Mechanical Properties of Additively Manufactured IN718

I’ve been diving deep into some exciting research lately, and I’m thrilled to share a bit about two projects I’ve had the chance to contribute to as a co-author. Both studies focus on Inconel 718 (IN718), a super tough nickel-based alloy that’s a big deal in industries like aerospace and energy because it can handle extreme conditions. What makes these projects cool is that we’re looking at how additive manufacturing (AM)—you know, 3D printing for metals—changes the way this material behaves, especially when you’re printing thin walls versus bulkier parts. I’ll walk you through what we did, what we found, and why it matters, all in a way that feels like we’re just chatting over coffee.

Study 1: Investigation of  Microstructure and Strength Differences

The first project I worked on was all about comparing thin-wall and bulk IN718 parts made using selective laser melting (SLM), which is a fancy way of saying we used a laser to melt metal powder layer by layer to build our samples. We printed tiny dogbone-shaped samples (think of them like mini dumbbells for testing) in two thicknesses—0.6 mm and 0.8 mm—using two different laser patterns: one where the laser follows the same path each time (aligned) and another where it rotates between layers. We also made a bigger “bulk” sample for comparison. The big question we wanted to answer was: does the shape and thickness of what you’re printing change how strong the material is?

Here’s the deal: when you print something with AM, the way the metal cools down as it’s being built matters a lot. Thin walls cool differently than bulkier parts because they’re surrounded by loose powder, which doesn’t conduct heat as well as solid metal. We thought this might make the thin walls have a different microstructure (the way the metal’s grains are arranged at a tiny scale) compared to the bulk, and that could affect how strong or stretchy the material is.

To figure this out, we used a technique called micro-tensile testing, where we pulled on those tiny dogbones until they broke, and we watched what happened using digital image correlation (DIC). It’s like giving the material a stress test while filming it to see how it stretches and deforms. We also peeked inside the samples with a scanning electron microscope (SEM) to check out their microstructure.

What we found was pretty fascinating. The thin-wall samples had much smaller grains—31.45 micrometers on average—compared to the bulk samples, which had grains bigger than 255.96 micrometers. Grains are like the tiny building blocks of the metal, and smaller grains often mean a more uniform structure. In the bulk samples, we saw these long, column-like grains that stretched across multiple layers of the print, which is called epitaxial growth. But in the thin walls, the faster cooling and surrounding powder broke up those long grains, giving us a finer, less textured microstructure, especially when we used the rotated laser pattern.

Here’s where it gets interesting: the bulk samples were about 30% more ductile, meaning they could stretch more before breaking, but they were over 25% weaker in terms of ultimate strength compared to the thin walls. Why? The thin walls’ smaller grains and more uniform structure made them stronger, but they were also more brittle. We also noticed that the thin walls had rougher surfaces in some spots, which could cause cracks to form sooner and lead to failure. The rotated laser pattern helped even things out a bit by breaking up those long grains even more, which was cool to see.

This all means that when you’re designing parts with thin walls—like intricate components in a jet engine—you can’t just assume they’ll behave like thicker parts, even if you use the same printing settings. The geometry of what you’re making changes the microstructure, and that changes how the part holds up under stress. It’s like baking cookies: the same dough can turn out differently depending on whether you make thin crisps or thick, chewy ones.

Scientific output: https://doi.org/10.1016/j.mtla.2023.101867

Study 2: Fatigue and Scan Strategies

The second study took things a step further by looking at how these thin-wall IN718 parts hold up under repeated stress, like the kind you’d see in parts that are constantly vibrating or flexing. This is called low-cycle fatigue (LCF), and it’s super important for things like turbine blades that have to last through thousands of cycles without cracking.

We printed more dogbone samples, all 0.8 mm thick, using the same contour + hatching strategy as before, with both a static (aligned) laser path and a rotating one (67° between layers). We also took some 0.8 mm dogbones from a 10 mm thick bulk sample to see how they compared. The twist here was that we left the surfaces of the thin-wall samples as-is—no polishing—because surface roughness and tiny defects can play a big role in how cracks start. We tested these samples by repeatedly stressing them until they failed, using our trusty DIC setup to track when and where cracks formed. We also used SEM to take a close look at the fracture surfaces and any defects.

Our hypothesis was that the rotated laser pattern would make the parts more resistant to fatigue because it creates a more uniform microstructure with smaller grains. And guess what? We were onto something. The rotated scan strategy did lead to a more consistent microstructure, with fewer of those long, column-like grains we saw in the bulk samples. This made the thin-wall samples with the rotated pattern hold up better under fatigue testing compared to the aligned ones.

But it wasn’t all smooth sailing. The thin-wall samples, especially the aligned ones, had more variability in their performance. Those rough surfaces and tiny subsurface pores (like little air bubbles trapped in the metal) acted as weak spots where cracks could start. The bulk samples, on the other hand, were more predictable but didn’t have the same fatigue resistance as the best thin-wall samples. The SEM images showed that while the fracture surfaces looked similar in terms of overall roughness, the lower-performing samples had spots with extra-rough patches that triggered multiple cracks, leading to earlier failure.

What we learned here is that the way you move the laser matters a ton. The rotated scan strategy helps break up the microstructure in a way that makes the part more resistant to fatigue, especially in thin walls. But you’ve also got to watch out for surface roughness and defects, which can throw a wrench in things if you’re not careful.

Scientific output: https://link.springer.com/chapter/10.1007/978-3-031-27447-3_34

Why This Matters

Both of these studies show that additive manufacturing isn’t just about hitting “print” and calling it a day. The way you design and print a part—especially when you’re dealing with complex shapes or thin walls—changes how the material behaves. For someone like me, who’s fascinated by how materials work at a tiny scale, this is like solving a puzzle. We’re figuring out how to make stronger, more reliable parts for things like jet engines or power turbines, where failure just isn’t an option.

Working on these projects was a blast because I got to team up with some brilliant folks and use cool tools like micro-tensile testers and SEMs to peek into the world of metals at a microscopic level. It’s also a reminder that every little detail in AM, from the laser path to the part’s shape, can make a big difference. For designers and engineers, this means you’ve got to think about the local geometry and microstructure when you’re planning a part, not just its overall shape.