Last time we took a look at all the different thermoplastics available on the AM market now we’re onto metals. We’ve limited ourselves to those that are commercially available to keep things simple, but later on we’ll have a look at how far the industry has come.
Nothing in the world is ever straight forward and metallic additive manufacturing processes are no different. For example: out of Aluminium and Titanium which metal do you think is easier to laser sinter? Aluminium melts at 660°C and Titanium 1668°C so you might think it would be Aluminium. However Aluminium has a thermal conductivity 12 times higher than Titanium which not only contributes to warping issues during the build but requires much more laser energy to effectively melt the target area. Frustratingly a large portion of the energy which is delivered is quickly conducted away to the surrounding material.
To boot, issues with how much laser energy fine metallic powders actually absorb can further complicate the scenario. Fine powders especially those with oxide layers have a nasty habit of scattering and reflecting laser energy. This is part of the reason much more success was had in the early days with titanium than aluminium and steels, but also because the value proposition for titanium was much more attractive. Titanium as we all know is much more difficult to machine than aluminium, it’s even a pain to cast, requiring vacuum investment casting techniques rather than simpler sand casting setups. The final nail in the coffin is ultimately the price of the two materials. Titanium comes in at about 12 times the price per kilo. Don’t think you’ll be seeing much of that money again when you try to cash in your swarf (chips) at the scrapyard either. With AM offering net or near net parts without the need for casting or machining from solid billets the cost-benefit of investing in Titanium R&D over Aluminium was a clear choice.
So how far have we come?
In fact it’s a little bit of a lie to claim that these metals are available for use when really it is only certain alloys that have fully been developed. No surprises that they are the most common alloys in use today or alloys with applications that lend themselves to additive manufacture. For example, Ti-6Al-4V, which is commonly used in Aerospace and Medical applications. The last year or so has seen the addition of copper, tungsten & silver to the metallic pallet.
Our picks for the future? Looking some years out it is conceivable that other precious metals such as Palladium will come to market. The benefits of digitally manufacturing expensive metals are already widely known and would have transferable advantages in applications such as high surface area catalysts using valuable platinum group metals. Lithium’s reactivity will represent a significant challenge to 3D printing technologies but could open up a lot of design complexity for future batteries. Higher cell surface areas and complex battery shapes designed to take up whatever left over space is available may enable more powerful and space efficient systems thanks to additive manufacturing.
Don’t forget if you’re looking for something even weirder and wonderful that isn’t yet commercially available there are still options using additively produced tooling such as lost wax casting processes. It’s also worth noting that the buck doesn’t stop with powder bed fusion or directed energy deposition and that metals are also being processed with sheet lamination or by sintering green parts made by binder jetting. Looking further afield we are also closely following the developments from XJET who are researching the deposition of metals in 3D by material jetting. Despite many exciting developments in extruded liquid metals and metal jetting the gold standard for Metal AM will remain powder bed and directed energy systems for some time to come.