Corrosion of Titanium Alloys - Laser Cladding of Titanium and its Alloys

CHAPTER 2 THE LITERATURE REVIEW

2.6 Laser Cladding of Titanium and its Alloys

2.6.3 Corrosion of Titanium Alloys

Corrosion studies for titanium alloys will be reviewed to gain a better understanding of how laser cladding could affect the corrosion properties of the titanium.

The first study under review compares the corrosion properties of pure titanium to that of its alloys and is titled: Comparison of the corrosion behaviour of pure titanium and its alloys in fluoride-containing sulphuric acid [72]. The alloys tested along with pure titanium are Ti-0.2Pd and Ti-0.3Mo-0.8N, which were as mentioned in the title tested using a fluoride containing sulphuric acid. In summary it was found that the alloying components did in fact have a positive effect on the corrosion resistance characteristics of the titanium, however this was only observed for concentrations of fluoride lower than 0.002M, higher concentrations showed similar corrosion characteristics throughout [72].

28 The study titled: Corrosion of titanium alloys in high temperature near anaerobic Seawater [73], investigates the corrosion properties of grades 2, 5 and 7 titanium alloys in seawater up to 200°C. It was found that all three grades were resistance to stress corrosion cracking and pitting corrosion, however grades 2 and 5 did start to show signs of crevice corrosion at 80°C and 200°C respectively with no signs of crevice corrosion at lower temperatures. The potentiodynamic polarization curves obtained for Ti6Al4V is shown in Figure 2.18.

Figure 2.18: Potentiodynamic polarization curve in seawater at 160 °C for Ti6Al4V [73].

A. Obadele investigated the effect of scanning speed on the corrosive properties of a commercially pure titanium laser cladded coating and compared this to the corrosion behaviour of Ti6Al4V in the study: Electrochemical behaviour of laser-clad Ti6Al4V with CP Ti in 0.1 M oxalic acid solution [74]. The scanning speed of the clad was varied between 0.4, 0.6 and 0.8m/min, while the laser power, gas and powder flow rates were kept constant. Results obtained in this study showed that the clad obtained at a scanning speed of 0.4m/min formed a good cladding layer with the least amount of defects in the form of porosities and cracks.

The corrosion potentials of the laser cladded samples was lower than Ti6Al4V alloy, however the potentiodynamic polarisation measurement (Figure 2.19) showed that the anodic current density was lower than that of Ti6Al4V alloy and also increases with increasing laser scanning speed, which relates to an increase in corrosion rate.

Figure 2.19: Potentiodynamic Polarisation of Ti6Al4V substrate and laser-clad samples in 0.1 M oxalic acid [74].

29 The next study reviewed focused on remelting titanium alloys using a high powered diode laser, while this approach is not actual laser cladding it does provide a good view of how various properties of the titanium evolve with the addition of localized laser energy. The study reviewed is titled: Microstructure, microhardness and corrosion resistance of remelted TiG2and Ti6Al4V by a high power diode laser [75]. For this study the laser power was varied between 2.0kW and 2.5kW while the scanning speed was varied between 0.25m/min and 8m/min. This large difference in the scanning speed means that the energy input during the laser remelting will vary significantly between the various samples. It was found through this study that through laser remelting Ti6Al4V samples, martensite was obtained with proper laser fluence, the martensitic microstructure displayed a significant microhardness increase and corrosion resistance improvement, as shown in Figure 2.20.

Figure 2.20: Potentiodynamic Polarisation curves of Ti6Al4V samples: BM=Base Material; T4=2.5kW + 1m/min;

T8=2kW + 8m/min [75].

2.7 Summary

The literature section investigated laser technologies, as well as the laser-material processing of metals, specifically titanium. The titanium alloy under investigation for this study was also mentioned in the literature, in order to better understand the physical characteristics of the alloy; and therefore enable a proper analysis of the obtained results in Chapter Four.

Section 2.6 exclusively focused on the review of past publications related to the laser processing and titanium alloys. The present study will focus on optimizing the scanning speed of the laser-cladding process, which would expand on the current literature available.

Furthermore, through this extensive review, the correlation between the various parameters used when laser cladding and the material / clad properties were clearly identified. The information in this chapter will further enable proper analysis of the results obtained; and these results will be discussed in Chapter four.

CHAPTER 3 EXPERIMENTAL DESIGN AND SET-UP

3.1 Introduction

In this chapter, the apparatus used and the methodology followed in order to successfully obtain the required data will be discussed. Firstly, the creation of the samples using the Laser-cladding process is discussed. Thereafter, the process and tests followed to do the material characterization; and ultimately to obtain the information required for the study.

3.2 Laser-Cladding of the Sample

3.2.1 Apparatus

The laser cladding was carried out at the national laser centre at the Council for Scientific and Industrial Research (CSIR) in Pretoria, South Africa. The apparatus comprised a 4kW Nd:YAG diode laser attached to a Kuka Robot arm. Figures 3.1, 3.2 and 3.3 show the apparatus used at the CSIR, along with annotations to the various parts of the Laser and the Robotic arm combination.

Figure 3.1: Robot Arm Setup at the CSIR Laser Centre

KUKA Robot Arm

Coaxial Nozzle End Effector - Laser Head

Figure 3.2: Coaxial Laser Head

Figure 3.3: Laser Power Source

3.2.2 Methodology

Seven cladding tracks were made in total, starting with a scanning speed of 3.5m/min and decreasing the scanning speed by 0.5m/min for each consecutive track down to 0.5m/min.

Argon Shield Gas

Powder Feed Line

32 The laser power was kept constant at 1000W, along with the gas flow rate and the powder flow rate. In this way, it was ensured that the data obtained only showed the effect the decreasing scanning speed had on the clad quality and its efficiency. Table 3.1 below summarises the operating parameters used for each sample.

Table 3.1: Laser-Cladding Process Parameters

3.3 Metallurgical Examination and Characterization

3.3.1 Sample Preparation

The sample preparation is discussed in detail in this section; the samples were prepared, in accordance with ASTM E3-11, as well as the application guide for the Struers mounting press [76, 77].

3.3.1.1 Cutting

The first step in preparing the laser-cladded samples is cutting the plate with the cladded lines into smaller pieces that can be further prepared to be characterised by using various techniques. The cutting of the titanium was done using the Mecatome T300 cutting machine.

It was operated manually; and the cutting disks used were specifically selected for the purpose of cutting titanium. The rotational speed of the cutter was set to 3800RPM, and to ensure the cutting disk and the material being cut stayed cool a constant stream of water was directed onto the material, where the disk and the material made contact.

In total, 32 samples were produced: 4 samples for each of the 7 laser-scanning speeds, and 4 samples for the base material. Figure 3.4 shows the cutting machine; and Figure 3.5 depicts the cut samples.

Figure 3.4: Mecatome T300 Cutting Machine

Figure 3.5: Cut Samples

34 3.3.1.2 Mounting

After cutting the samples; the next step was to mount the samples in the appropriate resin to enable ease of grinding and polishing; and also to later be able to observe the samples using SEM, OM – and to perform the microhardness testing. The resin chosen for mounting the samples is a thermosetting resin from Advanced Laboratory Solutions, called Aka Resin Phenolic SEM. This resin was chosen because of its conductive properties, which would later be vital in performing SEM analysis. The Mounting Press Machine used to mount the sample in the resin was a Struers mounting press (Figure 3.6). To ensure that there is no adhesion between the mount and the rams; a mould-release agent was first applied to the rams – to ensure that the mount is easily removable.

A graphical depiction of the process cylinder of this Struers mounting press is shown in Figure 3.7. The parameters for mounting the sample were obtained from the Application Guide Book that accompanied the press. This parameter’s used are as follows:

Table 3.2: Mounting Parameters [76].

Parameter Value

Sample Size Ø 30mm

Mounting Pressure 250 Bar Heating Temperature 180°C Heating Time 4 min Cooling Time 2 min

Figure 3.6: Struers Mounting Press

Figure 3.7: Struers Process Cylinder [78].

3.3.1.3 Grinding and Polishing

The samples will each undergo a three-step grinding process, followed by a two-step polishing process. Grinding was done with a downward force of 25N at 300RPM. The samples were ground using SiC bonded-grinding paper of 320, 800 and 1200 Grit. The grinding time varied.

For the first step; no time was allocated; instead the samples were ground until plane.

Thereafter, for both the 800 and 1200 grit paper, the samples were ground for a total of 10 to 12 minutes each. For the polishing of the samples; the same downward force of 25N was used; but the rotational speed of the machine was reduced to 150RPM. The first polishing step was done using 4000 grit grinding paper – for 5 minutes with water as the lubricant. The final polishing step was done using an MD-Chem polishing disk, with 0.4µm colloidal silica suspension (OP-S).

After polishing the samples were rinsed with distilled water; then Acetone; and they were immediately dried using a warm-air dryer. Figure 3.8 shows the Struers Polishing machine used; while Figure 3.9 shows the completed samples with a mirror-like finish.

Figure 3.8: Struers Polishing Machine

Figure 3.9: Finished Samples

3.3.2 Optical Microscopy

Before OM the samples were etched. Etching the sample is critical; since it allows for the viewing of the grain structure of the material, Different materials require different etchants for optimal etching; and in some cases, if the proper etchant is not used, the sample might not react at all; and etching would not be possible. Kroll’s regent was used; as it is suitable for titanium alloys. The etchant comprises 100ml distilled water, 3ml Hydrofluoric acid and 6ml Nitric acid. The samples were etched between 1min to 1min 30 seconds, depending on the visibility of the microstructure. After the etchant; the samples were again rinsed with distilled water and dried.

The Optical Microscopic Examination was carried out using the Olympus BX51M optical microscope (Figure 3.10), along with the Olympus stream software. These samples were analysed under various magnifications – to obtain a clear image of the microstructure; to identify the various zones, as well as to analyse the porosity of the cladded samples.

Figure 3.10: Olympus BX51M Optical Microscope

37 3.3.3 SEM

The Scanning Electron Microscope (SEM) is capable of magnifications much greater than that of the Optical Microscope. This high-resolution electron microscope uses a focused ion beam to produce an image of the sample – by scanning the surface with a high-energy beam of electrons. This technology allows for the observation of the sample at scales ranging from nanometres to hundreds of microns. This enables the observation of details, such as the elemental composition, microstructure, porosity and cracking in the sample. The Scanning-Electron Microscope used is situated at the Doornfontein Campus of the University of Johannesburg (UJ). Figure 3.11 shows the TESCAN microscope used.

Figure 3.11: TESCAN Scanning-Electron Microscope (UJ Doornfontein Campus)

38 3.3.4 Micro-Hardness Testing

For the micro-hardness testing, the same samples that were used for the OM and SEM can be used. However, before use, the etching will need to be destroyed. To achieve this, the final polishing step must be repeated for all the samples.

The hardness of the cladded samples was determined by using the TIME digital micro-hardness tester, as shown in Figure 3.12. The testing force was set to 500g or 4.9N; and the dwell time was 15 seconds. The hardness tests were conducted, according to the ASTM E384-11 standard [79]. Multiple tests were done for each sample: first horizontally across the clad;

and thereafter vertically passing through the clad, the HAZ and ending at the base material, as shown in Figure 3.13.

Figure 3.12: TIME Digital Microhardness Tester

Figure 3.13: Hardness testing pattern for Cladded Sample

3.4 Atomic-Force Microscopy

The Atomic-Force Microscopy (AFM) measurement is done to determine a variety of properties of the clad that was produced. For the purpose of this thesis, the AFM will be used to determine the surface roughness of the samples. The relevant standard was used for the size measurement of nanoparticles. This includes surface-roughness measurements by the ASTM E2859-11(2017) [80]. For this test, no prior preparation of the sample is required; the cut piece only has to be cleaned, and then placed with the clad facing upwards, Figure 3.15 shows how the sample is placed.

Figure 3.14: Atomic-Force Microscope Block Diagram [81].

40 Atomic Force Microscopes (AFMs) typically make use of the laser-beam deflection system, as shown in Figure 3.14 above. The Laser beam is reflected from the back of the reflective AFM lever and onto a photodiode, or to a position-sensitive detector. The AFM tip and Cantilever are typically micro-fabricated from Si or Si3N4 [82].

Traditionally, most AFMs use a laser-beam deflection system where, a laser is reflected from the back of the reflective AFM lever, and thence onto a position-sensitive detector. The tip is typically a 3-6µm tall pyramid with a 15-40nm tip radius [82].

For the AFM to produce the images, it relies on the force measured between the tip and the sample. This force is not measured directly; but it is calculated by measuring the deflection of the cantilever and knowing the stiffness of the cantilever. Hooke’s law is used to calculate the force, as follows [82]:

𝐹 = −𝑘𝑧 (1)

Where F is the force; k the stiffness of the lever; and Z is the distance the lever is bent.

To ensure that the cantilever does not strain too much and break; or permanently deform, the AFM has a built-in feedback loop that uses the laser deflection to control the force and the tip position. As the tip interacts with the surface of the sample, the laser position on the photo detector is used, along with the force measurement for the imaging and the measuring of the surface. The Atomic-Force Microscope at the University of the Witwatersrand (Wits). Figure 3.16 shows the Microscope used.

Figure 3.15: Sample Placement for AFM (Wits University)

Figure 3.16: Atomic Force Microscope

3.5 Corrosion Resistance Testing

Potentiodynamic Polarization, whereby the sample is placed in an electrolyte; and then a current is introduced to accelerate the corrosion process, was implemented in order to determine the corrosion resistance (or rate of corrosion) of the various cladded samples. This test was conducted, in accordance with ASTM G59 – 97 [83].

3.5.1 Apparatus

The apparatus used comprised the following components (Figure 3.17 and 3.18):

 Working Electrode (WE)

 Counter Electrode (CE)

 Reference Electrode (RE)

 Electrolyte

 Potentiostat

 Beaker

 Connecting cables

3.5.2 Sample Preparation

The first step in the sample preparation is to attach a piece of wire to the sample, in such a way that it can conduct electricity. To achieve this; the wire was first soldered to the back of the sample; but this does not provide sufficient bonding strength; and therefore, the wire is also glued to the sample by using a metal epoxy glue. After the glue has set in approximately 30 min (depending on the glue used); the next step can be performed. In order to achieve good results, the cladded section of each sample has to be insulated. This is done by applying tape around the block of material – until only the clad is exposed. This will ensure that the

42 results are not skewed; because the base material is also participating in the chemical reaction.

3.5.3 Methodology

Firstly, the Open Circuit Potential (OCP) for the sample has to be determined. Thereafter, the corrosion testing can commence, whereby the Tafel Plot will be generated. To determine the OCP the set-up is as follows:

 Place the electrolyte in a beaker;

 Connect working electrode (black wire) and the counter electrode (wire colour).

 Place the electrode in the beaker, ensuring that there is no contact between the different electrodes.

 The reference electrode must not be connected during this phase.

 Input the desired parameters into the software; and start the OCP measurement.

 This should run for 1800 seconds or 30 min – to ensure that the results have converged.

Figure 3.17: Corrosion Resistance Testing Setup Schematic [84].

Next, the corrosion rate is determined by generating the Tafel plot. Now the reference electrode is added to the electrolyte inside the beaker; and the experiment is run at a rate of 0.1mV/sec. The Tafel plots are obtained and placed onto one graph for comparison. Figure 3.17 shows the typical set-up for this experiment. In addition to the Tafel plot, the Corrosion Rate can be calculated by using the Electrochemical Measurements, obtained during the corrosion testing. The corrosion rate is calculated as follows:

First, the corrosion-current density is determined. This calculation can be expressed as follows [85]:

𝑖_𝑐𝑜𝑟=𝐼_𝑐𝑜𝑟

𝐴 (2)

43 Where:

icor = corrosion’s current density, μA/cm², Icor = total anodic current, μA, and

A = exposed specimen area, cm².

Thereafter, the alloy’s equivalent weight is determined by using the following approach:

𝑄 = ∑𝑛𝑖𝑓𝑖

𝑊𝑖 (3)

Where:

fi= the mass fraction of the i^th element in the alloy, Wi= the atomic weight of the i^th element in the alloy, and ni = the valence of the i^th element of the alloy.

Then, the equivalent weight can be determined by taking the reciprocal of this quantity (Q):

𝐸𝑊 =1

𝑄 (4)

With the corrosion-current density and the equivalent weight determined, the corrosion rate (CR) can now be calculated using:

𝐶𝑅 = 𝐾_𝑖𝑖_𝑐𝑜𝑟

𝜌 𝐸𝑊 (5)

Where:

CR is given in mm/yr, and

K1 = 3.27 x 10-3, mm g/μA cm yr ρ = density in g/cm³

Figure 3.18: Corrosion Resistance Test Set-up

3.6 Summary

All experiments, tests and steps taken to obtain the relevant data have been executed; while carefully ensuring that the quality of the data is maintained at a high standard. The experiments and measurement have also been conducted in conformity with the relevant standards. The equipment used have all been calibrated; and they are calibrated regularly by the relevant institution, in order to ensure accurate readings, and data for all experimentation.

This chapter has elaborated on the tests that would be done by explaining the process followed; and the parameters selected for acquiring the data, or preparing the samples. It is vital that the methodology followed be clearly communicated for future reference, as well as duplication of the tests by another party, if required. The results obtained are reported and discussed in Chapter Four.

CHAPTER 4 RESULTS AND DISCUSSION

4.1 Introduction

The samples were analysed by using various methods, including structural / geometrical analysis, Microstructural characterisation, SEM, AFM, Microhardness testing and corrosion testing. The results obtained from each of these methods were analysed; and they will discussed in this chapter. Analysis was done by comparing the results obtained for each sample; and by comparing these results with those found in the reviewed literature. This was done; in order to determine which processing parameters used were the most efficient and effective.

4.2 Physical Appearance

Visual inspection of the cladded samples revealed no obvious defects in the clad lines. Figure 4.1 shows an overview of the cladded-titanium alloy after cutting. The clads did appear to be well-bonded to the base material – with a constant tract thickness along the length of each clad. Initial inspection also revealed a clear distinction between the varying scanning speeds that were used – with the slower scanning speed leaving a wider deposit; and the faster

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