Design for Metal Additive Manufacturing – A Detailed Overview
Over the years, manufacturing techniques have constantly evolved. With the advent of technology and added research, new manufacturing processes are being deployed. The traditional manufacturing techniques are gradually making way for additive manufacturing (AM) as manufacturers are realizing its true potential. Additive manufacturing (AM) has offered manufacturers an excellent opportunity to produce complex designs and parts that have detailed features easily with advanced software such as CAD. Moreover, no additional tooling is required for additive manufacturing. Without the additive manufacturing technology, it was impossible to produce these complex parts by adopting the traditional methods. Most importantly, the parts manufactured by additive manufacturing are lighter, highly efficient and cost-effective, and best suited for the desired applications.
It is essential to understand that just like other manufacturing processes, additive manufacturing has a few limitations. For example, laser powder-bed fusion components are designed with several overhanging features. A sacrificial support may be needed to ensure success. The additional supports increase the overall costs and the time required to build and also require extra materials. One of the main reasons why design for additive manufacturing is important is to produce parts that offer high performance and cost-effective.
This article will focus on the various crucial factors that influence the success rates in additive manufacturing.
1. Residual stress
During the process of laser powder-bed fusion, residual stress is created owing to the rapid heating and cooling involved. By moving the focussed laser across the bed, a fresh layer is created and the top layer is melted and infused into the layer below it. At this time, a high amount of heat starts to flow into the solid metal from the hot weld pool. The molten metal then solidifies after it cools down and this process happens at a very rapid pace (microseconds).
When the newly formed layer cools and solidifies on top of the existing layer, it tends to contract. The solid structure underneath the new layer constrains the new metal. Hence, there is a large amount of force between the layers due to the aforementioned process. As new layers start adding up on one another, a lot of stress is built up and could make the parts defective. Hence, residual stress is could potentially be highly destructive. In some cases, when the stress is more than the strength of the part, it may crack the part.
Design tips for minimizing residual stress:
- Get rid of large sections of uninterrupted melt
- Be aware of the various changes in cross-section
- A hybrid build uses a base-plate that is thick
- Make use of build-plates that are robust when the stress is likely to be high
- Choose the most stable scan strategy
In the case of any additive layer process, the direction of the build is usually in the Z-axis i.e. vertical to the build plate. It is important to understand that general use orientation is not build orientation all the time. One should choose a build orientation that can produce the most robust, stable, and reliable build with minimum support material.
Melting process and overhangs
In the case of power-bed processes, shapes are build according to layers and it is important for the multiple layers to relate to each other. When the fresh layers are melted, it is largely dependent on the layer underneath it for physical support and also to find a path to conduct the heat. A strong weld is created underneath the area where the laser is melting powder as the heat flows from the weld pool into the structure present below. Moreover, the layer tends to re-melt. Once the heat is conducted efficiently after the laser source is removed, the weld pool solidifies at a rapid pace.
In most cases, overhangs that are less than 45 degrees from the build plate, need support. The overhanging surfaces are also called down-skins. The surfaces of the overhang are rough compared to the vertical walls and surfaces that face in the upward direction. This effect is mainly due to the partial sintering of the powder present underneath the overhang. Different types of orientations can be used to produce parts. Ideally, the orientation should be able to support itself as one can save the build costs and post-processing.
One of the fundamental principles of DfAM is to consider the orientation of the build during the design stage. Design engineers should keep in mind the following points:
- Large-sized overhangs often need a supporting material that is robust and extensive
- Utilizing tweaked designs along with tapered material to ensure supports are reduced
- Increasing the mass of parts and adopting post-process machining
- If angled at around 45 degrees, the down-skins and up-skins will have a rough surface finish
- To eliminate EDM, a solid attachment along with stock allowance is necessary
It is essential to understand that it is important to analyze the different build orientations in the initial stages of the design process to get a fair idea of what works best.
Tips for orientation design
- Develop orientation for a part that is specifically designed for additive manufacturing
- Designers should focus on creating self-supporting designs
- The primary factor should be the build success
- The key factors of orientation include surface finish and residual stress
- Essential to know that the orientation has an effect on the costs and build-times
- It could be difficult to orientate complicated geometries
As mentioned previously, it is not an appropriate practice to depend on the supports to solve issues related to orientation. Although to some extent, additional build time and post-processing are accepted while making a prototype, it is not acceptable to produce AM parts.
Function of supports
Although it is important to reduce supports by design, it is not practical to get rid of them completely. There are three main functions of a support:
1. Isolated material
One of the main reasons why supports are used is to 'anchor' the material which is not connected to the previous layers. Hence, under these circumstances, adding the support structures to the component design is recommended.
2. Residual stress
Sharp edges and large sections of material which are built on a build plate should be avoided to negate the residual stress. In such situations, in which the same is not possible, supports should be used to resist the stress in the part to stop the material from peeling off. However, it is should be avoided for production builds.
The un-melted powder acts as an insulator. To avoid burning, distortion, discoloration, and over-melt, the supports play a key role in transferring heat away from down-skin areas.
Tips for support design
- Important to remodel the holes that are over 10 mm
- Make full use of the chamfer radius to avoid taller supports
- Eliminate the areas overhanging less than 45 degrees to the build plate
- Eliminate the areas horizontal down-skin
With an intention to develop parts that are design-efficient, topological optimization and generative designs are widely used. The ability of additive manufacturing to manufacture complex shapes and designs makes it easier to understand these designs.
One of the main reasons why optimization is important is to maintain structural strength and rigidity while getting rid of unwanted material. It is essential to understand that a part that is functionally optimized may not be well-suited to AM as far as build orientation is concerned.
Tips for Optimization design
- Application of minimum thickness of the wall guidelines is important
- Note down the critical surfaces required for machining
- Important to develop for a certain orientation and modify the details based on that
- Understand whether the desired surface finish can be achieved
Additive manufacturing offers immense design freedom to develop efficient parts and parts that offer high performance. However, it is essential to understand that by implementing the various characteristics of AM processes, production parts can be produced in a cost-effective manner. Moreover, by integrating Dfam with the various design processes, the economics of the AM processes can be enhanced significantly. Hence, it is essential for designers to be aware of the various manufacturing processes to have a competitive edge in the industry.