Designing for Additive Manufacturing DMLS
This is part 5 of 5, see below for further reading:
As predominantly a welding (or sintering) process, metal 3D printing involves similar design guidelines and approaches with fewer restrictions and variables. Additive manufacturing allows more freedom of design with less constraint, the goal of design engineers since designs first revolved around available manufacturing technologies. With metal 3D printing as an option, design may shift towards a largely material science approach, more-so incorporating type of material, material defects (porosity, cracks, surface finish, etc.), thermal stresses, necessary post processing, and more.
Re-designing a component or an assembly to incorporate additive manufacturing requires complete overhauling of the features for better and for worse in order to comply with the short-list of AM design requirements. This process will involve the original design engineers as well as a design engineer unfamiliar with the part’s manufacturing history or design history. New perspective with AM in mind opens design considerations.
Designing a part from scratch with the understanding of an AM machine’s printing process, supporting features and orientation, and physical properties and features that can be printed using DMLS will greatly reduce the frustration and barriers of outdated specifications and manufacturing requirements. The orientation of a printed part will determine the necessary external and internal support structure patterns and density. Bridging gaps, overhangs, angled walls, internal features should be used while determining a proper orientation method. Support structure and orientation will be a determining factor in whether a printed part will hold expected physical test properties.
Accompanied design software with AM machines will provide feedback and recommendations regarding the orientation and physical details of a 3D model. The recommendations and automated support structures will not always be the best option. Manually designing support structures into a design or re-designing/re-orienting in order to benefit from more self-supporting features may be better options. Support structures are meant to anchor the printed part to the base plate securely throughout the processes. It is important to design the supports with factors such as thermal expansion in mind in order to potentially reduce the effects of stress buildup. The rule of thumb for thin walled features is a 40:1 height to wall thickness ratio. This is an important consideration when designing internal support structures that will not breakdown overtime or due to stresses.
Self-supporting features will reduce printing time and reduce post processing requirements. Angled walls and overhangs can be considered fully self-supported when under a 45-degree angle from vertical. Bridge overhangs and gaps can be considered fully supported if smaller than .08 inches. This said, completely hollow internal gaps should be designed in order to not collapse within itself by incorporating internal supports to a potentially fully supported feature. These features must be designed with drainage holes for residual byproducts and excess metal powder.
Open and Closed Atmosphere Additive Manufacturing
The primary contaminant observed during a printing cycle is an overabundance of oxygen. An open atmosphere AM machine contains no atmosphere control and relies on the air quality of the manufacturing facility. In order to reduce oxygen levels, argon or a substitute shielding gas can be used in the immediate working area. Steel, nickel, cobalt, bronze, and tungsten are recommended to be printed in open or closed systems.
Closed atmosphere AM machines are capable of creating a vacuum atmosphere controlled printing chamber with oxygen levels around 25 PPM. Titanium, aluminum, and magnesium are recommended to be printed in closed systems. Material selection as well as working in either an open or closed atmosphere machine will require redesigning of the printing process and laser power. The laser power will affect the surface finish as well as the bead size laid down, in turn affecting required post processing and machining time as well as porosity within the material.
Laser-Based Additive Manufacturing of Metal Parts: Modeling, Optimization, and Control of Mechanical Properties (Advanced and Additive Manufacturing Series)
Additive Manufacturing of Metals: From Fundamental Technology to Rocket Nozzles, Medical Implants, and Custom Jewelry (Springer Series in Materials Science)
The most influential force that will affect the outcome of a printed part is thermal stress buildup. Residual stresses from cooling areas of parts and the stationary base plate will cause stresses that must be overcome by support structures. Research is being conducted with aspirations of scientifically predicting thermal stresses using FEM (FEA) in association with 3D CAD models and software. Current AM software provides predictive printing visualization by slicing the 3D model into layers which are representative of the printing process and offers basic thermal stress warnings. Non-scientific predictive models use algorithmic interpolation methods to vaguely indicate potentially high stress areas.
American Makes, a national accelerator for additive manufacturing and 3D printing, is currently engaging in research sponsorship through colleges and universities in order to further the understanding of AM processes. Test results for scientifically based thermal stress simulations on 3D printed parts were promising, with an 8% observed error from live tests. Simulations for a disk test specimen before and after being cut from the substrate showed a bowl effect upwards. Simulations for a rectangular bar before and after removal from the substrate showed warped edges. Testing both scientifically and algorithmically, the plasticity and elasticity in the formed metal should be observed.
Continuous Metallurgy Monitoring
Taking predictability of thermal stresses and the printing process a step further, continuous metallurgy monitoring is the primary goal for AM software and EOMs. The ability to accurately monitor the internal physical properties and external physical properties of a printed part while monitoring the machine parameters will provide runway to produce modular and dynamic printing programs. Modular programs will automatically recognize patterns between one printing job and another printing job and based on sensor data will be able to recommend machine parameters and post processing for certain areas of the production part. Dynamic programs will be able to automatically change machine parameters throughout the printing process in order to optimize the printing based on the support structures needed, the orientation, necessary post processing, and the physical property requirements.
Current methods of continuous monitoring systems are non-scientific and will only provide a basic understanding of what is or what will occur during the printing process. Ultimately, continuous metallurgy monitoring will provide feedback for strain, surface finish (on external and internal faces), porosity, chemical analysis, and information regarding the physical properties of internal support structures.
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