Material characterization and analysis of laser cladding of titanium alloy

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(1)COPYRIGHT AND CITATION CONSIDERATIONS FOR THIS THESIS/ DISSERTATION. o Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. o NonCommercial — You may not use the material for commercial purposes.. o ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.. How to cite this thesis Surname, Initial(s). (2012) Title of the thesis or dissertation. PhD. (Chemistry)/ M.Sc. (Physics)/ M.A. (Philosophy)/M.Com. (Finance) etc. [Unpublished]: University of Johannesburg. Retrieved from: https://ujcontent.uj.ac.za/vital/access/manager/Index?site_name=Research%20Output (Accessed: Date)..

(2) Material characterization and analysis of laser cladding of Titanium alloy. R. Du Plooy BEng 201201825. Submission for Dissertation in fulfilment of the requirements for the degree M.Ing in Mechanical Engineering at the University of Johannesburg. Supervisor: Prof. E. T. Akinlabi. January, 2018.

(3) ABSTRACT This study focused on laser cladding of Ti6Al4V, used as both the substrate and the powder material. The goal was to determine the most effective parameters for laser cladding, when using this alloy. The criteria used were both efficiency (i.e. how quickly a certain amount of area / volume could be cladded) and quality. To determine the most efficient and the highest quality cladding, the following was done: firstly, the samples were prepared by cladding multiple tracks, using varying scanning speeds for each track. The scanning speed was varied from 3.5m/min to 0.5m/min in increments of 0.5m/min; while the laser power, powder flow rate and gas flow rate was kept constant. Thereafter, a geometrical analysis, porosity analysis, microscopic analysis in the form of optical microscopy and scanning electron microscopy, atomic force microscopic analysis, Vickers microhardness testing and corrosion testing were all performed. One observation made from the optical and scanning electron microscopy was that the microstructure of the cladded area was consistent for sample scanning speeds ranging from 0.5m/min to 2.0m/min. This consistency also showed in the microhardness testing with the average hardness values for the scanning speeds ranging from 0.5m/min to 2.0m/min being almost equal at approximately 340HV. Corrosion resistance testing confirmed that the corrosion resistance of the clad was related to the scanning speed at which the clad tack was produced. In general it was shown that the slower the scanning speed the better the corrosion resistance. Further geometrical analysis of the micrographs obtained through optical microscopy revealed that sample five, with a scanning speed of 1.5m/min, yielded the best all-round clad, being the most efficient clad for the quality produced. The clads with higher scanning speeds, between 2.0m/min and 3.5m/min, could achieve area cladding rates greater than 50 000mm2/min; however the quality of the clad was not acceptable. The clads produced with slower scanning speeds (0.5 and 1.0m/min), did not produce a significant (or any) increase in the quality of the clad, when being compared with sample five. Overall this work proved successful; and it also yielded a number of ways whereby the current understanding and capabilities when considering the Laser Cladding of Titanium Alloys can be further improved.. ii.

(4) ACKNOWLEDGEMENTS I would like to take the opportunity to express my sincere gratitude to all the people who helped make the completion of this dissertation possible. Their support and guidance was essential for the timely completion of this dissertation. . Prof. E.T. Akinlabi, my supervisor, has been guiding and motivating me to take the next step in my education since the final year of my BIng degree. She was the first to suggest that I pursue my Master’s degree; and her motivation, support and guidance throughout this endeavour have been invaluable.. . Mr. E. A. Marais, my manager. Pursuing further studies, while working full time is no easy feat. Mr. Marais supported my efforts from the very beginning and kept motivating me to continue and complete my dissertation. He was very understanding and supportive whenever I needed time off work to complete lab work etc. for my dissertation.. . My wife, Nadia du Plooy, for her constant support, patience and understanding. She always believed in me; and she motivated me continuously, which helped a great deal with the completion of this dissertation.. . My parents, for all their support and the sacrifices they made, in order to get me though school and university. Truly, I would not be where I am today without their unfailing support.. . Dr. Monnamme Tlotleng, for his assistance at the CSIR.. . Yinka Michael Abegunde, at the University of Johannesburg, for his assistance with the laboratory work for this dissertation.. iii.

(5) Table of Contents Plagiarism Declaration ........................................................................................................... i Abstract..................................................................................................................................ii Acknowledgements ............................................................................................................... iii Table of Contents..................................................................................................................iv List of Figures .......................................................................................................................ix List of Tables ........................................................................................................................xi CHAPTER 1 INTRODUCTION ............................................................................................. 1 1.1. Background............................................................................................................. 1. 1.2. Problem Statement ................................................................................................. 2. 1.3. Aim of Research ..................................................................................................... 3. 1.4. Research Objectives ............................................................................................... 3. 1.5. Scope of Work ........................................................................................................ 3. 1.6. Hypothesis .............................................................................................................. 4. 1.7. Summary ................................................................................................................ 4. CHAPTER 2 THE LITERATURE REVIEW ........................................................................... 5 2.1. Introduction ............................................................................................................. 5. 2.2. Introduction to Laser Technology ............................................................................ 5. 2.3. Laser-Process Technologies................................................................................... 7. 2.3.1. Laser Welding .................................................................................................. 7. 2.3.2. Laser-Metal Deposition .................................................................................... 7. 2.4. Laser-Surface Engineering ..................................................................................... 9. 2.4.1. Laser-Surface Melting .................................................................................... 10. 2.4.2. Laser-Surface Cladding ................................................................................. 11. 2.5. Titanium and its alloys .......................................................................................... 15. 2.5.1. Overview........................................................................................................ 15. 2.5.2. Metallurgy and Material Properties of Titanium Alloys .................................... 17. 2.6. Laser Cladding of Titanium and its Alloys ............................................................. 19. 2.6.1. Laser Processing of Titanium Alloys .............................................................. 19 iv.

(6) 2.6.2. Titanium Microstructure and Properties.......................................................... 24. 2.6.3. Corrosion of Titanium Alloys .......................................................................... 27. 2.7. Summary .............................................................................................................. 29. CHAPTER 3 EXPERIMENTAL DESIGN AND SET-UP ...................................................... 30 3.1. Introduction ........................................................................................................... 30. 3.2. Laser-Cladding of the Sample ............................................................................... 30. 3.2.1. Apparatus ...................................................................................................... 30. 3.2.2. Methodology .................................................................................................. 31. 3.3. Metallurgical Examination and Characterization .................................................... 32. 3.3.1. Sample Preparation ....................................................................................... 32. 3.3.2. Optical Microscopy......................................................................................... 36. 3.3.3. SEM............................................................................................................... 37. 3.3.4. Micro-Hardness Testing ................................................................................. 38. 3.4. Atomic-Force Microscopy...................................................................................... 39. 3.5. Corrosion Resistance Testing ............................................................................... 41. 3.5.1. Apparatus ...................................................................................................... 41. 3.5.2. Sample Preparation ....................................................................................... 41. 3.5.3. Methodology .................................................................................................. 42. 3.6. Summary .............................................................................................................. 44. CHAPTER 4 RESULTS AND DISCUSSION ....................................................................... 45 4.1. Introduction ........................................................................................................... 45. 4.2. Physical Appearance ............................................................................................ 45. 4.3. Macrostructural Analysis ....................................................................................... 45. 4.3.1. Porosity Analysis ........................................................................................... 47. 4.3.2. Clad Geometry............................................................................................... 47. 4.3.3. Cladding Efficiency ........................................................................................ 49. 4.4. Microstructural Characterisation............................................................................ 50. 4.5. Scanning Electron Microscopy .............................................................................. 54. 4.6. Atomic Force Microscopy ...................................................................................... 55 v.

(7) 4.7. Microhardness Testing .......................................................................................... 57. 4.8. Corrosion-Testing ................................................................................................. 59. 4.9. Summary .............................................................................................................. 60. CHAPTER 5 CONCLUSION AND RECOMMENDATIONS ................................................. 61 5.1. Introduction ........................................................................................................... 61. 5.2. Conclusions .......................................................................................................... 61. 5.3. Further Work ......................................................................................................... 62. References ......................................................................................................................... 64. vi.

(8) ABBREVIATIONS 3D. -. Three Dimensional. Al. -. Aluminium. AM. -. Additive Manufacturing. AMF. -. Additive Manufacturing File. AFM. -. Atomic Force Microscopy. α. -. Alpha Phase. BCC. -. Body-Centred Cubic. BIS. -. Beam-Interference Solidification. β. -. Beta Phase. C. -. Carbon. CO2. -. Carbon Dioxide. CAD. -. Computer-Aided Design. CP. -. Commercially Pure. DZ. -. Deposit Zone. FDM. -. Fused-Deposition Modelling. FMS. -. Flexible Manufacturing Systems. FS. -. Formative-Shaping Manufacturing. FZ. -. Fusion Zone. Fe. -. Iron. H. -. Hydrogen. HAZ. -. Heat-Affected Zone. HCP. -. Hexagonal Closed Packed. Kg. -. Kilogram. LAM. -. Laser-Additive Manufacturing. LENS. -. Laser-Engineered Net Shaping. LED. -. Light-Emitting Diode. LMD. -. Laser-Metal Deposition. LMIT. -. Laser-Metal Interaction Time. LOM. -. Laminated-Object Manufacturing. vii.

(9) LTP. -. Liquid Thermal Polymerization. Mo. -. Molybdenum. MZ. -. Melted Zone. N. -. Nitrogen. Nd:YAG. -. Neodymium-doped Yttrium Aluminium Garnet. O2. -. Oxygen. SL. -. Stereo Lithography. SLS. -. Selective Laser Sintering. Sn. -. Tin. Ti. -. Titanium. UMC. -. Unmelted Carbide. UV. -. Ultraviolet. V. -. Vanadium. viii.

(10) LIST OF FIGURES Figure 1.1: Coaxial Laser-Cladding Set-up [6] ....................................................................... 1 Figure 1.2: Lateral Laser-Cladding Set-up [4] ........................................................................ 2 Figure 2.1: Bohr Atom Model Emitting a Photon [15]. ............................................................ 6 Figure 2.2: Stimulated Emission [14]. .................................................................................... 6 Figure 2.3: a) LMD Wire Feed; b) LMD Lateral Powder Feed; c) LMD Symmetric Powder feed; d) Coaxial Powder Feed [21]. ....................................................................................... 8 Figure 2.4: Schematic diagram of DMD system with concurrently fed powder and wire material [22]. ......................................................................................................................... 9 Figure 2.5: Classification of Laser surface-engineering approaches [24]. ............................ 10 Figure 2.6: Schematic of Laser-Surface Melting process showing the melted region [26]. .. 11 Figure 2.7: Jet Engine, low pressure compressor and fan blades manufactured from Titanium Alloys [48]. ............................................................................................................ 16 Figure 2.8: Automotive components manufactured from Titanium Alloys [49]. .................... 16 Figure 2.9: Titanium-Frame Racing Bicycle [50]. ................................................................. 16 Figure 2.10: Crystal Structure of Titanium (a) β-Ti phase; (b) α-Ti phase [54]. .................... 17 Figure 2.11 Pseudo-binary equilibrium phase diagram (schematic) for Ti6Al4V [52]. .......... 18 Figure 2.12: Clad Microstructure for Laser Power a) 862W and b) 1477W [9]. .................... 20 Figure 2.13: Average microhardness as laser power increases [45]. ................................... 21 Figure 2.14: Structure - Property Correlations in Titanium Alloys [66]. ................................ 25 Figure 2.15: TEM Images a) CO2 Laser without α/β interface phase; b) Diode laser with α/β interface phase [67]. ........................................................................................................... 25 Figure 2.16: SEM Images Slip Deformation a) with external force; b) without external force [69]...................................................................................................................................... 26 Figure 2.17: Joint Microstructure – Crack arrest [71]. .......................................................... 27 Figure 2.18: Potentiodynamic polarization curve in seawater at 160 °C for Ti6Al4V [73]. .... 28 Figure 2.19: Potentiodynamic Polarisation of Ti6Al4V substrate and laser-clad samples in 0.1 M oxalic acid [74]. ......................................................................................................... 28 Figure 2.20: Potentiodynamic Polarisation curves of Ti6Al4V samples: BM=Base Material; T4=2.5kW + 1m/min; T8=2kW + 8m/min [75]. ..................................................................... 29 Figure 3.1: Robot Arm Setup at the CSIR Laser Centre ...................................................... 30 Figure 3.2: Coaxial Laser Head........................................................................................... 31 Figure 3.3: Laser Power Source .......................................................................................... 31 Figure 3.4: Mecatome T300 Cutting Machine ...................................................................... 33 Figure 3.5: Cut Samples ..................................................................................................... 33 Figure 3.6: Struers Mounting Press ..................................................................................... 34 ix.

(11) Figure 3.7: Struers Process Cylinder [78]. ........................................................................... 35 Figure 3.8: Struers Polishing Machine ................................................................................. 35 Figure 3.9: Finished Samples.............................................................................................. 36 Figure 3.10: Olympus BX51M Optical Microscope .............................................................. 36 Figure 3.11: TESCAN Scanning-Electron Microscope (UJ Doornfontein Campus) .............. 37 Figure 3.12: TIME Digital Microhardness Tester ................................................................. 38 Figure 3.13: Hardness testing pattern for Cladded Sample ................................................. 39 Figure 3.14: Atomic-Force Microscope Block Diagram [81]. ................................................ 39 Figure 3.15: Sample Placement for AFM (Wits University) .................................................. 40 Figure 3.16: Atomic Force Microscope ................................................................................ 41 Figure 3.17: Corrosion Resistance Testing Setup Schematic [84]. ...................................... 42 Figure 3.18: Corrosion Resistance Test Set-up ................................................................... 43 Figure 4.1: Cladded Samples Overview .............................................................................. 45 Figure 4.2: Macro-appearance of clad cross section: a) Sample 1 – 3.5m/min; b) Sample 2 – 3m/min; ............................................................................................................................... 46 Figure 4.3: Clad-Pore Measurement ................................................................................... 47 Figure 4.4: Sample Porosity Graph ..................................................................................... 48 Figure 4.5: Sample 1 – Clad & HAZ Geometrical Measurement .......................................... 48 Figure 4.6: Sample 5 – Clad & HAZ Geometrical Measurement .......................................... 48 Figure 4.7: Sample 7 – Clad & HAZ Geometrical Measurement .......................................... 49 Figure 4.8: Area and Volume cladding rate comparison ...................................................... 50 Figure 4.9: Clad Microstructure: a) Sample 1 – 3.5m/min; b) Sample 2 – 3m/min; .............. 52 Figure 4.10: HAZ Microstructure: a) Sample 1 – 3.5m/min; b) Sample 2 – 3m/min; ............ 53 Figure 4.11: 2000x CLAD, a) Sample 1 – 3.5m/min; b) Sample 2 – 3.0m/min; c) Sample 3 – 2.5m/min. ............................................................................................................................ 54 Figure 4.12: 1000x CLAD a) Sample 4 – 2.0m/min; b) Sample 5 – 1.5m/min ...................... 54 Figure 4.13: a) Sample 6 (CLAD 370x) – 1.0m/min; b) Sample 7 (CLAD 350x) – 0.5m/min 55 Figure 4.14: Surface Roughness Analysis - Sample 1......................................................... 55 Figure 4.15: Surface Roughness Analysis - Sample 4......................................................... 56 Figure 4.16: Surface Roughness Analysis - Sample 7......................................................... 56 Figure 4.17: Sample Surface Roughness Values ................................................................ 57 Figure 4.18: Vickers Microhardness .................................................................................... 58 Figure 4.19: Sample 4 after Vickers Microhardness Testing................................................ 58 Figure 4.20: Potentiodynamic Polarisation of Laser Cladded Ti6Al4V with differing feed rates.................................................................................................................................... 59 Figure 4.21: Sample Corrosion Rate ................................................................................... 60. x.

(12) LIST OF TABLES Table 2.1: Physical and optical properties of Nd:YAG [36]. ................................................. 13 Table 2.2 Composition of Ti6Al4V sheet Grade 5 in wt% [53] ............................................. 18 Table 2.3 Physical Properties of Ti6Al4V [53] ..................................................................... 18 Table 2.4 Mechanical Properties of Ti6Al4V Grade 5 [53, 55] ............................................. 19 Table 3.1: Laser-Cladding Process Parameters .................................................................. 32 Table 3.2: Mounting Parameters [76]. ................................................................................. 34 Table 4.1: Clad Geometry ................................................................................................... 47 Table 4.2: Efficiency Analysis .............................................................................................. 49. xi.

(13) GLOSSARY OF TERMS A Additive Manufacturing – fabrication of component using layer-by-layer addition of material. Alloy – a metallic substance comprising two or more chemical elements. B Body-Centred Cubic Structure – a cell unit in which one atom is at the centre of a cube; and there is an atom at each corner of the cube. Biocompatibility – the reaction of the body to the materials and the material degradation in the human body environment. C Columnar Grains – grains that are longer than they are wide. Corrosion – chemical reaction between metal and oxygen. F Fluence – Optical energy delivered per unit area, kJ/cm2. Fusion zone – the zone where the molten powder material and substrate mix together. G Grain Size – the average grain diameter, as measured from the cross section of a micrograph. Gas Flow Rate – the rate at which the gas is delivered to the deposition platform. H Hexagonal close packed – a unit cell, which has six atoms at its corners that form a hexagonal shape. Heat-affected zone – the area of the substrate adjacent to the fusion or melted zone that has a different microstructure to the rest of the substrate, due to the heat delivered from the lasercladding process. L Laser – acronym for Light Amplification by Stimulated Emission of Radiation. Laser-metal deposition – advanced manufacturing process where a metal powder is deposited onto a substrate and fused with the substrate; this is repeated continuously until a part surface is completely covered. Laser power – the amount of energy the beam contains, typically measured in kW. Laser-scanning speed – the rate at which the laser head traverses the surface being cladded. Laser-spot size – the diameter of the emitted-laser beam. M Macrograph – an image of an object that is typically the same size as the object or larger. xii.

(14) Martensite – a very hard crystalline structure. Melt pool – the molten powder and substrate mixture on the surface or the part before it solidifies. Melted zone – the solidified melt pool. Metallurgy – the material science domain focused on metallic elements. Micrograph – an image, typically of an object’s surface, taken through a microscope. Microhardness – this is a mechanical property of a material, which quantifies its resistance to localized plastic deformation. Microstructure – the structural features of a metal alloy, which are only visible when observed through a microscope. Morphology – the form and orientation (shape characteristics), of a specific phase or constituent. O Oxidation – a chemical reaction in which an element reacts with oxygen to form an oxide. P Parameter – a variable to be controlled. Phase – a physically distinct and homogeneous form of matter. Porosity – a cavity formed inside the melt pool due to gas entrapment, during the solidification of the clad. R Repeatability – reproducibility of results. T Titanium – the chemical element represented by Ti, with an atomic number of 22, silver-grey in appearance and used in strong, light weight, corrosion-resistant alloys. W Widmanstätten Structure – a structure characterized by a geometrical pattern resulting from the formation of a new phase along certain crystallographic planes of the prior phase. This structure is the transformation product of the beta phase.. xiii.

(15) CHAPTER 1 INTRODUCTION 1.1 Background Laser cladding, ‘laser-overlay welding’ or ‘laser-metal deposition’ can be defined as a process of applying a covering layer of filler material to a part, and then fusing it to the part using a laser beam as the source of power or heat [1, 2]. Laser cladding is a flexible and efficient method of building up specifically engineered coatings on a part’s surface by mixing different powder materials to achieve a desired hardness, corrosive resistance and other metallurgical properties [3]. Powder delivery to the substrate can be done in two primary ways: coaxial delivery or lateral delivery. Lateral delivery is easier; and it therefore allows for the cladding of inaccessible positions, like the bottom of a narrow groove [4]. Laser cladding through coaxial delivery causes the particle to arrive at the substrate at a higher temperature, due to longer exposure to the laser beam. This can lead to some of the particles being melted upon arrival. The particle flow in coaxial delivery has the advantage of partially protecting the surface from the action of the laser beam, which reduces the heat input to the substrate. The advantages to coaxial cladding are, therefore, the possibility of free-directional cladding, better protection from the ambient atmosphere; and a relatively small heat-affected zone [3, 5]. Figure 1.1 shows a typical coaxial laser-cladding set-up; and Figure 1.2 shows a lateral set-up.. Figure 1.1: Coaxial Laser-Cladding Set-up [6]. 1.

(16) Figure 1.2: Lateral Laser-Cladding Set-up [4]. The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation [7]. A laser can therefore be defined as a device that produces an intense, coherent and directional beam of light. As a result; this light can be focused to a specific point with great precision, which means the heat-affected zone size can be reduced significantly when using the laser-welding process [8]. It is, therefore, possible to form a coating in the exact desired location, for example, to repair or to restore a locally damaged or worn-out surface [3]. This makes laser cladding an attractive option when considering the repair of parts that experience wear. Parts that are of particular interest are those that are expensive to manufacture, either due to the size of the part, the process required on the material the part of which has been manufactured. Industries where laser cladding is applied as a repair method includes the aerospace industry as well as the turbo-machinery industry. In the aerospace industry alone there are many current and next-generation aircraft that have components made of Titanium alloys, of which the most common is Ti6Al4V [9]. Laser cladding of titanium alloys can be used as a repair process. This dissertation will not test the validity of laser cladding as a repair process; but it will aim to optimise the input parameters of laser cladding, specifically the scanning speed. This will be done in the following two ways: . by analysing how the base material/substrate properties are affected; and. . by analysing the cladded-surface properties.. 1.2 Problem Statement Implementing laser cladding in the aerospace and turbo-machinery industries to repair a variety of high value components that have been damaged due to general wear and tear could 2.

(17) prove to be very cost effective. When using laser cladding to repair or build up parts that have been manufactured using Ti6Al4V; special consideration needs to be taken to ensure the quality of the clad is sufficient for the purpose of the component; while keeping the cost of the repair as low as possible. This study will investigate the effect that various laser scanning speeds have on the Ti6Al4V substrate and the clad. The scanning speed has a direct impact on the repair time (and cost) as well as the quality of the clad. This study will endeavour to find the balance between these two criteria through comparative analysis.. 1.3 Aim of Research The aim of this research is to optimize the laser cladding of Ti6Al4V, by using Ti6Al4V as both the cladding material and the substrate. This optimization will only be for one parameter: that being the scanning speed. With the scanning speed optimized one should be able to produce good quality clads at a high efficiency which would also lead to reduced costs; since the processing time will also be optimized as a direct consequence.. 1.4 Research Objectives The objective of this study is to determine the properties of Ti6Al4V after it has undergone laser cladding, as well as the properties of the Ti6Al4V clad itself. The microstructure of the clad will be investigated using Optical Microscopy and Scanning Electron Microscopy. The surface finish will be determined using Atomic Force Measurement (AFM), while corrosion resistance will be determined through Potentiodinamic Polarisation. Micro-Hardness testing will be conducted to verify the hardness characteristics of the clad, HAZ and substrate. All the information gathered through the various tests will then be used to determine what scanning speed is the most efficient (through dimensional analysis of the clad geometry); while maintaining the required quality.. 1.5 Scope of Work To successfully achieve the objective of this research, the following work was carried out: . A literature study was conducted to give a comprehensive account of the laser process technologies used in industry, laser-surface engineering; and finally, laser cladding. Literature study was also conducted on titanium and its alloys, with the focus being on the properties of Ti6Al4V and how this specific alloy reacts to the laser-cladding process. The effects that different operating parameters have on the laser cladding of Ti6Al4V were investigated through a review of previous researchers’ work.. . Following the literature survey, the experimental set-up was prepared for the material characterization of the test pieces.. 3.

(18) . Seven tracks of Ti6Al4V were deposited, all of equal length, and keeping the laser power, gas flow rate and powder feed rate constant, but varying the scanning speed.. . Material characterization of the test pieces was carried out. This comprised microstructure comparison, microhardness testing, surface roughness, corrosionresistance testing and the physical dimensions of the deposit, i.e. height and width.. . The results of the study were collected, analyzed and discussed in the results, as well as in the discussion chapter.. . Finally, the study was concluded by comparing the obtained results with those obtained from the literature study.. 1.6 Hypothesis The laser-cladding process was employed to clad the substrate Ti6Al4V, using Ti6Al4V powder. Several clads have been completed – each consecutive track being cladded using a decreased scanning speed. The characteristics of these clads were determined by examining the microstructure, the hardness and the corrosion-resistance of each clad. It is expected that the properties; and therefore, the quality of these clads will differ slightly. The slower the scanning speed, the more heat input to the substrate – and the larger the heat-affected zone. The increased heat input will however produce clads with superior bonding to the substrate. However, it is probable that the best clad will be one of medium speed. This should still have sufficient bonding, a smaller HAZ; and it will be less expensive to implement for the repair of Ti-6Al-4V parts.. 1.7 Summary This chapter has provided a brief introduction to the research required in this dissertation. Outlined in Chapter one, there is the background of laser cladding, the problem statement, the aim of the research, the research objective, the scope of the work and the hypothesis. Also highlighted in this chapter is the significance of this study; and how it could benefit not only the academia, but also the industry. Thought has been put into the possible outcome of this study; and it was briefly discussed. The following section of the dissertation, Chapter Two, will critically discuss in greater detail the fundamental technology behind lasers, various laserprocess technologies, the laser cladding of titanium alloys, in addition to the mechanical and metallurgical properties of Ti6Al4V. Previous research done in the literature on the lasersurface engineering of titanium alloys will also be discussed.. 4.

(19) CHAPTER 2 THE LITERATURE REVIEW 2.1 Introduction In this section, various technologies focusing on laser processing and laser-surface engineering will be discussed, together with extensive details of the processing techniques, as well as the characteristics of the process. Ti6Al4V was used for the research work; therefore, Titanium (with the focus on Ti6Al4V) will be discussed in terms of its properties, crystal structure and phase diagrams, in order to assist in the understanding of the material characterisation. Since this study will focus on the laser cladding of Ti6Al4V, previous work on the subject will also be discussed; and it will be explored to gain an understanding of how this research fits into the current literature available on the laser processing of titanium alloys.. 2.2 Introduction to Laser Technology The laser industry has been showing steady growth; since the first working laser was produced by Theodore Maiman in 1960 [10]. This steady growth of the laser industry is driven by the increasing applications of laser light. Laser technology has been utilized in a variety of applications over the years, applications, such as metrology, photography, material processing, medicine, communication and electronics [10]. In the manufacturing industries; this laser light, a form of optical energy, is transformed into heat by focusing the laser beam – using a variety of optics (e.g. lenses) to obtain the required spot size and profile. The highly concentrated laser energy reacts with the material by exciting the material on a molecular scale; and therefore causing structural vibration on the material. This vibration is detected as heat; and if excited for a long enough period, with a high amount of optical energy, this heat could be high enough to cause the material to melt [10]. This exciting of the molecules / atoms to the point where phase change occurs is a common phenomenon that can be observed in nature on a daily basis [11]. This process of using the laser as the source of the energy has some advantages. One such advantage is that the laser energy can easily be automated, making it very flexible and useful for various applications. These applications in laser-manufacturing includes: welding, drilling, cladding, peening, cleaning of material surfaces, surface treatment, micromachining, bending, 3D marking and cutting [10]. LASER (Light Amplification by Stimulated Emission of Radiation) light is generated by exciting some active medium [12]. This medium can be in the form of a solid, liquid or gas, with an energy source, such as light, electricity, heat etc. When excited, the electrons of the active medium will absorb energy from the source; and they will move from the lower-energy level (inner shell) to the higher-energy level (outer shell). When the electrons relax, however, and 5.

(20) return to their original state; they emit the excess energy in the form of photons [13]. (Refer to Figure 2.1 below as a visual aid to this process). The photons are then made to reflect with the medium in one of two ways. For a CO2 laser; two mirrors are used, one being a fully reflecting mirror, and the other a partially reflecting mirror; whereas fibre lasers utilize two Bragg gratings. As the photons are reflected back onto the medium; they once again react with the excited atoms, which leads to a further increase in the amount of photons in the system. This process is known as stimulated emission (refer to Figure 2.2) [14]. The quantity of photons will continue to increase, until a certain wavelength is reached; and a coherent, monochromatic laser beam is generated that will pass through the partially reflective mirror or the Bragg grating. From this point on, the laser beam will be passed through multiple lenses; and/or it will be guided by using fibre cables – to be further focused – depending on the requirements for one of the various manufacturing processes, for which lasers are being used [13].. Figure 2.1: Bohr Atom Model Emitting a Photon [15].. Figure 2.2: Stimulated Emission [14].. 6.

(21) 2.3 Laser-Process Technologies 2.3.1 Laser Welding Laser light possesses several unique properties compared to conventional light sources like LED or fluorescent light, among which is the fact that it is highly concentrated (high energy densities), parallel, time and spatial coherence, monochromatic; and it has low divergence [16]. It can, therefore, be guided or conducted, using a combination of mirrors and glass fibres, to a welding position that is in an entirely different location than the power unit. This is beneficial; since the power unit can be quite large [16]. Lasers have been promoted as potentially useful welding tools for a variety of applications. Until the 1970s, however, laser welding had been restricted to relatively thin materials and low speeds; because of the limited continuous power available. By 1965, a variety of laser systems had been developed for making micro-welds inside vacuum tubes, in electronic circuit boards, and in other specialized applications; where conventional technology was unable to provide reliable joining [17]. The availability of highpower continuous-wave (CW) carbon dioxide (CO2) and neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers and the limitations of current welding technology have promoted interest in deep-penetration welding in the past 20 years by using these devices [17].. 2.3.2 Laser-Metal Deposition Direct laser deposition is a relatively new fabrication technique used for the manufacturing of metallic parts and the engineering of metallic surfaces. The same progress made in laser technology that enables laser welding also enables this fabrication technique. The process of metal deposition by using a laser as the source of power is similar to the already-widely used methods. The additive material (either wire, powder or both) is melted to deposit a single bead on the substrate. An entire area is coated then by simply making repeated bead passes with a specified overlap; and repeating this process over multiple layers will then yield a 3dimensional structure [18]. The main difference that this process has when compared to the more widely used welding processes is that the source of power / heat is a laser. Laser-metal deposition has been researched and developed by various individuals and organizations; and therefore, it is known by a variety of names. These include: direct-metal deposition (DMD), direct-light fabrication (DLF), laser-engineered net shaping (LENS), and laser-based flexible fabrication (LBFF) [19]. Each of these processes / systems works on the same freeform fabrication principle, which is rendered possible by using a focused laser beam with high enough energy density to melt the metallic powder and create the component in 3 dimensional space [20].. 7.

(22) Figure 2.3 shows four possible arrangements of the material feeding that could be used. It is also possible to have a combination of both wire and powder feeding. This is used for the development of functionally graded materials (Figure 2.4).. Figure 2.3: a) LMD Wire Feed; b) LMD Lateral Powder Feed; c) LMD Symmetric Powder feed; d) Coaxial Powder Feed [21].. 8.

(23) Figure 2.4: Schematic diagram of DMD system with concurrently fed powder and wire material [22].. 2.4 Laser-Surface Engineering Surface engineering can be defined as the process of designing and modifying of a surface to enhance / change properties like, hardness, wear resistance, heat resistance etc. This is useful in that it allows for the use of cost-effective materials for manufacturing components that are exposed to harsh and corrosive environments [23]. Laser-surface engineering can be described as a novel approach to modify the surface of a metal that is exposed to severe environmental and industrial conditions. The material surface is re-engineered by using concentrated laser energy. This re-engineering of the material surface, using the laser, can be deployed by using different approaches. These may include: laser-surface melting, alloying, and cladding amongst others. Figure 2.5 shows a diagram, which can be used to classify different laser surface-engineering approaches. Laser-Surface Melting will correspond to route A1-B1-C1, Laser-Surface Alloying to A1-B2-C3. A1-B (2 or 3)-C2; while A2-B1-C2 both corresponds to Laser-Surface Cladding, with any of the various feeding mechanisms [24].. 9.

(24) Figure 2.5: Classification of Laser surface-engineering approaches [24].. 2.4.1 Laser-Surface Melting Laser-surface melting is a simple surface-engineering method; since there is no material addition to the substrate. Rather, the modification process only involves irradiating the surface of a component in the presence of air, nitrogen, or an inert gas. When using sufficient laser energy a thin layer of the surface will melt; this is followed by a rapid solidification process, all within a short period of time. The thin layer will now have a nearly homogeneous and fine microstructure. Therefore, the component’s surface would now have enhanced surface hardness, corrosion, and wear properties, based on the resulting microstructure [25].. 10.

(25) Figure 2.6: Schematic of Laser-Surface Melting process showing the melted region [26].. 2.4.1.1 Laser-Surface Alloying Laser-surface alloying is another surface engineering method whereby the substrate surface is irradiated by using a laser beam; the alloying elements are simultaneously injected via a powder-feed system into the melt pool. The same effect can be achieved by preplacing the alloying element on the substrate surface, before laser scanning. The energy absorbed from the laser beam increases the temperature of both the substrate and the alloying element above their melting points. This results in an alloyed-melt pool composition (mixture of materials), which upon rapid cooling and solidification generate an alloyed surface. This newformed surface would have enhanced properties, which can vary, based on the alloying elements introduced [27].. 2.4.2 Laser-Surface Cladding Laser cladding is a laser-surfacing process, in which the main objective is to cover a particular part of the substrate (base material) with a homogeneous layer of material, which has superior properties – and thereby producing a fusion bond between the two – with minimal mixing (dilution) of the clad by the substrate. In other words, the mechanical properties, like the yield strength of the part will not be affected; but the surface properties, like hardness and corrosion resistance, would be that of the powder material used to clad the substrate [28, 29, 2]. The last 20 years has seen an increase in the popularity of laser-metal deposition – not only as a research topic, but also commercially in a range of industries, such as the automotive, mining and aerospace [2]. Laser cladding is mainly used in one of two ways in industry. Firstly, in the production of parts of composite materials, i.e. when the mechanical properties of one 11.

(26) material are required, but the corrosion resistance of another. Secondly, for the repair of worn parts; in this instance, the cladding can also act as an upgrade that will allow the part to stay in service for longer [30]. In the production of parts of composite materials, this technique is used to produce hard, wearresistant and/or corrosion-resistant surface layers [31]. There are various methods that can be implemented to improve the surface properties (corrosion and wear-resistance) of metallic materials. Laser cladding, however, is an attractive alternative to conventional techniques due to its intrinsic properties of laser radiation. The properties that are of interest are: the high input energy, low distortion, capability to avoid undesirable phase transformations – due to high controllability and minimum dilution between the substrate and the coating [32]. In addition laser-cladding has the advantage of having good flexibility, repeatability and the possibility of cladding small areas [32]. These advantages allow for better quality products and repair; but they also offer significant economic benefits [33]. Repair through laser cladding is a relatively standard and common practice in the mould-and-die industries, where the life of loaded die elements and vital tool parts can be successfully and easily extended by applying the laser-cladding repair timeously. Machining a new tool or die part is expensive and timeconsuming; whereas laser cladding has the advantage of enabling the repair within a shortlead time and minimal cost [34]. More and more industries are expanding their repair capabilities by implementing laser cladding, as a possible repair method. This method has shown some promise in the powergeneration industry with the in situ repair of turbine blades and shafts used in gas turbines, as well as the aerospace industry [35]. 2.4.2.1 Nd:YAG laser One of the most common of welding lasers is the Nd:YAG laser (solid-state); the other being the CO2 laser (gas-state). Nd:YAG lasers are typically used for the welding of thinner materials less than 6mm thick and the CO2 thicker materials equal to or greater than 6mm. The Nd:YAG lasers are also suitable for laser-cladding; and will be used for the purpose of this dissertation. The laser beam used for the cladding may be either pulsed, or continuously emitted [16]. The Nd:YAG laser is the most commonly used solid-state laser by far [36]. Neodymium (Nd) is the active substance in this laser; it is found in the form of a doping agent within a transparent rod of Yttrium Aluminium Garnet (YAG) [16]. A doping agent can be defined as an impurity element added to a crystal or semiconductor lattice in low concentrations. Doping is done, in order to alter the optical and/or the electrical properties of the crystal or semiconductor [37]. Energy is supplied to produce the laser through a flash tube; this is similar to the principle used in cameras. The laser light, which is produced, 12.

(27) has a wavelength of 1.06μm, which is considerably shorter when compared with that of the CO2 laser; but it is still within the invisible infra-red section of the spectrum. The shorter wavelength produced by using this method provides substantial practical benefits, due to the fact that the light can be carried by optical fibres; and it can be focused by using normal lenses. This also makes using the laser with a robot possible, which allows for robot-operated cladding and welding, which is highly accurate and repeatable. Additionally, metallic materials absorb short wavelength radiation more efficiently [16]. Problems due to the presence of absorbing plasma are less when welding with Nd:YAG lasers; and consequently, argon and argon/CO2 gas mixtures can be used. Acceptable and even good results can even be obtained without shielding gas, when welding spot welds or by altering the processing parameters (e.g. low power). The Nd:YAG laser is particularly suitable for welding difficult materials, such as titanium (Ti) and Inconel. However, there is one major disadvantage compared to the CO2 laser; and that is that the power outputs are significantly lower, meaning that the application is limited to thin sheet metal of no more than 6mm when welding. For cladding, the limited power is not really a disadvantage; since the layer thickness of the clad would typically not exceed 6mm [16]. Table 2.1 summarises some of the physical and optical properties of Nd:YAG. Table 2.1: Physical and optical properties of Nd:YAG [36].. Description Chemical formula Weight % Nd Atomic % Nd Nd atoms/cm3 Melting point Mechanical hardness Density Tensile strength Modulus of elasticity Poisson ratio Thermal-expansion coefficient [100] orientation [110] orientation [111] orientation Line width Fluorescence lifetime Photon energy at 1.06 μm Index of refraction. Value Nd:Y3Al5O12 0.725 1.0 1.38 × 1020 1970ºC 1320 kg/mm2 4.56 g/cm3 200 MPa 310 GPa 0.30 8.2 × 10−6/ºC 7.7 × 10−6/ºC 7.8 × 10−6/ºC 120 GHz 230 μs hv = 1.86 × 10−19 J 1.82 (at 1.0 μm). 13.

(28) 2.4.2.2 Continuous Wave vs Pulsed Wave When operating the laser during cladding or welding; there are two wave modes, in which the laser can be used. These are continuous-wave and pulsed-wave laser modes. The welding parameters normally associated with continuous wave lasers are laser power, beam diameter and welding speed. When operating pulsed-wave lasers; the parameters that are considered are pulse: duration, energy and beam diameter. The transitional mode region differs significantly from pulsed and continuous lasers, with the continuous-mode typically showing a larger region compared to that of the pulsed-mode laser [38, 39]. When comparing how the material reacts to the different laser types, keeping the conditions (power density, interaction time and beam diameter) constant, the pulsed-wave (PW) laser will have a much greater penetration depth when compared with the continuous-wave (CV) welds. This means that the PW laser has a higher-penetration efficiency; and it would, therefore, be able to produce a weld with lower heat input to the substrate, in order to achieve the same penetration depth as with CW. The combination of the following two effects is the main cause for the difference between the PW and the CW laser types [38]: . During the formation of the key hole, there exists a surface tension gradient driving the fluid flow to oppose the formation of the key hole. The transitional region originates because of this fluid flow. For the CW laser welding, there exists an additional fluid flow towards the key hole. This is caused by the translation of the weld pool through the material. Since the formation of the key hole is being opposed by the fluid flow; it is being delayed further, which leads to the wide transitional region that can be observed. This phenomenon also explains the differing penetration depths observed when comparing CW and PW. The size of the difference depends on the duration of the interaction time [38].. . A higher spatial peak power density is observed for PW compared to CW. This results in the 10ms interaction time when the penetration depth for CW is lower than that for the PW welds. However, this is again equalized for 20ms interaction times when the penetration depth is similar for both PW and CW welds [38].. 2.4.2.3 Powder Feeding Laser cladding, also known as laser-metal deposition requires some kind of consumable metal that will be deposited on the substrate to create the clad. This material can be introduced in the form of a powder or a wire. For the powder-based cladding process, the powder can be fed in one of two ways: either through a coaxial nozzle, or laterally through a separate feeding. 14.

(29) nozzle. When the powder is introduced, the laser has to melt the powder, as well as a thin layer of the substrate, in order for fusion to take place [40]. The powder used for the cladding can vary – based on the desired application. The hardness can be increased; wear and corrosion resistance can be improved, or the geometry of a worn part can simply be restored. In any case, it is important to not only consider the type of powder to use; but to also take care that the other parameters that influence the clad properties are thoroughly considered. These parameters include: the powder-feed rate, the process speed, the laser power, the beam diameter and the melting pool temperature [28]. It is also important to consider the powder quality and the particle size. The surface temperature can be influenced by the laser power and by the travelling speed. Higher power and slower speed will cause an increase in the surface temperature, and vice versa [41]. Lower temperature will cause incorrect bonding; while higher temperatures would cause deeper penetration, which would also lead to a change in material properties and greater dilution [42]. The potential advantages to powder feeding can be: low dilution between the substrate and the consumable; limited heat effects to the base material; improved metallurgical bonding between the powder and the substrate, minimum distortion of the part – due to the low heat input, the low possibility of cracking; and the possibility of automating the process [29].. 2.4.2.4 Pre-heating Preheating is a common practice for many welding procedures. By preheating, the residual stress in the part can be reduced. The residual stress could lead to crack formation, delamination, distortion and porosity [43, 44]. Preheating the part could also help limit the hardness increase in the substrate; if this is required.. 2.5 Titanium and its alloys 2.5.1 Overview Titanium alloy, Ti6Al4V, is the most widely used titanium alloy [45], it accounts for approximately 60% of the industrial use of titanium; and it can be subjected to various forms of processing to yield a wide variety of product forms like: forgings, castings, bar, foil, sheet, plate extrusions, fasteners and tubing [35]. Ti6Al4V alloy is good for applications at room temperature; and in high-temperature applications up to a temperature range of 315-400°C. Additionally, Ti6Al4V has a high strength-to-weight ratio, as well as excellent corrosion resistance, when compared with aluminium and steels [35, 45, 46]. This material is widely 15.

(30) used in the aerospace industry (Figure 2.7), in the manufacturing of low-pressure compressorengine blades, compressor spool, discs and airframes [46]. Other applications of Titanium alloys may include: power generation; chemical processing; automotive engineering (Figure 2.8), marine, oil gas and in petroleum processing, medicine, racing bicycles (Figure 2.9) and architectural structures [35, 47].. Figure 2.7: Jet Engine, low pressure compressor and fan blades manufactured from Titanium Alloys [48].. Figure 2.8: Automotive components manufactured from Titanium Alloys [49].. Figure 2.9: Titanium-Frame Racing Bicycle [50].. For all its uses, Ti6Al4V like most titanium alloys are difficult to machine, making the manufacturing process more expensive and time-consuming [51]. Therefore, investigating and. 16.

(31) developing new and innovative ways of not only manufacturing new titanium components, but also to repair existing components. This could yield substantial benefits [45].. 2.5.2 Metallurgy and Material Properties of Titanium Alloys In this section, the properties and metallurgical structure of Ti6Al4V will be investigated to obtain a base line for the comparison of the results later in the dissertation. Unalloyed titanium consists of two allotropic forms: The Alpha phase (α), which is a low temperature form that exists as a hexagonal-closed pack (HCP), a crystalline structure up to 882°C. Above this temperature, it transforms to the beta (β) phase, which has a body centred cube (BCC) crystal structure. Both these structures are displayed in Figure 2.10. When alloying Titanium, the behaviour of the alloying elements is defined by the effect the element has on the α and β phases of the titanium [52]. When adding elements that increase or maintain the temperature range at which the α-phase is stable those elements are called αstabilizers. The most important of these elements are aluminium, tin and zirconium. βstabilizers are then elements that do the same but only for the β-phase of the titanium, these include molybdenum, vanadium and iron [52]. Titanium alloys can now be classified in four categories, namely: α-alloys, near-α alloys, α-β alloys and β-alloys. This dissertation focuses on the alloy Ti6Al4V, which is classified as an αβ alloy; since it contains only limited amounts of β-stabilizers (Vanadium); and therefore, it also requires α-stabilizers (Aluminium). Figure 2.11 shows the phase diagram of Ti6Al4V. The mechanical properties of this specific alloy will then be determined by the relative amounts and distribution of the α and β phases, which can be controlled by processing and heat treatment [52]. Interstitial alloying elements are also introduced to the titanium to increase the strength of the alloy and decrease the ductility; these elements may include: carbon, hydrogen, oxygen and nitrogen [53]. In summary, Ti6Al4V is a non-magnetic, α-β phase type alloy of titanium that derives its strength and other properties from substitutional and interstitial alloying elements. The composition of Ti6Al4V can be found in Table 2.2, the physical properties in Table 2.3 and the mechanical properties in Table 2.4.. Figure 2.10: Crystal Structure of Titanium (a) β-Ti phase; (b) α-Ti phase [54].. 17.

(32) Figure 2.11 Pseudo-binary equilibrium phase diagram (schematic) for Ti6Al4V [52].. Table 2.2 Composition of Ti6Al4V sheet Grade 5 in wt% [53]. Element. C. Fe. N2. O2. Al. V. H2. Ti. wt. %. <0.08. <0.25. <0.05. <0.2. 5.5-6.76. 3.5-4.5. <0.015. Balance. Table 2.3 Physical Properties of Ti6Al4V [53]. Property. Typical Value. Density (kg/m3). 4420. Melting point (°C). 1649±15. Specific heat (J/kg.K). 560. Volume electrical resistivity (Ω.m). 1.7. Thermal conductivity (W/m.K). 7.2. Mean thermal expansion co-efficient (oC-1) 8.6 x 10−6 β-transus temperature (oC). 999±15. 18.

(33) Table 2.4 Mechanical Properties of Ti6Al4V Grade 5 [53, 55]. Property. Typical value. Tensile strength (UTS) (MPa). 950. Elongation (%). 14. Area reduction (%). 36. Elastic modulus (GPa). 114. Poisson’s ratio. 0.342. Fatigue strength (MPa). 240. Fracture toughness (MPa.m0.5) 75 Shear modulus (GPa). 44. Shear strength (MPa). 550. Vickers hardness (HV). 349. 2.6 Laser Cladding of Titanium and its Alloys Ti6Al4V, being such a commonly used titanium alloy, has been subjected to various studies in the past, many of which relating to the laser cladding of this alloy. In this section various studies will be referenced to obtain an understanding of how this alloy reacts when subjected to laser cladding or other heat input processes like welding. This will be vital to interpreting the results that will be obtained from experimentation and to also understand how this research contributes to the current literature on laser cladding of Ti6Al4V. The literature under review will be split into 4 major categories, these will be Laser Cladding of Titanium Alloys, Varying Laser Properties/Parameters and the Effect on the Clad, Titanium Microstructure and Properties, and Corrosion Properties of Titanium Alloys.. 2.6.1 Laser Processing of Titanium Alloys In this section literature will be reviewed with specific focus of laser process technologies and the various ways this technology can be applied using Titanium alloys. The review of the literature in this section will play an important role in understanding the relevance of the work being done for this dissertation. Various studies will be reviewed whereby the laser processing parameters where altered or optimized in some way, this will provide a critical understanding of how various parameters like scanning speed and laser power will affect the properties of the clad. The first study under review is titled: Direct laser cladding of layer-band-free ultrafine Ti6Al4V alloy [56]. This study is similar to the work being done for this dissertation, here Ti6Al4V alloy was fabricated on pure titanium substrate using laser cladding, and the effect of the scanning 19.

(34) speed on the microstructure, hardness and wear performance was investigated. This study varied the laser scanning speed between 300mm/min and 600mm/min, which is considerably slower than what will be considered for this dissertation. The laser power used was 650W, powder feed rate of 9.05g/min and argon shielding gas flow rate of 6L/min. It was concluded in this study that the high scanning speeds promote the formation of an ultra-fine martensitic microstructure which as a result yielded high hardness values (compared to the slower scanning speeds), high residual stress and fatigue strength. This difference in the microstructure and hardness is due to the decreased reheat cycle treatment temperature and the less heat accumulation in the substrate. In addition it was also found that the coefficient of friction is lower and the wear resistance is higher [56]. The study by R. Cottam titled: Laser Cladding of Ti6Al4V Powder on Ti6Al4V Substrate [9], focuses on keeping the clad thickness constant by varying the scanning speed, laser power and powder feed rate. This is done for two different melt pool depths. For this study the mechanical properties of the clads were not considered, rather the focus was on the microstructure and the hardness of the produced clads and the HAZ. With the increase in laser power the heat input into the clad is also increased which affects the cooling rate and also the achieved microstructure as can be seen in Figure 2.12.. Figure 2.12: Clad Microstructure for Laser Power a) 862W and b) 1477W [9].. When considering the HAZ, it was found that it shows refinement of the lath thickness as the laser processing speed decreases. This is in contrast to what was observed in the clad zone. This suggests that the cooling rate within the HAZ is increasing with a decrease in the laser processing speed [9]. Laser Metal Deposition of Ti6Al4V: A Study on effects of Laser Power on microstructure and Microhardness [45], is another study which focuses on the effect laser power has on the microstructure and the microhardness of Ti6Al4V. The scanning speed, powder flow rate and gas flow rate were all kept constant while the power was varied between 0.8kW and 3kW. 20.

(35) After investigating the samples using an Optical Microscope and preforming microhardness test the following was concluded [45]. . A study of the micrographs revealed that a layer band occurred for all the samples. This is typically a characteristic of a multi-layered deposition; this is due to the remelting of a previously deposited layer when depositing another layer on top. In this study only one layer was deposited, therefore the above reasoning is not applicable here. A possible explanation given for this phenomenon is that the banding could be as a result of shrinkage during solidification in the fusion zone.. . As the laser power increased the following was noted: o. Microstructure for HAZ ranged from fine to coarse globular primary alpha structure.. o. Columnar prior beta grain structure ranged from high density to low density.. o. Microstructure range between fine and thick martensite structure.. o. Finally the average microhardness also increased as the laser power increased (Figure 2.13).. Figure 2.13: Average microhardness as laser power increases [45].. Under consideration is a study titled: Laser Power and Powder flow rate influence on the metallurgy and microhardness of Laser metal Deposited Titanium alloy [57]. This study focused on the effect laser power and powder flow rate has on the properties of the laser metal deposited titanium alloy-Ti6Al4V. The laser power was varied between 1.8kW and 3.0kW and the powder flow rate between 2.88g/min and 5.67g/min. It was found that with an increase in laser power or a decrease in the powder flow rate would yield an increase in the production of Widmanstätten alpha micro structure, which also yielded a lower microhardness. In contrast when the laser power was decreased and the powder flow rate increased the microstructure would be more predominantly a martensitic alpha structure with a high microhardness [57].. 21.

(36) The optimization of the laser cladding process and clad properties was investigated by B. Carcel in the study titled: Laser cladding of TiAl intermetallic alloy on Ti6Al4V - Process optimization and properties [58]. In this study Ti6Al4V substrate was cladded using a TiAl intermetallic coating in order to improve the high temperature resistance and tribological properties of Ti6Al4V. In this study the laser power was varied between 700, 800 and 900W; the scanning speed varied between 300, 450 and 600mm/min and the powder feed rate between 2 and 4 g/min. In addition two preheating temperatures of 350°C and 450°C was also considered, this yielded a total of 36 cladding tracks for analysis. It was found that there is a direct correlation to the cooling rate of the clad and the achieved hardness, and between the cooling rate and the scanning speed. Therefore the slower scanning speed promoted decreased cooling rate and lower hardness, while the high scanning speed produced a higher hardness in the coating, but also increased defects in the form of cracks. The preheating temperature did not yield a significant difference in the mechanical properties of the coating, but the higher temperature did decrease the amount of crack defects observed [58]. Disk-laser welding of Ti6Al4V titanium alloy plates in T-joint configuration [59], focusses of laser welding of Titanium rather that cladding, however the process parameters are similar and the effect of the heat input on the substrate is similar to that of laser cladding. In this study three processing parameters were varied namely; power, scanning speed and tilt angle, for the purpose of this study only the power and scanning speed are relevant for review. It was found that for a higher laser power and slower scanning speed the heat input into the substrate material was higher, this led to the melted zone and heat affected zone of the weld to increase in size as well. Another observation made was that the peak hardness values were similar for the low power, fast scanning speed runs and the high power, slow scanning speeds. Therefore it can be concluded that the welding parameters do not affect the maximum hardness values achieved, but only the extent of the melted and heat affected zone and as a consequence the size of the area in which the maximum hardness has been observed [59]. This conclusion can be partially confirmed by another study titled: Laser beam welding of titanium additive manufactured parts [60]. In this paper the joinability of titanium additive manufactured (AM) parts was explored through keyhole welding of the titanium using a pulsed laser beam. The parameters varied were the power and the pulse length, which as a result affected the heat input into the welded material. The hardness across the achieved welds were not measured, however it can clearly be seen that again the higher heat input resulted in a much larger melted zone and HAZ [60]. The next study under review titled: Stitch welding of Ti−6Al−4V titanium alloy by fiber laser [61], again focuses not on laser cladding but rather stitch welding of Ti6Al4V titanium alloy using a 4kW ROFIN fibre laser. In this paper the influences of the laser welding parameters 22.

(37) on the macroscopic geometry and porosity are also investigated along with the microstructure and mechanical properties. The results to the study showed that using a three-pipe nozzle with shield gas flow rate in excess of 5L/min could avoid oxidation which shows a better shielding effect when compared to a single-pipe nozzle. It was also found that the laser power and scanning speed had an effect on the porosity of the weld. When reducing the laser power and the scanning speed the porosity also decreased significantly. Overall the maximum shear strength of the welding joint with minimal porosity was obtained by using a laser power of 1700W, a scanning speed of 1.5m/min and a defocussing distance on +8mm [61]. The effect of using a pulsed wave or continuous wave fibre laser will be considered by reviewing the study: Comparative study on continuous and pulsed wave fiber laser cladding in-situ titanium–vanadium carbides reinforced Fe-based composite layer [62]. This study investigated the effect of the type of laser by analysing the carbide particle size, cladding layer grain size and hardness. Results showed that there were no difference between pulsed wave and continuous wave with regards to the cladding layers final phases achieved. However the pulsed wave laser did refine the carbide and the cladding layer matrix grains significantly when compared to the continuous wave laser. The particle size distribution range became more concentrated and the hardness obtained using the pulsed wave laser was 210HV higher than that obtained using the continuous wave laser [62]. In order to better understand the difference between titanium parts produced through various processes the study titled: Comparative study of commercially pure titanium produced by laser engineered net shaping, selective laser melting and casting processes [63], will be reviewed. As mention in the title of the study commercially pure titanium was produced using LENS and SLM processes and the resulting properties were compared to that of a more traditional approach, namely casting. The first finding of the study was that a significantly higher laser power and energy density is required for the LENS process compared to SLM in order to gain near full density of 99.5%. The investigation further revealed that the LENS sample had an α phase microstructure with mixed plate-like, Widmanstätten and serrated morphologies somewhat similar to cast microstructures. In contrast, SLM showed only martensitic α' phase microstructure. In summary it was observed that the SLM sample has a significantly higher cooling rate compared to LENS. In addition due to the martensitic phase composition of the SLM sample it showed the highest mechanical properties from compression and hardness tests [63]. The next study titled: Process parameter interaction effect on the evolving properties of laser metal deposited titanium for biomedical applications [64], investigates the effect laser power has on the microstructure, microhardness, wear and corrosion properties of the clad. The. 23.

(38) parameters that were kept constant throughout the study were; spot size (4 mm), powder flow rate (2 g/min), gas flow rate (2 L/min) and the scanning speed (0.002 m/s), the laser power was varied between 400 and 1600W. The powder material used was commercially pure titanium and the substrate Ti6Al4V. The results obtained showed an increase in the clad geometry with an increase in laser power. The microstructure observed at low power settings showed increased martensitic structures, while the higher power settings yielded a Widmanstätten structure. It was also noted that the wear resistance decreased as the laser power increased, pitting and the presence of unmelted powder material also decreased with an increase in the laser power [64]. The final study to be considered in this section is titled: Non-destructive residual stress analysis and microstructural behaviour of laser deposited titanium and copper alloy [65]. The aim of this study was slightly different to the previous studies in that it focusses on how the laser power and powder composition affects the residual stresses within the clad in addition to the microstructure. It was found through analysis of the residual stresses in the laser metal deposit using the biaxial and shear-stressed model that the Ti6Al4V (no Cu) alloy had the highest compressive residual stress compared to the other depositions made with Cu alloys included. For the other samples it was found that an increase in laser power and therefore heat input resulted in a decrease of the residual compressive stresses [65].. 2.6.2 Titanium Microstructure and Properties To better understand how the microstructure of titanium alloys correlate to the physical properties of the material, numerous articles and papers will be reviewed which focus on identifying this correlation. The first study to be review is titled: Experimental and numerical investigation on the tensile properties of a titanium alloy disc with dual microstructure [66]. This study explores in depth the effect of the microstructure of a Titanium alloy has on the mechanical properties of said alloy. It was found in this study that titanium alloys with a fully transformed β microstructure as shown in Figure 2.14a provides better creep and fracture toughness while an equiaxed microstructure (Figure 2.14b), consisting of both globular α and transformed β structures provides better fatigue resistance and ductility. An optimum combination of static and dynamic properties can be obtained by a microstructure comprising of 10-15% globular α in a transformed β matrix, as shown in Figure 2.14c.. 24.

(39) Figure 2.14: Structure - Property Correlations in Titanium Alloys [66].. The next study under review also investigated how the phase structure effects the mechanical properties of the titanium alloy. This study is titled: Influence of α/β interface phase on the tensile properties of laser cladding deposited Ti–6Al–4V titanium alloy [67]. In this study Ti6Al4V is laser cladded using two different laser cladding deposition systems, one being a CO2 laser and the other a diode laser. Laser power and scanning speed was kept constant between the two systems at 2000W and 600mm/min. It was found through transmission electron microscopy (TEM) that the diode laser did produce the α/β interface phase (Figure 15) while the CO2 laser did not. The α/β interface phase acts as an inhibitor to dislocation motion which yielded increased mechanical strength. Tensile testing of the samples confirmed that the samples with the α/β interface phase had higher yield strength than the samples without the α/β interface. It was concluded that this work suggests an effective method of improving both the strength and plasticity of laser cladding deposited Ti6AlV based on the α/β interface phase [67].. Figure 2.15: TEM Images a) CO2 Laser without α/β interface phase; b) Diode laser with α/β interface phase [67].. 25.

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