Cracking: Another AM Challenge

Cracking: Another AM Challenge

In our previous article about Additive Manufacturing, we discussed the post-processing challenges associated with metals produced using LPBF-type techniques.

In this article we will talk about the issue of in-process cracking as it is a significant concern in the context of 3D printing of metals.

Heat Cycles

During the laser melting process in a 3D printer, metal powder is subjected to a series of intense heat cycles that are similar in some ways to traditional heat treatment processes.

  1. Powder Deposition: The metal powder is initially deposited in thin layers onto the build platform. The powder bed is preheated to a specific temperature, which can be considered similar to a preheating step in some heat treatment processes.
  2. Laser Scanning: A high-energy laser is precisely directed at specific points on the powder bed to selectively melt and fuse the metal particles. This localized melting process is used to build up the 3D object layer by layer.
  3. Rapid Cooling: As the laser moves away from a melted area, rapid cooling occurs, leading to the solidification of the molten metal. This rapid cooling can result in the formation of a fine and often non-equilibrium microstructure.

Difference from Traditional Heat Treatment

  1. Localized Heating: In laser metal printing, the heat is localized to the specific areas being melted by the laser. This means that different regions of the printed part can experience different thermal histories, leading to a complex and non-uniform microstructure.
  2. Localized Cooling: The localized and selective nature of the heating and cooling in laser metal printing processes results in a complex and heterogeneous microstructure within the printed part.
  3. Layered Structure: The additive manufacturing process builds the part layer by layer. This layering can influence the material’s anisotropy and mechanical properties, which differs from the more homogeneous nature of materials processed through traditional heat treatments.

Some Reasons for In-Process Cracking

  1. Residual Stresses: As pointed out above, the laser metal printing process is characterized by rapid localized heating and cooling. This can result in the development of significant residual stresses within the printed part. These stresses can arise from non-uniform thermal expansion and contraction and the formation of non-equilibrium microstructures.
  2. Non-Equilibrium Microstructures: As mentioned, the rapid solidification and cooling rates can lead to non-equilibrium microstructures, such as fine grains and metastable phases. These microstructural features can make the material more susceptible to cracking.
  3. Layer-by-Layer Building: Components are build layer by layer, and the interfaces between these layers can become critical locations for crack initiation and propagation. Thermal stresses at these interfaces can be significant.
  4. Complex Geometries: This is where the advantage of 3D printing presents a double-edged sword. The creation of complex and intricate geometries may have overhangs, sharp angles, or abrupt changes in geometry. These features can create stress concentration points where cracks are more likely to form.
  5. Metal Type: The choice of metal alloy can also influence crack susceptibility. Some alloys are more prone to cracking than others, particularly those with a high susceptibility to hot cracking.

Examples of Metals with Inclination towards Cracking

  1. High-carbon steels: High-carbon steels, especially those with carbon content significantly above 0.4%, are more prone to cracking. This is because the high-carbon content can promote the formation of brittle martensite during rapid cooling, increasing the risk of cracking. Proper preheating and post-processing heat treatment can help mitigate this issue.
  2. Superalloys: Certain nickel-based superalloys, commonly used in aerospace applications, can experience cracking. The reason behind this is the fact that the high melting temperatures and alloying elements present in superalloys can create challenges during solidification. This leads to susceptibility to hot cracking.

Mitigating In-Process Cracking

  1. Optimize Printing Parameters: Carefully control laser power, scan speed, layer thickness, and other parameters to minimize thermal stresses and ensure proper fusion between layers.
  2. Preheat the Build Plate: Preheating the build plate or powder bed can help reduce thermal gradients and minimize cracking risks during printing.
  3. Support Structures: Properly designed and placed support structures can prevent warping and deformation, reducing the likelihood of cracks.

As we can already derive from these interrelations and associations, the process of heating and cooling plays a pivotal role in shaping a metal’s microstructure, consequently influencing its thermo-mechanical properties. Planning to delve deeper into various aspects of metal part heat treatment in our next article…

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