N221-021 TITLE: Modeling and Process Planning Tool for Hybrid Metal Additive/Subtractive Manufacturing to Control Residual Stress and Reduce Distortion
OUSD (R&E) MODERNIZATION PRIORITY: General Warfighting Requirements (GWR)
TECHNOLOGY AREA(S): Materials / Processes
OBJECTIVE: Develop a modeling and process planning tool for hybrid metal additive manufacturing processes to predict and minimize the residual stress and distortion of a part.
DESCRIPTION: Additive Manufacturing (AM) technologies have become increasingly important for the rapid production of industrial products. However, AM processes also pose challenges with associated features such as residual stress. Many AM process parameters and post-processes may affect the final residual stress of the part. Laser scanning patterns during AM processing can significantly affect distortion and residual stress distribution in an AM process [Ref 1]. Residual stress caused by the thermal cycles in AM processing is a critical issue for the fabricated metal parts since the steep residual stress gradients generate part distortion which dramatically deteriorates the functionality of the end-use parts. Thus, the residual stress can degrade the AM partís quality, service life, precision, and fatigue performance. For example, after AM processing, a considerable amount of chip curl out of the cutting plane was observed, which was not observed when cutting wrought parts of the same material. This out-of-plane curl was attributed to the residual stress distribution in the part from an AM process, and indicated that residual stresses from the AM process can impact chip formation during machining [Ref 2].
Hybrid additive/subtractive manufacturing is a process that combines both AM and subtractive manufacturing, such as machining, to create parts with high complexity, tight tolerances, and good surface finish. The hybrid process integrates the AM capability of fabricating almost any complex geometry and the machining capability of offering high part quality and short processing times. Properly chosen tooling and cutting conditions may induce stresses along the outer surface to counteract those imposed from the preceding AM process [Ref 3]. Thus, if well planned, a hybrid process can potentially be used to produce a part with controlled stresses and minimum distortion.
Due to the complexity of the residual stresses, some researchers have investigated the modeling of dual processes or hybrid processes. For example, finite element modeling was used to predict the residual stresses developed during heat treatment processes and the distortion during machining operations [Ref 4]. Another finite element method, utilizing the level set method to define the cutting tool path, was able to predict results such as residual stresses and part distortion [Ref 5]. The results show that machining can partially eliminate the residual stresses and distortion caused by laser cladding. However, the entire part needs to be modelled to predict residual stress, making the analysis computationally very expensive. The challenge increases when using a modeling tool to plan and benchmark between different tool paths and deposition strategies. Thus, an efficient and effective modeling and planning tool for AM processes is needed.
The Navy requires a modeling and process planning system for a hybrid metal additive manufacturing process. The tool will integrate the effects of additive and subtractive processes. These results will be the basis for hybrid process planning in order to control the residual stress and minimize the distortion of the resulting parts. This modeling and planning system should also be computationally efficient.
PHASE I: Demonstrate the feasibility of a modeling and planning tool to predict the residual stress and distortion of an AM part based on key hybrid process parameters for both the additive and subtractive steps. This tool should be capable of predicting the residual stress in a Ti-6Al-4 coupon, which is repaired using a hybrid process. This coupon should be developed independently. The Phase I effort will include prototype plans to be developed under Phase II.
PHASE II: Develop a full-scale modeling and process planning tool prototype to efficiently predict the residual stress and distortion of hybrid additive/subtractive parts based on various process parameters, including, but not limited to, energy density, build orientation, build tool path, material properties, scan speed, layer thickness, part geometry, machining conditions, sequence between additive (conventional AM, cold spray, and welding) and subtractive processes, and post processing (stress relieving, normalization, etc.). Compare the predicted residual stress of the test cases by printing and machining Ti-6Al-4V samples to show the effectiveness of the modelís prediction capability and the computational efficiency of the planning capability. Demonstrate the solution(s) in a real-world AM processing scenario and its possible transition into both military and commercial applications. Note: No Government test facility should be needed.
PHASE III DUAL USE APPLICATIONS: Validate and demonstrate an aircraft ready AM part using a hybrid process. This part should conform to all design tolerances and strength requirements predicted by the physics-based modeling solution created in Phase II.
Metal AM component studies are being conducted in both the private and public sector for parts that might benefit from a hybrid additive/subtractive construction using AM. AM components can reduce weight, tooling costs, and material waste. By understanding the distortions and internal stresses of as-built and post-processed parts, a manufacturer can reduce material waste and time required to redesign components to meet requirements.
KEYWORDS: Additive Manufacturing; Design; Distortion; Hybrid Process; Residual Stress; Subtractive Manufacturing
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