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Title: Modelling and analysis of laser direct metal deposition of Ti-6Al-4V alloy
Author: Ahsan, Muhammad Naveed
ISNI:       0000 0004 2714 0425
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2011
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A major strength of the laser direct metal deposition (LDMD) rapid manufacturing technique is its ability to manufacture freeform shapes, to directly create different surface coatings on a part, and to produce parts from graded porous to fully dense solid structures. This technique is gaining popularity in the fields of aerospace and biomedical manufacture due to its flexibility and cost effectiveness. Surface coating and repair are its biggest application in industry. Previous models of surface layer coating predicted heat flow in the substrate and the resulting temperature distributions successfully, however layer geometry predictions were absent or incomplete. Here, an analytical modelling method of surface layer coating has been presented which accounts for track interactions and uses a novel powder mass affinity factor in multiple track laser direct metal deposition. The model has been validated by a series of surface layer coating experiments using Ti-6Al-4V alloy. Surface layer characteristics in terms of layer profile, layer thickness and layer roughness have been compared. The model is, then, extended from multiple tracks to multilayer structure fabrication. The multilayer porous structures have been fabricated by using the laser in continuous-wave and pulsed-wave modes for potential use in biomedical applications. In the second part of the work, a coupled analytical-numerical solution has been developed for the single track deposition model described earlier. Laser direct metal deposition is a complex process involving multiple interdependent processes which can be best simulated using a fully coupled mass-energy balance solution and the model removes the shortcoming of previous models where energy and mass balance equations are solved in a decoupled manner. The model has been applied to find out temperature distributions, track profile and microstructure scale, its experimental validation makes it convincing. The model is quite efficient as compared to finite element methods and a useful industrial aid for selecting the parameters to use for laser direct metal deposition when separate geometric and microstructural outcomes are required. In the third part of the work, a comparative study of LDMD characteristics using GA (gas-atomised) and PREP (plasma rotating electrode process) Ti-6Al-4V powders has been presented. The LDMD characteristics in terms of geometric dimensions, surface finish, microstructure, micro hardness and any defects such as intralayer porosity are compared and it is concluded that PREP powder deposition has some potential benefits when high value components are to be built, for example, aerospace applications. This study has also exposed the scientific understanding on the causes of intralayer porosity generation behaviour in laser direct metal deposition. To minimize the intralayer porosity, optimum combinations of processing parameters/ conditions and characteristics of the initial powder, have been proposed for practical engineering applications.
Supervisor: Pinkerton, Andrew Sponsor: DESTO, Pakistan
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available