Title page for ETD etd-11242004-211009

Type of Document Dissertation
Author Kelly, Shawn Michael
Author's Email Address smkellyvt@gmail.com
URN etd-11242004-211009
Title Thermal and Microstructure Modeling of Metal Deposition Processes with Application to Ti-6Al-4V
Degree PhD
Department Materials Science and Engineering
Advisory Committee
Advisor Name Title
Kampe, Stephen L. Committee Chair
Babu, Sudarsanam Suresh Committee Co-Chair
Aning, Alexander O. Committee Member
Christodoulou, Leonitus Committee Member
David, Stan A. Committee Member
Reynolds, William T. Jr. Committee Member
  • additive manufacturing
  • titanium alloy
  • metal deposition
  • ti-6al-4v
  • heat treatment
  • microstructure model
  • near net shape
  • thermal model
  • rapid prototyping
Date of Defense 2004-11-12
Availability unrestricted
Laser metal deposition (LMD) offers a unique combination of process flexibility, time savings, and reduced cost in producing titanium alloy components. The current challenge in processing titanium alloys using LMD methods is understanding the complex microstructure evolution as a part is fabricated layer by layer. The current work focuses on the characterization, thermal, and microstructural modeling of multilayered Ti-6Al-4V deposits. A thermal model has been developed using finite difference techniques to predict the thermal history of LMD processes. A microstructure model that predicts the alpha phase fraction and morphology evolution was constructed to quantify the effect of thermal cycling on the as-deposited microstructure evolution. Alpha dissolution and growth are modeled assuming one-dimensional plate dissolution according to a parabolic rate law, and a Johnson-Mehl-Avrami-Kolmorgorov (JMAK) nucleation and growth model, respectively. Alpha morphology (colony-alpha and basketweave-alpha) evolution is tracked using a simplistic approach.

Characterization of the deposit has shown that a complex microstructure evolves consisting of a two distinct regions: a transient region of undeveloped microstructure and a characteristic layer that is periodically repeated throughout the deposit. The transient region contains a fine basketweave and colony-alpha morphology. The characteristic layer contains a two phase mixture of alpha+beta, with the alpha phase exhibits regions of colony-alpha (layer band) and basketweave-alpha morphology.

The different regions of microstructural contrast in the deposit are associated with thermal cycling. The thermal model results show that a heat affected zone defined by the beta transus extends approximately 3 layers into the deposit. The phase fraction model predicts the greatest variation in microstructural evolution to occur in a layer n after the deposition of layer n+3. The results of the morphology model show that increased amounts of colony-alpha form near the top of a characteristic layer. It follows that a layer band (colony-alpha region) forms as a result of heating a region of material to a peak temperature just below the beta transus, where a large amount of primary-alpha dissolves. Upon cooling, colony-alpha forms intragranularly. The coupled thermal and microstructure models offer a way to quantitatively map microstructure during LMD processing of Ti-6Al-4V.

  Filename       Size       Approximate Download Time (Hours:Minutes:Seconds) 
 28.8 Modem   56K Modem   ISDN (64 Kb)   ISDN (128 Kb)   Higher-speed Access 
  Figure_4_10_Last_4_Layers_Movie.mov 27.56 Mb 02:07:34 01:05:36 00:57:24 00:28:42 00:02:26
  Figure_6_12_Temperature_Movie.mov 139.41 Mb 10:45:23 05:31:55 04:50:25 02:25:12 00:12:23
  Figure_8_4_Phase_Fraction_Movie.mov 163.76 Mb 12:38:08 06:29:53 05:41:09 02:50:34 00:14:33
  Figure_8_7_Morphology_Movie.mov 203.41 Mb 15:41:42 08:04:18 07:03:46 03:31:53 00:18:04
  SM_Kelly_Dissertation_Final.pdf 232.58 Mb 17:56:45 09:13:45 08:04:32 04:02:16 00:20:40

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