| Author(s) |
Dennis Harwig, William Mohr, Nick Kapustka, Jacob Hay, Michael Carney, Susan Hovanec, Evan Handler, Jeff Farren, Justin Rettaliata, Ryan Hayleck |
| Abstract Scope |
Introduction: Metal additive manufacturing (AM) technology is rapidly maturing, but shipbuilding implementation has been slow due to lack of fabrication standards. The standards development process is slow for many reasons including a large number of new AM process types and lack of proven qualification and certification schemes. Directed energy deposition (DED) processes, which include variants of arc, laser, and electron beam welding processes, have the least risk since they leverage mature welding technology. Standard requirements for DED AM parts are being developed for a new Technical Publication (Tech Pub) – NAVSEA Process Requirements for Metal Directed Energy Deposition Additive Manufacturing. Here, the Tech Pub requirements for DED AM applications are divided into two sections: procedure qualification and part verification. Procedure qualification establishes process-feature-property relationships and can support a range of part and component DED applications. Part verification demonstrates the ability to manufacture high quality DED AM parts and components that may have specific geometric features and process control requirements. A series of projects have been completed to develop a portfolio of DED procedure qualification schemes (ref. 1, 2). Nine different schemes have been developed. Three schemes were demonstrated with stainless steel consumables using different DED processes, on different standard qualification build (SQB) designs that cover a range of feature scale.
Experimental Approach: DED processes are used for digital manufacturing of structure, adding features to structure, and repair. As such, Tech Pub requirements need to accommodate both non-integrated and integrated build platform (for adding features or repair) conditions. DED procedure qualification schemes were developed for integrated and non-integrated build platforms and for both single-sided and double-sided build applications. The latter is widely preferred for distortion control and build productivity. These procedure qualification requirements were developed for arc, laser, and electron beam welding-based DED processes using wire or powder consumables. Each procedure qualification scheme included a standard qualification build design, nondestructive evaluation (NDE) test map, property test specimen matrix, and qualification records for each application and process combination. Since these metal AM processes cover a range of feature size capabilities that are defined by minimum deposit bead width, SQBs were designed for full-scale (~> 5 mm), sub-scale (~2 – 5 mm) and mini-scale (< 2mm) deposit width features. As a result, there are nine baseline SQB designs for plate platforms that include combinations of tensile, bend, Charpy, hardness, metallographic, and composition specimens depending on the material, scale, and application (integrated build platform (IBP) versus non-integrated build platform (NIBP)). In addition, each SQB evaluates the effects of thickness using thin (single pass wide wall) and thick (multi-pass wide block) features to extract property specimens in different directions and deposit conditions.
The buildability of these SQBs and process-feature-property relationships were demonstrated using stainless steel consumables. Here an ER308L gas metal arc pulse (GMA-P) full-scale NIBP SQB and a MIL-308L wire laser (WL) sub-scale IBP SQB were completed, and a powder laser (PL) 316L mini-scale IBP SQB was in-process. The specimen matrices within each SQB were configured to minimize build volume and costs. These builds were examined with visual, surface (dye penetrant testing), and volumetric (ultrasonic and radiographic testing) inspection methods, and a standard sets of property test specimens.
Results and Discussion: The ER308L GMA-P DED builds evaluated effects of deposit size (heat input) and preheat/inter-pass temperature ranges. A systematic parameter development method was used to develop parameters at a constant bead size by fixing the wire feed speed (WFS) to travel speed (TS) ratios at 15 for small beads and 30 for large beads. Preferred parameters were selected and used to make two SQBs at each bead size (WFS/TS ratio). For each bead size, an SQB was built with a 60°F to 350°F preheat/inter-pass temperature range, and a 500°F to 750°F preheat/inter-pass temperature range. The GMA-P process produced sound SQBs that met ultrasonic and radiographic inspection criteria.
The MIL-308L WL DED on 304L sub-scale single-sided IBP SQB was made using 6000-W laser power. The WL deposits had a WFS/TS ratio of 2.6 to 3.4 and were made at less than half the deposition rates of the GMA-P deposits. The WL process produced sound SQBs that met ultrasonic and radiographic inspection criteria.
All bend tests passed at 20% elongation and further demonstrated soundness of both wall and block deposits in different planes using both DED processes. Tensile properties of both the ER308L GMA-P DED SQB and the MIL-308L WL DED SQB exceeded all property requirements for MIL-308L filler material in x-, y- and z-directions. These builds exceeded property requirements for 304L base materials for yield and ultimate strength per ASME SA 240. Elongations were slightly lower than base metal requirement of 40% in y- and z-directions for the GMA-P deposit material. Yield strength decreased in ER308L GMA-P DED builds with increasing bead size and preheat/inter-pass temperature: the greatest strengths were in the small bead and low preheat/inter-pass build condition.
Conclusions:
1. A prescriptive qualification scheme provides a clear path to accelerate implementation of high impact DED AM technology. These DED procedure qualification schemes should be considered to harmonize requirements across industries and reduce manufacturing supply chain qualification and certification costs.
2. The GMA-P and WL DED processes produced sound builds that met ultrasonic and radiographic inspection criteria. Bend tests further demonstrated soundness of multi-bead, multi-layer deposits in different planes.
3. The tensile properties of the ER308L GMA-P DED full-scale single-sided NIBP SQBs exceeded all property requirements for MIL-308L filler wire in x- and y-directions. These builds exceeded property requirements for 304L base material for yield and ultimate strength per ASME SA 240. Elongations were slightly lower than base metal requirement of 40% in y- and z-directions.
4. The tensile properties for the MIL-308L WL DED on 304L sub-scale single-sided IBP SQB met the requirements of the corresponding referenced specifications. The deposit material and build platform HAZ interface tensile specimens for both the block and wall met the minimum tensile strength requirements of both the MIL-308L wire and the 304L build platform. |