Ti6Al4V is the most commonly used of the titanium alloys and is known for its high strength to weight ratio and superb corrosion resistance compared to conventional steels. Ti6Al4V is used in applications in the aerospace, biomedical, automotive, power generation and oil and gas fields. Laser metal deposition (LMD) is an additive manufacturing (AM) platform used to build 3-D metal shapes. LMD is one of the most researched topics within the laser processing field currently and is advancing continuously. The rapid growth in the AM field is driven by market demands to reduce manufacturing costs, shorter lead times and an increasing demand for customized products. One of the major challenges facing the production of Ti6Al4V components using LMD is the high resultant residual stresses, limiting build size due to delamination or distortion. At the commencement of this study, little data pertaining to the residual stress build up in larger LMD components was available. This research was conducted to create an understanding of the relationship between build height and surface residual stresses and how they influence the dimensional stability of a part. Additionally, the relationship between build height and static mechanical properties was analysed. The effects of laser power, scanning speed and powder mass flow rate on the deposition layer were evaluated. The number of defects and the deposition build height were evaluated to determine the optimum process parameters for multi-layer components. An increase in laser power resulted in an increase in build height for the parameter window selected for the study. Similarly, an increase in build height was observed with an increase in powder mass flow rate, while an increase in scanning speed resulted in a decrease in build height. As laser power and scanning speed had inverse effects on the build height, heat input was evaluated to determine the optimum combination of the 2 parameters. Multilayer samples were produced with a laser power setting of 1900 W, a scanning speed of 0.01 m/s and a powder mass flow rate of 8 g/min. Fully dense components were produced with no notable defects. These components were analysed to reveal the relationship between build height and surface residual stresses and showed that the minimum residual stress observed in a component was related to an actual height from the base and was not affected by the build height of the sample. Maximum residual stresses were observed closest to the base of the cylinder and the stresses were larger in larger samples for both hoop and longitudinal surface residual stress. The micro-hardness of the samples increased as build height increased. The tensile strength remained within constant range between 1080 MPa and 1050 MPa for all samples successfully tested. Brittle failures were observed on the upper sections of the larger samples, attributed to the high micro-hardness observed in these areas. The study successfully evaluated the relationship between build height and surface residual stresses as well as build height and static mechanical properties thereby increasing the knowledge within this field.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nmmu/vital:30577 |
| Date | January 2018 |
| Creators | Swan, Lindsay Jane |
| Publisher | Nelson Mandela University, Faculty of Engineering, The Built Environment and Information Technology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Masters, MEng |
| Format | xxii, 146 leaves : color illustrations, pdf |
| Rights | Nelson Mandela University |
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