DMLS Material Selection
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Our Insight video series will help you master digital manufacturing.
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Insight: DMLS Material Selection
Hi and welcome to this week’s Insight video.
Today I’m going to talk about material selection for one of the coolest production technologies out there, 3D printing of metal using direct metal laser sintering.
Now you might ask what is the difference between the same metal or alloy if it is cast, CNC machined or 3D printed.
Well, there are differences, because the exact material properties will depend not only on the material but how it is processed.
To select the right material, you need to define what mechanical or physical properties are key to your part and also identify critical to quality features. When you have done that, you can review what are your best material and manufacturing options.
With that said, let’s delve into DMLS materials. First the science bit.
DMLS uses metal powder to produce parts that are generally comparable to wrought metals. Because there is rapid melting and solidification of powder particles in a small, constantly moving spot, it may yield differences in grain size and boundaries, both of which can affect performance.
This can change according to the laser parameters, post build heat treatment and hot isostatic pressing. There’s loads or research going into this and eventually we will be able to manipulate the grain structure to offer different mechanical properties in a part. So, you will be able flex it to meet your specs – but that’s in the future.
Where are we right now?
Let’s compare some commonly used materials and their properties when we produce parts using different production processes.
I’ll start with defining which commonly specified properties we will be looking at, because there are hundreds that we can choose from.
We’ll begin with ultimate tensile strength, which is the maximum stress that a material can take before breaking.
Next on my list is the tensile modulus, or as it’s also known elastic modulus. This is how stiff a material is, the higher the modulus, the stiffer it is.
A material’s elongation percentage refers to its ductility. The higher its elongation percentage the more you can elongate it into a thinner shape. You will also come across elongation at break – which, you’ve guessed it, is how far you can stretch it before it breaks.
What’s next, hardness. This is measured and reported in HRC or HRB on the Rockwell Guide. The higher the number the harder the material is.
And the final one on my list is heat deflection temperature, or as its sometimes known the heat distortion temperature. This is the temperature at which deformation occurs when a rigid material is placed under a specific load. This is an important one for a lot of aerospace and automotive applications.
Okay, you can wake up now the physics lesson is over.
Let’s get onto the good stuff, how do different materials compare against each other when produced using different manufacturing technologies?
I’m going to pick some common metals and alloys to compare.
We’ll start with that great workhorse stainless steel and look at two of the most common alloys 17-4 PH and 316L. You would choose 17-4 PH for its much higher tensile and yield strength over 316L. But if you want something that is more flexible then, 316L is better with a much higher elongation at break.
If we compare both of these alloy’s mechanical and physical properties with their wrought and annealed counterparts you will see that there are some minor differences. For some properties the DMLS alloys come out on top and for others the wrought or annealed version have higher values.
If we have a look at aluminium, a favourite choice if you are after a great strength to weight ratio or thermal and electrical conductivity, then DMLS will give you a way better ultimate tensile strength, hardness and elongation percentage compared to a die cast option.
To finish this brief material and manufacturing comparison let’s have a look at that superalloy Inconel.
Now this is an interesting one. The alloy is used for applications when the going gets really tough. You can use it for components included in high heat aircraft engines right down to cryogenic applications.
The great thing is you can change the properties of Inconel 718 by treating it in a solution and aging it. In this case when its solution treated and aged, you’ll get a higher tensile strength and hardness and a reduction in elongation percentage.
Anyway, take a look for yourself at these graphs. For tensile strength and hardness, the solution and aged DMLS Inconel 718 part outperforms the Wrought Inconel 718. If, however you want a higher elongation percentage then you are better off with DMLS using the stress relieved version.
It really is a mix and match and depends on what properties you want.
So, when someone asks me which material and process, they should choose for a metal part then answer is it depends. You need to think through exactly how you want your part to perform and decide which one or two material properties are critical.
And with those final thoughts it’s time to say bye for now and see you again next Friday for another Insight video.
With special thanks to Natalie Constable.