes the piece impossible to use as part of a product which is regularly put under stress.

Finally, if the metal AM powder is exposed to contaminants- including oxygen during processing (which is likely) then the powders degrade. Although it isn’t impossible to recondition these powders a cost effective way of doing so is yet to be found.

The Future of 3D Printed Metal
Metal AM technology is increasing at a remarkable rate, which can be partly attributed to the competition within the industry. Currently, the demand is there in the aerospace industry (Airbus are known users of the method and even own a printing machine) and it will encompass Automotive manufacturing in the not so distant future- and not just in the prototyping phase.

It is also believed that Metal AM will play a big part in reducing the cost of space travel, in developing military missile technology, and also changing the way the military air force’s supply chain works.

Additive Manufacturing is here to stay, especially when bearing in mind the increasing number of methods, as well as predicted revenues of around $1.2 Billion for AM of metal powders for 2021 (reaching $2.4 Billion in 2025).

Here are the methods to look out for:

● Binder jetting technology by Digital Metals- This version can create microscopic details and some of the thinnest walls possible, something that powder bed fusion cannot.
● Directed Energy Deposition (AKA laser cladding)- This method can deposit a high quantity of materials in a short time, which means that it is fast.
● Wire-Based Additive Manufacturing- Although there is competition between wire and powder based approaches neither will have a negative effect on the growth of the other.
● Ultrasonic Additive Manufacturing- This process can create complex geometries such as hollowing, lattice and honeycombed internal structures.

Engineers are trying to work out a way to print objects from more than one material at a time, and in an ideal world they would like to give manufacturers the option to print a component one day, qualify it on the same day, and then repeat the process with a different component on the following day. However, the current speed at which Metal can be printed suggests that overall developments in the industry are slow, and so this won’t be possible for quite some time.