Wire arc directed energy deposition of 18Ni-350 maraging steel: process optimization, mechanical properties and aging response

Imagine a world where we can build giant, complex metal objects not by cutting them out of a big block or pouring molten metal into a mold, but by "printing" them layer by layer, like a 3D printer for heavy-duty machinery. This is the realm of Additive Manufacturing. While some 3D printers use lasers to melt tiny metal powders, others use a giant, robotic welding torch to melt a wire as it lays it down. This specific method is called Wire Arc Directed Energy Deposition (WA-DED). Think of it like a very precise, super-fast hot glue gun, but instead of glue, it's spitting out molten steel.

The material in question here is a special kind of steel called "maraging steel." The name sounds like a mix of "mar" (for martensite, a super-strong crystal structure) and "age" (because it gets even stronger when you heat it up and let it sit, a process called aging). It's the kind of metal used for rocket parts and high-performance tools because it's incredibly tough and strong. However, printing this specific type of steel with a welding torch is tricky. If the "glue gun" wobbles or the heat is wrong, the metal can end up weak or full of holes. Scientists have been trying to figure out the perfect recipe of speed, heat, and wire speed to make this process work reliably for large parts. This is the puzzle the researchers set out to solve.

The Great Metal Printing Puzzle

In this study, a team of researchers from Portugal decided to treat the metal printing process like a giant science experiment. They wanted to find the "Goldilocks" settings for printing 18Ni-350 maraging steel using a wire-fed welding torch. Their goal wasn't just to make a block of metal; they wanted to make a block that was strong, tough, and free of defects, and then see what happened when they gave it a special heat treatment.

Finding the Perfect Recipe
The team started by playing with three main knobs on their machine: how fast the torch moved (Travel Speed), how fast the wire was fed into the flame (Wire Feed Speed), and how much electrical power was used (Voltage). They used a clever statistical method called a "Design of Experiments" to test dozens of combinations without wasting months of time.

They measured "arc stability," which is basically how steady the welding flame is. Imagine trying to draw a straight line with a pen that keeps shaking; that's an unstable arc. If the pen is steady, you get a smooth line. The researchers found that the Voltage and the Wire Feed Speed were the most important ingredients. The speed of the torch didn't matter as much.

By crunching the numbers, they discovered the perfect setting to keep the flame steady:

• Travel Speed: 481 mm/min
• Wire Feed Speed: 5.9 m/min
• Voltage: 23.4 V

At these settings, they predicted the arc would be stable about 96.3% of the time. It's like finding the exact pressure to squeeze a toothpaste tube so it comes out in a perfect, steady stream without splattering.

Building the Walls
Once they had their perfect recipe, they built two types of metal walls. First, they made thin, single-layer walls to test how strong the metal was when pulled apart (tensile strength). Then, they built a much thicker wall by laying down four beads of metal per layer, like stacking bricks with a little overlap, to see how the metal behaved in a more realistic, chunky shape.

The Results: Stronger and Tougher
When they tested the thin walls, the results were impressive.

• Before Heat Treatment (As-Built): The metal was already quite strong, with a pulling strength (Ultimate Tensile Strength) of 1253 MPa to 1366 MPa, depending on which way you pulled it.
• After Heat Treatment (Aged): They heated the metal to 480 °C for 5 hours and let it cool. This is the "aging" part. Suddenly, the metal got even stronger, reaching 1613 MPa to 1687 MPa. Even better, it became more stretchy (elongation went from about 6% to 8.5–10.2%).

Usually, when metal gets stronger, it gets more brittle (like a dry twig that snaps). But here, the metal got stronger and more flexible. The researchers think this is because the heat treatment smoothed out some of the tiny imperfections left over from the printing process.

The Thick Wall Surprise
When they looked at the thick, multi-layer wall, they found something interesting about its hardness.

• Before Heat Treatment: It was already quite hard at 508 HV1.0.
• After Heat Treatment: It got even harder, reaching 625 HV1.0.

The fact that the "as-built" metal was already so hard suggests that the process of building the thick wall acted like a mini-heat treatment on its own. As each new layer of hot metal was added, it reheated the layers below, tempering them slightly before the final oven bake.

The Toughness Test
Finally, they tested how much energy the metal could absorb before breaking (impact toughness) using a swinging hammer test (Charpy V-notch).

• As-Built: 438 ± 2 J
• Aged: 440 ± 3 J

Here is the twist: The heat treatment made the metal harder and stronger, but it did not change how much energy it could absorb. The toughness stayed exactly the same. The researchers explain that the features controlling how the metal breaks were already set during the printing process itself, not during the final heating. It's like the "toughness" of a chocolate bar is determined by how you mix the ingredients, not by how you wrap it in the wrapper later.

What's Next?
The study concludes that this method is a viable way to make large, strong, and tough parts out of 18Ni-350 maraging steel. They found a sweet spot for the machine settings that works well for the "short-circuit" mode of welding. However, they note that this "sweet spot" is right at the edge of the settings they tested. To find a truly perfect global optimum, they would need to test even wider ranges of settings. They also suggest that future work should look deeper into the tiny crystals inside the metal and see if a different kind of heat treatment before the final aging could make the parts even better.

In short, these researchers figured out how to tune a giant metal 3D printer to make a super-strong steel that gets even better after a warm bath, proving that you can print complex, high-performance parts without needing a million-dollar laser machine.