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Non-Destructive Evaluation of Bonded Interface of Cold Spray Additive Repair



OBJECTIVE: The purpose of this effort is to provide a non-destructive evaluation (NDE) inspection process to verify the cold spray bond line, ensuring good adhesion to the aviation component.

DESCRIPTION: It is the intent for the offeror to demonstrate acceptable cold spray material properties with non-destructive evaluation (NDE) method developmentidentifying degraded cold spray bulk material properties and/or adhesion to a substrate. Qualified performers must demonstrate that they have a cold spray capability (in-house or partnership with a company).Samples must be provided containing intentional defects created by this carefully controlled cold spray bonding capability, and the NDE techniques to be developed must detect at least 90% of the defects.The existing NDE method of fluorescent penetrant inspection (FPI) only evaluates the surface.The requirement is for sub-surface NDE of the cold spray bonded interface for use in structural flight critical safety item (CSI) restoration. Since bulk material properties of the cold sprayed coating may influence bond condition, NDE methods must also determine acceptable bulk material properties of the cold spray coating.NDE methods shall be explored and a solution provided to characterize these conditions.There is a need for developing a nondestructive examination method that will determine if the interface bond is intact without destroying the part. This is critical for flight safety on Army aviation components. Defects within the deposit can be the weak areas allowing for fatigue crack initiation and growth.These defects need to be ascertained so the cold spray deposit onto the substrate has acceptable strength, elongation, fatigue resistant, and other characteristics to ensure it will give the same life expectancy as the original component.It is desired that the NDE capability will be developed from this SBIR to ensure that the cold spray interface bond line is not compromised when it is applied and when it is returned to the depot for overhaul to ensure continued flight safe operation after overhaul.Pre- and post-process with possible in-situ NDE development of the cold spray process shall be accomplished on cold sprayed coupons prior and after testing specifically ensuring no cracks, excessive porosity or contaminants, and good adhesion (25,000 psi).Use of cold spray process parameter adjustment or introduction of contaminants to establish a non-acceptable cold spray deposit is expected during research and development of the NDE method.Various NDE methods may include eddy current, ultrasonic, computed tomography (CT) radiology, or other techniques.The cold spray powder and process used should be sufficient to establish structural integrity on aluminum and magnesium substrates (Aluminum (Al) 7049 forged, Magnesium (MG) forged AZ80A, and Mg ZE41A) during the NDE development.Typical acceptable cold spray deposit are adherent to the substrate material, showing a uniform continuous surface free from blisters, voids, spalling, chipping, flaking, cracking, lumps (berries), loosely adherent spattered particles, and other objectionable imperfections.Microscopic examination of the cold spray does not exhibit any cracks, excessive or massive oxides or porosity when examined at a minimum magnification of 100X per ASTM E3, E407, and E1920.Oxide and porosity content are usually less than 2 percent when viewed at 100X minimum per ASTM E2109.Acceptable bond strength is near the 25000 pounds per square inch per plug bond testing.No de-bonds or delamination are to be present in the cold spray bond line.Hardness minimum is 70 Vickers minimum on as deposited 6061 cold spray powder.The CCDC AvMC Aviation Engineering Directorate (AED) has tapped Army Research Laboratory’s (ARL) cold spray research, however ARL recommended NDE process assessment of properties using frictional sliding (presented at cold spray action team (CSAT)) does not establish sub-surface evaluation at the degree of resolution needed in aviation CSI components.The success criteria is ability to detect degraded interface bond to an acceptable probability of detection (POD).

PHASE I: Develop NDE method(s) to determine an acceptable cold spray interface bond during the cold spray process, inclusive of cold spray coating bulk material properties. The innovation desired of phase I is to give the Army aviation the capability to detect subsurface (internal) cold spray flaws from the smallest critical size, interface delamination, bulk properties in thin, medium thick, and thick deposits.The offeror shall be able to inspect subsurface (internal) flaws for any linear size indications.(Note- current critical size visibly detectable on surface inspection via NDE method(s).)Comparison example of external surface inspection criteria (using NDE method fluorescent penetrant inspection (FPI)) on critical safety items (CSI) requires detection of cracks and corrosion (any linearity size visually detectable) and thru-wall indications (any linearity size visually detectable), of which no indications are allowed.Using NDE surface method FPI, non-CSI parts (lower quality) may allow indications of up to 0.010 inch in diameter of porosity/cold shuts/shrinkage/inclusions.The effort shall require cold spray of coupons with NDE development pre- and post-evaluation and in-situ of subsurface (internal).The cold spray ranges from the thin coatings (deposits) (0.010 -0.050 inch thick coatings), medium thick coatings (deposits) (0.1 – 0.5 inch thick coatings), and thick coatings (deposits) (0.5 – 1 inch thick coatings).This NDE developed method(s) should include the capability to detect the cold spray interface bond line for any linearity size delamination, cracks, porosity, contaminants, and weak cold spray bonded interface resulting from cold spray parameters (e.g. critical velocity, pressures, gas flow, etc.) while providing quantitative material condition results.The offeror will be required to develop detection methods for subsurface (internal) flaws in the thin and medium coatings, linear or diameter.If successful on the thin and medium thick coatings, then the thick coatings will be attempted as well.Metrics for phase I include for thin (0.010-0.050 inch) and medium thick (0.1-0.5 inch) coatings (evaluation based on NDE with performance testing via coupon testing (ASTM E8, E466, R.R. Moore (bend/rotate))- a. Demonstrate NDE method(s) capability to detect flaws (cracks, delamination, porosity, contaminants, weak bond interface). b. Demonstrate NDE method(s) smallest critical size of flaws detectable (cracks, delamination, porosity, contaminants, weak bond interface).(Critical indication flaw size should equate (similar) to a visual discernment on external surface for indications less than 0.010 inch linear or diameter using FPI method ) c. Demonstrate NDE method(s) capability to inspect bulk properties of the cold spray (densities, acoustics, etc. that are not influenced by interfaces (boundaries). d. Upon successful demonstration of thin and medium thick coatings, then thick coatings (0.5-1 inch) shall be demonstrated in subparagraphs a. through c.For NDE detection development it is expected that an unacceptable cold spray process will require seeding of faults (parameter adjustment, contaminant introduction, etc.).The material characterization of the cold spray shall be accomplished during the research effort and be inclusive of failure mechanisms, residual stress, microstructure, microhardness, mechanical properties, etc. The deliverable of the project includes recommendation of inspection equipment and NDE method(s) along with substantiated results.All research and development processes shall be documented and reported for potential replication. NDE methods with greatest promise shall be highlighted and recommended for Phase II demonstration.

PHASE II: Deliverable will be the design, development, and fabrication of a prototype NDE method(s) from phase I to include motion control, data acquisition system, data reduction (software), used to detect sub-surface cold spray interface bond and bulk material properties characterization during research and development of statistically sound repeatable results for aerospace application of the cold spray process.The success criteria will be an established probability of detection (POD) 90 % with 95% confidence.The offeror shall develop a NDE method to along with the examination of the cold spray interface bond.This phase II will assess, describe, and develop a NDE method that will establish the ability to detect a particular defect, (delamination, crack, porosity, contaminants, weak cold spray bonded interface) along with identifying the size, orientation, and location of the defect.Typical four options that constitute the probability matrix of include- 1. An item is flawed and the NDE method detects it (True Positive). 2. No flaw exists and the NDE method indicates a flow present (False Positive). 3. An item is flawed and the NDE method does not detect it (False Negative). 4. No flaw exists and the NDE method has no indication of a flaw (True Negative). Probability of detection (POD) studies such as this SBIR is requiring for development is to be done, possibly by plotting the accumulation of flaws detected by a newly developed NDE method against the flaw size of all flaws “detected” (or that produce a response over some threshold). Ideally all flaws over some critical size will be detected and flaws smaller than that are not “detected”.A common tool used for POD is the POD curve, probability of detection versus flaw height.A demo system will be developed incorporating the NDE method including all data processing methods developed in Phase I and Phase II.

PHASE III: Upon successful completion of Phases I and II, the actual NDE method will be implemented in a cold spray additive repair process for actual Army aviation components.The demo system built in Phase II will be modified and adapted for inspection of selected prototype part geometries.Finally, the cold spray repaired prototype components will undergo full scale fatigue testing as required by Aviation Engineering Directorate (AED) using the NDE demo system for inspection and quantifying degraded cold spray properties and adhesion.Upon successful completion of any further testing required (i.e. corrosion, etc.), a maintenance engineering order will be established allowing repair and overhaul production.

KEYWORDS: Cold Spray, Nondestructive Evaluation (NDE), eddy current, ultrasonic, computed tomography


Gheorghe Bunget, Adam Goff, Nathan K. Brown, Jeff Demo, Fritz Friedersdorf, (Luna Innovations);Anindya Ghoshal, Mark Pepi (U.S.Army Research Laboratory); Siddhant Datta, Aditi Chattopadhyay (Arizona State University), “Identification of Material Damage Precursors using Nonlinear Ultrasound”, Presented at American Institute of Aeronautics and Astronautics, circa 2014.; Lee H. Pearson, “Eddy Current Characterization of Fiber Integrity in Graphite Fiber Composite Structures,” Proceedings of the JANNAF NDES/RNTS/S&MBS Joint Meeting, 27-29 Mar 2001, Cocoa Beach, Florida.; ASTM E3, Standard Guide for Preparation of Metallographic Specimens; ASTM E8, Standard Test Methods for Tension Testing of Metallic Materials; ASTM B557, Standard Test Methods for Tension Testing Wrought and Cast Aluminum-and Magnesium-Alloy Products; ASTM E18, Standard Test Methods for Rockwell Hardness of Metallic Materials; ASTM E407, Standard Practice for Microetching Metals and Alloys; ASTM E2248, Standard Test Method for Impact Testing of Miniaturized Charpy V-Notch Specimens; ASTM E466, Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials; ASTM C633, Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings; ASTM E1097, Standard Guide for Determination of Various Elements by Direct Current Plasma Atomic Emission Spectrometry; ASTM E1920, Standard Guide for Metallographic Preparation of Thermal Sprayed Coatings; ASTM E 2109; MIL-STD-3021, Manufacturing Process Standard Materials Deposition, Cold Spray

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