Is additive manufacturing or 3D printing the best fabrication process for metal parts? Will the properties of the print metal parts meet the needs of application? How do the various 3D alloy printing processes compare? Are materials specifications and standards available for raw materials, processes and additive manufactured alloys?

Figure 1. Seven additive manufacturing processes according to ASTM Committee F42 on Additive Manufacturing. Source: Boeing/ASTM

Determining the best additive fabrication method for the materials and geometry of the specific parts under development is the first factor. The ASTM F42 Committee on Additive Manufacturing has issued a standard on process terminology, defining the seven F42 standard additive categories and the following four categories pertaining to metal additive manufacturing:

• Powder bed fusion (PBF), selective laser melting (SLM), electron beam melting (EBM)
• Direct energy deposition (DED) (Laser vs. e-beam; wire fed vs. powder fed)
• Binder jetting (infiltration, consolidation)
• Sheet lamination: ultrasonic additive manufacturing (UAM)

Metal additive processes are also grouped as direct and indirect. Direct metal additive manufacturing processes build parts using metal powder, wire or sheets bonded with laser, ultrasonic, plasma, arc or electron beam energy source. Indirect routes to metal parts use an additive process to produce wax or plastic patterns or sand molds, which allow casting of metal parts. Direct manufacturing via controlled melting (DMLM, SLM or EMD) of metal alloys can produce practically any shape imaginable straight from a CAD file. Direct manufacturing processes include direct metal laser melting (DMLM), selective laser melting (SLM), laser cusing, or electron beam melting (EBM).

Figure 2: Cost comparison of various 3D printing or additive manufacturing processes. Source: NIST/International Journal of Advanced Manufacturing

Indirect metal additive methods use plastics or binder and then sinter or cast the metal part. Another option is to 3D print plastic or wax part patterns and then sand or investment cast metal alloy parts at a foundry with short-run capabilities.

Directly 3D printing a metal casting mold uses a “sand printer” or powder binder jetting 3D printer such as S-Max™ from ExOne, VX500™ from voxeljet and Viridis3D™ from EnvisionTEC. Sheet lamination and ultrasonic additive manufacturing (UAM) bonds sheets of materials together and then excess, non-bonded areas are machined away. Cost comparison of several 3D printing and additive manufacturing processes is shown in Figure 2. Material, labor and machine costs vary with the additive method. The time to build or print parts is an important factor to consider when selecting a 3D printing or additive process.

Key Considerations

Additive manufacturing and 3D printing has some attractive capabilities compared to conventional fabrication processes. 3D printing is useful in applications where only a few parts are needed or where every part needs to be customized to an individual fit. In medical and dental applications, prostheses, implants and other devices ideally should be customized to the patient. While the parts can be customized material composition, geometry and compositional changes across dimensions, these alterations require analysis for structural integrity.

The ability to produce high-integrity, 3D-printed metal alloy parts would be useful in designing exploratory spacecraft, satellites, weapons and in other R&D fields where only one or few versions of a device will be built. Additive manufacturing of metals is also attractive for space stations, as well as remote military bases and research sites where delivery of critical replacement parts can take an inordinate amount of time.

Figure 3: Integration of unique light-weighting 3D lattice structures or honeycombing into part designs is possible with metal additive manufacturing. Source: AutodeskUnique light-weighting structures can be implemented via additive manufacturing, which cannot be manufactured by other methods. Aerospace designers use foam- or honeycomb-cored composite panels to reduce weights in structures. Honeycomb panels are constructed with adhesive bonds, brazes or are welded, usually from sheet components. Unique microtrusses, internal cavities, ribbing or honeycombing can be printed into the part, which provides a lighter component while maintaining stiffness and strength.

Wear resistant thermal spray deposits are applied to the surfaces of jet engine parts, grinding mills, mining machines and solids handling equipment to reduce wear and erosion. In some cases, additional bond enhancing or transition coatings are applied to prevent the wear coating from delaminating or flaking off a surface. Additive processes have the potential to transition from a tougher alloy core to a harder outer wear layer without a bond interface, which eliminates delamination concerns. Potentially, materials with unique compositions could be 3D printed, which cannot be cast, extruded or machined.

If short production runs, internal cavities or unique materials (i.e., gradients in properties) are not required, then perhaps conventional fabrication methods are a better choice. If the parts are simple or made of low-cost, easy-to-machine alloys, then non-additive fabrication like water jet or laser cutting with some additional machining or forming might suffice. Metal casting can be a good option for complex parts using binder jet AM patterns or molds until production levels justify hard tooling for die casting or injection-molded patterns.

The time to additively build or print a metal part can be quite long, which is acceptable in prototype and short run applications. Metal additive manufacturing can reduce material costs and scrap, which can provide savings when manufacturing costly titanium and nickel alloy parts. Figure 3 shows the factors to consider when comparing metal additive manufacturing method and conventional methods. Of course, the specific part and manufacturing options (specific AM and conventional processes) need to be examined in a more detailed analysis.

Figure 4. Selection factors in additive manufacturing versus conventional fabrication processes.

Challenges

3D printing raw material costs and availability are another factor to consider. Are the additive or 3D printing metal powders, wires or binder based on industry standards or are they proprietary to the 3D printer manufacturer? Ideally, the ability to source the additive raw materials from multiple sources could help control costs and assure availability. If the parts are going to be printed in-house then CAPEX and operating costs (cycle times, power, water, gases, labor, etc.) need to be evaluated.

Is the metal 3D printing process in the R&D, pilot or production stage? Over 3,000 U.S. additive manufacturing patents were issued over the last 5 years, so there are many new processes to choose from and evaluate. While new additive or 3D printing processes for metal alloys might have some patented advantages, bugs in the process might need to be ironed out. Feedstocks and printing procedures for specific alloys might need to be developed as well.

Schematic of residual stress induced warping of build plate during AM processing (a-c) and resulting lack of fusino defects (d). Source: ORNL/International Materials Reviews

Another factor to consider is the development time required to refine the additive process to remove any defects in geometry and materials properties. Even in conventional processing methods, like powder metallurgy or casting, shrinkage and residual stresses can cause warping and distortions. Molds and dies are adjusted to account for these problems. Part shrinkage in additive manufacturing must be considered. Software can automatically compensate for shrinkage in plastics where 3D printing temperatures are lower. For metal, shrinkage needs to be tackled on a case-by-case basis. Direct metal additive manufacturing temperatures vary with the metal and additive process. The residual stress in the additive metal part can cause the build plate to warp, resulting in lack of fusion defects in the part. Lack of fusion issues can be localized within the interior of the part and mitigated with post-processing such as hot isostatic pressing.

Rapid cooling, incomplete melting and residual stresses can also cause deposited layer delamination and these macroscopic defects cannot be repaired by post-processing. In “Seeking Larger Internal Channels using Ultrasonic Additive Manufacturing”, Travis Mayberry, senior mechanical engineer at Raytheon, aimed to develop an unsupported structure with internal cavities using an ultrasonic additive method where metal sheets are laminated. Bowing of the unsupported layers occurred, which was eventually solved with sacrificial supports, increased section thickness or higher strength materials.

Figure 6 - Relationship between the additive manufacturing operating input parameters and output parameters such as residual stress, microstructure, mechanical material properties, surface finish, feature size, and geometry. Source: ORNL

Not There Quite Yet

While additive manufacturing has been around for a while now, several issues still need to be resolved before more widespread use occurs in mission-critical applications. According to the article, “Embracing opportunity: additive technology for manufacturing,” researchers at the Air Force Research Laboratory’s Materials and Manufacturing Directorate are trying to overcome some deficiencies with additive manufacturing, such as the lack of standardized production processes, quality assurance methods, significant material variability and reduced material performance.

Materials engineers understand how conventional processes influence the safety, reliability and durability of critical aircraft parts, but they do not know the affect on parts made through additive processes. A better understanding of processing and structure property relationships for metal additive manufacturing is required to further expand AM for mission critical applications and wide scale production. ORNL’s process map in Figure 4 shows some of the complexities between processing parameters and their impact on properties and build quality.