The addition and removal of supports have long been a challenge during metal additive manufacturing (AM). Taking direct metal laser sintering (DMLS) as an example, the model needs to be pre-set with support structures before printing to avoid deformation caused by thermal stress and conduct heat away from the molten pool. These brackets are part of the design and manufacturing as a whole. After construction, the support structure was dismantled and discarded. Without supports, it is difficult to print cantilevered structures below a certain inclination angle (usually around 45°), which often limits the options for users of metal 3D printing systems, and also brings many equipment OEMs and additive manufacturing software companies. a great challenge.
To solve the above problems, experts from EOS company Additive Minds have now developed various process optimization techniques to produce 3D printed parts without support structures, such as stator rings, housings, turbo pumps, oil tanks, heat exchangers, valves and impellers, of which the closed impeller is one of the more typical cases. Through optimized design software and parameter packages, EOS enables users to print cantilevers and bridges at lower angles (sometimes even zero), requiring far fewer or no supports.
Unsupported additive manufacturing also saves a lot of time in the post-processing stage, as no additional supports need to be removed. In the case of manual removal, this also frees up employees' time and energy for other work. Manufacturing parts without a support structure also reduces material waste, as nothing is thrown away and all aspects of the part and support design are necessary. However, this is not an easy process, and software design experts and manufacturers have been working on the challenge of unsupported design for years.
In this article, it is mainly shown how the experts at EOS utilize the unsupported method to construct the impeller. Enclosed or shrouded impellers are used in many industries and they vary widely in size, shape, material, and performance requirements. Enclosed impellers are often exposed to extreme conditions such as high rotational speeds, highly corrosive media, and mechanical loads caused by extreme temperatures. For example, turbopump applications in space rockets, compression systems in micro-turbines, and seawater pumps in oil and gas applications.
Support design requirements in traditional metal 3D printing
Designing 3D-printed parts with supports has been a standard approach to additive manufacturing (AM). The number, size, and location of supports are determined by a number of factors:
Residual stresses during printing can cause deformation of the 3D model. Supports can be added to physically prevent this deformation;
Interruption of the recoater affecting the intermediate build of the part may vibrate the part or cause damage, resulting in an unsuccessful job. Brackets are used to protect the parts from any influence by the recoater;
Heat transfer through supports allows parts to cool and form faster and more successfully during the build process.
To ensure a 3D printer builds and successfully produces parts, a variety of reasons that affect support design need to be considered, including:
The part orientation determines how much of the part needs to be supported. Typically, if the parts are oriented so that a larger surface area is not on the build plate, more support is required to compensate for the above factors.
Overhangs of 45 degrees or less are generally considered to require support structures.
Channels and holes may deform without support, depending on their size and whether they are oriented ineffectively.
Model design
Armed with the right expertise and creative problem-solving skills, the team at EOS has successfully developed new ways to design and build models, shattering the preconceived notion of "low dips must add support", with excellent results. The impeller used in this article to demonstrate the unsupported structure and the capabilities of the DMLS process was designed by EOS Additive Minds with a diameter of 150 mm with 12 blades with overhang angles down to 10 degrees.
Member inclination direction and support structure
Impellers are usually printed in an inclined orientation to avoid internal supports as they are difficult to remove. However, this orientation typically results in longer build times, uneven surface quality, and the roundness of the part suffers. Planar orientation offers several advantages, such as shorter build times, better roundness and accuracy, and a more uniform surface quality across the part. However, low overhangs usually require a lot of support. For the current DMLS process, larger overhangs of less than 35° need to be supported. Supports are required to dissipate heat from the molten pool to compensate for recoating forces and internal part stress.
Unsupported design optimization
EOS significantly reduces the need to add internal support by leveraging advanced model design techniques. The design optimization of the additive manufacturing process is also another important aspect that is related to the success of printing. While internal support can be avoided primarily through the use of adjusted exposure strategies, external support structures are often still required.
In the impeller case of this article, instead of using solid fill, the bottom of the part was modified by using self-supporting arches and thin walls to ensure a strong platform connection and prevent deformation during construction. This allows the use of less material than conventional stents while providing high strength and improved machinability. The outer diameter of the impeller is closed to provide greater stiffness to the part when built and to prevent loss of geometric accuracy at the outlet edge. For this impeller, an advanced design enables a 15% material reduction, while being machine-optimized and self-supporting, with no internal support.
Process Optimization
The impeller is constructed using the so-called high-energy DownSkin method (the type of exposure used to build overhanging surfaces). Essentially, this method increases the energy density input of the DownSkin exposure by increasing the laser power while adjusting other DownSkin parameters. This produces a larger but more stable melt pool, especially when building overhangs on loose powders. This method has been successfully used for many materials often used to make impellers (eg Ti64, 316L, AlSi10Mg, In718, etc.).
Therefore, it can be ensured that all critical angles can benefit from this optimized parameter. Unlike other unsupported technologies, the high-energy DownSkin approach does not sacrifice build speed and therefore the business case to avoid support.
In the absence of any countermeasures, the high-energy DownSkin method can result in oversized parts in the z-direction in the DownSkin region due to the deep weld pool. Parts can be adjusted to the right size by post-processing or by tweaking the design. DownSkin is also relatively rough, but the roughness is uniform, which helps with bulk surface finishing techniques such as abrasive flow machining. There is also hardly any porosity (see image below), porosity is limited to DownSkin. Therefore, the overall mechanical properties are not affected and you can still rely on the high-quality InFill process developed by EOS. Therefore, a secondary process like hot isostatic pressing is also not required to obtain sufficient mechanical properties.
Post-processing (Abrasive Flow Machining, AM Metals)
Abrasive flow machining is a common surface finishing technique used for flow-related applications and internal geometries. The abrasive media is pushed through the part held in the fixture. The abrasive particles in the media grind and polish the surface along the flow path. As preparation for inner surface finishing, the closed outer diameter needs to be machined into an open, diameter and part height adjusted to the fixture for the AFM process. After pre-machining, the part is clamped and abrasive media is pushed through the part with the help of the clamp. After the AFM process, the impeller is machined to the final size.
The final part treated with Abrasive Flow Machining (AFM)
With the continuous advancement of 3D printing technology, metal 3D printed parts will continue to develop toward the end consumer market.