corrosion fatigue insitu ocp

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CHAPTER 1 INTRODUCTION Understanding corrosion processes as they relate to fatigue is becoming increasingly important as mechanical systems are consistently moving toward lighter builds with higher power densities in mechanical components. As the mechanisms of corrosion fatigue become more defined, engineered metals can be formulated specifically to increase resistance to corrosion accelerated fatigue. Standard materials data handbooks offer the design engineer information on fatigue through many different testing standards. Testing standards have not been designed to consider the use of Open Circuit Potential testing during cyclic fatigue. An idealized test rig configuration will pull together standards for Open Circuit Potential measurements and fatigue testing without compromising the control area under study. 1

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CHAPTER 1 INTRODUCTION Understanding corrosion processes as they relate to fatigue is becoming increasingly important as mechanical systems are consistently moving toward lighter builds with higher power densities in mechanical components. As the mechanisms of corrosion fatigue become more defined, engineered metals can be formulated specifically to increase resistance to corrosion accelerated fatigue. Standard materials data handbooks offer the design engineer information on fatigue through many different testing standards. Testing standards have not been designed to consider the use of Open Circuit Potential testing during cyclic fatigue. An idealized test rig configuration will pull together standards for Open Circuit Potential measurements and fatigue testing without compromising the control area under study.

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CHAPTER 2 LITERATURE REVIEW 2.1 Aluminum Corrosion: When looking at Aluminum from a chemistry standpoint, it is very reactive. Aluminum is the fourth most reactive metal in the Electromotive Force Series. Table 2a. Electromotive Force Series for selected metals [6]. Element Magnesium Aluminum Chromium Copper Copper Platinum Electrode Reaction Mg Mg++ + 2e Al Al+++ + 3e Cr Cr+++ + 3e Cu Cu++ + 2e Cu Cu+ + e Pt Pt++ + 2e Standard Hydrogen Electrode Potential -2.34 V -1.67 V -0.71 V +0.345 V +0.522 V +1.2 V

The reason Aluminum is resistant to corrosion is because it builds protective layers or passivates. Aluminum forms passive layers at a rapid rate in many environments. The rate or magnitude of passivation is dependent on the alloy system chosen and the environment it is subjected to. When passivation occurs, the aluminum forms an inner barrier layer made of Alumina or Al2O3 and a hydrated outer layer. This hydrated outer layer has different compositions depending on the temperature and environment. At low temperatures Aluminum forms an outer layer of Bayerite, or aluminum-trihydroxide Al(OH)3 [7].

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Figure 2a. Passive layer formed on Aluminum [7].

A Pourbaix Diagram can be constructed to determine conditions of passivation or corrosion in metals. Standard Hydrogen Electrode Potentials (SHEP) is needed to construct a Pourbaix Diagram. Often times they are difficult to find and can be calculated from known affinities of the compounds or extracted from electrochemical test data. Equation 2a relates Gibbs free energy to SHEP.

Equation 2a. G = nFE

G is the change in Gibbs free energy in Joules for n moles of electrons transferred. F is Faradays constant = 98485 C/mol e- and E is the Standard Hydrogen Electrode Potential in Volts. Once the SHEP is calculated for each possible reaction, the

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Nernst Equation 2b is then used to relate SHEP to pH. This relation is then plotted for each reaction to form a Pourbaix Diagram. [9]

Equation 2b. Ecell = E - (R*T*ln(Q))/(n*F)

Ecell is the Potential for any given point, E is the SHEP for the reaction, R is the gas constant = 8.314 Joule*K-1*mole-1, T is absolute temperature, n is the number of moles of electrons in the reaction, Q is the reaction quotient and F is Faradays constant = 9.65 x 104 Coulomb*mole-1. [9]

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Figure 2b. Pourbaix Diagram for Al-H2O at Room Temperature [8].

Information as to the nature of a corrosive environment can be extrapolated from a Pourbaix Diagram. Below the bottom dashed line, hydrogen evolution occurs. Above the top dashed line, oxygen evolution occurs. A pH value less than 5 indicates acidic corrosion while pH values greater than 9.5 indicate alkali corrosion regions. Passivation occurs between hydrogen and oxygen evolution and from 5 to 9.5 pH. When inside of the passive region, corrosion is still possible. Localized pitting corrosion can form.

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The Mg2Si system in AA 6061 has superior Stress-Crack-Corrosion (SCC) resistance. No known cases of service related SCC has been reported for 6XXX series alloys. The Mg2Si is anodic to aluminum and reactive in acidic environments. [11]

2.2 Aluminum Fatigue: Constant amplitude cyclic loading states are specified by the maximum and minimum stresses that are defined at cycle boundaries. Other standard variables used to define cyclic loading include Alternating Stress (Sa), Mean Stress (Sm) and stress ratios (R, A). These variables are defined as follows [1].

Sa = (Smax Smin) / 2

Sm = (Smax + Smin) / 2

R = Smin / Smax

A = Sa / Sm

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Figure 2c. Constant amplitude cyclic loading [23]

Constant Life Diagrams are commonly used to predict fatigue failures at conditions other than fully reversed. Construction of a Constant Life Diagrams requires basic knowledge of material properties and data from one fatigue failure point. This data is typically calculated at the fully reversed condition and can be found in fatigue data books. A Constant Life Diagram with a Modified Goodman line can be constructed with a materials Ultimate Tensile Strength and Yield Strength as design criteria [1]. For AA 6061-T6, Su = 45 ksi and Sy = 40 ksi [10]. Fatigue failure for 15,000 cycles in an aqueous environment is estimated at 23 ksi. This estimate was approximated from data found in a fatigue data book [22]. Figure 2c is a Constant Life Diagram constructed for this study. The Modified Goodman Line is indicated by 1 and the Yield Line is indicated by 2.

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50 Sa - Alternating Stress (ksi) 40 30 2 20 10 0 -40 -30 -20 -10 0 10 20 30 40 Sm - Mean Stress (ksi) 1

Figure 2d. Constant life Diagram for AA6061-T6 Fatigue failure without yielding can be achieved above the Goodman line and below the yield line. Below the Modified Goodman line, fatigue failure for the desired number of cycles will not likely occur. The Modified Goodman Equation is as follows [1].

Sa/Sf + Sm/Su = 1

Where Sa is the Alternating Stress, Sm is the Mean Stress, is the materials Ultimate Tensile Strength and Sf is the failure stress at the desired number of cycles in the fully reversed condition when the stress ratio (R) is -1.

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2.3 Aluminum Corrosion-Fatigue: Corrosion fatigue is based on the crack growth model where notches are created by intergranular corrosion or pitting. The notches create localized stress risers and forms crack tips. The crack growth rates are dependent on geometry and size of pits for pitting corrosion or grain size and distribution for inter-granular corrosion. [27]

Aluminum alloy 6061-T6 is susceptible to inter-granular corrosion in regular service. Susceptibility to inter-granular attack can be determined by following MIL-H-6088 G [7]. The cause of inter-granular attack in this alloy system is that the Mg2Si in the grain boundaries is anodic to aluminum [11]. Corrosion fatigue is a definite concern with this particular alloy system.

2.4 Electrochemistry: Open Circuit Potential (OCP) or equilibrium potential is a measure of a metals voltage compared to a known standard while no current is moving in the cell. The metal of interest is the working electrode, while the know standard is the reference electrode. To determine the metals placement in the electromotive force series, simply add the OCP reading to the reference electrodes known value within the series. A metals OCP value can be used to establish ranges to run Potentiodynamic tests. Changes in OCP of an aluminum alloy under fatigue conditions can help to identify crack formation and growth

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as the OCP for a passive layer differs from that of the base alloy. Changes in OCP curves during fatigue testing can be correlated to crack growth rates in the specimen.

Potentiodynamic polarization tests are used to extract important electrochemical data from metals. This type of test utilizes a working and reference electrode identical to the OCP test. The cell voltage is regulated through a range of values while the current is monitored. Tests can cover anodic, cathodic or both regions of a metal. Tests performed using aerated electrolyte give useful information as to passive and trans-passive behavior in oxide forming metals while tests performed in de-aerated electrolyte typically provide corrosion rates.

2.5 Welding of Aluminum Gas Metal Arc Welding (GMAW) is a fusion welding process in typical use for joining aluminum alloys. A consumable bare wire is used for the electrode and an inert gas is used to shield the weld from the atmosphere. The arc creates a molten pool of filler and base metal that is 100 to 200 degrees Celsius above the base metals liquidus temperature [12]. The heat generated during the welding process dissipates through the surrounding metal and adjoining surfaces.

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Figure 2e. Gas Metal Arc Welding [13] Several thermally affected zones form depending on the temperature profile of the area of interest. Grain growth, recrystallization and precipitation evolution can be identified through in-depth microstructural evaluations. The areas of interest in GMAW welding are the unmixed zone (UMZ), the partially melted zone (PMZ) and the heat affected zone (HAZ).

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2.6 Aluminum Alloy 6061-T6: AA 6061 was developed by the Aluminum Company of America in 1935. It is a solution heat treated, artificially aged, marine grade alloy. The use of AA 6061 is of wide spectrum. It is readily available in a variety of forms, easy to machine, easy to weld and has very good corrosion resistance properties [5].

Copper has been added for extra strength and Chromium for extra strength, grain refinement and corrosion resistance. Mg2Si is the main precipitate that forms when hardening while CuAl2 contributes slightly. An excess of Si present in the alloy increases corrosion resistance.

Table 2b. Nominal Chemical Composition of AA 6061 in percent [4]. Element Magnesium Copper Chromium Silicon Aluminum Composition 1.0 % 0.27 % 0.25 % 0.6 % Balance

AA 6061-T6 has very good SCC corrosion resistance properties. ASTM has rated AA 6061-T6 as an A class alloy for resistance to Stress-Corrosion Cracking using 3.5% NaCl alternate immersion testing in accordance with ASTM practice G44-99 [3]. Despite

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its excellent SCC resistance, AA 6061-T6 is susceptible to inter-granular attacks [7]. Corrosion fatigue can be a cause of concern with this alloy.

Annealing AA 6061 requires a temperature of 775 degrees F for 2-3 Hours. A solution treatment at 970 degrees F followed by a coldwater quench is needed to bring the alloy to T4 condition. The final stage of heat treatment for the T6 condition requires a precipitation heat treatment. A temperature of 320 degrees F for 16 to 20 hours or a temperature of 350 degrees F for 6 to 10 hours will accomplish this.[10]

Welded AA 6061 heats up to 577 degrees Celsius when Gas Metal Arc Welded (GMAW) [12] and from 300 to 475 degrees Celsius when Friction Stir Welded (FSW) [15]. The process temperatures for both GMAW and FSW generate well defined microstructural differences within and adjacent to the weld regions. GMAW welding of AA 6061 requires 4043 filler wire [14]. This is a bare wire consumable electrode.

2.7 Three-Point Bending: Three point bend tests are standard tests that can be used to extrapolate various mechanical property data for a particular material. A specimen is simply supported at two 13

points while adding a load to the middle. The maximum stress occurs on the opposite side of the specimen from loading, normal to the load direction and is tensile. The location of this maximum stress is commonly referred to as the critical fiber. Reversed three-point bending was the second choice for these investigations at UND. The center is simply supported, while the two outside points apply equal forces. The critical fiber is now located at the top of the sample, allowing placement of a fluid reservoir for aqueous tests.

Figure 2f. Reversed Three-point Bending

Using a sample that has a rectangular cross-section allows for simple calculations of stress at the critical fiber. This holds true for any stress that is below the materials yield point. Calculations of stress at the critical fiber for a specimen that is below the yield point are as follows. max = M x c / I M=L/2xF I = 1/12 x b x h3 Where max is the maximum stress at the critical fiber, M is the bending moment, c is the critical fibers distance from the neutral axis, I is the area moment of Inertia, L is the 14

distance between the two end supports, F is the total force applied to the specimen, b is the length of the specimens base and h is the specimens height.

Once yield has been exceeded, a plastic hinge will form and the model can not be assumed linear elastic. Removal of a force after plastic hinge will result in residual stresses in the specimen. It is important to avoid plastic deformation during fatigue testing to ensure the actual stress remains a normal tensile one. Severe deformations from plastic pileup can lead to complex stress tensors that can no longer be related to both S-N and -N curves.

2.8 Four Point Bending Four point bending gives an advantage over three point for the studied application because the loads that are applied are not in line with the critical fiber. This allows fully reversed cyclic fatigue testing and will give more access of maximum stress areas to instrumentation. This is critical if a reservoir is to be mounted on the specimen. Maximum deflection at the critical fiber can be found be the equation: [28] x = P(L-a)/(6LEI)[(L/(L-a))(x-a)^3-x^3 + (L^2-(L-a)^2)x] + Pa/6LEI[L/a(x-(L-a))^3-x^3 +(L^2-a^2)x] Where P is Loading, L is the Length of the beam, a is the distance between loading and support points, x is the measurement to the fiber of interest x=L/2 for center fiber, I is

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the Area moment of Inertia and E is the modulus of elasticity. Stress between Loading Points can be calculated as: = Pa/c Where c is the distance between the neutral axis and the critical fiber.

Figure 2g. Four-Point Bending [29]

As one could see from the equations, the 4-point bending presents an opportunity to exploit the uniform nature of stress between the loading points. This allows the study of an area under condition rather than a finite section in 3-point bending. To further restrict 16

the area of study for failure, one could machine the studied section down to reduce the distance to the critical fiber and thus increasing the tensile stress. Care should be taken in doing this as sharp geometrical changes in the specimen could cause notch effects and become the source of failure. Smooth, rounded transitions are recommended. This writer strongly recommends 4-point bending as an ideal load configuration for fatigue testing in aqueous conditions.

2.9 Flexural Bending Flexural bending involves the usage of rectangular specimens bent in a typical 3-point or 4-point fashion. Initial investigations and testing relating to this paper at UND were performed using a Shimadzu Autograph AG-IS machine. Load overshoot and undershoot can occur with machines that have low force control response. Other flexural bending methods involve displacement control. While displacement control will work to obtain fatigue data, it produces -N data curves instead of the desired and most commonly reported S-N data curves. It is this writers professional opinion that flexural bending methods are inferior to rotating bending methods for this type of investigation.

2.10 Rotating Bending Rotating bending fatigue operates similar to that of 3 point or 4 point flexural bending fatigue in that the critical fiber is located in the middle of the specimen and loads are 17

applied the same as well. Inherent advantages that rotating bending has over flexural bending is that specimen geometry is cylindrical, specimens are easy to manufacture and the applied load is constant and easy to control. The Aluminum Association has used rotating bending endurance limits to report fatigue resistance for over half a century. High cycle rotating bending fatigue tests are extremely useful to detect the effect of small variations of material properties on fatigue. [24]

2.11 Surface Roughness Characterization In making a surface measurement, two parameters are usually considered. The first parameter, amplitude; is a measure of vertical characteristics that surface deviations exhibit. The second parameter, Spacing; measures horizontal characteristics of the surface deviations. Hybrid parameters can be measured as well, but they are combinations of amplitude and spacing parameters that are used to find customized results. Both two and three dimensional measurements can be made, depending on the type of measurement device.

Several geometric factors may be involved with each type of measurement. These factors will vary from surface to surface, and a clear understanding of them is needed to interpret the results of surface measurements. Taking measurement of roughness without regard to waviness and lay would be disastrous to the end result. Refer to figure 1 for a picture of a 18

basic geometry of roughness, waviness and lay. This type of surface geometry is common with grinding operations.

Figure 2h. Common geometric factors that are used in surface measurements [17].

When calculating roughness, the most common approach is to reference the amplitude parameter to an averaged value for the entire measured surface [18].

Ym = ABS(yi ybar)

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Where Ym is the measured value, yi is the current amplitude and ybar is the average amplitude of the entire measured surface. When considering all measurement points, roughness parameters can be obtained.

Roughness parameter Ra is an averaged roughness over all points of measure. This is the most widely used parameter in the world [18].

Roughness parameter Rz is the 10 point height of the entire measured span [18].

Stylus-type profilometers are the most commonly used in industry. It utilizes a mechanical tip to physically drag across the surface to obtain the measurement. An example of this type of machine can be seen in figure 2f. The advantages of stylus-type profilometers are economy, speed, and they can be used on a wide range of roughness values. One disadvantage to using a stylus is that only two-dimensional measurements 20

are possible. The physical dimensions of the tip may also distort the recorded profile of the measurement. Sharp deviations may be depicted as transitional or missed altogether if the tip is larger than the pit that is measured. This makes stylus-style measurements poor for smaller roughness values. Stylus-type instruments are found widely throughout industry.

Figure 2i. Stylus-type Profilometer [19].

Laser Scanning Confocal Microscopes can offer high resolution, three dimensional renderings of surface topography. It does this by sectioning the surface into multiple focal planes. These focal planes are then rendered together to form a virtual three dimensional topographical surface. Some offer software packages capable of three dimensional profilometry readings and can even allow the user to make stylus type profilometry readings on these renderings. This feature proves to be a useful tool to get accurate 21

readings perpendicular to lay. UND currently has this capability at the Advanced Engineered Materials Center. It would be highly recommended to use such technology to report accurate surface roughness data of fatigue specimens.

Figure 2j. Laser Scanning Confocal Microscopy [25]

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Figure 2k. Virtual Profilometry [26]

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2.12 Design of Experiments Design of Experiments or DOE can be utilized to maximize the usefulness of information in testing. A Central Composite Design is typically used for process or operating point optimization where numerical modeling leaves linearity. The following is a template of a central composite design.

Table 2c. Central Composite Design [20]X1 -1 1 -1 1 -1.41 1.41 0 0 0 X2 -1 -1 1 1 0 0 -1.41 1.41 0 X1X2 1 -1 -1 1 0 0 0 0 0 X1^2 1 1 1 1 1.99 1.99 0 0 0 X2^2 1 1 1 1 0 0 1.99 1.99 0

For a successful DOE implementation, it is important to apply statistics and randomize experimental run orders. Replication is necessary to acquire statistical significance. Each repeated run adds a single Degree of Freedom to the system. It is also important to balance the DOE with equal runs at each factorial point [20].

Each run on the DOE results in a yield or numerical result. Multiple runs at the same point results in the ability to calculate statistical significance. The first step to calculating statistical significance is to calculate the variance of each point. 24

Variance = ((Y-Ybar)2) / DOF

Where the summation encompasses each measured value of Y. Ybar is the average value at the design point and DOF is the Degrees of freedom of the point or the number of replicates. Once the variances are established for each point, the Pooled Standard Deviation or Sp can be calculated [20].

Sp = ((Variance x DOF) / (DOF))1/2

The summations encompass each experimental point. Once the Pooled Standard Deviation is calculated, the Standard Deviation of Effect or SE can be determined [20].

SE = 2 x Sp / Nf(1/2)

Where Nf is the number of factorial points ran, including replicate runs. Effects and Interactions can now be formulated as shown below [20].

Ei = (Coded value of i x Ybari) / Nc

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Where Nc is the number of comparisons made or two times the number of variables under study. The summation encompasses all experimental runs. With effects calculated, a Signal-Noise-Ratio can be obtained [20].

tE = Ei / SE

The variable tE is the Signal-to-Noise Ratio. This value is compared to the critical value of the Students t distribution to determine statistical significance. Significant Effects and Interactions can be put into a mathematical model that encompasses the entire range of the variable field. This allows contour plots to be generated to give indication of relative maxima and minima criterion for process optimization points. The Coefficient of any variable in a DOE is the Effect or Interaction divided by two. Through the Law of Inheritance, any variable that is within a statistically significant Interaction becomes statistically significant itself [20].

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CHAPTER 3 EXPERIMENTAL METHODS

3.1 Specimen Preparation: Metallurgical specimens should be prepared to evaluate microstructural characteristics of the base alloy system, Friction Stir Weld samples and Metal Inert Gas weld samples. The base alloy is cut into several samples for screening different etchants. The samples are then prepared as hot-mounted as per table 3a and machine polished to mirror finish as per table 3b.

Table 3a. Sample Hot-mount preparation instructions. 1 2 3 4 5 6 7 8 9 Turn on Buehler Simplimet 1000 automatic mounting press Figure 3a. Verify machine settings are right for the resin system used 1 min heat time, 2.5 min cool time, 4200 psi pressure, 300 Deg. F Temperature Open mounting cylinder and raise until specimen can be placed on the head Apply release agent to cylinder head and place specimen face down Lower cylinder head and put 20 g of Buehler Phenocure resin in chamber Close the chamber and press start cycle After cycle is complete, open chamber and raise cylinder head Remove sample and move on to surface finishing procedures Table 3b. Metallurgical specimen preparation Machine polishing 1 2 3 4 5 6 Place 320 grit sandpaper in the Struers LaboPol-21 machine Figure 3b Adjust sample tensioning rods to apply moderate force to specimens Insert hot or cold mount specimens into the slots on polishing head. Lower polishing head until it is parallel with sanding surface Ensure the polishing head locks into place and will not move upward Turn on for 5 to 10 minutes. Adjust water flow as needed to reduce friction and 27

7 8 9 10 11

carry away polishing debris. Inspect specimen and verify it is uniformly polished Repeat steps 1-7 for 1200 grit and 6 micron diamond suspension. Rinse with distilled water 3 times Rinse with isopropyl alcohol 3 times Measure and record surface roughness using profilometer

Micro-structural characteristics of the samples can be obtained using etching techniques. For the general grain structure, Kellers etchant is used with a 20 to 60 second surface swab until micro-structural differences are visually apparent [16].

Table 3c. Kellers etchant 1 2 3 4 5 2.5 mL HNO3 1.5 mL HCl 1.0 mL HF 95 mL water For color tint etchant, mix 1 part of the above steps 1-4 with 4 parts water

To analyze Aerated and De-aerated Potentiodynamic Polarization Scans, electrochemical specimens must be prepared. Samples of the base alloy, FSW and GMAW should be examined. The samples first need a thin copper wire attached to the surface using 28

Locktite 3888 2-Part conductive adhesive (Part Number 29840). After curing, the sample needs to be coated with Ameron Americoat 90HS Pearl Grey Resin system. The system requires a 4 part resin to one part curing agent ratio. This is performed volumetrically. The Pearl Grey Americoat resin system prevents crevice corrosion of the samples. Once this resin cures, the samples can be cold mounted as per table 3c and hand polished as per table 3d.

Table 3d. Cold Mounting Procedure 1 2 3 4 5 6 7 8 9 Apply Release Agent to glass sheet. Buehler PN 626-500551 Drill a hole in the side of phenolic cold mount ring to allow glass tube inside Insert glass tube into the hole on the phenolic ring Place the sample inside the phenolic ring on the glass plate Route copper wire from sample through the glass tube Center the sample to the middle of the phenolic ring Mix Epoxicure resin and hardener. Buehler PN 20-8130-032 and 20-8132-08 respectively. Use a 5:1 resin to hardener weight ratio. Mix thoroughly. Fill the phenolic ring with prepared resin and allow time to cure. De-tool the glass plate and sand off excess resin flush with the ring.

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Table 3e. Hand polishing specimens 1 2 3 Put the sample on flat table and apply a moderate pressure. Start with 320 grit Move the sample away from you in a level forward motion while applying force After 5 min of repetitive strokes in the same direction, check the sample for uniform abrasion in the direction of motion. If uniform, continue to step 4. If not 4 5 6 7 8 9 uniform repeat from step 1. Rotate the sample 90 degrees so that the next step sands perpendicular Repeat steps 1 through 4 for 240, 0, 00 and 000 sandpapers. Apply diamond paste or desired abrasive suspension to polishing pad. Turn on polishing wheel and place sample with a light force on the pad. Rotate the sample CW while polishing for 5 minutes. Rinse 3 times with Distilled water then 3 times with isopropyl alcohol

Table 3f. Three-point-bending sample preparation 30

1 2 3 4 5 6

Cut Bar-stock material to 5 lengths. Chamfer cut edges. Cut .05 deep circular slot at the center of the sample using a 0.25 diameter ball mill. Sand specimen to appropriate roughness. Measure sample using the profilometer and ensure the sample conforms to statistical specifications in section 3.4 Designed Experiment Rinse sample 3 times with Distilled water then 3 times with isopropyl alcohol Coat a 0.375 inside diameter ring of Ameron Americoat 90HS Pearl Grey Resin system on the sample face located at the critical fiber. The system requires a 4

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part resin to one part curing agent volumetric ratio. Allow overnight cure. Glue 0.5 outside diameter, .375 inside diameter fluid reservoir to cured resin ring. Use 100% silicon adhesive and ensure glue does not contact the inside of

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grey resin ring. Allow overnight cure. Attach a thin copper wire one end of the sample using Locktite 3888 2-Part conductive adhesive. Allow overnight cure.

3.2 Experimental Setup: In-situ Open Circuit Potential (OCP) tests were performed using the Shimadzu Autograph AG-IS mechanical properties test machine and a Gamry Series G-750 potentiostat driven by Gamry Framework 5 software. The primary operation of the

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Shimadzu Autograph AG-IS machine is tensile testing. Exacting control of crosshead motion on the machine does not allow axial testing.

A reversed 3-Point Bending test was designed to perform fatigue tests. The sample was placed exactly center on the fixture. The Gamry Series G-750 potentiostat was utilized with connection to the sample as the working electrode. The reference electrode was placed in the electrolyte. The setup utilized a stand to fix the reference electrode into place.

Figure 3a. Specimen Configuration

3.3 Fatigue Design Criteria: Using the Modified Goodman Equation, various maximum and minimum forces were calculated for the Shimadzu Autograph AG-IS mechanical properties test machine as can be seen in table 3f. 32

Forces 1200 N and under were eliminated because they were below the minimum force criteria for crosshead-to-sample mating stability. Stresses that were above 40 ksi were eliminated for exceeding the yield strength of the material. The ideal operating condition was selected from the center of the remaining operating points.

The forces programmed into the Shimadzu Autograph AG-IS are Fmin = 1754 N and Fmax = 2959 N. The test machine moves alternately between these two forces to create a mean stress of 30 ksi and an alternating stress of 7.67 ksi. The corresponding stress ratio R is 0.59.

Table 3g. Potential operating conditions for fatigue testingSm (ksi) 7 8 9 10 Sa (ksi) 19.42 18.91 18.40 17.89 Smin (ksi) -12.42 -10.91 -9.40 -7.89 Smax (ksi) 26.42 26.91 27.40 27.89 Fmin (N) -976 -857 -738 -620 Fmax (N) 2075 2114 2152 2191 R -0.47 -0.41 -0.34 -0.28

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11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

17.38 16.87 16.36 15.84 15.33 14.82 14.31 13.80 13.29 12.78 12.27 11.76 11.24 10.73 10.22 9.71 9.20 8.69 8.18 7.67 7.16 6.64 6.13 5.62 5.11 4.60 4.09 3.58 3.07 2.56 2.04 1.53 1.02 0.51 0.00

-6.38 -4.87 -3.36 -1.84 -0.33 1.18 2.69 4.20 5.71 7.22 8.73 10.24 11.76 13.27 14.78 16.29 17.80 19.31 20.82 22.33 23.84 25.36 26.87 28.38 29.89 31.40 32.91 34.42 35.93 37.44 38.96 40.47 41.98 43.49 45.00

28.38 28.87 29.36 29.84 30.33 30.82 31.31 31.80 32.29 32.78 33.27 33.76 34.24 34.73 35.22 35.71 36.20 36.69 37.18 37.67 38.16 38.64 39.13 39.62 40.11 40.60 41.09 41.58 42.07 42.56 43.04 43.53 44.02 44.51 45.00

-501 -382 -264 -145 -26 93 211 330 449 567 686 805 923 1042 1161 1279 1398 1517 1635 1754 1873 1992 2110 2229 2348 2466 2585 2704 2822 2941 3060 3178 3297 3416 3535

2229 2267 2306 2344 2383 2421 2459 2498 2536 2575 2613 2651 2690 2728 2767 2805 2843 2882 2920 2959 2997 3035 3074 3112 3151 3189 3227 3266 3304 3343 3381 3419 3458 3496 3535

-0.22 -0.17 -0.11 -0.06 -0.01 0.04 0.09 0.13 0.18 0.22 0.26 0.30 0.34 0.38 0.42 0.46 0.49 0.53 0.56 0.59 0.62 0.66 0.69 0.72 0.75 0.77 0.80 0.83 0.85 0.88 0.91 0.93 0.95 0.98 1.00

3.4 Designed Experiment (DOE): Range of surface roughness variation was conducted using a Surfcom-480A stylus type profilometer. The low range of the experimental field was determined from 6 measured Ra values of a sample that was abraded on 320 grit paper with light pressure. The high range was determined from 6 measurements of a sample that was abraded on 240 grit 34

paper with high pressure. The mean and standard deviations of the measurements were calculated and are shown in table 3g. The mean to standard deviation ratio of both data sets are less than 1% different, indicating that linear interpolations of mean and standard deviation target points within this range are acceptable. The coded Ra values can be seen in table 3h. Table 3h. Calculated roughness experimental rangeMeas. # 1 2 3 4 5 6 Mean ==> Sd => LOW - 320L Ra in u" 17.68 15.51 16.82 15.9 18.13 14.01 16.34 1.52 HIGH - 240H Ra in u" 53.7 47.55 41.17 52.8 50.5 48.68 49.07 4.52

Table 3i. Coded Ra values for DOE in Coded ==> Mean ==> Sd => -1.41 16.34 1.52 -1 21.10 1.96 0 32.71 3.02 1 44.31 4.08 1.41 49.07 4.52

Every sample was checked on a statistical base for conformity to the roughness with a comparative t test at a 95% Confidence Level. A t distribution was used to determine acceptance of the sample [21].

Ho: 1 = 2

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t = (ybar1-ybar2) / ( Sp x (1/n1 + 1/n2)(1/2))

Where t is the test statistic, ybar1 and ybar2 are the comparative means, Sp is the standard deviation, n1 and n2 are the number of measurements taken. Three measurements were taken for each sample. Samples with standard deviations 10% or greater than the previously calculated standard deviation were automatically rejected. The comparative t* value for 7 DOF and 95% confidence is 2.365. Samples that did not conform were reworked and measured again.

The NaCl solution percent mass (%wt) experimental field ideally contained 0 to 3.5+ percent concentrations. The experimental key revolved the experiment around 3.5 percent. . Formulation of the mixture assumed that one milliliter of H2O equals one gram of H20 and the density of NaCl is 2.16 grams per cubic centimeter. One liter mixtures were made as shown in table 3j.

Table 3j. Experimental key for percentage composition of NaClCoded ==> % Conc. => -1.41 0 -1 1.02 0 3.50 1 5.98 1.41 7

Table 3k. Solution mass and volumesNaCl (g) 10.257 35.671 NaCl (cm^3) 4.75 16.51 H2O (g=ml) 995.25 983.49 Total (g) 1005.51 1019.16 wt% 1.02 3.50

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61.785 72.735

28.60 33.67

971.40 966.33

1033.18 1039.06

5.98 7.00

A randomized run order was generated. This was performed to prevent lurking variables that could negatively influence the experiment unknowingly. The coded experimental settings were decoded and reordered in Table 3k.

Table 3l. Experimental run orderrun # 1 2 3 4 5 6 7 8 9 Ra - u" 21.10 21.10 16.34 32.71 49.07 44.31 32.71 32.71 32.71 % Conc. 1.02 5.98 3.5 0 3.5 5.98 3.5 3.5 3.5

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10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

16.34 21.10 44.31 32.71 16.34 44.31 21.10 49.07 32.71 49.07 32.71 32.71 44.31 21.10 32.71 32.71 21.10 44.31 32.71 32.71 44.31

3.5 5.98 1.02 3.5 3.5 1.02 1.02 3.5 0 3.5 3.5 7 1.02 1.02 3.5 0 5.98 5.98 7 7 5.98

CHAPTER 4 RESULTS AND DISCUSSION 4.1 Three Point Bending: A sample of AA 6061-T6 was prepared for stand alone fatigue testing. The sample was loaded with Fmin = 1754 N and Fmax = 2959 N Shimadzu Autograph AG-IS machine and tested to failure. This occurred prematurely. Visual inspection clearly indicated severe 38

plastic deformation prior to failure. The failure mode was no longer tensile but complex. Plastic pileup had occurred and the test is invalid. Subsequent tests with variations of the loading were proven fruitless as well. It is clear that AA6061-T651 is too ductile to test in the original 3-point configuration for low cycle fatigue.

4.2 In-Situ Open Circuit Potential of Reversed 3-Point Cyclic Bending: The samples were tested in 0.5 M NaCl solution and de-ionized water. After a stable OCP reading was achieved, the Shimadzu Autograph AG-IS machine was started. Specimens lost fluid from the cell reservoir prior to failure. The sample experience plastic pileup and deformed. As this had occurred, the protective paint cracked and the reservoir failed to hold the solution. The OCP curves were as expected for the short duration the reservoirs were intact.

4.3 Micro-structural Evaluations Scanning Electron Microscopy (SEM) can be used to analyze failures and determine root cause. Below is an image of a steel specimen that was broke via cyclic torsion. Note the beach marks that can be seen on the surface that are indicative of classical fatigue failure.

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Figure 4a. SEM image of torsion fatigue in steel.

Grain structure, distribution, size, shape and even composition can be determined from the use of various etchants and optical microscopic techniques. Two samples of AA6061T651 were welded using a TIG welding process and FSW process. The samples were polished and treated with Kellers etchant solution as outlined in chapter 3.

Figure 4b. FSW and TIG welded samples treated with Kellers etchant

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As can be seen in the photographs, the TIG welded sample did not fully penetrate the barstock. The grain microstructure is much larger than in the FSW sample and the transition from HAZ to the base alloy very abrupt where in the FSW sample, the HAZ microstructure is not nearly as evolved with smaller grains and is more transitional to the base alloy. Kellers etchant works well for AA6061T651 as it attacks the alloy intergranularly. Grains that evolve will precipitate more into the grain boundaries thus leaving more distinct traces on the surface.

4.4 Potentiodynamic Testing Polarization curves were generated for aerated and de-aerated AA 6061-T651 specimens in 0.5 M NaCl solution. The de-aeration was performed by nitrogen percolation and aeration with compressed air through the solution for 30 minutes prior and throughout the test. The samples were prepared as described in chapter 3.

Figure 4c. AA 6061-T651 Sample prepared for potentiodynamic testing

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Figure 4d. Polarization curve for AA 6061-T651 base alloy

The de-aerated curve is as expected for the range of the test. The aerated sample does not include the needed range. If the sweep was adjusted to -500 mV for the upper range on both aerated and de-aerated samples, normal should be readily obtainable. [30] CHAPTER 5 STANDARDS 5.1 Electrochemical Aspects One of the primary concerns with electrochemical monitoring is that of solution chemistry. As a sample is put into a corrosive environment, metal dissolution into the solution can occur. The rate of dissolution can be affected by concentration of metal ion in solution. It is imperative to have an ample amount of solution for these experiments. In 42

addition to metal ion contamination of the solution, a saturated calomel electrode is used to monitor OCP readings. This electrode must be set to have 3 micro-liters per hour leakage to ensure proper function as per ASTM G5.

The previous configuration used solution reservoirs that held 1.25 ml of solution. This amount is too low. ASTM G5 proposes a test cell configuration with 900 ml of solution. As a standard practice, an electrochemical test cell should be designed to hold 1L of solution. Electrodes, samples and other objects will displace some of this solution.

ASTM G5 calls out the usage of Type IV reagent water as per ASTM D1193. This standard calls out for the use of DI water with the following properties.

Table 5a. ASTM D1193 callout for Type IV grade reagent water Conductivity < 5 uS/cm Resistivity < .2 M/cm pH of 5 to 8 Sodium < 50 ug/L Chloride < 50 ug/L

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Solutions are to be prepared in a 1 L volumetric flask. To prepare a proper solution, first add 58.44 x desired molarity in grams of NaCl to the empty flask. Add ASTM D1193 type IV reagent water to the solution in part. Heating and agitation may be needed to dissolve the salt crystals. When the crystals are dissolved, fill the flask to the point where the bottom of the meniscus is at the fill line. Ensue the final volume is filled at room temperature as the previous heating to encourage dissolution expands the water in the flask. This could give inaccurate solution concentrations.

The working electrode in this experiment differs from that of ASTM G5s sample experimental set-up. Ensure the working electrode is electrically connected to the sample under study outside of the tested solution.

Prior to testing, the solution should be percolated with 150 cm^2/min of Nitrogen gas as per ASTM G5 for de-aeration or the same flow rate of compressed air for aerated studies. This ensures a uniform amount of dissolved gas in the solution for standardization and reproducibility of the experimental data. This should be performed for a minimum of 0.5 hrs as per ASTM G5.

The solutions temperate should be 30 +/- 1 degree C as per ASTM G5. A convenient way to perform this is to mount a glass lined immersion heater inside the test cell.

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5.2 Fatigue Aspects When machining the test section, the radius of curvature for reducing thickness must be equal to or greater than 8 times the diameter of test section as per ASTM E466. This gradual transition from one diameter to another is intended to reduce the stress concentration factor in the necked areas that would ultimately lead to failures outside the designed area.

Once specimen geometry has been chosen, it is important not to change this geometry for the duration of the investigation. ASTM E466 indicates that the final material removal of the specimen is to be performed parallel to the long axis of the specimen. This can pose a time consuming task for specimens that are cylindrical and turned on a lathe. Extensive hand sanding may be needed to ensure the standard is followed. Test specimens are to be visually inspected prior to installation in the test machine as per ASTM E466. Abnormalities such as cracks, scratches and gauges in the area under study increase the risk of premature failure. This failure will influence the validity of the experimental data.

5.3 Reporting Data Specimen shape, size and dimensions are to be reported in accordance with ASTM E468. This information is to include test machine gripping and specimen orientation. It is important to record surface preparation technique, roughness measurements and the time 45

between preparation and immersion into solution for these experiments as aluminum alloys for passive layers that exhibit different OCP readings immediately after preparation compared to several hours after preparation. ASTM E468 stresses the importance of recording the details of the last material removal operation in particular.

Machine type, test type and details of the forces used and waveform shape are reported as per ASTM E468. This is important to duplicate results and claims. ASTM E468 suggests that S-N curves and constant life diagrams include the R value of testing, test frequency, environmental conditions and selected material properties.

Cycle counting is outlined in ASTM E1049. If an R value of -1 is chosen, peak reporting and counting is relatively easy. Complex loadings require a large degree of attention to this standard.

Refer to ASTM E739 for details on statistical analysis. DOE alone is not a substitution to standardized statistical analysis of data, but serves as an excellent compliment to reports.

Solution temperature should be recorded at the start of the experiment and periodically throughout it until the end. One half hour increments should suffice. ASTM E648 mentions to record the environmental conditions. Greater detail in environmental condition reporting is needed as the environment is engineered to influence the failure. 46

Reference electrode potentials other than saturated calomel and hydrogen are reported with conversion boxes provided to the reader as per ASTM G3. Where anodic and cathodic currents simultaneously exist, ASTM G3 recommends assignment of cathodic current densities a negative value for differentiation of data. Details for reporting various polarization plots can be obtained in ASTM G3.

CHAPTER 6 CONCLUSION

This writer has looked at countless test types and possible configurations to reiterate the original design into a better data gathering rig. UND does not currently have proper test equipment for the proposed rig. As this work is a literary review with recommendations, this writer is not limiting the proposed rig to resources at hand.

The proposed rig consists of a 4- point rotating-bending fatigue style test machine. Specimens are now under area of failure rather than a cross-section. Necking of the specimen will control the area of failure as will the location of the Americoat paint. The 47

load amplitude control with rotating bending fatigue machines is excellent. A few concerns arise with the use of this machine for aqueous testing.

The first concern involves the ability of the specimen to maintain electrical continuity to the working electrode of the potentiostat. As the rotating-bending test specimen is turning, it is difficult to physically maintain contact. A possible solution to this problem is to press a conductive lubricant packed bearing on the specimen. The case of the bearing can then be electrically connected to the potentiostat without adverse effects.

The second concern involves the problem of test machine iteration with the solution reservoir. One proposed solution to this problem involves the use of water tight bearings. These bearings will be placed on the specimen in both locations where the wall of the reservoir meets the specimen. These bearings must not support any load. To minimize this risk, a large hole can be cut in the sides of the reservoir and the hole is sealed with a flexible rubber matt with the bearing mounted to it.

One other concern involves the speed of the rotating bending machine. As the machine rotates, it acts as if the solution is a flowing liquid. A low speed will make this effect minimized, but experimentation is needed to see how much this actually affects the testing condition. It could possibly exacerbate frequency related fatigue effects.

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Figure 6a. Proposed Test Rig Configuration

With a new proposed test configuration and detailed knowledge on standards and statistical analysis complete with DOE, one could successfully implement a very worthy research endeavor into OCP during cyclic fatigue. Results from this style of testing can be used to help formulate optimal alloy systems for corrosion resistance.

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