N15A-T008 TITLE: In Situ, Nondestructive Inspection During Additive Manufacturing of Metallic Parts

TECHNOLOGY AREAS: Materials/Processes

OBJECTIVE: Develop an in situ, nondestructive inspection and monitoring method able to identify material defects for additive-manufactured, metallic parts during the build process.

DESCRIPTION: Additive manufacturing (AM) has the potential to provide rapid production of critical parts for Navy aircraft. However, the final material properties and defects in parts are not as well understood for AM parts as they are for traditionally manufactured forged, cast, and machined parts. These properties are highly dependent on parameters like part geometry, build path, cooling rate, etc. and are difficult to qualify.

Parts manufactured using traditional machining methods and heat treatment processes have been correlated to vast amounts of physical test data to establish material properties and statistically justified confidence bands for those properties. Detection of physical defects is done through multiple inspections throughout the manufacturing process to yield parts that have been 100 percent inspected for all physical defects that might be in the parent material or created by the manufacturing process. Direct (additive) manufacturing machines do not afford the same opportunities to inspect the raw material as a plate, sheet or bar, or, to inspect the raw forging before final machining. Parts are made from wire or powder in a single process and the final net-shape part is more difficult to inspect than those made with conventional manufacturing techniques.

One approach to qualifying an AM part�s suitability for use is to destructively evaluate a significant number of parts and measure the properties of interest and look for defects in the part. Typically, this approach also requires statistical sampling of parts for destructive testing to verify the process is still operating correctly. This is costly and time consuming and it negates some of the benefits of being able to rapidly produce different and new parts with this method. It also yields a certain amount of uncertainty that is inherent and any sampling approach to ensuring quality control. Traditional nondestructive inspection (NDI) methods can be used on the finished parts, but more often than not, it is not possible to get 100 percent coverage in these inspections due to the complexity of the geometry that can be obtained by AM. A replacement or supplemental final inspection of an AM part with one or more nondestructive, non-contact inspections that can be done concurrent with the AM build process is needed. Real-time inspection of a part as it is being manufactured will greatly reduce the amount of material that needs to be inspected and could even enable immediate correction of manufacturing defects.

This STTR is aimed at developing an inspection and monitoring method that can inspect and collect data during the build process. This approach to inspection will allow each layer to be inspected before the next layer is built on top of it. The final inspection system should, at a minimum, be able to detect physical defects that would be common to that AM process. Typical physical defects might include problems like porosity and lack of fusion. The ideal inspection system should not only be able to detect physical defects, but also capture information that can be used to accurately estimate material properties of the final part. For example, microstructural changes in the part (like grain size or grain orientation) might be estimated by monitoring parameters like the size, temperature and cooling rate of the melt pool. Eventually, it is envisioned that the system would be part of a feedback loop that helps correct the AM process as the build progresses to optimize build parameters and "prevent" these defects from materializing and to detect and repair defects before depositing the next layer.

The goal of the system is to collect information about all of the characteristics of the deposited material as it is deposited and before it is buried in an un-inspectable location in the part. Such a system will likely need to rely on multiple modes of data collection. Combinations of thermography, laser ultrasonics, eddy current, and/or other non-contact inspection techniques may allow this goal to be reached. The AM processes to be considered for this STTR are electron beam AM and laser-sintered powder bed AM processes. The materials of interest are Ti-6Al-4V or PH17-4 stainless steel. The effort should focus on a single AM process/material combination.

PHASE I: Provide proof of concept for proposed NDI process to detect and quantify possible AM process-induced defects such as lack of fusion and micro-porosity associated with that AM process/material. Consideration should be given to the AM process environment (i.e. inert atmosphere, high temperatures, etc.). Evaluate NDI method for applicability to the potential defects and characterizing material properties with minimal impact on the AM process and determine feasibility for incorporating the proposed NDI process into the selected AM process.

PHASE II: Construct a prototype inspection system that collects the data during the AM process based on the concept from Phase I. Demonstrate ability to collect the appropriate data during the AM build to model material properties and defect locations in the part. Include the development of a physics-based model that correlates the data collected with changes in the NDI response to a defect in the AM test parts. Validate models though additional test coupons, followed by destructive testing and metallography.

PHASE III: Refine system hardware and modeling software to maximize system utility. Identify limitations of the inspection system and probabilities of detection for critical defects. Use the defect model to identify pass/fail criteria for a general AM construction if given a strength or fatigue requirement. Investigate integrating the system into the process controls of the AM machine to correct defects on the fly. Prepare technology for military and commercial transition.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: AM is of wide interest across many industries and throughout the world. Quality control of AM parts is a critical component for facilitating the transition of AM into critical applications. This technology is expected to be of interest to many commercial industries, including aerospace, automotive, and medical.

REFERENCES:
1. Frazier, W. (2014). Metal Additive Manufacturing: A Review. ASM International Journal of Materials Engineering and Performance, 23(6), 1917-1928.

2. Liu, S., Liu, W., Harooni, M., Ma, J., & Kovacevic, R. (2014). Real-time monitoring of laser hot-wire cladding of Inconel 625. Optics & Laser Technology, 62, 124-134.

3. Gu, D., Meiners, W., Wissenbach, K. & Poprawe, R. (2012). Laser additive manufacturing of metallic components: materials, processes and mechanisms. International Materials Reviews 57, 3, 133-164.

4. Kruth, J.P., Mercelis, P. & Van Varenbergh, J. (2005). Binding mechanisms in selective laser sintering and selective laser melting. Rapid Prototyping Journal, 11(1), 26.

KEYWORDS: Porosity; Additive Manufacturing; defects; lack of fusion; inclusions; in situ nondestructive inspection

** TOPIC AUTHOR (TPOC) **

DoD Notice:   Between December 12, 2014 and January 14, 2015 you may talk directly with the Topic Authors (TPOC) to ask technical questions about the topics. For reasons of competitive fairness, direct communication between proposers and topic authors is not allowed starting January 15, 2015 , when DoD begins accepting proposals for this solicitation.

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