Metal additive manufacturing top choice for high value, low volume production
Additive manufacturing (AM or 3D printing) in metals is becoming a method of choice for some major manufacturers, particularly in high-value, low-volume industries such as specialist automotive, motorsport, aerospace, medical devices, dentistry and jewellery, writes Dr Michael Wilson, technology director at the 3M Buckley Innovation Centre in Huddersfield.
AM can create products that would be difficult or impossible to achieve with casting or subtractive techniques, such as machining. It can produce complex structures such as webs and honeycombs where the physical properties of the object are retained, but with a large saving in mass. It’s particularly beneficial to the aerospace and motorsports industries, where weight equates to higher fuel consumption and reduced speed.
There are two fundamental forms of metal AM, indirect and direct. Indirect techniques are relatively inexpensive and involve either the 3D printing of a wax impression of the desired object followed by a casting process. Another process is the 3D printing of an object from metal powder held together with a glue. This ‘green state’ object is fragile and is finished by infusing it with an alloy, usually bronze, or sintering it at high temperature (>1,000˚C) to produce a robust object which can be finished to the customer’s needs.
Many professional applications, however, rely on direct 3D printing, layer by layer, in fine metal powders, using the same technique that is common place with plastics, selective laser sintering (SLS) or the more powerful, selective laser melting (SLM). Sintering melts the surface of the powder grains to form the solid object, while SLM uses a higher power laser to melt the grains to form an object with metallurgical properties closer to that of a cast object. Finally, electron beam melting (EBM) is a powerful technique which is capable of producing objects almost indistinguishable from casting.
The list of metals that can be used in AM grows, from steels, such as stainless and high strength alloys to aluminium, titanium, gold, silver and platinum. But metal powders can be explosive and many direct AM machines operate in an inert gas environment.
Unlike self-supporting plastic SLS, direct metal AM involves a lot of pre- and post-printing handling. Object shape, size and orientation are vital in generating a successful print. Metal is heavy, so the placement in the build and the support of objects must be considered. After the print, the object(s) will be attached to a solid build plate/platform and this and any support structures must be cut away. Objects are often shot-blasted and tumbled with abrasives. In many cases the final object will also be machined to obtain the precise dimensions and surface condition required by the end-user.
As the properties of AM produced objects become better characterised and cost decreases, the use of the technology will become increasingly commonplace. However, questions remain to be answered before AM can properly be accepted as a manufacturing tool, especially for critical components. It is assumed that since the technology is ‘digital’ multiple copies of the same object are identical. But what is the object-to-object precision and repeatability?
And, if in order to produce a lighter structure the inside of a component is ‘honeycombed’, how can we reliably tell what the internal structure actually looks like? The answer to both these questions, and others, lies in metrology and the use of techniques such as X-ray computed tomography (X-ray CT scanning).
The advantages of AM such as rapid prototyping, manufacture of custom objects, and even on-site production, are multiplied when the ability to print in metals is added. Metal AM is certainly a major addition to the manufacturer’s arsenal.