Current State of the Art in Laser Powder Bed Systems

Benefits and Applications of AM

Until recently the most popular applications of AM was for prototyping, low-rate production, and complex dies including those with conformal cooling passages. However, over the last several years, large volumes of production parts have been realized. By taking advantage of the AM process, manufacturers have been able to justify the higher production cost (relative to other established metalworking processes) of AM primarily as a result of lower overall system production costs, lower weight and/or improved performance of the parts in service.

AM offers the opportunity to consolidate parts that would otherwise be produced by other means. For example, bolted, riveted and/or welded structures may be produced as a single-piece part, thereby eliminating assembly costs. This has been true in several well-publicized GE developments [4] .

  • A fuel nozzle for the LEAP jet engine consolidated 20 parts into a single AM component, which also was 25% lighter and had a projected life five times that of the legacy design.

  • An advanced turboprop engine consolidated 855 components into 12 AM parts, which reduced fuel consumption by as much as 20% while achieving a 10% boost in power

Much of the in-service benefits from these GE parts have been realized due to lower weight. Other improvements in performance have been realized as a result of part geometries that would not be practical to produce via traditional metalworking processes. In another instance, conformal cooling channels have been used in electronic chassis to improve its cooling characteristics [5] . The complex three-dimensional path that includes the cooling channels could not be manufactured by any other means than through AM. Conformal cooling channels have also been produced during AM of dies/molds for die and permanent mold castings, metal forming, plastic injection molding and other processes. Conformal cooling allows for more rapid production of components and reduced stresses as a result of better thermal management throughout the die/mold.

Another benefit of AM is weight reduction in parts. Given the practical limitations associated with traditional metal-working processes, components often have excess material. In these cases, components often take on a blocky geometry. Through advanced design methodologies, including topology optimization and generative design, engineers have developed complex geometries that do not include excess metal mass. The resulting structures often include features looking more like a labyrinth of ligaments rather than a solid blocky structure. In other instances, structural lattice structures are used to lighten components.

Figure 6. AM component for a compact line pipe repair tool.

Reentrant features are generally challenging and costly to achieve in practice. However, with the design flexibility offered by AM, such features may be included without the need for additional follow-up process steps. Figure 6 shows an example of a reentrant lip on the top of a L-PBF build. The part is shown as it was removed from the build plate. Note that the threads in the middle knob are as built. Though not close tolerance, they functioned well enough to allow for a tight connection to a mating part of the assembly.

This stainless steel component took approximately 35 hours to build and less than one hour to remove from the build plate. Lead time for the prototype component by other manufacturing methods, including machining and casting, exceeded the required delivery time for the component. Furthermore, the additive design permitted a significantly smaller axial length, which allowed for use in pipe repairs having 90° elbows, whereas the legacy design could not be used in such applications.

Current Hurdles to More Widespread Use of AM

Several hurdles exist to more widespread use of AM in metallic system components. Perhaps the most significant hurdle is cost. Parts produced by AM nearly always cost more to produce than shaped, mass produced parts. Given the processing steps required, AM feedstock is more expensive per unit weight (by a factor of 2 to 4) than wrought material of the same composition.

However, when powder recycling is used, L-PBF nearly always has a higher part-weight-to-feedstock usage rate. Furthermore, this argument assumes the geometry of the AM part is identical to that produced by alternative processes. Direct substitution of identically shaped parts may be suitable for replacement of unavailable parts; however, as a general rule, production of a geometrically identical AM substitute for parts produced by alternative processes is not economically viable. Parts designed for the AM process should take advantage of the capabilities offered by the AM process. Therefore, the best parts to consider for AM are those whose shape cannot be economically produced by well-established alternative processes such as machining, casting, or forging.

Production rates have been cited as an issue with some AM parts. Equipment manufacturers have recently introduced machines with multiple lasers to simultaneously sinter multiple areas within each build layer. Heat management of these builds does limit the number of lasers that can practically be applied in any given AM machine.

Build chamber size has excluded the use of powder bed machines to produce some large components as a single piece. The majority of commercially available production L-PBF machines have a build volume limit of approximately 12 in × 12 in × 12 in. Machines that can produce builds of approximately 24 in × 12 in × 12 in are available from equipment suppliers. Metal powder bed machines with build volumes of 1 m × 1 m × 1 m (39 in × 39 in × 39 in) have been announced, but such machines would only be used for niche applications by today’s demand for AM builds. Furthermore, machines of this size require additional automated powder metal handling systems and a significant investment in machine costs.

The most common defect in AM production is porosity. When operating at a low heat input, lack of fusion can occur among the powder particles; on the other hand, operating at a high heat input may lead to vaporization of low-melting-point alloy constituents, which may be trapped in solidified metal. Even when operating in the middle ground, some gas porosity is typically present in the as-built state. Typical levels of porosity are less than 1% by volume. This porosity generally does not hamper mechanical strength of metallic builds, but will detrimentally impact elongation and fatigue properties. Porosity is often reduced or eliminated through HIP.

Residual stresses and distortion may occur in builds as a result of the thermal response of the build associated with solidification and the transient temperature gradients throughout the build. These effects may manifest themselves in the part detaching from the build plate or lifting the top surface of the build above the top of the next powder layer. Cracks or slightly misshapen parts may also result from the transient temperature gradients. Guidance on how to avoid these conditions is an area of active research.

Part detachment and solid protruding beyond the next build layer can result in recoater blade damage, which will manifest itself in ridges throughout the unfused powder and potentially pieces of the blade breaking off into the powder bed. Cracks are rarely repairable. Misshapen parts are often corrected during final machining. These problems are overcome by reprinting the parts with improved part orientation, different support structures, part shape changes or different AM process conditions.

Properties of AM builds are not isotropic. Strength and elongation in the build direction are consistently lower than those in the build plane. While the differences in these directions are normally not more than 10–15%, engineers must be aware of – and account for – these effects during part design and development.

While surface finish of the builds is often acceptable, fatigue-critical regions of parts may need additional attention. A finer surface finish may be obtained by altering the process design of border contour passes during building and/or smoothing the surface of these fatigue-critical areas during post processing.

Industrial standards for AM have been developed by ASTM International, SAE International, ISO and others. However, many standards have yet to be developed to the point of application by a broad range of industries. These standards are useful, but they remain somewhat in flux as the industry continues in its current rapid development, expansion and evolution. As these standards are recognized and implemented by more industries, the application of AM will to continue to expand.

Inspection of parts produced by AM often pose a challenge. Many low-cost, high-volume inspection processes have limited application for the broad range of geometrically complex shapes produced by AM. Exterior part geometry is easy to confirm with laser scanning techniques. However, the most demanding applications require assurance of internal quality of AM builds.

Computed tomography offers a method of determining internal flaws in complex 3D components. However, the process is relatively expensive and slow. Furthermore, the information provided by the process is often difficult to relate to existing quality standards.

To combat this issue, a number of organizations are pursuing part certification and process/producer qualification through in-situ process monitoring and control. When coupled with software to predict the resulting microstructure and properties, these systems offer an alternative to post-process inspection. They also offer the potential of either stopping a defective build before its completion or repairing certain defects as they are detected. The need for in-situ process monitoring and control is also justified since each L-PBF system responds somewhat differently.

Performance differences also occur due to degradation of equipment, potential soot buildup on equipment, lot-to-lot variability in powder characteristics and other factors. Therefore, without in-situ process monitoring and control each machine has to be periodically tuned to achieve optimum part performance. In-situ monitoring and control will also correct for any differences brought on by dissimilarities in powder lots.

The future of AM is very bright according to published reports [2] . Double digit annual growth rates are predicted for the next five or more years. Current research and development will reduce or eliminate many of the hurdles identified above.

This article was written by Michael L. Tims, Advisor Engineer, and Kenneth Sabo, Senior Director of Manufacturing, Concurrent Technologies Corporation (CTC) (Johnstown, PA). For more information, visit here.

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