Masters thesis of engineering investigation of material and mechanical properties of laser beam welded (lbw) laser powder bed fusion (lpbf) and wrought titanium alloy samples

Investigation of Material and Mechanical Properties of Laser Beam Welded (LBW) Laser Powder Bed Fusion (LPBF) and Wrought Titanium Alloy Samples A thesis submitted in fulfilment of the requirement for the degree of Master of Engineering Ali Tamaddon B Sc (Mechanical Engineering), Azad University, Tehran P E (Institute of Engineers Australia), Canberra, ACT School of Engineering College of Science, Technology, Engineering and Mathematics RMIT University November 2021 Declaration I certify that except where due acknowledgement has been made, this research is that of the author alone; the content of this research submission is the result of work which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged; and, ethics procedures and guidelines have been followed In addition, I certify that this submission contains no material previously submitted for award of any qualification at any other university or institution, unless approved for a joint-award with another institution, and acknowledge that no part of this work will, in the future, be used in a submission in my name, for any other qualification in any university or other tertiary institution without the prior approval of the University, and where applicable, any partner institution responsible for the joint-award of this degree I acknowledge that copyright of any published works contained within this thesis resides with the copyright holder(s) of those works I give permission for the digital version of my research submission to be made available on the web, via the University’s digital research repository, unless permission has been granted by the University to restrict access for a period of time I acknowledge the support I have received for my research through the provision of an Australian Government Research Training Program Scholarship Ali Tamaddon, 19/11/2021 Ali Tamaddon Page | ii Acknowledgements: This is to acknowledge the start of a new chapter in my life and a career in research I would like to express my sincere gratitude to Professor Sabu John and Distinguished Professor Milan Brandt for their unequivocal support, guidance, and mentorship, which will never be forgotten Their invaluable knowledge and wisdom have been a beacon of reckoning throughout these years I would like to thank key RMIT staff that assisted me throughout this research: Mr Alan Jones of the Advanced Manufacturing Precinct (AMP) and Dr Matthew Field and Dr Edwin Mayes from the RMIT Microscopy and Microanalysis Facility (RMMF) and Dr Wei Qian Song from the RMIT Bundoora East Material Testing Laboratories I would also like to thank my dearest son Mr Daniel Tamaddon for his patience and understanding during this period and Dr Mohammad Mehdizadeh who encouraged me to start this journey A list of many good friends who supported me throughout this process would be too long for this limited space and includes but is not limited to Dr Joe Elambasseril and Dr Nabi Chowdhury to name a few Ali Tamaddon Page | iii Table of Contents Declaration ii Acknowledgements: iii Table of Contents iv List of Tables viii List of Figures ix Abstract 1 Introduction 1.1 Background 1.2 Motivation 1.3 Project Objectives 1.3.1 Aim 1.3.2 Research Questions 1.3.3 Research Scope 1.4 Thesis Outline 1.5 References Literature Review 10 2.1 Chapter overview 10 2.2 Overview of additive manufacturing (AM) 10 2.3 Metal additive manufacturing 13 2.4 Laser powder bed fusion (LPBF) 15 2.4.1 Powder feedstock 17 2.4.2 Laser fusion 18 2.4.3 Build shielding gas 20 2.5 Laser beam welding (LBW) 23 2.5.1 Introduction to lasers 23 2.5.2 Disk lasers 24 2.5.3 Laser welding 24 2.6 Welding considerations 28 2.7 Microscopy 29 2.8 Research Questions 30 2.9 References 31 Materials and Methodology 35 3.1 Introduction 35 3.2 First Phase: Pilot Samples 35 3.2.1 Design of experiments 35 Ali Tamaddon Page | iv 3.2.2 Material: 36 3.2.3 Fabrication of Laser Powder Bed Fusion (LPBF) parts 38 3.2.4 Laser Beam Welding 41 3.2.5 Cross section and metallographic preparation 44 3.2.6 Microscopy (area, shape) 47 3.2.7 Weld porosity studies 47 3.2.8 EDS (chemical composition) 49 3.2.9 Microscopy (EBSD) 50 3.2.10 Micro hardness 52 3.3 Second Phase: Main Samples 53 3.3.1 Design of experiments 53 3.3.2 Fabrication of parts 55 3.3.3 Pre-testing measurement and testing: 59 3.3.4 Tensile test set up: 59 3.3.5 Fatigue test set up: 62 3.3.6 Post-test examinations: 64 3.4 References 65 Results and Discussion 66 4.1 Introduction 66 4.2 Pilot samples 66 4.2.1 Macroscopy 66 4.2.1.1 Weld appearance and comparison to AWI standards 66 4.2.2 Optical Microscopy 68 4.2.2.1 Weld area measurement, shape, and penetration profile 68 4.2.3 Scanning Electron Microscopy 73 4.2.3.1 EDS and chemical composition results 73 4.2.3.2 Microstructure and EBSD results 75 4.2.4 Porosity analysis 79 4.2.5 Hardness profile 85 4.3 Main samples 89 4.3.1 Design of experiments 89 4.3.2 Tensile tests 90 4.3.2.1 Tensile test setup 90 4.3.2.2 Single piece wrought and LPBF tensile tests 90 4.3.2.3 Porosity analysis – Single piece wrought and LPBF 91 4.3.2.4 Welded wrought to LPBF assembly tensile tests 100 Ali Tamaddon Page | v 4.3.2.4.1 Pre-test porosity analysis – Wrought to LPBF 100 4.3.2.4.2 Tensile tests – Wrought to LPBF 103 4.3.2.4.3 Post-test CT scan – Wrought to LPBF 106 4.3.2.5 Welded LPBF to LPBF assembly tensile tests 109 4.3.2.6 Tensile tests – LPBF to LPBF 110 4.3.2.7 Post-test CT scans – LPBF to LPBF 111 4.3.3 Fatigue tests 116 4.3.3.1 Design of experiments 116 4.3.3.2 Single piece wrought fatigue tests 117 4.3.3.3 Pre-test porosity analysis – single piece wrought 118 4.3.3.4 Fatigue test – single piece wrought 118 4.3.3.5 Post-test SEM – single piece wrought 118 4.3.3.6 Single piece LPBF fatigue tests 121 4.3.3.7 Pre-test porosity analysis – single piece LPBF 121 4.3.3.8 Fatigue test – single piece LPBF 122 4.3.3.9 Post-test CT scan – single piece LPBF 122 4.3.3.10 Post-test SEM – single piece LPBF 123 4.3.3.11 Welded wrought to LPBF assembly fatigue tests 125 4.3.3.12 Pre-test porosity analysis – wrought to LPBF 125 4.3.3.13 Fatigue test – wrought to LPBF 131 4.3.3.14 Post-test CT scan – wrought to LPBF 136 4.3.3.15 Post-test SEM – wrought to LPBF 138 4.3.3.16 Welded LPBF to LPBF assembly fatigue tests 150 4.3.3.17 Pre-test porosity analysis – LPBF to LPBF 150 4.3.3.18 Fatigue test – LPBF to LPBF 151 4.3.3.19 Post-test CT scan – LPBF to LPBF 157 4.3.3.20 Post-test SEM – LPBF to LPBF 158 4.4 References 168 Conclusions 169 5.1 Chapter overview 169 5.2 Welding parameters 169 5.3 Tensile strength and fatigue performance 170 5.4 Microstructural parameters 171 5.5 Future work 172 5.5.1 Dissimilar alloys 172 5.5.2 Post weld processing 172 Ali Tamaddon Page | vi 5.5.3 Build parameters effect on weldability 172 5.5.4 Impact of geometry on weld performance 172 5.5.5 Enhance statistical confidence of results 172 Ali Tamaddon Page | vii List of Tables Table 2-1 (Page 15) Cost comparison of titanium vs steel and aluminium Table 3-1 (Page 36) Primary Design of Experiment for pilot sample weld parameters Table 3-2 (Page 37) Nominal chemical composition of Titanium alloy grade used for this research, as per ASTM F2924-14 Table 3-3 (Page 39) LPBF base material build parameters Table 3-4 (Page 53) Proposed design of experiment for tensile tests as per ASTM E8 Table 3-5 (Page 54) Proposed design of experiment for fatigue tests as per ASTM E466 Table 4-1 (Page 68) Surface colour in titanium welds; as per American Welding Society (AWS) G2.4M:2014 – guide for the fusion welding of titanium and titanium alloys Table 4-2 (Page 72) Summary of weld parameters, resulting energy density and welded joint fusion zone cross section area on pilot samples as measured using digital microscopy Table 4-3 (Page 75) Ti6Al4V raw material properties Table 4-4 (Page 77) Phase identification in fusion zone of pilot sample number Table 4-5 (Page 98) ASTM E8 tensile test results on wrought single piece samples Table 4-6 (Page 105) ASTM E8 tensile test results for LPBF to wrought welded samples Table 4-7 (Page 110) ASTM E8 tensile test results for LPBF-to-LPBF welded samples, note the close proximity of the UTS to breaking strength and relative short overall elongation prior to breakage Table 4-8 (Page 117) ASTM E466 fatigue test design of experiment (DOE) Table 4-9 (Page 118) ASTM E466 fatigue test results for single piece wrought samples Table 4-10 (Page 122) ASTM E466 fatigue test results for single piece LPBF samples Table 4-11 (Page 131) ASTM E466 fatigue test results for LPBF to wrought welded assemblies Table 4-12 (Page 152) ASTM E466 fatigue test results for LPBF to LPBF welded assemblies Ali Tamaddon Page | viii List of Figures Figure 2-1 (Page 11) Single step and multi-step AM process principles (Standard 2012) Figure 2-2 (Page 12) Production of AM parts from independent service providers (in millions of dollars) (Wohlers Report 2021) Figure 2-3 (Page 13) Experience of AM technologies per industry 2019 (%) Courtesy Ernest Young (EY_Global 2019b) Figure 2-4 (Page 14) Overview of single-step AM processing principles for metallic materials courtesy ISO ASTM 52900 (Standard 2012) Figure 2-5 (Page 16) Build rate comparison of small (blue), medium (red) and large (green) machines Courtesy (Khorasani et al 2020) Figure 2-6 (Page 20) Summary of stress (S) versus cycles to failure (N) (S-N) data for PBF (laser), PBF (E-beam), and wire (DED) at R = 0.1 Metallic Materials Properties Development and Standardization (MMPDS) data for cast, wrought machined data are shown for comparison (Lewandowski & Seifi 2016) Figure 2-7 (Page 22) Schematic representation of possible process by-products courtesy (Ladewig et al 2016) Figure 2-8 (Page 22) Schematic of defect induced by recoating during LPBF process Figure 2-9 (Page 23) Four major components of a laser (AWS 2010) Figure 2-10 (Page 24) Principles for thin disk laser (AWS 2010) Figure 2-11 (Page 25) Trump TruLaser Cell 7020 weld setup Laser beam comes from above and nozzle feeds the inert shielding gas to the work area Figure 2-12 (Page 27) Tensile test results related to the porosity ratio in welding area, according to the average power variation (Akman et al 2009) Figure 2-13 (Page 27) Microhardness distribution of workpieces for different average power (Akman et al 2009) Figure 3-1 (Page 37) SEM image of the used Titanium grade (Ti-6Al-4V) powder Figure 3-2 (Page 37) Titanium grade alloy wrought sheets Figure 3-3 (Page 39) SLM Solutions ™ SLM 250, metal additive manufacturing machine Figure 3-4 (Page 40) 20mm x 20mm x 4mm LPBF fabricated Ti64 plates for preliminary welding experiments Figure 3-5 (Page 41) Trumpf TruLaser Cell 7020 welding machine and interface Ali Tamaddon Page | ix Figure 3-6 (Page 42) Custom jig to hold the square samples in compression for welding and diagram of weld direction used Figure 3-7 (Page 42) Welding set up inside the Trumpf Trulaser ™ Cell 7020 robotic laser welding machine chamber Figure 3-8 (Page 43) Trial runs on provisional samples, to verify weld parameters Note the direction of welds marked on far-right sample in image Start (S) to finish (F) Figure 3-9 (Page 44) Work in progress of the pilot sample fabrication Figure 3-10 (Page 45) Struers Labotom-3 manual table-top lab size cutting machine Figure 3-11 (Page 46) Struers CitoPress phenolic resin mounting machine Figure 3-12 (Page 46) Struers RotoPol-21 and RotoForce-4 polishing machine Figure 3-13 (Page 47) Keyence VHX-5000 Digital optical microscope Figure 3-14 (Page 48) Phoenix v| Tome|x s CT scanner at RMIT Bundoora East campus Figure 3-15 (Page 49) Image of VG Studio 3.0 interface Figure 3-16 (Page 50) FEI Quanta 200 Figure 3-17 (Page 52) FEI Nova NanoSEM 200 Figure 3-18 (Page 52) Microhardness measurements performed on cross section Figure 3-19 (Page 55) Work in progress of main samples fabrication in the SLM® 250HL machine Figure 3-20 (left) (Page 56) Additively manufactured tensile testing half specimens as per ASTM E8 prior to welding, note elongated tabs the junction for welding, these are incorporated to accommodate any anomalies occurring at the start and finish of each weld These tabs are machined off and result in a fully compliant geometry as per ASTM E8 (right) wrought titanium alloy half specimens alongside baseline samples, these are used to establish the experiment reference data necessary for this research Figure 3-21 (Page 56) Tensile testing sample geometry (courtesy ASTM E8 standard) Figure 3-22 (Page 57) (left) Additively manufactured fatigue testing half specimens as per ASTM E466 prior to welding Note the provision for welding tabs, similar to tensile testing samples (right) wrought titanium alloy half specimens alongside baseline fatigue full samples Figure 3-23 (Page 57) Fatigue testing sample geometry (courtesy ASTM E466 standard) Figure 3-24 (Page 58) Custom jig to accommodate ASTM E8 and E466 autogenous welding in the Trumpf Trulaser ™ Cell 7020 robotic laser welding machine Ali Tamaddon Page | x Figure 4-91 Sample LL1, subjected to 200MPa of axial load, survived 117,055 cycles The perpendicular cut surface suggests the brittle nature of the fracture and the pores at the perimeter, are sources of crack initiation Results and Discussion Page | 159 Figure 4-92 Sample LL2, subjected to 200MPa of axial loading, underwent 161,869 cycles prior to fracture As typical in LPBF to LPBF welded assemblies in this research, the failure mode is demonstrated to be brittle, with multiple points of fracture initiation at the perimeter of the welded assembly Results and Discussion Page | 160 Figure 4-93 Sample LL3, subjected to 400MPa of axial stress loading, underwent 19,149 cycles prior to failure Bottom right picture shows the starting point of a microcrack that contributed to the brittle fracture of the part Results and Discussion Page | 161 Figure 4-94 Large pores pertaining to incompletely melted particles can be seen as dark patches in the images of sample LL4, which was subjected to 400MPa axial loading and maintained its integrity up to 16,544 cycles Results and Discussion Page | 162 Figure 4-95 Sample LL5, which was subjected to the highest axial loading of this research at 500MPa, withstood 10,746 cycles before showing multiple failures, including a secondary fracture on the side of the body, as seen in the bottom two images Results and Discussion Page | 163 Figure 4-96 LL6, close up image of the fusion zone (FZ) and heat affected zones (HAZ) Note that albeit surface anomalies, the welded assembly was able to maintain its integrity after 1,000,001 cycles of 100MPa axial loading Results and Discussion Page | 164 Figure 4-97 Sample LL7, which failed at the fusion zone at 300MPa axial loading after 4,094 cycles, shows a large cluster of porosities at the centre and perimeter of the fusion zone Results and Discussion Page | 165 Figure 4-98 Sample LL8 which also failed at the fusion zone at 300MPa axial loading after only 2,264 cycles, has multiple large pores on the surface and perimeter of the fractured area, contributing to the failure of the part Results and Discussion Page | 166 Figure 4-99 Sample LL9, which failed at the fusion zone after 3,965 cycles at 200MPa axial force, also demonstrates large pores scattered all over the face of the fractured surface and perimeter Results and Discussion Page | 167 4.4 References AWS, AWS- 2014, Guide for the Fusion Welding of Titanium and Titanium Alloys, G 2.4M:2014, Leuders, S, Vollmer, M, Brenne, F, Tröster, T & Niendorf, T 2015, 'Fatigue Strength Prediction for Titanium Alloy TiAl6V4 Manufactured by Selective Laser Melting', Metallurgical and Materials Transactions A, vol 46, no 9, pp 3816-3823 Liinalampi, S, Remes, H & Romanoff, J 2019, 'Influence of three-dimensional weld undercut geometry on fatigue-effective stress', Welding in the World, vol 63, no 2, pp 277-291 Ninh Nguyen, T & Wahab, MA 1995, 'A theoretical study of the effect of weld geometry parameters on fatigue crack propagation life', Engineering Fracture Mechanics, vol 51, no 1, pp 1-18 Yung, WKC, Ralph, B, Lee, WB & Fenn, R 1997, 'An investigation into welding parameters affecting the tensile properties of titanium welds', Journal of Materials Processing Technology, vol 63, no 1, pp 759-764 Results and Discussion Page | 168 Conclusions 5.1 Chapter overview Additive manufacturing provides many benefits to the application of titanium alloys, including the cost-effective fabrication of parts in complex geometrical shapes and with minimal material wastage These benefits are sometimes overshadowed by dimensional limitations, and relatively slow speeds of mass production, inherent to additive manufacturing, compared to traditional subtractive manufacturing methods This research explores the possibility of overcoming such limitations by welding additively manufactured titanium alloy parts to wrought and nominally larger components of same material and investigates the governing parameters that influence the success of such an endeavour To the date of this writing, no prior literature exists, to the best knowledge of the author, which addresses the study of laser beam welding of additively manufactured titanium alloys with wrought titanium alloys 5.2 Welding parameters Based on literature available on the fabrication of Ti6Al4V grade titanium alloys, a set of standard parameters were chosen for the fabrication of additively manufactured titanium alloy samples Also based on the literature available on welding of wrought titanium alloy sheets, various welding parameters were elected and included in a design of experiment at the pilot stage of this research The variation of such parameters and their effects of on weld morphology, porosity distribution and hardness profile were documented The main parameters effecting the characteristics of an autogenous welded LPBF (laser powder bed fusion), and wrought assembly were established to be energy density and the build quality of the LPBF parts Additively manufactured parts using laser powder bed fusion (LPBF) technology are prone to process parameter defects that are inherent to the nature of the build process Process induced defects such as lack of complete melting of the feedstock powder, inclusions, porosities and creation of microcracks being the main types of such defects The rough surface of additively manufactured parts due to the granular texture resulting from the layer-by-layer melting of powder feedstock is also another contributor to the overall quality of the LPBF manufactured parts In this research, it has been demonstrated that where the welded joint strength is sufficient and free from defects, failure of the assemblies is most likely to happen at areas within the LPBF base metal that have initial build Conclusions Page | 169 quality issues Energy density can be controlled via the power output of the laser and the scanning speed of the laser LPBF build quality depends on many parameters such as feedstock quality, laser power, and hatch spacing, that have been thoroughly examined by prior literature available High quality LPBF are homogeneous and dense and have minimal discontinuities, inclusions or microcracks Too little energy density will prevent the creation of the weld keyhole and consequently there will not be a full penetration weld across the entire depth of the welded assembly On the other hand, too much energy density during welding will result in the increased evaporation of the molten pool, consequently increasing the risk of porosities and increasing the width of the heat affected zones (HAZ), leading to a decrease in ductility and increase in embrittlement, thus heightening the risk of failure at the welded joint High energy density of laser beam welds creates narrow, concentrated, parallel walled fusion zones that are best optimised for full penetration, but minimised to limit the width of the heat affected zones Using statistical methods; it was established that there is a direct linear relationship between energy density and the width of the heat affected zones 5.3 Tensile strength and fatigue performance Laser powder bed fusion (LPBF) parts undergo cyclic thermal heating and cooling during the build process, which results in changes in the microstructure and eventual increase in overall tensile strength of the base metal, due to the creation of martensitic microstructure resulting from rapid cooling of the molten pool Martensite microstructures result in reduction in ductility Full penetration welding of LPBF and wrought parts results also contributes to the creation of further martensite microstructure and increase in the tensile strength of the joint The increase in tensile strength also is accompanied with the increase in material microhardness, with the highest microhardness being recorded at the fusion zone, then the LPBF and then wrought side of the assembly During this research it was demonstrated that LPBF to wrought welded assemblies have superior tensile performance with the failures mostly being recorded at the wrought and weaker side of the assembly This can be attributed to the distribution of higher microhardness and tensile strength at the LPBF base metal and fusion zone in the assemblies Conclusions Page | 170 Fatigue performance is highly dependent on the presence of material discontinuities, such as porosities, inclusions, microcracks and any other points of stress concentration, as well the presence of brittle alpha-prime martensite This research demonstrated that both LPBF to wrought and LPBF to LPBF welded assemblies are most vulnerable to porosities at the LPBF base material and LPBF HAZ, which has a high concentration of alpha-prime martensite 5.4 Microstructural parameters Welding LPBF and wrought titanium alloys induces further thermal stresses resulting in acicular (needle-shaped) alpha-prime crystals in the fusion zone and a mixture of martensitic alpha-prime, acicular alpha and primary alpha at the heat affected zones, this is due to quenching below the beta-transus temperature Brittle alpha prime martensite is most dominant at the HAZ of LPBF assembly and are most prone to fatigue failure Porosity and microstructure of the assembly define the mechanical tensile strength and fatigue performance of the welded structure, as evident in the failure modes witnessed during the tests Fusion zones and LPBF side of a welded assembly showed greater strength during tensile tests, due to the existence of martensite phases in such areas Excessive heat concentration and remelting of LPBF at welded joints with other LPBF created the greatest opportunity for failure via the creation of large pores and material discontinuity, resulting in points of high stress concentration and opportunity for crack growth and propagation High welding energy densities beyond what is required for the full penetration of the weld, will result in the increased share of elongated acicular alpha-prime martensite which exhibit brittle behaviour Excessive energy densities also result in the increase in the width of the heat affected zone (HAZ), which hosts a combination of primary alpha, hexagonal close packed (HCP) alpha-prime and acicular alpha and demonstrate brittle behaviour Excessive energy densities also increase the evaporation of material and increase the chance of porosities and eventual creation of microcracks Therefore, it is recommended that autogenous weld parameters be chosen such that optimise for minimal energy densities, whilst guaranteeing full depth penetration The welding parameters for this investigation were 2200 Watts at 1200mm/ for LPBF Ti6Al4V alloys of 4mm thickness Conclusions Page | 171 5.5 Future work The following work is proposed to enhance the understanding of challenges in manufacturing large hybrid additive and traditional fabricated assemblies: 5.5.1 Dissimilar alloys By investigating the prospects of having large assemblies made by joining titanium alloys in wrought and additively manufactured format together, new opportunities for more complex configurations come to mind, such as joining titanium Ti6Al4V alloys to other alloys such as Aluminium or copper-rich alloys in wrought or additively manufactured format 5.5.2 Post weld processing With the limitation of as-built welded assemblies documented in this research, it would be beneficial to investigate the effects of post processing activities, such as heat treatment on tensile properties and fatigue life of wrought and additively manufactured parts The economics and feasibility of such postprocessing should be taken into account 5.5.3 Build parameters effect on weldability Investigate effects of base metal build parameters on weldability, tensile and fatigue properties of the final assembly This study would expand the design of experiments to include the parameters used to fabricate the base metal (BM) additively manufactured parts and find their impact on the final mechanical and fatigue performance 5.5.4 Impact of geometry on weld performance Investigate effects of the geometry of LPBF and wrought parts on the weldability, tensile and fatigue properties of the final assembly This study would expand the design of experiments to include the geometry of the parts being welded together to find the impact of various orientations and shapes of the base metal on the final mechanical and fatigue performance 5.5.5 Enhance statistical confidence of results Increase the statical confidence of the research by increasing the number of parts fabricated and tests conducted Conclusions Page | 172 Conclusions Page | 173 ... 50.00 2.4 Laser powder bed fusion (LPBF) Laser powder bed fusion (LPBF) technology employs, as the naming suggests, a bed of powder that is selectively melted layer upon layer by high power laser, ... as laser power level and scanning speed can affect the overall performance of titanium alloy assemblies made using laser powder bed fused (LPBF) and wrought parts welded together using a disc laser. .. across several publications to compare the fatigue and mechanical properties of parts fabricated using laser powder bed fusion (LPBF) and electron beam fusion (EBF) methods Figure 2-6, graphically