Page 27 - 3D Metal Printing Spring 2016

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up front the impact on post-processing operations and cost considerations, com- bined with the design opportunities that additive manufacturing provides with regard to complexity, the ideal design can be found rapidly, significantly reducing the new-product development timeline.
Designing for the Additive Process
The design process for additive man- ufacturing often begins with an existing design, especially when the goal is to remove cost or weight from a current product. The part-selection process, mate- rial decision and technology down-select could represent an entire discussion and article on its own. But let’s fast-forward to after those decisions have been made when it’s time to open a CAD package and create the additive design.
tities would increase with labor costs for setup and tear down, build-to-build con- sumable costs and machine downtime between builds. The one-off development- piece design will be completely different than one that optimizes for a production volume that maximizes parts per build, nests multiple layers of parts and reduces post-processing steps via unique features.
Investigating each of these puzzle pieces separately is difficult because they are related by cause and effect. However, some basic guidelines and considerations should be drawn upon when in the design cycle.
Material Properties Differ
Properties of additive-manufactured alloys are not equivalent to those of wrought or cast alloys. Additively manu- factured materials have their own design system, and should be treated as such. Due to temperature gradients encoun- tered during the additive-manufacturing process, material properties and microstructure can be anisotropic in nature prior to post-process thermal treat- ment. Thermal processing can alter the microstructure to become more isotropic, but some orientation and geometric effects may remain. The effects may be seen on monotonic or dynamic properties.
It’s important to understand how sig- nificant the effects are within the material/ process combination chosen. If there are design features that would be limiting if not built in a certain direction, the design engi- neer should integrate that design rule into the beginning stage of orientation decisions.
Consider Post-Processing
Two types of CAD models exist for addi- tive manufacturing. One is the final geom- etry, to which the final produced part will be inspected. Datums are identified, machined tolerances called out, surface- finish requirements zoned, etc.
An equally important CAD model, the as-built model or the model input into
the additive machine, appears differently than the finished models. Holes may be filled in, support structure added, machin- ing stock added and more. To understand the considerations placed into the as-built model, a generic process map is provided.
The required post-processing steps appear slightly differently depending on the material and technology used. For example, electron-beam melting (EBM) does not require stress relief or mechanical removal of parts from the build plate, but does require removal of the partially sin- tered powder surrounding the parts. Laser powder-bed fusion (LPBF) requires stress relief and either a wire-EDM or a band- saw procedure to remove parts from the plate. Thermal processing for all 3D metal printing often includes hot isostatic press- ing (HIP) to reduce voids and porosity within the material, with parameters based upon material choice. Additionally, some materials may require heattreating to achieve the desired microstructure.
Advantages, Disadvantages in Build and Powder Removal
Additive technologies offer certain advantages and disadvantages related to build and powder removal.
Electron-beam melting (EBM). Best practice is to not build directly on the plate, especially if the parts have strict composition requirements. The stainless- steel plate may influence the first few lay- ers of the build, so start parts at least 3 to 5 mm from the top of the build plate. That 3 to 5-mm area can be filled either by sup- port structure or solid material to be later removed, whichever is more efficient for the part. One advantage of the EBM process: Due to low residual stress from the entire build area at elevated temper- ature, fewer supports are required and the parts usually pop off of the plate easily.
Powder removal represents the greatest consideration during part removal in an EBM build. On one hand, the preheat step in the EBM process provides better part
Build Thermal Surface Inspection &
Build Removal Processing Finish Machining Testing Method
Part Selection
Material
Post Processing
Orientation
Producidility
Cost
Materials Properties
Design Cycle
Technology
Additive Design Models
The design cycle is not linear. Decisions made during each design stage will affect and contribute to others.
The cycle has many possible outcomes depending on what parameters must be optimized. For example, a one-off devel- opment piece where lead time represents the most important factor may lead to a flat and low orientation, which reduces build height and does not consider the quantity of parts that can nest together in the build box.
This would be inefficient in a produc- tion environment because the number of builds required to meet production quan-
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