Stress Corrosion Cracking Resistance of Cold-Sprayed Al 6061 Deposits Using a Newly Developed Test Fixture

Article Stress Corrosion Cracking Resistance of Cold-Sprayed Al 6061 Deposits Using a Newly Developed Test Fixture

Mala Sharma 1,*, Jeremy Schreiber 2 , Timothy Eden 2 and Victor Champagne 3 1 Department of Mechanical Engineering, Bucknell University, Lewisburg, PA 17837, USA 2 Applied Research Laboratory, University Park, The Pennsylvania State University, State College, PA 16802, USA 3 U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, MD 21005, USA * Correspondence: [email protected]; Tel.: +1-570-577-1686

Received: 10 June 2019; Accepted: 11 July 2019; Published: 17 July 2019

Abstract: The stress corrosion cracking (SCC) response of Al 6061 bulk deposits produced by high-pressure cold spray (HPCS) was investigated and compared to commercial wrought Al 6061-T6 material. Representative tensile coupons were stressed to 25%, 65% and 85% of their respective yield strength and exposed to ASTM B117 salt fog for 90 days. After exposure, the samples were mechanically tested to failure, and subsequently investigated for stress corrosion cracking via optical and scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). The results were compared to the wrought Al 6061-T6 properties and correlated with the observed microstructures. Wrought samples showed the initiation of stress corrosion cracking, while the cold-sprayed deposits appeared to be unaffected or affected by general corrosion only. Optical microscopy revealed evidence of stress corrosion cracking in the form of intergranular corrosion in the wrought samples, while no significant corrosion was observed in the cold-sprayed deposits. Fractography revealed wrought samples failed due to multiple mechanisms, with predominant cleavage and intergranular failure, but cold-sprayed samples only failed by ductile dimple rupture. The difference in SCC response between the differently processed materials is attributed to the documented benefits of the cold spray process, which includes maintaining fine grain structure of the feedstock powder and high density after consolidation, low oxidation, and work hardening effect.

Keywords: aluminum alloys; cold spray; corrosion resistance; stress corrosion cracking

1. Introduction Recently, eﬀorts to produce highly dense, bulk components with high strength and ductility have shifted focus from traditional metallurgy methods, which have limitations, to the cold spray process [1–4]. Cold spray is a relatively new technology derived from other thermal spray processes. The cold spray process involves the acceleration of micron-sized particles in the solid state toward a target, upon which the surface of the powder particle undergoes high levels of plasticity. This plasticity helps to break down surface contaminants on both the powder and substrate (which can be the target or the prior deposited layers of cold spray) leading to a metallic and mechanical bond. The bulk of the powder also undergoes plasticity, but to a much lesser extent than the surface. Because the material is deposited in the solid state, the microstructure prior to spraying remains relatively intact after spraying, with the exception of some dynamic recrystallization due to high strain levels. Also, because the process requires high levels of plasticity, it is important to have a feedstock material that can undergo high levels of strain with low energy input, but also work-harden suﬃciently to obtain

Coatings 2019, 9, 445; doi:10.3390/coatings9070445 www.mdpi.com/journal/coatings Coatings 2019, 9, 445 2 of 19 the strength required. The cold spray process can also produce near-net shapes before machining to final components, making it a possible method for three-dimensional (3D) manufacturing. Unlike thermal spray methods or conventional manufacturing processes, the main advantages of using cold spray is that it can produce coatings and bulk materials with no recrystallization, thus maintaining the structure and size of starting powders. As a low-temperature process, it operates below the melting point of metals and results in very low porosity deposits without the need for combustible gasses. Additional motivation to use this process is the low oxide content, which can improve corrosion resistance, mechanical properties, and wear resistance by eliminating sites for localized corrosion and premature failure. There has been a great deal of research into corrosion mechanisms in metallic materials over the past century [5–17]. Atmospheric or environmentally assisted corrosion is one of the least understood areas of corrosion. Stress corrosion cracking is a type of atmospheric corrosion that is very difficult to detect and mitigate, since it only occurs in alloys that have been under stress in corrosive environments. Metallic materials used extensively in engineering and defense applications are susceptible to corrosion. Corrosion costs the US billions of dollars per year in preventative maintenance and product loss. There are many different types of corrosion that can attack metallic materials, which include uniform corrosion, galvanic corrosion, crevice corrosion, pitting and environmentally induced corrosion. The environment the material is exposed to has a major effect on how it corrodes. The three main types of environmental corrosion that lead to cracking are: corrosion fatigue, hydrogen induced, and stress corrosion (SCC). Stress corrosion cracking is the formation of cracks in a material that form due to corrosion while a part is exposed to tensile stresses. Pure metals are typically resistant to stress corrosion cracking, but metallic alloys such as stainless steel and aluminum alloys are susceptible due to the large amount of alloying elements in the matrix. Typically, alloys rely on intermetallic formation at the grain boundaries to increase strength and ductility. Consequently, the intermetallics cause an anodic condition at the grain boundaries, which in turn causes preferential intergranular corrosion in aluminum alloys. Dealloying is associated with stress corrosion cracking in aluminum alloys, steels, and noble metal alloys such as AgAu and CuAu [14–16]. Renner et al. [17] noted that cracks can form in CuAu alloys during local dealloying and depend on the crystallographic orientation of the substrate material and stability of applied inhibitor layers. Higher strength alloys such as the 7xxx and the 2xxx alloys are more susceptible to SCC [18–22] due to the high alloying additions. The 6xxx series alloys are usually not as susceptible to SCC when exposed to high tensile stresses and highly corrosive atmospheres [18,21–23]. For the most part, the service record of 6xxx alloys shows no reported cases of SCC. But in accelerated laboratory tests where high stresses and aggressive solutions are used, cracking has been observed [18,23]. These results were predominately demonstrated in alloys with high alloy content and containing silicon in excess of the Mg2Si ratio and/or high percentages of copper [23]. Aluminum alloy 6061-T6 (peak-aged) is typically not susceptible to this type of corrosion, but more understanding of this behavior is required, as published research in this area is often contradictory. As high-pressure cold spray (HPCS) continues to gain interest in various applications such as repair, wear and corrosion resistance, there is a necessity to understand the effects of the processing parameters and resulting microstructure and mechanical/corrosion performance of defense-related alloys produced by cold spray. Postdeposition heat treatments are often used in cold-sprayed materials to improve ductility [24–27]. In precipitation-hardened alloys, such as Al 6061 that is used in various defense applications, it is important to consider the effect the heat treatment may have on corrosion resistance and precipitation strengthening. Cold-sprayed materials which are synthesized by severe plastic deformation can result in varied precipitation strengthening performance when compared with that of conventionally processed materials. As a result, finding an optimized heat treatment for cold-sprayed materials of the same material class can be challenging. Thus, evaluating material performance in the as-sprayed condition can provide a useful starting point. Coatings 2019, 9, 445 3 of 19

This paper studies the effect of the high-pressure cold spray process on the SCC performance of AlCoatings 60601 2019 alloy, 9, x inFOR an PEER aggressive REVIEW corrosive environment. Mechanical properties of bulk as-sprayed3 of and 19 annealed cold-sprayed material were first evaluated and compared to wrought Al 6061-T6 values. Since preliminary tensiletensile datadata forfor the the cold-sprayed cold-sprayed 6061 6061 was was close close to to the the wrought wrought values values (other (other than than % elongation),% elongation), it wasit was decided decided that that the the SCC SCC performance performance of of the the materials materials wouldwould bebe comparedcompared inin these respective processingprocessing conditions. conditions. Furthermore, Furthermore, Rokni Rokni et al. et [24 al.] observed[24] observed that ultimate that ultimate tensile strength tensile didstrength not substantially did not substantially increase withincrease subsequent with subseq heatuent treatment heat treatment after deposition. after deposition. They noted They that noted the increasethat the inincrease ultimate in tensile ultimate strength tensile (UTS) strength for the (UTS) solution-treated for the solution-treated and peak-aged and cold-sprayed peak-aged Al cold- 6061 wassprayed relatively Al 6061 small. was relatively Knowing thatsmall. precipitates Knowing that can haveprecipitates negative can aff haveect on negative corrosion affect behavior, on corrosion it was decidedbehavior, to it comparewas decided cold to spray compare materials cold spray in the mate as-depositedrials in the as-deposited and annealed and conditions annealed to conditions wrought materialto wrought in the material peak-aged in the condition. peak-aged Resulting condition. microstructures Resulting microstructures and fracture surfaces and fracture in this studysurfaces were in evaluatedthis study through were evaluated scanning electronthrough microscopy scanning elec withtron energy microscopy dispersive with X-ray energy spectroscopy dispersive (EDS) X-ray and opticalspectroscopy microscopy (EDS) afterand optical corrosion microscopy exposure after and tensilecorrosion testing exposure to failure. and tensile testing to failure.

2. Materials and Methods

2.1. Feedstock Powders All powders were produced by Valimet Inc.Inc. (Stockton, CA,CA, USA) by inert gas atomization. Most production of aluminum powder powder uses uses a a process process of of in inertert gas gas atomization. atomization. In Inthis this process, process, an aninert inert or orsemi-inert semi-inert gas gas like like argon, argon, helium, helium, and and even even sometime sometimess nitrogen nitrogen is used is used to spray to spray a molten a molten stream stream of ofaluminum, aluminum, atomizing atomizing it into it into fine fine particles. particles. The The Al Al6061 6061 powder powder particles particles are are relatively relatively spherical spherical in inappearance appearance (Figure (Figure 1).1). Spherical Spherical aluminum aluminum part particleicle morphology morphology providesprovides goodgood consolidationconsolidation characteristics, with high deposit efficiency efficiency and results in a microstructure containing low porosity and high hardness/strength. The specified particle size cut was 325 mesh, which corresponds to and high hardness/strength. The specified particle size cut was –325− mesh, which corresponds to a amaximum maximum particle particle size size of approximately of approximately 44 μ 44m.µ Particlem. Particle size and size morphology and morphology was confirmed was confirmed using usinga Horiba a Horiba LA-910 LA-910 laser laserscattering scattering particle particle size sizedistribution distribution analyzer analyzer (Horiba, (Horiba, Kyoto, Kyoto, Japan) Japan) and and an anenvironmental environmental scanning scanning electron electron microscope microscope (ESE (ESEM,M, Quanta Quanta 200, 200, FEI, FEI, Hillsboro, Hillsboro, OR, OR, USA). The average Al 6061 feedstock particleparticle size was approximately 18 µμm (D50). Figure 22 shows shows thethe particleparticle size distribution for the 6061 powders.

(a) (b)

Figure 1. Al 6061 as-received powder at (a) lowlow magniﬁcationmagnification and ((b)) highhigh magniﬁcation;magnification; particlesparticles primarily exhibit a spherical morphology.

2.2. Sample Preparation The ﬁrst major technical challenge was development and optimization of the cold spray process parameters. Particle velocities and temperatures are two of the key variables in the cold spray process. Modeling these variables saved signiﬁcant costs by eliminating iterative and extraneous experimentation. Army Research Laboratory’s (Adelphi, MD, USA) propriety cold spray computer

Coatings 2019, 9, 445 4 of 19 model was used for this effort. Input variables include, but are not limited to, nozzle geometry, carrier gas type, particle density, gas viscosity, operating pressure, gas temperatures and particle size. Particle temperature and particle velocity at the substrate are the two of the variables which are actually modeled. The modeling program also provides estimates for deposition efficiency as well as coating cost per area. Both of these estimates are extremely practical tools which aid implementation. Other parameters such as robot raster speed, nozzle material and stand-off distance were optimized for aluminum coatings during the trials. Specific coating compositions, carrier gases and nozzle designs areCoatings provided 2019, 9, inx FOR Table PEER1. REVIEW 4 of 19

Figure 2. Particle size distribution (PSD)(PSD) for the Al 6061 feedstock powder.

Table 1. Cold spray processing parameters for Al 6061 bulk deposit. 2.2. Sample Preparation The first majorContents technical challenge was Parameters development and optimization Value of the cold spray process Feedstock Powder Powder Information Al6061 Valimet ( 270/+635 mesh) parameters. Particle velocities and temperatures are two of the key variables− in the cold spray process. Modeling these variables saved significant costs by eliminatingPBI (polybenzimidazole) iterative and extraneous Nozzle Type ® experimentation. Army Research Laboratory’s (Adelphi, MD, USA) proprietyCelazole cold spray computer Cold Spray Nozzle model was used for this effort. Input variablesThroat Diameter include, but are not limited 2 to, mm nozzle geometry, carrier Exit Diameter 4 mm gas type, particle density, gas viscosity, Lengthoperating pressure, gas temperatures 120 mm and particle size. Particle temperature and particle velocity at the substrate are the two of the variables which are Gas Type Helium actually modeled. The modeling programGas also Pressure provides estimates for deposition 25 bar efficiency as well as coating cost per area. Both of these estimatesGas Heater are extremely practical tools 400 which C aid implementation. Gas, Powder and Robot Other parameters such as robot raster Massspeed, Feed nozzle Rate material and 4.8 stand-off grams per distance minute were optimized Controls for aluminum coatings during the trials.Robot Specific Program coating compositions, Raster carrier gases and nozzle designs are provided in Table 1. Standoﬀ 1 inch Raster Speed 200 mm/sec

Table 1. Cold spray processing parameters for Al 6061 bulk deposit. The SCC specimens were fabricated from both bulk cold-sprayed and wrought material. The Contents Parameters Value wrought Al 6061-T6 samples were used as a baseline. All samples were fabricated using wire electrical Al6061 Valimet discharge machiningFeedstock (EDM). Powder For the wrought Powder Information samples, a 0.125” thick plate of Al 6061-T6 was cut into 4” 6” rectangular sections. Flat tensile bar specimens were machined(−270/+635 in mesh) accordance to ASTM E8, × PBI (polybenzimidazole) Standard Test Method for Tension TestingNozzle of Metallic Type Materials [28]. The samples were 4” in length ® by 0.375” in width in the grip section and 0.25” in the gage section.Celazole The samples were inspected for machining defects,Cold and Spray any Nozzle burrs were Throat removed Diameter using a ﬁle. 2 mm For the cold-sprayed bulk samples,Exit two Diameter blocks of cold-sprayed Al4 mm 6061 were provided by the Army Research Laboratory. They were manufacturedLength by depositing a120 cold-sprayed mm 0.375” thick layer of Al 6061 on to a 0.50” thick plate of wroughtGas Type Al 6061-T6. After deposition, Helium the aluminum coating was machined to a uniform thickness ofGas 0.20”. Pressure To accomplish this task, 25 bar the block was placed in a Gas Heater 400 C 3 Bridgeport verticalGas, mill Powder (Bridgeport, and CT, USA) and the coating was machined using a 4 ” diameter Mass Feed Rate 4.8 grams per minute titanium aluminumRobot nitride Controls coated carbide end mill (Niagara Cutter LLC, Reynoldsville, PA, USA). The Robot Program Raster Standoff 1 inch Raster Speed 200 mm/s

The SCC specimens were fabricated from both bulk cold-sprayed and wrought material. The wrought Al 6061-T6 samples were used as a baseline. All samples were fabricated using wire electrical discharge machining (EDM). For the wrought samples, a 0.125″ thick plate of Al 6061-T6 was cut into 4″ × 6″ rectangular sections. Flat tensile bar specimens were machined in accordance to ASTM E8, Standard Test Method for Tension Testing of Metallic Materials [28]. The samples were 4″ in length by 0.375″ in width in the grip section and 0.25″ in the gage section. The samples were inspected for machining defects, and any burrs were removed using a file.

Coatings 2019, 9, x FOR PEER REVIEW 5 of 19 Coatings 2019, 9, x FOR PEER REVIEW 5 of 19 For the cold-sprayed bulk samples, two blocks of cold-sprayed Al 6061 were provided by the For the cold-sprayed bulk samples, two blocks of cold-sprayed Al 6061 were provided by the Army Research Laboratory. They were manufactured by depositing a cold-sprayed 0.375″ thick layer Army Research Laboratory. They were manufactured by depositing a cold-sprayed 0.375″ thick layer of Al 6061 on to a 0.50″ thick plate of wrought Al 6061-T6. After deposition, the aluminum coating of Al 6061 on to a 0.50″ thick plate of wrought Al 6061-T6. After deposition, the aluminum coating was machined to a uniform thickness of 0.20″. To accomplish this task, the block was placed in a was machined to a uniform thickness of 0.20″. To accomplish this task, the block was placed in a Bridgeport vertical mill (Bridgeport, CT, USA) and the coating was machined using a ¾″ diameter Bridgeport vertical mill (Bridgeport, CT, USA) and the coating was machined using a ¾″ diameter titanium aluminum nitride coated carbide end mill (Niagara Cutter LLC, Reynoldsville, PA, USA). Coatingstitanium2019 aluminum, 9, 445 nitride coated carbide end mill (Niagara Cutter LLC, Reynoldsville, PA, USA).5 of 19 The cross section and isometric views of the aluminum blocks are shown in Figure 3. A wire electrical The cross section and isometric views of the aluminum blocks are shown in Figure 3. A wire electrical discharge machining (EDM) process was used to separate the cold spray coating from the substrate. discharge machining (EDM) process was used to separate the cold spray coating from the substrate. crossTensile section specimens and isometric were subsequently views of the cut aluminum from the blockscold spray are shownmaterial in following Figure3. AASTM wire electricalE8, to the Tensile specimens were subsequently cut from the cold spray material following ASTM E8, to the dischargesame dimensions machining and (EDM) method process as the was wrought used to separatespecimens. the coldThe spraygrip areas coating of fromthe samples the substrate. were same dimensions and method as the wrought specimens. The grip areas of the samples were Tensileanodized specimens to a type were 3 condition subsequently to minimize cut from galvanic the cold corrosion spray material between following the test ASTMspecimen E8, toand the the same test anodized to a type 3 condition to minimize galvanic corrosion between the test specimen and the test dimensionsfixture. and method as the wrought specimens. The grip areas of the samples were anodized to a typefixture. 3 condition to minimize galvanic corrosion between the test specimen and the test ﬁxture.

FigureFigure 3.3. Cross section andand isometricisometric renderingrendering ofof cold-sprayedcold-sprayed coatingcoating onon AlAl 6061-T66061-T6 plate.plate. Figure 3. Cross section and isometric rendering of cold-sprayed coating on Al 6061-T6 plate. 2.3.2.3.Test Test Procedure and Fixture Development 2.3.Test Procedure and Fixture Development FollowingFollowing ASTMASTM standard standard G64-99 G64-99 (2013), (2013), Standard Standard Classification Classification of Resistance of Resistance to Stress-Corrosion to Stress- Following ASTM standard G64-99 (2013), Standard Classification of Resistance to Stress- CrackingCorrosion of Cracking Heat-Treatable of Heat-Treatable Aluminum AlloysAluminum [29], theAlloys stress [29], levels the forstress the wroughtlevels for were the wrought 0%, 25%, were 65% Corrosion Cracking of Heat-Treatable Aluminum Alloys [29], the stress levels for the wrought were and0%, 85%25%, of 65% the and specified 85% of minimum the specified yield strengthminimum for yi wroughteld strength Al 6061-T6 for wrought (270 MPa). Al 6061-T6 The cold-sprayed (270 MPa). 0%, 25%, 65% and 85% of the specified minimum yield strength for wrought Al 6061-T6 (270 MPa). materialThe cold-sprayed was tested material at 25%, was 65% tested and 85% at 25%, of the 65% minimum and 85% yield of the strength, minimum 270-MPa, yield strength, since preliminary 270-MPa, The cold-sprayed material was tested at 25%, 65% and 85% of the minimum yield strength, 270-MPa, tensilesince preliminary data for the tensile cold-sprayed data for Al the 6061 cold-sprayed was close to Al the 6061 wrought was close values to the (other wrought than % values elongation). (other since preliminary tensile data for the cold-sprayed Al 6061 was close to the wrought values (other Thethan stress % elongation). levels were The achieved stress bylevels mounting were achiev the tensileed by bars mounting into a corrosion-resistantthe tensile bars into uniaxial a corrosion- stress than % elongation). The stress levels were achieved by mounting the tensile bars into a corrosion- fixture,resistant shown uniaxial in Figure stress4 .fixture The tensile, shown test in fixtures Figure were 4. The developed tensile bytest Pennsylvania fixtures were State developed University by resistant uniaxial stress fixture, shown in Figure 4. The tensile test fixtures were developed by (PSU,Pennsylvania State College, State University PA, USA)/ARL (PSU, and State were College, designed PA, toUSA)/ARL withstand and extended were designed periods of to time withstand in the Pennsylvania State University (PSU, State College, PA, USA)/ARL and were designed to withstand ASTMextended B117 periods [30] salt of fogtime environment. in the ASTM B117 [30] salt fog environment. extended periods of time in the ASTM B117 [30] salt fog environment.

(a) (b) (a) (b) FigureFigure 4.4. Corrosion-resistantCorrosion-resistant uniaxialuniaxial testtest fixturefixture designeddesigned atat PSUPSU/ARL,/ARL, usedused inin SCCSCC testingtesting loadedloaded Figure 4. Corrosion-resistant uniaxial test fixture designed at PSU/ARL, used in SCC testing loaded withwith aa sample sample (a )(a before) before coating coating to prevent to prevent galvanic galvanic couple couple and (b and) after (b coating) after coating with STOP-OFF with STOP-OFF lacquer. with a sample (a) before coating to prevent galvanic couple and (b) after coating with STOP-OFF lacquer. Thelacquer. fixtures were made from 316L stainless steel and have a polymer spring system that can be adjustedThe fixtures to apply were a made constant from force 316L on stainless the sample steel dependingand have a onpolymer the compression spring system of thethat spring.can be The fixtures were made from 316L stainless steel and have a polymer spring system that can be Anadjusted estimate to forapply the a required constant compression force on the amount sample can depending be made byon usingthe compression values from theof the yield spring. strength An adjusted to apply a constant force on the sample depending on the compression of the spring. An andestimate Hooke’s for the law. required Before testing compression occurred, amount the fixtures can be were made calibrated by using using values Fiber from Bragg the Gratingyield strength (FBG) sensorsestimate (Micron for the Optics,required Atlanta, compression GA, USA) amount to ensure can be that made the by compression using values was from accurate the yield and strength did not creep over time. Figure5 shows the results from a fixture loaded to 80% of the yield strength of Al 6061-T6. There was little change in the applied load, even during long-term testing. Coatings 2019, 9, x FOR PEER REVIEW 6 of 19

and Hooke’s law. Before testing occurred, the fixtures were calibrated using Fiber Bragg Grating (FBG) sensors (Micron Optics, Atlanta, GA, USA) to ensure that the compression was accurate and Coatingsdid not2019 creep, 9, 445over time. Figure 5 shows the results from a fixture loaded to 80% of the yield strength6 of 19 of Al 6061-T6. There was little change in the applied load, even during long-term testing.

Figure 5.5. Fiber Bragg grating data captured during ﬁxturefixture testing.testing. Units on the vertical axis are in microstrain, andand thethe horizontalhorizontal axisaxis isis time.time.

The movingmoving parts parts in in the the tension tension fixtures fixtures were were coated coated with anti-seizewith anti-seize lubricant lubricant to prevent to galvanicprevent corrosiongalvanic corrosion from occurring from betweenoccurring sample between and sample fixture. Eachand fixture. fixture wasEach subsequently fixture was compressedsubsequently to thecompressed desired percentage to the desired of the percentage Al 6061-T6 of yield the Al strength 6061-T6 by yield turning strength the nuts by on turning the threaded the nuts rod. on Thethe initialthreaded distance rod. The between initial the distance faces of between each stainless the fa steelces of clamp each were stainless noted steel at assembly. clamp were This noted distance at wasassembly. then compressed This distance to thewas calculated then compressed compression to the distance. calculated The compression sample was distance. then placed The intosample the fixture;was then the placed clamps into were the boltedfixture; down the clamps and torqued were bo tolted 35 ft-lb down in anand alternating torqued to pattern. 35 ft-lb Oncein an alternating the clamps werepattern. in placeOnce and the bolted,clamps the were threaded in place rod and was bolt loosened,ed, the applying threaded a constantrod was forceloosened, to the applying aluminum a sample.constant Theforce loaded to the fixtures aluminum were sample. then placed The loaded in the cyclicfixtures salt were fog (SF)then chamberplaced in for the test cyclic duration salt fog of 90(SF) days. chamber The samples for test duration were photographed of 90 days. The and samp inspectedles were periodically photographed during an testing.d inspected Test duration periodically was determinedduring testing. using Test ASTM duration G64 [29 was] and determined was the recommended using ASTM maximum G64 [29] exposureand was period the recommended to determine intergranularmaximum exposure SCC while period avoiding to dete excessivermine intergranular pitting. Test parameters SCC while for avoiding ASTM B117,excessive Standard pitting. Practice Test forparameters Operating for Salt ASTM Spray B117, (Fog) ApparatusStandard Practice were followed for Operating and are shownSalt Spray in Table (Fog)2. Apparatus were followed and are shown in Table 2. Table 2. Standard test parameters for B-117 testing. Table 2. Standard test parameters for B-117 testing. Salt Salt Solution Salt Solution Exposure Zone ConcentrationSalt SaltpH Solution TemperatureSalt Solution TemperatureExposure Zone Concentration4%–6% 6.5–7.2pH Temperature 35 C 35Temperature2 C ◦ ± ◦ 4%–6% 6.5–7.2 35 °C 35 ± 2 °C Samples were tensile-tested to failure after 90 days of exposure to ASTM B117 salt fog. Samples were tensile-tested to failure after 90 days of exposure to ASTM B117 salt fog. A screw- A screw-driven tensile frame (Instron 5866, Norwood, MA, USA) was used for all testing, which was driven tensile frame (Instron 5866, Norwood, MA, USA) was used for all testing, which was conducted following the ASTM E8 standard at a strain rate of 0.015 mm/mm/min. A 25 mm resolution conducted following the ASTM E8 standard at a strain rate of 0.015 mm/mm/min. A 25 mm resolution extensometer was mounted to each sample to measure strain, and a 10 kN load cell (Instron 5866, extensometer was mounted to each sample to measure strain, and a 10 kN load cell (Instron 5866, Norwood, MA, USA) was used to measure load. Each sample’s cross-sectional area was measured Norwood, MA, USA) was used to measure load. Each sample’s cross-sectional area was measured with digital calipers (Mitutoyo 500-753-100, Kawasaki, Japan) and used for stress calculation. After with digital calipers (Mitutoyo 500-753-100, Kawasaki, Japan) and used for stress calculation. After testing, the fracture surfaces were preserved for further microscopic analysis. All valid results using at testing, the fracture surfaces were preserved for further microscopic analysis. All valid results using least five specimens per condition (based on ASTM standard) were averaged and are presented for at least five specimens per condition (based on ASTM standard) were averaged and are presented for each stress condition. each stress condition. 2.4. Microstructural Characterization Both mechanical testing and microstructural analysis were performed to characterize the behavior of wrought Al 6061-T6 and cold-sprayed Al 6061 material in a saline environment. This included tensile testing, optical microscopy, and scanning electron microscopy (SEM, NanoSEM 630, FEI, Hillsboro, OR, Coatings 2019, 9, x FOR PEER REVIEW 7 of 19

2.4. Microstructural Characterization Both mechanical testing and microstructural analysis were performed to characterize the behavior of wrought Al 6061-T6 and cold-sprayed Al 6061 material in a saline environment. This Coatingsincluded2019 tensile, 9, 445 testing, optical microscopy, and scanning electron microscopy (SEM, NanoSEM7 630, of 19 FEI, Hillsboro, OR, USA) analysis of fracture surfaces with energy-dispersive X-ray spectroscopy USA)(EDS, analysisNanoSEM of 630, fracture FEI, Hillsboro, surfaces with OR, energy-dispersiveUSA). Specimens were X-ray analyzed spectroscopy using (EDS,a Nikon NanoSEM Epiphot 630,200 FEI,inverted Hillsboro, microscope OR, USA). (Tokyo, Specimens Japan) and were DS-Fil analyzed camera using head a Nikon system Epiphot to capture 200 inverted optical micrograph microscope (Tokyo,images. Japan)Samples and for DS-Fil grain-size camera analysis head systemwere etched to capture using optical Keller’s micrograph reagent (95 images. pct water, Samples 2.5 pct for HNO3, 1.5 pct HCl, 1.0 pct HF) for 10 s. The mean linear intercept method according to the ASTM- grain-size analysis were etched using Keller’s reagent (95 pct water, 2.5 pct HNO3, 1.5 pct HCl, 1.0 pct HF)E112-12 for 10standard s. The [31] mean was linear used intercept to measure method average according grain size to and the ASTM-E112-12was recorded using standard NIS Elements [31] was usedD (version to measure 4.6). average grain size and was recorded using NIS Elements D (version 4.6).

3. Results

3.1. Microstructural Characterization before B117 Salt Fog Exposure

3.1.1. As-Deposited Cold Spray Condition Figure 66a,ba,b showsshows thethe cross-sectionalcross-sectional opticaloptical microscopymicroscopy imagesimages ofof thethe coldcold sprayspray AlAl 60616061 bulkbulk deposit perpendicular to the spraying direction. The The representative representative microstructures microstructures were were etched, showing where where prior prior particle particle boundaries boundaries exist. exist. Upon Upon inspection, inspection, interparticle interparticle voids voids andand porosity porosity can canbe observed; be observed; lower lower particle-impact particle-impact velocity velocity and and subsequent subsequent lack lack of of local local deformation deformation in in some particles resultsresults inin these these types types of of imperfections imperfections forming forming in coldin cold spray spray deposits deposits [32– [32–34].34]. Rokni Rokni et al. et [ 24al.] observed[24] observed similar similar microstructure microstructure in their in their as-deposited as-deposited cold-sprayed cold-sprayed Al 6061 Al 6061 materials materials and and found found the imperfectionsthe imperfections to influenceto influence the the mechanical mechanical properties. properties. They They also also performedperformed electronelectron back-scatter didiffractionffraction (EBSD)(EBSD) mappingmapping in in order order to to characterize characterize the the grain grain structure, structure, and and in doingin doing so, Rokiso, Roki et al. et [ 24al.] identified[24] identified the presencethe presence of two of distincttwo distinct regions, regions, particle particle interiors interiors and prior and particleprior particle boundary boundary (PPB) regions.(PPB) regions. The firstThe wasfirst was characterized characterized by smallby small grain grain size, size, approximately approximately 1–10 1–10µ mμm in in size size andand was attributed to significantsignificant plastic deformation that occurs in thethe feedstockfeedstock powders.powders. The second was described as much smaller grain sizes and was believedbelieved to be the result of the pancaking mechanism that resultsresults from from the the continuous continuous buildup buildup of particles. of particles. The ultrafineThe ultrafine grain structuresgrain structures (UFG) that(UFG) resulted that 6 9 wereresulted attributed were attributed to the high to plasticthe high deformation plastic deformation at strain ratesat strain between rates 10 between/s–10 / s10 and6/s–10 moderate9/s and temperaturesmoderate temperatures during the consolidationduring the consolidation of the deposits. of the Ultrafine deposits. grain Ultr structuresafine grain in cold structures spray deposits in cold havespray been deposits observed have bybeen numerous observed researchers by numerous [35– 43rese], whoarchers attributed [35–43], the who result attrib touted two typesthe result of dynamic to two recrystallation,types of dynamic continuous recrystallation, and geometric. continuous In thisand study,geometric. the average In this grainstudy, size the of average the as-deposited grain size Alof 6061the as-deposited material was Al found 6061 tomaterial be 25 µ wasm (standard found todeviation be 25 μm of(standard 14.5). The deviation ultrafine of grain 14.5). sizes The can ultrafine easily begrain observed sizes can in the easily cold be spray observed material in atthe higher cold magnificationspray material and at werehigher observed magnification to range and from were 1 to 10observedµm, as to shown range in from Figure 1 to6b. 10 μm, as shown in Figure 6b.

(a)

Figure 6. Cont. Coatings 2019, 9, 445 8 of 19 Coatings 2019, 9, x FOR PEER REVIEW 8 of 19

(b)

Figure 6.6. Optical cross-sectional micrograph for cold spray Al 6061, etched using Keller’s reagent, showing the particle morphologiesmorphologies afterafter deposition.deposition. ( a) There is some evidence of interparticle voidsvoids and porosity, asas indicatedindicated byby arrowsarrows andand ((bb)) high-magniﬁcationhigh-magnification micrographmicrograph showingshowing smallsmall averageaverage grain size and ﬁnerfiner subsub grainsgrains resultingresulting fromfrom buildupbuildup ofof particles.particles.

3.1.2. As-Received Wrought Al 6061-T6 3.1.2. As-Received Wrought Al 6061-T6 Figure7 shows the cross-sectional optical micrograph of wrought Al 6061-T6 in the as-received Figure 7 shows the cross-sectional optical micrograph of wrought Al 6061-T6 in the as-received condition. Irregular distribution of course second-phase particles can be observed throughout the condition. Irregular distribution of course second-phase particles can be observed throughout the cross section. The eﬀect of these precipitates on the SCC behavior is discussed later in this paper. The cross section. The effect of these precipitates on the SCC behavior is discussed later in this paper. The average grain size was found to be 78 µm (standard deviation 24 µm). average grain size was found to be 78 μm (standard deviation 24 μm).

Figure 7. Optical cross-sectional micrograph of wrough wroughtt Al 6061-T6 samples in as-received condition and etchedetched using using Keller’s Keller’s reagent. reagen Theret. There is clear is evidenceclear evidence of randomly of randomly sized and sized irregularly and distributedirregularly second-phasedistributed second-phase precipitates. precipitates. 3.2. Tensile Testing and Residual Strength Calculations 3.2. Tensile Testing and Residual Strength Calculations None of the wrought or cold-sprayed samples fractured in the stress frames during the B117 None of the wrought or cold-sprayed samples fractured in the stress frames during the B117 exposure; all samples remained intact after 90 days of exposure until removed from ﬁxtures. Figure8 exposure; all samples remained intact after 90 days of exposure until removed from fixtures. Figure shows the representative (average of ﬁve samples) stress/strain behavior of the wrought Al 6061-T6 8 shows the representative (average of five samples) stress/strain behavior of the wrought Al 6061- samples that were tensile-tested to failure after 90 days of ASTM B117 salt fog exposure. The samples T6 samples that were tensile-tested to failure after 90 days of ASTM B117 salt fog exposure. The samples strained at 85% and 65% of the yield stress displayed similar stress/strain behavior, while the samples strained at 85% displayed a much shorter plastic region and lower strain at fracture. The

Coatings 2019, 9, x FOR PEER REVIEW 9 of 19 Coatings 2019, 9, 445 9 of 19 samplesCoatings 2019 prestrained, 9, x FOR PEER at 25% REVIEW of yield had the highest resulting yield and tensile stress after 90 9days of 19 exposurestrained atto 85%the salt and fog, 65% while of the the yield 85% stress had the displayed lowest. This similar result stress was/strain expected, behavior, since while higher the imposed samples pre-strainsstrainedsamples atprestrained generally 85% displayed resultat 25% ain of muchdecreased yield shorter had strength the plastic highest properties, region resulting and resulting lower yield strainand from tensile at more fracture. stress aggressive Theafter samples90 stress days corrosionprestrainedexposure attack. to atthe 25% salt of fog, yield while had the the 85% highest had the resulting lowest. yield This and result tensile was stressexpected, after since 90 days higher exposure imposed to thepre-strains salt fog, generally while the 85%result had in thedecreased lowest. strength This result properties, was expected, resulting since from higher more imposed aggressive pre-strains stress corrosion attack. generally result in decreased300 strength properties, resulting from more aggressive stress corrosion attack. 25% Strain

250300 65% Strain 25% Strain 200 85% Strain 250 65% Strain

150200 85% Strain Stress (MPa) 100150

Stress (MPa) 50100

050 00.511.52 Strain (%) 0

00.511.52 Figure 8. Stress/strain behavior of the wrought StrainAl 6061-T6 (%) samples after 90 days of B-117 salt fog exposure.Figure 8. Stress/strain behavior of the wrought Al 6061-T6 samples after 90 days of B-117 salt fogFigure exposure. 8. Stress/strain behavior of the wrought Al 6061-T6 samples after 90 days of B-117 salt fog Figureexposure. 9 shows the results from the cold spray Al 6061 samples that were tensile-tested (average of fiveFigure samples)9 shows after the 90 resultsdays of from ASTM the coldB117 spraysalt fog Al exposure. 6061 samples As thatexpected, were tensile-testedthe samples strained (average at of 85%five of samples)Figure the yield 9 aftershows strength 90 the days hadresults of ASTMthe from least B117 theamount saltcold fog sprayof exposure.plastic Al 6061 deformation As samples expected, andthat the thewere samples lowest tensile-tested yield strained and at(average tensile 85% of strength.theof five yield samples) The strength samples after had the90strained days least of amountat ASTM25% ofand B117 plastic 65% salt deformationof fog yield exposure. strength and As thehad expected, lowest simila yieldr thetensile andsamples tensilebehavior strained strength. after at exposure,The85% samplesof the with yield strained the strength 25% at displaying 25%had andthe least 65% a more amount of yield significant of strength plastic plastic deformation had similarregion, tensileanda higher the behavior lowest tensile yield afterstrength, and exposure, tensile and higherwithstrength. the strain 25% The at displaying samples fracture. strained aThere more is significantat no 25% significant and plastic 65% difference region,of yield a strength higherin the tensileslope had ofsimila strength, the relastic tensile and region, higher behavior strainfor afterthe at mostfracture.exposure, part, There butwith the isthe nobiggest 25% significant displaying change diff iserence a in more the in plasticsignificant the slope region of plastic the of elasticthe region, stress–strain region, a higher for the curvestensile most forstrength, part, the but cold- and the sprayedbiggesthigher strain changematerial at is fracture.and in the is plasticbelieved There region is to no be significant of related the stress–strain to differencethe type curves of in corrosive the for slope the atta cold-sprayedof ckthe (discussed elastic materialregion, later). for and For the is engineeringbelievedmost part, to but bealloys, related the this biggest to is thetypically change type of not is corrosive inan the issue, plastic attack since (discussedregion most designsof the later). stress–strain do Fornot engineeringcome curves close to alloys,for the the elastic this cold- is limittypicallysprayed of the material not material. an issue, and sinceis believed most designs to be related do not to come the closetype of to thecorrosive elastic atta limitck of (discussed the material. later). For engineering alloys, this is typically not an issue, since most designs do not come close to the elastic limit of the material. 300 25% Strain 250300 65% Strain

25% Strain 200250 65% Strain 85% Strain 150200 85% Strain

Stress (MPa) 100150

Stress (MPa) 50100

500 0 0.2 0.4 0.6 0.8 1 0 Strain (%) 0 0.2 0.4 0.6 0.8 1 Figure 9. Stress/strain behavior of the cold-sprayed Al 6061-T6 deposits after 90 days of B-117 salt fog exposure. Strain (%) Figure 9. Stress/strain behavior of the cold-sprayed Al 6061-T6 deposits after 90 days of B-117 salt fog exposure. Figure 9. Stress/strain behavior of the cold-sprayed Al 6061-T6 deposits after 90 days of B-117 salt fog exposure.

Coatings 2019, 9, x FOR PEER REVIEW 10 of 19 Coatings 2019, 9, 445 10 of 19 Figure 10 combines data from Figures 8 and 9 into bar graphs to compare the average values for ultimateFigure tensile 10 strength combines and data yield from strength Figures 8of and differently9 into bar prepared graphs to samples compare at the each average pre-stress values condition.for ultimate Average tensile values strength of yield and strength yield strengthranged from of di 227ﬀerently to 191 prepared MPa for the samples wrought at each Al 6061-T6 pre-stress andcondition. 251–193 MPa Average for the values cold-sprayed of yield strength Al 6061 ranged for all fromsamples 227 exposed to 191 MPa for for90 thedays. wrought The yield Al 6061-T6and tensileand strength 251–193 of MPa the for cold the spray cold-sprayed samples was Al 6061 very for close all samplesto that of exposed the wrought for 90 samples days. The and yield was and slightlytensile higher strength in most of thecases, cold even spray though samples they washad verynot been close heat-treated. to that of the This wrought improved samples strength and in was the slightlynon-heat-treated higher in condition most cases, can even be attributed though they to the had cold not beenspray heat-treated. process, where This particles improved experience strength in severethe non-heat-treatedplastic deformation condition during can deposition be attributed [1,28,34,37,38]. to the cold spray The process, grain refinement where particles and experience strain hardeningsevere plastic of the deformationprocess are mechanisms during deposition which [ 1assist,28,34 with,37,38 this]. The increase grain reﬁnementin strength andwhen strain compared hardening withof that the processof conventionally are mechanisms processed which Al assist6061 [1 with,28,38,41,42]. this increase Similar in strength behavior when hascompared been documented with that of by conventionallyresearchers who processed studied Al other 6061 [manufacturing1,28,38,41,42]. Similarprocesses behavior where has severe been documentedplastic deformation by researchers is involved,who studied such as other tube manufacturing channel pressing processes [43], equa wherel channel severe angular plastic deformation pressing (ECAP) is involved, [44], and such high- as tube pressurechannel torsion pressing [45], [ 43with], equal increases channel in UTS angular from pressing 39% to 92%. (ECAP) Depending [44], and on high-pressure heat treatment torsion and [stress45], with condition,increases the in UTScold-sprayed from 39% processing to 92%. Depending yields re onsults heat similar treatment or andimproved stress condition,over the theconventional cold-sprayed wroughtprocessing material, yields as resultsshown similarin Figure or improved10. While overthe results the conventional establish the wrought cold spray material, process as shownyields in improvedFigure 10ultimate. While tensile the results strength, establish the ductility the cold sprayof these process materials yields displayed improved a ultimatemarked reduction tensile strength, in ductilitythe ductility (Figure of 11). these This materials behavior displayed can be a markedexplained reduction by the inpresence ductility of (Figure interparticle 11). This voids behavior and can localizedbe explained porosity, by in the addition presence to of the interparticle increased cold voids work and during localized deposition, porosity,in as addition explained to in the Section increased 3.1.1.cold work during deposition, as explained in Section 3.1.1.

400 Yeild Strength 350 Tensile Strength 300 250 200 150

(MPa) 100 50 0

Figure 10. Tensile data for wrought (W) and cold-sprayed (CS) samples at various pre-stress levels Figureafter 10. 90 Tensile days of data B-117 for salt wrought fog exposure (W) and (SS). cold-sprayed (CS) samples at various pre-stress levels after 90 days of B-117 salt fog exposure (SS). The percentage of residual strength retained after exposure was calculated and is presented in Figure 11. Most notable is that the cold-sprayed deposits retained a higher percentage of their original The percentage of residual strength retained after exposure was calculated and is presented in tensile strength after 90 days exposure than the wrought samples at both the 25% and 65% of yield Figure 11. Most notable is that the cold-sprayed deposits retained a higher percentage of their original stress conditions. This reduction in residual strength is believed to be due to the stress corrosion tensile strength after 90 days exposure than the wrought samples at both the 25% and 65% of yield attack, which developed in the Al 6061-T6 and was the focus of the subsequent microscopic analysis. stress conditions. This reduction in residual strength is believed to be due to the stress corrosion Conversely, in the 85% of yield stress condition, the wrought material retained more strength than the attack, which developed in the Al 6061-T6 and was the focus of the subsequent microscopic analysis. cold-sprayed Al 6061 deposits. This behavior is attributed to the aggressive loading condition and the Conversely, in the 85% of yield stress condition, the wrought material retained more strength than predominately mechanical vs. corrosion response of the cold spray deposits, and can be rationalized the cold-sprayed Al 6061 deposits. This behavior is attributed to the aggressive loading condition and by understanding the precipitation behavior of the diﬀerently processed materials. the predominately mechanical vs. corrosion response of the cold spray deposits, and can be Rokni et al. [24,37] characterized cold-sprayed Al 6061 deposits in the as-deposited (AD), rationalized by understanding the precipitation behavior of the differently processed materials. stress-relief-annealed (SR) and peak-aged (T-6) conditions to understand the eﬀect of heat treatment on strength and ductility. They observed AD cold sprayed Al 6061 to possess moderate dislocation density and multiple low-angle grain boundaries (LAGB). The microstructure of conventionally processed Al 6061-T6 is well documented and commonly described as containing disc-shaped β, rod-shaped

Coatings 2019, 9, 445 11 of 19

β0, and needle-shaped β” particles, in addition to ﬁne spherical or needle-shaped Guinier-Preston (GP) zones approximately 5 nm in size [23]. These precipitates are known to be sites for failure and corrosiveCoatings 2019 attack, 9, x FOR [23 PEER,26,27 REVIEW]. 11 of 19

FigureFigure 11.11. StrainStrain atat fracturefracture andand strengthstrength retainedretained ofof wroughtwrought (W)(W) andand cold-sprayedcold-sprayed (CS)(CS) samplessamples atat variousvarious pre-stresspre-stress levelslevels afterafter 9090 daysdays ofof B-117B-117 salt salt fog fog exposure exposure (SF). (SF).

3.3. MicrostructuralRokni et al. [24,37] Analysis characterized after B117 Exposurecold-sprayed Al 6061 deposits in the as-deposited (AD), stress- relief-annealed (SR) and peak-aged (T-6) conditions to understand the effect of heat treatment on 3.3.1. Optical Microscopy strength and ductility. They observed AD cold sprayed Al 6061 to possess moderate dislocation densityOptical and microscopymultiple low-angle was performed grain boundaries after 90 days (LAGB). of B117 The exposure microstructure on all fractured of conventionally tensile cross sectionsprocessed to betterAl 6061-T6 understand is well thedocumented mechanisms and of commonly failure. The described samples as were containing polished disc-shaped and imaged β using, rod- ashaped Nikon Epiphotβ’, and needle-shaped 200 optical microscope β” particles, using in brightﬁeld addition microscopyto fine spherical in order or to needle-shaped identify any indication Guinier- ofPreston stress corrosion(GP) zones cracking. approximately Metallographic 5 nm in sectionssize [23]. of These stress precipitates corrosion cracking are known samples to be revealedsites for evidencefailure and of corrosive pitting and attack intergranular [23,26,27]. corrosion at all three stress conditions (Figures 12–14) for the wrought Al 6061-T6 samples. Stress corrosion cracking in aluminum alloys is typically intergranular in nature3.3. Microstructural [19]. At all three Analysis pre-stressed after B117 conditions, Exposure cracks nucleated at pits and were highly branched, indicative of stress corrosion cracking (Figures 12–14). Braun et al. [22,23] investigated Al 6061 sheet samples3.3.1. Optical using Microscopy bent-beam specimens (stress was applied in the transverse direction) in the natural and peak-agedOptical microscopy conditions was that performed were alternately after 90 immersed days of B117 in 3.5% exposure NaCl on solution all fractured and found tensile them cross to onlysections show to signsbetter of understand pitting. It isthe possible mechanisms that the of wrought failure. samplesThe samples in the were current polished study and show imaged signs ofusing SCC, a Nikon most likely Epiphot due 200 to optical the extremely microscope corrosive using atmospherebrightfield microscopy found in the in ASTMorder to B117 identify salt fog.any µ Braunindication [23] notedof stress pits corrosion reached acrac maximumking. Metallographic depth of 30 m,sections while of pits stress in the corrosion current studycracking were samples much deeper,revealed the evidence size increasing of pitting with and pre-stress intergranular condition. corrosion In the at all 25% three of yield stress stress conditions condition (Figures (Figure 12–14) 12), µ thefor wroughtthe wrought Al 6061-T6 Al 6061-T6 displayed samples. pits approximatelyStress corrosion 350 crackingm in size, in withaluminum a maximum alloys pitis depthtypically of µ µ 450intergranularm. Maximum in nature depths [19]. of attackAt all includingthree pre-stressed branching conditions, reached up cracks to 750 nucleatedm, with the at averagepits and depth were µ ofhighly combined branched, attack indicative being approximately of stress corrosion 400 m. In cracking the 65% of(Figures yield stress 12–14). condition, Braun maximumet al. [22,23] pit µ µ depthsinvestigated grew toAl 600 6061m, sheet with samples combined using depths bent-beam of attack specimens which ranged (stress from was 150applied to 750 in them (Figure transverse 13). Whendirection) wrought in the samples natural wereand stressedpeak-aged to conditions 85% of yield that stress, were the alternately maximum immersed pit depth in increased 3.5% NaCl to µ µ 750solutionm,with and averagefound them crack tobranching only show attack signs reachingof pitting. an It additional is possible 250 that m;the average wrought depths samples of attackin the µ rangedcurrent betweenstudy show 250 signs and 650 of SCC,m (Figure most likely 14). Underdue to staticthe extremely loading, corrosive Braun [23 atmosphere] tested Al 6061 found sheet in the in theASTM peak-aged B117 salt condition fog. Braun using [23] an noted aqueous pits chloride–bicarbonate reached a maximum solution. depth of After 30 μ 30m, days, while he pits observed in the µ pittingcurrent and study intergranular were much corrosion deeper, the extending size increasing up to 210 withm pre-stress in his samples condition. [23], In which the 25% is similar of yield to whatstress was condition observed (Figure in this 12), study. the wrought The occurrence Al 6061-T6 of stress displayed corrosion pits cracking approximately is also supported 350 μm in in size, the fractographywith a maximum of the pit failed depth specimen of 450 μm. and Maximum is discussed depths later. of attack including branching reached up to 750 μm, with the average depth of combined attack being approximately 400 μm. In the 65% of yield stress condition, maximum pit depths grew to 600 μm, with combined depths of attack which ranged from 150 to 750 μm (Figure 13). When wrought samples were stressed to 85% of yield stress, the maximum pit depth increased to 750 μm, with average crack branching attack reaching an additional 250 μm; average depths of attack ranged between 250 and 650 μm (Figure 14). Under static loading, Braun [23] tested Al 6061 sheet in the peak-aged condition using an aqueous chloride–bicarbonate

Coatings 2019, 9, x FOR PEER REVIEW 12 of 19

Coatings 2019, 9, x FOR PEER REVIEW 12 of 19 solution. After 30 days, he observed pitting and intergranular corrosion extending up to 210 μm in solution.his samples After [23], 30 days,which he is observedsimilar to pitting what anwasd intergranularobserved in thiscorrosion study. extending The occurrence up to 210 of μstressm in hiscorrosion samples cracking [23], which is also issupported similar toin whatthe fractograp was observedhy of the in failedthis study. specimen The and occurrence is discussed of stress later. corrosionResults cracking of optical is also analysis supported (Figure in the15a–c) fractograp of the coldhy of spray the failed samples specimen did not and show is discussed any evidence later. of stressResults corrosion of optical cracking analysis at any(Figure of the 15a–c) stress of conditions. the cold spray The samplessamples did showed not show negligible any evidence general corrosionof stress corrosion on the surface cracking (Figure at any 15a,b) of the in thestress 25% conditions. and 65% yield The samplesstress conditions. showed negligibleThis is in contrastgeneral corrosionto the wrought on the samples, surface (Figurewhich showed 15a,b) in evidence the 25% of and pitting 65% andyield intergranular stress conditions. corrosion This atis thein contrast surface toin the samewrought stress samples, condition. which At theshowed 85% yieldevidence stress of condition, pitting and the intergranular cold spray deposit corrosion did atshow the signssurface of some corrosive attack, although intergranular attack, pitting, or branching was not obvious (Figure 15c). Coatingsin the same2019, 9stress, 445 condition. At the 85% yield stress condition, the cold spray deposit did show signs12 of 19of some corrosive attack, although intergranular attack, pitting, or branching was not obvious (Figure 15c).

Figure 12. Metallographic section of wrought Al 6061-T6 stressed at 25% of yield strength after 90 days exposure to B117 salt fog testing. Various locations show pitting and intergranular crack Figure 12.12. MetallographicMetallographicsection section of of wrought wrought Al Al 6061-T6 6061-T6 stressed stressed at 25%at 25% of yield of yield strength strength after after 90 days 90 branching. exposuredays exposure to B117 to salt B117 fog salt testing. fog Varioustesting. locationsVarious showlocations pitting show and pitting intergranular and intergranular crack branching. crack branching.

Figure 13.13. MetallographicMetallographicsection section of of wrought wrought Al Al 6061-T6 6061-T6 stressed stressed at 65%at 65% of yield of yield strength strength after after 90 days 90 exposuredays exposure to B117 to salt B117 fog salt testing. fog Varioustesting. locationsVarious showlocations pitting show and pitting intergranular and intergranular crack branching. crack Figure 13. Metallographic section of wrought Al 6061-T6 stressed at 65% of yield strength after 90 branching. Resultsdays exposure of optical to B117 analysis salt (Figurefog testing. 15a–c) Variou of thes locations cold spray show samples pitting didand notintergranular show any crack evidence branching. of stress corrosion cracking at any of the stress conditions. The samples showed negligible general corrosion on the surface (Figure 15a,b) in the 25% and 65% yield stress conditions. This is in contrast to the wrought samples, which showed evidence of pitting and intergranular corrosion at the surface in the same stress condition. At the 85% yield stress condition, the cold spray deposit did show signs of some corrosive attack, although intergranular attack, pitting, or branching was not obvious (Figure 15c). CoatingsCoatings2019 2019, 9,, 4459, x FOR PEER REVIEW 13 of 1319 of 19 Coatings 2019, 9, x FOR PEER REVIEW 13 of 19

Figure 14. Metallographic section of wrought Al 6061-T6 stressed at 85% of yield strength after 90 FiguredaysFigure 14.exposure 14.Metallographic Metallographic to B117 salt section section fog oftesting. of wrought wrought Variou Al Al 6061-T6s locations6061-T6 stressed stressed showat pitting at 85% 85% of and of yield yield intergranular strength strength after aftercrack 90 90 days exposurebranching.days exposure to B117 saltto B117 fog testing. salt fog Various testing. locations Various showlocations pitting show and pitting intergranular and intergranular crack branching. crack branching.

(a) (b)

500 um

(c500) um

FigureFigure 15. 15.Metallographic Metallographic sectionssections of cold spray spray samples samples(c) atat (a) ( a25%) 25% yield yield strength strength and and (b) 65% (b) 65%yield yield strength after 90 days exposure to B117 salt fog testing. There is no indication of stress corrosion strengthFigure after 15. Metallographic 90 days exposure sections to B117 ofsalt cold fog spray testing. samples There at is(a no) 25% indication yield strength of stress and corrosion (b) 65% crackingyield cracking through branching or evidence of any other artifacts related to corrosive attack on failed throughstrength branching after 90 ordays evidence exposure of anyto B117 other salt artifacts fog testing. related There to corrosive is no indication attack on of failed stress samplecorrosion cross sample cross sections; (c) Metallographic sections of cold spray samples at 85% yield strength after 90 sections;cracking (c) Metallographicthrough branching sections or evidence of cold of spray any samplesother artifacts at 85% related yield strengthto corrosive after attack 90 days on exposurefailed days exposure to B117 salt fog testing. There is no indication of stress corrosion cracking through to B117sample salt cross fog sections; testing. ( Therec) Metallographic is no indication sections of stressof cold corrosion spray samples cracking at 85% through yield strength branching after on 90 the branching on the entire cross section. Evidence of slight corrosive attack is observed and identified entiredays cross exposure section. to EvidenceB117 salt offog slight testing. corrosive There is attack no indication is observed of stress and identified corrosion oncracking the figure. through on the figure. branching on the entire cross section. Evidence of slight corrosive attack is observed and identified 3.3.2. Fractography on the figure. 3.3.2. Fractography Scanning electron microscopy (SEM) was used to analyze the fracture surfaces of both the wrought and3.3.2. cold-sprayed Fractography samples to obtain more information about the failure mechanism. Figures 16 and 17 show the SEM micrographs of the fracture surfaces for wrought Al 6061-T6 samples after 90 days of ASTM B117 salt fog exposure. The samples appear to have failed due to multiple CoatingsCoatings 2019 2019, 9,, 9x, FORx FOR PEER PEER REVIEW REVIEW 1414 of of19 19

ScanningScanning electron electron microscopy microscopy (SEM) (SEM) was was used used to to analyze analyze the the fracture fracture surfaces surfaces of of both both the the Coatings 2019, 9, 445 14 of 19 wroughtwrought and and cold-sprayed cold-sprayed samples samples to to obtain obtain more more information information about about the the failure failure mechanism. mechanism. Figures Figures 1616 and and 17 17 show show the the SEM SEM micrographs micrographs of of the the fracture fracture surfaces surfaces for for wrought wrought Al Al 6061-T6 6061-T6 samples samples after after mechanisms,9090 days days of of ASTM ASTM as characterized B117 B117 salt salt fog fog by exposure. twoexposure. primary The The features:samples samples appear (1)appear few to fineto have have dimples failed failed showingdue due to to multiple microvoidmultiple coalescencemechanisms,mechanisms, with as as characterized associatedcharacterized tearing, by by two two and primary primary (2) smooth features: features: regions (1) (1) suggestingfew few fine fine dimples dimples particle-to-particle showing showing microvoid microvoid fracture. In thecoalescencecoalescence wrought with sample with associated associated strained tearing, to tearing, 25% yieldand and (2) (Figure (2) smooth smooth 16a), regions regions although suggesting suggesting few fine particle-to-particle dimples particle-to-particle exist, the fractographyfracture. fracture. In the wrought sample strained to 25% yield (Figure 16a), although few fine dimples exist, the predominatelyIn the wrought shows sample intergranular strained to fracture;25% yield Figure (Figure 16b shows16a), although the grain few boundaries fine dimples have beenexist, attacked. the fractography predominately shows intergranular fracture; Figure 16b shows the grain boundaries Thisfractography type of failure predominately is caused shows by elemental intergranular depletion fracture; at the Figure grain boundary16b shows or the weakening grain boundaries of the grain have been attacked. This type of failure is caused by elemental depletion at the grain boundary or boundarieshave been attacked. from oxidation, This type chemical of failure attack, is caus ored embrittlement by elemental depletion [17,27,46, 47at ].the Figure grain 17 boundarya,b also showsor weakeningweakening of of the the grain grain boundaries boundaries from from oxidation, oxidation, chemical chemical attack, attack, or or embrittlement embrittlement [17,27,46–47]. [17,27,46–47]. additional transgranular fracture. This fracture behavior corresponds with the ASM handbook on FigureFigure 17a,b 17a,b also also shows shows additional additional transgranular transgranular fracture. fracture. This This fracture fracture behavior behavior corresponds corresponds with with aluminum alloys evaluating stress corrosion cracking and compliments the optical analysis by previous thethe ASM ASM handbook handbook on on aluminum aluminum alloys alloys evaluating evaluating stress stress corrosion corrosion cracking cracking and and compliments compliments the the researchers [24,47]. The smooth regions indicate fast fracture through rapid crack propagation as a opticaloptical analysis analysis by by previous previous researchers researchers [24,47]. [24,47]. The The smooth smooth regions regions indicate indicate fast fast fracture fracture through through result of brittle behavior and low ductility (low strain to failure) and explains the similar and decreased rapidrapid crack crack propagation propagation as as a aresult result of of brittle brittle behavior behavior and and low low ductility ductility (low (low strain strain to to failure) failure) and and strain to failure when compared with that of the cold-sprayed deposits. Figure 18a–c shows the SEM explainsexplains the the similar similar and and decreased decreased strain strain to to failure failure when when compared compared with with that that of of the the cold-sprayed cold-sprayed micrographsdeposits.deposits. Figure Figure of the18a–c 18a–c fracture shows shows surfacesthe the SEM SEM formicrographs micrographs the cold sprayof of the the Alfracture fracture 6061 depositssurfaces surfaces for after for the the 90 cold cold days spray spray of ASTM Al Al 6061 6061 B117 saltdepositsdeposits fog exposure. after after 90 90 days Thesedays of of ASTM samples ASTM B117 B117 show salt salt evidence fog fog expo expo ofsure.sure. predominate These These samples samples failure show show by evidence ductile evidence dimple of of predominate predominate rupture and voidfailurefailure coalescence, by by ductile ductile dimplealthough dimple rupture rupture smooth and and regions void void coalescence, coalescence, attributed although to although particle–particle smooth smooth regions regions fracture attributed attributed can also to to beparticle– particle– observed andparticleparticle are fracture identified fracture can can byalso also arrows. be be observed observed and and are are identified identified by by arrows. arrows.

(a()a ) (b()b )

FigureFigure 16. 16. SecondarySecondary Secondary electron electronelectron images imagesimages of of wrought wrought Al Al 6061-T6 6061-T6 fracture fracture fracture surfaces. surfaces. surfaces. The The The fractography fractography fractography predominatelypredominately shows showsshows (a ((a)a )cleavage) cleavagecleavage as asas well wellwell as as inte inte intergranularrgranularrgranular fracture fracture fracture with with with (b ()b (theb) )the thegrain grain grain boundaries boundaries boundaries attackedattacked in in in the the the 25% 25% 25% yield yield yield stressstress stress condition. condition.condition.

(a)(a) (b)(b) Figure 17. Secondary electron images of wrought Al 6061-T6 fracture surfaces. The fractography predominately shows cleavage and transgranular fracture (circles), but intergranular facture is also present (arrows) in (a) 65% and (b) 85% stress conditions. Coatings 2019, 9, x FOR PEER REVIEW 15 of 19

Figure 17. Secondary electron images of wrought Al 6061-T6 fracture surfaces. The fractography predominately shows cleavage and transgranular fracture (circles), but intergranular facture is also present (arrows) in (a) 65% and (b) 85% stress conditions.

Coatings 2019, 9, x FOR PEER REVIEW 15 of 19

Figure 17. Secondary electron images of wrought Al 6061-T6 fracture surfaces. The fractography Coatingspredominately2019, 9, 445 shows cleavage and transgranular fracture (circles), but intergranular facture is also15 of 19 present (arrows)(a in) (a) 65% and (b) 85% stress conditions.(b) (c) Figure 18. Secondary electron images of cold-sprayed Al 6061 fracture surfaces. The fractography predominately shows the failure mode to be ductile in nature in (a) 25% stress, (b) 65% stress, and (c) 85% stress conditions. Smooth particle–particle fracture can be observed (arrows).

3.4. Stress Corrosion Cracking Research has determined that the effects of heat treatment, grain size, grain boundary segregation and precipitates, hydrogen embrittlement, as well as the loading mode, all have an influence on the SCC resistance of aluminum alloys [5–7]. Various studies have also shown that the stress corrosion cracking behavior of aluminum alloys in chloride environments is largely dependent on the specific immersion conditions [18,19,44–46]. Additionally, precipitates can cause areas of (a) (b) (c) localized corrosion because of differences in corrosion potential as compared with surrounding areas, resultingFigure in 18. hydrogen Secondary evolution, electron images promoting of cold-sprayed SCC [7–10,17,48–49]. Al 60616061 fracturefracture The surfaces.surfaces. build-up The of fractography hydrogen can eventuallypredominately embrittle shows the materialthe the failure failure and mode mode cause to to be becracking ductile ductile in in[11–13]. nature nature in Sincein (a) ( a25%) the 25% stress, cold stress, (sprayb) ( b65%) 65% deposit stress, stress, and was and (c in) the as-deposited(85%c) 85% stress stress condition, conditions. conditions. whileSmooth Smooth the particle–par wrought particle–particleticle Al fracture6061 fracture in canthe can be T6, observed be EDS observed was (arrows). (arrows).conducted to evaluate the 3.4.elemental Stress Corrosiondifferences Cracking in fracture surfaces between the two materials. Wrought fracture surfaces 3.4.showed Stress a presenceCorrosion ofCracking second-phase particles rich in Mg, Si and Fe, as shown in Figure 19. Conversely, the cold-sprayedResearch hashas determined depositdetermined fracture that that the surfaces etheffects effects ofshow heat of treatment,evenly heat treatment,distributed grain size, grainelemental grain size, boundary mappinggrain segregationboundary free of andsegregationsegregation. precipitates, andThere hydrogenprecipitates, were some embrittlement, randomhydrogen inclusions embrittlemen as well as observed, the loadingt, as aswell mode,shown as the all in haveFigureloading an 20. influence mode, all on have the SCC an resistanceinfluenceBased on of on aluminumthe the SCC microstructural resistance alloys [5– of7]. analysis,aluminum Various studiesit alloyscan be have [5–7]. concluded also Various shown that studies that the thewrought have stress also corrosionsamples shown failed crackingthat the by behaviorstress corrosioncorrosion of aluminum crackingcracking, alloys behavior while inchloride the of cold-sprayed aluminum environments allo samplesys in is largelychloride failure dependent environmentswas predominately on the is specificlargely by mechanicaldependent immersion conditionsonmeans. the specificIt has [18 ,been19 immersion,44 –well46]. established Additionally, conditions that precipitates[18,19,44–46]. powder metallurgy can Additionally, cause areas(PM) ofmethodsprecipitates localized can corrosion can produce cause because materials areas of dilocalizedwithfferences fine corrosionmetallurgical in corrosion because potential structures of differences as comparedand compositions, in corrosion with surrounding potentialwhich make areas,as compared such resulting materials with in hydrogen surrounding more resistant evolution, areas, to promotingresultingstress corrosion in SCC hydrogen [cracking7–10,17 evolution,,48 [5,6].,49]. The The promoting cold build-up spray ofSCC process hydrogen [7–10,17,48–49]. can can also eventually produce The builmaterials embrittled-up of thethat hydrogen material have a very andcan causeeventuallyfine grain cracking size, embrittle which [11–13 the is]. dictatedmaterial Since the byand cold the cause spraystarting cracking deposit feedstock [11–13]. was powder. in Since the as-deposited Thethe cold fine spraygrain condition, sizedeposit of the was while powder in the wroughtas-depositedis maintained Al 6061 condition, in inthe the bulk T6, while deposit. EDS wasthe This wrought conducted inherent Al to 6061characteristic evaluate in the the T6, elemental becomes EDS was dievenff conductederences more inpronounced fractureto evaluate surfaces when the betweenelementalproducing the differencesaluminum two materials. incold fracture Wroughtspray surfacesmaterials, fracture between becaus surfaces ethe there showed two can materials. a presencebe a great Wrought of disparity second-phase fracture in grain particlessurfaces size richshowedbetween in Mg, a thepresence Si cold and Fe, sprayof second-phase as shownAl 6061 in material Figure particles 19 (Figure. rich Conversely, in Mg,6) and Si theand its cold-sprayed Fe,wrought as shown counterpart depositin Figure fracture (Figure19. Conversely, surfaces 7). The showthedifference cold-sprayed evenly in average distributed deposit grain elemental fracturesize is approximately mappingsurfaces freeshow 53 of segregation.μevenlym, and distributedtherefore, There the were elemental SCC some results random mapping were inclusions expected free of observed,segregation.and substantiate as There shown previous were in Figure some research 20 random. of PM inclusions materials. observed, as shown in Figure 20. Based on the microstructural analysis, it can be concluded that the wrought samples failed by stress corrosion cracking, while the cold-sprayed samples failure was predominately by mechanical means. It has been well established that powder metallurgy (PM) methods can produce materials with fine metallurgical structures and compositions, which make such materials more resistant to stress corrosion cracking [5,6]. The cold spray process can also produce materials that have a very fine grain size, which is dictated by the starting feedstock powder. The fine grain size of the powder is maintained in the bulk deposit. This inherent characteristic becomes even more pronounced when producing aluminum cold spray materials, because there can be a great disparity in grain size between the cold spray Al 6061 material (Figure 6) and its wrought counterpart (Figure 7). The difference Figure in 19. averageEDS analysis grain of size wrought is approximately fracture surface 53 after μm, B117 and testing, therefore, showing the evidenceSCC results of second-phase were expected and substantiateparticle concentrations previous of research Mg, Si and of PM Fe. materials.

Based on the microstructural analysis, it can be concluded that the wrought samples failed by stress corrosion cracking, while the cold-sprayed samples failure was predominately by mechanical means. It has been well established that powder metallurgy (PM) methods can produce materials with fine metallurgical structures and compositions, which make such materials more resistant to stress corrosion cracking [5,6]. The cold spray process can also produce materials that have a very fine grain size, which is dictated by the starting feedstock powder. The fine grain size of the powder is maintained in the bulk deposit. This inherent characteristic becomes even more pronounced when producing aluminum cold spray materials, because there can be a great disparity in grain size between the cold spray Al 6061 material (Figure6) and its wrought counterpart (Figure7). The di fference in average Coatings 2019, 9, 445 16 of 19 Coatings 2019, 9, x FOR PEER REVIEW 16 of 19 grainFigure size is19. approximately EDS analysis of 53wroughtµm, and fracture therefore, surface the after SCC B117 results testing, were showing expected evidence and of substantiatesecond- previousphase research particle concentrations of PM materials. of Mg, Si and Fe.

Figure 20. EDSEDS analysis analysis of of cold-sprayed, cold-sprayed, stressed stressed 25% 25% fracture fracture surface surface after after B117 B117 testing, testing, showing showing the fracturethe fracture surface surface is predominately is predominately free free of ofprecip precipitatesitates or or element element segregation. segregation. Two Two second-phasesecond-phase particles are obvious, one an oxide inclusion and thethe otherother richrich inin Si.Si. 4. Conclusions 4. Conclusions In this preliminary study, the stress corrosion cracking response of cold spray deposits was In this preliminary study, the stress corrosion cracking response of cold spray deposits was compared to wrought counterparts that were peak-aged. Following ASTM standard G64-99 (2013), compared to wrought counterparts that were peak-aged. Following ASTM standard G64-99 (2013), Standard Classification of Resistance to Stress-Corrosion Cracking of Heat-Treatable Aluminum Alloys, Standard Classification of Resistance to Stress-Corrosion Cracking of Heat-Treatable Aluminum the stress levels for the wrought were 0%, 25%, 65% and 85% of the specified minimum yield strength Alloys, the stress levels for the wrought were 0%, 25%, 65% and 85% of the specified minimum yield for Al 6061-T6, while the cold-sprayed deposits were evaluated at 25%, 65% and 85% of the minimum strength for Al 6061-T6, while the cold-sprayed deposits were evaluated at 25%, 65% and 85% of the yield strength. SCC test fixtures were developed at PSU/ARL and were designed to withstand extended minimum yield strength. SCC test fixtures were developed at PSU/ARL and were designed to periods of time in the ASTM B117 salt fog environment without galvanic interaction. Nonfailed withstand extended periods of time in the ASTM B117 salt fog environment without galvanic samples that were exposed for 90 days were subsequently tensile tested to obtain residual strength. interaction. Nonfailed samples that were exposed for 90 days were subsequently tensile tested to obtainThe residual cold-sprayed strength. deposits displayed yield strengths similar to wrought peak-aged samples after • • exposure.The cold-sprayed Furthermore, deposits the displayed cold-sprayed yield deposits strengths retained similar a to higher wrought percentage peak-aged of tensile samples strength after afterexposure. 90 days Furthermore, exposure in samplesthe cold-sprayed pre-stressed deposit to 25%s andretained 65% of a yield higher strength percentage than the of wrought tensile counterpartsstrength after did. 90 days The reductionexposure in residualsamples strengthpre-stressed was to attributed 25% and to 65% the of development yield strength of stress than corrosionthe wrought cracking counterparts and was supporteddid. The reduction by metallographic in residual evidence, strength which was revealed attributed pitting to andthe intergranulardevelopment of branch stress cracking corrosion in cracking the wrought and wa samples,s supported with depths by metallographic of attack averaging evidence, between which 350revealed and 400 pittingµm. and intergranular branch cracking in the wrought samples, with depths of Atattack the averaging same stress between conditions, 350 theand cold-sprayed 400 μm. deposits showed negligible evidence of corrosion. • • WhenAt the samples same werestress pre-stressed conditions, to the 85% cold-spray of the yielded strength deposits and exposedshowed fornegligible 90 days, wroughtevidence and of coldcorrosion. spray When deposits samples retained were a similarpre-stressed percentage to 85% of of residual the yield strength. strength Metallographicand exposed for evidence 90 days, showedwrought average and cold depths spray of attack deposits reached retained 650 µm fora thesimilar wrought percentage material, of with residual some evidence strength. of corrosiveMetallographic attack evidence observed forshowed the cold aver sprayage depths deposits, of althoughattack reached not intergranular. 650 μm for the wrought Scanningmaterial, with electron some microscopy evidence of of corrosive the fractured attack wrought observed peak-aged for the cold Al6061 spray showed deposits, failure although from • multiplenot intergranular. mechanisms including cleavage, intergranular and transgranular fracture. Fractography • ofScanning the cold-sprayed electron microscopy deposits revealed of the fractur failureed predominately wrought peak-aged by ductile Al 6061 dimple showed rupture, failure although from brittlemultiple particle–particle mechanisms fractureincluding was alsocleavage, observed. intergranular Microstructural and analysistransgranular supports fracture. that the Fractography of the cold-sprayed deposits revealed failure predominately by ductile dimple rupture, although brittle particle–particle fracture was also observed. Microstructural analysis

Coatings 2019, 9, 445 17 of 19

wrought samples failed by stress corrosion cracking while the cold-sprayed samples failure predominately by mechanical means and general corrosion. The dissimilarities between the SCC response of the differently processed materials is attributed • to the benefits of the cold spray process, which includes maintaining fine grain structure of the feedstock powder, fine distribution of second phase precipitates, and low oxidation.

Author Contributions: Conceptualization, V.C. and T.E.; Methodology, V.C., J.S., M.S. and T.E.; Validation, M.S. and J.S.; Formal Analysis, M.S.; Investigation, J.S., M.S., T.E. and V.C.; Writing—Original Draft Preparation, Review and Editing, M.S. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Champagne, V.K. The Cold Spray Materials Deposition Process: Fundamentals and Applications, 1st ed.; Woodhead Publishing: Cambridge, UK, 2007. 2. Schmidt, T.; Gartner, F.; Assadi, H.; Kreye, H. Development of a generalized parameter window for cold spray deposition. Acta Mater. 2006, 54, 729–742. [CrossRef] 3. Assadi, H.; Gartner, F.; Stoltenhoff, T.; Kreye, H. Bonding mechanism in cold gas spraying. Acta Mater. 2003, 51, 4379–4394. [CrossRef] 4. Zou, Y.; Goldbaum, D.; Szpunar, J.A.; Yue, S. Microstructure and nanohardness of cold-sprayed coatings: Electron backscattered diffraction and nanoindentation studies. Scr. Mater. 2010, 62, 395–398. [CrossRef] 5. Lyle, J.P.; Cebulak, W.S. Powder metallurgy approach for control of microstructure and properties in high strength aluminum alloys. MTA 1975, 6, 685. [CrossRef] 6. Pickens, J.R.; Christodoulou, L. The stress-corrosion cracking behavior of high-strength aluminum powder metallurgy alloys. MTA 1987, 18, 135–149. [CrossRef] 7. Wang, S.D.; Xu, D.K.; Wang, B.J.; Sheng, L.Y.; Han, E.H.; Dong, C. Effect of Solution Treatment on Stress Corrosion Cracking Behavior of an As-Forged Mg-Zn-Y-Zr Alloy; Springer Nature: Basingstoke, UK, July 2016. [CrossRef] 8. Williams, G.; ap Llwyd Dafydd, H.; Grace, R. The localised corrosion of Mg alloy AZ31 in chloride containing electrolyte studied by a scanning vibrating electrode technique. Electrochim. Acta 2013, 109, 489–501. [CrossRef] 9. Lebouil, S.; Duboin, A.; Monti, F.; Tabeling, P.; Volovitch, P.; Ogle, K. A novel approach to on-line measurement of gas evolution kinetics: Application to the negative difference effect of Mg in chloride solution. Electrochim. Acta 2014, 124, 176–182. [CrossRef] 10. Ralston, K.D.; Williams, G.; Birbilis, N. Effect of pH on the grain size dependence of magnesium corrosion. Corrosion 2012, 68, 507–517. [CrossRef] 11. Winzer, N.; Atrens, A.; Song, G.; Ghali, E.; Dietzel, W.; Kainer, K.U.; Hort, N.; Blawert, C. A critical review of the stress corrosion cracking (SCC) of magnesium alloys. Adv. Eng. Mater. 2005, 7, 659–693. [CrossRef] 12. Jafari, S.; Harandi, S.E.; Singh Raman, R.K. A Review of stress-corrosion cracking and corrosion fatigue of magnesium alloys for biodegradable implant applications. J. Mater. 2015, 67, 1143–1153. [CrossRef] 13. Kannan, M.B.; Dietzel, W. Pitting-induced hydrogen embrittlement of magnesium-aluminium alloy. Mater. Des. 2012, 42, 321–326. [CrossRef] 14. Deakin, J.; Dong, Z.H.; Lynch, B.; Newman, R.C. De-alloying of type 316 stainless steel in hot, concentrated sodium hydroxide solution. Corros. Sci. 2004, 46, 2117–2133. [CrossRef] 15. Knight, S.P.; Birbilis, N.; Muddle, B.C.; Trueman, A.R.; Lynch, S.P. In situ X-ray tomography of intergranular corrosion of 2024 and 7050 aluminium alloys. Corros. Sci. 2010, 52, 3855–3860. [CrossRef] 16. Raja, V.S.; Shoji, T. Stress Corrosion Cracking: Theory and Practice (Woodhead Publishing Series in Metals and Surface Engineering); Elsevier: Cambridge, UK, 2011. 17. Renner, F.U.; Ankah, G.N.; Bashir, A.; Ma, D.; Biedermann, P.U.; Shrestha, B.R.; Nellessen, M.; Khorashadizadeh, A.; Losada-Pérez, P.; Duarte, M.J.; et al. Star-shaped crystallographic cracking of localized nanoporous defects. Adv. Mater. 2015, 27, 4877–4882. [CrossRef][PubMed] 18. Holroyd, N.J.H.; Vasudevan, A.K.; Christodoulou, L. Aluminum Alloys–Contemporary Research and Applications; Treatise on Materials Science and Technology; Academic Press: Boston, MA, USA, 1989; pp. 46–52. Coatings 2019, 9, 445 18 of 19

19. Holroyd, N.J.H. Environment-Induced Cracking of High-Strength Aluminum Alloys. In Proceedings of the First International Conference on Environmental Induced Cracking of Metals, Sheboygan, WI, USA, 2–7 October 1988; Gangloff, R.P., Ives, M.B., Eds.; National Association of Corrosion Engineers: Houston, TX, USA, 1990; pp. 311–345. 20. Gruhl, W. Stress corrosion cracking of high strength aluminum alloys. Z. Metalkd. 1984, 75, 819–826. 21. Burleigh, T.D. The postulated mechanisms for stress corrosion cracking of aluminum alloys: A review of the literature 1980–1989. Corrosion 1991, 47, 89–98. [CrossRef] 22. Braun, R. Environmentally assisted cracking of aluminium alloys. Materialwiss. Werkstofftech. 2007, 38, 674–689. [CrossRef] 23. Braun, R. On the stress corrosion cracking behaviour of 6XXX series aluminium alloys. Int. J. Mater. Res. 2010, 101, 657–668. [CrossRef] 24. Rokni, M.R.; Widener, C.A.; Ozdemir, O.C.; Crawford, G.A. Microstructure and mechanical properties of cold sprayed 6061 Al in As-sprayed and heat treated condition. Surf. Coat. Tech. 2017, 309, 641–650. [CrossRef] 25. Gärtner, F.; Stoltenhoff, T.; Voyer, J.; Kreye, H.; Riekehr, S.; Koçak, M. Mechanical properties of cold-sprayed and thermally sprayed copper coatings. Surf. Coat. Tech. 2006, 200, 6770–6782. [CrossRef] 26. Meng, X.-M.; Zhang, J.-B.; Han, W.; Zhao, J.; Liang, Y.-L.Influence of annealing treatment on the microstructure and mechanical performance of cold sprayed 304 stainless steel coating. Appl. Surf. Sci. 2011, 258, 700–704. [CrossRef] 27. AL-Mangour, B.; Vo, P.; Mongrain, R.; Irissou, E.; Yue, S. Effect of heat treatment on the microstructure and mechanical properties of stainless steel 316L coatings produced by cold spray for biomedical applications. J. Therm. Spray Technol. 2014, 23, 641–652. [CrossRef] 28. ASTM E8-04, Standard Test Methods for Tension Testing of Metallic Materials; ASTM International: West Conshohocken, PA, USA, 2004. 29. ASTM standard G64-99, Standard Classification of Resistance to Stress-Corrosion Cracking of Heat-Treatable Aluminum Alloys; ASTM International: West Conshohocken, PA, USA, 2013. 30. ASTM B117-18, Standard Practice for Operating Salt Spray (Fog) Apparatus; ASTM International: West Conshohocken, PA, USA, 2018. 31. ASTM-E112-12, Standard Test Methods for Determining Average Grain Size; ASTM International: West Conshohocken, PA, USA, 2013. 32. Rokni, M.R.; Widener, C.A.; Champagne, V.K.; Crawford, G.A. Microstructure and mechanical properties of cold sprayed 7075 deposition during non-isothermal annealing. Surf. Coat. Tech. 2015, 276, 305–315. [CrossRef] 33. Rokni, M.R.; Widener, C.A.; Nardi, A.T.; Champagne, V.K. Nano crystalline high energy milled 5083 Al powder deposited using cold spray. Appl. Surf. Sci. 2014, 305, 797–804. [CrossRef] 34. Ajdelsztajn, L.; Schoenung, J.M.; Jodoin, B.; Kim, G.E. Cold spray deposition of nanocrystalline aluminum alloys. Metall. Mater. Trans. A 2005, 36, 657–666. [CrossRef] 35. Rokni, M.R.; Widener, C.A.; Champagne, V.R. Microstructural stability of ultrafine grained cold sprayed 6061 aluminum alloy. Appl. Surf. Sci. 2014, 290, 482–489. [CrossRef] 36. Rokni, M.R.; Widener, C.A.; Crawford, G.A. Microstructural evolution of 7075 Al gas atomized powder and high-pressure cold sprayed deposition. Surf. Coat. Tech. 2014, 251, 254–263. [CrossRef] 37. Rokni, M.R.; Widener, C.A.; Champagne, V.K. Microstructural evolution of 6061 aluminum gas-atomized powder and high-pressure cold-sprayed deposition. J. Therm. Spray Tech. 2014, 23, 514–524. [CrossRef] 38. Zou, Y.; Qin, W.; Irissou, E.; Legoux, J.-G.; Yue, S.; Szpunar, A.J. Dynamic recrystallization in the particle/particle interfacial region of cold-sprayed nickel coating: Electron backscatter diffraction characterization. Scripta Mater. 2009, 61, 899–902. [CrossRef] 39. Borchers, C.; Gärtner, F.; Stoltenhoff, T.; Kreye, H. Formation of persistent dislocation loops by ultra-high strain-rate deformation during cold spraying. Acta Mater. 2005, 53, 2991–3000. [CrossRef] 40. Kim, K.H.; Watanabe, M.; Kawakita, J.; Kuroda, S. Grain refinement in single titanium powder particle impacted at high velocity. Scripta Mater. 2008, 59, 768–771. [CrossRef] 41. Henao, J.; Sharma, M.M. Characterization, deposition mechanisms and modeling of metallic glass powders for cold spray. In Cold-Spray Coatings: Recent Trends and Future Perspectives; Cavaliere, P., Ed.; Springer: Berlin, Germany, 2017; pp. 251–272. Coatings 2019, 9, 445 19 of 19

42. Sharma, M.M.; Eden, T.J.; Golesich, B.T. Eﬀect of surface preparation on the microstructure, adhesion and tensile properties of cold sprayed aluminum coatings on AA2024 substrates. J. Therm. Spray. Tech. 2015, 24, 410–422. [CrossRef] 43. Farshidi, M.H.; Kazeminezhad, M.; Miyamoto, H. Severe plastic deformation of 6061 aluminum alloy tube with pre and post heat treatments. Mater. Sci. Eng. A 2013, 563, 60–67. [CrossRef] 44. Kim, W.J.; Kim, J.K.; Park, T.Y.; Hong, S.I.; Kim, D.I.; Kim, Y.S.; Lee, J.D. Enhancement of strength and superplasticity in a 6061 Al alloy processed by equal-channel-angular-pressing. Metall. Mater. Trans. A 2002, 33, 3155–3164. [CrossRef] 45. Nurislamova, G.; Sauvage, X.; Murashkin, M.; Islamgaliev,R.; Valiev,R. Nanostructure and related mechanical properties of an Al–Mg–Si alloy processed by severe plastic deformation. Philos. Mag. Lett. 2008, 88, 459–466. [CrossRef] 46. Liu, X.; Frankel, G.S.; Zoofan, B.; Rokhlin, S.I. In-situ observation of intergranular stress corrosion cracking in AA2024-T3 under constant load conditions. Corros. Sci. 2007, 49, 139–148. [CrossRef] 47. Phull, B. Evaluating stress-corrosion cracking. In ASM Handbook Corrosion: Fundamentals, Testing, and Protection; ASM International: Russell Township, OH, USA, 2003; pp. 42–44. 48. Speidel, M.O. Stress corrosion cracking of aluminum alloys. Metall. Trans. A 1975, 6, 631–651. [CrossRef] 49. Huang, I.-W.; Hurley, B.L.; Yang, F.; Buhheit, R.G. Dependence on temperature, pH, and Cl in the uniform corrosion of aluminum alloys 2024-T3, 6061-T6, and 7075-T6. Electrochim. Acta 2016, 199, 242–253. [CrossRef]

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