How to address Process Development in metal Additive Manufacturing
by Amit Saini, Application Engineer at AMEXCI
The metal Additive Manufacturing industry has grown by leaps and bounds in the past decade and it is high time to reflect on understanding the process, as well as the many variables attached to it. New machines can take the AM production into next level, but the Industry remains skeptical about the availability of different materials and the process itself. Although it is a novel process, it can still be in a unique way linked to the conventional thermo manufacturing processes like casting, welding and metal forming. After all, these conventional techniques only involve processing the materials, which behave in their own unique way with respect to the type of manufacturing process. Using this knowledge along with the know-how of laser photonics, we can find ways to develop unique processing strategies. This approach can be applicable to many commercially available alloys as well as designing new alloys tailormade for AM.
Limitations and Possibilities:
Over the years the portfolio of materials that can be successfully processed by L-PBF has increased, but it is still nowhere close to the options we have available for the conventional manufacturing counterparts. One reason is that most of the alloy designs have been created to suit other manufacturing processes. Now, the same approach can be applied to AM. Another more important reason is a lack of availability of widescale processing strategies for existing alloys. There have been numerous examples of new materials being consistently adopted to L-PBF. However, the know-how of the developing process parameters is available to a limited few, who, working on the technology for several years, have developed their own unique approaches for process development. Here we try to generalize the approach for material selection and process development, but this may not be applicable to every material. Each material can have its own intrinsic properties that can lead to different complications.
Even though the possibilities can be endless, there are also limitations regarding the process. Having a clear know-how is important before investing into a materials development project for metal AM The right selection of alloy and understanding the materials behavior of laser interactions is the first step in developing the right process strategies for L-PBF. Depending on the specific material, the following steps can still be utilized for many alloy designs and adoptions in AM.
1. Understanding the materials behavior in manufacturing processes
First step in AM process development is to understand the materials behavior in different manufacturing processes, being a vast topic itself. Having a deep understanding of various other Thermo Manufacturing Processes is equally important while developing the know-how of metal AM process. When we talk about the manufacturing processes and materials behavior, there are many subtopics that are required to be taken into consideration. Some of them are mentioned below:
a) The type of manufacturing process, energy sources, material processing conditions, and other process variables.
b) The type of material, whether it is eutectic, inter-metallic, solid solution or any else.
c) The alloy melting range and its impact on a particular manufacturing process.
d) The solidification mechanism of the alloy
e) The phase changes and the impact of phase change upon solidification.
f) The thermo physical properties of materials
g) The targeted mechanical properties of the material
2. Understanding powder feedstock characteristics
Pre-alloyed powders are commonly used as feedstock materials in the laser beam assisted AM techniques. Manufacturing of high-quality feedstock materials in AM is a challenging process, as they are susceptible to oxidation due to high surface areas. There are few characteristics of metal powders that are required to be understood and controlled for producing better quality components.
a) Powder flow, apparent density, and tap Density.
b) Hausner ratio and Carr index: These are the indicators of flowability and compressibility in the powders.
c) Particle shape and size distribution: Spherical powders are ideal in shape due to the fact they have the best flowability and least surface area. Particle sizes of up to 10 μm – 100 μm are used currently in L-PBF due to limitations on laser processing and print resolutions. Particle size needs to be distributed in a way to get maximum packing density. This can be achieved by a gaussian distribution of powder particle sizes.
d) Surface morphology: The morphology of an individual particle is also of great significance to the overall process. Even a spherical particle on close inspection can reveal surface irregularities like porosities or roughness. These can lead to defects like oxidation and reduced flowability due to inter particle friction.
e) Proportion of impurity elements: The proportion of impurity elements in metal powders are very significant to both printing process and properties of the final components. It is required to find out whether such elements are present in their elemental form or in the form of a chemical compound.
3. Understanding the optics in L-PBF machines
The optics system of L-PBF machines consists of the following:
a) Laser source: Fiber lasers are the preferred ones in L-PBF over others. The advantages of using fiber lasers are high optical quality, low thermal distortion of the optical path, lower heat output, compact size, high operation life, and more.
b) Collimator: takes scattered particle waves from the laser source and makes them aligned or parallel.
c) Beam Expander: takes collimated beam and expand its size.
d) Scanner: Laser scanners use system of rotating mirrors to scan and measure objects systematically to position or steer the laser beam.
e) F-theta lens: These lenses are used to get nearly constant spot size across the build plane. This is achieved because in F-θ lenses the beam displacement is directly proportional to θ and depends on F x θ, where F is the focal length and θ denotes the angle of beam in Radians. Other advantage of these lenses is their compact design due that allows for reduced number of optical components required to produce a flat image plane.
It is also highly important to understand the type of beam profile and how to control it. Most L-PBF optics systems have a Gaussian beam profile with a large Rayleigh length. The reason for this is to enable the beam to propagate over large areas without diverging significantly. Controlling the laser focus with a desired beam spot are prerequisites for producing quality components.
4. Understanding the process parameters in L-PBF
There are more than 50 process variables involved in printing a successful component. We can divide those variables into two categories: bulk parameters and surface parameters. First step is to develop correct bulk parameters for the material which defines most of its properties. After that, the attention can be shifted to development of surface parameters. The most important bulk parameters are laser power, scan speed, hatch spacing and layer thickness but there are many other process variables attached to it are equally important. The surface parameters are required to produce components with difficult geometries, remove possible sub-surface porosities and improve surface quality.
5. Strengthening mechanisms and utilizing heat treatments
Post processing of the components plays a crucial role in completing the AM process lifecycle and delivering a final part. Post processing can include stress relieving/ heat treatment, surface finishing treatments or more. Heat treatment itself is a major step in completing the process development for a material. Many times we need to develop custom heat treatment cycles for AM components considering the unique input microstructure after printing. Also, they help by reducing the residual stresses generated in the components due to high thermal loads during the printing process.
If you are interested to learn more about the above-mentioned topics or have a project in mind, please feel free to reach out to us. In some of our education programs here at AMEXCI we share with Designers and Material experts, the dedicated approach when considering AM for their applications. At AMEXCI, we constantly work on process development of novel materials as well as improving the quality of existing printing materials. Finally, we will post more in-depth articles surrounding the above-mentioned topics in the future, so be sure to subscribe to our newsletter to receive the latest updates!