Digital Alloys’ Guide to Metal Additive Manufacturing - Part 3
Process Steps in the Metal Additive Manufacturing Workflow
December 13th, 2018
A production manufacturing tool does not operate in isolation; it is tightly integrated into the upstream and downstream processes, and is only valuable when this integration is successful. Many publications about metal AM focus on the printing process, ignoring the important process steps that precede and follow. Implementing a successful Metal Additive Manufacturing (AM aka 3D Printing) production process requires more than installing and operating a printer. The entire AM workflow must be considered in evaluating, selecting, testing, and implementing a metal 3D printing solution.
Metal AM process steps depend on many factors including the technology, equipment, industry, and application. This post describes a general workflow which applies in most cases. We have structured this post by five key sections: Design, Pre-processing, Printing, Post-processing and Quality Assurance.
Design is the first step in the workflow. The challenges and opportunities in design for metal AM vary with whether a pre-existent part is chosen for printing or if a new part design is created.
For an existing part design, the objective is usually to select a production process that requires few modifications to the part. This reduces the costs of redesign and requalification. The business value for 3D printing in these cases will rely on time and production cost savings, rather than improved product performance (see Digital Alloys’ Guide to Metal Additive Manufacturing – The Business Value of Metal AM ). The time-to-value for metal AM is generally shorter if an existing part design is selected.
For a new, clean sheet design, design balances product function with manufacturability. Designing (or modifying) parts for 3D printing opens improved product performance as a potential driver of business value. Each metal AM process technology has its own design rules that limit part geometry and dictate the design of any structures needed to support the part during printing (“supports”). Engineers are increasingly turning to generative design and topology optimization to design for 3D printing, and new tools are coming to market for this task.
Pre-processing encompasses the steps between design and printing. The first pre-processing step is to convert a 3D CAD file into instructions the printer uses to build each layer of the part. These instructions are created by a “slicer”, which slices the design file into layers of “voxels” (3D pixels) and generates a toolpath for the printing process. The toolpath incorporates both position information and print-process parameters (for example, power used to melt metal) for each voxel. There may be multiple parts in a single build, in which case organizing the parts efficiently on a build plate is an added step.
With some metal AM technologies, defining process parameters is a complex iterative process because the quality of metal printed and the accuracy of the print are highly sensitive to these parameters. In volume production, print parameters are often fixed and then maintained with ongoing machine calibration. In some technologies, process parameters are managed in real time with closed loop process control.
Once the software pre-processing and parameter development are complete, there is the physical setup of the machine. Physical setup includes:
- Loading and aligning the build plate or substrate,
- Preparing the printing chamber atmosphere (molten metal needs to be protected from oxygen), and
- Preparing and loading the feedstock for the printer. The complexity of these steps will depend on the feedstock type. As an example, metal powders require carefully handling due to their flammability, toxicity, and propensity to oxidize.
Printing, while cool to watch, should actually be the process step that requires the least attention. Ideally, the 3D printer is able to run “lights out” with no operator monitoring or intervention. This is true today for more mature and stable processes and equipment.
The printing step can take anywhere from minutes to many days, depending on the printing technology and size of the build. Most 3D printers heat the build plate or whole build envelope before printing. This can add considerable time which must be factored into cycle time calculations. Some processes require heat treatments for stress relief during the printing process which also adds time and cost to the printing step. Once a printing process is predictable enough that it doesn’t need monitoring, the operator can spend print time on other tasks, improving overall productivity.
Sometimes referred to as 3D Printing’s “dirty little secret”, post-processing is often more expensive and time consuming than the printing process itself. John Barnes of The Barnes Group Advisors states that post-processing is “typically where the battle is won or lost” and draws an analogy to the old golf adage: “Drive [print] for show, putt [post-process] for dough.”
Post-processing steps vary widely between various AM processes, equipment, applications, etc. The steps have to be designed carefully to meet all the part requirements (e.g. accuracy, surface roughness, strength, etc.) and this is another area where iteration and testing are typically needed for program validation. Most of the key steps are outlined below.
- Build Removal
- Removing excess material from the build inside the printer (e.g. in powder bed processes)
- Removing the build from the printer
- Inspection for accuracy, potential delamination, surface quality, support attachments, etc.
- Part Separation
- Removing parts from the build plate, using EDM, band saw or machining
- Removing parts from each-other if attached for nesting purposes
- Removing supports from individual parts, often requiring clippers, EDM, bandsaw or machining (support removal can also be completed in secondary machining steps)
- De-binding and Sintering (only for binder processes)
- Soaking parts in solution for up to a couple days to remove binding materials from the metal
- Sintering highly porous parts from previous step to reduce porosity
- Machining to remove remaining supports, smooth surfaces, add critical features, and hit critical tolerances
- Custom fixtures may need to be created to hold printed parts for secondary operations. If the part geometry is complex or organic, these fixtures can become resource intensive to design and manufacture
- Surface Finishing
- Polishing surfaces where surface roughness requirements were not achievable from machining
- Tumbling or Shot peening to smooth and/or work harden surfaces, or mitigate issues with loose powder on unfinished surfaces
- Heat Treatment (HT) - Often performed at multiple steps throughout the workflow but most frequently:
- HT after build removal to help relieve residual stresses
- Hot Isostatic Pressing (HIP) often used by powder processes after part separation to decrease porosity and further relieve stresses
- Furnace Sintering, required for powder binder processes after binder removal. It can result in a lot of shrinkage and drift in geometry. This must be compensated for upstream at the design stage and managed closely with QA.
- HT before machining to temper the material and reduce hardness (due to rapid cooling in most metal printing processes, the material can be in a highly hardened state that is difficult to machine)
- HT after machining to achieve final hardness requirements and desired metallurgy phases and grain structure
Quality Assurance (QA) for AM is not a single in step, but instead is a set of inspections, measurements, analyses and documentation performed throughout the workflow.
QA for metal AM is unique. Unlike most conventional manufacturing processes, the repeatability of most metal AM processes cannot be taken for granted. Certain processes are particularly sensitive to material input and process variables which are hard to control. This is what reinforces the need for a robust QA strategy that addresses software, hardware and materials. Processes which are able to directly measure and control the metal deposition (printing) process will have an advantage.
While the individual process steps are unique for every application, the below infographic summarizes the major end-to-end process steps in a metal AM workflow. Developing a custom metal AM workflow may seem like a daunting task, especially the first time a business implements metal AM in production, but there are many professional resources, AM consultants, service providers, and OEMs with the expertise to help.
Thanks to the following individuals for their contributions to this post,
John Barnes, The Barnes Group Advisors
Joy Forsmark, Ford
Eric Mutchler, Stratasys Direct Manufacturing
Our next blog post will dive into material properties and metallurgy that result from the different printing processes and how these can be manipulate with post-processing. Please leave your email here if you would like to be updated as we release future articles in our Guide to Metal Additive Manufacturing.
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Digital Alloys is committed to providing the technology and expertise manufacturers need to use metal additive manufacturing in production — enabling them to save time, shrink costs, and produce valuable new product.