Features

Current State of the Art in Laser Powder Bed Systems

Additive manufacturing (AM) – the process of building up solid layers of material to form a finished solid part — is an emerging and exciting technical discipline. Also referred to as “3D printing,” many misconceptions exist about the capabilities and promises of the technology.

Yes, AM has the potential to create customized parts. Yes, AM is able to create parts that cannot be practically produced by other traditional manufacturing methods. Yes, AM offers tremendous opportunities for part weight reduction and parts consolidation. And yes, AM will be and has been used to produce some large volume production parts. But AM does not offer infinite freedom in shape; complexity is not “free”; parts do not pass through the printer at the touch of a button or the speed of printing a page of text; and one must carefully design both preprocessing and post processing activities to achieve a finished metallic part from the AM process.

Although many material systems have been printed via AM (including polymers, metals, ceramics and biological tissue), this article will focus on metal AM and its benefits, limitations and applications.

Laser Powder Bed AM Process

Table 1. Classification of AM Processes (Metals and Nonmetals)[1]

A number of metal AM processes have been developed over the past 30 years. Table 1 highlights those recognized by ASTM International [1] . A thorough description of each of these processes may be found in the annual report produced by Wohlers [2] . Nearly all of these methods rely on a computer-controlled heat source (typically laser or electron beam) to melt or fuse the metal feedstock and deposit it point-by-point at precise 3D locations. Both wire and powder are commonly used for feedstock. (A notable exception is gaseous deposition [3] .) Here, we will only focus on the laser powder bed fusion (L-PBF) process, by far the most commonly available method for production of metal AM parts. Electron beam powder bed fusion systems are also available; in principle, they operate in much the same way as laser-based systems, except that the build is completed in a vacuum rather than in an inert gas environment.

Figure 1. Typical laser powder bed fusion system.

Figure 1 highlights the laser powder bed process where a laser is used to melt selected regions of a thin layer (typically 20–50 microns) of metal powder. To keep the molten metal from absorbing oxygen or nitrogen from the atmosphere, the system is enclosed in a chamber pressurized with a flowing inert gas – typically argon. Builds are completed on a build plate to anchor the solid material, thereby keeping it accurately positioned during the build process.

Considering this method of production, several steps must be completed prior to fusing metal powder. These steps are highlighted in Figure 2.

Figure 2. Preprocessing steps for production of AM parts.

One should consider several geometric factors when establishing the part shape and build orientation. First, overhanging features – like the bottom of a bridge – greater than a few millimeters in length require support structure. These features allow material to be anchored to a solid structure so that when the first few layers of fused metal are generated in the overhang they don’t sag or get displaced during construction of subsequent layers. Examples of inadequately supported overhangs are shown in Figure 3. To avoid these situations, parts are oriented in the build process to minimize overhangs.

When orientation selection is insufficient to eliminate overhangs, part geometry may be altered to ensure such overhanging surface is at least 45° from the horizontal. Shallower angles have resulted in overhanging features that consistently yield defects like those in Figure 3. When shallow overhang angles cannot be avoided, lattice support structures are typically added. These structures provide thin truss-like features onto which the overhanging material will be firmly supported to avoid delamination-like defects shown in Figure 3. However, these lattice support structures typically require removal as part of the post-processing of completed builds. Horizontally positioned holes, for example must either use support structure or the top of the hole must have flat sloping sides that do not exceed 45° from the horizontal. Therefore, beyond designing a part for form, fit and function, one must consider the manufacturing process in part design, not unlike designing parts to account for limitations of any other manufacturing process.

Figure 3. Examples of improperly supported overhanging features.

After defining the solid mass (i.e., the as-built shape) of the part, including lattice structures, one must determine the process conditions to be used to produce the part. Although some laser powder bed fusion manufacturers fix process conditions for a given alloy, other manufacturers offer open architectures that allow for user-defined process definition including build plate preheat temperature; laser power, focal point and travel speed; powder layer thickness; laser rastering pattern and angle increment; hatch spacing; and many other potential process details.

Optimum process conditions depend on the selected alloy as well as the powder size distribution and production process used to produce the powder. The powder layer thickness is used by slicing software to establish the region to be melted and fused for each build layer. This is commonly completed by software using the computer solid model of the as-built shape and orientation.

Figure 4. Powder bed fusion build parameters.

Also completed automatically is the laser rastering pattern, that is, the path the laser will take as it locally sinters small sections of the powder in each build layer. Multiple rastering patterns are available, as highlighted in Figure 4. At this point, the detailed build plan is completed.

Machine preparations include sieving and loading the metal powder. Sieved metal powders typically range in size from 15 to 55 microns. Several processes are used to produce metal powders. Argon atomized is the most commonly specified powder. Air-atomized powder is lower in cost, but may result in undesirable defects in finished parts. Plasma atomized powder has the highest quality resulting in minimal porosity in finished parts.

In essence, one must consider more than powder chemistry when selecting a metal powder for production of AM parts. Powder sizes outside this range are undesirable as small particles typically lead to clogged filters, easily become airborne and are extremely flammable due to surface area. Larger powder particles may not completely melt or may interfere with good powder spreading.

While the powders typically arrive from the powder producers in this size range, unfused powder that is reused from previous builds requires onsite sieving to eliminate satellites/irregularly shaped particles to ensure the sieved powder remains within the acceptable size range. Once ready, the powders are introduced into the powder handling system, which must also be kept under inert gas to avoid oxygen or nitrogen contamination from the atmosphere. After backfilling the system with an inert gas, the build plate is heated and made ready for the build.

The actual building process is characterized by four steps that repeat until the build is complete.

  1. From a moving hopper, deposit a layer of metal powder along the entire surface of the build plate.

  2. Level the powder layer to a uniform thickness by using either a solid recoater blade made of a metallic or elastomeric material, or a roller that traversed from one side of the build plate to the other and slightly compacts the powder in addition to leveling it.

  3. Fuse the powder via one or more lasers (each typically between 200 and 700 watts) that follow the pattern established in preprocessing; the unaffected powder remains in the build chamber and is often recycled for use in subsequent builds.

  4. Lower the build plate by an amount equal to the powder layer thickness.

To minimize directional property variations, the raster angle is modified from one layer to the next as illustrated in Figure 4. The part borders in each layer are typically scanned multiple times to improve geometric accuracy and to minimize surface roughness. As the molten metal solidifies and cools, it shrinks, resulting in residual stresses that if not properly accounted for may lead to severe part distortion or even cracking, as shown in Figure 5. In this case, the sharp corners were later replaced with ones having a small radius of curvature to eliminate the associated stress concentration at this location, and yield a crack-free build.

Figure 5. Effects of residual stresses during builds.

Post processing of the builds includes loose powder removal, separating components from the build plate, removal of support structure, thermal processing, final machining and surfacing (as needed). Operators typically wear personal protective equipment during this phase of the build to avoid breathing in fine metal particles. In addition, care must be exercised when removing loose powder (i.e., that powder that remains unfused during the build process) as the fine particles could spark a spontaneous, localized fire if a sufficient quantity of particles becomes airborne in the presence of an ignition source. Static-free mats are used in the vicinity of AM machines to minimize the probability of such an event. Typically, the loose powder is recycled after sieving. However, premium-quality components may always be made from virgin powder to avoid any contamination from recycled powder.

Once the loose powder has been removed, the build plate is removed from the machine and the builds taken off the build plates via wire electrical discharge machining (EDM) or band saw. The associated metal removed by these machining operations must be accounted for in the preprocess design phase mentioned above. In a few instances, parts can be pried off through use of a flathead screwdriver; however, this method is not recommended since it may damage the parts or build plate. After removal of the parts, the build plate is redressed, typically by machining a thin layer of material from its upper surface and ensuring the top surface is flat and parallel to the underside of the build plate. This is needed to ensure the next build has a predictable, uniformly thick initial powder layer.

The built parts require several additional processing steps. Additional powder removal may be required at this point. Powder from readily accessible pockets generally can be easily removed with a soft brush; powder remaining in small crevices and deep, small-diameter holes, however, may require use of ultrasonic vibration or pressurized air to remove loose powder. Typical as-built surface finish for metallic components is 100–250 microinch Ra. While this finish is often sufficient for exterior surfaces, machining is typically required to obtain the necessary finish for mating surfaces. Remaining build structure must also be removed by machining.

Thermal treatments typically include a stress relief while the components are still attached to the build plate. Additional post-build thermal treatments may include hot isostatic pressing (HIP) to consolidate minor porosity in the build and/or thermal treatments designed to tune mechanical properties. (Note that in some cases, all thermal treatments are completed prior to removal of parts from the build plate.) All thermal treatments are similar to those used for other metalworking processes such as casting or forging. Finally, any additional surfacing operations, such as tumbling (for surface finish and/or cleaning), painting, etc. may also be required. Inspection for dimensions, material flaws or other features is also typically completed at one or more stages of the process.

Common Metallic Materials

Table 2. Commonly Used Metallic Alloys within Laser Powder Bed Systems

Although some research is underway to develop a series of alloys specifically designed for additive processes, the most common metallic materials are those currently used and qualified for casting and/or welding. Though the list of alloys used to produce metallic materials is ever expanding, the most commonly used alloys are shown in Table 2. Optimum process conditions vary widely among the various alloys. In some instances, minor modifications are made to the starting alloy composition to account for evaporation of light alloying elements.

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